Author name code: hudson ADS astronomy entries on 2022-09-14 author:"Hudson, Hugh S." ------------------------------------------------------------------------ Title: Observations of Thomson Scattering from a Loop-prominence System Authors: Martínez Oliveros, Juan Carlos; Guevara Gómez, Juan Camilo; Saint-Hilaire, Pascal; Hudson, Hugh; Krucker, Säm Bibcode: 2022ApJ...936...56M Altcode: 2022arXiv220806007M We describe observations of the white-light structures in the low corona following the X8.2 flare SOL 2017-09-10, as observed in full Stokes parameters by the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. These data show both bright loops and a diffuse emission region above them. We interpret the loops as the white-light counterpart of a classical loop-prominence system, intermediate between the hot X-ray loops and coronal rain. The diffuse emission external to the loops is linearly polarized and has a natural interpretation in terms of Thomson scattering from the hot plasma seen prior to its cooling and recombination. The polarimetric data from HMI enable us to distinguish this contribution of scattering from the HMI pseudocontinuum measurement, and to make a direct estimation of the coronal mass in the polarized source. For a snapshot at 16:19 UT, we estimate a mass 8 × 1014 g. We further conclude that the volumetric filling factor of this source is near unity. Title: Fast prograde coronal flows in solar active regions Authors: Hudson, Hugh S.; Mulay, Sargam M.; Fletcher, Lyndsay; Docherty, Jennifer; Fitzpatrick, Jimmy; Pike, Eleanor; Strong, Morven; Chamberlin, Phillip C.; Woods, Thomas N. Bibcode: 2022MNRAS.515L..84H Altcode: 2022MNRAS.tmpL..75H; 2022arXiv220713461H We report the discovery and characterization of high-speed (>100 km s-1) horizontal flows in solar active regions, making use of the Sun-as-a-star spectroscopy in the range 5-105 nm provided by the EVE (Extreme Ultraviolet Variability Experiment) spectrometers on the Solar Dynamics Observatory. These apparent flows are persistent on time-scales of days, and are well observed in lines of Mg X, Si XII, and Fe XVI for example. They are prograde, as evidenced directly by blueshifts/redshifts peaking at the east/west limb passages of isolated active regions. The high-speed flow behaviour does not depend upon active-region latitude or solar cycle, with similar behaviour in Cycles 24 and 25. Title: The Eclipse Megamovie Project (2017) Authors: Hudson, Hugh S.; Peticolas, Laura; Johnson, Calvin; White, Vivian; Bender, Mark; Pasachoff, Jay M.; Martínez Oliveros, Juan Carlos; Collier, Braxton; Filippenko, Alexei V.; Filippenko, Noelle; Fraknoi, Andrew; Guevara Gómez, Juan Camilo; Koh, Justin; Konerding, David; Krista, Larisza; Kruse, Brian; McIntosh, Scott; Mendez, Brian; Ruderman, Igor; Yan, Darlene; Zevin, Dan Bibcode: 2022arXiv220713704H Altcode: The total solar eclipse of August 21, 2017, crossed the whole width of North America, the first occasion for this during the modern age of consumer electronics. Accordingly, it became a great opportunity to engage the public and to enlist volunteer observers with relatively high-level equipment; our program ("Eclipse Megamovie") took advantage of this as a means of creating a first-ever public database of such eclipse photography. This resulted in a large outreach program, involving many hundreds of individuals, supported almost entirely on a volunteer basis and with the institutional help of Google, the Astronomical Society of the Pacific, and the University of California, Berkeley. The project home page is \url{http://eclipsemegamovie.org}, which contains the movie itself. We hope that our comments here will help with planning for similar activities in the total eclipse of April 8, 2024. Title: NuSTAR observations of a quiet Sun minifilament eruption Authors: Hannah, Iain; Sterling, Alphonse; Grefenstette, Brian; Glesener, Lindsay; White, Stephen; Smith, David; Cooper, Kristopher; Krucker, Sam; Paterson, Sarah; Hudson, Hugh Bibcode: 2022cosp...44.2538H Altcode: We present a unique set of observations of a confined minifilament eruption from the quiet-Sun during solar minimum. The Nuclear Spectroscopic Telescope Array (NuSTAR) spotted a tiny, compact hard X-ray (HXR) flare on 2019 April 26, peaking about 02:06UT for a few minutes, finding brief emission >5MK. Observations with SDO/AIA and Hinode/XRT show this HXR emission was due to a tiny flare arcade underneath a confined minifilament eruption - behaviour similar to those seen in both major active-region filament eruptions and minifilament eruptions that lead to coronal jets. Line-of-sight magnetograms from SDO/HMI show that this eruption is due to opposite polarity flux moving together and cancelling and not due to flux emergence. This eruption occurred near disk-centre, so the Earth orbiting observatories provide a top-down view of the event, but fortuitously a side-on view is obtained from STEREO-A/SECCHI, giving a clearer sense of eruption geometry. We also explore the possibility of non-thermal emission due to accelerated electrons in the NuSTAR HXR observations of this small-scale phenomena in the quiet Sun. Title: Detecting stellar CMEs through post-flare coronal dimmings Authors: Veronig, Astrid; Hudson, Hugh S.; Odert, Petra; Leitzinger, Martin; Dissauer, Karin; Fleck, Nikolaus Bibcode: 2022cosp...44.1379V Altcode: Coronal dimmings are sudden decreases of the solar EUV and X-ray emission caused by coronal mass ejections (CMEs). Dimming regions map to the bipolar ends of closed magnetic field lines that become stretched or temporarily opened during an eruption, and are a result of the depletion of coronal plasma caused by the expansion and mass loss due to the CME. Recently available multi-point imagery from satellites at different locations in the heliosphere provided us with unprecedented observations of the three-dimensional evolution of solar CMEs and their coronal dimmings. These studies showed distinct correlations between CME mass and speed with key parameters of the associated coronal dimmings such as their spatial extent and intensity drop. While CMEs from our Sun are regularly imaged by white-light coronagraphs, and their speeds and masses are derived from these observations, for stars such direct imaging is not possible. Here, we present a new approach to detect stellar mass ejections through post-flare coronal dimmings. To this aim, we study Sun-as-a-star broad-band EUV light curves derived from SDO's Extreme ultraviolet Variability Experiment (EVE) as a testbed to investigate whether coronal dimmings can be also observed on stars and used for stellar CME detection. We demonstrate that large eruptive flares are with a high probability associated with a post-flare coronal dimming, with intensity drops in the 15-25 nm full-Sun light curves up to 5%. Searching for similar patterns of post-flare dimmings in the X-ray and EUV light curves of solar-like and late-type stars, we identify 21 stellar CME candidates. The derived intensity drops are an order of magnitude larger than for the Sun, suggesting that a substantial part of the stellar corona gets ejected by the CME. This study is published in: A.M. Veronig, P. Odert, M. Leitzinger, K. Dissauer, N. Fleck, H.S. Hudson, Indications of stellar coronal mass ejections through coronal dimmings, Nature Astronomy 5, 697-706 (2021). https://www.nature.com/articles/s41550-021-01345-9 Title: NuSTAR observations of small-scale phenomena in the quiet Sun Authors: Paterson, Sarah; Krucker, Samuel; Grefenstette, Brian; Glesener, Lindsay; Hannah, Iain; Smith, David; Hudson, Hugh Bibcode: 2022cosp...44.2561P Altcode: NuSTAR is a sensitive hard X-ray (HXR) focusing telescope capable of observing the faint emission from small-scale phenomena in the quiet Sun. During the recent solar minimum, NuSTAR was used several times to observe the quiet Sun, providing the unique opportunity to perform imaging spectroscopy on very faint solar HXR sources. We present analysis on several small features from the NuSTAR 28 September 2018 full disk solar mosaics, including X-ray/coronal bright points, a jet, and an emerging flux region that later went on to become an active region. This is the first time these features have been observed with an HXR imaging spectrometer. To investigate the contribution of these quiet Sun features to heating the solar atmosphere, we determine their thermal properties from their X-ray spectra. We combine the X-ray data from NuSTAR with EUV data from SDO/AIA and soft X-ray data from Hinode/XRT by reconstructing their differential emission measures in order to investigate the multithermal temperature evolution of these small-scale phenomena. Title: Variability of Lyman-alpha Emission During Solar Flares and Implications for Planetary Atmospheres Authors: Milligan, Ryan; Hudson, Hugh S.; Chamberlin, Phillip; Hayes, Laura Bibcode: 2022cosp...44..833M Altcode: Despite decades of observations of the Sun in the Lyman-alpha (Ly$\alpha$; 1216\AA) line of neutral hydrogen - the strongest emission line in the solar spectrum - few instruments have had the sensitivity, cadence, or duty cycle to measure changes in the solar irradiance at this wavelength on the timescales of solar flares. The few flare observations that previously existed were often contradictory, and the ionospheric impacts of enhanced Ly$\alpha$ irradiance were often deemed to be negligible in comparison with the corresponding increase in soft X-rays. With the availability of spatially and spectrally integrated Ly$\alpha$ flare observations from GOES/EUVS, SDO/EVE, MAVEN/EUM, and PROBA2/LYRA during Solar Cycle 24, there has been a resurgence of interest in this fundamental chromospheric emission line. Statistical studies by Milligan et al. (2020) and Milligan (2021) have shown that, despite increases in irradiance of only a few percent, Ly$\alpha$ emission may be responsible for inducing currents in the E-layer of the ionosphere (the so-called magnetic crochet, or solar flare effect, Sfe) due the ionoisation of NO, while weaker flares were found to produce greater enhancements of Ly$\alpha$ emission than some larger events when associated with failed filament eruptions, pointing to a possible coronal origin. This presentation will include an overview of Ly$\alpha$ flare observations to date, the outstanding questions, and how these may be resolved with the influx of Ly$\alpha$ flare observations anticipated from GOES/EXIS, Solar Orbiter, Solar-C, and ASO-S during Solar Cycle 25. Title: Solar Guidance for Stellar CMEs Authors: Hudson, Hugh Bibcode: 2022cosp...44.1356H Altcode: Solar influences on the Earth involve flares, CMEs, and SEPs in particular, but the latter involve coronagraphic and in-situ observational tools that we don't have in the stellar case. SEP ions ("solar cosmic rays") in particular have no remote-sensing astronomical signatures, since gamma-ray detection is so difficult. This also applies to extreme events analogous to those detectable in the terrestrial radioisotope history. One ray of light is that Sun-as-a-star EUV observations from the SDO/EVE instrument provide an excellent basis for assessing stellar CME occurrence via the dimming signature. I describe the solar observations and place them within the context of other solar (hence often spatially resolved) signatures, as discussed by Hudson & Cliver (2001JGR...10625199H) in terms of "calibrating'' the relationships of such proxies with direct CME observations. Recently dimming signatures of stellar CMEs by this method have been confirmed by Veronig et al. (2021NatAs...5..697V). Title: Revisting the Orrall-Zirker Effect: Identifying the suprathermal proton distribution during solar flares from Lyman line emission Authors: Kerr, Graham; Allred, Joel; Milligan, Ryan; Kowalski, Adam; Hudson, Hugh Bibcode: 2021AGUFMSH23B..04K Altcode: It is likely that ions are accelerated during solar flares. However, due in large part to a lack of observational constraints on the suprathermal ion population in flares, they are not usually considered in energy transport models, with the focus being on flare accelerated electrons. Gamma-ray observations are required to constrain the high energy (MeV) protons, but lower energy (deka-keV to 1 MeV) protons can potentially be detected through the Orrall-Zirker effect (Orrall & Zirker, 1976). Suprathermal protons undergo charge exchange with ambient neutral hydrogen, creating a population of suprathermal neutral hydrogen. These energetic neutrals can subsequently emit extremely Doppler shifted photons. The appearance of a very broad redshifted feature in the far red wings of certain spectral lines can indicate the presence of suprathermal ions, and the properties of the feature has diagnostic potential of the distribution of those ions (e.g. Brosius & Woodgate 1999). This effect is revisited here using modern state-of-the-art flare simulations that track the ionisation stratification and suprathermal proton distribution as a function of time in proton beam driven flares (RADYN+FP, Allred et al 2020), and using up-to-date charge exchange cross sections. We have developed a post-processing radiation transfer code (OrrallZirkerPy) that takes those flare atmospheres as input and makes time-dependent predictions of red-shifted features. The characteristics of non-thermal emission of Lyman alpha and Lyman beta, and their potential as diagnostics of flare accelerated protons, are presented. These predictions are of particular interest now that we have current and planned missions that can observe these lines during solar flares (e.g. SolO/SPICE, SDO/EVE, EUVST, SNIFS). Title: Sun-as-a-star Spectral Irradiance Observations of Transiting Active Regions: a Milestone for Characterization of Stellar Active Regions Authors: Toriumi, Shin; Airapetian, Vladimir; Hudson, Hugh; Schrijver, Karel; Cheung, Chun Ming Mark; DeRosa, Marc Bibcode: 2021AGUFM.U43B..05T Altcode: Recent observations have revealed that solar-type stars can produce massive "superflares". The strongest flares on the Sun are almost always associated with large, complex, rapidly-evolving active regions (ARs) including sunspots. Therefore, to understand why and how stellar flares and coronal eruptions occur, which may directly determine the circumstances of exoplanets, it is critically important to gain information on stellar ARs. One possible way to do so is to monitor the star in multiple wavelengths. In this study, we perform multi-wavelength irradiance monitoring of transiting solar ARs by using full-disk observational (i.e. Sun-as-a-star) data from four satellites. We find that the near UV light curves show strong correlations with photospheric total magnetic flux and that there are time lags between the coronal and photospheric light curves when ARs are close to the limb. Such time lags result from high-arching, bright coronal loops above stellar ARs being visible even when the AR is behind the limb. It is also found that the EUV light curves sensitive to transition-region temperatures are sometimes dimmed because of a reduction in the emission measure of 0.60.8 MK due to the plasma being heated to higher temperatures over a wide area around the AR. These results indicate that, by measuring the stellar light curves in multiple wavelengths, we may obtain information on the structures and evolution of stellar ARs. Title: NuSTAR Observations of a Repeatedly Microflaring Active Region Authors: Cooper, Kristopher; Hannah, Iain; Grefenstette, Brian; Glesener, Lindsay; Krucker, Sam; Hudson, Hugh; White, Stephen; Smith, David; Duncan, Jessie Bibcode: 2021AGUFMSH22B..03C Altcode: We present observations of microflares from Sep 9-10 2018 in X-rays with NuSTAR, EUV with SDO/AIA, and photosphere magnetic field with SDO/HMI. We investigate how the energy released in these small flares contributes to the heating of the solar atmosphere and the role flux cancellation/emergence plays during onset and occurrence of the microflares. 10 microflares were studied from AR12721, all equivalent to GOES <A1 Class, and we find that many contain plasma heated to 5-10 MK with energies of 10261028 erg. One particularly small microflare, equivalent to GOES Class A0.005, demonstrated emission from 6.7 MK plasma with a thermal energy of 1.11026 erg. Another microflare, equivalent to GOES Class A0.1, showed clear non-thermal emission in the X-ray spectra, with non-thermal energy of 1.31027 erg, making it one of the faintest non-thermal X-ray microflares on record. For 8 of the 10 microflares we can identify areas of magnetic flux cancellation/emergence at the footpoints, indicating the role these play in this active region repeatedly producing microflares. Title: The eclipse Megamovie Project (2017) Authors: Hudson, Hugh; Peticolas, Laura; Johnson, Calvin; White, Vivian; Bender, Mark; Pasachoff, Jay M.; Oliveros, Juan Carlos Martínez; Filippenko, Alexei V.; et al. Bibcode: 2021JAHH...24.1080H Altcode: The total solar eclipse of August 21, 2017, crossed the whole width of North America, the first occasion for this during the modern age of consumer electronics. Accordingly, it became a great opportunity to engage the public and to enlist volunteer observers with relatively high-level equipment; our program ('Eclipse Megamovie') took advantage of this as a means of creating a first-ever public database of such eclipse photography. This resulted in a large outreach program, involving more than one thousand individuals, supported almost entirely on a volunteer basis and with the institutional help of Google, the Astronomical Society of the Pacific, and the University of California, Berkeley. The project home page at eclipsemegamovie.org contains the finished movie itself. We hope that our comments here will help with planning for similar activities during the total eclipse of 8 April 2024. Title: Thomson Scattering in the Lower Corona in the Presence of Sunspots Authors: Saint-Hilaire, Pascal; Martínez Oliveros, Juan Carlos; Hudson, Hugh S. Bibcode: 2021ApJ...923..276S Altcode: Polarized scattered light from low (few tens of megameter altitudes) coronal transients has been recently reported in Solar Dynamics Observatory/Helioseismic and Magnetic Image (HMI) observations. In a classic paper, Minnaert (1930) provided an analytic theory of polarization via electron scattering in the corona. His work assumed axisymmetric input from the photosphere with a single-parameter limb-darkening function. This diagnostic has recently been used to estimate the free-electron number and mass of HMI transients near the solar limb, but it applies equally well to any coronal material, at any height. Here we extend his work numerically to incorporate sunspots, which can strongly effect the polarization properties of the scattered light in the low corona. Sunspot effects are explored first for axisymmetric model cases, and then applied to the full description of two sunspot groups as observed by HMI. We find that (1) as previously reported by Minnaert, limb darkening has a strong influence, usually increasing the level of linear polarization tangential to the limb; (2) unsurprisingly, the effects of the sunspot generally increase at the lower scatterer altitudes, and increase the larger the sunspot is and the closer to their center the scatterer subpoint is; (3) assuming the Stokes Q > 0 basis to be tangential to the limb, sunspots typically decrease the Stokes Q/I polarization and the perceived electron densities below the spotless case, sometimes dramatically; and (4) typically, a sizeable non-zero Stokes U/I polarization component will appear when a sunspot's influence becomes non-negligible. However, that is not true in rare cases of extreme symmetry (e.g., scattering mass at the center of an axisymmetric sunspot). The tools developed here are generally applicable to an arbitrary image input. Title: Hot onsets of solar flares Authors: Hudson, Hugh; Fletcher, Lyndsay; Hannah, Iain; Hayes, Laura; Simoes, Paulo Bibcode: 2021AGUFMSH22B..02H Altcode: The GOES/XRS data show low-level soft X-ray emissions prior to a flare in a "hot onset" precursor of the main flare development (2021MNRAS.501.1273H). This phenomenon provides clear evidence for energy release not identifiable with the impulsive phase, and apparently not mediated by non-thermal particles. The hot onset phase may last for tens of seconds to minutes, characterized by gradual and roughly linear growth of emission measure at characteristic isothermal temperatures of 10-15 MK and no clear pattern of temperature increase in the sense of dT/dt > 0. The figure shows an example of this behavior, from SOL2004-02-26 (X1.1). The newer GOES-R data also show this effect, providing higher time resolution (1 s) and better noise properties, although with higher background levels. Hot onsets occur in most if not all flares, but their properties do not appear to provide any guidance towards the magnitude of the flare that follows. Title: Finding Fast Gamma-ray Variability in Solar Flares Authors: Zeitohn, Hind; Briggs, Michael; Veres, Peter; Hudson, Hugh; Lesage, Stephen Bibcode: 2021AGUFMSH35E2122Z Altcode: Fast Radio Bursts (FRBs) are fast, bright, extragalactic transients, typically associated with magnetars. In early 2021, a very sensitive radio array observed a unique FRB-like event which localized to the Sun. This solar-FRB (sFRB) lasted for only a few milliseconds at 1.4 GHz and had a flux density of 9.1 Mega-Janskys (910 solar flux). This millisecond long sFRB was seen as a "spike" in the radio data, which raises the question, can sFRBs be detected in other wavelengths? Fermi-GBM has a 4 pi steradian field of view of the sky and measures gamma-rays down to 2 microseconds temporal resolution, which makes it the perfect instrument for detecting sFRBs. If such variability were to be found in the Fermi-GBM data, it would provide evidence that this unique energetic process spans multiple 9 orders of magnitude in energy. We performed a blind search of Solar Flares in the Fermi-GBM time-tagged event (TTE) data, binned at 10, 20, and 50 milliseconds in the 5 keV to 20 keV range starting from November 27th, 2012, and ending on April 23rd, 2021. Of the ~3500 solar flares that were examined, no statistically significant gamma-ray emission were detected. We derive upper limit density flux range for the gamma-ray emission to be 0.2 to 0.9 Janskys. Title: Detection of stellar CMEs through post-flare coronal dimmings Authors: Veronig, Astrid; Odert, Petra; Leitzinger, Martin; Dissauer, Karin; Fleck, Nikolaus; Hudson, Hugh Bibcode: 2021AGUFM.U43B..07V Altcode: Coronal mass ejections (CMEs) from our Sun are regularly imaged by white-light coronagraphs, and their speeds and masses are derived from these observations. However, for stars such direct imaging is not possible, and so far, only a few candidates for stellar CMEs have been reported. Here, we present a new approach to detect stellar mass ejections through post-flare coronal dimmings. Coronal dimmings are sudden decreases of the solar EUV and X-ray emission caused by CMEs. Dimming regions map to the bipolar ends of closed magnetic field lines that become stretched or temporarily opened during an eruption, and are a result of the depletion of coronal plasma caused by the expansion and mass loss due to the CME. Recently available multi-point imagery from satellites at different locations in the heliosphere provided us with unprecedented observations of the three-dimensional evolution of solar CMEs and their coronal dimmings. These studies showed distinct correlations between CME mass and speed with key parameters of the associated coronal dimmings such as their spatial extent and intensity drop. In this contribution, we present Sun-as-a-star broad-band EUV light curves derived from SDOs Extreme ultraviolet Variability Experiment (EVE) as a testbed to study whether coronal dimmings can be also observed on stars and used for stellar CME detection. We demonstrate that large flares associated with CMEs have a high probability to show a post-flare coronal dimming, with intensity drops in the 15-25 nm full-Sun light curves of up to 5%. Searching for similar patterns of post-flare dimmings in the X-ray and EUV light curves of solar-like and late-type stars, we identify 21 stellar CME candidates, which is more than all previous reports of stellar CMEs. The derived intensity drops are an order of magnitude larger than for the Sun, suggesting that a substantial part of the stellar corona gets ejected by the CME. This study paves the way for comprehensive detections and characterizations of CMEs on stars, which are important factors in planetary habitability and stellar evolution. This study is published in: A.M. Veronig, P. Odert, M. Leitzinger, K. Dissauer, N. Fleck, H.S. Hudson, Indications of stellar coronal mass ejections through coronal dimmings, Nature Astronomy 5, 697-706 (2021). https://www.nature.com/articles/s41550-021-01345-9 Title: NuSTAR observations of a repeatedly microflaring active region Authors: Cooper, Kristopher; Hannah, Iain G.; Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; White, Stephen M.; Smith, David M.; Duncan, Jessie Bibcode: 2021MNRAS.507.3936C Altcode: 2021arXiv210900263C; 2021MNRAS.tmp.2159C We investigate the spatial, temporal, and spectral properties of 10 microflares from AR12721 on 2018 September 9 and 10 observed in X-rays using the Nuclear Spectroscopic Telescope ARray and the Solar Dynamic Observatory's Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager. We find GOES sub-A class equivalent microflare energies of 1026-1028 erg reaching temperatures up to 10 MK with consistent quiescent or hot active region (AR) core plasma temperatures of 3-4 MK. One microflare (SOL2018-09-09T10:33), with an equivalent GOES class of A0.1, has non-thermal hard X-ray emission during its impulsive phase (of non-thermal power ~7 × 1024 erg s-1) making it one of the faintest X-ray microflares to have direct evidence for accelerated electrons. In 4 of the 10 microflares, we find that the X-ray time profile matches fainter and more transient sources in the extreme-ultraviolet, highlighting the need for observations sensitive to only the hottest material that reaches temperatures higher than those of the AR core (>5 MK). Evidence for corresponding photospheric magnetic flux cancellation/emergence present at the footpoints of eight microflares is also observed. Title: Carrington Events Authors: Hudson, Hugh S. Bibcode: 2021ARA&A..59..445H Altcode: The Carrington event in 1859, a solar flare with an associated geomagnetic storm, has served as a prototype of possible superflare occurrence on the Sun. Recent geophysical (14C signatures in tree rings) and precise time-series photometry [the bolometric total solar irradiance (TSI) for the Sun, and the broadband photometry from Kepler and Transiting Exoplanet Survey Satellite, for the stars] have broadened our perspective on extreme events and the threats that they pose for Earth and for Earth-like exoplanets. This review assesses the mutual solar and/or stellar lessons learned and the status of our theoretical understanding of the new data, both stellar and solar, as they relate to the physics of the Carrington event. The discussion includes the event's implied coronal mass ejection, its potential "solar cosmic ray" production, and the observed geomagnetic disturbances based on the multimessenger information already available in that era. Taking the Carrington event as an exemplar of the most extreme solar event, and in the context of our rich modern knowledge of solar flare and/or coronal mass ejection events, we discuss the aspects of these processes that might be relevant to activity on solar-type stars, and in particular their superflares. The Carrington flare of 1859, though powerful, did not significantly exceed the magnitudes of the greatest events observed in the modern era. Stellar "superflare" events on solar-type stars may share common paradigms, and also suggest the possibility of a more extreme solar event at some time in the future. We benefit from comparing the better-known microphysics of solar flares and coronal mass ejections with the diversity of related stellar phenomena. Title: Characterizing a "Solar FRB" Authors: Hudson, H.; Briggs, M.; Chitta, L.; Fletcher, L.; Gary, D.; Monstein, C.; Nimmo, K.; Saint-Hilaire, P.; White, S. Bibcode: 2021AAS...23812716H Altcode: A remarkable solar microwave (1.4 GHz) burst, SOL2019-05-06T17:47:35.385, has been reported by the STARE2 fast cosmic transient survey (Bochenek et al., 2020). Its behavior resembles that of the Fast Radio Burst (FRB) extragalactic events in having a relatively broad spectral bandwidth and brief (19-msec) duration. It also had no measureable dispersion. The associated flare, GOES class C1, had a relatively hard X-ray spectrum as observed by Fermi/GBM, but no temporal association at the msec time scale suggested by the microwaves. Although msec variability in the microwave domain has been known to solar radio astronomy since the 1970s, the brightness and isolation of this event (both spatial and temporal) suggests novelty. Accordingly we survey the available correlative data from many sources and discuss possible interpretations in terms of type III-like behavior, electron cyclotron masering, and gyrosynchrotron emission. We note that the radio data (e-Callisto and EOVSA) revealed abundant type III activity in the vicinity, and the related flares as observed by GOES had exceptionally short time scales, suggesting burst origins in the lower solar atmosphere. Title: Sun-as-a-star Spectral Irradiance Observations: Milestone For Characterizing The Stellar Active Regions Authors: Toriumi, S.; Airapetian, V.; Hudson, H.; Schrijver, C.; Cheung, M.; DeRosa, M. Bibcode: 2021AAS...23820503T Altcode: For understanding the physical mechanism behind the solar flares, it is crucial to measure the magnetic fields of active regions (ARs) from the photosphere to the corona and investigate their scale, complexity, and evolution. This is true for the stellar flares. However, it is still difficult to spatially resolve the starspots, and one possible way to probe their evolution and structure is to monitor the star in multiple wavelengths. To test this possibility with the solar data, we perform multi-wavelength irradiance monitoring of transiting solar ARs by using full-disk observation data from SDO, Hinode, GOES, and SORCE. As a result, we find, for instance, that the near UV light curves show strong correlations with photospheric total magnetic flux and that there are time lags between the coronal and photospheric light curves when ARs are close to the limb, which together may enable one to discern how high bright coronal loops extend above stellar ARs. It is also revealed that the sub-MK (i.e. transition-region temperature) EUV light curves are sometimes dimmed because the emission measure is reduced owing to the heating over a wide area around the AR. These results indicate that, by measuring the stellar light curves in multiple wavelengths, we may obtain information on the structure and evolution of stellar ARs. Title: Carrington's lost photograph Authors: Cliver, E. W.; Fletcher, Lyndsay; Hudson, H. S. Bibcode: 2021A&G....62.2.40C Altcode: Ed Cliver, Lyndsay Fletcher and Hugh Hudson are looking for a photograph of Richard Carrington. Can you help? Title: Carrington Events Authors: Hudson, Hugh Bibcode: 2021csss.confE.162H Altcode: Poster Title: Sun-as-a-star Multi-wavelength Observations: A Milestone for Characterization of Stellar Active Regions Authors: Toriumi, Shin; Airapetian, Vladimir S.; Hudson, Hugh S.; Schrijver, Carolus J.; Cheung, Mark C. M.; DeRosa, Marc L. Bibcode: 2021csss.confE..46T Altcode: It has been revealed that "superflares" can occur on solar-type stars. The magnetic energy of the flares is likely to be stored in active-region atmospheres. Therefore, to explain the energy storage and occurrence of the flares, it is important to monitor the evolutions of the active regions, not only in visible light but also in ultraviolet (UV) and X-rays. To demonstrate this, we perform multi-wavelength irradiance monitoring of transiting solar active regions by using full-disk observation data. As a result of this sun-as-a-star spectral irradiance analysis, we confirm that the visible continuum that corresponds to the photosphere becomes darkened when the spot is at the central meridian, whereas most of the UV, EUV and X-rays, which are sensitive to chromospheric to coronal temperatures, are brightened, reflecting the bright magnetic features above the starspots. The time lags between the coronal and photospheric light curves have the potential to probe the extent of coronal magnetic fields above the starspots. These results indicate that, by measuring the stellar light curves in multiple wavelengths, we may obtain information on the structures and evolution of stellar active regions. Title: Carrington Events Authors: Hudson, Hugh Bibcode: 2021csss.confE.149H Altcode: Haiku Title: Hot X-ray onsets of solar flares Authors: Hudson, Hugh S.; Simões, Paulo J. A.; Fletcher, Lyndsay; Hayes, Laura A.; Hannah, Iain G. Bibcode: 2021MNRAS.501.1273H Altcode: 2020MNRAS.tmp.3462H; 2020arXiv200705310H The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray 'onset' interval of enhanced isothermal plasma temperatures in the range of 10-15 MK over a period of time prior to the flare's impulsive phase. This 'hot onset' interval occurs during the initial soft X-ray increase and definitely before any detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite X-ray sensor, and confirmed with data from the Reuven Ramaty High Energy Solar Spectroscopic Imager, show no signs of gradual increase, and the 'hot onset' phenomenon occurs regardless of flare classification or configuration. In a small sample of four representative flare events, we tentatively identify this early hot onset soft X-ray emission to occur within footpoint and low-lying loop regions, rather than in coronal structures, based on images from the Atmospheric Imaging Assembly. We confirm this via limb occultation of a flaring region. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence challenge the standard modelling techniques. Title: NuSTAR Observation of Energy Release in 11 Solar Microflares Authors: Duncan, Jessie; Glesener, Lindsay; Grefenstette, Brian W.; Vievering, Juliana; Hannah, Iain G.; Smith, David M.; Krucker, Säm; White, Stephen M.; Hudson, Hugh Bibcode: 2021ApJ...908...29D Altcode: 2020arXiv201106651D Solar flares are explosive releases of magnetic energy. Hard X-ray (HXR) flare emission originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, giving insight into flare energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR) utilizes direct-focusing optics to attain much higher sensitivity in the HXR range than that of previous indirect imagers. This paper presents 11 NuSTAR microflares from two active regions (AR 12671 on 2017 August 21 and AR 12712 on 2018 May 29). The temporal, spatial, and energetic properties of each are discussed in context with previously published HXR brightenings. They are seen to display several "large flare" properties, such as impulsive time profiles and earlier peak times in higher-energy HXRs. For two events where the active region background could be removed, microflare emission did not display spatial complexity; differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high-energy excess over a single thermal model in all events. This excess was consistent with additional higher-temperature plasma volumes in 10/11 microflares and only with an accelerated particle distribution in the last. Previous NuSTAR studies focused on one or a few microflares at a time, making this the first to collectively examine a sizable number of events. Additionally, this paper introduces an observed variation in the NuSTAR gain unique to the extremely low livetime (<1%) regime and establishes a correction method to be used in future NuSTAR solar spectral analysis. Title: Indications of stellar coronal mass ejections through coronal dimmings Authors: Veronig, Astrid M.; Odert, Petra; Leitzinger, Martin; Dissauer, Karin; Fleck, Nikolaus C.; Hudson, Hugh S. Bibcode: 2021NatAs...5..697V Altcode: 2021arXiv211012029V; 2021NatAs.tmp...72V Coronal mass ejections (CMEs) are huge expulsions of magnetized matter from the Sun and stars, traversing space with speeds of millions of kilometres per hour. Solar CMEs can cause severe space weather disturbances and consumer power outages on Earth, whereas stellar CMEs may even pose a hazard to the habitability of exoplanets. Although CMEs ejected by our Sun can be directly imaged by white-light coronagraphs, for stars this is not possible. So far, only a few candidates for stellar CME detections have been reported. Here we demonstrate a different approach that is based on sudden dimmings in the extreme ultraviolet and X-ray emission caused by the CME mass loss. We report dimming detections associated with flares on cool stars, indicative of stellar CMEs, and which are benchmarked by Sun-as-a-star extreme ultraviolet measurements. This study paves the way for comprehensive detections and characterizations of CMEs on stars, which are important factors in planetary habitability and stellar evolution. Title: NuSTAR Observation of Eleven Solar Microflares Authors: Duncan, J. M.; Glesener, L.; Grefenstette, B.; Vievering, J. T.; Hannah, I. G.; Smith, D. M.; Krucker, S.; White, S. M.; Hudson, H. S. Bibcode: 2020AGUFMSH045..07D Altcode: This work presents eleven microflares observed by the Nuclear Spectroscopic Telescope ARray (NuSTAR), representing the first time that a sizable number of these events have been examined collectively. NuSTAR's direct focusing optics give it a dramatic increase in sensitivity over indirect imagers in the hard X-ray (HXR) range. HXR emission in solar flares originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, both of which are diagnostic of flare energy release. Therefore, NuSTAR solar observation campaigns can give unique insight into the energetics of faint microflares, including those that were unobservable with previous-generation HXR instruments. We discuss the temporal, spatial, and energetic properties of all eleven microflares in context with previously published HXR brightenings. They are seen to display several `large-flare' properties, such as impulsive time profiles and earlier peaktimes in higher energy HXRs. For two events where active region background could be removed, microflare emission did not display spatial complexity: differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high energy excess over a single thermal model in all events. This excess was found to most likely originate from additional higher-temperature plasma volumes in 10/11 microflares, and from an accelerated particle distribution in the last. Finally, we introduce an observed variation in the NuSTAR gain unique to the extremely low-livetime (< 1%) regime, and establish a correction method to be used in future NuSTAR solar spectral analysis. Title: Hot Onsets of Solar Flares Authors: Hudson, H. S.; Simoes, P. J. D. A.; Fletcher, L.; Hayes, L.; Hannah, I. G. Bibcode: 2020AGUFMSH0500003H Altcode: No abstract at ADS Title: Sun-as-a-star Spectral Irradiance Observations of Transiting Active Regions Authors: Toriumi, Shin; Airapetian, Vladimir S.; Hudson, Hugh S.; Schrijver, Carolus J.; Cheung, Mark C. M.; DeRosa, Marc L. Bibcode: 2020ApJ...902...36T Altcode: 2020arXiv200804319T Major solar flares are prone to occur in active-region (AR) atmospheres associated with large, complex, dynamically evolving sunspots. This points to the importance of monitoring the evolution of starspots, not only in visible but also in ultraviolet (UV) and X-rays, in understanding the origin and occurrence of stellar flares. To this end, we perform spectral irradiance analysis on different types of transiting solar ARs by using a variety of full-disk synoptic observations. The target events are an isolated sunspot, spotless plage, and emerging flux in prolonged quiet-Sun conditions selected from the past decade. We find that the visible continuum and total solar irradiance become darkened when the spot is at the central meridian, whereas it is bright near the solar limb; UV bands sensitive to the chromosphere correlate well with the variation of total unsigned magnetic flux in the photosphere; amplitudes of extreme ultraviolet (EUV) and soft X-ray increase with the characteristic temperature, whose light curves are flat-topped due to their sensitivity to the optically thin corona; the transiting spotless plage does not show the darkening in the visible irradiance, while the emerging flux produces an asymmetry in all light curves about the central meridian. The multiwavelength Sun-as-a-star study described here indicates that the time lags between the coronal and photospheric light curves have the potential to probe the extent of coronal magnetic fields above the starspots. In addition, EUV wavelengths that are sensitive to temperatures just below 1 MK sometimes show antiphased variations, which may be used for diagnosing plasmas around starspots. Title: Solar Flare Build-Up and Release Authors: Hudson, Hugh S. Bibcode: 2020SoPh..295..132H Altcode: Flares and coronal mass ejections should follow a pattern of build-up and release, with the build-up phase understood as the gradual addition of stress to the coronal magnetic field. Recently Hudson (Mon. Not. Roy. Astron. Soc.491, 4435, 2020) presented observational evidence for this pattern in two isolated active regions from 1997 and 2006, finding a correlation between the waiting time after the event, and the event magnitude. In this article we systematically search for related evidence in the largest 14 active regions of Solar Cycle 24, chosen as those with peak sunspot area exceeding 1000 millionths of the solar hemisphere (MSH). The smallest of these regions, NOAA 12673, produced the exceptional flares SOL2017-09-06 and SOL2017-09-10. None of these regions showed significant correlations of waiting times and flare magnitudes, although two hinted at such an interval-size relationship. Correlations thus appear to be non-existent or intermittent, depending on presently unknown conditions. Title: Solar Flare Energy Partitioning and Transport -- the Impulsive Phase (a Heliophysics 2050 White Paper) Authors: Kerr, Graham S.; Alaoui, Meriem; Allred, Joel C.; Bian, Nicholas H.; Dennis, Brian R.; Emslie, A. Gordon; Fletcher, Lyndsay; Guidoni, Silvina; Hayes, Laura A.; Holman, Gordon D.; Hudson, Hugh S.; Karpen, Judith T.; Kowalski, Adam F.; Milligan, Ryan O.; Polito, Vanessa; Qiu, Jiong; Ryan, Daniel F. Bibcode: 2020arXiv200908400K Altcode: Solar flares are a fundamental component of solar eruptive events (SEEs; along with solar energetic particles, SEPs, and coronal mass ejections, CMEs). Flares are the first component of the SEE to impact our atmosphere, which can set the stage for the arrival of the associated SEPs and CME. Magnetic reconnection drives SEEs by restructuring the solar coronal magnetic field, liberating a tremendous amount of energy which is partitioned into various physical manifestations: particle acceleration, mass and magnetic-field eruption, atmospheric heating, and the subsequent emission of radiation as solar flares. To explain and ultimately predict these geoeffective events, the heliophysics community requires a comprehensive understanding of the processes that transform and distribute stored magnetic energy into other forms, including the broadband radiative enhancement that characterises flares. This white paper, submitted to the Heliophysics 2050 Workshop, discusses the flare impulsive phase part of SEEs, setting out the questions that need addressing via a combination of theoretical, modelling, and observational research. In short, by 2050 we must determine the mechanisms of particle acceleration and propagation, and must push beyond the paradigm of energy transport via nonthermal electron beams, to also account for accelerated protons & ions and downward directed Alfven waves. Title: Solar Flare Energy Partitioning and Transport -- the Gradual Phase (a Heliophysics 2050 White Paper) Authors: Kerr, Graham S.; Alaoui, Meriem; Allred, Joel C.; Bian, Nicholas H.; Dennis, Brian R.; Emslie, A. Gordon; Fletcher, Lyndsay; Guidoni, Silvina; Hayes, Laura A.; Holman, Gordon D.; Hudson, Hugh S.; Karpen, Judith T.; Kowalski, Adam F.; Milligan, Ryan O.; Polito, Vanessa; Qiu, Jiong; Ryan, Daniel F. Bibcode: 2020arXiv200908407K Altcode: Solar flares are a fundamental component of solar eruptive events (SEEs; along with solar energetic particles, SEPs, and coronal mass ejections, CMEs). Flares are the first component of the SEE to impact our atmosphere, which can set the stage for the arrival of the associated SEPs and CME. Magnetic reconnection drives SEEs by restructuring the solar coronal magnetic field, liberating a tremendous amount of energy which is partitioned into various physical manifestations: particle acceleration, mass and magnetic-field eruption, atmospheric heating, and the subsequent emission of radiation as solar flares. To explain and ultimately predict these geoeffective events, the heliophysics community requires a comprehensive understanding of the processes that transform and distribute stored magnetic energy into other forms, including the broadband radiative enhancement that characterises flares. This white paper, submitted to the Heliophysics 2050 Workshop, discusses the flare gradual phase part of SEEs, setting out the questions that need addressing via a combination of theoretical, modelling, and observational research. In short, the flare gradual phase persists much longer than predicted so, by 2050, we must identify the characteristics of the significant energy deposition sustaining the gradual phase, and address the fundamental processes of turbulence and non-local heat flux. Title: Active Region Irradiance during Quiescent Periods: New Insights from Sun-as-a-star Spectra Authors: Kazachenko, Maria D.; Hudson, Hugh S. Bibcode: 2020ApJ...901...64K Altcode: 2020arXiv200802702K How much energy do solar active regions (ARs) typically radiate during quiescent periods? This is a fundamental question for storage and release models of flares and ARs, yet it is presently poorly answered by observations. Here we use the "Sun-as-a-point-source" spectra from the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory to provide a novel estimate of radiative energy losses of an evolving AR. Although EVE provides excellent spectral (5-105 nm) and temperature (2-25 MK) coverage for AR analysis, to our knowledge, these data have not been used for this purpose due to the lack of spatial resolution and the likelihood of source confusion. Here we present a way around this problem. We analyze EVE data time series, when only one large AR 11520 was present on the disk. By subtracting the quiet-Sun background, we estimate the radiative contribution in EUV from the AR alone. We estimate the mean AR irradiance and cumulative AR radiative energy losses in the 1-300 Å and astronomical standard ROSAT-PSPC, 3-124 Å, passbands and compare these to the magnetic energy injection rate through the photosphere, and to variations of the solar cycle luminosity. We find that while AR radiative energy losses are ∼100 times smaller than typical magnetic energy injection rates at the photosphere, they are an order of magnitude larger or similar to the bolometric radiated energies associated with large flares. This study is the first detailed analysis of AR thermal properties using EVE Sun-as-a-star observations, opening doors to AR studies on other stars. Title: NuSTAR X-ray Observations of a Minuscule Microflare from a Repeatedly Microflaring Active Region Authors: Cooper, K.; Hannah, I.; Grefenstette, B.; Glesener, L.; Krucker, S.; Hudson, H. Bibcode: 2020SPD....5121101C Altcode: Highly frequent, small flares are thought to contribute to heating the Sun's atmosphere, particularly in active regions. This impulsive energy release would heat plasma to at least 10 MK and accelerate electrons, producing weak thermal and non-thermal signatures that could be observed by a very sensitive X-ray telescope. No such solar telescope exists (yet) so we use Nuclear Spectroscopic Telescope Array (NuSTAR), an astrophysical X-ray telescope, with focusing optics imaging spectroscopy providing a unique sensitivity for observing the Sun above 2.5 keV. We present an overview of several microflares from the recently emerged active region AR12721 on 2018 September 9-10. Using NuSTAR's imaging spectroscopy and the Solar Dynamics Observatory's Atmospheric Imaging Assembly's (SDO/AIA) EUV imaging capabilities we can analyse the temporal, spatial, and spectral evolution of these microflares, determining the energy release and associated heating of the solar atmosphere. All microflares from AR12721 were below GOES A1 equivalent level and the heated coronal loops were all visible in an Fe XVIII proxy channel derived from SDO/AIA channels. In particular, we present our recently published analysis of the weakest microflare from AR12721 (Cooper et al. 2020 ApJL 893 2) finding it to be one of the smallest active region X-ray flares on record, with material heated up to 7MK and a thermal energy of just 1e26 erg. Title: Hot X-ray Onsets of Solar Flares Authors: Hayes, L.; Hudson, H.; Simoes, P.; Fletcher, L.; Hannah, I. Bibcode: 2020SPD....5121113H Altcode: The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray 'onset' interval of enhanced isothermal plasma temperatures in the range of 10-15 MK up to tens of seconds prior to the flare's impulsive phase. This 'hot onset' interval occurs during the pre-flare time during which elevated GOES soft X-ray flux is detected, but prior to detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite (GOES) X-ray sensor, and confirmed with data from RHESSI, show no signs of gradual increase, and occurs regardless of flare classification or configuration. In a small sample of four representative flare events we identify this early hot onset soft X-ray emission mainly within footpoint and low-lying loops, rather than with coronal structures, based on images from the Atmospheric Imaging Assembly (AIA) and the use of limb occultation. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence challenges the standard flare heating modeling techniques. Title: NuSTAR Observation of Quiet Sun X-ray Bright Points Authors: Paterson, S.; Hannah, I.; Grefenstette, B.; Hudson, H.; Krucker, S.; Glesener, L. Bibcode: 2020SPD....5121013P Altcode: NuSTAR is a focusing hard X-ray telescope designed for observing astrophysical sources, but it is also capable of being pointed at the Sun. NuSTAR's much greater sensitivity compared to RHESSI and the current minimum of the solar activity cycle provide a unique opportunity to investigate quiet Sun features that it has previously not been possible to with X-ray imaging spectroscopy. We present analysis from NuSTAR quiet Sun full disk mosaics from April 2019. With the absence of very bright sources, these mosaics show very small and faint X-ray bright points. We investigate the contribution of these small events to heating the solar atmosphere. The X-ray spectra of these features have been fit, allowing for estimates of their temperatures and emission measures to be obtained. The temperatures were found to lie in the range 1.2-3.5 MK. The temperatures and emission measures predicted by the spectral fits can be tested through comparison to SDO/AIA observations. Using the characteristics of the bright points found from their spectra, heating processes occurring in these features will be investigated. Title: Lyman-alpha Variability During Solar Flares Over Solar Cycle 24 Using GOES-15/EUVS-E Authors: Milligan, Ryan O.; Hudson, Hugh S.; Chamberlin, Phillip C.; Hannah, Iain G.; Hayes, Laura A. Bibcode: 2020SpWea..1802331M Altcode: 2019arXiv191001364M The chromospheric Lyman-alpha line of neutral hydrogen (Lyα; 1216 Å) is the strongest emission line in the solar spectrum. Fluctuations in Lyα are known to drive changes in planetary atmospheres, although few instruments have had the ability to capture rapid Lyα enhancements during solar flares. In this paper, we describe flare-associated emissions via a statistical study of 477 M- and X-class flares as observed by the Extreme UltraViolet Sensor on board the 15th Geostationary Operational Environmental Satellite, which has been monitoring the full-disk solar Lyα irradiance on 10-s timescales over the course of Solar Cycle 24. The vast majority (95%) of these flares produced Lyα enhancements of 10% or less above background levels, with a maximum increase of ∼30%. The irradiance in Lyα was found to exceed that of the 1-8 Å X-ray irradiance by as much as two orders of magnitude in some cases, although flares that occurred closer to the solar limb were found to exhibit less of a Lyα enhancement. This center-to-limb variation was verified through a joint, stereoscopic observation of an X-class flare that appeared near the limb as viewed from Earth, but close to disk center as viewed by the MAVEN spacecraft in orbit around Mars. The frequency distribution of peak Lyα was found to have a power-law slope of 2.8±0.27. We also show that increased Lyα flux is closely correlated with induced currents in the ionospheric E-layer through the detection of the solar flare effect as observed by the Kakioka magnetometer. Title: NuSTAR Observation of a Minuscule Microflare in a Solar Active Region Authors: Cooper, Kristopher; Hannah, Iain G.; Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; White, Stephen M.; Smith, David M. Bibcode: 2020ApJ...893L..40C Altcode: 2020arXiv200411176C We present X-ray imaging spectroscopy of one of the weakest active region (AR) microflares ever studied. The microflare occurred at ∼11:04 UT on 2018 September 9 and we studied it using the Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic Observatory's Atmospheric Imaging Assembly (SDO/AIA). The microflare is observed clearly in 2.5-7 keV with NuSTAR and in Fe XVIII emission derived from the hotter component of the 94 Å SDO/AIA channel. We estimate the event to be three orders of magnitude lower than a GOES A class microflare with an energy of 1.1 × 1026 erg. It reaches temperatures of 6.7 MK with an emission measure of 8.0 × 1043 cm-3. Non-thermal emission is not detected but we instead determine upper limits to such emission. We present the lowest thermal energy estimate for an AR microflare in literature, which is at the lower limits of what is still considered an X-ray microflare. Title: Accelerated Electrons Observed Down to <7 keV in a NuSTAR Solar Microflare Authors: Glesener, Lindsay; Krucker, Säm; Duncan, Jessie; Hannah, Iain G.; Grefenstette, Brian W.; Chen, Bin; Smith, David M.; White, Stephen M.; Hudson, Hugh Bibcode: 2020ApJ...891L..34G Altcode: 2020arXiv200312864G We report the detection of emission from a nonthermal electron distribution in a small solar microflare (GOES class A5.7) observed by the Nuclear Spectroscopic Telescope Array, with supporting observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The flaring plasma is well accounted for by a thick-target model of accelerated electrons collisionally thermalizing within the loop, akin to the "coronal thick-target" behavior occasionally observed in larger flares. This is the first positive detection of nonthermal hard X-rays from the Sun using a direct imager (as opposed to indirectly imaging instruments). The accelerated electron distribution has a spectral index of 6.3 ± 0.7, extends down to at least 6.5 keV, and deposits energy at a rate of ∼2 × 1027 erg s-1, heating the flare loop to at least 10 MK. The existence of dominant nonthermal emission in X-rays down to <5 keV means that RHESSI emission is almost entirely nonthermal, contrary to what is usually assumed in RHESSI spectroscopy. The ratio of nonthermal to thermal energies is similar to that of large flares, in contrast to what has been found in previous studies of small RHESSI flares. We suggest that a coronal thick target may be a common property of many small microflares based on the average electron energy and collisional mean free path. Future observations of this kind will enable understanding of how flare particle acceleration changes across energy scales, and will aid the push toward the observational regime of nanoflares, which are a possible source of significant coronal heating. Title: Cosmic ray interactions in the solar atmosphere Authors: Hudson, Hugh S.; MacKinnon, Alec; Szydlarski, Mikolaj; Carlsson, Mats Bibcode: 2020MNRAS.491.4852H Altcode: 2019arXiv191001186H; 2019MNRAS.tmp.3116H High-energy particles enter the solar atmosphere from Galactic or solar coronal sources, and produce 'albedo' emission from the quiet Sun that is now observable across a wide range of photon energies. The interaction of high-energy particles in a stellar atmosphere depends essentially upon the joint variation of the magnetic field and plasma density, which heretofore has been characterized parametrically as P ∝ Bα with P the gas pressure and B the magnitude of the magnetic field. We re-examine that parametrization by using a self-consistent 3D MHD model (Bifrost) and show that this relationship tends to P ∝ B3.5 ± 0.1 based on the visible portions of the sample of open-field flux tubes in such a model, but with large variations from point to point. This scatter corresponds to the strong meandering of the open-field flux tubes in the lower atmosphere, which will have a strong effect on the prediction of the emission anisotropy (limb brightening). The simulations show that much of the open flux in coronal holes originates in weak-field regions within the granular pattern of the convective motions seen in the simulations. Title: SmallSat Solar Axion and Activity X-ray Imager (SSAXI) Authors: Hong, J.; Romaine, S.; Kenter, A.; Moore, C.; Reeves, K.; Ramsey, B.; Kilaru, K.; Vogel, J.; Ruz Armendariz, J.; Hudson, H.; Perez, K. Bibcode: 2020AAS...23527101H Altcode: The axion is a promising dark matter candidate as well as a solution to the strong charge-parity (CP) problem in quantum chromodynamics (QCD). We describe a new concept for SmallSat Solar Axion and Activity X-ray Telescope (SSAXI) to search for solar axions or axion-like particles (ALPs) and to monitor solar activity over a wide dynamic range. SSAXI aims to unambiguously identify X-rays converted from axions in the solar magnetic field along the line of sight to the solar core, effectively imaging the solar core. SSAXI employs Miniature lightweight Wolter-I focusing X-ray optics (MiXO) and monolithic CMOS X-ray sensors in a compact package. The wide energy range (0.5 - 5 keV) of SSAXI can easily distinguish spectra of axion-converted X-rays from solar X-ray spectra, while encompassing the prime energy band (3 - 4.5 keV) of axion-converted X-rays. The high angular resolution (30 arcsec) and large field of view (40 arcmin) in SSAXI will easily resolve the enhanced X-ray flux over the 3 arcmin wide solar core while fully covering the X-ray activity over the entire solar disc. The fast readout in the inherently radiation tolerant CMOS X-ray sensors enables high resolution spectroscopy over a wide dynamic range with a broad range of operational temperatures. We present multiple mission implementation options for SSAXI under ESPA class. SSAXI will operate in a Sun-synchronous orbit for 1 yr preferably near a solar minimum to accumulate sufficient X-ray photon statistics. Title: A correlation in the waiting-time distributions of solar flares Authors: Hudson, Hugh S. Bibcode: 2020MNRAS.491.4435H Altcode: 2019arXiv190808749H; 2019MNRAS.tmp.2713H In a limited sample of isolated solar active regions, we find that the waiting times between flares may correlate well with flare magnitudes as determined by the Geostationary Operational Environmental Satellites (GOES) soft X-ray fluxes. A 'build-up and release' (BUR) scenario for magnetic energy storage in the solar corona suggests the existence of such a relationship, relating the slowly varying subphotospheric energy sources to the sudden coronal energy releases of flares and coronal mass ejections (CMEs). Substantial amounts of research effort had not previously found any obvious observational evidence for such a BUR process. This has posed a puzzle since coronal magnetic energy storage represents the consensus view of the basic flare mechanism. We have revisited the GOES soft X-ray flare statistics for any evidence of correlations, using two isolated active regions, and have found significant evidence for a 'saturation' correlation. Rather than a 'reset' form of this relaxation, in which the time before a flare correlates with its magnitude, the 'saturation' relationship results in the time after the flare showing the correlation. The observed correlation competes with the 'obscuration' effect of reduced GOES sensitivity following a strong event, by which weaker events can be under-reported systematically. This complicates the observed correlation, and we discuss several approaches to remedy this. Title: NuSTAR observations of the quietest Sun Authors: Hannah, I. G.; Cooper, K.; Grefenstette, B.; Glesener, L.; Krucker, S.; Smith, D. M.; Hudson, H. S.; White, S. M.; Kuhar, M. Bibcode: 2019AGUFMSH41F3335H Altcode: Observing X-rays (above a few keV) from the Sun provides a direct insight into energy release (heating and/or particle acceleration) in the solar atmosphere. Targeting the faintest X-ray emission allows the study of the smallest flares and eruption, and crucially their contribution to heating the solar atmosphere. NuSTAR is an astrophysics telescope that uses directly focusing X-rays optics to detect weak X-rays from the Sun. We have observed the Sun many times since the start of solar pointings in Sep 2014 through to our latest observations in 2019. See http://ianan.github.io/nsovr/ for an overview. During the current solar minimum, when the Sun is devoid of active regions and presenting the very quietest levels of activity, NuSTAR has targeted the Sun several times. We have detected X-rays from a variety of sources: large diffuse sources, steady compact sources, brief flares/brightenings and small eruptions. The NuSTAR X-ray images of these weak sources are related to features seen at other wavelengths, such as in softer X-rays with Hinode/XRT and EUV with SDO/AIA. Crucially, NuSTAR's imaging spectroscopy allows us to obtain and fit the X-ray spectrum from these small events determining their thermal (and potentially non-thermal) properties. We will present some of the latest solar observations with NuSTAR as we go through the current solar minimum. Title: Lyman-alpha Variability During Solar Flares Over Solar Cycle 24 Using GOES-15/EUVS-E Authors: Milligan, R. O.; Hudson, H. S.; Chamberlin, P.; Hannah, I. G. Bibcode: 2019AGUFMSA11B3221M Altcode: The chromospheric Lyman-alpha line of hydrogen (Lyα; 1216Å) is the strongest emission line in the solar spectrum. Fluctuations in Lyα are known to drive changes in the dynamics and composition of planetary atmospheres, and many space weather monitoring platforms have included Lyα photometers to measure such changes. However, few instruments have had the capacity to capture Lyα enhancements during solar flares. The EUV Sensor (EUVS) on board the 15th Geostationary Operational Environmental Satellite (GOES-15) has been monitoring the full-disk solar Lyα irradiance on 10s timescales over the course of Solar Cycle 24. These data, thus far little used in the astrophysics community, show variability on all time scales, and here we describe flare-associated emissions via a statistical study of 477 M- and X-class flares observed in Lyα emission by GOES-15/EUVS-E. We find a strong impulsive-phase component of the flare emission, but without as much of the rapid variability characteristic of the hard X-ray bremsstrahlung or microwave gyrosynchrotron emissions defining this phase. The vast majority (98%) of these flares produced Lyα enhancements of 10% or less above background levels, with a maximum increase of ~30%. However, the energy contained in this flare excess averages about 10 times more than that of the GOES soft X-ray energy (1-8Å; also a driver of atmospheric fluctuations), with as much as a factor of 100 in some events. Conversely, flares that occurred closer to the solar limb were found to exhibit less of a Lyα enhancement due to either absorption by the solar atmosphere along the line of sight, foreshortening of the flare ribbons, or occultation by the solar disk. This center-to-limb variation was verified through a joint observation of an X-class flare that appeared on the limb as viewed by GOES, but was closer to disk center as viewed by the MAVEN spacecraft in orbit around Mars, which also carries a Lyα photometer. The MAVEN data showed a ~60% higher flux increase relative to the GOES data. Title: Cosmic Rays Across the Rainbow Bridge: Particle Interactions in a Magnetized Plasma Atmosphere Authors: Hudson, H. S.; MacKinnon, A.; Szydlarski, M.; Carlsson, M. Bibcode: 2019AGUFMSH21B..02H Altcode: High-energy particles enter the solar atmosphere from Galactic or solar sources, producing gamma-ray "albedo'' radiations. These emissions come from the quiet Sun and from the large-scale corona, as well as from the vicinity of flares, and have been observed across a wide range of photon energies (MeV to GeV). The interaction of high-energy particles in the solar atmosphere depends essentially upon the joint variation of the magnetic field and the gas, and this has heretofore generally been characterized parametrically as P ~ Balpha (Zweibel & Haber 1983), with P the gas pressure and B the magnitude of the magnetic field. We have checked this relationship with a Bifrost 3D MHD model, approximating the particle transport as a guiding-center motion along "open'' (large-scale) fieldlines. We find alpha ~ 2.2 in the strongest (kG) fields in the simulation, but individual field structures have widely disparate relationships. The scatter corresponds to the strong meandering of the open-field flux tubes in the lower atmosphere and to their incessant dynamics. We discuss this and other aspects of the interactions of high-energy particles with the Sun (the "solar Størmer problem''). Title: Principles Of Heliophysics: a textbook on the universal processes behind planetary habitability Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer, Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David; Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen, Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry; Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George; Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo; Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert; Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp, Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel; Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav; Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George; Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley, Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan; Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian; Woods, Tom; Zapp, Neal Bibcode: 2019arXiv191014022S Altcode: This textbook gives a perspective of heliophysics in a way that emphasizes universal processes from a perspective that draws attention to what provides Earth (and similar (exo-)planets) with a relatively stable setting in which life as we know it can thrive. The book is intended for students in physical sciences in later years of their university training and for beginning graduate students in fields of solar, stellar, (exo-)planetary, and planetary-system sciences. Title: Joint X-Ray, EUV, and UV Observations of a Small Microflare Authors: Hannah, Iain G.; Kleint, Lucia; Krucker, Säm; Grefenstette, Brian W.; Glesener, Lindsay; Hudson, Hugh S.; White, Stephen M.; Smith, David M. Bibcode: 2019ApJ...881..109H Altcode: 2018arXiv181209214H We present the first joint observation of a small microflare in X-rays with the Nuclear Spectroscopic Telescope ARray (NuSTAR), in UV with the Interface Region Imaging Spectrograph (IRIS), and in EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). These combined observations allow us to study the hot coronal and cooler chromospheric/transition region emission from the microflare. This small microflare peaks from 2016 July 26 23:35 to 23:36 UT, in both NuSTAR, SDO/AIA, and IRIS. Spatially, this corresponds to a small loop visible in the SDO/AIA Fe XVIII emission, which matches a similar structure lower in the solar atmosphere seen by IRIS in SJI1330 and 1400 Å. The NuSTAR emission in both 2.5-4 and 4-6 keV is located in a source at this loop location. The IRIS slit was over the microflaring loop, and fits show little change in Mg II but do show intensity increases, slight width enhancements, and redshifts in Si IV and O IV, indicating that this microflare had most significance in and above the upper chromosphere. The NuSTAR microflare spectrum is well fitted by a thermal component of 5.1 MK and 6.2 × 1044 cm-3, which corresponds to a thermal energy of 1.5 × 1026 erg, making it considerably smaller than previously studied active region microflares. No non-thermal emission was detected but this could be due to the limited effective exposure time of the observation. This observation shows that even ordinary features seen in UV can remarkably have a higher-energy component that is clear in X-rays. Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener, Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Gburek, Szymon; Steslicki, Marek; Allred, Joel C.; Battaglia, Marina; Baumgartner, Wayne H.; Drake, James; Goetz, Keith; Grefenstette, Brian; Hannah, Iain; Holman, Gordon D.; Inglis, Andrew; Ireland, Jack; Klimchuk, James A.; Ishikawa, Shin-Nosuke; Kontar, Eduard; Massone, Anna-maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard A.; Woods, Thomas N.; Chen, Bin; Gary, Dale E.; Hudson, Hugh S.; Kowalski, Adam; Warmuth, Alexander; White, Stephen M.; Veronig, Astrid; Vilmer, Nicole Bibcode: 2019AAS...23422501C Altcode: The Focusing Optics X-ray Solar Imager (FOXSI), a SMEX mission concept in Phase A, is the first-ever solar-dedicated, direct-imaging, hard X-ray telescope. FOXSI provides a revolutionary new approach to viewing explosive magnetic-energy release on the Sun by detecting signatures of accelerated electrons and hot plasma directly in and near the energy-release sites of solar eruptive events (e.g., solar flares). FOXSI's primary science objective is to understand the mystery of how impulsive energy release leads to solar eruptions, the primary drivers of space weather at Earth, and how those eruptions are energized and evolve. FOXSI addresses three important science questions: (1) How are particles accelerated at the Sun? (2) How do solar plasmas get heated to high temperatures? (3) How does magnetic energy released on the Sun lead to flares and eruptions? These fundamental physics questions are key to our understanding of phenomena throughout the Universe from planetary magnetospheres to black hole accretion disks. FOXSI measures the energy distributions and spatial structure of accelerated electrons throughout solar eruptive events for the first time by directly focusing hard X-rays from the Sun. This naturally enables high imaging dynamic range, while previous instruments have typically been blinded by bright emission. FOXSI provides 20-100 times more sensitivity as well as 20 times faster imaging spectroscopy than previously available, probing physically relevant timescales (<1 second) never before accessible. FOXSI's launch in July 2022 is aligned with the peak of the 11-year solar cycle, enabling FOXSI to observe the many large solar eruptions that are expected to take place throughout its two-year mission. Title: Hard X-ray Spectroscopy of Six NuSTAR Microflares Authors: Duncan, Jessie McBrayer; Glesener, Lindsay; Hannah, Iain; Smith, David M.; Krucker, Sam; Hudson, Hugh S.; Grefenstette, Brian Bibcode: 2019AAS...23420404D Altcode: Hard X-ray (HXR) emission in solar flares can originate from regions of high temperature plasma, as well as from non-thermal particle populations. Both of these sources of HXR radiation make solar observation in this band important for study of flare energetics. NuSTAR is the first HXR telescope with direct focusing optics, giving it a dramatic increase in sensitivity over previous indirect imaging methods. Here we present NuSTAR observation of six microflares from one solar active region during a period of several hours on May 29th, 2018. Spectral fitting of emission at each flare time shows excess high energy emission over an isothermal spectral component in all six flares. The most likely origin of this excess could be either additional volumes of high-temperature plasma, or non-thermally accelerated particles. For each event, characterization of this excess is presented, including determination of upper limits on the non-thermal emission possible in events where it is not directly observed. Title: Eclipse Megamovie 2017 Successes and Potential For Future Work Authors: Peticolas, L.; Hudson, H.; Johnson, C.; Zevin, D.; White, V.; Oliveros, J. C. M.; Ruderman, I.; Koh, J.; Konerding, D.; Bender, M.; Cable, C.; Kruse, B.; Yan, D.; Krista, L.; Collier, B.; Fraknoi, A.; Pasachoff, J. M.; Filippenko, A. V.; Mendez, B.; McIntosh, S. W.; Filippenko, N. L. Bibcode: 2019ASPC..516..337P Altcode: In 2011, an "Eclipse Megamovie" was envisioned for the 2017 total solar eclipse that would be created using the public's photographs of the Sun's corona as frames in a movie illuminating dynamic changes in the chromosphere and corona. On August 21, 2017, our team collected photographs of the total solar eclipse from thousands of volunteers with telescopes, DSLR (Digital Single-Lens Reflex) cameras, and mobile device cameras setup across the path of totality. Our efforts resulted in 1,190 photographers contributing 50,016 DSLR photographs in a final open-source, public archive that is 766 GB in size. All photographs in this archive are Creative Commons zero (CC0), making them freely available for public use. From mobile devices, we obtained an archive of 60,000 images, 211 GB in size. The first Eclipse Megamovie video was compiled and made available to the public a few hours after the Moon's shadow left the U.S. East Coast. For two weeks, additional images were added to this video, as volunteers uploaded them to the project server. The project also resulted in a comprehensive website with 12,749 users sufficiently interested in the project to each create a user profile on the website, several short documentaries, 190 articles and press releases, open-source code for use in future related efforts, and hundreds of public presentations across the country prior to the eclipse. Information on how to access these resources is included in this paper. Title: First detection of non-thermal emission in a NuSTAR solar microflare Authors: Glesener, Lindsay; Krucker, Sam; Duncan, Jessie McBrayer; Hannah, Iain; Grefenstette, Brian; Chen, Bin; Smith, David M.; White, Stephen M.; Hudson, Hugh S. Bibcode: 2019AAS...23422503G Altcode: We report the detection of emission from a non-thermal electron distribution in a small solar microflare observed by the Nuclear Spectroscopic Telescope Array (NuSTAR). On 2017 August 21, NuSTAR observed a solar active region for approximately an hour before the region was eclipsed by the Moon. The active region emitted several small microflares of GOES class A and smaller. In this work, we present spectroscopy demonstrating evidence of electron acceleration in one of these microflares (GOES class A5.7) and we compare energetic aspects of the accelerated distribution to commonly studied larger flares. The flaring plasma observed by NuSTAR, with supporting observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is well accounted for by a thick-target model of accelerated electrons collisionally thermalizing within the loop, akin to the "coronal thick target" behavior occasionally observed in larger flares. Future observations of this kind will enable understanding of how flare particle acceleration changes across energy scales, and will aid the push toward the observational regime of nanoflares, which are a possible source of significant coronal heating. Title: Chapter 9 - High-Energy Solar Physics Authors: Hudson, H. S.; MacKinnon, A. L. Bibcode: 2019sgsp.book..301H Altcode: This chapter deals generally with the high-energy astrophysics of the Sun, specifically with solar flares and coronal mass ejections (CMEs), but it also touches on the whole range of nonthermality or departures from Maxwellian distributions in solar plasmas. Radio, x-ray, and γ-ray observations provide primary remote-sensing observations of these departures, but such signatures can be hidden by brighter thermal emissions that may not be as fundamental in physics events. The solar paradigm for flare/CME development appears to match many of the new stellar observations of similar phenomena, but the limitations of observational sensitivity mean that we have few direct observations of the expected hard x-rays and none at all of the γ-rays that could confirm this. Title: First high-resolution look at the quiet Sun with ALMA at 3mm Authors: Nindos, A.; Alissandrakis, C. E.; Bastian, T. S.; Patsourakos, S.; De Pontieu, B.; Warren, H.; Ayres, T.; Hudson, H. S.; Shimizu, T.; Vial, J. -C.; Wedemeyer, S.; Yurchyshyn, V. Bibcode: 2018A&A...619L...6N Altcode: 2018arXiv181005223N We present an overview of high-resolution quiet Sun observations, from disk center to the limb, obtained with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm. Seven quiet-Sun regions were observed at a resolution of up to 2.5″ by 4.5″. We produced both average and snapshot images by self-calibrating the ALMA visibilities and combining the interferometric images with full-disk solar images. The images show well the chromospheric network, which, based on the unique segregation method we used, is brighter than the average over the fields of view of the observed regions by ∼305 K while the intranetwork is less bright by ∼280 K, with a slight decrease of the network/intranetwork contrast toward the limb. At 3 mm the network is very similar to the 1600 Å images, with somewhat larger size. We detect, for the first time, spicular structures, rising up to 15″ above the limb with a width down to the image resolution and brightness temperature of ∼1800 K above the local background. No trace of spicules, either in emission or absorption, is found on the disk. Our results highlight the potential of ALMA for the study of the quiet chromosphere. Title: NuSTAR's observations of tiny flares and big eruptions Authors: Hannah, Iain; Grefenstette, Brian; Glesener, Lindsey; Krucker, Sam; Smith, David; Hudson, Hugh; White, Stephen; Kuhar, Matej Bibcode: 2018csc..confE.118H Altcode: NuSTAR is an astrophysics X-ray telescope, with direct imaging spectroscopy providing a unique sensitivity for observing the Sun above 2.5keV. Targeting the faintest X-ray emission from the solar atmosphere allows the study of the smallest flares, and their contribution to heating the corona. However, it can also be used to observe weak high-coronal sources that are associated with the energy release in large, but occulted, eruptions. NuSTAR has observed the Sun over a dozen times since Sep 2014, through to our latest observations in 2018: see http://ianan.github.io/nsovr/ for a quicklook overview of NuSTAR's solar observations. We will present some of the latest solar observations with NuSTAR and compare them to the emission seen at lower energy wavelengths, particularly in EUV with SDO/AIA and also the derived Fe18 emission. Title: The association of the Hale sector boundary with RHESSI solar flares and active longitudes Authors: Loumou, K.; Hannah, I. G.; Hudson, H. S. Bibcode: 2018A&A...618A...9L Altcode: 2018arXiv180805866L Context. The heliospheric magnetic field (HMF) is structured into large sectors of positive and negative polarity. The parts of the boundary between these sectors where the change in polarity matches that of the leading-to-following sunspot polarity in that solar hemisphere, are called Hale sector boundaries (HSB).
Aims: We investigate the flare occurrence rate near HSBs and the association between HSBs and active longitudes.
Methods: Previous work determined the times HSBs were at solar central meridian, using the detection of the HMF sector boundary crossing at the Earth. In addition to this, we use a new approach which finds the HSB locations at all times by determining them from potential field source surface (PFSS) extrapolations of photospheric magnetograms. We use the RHESSI X-ray flare list for comparison to the HSB as it provides accurate flare locations over 14 years, from February 2002 to February 2016, covering both Cycles 23 and 24. For the active longitude positions we use previously published work based on sunspot observations.
Results: We find that the two methods of determining the HSB generally agree and that 41% (Cycle 23) and 47% (Cycle 24) of RHESSI flares occur within 30° of the PFSS determined-HSB. The behaviour of the HSBs varies over the two Cycles studied, and as expected they swap in hemisphere as the Cycles change. The HSBs and active longitudes do overlap but not consistently. They often move at different rates relative to each other (and the Carrington solar rotation rate) and these vary over each Cycle. The HSBs provide a useful additional activity indicator, particularly during periods when active longitudes are difficult to determine. Title: The Relationship between Long-Duration Gamma-Ray Flares and Solar Cosmic Rays Authors: Hudson, Hugh S. Bibcode: 2018IAUS..335...49H Altcode: 2017arXiv171105583H A characteristic pattern of solar hard X-ray emission, first identified in SOL1969-03-30 by Frost & Dennis (1971), turns out to have a close association with the prolonged high-energy gamma-ray emission originally observed by Forrest et al. (1985). This identification has become clear via the observations of long-duration γ-ray flares by the Fermi/LAT experiment, for example in the event SOL2014-09-01. The distinctive features of these events include flat hard X-ray spectra extending well above 100 keV, a characteristic pattern of time development, low-frequency gyrosynchrotron peaks, CME association, and gamma-rays identifiable with pion decay originating in GeV ions. The identification of these events with otherwise known solar structures nevertheless remains elusive, in spite of the wealth of EUV imagery available from SDO/AIA. The quandary is that these events have a clear association with SEPs in the high corona, and yet the gamma-ray production implicates the photosphere itself, despite the strong mirror force that should focus the particles away from the Sun We discuss the morphology of these phenomena and propose a solution to this problem. Title: On the observation of a classical loop-prominence system during the 2017 September 10 flare Authors: Martinez Oliveros, Juan Carlos; Hudson, Hugh; Krucker, Säm; Guevara Gomez, Juan Camilo Bibcode: 2018shin.confE.258M Altcode: We report observations of white-light ejecta in the low corona after the 2017 September 10 flare, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. We report the observation of a classical loop-prominence system, but are brighter than expected and possibly seen here in the continuum rather than line emission. We studied the spatial and temporal relation between RHESSI X-ray and the white-light emissions. We also studied the HMI spectroscopic data to determine the most probable emission mechanism that can explain the observation of the loop-prominence system. Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener, Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel C.; Chen, Bin; Battaglia, Marina; Drake, James Frederick; Gary, Dale E.; Goetz, Keith; Gburek, Szymon; Grefenstette, Brian; Hannah, Iain G.; Holman, Gordon; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa, Shin-nosuke; Klimchuk, James A.; Kontar, Eduard; Kowalski, Adam F.; Massone, Anna Maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard; Steslicki, Marek; Ryan, Daniel; Warmuth, Alexander; Veronig, Astrid; Vilmer, Nicole; White, Stephen M.; Woods, Thomas N. Bibcode: 2018tess.conf40444C Altcode: We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer (SMEX) Heliophysics mission that is currently undergoing a Phase A concept study. FOXSI will provide a revolutionary new perspective on energy release and particle acceleration on the Sun. FOXSI's primary instrument, the Direct Spectroscopic Imager (DSI), is a direct imaging X-ray spectrometer with higher dynamic range and better than 10x the sensitivity of previous instruments. Flown on a 3-axis-stabilized spacecraft in low-Earth orbit, DSI uses high-angular-resolution grazing-incidence focusing optics combined with state-of-the-art pixelated solid-state detectors to provide direct imaging of solar hard X-rays for the first time. DSI is composed of a pair of X-ray telescopes with a 14-meter focal length enabled by a deployable boom. DSI has a field of view of 9 arcminutes and an angular resolution of better than 8 arcsec FWHM; it will cover the energy range from 3 up to 50-70 keV with a spectral resolution of better than 1 keV. DSI will measure each photon individually and will be able to create useful images at a sub-second temporal resolution. FOXSI will also measure soft x-ray emission down to 0.8 keV with a 0.25 keV resolution with its secondary instrument, the Spectrometer for Temperature and Composition (STC) provided by the Polish Academy of Sciences. Making use of an attenuator-wheel and high-rate-capable detectors, FOXSI will be able to observe the largest flares without saturation while still maintaining the sensitivity to detect X-ray emission from weak flares, escaping electrons, and hot active regions. This presentation will cover the data products and software that can be expected from FOXSI and how they could be used by the community. Title: GeV Particles in the Inner Heliosphere Authors: Hudson, Hugh S.; MacKinnon, Alec Bibcode: 2018tess.conf22301H Altcode: The Fermi/LAT observations of GeV solar gamma rays, both from flares and from two distingishable quiet-Sun sources, make the cosmic-ray environment of the inner heliosphere topically interesting. The Parker Solar Probe and Solar Orbiter will explore this territory in new ways. We discuss recent developments related both to flare/CME processes and to the quiet Sun, noting that the solar equivalent of the Stoermer theory remains to be worked out. We describe extensive shower products from the GeV particles in the solar atmosphere (and in the atmosphere of Venus) as another item of interest.

~ Title: Do "Last Best" Flares Conclude Solar Cycles? Authors: Hudson, Hugh S. Bibcode: 2018tess.conf31906H Altcode: The Sun has just produced two major X-class flares in the waning phase of Solar Cycle 24, to be thoroughly discussed at TESS 2018. This has happened in previous cycles:

Cycle 21: SOL1984-04-24 (X10; SMM on furlough),

Cycle 22: SOL1996-07-09 (X2.2 or maybe X2.6; first sunquake),

Cycle 23: SOL2006-12-13 (X3.4; Hinode),

Cycle 24: SOL2017-09-10 (X8.2; Fermi sustained gamma-rays).

These could be termed "last best" events, each one quite memorable and each one at the very end of its 11-year cycle. Above a certain magnitude, flare occurrence approaches a Poisson distribution and one can ignore the solar cycle. These considerations don't reflect any particular physics, and yet... Title: NuSTAR Detection of X-Ray Heating Events in the Quiet Sun Authors: Kuhar, Matej; Krucker, Säm; Glesener, Lindsay; Hannah, Iain G.; Grefenstette, Brian W.; Smith, David M.; Hudson, Hugh S.; White, Stephen M. Bibcode: 2018ApJ...856L..32K Altcode: 2018arXiv180308365K The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2-4.1 MK and emission measures of (0.6-15) × 1044 cm-3 describe their spectra well, resulting in thermal energies in the range (2-6) × 1026 erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares. Title: Exploring the Sun with ALMA Authors: Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; Pontieu, B. D.; Gary, D. E.; Fleishman, G. D.; Hales, A. S.; Iwai, K.; Hudson, H.; Kim, S.; Kobelski, A.; Loukitcheva, M.; Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S. M.; Yan, Y. Bibcode: 2018Msngr.171...25B Altcode: The Atacama Large Millimeter/submillimeter Array (ALMA) Observatory opens a new window onto the Universe. The ability to perform continuum imaging and spectroscopy of astrophysical phenomena at millimetre and submillimetre wavelengths with unprecedented sensitivity opens up new avenues for the study of cosmology and the evolution of galaxies, the formation of stars and planets, and astrochemistry. ALMA also allows fundamentally new observations to be made of objects much closer to home, including the Sun. The Sun has long served as a touchstone for our understanding of astrophysical processes, from the nature of stellar interiors, to magnetic dynamos, non-radiative heating, stellar mass loss, and energetic phenomena such as solar flares. ALMA offers new insights into all of these processes. Title: Eclipse Megamovie 2017: How did we do? Authors: Hudson, Hugh; Bender, Mark; Collier, Braxton; Johnson, Calvin; Koh, Justin; Konerding, David; Martinez Oliveros, Juan Carlos; Peticolas, Laura; White, Vivian; Zevin, Dan Bibcode: 2018AAS...23122003H Altcode: The Eclipse Megamovie program, as set up for the Great American Eclipse of 21 August 2017, achived a massive volunteer participation, making maximal use existing equipment but with coordinated training. Everything worked fine, and the archive entered the public domain on Friday, October 6. It comprises about 800 GB of data from DSLR cameras and telescopes. An additional 200 GB of data were obtained by smartphone cameras operating a dedicated free app. The massive oversampling made possible by the many (about 2500) volunteer observers has opened new parameter space for tracking coronal and chromospheric time development. Fortuitously some solar activity appeared during the 90-minute period of totality, including a C-class flare and an ongoing CME. At the smartphone level, with the advantage of precise GPS timing, we have data on solar structure via the timing of Baily's Beads at the 2nd and 3rd contacts. The Megamovie archive is an historical first, and we hope that it has already been a springboard for citizen-science projects. We discuss the execution of the program, presenting some of the 2017 science plans and results. We expect that the eclipse of 2024 will be better still. Title: Detection of 3-Minute Oscillations in Full-Disk Lyman-alpha Emission During A Solar Flare Authors: Milligan, R. O.; Ireland, J.; Fleck, B.; Hudson, H. S.; Fletcher, L.; Dennis, B. R. Bibcode: 2017AGUFMSH41A2739M Altcode: We report the detection of chromospheric 3-minute oscillations in disk-integrated EUV irradiance observations during a solar flare. A wavelet analysis of detrended Lyman-alpha (from GOES/EUVS) and Lyman continuum (from SDO/EVE) emission from the 2011 February 15 X-class flare revealed a 3-minute period present during the flare's main phase. The formation temperature of this emission locates this radiation to the flare's chromospheric footpoints, and similar behaviour is found in the SDO/AIA 1600A and 1700A channels, which are dominated by chromospheric continuum. The implication is that the chromosphere responds dynamically at its acoustic cutoff frequency to an impulsive injection of energy. Since the 3-minute period was not found at hard X-ray energies (50-100 keV) in RHESSI data we can state that this 3-minute oscillation does not depend on the rate of energization of, or energy deposition by, non-thermal electrons. However, a second period of 120 s found in both hard X-ray and chromospheric emission is consistent with episodic electron energization on 2-minute timescales. Our finding on the 3-minute oscillation suggests that chromospheric mechanical energy should be included in the flare energy budget, and the fluctuations in the Lyman-alpha line may influence the composition and dynamics of planetary atmospheres during periods of high activity. Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.; Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Gburek, S.; Grefenstette, B.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Schwartz, R.; Steslicki, M.; Turin, P.; Ryan, D.; Warmuth, A.; Veronig, A.; Vilmer, N.; White, S. M.; Woods, T. N. Bibcode: 2017AGUFMSH44A..07C Altcode: We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer (SMEX) Heliophysics mission that is currently undergoing a Phase A concept study. FOXSI will provide a revolutionary new perspective on energy release and particle acceleration on the Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic range and better than 10x the sensitivity of previous instruments. Flown on a 3-axis-stabilized spacecraft in low-Earth orbit, FOXSI uses high-angular-resolution grazing-incidence focusing optics combined with state-of-the-art pixelated solid-state detectors to provide direct imaging of solar hard X-rays for the first time. FOXSI is composed of a pair of x-ray telescopes with a 14-meter focal length enabled by a deployable boom. Making use of a filter-wheel and high-rate-capable solid-state detectors, FOXSI will be able to observe the largest flares without saturation while still maintaining the sensitivity to detect x-ray emission from weak flares, escaping electrons, and hot active regions. This mission concept is made possible by past experience with similar instruments on two FOXSI sounding rocket flights, in 2012 and 2014, and on the HEROES balloon flight in 2013. FOXSI's hard X-ray imager has a field of view of 9 arcminutes and an angular resolution of better than 8 arcsec; it will cover the energy range from 3 up to 50-70 keV with a spectral resolution of better than 1 keV; and it will have sub-second temporal resolution. Title: Anticipated Results from the FOXSI SMEX Mission Authors: Shih, A. Y.; Christe, S.; Krucker, S.; Glesener, L.; Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.; Drake, J. F.; Gary, D. E.; Gburek, S.; Goetz, K.; Grefenstette, B.; Gubarev, M.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Ryan, D.; Schwartz, R.; Steslicki, M.; Turin, P.; Veronig, A.; Vilmer, N.; Warmuth, A.; White, S. M.; Woods, T. N. Bibcode: 2017AGUFMSH43C..03S Altcode: While there have been significant advances in our understanding of impulsive energy release at the Sun since the advent of RHESSI observations, there is a clear need for new X-ray observations that can capture the full range of emission in flares (e.g., faint coronal sources near bright chromospheric sources), follow the intricate evolution of energy release and changes in morphology, and search for the signatures of impulsive energy release in even the quiescent Sun. The FOXSI Small Explorer (SMEX) mission, currently undergoing a Phase A concept study, combines state-of-the-art grazing-incidence focusing optics with pixelated solid-state detectors to provide direct imaging of hard X-rays for the first time on a solar observatory. FOXSI's X-ray observations will provide quantitative information on (1) the non-thermal populations of accelerated electrons and (2) the thermal plasma distributions at the high temperatures inaccessible through other wavelengths. FOXSI's major science questions include: Where are electrons accelerated and on what time scales? Where do escaping flare-accelerated electrons originate? What is the energy input of accelerated electrons into the chromosphere and corona? How much do flare-like processes heat the corona above active regions? Here we present examples with simulated observations to show how FOXSI's capabilities will address and resolve these and other questions. Title: Solar Coronal Events with Extended Hard X-ray and Gamma-ray Emission Authors: Hudson, H. S. Bibcode: 2017AGUFMSH44B..02H Altcode: A characteristic pattern of solar hard X-ray emission, first identified in SOL1969-03-31 by Frost & Dennis (1971) now has been linked to prolonged high-energy gamma-ray emission detected by the Fermi/LAT experiment, for example in SOL2014-09-01. The distinctive features of these events include flat hard X-ray spectra extending well above 100 keV, a characteristic pattern of time development, low-frequency gyrosynchrotron peaks, CME association, and gamma-rays identifiable with pion decay originating in GeV ions. The identification of these events with otherwise known solar structures nevertheless remains elusive, in spite of the wealth of imagery available from AIA. The quandary is that these events have a clear association with CMEs in the high corona, and yet the gamma-ray production implicates the photosphere itself. The vanishingly small loss cone in the nominal acceleration region makes this extremely difficult. I propose direct inward advection of a part of the SEP particle population, as created on closed field structures, as a possible resolution of this puzzle, and note that this requires retracting magnetic structures on long time scales following the flare itself. Title: The First ALMA Observation of a Solar Plasmoid Ejection from an X-Ray Bright Point Authors: Shimojo, M.; Hudson, H. S.; White, S. M.; Bastian, T.; Iwai, K. Bibcode: 2017AGUFMSH41A2754S Altcode: Eruptive phenomena are important features of energy releases events, such solar flares, and have the potential to improve our understanding of the dynamics of the solar atmosphere. The 304 A EUV line of helium, formed at around 10^5 K, is found to be a reliable tracer of such phenomena, but the determination of physical parameters from such observations is not straightforward. We have observed a plasmoid ejection from an X-ray bright point simultaneously with ALMA, SDO/AIA, and Hinode/XRT. This paper reports the physical parameters of the plasmoid obtained by combining the radio, EUV, and X-ray data. As a result, we conclude that the plasmoid can consist either of (approximately) isothermal ∼10^5 K plasma that is optically thin at 100 GHz, or a ∼10^4 K core with a hot envelope. The analysis demonstrates the value of the additional temperature and density constraints that ALMA provides, and future science observations with ALMA will be able to match the spatial resolution of space-borne and other high-resolution telescopes. Title: High-Energy Aspects of Small-Scale Energy Release at the Sun Authors: Glesener, L.; Vievering, J. T.; Wright, P. J.; Hannah, I. G.; Panchapakesan, S. A.; Ryan, D.; Krucker, S.; Hudson, H. S.; Grefenstette, B.; White, S. M.; Smith, D. M.; Marsh, A.; Kuhar, M.; Christe, S.; Buitrago-Casas, J. C.; Musset, S.; Inglis, A. R. Bibcode: 2017AGUFMSM33E..04G Altcode: Large, powerful solar flares have been investigated in detail for decades, but it is only recently that high-energy aspects of small flares could be measured. These small-scale energy releases offer the opportunity to examine how particle acceleration characteristics scale down, which is critical for constraining energy transfer theories such as magnetic reconnection. Probing to minuscule flare sizes also brings us closer to envisioning the characteristics of the small "nanoflares" that may be responsible for heating the corona. A new window on small-scale flaring activity is now opening with the use of focusing hard X-ray instruments to observe the Sun. Hard X-rays are emitted by flare-accelerated electrons and strongly heated plasma, providing a relatively direct method of measuring energy release and particle acceleration properties. This work will show the first observations of sub-A class microflares using the FOXSI sounding rocket and the NuSTAR astrophysics spacecraft, both of which directly focus hard X-rays but have limited observing time on the Sun. These instruments serve as precursors to a spacecraft version of FOXSI, which will explore energy release across the entire range of flaring activity. Title: First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events Authors: Marsh, Andrew J.; Smith, David M.; Glesener, Lindsay; Hannah, Iain G.; Grefenstette, Brian W.; Caspi, Amir; Krucker, Säm; Hudson, Hugh S.; Madsen, Kristin K.; White, Stephen M.; Kuhar, Matej; Wright, Paul J.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Harrison, Fiona A.; Stern, Daniel; Zhang, William W. Bibcode: 2017ApJ...849..131M Altcode: 2017arXiv171105385M We present the first results of a search for transient hard X-ray (HXR) emission in the quiet solar corona with the Nuclear Spectroscopic Telescope Array (NuSTAR) satellite. While NuSTAR was designed as an astrophysics mission, it can observe the Sun above 2 keV with unprecedented sensitivity due to its pioneering use of focusing optics. NuSTAR first observed quiet-Sun regions on 2014 November 1, although out-of-view active regions contributed a notable amount of background in the form of single-bounce (unfocused) X-rays. We conducted a search for quiet-Sun transient brightenings on timescales of 100 s and set upper limits on emission in two energy bands. We set 2.5-4 keV limits on brightenings with timescales of 100 s, expressed as the temperature T and emission measure EM of a thermal plasma. We also set 10-20 keV limits on brightenings with timescales of 30, 60, and 100 s, expressed as model-independent photon fluxes. The limits in both bands are well below previous HXR microflare detections, though not low enough to detect events of equivalent T and EM as quiet-Sun brightenings seen in soft X-ray observations. We expect future observations during solar minimum to increase the NuSTAR sensitivity by over two orders of magnitude due to higher instrument livetime and reduced solar background. Title: Formation of the thermal infrared continuum in solar flares Authors: Simões, Paulo J. A.; Kerr, Graham S.; Fletcher, Lyndsay; Hudson, Hugh S.; Giménez de Castro, C. Guillermo; Penn, Matt Bibcode: 2017A&A...605A.125S Altcode: 2017arXiv170609867S
Aims: Observations of the Sun with the Atacama Large Millimeter Array have now started, and the thermal infrared will regularly be accessible from the NSF's Daniel K. Inouye Solar Telescope. Motivated by the prospect of these new data, and by recent flare observations in the mid infrared, we set out here to model and understand the source of the infrared continuum in flares, and to explore its diagnostic capability for the physical conditions in the flare atmosphere.
Methods: We use the one-dimensional (1D) radiation hydrodynamics code RADYN to calculate mid-infrared continuum emission from model atmospheres undergoing sudden deposition of energy by non-thermal electrons.
Results: We identify and characterise the main continuum thermal emission processes relevant to flare intensity enhancement in the mid- to far-infrared (2-200 μm) spectral range as free-free emission on neutrals and ions. We find that the infrared intensity evolution tracks the energy input to within a second, albeit with a lingering intensity enhancement, and provides a very direct indication of the evolution of the atmospheric ionisation. The prediction of highly impulsive emission means that, on these timescales, the atmospheric hydrodynamics need not be considered in analysing the mid-IR signatures. Title: Microflare Heating of an Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA Authors: Wright, Paul James; Hannah, Iain; Grefenstette, Brian; Glesener, Lindsay; Krucker, Sam; Hudson, Hugh S.; Smith, David M.; Marsh, Andrew; White, Stephen M.; Kuhar, Matej Bibcode: 2017SPD....4810802W Altcode: We present the first joint observation of a GOES equivalent A0.2 microflare that occurred on the 29 Apr 2015 with Hinode/XRT and NuSTAR. During the three hours of combined observation we observe distinctive loop heating in the soft X-rays from Hinode/XRT, and the hottest channels from SDO/AIA. Crucially the impulsive phase of this microflare was also observed by NuSTAR, a highly sensitive hard X-ray (2.5-80 keV; Harrison et al. 2013) focussing optics imaging spectrometer. The NuSTAR spectrum before and after the microflare is well-fitted by a single thermal model of about 3.3 - 3.5 MK, but at the impulsive phase shows additional material up to 10 MK. This higher temperature emission is confirmed when we produce the DEM using a combination of SDO/AIA, Hinode/XRT, and NuSTAR data. During the impulsive phase of the microflare we determine the heating rate to be about 3 x 1025 erg s-1. Although non-thermal emission is not detected we find upper-limits that are consistent with the required heating rate. Title: The thermal infrared continuum in solar flares Authors: Fletcher, Lyndsay; Simoes, Paulo; Kerr, Graham Stewart; Hudson, Hugh S.; Gimenez de Castro, C. Guillermo; Penn, Matthew J. Bibcode: 2017SPD....4810821F Altcode: Observations of the Sun with the Atacama Large Millimeter Array have now started, and the thermal infrared will regularly be accessible from the NSF’s Daniel K. Inouye Solar Telescope. Motivated by the prospect of these new observations, and by recent flare detections in the mid infrared, we set out here to model and understand the source of the infrared continuum in flares, and to explore its diagnostic capability for the physical conditions in the flare atmosphere. We use the 1D radiation hydrodynamics code RADYN to calculate mid-infrared continuum emission from model atmospheres undergoing sudden deposition of energy by non-thermal electrons. We identify and characterise the main continuum thermal emission processes relevant to flare intensity enhancement in the mid- to far-infrared (2-200 micron) spectral range as free-free emission on neutrals and ions. We find that the infrared intensity evolution tracks the energy input to within a second, albeit with a lingering intensity enhancement, and provides a very direct indication of the evolution of the atmospheric ionization. The prediction of highly impulsive emission means that, on these timescales, the atmospheric hydrodynamics need not be considered in analysing the mid-IR signatures. Title: Solar Commissioning Observations of the Sun with ALMA Authors: White, Stephen M.; Shimojo, Masumi; Bastian, Timothy S.; Iwai, Kazumasa; Hales, Antonio; Brajsa, Roman; Skokic, Ivica; Kim, Sujin; Hudson, Hugh S.; Loukitcheva, Maria; Wedemeyer, Sven Bibcode: 2017SPD....4820402W Altcode: PI-led science observations have commenced with the Atacama Large Millimeter-submillimeter Array (ALMA) following an extensive commissioning effort. This talk will summarize that effort and discuss some of the scientific results derived from the commissioning data. As the solar cycle declines, ALMA observations will mainly address chromospheric science topics. Examples of data obtained during commissioning, both from the interferometer and from single-dish observations, will be presented. The temperatures of the layers that ALMA is most sensitive to have been determined for the two frequency bands currently used for solar observations. Curious behavior in a sunspot umbra and an observations of a small chromospheric ejection will be discussed. Title: Results from NuSTAR: Dynamics and time evolution in a sub-A class hard X-ray flare Authors: Glesener, Lindsay; Krucker, Sam; Hannah, Iain; Hudson, Hugh S.; Grefenstette, Brian; White, Stephen M.; Smith, David M.; Marsh, Andrew Bibcode: 2017SPD....4810803G Altcode: We report a NuSTAR observation of a solar microflare, SOL2015-09-01T04. Although it was too faint to be observed by the GOES X-ray Sensor, we estimate the flare to be an A0.2 class flare in brightness. This flare, with only ∼5 counts s-1 detector-1 observed by RHESSI, is fainter than any hard X-ray (HXR) flare in the existing literature. The flare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR flare spectra and images. The flare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peaking time with higher energy, similar spatial dimensions to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The flare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with its arising via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the flare to be 1.8×1027 ergs. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as “flarelike” properties. Title: Magnetic Properties of Solar Active Regions that Govern Large Solar Flares and Eruptions Authors: Toriumi, Shin; Schrijver, Carolus J.; Harra, Louise; Hudson, Hugh S.; Nagashima, Kaori Bibcode: 2017SPD....4820001T Altcode: Strong flares and CMEs are often produced from active regions (ARs). In order to better understand the magnetic properties and evolutions of such ARs, we conducted statistical investigations on the SDO/HMI and AIA data of all flare events with GOES levels >M5.0 within 45 deg from the disk center for 6 years from May 2010 (from the beginning to the declining phase of solar cycle 24). Out of the total of 51 flares from 29 ARs, more than 80% have delta-sunspots and about 15% violate Hale’s polarity rule. We obtained several key findings including (1) the flare duration is linearly proportional to the separation of the flare ribbons (i.e., scale of reconnecting magnetic fields) and (2) CME-eruptive events have smaller sunspot areas. Depending on the magnetic properties, flaring ARs can be categorized into several groups, such as spot-spot, in which a highly-sheared polarity inversion line is formed between two large sunspots, and spot-satellite, where a newly-emerging flux next to a mature sunspot triggers a compact flare event. These results point to the possibility that magnetic structures of the ARs determine the characteristics of flares and CMEs. In the presentation, we will also show new results from the systematic flux emergence simulations of delta-sunspot formation and discuss the evolution processes of flaring ARs. Title: NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare Authors: Glesener, Lindsay; Krucker, Säm; Hannah, Iain G.; Hudson, Hugh; Grefenstette, Brian W.; White, Stephen M.; Smith, David M.; Marsh, Andrew J. Bibcode: 2017ApJ...845..122G Altcode: 2017arXiv170704770G We report a Nuclear Spectroscopic Telescope Array (NuSTAR) observation of a solar microflare, SOL2015-09-01T04. Although it was too faint to be observed by the GOES X-ray Sensor, we estimate the event to be an A0.1 class flare in brightness. This microflare, with only ∼5 counts s-1 detector-1 observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is fainter than any hard X-ray (HXR) flare in the existing literature. The microflare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR microflare spectra and images. The microflare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peak time with higher energy, spatial dimensions similar to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The microflare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with an origin via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the microflare to be 2.4 × 1027 erg. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as “flare-like” properties. Title: The Eclipse Megamovie Project Authors: Hudson, Hugh; Bender, Mark Bibcode: 2017S&T...134b..20H Altcode: No abstract at ADS Title: Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA Authors: Wright, Paul J.; Hannah, Iain G.; Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; Smith, David M.; Marsh, Andrew J.; White, Stephen M.; Kuhar, Matej Bibcode: 2017ApJ...844..132W Altcode: 2017arXiv170606108W NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has observed several small microflares in its initial solar pointings. In this paper, we present the first joint observation of a microflare with NuSTAR and Hinode/XRT on 2015 April 29 at ∼11:29 UT. This microflare shows the heating of material to several million Kelvin, observed in soft X-rays with Hinode/XRT, and was faintly visible in the extreme ultraviolet with SDO/AIA. For three of the four NuSTAR observations of this region (pre-flare, decay, and post-flare phases), the spectrum is well fitted by a single thermal model of 3.2-3.5 MK, but the spectrum during the impulsive phase shows additional emission up to 10 MK, emission equivalent to the A0.1 GOES class. We recover the differential emission measure (DEM) using SDO/AIA, Hinode/XRT, and NuSTAR, giving unprecedented coverage in temperature. We find that the pre-flare DEM peaks at ∼3 MK and falls off sharply by 5 MK; but during the microflare’s impulsive phase, the emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of about 2.5× {10}25 erg s-1. As the NuSTAR spectrum is purely thermal, we determined upper limits on the possible non-thermal bremsstrahlung emission. We find that for the accelerated electrons to be the source of heating, a power-law spectrum of δ ≥slant 7 with a low-energy cutoff {E}c≲ 7 keV is required. In summary, this first NuSTAR microflare strongly resembles much more powerful flares. Title: Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric Imaging Authors: Shimojo, M.; Bastian, T. S.; Hales, A. S.; White, S. M.; Iwai, K.; Hills, R. E.; Hirota, A.; Phillips, N. M.; Sawada, T.; Yagoubov, P.; Siringo, G.; Asayama, S.; Sugimoto, M.; Brajša, R.; Skokić, I.; Bárta, M.; Kim, S.; de Gregorio-Monsalvo, I.; Corder, S. A.; Hudson, H. S.; Wedemeyer, S.; Gary, D. E.; De Pontieu, B.; Loukitcheva, M.; Fleishman, G. D.; Chen, B.; Kobelski, A.; Yan, Y. Bibcode: 2017SoPh..292...87S Altcode: 2017arXiv170403236S Observations of the Sun at millimeter and submillimeter wavelengths offer a unique probe into the structure, dynamics, and heating of the chromosphere; the structure of sunspots; the formation and eruption of prominences and filaments; and energetic phenomena such as jets and flares. High-resolution observations of the Sun at millimeter and submillimeter wavelengths are challenging due to the intense, extended, low-contrast, and dynamic nature of emission from the quiet Sun, and the extremely intense and variable nature of emissions associated with energetic phenomena. The Atacama Large Millimeter/submillimeter Array (ALMA) was designed with solar observations in mind. The requirements for solar observations are significantly different from observations of sidereal sources and special measures are necessary to successfully carry out this type of observations. We describe the commissioning efforts that enable the use of two frequency bands, the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum interferometric-imaging observations of the Sun with ALMA. Examples of high-resolution synthesized images obtained using the newly commissioned modes during the solar-commissioning campaign held in December 2015 are presented. Although only 30 of the eventual 66 ALMA antennas were used for the campaign, the solar images synthesized from the ALMA commissioning data reveal new features of the solar atmosphere that demonstrate the potential power of ALMA solar observations. The ongoing expansion of ALMA and solar-commissioning efforts will continue to enable new and unique solar observing capabilities. Title: The Solar X-Ray Limb Authors: Battaglia, Marina; Hudson, Hugh S.; Hurford, Gordon J.; Krucker, Säm; Schwartz, Richard A. Bibcode: 2017ApJ...843..123B Altcode: 2017arXiv170511044B We describe a new technique to measure the height of the X-ray limb with observations from occulted X-ray flare sources as observed by the RHESSI (the Reuven Ramaty High-Energy Spectroscopic Imager) satellite. This method has model dependencies different from those present in traditional observations at optical wavelengths, which depend upon detailed modeling involving radiative transfer in a medium with complicated geometry and flows. It thus provides an independent and more rigorous measurement of the “true” solar radius, which means that of the mass distribution. RHESSI’s measurement makes use of the flare X-ray source’s spatial Fourier components (the visibilities), which are sensitive to the presence of the sharp edge at the lower boundary of the occulted source. We have found a suitable flare event for analysis, SOL2011-10-20T03:25 (M1.7), and report a first result from this novel technique here. Using a four-minute integration over the 3-25 keV photon energy range, we find {R}{{X} - {ray}}=960.11+/- 0.15+/- 0.29 arcsec, at 1 au, where the uncertainties include statistical uncertainties from the method and a systematic error. The standard VAL-C model predicts a value of 959.94 arcsec, which is about 1σ below our value. Title: Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping Authors: White, S. M.; Iwai, K.; Phillips, N. M.; Hills, R. E.; Hirota, A.; Yagoubov, P.; Siringo, G.; Shimojo, M.; Bastian, T. S.; Hales, A. S.; Sawada, T.; Asayama, S.; Sugimoto, M.; Marson, R. G.; Kawasaki, W.; Muller, E.; Nakazato, T.; Sugimoto, K.; Brajša, R.; Skokić, I.; Bárta, M.; Kim, S.; Remijan, A. J.; de Gregorio, I.; Corder, S. A.; Hudson, H. S.; Loukitcheva, M.; Chen, B.; De Pontieu, B.; Fleishmann, G. D.; Gary, D. E.; Kobelski, A.; Wedemeyer, S.; Yan, Y. Bibcode: 2017SoPh..292...88W Altcode: 2017arXiv170504766W The Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12 m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that uses the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions, we derive quiet-Sun values at disk center of 7300 K at λ =3 mm and 5900 K at λ =1.3 mm. These values have statistical uncertainties of about 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of about 25, the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range. Active regions and plages are among the hotter features, while a large sunspot umbra shows up as a depression, and filament channels are relatively cool. Prominences above the solar limb are a common feature of the single-dish images. Title: The First ALMA Observation of a Solar Plasmoid Ejection from an X-Ray Bright Point Authors: Shimojo, Masumi; Hudson, Hugh S.; White, Stephen M.; Bastian, Timothy S.; Iwai, Kazumasa Bibcode: 2017ApJ...841L...5S Altcode: 2017arXiv170404881S Eruptive phenomena such as plasmoid ejections or jets are important features of solar activity and have the potential to improve our understanding of the dynamics of the solar atmosphere. Such ejections are often thought to be signatures of the outflows expected in regions of fast magnetic reconnection. The 304 Å EUV line of helium, formed at around 105 K, is found to be a reliable tracer of such phenomena, but the determination of physical parameters from such observations is not straightforward. We have observed a plasmoid ejection from an X-ray bright point simultaneously at millimeter wavelengths with ALMA, at EUV wavelengths with SDO/AIA, and in soft X-rays with Hinode/XRT. This paper reports the physical parameters of the plasmoid obtained by combining the radio, EUV, and X-ray data. As a result, we conclude that the plasmoid can consist either of (approximately) isothermal ∼105 K plasma that is optically thin at 100 GHz, or a ∼104 K core with a hot envelope. The analysis demonstrates the value of the additional temperature and density constraints that ALMA provides, and future science observations with ALMA will be able to match the spatial resolution of space-borne and other high-resolution telescopes. Title: Science Objective: Understanding Energy Transport by Alfvénic Waves in Solar Flares Authors: Reep, Jeffrey W.; Warren, Harry P.; Leake, James E.; Tarr, Lucas A.; Russell, Alexander J. B.; Kerr, Graham S.; Hudson, Hugh S. Bibcode: 2017arXiv170201667R Altcode: Solar flares are driven by the release of magnetic energy from reconnection events in the solar corona, whereafter energy is transported to the chromosphere, heating the plasma and causing the characteristic radiative losses. In the collisional thick-target model, electrons accelerated to energies exceeding 10 keV traverse the corona and impact the chromosphere, where they deposit their energy through collisions with the much denser plasma in the lower atmosphere. While there are undoubtedly high energy non-thermal electrons accelerated in flares, it is unclear whether these electron beams are the sole mechanism of energy transport, or whether they only dominate in certain phases of the flare's evolution. Alfvénic waves are generated during the post-reconnection relaxation of magnetic field lines, so it is important to examine their role in energy transport. Title: Exploring impulsive solar magnetic energy release and particle acceleration with focused hard X-ray imaging spectroscopy Authors: Christe, Steven; Krucker, Samuel; Glesener, Lindsay; Shih, Albert; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel; Battaglia, Marina; Chen, Bin; Drake, James; Dennis, Brian; Gary, Dale; Gburek, Szymon; Goetz, Keith; Grefenstette, Brian; Gubarev, Mikhail; Hannah, Iain; Holman, Gordon; Hudson, Hugh; Inglis, Andrew; Ireland, Jack; Ishikawa, Shinosuke; Klimchuk, James; Kontar, Eduard; Kowalski, Adam; Longcope, Dana; Massone, Anna-Maria; Musset, Sophie; Piana, Michele; Ramsey, Brian; Ryan, Daniel; Schwartz, Richard; Stęślicki, Marek; Turin, Paul; Warmuth, Alexander; Wilson-Hodge, Colleen; White, Stephen; Veronig, Astrid; Vilmer, Nicole; Woods, Tom Bibcode: 2017arXiv170100792C Altcode: How impulsive magnetic energy release leads to solar eruptions and how those eruptions are energized and evolve are vital unsolved problems in Heliophysics. The standard model for solar eruptions summarizes our current understanding of these events. Magnetic energy in the corona is released through drastic restructuring of the magnetic field via reconnection. Electrons and ions are then accelerated by poorly understood processes. Theories include contracting loops, merging magnetic islands, stochastic acceleration, and turbulence at shocks, among others. Although this basic model is well established, the fundamental physics is poorly understood. HXR observations using grazing-incidence focusing optics can now probe all of the key regions of the standard model. These include two above-the-looptop (ALT) sources which bookend the reconnection region and are likely the sites of particle acceleration and direct heating. The science achievable by a direct HXR imaging instrument can be summarized by the following science questions and objectives which are some of the most outstanding issues in solar physics (1) How are particles accelerated at the Sun? (1a) Where are electrons accelerated and on what time scales? (1b) What fraction of electrons is accelerated out of the ambient medium? (2) How does magnetic energy release on the Sun lead to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI) instrument, which can be built now using proven technology and at modest cost, would enable revolutionary advancements in our understanding of impulsive magnetic energy release and particle acceleration, a process which is known to occur at the Sun but also throughout the Universe. Title: Magnetic Properties of Solar Active Regions That Govern Large Solar Flares and Eruptions Authors: Toriumi, Shin; Schrijver, Carolus J.; Harra, Louise K.; Hudson, Hugh; Nagashima, Kaori Bibcode: 2017ApJ...834...56T Altcode: 2016arXiv161105047T Solar flares and coronal mass ejections (CMEs), especially the larger ones, emanate from active regions (ARs). With the aim of understanding the magnetic properties that govern such flares and eruptions, we systematically survey all flare events with Geostationary Orbiting Environmental Satellite levels of ≥M5.0 within 45° from disk center between 2010 May and 2016 April. These criteria lead to a total of 51 flares from 29 ARs, for which we analyze the observational data obtained by the Solar Dynamics Observatory. More than 80% of the 29 ARs are found to exhibit δ-sunspots, and at least three ARs violate Hale’s polarity rule. The flare durations are approximately proportional to the distance between the two flare ribbons, to the total magnetic flux inside the ribbons, and to the ribbon area. From our study, one of the parameters that clearly determine whether a given flare event is CME-eruptive or not is the ribbon area normalized by the sunspot area, which may indicate that the structural relationship between the flaring region and the entire AR controls CME productivity. AR characterization shows that even X-class events do not require δ-sunspots or strong-field, high-gradient polarity inversion lines. An investigation of historical observational data suggests the possibility that the largest solar ARs, with magnetic flux of 2 × 1023 Mx, might be able to produce “superflares” with energies of the order of 1034 erg. The proportionality between the flare durations and magnetic energies is consistent with stellar flare observations, suggesting a common physical background for solar and stellar flares. Title: Evidence of Significant Energy Input in the Late Phase of a Solar Flare from NuSTAR X-Ray Observations Authors: Kuhar, Matej; Krucker, Säm; Hannah, Iain G.; Glesener, Lindsay; Saint-Hilaire, Pascal; Grefenstette, Brian W.; Hudson, Hugh S.; White, Stephen M.; Smith, David M.; Marsh, Andrew J.; Wright, Paul J.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Harrison, Fiona A.; Stern, Daniel; Zhang, William W. Bibcode: 2017ApJ...835....6K Altcode: 2017arXiv170107759K We present observations of the occulted active region AR 12222 during the third Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaign on 2014 December 11, with concurrent Solar Dynamics Observatory (SDO)/AIA and FOXSI-2 sounding rocket observations. The active region produced a medium-size solar flare 1 day before the observations, at ∼18 UT on 2014 December 10, with the post-flare loops still visible at the time of NuSTAR observations. The time evolution of the source emission in the SDO/AIA 335 Å channel reveals the characteristics of an extreme-ultraviolet late-phase event, caused by the continuous formation of new post-flare loops that arch higher and higher in the solar corona. The spectral fitting of NuSTAR observations yields an isothermal source, with temperature 3.8-4.6 MK, emission measure (0.3-1.8) × 1046 cm-3, and density estimated at (2.5-6.0) × 108 cm-3. The observed AIA fluxes are consistent with the derived NuSTAR temperature range, favoring temperature values in the range of 4.0-4.3 MK. By examining the post-flare loops’ cooling times and energy content, we estimate that at least 12 sets of post-flare loops were formed and subsequently cooled between the onset of the flare and NuSTAR observations, with their total thermal energy content an order of magnitude larger than the energy content at flare peak time. This indicates that the standard approach of using only the flare peak time to derive the total thermal energy content of a flare can lead to a large underestimation of its value. Title: NuSTAR's X-ray search for high energy emission from weakly flaring active regions Authors: Hannah, I. G.; Grefenstette, B.; Glesener, L.; Krucker, S.; Hudson, H. S.; Smith, D. M.; White, S.; Marsh, A.; Wright, P. J.; Kuhar, M. Bibcode: 2016AGUFMSH11D..07H Altcode: The NuSTAR X-ray focusing optics telescope, provides highly sensitivity imaging spectroscopy over 2-78 keV. Most of NuSTAR's time is spent on targets outside of the solar system but some is devoted to the Sun; solar observations began in late 2014 (Grefenstette et al. 2016 ApJ). Although not optimized for solar observations it is highly capable of searching for the weak X-ray emission from high temperature (> 5MK) or non-thermal components from the weakly or non-flaring Sun. Such emission provides strong constraints on the nature of energy release during these more quiescent times. NuSTAR has observed quiescent/non-flaring active regions from its first observations late in 2014, finding sources between 3.1-4.4 MK. These data placed strict upper limits on higher temperature emission (Hannah et al. 2016 ApJL). These observations had limited spectral dynamic range due to short effective exposures (duration and high deadtime), restricting our ability to detect higher temperature or non-thermal emission. With weakening solar activity since then we present further observations during 2015 and 2016 of microflares and non-flaring active regions with longer exposures (in part due to the decreasing detector deadtime). We also present robust multi-thermal emission measure distributions obtained by combining our NuSTAR observations with EUV data from SDO/AIA and softer X-rays from Hinode/XRT. Title: Focusing Solar Hard X-rays: Expected Results from a FOXSI Spacecraft Authors: Glesener, L.; Christe, S.; Shih, A. Y.; Dennis, B. R.; Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Ryan, D.; Inglis, A. R.; Hannah, I. G.; Caspi, A.; Klimchuk, J. A.; Drake, J. F.; Kontar, E.; Holman, G.; White, S. M.; Alaoui, M.; Battaglia, M.; Vilmer, N.; Allred, J. C.; Longcope, D. W.; Gary, D. E.; Jeffrey, N. L. S.; Musset, S.; Swisdak, M. Bibcode: 2016AGUFMSH13A2282G Altcode: Over the course of two solar cycles, RHESSI has examined high-energy processes in flares via high-resolution spectroscopy and imaging of soft and hard X-rays (HXRs). The detected X-rays are the thermal and nonthermal bremsstrahlung from heated coronal plasma and from accelerated electrons, respectively, making them uniquely suited to explore the highest-energy processes that occur in the corona. RHESSI produces images using an indirect, Fourier-based method and has made giant strides in our understanding of these processes, but it has also uncovered intriguing new mysteries regarding energy release location, acceleration mechanisms, and energy propagation in flares. Focusing optics are now available for the HXR regime and stand poised to perform another revolution in the field of high-energy solar physics. With two successful sounding rocket flights completed, the Focusing Optics X-ray Solar Imager (FOXSI) program has demonstrated the feasibility and power of direct solar HXR imaging with its vastly superior sensitivity and dynamic range. Placing this mature technology aboard a spacecraft will offer a systematic way to explore high-energy aspects of the solar corona and to address scientific questions left unanswered by RHESSI. Here we present examples of such questions and show simulations of expected results from a FOXSI spaceborne instrument to demonstrate how these questions can be addressed with the focusing of hard X-rays. Title: Arcade Implosion Caused by a Filament Eruption in a Flare Authors: Wang, Juntao; Simões, P. J. A.; Fletcher, L.; Thalmann, J. K.; Hudson, H. S.; Hannah, I. G. Bibcode: 2016ApJ...833..221W Altcode: 2016arXiv161005931W Coronal implosions—the convergence motion of plasmas and entrained magnetic field in the corona due to a reduction in magnetic pressure—can help to locate and track sites of magnetic energy release or redistribution during solar flares and eruptions. We report here on the analysis of a well-observed implosion in the form of an arcade contraction associated with a filament eruption, during the C3.5 flare SOL2013-06-19T07:29. A sequence of events including the magnetic flux-rope instability and distortion, followed by a filament eruption and arcade implosion, lead us to conclude that the implosion arises from the transfer of magnetic energy from beneath the arcade as part of the global magnetic instability, rather than due to local magnetic energy dissipation in the flare. The observed net contraction of the imploding loops, which is found also in nonlinear force-free field extrapolations, reflects a permanent reduction of magnetic energy underneath the arcade. This event shows that, in addition to resulting in the expansion or eruption of an overlying field, flux-rope instability can also simultaneously implode an unopened field due to magnetic energy transfer. It demonstrates the “partial opening of the field” scenario, which is one of the ways in 3D to produce a magnetic eruption without violating the Aly-Sturrock hypothesis. In the framework of this observation, we also propose a unification of three main concepts for active region magnetic evolution, namely the metastable eruption model, the implosion conjecture, and the standard “CSHKP” flare model. Title: RHESSI/SAS Observations of the Optical Solar Limb Over More Than 15 Years Authors: Fivian, M. D.; Hudson, H. S.; Krucker, S. Bibcode: 2016AGUFMSH42B..08F Altcode: The Solar Aspect System (SAS) of the RHESSI satellite measures the optical solar limb with a cadence typically set at 100 samples/s.RHESSI has observed the Sun continuously since its launch in early 2002, and we have acquired a unique data set ranging over more than a full 11-year solar cycle and consisting of about 4x10^10 single data points.The optics has a point spread of about 4.5 arcsec FWHM imaging the red continuum onto three linear CCD sensors with a pixel resolution of 1.7 arcsec.However, careful study of systematics, masking of contaminated data, and accumulation of data over appropriate time intervals has led to measurementswith sub-milli arcsec accuracy.Analyzing data for an initial period in 2004, these measurements have led to the most accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian et al., 2008), a value consistent with models predicting an oblateness from surface rotation.An excess oblateness term can be attributed to magnetic elements possibly located in the enhanced network.We also study photometric properties of our data. Previous observations of latitude-dependent brightness variations at the limb had suggested the presence of a polar temperature excess as large as 1.5 K.The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation.Our measurements of latitude-dependent brightness variations at the limb lead to a quadrupolar term (a pole-to-equator temperature variation) of the order of 0.1 K, an order of magnitude smaller than previously reported.We present the analysis of these unique data, an overview of some results and we report on our progress as we apply our developed analysis method to the whole 15 years of data. Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.; Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Grefenstette, B.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Gubarev, M.; Schwartz, R. A.; Steslicki, M.; Ryan, D.; Turin, P.; Warmuth, A.; White, S. M.; Veronig, A.; Vilmer, N.; Dennis, B. R. Bibcode: 2016AGUFMSH13A2281C Altcode: We present FOXSI (Focusing Optics X-ray Solar Imager), a recently proposed Small Explorer (SMEX) mission that will provide a revolutionary new perspective on energy release and particle acceleration on the Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic range and better than 10x the sensitivity of previous instruments. Flown on a 3-axis stabilized spacecraft in low-Earth orbit, FOXSI uses high-angular-resolution grazing-incidence focusing optics combined with state-of-the-art pixelated solid-state detectors to provide direct imaging of solar hard X-rays for the first time. FOXSI is composed of two individual x-ray telescopes with a 14-meter focal length enabled by a deployable boom. Making use of a filter-wheel and high-rate-capable solid-state detectors, FOXSI will be able to observe the largest flares without saturation while still maintaining the sensitivity to detect x-ray emission from weak flares, escaping electrons, and hot active regions. This SMEX mission is made possible by past experience with similar instruments on two sounding rocket flights, in 2012 and 2014, and on the HEROES balloon flight in 2013. FOXSI will image the Sun with a field of view of 9 arcminutes and an angular resolution of better than 8 arcsec; it will cover the energy range from 3 to 100 keV with a spectral resolution of better than 1 keV; and it will have sub-second temporal resolution. Title: EVE Doppler Signatures in X-class Flares Authors: Hudson, H. S.; Chamberlin, P. C.; MacKinnon, A.; Woods, T. N. Bibcode: 2016AGUFMSH13A2291H Altcode: The MEGS-A spectroscopy of the Sun as a star has produced a greatdeal of interesting solar physics, but without much exploitationyet of its Doppler capabilities (see Chamberlin, 2016 for details).In this presentation we examine the He II 304 line specifically andin specifically on the database of X-class flares from MEGS-A'sactive lifetime (34 events). The gradual-phase emission allows usto obtain a local reference for the rest wavelength to good accuracy,and the impulsive-phase centroid wavelengths typically shift to thered with values consistent with those of Milligan and Dennis (2009),but with smaller uncertainties. The exception events include SOL2014-02-25 (X4.9), which we find to have a blueshifted He IIline at an apparent 105 +- 5 km/s relative to its rest value. Thisevent also includes an unusual coronal hard X-ray source and aFermi/LAT high-energy gamma-ray emission. The EVE Doppler shiftanalysis can also be done at many other emission lines, includingthose covered by MEGS-B in the 33-106 nm range. Title: Sunquake Generation by Coronal Magnetic Restructuring Authors: Russell, A. J. B.; Mooney, M. K.; Leake, J. E.; Hudson, H. S. Bibcode: 2016ApJ...831...42R Altcode: 2016arXiv160208245R Sunquakes are the surface signatures of acoustic waves in the Sun’s interior that are produced by some but not all flares and coronal mass ejections (CMEs). This paper explores a mechanism for sunquake generation by the changes in magnetic field that occur during flares and CMEs, using MHD simulations with a semiempirical FAL-C atmosphere to demonstrate the generation of acoustic waves in the interior in response to changing magnetic tilt in the corona. We find that Alfvén-sound resonance combined with the ponderomotive force produces acoustic waves in the interior with sufficient energy to match sunquake observations when the magnetic field angle changes of the order of 10° in a region where the coronal field strength is a few hundred gauss or more. The most energetic sunquakes are produced when the coronal field is strong, while the variation of magnetic field strength with height and the timescale of the change in tilt are of secondary importance. Title: Flare differentially rotates sunspot on Sun's surface Authors: Liu, Chang; Xu, Yan; Cao, Wenda; Deng, Na; Lee, Jeongwoo; Hudson, Hugh S.; Gary, Dale E.; Wang, Jiasheng; Jing, Ju; Wang, Haimin Bibcode: 2016NatCo...713104L Altcode: 2016arXiv161002969L Sunspots are concentrations of magnetic field visible on the solar surface (photosphere). It was considered implausible that solar flares, as resulted from magnetic reconnection in the tenuous corona, would cause a direct perturbation of the dense photosphere involving bulk motion. Here we report the sudden flare-induced rotation of a sunspot using the unprecedented spatiotemporal resolution of the 1.6 m New Solar Telescope, supplemented by magnetic data from the Solar Dynamics Observatory. It is clearly observed that the rotation is non-uniform over the sunspot: as the flare ribbon sweeps across, its different portions accelerate (up to ~50° h-1) at different times corresponding to peaks of flare hard X-ray emission. The rotation may be driven by the surface Lorentz-force change due to the back reaction of coronal magnetic restructuring and is accompanied by a downward Poynting flux. These results have direct consequences for our understanding of energy and momentum transportation in the flare-related phenomena. Title: Properties and Developments of Flaring Active Regions Authors: Toriumi, Shin; Schrijver, Carolus J.; Harra, Louise K.; Hudson, Hugh; Nagashima, Kaori Bibcode: 2016usc..confE..15T Altcode: Larger flares and CMEs are often produced from active regions (ARs). In order to better understand the magnetic properties and evolutions of such ARs, we picked up all flare events with GOES levels >M5.0 with heliocentric angles of <45 deg in the period of May 2010 to April 2016, which led to a total of 29 ARs with 51 flares. We analyzed the observational data obtained by SDO and found that more than 80% of the 29 ARs have delta-sunspots. Most of them can be classified depending on their magnetic structures into (1) spot-spot, where a long sheared polarity inversion line (PIL: characterized by flare ribbons) is formed between two major sunspots, and (2) spot-satellite, where a newly-emerging minor bipole next to a pre-existing spot creates a compact PIL. The remaining minor groups are (3) quadrupole, where two emerging bipoles produce a PIL in between, and (4) inter-AR, which produces flares not from delta-spots but from between two separated ARs. From statistical investigations we found for example that the spot-spot group generally shows long-duration events due to large coronal structures, while the spot-satellite has impulsive events because of their compact magnetic nature. We will also present flux emergence simulations and discuss their formation processes. Title: The Characteristics of Solar X-Class Flares and CMEs: A Paradigm for Stellar Superflares and Eruptions? Authors: Harra, Louise K.; Schrijver, Carolus J.; Janvier, Miho; Toriumi, Shin; Hudson, Hugh; Matthews, Sarah; Woods, Magnus M.; Hara, Hirohisa; Guedel, Manuel; Kowalski, Adam; Osten, Rachel; Kusano, Kanya; Lueftinger, Theresa Bibcode: 2016SoPh..291.1761H Altcode: 2016SoPh..tmp..111H This paper explores the characteristics of 42 solar X-class flares that were observed between February 2011 and November 2014, with data from the Solar Dynamics Observatory (SDO) and other sources. This flare list includes nine X-class flares that had no associated CMEs. In particular our aim was to determine whether a clear signature could be identified to differentiate powerful flares that have coronal mass ejections (CMEs) from those that do not. Part of the motivation for this study is the characterization of the solar paradigm for flare/CME occurrence as a possible guide to the stellar observations; hence we emphasize spectroscopic signatures. To do this we ask the following questions: Do all eruptive flares have long durations? Do CME-related flares stand out in terms of active-region size vs. flare duration? Do flare magnitudes correlate with sunspot areas, and, if so, are eruptive events distinguished? Is the occurrence of CMEs related to the fraction of the active-region area involved? Do X-class flares with no eruptions have weaker non-thermal signatures? Is the temperature dependence of evaporation different in eruptive and non-eruptive flares? Is EUV dimming only seen in eruptive flares? We find only one feature consistently associated with CME-related flares specifically: coronal dimming in lines characteristic of the quiet-Sun corona, i.e. 1 - 2 MK. We do not find a correlation between flare magnitude and sunspot areas. Although challenging, it will be of importance to model dimming for stellar cases and make suitable future plans for observations in the appropriate wavelength range in order to identify stellar CMEs consistently. Title: The First Focused Hard X-ray Images of the Sun with NuSTAR Authors: Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh; Hannah, Iain G.; Smith, David M.; Vogel, Julia K.; White, Stephen M.; Madsen, Kristin K.; Marsh, Andrew J.; Caspi, Amir; Chen, Bin; Shih, Albert; Kuhar, Matej; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Forster, Karl; Hailey, Charles J.; Harrison, Fiona A.; Miyasaka, Hiromasa; Stern, Daniel; Zhang, William W. Bibcode: 2016ApJ...826...20G Altcode: 2016arXiv160509738G We present results from the the first campaign of dedicated solar observations undertaken by the Nuclear Spectroscopic Telescope ARray (NuSTAR) hard X-ray (HXR) telescope. Designed as an astrophysics mission, NuSTAR nonetheless has the capability of directly imaging the Sun at HXR energies (>3 keV) with an increase in sensitivity of at least two magnitude compared to current non-focusing telescopes. In this paper we describe the scientific areas where NuSTAR will make major improvements on existing solar measurements. We report on the techniques used to observe the Sun with NuSTAR, their limitations and complications, and the procedures developed to optimize solar data quality derived from our experience with the initial solar observations. These first observations are briefly described, including the measurement of the Fe K-shell lines in a decaying X-class flare, HXR emission from high in the solar corona, and full-disk HXR images of the Sun. Title: Implications Of The Mid-IR For ALMA Flare Observations Authors: Hudson, Hugh; Krucker, Sam; Penn, Matt; Simoes, Paulo Bibcode: 2016csss.confE..49H Altcode: Poster presented at the Cool Stars 19 meeting in Uppsala, Sweden, June 2016 Title: The smallest hard X-ray flare? Authors: Glesener, Lindsay; Krucker, Sam; Hannah, Iain; Smith, David M.; Grefenstette, Brian; Marsh, Andrew; Hudson, Hugh S.; White, Stephen M.; Chen, Bin Bibcode: 2016SPD....4740302G Altcode: We report a NuSTAR observation of a small solar flare on 2015 September 1, estimated to be on the order of a GOES class A.05 flare in brightness. This flare is fainter than any hard X-ray (HXR) flares in the existing literature, and with a peak rate of only ∼5 counts s-1 detector-1 observed by RHESSI, is effectively the smallest that can just barely be detected by the current standard (indirectly imaging) solar HXR instrumentation, though we expect that smaller flares will continue to be discovered as instrumental and observational techniques progress. The flare occurred during a solar observation by the highly sensitive NuSTAR astrophysical HXR spacecraft, which used its direct focusing optics to produce detailed flare spectra and images. The flare exhibits properties commonly observed in larger flares, including a fast rise and more gradual decay, and similar spatial dimensions to the RHESSI microflares. We will discuss the presence of non-thermal (flare-accelerated) electrons during the impulsive phase. The flare is small in emission measure, temperature, and energy, though not in physical dimensions. Its presence is an indication that flares do indeed scale down to smaller energies and retain what we customarily think of as “flarelike” properties. Title: BBSO/NST Observations of the Sudden Differential Rotation of a Sunspot Caused by a Major Flare Authors: Liu, Chang; Xu, Yan; Deng, Na; Cao, Wenda; Lee, Jeongwoo; Hudson, Hugh S.; Gary, Dale E.; Wang, Jiasheng; Jing, Ju; Wang, Haimin Bibcode: 2016SPD....47.0615L Altcode: Sunspots are concentrations of magnetic field visible on the solar surface (photosphere), from which the field extends high into the corona. Complex plasma motions that drag field in the photosphere can build up free energy in the corona that powers solar eruptions. It is known that solar flares and the often associated coronal ejections (CMEs) can produce various radiations in the low atmosphere. However, it was considered implausible that disturbances created in the tenuous corona would cause a direct perturbation of the dense photosphere involving bulk motion. Here we report the sudden rotational motion of a sunspot clearly induced by a major solar flare (SOL2015-06-22T18:23 M6.6), using the unprecedented spatiotemporal resolution of the 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO). It is particularly striking that the rotation is not uniform over the sunspot: as the flare ribbon sweeps across, its different portions accelerate (up to ~50 degree per hour) at different times corresponding to peaks of flare hard X-ray emission. The intensity and magnetic field of the sunspot also change significantly associated with the flare. Our results reveal an intrinsic relationship between the photospheric plasma bulk motions and coronal energy release, with direct consequences for our understanding of energy and momentum balance in the flare/CME phenomenon. This work is mainly supported by NASA grants NNX13AF76G and NNX13AG13G (LWS), and NNX16AF72G, and NSF grants AGS 1250818 and 1408703. Title: Science Objectives of the FOXSI Small Explorer Mission Concept Authors: Shih, Albert Y.; Christe, Steven; Alaoui, Meriem; Allred, Joel C.; Antiochos, Spiro K.; Battaglia, Marina; Buitrago-Casas, Juan Camilo; Caspi, Amir; Dennis, Brian R.; Drake, James; Fleishman, Gregory D.; Gary, Dale E.; Glesener, Lindsay; Grefenstette, Brian; Hannah, Iain; Holman, Gordon D.; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa, Shin-Nosuke; Jeffrey, Natasha; Klimchuk, James A.; Kontar, Eduard; Krucker, Sam; Longcope, Dana; Musset, Sophie; Nita, Gelu M.; Ramsey, Brian; Ryan, Daniel; Saint-Hilaire, Pascal; Schwartz, Richard A.; Vilmer, Nicole; White, Stephen M.; Wilson-Hodge, Colleen Bibcode: 2016SPD....47.0814S Altcode: Impulsive particle acceleration and plasma heating at the Sun, from the largest solar eruptive events to the smallest flares, are related to fundamental processes throughout the Universe. While there have been significant advances in our understanding of impulsive energy release since the advent of RHESSI observations, there is a clear need for new X-ray observations that can capture the full range of emission in flares (e.g., faint coronal sources near bright chromospheric sources), follow the intricate evolution of energy release and changes in morphology, and search for the signatures of impulsive energy release in even the quiescent Sun. The FOXSI Small Explorer (SMEX) mission concept combines state-of-the-art grazing-incidence focusing optics with pixelated solid-state detectors to provide direct imaging of hard X-rays for the first time on a solar observatory. We present the science objectives of FOXSI and how its capabilities will address and resolve open questions regarding impulsive energy release at the Sun. These questions include: What are the time scales of the processes that accelerate electrons? How do flare-accelerated electrons escape into the heliosphere? What is the energy input of accelerated electrons into the chromosphere, and how is super-heated coronal plasma produced? Title: Chasing White-Light Flares Authors: Hudson, H. S. Bibcode: 2016SoPh..291.1273H Altcode: 2016SoPh..tmp...77H In this memoir I describe my life in research, mostly in the area of solar physics. The recurring theme is "white-light flares," and several sections of this paper deal with this and related phenomena; I wind up describing how I see the state of the art in this still-interesting and crucially important (as it has been since 1859) area of flare research. I also describe my participation in two long-lived satellite programs dedicated to solar observations (Yohkoh and RHESSI) and elaborate on their discoveries. These have both helped with white-light flares both directly and also with closely related X-ray and γ -ray emissions), with the result that this article leans heavily in that direction. Title: RHESSI/SAS Observations of the Optical Solar Limb Over More Than 14 Years Authors: Fivian, Martin; Hudson, Hugh S.; Krucker, Sam Bibcode: 2016SPD....47.1204F Altcode: The Solar Aspect System (SAS) of the RHESSI satellite measures the optical solar limb with a cadence typically set at 100 samples/s.RHESSI has observed the Sun continuously since its launch in early 2002, and we have acquired a unique data set ranging over more than a full 11-year solar cycle and consisting of about 4x10^10 single data points.The optics has a point spread of about 4.5 arcsec FWHM imaging the red continuum onto three linear CCD sensors with a pixel resolution of 1.7 arcsec.However, careful study of systematics, masking of contaminated data, and accumulation of data over appropriate time intervals has led to measurementswith sub-milli arcsec accuracy.Analyzing data for an initial period in 2004, these measurements have led to the most accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian et al., 2008), a value consistent with models predicting an oblateness from surface rotation.An excess oblateness term can be attributed to magnetic elements possibly located in the enhanced network.We also study photometric properties of our data. Previous observations of latitude-dependent brightness variations at the limb had suggested the presence of a polar temperature excess as large as 1.5 K.The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation.Our measurements of latitude-dependent brightness variations at the limb lead to a quadrupolar term (a pole-to-equator temperature variation) of the order of 0.1 K, an order of magnitude smaller than previously reported.We present the analysis of these unique data, an overview of some results and we report on our progress as we apply our developed analysis method to the whole 14 years of data. Title: White-light flares, Hard X-Rays, and Heights Authors: Martinez Oliveros, Juan Carlos; Hudson, Hugh S.; Krucker, Sam Bibcode: 2016SPD....47.0617M Altcode: The white-light continuum of a solar flare was the first manifestation of a solar flare ever detected. Nevertheless, its mechanisms remain unknown, even today. Improved observations confirm the identification of white-light continuum emission and hard X-rays during the impulsive phase of a solar flare, both in space and in time, to within the observational limits. Two events observed near the limb, but not occulted by it (SOL2011-02-24 and SOL2012-02-18), show that these emissions appear to have physical heights lower than predicted by models by hundreds of kms, referring height to the location of optical-depth unity at disk center in the 500 nm continuum. We describe these results and place them in the context of the three extreme-limb events (within about 1o) reported by Krucker et al. (2015). The electrons responsible for hard X-ray bremsstrahlung coincide with the most intense flare energy release, but we do not presently understand the physics of energy transport nor the nature of particle acceleration apparently taking place at heights below the preflare temperature minimum. Title: Solar Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) Authors: Kobelski, A.; Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; De Pontieu, B.; Fleishman, G.; Gary, D.; Hales, A.; Hills, R.; Hudson, H.; Hurford, G.; Loukitcheva, M.; Iwai, K.; Krucker, S.; Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S.; Yan, Y.; ALMA Solar Development Team Bibcode: 2016ASPC..504..327K Altcode: The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint North American, European, and East Asian project that opens the mm-sub mm wavelength part of the electromagnetic spectrum for general astrophysical exploration, providing high-resolution imaging in frequency bands currently ranging from 84 GHz to 950 GHz (300 microns to 3 mm). It is located in the Atacama desert in northern Chile at an elevation of 5000 m. Despite being a general purpose instrument, provisions have been made to enable solar observations with ALMA. Radiation emitted at ALMA wavelengths originates mostly from the chromosphere, which plays an important role in the transport of matter and energy, and the in heating the outer layers of the solar atmosphere. Despite decades of research, the solar chromosphere remains a significant challenge: both to observe, owing to the complicated formation mechanisms of currently available diagnostics; and to understand, as a result of the complex nature of the structure and dynamics of the chromosphere. ALMA has the potential to change the scene substantially as it serves as a nearly linear thermometer at high spatial and temporal resolution, enabling us to study the complex interaction of magnetic fields and shock waves and yet-to-be-discovered dynamical processes. Moreover, ALMA will play an important role in the study of energetic emissions associated with solar flares at sub-THz frequencies. Title: Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu, B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin, P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz, A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.; Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary, D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.; Selhorst, C. L.; Barta, M. Bibcode: 2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere—a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases. Title: Spectral and Imaging Observations of a White-light Solar Flare in the Mid-infrared Authors: Penn, Matt; Krucker, Säm; Hudson, Hugh; Jhabvala, Murzy; Jennings, Don; Lunsford, Allen; Kaufmann, Pierre Bibcode: 2016ApJ...819L..30P Altcode: 2015arXiv151204449P We report high-resolution observations at mid-infrared wavelengths of a minor solar flare, SOL2014-09-24T17:50 (C7.0), using Quantum Well Infrared Photodetector cameras at an auxiliary of the McMath-Pierce telescope. The flare emissions, the first simultaneous observations in two mid-infrared bands at 5.2 and 8.2 μ {{m}} with white-light and hard X-ray coverage, revealed impulsive time variability with increases on timescales of ∼4 s followed by exponential decay at ∼10 s in two bright regions separated by about 13\prime\prime . The brightest source is compact, unresolved spatially at the diffraction limit (1\_\_AMP\_\_farcs;72 at 5.2 μ {{m}}). We identify the IR sources as flare ribbons also seen in white-light emission at 6173 Å observed by SDO/HMI, with twin hard X-ray sources observed by Reuven Ramaty High Energy Solar Spectroscopic Imager, and with EUV sources (e.g., 94 Å) observed by SDO/AIA. The two infrared points have nearly the same flux density (fν, W m-2 Hz) and extrapolate to a level of about an order of magnitude below that observed in the visible band by HMI, but with a flux of more than two orders of magnitude above the free-free continuum from the hot (∼15 MK) coronal flare loop observed in the X-ray range. The observations suggest that the IR emission is optically thin; this constraint and others suggest major contributions from a density less than about 4× {10}13 cm-3. We tentatively interpret this emission mechanism as predominantly free-free emission in a highly ionized but cool and rather dense chromospheric region. Title: The First X-Ray Imaging Spectroscopy of Quiescent Solar Active Regions with NuSTAR Authors: Hannah, Iain G.; Grefenstette, Brian W.; Smith, David M.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; Madsen, Kristin K.; Marsh, Andrew; White, Stephen M.; Caspi, Amir; Shih, Albert Y.; Harrison, Fiona A.; Stern, Daniel; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Zhang, William W. Bibcode: 2016ApJ...820L..14H Altcode: 2016arXiv160301069H We present the first observations of quiescent active regions (ARs) using the Nuclear Spectroscopic Telescope Array (NuSTAR), a focusing hard X-ray telescope capable of studying faint solar emission from high-temperature and non-thermal sources. We analyze the first directly imaged and spectrally resolved X-rays above 2 keV from non-flaring ARs, observed near the west limb on 2014 November 1. The NuSTAR X-ray images match bright features seen in extreme ultraviolet and soft X-rays. The NuSTAR imaging spectroscopy is consistent with isothermal emission of temperatures 3.1-4.4 MK and emission measures 1-8 × 1046 cm-3. We do not observe emission above 5 MK, but our short effective exposure times restrict the spectral dynamic range. With few counts above 6 keV, we can place constraints on the presence of an additional hotter component between 5 and 12 MK of ∼ {10}46 cm-3 and ∼ {10}43 cm-3, respectively, at least an order of magnitude stricter than previous limits. With longer duration observations and a weakening solar cycle (resulting in an increased livetime), future NuSTAR observations will have sensitivity to a wider range of temperatures as well as possible non-thermal emission. Title: Correlation of Hard X-Ray and White Light Emission in Solar Flares Authors: Kuhar, Matej; Krucker, Säm; Martínez Oliveros, Juan Carlos; Battaglia, Marina; Kleint, Lucia; Casadei, Diego; Hudson, Hugh S. Bibcode: 2016ApJ...816....6K Altcode: 2015arXiv151107757K A statistical study of the correlation between hard X-ray and white light emission in solar flares is performed in order to search for a link between flare-accelerated electrons and white light formation. We analyze 43 flares spanning GOES classes M and X using observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager and Helioseismic and Magnetic Imager. We calculate X-ray fluxes at 30 keV and white light fluxes at 6173 Å summed over the hard X-ray flare ribbons with an integration time of 45 s around the peak hard-X ray time. We find a good correlation between hard X-ray fluxes and excess white light fluxes, with a highest correlation coefficient of 0.68 for photons with energy of 30 keV. Assuming the thick target model, a similar correlation is found between the deposited power by flare-accelerated electrons and the white light fluxes. The correlation coefficient is found to be largest for energy deposition by electrons above ∼50 keV. At higher electron energies the correlation decreases gradually while a rapid decrease is seen if the energy provided by low-energy electrons is added. This suggests that flare-accelerated electrons of energy ∼50 keV are the main source for white light production. Title: ALMA Observations of the Sun in Cycle 4 and Beyond Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.; Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.; Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson, M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.; Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov, V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões, P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig, A.; White, S.; Yagoubov, P.; Zaqarashvili, T. Bibcode: 2016arXiv160100587W Altcode: This document was created by the Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) in preparation of the first regular observations of the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), which are anticipated to start in ALMA Cycle 4 in October 2016. The science cases presented here demonstrate that a large number of scientifically highly interesting observations could be made already with the still limited solar observing modes foreseen for Cycle 4 and that ALMA has the potential to make important contributions to answering long-standing scientific questions in solar physics. With the proposal deadline for ALMA Cycle 4 in April 2016 and the Commissioning and Science Verification campaign in December 2015 in sight, several of the SSALMON Expert Teams composed strategic documents in which they outlined potential solar observations that could be feasible given the anticipated technical capabilities in Cycle 4. These documents have been combined and supplemented with an analysis, resulting in recommendations for solar observing with ALMA in Cycle 4. In addition, the detailed science cases also demonstrate the scientific priorities of the solar physics community and which capabilities are wanted for the next observing cycles. The work on this White Paper effort was coordinated in close cooperation with the two international solar ALMA development studies led by T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be further updated until the beginning of Cycle 4 in October 2016. In particular, we plan to adjust the technical capabilities of the solar observing modes once finally decided and to further demonstrate the feasibility and scientific potential of the included science cases by means of numerical simulations of the solar atmosphere and corresponding simulated ALMA observations. Title: EUV & X-ray observations of microflare heating of AR12333 Authors: Hannah, I. G.; Wright, P. J.; Grefenstette, B.; Glesener, L.; Hudson, H. S.; Smith, D. M.; Krucker, S.; Marsh, A.; White, S. M. Bibcode: 2015AGUFMSH13B2442H Altcode: We present a study of the heating in AR12333 due to small microflares between 10:30 and 13:30UT on 29 April 2015. This region is well observed in EUV by the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) as well as Hinode's X-ray Telescope (XRT) operating in a higher cadence mode, switching through the five thicker filters (sensitive to the higher temperature range). The Hinode observations were a coordinated campaign with the NuSTAR hard X-ray focusing optics telescope (Harrison et al. 2013). NuSTAR was conducting a full disk mosaic observation of the Sun and caught AR12333 several times, providing imaging spectroscopy >2 keV. We investigate the heating in the active region due to several small microflares (about A1-Class). These were visible with the thicker XRT filters and only clear in EUV once the FeXVIII component was extracted from SDO/AIA 94Å, indicating heating primarily >3MK. Using the regularized inversion method of Hannah & Kontar 2012, we recover the DEM from the SDO/AIA and Hinode/XRT data and compare this to the thermal characteristics derived from NuSTAR. Title: NuSTAR X-ray observations of small flares and non-flaring active regions Authors: Hannah, I. G.; Grefenstette, B.; Smith, D. M.; Marsh, A.; Glesener, L.; Krucker, S.; Hudson, H. S.; White, S.; Madsen, K.; Caspi, A.; Vogel, J.; Shih, A. Bibcode: 2015AGUFMSH31D..03H Altcode: We present imaging spectroscopy of the Sun with the NuSTAR hard X-ray (HXR) telescope, an astrophysics mission that uses focusing optics to directly image X-rays between ~2-80 keV. Although not optimized for solar observations, NuSTAR's high sensitivity can probe previously inaccessible X-ray emission from the Sun - crucial for searching for high temperature and non-thermal emission from "non-flaring" active regions. We present analysis of the first NuSTAR solar observations, that began in late 2014 and continued through 2015. These include using its imaging spectroscopy capabilities to derive the thermal characteristics of several "non-flaring" active regions, providing limits to the high temperature emission. We also show NuSTAR observations of several small microflares that were also observed by Hinode/XRT (in multiple thicker filters sensitive to higher temperatures) and RHESSI. This combination of three separate X-ray telescopes provides a broad observational characterization of active region heating by these very small microflares. Title: SSALMON - The Solar Simulations for the Atacama Large Millimeter Observatory Network Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu, B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.; Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.; Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier, A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M. Bibcode: 2015AdSpR..56.2679W Altcode: 2015arXiv150205601W The Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) was initiated in 2014 in connection with two ALMA development studies. The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful new tool, which can also observe the Sun at high spatial, temporal, and spectral resolution. The international SSALMONetwork aims at co-ordinating the further development of solar observing modes for ALMA and at promoting scientific opportunities for solar physics with particular focus on numerical simulations, which can provide important constraints for the observing modes and can aid the interpretation of future observations. The radiation detected by ALMA originates mostly in the solar chromosphere - a complex and dynamic layer between the photosphere and corona, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere. Potential targets include active regions, prominences, quiet Sun regions, flares. Here, we give a brief overview over the network and potential science cases for future solar observations with ALMA. Title: Fast Single-Dish Scans of the Sun Using ALMA Authors: Phillips, N.; Hills, R.; Bastian, T.; Hudson, H.; Marson, R.; Wedemeyer, S. Bibcode: 2015ASPC..499..347P Altcode: 2015arXiv150206122P We have implemented control and data-taking software that makes it possible to scan the beams of individual ALMA antennas to perform quite complex patterns while recording the signals at high rates. We conducted test observations of the Sun in September and December, 2014. The data returned have excellent quality; in particular they allow us to characterize the noise and signal fluctuations present in this kind of observation. The fast-scan experiments included both Lissajous patterns covering rectangular areas, and “double-circle” patterns of the whole disk of the Sun and smaller repeated maps of specific disk-shaped targets. With the latter we find that we can achieve roughly Nyquist sampling of the Band 6 (230 GHz) beam in 60 s over a region 300” in diameter. These maps show a peak-to-peak brightness-temperature range of up to 1000 K, while the time-series variability at any given point appears to be of order 0.5% RMS over times of a few minutes. We thus expect to be able to separate the noise contributions due to transparency fluctuations from variations in the Sun itself. Such timeseries have many advantages, in spite of the non-interferometric observations. In particular such data should make it possible to observe microflares in active regions and nanoflares in any part of the solar disk and low corona. Title: Solar ALMA Observations - A New View of Our Host Star Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Shimojo, M.; Hales, A.; Yagoubov, P.; Hudson, H. Bibcode: 2015ASPC..499..345W Altcode: 2015arXiv150206397W ALMA provides the necessary spatial, temporal and spectral resolution to explore central questions in contemporary solar physics with potentially far-reaching implications for stellar atmospheres and plasma physics. It can uniquely constraint the thermal and magnetic field structure in the solar chromosphere with measurements that are highly complementary to simultaneous observations with other ground-based and space-borne instruments. Here, we highlight selected science cases. Title: Capabilities of a FOXSI Small Explorer Authors: Inglis, A. R.; Christe, S.; Glesener, L.; Krucker, S.; Dennis, B. R.; Shih, A.; Wilson-Hodge, C.; Gubarev, M.; Hudson, H. S.; Kontar, E.; Buitrago Casas, J. C.; Drake, J. F.; Caspi, A.; Holman, G.; Allred, J. C.; Ryan, D.; Alaoui, M.; White, S. M.; Saint-Hilaire, P.; Klimchuk, J. A.; Hannah, I. G.; Antiochos, S. K.; Grefenstette, B.; Ramsey, B.; Jeffrey, N. L. S.; Reep, J. W.; Schwartz, R. A.; Ireland, J. Bibcode: 2015AGUFMSH43B2456I Altcode: We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer (SMEX) concept, a mission dedicated to studying particle acceleration and energy release on the Sun. FOXSI is designed as a 3-axis stabilized spacecraft in low-Earth orbit making use of state-of-the-art grazing incidence focusing optics, allowing for direct imaging of solar X-rays. The current design being studied features three telescope modules deployed in a low-inclination low-earth orbit (LEO). With a 15 meter focal length enabled by a deployable boom, FOXSI will observe the Sun in the 3-50 keV energe range. The FOXSI imaging concept has already been tested on two sounding rocket flights, in 2012 and 2014 and on the HEROES balloon payload flight in 2013. FOXSI will image the Sun with an angular resolution of 5'', a spectral resolution of 0.5 keV, and sub-second temporal resolution using CdTe detectors. In this presentation we investigate the science objectives and targets which can be accessed from this mission. Because of the defining characteristic of FOXSI is true imaging spectroscopy with high dynamic range and sensitivity, a brand-new perspective on energy release on the Sun is possible. Some of the science targets discussed here include; flare particle acceleration processes, electron beams, return currents, sources of solar energetic particles (SEPs), as well as understanding X-ray emission from active region structures and the quiescent corona. Title: Švestka's Research Then and Now. Invited Review Authors: Hudson, H. S. Bibcode: 2015SoPh..290.3383H Altcode: 2015SoPh..tmp...36H Zdeněk Švestka's research work influenced many fields of solar physics, especially in the area of flare research. In this article I take five of the areas that particularly interested him and assess them in a "then and now" style. His insights in each case were quite sound, although of course in the modern era we have learned things that he could not readily have envisioned. His own views about his research life have been published recently in this journal, to which he contributed so much, and his memoir contains much additional scientific and personal information (Švestka in Solar Phys.267, 235, 2010). Title: SDO/HMI -- RHESSI White-Light Flare Catalog: Matsushita Analysis Authors: Martinez Oliveros, J. C.; Hudson, H. S.; Krucker, S. Bibcode: 2015AGUFMSH52A..05M Altcode: In recent years several observation of white-light flare features in the low corona using data from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory have been reported. We present the first results of a white-light flare catalog based on SDO/HMI 6173A Intensity observations and the RHESSI flare catalog. We selected flares during 2011 and 2013 with GOES classification above M1.0 that were fully or partially observed by RHESSI as reported in the RHESSI flare catalog. We found that at least one third of the flares present white-light enhancement in the 6173A line and at least one fifth of the events show above the limb white-light sources. We will also discuss the results of a comparative analysis between the Hard X-ray and white-light positions. This analysis show the statistical average-height variation using the analysis technique described by Matsushita et al. 1992. Title: Sunquake Generation by Coronal Magnetic Restructuring Authors: Russell, A. J. B.; Mooney, M.; Leake, J. E.; Hudson, H. S. Bibcode: 2015AGUFMSH22A..05R Altcode: Solar flares and coronal mass ejections are powered by major restructurings of the coronal magnetic field, which appear to strongly perturb the magnetic field in the photosphere as well. Could the associated Lorentz forces generate sunquakes, as suggested by Hudson et al. 2008? Here, we present the first MHD simulations of sunquake generation by magnetic field perturbations, and explore the details of this mechanism. The downgoing magnetic field change is modelled as an Alfven wave, which propagates into the lower atmosphere. When it reaches the vicinity of the beta=1 layer (where the Alfven and sound speeds are equal), non-linear coupling excites a downgoing acoustic wave, which we interpret as a sunquake. The amplitude of the acoustic wave increases nonlinearly with the amplitude of the magnetic perturbation, reaching a limit where around 35% of the injected Poynting flux is transferred to the seismic wave - enough energy to match sunquake observations. Title: The NuSTAR Sensitivity to Quiet-Sun Transient Events Authors: Marsh, A.; Smith, D. M.; Glesener, L.; Hannah, I. G.; Krucker, S.; Hudson, H. S.; Grefenstette, B.; Madsen, K.; Caspi, A. Bibcode: 2015AGUFMSH13B2441M Altcode: We present the NuSTAR sensitivity to quiet Sun (QS) transient events, which have been seen in wavelengths from UV to soft X-rays. Although not optimized for solar observations, NuSTAR can observe X-ray emission from the Sun with unprecedented sensitivity in the hard X-ray range; this is crucial for detecting individual events in the quiet corona. While NuSTAR has not yet detected any such events, we use QS data from the 01-November-2014 observations (at a GOES-level ~B4) to determine what types of events we could have detected. In particular, we place lower limits on the detectable flare emission measure for isothermal temperatures between 2 - 10 MK. While our sensitivity to date has been limited by noise due to active regions outside the field of view, these limits are still >2 orders of magnitude below the RHESSI detection limits at corresponding temperatures. We expect to increase our sensitivity by at least an order of magnitude with future observations at lower solar flux levels and with fewer active regions on the disk. Title: Soft X-Ray Pulsations in Solar Flares Authors: Simões, P. J. A.; Hudson, H. S.; Fletcher, L. Bibcode: 2015SoPh..290.3625S Altcode: 2014arXiv1412.3045S; 2015SoPh..tmp...50S The soft X-ray emissions (hν>1.5 keV) of solar flares mainly come from the bright coronal loops at the highest temperatures normally achieved in the flare process. Their ubiquity has led to their use as a standard measure of flare occurrence and energy, although the overwhelming bulk of the total flare energy goes elsewhere. Recently Dolla et al. (Astrophys. J. Lett.749, L16, 2012) noted quasi-periodic pulsations (QPP) in the soft X-ray signature of the X-class flare SOL2011-02-15, as observed by the standard photometric data from the GOES (Geostationary Operational Environmental Satellite) spacecraft. In this article we analyse the suitability of the GOES data for this type of analysis and find them to be generally valuable after September, 2010 (GOES-15). We then extend the result of Dolla et al. to a complete list of X-class flares from Cycle 24 and show that most of them (80 %) display QPPs in the impulsive phase. The pulsations show up cleanly in both channels of the GOES data, making use of time-series of irradiance differences (the digital time derivative on the 2-s sampling). We deploy different techniques to characterise the periodicity of GOES pulsations, considering the red-noise properties of the flare signals, finding a range of characteristic time scales of the QPPs for each event, but usually with no strong signature of a single period dominating in the power spectrum. The QPP may also appear on somewhat longer time scales during the later gradual phase, possibly with a greater tendency towards coherence, but the sampling noise in GOES difference data for high irradiance values (X-class flares) makes these more uncertain. We show that there is minimal phase difference between the differenced GOES energy channels, or between them and the hard X-ray variations on short time scales. During the impulsive phase, the footpoints of the newly forming flare loops may also contribute to the observed soft X-ray variations. Title: The Atacama Large Millimeter/Submillimeter Array: a New Asset for Solar and Heliospheric Physics Authors: Bastian, Timothy S.; Barta, Miroslav; Brajsa, Roman; Chen, Bin; De Pontieu, Bart; Fleishman, Gregory; Gary, Dale; Hales, Antonio; Hills, Richard; Hudson, Hugh; Iwai, Kazamasu; Shimojo, Masumi; White, Stephen; Wedemeyer, Sven; Yan, Yihua Bibcode: 2015IAUGA..2257295B Altcode: The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint North American, European, and East Asian interferometric array that opens the mm-submm wavelength part of the electromagnetic spectrum for general astrophysical exploration, providing high-resolution imaging in frequency bands ranging from 86 to 950 GHz. Despite being a general purpose instrument, provisions have been made to enable solar observations with ALMA. Radiation emitted at ALMA wavelengths originates mostly from the chromosphere, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere. In this paper we describe recent efforts to ensure that ALMA can be usefully exploited by the scientific community to address outstanding questions in solar physics. We summarize activities under North American and European ALMA development studies, including instrument testing, calibration and imaging strategies, a science simulations. With the support of solar observations, ALMA joins next-generation groundbased instruments that can be used alone or in combination with other ground-based and space-based instruments to address outstanding questions in solar and heliospheric physics. Opportunities for the wider community to contribute to these efforts will be highlighted. Title: Solar ALMA observations - A revolutionizing new view at our host star Authors: Wedemeyer, Sven; Brajsa, Roman; Bastian, Timothy S.; Barta, Miroslav; Hales, Antonio; Yagoubov, Pavel; Hudson, Hugh; Loukitcheva, Maria; Fleishman, Gregory Bibcode: 2015IAUGA..2256732W Altcode: Observations of the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA) have a large potential for revolutionizing our understanding of our host star with far reaching implications for stars in general. The radiation emitted at ALMA wavelengths originates mostly from the chromosphere - a complex and dynamic layer between the photosphere and the corona, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere.Despite decades of intensive research, the chromosphere is still elusive and challenging to observe owing to the complicated formation mechanisms of currently available diagnostics. ALMA will change the scene substantially as it serves as a nearly linear thermometer at high spatial, temporal, and spectral resolution, enabling us to study the complex interaction of magnetic fields and shock waves and yet-to-be-discovered dynamical processes. Furthermore, radio recombination and molecular lines may have great diagnostic potential but need to be investigated first. These unprecedented capabilities promise important new findings for a large range of topics in solar physics including the structure, dynamics and energy balance of quiet Sun regions, active regions and sunspots, flares and prominences. As a part of ongoing development studies, an international network has been initiated, which aims at defining and preparing key solar science with ALMA through simulation studies: SSALMON -- Solar Simulations for the Atacama Large Millimeter Observatory Network (http://ssalmon.uio.no). Here, we give an overview of potential science cases. Title: Solar extreme events Authors: Hudson, Hugh S. Bibcode: 2015JPhCS.632a2058H Altcode: 2015arXiv150404755H Solar flares and CMEs have a broad range of magnitudes. This review discusses the possibility of “extreme events,” defined as those with magnitudes greater than have been seen in the existing historical record. For most quantitative measures, this direct information does not extend more than a century and a half into the recent past. The magnitude distributions (occurrence frequencies) of solar events (flares/CMEs) typically decrease with the parameter measured or inferred (peak flux, mass, energy etc. Flare radiation fluxes tend to follow a power law slightly flatter than S-2, where S represents a peak flux; solar particle events (SPEs) follow a still flatter power law up to a limiting magnitude, and then appear to roll over to a steeper distribution, which may take an exponential form or follow a broken power law. This inference comes from the terrestrial 14C record and from the depth dependence of various radioisotope proxies in the lunar regolith and in meteorites. Recently major new observational results have impacted our use of the relatively limited historical record in new ways: the detection of actual events in the 14C tree-ring records, and the systematic observations of flares and “superflares” by the Kepler spacecraft. I discuss how these new findings may affect our understanding of the distribution function expected for extreme solar events. Title: Division II: Commission 10: Solar Activity Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk, James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul; Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi; Vršnak, Bojan; Yan, Yihua Bibcode: 2015IAUTB..28..106V Altcode: The Business Meeting of Commission 10 was held as part of the Business Meeting of Division II (Sun and Heliosphere), chaired by Valentin Martínez-Pillet, the President of the Division. The President of Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took the chair for the business meeting of C10. She summarised the activities of C10 over the triennium and the election of the incoming OC. Title: High-sensitivity search for transient solar X-ray emission with NuSTAR Authors: Marsh, Andrew; Hannah, Iain; Glesener, Lindsay; Smith, David M.; Grefenstette, Brian; Madsen, Kristin; Krucker, Sam; Hudson, Hugh; White, Stephen; Caspi, Amir; Christe, Steven; Shih, Albert; Mewaldt, Richard; Pivovaroff, Michael; Vogel, Julia Bibcode: 2015TESS....121302M Altcode: We present the first results of a search for transient X-ray emission in quiet solar regions with the NuSTAR astrophysics satellite. Transient brightenings of 1024-1027 ergs, or "nanoflares," have been observed as thermal emission in EUV and soft X-rays, but never in hard X-rays (HXRs) due to lack of sensitivity. Frequent nanoflares could account for a significant fraction of the energy release needed to heat the corona to >1 MK. NuSTAR directly images X-rays from ~2-80 keV, with much higher sensitivity than dedicated solar HXR instruments. More importantly it can point at the Sun without suffering damage, a rare capability for an astrophysics instrument. We have developed an algorithm to search the NuSTAR data in space and time for transient events, while taking into account instrumental and systematic effects. Preliminary analysis yields a sensitivity to events ~0.001 times as bright as an “typical” RHESSI microflare (Hannah et al. 2008), for linear scaling and event duration of 10 seconds. Future observations at full-Sun flux levels below GOES ~B5 will increase our sensitivity by an order of magnitude or more. Title: The solar magnetic activity band interaction and instabilities that shape quasi-periodic variability Authors: McIntosh, Scott W.; Leamon, Robert J.; Krista, Larisza D.; Title, Alan M.; Hudson, Hugh S.; Riley, Pete; Harder, Jerald W.; Kopp, Greg; Snow, Martin; Woods, Thomas N.; Kasper, Justin C.; Stevens, Michael L.; Ulrich, Roger K. Bibcode: 2015NatCo...6.6491M Altcode: 2015NatCo...6E6491M Solar magnetism displays a host of variational timescales of which the enigmatic 11-year sunspot cycle is most prominent. Recent work has demonstrated that the sunspot cycle can be explained in terms of the intra- and extra-hemispheric interaction between the overlapping activity bands of the 22-year magnetic polarity cycle. Those activity bands appear to be driven by the rotation of the Sun's deep interior. Here we deduce that activity band interaction can qualitatively explain the `Gnevyshev Gap'--a well-established feature of flare and sunspot occurrence. Strong quasi-annual variability in the number of flares, coronal mass ejections, the radiative and particulate environment of the heliosphere is also observed. We infer that this secondary variability is driven by surges of magnetism from the activity bands. Understanding the formation, interaction and instability of these activity bands will considerably improve forecast capability in space weather and solar activity over a range of timescales. Title: The Radiated Energy Budget Of Chromospheric Plasma In A Major Solar Flare Deduced From Multi-Wavelength Observations Authors: Milligan, Ryan; Kerr, Graham Stewart; Dennis, Brian; Hudson, Hugh; Fletcher, Lyndsay; Allred, Joel; Chamberlin, Phillip; Ireland, Jack; Mathioudakis, Mihalis; Keenan, Francis Bibcode: 2015TESS....130209M Altcode: The response of the lower solar atmosphere is an important diagnostic tool for understanding energy transport during solar flares. The 15 February 2011 X-class flare was fortuitously observed by a host of space-based instruments that sampled the chromospheric response over a range of lines and continua at <20s cadence. These include the free-bound EUV continua of H I (Lyman), He I, and He II, plus the emission lines of He II at 304Å and H I (Lyα) at 1216Å by SDO/EVE, the UV continua at 1600Å and 1700Å by SDO/AIA, and the white light continuum at 4504Å, 5550Å, and 6684Å, along with the Ca II H line at 3968Å using Hinode/SOT. RHESSI also observed the entire event at energies up to ~100keV, making it possible to determine the properties of the nonthermal electrons deemed to be responsible for driving the enhanced chromospheric emission under the assumption of thick-target collisions. Integrating over the duration of the impulsive phase, the total energy contained in the nonthermal electrons was found to be >2×1031 erg. By comparison, the summed energy detected by instruments onboard SDO and Hinode amounted to ~3×1030 erg; about 15% of the total nonthermal energy. The Lyα line was found to dominate the measured radiative losses in contrast to the predictions of numerical simulations. Parameters of both the driving electron distribution and the resulting chromospheric response are presented in detail to encourage the numerical modeling of flare heating for this event to determine the depth of the solar atmosphere at which these line and continuum processes originate, and the mechanism(s) responsible for their generation. Title: Hard X-ray imaging spectroscopy of hot coronal sources and active regions with NuSTAR Authors: Hannah, Iain; Marsh, Andrew; Glesener, Lindsay; Smith, David; Grefenstette, Brian; Madsen, Kristin; Krucker, Sam; Hudson, Hugh; White, Stephen; Shih, Albert Y. Bibcode: 2015TESS....120402H Altcode: We present imaging spectroscopy of the Sun with the NuSTAR hard X-ray (HXR) telescope, searching for high temperature and non-thermal emission in the “non-flaring” Sun. Launched in 2012, NASA's astrophysics mission NuSTAR uses focusing optics to directly image X-rays between ~2-80 keV. In the band below ~50 keV the field of view is 12'x12' and the instrument has an energy resolution of ~0.4 keV. Although not optimized for solar observations, NuSTAR’s high sensitivity can probe previously inaccessible X-ray emission from the Sun. NuSTAR observed the Sun three times during late 2014 and we present these first directly imaged hard X-rays from non-flaring active regions. Using NuSTAR’s imaging spectroscopy capabilities we are able to derive the active region’s multi-thermal characteristics. We will also discuss a hot (>3MK) source that appears to linger high in the corona and could be associated with the occulted active region AR12192. Title: Sub-Milli Arcsecond Resolution Observations of the Optical Solar Limb with RHESSI/SAS Authors: Fivian, Martin D.; Hudson, Hugh; Krucker, Sam Bibcode: 2015TESS....120325F Altcode: The Solar Aspect System (SAS) of the RHESSI satellite measures the optical solar limb with a cadence typically set at 100 samples/s. RHESSI has observed the Sun continuously since its launch in early 2002, and we have acquired a unique data set ranging over more than a full 11-year solar cycle and consisting of about 4x10^10 single data points. The optics has a point spread of about 4.5 arcsec FWHM imaging the red continuum onto three linear CCD sensors with a pixel resolution of 1.7 arcsec. However, careful study of systematics, masking of contaminated data, and accumulation of data over appropriate time intervals has led to measurements with sub-milli arcsec accuracy. Analyzing data for an initial period in 2004, these measurements have led to the most accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian et al., 2008), a value consistent with models predicting an oblateness from surface rotation. An excess oblateness term can be attributed to magnetic elements possibly located in the enhanced network. We also study photometric properties of our data. Previous observations of latitude-dependent brightness variations at the limb had suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. Our measurements of latitude-dependent brightness variations at the limb lead to a quadrupolar term (a pole-to-equator temperature variation) of the order of 0.1 K, an order of magnitude smaller than previously reported. We present the analysis of these unique data, an overview of some results and we report on our progress as we apply our developed analysis method to the whole 13 years of data. Title: SDO/HMI - RHESSI White-Light Flare Catalog: First Results Authors: Martinez Oliveros, Juan Carlos; Hudson, Hugh; Saint Hilaire, Pascal Bibcode: 2015TESS....130904M Altcode: In recent years several observation of white-light flare features in the low corona using data from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory have been reported. We present the first results of a white-light flare catalog based on SDO/HMI 6173A Intensity observations and the RHESSI flare catalog. We selected flares during 2011 and 2012 with GOES classification above M1.0 that were fully or partially observed by RHESSI as reported in the RHESSI flare catalog. We found that at least one third of the flares present white-light enhancement in the 6173A line and at least one fifth of the events show above the limb white-light sources. We will also discuss the physical implications of these observations. Title: Radio Observations of the CME-poor region AR2192: a type II burst with no CME driver Authors: Hudson, Hugh; Vilmer, Nicole; Wakeford, Peter Bibcode: 2015TESS....140803H Altcode: The remarkable sunspot group NOAA AR 2192 (October 2014) produced X-class flares without CMEs, and in general was large and powerful but with little heliospheric interaction. We discuss radio perspectives on the development of this region. In particular there were decametric type II bursts observed in association with jet-like flares SOL2014-10-21T12:28 (C4.4) and SOL2014-10-21T13:38 (M1.2), as first noted in the Glasgow Callisto observatory and confirmed via the Meudon decametric array. In cases such as this, the global coronal wave responsible for the type II emission seems to originate from an ejection of material flowing along a previously established field structure, rather than perpendicular to it as in a CME. Title: Co-Spatial White Light and Hard X-Ray Flare Footpoints Seen Above the Solar Limb Authors: Krucker, Säm; Saint-Hilaire, Pascal; Hudson, Hugh S.; Haberreiter, Margit; Martinez-Oliveros, Juan Carlos; Fivian, Martin D.; Hurford, Gordon; Kleint, Lucia; Battaglia, Marina; Kuhar, Matej; Arnold, Nicolas G. Bibcode: 2015ApJ...802...19K Altcode: We report analysis of three solar flares that occur within 1° of limb passage, with the goal to investigate the source height of chromospheric footpoints in white light (WL) and hard X-rays (HXR). We find the WL and HXR (≥30 keV) centroids to be largely co-spatial and from similar heights for all events, with altitudes around 800 km above the photosphere or 300-450 km above the limb height. Because of the extreme limb location of the events we study, emissions from such low altitudes are influenced by the opacity of the atmosphere and projection effects. STEREO images reveal that for SOL2012-11-20T12:36 the projection effects are smallest, giving upper limits of the absolute source height above the nominal photosphere for both wavelengths of ∼1000 km. To be compatible with the standard thick target model, these rather low altitudes require very low ambient densities within the flare footpoints, in particular if the HXR-producing electrons are only weakly beamed. That the WL and HXR emissions are co-spatial suggests that the observed WL emission mechanism is directly linked to the energy deposition by flare accelerated electrons. If the WL emission is from low-temperature (≤slant {{10}4} K) plasma as currently thought, the energy deposition by HXR-producing electrons above ∼30 keV seems only to heat chromospheric plasma to such low temperatures. This implies that the energy in flare-accelerated electrons above ∼30 keV is not responsible for chromospheric evaporation of hot (\gt {{10}6} K) plasma, but that their energy is lost through radiation in the optical range. Title: Svestka's Research: Then and Now Authors: Hudson, Hugh S. Bibcode: 2015arXiv150304452H Altcode: Zdenek Svestka's research work influenced many fields of solar physics, especially in the area of flare research. In this article I take five of the areas that particularly interested him and assess them in a "then and now" style. His insights in each case were quite sound, although of course in the modern era we have learned things that he could not readily have envisioned. His own views about his research life have been published recently in this journal, to which he contributed so much, and his memoir contains much additional scientific and personal information (Svestka, 2010). Title: Electron Energy Partition in the Above-the-looptop Solar Hard X-Ray Sources Authors: Oka, Mitsuo; Krucker, Säm; Hudson, Hugh S.; Saint-Hilaire, Pascal Bibcode: 2015ApJ...799..129O Altcode: Solar flares produce non-thermal electrons with energies up to tens of MeVs. To understand the origin of energetic electrons, coronal hard X-ray (HXR) sources, in particular above-the-looptop sources, have been studied extensively. However, it still remains unclear how energies are partitioned between thermal and non-thermal electrons within the above-the-looptop source. Here we show that the kappa distribution, when compared to conventional spectral models, can better characterize the above-the-looptop HXRs (gsim15 keV) observed in four different cases. The widely used conventional model (i.e., the combined thermal plus power-law distribution) can also fit the data, but it returns unreasonable parameter values due to a non-physical sharp lower-energy cutoff Ec. In two cases, extreme-ultraviolet data were available from SDO/AIA and the kappa distribution was still consistent with the analysis of differential emission measure. Based on the kappa distribution model, we found that the 2012 July 19 flare showed the largest non-thermal fraction of electron energies about 50%, suggesting equipartition of energies. Considering the results of particle-in-cell simulations, as well as density estimates of the four cases studied, we propose a scenario in which electron acceleration is achieved primarily by collisionless magnetic reconnection, but the electron energy partition in the above-the-looptop source depends on the source density. In low-density above-the-looptop regions (few times 109 cm-3), the enhanced non-thermal tail can remain and a prominent HXR source is created, whereas in higher-densities (>1010 cm-3), the non-thermal tail is suppressed or thermalized by Coulomb collisions. Title: Introduction to the Solar Activity Cycle: Overview of Causes and Consequences Authors: Balogh, A.; Hudson, H. S.; Petrovay, K.; von Steiger, R. Bibcode: 2015sac..book....1B Altcode: No abstract at ADS Title: The Solar Activity Cycle Authors: Balogh, André; Hudson, Hugh; Petrovay, Kristóf; von Steiger, Rudolf Bibcode: 2015sac..book.....B Altcode: No abstract at ADS Title: Solar Sector Structure Authors: Hudson, Hugh S.; Svalgaard, Leif; Hannah, Iain G. Bibcode: 2015sac..book...17H Altcode: No abstract at ADS Title: Introduction to the Solar Activity Cycle: Overview of Causes and Consequences Authors: Balogh, A.; Hudson, H. S.; Petrovay, K.; von Steiger, R. Bibcode: 2014SSRv..186....1B Altcode: 2014SSRv..tmp...60B The 11-year activity cycle is a dominant characteristic of the Sun. It is the result of the evolution in time the solar dynamo that generates the solar magnetic field. The nearly periodic variation in the sunspot number has been known since the mid-1800s; as the observations of the Sun broadened to cover an increasing number of phenomena, the same 11-year periodicity was noted in most of them. The discovery of solar magnetic fields introduced a 22-year periodicity, as the magnetic polarities of the polar regions change sign every 11 years. Correlations have been identified and quantified among all the measured parameters, but in most cases such correlations remain empirical rather than grounded in physical processes. This introductory paper and the reviews in the volume describe and discuss the current state of understanding of the causal chains that lead from the variable nature of the solar magnetic fields to the variability of solar phenomena. The solar activity cycle is poorly understood: predictions made for the current Cycle 24 have proved to be generally wrong. However, the re-evaluation of the relationships in the light of unexpected shortcomings is likely to lead to a better physical understanding of solar physics. This will help in the systematic reassessment of solar activity indices and their usefulness in describing and predicting the solar activity cycle. Title: Hmi and Rhessi Measurements of the Radial Location of Solar Flare Footpoints to Subarcsecond Accuracy Authors: Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Haberreiter, M.; Kleint, L.; Hurford, G. J.; Fivian, M. D.; Battaglia, M.; Martinez Oliveros, J. C. Bibcode: 2014AGUFMSH31C..05K Altcode: We report analysis of three solar flares that occur within one degree of limb passage, with the goal to investigate the source height of chromospheric footpoints in white light (WL) and hard X-rays (HXR). The optical observations are from the Helioseismic and Magnetic Imager (HMI) around 617.3 nm, providing high precision observations with an absolute positional accuracy in the radial direction below 0.1 arcsec (~70 km), as referred to the adjacent limb. The Reuven Ramaty Higher Energy Solar Spectroscopic Imager (RHESSI) gives HXR source centroids to a similar accuracy depending on counting statistics. The observed height of the emissions at either wavelength is influenced by the opacity of the atmosphere at that wavelength and the height must correspond to a radial distance from Sun center that is greater than the solar limb at that wavelength (~350 km for WL and ~450 km for HXR). We find the WL and HXR (~30 keV) centroids to be largely co-spatial and from similar heights for all events, with altitudes around 800 km above the height of the photosphere. The observed altitudes are limited by the uncertainty of the precise heliographic locations near the limb and the resulting projection effects. STEREO images reveal that for SOL2012-11-20T12:36 the projection effects are smallest, giving upper limits of the absolute source height of 979+-70 km for the WL emission and 926+-51 km for HXR source. Hence, the peak of the WL and HXR must be below 1000 km. To be compatible with the standard thick target model, these rather low altitudes require low ambient densities within the flare footpoints, in particular if the HXR-producing electrons are only weakly beamed. That the WL and HXR emissions are co-spatial suggests that the observed WL emission mechanism is directly linked to the energy deposition by flare accelerated electrons with energies above ~30 keV. If the WL emission is from low-temperature (~10 000 K) plasma as currently thought, the energy deposition by HXR-producing electrons above ~30 keV seems only to heat chromospheric plasma to such low temperatures. This implies that the energy in flare-accelerated electrons above ~30 keV is lost through radiation in the optical range rather than heating chromospheric plasma to coronal (> MK) temperatures. Title: Electron Energy Partition in the Above-the-looptop Solar Hard X-ray Sources Authors: Oka, M.; Krucker, S.; Hudson, H. S.; Saint-Hilaire, P. Bibcode: 2014AGUFMSH23A4147O Altcode: Solar flares produce non-thermal electrons with energies up to tens of MeVs. To understand the origin of energetic electrons, coronal hard X-ray sources in particular 'above-the-looptop' sources have been studied extensively. However, it still remains unclear how energies are partitioned between thermal and non-thermal electrons within the above-the-looptop source. Here we show that the kappa distribution, when compared to conventional spectral models, can better characterize the above-the- looptop hard X-rays (>~15 keV) observed in four different cases. The combined thermal plus power-law distribution can also fit the data, but it returns unreasonable parameter values due to the artifact of its sharp, lower-energy cutoff Ec. In two cases with extreme-ultraviolet data from SDO/AIA, the analysis of differential emission measure (DEM) did not rule out the kappa distribution model. Based on the kappa distribution model, we found that the 2012 July 19 flare showed the largest non-thermal fraction of electron energies about 50%, suggesting equipartition. Considering results of particle-in-cell (PIC) simulations as well as density estimates of the four cases studied, we propose a scenario in which electron energization is achieved primarily by collisionless magnetic reconnection but the non-thermal tail can be suppressed or thermalized by Coulomb collisions. Title: NuSTAR's First Solar Observations: Search for Transient Brightenings / Nanoflares Authors: Marsh, A.; Hannah, I. G.; Glesener, L.; Smith, D. M.; Grefenstette, B.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White, S.; Caspi, A.; Christe, S.; Shih, A.; Mewaldt, R. A.; Pivovaroff, M.; Vogel, J. Bibcode: 2014AGUFMSH13C4129M Altcode: We present a timing analysis of the Sun with the NuSTAR hard X-ray (HXR) telescope, searching for transient brightenings / nanoflares in the quiet Sun and active regions. A substantial amount of flare energy goes into accelerating electrons. HXR observations are a crucial tool for understanding this non-thermal emission and the energy release in flares. RHESSI is able to study this emission over many orders of magnitude (active region flares from X-class to A-class microflares), but it cannot detect the emission from smaller events. Such "nanoflares" have been postulated as a possible source of coronal heating and their existence and relationship to larger flares is still uncertain. In order to detect these events in HXRs, instruments more sensitive than RHESSI are required. Launched in 2012, the astrophysics mission NuSTAR uses focusing optics to directly image X-rays between ~2-80 keV. Although not optimized for solar observations, NuSTAR's highly sensitive imaging spectroscopy will be used to search for the faintest X-ray emission from the Sun. These solar observations will begin in September 2014; here we present the first results of our search for transient brightenings that could relate to nanoflares. Title: Wavelengths for EVE coronal dimming signatures Authors: Hudson, H. S.; Simoes, P. J. D. A.; Kukstas, E. Bibcode: 2014AGUFMSH13B4111H Altcode: The EVE instrument on SDO detects post-flare dimmings, mainly in the spectral regions of Fe IX-XII in its MEGS-A range. We have shown that dimmings occurred in most of the 31 X-class flares that occurred between SDO launch and the end of April 2014. Based upon earlier X-ray observations, we interpret these dimmings as the result of CME mass ejections from the low corona. We can estimate the masses involved in these dimmings by deriving a best pre-event temperature and emission measure in the dimmed region from EVE, and a source volume from AIA images. The dimming for SOL2011-02-15, the first of these events, "peaked" at -3.4% in Fe IX in terms of the pre-event emission from the whole Sun, with smaller relative depletions in higher ionization states of Fe. Because of its high photon throughput, EVE data determine line centroids with precisions of a few km/s equivalent. In the present study we analyze the wavelengths of the dimmed regions, characterizing their displacements from the mean wavelengths as functions of heliographic position, time, event magnitude, and excitation state of Fe. Title: Statistical Study of Coronal Hard X-ray Source Heights and Fluxes Authors: Glesener, L.; Oka, M.; Krucker, S.; Hudson, H. S. Bibcode: 2014AGUFMSH11D..03G Altcode: Hard X-ray observation of partly occulted flares has proven useful for studying flare-accelerated electrons in the solar corona. These nonthermal electrons emit bremsstrahlung hard X-rays (HXRs), but are difficult to observe for on-disk flares because the much brighter chromospheric footpoints tend to dominate HXR images. Previous research using the RHESSI spacecraft has performed individual and statistical study of HXR sources in partly occulted flares to investigate, for example, spectral characteristics and the relationship between nonthermal coronal sources and thermal loops. Source heights are not usually measured in these cases because of the difficulty in determining heliographic locations of flares beyond the limb. Occasionally, multi-spacecraft observation will identify a source location and thus calculate an absolute HXR source height. Microflare source heights have also been studied statistically by fitting distribution functions to the observed projected locations. But so far, a statistical study of coronal HXR sources in which source heights are determined individually for each flare has not been performed. In this work, we study flares jointly observed by RHESSI and STEREO/EUVI over a 2+ year time range. From RHESSI data, we obtain coronal HXR source positions and fluxes. The alternate viewing angle offered by STEREO provides flare locations, enabling RHESSI source positions to be translated into absolute heights above the photosphere. We will present the distribution of coronal HXR source heights and will discuss their possible corresponding features in the standard flare model, i.e. thermal loops, nonthermal looptop sources, above-the-looptop sources, and ejecta. Title: RHESSI/SAS Observations of the Optical Solar Limb Over a Full Solar Cycle Authors: Fivian, M. D.; Hudson, H. S. Bibcode: 2014AGUFMSH41C4156F Altcode: The Solar Aspect System (SAS) of the RHESSI satellite measures the optical solar limb in the red continuum with a cadence typically set at 16 samples/s in each of three linear CCD sensors. RHESSI has observed the Sun continuously since its launch in early 2002, and we have acquired a unique data set ranging over a full 11-year solar cycle and consisting of about 3x10^10 single data points. Analyzing data for an initial period in 2004, these measurements have led to the most accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian et al., 2008), a value consistent with models predicting an oblateness from surface rotation. An excess oblateness term can be attributed to magnetic elements possibly located in the enhanced network. We have started to also study photometric properties of our data. Previous observations of latitude-dependent brightness variations at the limb had suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. Our new measurements of latitude-dependent brightness variations at the limb lead to a quadrupolar term (a pole-to-equator temperature variation) of the order of 0.1 K, an order of magnitude smaller than previously reported. We present the analysis of these unique data, an overview of some results and we report on our progress as we apply our developed analysis method to the whole 12 years of data. Title: NuSTAR's first solar observations: Search for a high energy X-ray component to the "non-flaring" Sun Authors: Marsh, A.; Hannah, I. G.; Glesener, L.; Smith, D. M.; Grefenstette, B.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White, S.; Caspi, A.; Christe, S.; Shih, A.; Mewaldt, R. A.; Pivovaroff, M.; Vogel, J. Bibcode: 2014AGUFMSH12A..04M Altcode: We present spectroscopy of the Sun with the NuSTAR hard X-ray (HXR) telescope, searching for high temperature and non-thermal emission in the "non-flaring" Sun. A substantial amount of flare energy goes into accelerating electrons. HXR observations are a crucial tool for understanding this non-thermal emission and the energy release in flares. RHESSI is able to study this emission over many orders of magnitude (active region flares from X-class to A-class microflares), but it cannot detect the emission from smaller events. Such "nanoflares" have been postulated as a possible source of coronal heating and their existence and relationship to larger flares is still uncertain. In order to detect these events in HXRs, instruments more sensitive than RHESSI are required. Launched in 2012, the astrophysics mission NuSTAR uses focusing optics to directly image X-rays between ~2-80 keV. Although not optimized for solar observations, NuSTAR's highly sensitive imaging spectroscopy will be used to search for the faintest X-ray emission from the Sun. These solar observations will begin in September 2014. Here we present the first results of our search for transient brightenings in active and quiet Sun regions with NuSTAR. Title: Solar Sector Structure Authors: Hudson, Hugh S.; Svalgaard, Leif; Hannah, Iain G. Bibcode: 2014SSRv..186...17H Altcode: 2014SSRv..tmp...56H; 2015arXiv150304477H The interplanetary magnetic field near 1 AU has a characteristic "sector" structure that reflects its polarity relative to the solar direction. Typically we observe large-scale coherence in these directions, with two or four "away" or "towards" sectors per solar rotation, from any platform in deep space and near the ecliptic plane. In a simple picture, this morphology simply reflects the idea that the sources of the interplanetary field lie mainly in or near the Sun, and that the solar-wind flow enforces a radial component in this field. The sector boundaries are sharply defined in the interplanetary field near one AU, but have more complicated sources within the Sun itself. Recent evidence confirms that the origins of this pattern also appear statistically at the level of the photosphere, with signatures found in the highly concentrated fields of sunspots and even solar flares. This complements the associations already known between the interplanetary sectors and large-scale coronal structures (i.e., the streamers). This association with small-scale fields strengthens at the Hale sector boundary, defining the Hale boundary as the one for which the polarity switch matches that of the leading-to-following polarity alternation in the sunspots of a given hemisphere. Surface features that appear 4.5 days prior to the sector crossings observed at 1 AU correlate with this sense of polarity reversal. Title: The Radiated Energy Budget of Chromospheric Plasma in a Major Solar Flare Deduced from Multi-wavelength Observations Authors: Milligan, Ryan O.; Kerr, Graham S.; Dennis, Brian R.; Hudson, Hugh S.; Fletcher, Lyndsay; Allred, Joel C.; Chamberlin, Phillip C.; Ireland, Jack; Mathioudakis, Mihalis; Keenan, Francis P. Bibcode: 2014ApJ...793...70M Altcode: 2014arXiv1406.7657M This paper presents measurements of the energy radiated by the lower solar atmosphere, at optical, UV, and EUV wavelengths, during an X-class solar flare (SOL2011-02-15T01:56) in response to an injection of energy assumed to be in the form of nonthermal electrons. Hard X-ray observations from RHESSI were used to track the evolution of the parameters of the nonthermal electron distribution to reveal the total power contained in flare accelerated electrons. By integrating over the duration of the impulsive phase, the total energy contained in the nonthermal electrons was found to be >2 × 1031 erg. The response of the lower solar atmosphere was measured in the free-bound EUV continua of H I (Lyman), He I, and He II, plus the emission lines of He II at 304 Å and H I (Lyα) at 1216 Å by SDO/EVE, the UV continua at 1600 Å and 1700 Å by SDO/AIA, and the white light continuum at 4504 Å, 5550 Å, and 6684 Å, along with the Ca II H line at 3968 Å using Hinode/SOT. The summed energy detected by these instruments amounted to ~3 × 1030 erg about 15% of the total nonthermal energy. The Lyα line was found to dominate the measured radiative losses. Parameters of both the driving electron distribution and the resulting chromospheric response are presented in detail to encourage the numerical modeling of flare heating for this event, to determine the depth of the solar atmosphere at which these line and continuum processes originate, and the mechanism(s) responsible for their generation. Title: Relationship between the photospheric Poynting flux and the active region luminosity Authors: Kazachenko, Maria D.; Canfield, Richard C.; Fisher, George H.; Hudson, Hugh S.; Welsch, Brian Bibcode: 2014AAS...22412349K Altcode: How does energy radiated by active regions compare with magnetic energy that propagates lower across the photosphere? This is a fundamental question for energy storage and release in active regions, yet it is presently poorly understood. In this work we quantify and compare both energy terms using SDO observations of the active region (AR) 11520. To quantify the magnetic energy crossing the photosphere, or the Poynting flux, we need to know both the magnetic field vector B and electric field vector E as well. Our current electric field inversion technique, PDFI, combines the Poloidal-Toroidal-Decomposition method with information from Doppler measurements, Fourier local correlation tracking (FLCT) results, and the ideal MHD constraint, to determine the electric field from vector magnetic field and Doppler data. We apply the PDFI method to a sequence of Helioseismic and Magnetic Imager (HMI/SDO) vector magnetogram data, to find the electric-field and hence the Poynting-flux evolution in AR 11520. We find that most of the magnetic energy in this AR is injected in the range of $10^7$ to $10^8$ $ergs/{cm^2 s}$, with the largest fluxes reaching $10^{10}$ $ergs/{cm^2 s}$. Integrating over the active region this yields a total energy of order $10^{28}$ ergs/s. To quantify the active region luminosity, we use EUV Variability Experiment (EVE) and Atmospheric Imaging Assembly (AIA) spectrally resolved observations. We find the active region luminosity of order $10^{28}$ ergs/s. We compare derived magnetic and radiated energy fluxes on different temporal and spatial scales and estimate their uncertainties. We also discuss the roles that potential/non-potential and emerging/shearing terms play in the total magnetic energy budget. Title: Current and future solar observation using focusing hard X-ray imagers Authors: Glesener, Lindsay; Caspi, Amir; Christe, Steven; Hannah, Iain; Hudson, Hugh S.; Hurford, Gordon J.; Grefenstette, Brian; Krucker, Sam; Marsh, Andrew; Mewaldt, Richard A.; Pivovaroff, Michael; Shih, Albert Y.; Smith, David M.; Vogel, Julia; White, Stephen M. Bibcode: 2014AAS...22412364G Altcode: The efficient processes that accelerate particles in solar flares are not currently understood. Hard X-rays (HXRs) are one of the best diagnostics of flare-accelerated electrons, and therefore of acceleration processes. Past and current solar HXR observers rely on indirect Fourier imaging and thus lack the necessary sensitivity and imaging dynamic range to make detailed studies of faint HXR sources in the solar corona (where particle acceleration is thought to occur). A future generation of solar HXR observers will instead likely rely on direct HXR focusing, which can provide far superior sensitivity and imaging dynamic range.The first wave of focused solar HXR studies is already underway, including sounding rocket and high-altitude balloon payloads, and, in the near future, solar observation by the NuSTAR astrophysics observatory. This poster will (1) summarize the capabilities of current solar HXR instruments, comparing the science that can be done from each platform, and (2) discuss the scientific power of a future, dedicated, spaceborne observatory optimized to observe HXRs from the Sun. Title: MESSENGER soft X-ray observations of the quiet solar corona Authors: Schwartz, Richard A.; Hudson, Hugh S.; Tolbert, Anne K; Dennis, Brian R. Bibcode: 2014AAS...22432349S Altcode: In a remarkable result from their "SphinX" experiment, Sylwester et al. (2012) found a non-varying base level of soft X-ray emission at the quietest times in 2009. We describe comparable data from the soft X-ray monitor on board MESSENGER (en route to Mercury) which had excellent coverage both in 2009 and during the true solar minimum of 2008. These observations overlap SphinX's and also are often exactly at Sun-MESSENGER-Earth conjunctions. During solar minimum the Sun-MESSENGER distance varied substantially, allowing us to use the inverse-square law to help distinguish the aperture flux (ie, solar X-rays) from that due to sources of background in the 2-5 keV range. The MESSENGER data show a non-varying background level for many months in 2008 when no active regions were present. We compare these data in detail with those from SphinX. Both sets of data reveal a different behavior when magnetic active regions are present on the Sun, and when they are not.Reference: Sylwester et al., ApJ 751, 111 (2012) Title: RHESSI/SAS Observations of the Optical Solar Limb Over a Full Solar Cycle Authors: Fivian, Martin; Hudson, Hugh S. Bibcode: 2014AAS...22421827F Altcode: The Solar Aspect System (SAS) of the RHESSI satellite measures the optical solar limb in the red continuum with a cadence typically set at 16 samples/s in each of three linear CCD sensors. RHESSI has observed the Sun continuously since its launch in early 2002, and we have acquired a unique data set ranging over a full 11-year solar cycle and consisting of about 3x10^10 single data points. Analyzing data for an initial period in 2004, these measurements have led to the most accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian et al., 2008), a value consistent with models predicting an oblateness from surface rotation. An excess oblateness term can be attributed to magnetic elements possibly located in the enhanced network. We have started to also study photometric properties of our data. Previous observations of latitude-dependent brightness variations at the limb had suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. Our new measurements of latitude-dependent brightness variations at the limb lead to a quadrupolar term (a pole-to-equator temperature variation) of the order of 0.1 K, an order of magnitude smaller than previously reported. We present the analysis of these unique data, an overview of some results and we report on our progress as we apply our developed analysis method to the whole 12 years of data. Title: Chromospheric and Coronal HMI Flare Sources Authors: Martinez Oliveros, Juan Carlos; Saint-Hilaire, Pascal; Couvidat, Sebastien; Hudson, Hugh S.; Krucker, Sam Bibcode: 2014AAS...22412331M Altcode: We present observations of white-light features in the low corona, for three flares SOL20110308T1935, SOL20110308T0230 and SOL2013-05-13T16:01, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least two distinct kinds of sources appear (chromospheric and coronal) in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. The gradual emissions have a clear identification with the classical loop-prominence system, with emission contributions from electron scattering and from the free-free continuum (as seen in soft X-rays). These sources may also contain other continuum and/or line emissions and lead clearly to coronal rain in some cases observed Title: CME Mass Estimates via EVE Coronal Dimmings for X-class Flares Authors: Hudson, Hugh S.; Hannah, Iain; Schrijver, Karel Bibcode: 2014AAS...22421810H Altcode: The EVE instrument on SDO detects post-flare dimmings, mainly in the spectral regions of Fe IX-XII in its MEGS-A range, which is available for most of the 29 X-class flares that have occurred between SDO launch and the end of April 2014. Based upon earlier X-ray observations we interpret these dimmings as the result of CME mass ejection from the low corona. We estimate the masses involved in these dimmings by deriving a best pre-event temperature and emission measure in the dimmed region from EVE, and a source volume from AIA images. The dimming for SOL2011-02-15, the first of these events, "peaked"at -3.4% in Fe IX in terms of the pre-event emission from the whole Sun, with smaller relative depletions in higher ionization states of Fe. The "maximum" occurred more than one hour after GOES peak. The dimming signature is generally cleanly measurable in the EVE/MEGS-A spectral samples at10 s cadence, with the dominant source of uncertainty stemming from the "sun-as-a-star" integrations; for example flare-related excess emission at a given wavelength tends to compensate for the dimming,and in this sense the mass estimate must be considered a lower limit. We address these uncertainties for the solar case by appealing to the AIA images, but for analogous processes in stellar flares one would not have this luxury. Title: Helioseismic and Magnetic Imager Observations of Linear Polarization from a Loop Prominence System Authors: Saint-Hilaire, Pascal; Schou, Jesper; Martinez Oliveros, Juan Carlos; Hudson, Hugh S.; Krucker, Sam; Bain, Hazel; Couvidat, Sebastien Bibcode: 2014AAS...22412311S Altcode: White-light observations by the Solar Dynamics Observatory's Helioseismic and Magnetic Imager of a loop-prominence system occurring in the aftermath of an X-class flare on 2013 May 13 near the eastern solar limb show a linearly polarized component, reaching up to 20% at an altitude of 33 Mm, about the maximal amount expected if the emission were due solely to Thomson scattering of photospheric light by the coronal material. The mass associated with the polarized component was 8.2x10^14 g. At 15 Mm altitude, the brightest part of the loop was 3(+/-0.5)% linearly polarized, only about 20% of that expected from pure Thomson scattering, indicating the presence of an additional unpolarized component at wavelengths near Fe I (617.33 nm), probably thermal emission. We estimated the free electron density of the white-light loop system to possibly be as high as 1.8x10^12 cm^-3. Title: Observations of Linear Polarization in a Solar Coronal Loop Prominence System Observed near 6173 Å Authors: Saint-Hilaire, Pascal; Schou, Jesper; Martínez Oliveros, Juan-Carlos; Hudson, Hugh S.; Krucker, Säm; Bain, Hazel; Couvidat, Sébastien Bibcode: 2014ApJ...786L..19S Altcode: 2014arXiv1402.7016S White-light observations by the Solar Dynamics Observatory's Helioseismic and Magnetic Imager of a loop-prominence system occurring in the aftermath of an X-class flare on 2013 May 13 near the eastern solar limb show a linearly polarized component, reaching up to ~20% at an altitude of ~33 Mm, about the maximum amount expected if the emission were due solely to Thomson scattering of photospheric light by the coronal material. The mass associated with the polarized component was 8.2 × 1014 g. At 15 Mm altitude, the brightest part of the loop was 3(±0.5)% linearly polarized, only about 20% of that expected from pure Thomson scattering, indicating the presence of an additional unpolarized component at wavelengths near Fe I (617.33 nm). We estimate the free electron density of the white-light loop system to possibly be as high as 1.8 × 1012 cm-3. Title: The Role of Magnetic Fields in Transient Seismic Emission Driven by Atmospheric Heating in Flares Authors: Lindsey, C.; Donea, A. -C.; Martínez Oliveros, J. C.; Hudson, H. S. Bibcode: 2014SoPh..289.1457L Altcode: 2013arXiv1303.3299L; 2014SoPh..tmp....9L Transient seismic emission in flares remains largely mysterious. Its discoverers proposed that seismic transients are driven by impulsive heating of the flaring chromosphere. Simulations of such heating show strong shocks, but these are damped by heavy radiative losses as they proceed downward. Because compression of the gas the shock enters both heats it and increases its density, the radiative losses increase radically with the strength of the shock, leaving doubt that sufficient energy can penetrate into the solar interior to explain helioseismic signatures. We note that simulations to date have no account for strong, inclined magnetic fields characteristic of transient-seismic-source environments. A strong horizontal magnetic field, for example, greatly increases the compressional modulus of the chromospheric medium, greatly reducing compression of the gas, hence radiative losses. Inclined magnetic fields, then, must be fundamental to the role of impulsive heating in transient seismic emission. Title: Cycle 23 Variation in Solar Flare Productivity Authors: Hudson, Hugh; Fletcher, Lyndsay; McTiernan, Jim Bibcode: 2014SoPh..289.1341H Altcode: 2014arXiv1401.6474H The NOAA listings of solar flares in cycles 21 - 24, including the GOES soft X-ray magnitudes, enable a simple determination of the number of flares each flaring active region produces over its lifetime. We have studied this measure of flare productivity over the interval 1975 - 2012. The annual averages of flare productivity remained approximately constant during cycles 21 and 22, at about two reported M- or X-flares per region, but then increased significantly in the declining phase of cycle 23 (the years 2004 - 2005). We have confirmed this by using the independent RHESSI flare catalog to check the NOAA events listings where possible. We note that this measure of solar activity does not correlate with the solar cycle. The anomalous peak in flare productivity immediately preceded the long solar minimum between cycles 23 and 24. Title: Transient Artifacts in a Flare Observed by the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory Authors: Martínez Oliveros, J. C.; Lindsey, C.; Hudson, H. S.; Buitrago Casas, J. C. Bibcode: 2014SoPh..289..809M Altcode: 2013arXiv1307.5097M The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) provides a new tool for the systematic observation of white-light flares, including Doppler and magnetic information as well as continuum. In our initial analysis of the highly impulsive -ray flare SOL2010-06-12T00:57 (Martínez Oliveros et al., Solar Phys.269, 269, 2011), we reported the signature of a strong blueshift in the two footpoint sources. Concerned that this might be an artifact due to aliasing peculiar to the HMI instrument, we undertook a comparative analysis of Global Oscillation Network Group (GONG++) observations of the same flare, using the PArametric Smearing Correction ALgorithm (PASCAL) algorithm to correct for artifacts caused by variations in atmospheric smearing. This analysis confirms the artifactual nature of the apparent blueshift in the HMI observations, finding weak redshifts at the footpoints instead. We describe the use of PASCAL with GONG++ observations as a complement to the SDO observations and discuss constraints imposed by the use of HMI far from its design conditions. With proper precautions, these data provide rich information on flares and transients. Title: Chromospheric and Coronal Observations of Solar Flares with the Helioseismic and Magnetic Imager Authors: Martínez Oliveros, Juan-Carlos; Krucker, Säm; Hudson, Hugh S.; Saint-Hilaire, Pascal; Bain, Hazel; Lindsey, Charles; Bogart, Rick; Couvidat, Sebastien; Scherrer, Phil; Schou, Jesper Bibcode: 2014ApJ...780L..28M Altcode: 2013arXiv1311.7412M We report observations of white-light ejecta in the low corona, for two X-class flares on 2013 May 13, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least two distinct kinds of sources appeared (chromospheric and coronal), in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. The gradual emissions have a clear identification with the classical loop-prominence system, but are brighter than expected and possibly seen here in the continuum rather than line emission. We find the HMI flux exceeds the radio/X-ray interpolation of the bremsstrahlung produced in the flare soft X-ray sources by at least one order of magnitude. This implies the participation of cooler sources that can produce free-bound continua and possibly line emission detectable by HMI. One of the early sources dynamically resembles "coronal rain", appearing at a maximum apparent height and moving toward the photosphere at an apparent constant projected speed of 134 ± 8 km s-1. Not much literature exists on the detection of optical continuum sources above the limb of the Sun by non-coronagraphic instruments and these observations have potential implications for our basic understanding of flare development, since visible observations can in principle provide high spatial and temporal resolution. Title: Prominences in SDO/EVE spectra: contributions from large solar structures Authors: Labrosse, Nicolas; Hudson, Hugh; Kazachenko, Maria Bibcode: 2014IAUS..300..439L Altcode: The EVE instrument on SDO is making accurate measurements of the solar spectral irradiance in the EUV between 30 and 1069 Å, with 1 Å spectral resolution and 10 s sampling rate. These data define solar variability in the ``Sun-as-a-star'' mode and reveal many interesting kinds of variation. Its high sensitivity also makes it suitable for spectroscopic diagnostics of solar features such as flares. Here we present EVE's potential contribution to the diagnostics of large-scale, slowly evolving features such as prominences and active regions, and what we can learn from this. Title: Solar Eruptive Events (SEE) 2020 Mission Concept Authors: Lin, R. P.; Caspi, A.; Krucker, S.; Hudson, H.; Hurford, G.; Bandler, S.; Christe, S.; Davila, J.; Dennis, B.; Holman, G.; Milligan, R.; Shih, A. Y.; Kahler, S.; Kontar, E.; Wiedenbeck, M.; Cirtain, J.; Doschek, G.; Share, G. H.; Vourlidas, A.; Raymond, J.; Smith, D. M.; McConnell, M.; Emslie, G. Bibcode: 2013arXiv1311.5243L Altcode: Major solar eruptive events (SEEs), consisting of both a large flare and a near simultaneous large fast coronal mass ejection (CME), are the most powerful explosions and also the most powerful and energetic particle accelerators in the solar system, producing solar energetic particles (SEPs) up to tens of GeV for ions and hundreds of MeV for electrons. The intense fluxes of escaping SEPs are a major hazard for humans in space and for spacecraft. Furthermore, the solar plasma ejected at high speed in the fast CME completely restructures the interplanetary medium (IPM) - major SEEs therefore produce the most extreme space weather in geospace, the interplanetary medium, and at other planets. Thus, understanding the flare/CME energy release process(es) and the related particle acceleration processes are major goals in Heliophysics. To make the next major breakthroughs, we propose a new mission concept, SEE 2020, a single spacecraft with a complement of advanced new instruments that focus directly on the coronal energy release and particle acceleration sites, and provide the detailed diagnostics of the magnetic fields, plasmas, mass motions, and energetic particles required to understand the fundamental physical processes involved. Title: Implosion of Coronal Loops during the Impulsive Phase of a Solar Flare Authors: Simões, P. J. A.; Fletcher, L.; Hudson, H. S.; Russell, A. J. B. Bibcode: 2013ApJ...777..152S Altcode: 2013arXiv1309.7090S We study the relationship between implosive motions in a solar flare, and the energy redistribution in the form of oscillatory structures and particle acceleration. The flare SOL2012-03-09T03:53 (M6.4) shows clear evidence for an irreversible (stepwise) coronal implosion. Extreme-ultraviolet (EUV) images show at least four groups of coronal loops at different heights overlying the flaring core undergoing fast contraction during the impulsive phase of the flare. These contractions start around a minute after the flare onset, and the rate of contraction is closely associated with the intensity of the hard X-ray and microwave emissions. They also seem to have a close relationship with the dimming associated with the formation of the coronal mass ejection and a global EUV wave. Several studies now have detected contracting motions in the corona during solar flares that can be interpreted as the implosion necessary to release energy. Our results confirm this, and tighten the association with the flare impulsive phase. We add to the phenomenology by noting the presence of oscillatory variations revealed by Geostationary Operational Environmental Satellite soft X-rays (SXR) and spatially integrated EUV emission at 94 and 335 Å. We identify pulsations of ≈60 s in SXR and EUV data, which we interpret as persistent, semi-regular compressions of the flaring core region which modulate the plasma temperature and emission measure. The loop oscillations, observed over a large region, also allow us to provide rough estimates of the energy temporarily stored in the eigenmodes of the active-region structure as it approaches its new equilibrium. Title: Estimating active region luminosity using EVE/SDO observations Authors: Kazachenko, Maria D.; Hudson, H. S.; Fisher, G. H.; Canfield, R. C. Bibcode: 2013SPD....44...44K Altcode: Do solar active regions typically radiate more coronal energy during flares than the quiescent periods between them? This is a fundamental question for storage and release models of flares and active regions, yet it is presently poorly answered by observations. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of the Sun in the "Sun-as-a-point source" mode. It covers a wide range of temperatures and thus allows a detailed study of thermal emissions. Here we present two approaches for computing the active region luminosity, using EVE observations of fourteen Fe lines (FeIX-FeXXIV). In the first approach, we analyze EVE data in a time-series sense, when only one active region is present on the disk; this allows us to subtract the background due to the quiet sun and get the contribution from the active region alone. In the second approach, we analyze correlations of the radiative signatures with proxy indices (total solar magnetic and Poynting fluxes) during several months of data, when multiple active regions are present on the solar disk. We discuss capabilities of the two approaches, and what we can learn from them.Abstract (2,250 Maximum Characters): Do solar active regions typically radiate more coronal energy during flares than the quiescent periods between them? This is a fundamental question for storage and release models of flares and active regions, yet it is presently poorly answered by observations. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of the Sun in the "Sun-as-a-point source" mode. It covers a wide range of temperatures and thus allows a detailed study of thermal emissions. Here we present two approaches for computing the active region luminosity, using EVE observations of fourteen Fe lines (FeIX-FeXXIV). In the first approach, we analyze EVE data in a time-series sense, when only one active region is present on the disk; this allows us to subtract the background due to the quiet sun and get the contribution from the active region alone. In the second approach, we analyze correlations of the radiative signatures with proxy indices (total solar magnetic and Poynting fluxes) during several months of data, when multiple active regions are present on the solar disk. We discuss capabilities of the two approaches, and what we can learn from them. Title: Using X-ray absorption to measure the height of the solar atmosphere Authors: Hudson, Hugh S.; Battaglia, M.; Hurford, G. J.; Krucker, S.; Schwartz, R. A. Bibcode: 2013SPD....44..117H Altcode: The X-ray image of a partially-occulted solar flare, one occurring just behind the limb of the Sun, can have a sharply defined X-ray edge resulting from attenuation in the atmosphere of the quiet Sun in the foreground. Our analysis makes use of RHESSI's direct measurement of image Fourier visibilities, and we estimate that the ultimate precision of the limb height will on the order of the photospheric scale height in the region of dominant absorption. This occurs at an altitude depending on the X-ray photon energies used for the measurement, but generally in the upper photosphere and chromosphere. We give a preliminary report on analysis of one suitable event, the flare SOL2002-04-04T15:32 (M6.1), where we find a clean signature of this attenuation up to the RHESSI hard X-ray range 12-25 keV. At this energy Compton scattering begins to dominate the attenuation, greatly reducing the model-dependence of the result; at lower photon energies photoelectric absorption becomes more important. These data determine the physical altitude of the mean atmospheric density, with minimal model dependence, and therefore provide an independent calibration of the atmospheric height scale. Title: White-light and Hard X-ray source heights of the SOL2011-01-28T00:24 solar flare Authors: Martinez Oliveros, Juan Carlos; Glesener, L.; Hudson, H. S.; Krucker, S.; Hurford, G. J. Bibcode: 2013SPD....44...86M Altcode: White-light continuum and hard X-ray emission in flares have strong correlations in time, but at present we do not have a clear idea about their height structures. Recently, several studies of the relative positions of the white-light and hard X-ray sources have been made using observations of flares near the solar limb. However, these results are still inconclusive due to the small number of flares observed. On 28 January 2011 a white-light flare (SOL2011-01-28T07:35) was observed on the western limb, observed simultaneously by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Solar TErrestrial RElations Observatory (STEREO). This observation provides the heights of these emissions directly, limited only by the limb references for the two spacecraft, with almost no projection uncertainty. We report the results of this analysis and discuss our findings in terms of present models of particle acceleration and energy transport in the impulsive phase.Abstract (2,250 Maximum Characters): White-light continuum and hard X-ray emission in flares have strong correlations in time, but at present we do not have a clear idea about their height structures. Recently, several studies of the relative positions of the white-light and hard X-ray sources have been made using observations of flares near the solar limb. However, these results are still inconclusive due to the small number of flares observed. On 28 January 2011 a white-light flare (SOL2011-01-28T07:35) was observed on the western limb, observed simultaneously by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Solar TErrestrial RElations Observatory (STEREO). This observation provides the heights of these emissions directly, limited only by the limb references for the two spacecraft, with almost no projection uncertainty. We report the results of this analysis and discuss our findings in terms of present models of particle acceleration and energy transport in the impulsive phase. Title: Photospheric Temperature Variations near the Solar Limb II Authors: Fivian, Martin; Hudson, H. S. Bibcode: 2013SPD....44..121F Altcode: We use observations from the Solar Aspect Sensor (SAS) of RHESSI to characterize the latitude dependence of the temperature of the photosphere near the solar limb. Previous observations had suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. This photometry is differential, i.e. relative to a mean limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM) at 670 nm. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its average solar pointing. We fit an average limb-darkening function and reassemble the residuals into synoptic maps of differential intensity variations as a function of position angle. We further mask these images against EUV images (SOHO/EIT 284A for older data and SDO/AIA for more recent data) in order to eliminate magnetic regions. We present results from our new analysis which shows significantly larger signals of latitude-dependent temperature variations than what has been presented earlier in our preliminary analysis and interpretation.Abstract (2,250 Maximum Characters): We use observations from the Solar Aspect Sensor (SAS) of RHESSI to characterize the latitude dependence of the temperature of the photosphere near the solar limb. Previous observations had suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. This photometry is differential, i.e. relative to a mean limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM) at 670 nm. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its average solar pointing. We fit an average limb-darkening function and reassemble the residuals into synoptic maps of differential intensity variations as a function of position angle. We further mask these images against EUV images (SOHO/EIT 284A for older data and SDO/AIA for more recent data) in order to eliminate magnetic regions. We present results from our new analysis which shows significantly larger signals of latitude-dependent temperature variations than what has been presented earlier in our preliminary analysis and interpretation. Title: Flare Ribbon Energetics in the Early Phase of an SDO Flare Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Innes, D. E. Bibcode: 2013ApJ...771..104F Altcode: 2014arXiv1401.6538F The sites of chromospheric excitation during solar flares are marked by extended extreme ultraviolet ribbons and hard X-ray (HXR) footpoints. The standard interpretation is that these are the result of heating and bremsstrahlung emission from non-thermal electrons precipitating from the corona. We examine this picture using multi-wavelength observations of the early phase of an M-class flare SOL2010-08-07T18:24. We aim to determine the properties of the heated plasma in the flare ribbons, and to understand the partition of the power input into radiative and conductive losses. Using GOES, SDO/EVE, SDO/AIA, and RHESSI, we measure the temperature, emission measure (EM), and differential emission measure of the flare ribbons, and deduce approximate density values. The non-thermal EM, and the collisional thick target energy input to the ribbons are obtained from RHESSI using standard methods. We deduce the existence of a substantial amount of plasma at 10 MK in the flare ribbons, during the pre-impulsive and early-impulsive phase of the flare. The average column EM of this hot component is a few times 1028 cm-5, and we can calculate that its predicted conductive losses dominate its measured radiative losses. If the power input to the hot ribbon plasma is due to collisional energy deposition by an electron beam from the corona then a low-energy cutoff of ~5 keV is necessary to balance the conductive losses, implying a very large electron energy content. Independent of the standard collisional thick-target electron beam interpretation, the observed non-thermal X-rays can be provided if one electron in 103-104 in the 10 MK (1 keV) ribbon plasma has an energy above 10 keV. We speculate that this could arise if a non-thermal tail is generated in the ribbon plasma which is being heated by other means, for example, by waves or turbulence. Title: The properties of flare kernels observed by the Dunn Solar Telescope Authors: Fletcher, Lyndsay; Kowalski, A.; Cauzzi, G.; Hawley, S. L.; Hudson, H. S. Bibcode: 2013SPD....44...67F Altcode: We report on a campaign at the Dunn Solar Telescope which resulted in successful imaging and spectroscopic observations of a C1.1 solar flare on 18th August 2011. This flare exhibited ribbons with complicated fine structure at the resolution of the DST/IBIS instrument, and a number of bright kernels with sizes comparable to the smallest scales sampled by IBIS, around 2-4 pixels (0."3-0."6) FWHM. We focus on these bright kernels, describing their spatial characteristics in the core and wing of H alpha and Ca II 8542, and in the UV and EUV with SDO. We also show preliminary broad-band spectroscopy of the kernels which may demonstrate the presence of an optical continuum in this small flare. Title: Remote sensing of low-energy SEPs via charge exchange Authors: Hudson, H. S.; MacKinnon, A. L.; Badnell, N. R. Bibcode: 2013AIPC.1539...19H Altcode: Charge-exchange reactions at high energies provide new channels for the remote sensing of solar high-energy particles, as demonstrated by the recent detection of 1.8-5 MeV hydrogen atoms from a solar flare [1]. Orrall and Zirker [2] had earlier proposed the detection of low-energy protons via charge-exchange atomic reactions in the solar atmosphere, leading in the simplest case to extended red-wing emission in the Lyman-alpha line. We discuss the analogous process for the He II 304 A˚ line (for alpha particles) and also assess the feasibility of the analogous process in the solar wind, whereby ambient He and (C, N, O) ions allow low-energy alpha particles to undergo resonant charge exchange in the ambient corona and thereby produce 304 A˚ wing emission close to the acceleration region. Title: Solar flares at submillimeter wavelengths Authors: Krucker, Säm; Giménez de Castro, C. G.; Hudson, H. S.; Trottet, G.; Bastian, T. S.; Hales, A. S.; Kašparová, J.; Klein, K. -L.; Kretzschmar, M.; Lüthi, T.; Mackinnon, A.; Pohjolainen, S.; White, S. M. Bibcode: 2013A&ARv..21...58K Altcode: We discuss the implications of the first systematic observations of solar flares at submillimeter wavelengths, defined here as observing wavelengths shorter than 3 mm (frequencies higher than 0.1 THz). The events observed thus far show that this wave band requires a new understanding of high-energy processes in solar flares. Several events, including observations from two different observatories, show during the impulsive phase of the flare a spectral component with a positive (increasing) slope at the highest observable frequencies (up to 405 GHz). To emphasize the increasing spectra and the possibility that these events could be even more prominent in the THz range, we term this spectral feature a "THz component". Here we review the data and methods, and critically assess the observational evidence for such distinct component(s). This evidence is convincing. We also review the several proposed explanations for these feature(s), which have been reported in three distinct flare phases. These data contain important clues to flare development and particle acceleration as a whole, but many of the theoretical issues remain open. We generally have lacked systematic observations in the millimeter-wave to far-infrared range that are needed to complete our picture of these events, and encourage observations with new facilities. Title: Coronal Post-Flare Dynamics for 14 August 2010 Late Phase of the Coronal Dimming Event Authors: Didkovsky, Leonid; Judge, Darrell; Wieman, Seth; Woods, Tom; Hock, Rachel; Chamberlin, Phillip; Tobiska, Kent; Hudson, Hugh Bibcode: 2013enss.confE..22D Altcode: Coronal irradiance dynamics related to the 14 August 2010 C4.4 post-flare event was analyzed using EUV spectral emission lines in the range of logT from 5.8 to 6.4 (0.7 to 2.6 MK). Temporal changes of high-resolution spectral irradiance for different thermal layers of the Corona from the Extreme ultraviolet Variability Experiment (EVE) onboard the Solar Dynamics Observatory (SDO) were compared to study both a decrease and an increase of the spectral irradiance during the late phase of the coronal dimming. We found an irradiance decrease which propagated from the low-temperature layer to the higher temperature layers with much lower speed than the speed of the `horizontal' dimming wave inside the low-temperature layer. This upward decrease of the irradiance in the coronal layers may represent a `vertical' spatial perturbation of the plasma within these layers in response to the plasma density and temperature decreases of the dimming wave. The perturbation may cause a redistribution of the coronal loops and trigger the late phase of the flare. The increase of the irradiance detected in the high-temperature coronal layers where the late phase of the flare occurred after 12 UT was propagating in part downward, restoring the pre-dimming plasma conditions in these Coronal layers. Title: Non-thermal processes in coronae and beyond Authors: Poppenhaeger, K.; Günther, H. M.; Beiersdorfer, P.; Brickhouse, N. S.; Carter, J. A.; Hudson, H. S.; Kowalski, A.; Lalitha, S.; Miceli, M.; Wolk, S. J. Bibcode: 2013AN....334..101P Altcode: 2013csss...17..101P; 2012arXiv1210.2960P This contribution summarizes the splinter session ``Non-thermal processes in coronae and beyond'' held at the Cool Stars 17 workshop in Barcelona in 2012. It covers new developments in high energy non-thermal effects in the Earth's exosphere, solar and stellar flares, the diffuse emission in star forming regions and reviews the state and the challenges of the underlying atomic databases. Title: GRB 130427A: RHESSI observations. Authors: Smith, D. M.; Csillaghy, A.; Hurley, K.; Hudson, H.; Boggs, S.; Inglis, A. Bibcode: 2013GCN.14590....1S Altcode: 2013GCN..14590...1S No abstract at ADS Title: Opportunities for Solar Science with NuSTAR Authors: Glesener, Lindsay; Boggs, S. E.; Christensen, F.; Craig, W. W.; Hailey, C. J.; Grefenstette, B.; Harrison, F.; Hudson, H. S.; Hurford, G. J.; Krucker, S.; Marsh, A.; Mewaldt, R. A.; Pivovaroff, M.; Smith, D. M.; Stern, D.; Vogel, J.; White, S. M.; Zhang, W.; NuSTAR Team Bibcode: 2013AAS...22124423G Altcode: While NuSTAR was designed to observe faint cosmic sources in hard X-rays (HXR), its unprecedented sensitivity can also be used to address several outstanding questions in high energy solar physics. Medium- and large-sized solar flares have been well -studied in HXR by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), launched in 2002. These flares are always found in active regions and usually emit nonthermal HXR from accelerated electrons, along with thermal bremsstrahlung as those electrons lose their energy and heat the ambient plasma. To date, no HXR flares outside active regions have been observed, though thermal brightenings in soft X-rays and EUV suggest that small "nanoflares" may occur frequently across the entire solar disk, even at quiet times when no active regions are present. Even a few minutes of NuSTAR solar observations will allow a search for HXR from quiet-Sun nanoflares with better sensitivity than any previous study. These observations will have important implications for the role of flares in supplying the corona with its surprisingly hot temperature (1--2 MK, as compared with the photospheric temperature of 5800 K). NuSTAR will also make the first observations of escaping flare electrons associated with Type III radio emission, can image faint coronal sources in partially occulted flares that are below RHESSI's sensitivity, and, combined with RHESSI data, could study the faint, earliest phase of flares, where direct signatures of particle acceleration are most likely to be observed. Title: A Coral Sea Rehearsal for the Eclipse Megamovie Authors: Hudson, H. S.; Davey, A. R.; Ireland, J.; Jones, L.; Mcintosh, S. W.; Paglierani, R.; Pasachoff, J. M.; Peticolas, L. M.; Russell, R. M.; Suarez Sola, F. I.; Sutherland, L.; Thompson, M. J. Bibcode: 2012AGUFMSH11C..06H Altcode: The "Eclipse on the Coral Sea" - 13/14 November 2012 (GMT/Australia) - will have happened already. Our intention is to have used this opportunity as a trial run for the eclipse in 2017, which features 1.5 hours of totality across the whole width of the continental US. Conceived first and foremost as an education and public outreach activity, the plan is to engage the public in solar science and technology by providing a way for them to include images they have taken of the solar eclipse, into a movie representation of coronal evolution in time. This project will assimilate as much eclipse photography as possible from the public. The resulting movie(s) will cover all ranges of expertise, and at the basic smartphone or hand-held digital camera level, we expect to have obtained a huge number of images in the case of good weather conditions. The capability of modern digital technology to handle such a data flow is new. The basic purpose of this and the 2017 Megamovie observations is to explore this capability and its ability to engage people from many different communities in the solar science, astronomy, mathematics, and technology. The movie in 2017, especially, may also have important science impact because of the uniqueness of the corona as seen under eclipse conditions. In this presentation we will describe our smartphone application development (see the "Transit of Venus" app for a role model here). We will also summarize data acquisition via both the app and more traditional web interfaces. Although for the Coral Sea eclipse event we don't expect to have a movie product by the time of the AGU, for the 2017 event we do intend to assemble the heterogenous data into beautiful movies within a short space of time after the eclipse. These movies may have relatively low resolution but would extend to the base of the corona. We encourage participation in the 2012 observations, noting that no total eclipse, prior to 2017, will occur in a region with good infrastructure for extended observations. The National Center for Atmospheric Research is sponsored by the National Science Foundation. The Megamovie project is supported by NSF grant AGS-1247226, and JMP's eclipse work about the eclipses of 2012 is supported by NSF grant AGS-1047726. Title: New Coronal Developments in Flare Research Authors: Hudson, H. S. Bibcode: 2012AGUFMSH42A..04H Altcode: The past decade has produced spectacular new observations that have affected our views of the relationships between flares and coronal phenomena. We have substantial new clues about the nature of the energy release that results from the restructuring of the coronal field. The energy partitioning in a flare/CME event strongly suggests a close relationship between the microphysics of particle acceleration and the global scales of the necessary coronal restructuring. Among the sources of information about the accelerated particles we can now list hard X-rays, gamma-rays, SEPs, and ENAs, as well as the radio spectrum. Unexpectedly, the EUV spectroscopy from SDO/EVE shows that the Orrall-Zirker mechanism (Doppler-shifted He II 304 A, resulting from charge-exchange reactions) cannot easily be used to link these scales. In this presentation I synthesize the available data and suggest some desirable future developments of theory and observation. Title: Magnetism of Solar Flares and Prominences Authors: Heinzel, P.; Hudson, H. S. Bibcode: 2012ASPC..463..121H Altcode: We give an overview of magnetic fields in solar flares and prominences. Magnetic fields related to flares play a crucial role in the process of energy release and transport to the lower atmosphere, and thus magnetometry under the coronal and chromospheric conditions is extremely challenging. Magnetic fields in prominences are supposed to keep the prominence plasma at coronal heights against the gravity. Their measurements have been numerous, but high-resolution mapping is still missing. We discuss various flare and prominence models in connection to current and future high-resolution observations. Title: The Solar Oblateness at Solar Minimum as Observed by RHESSI/SAS II Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P. Bibcode: 2012AGUFMSH13C2264F Altcode: The Solar Aspect System (SAS) of the RHESSI satellite measures the optical solar limb in the red continuum with a cadence typically set at 32 samples/s in each of three linear CCD sensors. RHESSI has observed the Sun continuously now for more than 10 years, and we have acquired a unique data set ranging almost over a full solar cycle and consisting of about 25x10^9 single data points. For a three month period during the active phase of the last solar cycle in 2004, the shape of the solar disk has been measured discovering an apparent excess oblateness which we attributed to the enhanced network. These measurements have led to the most accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian et al., 2008), a value consistent with models predicting an oblateness from surface rotation. In order to avoid confusion between magnetic activity and a correlated brightness enhancement in the SAS signal, the SAS data has been masked using the SOHO/EIT284A data, and SDO/AIA for more recent data. The measured oblateness as function of the masking level is then extrapolated for a value of the underlaying, presumably non-magnetic sun. A recent and significantly improved calibration of the SAS data have allowed a new access to a measurement of the solar oblateness during the last, extended solar minimum. Here, we present the analysis of the RHESSI/SAS data during the solar minimum with the inferred interpretation for the oblateness signal. Title: Magneto-Acoustic Energetics Study of the Seismically Active Flare of 15 February 2011 Authors: Alvarado-Gómez, J. D.; Buitrago-Casas, J. C.; Martínez-Oliveros, J. C.; Lindsey, C.; Hudson, H.; Calvo-Mozo, B. Bibcode: 2012SoPh..280..335A Altcode: 2012arXiv1203.3907A; 2012SoPh..tmp..131A Multi-wavelength studies of energetic solar flares with seismic emissions have revealed interesting common features between them. We studied the first GOES X-class flare of Solar Cycle 24, as detected by the Solar Dynamics Observatory (SDO). For context, seismic activity from this flare (SOL2011-02-15T01:55-X2.2, in NOAA AR 11158) has been reported by Kosovichev (Astrophys. J. Lett., 734, L15, 2011) and Zharkov et al. (Astrophys. J. Lett., 741, L35, 2011). Based on Dopplergram data from the Helioseismic and Magnetic Imager (HMI), we applied standard methods of local helioseismology in order to identify the seismic sources in this event. RHESSI hard X-ray data are used to check the correlation between the location of the seismic sources and the particle-precipitation sites in during the flare. Using HMI magnetogram data, the temporal profile of fluctuations in the photospheric line-of-sight magnetic field is used to estimate the magnetic-field change in the region where the seismic signal was observed. This leads to an estimate of the work done by the Lorentz-force transient on the photosphere of the source region. In this instance, this is found to be a significant fraction of the acoustic energy in the attendant seismic emission, suggesting that Lorentz forces can contribute significantly to the generation of sunquakes. However, there are regions in which the signature of the Lorentz force is much stronger, but from which no significant acoustic emission emanates. Title: Estimating the frequency of extremely energetic solar events, based on solar, stellar, lunar, and terrestrial records Authors: Schrijver, C. J.; Beer, J.; Baltensperger, U.; Cliver, E. W.; Güdel, M.; Hudson, H. S.; McCracken, K. G.; Osten, R. A.; Peter, T.; Soderblom, D. R.; Usoskin, I. G.; Wolff, E. W. Bibcode: 2012JGRA..117.8103S Altcode: 2012arXiv1206.4889S; 2012JGRA..11708103S The most powerful explosions on the Sun - in the form of bright flares, intense storms of solar energetic particles (SEPs), and fast coronal mass ejections (CMEs) - drive the most severe space-weather storms. Proxy records of flare energies based on SEPs in principle may offer the longest time base to study infrequent large events. We conclude that one suggested proxy, nitrate concentrations in polar ice cores, does not map reliably to SEP events. Concentrations of select radionuclides measured in natural archives may prove useful in extending the time interval of direct observations up to ten millennia, but as their calibration to solar flare fluences depends on multiple poorly known properties and processes, these proxies cannot presently be used to help determine the flare energy frequency distribution. Being thus limited to the use of direct flare observations, we evaluate the probabilities of large-energy solar events by combining solar flare observations with an ensemble of stellar flare observations. We conclude that solar flare energies form a relatively smooth distribution from small events to large flares, while flares on magnetically active, young Sun-like stars have energies and frequencies markedly in excess of strong solar flares, even after an empirical scaling with the mean coronal activity level of these stars. In order to empirically quantify the frequency of uncommonly large solar flares extensive surveys of stars of near-solar age need to be obtained, such as is feasible with the Kepler satellite. Because the likelihood of flares larger than approximately X30 remains empirically unconstrained, we present indirect arguments, based on records of sunspots and on statistical arguments, that solar flares in the past four centuries have likely not substantially exceeded the level of the largest flares observed in the space era, and that there is at most about a 10% chance of a flare larger than about X30 in the next 30 years. Title: SDO/EVE spectra of solar flares Authors: Heinzel, Petr; Avrett, Eugene; Dzifcakova, Elena; Hudson, Hugh S. Bibcode: 2012cosp...39..743H Altcode: 2012cosp.meet..743H For selected flare events we present the SDO/EVE spectra of hydrogen and helium resonance continua and compare them with the results of non-LTE transfer computations based on various flare models. We discuss the formation of these continua and their diagnostic potential for determination of the temperature structure of the flaring atmosphere. Under the optically-thin conditions, we synthesize these resonance continua using the CHIANTI database where we added the relevant non-thermal atomic rates. This shows how the electron or proton beams may affect the continua during an impulsive phase of solar flares. As a benchmark we present our comparison of the quiet-Sun EVE spectra with the flux synthesized from the model C6 of Avrett and Loeser (2008). Our study has a more general relevance to the physics of stellar flares. Title: The Height of a White-light Flare and Its Hard X-Ray Sources Authors: Martínez Oliveros, Juan-Carlos; Hudson, Hugh S.; Hurford, Gordon J.; Krucker, Säm; Lin, R. P.; Lindsey, Charles; Couvidat, Sebastien; Schou, Jesper; Thompson, W. T. Bibcode: 2012ApJ...753L..26M Altcode: 2012arXiv1206.0497M We describe observations of a white-light (WL) flare (SOL2011-02-24T07:35:00, M3.5) close to the limb of the Sun, from which we obtain estimates of the heights of the optical continuum sources and those of the associated hard X-ray (HXR) sources. For this purpose, we use HXR images from the Reuven Ramaty High Energy Spectroscopic Imager and optical images at 6173 Å from the Solar Dynamics Observatory. We find that the centroids of the impulsive-phase emissions in WL and HXRs (30-80 keV) match closely in central distance (angular displacement from Sun center), within uncertainties of order 0farcs2. This directly implies a common source height for these radiations, strengthening the connection between visible flare continuum formation and the accelerated electrons. We also estimate the absolute heights of these emissions as vertical distances from Sun center. Such a direct estimation has not been done previously, to our knowledge. Using a simultaneous 195 Å image from the Solar-Terrestrial RElations Observatory spacecraft to identify the heliographic coordinates of the flare footpoints, we determine mean heights above the photosphere (as normally defined; τ = 1 at 5000 Å) of 305 ± 170 km and 195 ± 70 km, respectively, for the centroids of the HXR and WL footpoint sources of the flare. These heights are unexpectedly low in the atmosphere, and are consistent with the expected locations of τ = 1 for the 6173 Å and the ~40 keV photons observed, respectively. Title: Charge-exchange Limits on Low-energy α-particle Fluxes in Solar Flares Authors: Hudson, H. S.; Fletcher, L.; MacKinnon, A. L.; Woods, T. N. Bibcode: 2012ApJ...752...84H Altcode: 2014arXiv1401.6477H This paper reports on a search for flare emission via charge-exchange radiation in the wings of the Lyα line of He II at 304 Å, as originally suggested for hydrogen by Orrall & Zirker. Via this mechanism a primary α particle that penetrates into the neutral chromosphere can pick up an atomic electron and emit in the He II bound-bound spectrum before it stops. The Extreme-ultraviolet Variability Experiment on board the Solar Dynamics Observatory gives us our first chance to search for this effect systematically. The Orrall-Zirker mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV nucleon-1. We study 10 events in total, including the γ-ray events SOL2010-06-12 (M2.0) and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class flares, and one prominent M-class event that produced solar energetic particles. The absence of charge-exchange line wings may point to a need for more complete theoretical work. Some of the events do have broadband signatures, which could correspond to continua from other origins, but these do not have the spectral signatures expected from the Orrall-Zirker mechanism. Title: Flare Ribbons In The Early Phase Of An SDO Flare: Emission Measure And Energetics Authors: Fletcher, Lyndsay; Hannah, I. G.; Hudson, H. S.; Innes, D. E. Bibcode: 2012AAS...22050902F Altcode: We report on the M1.0 flare of 7th August 2010, which displayed extended early phase chromospheric ribbons, well observed by SDO/AIA and RHESSI. Most large flares saturate rapidly in the high-temperature AIA channels, however this event could be followed in unsaturated AIA images for ten minutes in the build-up to and first few minutes of the impulsive phase. Analysis of GOES, RHESSI and SDO/AIA demonstrates the presence of high temperature ( 10MK), compact plasma volumes in the chromospheric flare ribbons, with a column emission measure of on average 3-7 x 1028 cm-5. We construct a time-resolved energy budget for the ribbon plasma, including also SDO/EVE data, and discuss the implications of the observed ribbon properties for flare energisation.

This work was supported by the UK’s Science and Technology Facilities Council (ST/1001801), and by the European Commission through the FP7 HESPE project (FP7-2010-SPACE-263086). Title: The Journey of Sungrazing Comet Lovejoy Authors: Bryans, Paul; A'Hearn, M.; Battams, K.; Biesecker, D.; Bodewits, D.; Boice, D.; Brown, J.; Caspi, A.; Chodas, P.; Hudson, H.; Jia, Y.; Jones, G.; Keller, H. U.; Knight, M.; Linker, J.; Lisse, C.; Liu, W.; McIntosh, S.; Pesnell, W. D.; Raymond, J.; Saar, S.; Saint-Hilaire, P.; Schrijver, C.; Snow, M.; Tarbell, T.; Thompson, W.; Weissman, P.; Comet Lovejoy Collaboration Team Bibcode: 2012AAS...22052507B Altcode: Comet Lovejoy (C/2011 W3) was the first sungrazing comet, observed by space-based instruments, to survive perihelion passage. First observed by ground-based telescopes several weeks prior to perihelion, its journey towards the Sun was subsequently recorded by several solar observatories, before being observed in the weeks after perihelion by a further array of space- and ground-based instruments. Such a surfeit of wide-ranging observations provides an unprecedented insight into both sungrazing comets themselves, and the solar atmosphere through which they pass. This paper will summarize what we have learnt from the observations thus far and offer some thoughts on what future sungrazing comets may reveal about comets, the Sun, and their interaction. Title: Solar Flare Observations of the EUV Continua Authors: Milligan, Ryan O.; Chamberlin, P.; Hudson, H.; Woods, T.; Mathioudakis, M.; Fletcher, L.; Kowalski, A.; Keenan, F. Bibcode: 2012AAS...22052105M Altcode: Recent solar flare simulations suggest that the energy deposited in the chromosphere by nonthermal electrons during a flare's impulsive phase is re-emitted in the form of recombination (free-bound) continua, in particular, the Lyman, Balmer, and Paschen continua of hydrogen, and the He I and He II continua (Allred et al. 2005). However, definitive observations of free-bound emission during solar flares have been scarce in recent years as many modern, space-based instruments do not have the required sensitivity, wavelength coverage, or duty cycle. With the launch of SDO, these observations are now routinely available thanks to the EUV Variability Experiment (EVE) instrument. Here we present unambiguous, spectrally and temporally resolved detections of enhanced free-free and free-bound continua during the first X-class solar flare of Solar Cycle 24. While we find that the flare energy in the EVE spectral range amounts to at most a few percent of the total flare energy, these findings highlight the capability of EVE in giving us the first comprehensive look at these diagnostically important continuum components. Title: Global Forces in Eruptive Solar Flares: The Lorentz Force Acting on the Solar Atmosphere and the Solar Interior Authors: Fisher, George H.; Bercik, D. J.; Welsch, B. T.; Hudson, H. S. Bibcode: 2012AAS...22020440F Altcode: We compute the change in the Lorentz force integrated over the outer solar atmosphere implied by observed changes in vector magnetograms that occur during large, eruptive solar flares. This force perturbation should be balanced by an equal and opposite force perturbation acting on the solar photosphere and solar interior. The resulting expression for the estimated force change in the solar interior generalizes the earlier expression presented by Hudson, Fisher, and Welsch, providing horizontal as well as vertical force components, and provides a more accurate result for the vertical component of the perturbed force. We show that magnetic eruptions should result in the magnetic field at the photosphere becoming more horizontal, and hence should result in a downward (toward the solar interior) force change acting on the photosphere and solar interior, as recently argued from an analysis of magnetogram data by Wang and Liu. We suggest the existence of an observational relationship between the force change computed from changes in the vector magnetograms, the outward momentum carried by the ejecta from the flare, and the properties of the helioseismic disturbance driven by the downward force change. We use the impulse driven by the Lorentz-force change in the outer solar atmosphere to derive an upper limit to the mass of erupting plasma that can escape from the Sun. Finally, we compare the expected Lorentz-force change at the photosphere with simple estimates from flare-driven gasdynamic disturbances and from an estimate of the perturbed pressure from radiative backwarming of the photosphere in flaring conditions. Title: A RHESSI and SDO Campaign Measuring Latitude-dependent Limb Profiles and Oblateness of the Optical Solar Disk II Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.; Bush, R. I.; Emilio, M.; Kuhn, J. R.; Scholl, I. F. Bibcode: 2012AAS...22020511F Altcode: The SDO spacecraft conducts special roll maneuvers every 6 months. These SDO maneuvers enable its HMI instrument to obtain precise observations of the global structure of the limb. During the SDO roll on 2011 April 6 05:50-12:30 UT, we also successfully obtained RHESSI optical observations at very high cadence, 128 samples per sec for each of the three linear CCDs. A second coordinated observation with optimized parameter settings for RHESSI is planned for the time of the SDO roll maneuver in April 2012. The data from the two instruments (RHESSI/SAS and SDO/HMI), give different means for the investigation of the variation of the solar limb properties as a function of position angle (latitude). At the normal RHESSI cadence very long integrations (of order 3 months) are needed to obtain precise limb measurements, but in this case we are able to report results within the exact time frame of the SDO roll maneuver. The special RHESSI data rate was about 10,000 times larger than the standard rate and will achieve high precision in a relatively short time. We will compare these results with our earlier RHESSI observations (Fivian et al., 2008) and those obtained by Kuhn et al. (1998) and Emilio et al. (2007) with the earlier MDI roll maneuvers, and as well as with the most recent analysis of HMI data. Title: On The Energetics Of Seismic Excitation Mechanisms Authors: Martinez Oliveros, Juan Carlos; Bain, H.; Krucker, S.; Donea, A.; Hudson, H.; Lin, R. P.; Lindsey, C. Bibcode: 2012AAS...22020503M Altcode: Some solar flares emit strong acoustic transients into the solar interior during their impulsive phases (Kosovichev and Zharkova, 1998). These transients penetrate thousands of kilometers beneath the active region photosphere and refract back to the surface, where they produce a characteristic helioseismic signature tens of thousands of kilometers from their origin over the succeeding hour. Several mechanisms of seismic excitation have been proposed, ranging from hydrodynamic shocks to Lorentz force perturbations. However, regardless of the mechanism of generation, it is clear that not all flares induce an acoustic response in the interior of the Sun. A concrete hypothesis or theory about the nature of this is still a topic of ongoing investigations. For some particular flares, we present a comparative study between the energy deposited by the proposed mechanisms of seismic excitation and the acoustic energy deduced using holographic techniques. Title: Solar Hard X-ray Observations with NuSTAR Authors: Marsh, Andrew; Smith, D. M.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White, S. M.; Mewaldt, R. A.; Harrison, F. A.; Grefenstette, B. W.; Stern, D. Bibcode: 2012AAS...22052112M Altcode: High-sensitivity imaging of coronal hard X-rays allows detection of freshly accelerated nonthermal electrons at the acceleration site. A few such observations have been made with Yohkoh and RHESSI, but a leap in sensitivity could help pin down the time, place, and manner of reconnection. Around the time of this meeting, the Nuclear Spectroscopic Telescope ARray (NuSTAR), a NASA Small Explorer for high energy astrophysics that uses grazing-incidence optics to focus X-rays up to 80 keV, will be launched. Three weeks will be dedicated to solar observing during the baseline two-year mission. NuSTAR will be 200 times more sensitive than RHESSI in the hard X-ray band. This will allow the following new observations, among others: 1) Extrapolation of the micro/nanoflare distribution by two orders of magnitude down in flux; 2) Search for hard X-rays from network nanoflares (soft X-ray bright points) and evaluation of their role in coronal heating; 3) Discovery of hard X-ray bremsstrahlung from the electron beams driving type III radio bursts, and measurement of their electron spectrum; 4) Hard X-ray studies of polar soft X-ray jets and impulsive solar energetic particle events at the edge of coronal holes; 5) Study of coronal bremsstrahlung from particles accelerated by coronal mass ejections as they are first launched; 6) Study of particles at the coronal reconnection site when flare footpoints and loops are occulted; 7) Search for weak high-temperature coronal plasmas in active regions that are not flaring; and 8) Search for hypothetical axion particles created in the solar core via the hard X-ray signal from their conversion to X-rays in the coronal magnetic field. NuSTAR will also serve as a pathfinder for a future dedicated space mission with enhanced capabilities, such as a satellite version of the FOXSI sounding rocket. Title: Energetic Neutral Atoms (ENAs): A new window on Solar Energetic Particle (SEP) acceleration Authors: Lin, Robert P.; Wang, L.; Hudson, H.; Hurford, G.; Duncan, N.; Li, G.; Shih, A. Y.; Mewaldt, R. A. Bibcode: 2012AAS...22042406L Altcode: Large solar eruptive events accelerate ions up to GeV energies in both flares and fast coronal mass ejections (CMEs). Measurements of gamma-ray line emission show that up to 10-50% of the total energy released in the flare is contained in the few to 100 MeV ions accelerated in the flare. Observations in the interplanetary medium near 1 AU indicate that the number of SEP (Solar Energetic Particles) ions can be comparable or even up to 1-2 orders of magnitude larger than in flares. These SEPs are believed to be accelerated by shocks driven by fast CMEs at altitudes of from 1.5 to tens of solar radii, with the total energy in the SEPs of order 10% of the total kinetic energy of the CME. The ground-breaking discovery of 1.6-15 MeV energetic neutral atoms (ENAs, produced by charge exchange of SEPs with the ambient corona) from the Sun just prior to an SEP event (Mewaldt et al 2009) suggest the possibility that, for the first time, ions being accelerated by CME shocks close to the Sun can be remotely sensed and imaged, providing a completely new window on SEPs. We present model simulations of the SEP ENA production, and possible techniques for remote sensing and mapping SEP ions from seed particle energies ( 5 keV) to 10s of MeV.

R. A. Mewaldt, et al, Astrophys. J., 693: L11-15, 2009. Title: Results and Analysis of 10 Years of RHESSI/SAS Observations of the Optical Solar Limb Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P. Bibcode: 2012AAS...22042306F Altcode: The Solar Aspect System (SAS) of the RHESSI satellite measures the optical solar limb in the red continuum with a cadence typically set at 16 samples/s in each of three linear CCD sensors. RHESSI has observed the Sun continuously now for more than 10 years, and we have acquired a unique data set ranging almost over a full solar cycle and consisting of about 25x10^9 single data points. These measurements have led to the most accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian et al., 2008), a value consistent with models predicting an oblateness from surface rotation. An excess oblateness term can be attributed to magnetic elements possibly located in the enhanced network. New measurements of latitude-dependent brightness variations at the limb lead to a quadrupolar term (a pole-to-equator temperature variation) of 0.04+-0.02 K. We present the analysis of these unique data, an overview of some results and we report on our progress as we apply our developed analysis method to the whole 10 years of data. Title: X-Ray Searches for Solar Axions Authors: Hudson, H. S.; Acton, L. W.; DeLuca, E. E.; Hannah, I. G.; Reardon, K.; Van Bibber, K. Bibcode: 2012ASPC..455...25H Altcode: 2012arXiv1201.4607H Axions generated thermally in the solar core can convert nearly directly to X-rays as they pass through the solar atmosphere via interaction with the magnetic field. The result of this conversion process would be a diffuse centrally-concentrated source of few-keV X-rays at disk center; it would have a known dimension, of order 10% of the solar diameter, and a spectral distribution resembling the blackbody spectrum of the solar core. Its spatial structure in detail would depend on the distribution of mass and field in the solar atmosphere. The brightness of the source depends upon these factors as well as the unknown coupling constant and the unknown mass of the axion; this particle is hypothetical and no firm evidence for its existence has been found yet. We describe the solar magnetic environment as an axion/photon converter and discuss the upper limits obtained by existing and dedicated observations from three solar X-ray observatories: Yohkoh, RHESSI, and Hinode. Title: Direct Measurement Of The Height Of A White-light Flare Authors: Hudson, Hugh S.; Martinez-Oliveros, J.; Krucker, S.; Hurford, G.; Thompson, W.; Schou, J.; Couvidat, S.; Lindsey, C. Bibcode: 2012AAS...22020441H Altcode: We have used RHESSI and HMI observations to observe hard X-ray and white-light continuum sources of the limb flare SOL2011-02-24, and find the source centroids to coincide within errors of about 0.2 arc s, with the conclusion that the emissions form at the same height in the atmosphere. This greatly strengthens the known association between non-thermal electrons and white-light continuum formation. We also use STEREO observations to find the heliographic coordinates of the flare. This determines the projected height of the photosphere directly below the flare emissions. With this information, the RHESSI metrology determines the absolute height of the sources to be remarkably low in the solar atmosphere: the two footpoints have comparable heights, which we estimate at about 290 +- 138 km above the photosphere. This location lies significantly below the visible-light limb height, estimated at 500 km by Brown & Christensen-Dalsgaard (1998), and the height of optical depth unity to Thomson scattering, estimated at a higher altitude. The results are not consistent with any current models of these processes. Title: Generation of electric currents via neutral-ion drag in the chromosphere and ionosphere Authors: Krasnoselskikh, V.; Abbett, W. P.; Hudson, H.; Vekstein, G.; Bale, S. D. Bibcode: 2012AIPC.1439...42K Altcode: We consider the generation of electric currents in the solar chromosphere. The ionization level in this region is generally supposed to be low. We show that the ambient electrons are magnetized even for weak magnetic fields (30 G), i.e. their gyrofrequency is larger than the collision frequency; ion motions continue to be dominated by ion-neutral collisions in this region. Under such conditions the ions are dragged by neutrals. As a result, the dynamics of magnetic field resembles frozen-in motion of the field with the neutral gas. On the other hand magnetized electrons drift under the action of the electric and magnetic fields induced in the reference frame of ions moving with the neutral gas. This relative motion of electrons and ions results in the generation of quite intense electric currents. The dissipation of these currents leads to the resistive electron heating and efficient gas ionization. Ionization by electron-neutral impact does not alter the dynamics of the heavy particles; thus the gas turbulent motions persist even when the plasma becomes fully ionized and the resistive current dissipation continues to heat electrons and ions. This heating process is so efficient that it can result in typical temperature increases with altitude as large as 0.1-0.3 eV/km. We conclude that this process can play a major role in the heating of the chromosphere and corona. We show that the physical conditions in the solar chromosphere, in particular the neutral and ion density dependencies upon altitude, are very similar to those in the lower ionosphere of the Earth. A very similar process of current generation occurs in the ionosphere after strong earthquakes, resulting in the generation of strong perturbations in the ionosphere. We then present well-known results of the observations of such perturbations, which allow an evaluation of the increment of the growth of the perturbations with altitude, making use of ionospheric sounding. These results are in perfect agreement with estimates obtained making use a model similar to ours. We consider that these observations clearly show the efficiency of the physical mechanisms discussed, and thus provide strong support for our ideas. Title: Commission 10: Solar Activity Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk, James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul; Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi; Vršnak, Bojan; Yan, Yihua Bibcode: 2012IAUTA..28...69V Altcode: Commission 10 of the International Astronomical Union has more than 650 members who study a wide range of activity phenomena produced by our nearest star, the Sun. Solar activity is intrinsically related to solar magnetic fields and encompasses events from the smallest energy releases (nano- or even picoflares) to the largest eruptions in the Solar System, coronal mass ejections (CMEs), which propagate into the Heliosphere reaching the Earth and beyond. Solar activity is manifested in the appearance of sunspot groups or active regions, which are the principal sources of activity phenomena from the emergence of their magnetic flux through their dispersion and decay. The period 2008-2009 saw an unanticipated extended solar cycle minimum and unprecedentedly weak polar-cap and heliospheric field. Associated with that was the 2009 historical maximum in galactic cosmic rays flux since measurements begun in the middle of the 20th Century. Since then Cycle 24 has re-started solar activity producing some spectacular eruptions observed with a fleet of spacecraft and ground-based facilities. In the last triennium major advances in our knowledge and understanding of solar activity were due to continuing success of space missions as SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched by the breathtaking images of the solar atmosphere produced by the Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the framework of NASA's Living with a Star program. In August 2012, at the time of the IAU General Assembly in Beijing when the mandate of this Commission ends, we will be in the unique position to have for the first time a full 3-D view of the Sun and solar activity phenomena provided by the twin STEREO missions about 120 degrees behind and ahead of Earth and other spacecraft around the Earth and ground-based observatories. These new observational insights are continuously posing new questions, inspiring and advancing theoretical analysis and modelling, improving our understanding of the physics underlying magnetic activity phenomena. Commission 10 reports on a vigorously evolving field of research produced by a large community. The number of refereed publications containing `Sun', `heliosphere', or a synonym in their abstracts continued the steady growth seen over the preceding decades, reaching about 2000 in the years 2008-2010, with a total of close to 4000 unique authors. This report, however, has its limitations and it is inherently incomplete, as it was prepared jointly by the members of the Organising Committee of Commission 10 (see the names of the primary contributors to the sections indicated in parentheses) reflecting their fields of expertise and interest. Nevertheless, we believe that it is a representative sample of significant new results obtained during the last triennium in the field of solar activity. Title: Observations of Enhanced Extreme Ultraviolet Continua during an X-Class Solar Flare Using SDO/EVE Authors: Milligan, Ryan O.; Chamberlin, Phillip C.; Hudson, Hugh S.; Woods, Thomas N.; Mathioudakis, Mihalis; Fletcher, Lyndsay; Kowalski, Adam F.; Keenan, Francis P. Bibcode: 2012ApJ...748L..14M Altcode: 2012arXiv1202.1731M Observations of extreme ultraviolet (EUV) emission from an X-class solar flare that occurred on 2011 February 15 at 01:44 UT are presented, obtained using the EUV Variability Experiment (EVE) on board the Solar Dynamics Observatory. The complete EVE spectral range covers the free-bound continua of H I (Lyman continuum), He I, and He II, with recombination edges at 91.2, 50.4, and 22.8 nm, respectively. By fitting the wavelength ranges blueward of each recombination edge with an exponential function, light curves of each of the integrated continua were generated over the course of the flare, as was emission from the free-free continuum (6.5-37 nm). The He II 30.4 nm and Lyα 121.6 nm lines, and soft X-ray (SXR; 0.1-0.8 nm) emission from GOES are also included for comparison. Each free-bound continuum was found to have a rapid rise phase at the flare onset similar to that seen in the 25-50 keV light curves from RHESSI, suggesting that they were formed by recombination with free electrons in the chromosphere. However, the free-free emission exhibited a slower rise phase seen also in the SXR emission from GOES, implying a predominantly coronal origin. By integrating over the entire flare the total energy emitted via each process was determined. We find that the flare energy in the EVE spectral range amounts to at most a few percent of the total flare energy, but EVE gives us a first comprehensive look at these diagnostically important continuum components. Title: Global Forces in Eruptive Solar Flares: The Lorentz Force Acting on the Solar Atmosphere and the Solar Interior Authors: Fisher, G. H.; Bercik, D. J.; Welsch, B. T.; Hudson, H. S. Bibcode: 2012SoPh..277...59F Altcode: 2010arXiv1006.5247F; 2011SoPh..tmp..419F; 2011SoPh..tmp..415F We compute the change in the Lorentz force integrated over the outer solar atmosphere implied by observed changes in vector magnetograms that occur during large, eruptive solar flares. This force perturbation should be balanced by an equal and opposite force perturbation acting on the solar photosphere and solar interior. The resulting expression for the estimated force change in the solar interior generalizes the earlier expression presented by Hudson, Fisher, and Welsch (Astron. Soc. Pac. CS-383, 221, 2008), providing horizontal as well as vertical force components, and provides a more accurate result for the vertical component of the perturbed force. We show that magnetic eruptions should result in the magnetic field at the photosphere becoming more horizontal, and hence should result in a downward (toward the solar interior) force change acting on the photosphere and solar interior, as recently argued from an analysis of magnetogram data by Wang and Liu (Astrophys. J. Lett. 716, L195, 2010). We suggest the existence of an observational relationship between the force change computed from changes in the vector magnetograms, the outward momentum carried by the ejecta from the flare, and the properties of the helioseismic disturbance driven by the downward force change. We use the impulse driven by the Lorentz-force change in the outer solar atmosphere to derive an upper limit to the mass of erupting plasma that can escape from the Sun. Finally, we compare the expected Lorentz-force change at the photosphere with simple estimates from flare-driven gasdynamic disturbances and from an estimate of the perturbed pressure from radiative backwarming of the photosphere in flaring conditions. Title: Momentum Distribution in Solar Flare Processes Authors: Hudson, H. S.; Fletcher, L.; Fisher, G. H.; Abbett, W. P.; Russell, A. Bibcode: 2012SoPh..277...77H Altcode: We discuss the consequences of momentum conservation in processes related to solar flares and coronal mass ejections (CMEs), in particular describing the relative importance of vertical impulses that could contribute to the excitation of seismic waves ("sunquakes"). The initial impulse associated with the primary flare energy transport in the impulsive phase contains sufficient momentum, as do the impulses associated with the acceleration of the evaporation flow (the chromospheric shock) or the CME itself. We note that the deceleration of the evaporative flow, as coronal closed fields arrest it, will tend to produce an opposite impulse, reducing the energy coupling into the interior. The actual mechanism of the coupling remains unclear at present. Title: What is there before a flare? Authors: Hudson, Hugh S. Bibcode: 2012decs.confE.119H Altcode: The physical parameters in a region about to flare (or to make a micro-event of any sort) should be of interest to many people, and should be a suitable topic for (serendipitous) IRIS observations. Flaring is associated generally with magnetic fields, but apparently only infrequently with pre-existing coronal structures at high temperatures. This poster reviews what is known and aims at eliciting discussion of what could be observed with IRIS. Title: Thermal Properties of a Solar Coronal Cavity Observed with the X-Ray Telescope on Hinode Authors: Reeves, Katharine K.; Gibson, Sarah E.; Kucera, Therese A.; Hudson, Hugh S.; Kano, Ryouhei Bibcode: 2012ApJ...746..146R Altcode: Coronal cavities are voids in coronal emission often observed above high latitude filament channels. Sometimes, these cavities have areas of bright X-ray emission in their centers. In this study, we use data from the X-ray Telescope (XRT) on the Hinode satellite to examine the thermal emission properties of a cavity observed during 2008 July that contains bright X-ray emission in its center. Using ratios of XRT filters, we find evidence for elevated temperatures in the cavity center. The area of elevated temperature evolves from a ring-shaped structure at the beginning of the observation, to an elongated structure two days later, finally appearing as a compact round source four days after the initial observation. We use a morphological model to fit the cavity emission, and find that a uniform structure running through the cavity does not fit the observations well. Instead, the observations are reproduced by modeling several short cylindrical cavity "cores" with different parameters on different days. These changing core parameters may be due to some observed activity heating different parts of the cavity core at different times. We find that core temperatures of 1.75 MK, 1.7 MK, and 2.0 MK (for July 19, July 21, and July 23, respectively) in the model lead to structures that are consistent with the data, and that line-of-sight effects serve to lower the effective temperature derived from the filter ratio. Title: Destruction of Sun-Grazing Comet C/2011 N3 (SOHO) Within the Low Solar Corona Authors: Schrijver, C. J.; Brown, J. C.; Battams, K.; Saint-Hilaire, P.; Liu, W.; Hudson, H.; Pesnell, W. D. Bibcode: 2012Sci...335..324S Altcode: Observations of comets in Sun-grazing orbits that survive solar insolation long enough to penetrate into the Sun's inner corona provide information on the solar atmosphere and magnetic field as well as on the makeup of the comet. On 6 July 2011, the Solar Dynamics Observatory (SDO) observed the demise of comet C/2011 N3 (SOHO) within the low solar corona in five wavelength bands in the extreme ultraviolet (EUV). The comet penetrated to within 0.146 solar radius (~100,000 kilometers) of the solar surface before its EUV signal disappeared. Before that, material released into the coma - at first seen in absorption - formed a variable EUV-bright tail. During the final 10 minutes of observation by SDO's Atmospheric Imaging Assembly, ~6 × 10^8 to 6 × 10^10 grams of total mass was lost (corresponding to an effective nucleus diameter of ~10 to 50 meters), as estimated from the tail's deceleration due to interaction with the surrounding coronal material; the EUV absorption by the comet and the brightness of the tail suggest that the mass was at the high end of this range. These observations provide evidence that the nucleus had broken up into a family of fragments, resulting in accelerated sublimation in the Sun's intense radiation field. Title: Suzaku/WAM and RHESSI observation of non-thermal electrons in solar microflares Authors: Ishikawa, S.; Krucker, S.; Ohno, M.; Hudson, H. S.; Christe, S.; Lin, R. P. Bibcode: 2011AGUFMSH41A1908I Altcode: We report on hard X-ray spectroscopy of solar microflares observed by the Wide-band All-sky Monitor (WAM), onboard the Suzaku satellite, and by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). WAM transient data provide wide energy band (50 keV - 5 MeV) spectrum over a large field of view (~2π str) with a time resolution of 1 s. While WAM was mainly designed for gamma-ray bursts and other bursts of hard X-ray and gamma-ray from transient non-solar objects, it is also attractive as a hard X-ray solar flare monitor thanks to its large effective area (~800 cm2 at 100 keV, ~13 times larger than that of RHESSI). Hard X-ray (>50 keV) emissions from 17 GOES B-class flares were detected by WAM by Febrary 2010, and 7 of them were also observed by RHESSI. The GOES classes of these events range from B1.3 to B9.5, and the RHESSI non-thermal spectra are well-fit by power-laws with photon spectral indices between 3 and 5. The durations of both the WAM and RHESSI non-thermal emissions are ~1 minute, and the detected WAM fluxes are more than ~20 times smaller than RHESSI backgrounds at energies above ~100 keV. The WAM spectra show the high-energy extension of the non-thermal power-law distribution seen by RHESSI, showing that microflares, similar to regular flares, accelerate electrons to energies above 50 keV. We discuss high-energy (>50 keV) particle acceleration in solar microflares and its relation to large flares. Title: The EUV Emission in Comet-Solar Corona Interactions Authors: Bryans, P.; Pesnell, W. D.; Schrijver, C. J.; Brown, J. C.; Battams, K.; Saint-Hilaire, P.; Liu, W.; Hudson, H. S. Bibcode: 2011AGUFMSH34B..05B Altcode: The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) viewed a comet as it passed through the solar corona on 2011 July 5. This was the first sighting of a comet by a EUV telescope. For 20 minutes, enhanced emission in several of the AIA wavelength bands marked the path of the comet. We explain this EUV emission by considering the evolution of the cometary atmosphere as it interacts with the ambient solar atmosphere. Water ice in the comet rapidly sublimates as it approaches the Sun. This water vapor is then photodissociated, primarily by Ly-α, by the solar radiation field to create atomic H and O. Other molecules present in the comet also evaporate and dissociate to give atomic Fe and other metals. Subsequent ionization of these atoms can be achieved by a number of means, including photoionization, electron impact, and charge exchange with coronal protons and other highly-charged species. Finally, particles from the cometary atmosphere are thermalized to the background temperature of the corona. Each step could cause emission in the AIA bandpasses. We will report here on their relative contribution to the emission seen in the AIA telescopes. Title: Non-Observation of the He II 304 A Charge-Exchange Continuum in Major Solar Flares Authors: Hudson, H. S.; Fletcher, L.; MacKinnon, A.; Woods, T. N. Bibcode: 2011AGUFMSH41A1909H Altcode: We report on a search for flare emission via charge-exchange continuum radiation in the wings of the Lyman-alpha line of He ii at 304 A, as originally suggested for hydrogen by Orrall and Zirker (1976). Via this mechanism a primary alpha particle that penetrates into the neutral chromosphere can pick up an atomic electron and radiate recombination continuum before it stops. The Extreme-ultraviolet Variability Experiment (EVE) on board the Solar Dynamics Observatory (SDO) gives us our first chance to search for this effect systematically. The Orrall-Zirker charge-exchange mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV/nucleon. We study four EVE events: the gamma-ray events SOL2010-06-12 (M2.0) and SOL20 11-02-24 (M3.5), the latter a limb flare, and the X-class flares SOL2010-02-15 (X2.2) and SOL2011-03-09 (X1.2). No clear signature of the charge-exchange continuum appears, but SOL2010-02-15 (X2.2) does reveal a gradual broad-band signature that we tentatively interpret as due to unresolved emission lines or instrumental scattering. Title: Splinter Session "Solar and Stellar Flares" Authors: Fletcher, L.; Hudson, H.; Cauzzi, G.; Getman, K. V.; Giampapa, M.; Hawley, S. L.; Heinzel, P.; Johnstone, C.; Kowalski, A. F.; Osten, R. A.; Pye, J. Bibcode: 2011ASPC..448..441F Altcode: 2011csss...16..441F; 2012arXiv1206.3997F This summary reports on papers presented at the Cool Stars-16 meeting in the splinter session "Solar and Stellar flares." Although many topics were discussed, the main themes were the commonality of interests, and of physics, between the solar and stellar flare communities, and the opportunities for important new observations in the near future. Title: Using the EUV to Weigh a Sun-grazing Comet as it Disappears in the Solar Corona Authors: Pesnell, W. D.; Schrijver, C. J.; Brown, J. C.; Battams, K.; Saint-Hilaire, P.; Hudson, H. S.; Lui, W. Bibcode: 2011AGUFMSH33A2040P Altcode: On July 6, 2011, the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) observed a comet in most of its EUV passbands. The comet disappeared while moving through the solar corona. The comet penetrated to 0.146 solar radii ( ∼~100,000 km) above the photosphere before its EUV faded. Before then, the comet's coma and a tail were observed in absorption and emission, respectively. The material in the variable tail quickly fell behind the nucleus. An estimate of the comet's mass based on this effect, one derived from insolation, and one using the tail's EUV brightness, all yield ∼ 50 giga-grams some 10 minutes prior to the end of its visibility. These unique first observations herald a new era in the study of Sun-grazing comets close to their perihelia and of the conditions in the solar corona and solar wind. We will discuss the observations and interpretation of the comet by SDO as well as the coronagraph observations from SOHO and STEREO. A search of the SOHO comet archive for other comets that could be observed in the SDO/AIA EUV channels will be described. Title: Results and Analysis of the RHESSI/SAS Observations of the Optical Solar Limb Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P. Bibcode: 2011AGUFMSH34B..03F Altcode: The Solar Aspect System (SAS) of the RHESSI satellite measures the optical solar limb in the red continuum with a cadence typically set at 16 samples/s in each of three linear CCD sensors. RHESSI has observed the Sun continuously now for more than 9 years, and we have acquired a unique data set ranging almost over a full solar cycle and consisting of about 25x10^9single data points. These measurements have led to the most accurate oblateness measurement to date, 8.01±0.14 milliarcsec(Fivian et al., 2008), a value consistent with models predicting an oblateness from surface rotation. An excess oblateness term can be attributed to the enhanced network. New measurements of latitude-dependent brightness variations at the limb lead to a quadrupolarterm (a pole-to-equator temperature variation) of 0.04±0.02 K. We present the analysis of these unique data and an overview of some results. Title: The EVE Doppler Sensitivity and Flare Observations Authors: Hudson, H. S.; Woods, T. N.; Chamberlin, P. C.; Fletcher, L.; Del Zanna, G.; Didkovsky, L.; Labrosse, N.; Graham, D. Bibcode: 2011SoPh..273...69H Altcode: 2011SoPh..tmp..362H The Extreme-ultraviolet Variability Experiment (EVE; see Woods et al., 2009) obtains continuous EUV spectra of the Sun viewed as a star. Its primary objective is the characterization of solar spectral irradiance, but its sensitivity and stability make it extremely interesting for observations of variability on time scales down to the limit imposed by its basic 10 s sample interval. In this paper we characterize the Doppler sensitivity of the EVE data. We find that the 30.4 nm line of He II has a random Doppler error below 0.001 nm (1 pm, better than 10 km s−1 as a redshift), with ample stability to detect the orbital motion of its satellite, the Solar Dynamics Observatory (SDO). Solar flares also displace the spectrum, both because of Doppler shifts and because of EVE's optical layout, which (as with a slitless spectrograph) confuses position and wavelength. As a flare develops, the centroid of the line displays variations that reflect Doppler shifts and therefore flare dynamics. For the impulsive phase of the flare SOL2010-06-12, we find the line centroid to have a redshift of 16.8 ± 5.9 km s−1 relative to that of the flare gradual phase (statistical errors only). We find also that high-temperature lines, such as Fe XXIV 19.2 nm, have well-determined Doppler components for major flares, with decreasing apparent blueshifts as expected from chromospheric evaporation flows. Title: Momentum Balance in Eruptive Solar Flares: The Lorentz Force Acting on the Solar Atmosphere and the Solar Interior Authors: Fisher, George H.; Bercik, David J.; Welsch, Brian T.; Hudson, Hugh S. Bibcode: 2011sdmi.confE...9F Altcode: We compute the change in the Lorentz force integrated over the outer solar atmosphere implied by observed changes in vector magnetograms that occur during large, eruptive solar flares. This force perturbation should be balanced by an equal and opposite force perturbation acting on the solar photosphere and solar interior. The resulting expression for the estimated force change in the solar interior generalizes the earlier expression presented by Hudson, Fisher & Welsch (2008), providing horizontal as well as vertical force components, and provides a more accurate result for the vertical component of the perturbed force. We show that magnetic eruptions should result in the magnetic field at the photosphere becoming more horizontal, and hence should result in a downward (towards the solar interior) force change acting on the photosphere and solar interior, as recently argued from an analysis of magnetogram data by Wang & Liu. We suggest that there should be an observational relationship between the force change computed from changes in the vector magnetograms, the outward momentum carried by the ejecta from the flare, and the amplitude of the helioseismic disturbance driven by the downward force change. We use the impulse driven by the Lorentz force change in the outer solar atmosphere to derive an upper limit to the mass of erupting plasma that can escape from the Sun. Finally, we compare the expected Lorentz force change at the photosphere with simple estimates from flare-driven gasdynamic disturbances and from an estimate of the perturbed pressure from radiative backwarming of the photosphere in flaring conditions. Title: High-resolution Imaging of Solar Flare Ribbons and Its Implication on the Thick-target Beam Model Authors: Krucker, Säm; Hudson, H. S.; Jeffrey, N. L. S.; Battaglia, M.; Kontar, E. P.; Benz, A. O.; Csillaghy, A.; Lin, R. P. Bibcode: 2011ApJ...739...96K Altcode: We report on high-resolution optical and hard X-ray observations of solar flare ribbons seen during the GOES X6.5 class white-light flare of 2006 December 6. The data consist of imaging observations at 430 nm (the Fraunhofer G band) taken by the Hinode Solar Optical Telescope with the hard X-rays observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager. The two sets of data show closely similar ribbon structures, strongly suggesting that the flare emissions in white light and in hard X-rays have physically linked emission mechanisms. While the source structure along the ribbons is resolved at both wavelengths (length ~ 30''), only the G-band observations resolve the width of the ribbon, with values between ~0farcs5 and ~1farcs8. The unresolved hard X-ray observations reveal an even narrower ribbon in hard X-rays (the main footpoint has a width perpendicular to the ribbon of <1farcs1 compared to the G-band width of ~1farcs8) suggesting that the hard X-ray emission comes from the sharp leading edge of the G-band ribbon. Applying the thick-target beam model, the derived energy deposition rate is >5 × 1012 erg s-1 cm-2 provided by an electron flux of 1 × 1020 electrons s-1 cm-2 above 18 keV. This requires that the beam density of electrons above 18 keV be at least 1 × 1010 cm-3. Even if field lines converge toward the chromospheric footpoints, the required beam in the corona has too high a density to be described as a dilute tail population on top of a Maxwellian core. We discuss this issue and others associated with this extreme event, which poses serious questions to the standard thick target beam interpretation of solar flares. Title: Flare Seismology from SDO Observations Authors: Lindsey, Charles; Martinez Oliveros, Juan Carlos; Hudson, Hugh Bibcode: 2011sdmi.confE..10L Altcode: Some flares release intense seismic transients into the solar interior. These transients are the sole instance we know of in which the Sun's corona exerts a conspicuous influence on the solar interior through flares. The desire to understand this phenomenon has led to ambitious efforts to model the mechanisms by which energy stored in coronal magnetic fields drives acoustic waves that penetrate deep into the Sun's interior. These mechanisms potentially involve the hydrodynamic response of the chromosphere to thick-target heating by high-energy particles, radiative exchange in the chromosphere and photosphere, and Lorentz-force transients to account for acoustic energies estimated up to at 5X10^27 erg and momenta of order 6X10^19 dyne sec. An understanding of these components of flare mechanics promises more than a powerful diagnostic for local helioseismology. It could give us fundamental new insight into flare mechanics themselves. The key is appropriate observations to match the models. Helioseismic observations have identified the compact sources of transient seismic emission at the foot points of flares. The Solar Dynamics Observatory is now giving us high quality continuum-brightness and Doppler observations of acoustically active flares from HMI concurrent with high-resolution EUV observations from AIA. Supported by HXR observations from RHESSI and a broad variety of other observational resources, the SDO promises a leading role in flare research in solar cycle 24. Title: An Observational Overview of Solar Flares Authors: Fletcher, L.; Dennis, B. R.; Hudson, H. S.; Krucker, S.; Phillips, K.; Veronig, A.; Battaglia, M.; Bone, L.; Caspi, A.; Chen, Q.; Gallagher, P.; Grigis, P. T.; Ji, H.; Liu, W.; Milligan, R. O.; Temmer, M. Bibcode: 2011SSRv..159...19F Altcode: 2011SSRv..tmp..261F; 2011arXiv1109.5932F We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era. Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections. We also discuss flare soft X-ray spectroscopy and the energetics of the process. The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory. The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations. Title: Microflares and the Statistics of X-ray Flares Authors: Hannah, I. G.; Hudson, H. S.; Battaglia, M.; Christe, S.; Kašparová, J.; Krucker, S.; Kundu, M. R.; Veronig, A. Bibcode: 2011SSRv..159..263H Altcode: 2011SSRv..tmp..262H; 2011SSRv..tmp...87H; 2011arXiv1108.6203H; 2011SSRv..tmp..243H; 2011SSRv..tmp..163H This review surveys the statistics of solar X-ray flares, emphasising the new views that RHESSI has given us of the weaker events (the microflares). The new data reveal that these microflares strongly resemble more energetic events in most respects; they occur solely within active regions and exhibit high-temperature/nonthermal emissions in approximately the same proportion as major events. We discuss the distributions of flare parameters (e.g., peak flux) and how these parameters correlate, for instance via the Neupert effect. We also highlight the systematic biases involved in intercomparing data representing many decades of event magnitude. The intermittency of the flare/microflare occurrence, both in space and in time, argues that these discrete events do not explain general coronal heating, either in active regions or in the quiet Sun. Title: Overview of the Volume Authors: Dennis, B. R.; Emslie, A. G.; Hudson, H. S. Bibcode: 2011SSRv..159....3D Altcode: 2011SSRv..tmp..231D; 2011arXiv1109.5831D; 2011SSRv..tmp..277D; 2011SSRv..tmp..248D; 2011SSRv..tmp..155D In this introductory chapter, we provide a brief summary of the successes and remaining challenges in understanding the solar flare phenomenon and its attendant implications for particle acceleration mechanisms in astrophysical plasmas. We also provide a brief overview of the contents of the other chapters in this volume, with particular reference to the well-observed flare of 2002 July 23. Title: Preface Authors: Emslie, A. Gordon; Dennis, Brian R.; Hudson, Hugh; Lin, Robert P. Bibcode: 2011SSRv..159....1E Altcode: 2011SSRv..tmp..161E; 2011SSRv..tmp..288E; 2011SSRv..tmp..257E; 2011SSRv..tmp..239E No abstract at ADS Title: Solar Hard X-ray Observations with NuSTAR Authors: Smith, David M.; Krucker, S.; Hurford, G.; Hudson, H.; White, S.; Mewaldt, R.; Grefenstette, B.; Harrison, F.; NuSTAR Science Team Bibcode: 2011HEAD...12.4309S Altcode: High-sensitivity imaging of solar hard X-rays allows detection of freshly accelerated nonthermal electrons at the acceleration site. A few such observations have been made with Yohkoh and RHESSI, but a leap in sensitivity could help pin down the time, place, and manner of reconnection. The Nuclear Spectroscopic Telescope Array (NuSTAR) Small Explorer will be capable of solar pointing, and three weeks will be dedicated to solar observing during the baseline two-year mission. NuSTAR will be 200 times more sensitive than RHESSI in the hard X-ray band. This will allow the following new solar observations, among others:

1) Extrapolation of the micro/nanoflare distribution by two orders of magnitude down in flux

2) Search for hard X-rays from network nanoflares (soft X-ray bright points) and evaluation of their role in coronal heating

3) Discovery of hard X-ray bremsstrahlung from the electron beams driving type III radio bursts, and measurement of their electron spectrum

4) Hard X-ray studies of polar soft X-ray jets and impulsive solar energetic particle events at the edge of coronal holes, and comparison of these events with observations of 3He and other particles in interplanetary space

5) Study of coronal bremsstrahlung from particles accelerated by coronal mass ejections as they are first launched

6) Study of particles at the coronal reconnection site when flare footpoints are occulted; and

7) Search for hypothetical axion particles created in the solar core via the hard X-ray signal from their conversion to X-rays in the coronal magnetic field. Title: The U.S. Eclipse Megamovie in 2017: a white paper on a unique outreach event Authors: Hudson, Hugh S.; McIntosh, Scott W.; Habbal, Shadia R.; Pasachoff, Jay M.; Peticolas, Laura Bibcode: 2011arXiv1108.3486H Altcode: Totality during the solar eclipse of 2017 traverses the entire breadth of the continental United States, from Oregon to South Carolina. It thus provides the opportunity to assemble a very large number of images, obtained by amateur observers all along the path, into a continuous record of coronal evolution in time; totality lasts for an hour and a half over the continental U.S. While we describe this event here as an opportunity for public education and outreach, such a movie -with very high time resolution and extending to the chromosphere - will also contain unprecedented information about the physics of the solar corona. Title: Estimates of Densities and Filling Factors from a Cooling Time Analysis of Solar Microflares Observed with RHESSI Authors: Baylor, R. N.; Cassak, P. A.; Christe, S.; Hannah, I. G.; Krucker, Säm; Mullan, D. J.; Shay, M. A.; Hudson, H. S.; Lin, R. P. Bibcode: 2011ApJ...736...75B Altcode: 2011arXiv1107.3997B We use more than 4500 microflares from the RHESSI microflare data set to estimate electron densities and volumetric filling factors of microflare loops using a cooling time analysis. We show that if the filling factor is assumed to be unity, the calculated conductive cooling times are much shorter than the observed flare decay times, which in turn are much shorter than the calculated radiative cooling times. This is likely unphysical, but the contradiction can be resolved by assuming that the radiative and conductive cooling times are comparable, which is valid when the flare loop temperature is a maximum and when external heating can be ignored. We find that resultant radiative and conductive cooling times are comparable to observed decay times, which has been used as an assumption in some previous studies. The inferred electron densities have a mean value of 1011.6 cm-3 and filling factors have a mean of 10-3.7. The filling factors are lower and densities are higher than previous estimates for large flares, but are similar to those found for two microflares by Moore et al. Title: Temperature and Density Estimates of Extreme-ultraviolet Flare Ribbons Derived from TRACE Diffraction Patterns Authors: Krucker, Säm; Raftery, Claire L.; Hudson, Hugh S. Bibcode: 2011ApJ...734...34K Altcode: We report on Transition Region And Coronal Explorer 171 Å observations of the GOES X20 class flare on 2001 April 2 that shows EUV flare ribbons with intense diffraction patterns. Between the 11th to 14th order, the diffraction patterns of the compact flare ribbon are dispersed into two sources. The two sources are identified as emission from the Fe IX line at 171.1 Å and the combined emission from Fe X lines at 174.5, 175.3, and 177.2 Å. The prominent emission of the Fe IX line indicates that the EUV-emitting ribbon has a strong temperature component near the lower end of the 171 Å temperature response (~0.6-1.5 MK). Fitting the observation with an isothermal model, the derived temperature is around 0.65 MK. However, the low sensitivity of the 171 Å filter to high-temperature plasma does not provide estimates of the emission measure for temperatures above ~1.5 MK. Using the derived temperature of 0.65 MK, the observed 171 Å flux gives a density of the EUV ribbon of 3 × 1011 cm-3. This density is much lower than the density of the hard X-ray producing region (~1013 to 1014 cm-3) suggesting that the EUV sources, though closely related spatially, lie at higher altitudes. Title: Transient Artifacts in SDO/HMI Flare Observations Authors: Martinez Oliveros, Juan Carlos; Lindsey, C.; Hudson, H.; Schou, J.; Couvidat, S. Bibcode: 2011SPD....42.2123M Altcode: 2011BAAS..43S.2123M The Helioseismic and Magnetic Imager (HMI) on SDO provides a new tool for the systematic observation of white-light flares, including Doppler and magnetic information as well as continuum. In our initial analysis of the highly impulsive gamma-ray flare SOL2010-06-12T00:57 (Martinez-Oliveros et al. 2011), we detected an apparently artifactual blue shift in the two footpoint sources. We have now deployed the PASCAL algorithm for the same flare as viewed in GONG++ data. This algorithm makes it possible to obtain much better photometry (plus Doppler and magnetic measurements) from the ground-based data. Using GONG++ we have demonstrated the artifactual nature of the apparent blueshift, finding instead weak redshifts at the foopoints. We discuss the flare physics associated with these observations and describe the use of PASCAL (with GONG++ or other ground-based data) as a complement to the systematic SDO data. Title: Solar Hard X-ray Observations with NuSTAR Authors: Smith, David M.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White, S. M.; Mewaldt, R. A.; Stern, D.; Grefenstette, B. W.; Harrison, F. A. Bibcode: 2011SPD....42.1501S Altcode: 2011BAAS..43S.1501S High-sensitivity imaging of coronal hard X-rays allows detection of freshly accelerated nonthermal electrons at the acceleration site. A few such observations have been made with Yohkoh and RHESSI, but a leap in sensitivity could help pin down the time, place, and manner of reconnection.

In 2012, the Nuclear Spectroscopic Telescope Array (NuSTAR), a NASA Small Explorer for high energy astrophysics that uses grazing-incidence optics to focus X-rays up to 80 keV, will be launched. NuSTAR is capable of solar pointing, and three weeks will be dedicated to solar observing during the baseline two-year mission. NuSTAR will be 200 times more sensitive than RHESSI in the hard X-ray band. This will allow the following new observations, among others:

1) Extrapolation of the micro/nanoflare distribution by two orders of magnitude down in flux

2) Search for hard X-rays from network nanoflares (soft X-ray bright points) and evaluation of their role in coronal heating

3) Discovery of hard X-ray bremsstrahlung from the electron beams driving type III radio bursts, and measurement of their electron spectrum

4) Hard X-ray studies of polar soft X-ray jets and impulsive solar energetic particle events at the edge of coronal holes, and comparison of these events with observations of 3He and other particles in interplanetary space

5) Study of coronal bremsstrahlung from particles accelerated by coronal mass ejections as they are first launched

6) Study of particles at the coronal reconnection site when flare footpoints are occulted; and

7) Search for hypothetical axion particles created in the solar core via the hard X-ray signal from their conversion to X-rays in the coronal magnetic field.

NuSTAR will also serve as a pathfinder for a future dedicated space mission with enhanced capabilities, such as a satellite version of the FOXSI sounding rocket. Title: Flaring Solar Hale Sector Boundaries Authors: Svalgaard, L.; Hannah, I. G.; Hudson, H. S. Bibcode: 2011ApJ...733...49S Altcode: 2010arXiv1010.2710S The sector structure that organizes the magnetic field of the solar wind into large-scale domains has a clear pattern in the photospheric magnetic field as well. The rotation rate, 27-28.5 days, implies an effectively rigid rotation originating deeper in the solar interior than the sunspots. The photospheric magnetic field is known to be concentrated near that portion (the Hale boundary) in each solar hemisphere, where the change in magnetic sector polarity matches that between the leading and following sunspot polarities in active regions in the respective hemispheres. We report here that flares and microflares also concentrate at the Hale boundaries, implying that flux emergence and the creation of free magnetic energy in the corona also have a direct cause in the deep interior. Title: The Height of White-light Flare Continuum Formation Authors: Martinez Oliveros, J.; Hudson, Hugh; Krucker, S.; Schou, J.; Couvidat, S. Bibcode: 2011SPD....42.2211M Altcode: 2011BAAS..43S.2211M White-light continuum and hard X-ray emission in flares have strong correlations in time and in horizontal position, but at present we do not have a clear idea about their height structures. On 24 February 2011 a white-light flare (SOL2011-02-24T07:35) was observed on the east limb, simultaneously by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), and by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). This observation gives us the opportunity to determine the heights of these emissions directly, limited only by the limb references for the two spacecraft, with almost no projection undertainty. HMI obtained clear images in the pseudo-continuum around 6173A, and RHESSI obtained hard X-ray images. For both data sets, the precision of centroid determination is of order 0.1 arc s. We believe that the position of the white-light limb, as a local reference, can also be understood at a corresponding level of accuracy for the two data sets. We report the results of this analysis and discuss our findings in terms of present models of particle acceleration and energy transport in the impulsive phase. Title: UV Diagnostics of Stellar and Solar Flares Authors: Kowalski, Adam; Hawley, S. L.; Hudson, H. S. Bibcode: 2011AAS...21821303K Altcode: 2011BAAS..43G21303K The UV spectral regime provides a comprehensive view of the plasma dynamics and atmospheric temperature structure during stellar flares. We review the major developments in UV spectroscopy of flares on low mass stars that shape our understanding of the flare process and challenge the predictions of current radiative hydrodynamic models. We put the ultraviolet properties in context with the radiation in the neighboring X-ray and visible wavelength regimes. We also show how SDO/EVE data of several Cycle 24 solar flares allow for new comparisons to be made between solar and stellar flares. Title: A RHESSI And SDO Campaign Measuring Latitude-dependent Limb ProfilesAnd Oblateness Of The Optical Solar Disk Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.; Bush, R. I.; Emilio, M.; Kuhn, J. R.; Scholl, I. F. Bibcode: 2011SPD....42.1706F Altcode: 2011BAAS..43S.1706F The SDO spacecraft conducted a special roll maneuver, 2011 April 6 05:50-12:30 UT, to enable its HMI instrument to obtain precise observations of the global structure of the limb. Similar maneuvers had been carried out with SOHO for MDI in the past. On this occasion we also successfully obtained RHESSI optical observations at very high cadence, 128 samples per sec for each of the three linear CCDs. The data from the two instrument (RHESSI/SAS and SDO/HMI), give different means for the investigation of the variation of the solar limb properties as a function of position angle (latitude). At the normal RHESSI cadence very long integrations (of order 3 months) are needed to obtain precise limb measurements, but in this case we expect to be able to report results within the exact time frame of the SDO roll maneuver. The special RHESSI data rate was about 10,000 times larger than the standard rate and will achieve high precision in a relatively short time. We will compare these results with our earlier RHESSI observations (Fivian et al., 2008) and those obtained by Kuhn et al. (1998) and Emilio et al. (2007) with the earlier MDI roll maneuvers. Title: Doppler Signatures In EVE Spectra Authors: Hudson, Hugh S.; Chamberlin, P.; Woods, T.; Fletcher, L.; Graham, D. Bibcode: 2011SPD....42.2124H Altcode: 2011BAAS..43S.2124H The Extreme-ultraviolet Variability Experiment (EVE) on SDO is providing a comprehensive set of EUV spectra of the Sun as a star. The routine sampling is with 10 s integrations at a resolution of 0.1 nm. Although this resolution corresponds to only some 1000 km/s in velocity space, we demonstrate that the instrument is stable enough to detect the SDO orbital motion of a few km/s readily in the bright He II line at 30.4 nm. We find the random error in the centroid location of this line to be less than one pm (less than 1 km/s) per 10 s integration. We also note systematic effects from a variety of causes. For flare observations, the line centroid position depends on the flare position. We discuss the calibration of this effect and show that EVE can nonetheless provide clear Doppler signatures that may be interpreted in terms of flare dynamics. This information has some value in and of itself, because of EVE's sensitivity, but we feel that it will be of greatest importance when combined with imagery (e.g., via AIA) a modeling. We discuss flare signatures in several events, e.g. the gamma-ray flare SOL2010-06-12 and SOL2011-02-16T:07:44, taking advantage of AIA image comparisons. Title: Results and Analysis of the RHESSI/SAS Observations of the Optical Solar Limb Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P. Bibcode: 2011SPD....42.1724F Altcode: 2011BAAS..43S.1724F The Solar Aspect System (SAS) of the RHESSI satellite measures the optical solar limb in the red continuum with a cadence typically set at 16 samples/s in each of three linear CCD sensors. RHESSI has observed the Sun continuously now for more than 9 years, and we have acquired a unique data set ranging almost over a full solar cycle and consisting of about 25x109 single data points. These measurements have led to the most accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian et al., 2008), a value consistent with models predicting an oblateness from surface rotation. An excess oblateness term can be attributed to the enhanced network. New measurements of latitude-dependent brightness variations at the limb lead to a quadrupolar term (a pole-to-equator temperature variation) of 0.04+-0.02 K. We present the analysis of these unique data and an overview of some results. Title: On the Magnetic Field Variations and HXR Emission of the First X-class Flare in the 24th Solar Cycle Authors: Martinez Oliveros, Juan Carlos; Alvarado Gomez, J.; Buitrago Casas, J.; Lindsey, C.; Hudson, H.; Calvo-Mozo, B. Bibcode: 2011SPD....42.2225M Altcode: 2011BAAS..43S.2225M Multi-wavelength studies of energetic solar flares with seismic emissions have revealed interesting common features between them. We studied the first seismically active flare of the 24th solar cycle

(SOL2011-02-15T01:52 X2.2) detected by HMI/SDO (Kosovichev, 2011) using a pixel-by-pixel light-curve characterization of the fluctuations of the photospheric longitudinal magnetic field based on HMI data. For context we used HXR RHESSI data to find a correlation between these sources and the spatial location of the transient longitudinal magnetic field changes in the photospheric region where this flare took place. Title: Magnetic Oscillations Mark Sites of Magnetic Transients in an Acoustically Active Flare Authors: Lindsey, Charles A.; Donea, A.; Hudson, H. S.; Martinez Oliveros, J.; Hanson, C. Bibcode: 2011SPD....42.2207L Altcode: 2011BAAS..43S.2207L The flare of 2011 February 15, in NOAA AR11158, was the first acoustically active flare of solar cycle 24, and the first observed by the Solar Dynamics Observatory (SDO). It was exceptional in a number of respects (Kosovichev 2011a,b). Sharp ribbon-like transient Doppler, and magnetic signatures swept over parts of the active region during the impulsive phase of the flare. We apply seismic holography to a 2-hr time series of HMI observations encompassing the flare. The acoustic source distribution appears to have been strongly concentrated in a single highly compact penumbral region in which the continuum-intensity signature was unusually weak. The line-of-sight magnetic transient was strong in parts of the active region, but relatively weak in the seismic-source region. On the other hand, the neighbourhoods of the regions visited by the strongest magnetic transients maintained conspicuous 5-minutes-period variations in the line of sight magnetic signature for the full 2-hr duration of the time series, before the flare as well as after. We apply standard helioseismic control diagnostics for clues as to the physics underlying 5-minute magnetic oscillations in regions conducive to magnetic transients during a flare and consider the prospective development of this property as an indicator of flare potentiality on some time scale. We make use of high-resolution data from AIA, using diffracted images where necessary to obtain good photometry where the image is otherwise saturated. This is relevant to seismic emission driven by thick-target heating in the absence of back-warming. We also use RHESSI imaging spectroscopy to compare the source distributions of HXR and seismic emission. Title: Solar and Stellar Flares Authors: Hudson, Hugh Bibcode: 2011APS..APR.L3001H Altcode: Flares in the atmosphere of the Sun and of many other stars appear to result from the sudden conversion of electromagnetic field energy into a wide array of observable forms. Of these products the definitive modern observations are the X-rays and γ-rays, signifying the common occurrence of particle acceleration to mildly relativistic or higher energies. Abundant direct (the radiation) and indirect information confirms that this particle acceleration is energetically significant, as well as common. We are thus led to the physics of particle distribution functions that may deviate radically from Maxwellian distributions. Stellar observations allow us to study these phenomena across a wide variety of environments, whereas solar and planetary observations allow us to do imaging spectroscopy and thereby get a better understanding of the global structures of the processes. In particular we have spectacular new data from satellite solar observatories such as RHESSI (hard X-rays and γ-rays) and others, most recently the Solar Dynamics Observatory. Of particular interest from the point of view of plasma physics is the flare environment: a low-beta corona linked to a massive body through an intermediate weakly-ionized layer (the chromosphere). The chromosphere is extraordinarily complicated; its behavior is coming again to be recognized as fundamental to the overall flare process, and in this presentation I will attempt to clarify its role. Title: Imaging Spectroscopy of a White-Light Solar Flare Authors: Martínez Oliveros, J. C.; Couvidat, S.; Schou, J.; Krucker, S.; Lindsey, C.; Hudson, H. S.; Scherrer, P. Bibcode: 2011SoPh..269..269M Altcode: 2011SoPh..tmp....7M; 2010arXiv1012.0344M We report observations of a white-light solar flare (SOL2010-06-12T00:57, M2.0) observed by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The HMI data give us the first space-based high-resolution imaging spectroscopy of a white-light flare, including continuum, Doppler, and magnetic signatures for the photospheric Fe I line at 6173.34 Å and its neighboring continuum. In the impulsive phase of the flare, a bright white-light kernel appears in each of the two magnetic footpoints. When the flare occurred, the spectral coverage of the HMI filtergrams (six equidistant samples spanning ±172 mÅ around nominal line center) encompassed the line core and the blue continuum sufficiently far from the core to eliminate significant Doppler crosstalk in the latter, which is otherwise a possibility for the extreme conditions in a white-light flare. RHESSI obtained complete hard X-ray and γ-ray spectra (this was the first γ-ray flare of Cycle 24). The Fe I line appears to be shifted to the blue during the flare but does not go into emission; the contrast is nearly constant across the line profile. We did not detect a seismic wave from this event. The HMI data suggest stepwise changes of the line-of-sight magnetic field in the white-light footpoints. Title: Global Properties of Solar Flares Authors: Hudson, Hugh S. Bibcode: 2011SSRv..158....5H Altcode: 2011SSRv..tmp....7H; 2011arXiv1108.3490H This article broadly reviews our knowledge of solar flares. There is a particular focus on their global properties, as opposed to the microphysics such as that needed for magnetic reconnection or particle acceleration as such. Indeed solar flares will always remain in the domain of remote sensing, so we cannot observe the microscales directly and must understand the basic physics entirely via the global properties plus theoretical inference. The global observables include the general energetics—radiation in flares and mass loss in coronal mass ejections (CMEs)—and the formation of different kinds of ejection and global wave disturbance: the type II radio-burst exciter, the Moreton wave, the EIT "wave", and the "sunquake" acoustic waves in the solar interior. Flare radiation and CME kinetic energy can have comparable magnitudes, of order 1032 erg each for an X-class event, with the bulk of the radiant energy in the visible-UV continuum. We argue that the impulsive phase of the flare dominates the energetics of all of these manifestations, and also point out that energy and momentum in this phase largely reside in the electromagnetic field, not in the observable plasma. Title: Three-dimensional morphology of a coronal prominence cavity Authors: Gibson, S. E.; Kucera, T. A.; Rastawicki, D.; Dove, J.; de Toma, G.; Hao, J.; Hill, S. M.; Hudson, H. S.; Marque, C.; McIntosh, P. S.; Rachmeler, L.; Reeves, K. K.; Schmieder, B.; Schmit, D. J.; Sterling, A.; Tripathi, D.; Williams, D. R.; Zhang, M. Bibcode: 2010AGUFMSH51A1667G Altcode: We present a three-dimensional density model of coronal prominence cavities, and a morphological fit that has been tightly constrained by a uniquely well-observed cavity. Observations were obtained as part of an International Heliophysical Year campaign by instruments from a variety of space- and ground-based observatories, spanning wavelengths from radio to soft-X-ray to integrated white light. From these data it is clear that the prominence cavity is the limb manifestation of a longitudinally-extended polar-crown filament channel, and that the cavity is a region of low density relative to the surrounding corona. As a first step towards quantifying density and temperature from campaign spectroscopic data, we establish the three-dimensional morphology of the cavity. This is critical for taking line-of-sight projection effects into account, since cavities are not localized in the plane of the sky and the corona is optically thin. We have augmented a global coronal streamer model to include a tunnel-like cavity with elliptical cross-section and a Gaussian variation of height along the tunnel length. We have developed a semi-automated routine that fits ellipses to cross-sections of the cavity as it rotates past the solar limb, and have applied it to Extreme Ultraviolet Imager (EUVI) observations from the two Solar Terrestrial Relations Observatory (STEREO) spacecraft. This defines the morphological parameters of our model, from which we reproduce forward-modeled cavity observables. We find that cavity morphology and orientation, in combination with the viewpoints of the observing spacecraft, explains the observed variation in cavity visibility for the east vs. west limbs. Title: Three-dimensional Morphology of a Coronal Prominence Cavity Authors: Gibson, S. E.; Kucera, T. A.; Rastawicki, D.; Dove, J.; de Toma, G.; Hao, J.; Hill, S.; Hudson, H. S.; Marqué, C.; McIntosh, P. S.; Rachmeler, L.; Reeves, K. K.; Schmieder, B.; Schmit, D. J.; Seaton, D. B.; Sterling, A. C.; Tripathi, D.; Williams, D. R.; Zhang, M. Bibcode: 2010ApJ...724.1133G Altcode: We present a three-dimensional density model of coronal prominence cavities, and a morphological fit that has been tightly constrained by a uniquely well-observed cavity. Observations were obtained as part of an International Heliophysical Year campaign by instruments from a variety of space- and ground-based observatories, spanning wavelengths from radio to soft X-ray to integrated white light. From these data it is clear that the prominence cavity is the limb manifestation of a longitudinally extended polar-crown filament channel, and that the cavity is a region of low density relative to the surrounding corona. As a first step toward quantifying density and temperature from campaign spectroscopic data, we establish the three-dimensional morphology of the cavity. This is critical for taking line-of-sight projection effects into account, since cavities are not localized in the plane of the sky and the corona is optically thin. We have augmented a global coronal streamer model to include a tunnel-like cavity with elliptical cross-section and a Gaussian variation of height along the tunnel length. We have developed a semi-automated routine that fits ellipses to cross-sections of the cavity as it rotates past the solar limb, and have applied it to Extreme Ultraviolet Imager observations from the two Solar Terrestrial Relations Observatory spacecraft. This defines the morphological parameters of our model, from which we reproduce forward-modeled cavity observables. We find that cavity morphology and orientation, in combination with the viewpoints of the observing spacecraft, explain the observed variation in cavity visibility for the east versus west limbs. Title: Generation of Electric Currents in the Chromosphere via Neutral-Ion Drag Authors: Krasnoselskikh, V.; Vekstein, G.; Hudson, H. S.; Bale, S. D.; Abbett, W. P. Bibcode: 2010ApJ...724.1542K Altcode: 2010arXiv1011.5834K We consider the generation of electric currents in the solar chromosphere where the ionization level is typically low. We show that ambient electrons become magnetized even for weak magnetic fields (30 G); that is, their gyrofrequency becomes larger than the collision frequency while ion motions continue to be dominated by ion-neutral collisions. Under such conditions, ions are dragged by neutrals, and the magnetic field acts as if it is frozen-in to the dynamics of the neutral gas. However, magnetized electrons drift under the action of the electric and magnetic fields induced in the reference frame of ions moving with the neutral gas. We find that this relative motion of electrons and ions results in the generation of quite intense electric currents. The dissipation of these currents leads to resistive electron heating and efficient gas ionization. Ionization by electron-neutral impact does not alter the dynamics of the heavy particles; thus, the gas turbulent motions continue even when the plasma becomes fully ionized, and resistive dissipation continues to heat electrons and ions. This heating process is so efficient that it can result in typical temperature increases with altitude as large as 0.1-0.3 eV km-1. We conclude that this process can play a major role in the heating of the chromosphere and corona. Title: Morphology of a hot coronal cavity core as observed by Hinode/XRT Authors: Reeves, K. K.; Gibson, S. E.; Kucera, T. A.; Hudson, H. S. Bibcode: 2010AGUFMSH51A1669R Altcode: We follow a coronal cavity that was observed by Hinode/XRT during the summer of 2008. This cavity has a persistent area of relatively bright X-ray emission in its center. We use multifilter data from XRT to study the thermal emission from this cavity, and find that the bright center is hotter than the surrounding cavity plasma with temperatures of about 1.6 MK. We follow the morphology of this hot feature as the cavity structure rotates over the limb during the several days between July 19 - 23 2008. We find that the hot structure at first looks fairly circular, then appears to expand and elongate, and then shrinks again to a compact circular shape. We interpret this apparent change in shape as being due to the morphology of the filament channel associated with the cavity, and the change in viewing angle as the structure rotates over the limb of the Sun. Title: A "black light flare" observed by HMI? Authors: Martinez Oliveros, J.; Hudson, H. S.; Krucker, S. Bibcode: 2010AGUFMSH23A1827M Altcode: We report the observation of a negative precursor to the white-light flare SOL2010-06-12T00:57 (M2.0), as observed via SDO/HMI in narrow bands near 6173.3 A. RHESSI and Fermi hard X-ray and gamma-ray observations for this flare are also available and reveal an unusually hard spectrum in the hard X-ray range. The "black light flare" locations are in both of the two white-light flare emission regionsand precede them by about one HMI time step of 45 s. The timing thus resembles that predicted by Henoux et al. (1990) based on the interplay between ionization (leading to opacity) and heating (leading to emission). We discuss this remarkable event in its full observational scope. Title: The Solar Oblateness at Solar Minimum as Observed by RHESSI/SAS Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P. Bibcode: 2010AGUFMSH53B..05F Altcode: The RHESSI solar aspect sensor (SAS) has provided oblateness measurements of the optical solar disk with unprecedented precision. SAS measures the optical solar profile at 670 nm in the red continuum. SAS consists of three spacially uniformly distributed linear CCDs mounted on a rotating spacecraft; a crucial ingredient in access of calibration parameters. From the SAS data, differential radius measurements can be derived at about 100 Hz including about 1000 full CCD readouts per day for calibration purposes. For a three month period during the active phase of the solar cycle in 2004, the shape of the solar disk has been measured discovering an apparent excess oblateness which we attributed to the enhanced network. In order to avoid confusion between magnetic activity and a correlated brightness enhancement in the SAS signal at 670 nm, the SAS data has been masked using the SOHO/EIT284A data. The measured oblateness as function of the masking level is then extrapolated for a value of the underlaying, presumably non-magnetic sun. Here, we present the analysis of the RHESSI/SAS data during the solar minimum with the inferred interpretation for the oblateness signal. Title: Generation of electric currents in the chromosphere via neutral-ion drag Authors: Krasnoselskikh, V.; Vekstein, G.; Hudson, H. S.; Bale, S.; Abbett, W. P. Bibcode: 2010AGUFMSH31C1810K Altcode: We consider the generation of electric currents in the solar chromosphere. The ionization level in this region is generally supposed to be low. We show that the ambient electrons become magnetized even for weak magnetic fields (30 G), i.e. their gyrofrequency becomes larger than the collision frequency; ion motions continue to be dominated by ion-neutral collisions in this region. Under such conditions the ions are dragged by neutrals and magnetic field dynamics resembles frozen-in motion of the field with the neutral gas. On the other hand magnetized electrons drift under the action of the electric and magnetic fields induced in the reference frame of ions moving with the neutral gas. This relative motion of electrons and ions results in the generation of quite intense electric currents. The dissipation of these currents leads to the resistive electron heating and efficient gas ionization. Ionization by electron-neutral impact does not alter the dynamics of the heavy particles; thus the gas turbulent motions continue even when the plasma becomes fully ionized and the resistive current dissipation continues to heat electrons and ions. This heating process is so efficient that it can result in typical temperature increases with altitude as large as 0.1-0.3 eV/km. We conclude that this process can play a major role in the heating of the chromosphere and corona. Title: Uncovering Mechanisms of Coronal Magnetism via Advanced 3D Modeling of Flares and Active Regions Authors: Fleishman, Gregory; Gary, Dale; Nita, Gelu; Alexander, David; Aschwanden, Markus; Bastian, Tim; Hudson, Hugh; Hurford, Gordon; Kontar, Eduard; Longcope, Dana; Mikic, Zoran; DeRosa, Marc; Ryan, James; White, Stephen Bibcode: 2010arXiv1011.2800F Altcode: The coming decade will see the routine use of solar data of unprecedented spatial and spectral resolution, time cadence, and completeness. To capitalize on the new (or soon to be available) facilities such as SDO, ATST and FASR, and the challenges they present in the visualization and synthesis of multi-wavelength datasets, we propose that realistic, sophisticated, 3D active region and flare modeling is timely and critical, and will be a forefront of coronal studies over the coming decade. To make such modeling a reality, a broad, concerted effort is needed to capture the wealth of information resulting from the data, develop a synergistic modeling effort, and generate the necessary visualization, interpretation and model-data comparison tools to accurately extract the key physics. Title: Constraining the Hard X-ray Properties of the Quiet Sun with New RHESSI Observations Authors: Hannah, I. G.; Hudson, H. S.; Hurford, G. J.; Lin, R. P. Bibcode: 2010ApJ...724..487H Altcode: 2010arXiv1009.2918H We present new RHESSI upper limits in the 3-200 keV energy range for solar hard X-ray emission in the absence of flares and active regions, i.e., the quiet Sun, using data obtained between 2005 July and 2009 April. These new limits, substantially deeper than any previous ones, constrain several physical processes that could produce hard X-ray emission. These include cosmic-ray effects and the generation of axions within the solar core. The data also limit the properties of "nanoflares," a leading candidate to explain coronal heating. We find it unlikely for nanoflares involving nonthermal effects to heat the corona because such events would require a steep electron spectrum E with index δ>5 extending to very low energies (<1 keV), into the thermal energy range. We also use the limits to constrain the parameter space of an isothermal model and coronal thin-target emission models (power-law and kappa distributions). Title: On the Origin of the Solar Moreton Wave of 2006 December 6 Authors: Balasubramaniam, K. S.; Cliver, E. W.; Pevtsov, A.; Temmer, M.; Henry, T. W.; Hudson, H. S.; Imada, S.; Ling, A. G.; Moore, R. L.; Muhr, N.; Neidig, D. F.; Petrie, G. J. D.; Veronig, A. M.; Vršnak, B.; White, S. M. Bibcode: 2010ApJ...723..587B Altcode: We analyzed ground- and space-based observations of the eruptive flare (3B/X6.5) and associated Moreton wave (~850 km s-1 ~270° azimuthal span) of 2006 December 6 to determine the wave driver—either flare pressure pulse (blast) or coronal mass ejection (CME). Kinematic analysis favors a CME driver of the wave, despite key gaps in coronal data. The CME scenario has a less constrained/smoother velocity versus time profile than is the case for the flare hypothesis and requires an acceleration rate more in accord with observations. The CME picture is based, in part, on the assumption that a strong and impulsive magnetic field change observed by a GONG magnetograph during the rapid rise phase of the flare corresponds to the main acceleration phase of the CME. The Moreton wave evolution tracks the inferred eruption of an extended coronal arcade, overlying a region of weak magnetic field to the west of the principal flare in NOAA active region 10930. Observations of Hα foot point brightenings, disturbance contours in off-band Hα images, and He I 10830 Å flare ribbons trace the eruption from 18:42 to 18:44 UT as it progressed southwest along the arcade. Hinode EIS observations show strong blueshifts at foot points of this arcade during the post-eruption phase, indicating mass outflow. At 18:45 UT, the Moreton wave exhibited two separate arcs (one off each flank of the tip of the arcade) that merged and coalesced by 18:47 UT to form a single smooth wave front, having its maximum amplitude in the southwest direction. We suggest that the erupting arcade (i.e., CME) expanded laterally to drive a coronal shock responsible for the Moreton wave. We attribute a darkening in Hα from a region underlying the arcade to absorption by faint unresolved post-eruption loops. Title: Solar Flares and the Chromosphere Authors: Fletcher, L.; Turkmani, R.; Hudson, H. S.; Hawley, S. L.; Kowalski, A.; Berlicki, A.; Heinzel, P. Bibcode: 2010arXiv1011.4650F Altcode: A white paper prepared for the Space Studies Board, National Academy of Sciences (USA), for its Decadal Survey of Solar and Space Physics (Heliophysics), reviewing and encouraging studies of flare physics in the chromosphere. Title: The Optical Depth of White-light Flare Continuum Authors: Potts, Hugh; Hudson, Hugh; Fletcher, Lyndsay; Diver, Declan Bibcode: 2010ApJ...722.1514P Altcode: 2010arXiv1004.1039P The white-light continuum emission of a solar flare remains a puzzle as regards its height of formation and its emission mechanism(s). This continuum and its extension into the near-UV contain the bulk of the energy radiated by a flare, and so its explanation is a high priority. We describe a method to determine the optical depth of the emitting layer and apply it to the well-studied flare of 2002 July 15, making use of MDI pseudo-continuum intensity images. We find the optical depth of the visible continuum in all flare images, including an impulsive ribbon to be small, consistent with the observation of Balmer and Paschen edges in other events. Title: Solar Flares and the Chromosphere: A white paper for the Decadal Survey Authors: Hudson, H. S.; Fletcher, L.; Turkmani, R.; Hawley, S. L.; Kowalski, A. F.; Berlicki, A.; Heinzel, P. Bibcode: 2010helio2010....1H Altcode: No abstract at ADS Title: Solar physics: Solar flares add up Authors: Hudson, Hugh S. Bibcode: 2010NatPh...6..637H Altcode: Solar flares are the most energetic events in our Solar System, but relatively little is known about their contribution to the total energy the Earth receives from the Sun. The detection of a moderate solar flare in the total solar irradiance suggests their impact on the variability of the Sun's output could be larger than expected. Title: Flare and CME Properties and Rates at Sunspot Minimum Authors: Hudson, H. S.; Li, Y. Bibcode: 2010ASPC..428..153H Altcode: 2010arXiv1003.4289H The corona at solar minimum generally differs greatly from that during active times. We discuss the current Cycle 23/24 minimum from the point of view of the occurrence of flares and CMEs (coronal mass ejections). By comparison with the previous minimum, the flare/CME ratio diminished by almost an order of magnitude. This suggests that the environmental effect in flare/CME association differed in the sense that the Cycle 23/24 minimum corona was relatively easy to disrupt. Title: The Solar Microwave Flux and the Sunspot Number Authors: Svalgaard, L.; Hudson, H. S. Bibcode: 2010ASPC..428..325S Altcode: 2010arXiv1003.4281S The solar F10.7 index is has been a reliable and sensitive activity index since 1947. As with other indices, it has been showing unusual behavior in the Cycle 23/24 minimum. The origins of the solar microwave flux lie in a variety of features, and in two main emission mechanisms: free-free and gyroresonance. In past solar cycles, F10.7 has correlated well with the sunspot number SSN. We find that this correlation has broken down in Cycle 23, confirming this with Japanese fixed-frequency radiometric microwave data. Title: The Signature of Flares in VIRGO Total Solar Irradiance Measurements Authors: Quesnel, A.; Dennis, B. R.; Fleck, B.; Fröhlich, C.; Hudson, H. S.; Tolbert, A. K. Bibcode: 2010ASPC..428..133Q Altcode: 2010arXiv1003.4194Q We use Total Solar Irradiance (TSI) measurements from the VIRGO (Variability of solar Irradiance and Gravity Oscillations) instrument on board SOHO to obtain preliminary estimates of the mean total radiative energy emitted by X-class solar flares. The basic tool is that of summed-epoch analysis, which has also enabled us to detect and partially characterize systematic errors present in the basic data. We describe these errors, which significantly degrade the photometry at high frequencies. We find the ratio of GOES 1-8 Å luminosity to total bolometric luminosity to be of order 0.01. Title: Thermal Properties of Coronal Cavities as Observed by the X-Ray Telescope on Hinode Authors: Reeves, Kathy; Gibson, S. E.; Kucera, T. A.; Hudson, H. S.; Tripathi, D. Bibcode: 2010AAS...21640511R Altcode: 2010BAAS...41..891R Coronal cavities are voids in coronal emission often observed above high latitude filament channels. Sometimes, these cavities have areas of bright X-ray emission in their centers (i.e. Hudson et al

1999). In this study, we use data from the X-ray Telescope (XRT) on Hinode to examine the thermal emission properties of two kinds of coronal cavities, those with and without enhanced emission at their centers. For cavities with bright X-ray emission in their centers, we find evidence for elevated temperatures in the cavity center. We find no obvious correlation between the presence of

bright cavity cores and filament presence or eruption.

This work is part of the effort of the International Space Science Institute International Team on Prominence Cavities Title: Observations of solar and stellar eruptions, flares, and jets Authors: Hudson, Hugh Bibcode: 2010hssr.book..123H Altcode: No abstract at ADS Title: Flare Global Waves Of Three Kinds Authors: Hudson, Hugh S.; Martinez-Oliveros, J. C. Bibcode: 2010AAS...21640429H Altcode: 2010BAAS...41..904H Flares produce at least three kinds of global waves: Moreton, metric type II, and seismic. In addition, EIT waves, coronal dimmings, and CMES may also have wave-like properties. Each of these three global waves arises in the impulsive phase of a flare. This is also the time of sudden stepwise changes in the photospheric line-of-sight field, and the time of the CME acceleration phase. We review the observational material, starting with the published seismic events, and ask whether or not a common origin is consistent with the physical parameters in the likely region of origin. Title: Measurements of the Coronal Acceleration Region of a Solar Flare Authors: Krucker, Sam; Hudson, H. S.; White, S. M.; Masuda, S.; Lin, R. P. Bibcode: 2010AAS...21630603K Altcode: The most discussed coronal hard X-ray source has been the above-the-loop-top source observed in the Masuda flare. We present new RHESSI hard X-ray observations of a similar event with simultaneous microwave observations by NoRH. These observations clearly establish the non-thermal nature of the above-the-loop-top source and triggered a new interpretation. To account for the extremely bright hard X-ray source in a rather low ambient density plasma, all electrons in the above-the-loop-top source seem to be accelerated, suggesting that the above-the-loop-top source is itself the electron acceleration region. Title: G-band and Hard X-ray Emissions of the 2006 December 14 Flare Observed by Hinode/SOT and Rhessi Authors: Watanabe, Kyoko; Krucker, Säm; Hudson, Hugh; Shimizu, Toshifumi; Masuda, Satoshi; Ichimoto, Kiyoshi Bibcode: 2010ApJ...715..651W Altcode: 2010arXiv1004.4259W We report on G-band emission observed by the Solar Optical Telescope on board the Hinode satellite in association with the X1.5-class flare on 2006 December 14. The G-band enhancements originate from the footpoints of flaring coronal magnetic loops, coinciding with nonthermal hard X-ray bremsstrahlung sources observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager. At the available 2 minute cadence, the G-band and hard X-ray intensities are furthermore well correlated in time. Assuming that the G-band enhancements are continuum emission from a blackbody, we derived the total radiative losses of the white-light flare (white-light power). If the G-band enhancements additionally have a contribution from lines, the derived values are overestimates. We compare the white-light power with the power in hard X-ray producing electrons using the thick-target assumption. Independent of the cutoff energy of the accelerated electron spectrum, the white-light power and the power of accelerated electrons are roughly proportional. Using the observed upper limit of ~30 keV for the cutoff energy, the hard X-ray producing electrons provide at least a factor of 2 more power than needed to produce the white-light emission. For electrons above 40 keV, the powers roughly match for all four of the time intervals available during the impulsive phase. Hence, the flare-accelerated electrons contain enough energy to produce the white-light flare emissions. The observed correlation in time, space, and power strongly suggests that electron acceleration and white-light production in solar flares are closely related. However, the results also call attention to the inconsistency in apparent source heights of the hard X-ray (chromosphere) and white-light (upper photosphere) sources. Title: Geometric Model of a Coronal Cavity Authors: Kucera, Therese A.; Gibson, S. E.; Rastawicki, D.; Dove, J.; de Toma, G.; Hao, J.; Hudson, H. S.; Marque, C.; McIntosh, P. S.; Reeves, K. K.; Schmidt, D. J.; Sterling, A. C.; Tripathi, D. K.; Williams, D. R.; Zhang, M. Bibcode: 2010AAS...21640510K Altcode: 2010BAAS...41..890K We observed a coronal cavity from August 8-18 2007 during a multi-instrument observing campaign organized under the auspices of the International Heliophysical Year (IHY). Here we present initial efforts to model the cavity with a geometrical streamer-cavity model. The model is based the white-light streamer model of Gibson et al. (2003), which has been enhanced by the addition of a cavity and the capability to model EUV and X-ray emission. The cavity is modeled with an elliptical cross-section and Gaussian fall-off in length and width inside the streamer. Density and temperature can be varied in the streamer and cavity and constrained via comparison with data. Although this model is purely morphological, it allows for three-dimensional, multi-temperature analysis and characterization of the data, which can then provide constraints for future physical modeling. Initial comparisons to STEREO/EUVI images of the cavity and streamer show that the model can provide a good fit to the data. This work is part of the effort of the International Space Science Institute International Team on Prominence Cavities. Title: Photospheric Temperature Variations near the Solar Limb Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.; Zahid, H. J. Bibcode: 2010AAS...21631305F Altcode: 2010BAAS...41R.888F We use observations from the solar aspect sensor of RHESSI to characterize the latitude dependence of the temperature of the photosphere near the solar limb. Previous observations had suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. This photometry is differential, i.e. relative to a mean limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM) at 670 nm. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its average solar pointing. We fit an average limb-darkening function and reassemble the residuals into synoptic maps of differential intensity variations as a function of position angle. We further mask these images against SOHO/EIT 284A images in order to eliminate magnetic regions. The analysis establishes a limit on the quadrupole dependence of temperature (brightness) on position angle of order 0.04 K, with a comparable uncertainty. Title: Measurements of the Coronal Acceleration Region of a Solar Flare Authors: Krucker, Säm; Hudson, H. S.; Glesener, L.; White, S. M.; Masuda, S.; Wuelser, J. -P.; Lin, R. P. Bibcode: 2010ApJ...714.1108K Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph (NoRH) are used to investigate coronal hard X-ray and microwave emissions in the partially disk-occulted solar flare of 2007 December 31. The STEREO mission provides EUV images of the flare site at different viewing angles, establishing a two-ribbon flare geometry and occultation heights of the RHESSI and NoRH observations of ~16 Mm and ~25 Mm, respectively. Despite the occultation, intense hard X-ray emission up to ~80 keV occurs during the impulsive phase from a coronal source that is also seen in microwaves. The hard X-ray and microwave source during the impulsive phase is located ~6 Mm above thermal flare loops seen later at the soft X-ray peak time, similar in location to the above-the-loop-top source in the Masuda flare. A single non-thermal electron population with a power-law distribution (with spectral index of ~3.7 from ~16 keV up to the MeV range) radiating in both bremsstrahlung and gyrosynchrotron emission can explain the observed hard X-ray and microwave spectrum, respectively. This clearly establishes the non-thermal nature of the above-the-loop-top source. The large hard X-ray intensity requires a very large number (>5 × 1035 above 16 keV for the derived upper limit of the ambient density of ~8 × 109 cm-3) of suprathermal electrons to be present in this above-the-loop-top source. This is of the same order of magnitude as the number of ambient thermal electrons. We show that collisional losses of these accelerated electrons would heat all ambient electrons to superhot temperatures (tens of keV) within seconds. Hence, the standard scenario, with hard X-rays produced by a beam comprising the tail of a dominant thermal core plasma, does not work. Instead, all electrons in the above-the-loop-top source seem to be accelerated, suggesting that the above-the-loop-top source is itself the electron acceleration region. Title: On The Brightness and Waiting-Time Distributions of a Type III Radio Storm Observed By Stereo/Waves Authors: Eastwood, J. P.; Wheatland, M. S.; Hudson, H. S.; Krucker, S.; Bale, S. D.; Maksimovic, M.; Goetz, K.; Bougeret, J. -L. Bibcode: 2010ApJ...708L..95E Altcode: 2009arXiv0911.4131E Type III solar radio storms, observed at frequencies below ~16 MHz by space-borne radio experiments, correspond to the quasi-continuous, bursty emission of electron beams onto open field lines above active regions. The mechanisms by which a storm can persist in some cases for more than a solar rotation whilst exhibiting considerable radio activity are poorly understood. To address this issue, the statistical properties of a type III storm observed by the STEREO/WAVES radio experiment are presented, examining both the brightness distribution and (for the first time) the waiting-time distribution (WTD). Single power-law behavior is observed in the number distribution as a function of brightness; the power-law index is ~2.1 and is largely independent of frequency. The WTD is found to be consistent with a piecewise-constant Poisson process. This indicates that during the storm individual type III bursts occur independently and suggests that the storm dynamics are consistent with avalanche-type behavior in the underlying active region. Title: The white-light continuum in the impulsive phase of a solar flare. Authors: Hudson, H. S.; Fletcher, L.; Krucker, S. Bibcode: 2010MmSAI..81..637H Altcode: 2010arXiv1001.1005H We discuss the IR/visible/VUV continuum emission of the impulsive phase of a solar flare, using TRACE UV and EUV images to characterize the spectral energy distribution. This continuum has been poorly observed but energetically dominates the radiant energy output . Recent bolometric observations of solar flares furthermore point to the impulsive phase as the source of a major fraction of the radiant energy. This component appears to exhibit a Balmer jump and thus must originate in an optically thin region above the quiet photosphere, with an elevated temperature and strong ionization. Title: The RHESSI Microflare Height Distribution Authors: Christe, Steven; Krucker, Samuel; Hudson, Hugh Bibcode: 2010cosp...38.2963C Altcode: 2010cosp.meet.2963C We present the first in-depth statistical survey of flare source heights observed by RHESSI between March 2002 and March 2007, a total of 25,705 events. These flares were found using a new flare-finding algorithm designed to search the 6-12 keV count-rate when RHESSI's full sensitivity was available in order to find the smallest events. Thermal (4-10 keV) and nonther-mal (15-25 keV) images were made for all microflares and source centroid locations were found for each event. In order to extract the height information from source positions, a Monte-Carlo model was developed with an assumed source height distribution where height is measured from the photosphere. We find that the best source height model is given by an exponential distri-bution with a scale height of 2.1 (0.3) Mm and a minimum height of 3.1 (0.3) Mm. Comparing with previously published loop length measurements, we find that the average loop tilt is 44 degrees as measured from the vertical. Title: The McClymont Jerk: A driver of solar seismicity Authors: Martinez Oliveros, J. C.; Hudson, H. S.; Krucker, S. Bibcode: 2009AGUFMSH13A1508M Altcode: Our Sun is a vibrating and amazing celestial object, full of little-understood phenomena. One such phenomenon is the so-called Sunquake, originally observed by Kosovichev and Zharkova (1998). Studies by Donea and Lindsey (2005) and Besliu et al. (2008) now show that the sunquake is not a rare phenomenon. They are powerful events, hardly visible on the solar surface, and result directly from energy release. They can be detected using acoustic techniques as high frequencies oscillations in the sun. The first models proposed to explain sunquakes involved pressure pulses, perhaps associated with heating revealed by white-light flare emission. Hudson, Fisher and Welsh (2008) proposed a mechanism to generate seismic waves based on the dynamical behavior of the solar magnetic field during flares. In this poster we study the variations of the magnetic field as an alternative mechanism for the generation of seismic waves. Title: Ulysses/GRB Measurements of Hard X-Ray Flares on the Far Side of the Sun Authors: Tranquille, C.; Hurley, K.; Hudson, H. S. Bibcode: 2009AGUFMSH21C..03T Altcode: The Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) on Ulysses provided continuous measurements of energetic (25-150 keV) solar X-ray activity from launch in October 1990 to November 2003, when it was temporarily switched off due to instrument power sharing requirements. The unique high latitude orbit of the spacecraft made it possible to monitor solar X-ray flare activity on the far side of the Sun during extended periods of the mission. We correlate all X-class flares measured by the GOES fleet of satellites listed in the NOAA reports, with count rate increases measured by GRB to confirm that energetic flares have hard non-thermal components which extend into the energy range above 25 keV. By comparing peak GRB count rates with the GOES flare magnitudes, we establish a scaling law between the two sets of measurements, characterized by a power-law fit. Having accounted for all the X-class flares seen simultaneously by both GRB and GOES, we are able to identify signatures of intense X-ray activity in the GRB data set which must originate from flares on the hidden face of the Sun. In total, we list 82 such events during the 13 years of GRB operation. We provide timing information for each event and also coarse flare site locations based on the geometry of the Ulysses orbit. Estimates of the flare intensity are made using the scaling law derived from the correlated GRB and GOES measurements. Global monitoring of flare activity on the complete surface of the Sun can provide useful information to validate and refine models and numerical simulations of heliospheric and space weather processes. Title: High Resolution Imaging of Solar Flare Footpoints in White Light and Hard X-rays Authors: Krucker, S.; Hudson, H. S.; Lin, R. P. Bibcode: 2009AGUFMSH21C..01K Altcode: We test the standard thick target beam scenario of solar flares using high resolution G-band (430 nm) observations (~0.2 arcsec) taken by HINODE/SOT and hard X-ray observations (2.3 arcsec) from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) of the December 6, 2006 GOES X9 flare. At both wavelengths, several co-spatial footpoint sources are seen on the flare ribbons that show similar relative intensities. This excellent correlation suggests that the suprathermal electrons producing the hard X-ray emission are also the source of energy for the white light emission, excluding energetic protons as a possible source. If both emissions indeed come from the same location, the higher resolution G-band observations suggest that the individual hard X-ray sources are unresolved. Using the footpoint area from the G-band images, the energy deposition rate by the hard X-ray producing electron beam in cold thick target approximation become enormous with values of 2x10 12 erg/s/cm2 for 25 keV (9x1012 erg/s/cm2 for 10 keV). This corresponds to a giant electron beam density within the hard X-ray source of 0.3x1010 cm-3 above 25 keV (5x1010 cm-3 above 10 keV). These estimates pose serious questions for the thick-target beam interpretation. We will discuss alternative scenarios, including the idea of a purely non-thermal electron distribution as the source of the hard X-ray emission. Title: The Solar Radio Microwave Flux and the Sunspot Number Authors: Svalgaard, L.; Hudson, H. S. Bibcode: 2009AGUFMSH13C..03S Altcode: Since 1947 the flux of microwaves from the Sun at wavelengths between 3 and 30 cm [frequencies between 10 and 1 GHz] has been routinely measured. This emission comes from both thromosphere and the corona and has two main sources: thermal bremsstrahlung (free-free emission) and thermal gyroradiation. These mechanisms give rise to enhanced radiation when the density and magnetic field increase, so the microwave radiation is a good measure of general solar activity. Strong magnetic fields occur in the network and can persist for weeks or longer; hence there is a strong rotational signal in the emission superposed on a solar cycle variation of the background coronal signal. The radio flux measurements can be calibrated absolutely and are not very sensitive to observing conditions, and in principle have no personal equation. They may thus be the most objective measure of solar activity, and our many decades-long flux record could throw light on the important issue of the long-term variation of solar activity. The longest series of observations F10.7, begun by Covington in Ottawa, Canada in April 1947 and maintained to this day. Other observatories also have long and continuing series of measurements of the microwave flux. One can now ask how this measure of solar activity compares to other measures, in particular the sunspot number. We correlate the sunspot number against the F10.7 flux for the interval 1951-1988, and obtain a good polynomial fit (R^2 = 0.977) up until ~1989.0 after which time the observed sunspot number falls progressively below the fitted number. Three obvious hypotheses present themselves: 1) The sunspot counting procedure or observers have changed, with resulting artificial changes of the sunspot number as they have in the past. 2) Physical changes in the corona or chromosphere have occurred. 3) Livingston & Penn’s observations that the sunspots are getting warmer during the last decade, leading to a decreased contrast with the surrounding photosphere and hence lessened visibility, possibly resulting in an undercount of sunspots. The near constancy of the flux at minima since 1954 argues against a change of the physical conditions at the source locations, leaving the exciting possibility that Livingston & Penn may be correct. Title: Latitude-Dependent Temperature Variations at the Solar Limb Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.; Zahid, H. J. Bibcode: 2009AGUFMSH23B1549F Altcode: We use observations from the solar aspect sensor of RHESSI to characterize the latitude dependence of the temperature of the photosphere at the solar limb. Previous observations have suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. This photometry is differential, i.e. relative to a mean limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM) at 670 nm. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its average solar pointing. We fit an average limb-darkening function and reassemble the residuals into synoptic maps of differential intensity variations as function of position angle. We further mask these images against SOHO/EIT 284A images in order to eliminate magnetic regions. The analysis establishes a limit on the quadrupole dependence of temperature (brightness) on position angle of 0.04 +/- 0.02 K. This results in a possible correction of our precise measurement of the solar oblateness which is smaller than its rms error of 0.14 mas. Title: The Ulysses Catalog of Solar Hard X-Ray Flares Authors: Tranquille, C.; Hurley, K.; Hudson, H. S. Bibcode: 2009SoPh..258..141T Altcode: Ulysses was launched in October 1990, and its Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) has provided more than 13 years of uninterrupted observations of solar X-ray flare activity. Due to the large variation of the relative solar latitude and longitude of the spacecraft orbit with respect to the Earth, the perspective of the GRB instrument often differed significantly from that of X-ray instruments on Earth-orbiting satellites. During extended periods the GRB experiment made direct observations of flares on the hidden face of the Sun, providing a unique record of events not visible to other instruments. The small detector area of GRB and its optimization for very high counting rates minimized the effects of pulse pile-up. We interpret the spectra, time histories, and occurrence distribution patterns of GRB data in terms of "thermal feed-through", the confusion of thermal soft X-rays and non-thermal hard X-rays. This effect is a systematic problem for scintillation-counter spectrometers observing the solar hard X-ray spectrum. This paper provides a definitive catalog of the Ulysses X-ray flare observations and discusses various features of this unique database. For the equivalent GOES range X2 - X25, we find a power-law fit for the (differential) occurrence frequency at >25 keV with slope −1.61±0.04, with no evidence for a downturn at the highest event magnitudes (for the relatively small sample of such events available in this study). If the nine most intense events are excluded because of concerns about the effects of pulse pile-up, the slope steepens to −1.75±0.08. Title: X-ray Behavior of the CMEless X-class flares Authors: Hudson, Hugh S. Bibcode: 2009shin.confE.179H Altcode: The RHESSI data reveal a wealth of coronal hard X-ray sources, both in the impulsive phase and in the extended nonthermal phase. The latter are strongly associated with CME occurrence and the related global shock wave. Major flare events without CMEs have significantly different morphology: no soft-hard-harder evolution in the hard X-ray spectrum and weak soft X-ray precursors. This presentation reviews the data for the CMEless X-class flare list of Gopalswamy et al. (2009) and discusses interpretations of these and of the phenomena associated with CME flares. Time permitting we will also discuss the impulsive-phase coronal hard X-ray sources (e.g. Masuda). Title: Coronal Radiation Belts Authors: Hudson, H. S.; MacKinnon, A. L.; De Rosa, M. L.; Frewen, S. F. N. Bibcode: 2009ApJ...698L..86H Altcode: 2009arXiv0905.3824H The magnetic field of the solar corona has a large-scale dipole character, which maps into the bipolar field in the solar wind. Using standard representations of the coronal field, we show that high-energy ions can be trapped stably in these large-scale closed fields. The drift shells that describe the conservation of the third adiabatic invariant may have complicated geometries. Particles trapped in these zones would resemble the Van Allen belts and could have detectable consequences. We discuss potential sources of trapped particles. Title: Flares and the chromosphere Authors: Hudson, Hugh S.; Fletcher, Lyndsay Bibcode: 2009EP&S...61..577H Altcode: 2009EP&S...61L.577H The chromosphere (the link between the photosphere and the corona) plays a crucial role in flare and CME development. In analogies between flares and magnetic substorms, it is normally identified with the ionosphere, but we argue that the correspondence is not exact. Much of the important physics of this interesting region remains to be explored. We discuss chromospheric flares in the context of recent observations of white-light flares and hard X-rays as observed by TRACE and RHESSI, respectively. We interpret key features of these observations as results of the stepwise changes a flare produces in the photospheric magnetic field. Title: Gamma Rays and Energetic Neutral Atoms from Solar Flares Authors: Hudson, Hugh S.; Lin, R. P.; MacKinnon, A. N.; Raymond, J. C.; Shih, A. Y.; Wang, L. Bibcode: 2009SPD....40.3604H Altcode: The recent discovery of energetic (1.6-5.0 MeV) neutral hydrogen atoms (ENAs) from an X9 solar flare on 2006 Dec. 5 (Mewaldt et al. 2009) raises the exciting possibility that they could represent a lower-energy range of the same population of accelerated ions responsible for flare gamma-ray emission. If so the neutralization would take place at low altitudes in the flaring loop, and the neutral atoms might need to escape through multiple epochs of re-ionization and neutralization. The probability for eventual escape is high because of the large mirror ratios of the coronal fields and because of small energy losses of the ENAs and the ions they become. Using a standard density model, we have shown that the collisional lifetime of re-ionized particles exceeds the observed injection time. We use Monte Carlo simulations of particle trajectories, including both neutral and ionized states, to estimate escape probabilities and directivity more generally within the context of a standard PFSS (potential-field source surface) model for the field above the Mewaldt et al. flare, and compare the results with RHESSI gamma-ray observations of this flare. Title: High Resolution Imaging of Solar Flare Footpoints in White Light and Hard X-rays Authors: Krucker, Sam; Hudson, H. S.; Lin, R. P. Bibcode: 2009SPD....40.1919K Altcode: High resolution G-band (430 nm) observations ( 0.2 arcsec) taken by HINODE/SOT and hard X-ray observations (2.3 arcsec) from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are used to study footpoint sources of the December 6, 2006 GOES X9 class solar flare. At both wavelengths, several co-spatial footpoint sources are seen on the flare ribbons that show similar relative intensities. This excellent correlation suggests that the energetic electrons producing the hard X-ray emission are also the source of energy for the white light emission, excluding energetic protons as a possible source. If both emissions indeed come from the same location, the higher resolution G-band observation suggest that the individual hard X-ray sources are unresolved. Using the area from the G-band images, the cold thick target model gives the enormous energy deposition rate of 2e12 erg/s/cm2 for 25 keV (9e12 erg/s/cm2 for 10 keV) and huge densities of beam electrons within the hard X-ray source of 0.3e10 cm-3 above 25 keV (5d10 cm-3 above 10 keV). These estimates pose serious questions for the thick-target interpretation. We will discuss alternative scenarios, including the idea of a purely non-thermal electron distribution as the source of the hard X-ray emission. Title: Latitude-Dependent Temperature Variations at the Solar Limb Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.; Zahid, H. J. Bibcode: 2009SPD....40.0925F Altcode: We use observations from the solar aspect sensor of RHESSI to characterize the latitude dependence of the temperature of the photosphere at the solar limb. Previous observations have suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. This photometry is differential, i.e. relative to a mean limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM) at 670 nm. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its average solar pointing. We fit an average limb-darkening function and reassemble the residuals into synoptic maps of differential intensity variations as function of position angle. We further mask these images against SOHO/EIT 284A images in order to eliminate magnetic regions. The analysis establishes limits on the quadrupole dependence of brightness (temperature) on position angle, a crucial unknown in our precise measurement of the solar oblateness. Title: Direct Observations of the Coronal Acceleration Region of a Solar Flare Authors: Krucker, Sam; Hudson, H. S.; White, S. M.; Lin, R. P. Bibcode: 2009SPD....40.3601K Altcode: Solar flares essentially convert the intrinsic energy of coronal magnetic field into the kinetic energy of accelerated particles. Hard X-ray emission from flare-accelerated electrons produced by the bremsstrahlung mechanism provides the most direct diagnostics of electron acceleration. The most discussed coronal hard X-ray source has been the above-the-loop-top source observed in the Masuda flare. The poor spectral resolution of these observations, however, made an interpretation ambiguous, and the exact location of the acceleration remained elusive. We present high spatial and spectral resolution RHESSI hard X-ray observations of an above-the-loop-top source with simultaneous microwave observations from NoRH. These observations provide a unambiguous interpretation of above-the-loop-top sources: The above-the-loop-top source itself is the acceleration region, where all electrons within an extended volume (1e27 cm3) are accelerated. The distribution of the accelerated electrons is definitely non-thermal, with a power law distribution extending from 10 keV up to the relativistic range (few MeV). The plasma beta in the acceleration region changes from the pre-flare value of 0.01 to 1, indicating that roughly half of the magnetic energy has been transformed into kinetic energy. Title: Solar Trapped Particles Authors: Hudson, Hugh S.; MacKinnon, A. L.; Frewen, S. F. Bibcode: 2009SPD....40.1205H Altcode: High-energy particles (>10 MeV protons) can be trapped in large-scale coronal magnetic fields for periods of days to weeks. We model this trapping by following the adiabatic motions of particles in test fields, including the Schrijver-DeRosa PFSS models. These are available in a SolarSoft interface for the entire duration of the SOHO mission thus far. In spite of the complexity of the field, we find drift shells in which particles can circulate completely around the Sun, and thus conserve the third adiabatic invariant of motion well. In this work we study the morphology of the these drift shells, including their appearance as a function of phase in the solar cycle. Title: The RHESSI Microflare Height Distribution Authors: Christe, S.; Krucker, Sam; Hudson, H.; Lin, R. Bibcode: 2009SPD....40.1903C Altcode: We present the first in-depth statistical survey of flare source heights observed by RHESSI between March 2002 and March 2007, a total of 25,705 events. These flares were found using a new flare-finding algorithm designed to search the 6-12 keV count-rate when RHESSI's full sensitivity was available in order to find the smallest events. Thermal (4-10 keV) and nonthermal (15-25 keV) images were made for all microflares and source centroid locations were found for each event. In order to extract the height information from source positions, a Monte-Carlo model was developed with an assumed source height distribution where height is measured from the photosphere. We find that the best source height model is given by an exponential distribution with a scale height of 2.1 (0.3) Mm and a minimum height of 3.1 (0.3) Mm. Comparing with previously published loop length measurements, we find that the average loop tilt is 44 degrees as measured from the vertical. Title: Particle Acceleration and Transport on the Sun Authors: Bastian, T. S.; Emslie, G.; Fleishman, G.; Gary, D. E.; Holman, G.; Hudson, H.; Hurford, G.; Krucker, S.; Lee, J.; Miller, J.; White, S. Bibcode: 2009astro2010S..13B Altcode: No abstract at ADS Title: RHESSI Observations of Facular Limb Darkening at 670 nm Authors: Zahid, H. J.; Chapman, G.; Fivian, M. D.; Hudson, H. Bibcode: 2008AGUFMSH23A1623Z Altcode: We use observations from the solar aspect sensor of RHESSI to characterize the facular limb brightening function. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. The facular photometry is differential relative to a mean background limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band at 670 nm. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its nominal solar pointing, with amplitude of a few arc sec. We reassemble these line images into synoptic images with a relatively low time cadence but an almost full coverage of more than six years. We further mask these images against SOHO/EIT 284A images in order to select magnetic regions. The resulting mean limb-darkening function is clearly resolved in radius and has a maximum at mu = 0.24 and approaches zero at the limb, consistent with Spruit's "hot wall" model. The contrast is positive at disk center, and we discuss explanations for this. Title: Spatial and Temporal Relationships Between WL/UV Continuum and hard X-ray Footpoints in Solar Flares Authors: Hudson, H. S.; Fletcher, L.; McTiernan, J. Bibcode: 2008AGUFMSH41A1605H Altcode: Hard X-rays show the presence of energetic electrons in the impulsive phase of a solar flare. According to standard models, these electrons contain a large fraction of the total flare energy. We show that comparable amounts of energy are present in the compact, rapidly variable WL and UV bright points that constitute white-light flares. This suggests that these structures can be identified with each other, and indeed the image centroids and time variations match well. There are image differences that we believe mainly to be due to the different resolving powers of Hinode and TRACE WL/UV imaging on the one hand, and RHESSI hard X-rays on the other. We therefore also use RHESSI modeling software to simulate hard X-ray images using TRACE and Hinode data as templates to understand this relationship more precisely. Title: The Global Photospheric Temperature Field Authors: Fivian, M. D.; Zahid, H. J.; Hudson, H. S.; Lin, R. P. Bibcode: 2008AGUFMSH23A1624F Altcode: We use observations from the solar aspect sensor of RHESSI to characterize the large-scale temperature variation across the solar disk. Previous observations have suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. This photometry is differential relative to a mean limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band at 670 nm. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its nominal solar pointing. We reassemble these line images into synoptic images with a relatively low time cadence but an almost full coverage of more than six years. We further mask these images against SOHO/EIT 284A images in order to eliminate magnetic regions. The analysis establishes limits on the quadrupole dependence of brightness (temperature) on position angle, a crucial unknown in our precise measurement of the solar oblateness. Title: Modeling of Solar Radiation Belts Authors: Frewen, S. S.; De Rosa, M.; Hudson, H.; MacKinnon, A. Bibcode: 2008AGUFMSH13B1526F Altcode: Stable particle trapping in the complicated magnetic field of the solar corona -- "solar radiation belts" -- at first seems unlikely in the face of the Sun's complex, variable magnetic field. By integrating particle orbit equations in the guiding-center approximation, we investigate the fates of energetic ions in model coronal magnetic fields. We use both PFSS (Potential Field Source Surface) and simple analytic field models. Contrary to naive expectation, we find that significant numbers of particles remain trapped more than long enough to circumnavigate the Sun, neither precipitating to the surface nor attaining open field lines. The drift "shells" corresponding to conservation of the third adiabatic invariant may be complicated in form. A close look at the dependence of the cross-field drift speed on magnetic field strength and topology accounts for this finding. Title: A Large Excess in Apparent Solar Oblateness Due to Surface Magnetism Authors: Fivian, Martin D.; Hudson, Hugh S.; Lin, Robert P.; Zahid, H. Jabran Bibcode: 2008Sci...322..560F Altcode: The shape of the Sun subtly reflects its rotation and internal flows. The surface rotation rate, ~2 kilometers per second at the equator, predicts an oblateness (equator-pole radius difference) of 7.8 milli arc seconds, or ~0.001%. Observations from the Reuven Ramaty High-Energy Solar Spectroscopic Imager satellite show unexpectedly large flattening, relative to the expectation from surface rotation. This excess is dominated by the quadrupole term and gives a total oblateness of 10.77 ± 0.44 milli arc seconds. The position of the limb correlates with a sensitive extreme ultraviolet proxy, the 284 angstrom limb brightness. We relate the larger radius values to magnetic elements in the enhanced network and use the correlation to correct for it as a systematic error term in the oblateness measurement. The corrected oblateness of the nonmagnetic Sun is 8.01 ± 0.14 milli arc seconds, which is near the value expected from rotation. Title: Hard X-ray emission from the solar corona Authors: Krucker, S.; Battaglia, M.; Cargill, P. J.; Fletcher, L.; Hudson, H. S.; MacKinnon, A. L.; Masuda, S.; Sui, L.; Tomczak, M.; Veronig, A. L.; Vlahos, L.; White, S. M. Bibcode: 2008A&ARv..16..155K Altcode: 2008A&ARv.tmp....8K This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even γ-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual “footpoint” emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration. Title: Impulsive Flare Energy Transport by Large-Scale Alfven Waves, and Flare Electron Acceleration Authors: Fletcher, L.; Hudson, H. S. Bibcode: 2008ESPM...12.3.62F Altcode: The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the pre-flare coronal magnetic field. Hard X-ray observations imply that a substantial fraction of flare energy released during the impulsive phase is converted to the kinetic energy of mildly relativistic electrons (10-100 keV). The liberation of the magnetic free energy can occur as the coronal magnetic field reconfigures and relaxes following reconnection. Motivated by observations pointing to a high local Alfven speed in parts of the corona, and by considerations from magnetospheric physics, we investigate a scenario in which products of the reconfiguration - large-scale Alfven wave pulses - transport the energy and magnetic-field changes rapidly through the corona to the lower atmosphere. We investigate the opportunities that such a scenario offers for heating of the chromospheric plasma in flare footpoints, and for electron acceleration, and confront our findings with observational constraints, including energetics, HXR timing, and radio signatures. Title: Constraining the Properties of Hard X-ray Nanoflares with RHESSI Observations of the Quiet Sun Authors: Hannah, I. G.; Hudson, H. S.; Hurford, G. J.; Lin, R. P. Bibcode: 2008ESPM...12.2.83H Altcode: We present new results from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) of solar X-ray emission not associated with active regions, sunspots, or flares (the quiet Sun). RHESSI has greater sensitivity in the 5-25 keV range than previous missions, but since the quiet Sun sources may be well-dispersed spatially across the disk, RHESSI's normal imaging technique is not well suited to the task. Instead, we obtain observations in a special mode ("fan-beam modulation," Hannah et al. RSI 78, 024501, 2007) to "chop" the quiet solar signal. This technique has been used fourteen times between between June 2005 and June 2008, obtaining limits to the emission between 3-200 keV. These limits improve on those previously reported (Hannah et al. ApJ 659L, 77, 2007). They are both lower and also extend the energy range covered by the pre-RHESSI results. We use the new limits to constrain the possible properties of the thermal and non-thermal emission of the quiet Sun. In particular we discuss the possible properties of hard X-ray nanoflares and the implications for nanoflare coronal heating models. Title: Facular Contrast at 670 nm Authors: Hudson, H. S.; Zahid, H. J.; Fivian, M. D. Bibcode: 2008ESPM...12.2.59H Altcode: The Solar Aspect System (SAS) on the RHESSI satellite consists of three simple lenses with linear CCD detectors, whose data we use to synthesize images of solar photospheric brightness. The three independent detectors record about 1,000 line images per day each, with chord locations at roughly random positions on the disk. The synthesized images allow us to study the mean distribution of faculae and enhanced network. RHESSI has been observing since launch in 2002. Here we report an analysis of three years (2003-2005) of the these data. We find strong correlations between the facular excess signals in these data, and EUV brightness as derived from SOHO/EIT. The data interior to mu of about 0.24 fit a generalization of the "hot wall" model, but this fails closer to the limb. We discuss the fit and its model discrepancy in terms of the geometry of the observation, including the possibility of a "cloud" model, ie one involving emitting material above the height of the photosphere. Title: Microflares with RHESSI and Hinode/XRT Authors: Hannah, I. G.; Krucker, S.; Christe, S.; Hudson, H. S.; Lin, R. P. Bibcode: 2008ASPC..397..169H Altcode: In this article we discuss the opportunities for analyzing microflares with RHESSI and Hinode/XRT. We present analysis of one microflare, using the RHESSI to obtain the thermal and non-thermal spectral parameters and compare the RHESSI images of the thermal (4-8 keV) and non-thermal (12-50 keV) emission with the Hinode/XRT images. The RHESSI non-thermal emission in this event matches spatial and temporally the initial brightest emission from XRT. Title: Physical Conditions in Coronal Structures About to Flare Authors: Hudson, H. S.; Hannah, I. G.; Deluca, E. E.; Weber, M. Bibcode: 2008ASPC..397..130H Altcode: We use Hinode observations to study coronal structures about to flare, based on their apparent footpoints as a guide to identification. The high resolution and excellent stability of the Hinode observations makes the identifications much more precise than those done with the soft X-ray telescope (SXT) on board Yohkoh. The physical conditions in the coronal structure about to flare are important in understanding the nature of the plasma processes leading to the eruption. We find examples of soft X-ray microflares that agree with the SXT conclusions: the structure is essentially invisible prior to the flare in most cases. We present an estimation of preflare temperature and density and find that in these cases, the flare appears to happen in flux tubes with undetectably low electron density, less than ∼10^{8} cm^{-3}. A similar program with the full instrument set of Hinode would be extremely powerful, owing to the broad temperature coverage available. Title: How Solar Flares Work Authors: Hudson, H. S. Bibcode: 2008AGUSM.U22A..01H Altcode: Geophysics, radio astronomy, Japan, the ionosphere, X and gamma rays: all have contributed to my view of how solar flares and their partner coronal mass ejections (CMEs) work. A solar flare (and a CME, it turns out) has an "impulsive phase" in which catastrophic and dominant energy release from magnetic storage takes place, resulting in particle acceleration. The impulsive restructuring of the coronal currents and fields leads directly (if still mysteriously) to the many observable phenomena, which can reach the surface of the Earth. In particular we now recognize that the term "impulsive phase" correctly captures the basic morphology of the process: it is highly intermittent in both space and time, even though it underlies large-scale phenomena such as CME eruptions. We still cannot resolve the scales of the flare intermittency, but we can use in-situ observations of possibly analogous processes in the solar wind and magnetosphere for guidance. Title: The role of large-scale Alfvén waves in solar flare energy release and particle acceleration Authors: Fletcher, L.; Hudson, H. S. Bibcode: 2008AGUSMSH51C..06F Altcode: The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the pre-flare coronal magnetic field. Hard X-ray observations imply that a substantial fraction of flare energy released during the impulsive phase is converted to the kinetic energy of mildly relativistic electrons (10-100 keV). The liberation of the magnetic free energy can occur as the coronal magnetic field reconfigures and relaxes following reconnection. We investigate a scenario, inspired in part by magnetospheric physics, in which products of the reconfiguration - large-scale Alfvén wave pulses - transport the energy and magnetic-field changes rapidly through the corona to the lower atmosphere. This offers two possibilities for electron acceleration. Firstly, in a coronal plasma with E < me/mp, the waves propagate as inertial Alfvén waves. In the presence of strong spatial gradients, these generate field-aligned electric fields that can accelerate electrons to energies on the order of 10 keV and above, including by repeated interactions between electrons and wavefronts. Secondly, when they reflect in the chromosphere, a cascade to high wave numbers may develop. This will also accelerate electrons by turbulence, in a medium with a locally high electron number density. This concept, which bridges MHD-based and particle- based views of a flare, provides an interpretation of the recently-observed rapid variations of the line-of-sight component of the photospheric magnetic field across the flare impulsive phase, and offers solutions to some perplexing flare problems, such as the flare "number problem" of finding and resupplying sufficient electrons to explain impulsive-phase hard X-ray emission. Title: Limb Darkening at 670 nm Measured from RHESSI Authors: Zahid, H. J.; Fivian, M. D.; Hudson, H. S. Bibcode: 2008AGUSMSP21B..04Z Altcode: The Solar Aspect System (SAS) on RHESSI uses linear CCD detectors to make chord images across the solar disk. The three independent detectors record about 1,000 images per day each, and RHESSI has been observing since launch in 2002. Here we report on the these data, including models of the limb-darkening and point spread function, our methods for calibration, and a survey of the observations. We will present results on possible time and latitude dependences of the limb-darkening coefficients, and discuss sources of contamination to a 'quiet-sun' limb-darkening model. Title: Masking RHESSI radius measures against EUV brightness Authors: Hudson, H. S.; Zahid, H. J.; Fivian, M. D. Bibcode: 2008AGUSMSP21B..05H Altcode: The RHESSI radius observations, made with its Solar Aspect Sensor (SAS), are providing our best characterization of the shape of the Sun. In spite of the differential nature of our measurement, which somewhat resembles that of Dicke's original (ground-based) Solar Oblateness Telescope, we still have a sensitivity in the radius measure to the presence of faculae and other small-scale magnetic features in the solar atmosphere. We find an clear positive correlation between radius and EUV brightness, as obtained from SOHO/EIT images; in addition we clearly see the Wilson Depression as a negative correlation. The facular correlation has been successfully used to screen RHESSI data from our initial study interval in 2004. We describe the nature of the correlation and discuss its interpretation. Title: RHESSI Observations of a Large Excess Solar Oblateness and its Identification as Magnetic in Nature Authors: Fivian, M. D.; Hudson, H. S.; Zahid, H. J.; Lin, R. P. Bibcode: 2008AGUSMSP21B..06F Altcode: The RHESSI solar aspect sensors (SAS) serendipitously provide precise measures of the shape of the Sun at 670 nm. The data rate is high (more than 106 points since launch in 2002), and each point has a statistical precision of the order of 10 mas. We present reduced data from a three-month interval in 2004. The full data gives an oblateness (axisymmetric quadrupole shape term, expressed as the difference between equatorial and polar radii) of 10.74 ± 0.44 mas. For comparison, Dicke's 1970 estimate, based on uniform rotation, predicted 8.10 mas. The apparent radius strongly correlates with the EUV limb brightness. Accordingly, by restricting the data base to avoid faculae, including a component outside the active regions, we obtain a lower value for the oblateness. We find a value of 7.98 ± 0.14 mas. Based on a comparison of our results with previous balloon and satellite observations, the apparent excess oblateness may have a positive correlation with the solar cycle. Title: How Solar Flares Work Authors: Hudson, Hugh S. Bibcode: 2008AAS...212.6501H Altcode: 2008BAAS...40..256H Geophysics, radio astronomy, Japan, the ionosphere, X and gamma rays: all have contributed to my view of how solar flares and their partner coronal mass ejections (CMEs) work. A solar flare (and a CME, it turns out) has an "impulsive phase'' in which catastrophic and dominant energy release from magnetic storage takes place, resulting in particle acceleration. The impulsive restructuring of the coronal currents and fields leads directly (if still mysteriously) to the many observable phenomena, which canreach the surface of the Earth. In particular we now recognize that the term "impulsive phase'' correctly captures the basic morphology of the process: it is highly intermittent in both space and time, even though it underlies large-scale phenomena such as CME eruptions. We still cannot resolve the scales of the flare intermittency, but we can use in-situ observations of possibly analogous processes in the solar wind and magnetosphere for guidance. Title: Searching the X-ray Sun For Solar Axions Authors: Hudson, Hugh S.; Acton, L. W.; DeLuca, E. E.; Hannah, I. G.; Hurford, G. J.; Lin, R. P.; Reardon, K. P.; van Bibber, K. Bibcode: 2008AAS...212.0402H Altcode: 2008BAAS...40..193H The axion is a hypothetical weakly-interacting elementary particle. The solar core may produce a copious axion flux via the Primakoff effect. This same process can also convert a tiny fraction of the axions back into photons via interaction with the magnetic field threading the solar atmosphere. The spectral signature of the emitted X-rays is determined mainly by the temperature of the solar core, and the spatial distribution also depends strongly on the solar magnetic field in the back-conversion process. The X-ray intensity thus varies as Gaγγ(&#8747BperpdL)2, where Bperp is the perpendicular component of the chromospheric and coronal magnetic field in the appropriate zone for interaction and photon escape, and Gaγγ is the (unknown) coupling constant, dependent on the (unknown) mass of the axion. We describe observational tests suitable for solar X-ray imagers and discuss projects now under way with Yohkoh/SXT, RHESSI, and Hinode/XRT. The successful detection of axions would have implications for basic physics and for cosmological dark matter. It would also help us to characterize the ill-understood extension of the solar magnetic field into the chromosphere and corona. We specifically focus on applying the existing understanding of solar (and stellar) magnetism to this problem. Title: An intriguing solar microflare observed with RHESSI, Hinode, and TRACE Authors: Hannah, I. G.; Krucker, S.; Hudson, H. S.; Christe, S.; Lin, R. P. Bibcode: 2008A&A...481L..45H Altcode: 2007arXiv0712.0369H Aims:We investigate particle acceleration and heating in a solar microflare.
Methods: In a microflare with non-thermal emission to remarkably high energies (>50 keV), we investigate the hard X-rays with RHESSI imaging and spectroscopy and the resulting thermal emission seen in soft X-rays with Hinode/XRT and in EUV with TRACE.
Results: The non-thermal footpoints observed with RHESSI spatially and temporally match bright footpoint emission in soft X-rays and EUV. There is the possibility that the non-thermal spectrum extends down to 4 keV. The hard X-ray burst clearly does not follow the expected Neupert effect, with the time integrated hard X-rays not matching the soft X-ray time profile. So, although this is a simple microflare with good X-ray observation coverage it does not fit the standard flare model. Title: RHESSI Microflare Statistics. II. X-Ray Imaging, Spectroscopy, and Energy Distributions Authors: Hannah, I. G.; Christe, S.; Krucker, S.; Hurford, G. J.; Hudson, H. S.; Lin, R. P. Bibcode: 2008ApJ...677..704H Altcode: 2007arXiv0712.2544H We present the first statistical analysis of the thermal and nonthermal X-ray emission of all 25,705 microflares (RHESSI) observed between 2002 March and 2007 March. These events were found by searching the 6-12 keV energy range (see Paper I) and are small active region flares, from low (GOES) C class to below A class. Each microflare is automatically analyzed at the peak time of the 6-12 keV emission: the thermal source size is found by forward-fitting the complex visibilities for 4-8 keV, and the spectral parameters (temperature, emission measure, power-law index) are found by forward-fitting a thermal plus nonthermal model. The resulting wealth of information we determine about the events allows a range of the thermal and nonthermal properties to be investigated. In particular, we find that there is no correlation between the thermal loop size and the flare magnitude, indicating that microflares are not necessarily spatially small. We present the first thermal energy distribution of RHESSI flares and compare it to previous thermal energy distributions of transient events. We also present the first nonthermal power distribution of RHESSI flares and find that a few microflares have unexpectedly large nonthermal powers up to 1028 erg s-1. The total microflare nonthermal energy, however, is still small compared to that of large flares as it occurs for shorter durations. These large energies and difficulties in analyzing the steep nonthermal spectra suggest that a sharp broken power law and thick-target bremsstrahlung model may not be appropriate for microflares. Title: Impulsive Phase Flare Energy Transport by Large-Scale Alfvén Waves and the Electron Acceleration Problem Authors: Fletcher, L.; Hudson, H. S. Bibcode: 2008ApJ...675.1645F Altcode: 2007arXiv0712.3452F The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the preflare coronal magnetic field. Hard X-ray observations imply that a substantial fraction of flare energy released during the impulsive phase is converted to the kinetic energy of mildly relativistic electrons (10-100 keV). The liberation of the magnetic free energy can occur as the coronal magnetic field reconfigures and relaxes following reconnection. We investigate a scenario in which products of the reconfiguration—large-scale Alfvén wave pulses—transport the energy and the magnetic field changes rapidly through the corona to the lower atmosphere. This offers two possibilities for electron acceleration. First, in a coronal plasma with β < me/mp, the waves propagate as inertial Alfvén waves. In the presence of strong spatial gradients, these generate field-aligned electric fields that can accelerate electrons to energies on the order of 10 keV and above, including by repeated interactions between electrons and wave fronts. Second, when they reflect and mode-convert in the chromosphere, a cascade to high wavenumbers may develop. This will also accelerate electrons by turbulence, in a medium with a locally high electron number density. This concept, which bridges MHD-based and particle-based views of a flare, provides an interpretation of the recently observed rapid variations of the line-of-sight component of the photospheric magnetic field across the flare impulsive phase, and offers solutions to some perplexing flare problems, such as the flare "number problem" of finding and resupplying sufficient electrons to explain the impulsive-phase hard X-ray emission. Title: An universal flaring mechanism from the Sun to the stars? . Authors: Isola, C.; Favata, F.; Micela, G.; Hudson, H. Bibcode: 2008MmSAI..79..255I Altcode: We present an analysis of solar flares and its extension to active stars with the goal to investigate the observability of non-thermal components of stellar flares with future instrumentation, in particular with Simbol-X. We derive a scaling law for the relationship between soft (thermal) and hard (non-thermal) peak X-ray fluxes which we extrapolate from the solar case to energetic stellar flares. Title: Flare Energy and Magnetic Field Variations Authors: Hudson, H. S.; Fisher, G. H.; Welsch, B. T. Bibcode: 2008ASPC..383..221H Altcode: We describe ways in which the photospheric vector magnetic field might vary across the duration of a solar flare or CME. We also quantitatively assess the back reaction on the photosphere and solar interior by the coronal field evolution required to release flare energy. Our estimates suggest that the work done by Lorentz forces in this back reaction could supply enough energy to explain observations of flare-driven seismic waves. Title: Energetic Particles in the Quiet Corona Authors: MacKinnon, A. L.; Hudson, H. S. Bibcode: 2007AGUFMSH41A0303M Altcode: We address the fates of energetic particles in typical solar minimum coronal magnetic fields. A baseline density of such particles will be set by cosmic ray albedo neutron decay. Other, electromagnetic mechanisms may augment this. PFSS extrapolation is combined with synoptic magnetogram data to constrain the coronal fields, and particle trajectories are determined in the guiding center approximation. We discuss the extent to which solar analogues of terrestrial magnetospheric phenomena (radiation belts, ring current) may exist near solar minimum. Title: X-ray Microflares with Hinode and RHESSI. Authors: Hannah, I. G.; Christe, S.; Krucker, S.; Hudson, H.; Lin, R. P.; Deluca, E. Bibcode: 2007AGUFMSH52C..07H Altcode: We present analysis of microflares (small active region associated flares below GOES C class) using RHESSI and Hinode/XRT. RHESSI has observed well over 1,000 microflares since Hinode launched late in 2006 and of these over 150 have good Hinode/XRT coverage. We use RHESSI to obtain the temperature, emission measure and non-thermal power-law parameters from spectral fitting. We compare RHESSI and Hinode/XRT images to locate the thermal and non-thermal emissions. Taking advantage of the sensitive high-resolution capability of XRT for the softer X-rays, we investigate the resulting heating and evaporation from the accelerated electrons observed via the non-thermal emission by RHESSI. Title: Solar Shape Measurements from RHESSI: A Large Excess Oblateness Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.; Zahid, H. J. Bibcode: 2007AGUFMSH53A1076F Altcode: The Solar Aspect System of the RHESSI spacecraft scans the limb at the ~4 sec rotation period of the spacecraft, producing a large quantity of precise differential measurements of the solar radius at optical wavelengths (monochromatic at 670 nm). These data provide the most precise determinations of the oblateness prior in particular to the launch of the Picard mission in 2008. The observation of standing waves in the body of the Sun (helioseismology) provided the first direct way to study the interior of a star. The astrometric shape of the solar limb gives independent constraints on interior structures and flows; the surface rotation predicts an oblate ellipsoid with an equator-pole radius difference of some 8 mas (~0.001%). Here we report the most accurate observations to date of the solar shape, which show a much larger apparent oblateness with an equator-pole radius difference of 13.72± 0.44 mas. This new component can easily be distinguished spatially from the effects of faculae in the active latitude zones. Comparison with earlier observations suggests that this excess oblateness results from solar magnetic activity, as do the frequency variations of the helioseismic modes. Title: Impulsive Flare Energy Transport by Large-Scale Alfven Waves and the Electron Acceleration Problem Authors: Fletcher, L.; Hudson, H. S. Bibcode: 2007AGUFMSM53B1278F Altcode: In this poster, we investigate an alternative scenario for solar flare energy transport. Usually, the energy stored in the solar corona and released during a flare is thought to be transported to the lower atmosphere by electron beams. However, based on microwave observations, magnetic fields of the order of a few 100 Gauss to a kilogauss are measured in the corona above the core of an active region. This implies an Alfvén speed on the order of 0.1c, meaning that Alfvén wave pulses become plausible agents for transporting the stored energy of the flare. We investigate this scenario, and the opportunities it presents for both heating the lower chromosphere and accelerating electrons to HXR-emitting energies. Title: Investigation of Ion Acceleration in Small RHESSI Flares Authors: Shih, A. Y.; Hudson, H. S.; Smith, D. M.; Lin, R. P. Bibcode: 2007AGUFMSH44C..01S Altcode: Solar flares accelerate both ions and electrons, and the energy content in accelerated ions may be comparable to that in accelerated electrons. The signature of ion acceleration is the emission of gamma-ray lines that result from nuclear interactions in the ambient solar atmosphere, and the gamma-ray line most easily observed by the high-resolution detectors on the Reuven Ramaty High-Energy Solar Spectroscopic Imager ( RHESSI) is the 2.223~MeV neutron-capture line. Unfortunately, gamma-ray lines are often too weak to observe in all but the largest flares, but it is of interest to be able to put constraints on ion acceleration in the more frequently occurring smaller flares. We combine the spectra of many small flares to obtain limits on the average neutron-capture line flux for small flares and discuss the implications of these limits relative to previous studies of larger flares. The work at the University of California, Berkeley, was supported by NASA contract NAS 5-98033. Title: Synoptic Views of the Solar Limb: RHESSI Radius and SOHO Images Authors: Zahid, H. J.; Fivian, M. D.; Hudson, H. S. Bibcode: 2007AGUFMSH32A0792Z Altcode: The RHESSI mission includes precise astrometric measurements of the solar limb shape at optical wavelengths as a part of its aspect-determination system. These data have precisions below 1 mas and extend over the full lifetime of the mission (from February, 2002). Synoptic maps of the limb shape reveal facular regions as increases, and sunspots as decreases, in the apparent radius. We compare these signatures with synoptic SOHO images for a 3-month period in 2004. The patterns are strongly similar, but the EUV synoptic maps have contributions from features not at the exact limb, which dominates the RHESSI data. This study anticipates making use of such high-contrast coronal or chromospheric measurements to provide a masking function to screen against these features in determinations of the true solar oblateness and higher-order permanent shape features. We also explore the possibility of cross-correlating RHESSI sunspot images against those of other optical telescopes, such as MDI, as a means of calibrating the roll coordinate of the telescope pointing. Title: What is There Before a Flare? Authors: Hudson, H. S.; Hannah, I. G.; Krucker, S.; Watanabe, K. Bibcode: 2007AGUFMSH53A1055H Altcode: The physical parameters in a coronal volume prior to the occurrence of a flare are generally unknown, but may play an important role in identifying the processes involved in flaring or eruption. Now we have observations from Hinode at very high resolution that can provide the best possible values for preflare temperature and density, for example. We make use of Hinode XRT observations of the preflare magnetic structure with the same footpoint locations as a flaring loop, as identified in RHESSI images. We additionally introduce a method based on conductive equilibrium (RTV scaling) to reduce the uncertainty on estimates of physical parameters due to lack of knowledge of the detailed geometry. Preliminary results are consistent with the finding at lower resolution from Yohkoh: in a majority of cases, the preflare conditions are not observable in soft X-rays. We discuss the upper limits that result, which point to low temperatures, densities, and plasma beta values, but high Alfven speeds. We hope also to be able to extend this conclusion with EIS observations. Title: The correlation between soft and hard X-rays component in flares: from the Sun to the stars Authors: Isola, C.; Favata, F.; Micela, G.; Hudson, H. S. Bibcode: 2007A&A...472..261I Altcode: 2007arXiv0707.2322I Aims:We study the correlation between the soft (1.6-12.4 keV, mostly thermal) and the hard (20-40 and 60-80 keV, mostly non-thermal) X-ray emission in solar flares up to the most energetic events, spanning about 4 orders of magnitude in peak flux, establishing a general scaling law and extending it to the most intense stellar flaring events observed to date.
Methods: We used the data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, a NASA Small Explorer launched in February 2002. RHESSI has good spectral resolution (≃1 keV in the X-ray range) and broad energy coverage (3 keV-20 MeV), which makes it well suited to distinguish the thermal from non-thermal emission in solar flares. Our study is based on the detailed analysis of 45 flares ranging from the GOES C-class, to the strongest X-class events, using the peak photon fluxes in the GOES 1.6-12.4 keV and in two bands selected from RHESSI data, i.e. 20-40 keV and 60-80 keV.
Results: We find a significant correlation between the soft and hard peak X-ray fluxes spanning the complete sample studied. The resulting scaling law has been extrapolated to the case of the most intense stellar flares observed, comparing it with the stellar observations.
Conclusions: Our results show that an extrapolation of the scaling law derived for solar flares to the most active stellar events is compatible with the available observations of intense stellar flares in hard X-rays. Title: The Unpredictability of the Most Energetic Solar Events Authors: Hudson, Hugh S. Bibcode: 2007ApJ...663L..45H Altcode: 2007arXiv0707.1118H Observations over the past two solar cycles show a highly irregular pattern of occurrence for major solar flares, γ-ray events, and solar energetic particle (SEP) fluences. Such phenomena do not appear to follow the direct indices of solar magnetic activity, such as the sunspot number. I show that this results from the non-Poisson occurrence for the most energetic events. This Letter also points out a particularly striking example of this irregularity in a comparison between the declining phases of the recent two solar cycles (1993-1995 and 2004-2006, respectively) and traces it through the radiated energies of the flares, the associated SEP fluences, and the sunspot areas. These factors suggest that processes in the solar interior involved with the supply of magnetic flux up to the surface of the Sun have strong correlations in space and time, leading to a complex occurrence pattern that is presently unpredictable on timescales longer than active region lifetimes (weeks) and not correlated well with the solar cycle itself. Title: Solar Magnetic Activity Observed as Distortions of the Apparent Radius Authors: Zahid, H. J.; Fivian, M. D.; Hudson, H. S. Bibcode: 2007AAS...210.2223Z Altcode: 2007BAAS...39..323Z The RHESSI spacecraft carries a high-resolution X-ray and gamma-ray telescope. To create images and to align them requires a high-resolution limb aspect sensor with high time resolution. This system has a random error of less than 20 mas per point, and accumulates about 100 samples per second in normal operation. The data clearly show faculae and spots (including the Wilson depression) as they cross the limb. We characterize these signatures and discuss their significance. Title: Energy Deposition in White Light Flares with TRACE and RHESSI Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2007ASPC..368..423F Altcode: In Fletcher et al. (2007) we investigated the white light (WL) continuum during solar flares and its relationship to energy deposition by electron beams. In 9 flare events, spanning GOES classifications from C4.8 to M9.1, we have high cadence TRACE WL and RHESSI hard X-ray observations, and compare the WL radiative power output with that provided by flare electrons. Under the thick--target model assumptions, we find that the electron beam must extend down to 15--20 keV, and the energy input to the chromosphere should occur within the collisional stopping depth of these electrons - approximately 2× 10-4 g cm-2. In this short paper, we discuss some ideas on flare WL emission, summarise the results of the Fletcher et al. (2007) study and discuss their implications for chromospheric heating and white light flare emission. Title: Chromospheric Flares Authors: Hudson, H. S. Bibcode: 2007ASPC..368..365H Altcode: 2007arXiv0704.0823H In this topical review I revisit the ``chromospheric flare''. This should currently be an outdated concept, because modern data seem to rule out the possiblity of a major flare happening independently in the chromosphere alone, but the chromosphere still plays a major observational role in many ways. It is the source of the bulk of a flare's radiant energy -- in particular the visible/UV continuum radiation. It also provides tracers that guide us to the coronal source of the energy, even though we do not yet understand the propagation of the energy from its storage in the corona to its release in the chromosphere. The formation of chromospheric radiations during a flare presents several difficult and interesting physical problems. Title: Electron Acceleration By Inertial Alfven Waves In The Impulsive Phase Of A Solar Flare Authors: Hudson, Hugh S.; Fletcher, L. Bibcode: 2007AAS...210.9303H Altcode: 2007BAAS...39R.211H The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the pre-flare coronal magnetic field. Hard X-ray observations imply that a substantial fraction of flare energy released during the impulsive phase is converted to the kinetic energy of mildly relativistic electrons (10-100 keV). The liberation of the magnetic free energy occurs as the coronal magnetic field reconfigures and relaxes following reconnection. Therefore, we investigate a scenario in which the electron acceleration is caused by the agents of this reconfiguration -- the large-scale Alfven waves which propagate the field changes throughout the atmosphere. In a plasma with β < me/mp, these waves propagate as inertial Alfven waves which, in the presence of strong spatial gradients, generate field-aligned electric fields that can accelerate electrons to energies on the order of 10 keV and above. This novel view also provides an interpretation of the recently-observed rapid variations of the line-of-sight component of the photospheric magnetic field during the flare impulsive phase, and offers solutions to some perplexing flare problems, such as the flare “number problem” of finding sufficient and resupplying sufficient electrons to explain the impulsive-phase hard X-ray emission. Title: Variations Of The Optical Solar Limb As Observed By RHESSI Authors: Fivian, Martin; Hudson, H. S.; Zahid, H. J. Bibcode: 2007AAS...210.2222F Altcode: 2007BAAS...39..128F The Solar Aspect System of the RHESSI spacecraft scans the limb at the 4 sec rotation period of the spacecraft, producing a large quantity of precise differential measurements of the solar radius at optical wavelengths (monochromatic at 670 nm). These data provide the most precise determinations of the oblateness prior in particular to the launch of the Picard mission in 2008. The statistical error of an individual data point is now below 20 mas (milli arc sec) and we hope to improve on this still further. The database of about 100 samples per second now exceeds five years, following RHESSI's launch in February 2002. We present results on solar shape and its time variations, including several signatures of solar magnetic activity (spots and faculae). We report new improvements made possible by an analysis of subtle systematic errors due to temperature variations in the spacecraft and telescope and extensive modeling of the solar limb shape. Title: First Limits on the 3-200 keV X-Ray Spectrum of the Quiet Sun Using RHESSI Authors: Hannah, I. G.; Hurford, G. J.; Hudson, H. S.; Lin, R. P.; van Bibber, K. Bibcode: 2007ApJ...659L..77H Altcode: 2007astro.ph..2726H We present the first results using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) to observe solar X-ray emission not associated with active regions, sunspots, or flares (the quiet Sun). Using a newly developed chopping technique (fan-beam modulation) during seven periods of offpointing between 2005 June and 2006 October, we obtained upper limits over 3-200 keV for the quietest times when the GOES 12 1-8 Å flux fell below 10-8 W m-2. These values are smaller than previous limits in the 17-120 keV range and extend them to both lower and higher energies. The limit in 3-6 keV is consistent with a coronal temperature <=6 MK. For quiet-Sun periods when the GOES 12 1-8 Å background flux was between 10-8 and 10-7 W m-2, the RHESSI 3-6 keV flux correlates to this as a power law, with an index of 1.08+/-0.13. The power-law correlation for microflares has a steeper index of 1.29+/-0.06. We also discuss the possibility of observing quiet-Sun X-rays due to solar axions and use the RHESSI quiet-Sun limits to estimate the axion-to-photon coupling constant for two different axion emission scenarios. Title: A TRACE White Light and RHESSI Hard X-Ray Study of Flare Energetics Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2007ApJ...656.1187F Altcode: In this paper we investigate the formation of the white-light (WL) continuum during solar flares and its relationship to energy deposition by electron beams inferred from hard X-ray emission. We analyze nine flares spanning GOES classifications from C4.8 to M9.1, seven of which show clear cospatial RHESSI hard X-ray and TRACE WL footpoints. We characterize the TRACE WL/UV continuum energy under two simplifying assumptions: (1) a blackbody function, or (2) a Paschen-Balmer continuum model. These set limits on the energy in the continuum, which we compare with that provided by flare electrons under the usual collisional thick-target assumptions. We find that the power required by the white-light luminosity enhancement is comparable to the electron beam power required to produce the HXR emission only if the low-energy cutoff to the spectrum is less than 25 keV. The bulk of the energy required to power the white-light flare (WLF) therefore resides at these low energies. Since such low-energy electrons cannot penetrate deep into a collisional thick target, this implies that the continuum enhancement is due to processes occurring at moderate depths in the chromosphere. Title: A new method of observing weak extended x-ray sources with the Reuven Ramaty High-Energy Solar Spectroscopic Imager Authors: Hannah, Iain G.; Hurford, Gordon J.; Hudson, Hugh S.; Lin, Robert P. Bibcode: 2007RScI...78b4501H Altcode: 2007astro.ph..2217H We present a new method, fan-beam modulation, for observing weak extended x-ray sources with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). This space-based solar x-ray and γ-ray telescope has much greater sensitivity than previous experiments in the 3-25 keV range, but is normally not well suited to detecting extended sources since their signal is not modulated by RHESSI's rotating grids. When the spacecraft is offpointed from the target source, however, the fan-beam modulation time-modulates the transmission by shadowing resulting from exploiting the finite thickness of the grids. In this article we detail how the technique is implemented and verify its consistency with sources with clear known signals that have occurred during RHESSI offpointing: microflares and the Crab Nebula. In both cases the results are consistent with previous and complementary measurements. Preliminary work indicates that this new technique allows RHESSI to observe the integrated hard x-ray spectrum of weak extended sources on the quiet Sun. Title: Small Scale Energy Release and the Acceleration and Transport of Energetic Particles Authors: Hudson, Hugh; Vilmer, Nicole Bibcode: 2007LNP...725...81H Altcode: We report on results presented at the sessions of Working Group~1 at CESRA 2004, which covered the topic area of the title of this paper. The working-group participants are listed in the Appendix, and the topics discussed have been brought together in several general areas of focus. The emphasis on the discussion is from the point of view of radiophysics. We organize the material by presenting new constraints imposed by the recent high-energy and radio observations. We note though that multi-wavelength knowledge is generally vital in understanding all of the phenomena involved. The new constraints include exciting new millimeter-wave discoveries, among others. We then place these observations into the framework of our knowledge of the acceleration and propagation of high-energy particles, and of their radio emission mechanisms. The RHESSI1 results are the most distinctive in this time frame, and they have made possible several new advances. Title: Review of Selected RHESSI Solar Results Authors: Dennis, Brian R.; Hudson, Hugh S.; Krucker, Säm Bibcode: 2007LNP...725...33D Altcode: We review selected science results from RHESSI solar observations made since launch on 5 February 2002. A brief summary of the instrumentation is given followed by a sampling of the major science results obtained from the soft X-ray, hard X-ray, and gamma-ray energy domains. The thermal continuum measurements and detection of Fe-line features are discussed as they relate to parameters of the thermal flare plasma for several events, including microflares. Observations of X-ray looptop, and rising above-the-loop sources are discussed as they relate to standard models of eruptive events and the existence of a current sheet between the two. Hard X-ray spectra and images of footpoints and coronal sources are presented, showing how they can be used to separate thermal and nonthermal sources and determine the magnetic reconnection rate. Gamma-ray line images and spectra are presented as they relate to determining the location, spectra, and angular distribution of the accelerated ions and the temperature of the chromospheric target material. Finally, we discuss the overall energy budget for two of the larger events seen with RHESSI. Title: RHESSI observations of the solar radius Authors: Fivian, M. D.; Hudson, H. S.; Zahid, H. J. Bibcode: 2006AGUFMSH23B0368F Altcode: The Solar Aspect System of the RHESSI spacecraft scans the limb at the ~4 sec rotation period of the spacecraft, producing a large quantity of precise differential measurements of the solar radius at optical wavelengths (monochromatic at 670~nm). These data provide the most precise determinations of the oblateness prior in particular to the launch of the Picard mission in 2008. The statistical error of an individual data point is now below 20 mas (milli arc sec) and we hope to improve on this still further. Data accumulate at the rate of about 100 points per sec, beginning at launch in February 2002. We present results on solar shape and its time variations, including several signatures of solar magnetic activity (spots and faculae). We report new improvements made possible by an analysis of subtle systematic errors due to temperature variations in the spacecraft and telescope. Title: High Energy Solar Physics from Lunar-Based Observatories Authors: Emslie, G.; Dennis, B. R.; Holman, G. D.; Hudson, H. S.; Krucker, S.; Lin, R. P.; Murphy, R.; Ryan, J. M.; Share, G. H. Bibcode: 2006AGUFMSM52A..05E Altcode: High-energy and optical solar observations from large telescopes will greatly improve our understanding of the physical processes responsible for particle acceleration in the Sun and in other astrophysical sources. In addition, such observations will allow us to identify the conditions preceding solar eruptive events that are potentially hazardous to astronauts and equipment on the Moon and on interplanetary flights, and to power transmission and communications on the Earth. The moon provides an exceptionally large and stable platform on which to position instruments that can be used to observe the Sun (and other astrophysical sources) at photon energies from < 1 keV to > 100 MeV. Uninterrupted measurements over extended periods of time (1/2 lunar day or 14 days, which is also the duration of the East-West passage of an active region on the Sun) would be possible, and the gradual rotation rate of the Moon also allows horizon occultation measurements (at a drift rate ~ 0.5 arc seconds/second) to be made. Title: Coronal particle trapping revisited Authors: Hudson, H. S.; MacKinnon, A.; De Rosa, M. Bibcode: 2006AGUFMSH54A..07H Altcode: We re-examine the idea of long-term particle storage in the solar corona in the context of modern PFSS (potential-field source surface) magnetic models. As pointed out by H. Elliot in 1964 and others since then, such particles could be energetically important, at the level of some large fraction of the magnetic energy density B2/8π. We estimate the distribution and time scales of particle trapping by using representative PFSS coronal models from the Schrijver-De Rosa SolarSoft code. As the coronal field simplifies during solar minimum, it approaches axisymmetry and thus contains volumes inaccessible to charged particles under the guiding-center approximation. We conclude that time scales can be sufficiently long, so long in fact that the azimuthal drift time scale (third adiabatic invariant of guiding-center motion), for the large-scale dipolar configuration characteristic of solar minimum, can exceed one solar cycle. We discuss the possible sources of trapped particles, starting with the basic CRAND (cosmic-ray albedo neutron decay) mechanism, and relate their X-ray and γ-ray signatures to future observational capabilities including the Sentinels spacecraft. Title: Occulted Hard X-ray Flare Observations with Sentinels Authors: Krucker, S.; Hannah, I.; Hudson, H. S.; Hurford, G.; Lin, R. P. Bibcode: 2006AGUFMSH53C..07K Altcode: Multi-spacecraft hard X-ray observations provided by the Sentinel mission will give different view angles of solar flares, including the possibility of partially occulted observations. Occultation of the main flare emission by the solar limb allows us to study fainter coronal emission that otherwise would be hidden by the limited dynamic range of the observations. Furthermore, it will allow to measure directivity and reconstruct the 3 dimensional geometry of X-ray sources. We use statistical results from RHESSI observations to derive the probability of occulted flare observations with Sentinels. A statistical study of 19 giant flares with fast CMES (v>1500 km/s) occurring behind the solar limb is presented. We find that all events occurring 45 degrees or less behing the limb (corresponding to an occultation height h < 0.4~Rsun) show X-ray enhancements, while the three events occurring more than 50 degrees (h>1~Rsun) behind the limb do not. This result indicates that on the average, two Sentinels will see the total flare, while one spacecraft will see the flare partially occulted. The most prominent event in our RHESSI survey will be discussed in detail. It has an occultation high of 0.3 solar radius, but still shows emission up to 80 keV. The observed hard X-ray source is extended, with a size of about 200", and moves upwards with a velocity of ~1000 km/s. Title: Gamma Rays and the Evolving, Compact Structures of the 2003 October 28 X17 Flare Authors: Schrijver, C. J.; Hudson, H. S.; Murphy, R. J.; Share, G. H.; Tarbell, T. D. Bibcode: 2006ApJ...650.1184S Altcode: The X17 flare on 2003 October 28 was observed by high-resolution imaging or spectroscopic instruments on CORONAS, GOES, INTEGRAL, RHESSI, SOHO, and TRACE. These spacecraft observed the temporal evolution of the γ-ray positron-annihilation and nuclear de-excitation line spectra, imaged the hard X-ray bremsstrahlung and EUV and UV emission, and measured the surface magnetic field and subphotospheric pressure perturbations. In the usual pattern, the onset of the flare is dominated by particle acceleration and interaction, and by the filling of coronal magnetic structures with hot plasma. The associated positron-annihilation signatures early in the impulsive phase from 11:06 to 11:16 UT have a line-broadening temperature characteristic of a few hundred thousand kelvins. The most intense precipitation sites within the extended flare ribbons are very compact, with diameters of less than 1400 km, and a 195 Å TRACE intensity that can exceed 7500 times the quiescent active-region value. These regions appear to move at speeds of up to 60 km s-1. The associated rapidly evolving, compact perturbations of the photosphere below these sites excite acoustic pulses that propagate into the solar interior. Less intense precipitation sites typically persist for several minutes behind the advancing flare ribbons. After ~1 ks, the flare enters a second phase, dominated by coronal plasma cooling and downflows and by annihilation-line radiation characteristic of a photospheric environment. We point out (1) that these detailed observations underscore that flare models need to explicitly incorporate the multitude of successively excited environments whose evolving signals differ at least in their temporal offsets and energy budgets, if not also in the exciting particle populations and penetration depths, and (2) that the spectral signatures of the positron annihilation do not fit conventional model assumptions. Title: The Optical Solar Limb As Observed By RHESSI Authors: Fivian, Martin D.; Hudson, H. S.; Zahid, H. J. Bibcode: 2006SPD....37.3005F Altcode: 2006BAAS...38Q.257F The RHESSI limb sensors routinely make precise differentialobservations of the solar limb shape and its time variations. Therandom error for a single point is less than 40 milli-arcsecondsand in regular operation we obtain about 100 samples per second.The database now exceeds four years, following RHESSI's launch inFebruary 2002. Because of RHESSI's rotation (nominally at 4 secperiod) the basic measurement resembles that pioneered by Dicke's"Oblateness Telescope". The data clearly show several solar features,including the oblateness as well as sunspots, faculae, and p-modes.We give a general description and report preliminary values for theoblateness. We find the oblateness measurement to depend sensitivelyon the level of magnetic activity, and give a representative value of9.72 +/- 0.19 MAS (statistical error only) for a 60-hour intervalstarting 10:15 UT 14 August 2004. Title: Limb Distortions Related To Solar Magnetic Activity Authors: Hudson, Hugh S.; Fivian, M. D.; Wilson, B. M.; Zahid, H. J. Bibcode: 2006SPD....37.0714H Altcode: 2006BAAS...38..230H We use the RHESSI aspect-sensor observations of the solar limb tocharacterize the effects of solar magnetic activity on the apparentradius as a function of position angle. Sunspots at the exact limbresult in a depression (the Wilson effect), which we observe directly.The results are interpreted as lower limits of a few hundred km,as limited by the umbral diameter, based on simple geometricalmodels. Faculae produce clear signatures with a presently ambiguousinterpretation because of lack of knowledge of the limb-darkeningfunction and fine structure. We display the limb data as stackplotsof radius as a function of position angle and compare these with othermeasures of solar activity. Title: Searching For Quiet Sun X-ray Emission With RHESSI Authors: Hannah, Iain G.; Hurford, G. J.; Hudson, H. S.; Lin, R. P. Bibcode: 2006SPD....37.1402H Altcode: 2006BAAS...38R.243H In this work we are searching for X-ray emission from the Sun that is not associated with active regions. RHESSI has greater sensitivity in the 5-25 keV range than previous missions, but since the quiet Sun sources may be well-dispersed spatially across the disk, RHESSI's normal imaging technique is not well suited to the task. An alternate observational technique involves pointing RHESSI slightly away from the Sun. The resulting time-dependent transmission of the signal through RHESSI's rotating modulation collimators allows the quiet Sun signal to be distinguishable from the background. However, a large sample of data is required in this mode so that we can determine whether the observations are noise limited or quiet Sun signal. This novel offpointing technique has been used 4 times since July 2005, during which the GOES background level dropped below A1. From the data acquired we have been able to obtain limits to the quiet Sun X-ray flux between 3-100 keV. At the lowest energies, below 7 keV, the limits correlate with the observed GOES flux, suggesting a signal. We will discuss the consequences of interpreting this emission as originating from either a thermal or non-thermal electron population, and the relevance to coronal heating. Title: Coronal Mass Ejections: Overview of Observations Authors: Hudson, H. S.; Bougeret, J. -L.; Burkepile, J. Bibcode: 2006SSRv..123...13H Altcode: 2006SSRv..tmp...63H We survey the subject of Coronal Mass Ejections (CMEs), emphasizing knowledge available prior to about 2003, as a synopsis of the phenomenology and its interpretation. Title: Coronal Observations of CMEs. Report of Working Group A Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.; Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.; Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.; Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J. Bibcode: 2006SSRv..123..127S Altcode: 2006SSRv..tmp...58S CMEs have been observed for over 30 years with a wide variety of instruments. It is now possible to derive detailed and quantitative information on CME morphology, velocity, acceleration and mass. Flares associated with CMEs are observed in X-rays, and several different radio signatures are also seen. Optical and UV spectra of CMEs both on the disk and at the limb provide velocities along the line of sight and diagnostics for temperature, density and composition. From the vast quantity of data we attempt to synthesize the current state of knowledge of the properties of CMEs, along with some specific observed characteristics that illuminate the physical processes occurring during CME eruption. These include the common three-part structures of CMEs, which is generally attributed to compressed material at the leading edge, a low-density magnetic bubble and dense prominence gas. Signatures of shock waves are seen, but the location of these shocks relative to the other structures and the occurrence rate at the heights where Solar Energetic Particles are produced remains controversial. The relationships among CMEs, Moreton waves, EIT waves, and EUV dimming are also cloudy. The close connection between CMEs and flares suggests that magnetic reconnection plays an important role in CME eruption and evolution. We discuss the evidence for reconnection in current sheets from white-light, X-ray, radio and UV observations. Finally, we summarize the requirements for future instrumentation that might answer the outstanding questions and the opportunities that new space-based and ground-based observatories will provide in the future. Title: Multi-Wavelength Observations of CMEs and Associated Phenomena. Report of Working Group F Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.; Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen, A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic, Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.; Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.; Sheeley, N. R.; Marqué, C. Bibcode: 2006SSRv..123..341P Altcode: 2006SSRv..tmp...60P This chapter reviews how our knowledge of CMEs and CME-associated phenomena has been improved, since the launch of the SOHO mission, thanks to multi-wavelength analysis. The combination of data obtained from space-based experiments and ground based instruments allows us to follow the space-time development of an event from the bottom of the corona to large distances in the interplanetary medium. Since CMEs originate in the low solar corona, understanding the physical processes that generate them is strongly dependant on coordinated multi-wavelength observations. CMEs display a large diversity in morphology and kinematic properties, but there is presently no statistical evidence that those properties may serve to group them into different classes. When a CME takes place, the coronal magnetic field undergoes restructuring. Much of the current research is focused on understanding how the corona sustains the stresses that allow the magnetic energy to build up and how, later on, this magnetic energy is released during eruptive flares and CMEs. Multi-wavelength observations have confirmed that reconnection plays a key role during the development of CMEs. Frequently, CMEs display a rather simple shape, exhibiting a well known three-part structure (bright leading edge, dark cavity and bright knot). These types of events have led to the proposal of the ‘`standard model’' of the development of a CME, a model which predicts the formation of current sheets. A few recent coronal observations provide some evidence for such sheets. Other more complex events correspond to multiple eruptions taking place on a time scale much shorter than the cadence of coronagraph instruments. They are often associated with large-scale dimming and coronal waves. The exact nature of these waves and the physical link between these different manifestations are not yet elucidated. We also discuss what kind of shocks are produced during a flare or a CME. Several questions remain unanswered. What is the nature of the shocks in the corona (blast-wave or piston-driven?) How they are related to Moreton waves seen in Hα? How they are related to interplanetary shocks? The last section discusses the origin of energetic electrons detected in the corona and in the interplanetary medium. “Complex type III-like events,”which are detected at hectometric wavelengths, high in the corona, and are associated with CMEs, appear to originate from electrons that have been accelerated lower in the corona and not at the bow shock of CMEs. Similarly, impulsive energetic electrons observed in the interplanetary medium are not the exclusive result of electron acceleration at the bow shocks of CMEs; rather they have a coronal origin. Title: White-Light Flares: A TRACE/RHESSI Overview Authors: Hudson, H. S.; Wolfson, C. J.; Metcalf, T. R. Bibcode: 2006SoPh..234...79H Altcode: The Transition Region and Coronal Explorer (TRACE) instrument includes a "white light" imaging capability with novel characteristics. Many flares with such white-light emission have been detected, and this paper provides an introductory overview of these data. These observations have 0.5″ pixel size and use the full broad-band response of the CCD sensor; the images are not compromised by ground-based seeing and have excellent pointing stability as well as high time resolution. The spectral response of the TRACE white-light passband extends into the UV, so these data capture, for the first time in images, the main radiative energy of a flare. This initial survey is based on a sample of flares observed at high time resolution for which the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) had complete data coverage, a total of 11 events up to the end of 2004. We characterize these events in terms of source morphology and contrast against the photosphere. We confirm the strong association of the TRACE white-light emissions - which include UV as well as visual wavelengths - with hard X-ray sources observed by RHESSI. The images show fine structure at the TRACE resolution limit, and often show this fine structure to be extended over large areas rather than just in simple footpoint sources. The white-light emission shows strong intermittency both in space and in time and commonly contains features unresolved at the TRACE resolution. We detect white-light continuum emission in flares as weak as GOES C1.6. limited by photon statistics and background solar fluctuations, and support the conclusion of Neidig (1989) that white-light continuum occurs in essentially all flares. Title: Preflare Nonthermal Emission Observed in Microwaves and Hard X-Rays Authors: Asai, Ayumi; Nakajima, Hiroshi; Shimojo, Masumi; White, Stephen M.; Hudson, Hugh S.; Lin, Robert P. Bibcode: 2006PASJ...58L...1A Altcode: We present a detailed examination on nonthermal emissions during the preflare phase of the X4.8 flare that occurred on 2002 July 23. The microwave (17GHz and 34GHz) data obtained with the Nobeyama Radioheliograph at Nobeyama Solar Radio Observatory and the hard X-ray data taken with the Reuven Ramaty High Energy Solar Spectroscopic Imager obviously showed nonthermal features in the preflare phase. We also found a faint ejection associated with the flare in the EUV images taken with the Transition Region and Coronal Explorer. We discuss the temporal and spatial features of the nonthermal emissions in the preflare phase, and their relation with the ejection. Title: Multi-Wavelength Observations of CMEs and Associated Phenomena Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.; Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen, A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic, Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.; Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.; Sheeley, N. R.; Marqué, C. Bibcode: 2006cme..book..341P Altcode: This chapter reviews how our knowledge of CMEs and CME-associated phenomena has been improved, since the launch of the SOHO mission, thanks to multi-wavelength analysis. The combination of data obtained from space-based experiments and ground based instruments allows us to follow the space-time development of an event from the bottom of the corona to large distances in the interplanetary medium. Since CMEs originate in the low solar corona, understanding the physical processes that generate them is strongly dependant on coordinated multi-wavelength observations. CMEs display a large diversity in morphology and kinematic properties, but there is presently no statistical evidence that those properties may serve to group them into different classes. When a CME takes place, the coronal magnetic field undergoes restructuring. Much of the current research is focused on understanding how the corona sustains the stresses that allow the magnetic energy to build up and how, later on, this magnetic energy is released during eruptive flares and CMEs. Multiwavelength observations have confirmed that reconnection plays a key role during the development of CMEs. Frequently, CMEs display a rather simple shape, exhibiting a well known three-part structure (bright leading edge, dark cavity and bright knot). These types of events have led to the proposal of the "standard model" of the development of a CME, a model which predicts the formation current sheets. A few recent coronal observations provide some evidence for such sheets. Other more complex events correspond to multiple eruptions taking place on a time scale much shorter than the cadence of coronagraph instruments. They are often associated with large-scale dimming and coronal waves. The exact nature of these waves and the physical link between these different manifestations are not yet elucidated. We also discuss what kind of shocks are produced during a flare or a CME. Several questions remain unanswered. What is the nature of the shocks in the corona (blast-wave or piston-driven?) How they are related to Moreton waves seen in Hα? How they are related to interplanetary shocks? The last section discusses the origin of energetic electrons detected in the corona and in the interplanetary medium. "Complex type III-like events," which are detected at hectometric wavelengths, high in the corona, and are associated with CMEs, appear to originate from electrons that have been accelerated lower in the corona and not at the bow shock of CMEs. Similarly, impulsive energetic electrons observed in the interplanetary medium are not the exclusive result of electron acceleration at the bow shocks of CMEs; rather they have a coronal origin. Title: Coronal Observations of CMEs Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.; Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.; Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.; Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J. Bibcode: 2006cme..book..127S Altcode: CMEs have been observed for over 30 years with a wide variety of instruments. It is now possible to derive detailed and quantitative information on CME morphology, velocity, acceleration and mass. Flares associated with CMEs are observed in X-rays, and several different radio signatures are also seen. Optical and UV spectra of CMEs both on the disk and at the limb provide velocities along the line of sight and diagnostics for temperature, density and composition. From the vast quantity of data we attempt to synthesize the current state of knowledge of the properties of CMEs, along with some specific observed characteristics that illuminate the physical processes occurring during CME eruption. These include the common three-part structures of CMEs, which is generally attributed to compressed material at the leading edge, a low-density magnetic bubble and dense prominence gas. Signatures of shock waves are seen, but the location of these shocks relative to the other structures and the occurrence rate at the heights where Solar Energetic Particles are produced remains controversial. The relationships among CMEs, Moreton waves, EIT waves, and EUV dimming are also cloudy. The close connection between CMEs and flares suggests that magnetic reconnection plays an important role in CME eruption and evolution. We discuss the evidence for reconnection in current sheets from white-light, X-ray, radio and UV observations. Finally, we summarize the requirements for future instrumentation that might answer the outstanding questions and the opportunities that new space-based and ground-based observatories will provide in the future. Title: Preflare Nonthermal Emission Observed in Microwaves and Hard X-Rays Authors: Asau, A.; Nakajima, H.; Shimojo, M.; White, S. M.; Hudson, H. S. Bibcode: 2006apri.meet...46A Altcode: No abstract at ADS Title: Coronal Mass Ejections: Overview of Observations Authors: Hudson, H. S.; Bougeret, J. -L.; Burkepile, J. Bibcode: 2006cme..book...13H Altcode: We survey the subject of Coronal Mass Ejections (CMEs), emphasizing knowledge available prior to about 2003, as a synopsis of the phenomenology and its interpretation. Title: Variations of Solar Radius: Observations with Rhessi Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P. Bibcode: 2005ESASP.600E...4F Altcode: 2005dysu.confE...4F; 2005ESPM...11....4F No abstract at ADS Title: Late-phase hard X-ray emission from flares Authors: Hudson, H. S.; Krucker, S. Bibcode: 2005AGUFMSH13A0290H Altcode: In a few major flare events observed by RHESSI, we see hard X-ray signatures long after (tens of minutes to more than one hour) the impulsive-phase onset. A prototype for such phenomena was the flare of March 31, 1969 (Frost and Dennis, 1971). Recent RHESSI examples include January 19, 2005, a GOES X1.5 event with gradual variations, and September 7, 2005 an X17 event with more impulsive variability. RHESSI images show footpoint emissions in both cases; the spectra are hard and become harder with time as in the Frost-Dennis event. The existence of impulsive variability and footpoint emission allows us to discuss trapping and injection. We discuss the morphology of these events, including other examples such as April 21, 2002 (X1.5). Title: RHESSI observations of solar radius Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P. Bibcode: 2005AGUFMSH11A0241F Altcode: The Solar Aspect System (SAS) of the rotating (at 15 rpm) RHESSI spacecraft has three simple solar optical telescopes. Each of these measures the position of the limb by sampling the solar chord profile with a linear CCD using a narrow bandwidth filter at 670 nm. With a resolution of each CCD of 1.7 arcsec/pixel, the precision of each of the 6 limb positions is observed to be better than 50 mas using 4 pixels at each limb. Since the launch of RHESSI early 2002, data have been obtained at a rate of at least 100 samples/sec. This has provided a database of currently 7~×~109 individual measurements. The main function of SAS is to determine the RHESSI pointing relative to Sun center. Using these data we observe the solar oblateness and see signatures of magnetic activity (spots and faculae). Title: A Search for Hard X-Ray Emission from the Quiet Spotless Sun Authors: Hannah, I. G.; Hurford, G. J.; Hudson, H. S. Bibcode: 2005AGUFMSH11A0242H Altcode: RHESSI observations often show hard x-ray emission from non-flaring active regions. The objective of this work is to search for hard x-ray emission that is not associated with active regions. There are a number of potential sources for such emission, some of which are relevant to coronal heating. With its shutters open RHESSI has greater sensitivity in the 5-25 keV range than previous missions. It uses a set of 9 rotating modulation collimators to image sources with size scales from 2 arcseconds to 3 arcminutes. However, since the quiet Sun sources may be well-dispersed spatially across the disk, RHESSI's normal imaging technique is not well suited to the task. An alternate observational technique involves pointing RHESSI slightly away from the Sun. Then the time-dependent transmission of the thick individual grids (whose field of view is 1 degree) chops the integrated solar signal at ~0.5 Hz to provide effective background suppression. This paper describes the technique and initial results from a period of offpointing observations (19-July-05 to 25-July-05) acquired when the GOES background level dropped to below A2 and there were no active regions visible on the disk. Title: TRACE white light and RHESSI hard X-rays Authors: Fletcher, L.; Allred, J.; Hannah, I.; Hudson, H.; Metcalf, T. Bibcode: 2005AGUFMSH13A0286F Altcode: We study the energetics of RHESSI and TRACE observations for a sample of 11 solar flares well observed in the TRACE ``white light" channel. In general, the data show excellent correlations between hard X-rays and white light. We discuss the energetics of this relationship based upon simple models for the visible/UV continuum, namely (i) a non-thermal approximation (Balmer and Paschen continuum), (ii) a thermal approximation (blackbody), and (iii) model spectra derived from radiation-hydrodynamic modeling. We relate the white-light energy and the low-energy cutoff energy of the primary electron spectrum required for energetic equivalence. This comparison will be made in the context of the flare WL morphology and in comparison with RHESSI hard X-ray images. Title: Refinements to flare energy estimates: A followup to ``Energy partition in two solar flare/CME events'' by A. G. Emslie et al. Authors: Emslie, A. G.; Dennis, B. R.; Holman, G. D.; Hudson, H. S. Bibcode: 2005JGRA..11011103E Altcode: Emslie et al. (2004) reported estimates of the energy in the different flare and coronal mass ejection (CME) components of two major solar events with unprecedented observational coverage, one on 21 April 2002 and the other on 23 July 2002. On the basis of these estimates, it appeared that the summed energy content of the different flare components was significantly lower than the total energy of the CME, leading them to reach the "cautious" conclusion that "in both events the coronal mass ejection has the dominant component of the released energy," amounting to approximately 30% of the available magnetic energy. In this note we present revised estimates of the flare thermal energies in the two events and also add a consideration of the total radiant energy of the events obtained by scaling the measured soft X-ray luminosity based on Solar Radiation and Climate Experiment (SORCE) total solar irradiance measurements for the 28 October 2003 event. Recognizing that many of these energetic components are interrelated, we also take care to distinguish between "primary" components of energy (e.g., the magnetic field), "intermediate" components (e.g., accelerated particles and thermal plasma), and "final" components (e.g., kinetic energy of ejecta, radiant energy in various wave bands). We note that since the values of these components are not all independent, careful tallying is necessary to arrive at an overall energy budget for the event. The best estimates for the energies of the various components still show that the CME contains the greatest fraction of the released energy in both events. However, given the large uncertainties in the energies of the different flare components and the higher estimates of radiant energy obtained by scaling from the SORCE measurements, the results are also consistent with the flare and CME energies in both events being comparable, with a common value of ∼1032 ergs. Title: Coronal shock waves observed in images Authors: Hudson, H. S. Bibcode: 2005AIPC..781..336H Altcode: The large-scale coronal shock waves observed from radio type II bursts and from Moreton waves have proven surprisingly difficult to detect in coronal images. I review the evidence for such waves in radio, optical, EUV, and soft X-ray images. The data generally support the conclusion that the metric type II bursts can be identified with weak fast-mode shock waves launched at the impulsive phase of the associated flares. Other coronal waves, well seen by EIT, are more closely related to CMEs. Title: Outburst Evolution and Pulse Period of 1A 0535+262 Authors: Smith, D. M.; Hazelton, B.; Coburn, W.; Boggs, S. E.; Fivian, M.; Hurford, G. J.; Hudson, H. S.; Grefenstette, B.; Gilmore, R. Bibcode: 2005ATel..557....1S Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft was pointed to the accreting pulsar 1A 0535+262 after the discovery by Tueller et al. (ATEL #504) that it was in a major outburst for the first time since 1994. Operations to point away from the Sun began on 3 June and the pulsar gradually moved into our field of view over the next week (RHESSI repointing is very slow since the spacecraft spins near 15 rpm and has only magnetic torquers to repoint it). Title: Energetics of RHESSI X-Class Flares Authors: Dennis, B. R.; Holman, G. D.; Haga, L.; Hudson, H. S. Bibcode: 2005AGUSMSP21A..01D Altcode: The thermal and nonthermal energies of several X-class flares seen with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) will be presented. The same techniques described by Emslie et al. (JGR, 109, A10104, 2004) are used to take the RHESSI imaging spectroscopic observations and compute the energies in the thermal plasma and in the nonthermal electrons as a function of time throughout the flares. Radiative and conductive cooling rates are estimated and total thermal and nonthermal energies are computed for each flare. Typically, the energy in nonthermal electrons integrated up to the time of peak soft X-ray emission is equal to or exceeds the energy in the thermal plasma at that time. This suggests that energy must have been converted into a form not visible with RHESSI and that the total energy released by the flares may be significantly greater than the sum of energies calculated from the RHESSI observations alone. This conclusion is supported by the high radiative energy seen with SORCE during the impulsive phase of the 28 October 2003 flare. The peak increase in total solar irradiance of 270 mW m-2 measured with SORCE was over two orders of magnitude higher than the peak soft X-ray flux seen with GOES or RHESSI. The implications of this new observation as compared to the energetics derived from the X-ray observations of that flare will be discussed along with the energetics analysis of most of the other X-class flares in October/November 2003. Title: RHESSI Observations of Relativistic Electron Precipitation at L=1.0-2.6 Following Large Solar Energetic Particle Events Authors: Goodhue, A. C.; Lin, R. P.; Smith, D. M.; Hudson, H. S. Bibcode: 2005AGUSMSM43B..06G Altcode: The past eighteen months have produced the largest solar energetic particle events in the current solar cycle. We present a preliminary investigation of the effects of large solar storms on trapped electron populations in the magnetosphere using observations from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) following the 28 October 2003, 27 July 2004, and 10 November 2004 storms. Although RHESSI's germanium detectors were designed to measure solar hard x-rays and gamma rays, they are large enough (7.1 cm diameter x 8.5 cm) to stop penetrating electrons and measure their energy up to 17 MeV, as well as detect bremsstrahlung radiation from the precipitating electrons. We first obtain raw counts spectra, which are a superposition of bremsstrahlung and direct electron detection signatures. We then apply a GEANT simulation model of the spacecraft and instrument in order to infer the original spectra of incoming electrons from the measured spectra. After the October 2003 storm, RHESSI detected electrons up to ~5 MeV that were transported into the slot region (below L=2.6), and subsequently began to diffuse away. In the month following the storm, the spectrum became softer as high-energy electrons receded from the L=1.0-2.6 region. By the end of November 2003, the highest-energy electrons detected dropped to ~2 MeV. At the beginning of June 2004, six months after the original storm, the relativistic electrons disappeared completely. A similar spectral analysis is done for the July 2004 and November 2004 storms. Title: RHESSI soft X-ray imaging spectroscopy of a flare Authors: Hudson, H.; Caspi, A.; Dennis, B.; Phillips, K. Bibcode: 2005AGUSMSH31A..03H Altcode: We apply RHESSI imaging spectroscopy to a well-observed solar flare on 26~April~2003. This GOES M2-class flare (N20W69) exercised all three of the RHESSI shutter states, and was simultaneously observed by the RESIK high-resolution X-ray spectrometer on board the CORONAS-F spacecraft (Dennis et al., 2005). It thus provides an excellent opportunity to study the behavior of the high-throughput RHESSI observations of the Fe~emission-line complex at ~6.7~keV. The equivalent width of this feature has a unique dependence on the assumed isothermal temperature of the source (Phillips, 2004). Comparing this feature-derived temperature between the onset and decay phases of the event, we note a discrepancy between it and the temperature derived directly from the continuum. We analyze the causes of this discrepancy in terms of image morphology, non-isothermality, non-equilibrium excitation, and instrument properties. Title: Initial localization and kinematic characteristics of the structural components of a coronal mass ejection Authors: Uralov, A. M.; Grechnev, V. V.; Hudson, H. S. Bibcode: 2005JGRA..110.5104U Altcode: 2005JGRA..11005104U The leading component of a coronal mass ejection (CME), its observed frontal structure (FS), has been detected close to the solar surface in a few near-the-limb events only. Thus far, no manifestations of such a frontal structure have been reported in reasonable proximity to a preeruptive filament located away from the solar limb. Thus the identification of the FS with preevent coronal structures remains unclear. We propose a method to estimate the parameters of the initial volume of a CME, using comparative measurements of the spatial locations of the erupting filament and FS with a self-similar solution of the magnetohydrodynamic equations describing the expansion of the CME. We develop this method by analyzing observations of a large eruptive filament on the solar disk on 4 September 2000, using data acquired with the Solar Heliospheric Observatory (SOHO), Large-Angle Spectroscopic Coronagraph (LASCO), and EUV Imaging Telescope (EIT) instruments and the Siberian Solar Radio Telescope. We show that if a magnetic structure corresponding to the FS prior to the filament eruption does exist, then it is localized at a relatively low height (here, about 100-150 Mm above the filament). At the initial stage of the motion, the shape of the hot FS approximately reproduces the configuration of the cool eruptive filament. In addition, we conclude that the coronal dimming observed in this event could be also due to CME-caused suppression of the heating and/or mass supply of the dimmed structures rather than due to their opening only. We also obtain in a simple way an exact self-similar solution of MHD equations in a form suitable for analyses of experimental data. Title: An exceptionally bright flare from SGR 1806-20 and the origins of short-duration γ-ray bursts Authors: Hurley, K.; Boggs, S. E.; Smith, D. M.; Duncan, R. C.; Lin, R.; Zoglauer, A.; Krucker, S.; Hurford, G.; Hudson, H.; Wigger, C.; Hajdas, W.; Thompson, C.; Mitrofanov, I.; Sanin, A.; Boynton, W.; Fellows, C.; von Kienlin, A.; Lichti, G.; Rau, A.; Cline, T. Bibcode: 2005Natur.434.1098H Altcode: 2005astro.ph..2329H Soft-γ-ray repeaters (SGRs) are galactic X-ray stars that emit numerous short-duration (about 0.1s) bursts of hard X-rays during sporadic active periods. They are thought to be magnetars: strongly magnetized neutron stars with emissions powered by the dissipation of magnetic energy. Here we report the detection of a long (380s) giant flare from SGR 1806-20, which was much more luminous than any previous transient event observed in our Galaxy. (In the first 0.2s, the flare released as much energy as the Sun radiates in a quarter of a million years.) Its power can be explained by a catastrophic instability involving global crust failure and magnetic reconnection on a magnetar, with possible large-scale untwisting of magnetic field lines outside the star. From a great distance this event would appear to be a short-duration, hard-spectrum cosmic γ-ray burst. At least a significant fraction of the mysterious short-duration γ-ray bursts may therefore come from extragalactic magnetars. Title: Rhessi Microflare Statistics Authors: Hannah, I. G.; Christe, S.; Krucker, S.; Hudson, H. S.; Fletcher, L.; Hendry, M. A. Bibcode: 2004ESASP.575..259H Altcode: 2004soho...15..259H No abstract at ADS Title: RHESSI: First Results Authors: Hudson, H. S. Bibcode: 2004ASPC..325..335H Altcode: The RHESSI observations consist of imaging spectroscopy in the γ-ray, hard X-ray, and ``firm X-ray'' (3--20 keV) bands. These data are now the most extensive and capable solar high-energy observations at high spectral and spatial resolution. The low-energy hard X-ray spectrum bridges the thermal and non-thermal ranges of solar electron distributions in flares systematically for the first time. In this presentation I survey some results from the first 18 months of observation, including findings on image and spectral morphology (both hard X-ray and γ-ray), and the behavior of microflares in the 3--20 keV band. Title: Overview of Early Rhessi Results Authors: Krucker, S.; Hudson, H. S. Bibcode: 2004ESASP.575..247K Altcode: 2004soho...15..247K No abstract at ADS Title: The hard X-ray spectral structure of flare ribbons Authors: Hudson, H.; Fletcher, L.; Krucker, S.; Pollock, J. Bibcode: 2004AGUFMSH24A..02H Altcode: We examine the spatial distribution of hard X-ray spectral parameters in flares exhibiting the classic two-ribbon structure using RHESSI observations. The flares studied include July~15 and July~17, 2002, and October~29, 2003. We confirm the existence of a tendency for the localization of the hard X-ray sources into dominant bright ``footpoint'' regions which do not show ribbon structure as extensive as that seen in Hα or UV~images. As a part of the study we characterize the ribbons photometrically in the EUV as observed by TRACE, confirming earlier results that find complicated relationships between EUV and hard X-rays. We seek an empirical explanation for the restricted hard X-ray footpoints in terms of a spatial analog of the well-known ``soft-hard-soft'' morphology: the regions of weaker hard X-ray emission correspond to steeper X-ray energy spectra and hence to softer electron precipitation spectra. This relationship may be as predicted by the 1D radiation hydrodynamics models of flaring loops. Title: The Halloween 2003 Storm's Effect on Trapped Electron Populations Authors: Goodhue, A. C.; Hudson, M. K.; Hudson, H. S.; McNab, M. C. Bibcode: 2004AGUFMSM41A1110G Altcode: We have investigated the effects of the October and November 2003 solar energetic particle (SEP) events on trapped electron populations using data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) in conjunction with the findings from the Solar, Anomalous, Magnetospheric Particle Explorer (SAMPEX). Immediately after the Halloween storm, RHESSI, saw an order of magnitude increase in the electron population in its low particle energy bin (less than 600 KeV); the population lingered for more than thirty days at L=1.9-2.2. The increase is caused by an inward transportation of trapped magnetospheric electrons from higher L values, and the decay is produced by pitch angle diffusion. Similar effects were seen by SAMPEX. However, the appearance of SEP's, whose energies are much greater and range up to 20 MeV, does not occur until months later, as these electrons are not pitch angle diffused as quickly as the lower energy population. Title: Coronal Loop Oscillations and Flare Shock Waves Authors: Hudson, H. S.; Warmuth, A. Bibcode: 2004ApJ...614L..85H Altcode: A statistical analysis of coronal loop oscillations observed by the Transition Region and Coronal Explorer (TRACE) shows that 12 of 28 cases were associated with metric type II bursts. The timing is consistent with the idea that in many cases the loop oscillations result from the passage of a large-scale wave disturbance originating in a flare in the nearby active region. The GOES classifications for these flares range from C4.2 to X20. Typically, the oscillating structures are not disrupted, implying that the disturbance has passed through the medium, which has returned to an equilibrium near that seen prior to the event. This is consistent with the Uchida interpretation of the disturbance as a weak fast-mode blast wave (i.e., a simple wave at a low Alfvénic Mach number) propagating in the ambient corona. We note that all 12 of the associated events were also associated with coronal mass ejections (CMEs) and conclude that the CME eruptions in these cases corresponded to only partial openings of the active-region magnetic fields. Title: Overview of Solar Flares Authors: Hudson, Hugh; Fletcher, Lyndsay; Khan, Josef I.; Kosugi, Takeo Bibcode: 2004ASSL..314..153H Altcode: This chapter reviews the physics of solar flares, with special emphasis on the past decade. During this decade first Yohkoh and then TRACE have drastically improved our observational capabilities for flares, with contributions also from the essentially non-flare instrumentation on SOHO and of course the ground-based observatories. In this review we assess how these new observations have changed our understanding of the basic physics of flares and consider the implications of these results for future observations with FASR. The discussion emphasizes flaring loops, flare ejecta, particle acceleration, and microflares. Title: Analysis of the impulsive phase of a solar flare at submillimeter wavelengths Authors: Raulin, Jean Pierre; Makhmutov, Vladimir S.; Kaufmann, Pierre; Pacini, Alessandra Abe; Lüthi, Thomas; Hudson, Hugh S.; Gary, Dale E. Bibcode: 2004SoPh..223..181R Altcode: We present a report on the strong X5.3 solar flare which occurred on 25 August 2001, producing high-level γ-ray activity, nuclear lines and a dramatic long-duration white-light continuum. The bulk of millimeter radio fluxes reached a peak of ∼100 000 solar flux units at 89.4 GHz, and a few thousands of solar flux units were detected in the submillimeter range during the impulsive phase. In this paper we focus on and discuss (i) the implications inferred from high frequency radio observations during the impulsive phase; (ii) the dynamics of the low corona active region during the impulsive phase. In particular we found that 4-5 × 1036 accelerated (>20 keV) electrons s−1 radiating in a 1000-1100 G region, are needed to explain the millimeter to submillimeter-wave emissions. We present evidence that the magnetic field in the active region was very dynamic, and that strong non-thermal processes were triggered by the appearance of new, compact, low-lying (few thousand kilometers) loop systems, suggesting the acceleration site(s) were also located in the low solar atmosphere. Title: Total Solar Irradiance Variation During Rapid Sunspot Growth Authors: Zahid, H. Jabran; Hudson, Hugh S.; Fröhlich, Claus Bibcode: 2004SoPh..222....1Z Altcode: Large sunspot areas correspond to dips in the total solar irradiance (TSI), a phenomenon associated with the local suppression of convective energy transport in the spot region. This results in a strong correlation between sunspot area and TSI. During the growth phase of a sunspot other physics may affect this correlation; if the physical growth of the sunspot resulted in surface flows affecting the temperature, for example, we might expect to see an anomalous variation in TSI. In this paper we study NOAA active region 8179, in which large sunspots suddenly appeared near disk center, at a time (March 1998) when few competing sunspots or plage regions were present on the visible hemisphere. We find that the area/TSI correlation does not significantly differ from the expected pattern of correlation, a result consistent with a large thermal conductivity in solar convection zone. In addition we have searched for a smaller-scale effect by analyzing white-light images from MDI (the Michelson Doppler Imager) on SOHO. A representative upper-limit energy consistent with the images is on the order of 3×1031 ergs, assuming the time scale of the actual spot area growth. This is of the same order of magnitude as the buoyant energy of the spot emergence even if it is shallow. We suggest that detailed image analyses of sunspot growth may therefore show `transient bright rings' at a detectable level. Title: The hard X-ray spectral structure of flare ribbons Authors: Fletcher, L.; Hudson, H. S.; Krucker, S.; Pollock, J. A. Bibcode: 2004AAS...204.5403F Altcode: 2004BAAS...36..758F We examine the spatial distribution of hard X-ray spectral parameters in flares exhibiting the classic two-ribbon structure using RHESSI observations. The flares studied include July 15 and July 17, 2002, and October 29, 2003. We confirm the existence of a tendency for the localization of the hard X-ray sources into dominant bright ``footpoint'' regions which do not show ribbon structure as extensive as that seen in Hα or UV images. As a part of the study we characterize the ribbons photometrically in the EUV as observed by TRACE, confirming earlier results that find complicated relationships between EUV and hard X-rays. We seek an empirical explanation for the restricted hard X-ray footpoints in terms of a spatial analog of the well-known ``soft-hard-soft'' morphology: the regions of weaker hard X-ray emission correspond to steeper X-ray energy spectra and hence to softer electron precipitation spectra. This relationship may be as predicted by the 1D radiation hydrodynamics models of flaring loops. Title: Remarkable Low Temperature Emission of the 4 November 2003 Limb Flare Authors: Leibacher, J. W.; Harvey, J. W.; Kopp, G.; Hudson, H.; GONG Team Bibcode: 2004AAS...204.0213L Altcode: 2004BAAS...36..669L Strong (> 1.5 times normal intensity) continuum and photospheric line emission of the 4 November 2003 X28 flare was recorded simultaneously by three widely separated GONG instruments. Emission was seen from on the disk to > 20" above the limb for nearly one hour, likely making this event the longest duration white light flare observed to date. GONG observations are one-minute duration integrations of intensity averaged across a Lyot filter bandpass of about 90 pm FWHM centered on the Ni I line at 676.8 nm with 2.5" instrument pixel size. Spatial resolution is limited by diffraction and seeing to greater than 5". Additional measurements include the Doppler shift and strength of the spectrum line. These latter measurements indicate that continuum and line emission contributed about equally to the observed intensity signal. Light curves and images of the flare show a notable two-kernel disk event starting at about 19:33 UTC followed by a much stronger event that peaked at about 19:44. Rare, white-light prominences were visible above the limb after 19:34. Comparison of total solar irradiance measurements from the TIM instrument on board the SORCE spacecraft with full-disk integrated GONG intensities shows the global five-minute oscillation and the white light flare. The latter is much weaker in the GONG data, suggesting that most of the TIM flare signal arises from other, most likely shorter, wavelengths.

This work utilizes data obtained by the Global Oscillation Network Group (GONG) Program, managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. SORCE is supported by NASA NAS5-97045 Title: Nonlinear Force Free Field Models of AR 0486 Authors: McTiernan, J. M.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2004AAS...204.0204M Altcode: 2004BAAS...36Q.668M In this work we present nonlinear force free field (NLFFF) extrapolations from vector magnetograms of AR0486 taken before, during and after the X11 flare of 29 October 2003. The extrapolations will be used to interpret the different source patterns and motions seen in RHESSI and TRACE images of the flare. In particular we will examine the footpoint motions observed in the flare by RHESSI, and also the expansion and other changes in the high temperature thermal sources visible in low energy (less than 20 keV) RHESSI images and TRACE 195 A images.

This work is funded by NASA contract NAS 5-98033. Title: Variations of Solar Radius: Observations with RHESSI Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P. Bibcode: 2004AAS...204.3719F Altcode: 2004BAAS...36..712F The Solar Aspect System (SAS) of the rotating (at 15 rpm) RHESSI spacecraft has three subsystems. Each of these measures the position of the limb by sampling the full solar chord profile with a linear CCD using a narrow-band filter at 670 nm. With a CCD pixel size of 1.7 arcsec, the accuracy of each of the six limb positions is theoretically better than 50 mas using four pixels at each limb. Since the launch of RHESSI early 2002, solar limbs have been sampled with at least 100 Hz. That provides database currently containin 4 × 109 single radius measurements. The main function of SAS is to determine the RHESSI pointing relative to Sun center. The observed precision of this determination has a typical instantaneous (16 Hz) value of the order of 50 mas (rms). We present initial RHESSI observations of the radius, including signatures of oblateness and of magnetic activity (spots and faculae). Title: Coronal Loop Oscillations and Flare Shock Waves Authors: Hudson, H. S.; Warmuth, A. Bibcode: 2004AAS...204.5411H Altcode: 2004BAAS...36..759H A preliminary statistical analysis of coronal loop oscillations observed by TRACE shows that 12/31 cases were associated with metric type II bursts. The timing is consistent with the idea that the loop oscillations represent the passage of a disturbance launched by an eruption in an active region; the GOES classifications for these flares range from C4.2 to X20. Typically the oscillating structures are not disrupted, implying that the disturbance has passed through the medium. This is consistent with the Uchida interpretation of the disturbance as a weak fast-mode blast wave propagating in the ambient corona. We note that all twelve of the events were also associated with CMEs. Title: Photospheric field variations during the Oct. 28 and 29 solar events Authors: Li, Y.; Welsch, B.; Fisher, G.; Luhmann, J.; Hudson, H. Bibcode: 2004AGUSMSH51A..01L Altcode: Solar photospheric magnetic field variations around the Oct. 28th and 29th, 2003 large flares and CMEs are investigated. The essential data for the study are the high cadence MDI full disk line of sight magnetograms. Abrupt and permanent changes of the field strength occur at the times of both X-flares (Oct. 28, ~10:30UT and Oct. 29, 20:45UT). GONG+ magnetograms are used to provide a possible confirmation of the observed field changes. Velocity fields in the CME related active region and their evolution around the time of the events are obtained using the Local Correlation Tracking (LCT) technique on the MDI magnetograms. Some Mees/IVM vector magnetic field data analysis results may also likely be available.This level of observation of major active region fields spawning superstorm conditions is unprecedented. Title: Soft X-ray analysis of a loop flare on the Sun Authors: Khan, J. I.; Hudson, H. S.; Mouradian, Z. Bibcode: 2004A&A...416..323K Altcode: We present the results of an analysis of soft X-ray images for a solar flare which occurred on 1992 July 11. This flare, as seen in Yohkoh Soft X-ray Telescope (SXT) images was of comparatively simple geometry, consisting of two bright footpoints early in the flare with a bright loop seen later in the flare. We examine how closely this flare compares with the supposed paradigm of a confined simple-loop flare. Closer examination of the SXT images reveals that the flare structure consisted of at least two adjacent loops, one much fainter than the other. We examine the brighter of the two soft X-ray loops. The SXT images reveal an apparent slow, northward motion of this loop (roughly transverse to its major axis). Examination of derived emission measure and temperature images also indicate an apparent northward motion. In addition, we find an increase in the cross-sectional width at the top of the loop with time. Emission measure maps derived from the SXT images also indicates an apparent broadening of the loop-top region. We infer that the apparent northward motion and the apparent broadening of the soft X-ray emission can be explained in a reconnection scenario where successive magnetic field structures do not lie in a plane but are tilted to the south of the line of sight but with successively brightening loops oriented at less tilted angles. Hα images for this flare reveal an evolution from a few brilliant points to a short two-ribbon-like appearance. Comparison of the SXT images with the Hα images shows that the Hα patches are aligned with the footpoints of the soft X-ray loops, suggesting the presence of a small arcade structure. There is no clear evidence for an eruptive signature in our observations nor in reports from other observations. The lack of an eruptive signature could suggest that the flare may have been a confined simple-loop flare, but this is not compelling due to a gap in the coronal observations prior to and early in the event. Analysis of our observations indicate that the flare exhibited characteristics suggesting that it may be better understood as a mini-arcade flare. These results casts doubt on the validity of the supposed paradigm of a confined simple-loop flare, at least for this flare. They indicate that even an apparently simple-loop flare may be considered to be a variety of arcade flare. We also find an effect which, to our knowledge, has not been reported before: the hot flaring regions later become cooler than the surrounding quiescent corona. That is, the flare loops do not evolve into bright active region loops, but into cooler loops. This may indicate an increase in the efficiency of the cooling mechanism or a transformed equilibrium state within the flaring loops. Title: Homologous large-scale activity in solar eruptive events of 24-26 November 2000 Authors: Chertok, I. M.; Grechnev, V. V.; Hudson, H. S.; Nitta, N. V. Bibcode: 2004JGRA..109.2112C Altcode: We study large-scale activity on the solar disk associated with a 24-26 November 2000 series of six recurrent major flares and "halo" coronal mass ejections (CMEs). The analysis is based mainly on the SOHO/EIT data, particularly by using properly rotated difference full-disk images with 12-min intervals at 195 Å as well as with 6-hour intervals at 171, 195, 284, and 304 Å. We demonstrate that these eruptive events were homologous not only by their flare and CME characteristics, as [2001] showed, but also in terms of their large-scale CME-associated manifestations in the EUV corona. These include long and narrow channeled dimmings, some transequatorial; anisotropic coronal waves, propagating in a restricted angular sector; and additional quasi-stationary emitting fronts. As a whole, in all of these six events, the homologous CME-associated disturbances covered a considerable portion of the solar disk. The homology tendency appears to be due to significant disturbance, partial eruption, and relatively fast restoration of the same large-scale structures involved in the repeating CME events. We briefly discuss the implications of the analysis in connection with the nature of coronal equilibrium. Title: Solar Energy Flux Variations Authors: Hudson, Hugh S. Bibcode: 2004GMS...141...85H Altcode: The chapters in this section of the monograph deal with the basic raw material of solar variability, namely the measurements themselves. With complete characterization of the spectral components of the irradiance, one might imagine an easy task in putting them all together to determine the energy flux from the Sun. As the length and depth of these chapters shows, however, the simple characterization of a spectral irradiance at the accuracies permitted by the available technology already becomes a sophisticated business. The scope of the problem becomes apparent when one realizes that the observed bolometric variability of the total irradiance does not exceed fractions of a percent, and has well-measured components with amplitudes below one part per million (Fröhlich)! Title: Total solar irradiance variation during rapid sunspot growth Authors: Zahid, H. J.; Hudson, H. S.; Frohlich, C. Bibcode: 2004cosp...35.2769Z Altcode: 2004cosp.meet.2769Z Large sunspot areas correspond to dips in the total solar irradiance (TSI), a phenomenon associated with the local suppression of convective energy transport in the spot region. During the growth of a sunspot other physics might conceivably affect the resulting correlation between sunspot area and TSI. We study NOAA active region 8179, in which large sunspots suddenly appeared near disk center, at a time (March 1998) when few competing sunspots or plage regions were present on the visible hemisphere. We find that the area/TSI correlation does not significantly differ from the expected pattern of correlation, a result consistent with the expected thermal conductivity of the solar convection zone. In addition we have searched for a smaller-scale effect by analyzing white-light images from MDI (the Michelson Doppler Imager) on SOHO. A representative upper-limit energy consistent with the images is on the order of 3 X 1031 ergs, assuming the time scale of the actual spot area growth. This is the same order of magnitude as the buoyant energy of the spot emergence even if it is shallow. We suggest that detailed image analyses of sunspot growth may therefore show "transient bright rings" at a detectable level. Title: Solar Variability and its Effects on Climate. Geophysical Monograph 141 Authors: Pap, Judit M.; Fox, Peter; Frohlich, Claus; Hudson, Hugh S.; Kuhn, Jeffrey; McCormack, John; North, Gerald; Sprigg, William; Wu, S. T. Bibcode: 2004GMS...141.....P Altcode: This monograph presents a state-of-the-art description of the most recent results on solar variability and its possible influence on the Earth's climate and atmosphere. Our primary goal in doing so is to review solar energy flux variations (both electromagnetic and particle) and understand their relations to solar magnetic field changes and global effects, their impact on different atmospheric layers, and—as a collaboration of scientists working on solar-terrestrial physics—to note unresolved questions on an important interdisciplinary area.

One of the highest-level questions facing science today is whether the Earth's atmosphere and climate system changes in a way that we can understand and predict. The Earth's climate is the result of a complex and incompletely understood system of external inputs and interacting parts. Climate change can occur on various time scales as a consequence of natural variability—including solar variability—or anthropogenic causes, or both. The Sun's variability in the form of sunspots and related magnetic activity has been the subject of careful study ever since the earliest telescopic observations. High precision photometric observations of solar-type stars clearly show that year-to-year brightness variations connected with magnetic activity are a widespread phenomenon among such stars. As our nearest star, the Sun is the only star where we can observe and identify a variety of structures and processes which lead to variations in the solar energy output, in both radiative and particle fluxes. Studying event tiny changes in solar energy flux variations may teach us about internal processes taking place in the Sun's convective zone and below. Title: Preface Authors: Pap, Judit M.; Fox, Peter; Fröhlich, Claus; Hudson, Hugh S.; Kuhn, Jeffrey; McCormack, John; North, Gerald; Sprigg, William; Wu, S. T. Bibcode: 2004GMS...141D...7P Altcode: This monograph presents a state-of-the-art description of the most recent results on solar variability and its possible influence on the Earth's climate and atmosphere. Our primary goal in doing so is to review solar energy flux variations (both electromagnetic and particle) and understand their relations to solar magnetic field changes and global effects, their impact on different atmospheric layers, and—as a collaboration of scientists working on solar-terrestrial physics—to note unresolved questions on an important interdisciplinary area.

One of the highest-level questions facing science today is whether the Earth's atmosphere and climate system changes in a way that we can understand and predict. The Earth's climate is the result of a complex and incompletely understood system of external inputs and interacting parts. Climate change can occur on various time scales as a consequence of natural variability—including solar variability—or anthropogenic causes, or both. The Sun's variability in the form of sunspots and related magnetic activity has been the subject of careful study ever since the earliest telescopic observations. High precision photometric observations of solar-type stars clearly show that year-to-year brightness variations connected with magnetic activity are a widespread phenomenon among such stars. As our nearest star, the Sun is the only star where we can observe and identify a variety of structures and processes which lead to variations in the solar energy output, in both radiative and particle fluxes. Studying even tiny changes in solar energy flux variations may teach us about internal processes taking place in the Sun's convective zone and below. Title: A scenario for three "homologous" CMEs from AR 8038 in May 1997 Authors: Li, Y.; Luhmann, J. G.; Hudson, H. Bibcode: 2003AGUFMSH42B0522L Altcode: The halo CME event of May 12, 1997 has been exceptionally well-documented, in part because of the Sun-to-Earth coverage afforded by the combination of comprehensive SOHO imaging, ground-based observations, and WIND spacecraft in-situ measurements, but also because it exhibited what some regard as a classic set of event signatures with exceptional clarity: C-class flare, Halo CME, coronal double dimming, EIT wave, type II radio burst, energetic particles, magnetic cloud, and moderate geomagnetic storm. One problem with interpreting halo CMEs is the head-on view. While the STEREO twin-spacecraft mission is aimed at resolving the question of what a halo event looks like from the side, and visualizations based on numerical simulations suggest a number of possible interpretations, nature has provided an opportunity in the form of nearly homologous events initiated at the same active region AR 8038. One is at the east limb on May 5, another near the west limb on May 16. Being shortly after the solar minimum, the Sun is very quiet and the coronal magnetic field is simple. During the entire disk passage of AR8038, it is the only active region present. We take advantage of this period to make a first comparison of the three similar events at different view angles. We also model the geometry of the CME in 3D, and discuss improved physical parameters such as the speed. Title: Variations of Solar Radius Observed with RHESSI Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P. Bibcode: 2003AGUFMSH32A1103F Altcode: The Solar Aspect System (SAS) of the rotating (at 15 rpm) RHESSI spacecraft has three subsystems. Each of these measures the position of the limb by sampling the full solar chord profile with a linear CCD using a narrow bandwidth filter at 670 nm. With a resolution of each CCD of 1.7 arcsec/pixel, the accuracy of each of the 6 limb positions is theoretically better than 50 mas using 4 pixels at each limb. Since the launch of RHESSI early 2002, solar limbs are sampled with at least 100 Hz. That provides a database of currently 4 x 109 single radius measurements. The main function of SAS is to determine the RHESSI pointing relative to Sun center. The observed precision of this determination has a typical instantaneous (16 Hz) value of about 200 mas (rms). We show and discuss first results of variations of solar radius observed with RHESSI. Title: Radiative Hydrodynamic Models of Solar White Light Flares Authors: Allred, J. C.; Hawley, S. L.; Abbett, W. P.; Fisher, G. H.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2003AGUFMSH22A0175A Altcode: We report on theoretical radiative hydrodynamic simulations of solar white light flares. The solar atmosphere is modeled in detail from the transition region to the photosphere. The coronal pressure and X-ray backheating are included self-consistently. Flare heating is assumed to be from an electron beam which is modeled for several white light flares using data from RHESSI, TRACE and Yohkoh. We also investigate the possibility that the 511 keV line width is produced from a significant column depth of atmosphere at transition region temperatures. We compare our new solar flare models to previous results, and to models of M dwarf stellar flares. Title: Solar irradiance variation during rapid sunspot growth Authors: Zahid, H.; Frohlich, C.; Hudson, H. Bibcode: 2003AGUFMSH32A1102Z Altcode: Large sunspot areas correspond to dips in the total solar irradiance (TSI), a phenomenon associated with the local suppression of convective energy transport in the spot region. Observations show a strong correlation between spot area and TSI. During the growth phase of a sunspot, though, other physics might conceivably affect this correlation. In this study we analyze the growth phase of NOAA active region 8179, in which large sunspots suddenly appeared near disk center at a time when few competing sunspots or plage regions were present on the visible hemisphere. The spot area of AR 8179 increased by a factor of two in less than 12 hours on March 13, 1998. The study makes use of the continuum images from SOHO/MDI and total-irradiance data from SOHO/VIRGO. We find that the area/TSI correlation does not significantly differ from the expected pattern of correlation, a result consistent with high thermal conductivity even at small depths below the photosphere. We have also searched for localized irradiance variations (transient bright rings) associated with the period of rapid growth. We discuss the significance of this lack of irradiance signatures of spot formation. Title: Gamma-ray flare occurrence patterns Authors: Hudson, H. S.; Lin, R. P.; Smith, D. M. Bibcode: 2003AGUFMSH22A0168H Altcode: As of 2003 September 4, RHESSI (the Reuven Ramaty High-Energy Spectroscopic Imager) had obtained coverage for the entire GOES duration (>95%) for 98 M- and 6 X-class flares, and for each of these we estimate the ratio of the 2.223 MeV line fluence to the GOES soft X-ray fluence. All are upper limits except for one M-class event and one X-class event. The GOES fluence is known to scale well with total flare energy. The statistics of these observations, considering as well the solar gamma-ray line observations from other spacecraft plus the statistics of proton events in the heliosphere, are not consistent with the hypothesis that ion acceleration scales proportionally with total flare energy. Title: Homologous large-scale activity in solar eruptive events of November 24-26, 2000 Authors: Chertok, I. M.; Grechnev, V. V.; Hudson, H. S.; Nitta, N. V. Bibcode: 2003AGUFMSH22A0180C Altcode: We study large-scale activity on the solar disk associated with a November 24-26, 2000 series of six recurrent major flares and ``halo'' coronal mass ejections (CMEs). The analysis is based mainly on the SOHO/EIT data, particularly by using properly rotated difference full-disk images with 12-min intervals at 195Å~ as well as with 6-hour intervals at 171, 195, 284, and 304Å. We demonstrate that these eruptive events were homologous not only by their flare and CME characteristics, as Nitta and Hudson [2001] showed, but also in terms of their large-scale CME-associated manifestations in the EUV corona. These include long and narrow channeled dimmings, some transequatorial; anisotropic coronal waves, propagating in a restricted angular sector; and additional quasi-stationary emitting fronts. As a whole, in all of these six events, the homologous CME-associated disturbances covered a considerable portion of the solar disk. The homology tendency appears to be due to significant disturbance, partial eruption and relatively fast restoration of the same large-scale structures involved in the repeating CME events. We briefly discuss the implications of the analysis in connection with the coronal equilibrium as indicated by recent TRACE observations of oscillating loop systems. Title: A catalogue of white-light flares observed by Yohkoh Authors: Matthews, S. A.; van Driel-Gesztelyi, L.; Hudson, H. S.; Nitta, N. V. Bibcode: 2003A&A...409.1107M Altcode: The aspect camera of the Soft X-ray Telescope (SXT) on Yohkoh provided the first systematic survey of white-light flares from an observatory in space. The observations were made in the Fraunhofer g-band at a pixel size of 2.46 arcsec and a typical sample interval on the order of ten seconds. A total of 28 flares with clear white-light signatures were detected, corresponding to GOES events down to the C7.8 level in one case. Above the X-class threshold, all 5 events observed by SXT were observed in white light, and the maximum average contrast observed was 30% relative to the pre-flare continuum brightness of the flare location. We have made comprehensive comparisons of Yohkoh soft X-ray and hard X-ray data for this list of flares. In addition we compare the properties of the WLF sample to a sample of 31 flares that showed no white-light emission. These comparisons show that while white-light continuum emission has a strong association with hard X-ray emission it is also strongly related to coronal overpressure, as determined from the soft X-ray spectrum, indicating a component with a thermal, rather than non-thermal origin.

Appendices are only available in electronic form at http://www.edpsciences.org Title: High-Resolution Observation of the Solar Positron-Electron Annihilation Line Authors: Share, Gerald H.; Murphy, Ronald J.; Skibo, Jeffrey G.; Smith, David M.; Hudson, Hugh S.; Lin, Robert P.; Shih, Albert Y.; Dennis, Brian R.; Schwartz, Richard A.; Kozlovsky, Benzion Bibcode: 2003ApJ...595L..85S Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed the positron-electron annihilation line at 511 keV produced during the 2002 July 23 solar flare. The shape of the line is consistent with annihilation in two vastly different solar environments. It can be produced by formation of positronium by charge exchange in flight with hydrogen in a quiet solar atmosphere at a temperature of ~6000 K. However, the measured upper limit to the 3γ/2γ ratio (ratio of annihilation photons in the positronium continuum to the number in the line) is only marginally consistent with what is calculated for this environment. The annihilation line can also be fitted by a thermal Gaussian having a width of 8.1+/-1.1 keV (FWHM), indicating temperatures of ~(4-7)×105 K. The measured 3γ/2γ ratio does not constrain the density when the annihilation takes place in such an ionized medium, although the density must be high enough to slow down the positrons. This would require the formation of a substantial mass of atmosphere at transition-region temperatures during the flare. Title: RHESSI Observations of Particle Acceleration and Energy Release in an Intense Solar Gamma-Ray Line Flare Authors: Lin, R. P.; Krucker, S.; Hurford, G. J.; Smith, D. M.; Hudson, H. S.; Holman, G. D.; Schwartz, R. A.; Dennis, B. R.; Share, G. H.; Murphy, R. J.; Emslie, A. G.; Johns-Krull, C.; Vilmer, N. Bibcode: 2003ApJ...595L..69L Altcode: We summarize Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) hard X-ray (HXR) and γ-ray imaging and spectroscopy observations of the intense (X4.8) γ-ray line flare of 2002 July 23. In the initial rise, a new type of coronal HXR source dominates that has a steep double-power-law X-ray spectrum and no evidence of thermal emission above 10 keV, indicating substantial electron acceleration to tens of keV early in the flare. In the subsequent impulsive phase, three footpoint sources with much flatter double-power-law HXR spectra appear, together with a coronal superhot (T~40 MK) thermal source. The north footpoint and the coronal source both move systematically to the north-northeast at speeds up to ~50 km s-1. This footpoint's HXR flux varies approximately with its speed, consistent with magnetic reconnection models, provided the rate of electron acceleration varies with the reconnection rate. The other footpoints show similar temporal variations but do not move systematically, contrary to simple reconnection models. The γ-ray line and continuum emissions show that ions and electrons are accelerated to tens of MeV during the impulsive phase. The prompt de-excitation γ-ray lines of Fe, Mg, Si, Ne, C, and O-resolved here for the first time-show mass-dependent redshifts of 0.1%-0.8%, implying a downward motion of accelerated protons and α-particles along magnetic field lines that are tilted toward the Earth by ~40°. For the first time, the positron annihilation line is resolved, and the detailed high-resolution measurements are obtained for the neutron-capture line. The first ever solar γ-ray line and continuum imaging shows that the source locations for the relativistic electron bremsstrahlung overlap the 50-100 keV HXR sources, implying that electrons of all energies are accelerated in the same region. The centroid of the ion-produced 2.223 MeV neutron-capture line emission, however, is located ~20''+/-6'' away, implying that the acceleration and/or propagation of the ions must differ from that of the electrons. Assuming that Coulomb collisions dominate the energetic electron and ion energy losses (thick target), we estimate that a minimum of ~2×1031 ergs is released in accelerated >~20 keV electrons during the rise phase, with ~1031 ergs in ions above 2.5 MeV nucleon-1 and about the same in electrons above 30 keV released in the impulsive phase. Much more energy could be in accelerated particles if their spectra extend to lower energies. Title: TRACE and Yohkoh Observations of a White-Light Flare Authors: Metcalf, Thomas R.; Alexander, David; Hudson, Hugh S.; Longcope, Dana W. Bibcode: 2003ApJ...595..483M Altcode: We present observations of a large solar white-light flare observed on 2001 August 25, using data from the Transition Region and Coronal Explorer (TRACE) white-light channel and Yohkoh/HXT. These emissions are consistent with the classic type I white-light flare mechanism, and we find that the enhanced white-light emission observed by TRACE originates in the chromosphere and temperature minimum regions via nonequilibrium hydrogen ionization induced by direct collisions with the electron beam and by back-warming of the lower atmosphere. The three flare kernels observed in hard X-rays and white light are spatially associated with magnetic separatrices, and one of the kernels is observed to move along a magnetic separatrix at 400 km s-1. This is evidence in favor of particle acceleration models, which energize the electrons via magnetic reconnection at magnetic separators. Title: Narrow coronal holes in Yohkoh soft X-ray images and the slow solar wind Authors: Arge, C. N.; Harvey, K. L.; Hudson, H. S.; Kahler, S. W. Bibcode: 2003AIPC..679..202A Altcode: Soft X-ray images of the solar corona sometimes show narrow dark features not obviously present in HE I 10830Å images. We term these ``narrow coronal holes'' (NCHs). A prototype for this type of structure crossed solar central meridian on October 29, 2001. Standard source-surface models showed open magnetic field lines in this feature, tending to confirm its identification as a coronal hole. The magnetic field in this example is relatively strong (above 100 G in the low-resolution Kitt Peak magnetograms), and the boundaries of the open-field domain fall within the unipolar area as expected. We have surveyed the Yohkoh SXT data for other examples of this phenomenon, and have found several candidates. From observations of the associated solar wind, and from modeling, we find these regions to be sources of slow solar wind. Title: Yutaka Uchida Authors: Hudson, Hugh S.; Kosugi, Takeo Bibcode: 2003PhT....56i..78H Altcode: No abstract at ADS Title: Detecção da fase impulsiva de uma explosão solar gigante até 405 GHz Authors: Raulin, J. -P.; Makhmutov, V.; Kaufmann, P.; Pacini, A. A.; Luethi, T.; Hudson, H. S.; Gary, D. E.; Yoshimori, M. Bibcode: 2003BASBr..23..179R Altcode: A explosão ocorrida no dia 25/08/2001 foi uma das mais intensas do presente ciclo solar em ondas de rádio de altas frequências. Foram medidas em ondas milimétricas e submilimétricas, aproximadamente, 105 e vários milhares de unidades de fluxo solar, respectivamente. Apresentamos um estudo deste evento em múltiplas frequências, desde microondas (1GHz), até ondas submilimétricas (405 GHz) detectadas pelo Telescópio Solar para ondas Submilimétricas (SST). Esta base de dados foi complementada utilizando-se o experimento Yohkoh, incluindo a emissão em raios-X duros e raios-g (até 100 MeV), e imagens em raios-X moles da região ativa envolvida. Enfocamos e discutimos principalmente os seguintes aspectos da fase impulsiva do evento: (i) as implicações deduzidas do espectro eletromagnético, obtido pela primeira vez até 405 GHz; (ii) a dinâmica da região ativa. Os resultados mostram que para explicar o espectro rádio observado, são necessários entre 3.5×1037 e 1.5×1039 elétrons acelerados acima de 20 keV em uma região de campo magnético entre 300 e 800 Gauss. A estimativa do fluxo de fótons que seria produzido por estes elétrons, mostra que grande parte deles não precipitou na baixa atmosfera. A evolução temporal da emissão em raios-X moles revela que a configuração magnética da região ativa foi muito dinâmica durante a fase impulsiva da explosão. Em particular, mostramos que a produção dos elétrons altamente energéticos foi iniciada junto com a aparição, na baixa coroa solar, de um novo sistema compacto de estruturas magnéticas. Este fato sugere que os locais de aceleração estão localizados na baixa atmosfera do Sol, como resultado da interação entre o novo sistema compacto e o campo magnético ambiente da região ativa. Title: RHESSI Observation of the Solar Annihilation Line Authors: Share, Gerald H.; Murphy, R. J.; Skibo, Jeffrey G.; Smith, David M.; Hudson, Hugh S.; Lin, Robert P.; Shih, Albert Y.; Dennis, Brian R.; Schwartz, Richard A. Bibcode: 2003ICRC....6.3199S Altcode: 2003ICRC...28.3199S; 2003ICRC....6.3199M RHESSI has observed the positron-electron annihilation line at 511 keV produced during the 2002 July 23 solar flare. The shape of the line is consistent with formation of positronium by charge exchange in flight with hydrogen in a quiet solar atmosphere at a temperature of ∼ 6000 K. However, the measured upper limit to the 3γ /2γ ratio (ratio of annihilation photons in the positronium continuum to the number in the line) is only marginally consistent with what is calculated for this environment. The annihilation line can also be fit by a thermal Gaussian having a width of 8.1 ± 1.1 keV (FWHM), indicating temperatures of ∼ 4 - 7 × 105 K. This would require the formation of a substantial mass of atmosphere at transition-region temperatures during the flare. Title: A Study of Extremely Low Density Solar Wind Detected on Three Successive Solar Rotations Authors: Wang, L. -H.; Lin, R. P.; Larson, D. E.; Hudson, H. S. Bibcode: 2003SPD....34.0611W Altcode: 2003BAAS...35..819W Three periods of extremely low density solar wind (n < 1 cm-3) were detected by the WIND 3D plasma and Energetic Particle (3DP) instrument on successive solar rotations, July 4, July 31 and Aug 28,1999. The measurements show that the halo electron pitch angle distributions were far narrower than those in normal solar wind. With the increasing distance from the Sun, adiabactic focusing narrows the pitch angle distribution, while Coulomb collisions, which are proportional to the electron number density, widens the pitch angle distribution.In the low density events, fewer Coulomb collisions lead to a more anisotropic halo electron distribution. The observations are compared to numerical results from a 1-dimensional simulation based on the Fokker-Planck equation. We also examine solar observations from SOHO and YOHKOH to search for the source of the low density solar wind. Title: High-Resolution Observation of the Solar Positron-Electron Annihilation Line Authors: Share, G. H.; Murphy, R. J.; Skibo, J. G.; Smith, D. M.; Hudson, H. S.; Lin, R. P.; Shih, A. Y.; Schwartz, R. A.; Kozlovsky, B. Bibcode: 2003SPD....34.1408S Altcode: 2003BAAS...35R.830S RHESSI has observed the positron-electron annihilation line at 511 keV produced during the 2002 July 23 solar flare. The shape of the line is consistent with annihilation in two vastly different solar environments. It can be produced by formation of positronium by charge exchange in flight with hydrogen in a quiet solar atmosphere at a temperature of ∼ 6000 K. However, the measured upper limit to the 3γ /2γ ratio (ratio of annihilation photons in the positronium continuum to the number in the line) is only marginally consistent with what is calculated for this environment. The annihilation line can also be fit by a thermal Gaussian having a width of 8.1 +/- 1.1 keV (FWHM), indicating temperatures of ∼ 4 - 7 x 105 K. The measured 3γ /2γ ratio does not constrain the density when the annihilation takes place in such an ionized medium, however the density must be high enough to slow down the positrons. This would require the formation of a substantial mass of atmosphere at transition-region temperatures during flares. Title: Coronal scattering as a source of flare-associated polarized hard X-rays Authors: Hudson, H. S.; Hurford, G. J.; Brown, J. C. Bibcode: 2003SoPh..214..171H Altcode: We consider the scattering of flare-associated X-rays above 1 keV at coronal heights, particularly from regions of enhanced density. This includes a discussion of the polarization of the scattered X-rays. Although the scattered radiation would not be bright by comparison with the total hard X-ray flux from a flare, its detectability would be enhanced for events located a few degrees behind the limb for which the dominant `footpoint' hard X-ray sources are occulted. Thus we predict that major flares occurring beyond the solar limb may be detectable via scattering in density enhancements that happen to be visible above the limb, and that such sources may be strongly polarized. Since thin-target bremsstrahlung will generally greatly exceed the scattered thick-target flux in flare loops themselves, these considerations apply only to coronal structures that do not contain significant populations of non-thermal electrons. Title: The nature of impulsive solar energetic particle events Authors: Nitta, N. V.; Hudson, H. S.; De Rosa, M. L. Bibcode: 2003SPD....34.1606N Altcode: 2003BAAS...35..833N Impulsive solar energetic particle (SEP) events, as opposed to gradual SEP events, are usually thought to originate from flares in the well-connected regions. In order to test this idea, we compute the solar locations of the field lines that cross the spacecraft encountering particles from SEP events, and compare them with the flare locations. We combine two frequently used techniques, i.e., the ballistic approximation for the interplanetary magnetic field and the potential-field source-surface model for the coronal field. Such comparisons are made for selected impulsive SEP events during 1995-2001. We check the validity of the techniques using Yohkoh SXT and SOHO EIT images, which often show coronal holes on the disk. Furthermore, we study the properties of the flares that are identified with impulsive SEP events, and compare them with those of other flares in the well-connected areas but without impulsive SEP events. This will give us a clue as to the importance of the flare processes relative to the magnetic field connectivity on the detection of impulsive SEP events. Title: Thermal and Nonthermal Contributions to the Flare X-ray Flux Authors: Dennis, B. R.; Phillips, K. J. H.; Schwartz, R. A.; Tolbert, A. K.; Hudson, H. S.; RHESSI Team Bibcode: 2003SPD....34.1809D Altcode: 2003BAAS...35..841D The relative thermal and nonthermal contributions to the total flare energy budget are being determined through analysis of RHESSI imaging and spectroscopy X-ray observations in the critical energy range from a few keV to a few tens of keV. The classic ways of differentiating between the thermal and nonthermal components - exponential vs. power-law spectra, impulsive vs. gradually varying flux, compact vs. extended sources - can now be combined together for individual flares. In addition, RHESSI's sensitivity and fine energy resolution of 1 keV FWHM allow the intensity and mean energy of the iron-line complex between 6 and 7 keV to be measured as a function of time. Thus, the temperature and emission measure of the thermal plasma can be determined in a consistent manner assuming only the abundance of iron and the thermal origin of the emission. This then defines the thermal continuum spectrum and allows it to be cleanly separated from any nonthermal component that may be required to fit the measured spectrum at higher energies. Examples of such separations of the thermal and nonthermal components will be presented for several flares. The intensity of the iron line complex is consistent with an iron abundance equal to the solar coronal abundance as given by Feldman (1992), i.e., three times higher than the photospheric abundance Title: Characterization of the RHESSI 3-10 keV spectrum Authors: Hudson, H. S.; Dennis, B. R.; Phillips, K. J. H.; Schwartz, R. A.; Smith, D. M. Bibcode: 2003SPD....34.1806H Altcode: 2003BAAS...35..840H We describe an empirical characterization of the RHESSI X-ray spectrum in the 3-10 keV range, using a sample of flares for which the attenuating shutters remained open. The main spectral features comprise the continuum, which we study in two broad-band ranges (F3: 2.83-5.5 keV, and F8: 7.5-10.17 keV). The lower-energy band F3 normally shows time profiles resembling those of the GOES hard channel, and the F8/F3 ratio exhibits a heating/cooling variation similar to that of the two GOES channels; in addition an impulsive component appears in the F8 channel. In between these bands lies the prominent Fe line complex. We fit the Fe excess emission to a Gaussian, using 2-sec integrations, and find values of sigma on the order of 0.3-0.4 keV for most of the RHESSI detectors. This emission feature embraces the 2-1 transitions for all of the Fe ionization states. The centroid of the Fe line feature varies significantly in energy as the flare evolves; the time variability resembles the F8 continuum more than the F3 continuum. We present statistical results for these properties of the 3-10 keV spectrum, including the systematic variation of Fe feature equivalent width. Title: Observations of CMEs associated with trans-equatorial loop systems Authors: Khan, J. I.; Hudson, H. S. Bibcode: 2003SPD....34.1004K Altcode: 2003BAAS...35Q.825K Soft X-ray observations frequently show the existence of trans-equatorial loop systems, often rooted near an active region. The visibility in soft X-rays implies relatively high temperature, a fact that has no ready explanation. The Yohkoh SXT observations from April-May 1998 showed the persistent formation and re-formation of trans-equatorial loops associated with NOAA AR 8210. At least four such events happened: May 2, 6, 8, and 9. In these events the erupting loop systems become parts of the associated CMEs. These events were also associated with major flares and with global waves visible in soft X-rays and type II bursts. We suggest that this morphology, even in cases in which the trans-equatorial structure was not hot enough to be visible in soft X-rays, could explain the occurrence of the larger angular scales of some CMEs as opposed to their associated flares. Title: RHESSI and non-thermal solar physics in the IHY Authors: Hudson, H.; Davila, J.; Dennis, B.; Emslie, G.; Lin, R.; Ryan, J.; Share, G. Bibcode: 2003EAEJA.....7939H Altcode: The signatures of non-thermal activity on the Sun - X-rays, gamma-rays, and high-energy particles - present us with the closest possible view of the essential physics underlying solar activity and its heliospheric consequences. During the International Heliophysical Year (2007) we will have a rich harvest of measurements from current sunspot maximum from an unprecedented array of observations from space. This poster will present the most recent observations from the newest spacecraft, RHESSI (the Reuven Ramaty High-Energy Solar Spectroscopic Imager) in the context of the IHY and possible future programs. The RHESSI data discussed emphasize the gamma-ray line flare of July 23, 2002 as well as discoveries made in the April 21, 2002 and other events. Title: X-ray and radio observations of the activation stages of an X-class solar flare Authors: Fárník, F.; Hudson, H. S.; Karlický, M.; Kosugi, T. Bibcode: 2003A&A...399.1159F Altcode: We report interesting developments prior to the impulsive phase of an X-class solar flare that occurred on September 24, 2001. Our multiwavelength study makes use of X-ray data from the Yohkoh satellite, the Ondřejov radio spectral observations in the decimetric band, and the new Hard X-Ray Spectrometer instrument (HXRS) on board the MTI satellite. The GOES time history of this event showed a ``precursor'' phase starting as early as two hours prior to the impulsive phase, and we have used various data sets to identify what parts of this development could be associated with the flare itself. The most interesting time interval was identified roughly one hour before the main peak when an unusual drifting radio continuum was observed together with two radio sources (at 327 and 164 MHz) in positions corresponding to expanding loops seen in Yohkoh/SXT and SOHO/EIT images, accompanied by a filament disappearence during the same period. Hard X-ray observations revealed a soft spectrum that we interpret as non-thermal, located within loop structures observed in soft X-rays along the magnetic neutral line. The hard X-ray emission continued for more than one hour, as observed in turn by the two spacecraft. In the initial phase of the flare itself, the hard X-ray emission arose in structures closely identifiable with the early soft X-ray loops, which appeared to evolve smoothly into the post-flare loop system of the flare maximum. The decimeter spectra showed loosely-correlated spiky emission at frequencies consistent with the densities inferred from soft X-rays, but with rapid drifts implying motions along field lines. From all these data we infer that the initiation of the flare involved non-thermal processes extending along the neutral line in the photosphere, systematically including open magnetic field lines as shown by the occurrence of interplanetary Type III bursts observed by the WAVES spectrometer on board the WIND spacecraft. Title: Soft X-ray observation of a large-scale coronal wave and its exciter Authors: Hudson, Hugh S.; Khan, Josef I.; Lemen, James R.; Nitta, Nariaki V.; Uchida, Yutaka Bibcode: 2003SoPh..212..121H Altcode: Recent extreme ultraviolet (EUV) observations from SOHO have shown the common occurrence of flare-associated global coronal waves strongly correlated with metric type II bursts, and in some cases with chromospheric Moreton waves. Until now, however, few direct soft X-ray detections of related global coronal waves have been reported. We have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to understand this apparent discrepancy, and describe the problems in this paper. We have found good X-ray evidence for a large-scale coronal wave associated with a major flare on 6 May 1998. The earliest direct trace of the wave motion on 6 May consisted of an expanding volume within 20 Mm (projected) of the flare-core loops, as established by loop motions and a dimming signature. Wavefront analyses of the soft X-ray observations point to this region as the source of the wave, which began at the time of an early hard X-ray spike in the impulsive phase of the flare. The emission can be seen out to a large radial distance (some 220 Mm from the flare core) by SXT, and a similar structure at a still greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on SOHO. The radio dynamic spectra confirm that an associated disturbance started at a relatively high density, consistent with the X-ray observations, prior to the metric type II burst emission onset. The wavefront tilted away from the vertical as expected from refraction if the Alfvén speed increases with height in the corona. From the X-ray observations we estimate that the electron temperature in the wave, at a distance of 120 Mm from the flare core, was on the order of 2-4 MK, consistent with a Mach number in the range 1.1-1.3. Title: Modeling large solar flares Authors: Somov, B. V.; Kosugi, T.; Hudson, H. S.; Sakao, T.; Masuda, S. Bibcode: 2003AdSpR..32.2439S Altcode: The basic ideas to model the large solar flares are reviewed and illustrated. Some fundamental properties of potential and non-potential fields in the solar atmosphere are recalled. In particular, we consider a classification of the non-potential fields or, more exactly, related electric currents, including reconnecting current layers. The so-called 'rainbow reconnection' model is presented with its properties and predictions. This model allows us to understand main features of large flares in terms of reconnection. We assume that in the two-ribbon flares, like the Bastille-day flare, the magnetic separatrices are involved in a large-scale shear photospheric flow in the presence of reconnecting current layers generated by a converging flow. Title: Unusual large-scale flaring structure Authors: Chertok, I. M.; Hudson, H. S.; Kahler, S. W. Bibcode: 2002ESASP.506..569C Altcode: 2002ESPM...10..569C; 2002svco.conf..569C The Yohkoh/SXT data of August 21, 1999 displayed a spectacular transient brightening of a large-scale double whip-like structure extending through the corona to a length greater than 950 Mm. The transient originated at a relatively small middle-latitude soft X-ray bright point (XBP), which was also visible in the EUV range and had a small underlying Hα plage with a bipolar magnetic configuration. The structure developed in the high-temperature (>2.5 MK) soft X-ray emission to both sides of the XBP during a few tens of minutes and could be recognized for about two hours. The observations suggest sudden energy release during the interaction of the magnetic structures of the XBP and the filament channel. Title: Two wave morphologies in SOHO/EIT - EIT waves and Moreton waves Authors: Biesecker, D. A.; Thompson, B. J.; Hudson, H. S.; Warmuth, A.; White, S. Bibcode: 2002AGUFMSH52A0450B Altcode: EIT waves are global waves observed to propagate across quiet coronal field regions in the SOHO/EIT data. The waves are initiated in association with other transient activity. The literature now contains many references to EIT waves and observers have published observations of associated waves at a variety of wavelengths. We show in this poster that there is confusion in the literature as to what an EIT wave is. We use YOHKOH SXT, Nobeyama Radioheliograph and He I 10830A observations to show that the EIT instrument observes waves with two distinct properties and morphologies. These two morphologies correspond to the classical Moreton wave and to what we call the EIT wave. The Moreton waves in EIT appear as a sharp, bright feature, travelling at super-Alfvenic velocities. The EIT waves instead appear as a diffuse, faint feature, moving at relatively slower velocities. The EIT waves appear much more frequently than the Moreton waves and Moreton waves are usually seen in tandem with EIT waves. Both types of waves have been modelled by various researchers as MHD waves. Title: Spectral variations of flare hard X-rays Authors: Hudson, H. S.; Fárník, F. Bibcode: 2002ESASP.506..261H Altcode: 2002ESPM...10..261H; 2002svco.conf..261H We report on a set of eight solar hard X-ray bursts corresponding to M-class flares and well-observed with the RHESSI and HXRS instruments. We find, as previously reported, that the impulsive phases of these events invariably show the soft-hard-soft spectral pattern (SHS) in which the power-law spectral index anticorrelates with the flux. The RHESSI data have higher spectral resolution but give the same result. The SHS pattern occurs in the spectral domain above the typical break energy of double power-law fits as well as below. Some of the events also show other patterns, including the soft-hard-harder evolutions seen in eruptive events. We discuss the physical interpretation of the spectral behavior. Title: Mapping RHESSI footpoints with potential-field models Authors: Fletcher, L.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2002AGUFMSH52A0452F Altcode: RHESSI hard X-ray observations help us to identify the locations of magnetically conjugate footpoints, and to study their apparent motions during the evolution of the impulsive phase of a flare. We put this information into the context of an M-class flare that occurred 2002 March 14 01:50 UT (GOES peak time) at S12, E23 (NOAA region 9866) by making potential-field mappings of the coronal magnetic structure. In principle the hard X-ray sources (plus the mapping) constrain the site of magnetic energy release, and the maps reveal the location of the stored energy. The RHESSI source centroids can be determined to better than 1'' (rms) for an M-class flare. This analysis is an exploration of the feasibility of such an approach, since full success would require understanding the magnetic restructuring in detail. If suitable X-class RHESSI flares occur we will be able to present data with better precision. Title: Intense Flares Without Solar Energetic Particle Events Authors: Nitta, N. V.; Cliver, E. W.; Hudson, H. S. Bibcode: 2002AGUFMSH61A0437N Altcode: We study favorably located (western hemisphere) X-class flares that were not associated with solar energetic particle (SEP) events. Three of the four such flares that occurred during the present cycle lacked coronal mass ejections (CMEs), consistent with the current paradigm. Soft X-ray data for these three events show either outward moving loops above the main flare loop or a much slower rise of the flare loop itself, as predicted in bipolar reconnection models. However, unlike fully eruptive events, the outward moving loops appear to stop at a certain distance. We speculate that they are held back by overlying magnetic field, as implied in soft X-ray images. The flare that was associated with a CME (but not an SEP event) produced metric and decametric type III bursts, but those without CMEs did not. Other characteristics for the flares not associated with SEP events include relatively short decay times of hard X-ray emission at 30--50 keV. We are extending our study to include additional (somewhat smaller) events to try to identify key parameters that keep intense flares from erupting and accompanying SEP events. Title: Five years of Yohkoh science nuggets Authors: Hudson, H. S.; McKenzie, D. E.; Nitta, N. V. Bibcode: 2002AGUFMSH52A0453H Altcode: The Yohkoh "science nuggets", weekly Web-based reports emphasizing current Yohkoh observations, began October 24, 1997. Many writers (the SXT "chief observers" in particular) contributed, and over the years a characteristic style developed: these were educational pieces rather than public-relations puffs, and they each attempted to describe a particular item for a technically savvy non-specialist. In this poster we summarize the nugget philosophy and point out some of our favorites, such as the remarkable "triple jet." This and many other interesting observations have not yet otherwise been published. Since December 2001 we have gradually broadened our science basis to include SOHO, TRACE, and now RHESSI input. The topic index lists more than 60 categories, and there is a general search facility. We present this poster partly to encourage discussion of the future development of the series. Title: Coronal blast waves detected in soft X-rays Authors: Khan, J.; Hudson, H. S.; Nitta, N. V. Bibcode: 2002AGUFMSH52A0487K Altcode: Several examples of rather clear X-ray detection of coronal blast waves have now been reported (e.g., Khan and Aurass, A&A 383, 1018, 2002). Interestingly, most of them have been reported from two particular active regions -- NOAA 8100 (November 1997) and 8210 (April-May 1998). The soft X-ray observations in some cases have high time resolution and the ability to look close to the core of the flare responsible for the wave. We summarize the observations to date, emphasizing the distinction between ejecta (magnetic loops) and freely running waves. Related observations now exist at metric and centimetric wavelengths, in the EUV, and in chromospheric lines (H-alpha and He 10830), and we describe the observational relationships among these different observations. Title: Spectral and Spatial Variations of Flare Hard X-ray Footpoints Authors: Fletcher, L.; Hudson, H. S. Bibcode: 2002SoPh..210..307F Altcode: In a sample of strong RHESSI M-class flares we have made a study of the relationship between the `hardness' of the HXR spectrum and the intensity in the 30-50 keV energy range. In all events we find clear evidence for a `soft-hard-soft' pattern of correlation between hardness and flux, on time scales as short as 10 s. We investigate whether or not this pattern is intrinsic to the acceleration mechanism. The RHESSI images in this energy range are dominated by footpoint brightenings, and we have searched for a correlation between footpoint separation velocity and spectral hardness, to be compared qualitatively with theoretical flare models. We find quite systematic footpoint motions, and also note that episodes in which footpoint separation varies rapidly often correspond with episodes of significant change in the flare spectral index, though not as the simplest flare models would predict. We report also on one of our events, on 14 March 2002, which exhibits highly sheared HXR footpoint ribbons extending over a scale of 100 arc sec. For this flare we find a correlation between footpoint motion and hard X-ray flux. Title: Magnetic Reconnection Scenario of the Bastille Day 2000 Flare Authors: Somov, Boris V.; Kosugi, Takeo; Hudson, Hugh S.; Sakao, Taro; Masuda, Satoshi Bibcode: 2002ApJ...579..863S Altcode: On the basis of Yohkoh Hard X-Ray Telescope data and the magnetograms taken by the SOHO Michelson Doppler Imager and the Solar Magnetic Field Telescope at Huairou Solar Observing Station, we suggest an interpretation of the well-observed ``Bastille Day 2000'' flare. The large-scale structure and dynamics of the flare, as seen in hard X-rays, can be explained in terms of the three-dimensional reconnection at a separator in the corona. More specifically, we suggest that before occurrence of two-ribbon flares with significant decrease of a distance between the hard X-ray (HXR) footpoints, like the Bastille Day flare, the bases of magnetic field separatrices are moved by the large-scale photospheric flows of two types. First, the shear flows, which are parallel to the photospheric neutral line, increase the length of field lines in the corona and produce an excess of magnetic energy. Second, the converging flows, i.e., the flows directed to the neutral line, create the preflare current layers in the corona and provide an excess of energy sufficient to produce a large flare. During the flare, both excesses of magnetic energy are released completely or partially. In the Bastille Day flare, the model describes two kinds of apparent motions of the HXR kernels. One is an increase of a distance between the flare ribbons in which the HXR kernels appear. The effect results from fast reconnection in a coronal current layer. The second effect is a decrease of the distance between the kernels moving to each other as a result of relaxation of magnetic tensions generated by the photospheric shear flows. Title: The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) Authors: Lin, R. P.; Dennis, B. R.; Hurford, G. J.; Smith, D. M.; Zehnder, A.; Harvey, P. R.; Curtis, D. W.; Pankow, D.; Turin, P.; Bester, M.; Csillaghy, A.; Lewis, M.; Madden, N.; van Beek, H. F.; Appleby, M.; Raudorf, T.; McTiernan, J.; Ramaty, R.; Schmahl, E.; Schwartz, R.; Krucker, S.; Abiad, R.; Quinn, T.; Berg, P.; Hashii, M.; Sterling, R.; Jackson, R.; Pratt, R.; Campbell, R. D.; Malone, D.; Landis, D.; Barrington-Leigh, C. P.; Slassi-Sennou, S.; Cork, C.; Clark, D.; Amato, D.; Orwig, L.; Boyle, R.; Banks, I. S.; Shirey, K.; Tolbert, A. K.; Zarro, D.; Snow, F.; Thomsen, K.; Henneck, R.; Mchedlishvili, A.; Ming, P.; Fivian, M.; Jordan, John; Wanner, Richard; Crubb, Jerry; Preble, J.; Matranga, M.; Benz, A.; Hudson, H.; Canfield, R. C.; Holman, G. D.; Crannell, C.; Kosugi, T.; Emslie, A. G.; Vilmer, N.; Brown, J. C.; Johns-Krull, C.; Aschwanden, M.; Metcalf, T.; Conway, A. Bibcode: 2002SoPh..210....3L Altcode: RHESSI is the sixth in the NASA line of Small Explorer (SMEX) missions and the first managed in the Principal Investigator mode, where the PI is responsible for all aspects of the mission except the launch vehicle. RHESSI is designed to investigate particle acceleration and energy release in solar flares, through imaging and spectroscopy of hard X-ray/gamma-ray continua emitted by energetic electrons, and of gamma-ray lines produced by energetic ions. The single instrument consists of an imager, made up of nine bi-grid rotating modulation collimators (RMCs), in front of a spectrometer with nine cryogenically-cooled germanium detectors (GeDs), one behind each RMC. It provides the first high-resolution hard X-ray imaging spectroscopy, the first high-resolution gamma-ray line spectroscopy, and the first imaging above 100 keV including the first imaging of gamma-ray lines. The spatial resolution is as fine as ∼ 2.3 arc sec with a full-Sun (≳ 1°) field of view, and the spectral resolution is ∼ 1-10 keV FWHM over the energy range from soft X-rays (3 keV) to gamma-rays (17 MeV). An automated shutter system allows a wide dynamic range (>107) of flare intensities to be handled without instrument saturation. Data for every photon is stored in a solid-state memory and telemetered to the ground, thus allowing for versatile data analysis keyed to specific science objectives. The spin-stabilized (∼ 15 rpm) spacecraft is Sun-pointing to within ∼ 0.2° and operates autonomously. RHESSI was launched on 5 February 2002, into a nearly circular, 38° inclination, 600-km altitude orbit and began observations a week later. The mission is operated from Berkeley using a dedicated 11-m antenna for telemetry reception and command uplinks. All data and analysis software are made freely and immediately available to the scientific community. Title: Solar Submillimeter and Gamma-Ray Burst Emission Authors: Kaufmann, P.; Raulin, J. -P.; Melo, A. M.; Correia, E.; Costa, J. E. R.; de Castro, C. G. Giménez; Silva, A. V. R.; Yoshimori, M.; Hudson, H. S.; Gan, W. Q.; Gary, D. E.; Gallagher, P. T.; Levato, H.; Marun, A.; Rovira, M. Bibcode: 2002ApJ...574.1059K Altcode: Solar flare emission was measured at 212 GHz in the submillimeter range by the Submillimeter Solar Telescope in the 1.2-18 GHz microwave range by the Owens Valley Solar Array and in the gamma-ray energy range (continuum) by experiments on board the Yohkoh (>1.2 MeV) and Shenzhou 2 (>0.2 MeV) satellites. At the burst onset, the submillimeter and microwave time profiles were well correlated with gamma rays to the limit of the temporal resolution (<=10 s). At 212 GHz, fast pulses (<1 s), defined as time structures in excess of the bulk emission, were identified as the flux increased. Their spatial positions were scattered by tens of arcseconds with respect to the main burst emission position. Correlation of submillimeter emission with gamma-ray fast time structures shorter than 500 ms is suggested at the gamma-ray maximum. The time variation of the rate of occurrence of the submillimeter rapid pulses was remarkably well correlated with gamma-ray intensities in the energy range (>1.2 MeV), attaining nearly 50 pulses per minute at the maximum. These results suggest that gamma rays might be the response to multiple rapid pulses at 212 GHz and might be produced at different sites within the flaring region. Title: Boundary Structures and Changes in Long-lived Coronal Holes Authors: Kahler, S. W.; Hudson, H. S. Bibcode: 2002ApJ...574..467K Altcode: We report a first systematic morphological study of the boundaries of coronal holes (CHs) as viewed in soft X-ray images from the Yohkoh Soft X-Ray Telescope. The special emphasis is on long-lived (several rotations) CHs that extend from the solar polar regions to midlatitudes. As shown earlier, such equatorward extensions tend to show rigid, rather than differential, rotation. Magnetic reconnection must occur at the ``closing'' boundary, in such a case, to maintain the CH integrity. We find three kinds of CH boundaries in the soft X-ray observations. The majority are generally ragged and not sharply defined; we also find smooth boundaries to occur near a matching-polarity active region (AR), and loopy boundaries to occur near an opposite-polarity AR. In this latter case the loops clearly do not extend far enough to reach another CH but instead end in normal corona. The CH boundaries evolve slowly, and neither large-scale transient X-ray events nor coronal bright points appeared significant factors in long-term CH boundary development. No direct evidence for magnetic reconnection is seen. We compare these results with those expected from current models, derived largely from considerations of heliospheric conditions rather than the detailed appearance of CHs in the low corona. Title: X-ray and radio observations in the initial development of an X-class solar flare Authors: Fárnik, F.; Karlický, M.; Hudson, H.; Kosugi, T. Bibcode: 2002ESASP.508..441F Altcode: 2002soho...11..441F The EIT/SOHO and SXT/YOHKOH plasma ejecta accompanied by an unusual drifting radio continuum and early hard X-ray emission observed prior to the impulsive phase of the September 24, 2001, X-class flare are analyzed. The paper presents some of the first reported observations from the new Hard X-ray Spectrometer instrument (HXRS), as well as imaging data from YOHKOH plus radio spectral observations in the decimetric band. The early hard X-ray observations revealed a soft spectrum that we interpret as non-thermal, located within loop structures observed in soft X-rays along the magnetic neutral line. The hard X-ray emission continued for more than one hour. In the initial phase of the flare, the hard X-ray emission arose in structures closely identifiable with the early soft X-ray loops, which appeared to evolve smoothly into the post-flare loop system of the flare maximum. At this time the decimeter spectra showed loosely-correlated spiky emission at frequencies consistent with the densities inferred from soft X-rays, but with rapid drifts implying motions along magnetic field lines. Title: Coronal holes as seen in soft X-rays by Yohkoh Authors: Hudson, H. S. Bibcode: 2002ESASP.508..341H Altcode: 2002soho...11..341H This paper reviews literature on the 10-plus years of Yohkoh soft X-ray observations of coronal holes (CHs), which span almost half a Hale cycle. They thus extend and generalize the results of Skylab, providing better sampling and duration. These modern X-ray data confirm the tendency towards rigid rotation of equatorward extensions of polar holes, but show no clear sign of the mechanism of magnetic reconnection that makes this possible. Coronal-hole boundary evolution does not seem to depend upon transient magnetic activity such as X-ray bright points or large-scale arcade events associated with transient coronal holes. The morphology of the coronal holes as seen in X-rays differs from that seen in He I λ10830: X-rays generally do not do so well with the polar holes or other holes near the limb, because of foreground confusion, but they show narrow features better. Title: Simultaneous Observation of a Moreton Wave on 1997 November 3 in Hα and Soft X-Rays Authors: Narukage, N.; Hudson, H. S.; Morimoto, T.; Akiyama, S.; Kitai, R.; Kurokawa, H.; Shibata, K. Bibcode: 2002ApJ...572L.109N Altcode: We report the observation of a Moreton wave in Hα (line center and +/-0.8 Å) with the Flare Monitoring Telescope at the Hida Observatory of Kyoto University at 4:36-4:41 UT on 1997 November 3. The same region (NOAA Active Region 8100) was simultaneously observed in soft X-rays with the soft X-ray telescope on board Yohkoh, and a wavelike disturbance (``X-ray wave'') was also found. The position of the wave front as well as the direction of propagation of the X-ray wave roughly agree with those of the Moreton wave. The propagation speeds of the Moreton wave and the X-ray wave are about 490+/-40 and 630+/-100 km s-1, respectively. Assuming that the X-ray wave is an MHD fast-mode shock, we can estimate the propagation speed of the shock, on the basis of MHD shock theory and the observed soft X-ray intensities ahead of and behind the X-ray wave front. The estimated fast shock speed is 400-760 km s-1, which is in rough agreement with the observed propagation speed of the X-ray wave. The fast-mode Mach number of the X-ray wave is also estimated to be about 1.15-1.25. These results suggest that the X-ray wave is a weak MHD fast-mode shock propagating through the corona and hence is the coronal counterpart of the Moreton wave. Title: Imaging Spectroscopy of the February 20, 2002 flare: RHESSI Observations Authors: Krucker, S.; Lin, R. P.; Caspi, A.; Hudson, H.; Schwartz, R. S.; Johns-Krull, C. M.; RHESSI Team Bibcode: 2002AAS...200.7607K Altcode: 2002BAAS...34..776K First RHESSI imaging spectroscopy results of X-ray bursts from solar flares are presented. In a first step, different spatial features such as footpoints, loops in-between footpoints, etc. are identified in the X-ray images. By reconstructing images at different energies (with a spectral resolution down to 1 keV) the X-ray photon spectrum for each of the spatial features can be extracted independently of each other. Here, images at high cadence (4 seconds) are analyzed allowing to follow the temporal evolution of the spectra of different spatial features. The results from imaging spectroscopy of individual spatial features are compared with the total spectrum and the differences are discussed. For the event of February 20, 11:05 UT, in-situ observations at 1 AU of 1-300 keV electron taken by the 3DP instrument on the WIND spacecraft show an impulsive electron event of solar origin. The electron onset times at different energies reveal that the solar release time of these escaping electrons coincides with the HXR burst seen with RHESSI. Therefore, a direct comparison of the different X-ray photon spectra and the electron spectrum observed at 1 AU is possible. Title: Investigation of the Sources of Irradiance Variation on the Sun (ISIS) Authors: LaBonte, B. J.; Bernasconi, P. N.; Rust, D.; Foukal, P.; Hudson, H.; Spruit, H. Bibcode: 2002AAS...200.5608L Altcode: 2002BAAS...34..736L There is a persistent correlation of the longterm climate change and solar irradiance. ISIS is designed to understand the physical basis of this correlation. ISIS combines an innovative bolometric imager and a multiband CCD imager. The bolometric imager has uniform response from 200 nm to 3000 nm, spatial resolution < 5 arcseconds, and precision of < 0.1% in a one minute integration. The multiband imager records ultraviolet irradiance variation in the band from 200 to 350 nm, measures photospheric temperature structure, and provides chromospheric structure in Ca II K and H-alpha, with spatial resolution <1.0 arcsecond. Designed for flight on the Solar Dynamics Observatory, ISIS will provide the comprehensive photometric measurements needed to characterize the irradiance variation from identifiable structures and challenge theoretical models of convection and the solar dynamo. Title: Coronal Dimming Associated with Coronal Mass Ejections Authors: Nitta, N. V.; Hudson, H. S.; Newmark, J. S. Bibcode: 2002AAS...200.2905N Altcode: 2002BAAS...34..681N Observations from Yohkoh SXT have identified different patterns of coronal dimming around the times of coronal mass ejections (CMEs), but the mapping between the dimming and the CME is still not well understood. There is always a possibility that the observed dimming may simply reflect cooling rather than mass depletion due to ejection. Empirically, dimming seen in SOHO EIT images appears to be more intimately associated with CMEs, although EIT images with narrower sensitivities should be more susceptible to changes in temperature. In this work, we compare SXT and EIT images systematically for various patterns of dimming, in order to understand possibly different origins of CMEs. We use new calibrations for both sets of images that allow us to more accurately estimate the changes of temperature/density that account for the observed level of dimming. We concentrate on events with good coverage to study the time dependence of coronal dimming at different temperatures. Title: Soft-hard-soft spectral evolution observed at HESSI resolution Authors: Hudson, H. S.; Dennis, B. R.; Krucker, S.; Lin, R. P.; Sato, J.; Schwartz, R. A.; Smith, D. M. Bibcode: 2002AAS...200.6905H Altcode: 2002BAAS...34..758H Hard X-ray bursts from solar flares show a strong correlation between spectral index and flux level in the 20-100 keV hard X-ray range. Prior to HESSI this pattern could mainly be studied only with scintillation counters, at relatively low resolution. HESSI has observed more than ten M-class flares, and we will report how this morphology appears at ~1-keV resolution for these events. Specifically, we address the question of whether the time evolution of the break energy in a double power-law fit can play a role in defining the soft-hard-soft morphology. Title: Hard X-ray and White Light Observations of the August 25, 2001 X Flare Authors: Metcalf, T. R.; Alexander, D.; Hudson, H. S.; Longcope, D.; Myers, D. Bibcode: 2002AAS...200.6803M Altcode: 2002BAAS...34..755M An X5.3 flare occurred about 16:31 UT on 2001 August 25 and was well observed by the Yohkoh and TRACE spacecraft. The flare showed gamma-ray emission, nuclear lines, and was a dramatic white light flare seen in TRACE data. A preliminary analysis of the hard X-ray images from the Yohkoh/HXT instrument shows two clear footpoints and a moving HXR source in this very energetic flare. The moving hard X-ray source appears to move along a magnetic separatrix at 400 km/sec. We will discuss the hard X-ray and white light structure of this flare and discuss the energetics and possible mechanisms for the formation of the TRACE white light emission. Title: Solar Disappearing Filament Inside a Coronal Hole Authors: Chertok, I. M.; Obridko, E. I.; Mogilevsky, V. N.; Shilova, N. S.; Hudson, H. S. Bibcode: 2002ApJ...567.1225C Altcode: Based on Yohkoh/SXT, SOHO/EIT images and movies, as well as on Hα, He I 10830 Å heliograms and other relevant data, we analyze an event of 1999 December 28, which is interesting in at least two aspects. (1) A major horseshoe-shaped Hα filament appeared to be located within a large transequatorial coronal hole (CH) in the eastern hemisphere. (2) This filament subsequently disappeared, with its eruption combined with a number of dynamic phenomena, including large-scale ones. The probable location of the filament inside the CH was confirmed in detail by calculations of the open field regions and, for the first time, the quasi-separatrix layers in the global solar magnetic field. The filament eruption was accompanied by significant evolution of the soft X-ray and EUV-emitting structures inside the CH as well as by a coronal mass ejection. The analysis indicates that CHs need not have the simple and uniform structure normally assumed and can sometimes contain local areas with low-altitude closed magnetic fields. It demonstrates also that the erupting filament inside the CH was a part of a much more global, evolving magnetic structure associated with activity extending through at least the entire eastern half of the disk. Title: Energetic particles HXR emission as a diagnostic for energy release Authors: Hudson, Hugh Bibcode: 2002ocnd.confE..15H Altcode: No abstract at ADS Title: RHESSI and TRACE Observations of an X-class Flare Authors: Hudson, H.; Dennis, B.; Gallagher, P.; Krucker, S.; Reeves, K.; Warren, H. Bibcode: 2002cosp...34E3101H Altcode: 2002cosp.meetE3101H RHESSI and TRACE both obtained excellent observations of an X1.5 flare on April 21, 2002. In this paper we provide an overview of the flare and discuss the high- energy imaging and spectra in detail. The TRACE images in the 195A passband (Fe XII and FeXXIV) reveal this flare to have a spiky arcade with post-flare flow field in the "supra-arcade downflow" pattern discovered by Yohkoh. Below the spikes, but above the FeXII loops, TRACE observes a region with complex motions and fine structure. We confirm with RHESSI that this region has an elevated temperature and discuss the transition between thermal and non-thermal sources. RHESSI also observes footpoint emission distributed along the flare ribbons. Title: Multi-Wavelength Observation of A Moreton Wave on November 3, 1997 Authors: Narukage, N.; Shibata, K.; Hudson, H. S.; Eto, S.; Isobe, H.; Asai, A.; Morimoto, T.; Kozu, H.; Ishii, T. T.; Akiyama, S.; Kitai, R.; Kurokawa, H. Bibcode: 2002mwoc.conf..295N Altcode: No abstract at ADS Title: Multi-Wavelength Observations of Yohkoh White-Light Flares Authors: Matthews, S. A.; van Driel-Gesztelyi, L.; Hudson, H. S.; Nitta, N. V. Bibcode: 2002mwoc.conf..289M Altcode: The problem of accounting for the continuum emission that is observed in solar flares is still one which is largely unresolved. These white-light flares place severe constraints on the energy requirements and transport mechanisms operating in the flare, raising the question of whether partial or total in-situ heating is required to account for this deep atmospheric heating. Since it is widely believed that the energy release in solar flares occurs in the corona and that energy is then transported to the low chromosphere where the optical emission is produced, most attempts to explain the origin of white-light flares have centred on canonical mechanisms. However, it has become clear that the spatial and temporal correspondence between white-light and HXR is not one-to one. In order to further our understanding of these events we study the temporal and spatial relationships between emission in the visible, SXR and HXR regimes in all of the white-light flares observed by Yohkoh prior to the failure of the Soft X-ray Telescope (SXT) Aspect Camera in 1992; a total of approximately 30 events. Title: Simultaneous observations of Moreton waves in Hα and Soft X-ray Authors: Narukage, N.; Hudson, H.; Morimoto, T.; Kitai, R.; Kurokawa, H.; Shibata, K. Bibcode: 2002cosp...34E1337N Altcode: 2002cosp.meetE1337N Moreton waves are flare-associated waves observed to propagate across the solar disk in H (Moreton, 1960). Such waves have been identified as the intersections of a coronal fast-mode shock fronts and the chromosphere (Uchida, 1967). We report the two observations of Moreton waves in H (line center and +/- 0.8 A) with the Flare Monitoring Telescope (FMT) at the Hida Observatory of Kyoto University. The both events were simultaneously observed in soft X-rays with the Soft X-ray Telescope (SXT) on board Yohkoh, and wave-like disturbances ("X-ray wave") were also found. One event occurred in solar-disk on November 3, 1997, the other near solar limb on March 3, 2000. Assuming that the X-ray waves are the MHD fast shocks, we can estimate the propagation speeds of the shocks, based on the MHD shock theory and the observed soft X-ray intensities ahead and behind the X-ray wave fronts. It is found that the estimated fast shock speeds are in rough agreement with the observed propagation speeds of the X-ray waves. The fast mode Mach numbers of the X-ray waves are also estimated. These results suggest that the X-ray waves are MHD fast shocks propagating through the corona and hence are the coronal counterparts of the Moreton waves. Title: Observations of Moreton Waves and EIT Waves Authors: Shibata, K.; Eto, S.; Narukage, N.; Isobe, H.; Morimoto, T.; Kozu, H.; Asai, A.; Ishii, T.; Akiyama, S.; Ueno, S.; Kitai, R.; Kurokawa, H.; Yashiro, S.; Thompson, B. J.; Wang, T.; Hudson, H. S. Bibcode: 2002mwoc.conf..279S Altcode: The Moreton wave is a flare-associated wave observed in H alpha, and is now established to be a fast mode MHD shock emitted from the flare, but the physical mechanism to create the wave is still puzzling. On the other hand, the EIT wave is a newly discovered flare-associated wave observed in EUV with the Extreme ultraviolet Imaging Telescope (EIT) aboard SOHO, and in this case, not only its origin but also its physical property are both puzzling. We study the relationship of these two flare-associated waves, Moreton waves and EIT waves, by analyzing 4 events observed on Nov. 3 and 4, 1997, Aug. 8, 1998, and Mar. 3, 2000 (Narukage et al. 2001). The Moreton waves were observed in Ha, Ha+0.8A and Ha-0.8A with the Flare Monitoring Telescope (FMT) at the Hida Observatory of Kyoto University, while the EIT waves were observed with SOHO/EIT. In the typical case associated with an X-class flare in AR 8100 on 4 November 1997 (Eto et al. 2001) the propagation speeds of the Moreton wave and the EIT wave were approximately 780 km/s and 200 km/s respectively. The data on speed and location show clearly that the Moreton wave differs physically from the EIT wave in this case. The detailed analyses of the other events (Nov. 3, 1997, Aug. 8, 1998, and Mar. 3, 2000) will also be presented, with Yohkoh/SXT data in the lucky case. Title: Modeling big flares: Principles and practice Authors: Somov, B.; Kosugi, T.; Hudson, H.; Sakao, T.; Masuda, S. Bibcode: 2002cosp...34E..43S Altcode: 2002cosp.meetE..43S The basic ideas, which are under current use to model the large-scale solar flares, are reviewed and illustrated. The so-called `rainbow reconnection' model is applied to the observations of flares with the HXT on board Yohkoh, the MDI instrument on the SOHO, the TRACE satellite, and the Solar Magnetic Field Telescope (SMFT) of the Beijing Astronomical Observatory. This allows us to improve a theory of large solar flare (Somov, B., Cosmic Plasma Physics, 2000, Dordrect: Kluwer). In particular, the famous Bastille day 2000 flare is studied and interpreted. It is shown that the main large-scale structure and dynamics of this flare can be explained in terms of the collisionless 3D reconnection. Title: CMEs: How do the puzzle pieces fit together? Authors: Cliver, E. W.; Hudson, H. S. Bibcode: 2002JASTP..64..231C Altcode: 2002JATP...64..231C This review consists of questions to participants in the S-RAMP Symposium (S3) on CMEs and Coronal Holes, as well as to a few others, and their responses in a ``town meeting'' format (originally conducted on Hugh Hudson's website). Here we deal only with CMEs. The questions we ask aim at probing the weaknesses of existing models and highlighting controversies, thereby providing guidance toward a more complete view of solar eruptions. Topics covered include: the ``solar flare myth'', flux ropes, new phenomena (EIT waves, dimmings, global brightenings), helicity and sigmoids, and transequatorial loops (as sources of CMEs). Although this is a review, we're more concerned here with what is not known than what is already agreed upon. We asked people to speculate freely in advance of the observational, analytical, and theoretical work that will provide definitive answers-this is not the standard Scientific Method at work! Title: A Rapidly Moving Hard X-Ray Source in a CME Authors: Hudson, H. S. Bibcode: 2002mwoc.conf..379H Altcode: We have observed a high-speed coronal ejection in hard X-rays, detectable to an altitude of some 2 times 105 km in the Yohkoh 33-53 keV energy bands. Simultaneous imaging at 17 and 34 GHz from the Nobeyama Radio Observatory shows complex moving features simultaneous with the ejection, including a compact source that we identify with the rapid X-ray source motion. The hard X-ray and microwave observations agree upon ejection velocities in the vicinity of 1000 km s-1. The hard X-ray sources also corresponded in position angle to a bright coronal mass ejection detected about 15 minutes later, and to both fast-drift and slow-drift radio bursts in the decimeter-meter bands. Other components of coronal hard X-ray emission were also detected, including an extended long-duration event with a hard spectrum. We suggest that a major eruptive flare occurred in NOAA region 9415, approximately 26circ beyond the W limb at the time of the event. Estimating a source density of 4 times 109 cm-3 from the compact source observed at 17 GHz, we find a total electron number (> 20 keV) of approximately N20 ~2.5 times 1035 for the compact part of the source. Title: Origin and development of transient coronal holes Authors: Kahler, S. W.; Hudson, H. S. Bibcode: 2001JGR...10629239K Altcode: Solar transient coronal holes (TCHs) are short-lived (<=2 days) regions of dimmed X-ray intensity sometimes observed in association with coronal mass ejections. These features, first discovered from Skylab observations, can occur in magnetic unipolar regions on either side of the X-ray posteruptive arcades. They have been suggested as the magnetically open footpoints of associated transient flux ropes observed at 1 AU. We have used images from the Yohkoh Soft X-ray Telescope (SXT) to study the development of 19 TCH events obtained in a survey of 9 years of Yohkoh observations. We find that the boundaries of the TCHs are never static. The boundaries closer to the magnetic neutral line generally move away from it as the closed-loop X-ray arcades expand. In addition, previously closed coronal loops at the ends of the arcades often continue to expand and open on the outer boundaries of the TCHs. These processes typically last for hours. The arcade brightenings do not extend into the full areas of the TCHs. The TCHs tend to disappear only by a net contraction of the boundaries, rather than by brightening within their boundaries. The location of a TCH appears to coincide with a large-scale curvature of the magnetic neutral line or the occurrence of a nearby active region at one end of the coronal eruption. This distinguishes the formation of TCHs from the arcade development, suggesting that there is no requirement for a pair of TCHs or even any TCH to be formed in an eruptive event. The moving magnetic boundaries, uniformly dark interiors, and short lifetimes of TCHs pose significant problems for the interpretation that TCHs are footpoints of interplanetary magnetic flux ropes. Title: High-Energy Aspects of CMEs Associated with X-Class Flares in the Present Cycle Authors: Nitta, N. V.; Hudson, H. S. Bibcode: 2001AGUFMSH32B..04N Altcode: We review the properties of X-class flares as observed by Yohkoh in the present solar cycle. More than a half of about four dozen X-class flares were caught by Yohkoh from an early phase. Most of these flares have durations not as long as long decay events (LDEs), and yet their association with coronal mass ejections (CMEs) is very high. They often correspond to extended or halo events. The flares not associated with CMEs do not show ejecta in soft X-rays. The flare core usually shows compact morphology. Within the sensitivity of the Yohkoh Hard X-ray Spectrometer, the hard X-ray spectra extend to the MeV range only in 20% of these flares, and their temporal variations are typically soft-hard-soft. Concerning their association with interplanetary proton events at 20 MeV, not all the proton-associated flares are associated with CMEs or located close to the well-connected longitudes. We plan to incorporate analysis of additional data such as metric/kilometric radio spectra to study when and where the shocks form. Title: A Hard X-ray Two-Ribbon Flare Observed with Yohkoh/HXT Authors: Masuda, S.; Kosugi, T.; Hudson, H. S. Bibcode: 2001SoPh..204...55M Altcode: The Yohkoh hard X-ray telescope (HXT) observed hard X-rays from the impulsive phase of a long-duration event (LDE) occurring on 14 July 2000. The Yohkoh soft X-ray telescope (SXT) and other instruments observed a large arcade, with width and length ∼30 000 km and ∼120 000 km, respectively. In hard X-rays, for the first time, a two-ribbon structure was clearly observed in the energy range above 30 keV. This result suggests that electrons are in fact accelerated in the whole system of this arcade, not merely in a particular dominant loop. We analyzed the motions of bright kernels in the two hard X-ray ribbons in detail. Assuming these bright kernels to be footpoints of newly reconnected loops, we infer from their motions that the loops reconnecting early are highly sheared, while the loops reconnecting later are less sheared. We have also analyzed the hard X-ray spectra of the two ribbons independently. At the outer edge of a ribbon, the spectrum tends to be harder than that in the inner edge. This suggests that higher-energy electrons precipitate at the footpoints of outer loops and lower ones do at those of inner loops. We discuss what kind of model can support this tendency. Title: The Magnetic Structure and Generation of EUV Flare Ribbons Authors: Fletcher, L.; Hudson, H. Bibcode: 2001SoPh..204...69F Altcode: The `ribbons' of two-ribbon flares show complicated patterns reflecting the linkages of coronal magnetic field lines through the lower solar atmosphere. We describe the morphology of the EUV ribbons of the July 14, 2000 flare, as seen in SOHO, TRACE, and Yohkoh data, from this point of view. A successful co-alignment of the TRACE, SOHO/MDI and Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on the underlying field to within ∼ 2'', and thus to investigate the relationship between the ribbons and the field, and also the sites of electron precipitation. We have also made a determination of the longitudinal magnetic flux involved in the flare reconnection event, an important parameter in flare energetic considerations. There are several respects in which the observations differ from what would be expected in the commonly-adopted models for flares. Firstly, the flare ribbons differ in fine structure from the (line-of-sight) magnetic field patterns underlying them, apparently propagating through regions of very weak and probably mixed polarity. Secondly, the ribbons split or bifurcate. Thirdly, the amount of line-of-sight flux passed over by the ribbons in the negative and positive fields is not equal. Fourthly, the strongest hard X-ray sources are observed to originate in stronger field regions. Based on a comparison between HXT and EUV time-profiles we suggest that emission in the EUV ribbons is caused by electron bombardment of the lower atmosphere, supporting the hypothesis that flare ribbons map out the chromospheric footpoints of magnetic field lines newly linked by reconnection. We describe the interpretation of our observations within the standard model, and the implications for the distribution of magnetic fields in this active region. Title: Numerical Modelling of Trapped Electrons in an Expanding Solar Loop Authors: Fletcher, L.; Hudson, H. Bibcode: 2001AGUFMSH42A0777F Altcode: Recent observations with the Yohkoh Hard X-ray telescope and the Nobeyama Radioheliograph [Hudson et al., 2001] have shown a moving hard X-ray coronal source, associated in space and time with moving microwave and decimeter sources. Because of the electron energies involved in producing these emissions, we hypothesize that the radiation indicates the presence of a coronal population of high energy electrons, trapped in a magnetic loop which is expanding outward. The expansion will lead to a `betatron' deceleration effect, which, combined with the normal Coulomb scattering and energy losses, will lead to an evolution of the particle energy density, spectrum and pitch angle distribution. We model this process using a stochastic test particle simulation. >http://isass1.solar.isas.ac.jp/ ~hudson/drafts/apr18.pdf</a> Title: Hard X-rays Associated with CMEs Authors: Hudson, H. S. Bibcode: 2001AGUFMSH32B..03H Altcode: In principle we can use hard X-ray bremsstrahlung (> few keV) to study the non-thermal tail of the electron distribution function in the solar corona. This approach is less sensitive than the techniques of radio astronomy, because of the low emissivity and low detection sensitivity for hard X-rays, but observations of major events are now possible; HESSI should greatly improve the breadth of such observations. These highly energetic flare events commonly are associated with CMEs. The view offered by such observations complements the radio techniques in interesting ways. This talk reviews data from several coronal hard X-ray events observed by the Yohkoh Hard X-ray Telescope and other instruments in the context of the radio observations. Title: Hard X-Radiation from a Fast Coronal Ejection Authors: Hudson, H. S.; Kosugi, T.; Nitta, N. V.; Shimojo, M. Bibcode: 2001ApJ...561L.211H Altcode: We have observed a high-speed coronal ejection in hard X-rays, detectable to an altitude of some 2×105 km in the Yohkoh 23-53 keV energy bands. Simultaneous imaging at 17 and 34 GHz from the Nobeyama radioheliograph shows complex moving features simultaneous with the ejection, including a compact source that we identify with the rapid X-ray source motion. The hard X-ray and microwave observations agree on ejection velocities in the vicinity of 1000 km s-1. The hard X-ray sources also corresponded in position angle to a bright coronal mass ejection (CME) detected about 15 minutes later and temporally to both fast-drift and slow-drift radio bursts in the decimeter-meter bands. Other components of coronal hard X-ray emission were also detected, including an extended long-duration event with a nonthermal spectrum. We suggest that a major eruptive flare occurred in NOAA Active Region 9415, approximately 26° beyond the west limb at the time of the event. Estimating a source density of 4×109 cm-3 from the compact source observed at 17 GHz, we find a total electron number (>20 keV) of approximately N20~1.3×1036 for the compact part of the source. We infer that these electrons were trapped in expanding loops forming a part of the CME and may have contributed substantial pressure within these loops. Title: Observing coronal mass ejections without coronagraphs Authors: Hudson, H. S.; Cliver, E. W. Bibcode: 2001JGR...10625199H Altcode: A coronal mass ejection (CME), strictly speaking, is a phenomenon observed via a white-light coronal imager. In addition to coronagraphs, a wide variety of other instruments provide independent observations of CMEs, in regimes ranging from the chromosphere to interplanetary space. In this paper we list the most important of these noncoronagraphic signatures, many of which had been known even before CMEs were first identified in coronagraph observations about 30 years ago. We summarize the new aspects of CMEs discovered in the past several years, primarily with instruments on the Yohkoh and SOHO satellites. We emphasize the need for detailed statistically based comparisons between SOHO CMEs and their noncoronagraphic manifestations. We discuss how the various aspects of CMEs fit into the current standard model (sigmoids, flux rope, double dimming, arcade). While a class of CMEs follows this pattern, it does not appear to work for all events. In particular, some CMEs involve extended dimming regions and erupting transequatorial X-ray loops, indicating a more complex geometry than a simple bipolar magnetic configuration. Title: Recurrent flare/CME events from an emerging flux region Authors: Nitta, Nariaki V.; Hudson, Hugh S. Bibcode: 2001GeoRL..28.3801N Altcode: We report on six recurrent ‘halo’ coronal mass ejections (CMEs) that occurred (in November 2000) during a 60-hour period in clear association with major flares in an active region on the solar disk. The region was undergoing dynamic restructuring due to flux emergence. The flares were not long-decay events (LDEs) in terms of soft X-ray light curves and morphologies, although, in the impulsive phase, they produced ejections in soft X-rays that are characteristic of CMEs. We do not detect global changes in EUV and X-ray full-disk images prior to these flares. We suggest that emerging magnetic flux in the core of an active region may be responsible for the occurrence of such repeated flare/CME events. Title: Chromospheric Damping of Alfvén Waves Authors: De Pontieu, B.; Martens, P. C. H.; Hudson, H. S. Bibcode: 2001ApJ...558..859D Altcode: We analytically study the damping of Alfvén mode oscillations in the chromosphere and in coronal loops. In the partially ionized chromosphere the dominant damping process of Alfvén waves is due to collisions between ions and neutrals. We calculate the damping time for Alfvén waves of a given frequency, propagating through model chromospheres of various solar structures such as active region plage, quiet sun, and the penumbra and umbra of sunspots. For a given wave frequency, the maximum damping always occurs at temperature minimum heights and in the coldest structure(s), i.e., the umbra of sunspots. Energy dissipation due to ion-neutral damping of Alfvén waves with an energy flux of 107 ergs cm-3 s- 1 can play a considerable role in the energy balance of umbrae, quiet sun, and plage for Alfvén wave periods of the order, respectively, 50, 5, and 0.5 s. We also consider Alfvén waves in coronal loops and the leakage of wave energy through the footpoints. We assume a three-layer model of coronal loops with constant Alfvén speed vA (and no damping) in the corona, vA varying exponentially with height in the dissipative chromosphere, and vA again constant in the photosphere at the end of the loop. We find an exact analytical solution in the chromospheric part. Using these solutions, we estimate the leakage of wave energy from the coronal volume through the footpoint regions of the loop and find that the presence of a moderate amount of chromospheric damping can enhance the footpoint leakage. We apply this result to determine the damping time of standing waves in coronal loops. The enhanced footpoint leakage also has implications for theories of coronal heating based on resonant absorption. Finally, we find exact expressions for the damping of Alfvén waves launched in the photosphere and upward propagating through the chromosphere and into the corona. The partially ionized chromosphere presents an effective barrier for upward propagating Alfvén waves with periods less than a few seconds. Title: Hard X-rays from Slow Flares Authors: Hudson, H. S.; McKenzie, D. E. Bibcode: 2001EP&S...53..581H Altcode: "Slow LDEs" are those for which the rise phase is slow, as well as the decay phase. Such flares follow the Neupert effect, which implies that the non-thermal energy release has a similar relationship to heating as in a normal impulsive flare. Based on a sample of 53 slow LDEs during the first nine years of Yohkoh observations, we find 19 for which substantial overlap occurs with BATSE hard X-ray observations. These events tend strongly to have extended hard X-ray emission even though their hard X-ray emission does not tend to be "impulsive" in the sense of rapid variation. The hard X-ray fluences for these 19 events correlate with the soft X-ray peak fluxes, implying strong non-thermal particle acceleration even for these relatively slow energy-release rates. These events often correspond to the occurrence of "supra-arcade downflows," a phenomenon consistent with the classical reconnection model for gradual-phase flare energy release. This correspondence suggests a close relationship, not depending strongly upon time scale, between large-scale reconnection and the acceleration of non-thermal electrons. Title: Energetics of an Active Region Observed from Helium-Like Sulphur Lines Authors: Watanabe, Tetsuya; Sterling, Alphonse C.; Hudson, Hugh S.; Harra, Louise K. Bibcode: 2001SoPh..201...71W Altcode: We report temperature diagnostics derived from helium-like ions of sulphur for an active region NOAA 7978 obtained with Bragg Crystal Spectrometer (BCS) on board the Yohkoh satellite. For the same region we estimate conductive flux downward to the chromosphere by the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO) satellite. This region appeared as a region of soft X-ray enhancement in May 1996, underwent a period of enhanced activity coinciding with flux emergence between 6 July and 12 July, and then continued to exist in a nearly flareless state for several solar rotations until November 1996. Energy balance of the non-flaring active region is basically consistent with a model of an arcade of coronal loops having an average loop-top temperature of 4×106 K. Energy from flare activity during a period of flux emergence is comparable to the energy requirements of the non-flaring active region. However, the non-flaring energy is roughly constant for the subsequent solar rotations following the birth of the active region even after the flare activity essentially subsided. Energy partition between flare activity and steady active-region heating thus varies significantly over the lifetime of the active region, and active-region emission cannot always be identified with flaring. Title: Downflows and structure above LDE arcades: Possible signatures of reconnection? Authors: McKenzie, D. E.; Hudson, H. S. Bibcode: 2001EP&S...53..577M Altcode: We will introduce a set of observations made during the decay phase of long duration event (LDE) flares on the Sun. In a number of events, the soft X-ray images from Yohkoh SXT indicate a downward directed flow field in the region immediately above the flare arcade. These are tentatively identified as evidence of reconnection outflows. The prototypical event is the M5 flare of 20 January 1999, presented by McKenzie and Hudson (1999); since the time that paper was completed, sixteen more events have been found in the interval April 1998 to February 2000. As in the 20 January event, the speeds of downflow are 35-500 km s-1, lower than both the freefall speed and the typically assumed Alfvén speed. The motion is evidenced by both dark and bright (i.e., X-ray emitting) features, some of which may have a looplike morphology; no cool counterparts have been detected in Hα or EUV observations. Movies depicting some of the LDE downflows were presented at this meeting; these also appear in the journal Solar Physics (McKenzie, 2000), since they cannot be presented in these Proceedings. Title: Onset of the Magnetic Explosion in Solar Flares and Coronal Mass Ejections Authors: Moore, Ronald L.; Sterling, Alphonse C.; Hudson, Hugh S.; Lemen, James R. Bibcode: 2001ApJ...552..833M Altcode: We present observations of the magnetic field configuration and its transformation in six solar eruptive events that show good agreement with the standard bipolar model for eruptive flares. The observations are X-ray images from the Yohkoh soft X-ray telescope (SXT) and magnetograms from Kitt Peak National Solar Observatory, interpreted together with the 1-8 Å X-ray flux observed by GOES. The observations yield the following interpretation. (1) Each event is a magnetic explosion that occurs in an initially closed single bipole in which the core field is sheared and twisted in the shape of a sigmoid, having an oppositely curved elbow on each end. The arms of the opposite elbows are sheared past each other so that they overlap and are crossed low above the neutral line in the middle of the bipole. The elbows and arms seen in the SXT images are illuminated strands of the sigmoidal core field, which is a continuum of sheared/twisted field that fills these strands as well as the space between and around them. (2) Although four of the explosions are ejective (appearing to blow open the bipole) and two are confined (appearing to be arrested within the closed bipole), all six begin the same way. In the SXT images, the explosion begins with brightening and expansion of the two elbows together with the appearance of short bright sheared loops low over the neutral line under the crossed arms and, rising up from the crossed arms, long strands connecting the far ends of the elbows. (3) All six events are single-bipole events in that during the onset and early development of the explosion they show no evidence for reconnection between the exploding bipole and any surrounding magnetic fields. We conclude that in each of our events the magnetic explosion was unleashed by runaway tether-cutting via implosive/explosive reconnection in the middle of the sigmoid, as in the standard model. The similarity of the onsets of the two confined explosions to the onsets of the four ejective explosions and their agreement with the model indicate that runaway reconnection inside a sheared core field can begin whether or not a separate system of overlying fields, or the structure of the bipole itself, allows the explosion to be ejective. Because this internal reconnection apparently begins at the very start of the sigmoid eruption and grows in step with the explosion, we infer that this reconnection is essential for the onset and growth of the magnetic explosion in eruptive flares and coronal mass ejections. Title: The Physical Nature of the Loop-Top X-Ray Sources in the Gradual Phase of Solar Flares Authors: Nitta, Nariaki V.; Sato, Jun; Hudson, Hugh S. Bibcode: 2001ApJ...552..821N Altcode: We have analyzed Yohkoh soft and hard X-ray images of 36 flares, primarily to study the loop-top source that often prevails in these wavelengths during and following the impulsive phase. There are typically two patterns for the location of the low-energy (15-30 keV) hard X-ray (HXR) source with respect to the soft X-ray (SXR) loop. In a quarter of the flares, the HXR source lies in an extended structure separate from the brightest SXR loop. In other flares, the HXR source appears to be part of the same bipolar structure as the SXR loop, but its centroid is often displaced from the SXR loop-top source. The fact that the HXR source is not cospatial with the SXR source may reflect the presence of a distinct hotter structure. According to Yohkoh X-ray emission-line spectroscopy, the ~20 MK plasma accounts for only a fraction of the HXR counts. The temperature maps obtained from the SXR broadband photometry occasionally reveal high-temperature areas outside the bright loop, but they also tend to be displaced from the HXR source, indicating that they do not represent the superhot (>~30 MK) plasma. We discuss possible distributions of plasma of different temperatures that could be consistent with the data. Title: Ribbons and field at high resolution Authors: Fletcher, L.; Hudson, H. S. Bibcode: 2001AGUSM..SH31D10F Altcode: The TRACE data let us define the geometry of flare ribbons with great precision, as for example in the Bastille Day 2000 flare. We study the UV and EUV ribbon motions for this event in comparison with the loops seen by SXT and in the TRACE high-temperature response, and compare these also with the photospheric magnetic field as observed by MDI. In this work we assume the standard model for large-scale coronal magnetic reconnection as a source of flare energy, and search for a correlation between footpoint locations, magnetic field strength and flare energy release as measured by hard X-ray emission. Title: A Search for Discrete Changes in Coronal Hole Boundaries Authors: Kahler, S. W.; Hudson, H. S. Bibcode: 2001AGUSM..SH51C05K Altcode: Coronal hole boundaries separate the large-scale open and closed magnetic fields of the solar corona. Mid-latitude coronal holes often last for several rotations, maintain their structure and rotate more slowly than photospheric magnetic structures such as active regions. Thus it is assumed that magnetic reconnection between open and closed field lines must occur at the boundaries of holes to maintain their shapes and rotation rates. We use Yohkoh SXT difference images to study the changes in coronal hole boundaries as long and short-lived holes cross the disk near central meridian. Specifically we are interested in learning whether the reconnection proceeds smoothly or in a stepwise fashion. We focus on the equatorial extensions of polar coronal holes observed in 1992 and in 2000. We use similar techniques to study the evolution of the boundaries of transient coronal holes. Title: Automated Search for Limb-Occulted Flares Authors: Freeland, S.; Hudson, H. Bibcode: 2001AGUSM..SP51A07F Altcode: Limb-occulted flares give instruments with limited spatial resolution or limited dynamic range an opportunity to study coronal processes without the competition of bright sources in the lower atmosphere, for example at the footpoints of coronal loops. We have created an automated search procedure for Yohkoh soft X-ray and hard X-ray data and have tested it for the 1999 data, finding a total of 59 candidate events that occurred quite near the limb. The procedure produces a Web page for each candidate event, including images and lightcurves plus a set of parameters intended to guide the distinction between front-side and back-side events. The initial application of the search will be to develop a comprehensive list of coronal hard X-ray events from the first 11 years of Yohkoh observations. Title: Coronal Structure from the Yohkoh Perspective Authors: Hudson, H. Bibcode: 2000SPD....31.1302H Altcode: 2000BAAS...32..845H The soft X-ray observations from Yohkoh delineate the higher-temperature regions of the solar corona. In general the large-scale corona generally appears diffuse outside of coronal holes and active regions. However trans-equatorial loop structures often appear, and appear to be especially prominent during the current rise to maximum. Title: Electron Trapping and Precipitation in Asymmetric Solar Flare Loops Authors: Aschwanden, M. J.; Fletcher, L.; Sakao, T.; Kosugi, T.; Hudson, H. Bibcode: 2000IAUS..195..375A Altcode: Acceleration, propagation, and energy loss of particles energized in solar flares cannot be studied separately because their radiative signatures observed in the form of hard X-ray bremsstrahlung or radio gyrosynchrotron emission represent a convolution of all these processes. We analyze hard X-ray emission from solar flares using a kinematic model that includes free-streaming electrons (having an energy-dependent time-of-flight delay) as well as temporarily trapped electrons (which are pitch-angle scattered by Coulomb collisional scattering) to determine various physical parameters (trapping times, flux asymmetry, loss-cone angles, magnetic mirror ratios) in flare loops with asymmetric magnetic fields. Title: Chromospheric Damping of Alfvén Waves Authors: De Pontieu, B.; Martens, P. C. H.; Hudson, H. S. Bibcode: 2000SPD....31.0131D Altcode: 2000BAAS...32..806D We study the damping of Alfvén mode oscillations on coronal loops and in the chromosphere. First we consider damping of standing waves on coronal loops, such as those observed in the aftermath of a flare with the Transition Region and Coronal Explorer (TRACE). We calculate the leakage of wave energy from the coronal volume through the footpoints of a coronal loop, assuming constant Alfvén speed vA in the corona and vA varying exponentially with height in the photosphere/chromosphere at both ends of the loop. We study analytically the influence of chromospheric damping of standing waves on a coronal loop and find that, for a moderate amount of chromospheric damping, the footpoint leakage can be enhanced. The damping in the partially ionized chromosphere is mostly due to collisions between ions and neutrals. In a second part we calculate the damping time for Alfvén waves of a given frequency, propagating through (model) chromospheres of various solar structures such as active region plage, quiet sun and the penumbra and umbra of sunspots. For a given wave frequency, the maximum damping always occurs at temperature minimum heights and in the coldest structure(s), i.e. the umbra of a sunspot. Energy dissipation due to ion-neutral damping of Alfvén waves could play a considerable role in the energy balance of umbrae, quiet sun and plage for wave periods of the order, respectively, 100, 10 and 1 s. Title: Max Millennium/Whole Sun Month Observations of a Sigmoid Region (AR 8668) Authors: Zarro, D. M.; Canfield, R. C.; Nitta, N.; Myers, D. C.; Gregory, S. E.; Qiu, J.; Alexander, D.; Hudson, H. S.; Thompson, B. J.; LaBonte, B. J. Bibcode: 2000SPD....31.0236Z Altcode: 2000BAAS...32..817Z We report on observations of a sigmoidal region AR 8668 obtained during the Whole Sun Month #3 campaign and Max Millennium Coordinated Observing Program #2. The observations pertain to the period 1999 August 16-17 during which several GOES B and C class flares occurred in AR 8668. Near simultaneous observations were obtained by SOHO (EIT 195 Angstroms/ and MDI full-disk magnetograms), TRACE 171 Angstroms/, Yohkoh SXT, Big Bear (Hα ), and Mees (IVM vector magnetograms). The multi-wavelength nature of these data, combined with their overlapping spatial and temporal coverages, provide a unique opportunity to study the magnetic topology and flaring evolution of twisted flux structures associated with sigmoids. An objective of this study is to co-align images and magnetograms obtained before and during the observed flares, and compare the results with inferences from the topological model of Titov and Demoulin, A&A 351, 707 (1999). We will present examples of these coalignments and identify sites of magnetic energy release that are associated with topological features (e.g. separatrices) predicted by this model. Title: Observations of the 24 September 1997 Coronal Flare Waves Authors: Thompson, B. J.; Reynolds, B.; Aurass, H.; Gopalswamy, N.; Gurman, J. B.; Hudson, H. S.; Martin, S. F.; St. Cyr, O. C. Bibcode: 2000SoPh..193..161T Altcode: We report coincident observations of coronal and chromospheric `flare wave' transients in association with a flare, large-scale coronal dimming, metric radio activity and a coronal mass ejection. The two separate eruptions occurring on 24 September 1997 originate in the same active region and display similar morphological features. The first wave transient was observed in EUV and Hα data, corresponding to a wave disturbance in both the chromosphere and the solar corona, ranging from 250 to approaching 1000 km s−1 at different times and locations along the wavefront. The sharp wavefront had a similar extent and location in both the EUV and Hα data. The data did not show clear evidence of a driver, however. Both events display a coronal EUV dimming which is typically used as an indicator of a coronal mass ejection in the inner corona. White-light coronagraph observations indicate that the first event was accompanied by an observable coronal mass ejection while the second event did not have clear evidence of a CME. Both eruptions were accompanied by metric type II radio bursts propagating at speeds in the range of 500-750 km s−1, and neither had accompanying interplanetary type II activity. The timing and location of the flare waves appear to indicate an origin with the flaring region, but several signatures associated with coronal mass ejections indicate that the development of the CME may occur in concert with the development of the flare wave. Title: Homologous sudden disappearances of transequatorial interconnecting loops in the solar corona Authors: Khan, Josef I.; Hudson, Hugh S. Bibcode: 2000GeoRL..27.1083K Altcode: We have found a remarkable sequence of homologous disappearances of transequatorial X-ray loops linking active regions. Each disappearance was closely associated with a major flare and coronal mass ejection (CME). In each case the flarings precede the disappearances and the CMEs. Mass estimates for the X-ray loops are similar to CME masses. This, the timing of the disappearances, their morphology, and the homology of the events in the sequence, provide direct evidence for a new class of CME origins in the low corona. We also briefly report observations of features which we infer to be the soft X-ray counterparts of shock waves emanating from the flare region. The inferred shocks appeared to play a vital role in the disappearances. Our results suggest that flare-generated shock waves may destabilize large transequatorial loops, causing them to erupt. Title: Implosions in Coronal Transients Authors: Hudson, H. S. Bibcode: 2000ApJ...531L..75H Altcode: Coronal events such as flares or coronal mass ejections derive their energy from the energy stored locally in the magnetic field. This leads to the conjecture that a magnetic implosion must occur simultaneously with the energy release. The site of the implosion would show the location of preflare energy storage, and its detection should have a high priority. The Transition Region and Coronal Explorer EUV observations, for example, have sufficient resolution to show the geometry of a flare implosion by following the motions of tracers in the images. Title: Yohkoh SXT and SOHO EIT Observations of Sigmoid-to-Arcade Evolution of Structures Associated with Halo Coronal Mass Ejections Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Thompson, Barbara J.; Zarro, Dominic M. Bibcode: 2000ApJ...532..628S Altcode: A subset of the solar-disk counterparts to halo coronal mass ejections (CMEs) displays an evolution in soft X-rays (SXR) that is characterized by a preflare S-shaped structure, dubbed a ``sigmoid,'' which evolves into a postflare cusp or arcade. We examine the morphological properties of the evolution of sigmoids into cusps and arcades for four such regions associated with SXR flares, using the Soft X-Ray Telescope (SXT) on the Yohkoh satellite and the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) satellite. Most of our EIT observations are with the 1.5 MK 195 Å Fe XII channel. At most, there is only a weak counterpart to the SXR sigmoid in the preflare 195 Å EUV images, indicating that the preflare sigmoid has a temperature greater than 1.5 MK. While more identifiable than in the 195 Å channel, a clear preflare sigmoid is also not observed in the 2.0 MK EIT 284 Å Fe XV channel. During the time of the flare, however, an EUV sigmoid brightens near the location of the SXR preflare sigmoid. Initially the SXR sigmoid lies along a magnetic neutral line. As the SXR flare progresses, new field lines appear with orientation normal to the neutral line and with footpoints rooted in regions of opposite polarity; these footpoints are different from those of the preflare sigmoid. The cusp structures in SXRs develop from these newly ignited field lines. In EIT images, the EUV sigmoid broadens as the flare progresses, forming an arcade beneath the SXR cusp. Our findings are consistent with a standard picture in which the origin of the flare and CME is caused by the eruption of a filament-like feature, with the stretching of field lines producing a cusp. We infer that the cusp-producing fields may be overlying the sigmoid fields in the preflare phase, but we do not directly observe such preflare overlying fields. Title: Solar Activity and the Formation of Coronal Holes Authors: Harvey, Karen L.; Hudson, Hugh S. Bibcode: 2000AdSpR..25.1735H Altcode: We describe the evolution of a complex of active regions belonging to cycle 22 and how its interaction with two new-cycle (23) regions resulted in the formation of several isolated coronal holes, in the development of the large coronal hole extending from the north polar hole observed in late August 1996, and in significant changes in both polar coronal holes Title: Hard X-rays from "Slow LDEs" Authors: Hudson, H. S.; McKenzie, D. E. Bibcode: 2000ASPC..206..221H Altcode: 2000hesp.conf..221H No abstract at ADS Title: Yohkoh Observations of White-Light Flares Authors: Matthews, S.; van Driel-Gesztelyi, L.; Hudson, H.; Nitta, N. Bibcode: 2000ASPC..206..239M Altcode: 2000hesp.conf..239M No abstract at ADS Title: The Sun as an X-Ray Star. II. Using theYohkoh/Soft X-Ray Telescope-derived Solar Emission Measure versus Temperature to Interpret Stellar X-Ray Observations Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 2000ApJ...528..537P Altcode: This paper is the second of a project dedicated to using solar Yohkoh/SXT data as a guide and a template to interpret data on stellar coronae. In the light of the large differences in scope and approach between solar and stellar studies, we have developed a method to translate Yohkoh/SXT data of the whole solar corona into stellar-like data, i.e., to put them in the same format and context as the stellar ones. First from the Yohkoh/SXT images we derive the whole-Sun X-ray emission measure versus temperature [EM(T)], in the range 105.5-108 K, during the specific observation. Then, we synthesize the solar X-ray spectrum; finally, we fold the spectrum through the instrumental response of nonsolar X-ray observatories, for instance, ROSAT/PSPC and ASCA/SIS. Finally, we analyze such solar coronal data in the same band and with the same methods used for stellar observations, allowing a direct and homogeneous comparison with them. In this paper we present in detail our method and, as an example of results, we show and discuss EM(T) and stellar-like spectra for three phases of the solar cycle: maximum, intermediate phase, and minimum. The total amount and the distribution of the emission measure change dramatically during the cycle, in particular at temperatures above 106 K. We also show the EM(T) of the whole solar corona during a large flare. The ROSAT/PSPC- and ASCA/SIS-like X-ray spectra of the Sun as a star that we obtain are discussed in the context of stellar coronal physics. The Sun's coronal total luminosity in the ROSAT/PSPC band ranges from ~2.7×1026 ergs s-1 (at minimum) to ~4.7×1027 ergs s-1 (at maximum). We discuss future developments and possible applications of our method. Title: Hot Cores in Coronal Filament Cavities Authors: Hudson, H.; Schwenn, R. Bibcode: 2000AdSpR..25.1859H Altcode: Filaments represent cold intrusions in the corona, embedded in magnetic configurations termed ``filament cavities.'' Such cavities may occur without actually containing prominence material. A cavity then may erupt, leading to a coronal mass ejection (CME). Studies of Yohkoh soft X-ray images have previously shown that such eruptions may contain elongated high-temperature regions closely aligned with the Hα filament material. We report in this paper multi-wavelength observations of an extremely stable filament cavity, observed by Yohkoh and SOHO during July-September 1997. Hot multi-thermal structures persistently occupied the core of this large-scale polar-crown cavity Title: Global Coronal Waves: Implications for HESSI Authors: Hudson, H. S.; Karlický, M. Bibcode: 2000ASPC..206..268H Altcode: 2000hesp.conf..268H No abstract at ADS Title: X-ray spectra of the Sun as a star: how different coronal regions contribute to the observed X-ray spectrum Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 2000ASPC..198..479O Altcode: 2000scac.conf..479O No abstract at ADS Title: Structure and Dynamics of the Corona Surrounding an Eruptive Prominence Authors: Gopalswamy, N.; Hanaoka, Y.; Hudson, H. S. Bibcode: 2000AdSpR..25.1851G Altcode: We report on the 1997 December 14 prominence eruption event that was accompanied by eruptive signatures in X-rays, EUV and white light: coronal dimming, X-ray arcade formation , X-ray brightenings, EUV eruption, and a white light CME. The data used were obtained by the Nobeyama Radioheliograph, Yohkoh Soft X-ray Telescope (SXT) and SOHO/LASCO and EIT. We identified various substructures of the eruption and their inter-relationships. We found that the pre-disruption swelling of the equatorial streamer was caused by the outward displacement of the coronal material around the prominence location. The dynamical behaviors of the CME and the accompanying eruptive prominence seem to be very different Title: TRACE and Yohkoh Observations of High-Temperature Plasma in a Two-Ribbon Limb Flare Authors: Warren, H. P.; Bookbinder, J. A.; Forbes, T. G.; Golub, L.; Hudson, H. S.; Reeves, K.; Warshall, A. Bibcode: 1999ApJ...527L.121W Altcode: The ability of the Transition Region and Coronal Explorer (TRACE) to image solar plasma over a wide range of temperatures (Te~104-107 K) at high spatial resolution (0.5" pixels) makes it a unique instrument for observing solar flares. We present TRACE and Yohkoh observations of an M2.4 two-ribbon flare that began on 1999 July 25 at about 13:08 UT. We observe impulsive footpoint brightenings that are followed by the formation of high-temperature plasma (Te>~10 MK) in the corona. After an interval of about 1300 s, cooler loops (Te<2 MK) form below the hot plasma. Thus, the evolution of the event supports the qualitative aspects of the standard reconnection model of solar flares. The TRACE and Yohkoh data show that the bulk of the flare emission is at or below 10 MK. The TRACE data are also consistent with the Yohkoh observations of hotter plasma (Te~15-20 MK) existing at the top of the arcade. The cooling time inferred from these observations is consistent with a hybrid cooling time based on thermal conduction and radiative cooling. Title: The Global Dynamics of the High-Temperature Corona Authors: Hudson, Hugh Bibcode: 1999SoPh..190...91H Altcode: This paper surveys coronal motions detected by the Yohkoh soft X-ray telescope SXT, emphasizing `global restructuring'. Large-scale structures in the solar corona can persist for time scales much longer than those of the supergranulation, and may have larger spatial scales. Flares and coronal mass ejections (CMEs) can disrupt these structures. A grazing-incidence telescope such as SXT provides a view of the corona biased in temperature towards the hotter components, but in a thick `observing slice' (spatial contribution function). This helps in seeing excitation (energy release) but may conceal some of the structural changes. The observations of restructuring largely appear to consist of expanding or outwards magnetic motions, which are endoergic. This suggests a conjecture regarding the existence of magnetic implosions on scales not yet detected, as a source of free energy. Title: Multi-Wavelength Observations of a Large-Scale Jet and an Eruptive- Prominence on 28 August 1992 Authors: Watanabe, Ta.; Ashizawa, K.; Nakagawa, Y.; Miyazaki, H.; Irie, M.; Ichimoto, K.; Kurokawa, H.; Hudson, H.; Yatagai, H. Bibcode: 1999spro.proc..171W Altcode: An eruption of a large (15o) north-south aligned quiescent prominence and associated coronal disturbance, which took place above the eastern solar limb near the equator on 28 August 1992, were observed at a wide range of wavelengths ranging from soft X-rays (Yohkoh) to microwaves (Nobeyama). The eruption was preceded by the formation of a large-scale jet which was apparently ejected near the root of the southern leg of the prominence. The characteristic outward speed of the jet was 450 km s-1. A potential-field presentation of the coronal magnetic field suggests that the jet was formed along the open field which was located immediately to the west of the magnetic arcade, originally surrounding the eruptive prominence. The temperature of the jet is suggested to be comparable to that of the nearby quiet corona (2 × 106 K). In the course of the prominence eruption, helically twisted loops surrounding the prominence were observed. This suggests that magnetic reconnection of the sheared arcade took place underneath the erupting prominence. Title: Coronal Mass Ejections at High Temperatures Authors: Hudson, H. Bibcode: 1999spro.proc..159H Altcode: We now have extensive X-ray (Yohkoh) and EUV observations of the behavior of coronal mass ejections (CMEs) at high temperatures in the lower corona. We also now have coronagraph observations from space with which to make identifications of the related phenomena. This paper reviews theories and observations of CMEs in this new context. Title: Reply Authors: Kahler, S. W.; Cane, H. V.; Hudson, H. S.; Kurt, V. G.; Gotselyuk, Y. V.; MacDowall, R. J.; Bothmer, V. Bibcode: 1999JGR...10422411K Altcode: No abstract at ADS Title: Transequatorial Interconnecting Loops and Increase of Their Length in the New Cycle Authors: Fárnik, F.; Ŝvestka, Z.; Karlický, M.; Hudson, H. S. Bibcode: 1999ESASP.446..305F Altcode: 1999soho....8..305F Using Yohkoh SXT data, we show that active regions of the new solar cycle, appearing at high latitudes, can be connected across the solar equator by loops as long as 60 heliographic degrees (730000 km). This length greatly exceeds the limit of 37 degrees found for transequatorial interconnecting loops on Skylab and implies that the Skylab limit was simply due to the low latitudes of active regions during the period of Skylab observations. By modelling the loops in force-free approximation using Kitt Peak magnetograms, we find strong support for the interpretation that these long interconnecting loops originate through reconnection of magnetic field lines which extend from the two active regions towards and beyond the equator, and confirm the earlier finding by Canfield, Pevtsov, and McClymont that a favourable condition for the reconnection is the same chirality (i.e., same sign of the force-free parameter alpha and same direction of current flow) in the two active regions. As we were unable to find any longitudinal (i.e., east-west) loops of a comparable length, we suggest that an important component of the driving force for the reconnection of transequatorial interconnecting loops may be the differential solar rotation. It can help to drive the reconnection of loops extending in the north- south direction, whereas it does not help in the case of longitudinal loops. These conclusions are based on loop observations in December 1997 and May 1998, when only short-lived transequatorial loops connected the high-latitude active regions on the northern and southern hemispheres. However, in February 1999 a rich system of transequatorial loops as long as 550 000 km could be observed during its whole transit across the visible solar disk. We are now studying this system (not all supporting data have been available yet when writing this Abstract), comparing it with a similar system of much shorter transequatorial loops which crossed the solar disk in March/April 1992 during the preceeding cycle. We again try to model these long-lived loop systems in current-free approximation, using both Kitt Peak and Big Bear magnetograms and, for the February 1999 transit, also the more frequent, high-resolution full-disk magnetic maps from SOHO. This modelling verifies the location of the footpoints of interconnecting loops in low magnetic fields at peripheries of the interconnected active regions, indicates how some of the loops have been formed, and provides information about some of the reasons which lead to the shape and brightness variations in the loops system. Title: SOHO EIT Observations of Extreme-Ultraviolet ``Dimming'' Associated with a Halo Coronal Mass Ejection Authors: Zarro, Dominic M.; Sterling, Alphonse C.; Thompson, Barbara J.; Hudson, Hugh S.; Nitta, Nariaki Bibcode: 1999ApJ...520L.139Z Altcode: A solar flare was observed on 1997 April 7 with the Soft X-ray Telescope (SXT) on Yohkoh. The flare was associated with a ``halo'' coronal mass ejection (CME). The flaring region showed areas of reduced soft X-ray (SXR) brightness--``dimmings''--that developed prior to the CME observed in white light and persisted for several hours following the CME. The most prominent dimming regions were located near the ends of a preflare SXR S-shaped (sigmoid) feature that disappeared during the event, leaving behind a postflare SXR arcade and cusp structure. Based upon these and similar soft X-ray observations, it has been postulated that SXR dimming regions are the coronal signatures (i.e., remnants) of magnetic flux ropes ejected during CMEs. This Letter reports new observations of coronal dimming at extreme-ultraviolet (EUV) wavelengths obtained with the Extreme-ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO). A series of EIT observations in the 195 Å Fe XII wavelength band were obtained simultaneously with SXT during the 1997 April 7 flare/CME. The EIT observations show that regions of reduced EUV intensity developed at the same locations and at the same time as SXR dimming features. The decrease in EUV intensity (averaged over each dimming region) occurred simultaneously with an increase in EUV emission from flaring loops in the active region. We interpret these joint observations within the framework of flux-rope eruption as the cause of EUV and SXR coronal dimmings, and as the source of at least part of the CME. Title: How the Sun's Corona Gets Hot Authors: Hudson, H. S.; Kosugi, T. Bibcode: 1999Sci...285..849H Altcode: No abstract at ADS Title: X-Ray Observations of Motions and Structure above a Solar Flare Arcade Authors: McKenzie, D. E.; Hudson, H. S. Bibcode: 1999ApJ...519L..93M Altcode: In this Letter, we describe a solar flare that was observed by Yohkoh in 1999 January 20. This long-duration event is notable because the Yohkoh images show not only the formation of the arcade associated with the coronal mass ejection but also a considerable amount of motion above the arcade in the region normally identified with a large-scale current sheet or the outflow from magnetic reconnection in the current sheet. A number of arcade events of this morphological type (i.e., a fan of spikelike ``rays'' above the posteruption loops) have been seen by Yohkoh, but in this case we have a much clearer view of mass motions in the region above the arcade. The motions indicate field-line retraction without the formation of long-lasting cusps during the rise phase of the flare, and a downward flow above the arcade during the decay phase. The late-phase downward motion is in the form of X-ray dark voids moving at 100-200 km s-1, i.e., at velocities much smaller than the free-fall speed or the assumed Alfvén speed. We interpret the voids as cross sections of evacuated flux tubes resulting from intermittent reconnection following the associated coronal mass ejection. We believe these data represent the first direct evidence of high-speed flows in the region immediately above the flare loops. Title: Deconvolution of Directly Precipitating and Trap-precipitating Electrons in Solar Flare Hard X-Rays. III.Yohkoh Hard X-Ray Telescope Data Analysis Authors: Aschwanden, Markus J.; Fletcher, Lyndsay; Sakao, Taro; Kosugi, Takeo; Hudson, Hugh Bibcode: 1999ApJ...517..977A Altcode: We analyze the footpoint separation d and flux asymmetry A of magnetically conjugate double footpoint sources in hard X-ray images from the Yohkoh Hard X-Ray Telescope (HXT). The data set of 54 solar flares includes all events simultaneously observed with the Compton Gamma Ray Observatory (CGRO) in high time resolution mode. From the CGRO data we deconvolved the direct-precipitation and trap-precipitation components previously (in Paper II). Using the combined measurements from CGRO and HXT, we develop an asymmetric trap model that allows us to quantify the relative fractions of four different electron components, i.e., the ratios of direct-precipitating (qP1, qP2) and trap-precipitating electrons (qT1, qT2) at both magnetically conjugate footpoints. We find mean ratios of qP1=0.14+/-0.06, qP2=0.26+/-0.10, and qT=qT1+qT2=0.60+/-0.13. We assume an isotropic pitch-angle distribution at the acceleration site and double-sided trap precipitation (qT2/qT1=qP2/qP1) to determine the conjugate loss-cone angles (α1=42deg+/-11deg and α2=52deg+/-10deg) and magnetic mirror ratiosat both footpoints (R1=1.6,...,4.0 and R2=1.3,...,2.5). From the relative displacement of footpoint sources we also measure altitude differences of hard X-ray emission at different energies, which are found to decrease systematically with higher energies, with a statistical height difference of hLo-hM1=980+/-250 km and hM1-hM2=310+/-300 km between the three lower HXT energy channels (Lo, M1, M2). Title: Quiet solar wind signatures above active regions observed in X-rays Authors: Hick, P.; Svestka, Z.; Jackson, B. V.; Farnik, F.; Hudson, H. Bibcode: 1999AIPC..471..231H Altcode: 1999sowi.conf..231H X-ray images from the Yohkoh satellite, obtained following occurrences of limb flares sometimes show coronal fan-like structures extending above a growing post-flare loop system. We show one such event observed in AR 7270 on the east limb of the Sun on 28/29 August 1992. We suggest that these rays are `ministreamers,' formed as a result of the re-structuring of the corona following the occurrence of a flare-associated CME. Synoptic maps of the solar wind density, constructed from a tomographic analysis of interplanetary scintillation (IPS) measurements, show enhanced scintillation matching the position of AR 7270 if we assume a radial outflow at a reasonable slow solar wind speed of 400 km s-1. From this agreement we argue that outflow of mass occurs from the active region into interplanetary space. Title: Modeling CMEs in three dimensions using an analytic MHD model Authors: Gibson, Sarah E.; Alexander, David; Biesecker, Doug; Fisher, Richard; Guhathakurta, Madhulika; Hudson, Hugh; Thompson, B. J. Bibcode: 1999AIPC..471..645G Altcode: 1999sowi.conf..645G Because coronal mass ejections (CMEs) are viewed in projection, it is difficult to determine their three-dimensional nature. We use an analytic model of CMEs as an example of a fully three-dimensional magnetic field structure in MHD force balance with an emerging CME. We present the CME magnetic field and its associated density structure, seen projected at the limb from two viewing angles perpendicular to the plane of the sky, and emerging from disk center representing ``earth-directed'' CME events. The range of CME structures thus produced compares well to existing CME white-light coronagraph and full disk EUV and X-ray observations. In particular, we find that both 3-part ``front-cavity-core'' and ``U-shaped'' white light CMEs, as well as the twin dimmings (also referred to as transient coronal holes) observed in X-ray and EUV, can successfully be reproduced by the CME model. All of these structures are a direct consequence of a single three-dimensional magnetic field topology, viewed from different directions. Title: Soft X-ray Observation of a Flare-Associated Coronal Wave Authors: Hudson, H. S.; Lemen, J. R.; Thompson, B.; Uchida, Y. Bibcode: 1999AAS...194.2205H Altcode: 1999BAAS...31..860H Recent EUV observations from the EIT instrument of SOHO have shown the common occurrence of flare-associated global coronal waves, allies of Type II burst exciters and chromospheric Moreton waves. Until now, however, no direct soft X-ray detections have been reported. We have studied Yohkoh SXT observations to understand this apparent discrepancy between EUV and soft X-ray observations,and have now found good X-ray evidence for a large-scale coronal wave launched during an X-class flare of May 6, 1998. During the impulsive phase of this flare, a rapid ( ~ 10(3) km s(-1) ) bright front appeared to the north of the flare core; later a more normal loop-like ejection emerged to the west at a lower projected speed. We identify the rapid front with enhanced X-ray emission from a global coronal wave. Wave signatures also appear in SOHO EIT images and in the meter-wave dynamic spectrum from Hiraiso, and SOHO LASCO detected a coronal mass ejection. NASA supported this work under contract NAS 8-37334. Title: YOHKOH SXT and SOHO EIT Observations of ``Sigmoid-to-Arcade'' Evolution of Structures Associated with Halo CMEs Authors: Sterling, A. C.; Hudson, H. S.; Thompson, B. J.; Zarro, D. M. Bibcode: 1999AAS...19410107S Altcode: 1999BAAS...31..999S A subset of the solar-disk counterparts to halo coronal mass ejections (CMEs) display an evolution in soft X-rays (SXR) characterized by a preflare ``S''-shaped structure, dubbed a ``sigmoid,'' evolving into a postflare cusp or arcade. We examine the morphological properties of the evolution of sigmoids into cusps and arcades for four such regions associated with SXR flares, using the Soft X-ray Telescope (SXT) on Yohkoh and the 195 Angstroms Fe xii\ channel of the EUV Imaging Telescope (EIT) on SOHO. There is, at most, only a weak counterpart to the SXR sigmoid in the preflare EUV images, indicating that the preflare sigmoid has a temperature >1.5 MK\@. During the time of the flare itself, however, an EUV sigmoid brightens near the location of the SXR preflare sigmoid. Initially the SXR sigmoid lies along a magnetic neutral line. As the SXR flare progresses new field lines appear with orientation normal to the neutral line and with footpoints rooted in opposite polarity regions; these footpoints are different from those of the preflare sigmoid. The cusp structures in SXRs develop from these newly-ignited field lines. In EIT images the EUV sigmoid broadens out as the flare progresses, forming an arcade which resides beneath the SXR cusp. In many respects, our findings are consistent with a standard picture where the origin of the flare and CME is due to the eruption of a filament-like feature, and the stretching of overlying preflare fields produces the cusp. We do not, however, observe these preflare overlying fields prior to flare onset. This work was supported by the NRL Naval basic research program and NASA. Title: Arcade Structure and Dynamics in the 20-Jan-99 M5 Solar Flare Authors: McKenzie, D. E.; Hudson, H. S. Bibcode: 1999AAS...19410108M Altcode: 1999BAAS...31..999M We present observations of a remarkable solar flare which was observed by Yohkoh on 20-Jan-99. This long duration event is notable because the Yohkoh images show not only the formation of the arcade associated with the coronal mass ejection, but also a considerable amount of structure and motion in the current sheet above the arcade. Though not unique, the structure of this arcade is different from the majority of flare arcades observed by Yohkoh. The motions indicate field line shrinkage during the rise phase of the flare, and downward flow above the arcade during the decay phase. The late-phase downward motion suggests the possibility either of a high-Beta regime in the current sheet, or of shrinkage of evacuated flux tubes downward through the current sheet. This research is supported by NASA under MSFC contract NAS8-40801. Title: A Stable Filament Cavity with a Hot Core Authors: Hudson, H. S.; Acton, L. W.; Harvey, K. L.; McKenzie, D. E. Bibcode: 1999ApJ...513L..83H Altcode: We present observations of a long-lived solar filament cavity with soft X-ray sources along its axis. This structure appeared above the southern polar crown polarity-inversion line for approximately three rotations during 1997 June-August, centered at a west-limb passage on approximately July 3. At the limb, the Yohkoh soft X-ray data showed a bright region situated above and around the projected filament location but near the axis of the cavity. We describe measurements of the geometry of the cavity, which we interpret as a flux rope that is partially embedded in the photosphere, and use the Yohkoh data to describe the physical parameters of the structure. We find that the core consists of an unresolved mass of filamentary substructures, with a volume filling factor significantly less than unity for the soft X-ray telescope (SXT) resolution. The core has a higher temperature than the cavity surrounding it, ruling out explanations in terms of a transition region supported by thermal conduction. Transient activity occurred in the polar crown region, but no detectable destabilization or eruption of the cavity structure resulted from it. We suggest that the bright structure at the core of the cavity corresponds to higher altitude coronal segments of the field lines that support the filament material. Title: Sigmoidal morphology and eruptive solar activity Authors: Canfield, Richard C.; Hudson, Hugh S.; McKenzie, David E. Bibcode: 1999GeoRL..26..627C Altcode: Soft X-ray images of solar active regions frequently show S- or inverse-S (sigmoidal) morphology. We have studied the Yohkoh Soft X-Ray Telescope video movie for 1993 and 1997. We have classified active regions according to morphology (sigmoidal or non-sigmoidal) and nature of activity (eruptive or non-eruptive). As well, we have used NOAA sunspot areas for each region as a measure of size. We find that regions are significantly more likely to be eruptive if they are either sigmoidal or large. Title: The Solar-Stellar Connection in X-rays: How to Take Advantage of the YOHKOH data Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1999ASPC..158..391P Altcode: 1999ssa..conf..391P No abstract at ADS Title: Observational Tests of a Double Loop Model for Solar Flares Authors: Hardy, S. J.; Melrose, D. B.; Hudson, H. S. Bibcode: 1998PASA...15..318H Altcode: 1998PASA...15..317H A model for the energetics of solar flares, developed by Melrose (1997), is based on magnetic reconnection between two current-carrying magnetic loops. A detailed numerical investigation of the model has been made to identify those configurations that lead to energy release in a flare. Our results predict a strong relation between the ratio of currents in the interacting loops for a favoured flare configuration, and provide further support for a proposed method of generating long loops connecting different active regions. Both of these predictions are amenable to observational verification. Title: High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission for the next (2000) solar maximum Authors: Lin, Robert P.; Hurford, Gordon J.; Madden, Norman W.; Dennis, Brian R.; Crannell, Carol J.; Holman, Gordon D.; Ramaty, Reuven; von Rosenvinge, Tycho T.; Zehnder, Alex; van Beek, H. Frank; Bornmann, Patricia L.; Canfield, Richard C.; Emslie, A. Gordon; Hudson, Hugh S.; Benz, Arnold; Brown, John C.; Enome, Shinzo; Kosugi, Takeo; Vilmer, Nicole; Smith, David M.; McTiernan, Jim; Hawkins, Isabel; Slassi-Sennou, Said; Csillaghy, Andre; Fisher, George; Johns-Krull, Chris; Schwartz, Richard; Orwig, Larry E.; Zarro, Dominic; Schmahl, Ed; Aschwanden, Markus; Harvey, Peter; Curtis, Dave; Pankow, Dave; Clark, Dave; Boyle, Robert F.; Henneck, Reinhold; Michedlishvili, Akilo; Thomsen, K.; Preble, Jeff; Snow, Frank Bibcode: 1998SPIE.3442....2L Altcode: The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum. Title: Large-Scale Active Coronal Phenomena in Yohkoh SXT Images IV. Solar Wind Streams from Flaring Active Regions Authors: Švestka, Zdeněk; Fárník, František; Hudson, Hugh S.; Hick, Paul Bibcode: 1998SoPh..182..179S Altcode: We demonstrate limb events on the Sun in which growing flare loop systems are embedded in hot coronal structures looking in soft X-rays like fans of coronal rays. These structures are formed during the flare and extend high into the corona. We analyze one of these events, on 28-29 August 1992, which occurred in AR 7270 on the eastern limb, and interpret these fans of rays either as temporary multiple ministreamers or plume-like structures formed as a result of restructuring due to a CME. We suggest that this configuration reflects mass flow from the active region into interplanetary space. This suggestion is supported by synoptic maps of solar wind sources constructed from scintillation measurements which show a source of enhanced solar wind density at the position of AR 7270, which disappears when 5 days following the event are removed from the synoptic map data. We also check synoptic maps for two other active regions in which existence of these fan-like structures was indicated when the active regions crossed both the east and west limbs of the Sun, and both these regions appear to be sources of a density enhancement in the solar wind. Title: On the relationship between coronal mass ejections and magnetic clouds Authors: Gopalswamy, N.; Hanaoka, Y.; Kosugi, T.; Lepping, R. P.; Steinberg, J. T.; Plunkett, S.; Howard, R. A.; Thompson, B. J.; Gurman, J.; Ho, G.; Nitta, N.; Hudson, H. S. Bibcode: 1998GeoRL..25.2485G Altcode: We compare the substructures of the 1997 February 07 coronal mass ejection (CME) observed near the Sun with a corresponding event in the interplanetary medium to determine the origin of magnetic clouds (MCs). We find that the eruptive prominence core of the CME observed near the Sun may not directly become a magnetic cloud as suggested by some authors and that it might instead become the ”pressure pulse” following the magnetic cloud. We substantiate our conclusions using time of arrival, size and composition estimates of the CME-MC substructures obtained from ground based, SOHO and WIND observations. Title: X-ray coronal changes during Halo CMEs Authors: Hudson, H. S.; Lemen, J. R.; St. Cyr, O. C.; Sterling, A. C.; Webb, D. F. Bibcode: 1998GeoRL..25.2481H Altcode: Using the Yohkoh soft X-ray images, we examine the coronal structures associated with “halo” coronal mass ejections (CMEs). These may correspond to events near solar disk center. Starting with a list of eleven confirmed halo CMEs over the time range from December 1996 through May 1997, we find seven with surface features identifiable in soft X-rays, with GOES classifications ranging from A1 to M1.3. These have a characteristic pattern of sigmoid → arcade development. In each of these events, the pre-flare structure disrupted during the flare, leaving the appearance of compact transient coronal holes. The four remaining events had weak or indistinguishable signatures in the X-ray images. For the events for which we could see well-defined coronal changes, we confirm our previous result that the estimated mass loss inferred from the soft X-ray dimming is a small fraction of typical CME masses [Sterling & Hudson 1997]. Title: The solar origin of the January 1997 coronal mass ejection, magnetic cloud and geomagnetic storm Authors: Webb, D. F.; Cliver, E. W.; Gopalswamy, N.; Hudson, H. S.; St. Cyr, O. C. Bibcode: 1998GeoRL..25.2469W Altcode: The magnetic cloud and geomagnetic storm on January 10-11, 1997 were associated with a halo-type Coronal Mass Ejection (CME) observed by the SOHO/LASCO coronagraphs near the sun on January 6. We summarize the solar activity related to this CME and the subsequent storm at Earth. This solar activity was remarkably weak and unimpressive. If the wide CME had not been observed, the storm would not have been forecast. Thus this case represents an extreme example of so-called “problem” magnetic storms that lack obvious surface signatures of eruptive solar activity. It supports the view that CMEs involve the destabilization of large-scale coronal structures which may or may not have associated surface activity, and that CMEs, not the surface activity, are the key causal link between solar eruptions and space weather at Earth. Title: The solar energetic particle event of April 14, 1994, as a probe of shock formation and particle acceleration Authors: Kahler, S. W.; Cane, H. V.; Hudson, H. S.; Kurt, V. G.; Gotselyuk, Y. V.; MacDowall, R. J.; Bothmer, V. Bibcode: 1998JGR...10312069K Altcode: Gradual solar energetic particle (SEP) events observed at 1 AU are associated with coronal mass ejections (CME) that drive shocks which are presumed to accelerate the ions and electrons to suprathermal energies. However, high-energy (>30 MeV) proton and (>1 MeV) electron events are nearly always associated with both CMEs and flares, suggesting that the acceleration of those particles, particularly the electrons, could be attributed to the associated flares. Only one clear example of a high-energy SEP event without an active region flare association has been reported previously. We discuss a second such SEP event, on April 14, 1994, associated with a well-observed solar X ray arcade structure spanning ~150° of solar longitude. The SEP event, observed by detectors on the IMP 8 and Koronas I spacecraft, began ~10 hrs after the beginning of the X ray event and was temporally and spatially associated with the last of three weak interplanetary type III radio bursts observed by the Ulysses low-frequency radio experiment. The delayed onset and rapid rise of the SEP intensities preclude a recent interpretation in which SEPs were presumed to be accelerated by a shock driven by a CME which erupted at the onset of the X ray event. Yohkoh soft X ray subtracted images show a large-scale arcade brightening west of ~E10° beginning about 8 hours after the initial brightening near the east limb. We suggest that the April 14 SEP event at Earth was produced by a shock driven by a CME associated with the later brightening near central meridian. The initial X ray brightening may also have been associated with an earlier CME. Title: Precise Determination of the Coordinate Systems for the YOHKOH Telescopes and the Application of a Transit of Mercury Authors: Wülser, J. -P.; Hudson, H. S.; Nishio, M.; Kosugi, T.; Masuda, S.; Morrison, M. Bibcode: 1998SoPh..180..131W Altcode: The Yohkoh solar X-ray observatory carries two telescopes that require coalignment at a level better than the minimum pixel size of 2_45″. This coalignment is needed both internally within Yohkoh and for many scientific applications involving data from ground-based radio and optical observatories. We describe the methods successfully developed for this purpose and now incorporated in the Yohkoh software. Soft X-ray observations of the 1993 transit of Mercury across the solar disk provided key information for the calibration of the coalignment procedures. Title: Fan-Like coronal X-ray Structures as Sources of Solar Wind Authors: Hick, P.; Svestka, Z.; Farnik, F.; Hudson, H. S.; Jackson, B. V. Bibcode: 1998AAS...192.1503H Altcode: 1998BAAS...30..840H We show coronal soft X-ray images from the Yohkoh satellite, obtained following occurrences of limb flares. These images show rising post-flare loops, which are embedded in hot coronal structures looking like fans of coronal rays. We analyze the event on 28/29 August 1992, which occurred in AR 7270 on the east limb of the Sun. We suggest that these rays are multiple 'ministreamers', which apparently are formed as a result of the restructuring of the corona following the occurrence of a flare-associated CME. We argue that this configuration allows outflow of mass from the active region into interplanetary space. This is supported by synoptic maps of solar wind sources constructed from scintillation measurements showing a source of enhanced scintillation at the position of AR 7270. Title: 3-Dimensional Models of Active Region Loops Authors: Aschwanden, M. J.; Neupert, W. M.; Newmark, J.; Thompson, B. J.; Brosius, J. W.; Holman, G. D.; Harrison, R. A.; Bastian, T. S.; Nitta, N.; Hudson, H. S.; Zucker, A. Bibcode: 1998ASPC..155..145A Altcode: 1998sasp.conf..145A No abstract at ADS Title: An Interdisciplinary Study of the Eruptive Prominence of 28 August 1992 Authors: Watanabe, Ta.; Yamamoto, M.; Hudson, H.; Irie, M.; Ichimoto, K.; Kurokawa, H.; Yatagai, H. Bibcode: 1998ASSL..229..101W Altcode: 1998opaf.conf..101W No abstract at ADS Title: Solar-Stellar Connection: Relevance of YOHKOH Data Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1998ASPC..154.1130O Altcode: 1998csss...10.1130O The similarity of late-type stars to the Sun is often assumed when studying the physical conditions in their coronae. In order to explore better such a ``solar-stellar'' connection we use the Yohkoh/SXT X-ray images to generate the distribution of the emission measure vs. temperature of the Sun and, from that, the expected emission, as it would be observed by non-solar X-ray telescopes such as ROSAT/PSPC and ASCA/SIS. We discuss the role of the various solar structures in determining the total distribution of the emission measure vs. temperature and in determining the stellar-like synthesized X-ray spectra. Title: Birth Place of the 1998 January 21 CME Authors: Gopalswamy, N.; Hanaoka, Y.; Kaiser, M.; Gurman, J.; Hudson, H.; Howard, R. A. Bibcode: 1998cee..workE..40G Altcode: The 1998 January 21 halo coronal mass ejection was launched from high southern latitudes in association with a filament disappearance observed by the Nobeyama Radioheliograph. Signatures of the initial destabilization of the filament were observed by the Extreme-ultraviolaet Imaging Telescope (EIT) on board the SOHO spacecraft and by the Soft X-ray Telescope (SXT) on board Yohkoh. The Wind/WAVES experiment observed a type II burst in the 600-300 kHz range. The data coverage for this event is unusually high and we make use of it to understand the origin and evolution of the eruption. We address several issues based on these data: (i) relation between filament eruption and arcade formation beneath the filament, (ii) comparison between the hot arcade formation in X-rays and EUV, (iii) relation between the filament eruption and the white light CME, (iv) relation between the CME and the interplanetary shock inferred from the WAVES data. A summary of near-surface activities associated with the eruption can be seen in the Figure 1. Figure 1. SOHO/MDI longitudinal magnetogram, with radio filament (white contours) and X-ray emission (enclosed by dark lines) are overlaid. The thick white line from E to W is the neutral line over which the eruption took place. North is to the top and east is to the left. F is the location where the filament split at the time of eruption. Title: The Formation and Evolution of the Coronal Holes Associated with NOAA Region 7978 Authors: Harvey, K. L.; Hudson, H. S. Bibcode: 1998ASSL..229..315H Altcode: 1998opaf.conf..315H No abstract at ADS Title: NOAA 7978: the Last best Old-Cycle Region Authors: Hudson, H. S.; Labonte, B. J.; Sterling, A. C.; Watanabe, Te. Bibcode: 1998ASSL..229..237H Altcode: 1998opaf.conf..237H No abstract at ADS Title: Soft X-ray Observations of Eruptive Prominences Authors: Watanabe, T.; Yamamoto, M.; Hudson, H.; Irie, M.; Ichimoto, K.; Kurokawa, H.; Yatagai, H. Bibcode: 1998ASPC..150..376W Altcode: 1998IAUCo.167..376W; 1998npsp.conf..376W No abstract at ADS Title: The Sun as an X-Ray Star: Overview of the Method Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1998ASSL..229...29P Altcode: 1998opaf.conf...29P No abstract at ADS Title: Coronal and Interplanetary Disturbances Associated with an Eruptive Prominence of 28 August 1992 Authors: Watanabe, T.; Yamamoto, M.; Hudson, H.; Irie, M.; Ichimoto, K.; Kurokawa, H.; Yatagai, H. Bibcode: 1998asct.conf..313W Altcode: No abstract at ADS Title: The Evolution of a Rapidly-Expanding Active Region Loop into a Trans-Equatorial Coronal Mass Ejection Authors: Simnett, G. M.; Hudson, H. S. Bibcode: 1997ESASP.415..437S Altcode: 1997cpsh.conf..437S No abstract at ADS Title: Large-Scale Active Coronal Phenomena in YOHKOH SXT Images. III. Enhanced Post-Flare Streamer Authors: Švestka, Zdeněk; Fárník, František; Hick, Paul; Hudson, Hugh S.; Uchida, Yutaka Bibcode: 1997SoPh..176..355S Altcode: We demonstrate several events where an eruptive flare close to the limb gave rise to a transient coronal streamer visible in X-rays in Yohkoh SXT images, and analyze one of these events, on 28-29 October 1992, in detail. A coronal helmet streamer began to appear 2 hours after the flare, high above rising post-flare loops; the streamer became progressively narrower, reaching its minimum width 7-12 hours after the flare, and widened again thereafter, until it eventually disappeared. Several other events behaved in a similar way. We suggest that the minimum width indicates the time when the streamer became fully developed. All the time the temperature in the helmet streamer structure was decreasing, which can explain the subsequent fictitious widening of the X-ray streamer. It is suggested that we may see here two systems of reconnection on widely different altitudes, one giving rise to the post-flare loops while the other creates (or re-forms) the coronal helmet streamer. A similar interpretation was suggested in 1990 by Kopp and Polettofor post-flare giant arches observed on board the SMM; indeed, there are some similarities between these post-flare helmet streamers and giant arches and, with the low spatial resolution of SMM instruments, it is possible that some helmet streamers could have been considered to be a kind of a giant arch. Title: The High Energy Solar Spectroscopic Imager (HESSI): A Small Explorer for the Start of the New Millennium Authors: Holman, G. D.; Lin, R. P.; Dennis, B. R.; Crannell, C. J.; Ramaty, R. R.; Rosenvinge, T. T.; Canfield, R. C.; Emslie, A. G.; Hudson, H. S.; Hurford, G. J.; Madden, N. W.; van Beek, H. F.; Benz, A.; Bornmann, P. L.; Brown, J. C.; Enome, S.; Kosugi, T.; Vilmer, N.; Zehnder, A. Bibcode: 1997AAS...191.7416H Altcode: 1997BAAS...29R1326H The High Energy Solar Spectroscopic Imager (HESSI) has been selected for launch in mid 2000, at the peak of the solar activity cycle. The primary scientific objective of HESSI is to understand particle acceleration and explosive energy release in the magnetized plasma at the Sun. HESSI will provide the first high-spectral-resolution x-ray and gamma -ray images of the Sun. It will obtain the first imaging above 100 keV, the first imaging of solar gamma -ray lines, and the first high-resolution spectroscopy of solar gamma -ray lines, including the first determination of line shapes. In two years HESSI is expected to obtain observations of tens of thousands of microflares, thousands of hard x-ray flares, and of order a hundred gamma -ray line flares. HESSI will also monitor and provide high-spectral-resolution observations of cosmic and terrestrial hard x-ray and gamma -ray transients, as well as imaging of the Crab Nebula. HESSI's high spectral, spatial, and temporal resolution and dynamic range will allow the first detailed studies of the evolution of both accelerated particles and hot, thermal plasma in solar flares. Title: Solar Energetic Particle Events and Coronal Mass Ejections: New Insights from SOHO Authors: Bothmer, V.; Posner, A.; Kunow, H.; Müller-Mellin, R.; Herber, B.; Pick, M.; Thompson, B. J.; Delaboudinière, J. -P.; Brueckner, G. E.; Howard, R. A.; Michels, D. J.; Cyr, C. St.; Szabo, A.; Hudson, H. S.; Mann, G.; Classen, H. -T.; McKenna-Lawlor, S. Bibcode: 1997ESASP.415..207B Altcode: 1997cpsh.conf..207B No abstract at ADS Title: Yohkoh SXT Observations of X-Ray ``Dimming'' Associated with a Halo Coronal Mass Ejection Authors: Sterling, Alphonse C.; Hudson, Hugh S. Bibcode: 1997ApJ...491L..55S Altcode: A sudden depletion or intensity ``dimming'' of the X-ray corona sometimes accompanies a solar eruptive flare or coronal mass ejection (CME). We have identified a dimming that occurred just prior to a ``halo'' CME, observed on 1997 April 7 using the Soft X-ray Telescope (SXT) on Yohkoh. Halo CMEs are prime candidates for ``space weather'' effects. The dimming occurred in compact regions near a flare of 14 UT on April 7, over a projected area of about 1020 cm-2, and indicate that a mass of a few times 1014 g was ejected. This is a lower limit imposed by the obscuration of the dimming volume by the brightness of the accompanying flare and other factors. Most of the mass deficit comes from two regions close to the ends of a preflare S-shaped active-region structure, and the resulting dimmings in these regions persisted for more than three days following the flare. A cusp-shaped loop--not apparent prior to the flare--dominates the emission in the flare decay phase, and has a mass comparable to that lost in the dimming regions. Our findings are consistent with the source of the CME being a flux rope that erupted, leaving behind the dimming regions. The cusp-shaped loop probably represents magnetic fields reconfigured or reconnected by the eruption. We do not see an X-ray analog of the wavelike disturbance evident in SOHO EUV images. Title: An Overview of IACG Campaign 4: Solar Sources of Heliospheric Structure Observed Out of the Ecliptic Authors: Galvin, A. B.; Hudson, H. S. Bibcode: 1997ESASP.415...39G Altcode: 1997cpsh.conf...39G No abstract at ADS Title: Post-Flare Loops Embedded in a Hot Coronal Fan-Like Structure Authors: Švestka, Z.; Fárnik; Hudson, H. S.; Hick, P. Bibcode: 1997ESASP.415..139S Altcode: 1997cpsh.conf..139S No abstract at ADS Title: The Solar Energetic Particle Event of 14 April 1994 as a Probe of Shock Formation and Particle Acceleration Authors: Kahler, S. W.; Cane, H. V.; Hudson, H. S.; Kurt, V. G.; MacDowall, R. J.; Bothmer, V. Bibcode: 1997AAS...191.7412K Altcode: 1997BAAS...29.1326K Gradual solar energetic particle (SEP) events observed at 1 AU are associated with coronal mass ejections (CMEs) that drive shocks which accelerate the ions and electrons to suprathermal energies. However, high energy (> 30 MeV) proton and (> 1 MeV) electron events are nearly always associated with both CMEs and flares, suggesting that the acceleration of those particles, particularly the electrons, could be attributed to the associated flares. Only one clear example of a high energy SEP event without a flare association has been reported previously. We discuss a second such SEP event, on 14 April 1994, associated with a well observed solar X-ray arcade structure spanning ~ 150deg of solar longitude. The SEP event, observed by detectors on the IMP-8 and Koronas-I space craft, began about 10 hrs after the beginning of the X-ray event and was temporally and spatially associated with the last of three weak interplanetary type III radio bursts observed by the Ulysses low frequency radio experiment. The delayed onset and rapid rise of the SEP intensities preclude a recent interpretation in which SEPs were accelerated by a shock driven by a CME which erupted at the onset of the X-ray event. Yohkoh soft X-ray subtracted images show a large-scale arcade brightening west of ~ E10deg beginning about 8 hours after the initial brightening near the east limb. We suggest that the 14 April event consisted of at least two CMEs with progressively westward source regions and that the SEP event was produced in a second shock driven by a later CME. Title: Correlated Studies at Activity Maximum: the Sun and the Solar Wind Authors: Hudson, H. S.; Galvin, A. B. Bibcode: 1997ESASP.415..275H Altcode: 1997cpsh.conf..275H No abstract at ADS Title: Electron Trapping Times and Trap Densities in Solar Flare Loops Measured with COMPTON and YOHKOH Authors: Aschwanden, Markus J.; Bynum, Robert M.; Kosugi, Takeo; Hudson, Hugh S.; Schwartz, Richard A. Bibcode: 1997ApJ...487..936A Altcode: We measure energy-dependent time delays of ~20-200 keV hard X-ray (HXR) emission from 78 flares observed simultaneously with the Compton Gamma Ray Observatory and Yohkoh. Fast time structures (<~1 s) are filtered out, because their time delays have been identified in terms of electron time-of-flight (TOF) differences from directly precipitating electrons (Aschwanden et al.). For the smooth HXR flux, we find systematic time delays in the range of τS = t50 keV-t200 keV ~ -(1 ... 10) s, with a sign opposite to TOF delays, i.e., the high-energy HXRs lag the low-energy HXRs.

We interpret these time delays of the smooth HXR flux in terms of electron trapping, and we fitted a model of the collisional deflection time tDefl(E)~E3/2n-1e to the observed HXR delays in order to infer electron densities nTrape in the trap. Independently, we determine the electron density nSXRe in flare loops from soft X-ray (SXR) peak emission measures EM= n2edh, using loop width (w) measurements to estimate the column depth dh ~ w. Comparing the two independent density measurements in HXR and SXR, we find a mean ratio of qe=nTrape/nSXRe~1, with a relatively small scatter by a factor of ~2. Generally, it is likely that the SXR-bright flare loops have a higher density than the trapping regions (when qe < 1), but they also are subject to filling factors less than unity (when qe > 1). Our measurements provide comprehensive evidence that electron trapping in solar flares is governed in the weak-diffusion limit, i.e., that the trapping time corresponds to the collisional deflection time, while pitch-angle scattering by resonant waves seems not to be dominant in the 20-200 keV energy range. The measurements do not support a second-step acceleration scenario for energies <=200 keV. Title: The IACG Campaign IV: solar sources of heliospheric structure observed out of the ecliptic Authors: Galvin, A. B.; Hudson, H. S. Bibcode: 1997AdSpR..20..631G Altcode: The Inter-Agency Consultative Group, representing the four space agencies with contributing missions (the European Space Agency, the Japanese Institute of Space and Astronautical Science, the Russian Space Agency, and the United States National Aeronautics and Space Administration), has initiated a data analysis campaign ``Solar Sources of Heliospheric Structure Observed out of the Ecliptic'' (IACG Campaign IV) based on the unique capabilities and opportunities afforded by the core campaign missions, Ulysses and Yohkoh. The campaign is strongly benefited by data sets from solar ground observatories and other spacecraft missions (SPARTAN 201, IMP, WIND, SoHO). The campaign themes emphasize the joint analysis of these remote-sensing and in-situ data sets for features such as coronal holes, coronal mass ejections, and other solar sources of heliospheric structure. Title: (Erratum) YOHKOH observations of flares with flat hard X-ray spectra. Authors: Farnik, F.; Hudson, H.; Watanabe, T. Bibcode: 1997A&A...324..433F Altcode: Erratum to Astron. Astrophys. 320, 620 (1997) Title: Statistics of Fluctuations in the Solar Soft X-Ray Emission Authors: UeNo, S.; Mineshige, S.; Negoro, H.; Shibata, K.; Hudson, H. S. Bibcode: 1997ApJ...484..920U Altcode: X-ray emission from the Sun fluctuates as a result of occasional flare events. We have calculated the power spectra of the solar soft X-ray variations using the photometric data of the GOES 6 satellite. The data cover the period 1991 September to 1994 April, about 32 months in total, and we have worked with 10 minute averages. We find that the total power spectral densities (PSDs) have three distinct components; a flat or slightly decreasing low-frequency section, a 1/f-like moderate decline medium-frequency section, and steep decline high-frequency parts. The break frequencies separating three parts are fbreak ~= 10-4.7 Hz and 10-3.8 Hz , respectively. Such downward breaks are expected from the shot-noise (flarelike) character of solar X-ray emission, and we interpret the lower break frequency as indicating an upper limit on flare timescales. These break frequencies do not vary appreciably with activity level. This suggests the existence of a universal mechanism for triggering flares in the solar corona. Moreover, the power-law index (where we assume PSD ~ f, f being frequency) of each part does not vary appreciably with the level of activity either; its average is β ~= 0.45, 0.95, and 1.5, respectively. The overall shape of the PSD is quite similar to those of other astrophysical objects such as black hole candidate stars and active galactic nuclei, albeit on a vastly different scale. Title: The Solar-B Mission Authors: Antiochos, Spiro; Acton, Loren; Canfield, Richard; Davila, Joseph; Davis, John; Dere, Kenneth; Doschek, George; Golub, Leon; Harvey, John; Hathaway, David; Hudson, Hugh; Moore, Ronald; Lites, Bruce; Rust, David; Strong, Keith; Title, Alan Bibcode: 1997STIN...9721329A Altcode: Solar-B, the next ISAS mission (with major NASA participation), is designed to address the fundamental question of how magnetic fields interact with plasma to produce solar variability. The mission has a number of unique capabilities that will enable it to answer the outstanding questions of solar magnetism. First, by escaping atmospheric seeing, it will deliver continuous observations of the solar surface with unprecedented spatial resolution. Second, Solar-B will deliver the first accurate measurements of all three components of the photospheric magnetic field. Solar-B will measure both the magnetic energy driving the photosphere and simultaneously its effects in the corona. Solar-B offers unique programmatic opportunities to NASA. It will continue an effective collaboration with our most reliable international partner. It will deliver images and data that will have strong public outreach potential. Finally, the science of Solar-B is clearly related to the themes of origins and plasma astrophysics, and contributes directly to the national space weather and global change programs. Title: The Sun as AN X-Ray Star: Overview of the Method Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1997SoPh..172..239P Altcode: 1997ESPM....8..239P We present a method to study the solar-stellar connection, i.e., the close similarity of the physical phenomena occurring on the Sun and on late-type active stars, by taking advantage of Yohkoh/SXT X-ray images. From such images, we first generate distribution functions of the whole disk differential emission measure, and then synthesize from these spectra analogous to those collected by X-ray telescope instruments aimed at stars other than the Sun. Here we illustrate the application of this method to the ROSAT/PSPC and ASCA/SIS, and discuss test cases as well as future applications. Title: Tracking a CME from Cradle to Grave: A Multi-wavelength Analysis of the February 6-7, 1997 Event Authors: Gopalswamy, N.; Kundu, M. R.; Hanaoka, Y.; Kosugi, T.; Hudson, H.; Nitta, N.; Thompson, B.; Gurman, J.; Plunkett, S.; Howard, R.; Burkepile, J. Bibcode: 1997SPD....28.0501G Altcode: 1997BAAS...29..908G The partially earth-directed coronal mass ejection (CME) event of 1997 February 6-7 originated from the southwest quadrant of the sun. The CME accelerated from 170 km/s to about 830 km/s when it reached a distance of 25 solar radii. The CME was an arcade eruption followed by bright prominence core structures. The prominence core was tracked continuously from the solar surface to the interplanetary medium by combining data from the Nobeyama radioheliograph (microwaves), Mauna Loa Solar Observatory (He 10830 { Angstroms}), SOHO/EIT (EUV) and SOHO/LASCO (white light). The CME was accompanied by an arcade formation, fully observed by the YOHKOH/SXT (soft X-rays) and SOHO/EIT (EUV). The X-ray and EUV observations suggest that the reconnection proceeded from the northwest end to the southeast end of a filament channel. In the SOHO/EIT images, the the feet of the soft X-ray arcade were observed as EUV ribbons. The CME event also caused a medium sized geomagnetic storm: The hourly equatorial Dst values attained storm level during 18:00-19:00 UT on February 09. This means the disturbance took about 2.25 days to reach the Earth. The first signatures of an IP shock was a pressure jump in the WIND data around 13:00 UT on Feb 09, 1997 which lasted for about 14 hours, followed by flux rope signatures. This CME event confirms a number of ideas about CMEs: The three part structure (frontal bright arcade, dark cavity and prominence core), disappearing filament, elongated arcade formation, and terrestrial effects. We make use of the excellent data coverage from the solar surface to the Earth to address a number of issues regarding the origin and propagation of the geoeffective solar disturbances. We benefited from discussions at the first SOHO-Yohkoh Coordinated Data Analysis Workshop, held March 3-7, 1997, at Goddard Space Flight Center. Title: The Solar Source of the January 1997 CME/Magnetic Cloud; Recurrent Activity on a Polar Crown Filament Channel Authors: Webb, David; Cliver, E.; McIntosh, P.; Gopalswamy, N.; Hudson, H. Bibcode: 1997SPD....28.1501W Altcode: 1997BAAS...29..918W The magnetic cloud and geomagnetic storm on 10-11 January 1997 was associated with a halo-type CME observed by LASCO near the sun on 6 January. As part of the ISTP and SHINE collaboration on this event we are studying the solar source region of the CME and its propagation through the interplanetary medium to Earth. We summarize the rather weak solar activity that apparently was associated with this CME, and contrast it with the strong surface and coronal activity associated with the 9-11 February 1997 magnetic cloud/storm. We present evidence that the solar sources of both of these events occurred over an area where the southern polar crown filament channel diverted sharply to the north and in the decaying remnants of the first large-scale active region to form during the new solar cycle. Our results also suggest that this region was the site of earlier CMEs; i.e., it was a key site of recurrent activity during 1996-97 which, when aimed toward Earth, produced recurrent magnetic clouds and storms. Title: Observations of Superhot Plasma in Solar Flares Authors: Nitta, N.; Hudson, H. S. Bibcode: 1997SPD....28.0162N Altcode: 1997BAAS...29..890N We have shown in an M1-class flare that a structure away from the main bright loop filled with superhot ( ~ 30 MK) plasma. The analysis incorporated Yohkoh SXT measurements in three filters and BCS Fe XXV and Ca XIX spectra. We made use of the fact that even the thickest filter of SXT is sensitive to plasma of ~ 10 MK, biasing the temperature determination towards lower values. This resulted in an estimated temperature of the imaged superhot plasma consistent with that derived from hard X-rays (the Yohkoh HXT M1/L channel ratio). We now apply the same technique to several other flares to study the existence of superhot plasma, its timing and location with respective to the impulsive component and its range of parameters. We identify flares with superhot plasma irrespective of the results from the BCS Fe XXVI diagnostic (which has large uncertainty), suggesting that the latter should not be the only method to determine whether a flare has superhot plasma. We also discuss the data in terms of the true differential emission measure (a local property of the plasma) rather than the spatial-composite emission measure (the average along the line of sight). Title: Electron Temperatures of the Corona Above a Solar Active Region Determined from S XV Spectra Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Watanabe, Tetsuya Bibcode: 1997SPD....28.0136S Altcode: 1997BAAS...29..885S We present high resolution soft X-ray spectral observations of the corona above an active region, using data from the Bragg crystal spectrometer (BCS) on board the Yohkoh satellite. We observed NOAA AR 7978 as it rotated beyond the solar limb so that the lower portions of the region were occulted. Long integrations from times after the region had totally disappeared some days later show a substantial background in S xv. Since the background spectrum is featureless, spectral lines obtained from the time of occultation must originate from the upper corona of the active region. Our results support previous findings that the corona consists of two components: a cooler, steady component with T_e ~ 3 MK, and a hotter, transient component in excess of 5 MK. This hotter component is due to microflares; outside the time of microflares there is relatively little or no active region upper coronal plasma with T_e gtrsim 3.5 MK. There is evidence for a decrease in T_e with height for the cool component. Title: Interacting X-ray Loops in a Coronal Mass Ejection Authors: Hudson, Hugh S. Bibcode: 1997SPD....28.0137H Altcode: 1997BAAS...29..886H The Yohkoh soft X-ray telescope observed a part of a large coronal mass ejection on 5 October 1996. The event originated beyond the west limb, probably by at least two days' rotation to be consistent with the location of the successor of AR 7978. Coronal soft X-ray dimming accompanied the ejection, suggesting that the X-ray brightness of the source region extended to high altitude, again consistent with the identification with the successor of AR 7978. Following the dimming, a network of filamentary structures rose outward, giving the appearance of an opening flux rope that twisted as it rose. The south end of the flux rope appeared to be confined by an arcade of loops. There was no evidence for loop-loop reconnection between the two sets of filamentary structures. Title: Temporal Variations of Solar Flare Spectral Properties: Hard X-Ray Fluxes and Fe XXV, Ca XIX, and Wide-Band Soft X-Ray Fluxes, Temperatures, and Emission Measures Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Lemen, James R.; Zarro, Dominic A. Bibcode: 1997ApJS..110..115S Altcode: We present fluxes, temperatures, and emission measures for nine solar flares, using data from both the Fe XXV and Ca XIX channels of the Bragg Crystal Spectrometer (BCS) experiment on the Yohkoh satellite and from the wide-band soft X-ray spectrometers on the GOES spacecraft. We also present hard X-ray fluxes from the Hard X-ray Telescope (HXT) on Yohkoh and the BATSE spectrometer on the Compton Gamma-Ray Observatory (CGRO). All events occurred during 1992 and ranged in size from GOES class C5 to M2. Three of the events occurred near the solar limb. For each flare we give two sets of plots. The first set shows flux, electron temperature, and emission measures for Fe XXV, Ca XIX, and GOES as functions of time. The second set of plots gives log electron temperature as functions of log (emission measure)1/2 for these three wavelength ranges; we refer to these plots as E1/2-T diagrams. Hard X-ray flux information is included in both sets of plots. Our observations indicate that (1) cooler plasmas are located along the legs of, or are evenly distributed along, the flaring loops, while hotter plasmas are concentrated near the loop tops, (2) peaks in temperature in each of the wavelength bands are closely associated with hard X-ray enhancements, and (3) the emission from both relatively hot and relatively cool flaring plasmas emanates from the same loop or from closely related loops. Title: YOHKOH observations of flares with flat hard X-ray spectra. Authors: Farnik, F.; Hudson, H.; Watanabe, T. Bibcode: 1997A&A...320..620F Altcode: A series of flares with exceptionally hard spectral indices in the hard X-ray band occurred on 3 October 1993. The non-thermal bremsstrahlung spectra may extend to a few keV in these events, one of which was detectable in the Yohkoh Bragg Crystal Spectrometer at 7keV as well as by the hard X-ray instruments at higher energies. We present Yohkoh soft and hard X-ray imaging, spectroscopy and energetics analysis of these events, with the idea that flares with such flat spectra (power-law as hard as 1.98 below 33keV) might differ appreciably from ordinary flares. The series of events is strongly homologous, with no systematic variations in structure over a period of 3.5-hours except for jet-like ejecta accompanying Type III/V bursts. Unlike other hard events, these flares are large (footpoint separation about 3x10^4^km) and therefore well resolved by the Yohkoh imaging instruments. The time variations match the Neupert effect. The hard and soft X-ray images also show footpoint brightening and loop filling. The spikes with the flattest spectra have the weakest Neupert-effect signature, but no perceptible time delays between the hard X-ray time profile and the soft X-ray time derivative. These events do not produce superhot emission but are probably microwave-rich. We find no evidence for large-scale magnetic reconnection in the development of these flares. We note two discrepancies between the observations and the existing numerical hydrodynamic models of flare energetics, and suggest that rapid spike events of this type provide good tests of such models. Title: Electron Temperatures of the Corona Above a Solar Active Region Determined from S XV Spectra Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Watanabe, Tetsuya Bibcode: 1997ApJ...479L.149S Altcode: We present the first high-resolution soft X-ray spectral observations of the corona above an active region, using the Bragg crystal spectrometer (BCS) on board the Yohkoh satellite. We observed NOAA AR 7978 as it rotated beyond the solar limb so that the lower portions of the region were occulted. Long integrations from times after the region had totally disappeared some days later show a substantial, variable background in S XV. Since the background spectrum is featureless, spectral lines obtained from the time of occultation must originate from the upper corona of the active region. Our results support previous findings that the active region corona consists of two components: a cooler, steady component with Te ~ 3 MK and a hotter, transient component in excess of 5 MK. This hotter component is due to microflares; outside the time of microflares there is relatively little or no active region upper coronal plasma with Te >~ 3.5 MK. There is evidence for a decrease in Te with height for the cool component. Title: Isolating the Footpoint Characteristics of a Solar Flare Loop Authors: Harra-Murnion, L. K.; Culhane, J. L.; Hudson, H. S.; Fujiwara, T.; Kato, T.; Sterling*, A. C. Bibcode: 1997SoPh..171..103H Altcode: We analyse the physical characteristics of a C5.7 class flare which was observed on 27 September, 1993 using data from the soft X-ray telescope (SXT), the Bragg crystal spectrometer (BCS), and the hard X-ray telescope (HXT) on Yohkoh. The flare takes the form of a simple loop which is much brighter at one of its footpoints than anywhere else for a period of 2 min. During this time there is an increase in the soft X-ray fluxes, and a corresponding peak in hard X-rays. The parameters derived from the hard X-ray and soft X-ray spectra and images are assumed to be from the footpoint. This flare showed two peaks in the non-thermal velocity, the first one simultaneous with the footpoint brightening. The non-thermal velocity corresponding to these first few minutes is unusually large - by a factor of 80%, 68%, and 26% relative to the second peak in the Fexxv, Caxix, and Sxv channels respectively. Title: A Multi-Wavelength Analysis of the February 6/7, 1997 Coronal Mass Ejection Authors: Plunkett, S. P.; Gopalswamy, N.; Kundu, M. R.; Howard, R. A.; Thompson, B. J.; Gurman, J. B.; Lepping, R. P.; Hudson, H. S.; Nitta, N.; Hansoka, Y.; Kosugi, T.; Burkepile, J. T. Bibcode: 1997ESASP.404..615P Altcode: 1997cswn.conf..615P No abstract at ADS Title: Active Region Coronal Temperatures from YOHKOH BCS Sxv Spectra Authors: Sterling, A. C.; Hudson, H. S.; Watanabe, T. Bibcode: 1997IAUJD..19E...9S Altcode: Using the Bragg crystal spectrometer (BCS) on board the Yohkoh satellite, we present high resolution soft X-ray spectral observations of the corona above an active region. We observed NOAA AR 7978 as it rotated beyond the solar limb so that the lower portions of the region were occulted. Long integrations from times after the region had totally disappeared some days later show a substantial background in Sxv. Since the background spectrum is featureless, spectral lines obtained from the time of occultation must originate from the upper corona of the active region. Our results support previous findings that the active region corona consists of two components: a cooler, steady component with electron temperature T_e ~3 MK, and a hotter, transient component in excess of 5 MK @. This hotter component is due to micro-flares; outside the time of micro-flares there is relatively little or no active region upper coronal plasma with T_e higher than about 3.5 MK @. There is evidence for a decrease in T_e with height for the cool component. Title: Active Region Energetics via Yohkoh/BCS and SOHO/CDS Authors: Watanabe, T.; Sterling, Alphonse C.; Hudson, Hugh S.; Harra-Murnion, Louise K. Bibcode: 1997ESASP.404..723W Altcode: 1997cswn.conf..723W No abstract at ADS Title: Limb Events Observed by YOHKOH and Coronal Mass Ejections: A Filamentary Soft X-ray Structure on 5 October 1996 Authors: Watari, S.; Watanabe, Takashi; Acton, L. W.; Hudson, H. S. Bibcode: 1997ESASP.404..725W Altcode: 1997cswn.conf..725W No abstract at ADS Title: The Sun as an X-ray Star: Overview of the Method Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1997IAUJD..19E..37P Altcode: We present a method to study the solar-stellar connection, i.e., the close similarity of the physical phenomena occurring on the Sun and on late-type active stars, by taking advantage of Yohkoh/SXT X-ray images. From such images, we first generate distribution functions of the whole disk differential emission measure, and then synthesize from these spectra analogous to those collected by X-ray telescope instruments aimed at stars other than the Sun. Here we illustrate the application of this method to the ROSAT/PSPC and ASCA/SIS, and discuss test cases as well as future applications. For a more detailed discussion, please refer to a paper, by the same authors in "OBSERVATIONAL PLASMA ASTROPHYSICS: FIVE YEARS OF YOHKOH AND BEYOND", T. Watanabe, T. Kosugi, and A. C. Sterling, eds., Kluwer Academic Publishers, the Proceedings of the Yohkoh 5th Anniversary Symposium, held in November 1996, in Yoyogi, Tokyo, Japan. Title: Comparison between arch filaments and coronal loops. Authors: Yoshimura, K.; Kurokawa, H.; Sano, S.; Hudson, H. Bibcode: 1997IAUJD..19E..60Y Altcode: Kawai et al. (1992) presented preliminary results from comparison between arch filament system (AFS) and soft X-ray (SXR) loops. They found the AFS was covered with the SXR bright features. Coordinated observations with YOHKOH enable us to make more detail comparison between arch filament(AFS) and SXR loops. (With small brightening points both in SXR and H alpha images, we can co-align those images accurately enough.) We intended to see how each AFS contributes to SXR brightenings. We will show examples of AFS which no particular SXR brightening occurred around. And we will present observations of SXR major brightenings of loops, which was related to H alpha dark features. The dark features did not seem to be AFS. They may be small active region filaments which indicates magnetic sheared structure (evidence for storages of extra magnetic energy). Title: X-ray Observations of Coronal Mass Ejections Authors: Hudson, H. S. Bibcode: 1997IAUJD..19E..24H Altcode: X-ray and XUV telescopes on Yohkoh and SOHO show the full coronal structure, both over the limbs and in front of the solar disk. A coronal mass ejection (CME) must result in a perceptible alteration of the coronal structure, and indeed ``depletions'' have long been known from coronagraph and corona-meter data. The X-ray observations also show depletion effects, which we term ``dimming'', including the transient coronal holes observed by the X-ray telescopes on Skylab. The new X-ray observations allow us to study the actual outward flow of the coronal material in some cases; we find both diffuse clouds and highly filamented loop-like structures in the ``dimming'' volumes and can estimate the CME masses. The observations are generally consistent with the standard model of magnetic reconnection used to explain CME's and long-duration flares, except that there appears to be little delay between the eruption and the flaring. Title: The Solar Flare of 1992 August 17 23:58 UT} % Authors: Takahashi, Masaaki; Watanabe, Tetsuya; Sakai, Jun-Ichi; Sakao, Taro; Kosugi, Takeo; Sakurai, Takashi; Enome, Shinzo; Hudson, Hugh S.; Hashimoto, Shizuyo; Nitta, Nariaki Bibcode: 1996PASJ...48..857T Altcode: A small flare (C4.3 in the GOES X-ray class) was well observed by all of the instruments on board Yohkoh. The X-ray light curves have double peaks which are about 5 min apart. Until the first peak from flare onset, four compact areas brighten up in the soft X-ray region, which are aligned almost on one straight line. We regard them as being footpoints of two sets of loops, which are identifiable in soft X-ray images, since their locations match those of hard X-ray sources. Indeed, after the second peak, the temporal behavior of the temperature and emission measure at each point is consistent with the existence of two such loops. Comparing our results with recent MHD simulations, we propose a possible scenario for this flare that is based on the coalescence of two loops. Title: Comprehensive Multiwavelength Observations of the 1992 January 7 Solar Flare Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.; de Pater, Imke; Gary, Dale E.; McTiernan, James M.; Hudson, Hugh S.; Doyle, J. Gerry; Hagyard, Mona J.; Kundu, Mukul R. Bibcode: 1996ApJS..106..621S Altcode: Observations of a solar flare that occurred at 2022 UT on 1992 January 7, during the 1991 December/1992 January Max `91 campaign, are presented. This flare was observed simultaneously in Hα, radio (at microwave and millimeter wavelengths), and soft and hard X-rays (by the Yohkoh spacecraft) with high spatial and moderate spectral resolution. A comparison of magneto grams before and after the flare shows evidence of the emergence of new magnetic flux of opposite polarity at the flare site. Although this flare was only of moderate size (GOES classification C8.9 and Hα importance SF), it exhibited several distinct bursts and at least 10 spatially distinct hard/soft X-ray sources. Cospatial Hα brightenings suggest that most of the X-ray sources are located at footpoints of magnetic loops. Two of the hard X-ray sources have no Hα counterparts and are therefore believed to be located at loop tops. The flare consisted of three bursts of particle acceleration followed by a purely thermal phase. High spectral resolution Ca XIX line profiles indicate upflows shortly after the second acceleration phase. Analysis of the microwave/hard X-ray/soft X-ray emission from individual sources provides information on the radio emission mechanisms, the energetic electron population, the magnetic field strength, and the plasma density. These parameters were estimated for the two microwave sources observed during the third acceleration burst; these sources were simultaneously detected in soft X-rays, and one of the sources is also seen in hard X-ray maps. Although the microwave emission is consistent with the gyro synchrotron mechanism, the millimeter emission, which peaks during the thermal phase when all nonthermal activity has ceased, is likely due to thermal bremsstrahlung from the soft X-ray emitting hot plasma. The energy lost to collisions by the energetic (>15 keV) electrons and the energy contained in the thermal plasma are calculated for each source. The energy injected by the nonthermal electrons from all sources is estimated to be 1030 ergs. Only the soft X-ray sources with gradual time profiles seem to show the Neupert effect. Title: A Long-Duration Solar Flare with Mass Ejection and Global Consequences Authors: Hudson, H. S.; Acton, L. W.; Freeland, S. L. Bibcode: 1996ApJ...470..629H Altcode: We report observations of a long-duration flare with mass ejection from the corona, using the Yohkoh soft X-ray telescope (SXT). This flare occurred 1994 November 13 near disk center during quiet solar conditions, with excellent temporal coverage of both the core activity in the active region itself and of the global corona. The initial X-ray images reveal two arcades of cusped magnetic loops, connected via a series of thin loops. These loops rise rapidly during the increasing phase of soft X-ray flare brightness. In its final state, the flare has the configuration of postflare loops with a cusp. Large regions of the X-ray corona appear to empty during the evolution of the event. We suggest that this corresponds a coronal mass ejection (CME) seen in soft X-rays. Its detection in the SXT images is consistent with the finding that material participating in a CME exists at elevated coronal temperatures (2.8 x 106 K in this case) before the ejection. We estimate a mass >4 x 1014 g for the ejected material. The X-ray morphology of the event has strong points of similarity with the classical reconnection picture of long-duration event (LDE) formation, but there are significant discrepancies: there is no observed inward flow during the rise phase, the expansions are multiple and appear to be nonradial, and none of the observed motions suggest a reconnection jet. We note the subsequent occurrence of very large scale coronal disturbances, including regions near the boundaries of coronal holes at both poles. We suggest that this global disturbance implies a perturbation reaching as far outward as the heliospheric neutral sheet. The exciter would require a horizontal velocity of approximately 200 km s-1 in such a case, consistent with the projected velocity of the plasma cloud that we identify with a CME in the process of launching. Title: The Scaling Law between Electron Time-of-Flight Distances and Loop Lengths in Solar Flares Authors: Aschwanden, Markus J.; Kosugi, Takeo; Hudson, Hugh S.; Wills, Meredith J.; Schwartz, Richard A. Bibcode: 1996ApJ...470.1198A Altcode: From the complete data set of solar flares simultaneously observed with the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) in the high-time resolution mode (64 ms) and the Hard X-ray Telescope (HXT) on board Yohkoh, we were able to determine the electron time-of-flight (TOF) distance l' and the flare loop geometry in 42 events. The electron TOF distances were determined from time delays (of ≍ 10-100 ms) of hard X-ray (HXR) pulses (measured in 16 channel spectra over ≍ 20-200 keV), produced by the velocity difference of the HXR-producing electrons. The flare loops were mostly identified from double footpoint sources in ≥30 keV HXT images, with radii in the range r = 3000-25,000 km. We find a scaling law between the electron TOF distance l' and the flare loop half-length 5 = r(π/2), having a mean ratio (and standard deviation) of l'/s = 1.4±0.3. In five flares, we observe coronal ≥ 30 keV HXR sources of the Masuda type in the cusp region above the flare loop and find that their heights are consistent with the electron TOF distance to the footpoints. These results provide strong evidence that particle acceleration in solar flares occurs in the cusp region above the flare loop and that the coronal HXR sources discovered by Masuda et al. are a signature of the acceleration site, probably controlled by a magnetic reconnection process. Title: Large-scale active coronal phenomena in Yohkoh SXT images Authors: Fárník, František; Švestka, Zdeněk; Hudson, Hugh S.; Uchida, Yutaka Bibcode: 1996SoPh..168..331F Altcode: 1996SoPh..168..331U We discuss Yohkoh SXT observations of stationary giant post-flare arches which occurred on 3-6 May, 1992 and study in detail the last arch, associated with the flare at 19:02 UT on 5 May, which extended above the west limb. The arch was similar to the first giant arch discovered on board the SMM, on 21-22 May, 1980. We demonstrate that the long lifetimes of these structures necessarily imply additional energy input from the underlying active region: otherwise, conduction would cool these arches in less than one hour and even with the unlikely assumption of conduction inhibited, pure radiative cooling would not produce the temperature decrease observed. All arch tops, although varying in brightness, stayed for several days at a fairly constant altitude of ∼ 100 000 km, and the arch studied, on 5-6 May, was just a new brightening of the pre-existing decaying structure. The brightening was apparently due to inflow of hot plasma from the flare region. Yohkoh data confirm that these stationary arches are rare phenomena when compared with the rising arches studied in Paper I and with Uchida et al.'s expanding active regions. Title: Electron Time-of-Flight Distances and Flare Loop Geometries Compared from CGRO and YOHKOH Observations Authors: Aschwanden, Markus J.; Wills, Meredith J.; Hudson, Hugh S.; Kosugi, Takeo; Schwartz, Richard A. Bibcode: 1996ApJ...468..398A Altcode: The distance between the coronal acceleration site and the chromospheric hard X-ray (HXR) emission site can be determined from velocity-dependent electron time-of-flight (TO F) differences in the framework of the thick-target model. We determine these electron TOF distances 1 with relative time delay measurements in the 30-300 keV energy range, using 16 channel data from BATSE/CGRO for the eight largest flares simultaneously observed with Yohkoh. We filter the HXR fine structure from the smoothly varying HXR flux with a Fourier filter in order to separate competing time delays. In the Yohkoh/HXT images we identify the corresponding flare loops that show ≥30 keV HXR footpoint emission and project the electron TOF distances into the loop plane, assuming a semicircular shape (with radius r). The flare loop radii vary in the range of r = 5600-17,000 km. In all eight flares we find that the projected electron TOF distance l' exceeds the loop half-length s = r(π/2), with a scale-invariant ratio of l'/s = 1.3±0.2. Projecting the electron TOF distances onto an open field line that extends to the cusp region above the flare loop, we find an average ratio of h/r = 1.7±0.4 for the height h of the acceleration site. This geometry is compatible with acceleration mechanisms operating in the cusp region, perhaps associated with magnetic reconnection processes above the flare loop. Alternatively, acceleration sites inside the flare loop cannot be ruled out (since l'/s < 2), but they do not provide a natural explanation for the observed length ratio l'/s. Large-scale electric DC field acceleration mechanisms are found to be less suitable to explain the observed HXR timing and pulse durations. Title: Solar identification of solar-wind disturbances observed at Ulysses Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.; Harvey, K. L.; Hoeksema, J. T.; Zhao, X.; Hudson, H. S. Bibcode: 1996AIPC..382...92L Altcode: The Ulysses polar passages are producing a unique set of observations of solar-wind disturbances at high heliographic latitudes. In this paper we use the Yohkoh soft X-ray telescope (SXT) to locate some of these events, as defined by the Ulysses/SWICS data, in the solar corona. Of 8 events, we identify two with flares, three with front-side large arcade events, two with far-side events, and one was not seen in the Ulysses data. The arcade events generally resemble long-duration flares seen in active regions, but are larger, slower, and cooler. We present Yohkoh images of each of these events. In the large arcade events (see Alexander et al., 1996, for a detailed look at one of them) the magnetic morphology at the location of the Yohkoh arcade is generally consistent with the development of a large system of loops. Some of the identifications are ambiguous, and we summarize the reasons for this. From the SWICS data we have obtained ionization temperatures for several events, and find that they have no obvious pattern in relation to the X-ray temperatures; this may be expected on the basis that the interplanetary plasma cloud is physically distinct from the plasma trapped in the corona. Soft X-ray observations of the solar corona show occasional occurrences of large-scale brightenings in the form of arcades of loops. Such structures have been known since Skylab (e.g., Sturrock, 1980), and have a clear relationship with coronal mass ejections (e.g., Kahler, 1977). We now may study this phenomenon statistically with the much more comprehensive Yohkoh observations; with Yohkoh movies we can also begin to extend our knowledge to the three-dimensional development of the structures. At the same time Ulysses has sampled the latitude dependence of the interplanetary effects. With this paper we introduce this subject and provide a preliminary listing of events from the passage of Ulysses through high heliographic latitudes. The starting point of the present survey is a list of interplanetary plasma clouds (IPC's) derived from Ulysses/SWICS data. These are essentially the same as the events termed CMEs by Gosling et al. (1994a, 1994b). For this identification the presence of bidirectional streaming in the suprathermal electron distribution is one of the main criteria. We note that there are no direct coronagraph observations, however. The Yohkoh observations were examined at the apparent time of origin of each Ulysses event, resulting in some clear and some less-certain identifications. We also studied the ionization temperatures of the IPC material as a beginning step to give the identifications a physical basis. There has been little study thus far of the Yohkoh soft X-ray observations in relationship to CMEs, which we believe to be closely related to the interplanetary disturbances. Hiei et al. (1993) reported the only Yohkoh event yet studied in conjunction with white-light coronagraph observations. However Klimchuk et al. (1994) showed that X-ray eruptive phenomena with parameters similar to those of CMEs occur frequently at the limb, and there have been several studies of individual eruptive events (e.g., Watanabe et al., 1992). Presently there is no systematic knowledge of the X-ray coronal counterparts of CMEs, and the survey represented here is part of the effort to rectify this situation. Title: Yohkoh/SXT soft x-ray observations of sudden mass loss from the solar corona Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Freeland, S. L.; Lemen, J. R.; Harvey, K. L. Bibcode: 1996AIPC..382...88H Altcode: With soft X-ray imaging we can study the entire coronal volume, except for cold inclusions such as prominences, as a function of time. This should allow us to observe the origins of coronal mass ejections. We report here an initial survey of the Yohkoh/SXT observations at the times of reported or apparent mass ejections: three LDE flare events and two large-scale arcade formations. For each of the events we can easily detect sudden coronal dimming, which we interpret as the launch interval of a CME. In one of the flare events we have found a well-defined plasma cloud, apparently formed from a set of loop structures, which rises and disappears during the growth phase of the flare emission. Its mass amounted to some 4×1014 g with a density of 3×108 cm-3 and a temperature of 2.8 MK before its disappearance. Title: Energetic particles and coronal mass ejections in the high latitude heliosphere: Ulysses-LET observations Authors: Bothmer, V.; Marsden, R. G.; Sanderson, T. R.; Trattner, K. J.; Wenzel, K. -P.; Balogh, A.; Forsyth, R. J.; Goldstein, B. E.; Uchida, Y.; Hudson, H. S. Bibcode: 1996AIPC..382..445B Altcode: We have investigated energetic ions of non-corotating nature in the high latitude heliosphere. Major particle events were observed by Ulysses up to latitudes of 60 °S. All were associated with passage of coronal mass ejections (CMEs) over the spacecraft. The relationship of these events with solar activity was investigated using Yohkoh soft X-ray images. Title: The large scale eruptive event of 1994 April 14 Authors: Alexander, David; Harvey, K. L.; Hudson, H. S.; Hoeksema, J. T.; Zhao, X. Bibcode: 1996AIPC..382...80A Altcode: The polar crown event of 1994 April 14 is one of the largest scale eruptive events observed by the Yohkoh/SXT. Associated with the formation of an arcade of soft X-ray loops at the Sun was the detection of an interplanetary forward/reverse shock event by the Ulysses spacecraft some 7 days later. The relationship between the coronal and interplanetary signatures of these events is important if we are to address fully the initialisation and consequent development of interplanetary phenomena, such as CMEs and counter-streaming electrons, originating at the Sun. We investigate the development of the energetics of the 1994 April 14 event and their relationship to the dynamics of the eruption are investigated. The arcade formation, together with the eruption of material into interplanetary space, suggests a large-scale temporary reconfiguration of the coronal magnetic field. We examine the effects of the formation of such a coronal arcade structure on the HCS and discuss the dynamics involved with the passage of a large scale disturbance through the interplanetary magnetic field. Title: The solar origins of two high-latitude interplanetary disturbances Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.; Kahler, S. W.; Kurokawa, H.; Lemen, J. R. Bibcode: 1996AIPC..382...84H Altcode: Two extremely similar interplanetary forward/reverse shock events, with bidirectional electron streaming, were detected by Ulysses in 1994 [Gosling et al., 1994]. Both events resulted in geomagnetic storms and presumably were associated with coronal mass ejections. In this paper we use the Yohkoh soft X-ray observations to characterize the conditions in the lower corona at the times appropriate for the launching of these two events. We find two strikingly different solar events to be the likeliest candidates: an LDE flare on 20 Feb. 1994, and a extremely large-scale arcade event on 14 April 1994. Title: Pixon-based Multiresolution Image Reconstruction for Yohkoh's Hard X-Ray Telescope Authors: Metcalf, Thomas R.; Hudson, Hugh S.; Kosugi, Takeo; Puetter, R. C.; Pina, R. K. Bibcode: 1996ApJ...466..585M Altcode: We present results from the application of pixon-based multiresolution image reconstruction to real and simulated data from Yohkoh's Hard X-ray Telescope (HXT). The goal of the pixon algorithm is to minimize the number of degrees of freedom used to describe an image within the accuracy allowed by the noise. This leads to a reconstruction that is optimally constrained. We apply the pixon code to two solar flares in the HXT database and compare the results of the pixon reconstruction to the results of a direct, linear, smoothed inversion of the HXT Fourier synthesis data and to a maximum entropy reconstruction. The maximum entropy reconstruction is vastly better than the direct inversion, but the pixon reconstruction gives superior noise suppression and photometry. Further, the pixon reconstruction does not suffer from overresolution of the images. Title: Electron Time-of-Flight Measurements during the Masuda Flare, 1992 January 13 Authors: Aschwanden, Markus J.; Hudson, Hugh; Kosugi, Takeo; Schwartz, Richard A. Bibcode: 1996ApJ...464..985A Altcode: The solar flare of 1992 January 13, 1729 UT, has become famous because Masuda's discovery of a hard X-ray loop-top source (Masuda 1994). Here we analyze energy-dependent time delays occurring in 30-120 keV hard X-ray (HXR) emission during this flare, observed by BATSE on board the Compton Gamma Ray Observatory with a time resolution of 64 ms. The purpose of this study is to reconstruct the kinematics of HXR-producing electrons from energy-dependent HXR delays, and to relate the inferred time-of-flight distance to the spatial geometry of the flare loop, as observed by SXT and HXT on board Yohkoh. The findings are the following:

1. The HXR flux can be decomposed into a sequence of pulses with ≍2-3 s duration and into a smoothly varying envelope that accounts for 90% of the ≥ 30 keV. flux. Cross-correlating the pulses between five different energy channels in the 30-120 keV range, we find that the HXR pulses are delayed (τP = 40-220 ms) at the lower energies with respect to the higher energies. For the HXR envelopes, we find much larger delays (-τE = 2.1-6.6 s) of opposite sign.

2. We fit kinematic models that quantify electron acceleration and propagation times to the observed HXR timing, for small-scale and large-scale accelerating fields, in semicircular and cusplike flare loop geometries. We find that the acceleration site is most likely located in an altitude of h = 44,000±6000 km, in the cusp region above the SXR-emitting flare loop (h = 12,500 km), and also significantly above Masuda's coronal HXR source (h = 22,100 km). This finding offers an interpretation of Masuda's HXR source in terms of nonthermal bremsstrahlung by electrons partially confined in the cusp region either by magnetic mirroring or by wave turbulence in the reconnection outflow.

3.The delay of the smoothly varying HXR flux is found to be consistent with trapping time differences in terms of collisional deflection, based on estimates of the electron density (ne ≤ 2 x 1011 cm-3) from SXT emission measure maps.

This study provides the first quantitative localization of the electron acceleration site in a solar flare, and demonstrates that energy-dependent HXR delays offer a sensitive diagnostic for electron acceleration, propagation, and trapping in solar flares. Title: YOHKOH Observations of an Over-the-Limb Solar Flare with Large Spectral Line Shifts Authors: Sterling, Alphonse C.; Harra-Murnion, Louise K.; Hudson, Hugh S.; Lemen, James R. Bibcode: 1996ApJ...464..498S Altcode: We present observations of a solar flare of 1993 April 15 near 9 UT, using data from the Yohkoh Bragg crystal spectrometer (BC S) and soft X-ray telescope (SXT). Observations from SXT indicate that the flare occurred well beyond the solar limb, meaning that our observations are restricted to the uppermost portions of the flaring structure. BCS spectra show strong bulk blueshifts of the spectral line profiles for a short period near the start of the event, followed by an extended period of strong bulk redshifts of the line profiles. Concurrent with these bulk line shifts, the spectra show "blue wing" asymmetries. Both bulk line shifts and blue wings are infrequent characteristics of flares observed near the solar limb. Our observations are consistent with strong upward mass motions occurring on a high-altitude flaring loop oriented edge-on with the Earth. We find nonthermal line broadenings in the spectra which are qualitatively and quantitatively similar to line broadenings in spectra of disk flares. Near peak intensity of the flare, ≤10% of the residual nonthermal broadening can be explained by the spatial distribution of the soft X-ray flaring structure. Title: Yohkoh observations of flares with superhot properties Authors: Hudson, H. S.; Nitta, N. Bibcode: 1996AIPC..374..285H Altcode: 1996hesp.conf..285H Solar flares, almost as their defining property, fill coronal magnetic flux tubes with hot plasma. When the temperature of a significant fraction of this plasma exceeds about 30×106 K, we call the event ``superhot'', following the initial observation of the hard X-ray continuum of such an event by Lin et al. (11). The Yohkoh observations include many examples of similar events, of which three have been published thus far. This paper reports a survey of the Yohkoh observations, based mainly on the hard X-ray spectra obtained by the HXT instrument. While comprehensive conclusions will not be possible until the survey includes the Yohkoh imaging observations, we make tentative suggestions here about the nature of flares with superhot properties. Title: Temporal Variations of Solar Flare Spectral Properties in CA XIX and GOES Authors: Sterling, A. C.; Hudson, H. S.; Lemen, J. R.; Zarro, D. A. Bibcode: 1996AAS...188.1905S Altcode: 1996BAAS...28..850S Since the advent of space borne X-ray observations, there has been a strong interest in the nature of the X-ray solar flare. Examining the relationships between radiations produced in different portions of the X-ray spectrum gives us information on the properties of the constituent flaring plasmas. We have studied the joint variation of electron temperatures and emission measures for a number of flares in two different wavelength ranges, using data from the narrow band Ca xix channel (near 3.18 Angstroms) of the Bragg crystal spectrometer (BCS) experiment on board the Yohkoh spacecraft, and data from the wide band X-ray monitors on the Geostationary Operational Environmental Satellites (GOES, covering 0.5---8 Angstroms). A power law relationship often describes the relationship between temperature and emission measure during the decay phase in both wavelength ranges. According to work of Sylwester et al. (1993, A&A 267, 586), energy input parameters and physical properties of the flaring loop(s) determine the slope of this power law. We find that ratios of Ca xix to GOES slopes generally fall between .6 and 1.0, when slopes in both channels are measured during the flare decay in each respective wavelength range. This relatively good agreement between slopes in the two channels suggests that emissions in both wavelength ranges originate from either the same flaring loop, or differing loops with similar global properties. Title: A Signature of CME Onsets in Soft X-rays Authors: Webb, D.; Hudson, H.; Lemen, J. Bibcode: 1996AAS...188.7008W Altcode: 1996BAAS...28..939W Some CME models involve the opening of previously closed magnetic field lines as the plasma erupts through the lower corona. However, the identification of observational signatures of this process has proven illusive. We report on Yohkoh/SXT observations of large-scale arcade events which reveal the dimming, or depletion, of coronal material above the bright long-duration arcade region. Viewed in video form, the impression is of an opening up and evacuation of a local portion of the lower corona. We interpret this as the process of field line opening in the initial phase of a CME and, therefore, as the first direct signature of the mass ejection in soft X-rays. The dimming appears as a reduction of the soft X-ray intensity by about a factor of 2-3. We are studying several events of this type which reveal different aspects of the geometry and kinematics of this process. Highly structured loops can be seen expanding in the dimming regions of some events and can be used to characterize the velocity field of the initial CME expansion. Title: The Scaling Law between Electron Time-of-Flight Distances and Loop Lengths in Solar Flares Authors: Aschwanden, M. J.; Kosugi, T.; Hudson, H. S.; Wills, M. J.; Schwartz, R. A. Bibcode: 1996AAS...188.2608A Altcode: 1996BAAS...28..858A ;}} ;}} From the complete dataset of solar flares simultaneously observed with the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory (CGRO) in the high-time resolution mode (64 ms) and the Hard X-Ray Telescope (HXT) onboard Yohkoh we were able to determine the electron time-of-flight (TOF) distance and the flare loop geometry in 42 events. The electron TOF distances l' were determined from hard X-ray (HXR) time delays (~ 10-100 ms) occurring in the 16-channel spectra (at ~ 20-200 keV), produced by the velocity difference of the HXR-producing electrons. The flare loops were mostly identified from double footpoint sources in >~ 30 keV HXT images, with radii ranging from r=3000 to r=25,000 km. We find a scaling law between the electron TOF distance l' and the flare loop half length s=r(pi /2), having a mean ratio (and standard deviation) of l'/s=1.41+/- 0.29. In 5 flares we observe coronal >~ 30 keV HXR sources of the Masuda-type in the cusp region above the flare loop, and find that their heights are consistent with the electron TOF distance to the footpoints. These results provide strong evidence that particle acceleration in solar flares occurs in the cusp region above the flare loop and that the coronal HXR sources discovered by Masuda are a signature of the acceleration site, probably controlled by a magnetic reconnection process. Title: Observations of Coronal Depletion and Ejection Authors: Lemen, J. R.; Hudson, H.; Webb, D.; Tsuneta, S. Bibcode: 1996AAS...188.7007L Altcode: 1996BAAS...28..939L We present the analysis of the Yohkoh/SXT observations of a long-decay event that was observed on 1992 February 21. This event, previously reported by Tsuneta et al. \ 1992, had a clearly observed compact ejection at its onset. There is also evidence for a depletion of material in the corona above the flare site (coronal dimming) accompanying the onset of flare brightening. We find a lower limit of 3 x 10(14) g for this mass depletion. This event was observed near the limb as an arcade viewed almost end-on. Its morphology resembles the classical reconnection model for a solar flare. Here we describe the early phase of this event when the outward motions as observed in the SXT images are interpreted as the X-ray signature of a coronal mass ejection (no white-light images are available). The coronal depletion marks the beginning of the outward motion and occurs simultaneously with hard X-ray emission. If this marks the beginning of a CME, then this event provides support for a close connection between of X-ray flares and coronal mass ejections. Title: The High Energy Solar Spectroscopic Imager - HESSI Authors: Dennis, B. R.; Crannell, C. J.; Holman, G. D.; Ramaty, R.; von Rosenvinge, T. T.; Benz, A.; Bornmann, P. L.; Brown, J. C.; Canfield, R. C.; Emslie, A. G.; Enome, S.; Kosugi, T.; Hudson, H. S.; Hurford, G. J.; Lin, R. P.; Ling, J. C.; Madden, N. W.; van Beek, H. F.; Vilmer, N. Bibcode: 1996AAS...188.7016D Altcode: 1996BAAS...28..941D HESSI will investigate the physics of particle acceleration and energy release in solar flares through an unprecedented combination of high resolution imaging and spectroscopy of X-rays and gamma rays from 2 keV to 20 MeV during the next solar maximum. It uses Fourier-transform imaging with 12 bi-grid modulation collimators and cooled germanium and silicon detectors mounted on a Sun-pointed spin-stabilized spacecraft in a low-altitude equatorial orbit. HESSI will carry out the first imaging spectroscopy in hard X-rays with 2 arcseconds angular resolution, time resolution to tens of ms, and ~ 1 keV energy resolution; the first gamma-ray line spectroscopy from a spacecraft with ~ 1 keV energy resolution; and the first gamma-ray line and continuum imaging with 20 arcseconds angular resolution. Title: X-ray Images of Two Type II Bursts Authors: Hudson, Hugh S. Bibcode: 1996AAS...188.1907H Altcode: 1996BAAS...28Q.851H The Yohkoh soft X-ray telescope has been providing high-resolution images of the counterparts of various meter-wave phenomena. We report here on clear examples of Type II emission in flares of 13 and 20 October, 1995, based upon Culgoora spectral observations. The starting frequencies of the Type II bursts differed greatly, and both showed accompanying Type III and continuum structures. The Type II traces were highly structured. The X-ray images show no sign of concentric wave fronts, as expected from the standard interpretation of the Type II burst phenomenon. Instead we see a series of loops emerging from the flare regions. We interpret the fine structure in the X-ray images as the origin of the patterns in the dynamic radio spectra, and attempt thereby to compare the densities independently inferred from the radio and X-ray observations. Title: Spatial Relations between Preflares and Flares Authors: Fárník, František; Hudson, Hugh; Watanabe, Tetsuya Bibcode: 1996SoPh..165..169F Altcode: We have conducted an initial search for discrete preflare brightenings as observed in soft X-radiation by Yohkoh. The Yohkoh images allow us to identify, to within a few arc seconds, the location of a preflare event relative to the succeeding flare. Our initial motivation in this study was to search for early coronal brightenings leading to flare effects, as had been suggested by earlier studies; thus we concentrated on Yohkoh limb events. We find no evidence for such early coronal brightenings. Between 15% and 41% of the 131 suitable events matched our criteria for preflare brightening: the same active region; brightening within one hour of the flare peak; preflare brightness less than 30% of the flare peak. In the great majority of the preflare cases, we found that physically separate nearby structures brightened initially. Often these structures appeared to share a common footpoint location with the flare brightening itself. In a few cases the preflare could have occurred in exactly the same structure as the flare. Title: Metric Type III bursts associated with soft X-ray jets. Authors: Raulin, J. P.; Kundu, M. R.; Hudson, H. S.; Nitta, N.; Raoult, A. Bibcode: 1996A&A...306..299R Altcode: From soft X-ray and metric radio observations with high temporal and spatial resolution, we show that electron acceleration in the form of Type III bursts occurs in association with coronal jets observed by the Yohkoh soft X-ray telescope. The excellent correspondence between the positions of the radio sources observed at different frequencies and the X-ray jets strongly suggests that electron beams propagate along the relatively dense paths formed by the jets. Assuming a constant temperature for the jets, one can estimate the electron density from the soft X-ray measurements. These computed electron densities agree well with the values derived from Type III bursts produced by the plasma emission process. The observations are consistent with the idea that strong particle acceleration accompanies magnetic reconnection in these events as well as in solar flares. Title: First Images of a Solar Flare at Millimeter Wavelengths Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.; de Pater, Imke; Shibasaki, K.; Hudson, Hugh S.; Kundu, Mukul R. Bibcode: 1996ApJ...458L..49S Altcode: We present the first high spatial resolution images of a solar flare at millimeter wavelengths. On 1994 August 17, a GOES soft X-ray class M1 flare was observed by the Berkeley-Illinois-Maryland Array at 86 GHz by the Nobeyama 17 GHz array and by the Yohkoh spacecraft. The flare displayed both a prominent impulsive phase in microwaves and a gradual phase that lasted over 30 minutes. The millimeter data were taken only during the gradual phase. The millimeter images show a source with a size of ~8", a peak brightness temperature of ~106 K, and maximum optical depth of 0.09. At both X-ray and radio wavelengths, the emitting region appeared to be compact (<~20"). In soft X-ray, the images are resolved into two sources: one located at a footpoint and the other at the top of the flaring loop. The millimeter emission is consistent with the predicted free-free flux from an isothermal temperature (~14 MK) loop-top source, a multitemperature footpoint source with a hot (~22 MK), and a cold (~12 MK) component. Most (80%) of the millimeter flux density originates from the top of the magnetic loop, and the footpoint contribution is only 20%. Title: Statistical Study of Solar X-Ray Jets Observed with the YOHKOH Soft X-Ray Telescope Authors: Shimojo, Masumi; Hashimoto, Shizuyo; Shibata, Kazunari; Hirayama, Tadashi; Hudson, Hugh S.; Acton, Loren W. Bibcode: 1996PASJ...48..123S Altcode: We have found 100 X-ray jets in the database of full Sun images taken with the Soft X-ray Telescope (SXT) aboard Yohkoh during the period from 1991 November through 1992 April. A statistical study for these jets results in the following characteristics: 1)\ Most are associated with small flares (microflares--subflares) at their footpoints. 2)\ The lengths lie in the range of a few times 10(4) --4 times 10(5) km. 3)\ The widths are 5 times 10(3) --10(5) km. 4)\ The apparent velocities are 10--1000 km s(-1) with an average velocity of about 200 km s(-1) . 5)\ The lifetime of the jet extends to ~ 10 hours and the distribution of the observed lifetime is a power law with an index of ~ 1.2. 6)\ 76% of the jets show constant or converging shapes; the width of the jet is constant or decreases with distance from the footpoint. The converging type tends to be generated with an energetic footpoint event and the constant type by a wide energy range of the footpoint event. 7)\ Many jets ( ~ 68%) appear in or near to active regions (AR). Among the jets ejected from bright-point like features in ARs, most ( ~ 86%) are observed to the west of the active region. 8)\ 27% of the jets show a gap ( > 10(4) km) between the exact footpoint of the jet and the brightest part of the associated flare. 9)\ The X-ray intensity distribution along an X-ray jet often shows an exponential decrease with distance from the footpoint. This exponential intensity distribution holds from the early phase to the decay phase. Title: Comparison and Relation of HeI 1083 NM Two-Ribbon Flares and Large-Scale Coronal Arcades Observed by YOHKOH Authors: Harvey, Karen L.; McAllister, Alan; Hudson, Hugh; Alexander, David; Lemen, James R.; Jones, Harrison P. Bibcode: 1996ASPC...95..100H Altcode: 1996sdit.conf..100H No abstract at ADS Title: Broadband Imaging Spectroscopy with the Solar Radio Telescope Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Hudson, H. S.; Klimchuk, J. A.; Petrosian, V.; White, S. M. Bibcode: 1996ASPC...93..430B Altcode: 1996ress.conf..430B No abstract at ADS Title: YOHKOH SXT and BCS Observations of the "Reconnection Region" of a Solar Flare Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Lemen, James R. Bibcode: 1996ASPC..111..177S Altcode: 1997ASPC..111..177S The authors find strong line shifts in Bragg crystal spectrometer (BCS) spectra of a flare which occurred well beyond the solar limb on 1993 April 15. Since the flare is beyond the limb, only the uppermost regions of the flare are visible. If reconnection is acting in flares, than one may expect that the line shifts from this event are due to reconnection jets emanating from above the region of the main flaring loops. The authors show, however, that details of the line shifts are not consistent with this picture. Rather than being a result of reconnection jets, it is more likely that the line shifts are due to plasma motions on a flaring loop oriented edge on with respect to the Earth. Title: Large-Scale Active Coronal Phenomena in YOHKOH SXT Images Authors: Svestka, Z.; Farnik, F.; Hudson, H. S.; Uchida, Y.; Hick, P. Bibcode: 1996ASPC..111..388S Altcode: 1997ASPC..111..388S The authors have checked in Yohkoh SXT images the appearance of giant post-flare arches which were discovered in hard X-ray images from the HXIS and FCS instruments onboard the SMM. They have verified the existence of both the rising and stationary arches. In addition to these two kinds of giant post-flare arches, known before from SMM observations, Yohkoh also reveals other large post-flare coronal structures which might have been considered to be giant arches by the low-resolution SMM instruments. These include coronal helmet streamers above rising flare loops or fans of hot structures in which the rising loops are embedded. Title: Magnetodynamic phenomena in the solar atmosphere. Prototypes of stellar magnetic activity Authors: Uchida, Yutaka; Kosugi, Takeo; Hudson, Hugh S. Bibcode: 1996mpsa.conf.....U Altcode: 1996IAUCo.153.....U No abstract at ADS Title: Large-Scale Active Coronal Phenomena in YOHKOH SXT Images Authors: Svestka, Z.; Farnik, F.; Hudson, H. S.; Uchida, Y.; Hick, P.; Lemen, J. R. Bibcode: 1996mpsa.conf..609S Altcode: 1996IAUCo.153..609S No abstract at ADS Title: Pixon Reconstruction and the Masuda Event of 1992 January 13 Authors: Alexander, D.; Metcalf, T.; Hudson, H. S. Bibcode: 1996ASPC..111..253A Altcode: 1997ASPC..111..253A The data set of localised hard X-ray sources observed by the Yohkoh/HXT has been analysed using maximum entropy methods (MEM) to reconstruct HXT images. Recently, an alternative method, that of fractal pixon reconstruction, has been developed for use with the HXT. The authors have reanalysed the event of 13 January 1992 (the Masuda event), comparing the MEM and pixon methods. There are distinct differences in the two sets of results. The pixon method, favoured by the authors, indicates a less impulsive coronal source than the MEM reconstruction and also a relatively weaker coronal/footpoint emission ratio. Title: Observational Problems for Flare Models Based on Large-Scale Magnetic Reconnection (Invited) Authors: Hudson, Hugh S.; Khan, Josef I. Bibcode: 1996ASPC..111..135H Altcode: 1997ASPC..111..135H The authors examine the observable consequences of flare models which describe the energetics in terms of large-scale magnetic reconnection. The Yohkoh data offer many ways to check whether the phenomena expected from large-scale reconnection models actually occur. There are apparent successes, as for example in the morphology of flares with cusp geometry. Puzzling observational discrepancies also exist and are discussed: the lack of inward flows toward the reconnection site; the lack of outward jet flows from the reconnection site; the implicit existence of open field lines prior to flare onset; the dominance of apparently single loops; the need for efficient acceleration of non-thermal particles; the existence of homologous flares; the existence of intense compact events; the existence of energetic flares essentially without mass motion; and the rarity of concave-up field lines in the corona. The authors propose an observational technique, "conjugate variability", which if successful could establish reconnection relatively unambiguously. Title: Causal Relation between H alpha Arch Filament Loops and Soft X-ray Coronal Loops Authors: Yoshimura, K.; Kurokawa, H.; Sano, S.; Hudson, H. Bibcode: 1996mpsa.conf..457Y Altcode: 1996IAUCo.153..457Y No abstract at ADS Title: Isolating the Footpoint Characteristics of a Solar Flare Loop Authors: Harra-Murnion, L. K.; Culhane, J. L.; Fujiwara, T.; Hudson, H. S.; Kato, T.; Sterling, A. C. Bibcode: 1996mpsa.conf..527H Altcode: 1996IAUCo.153..527H No abstract at ADS Title: Coronal X-Ray Dimming in Two Limb Flares Authors: Hudson, Hugh S.; Lemen, James R.; Webb, David F. Bibcode: 1996ASPC..111..379H Altcode: 1997ASPC..111..379H Yohkoh SXT observations of flares and large-scale arcade events frequently show coronal dimming accompanying X-ray brightening in long-duration events. The authors tentatively identify this with the process of field-line opening in the initial phase of a coronal mass ejection (CME), although few simultaneous coronagraph and soft X-ray observations have yet been described. The dimming signature may reduce the coronal soft X-ray intensity by as much as a factor of 2 - 3, and thus has a higher contrast than the cavity often seen in white-light CME observations. In the cases examined thus far, the authors find a close match between the onsets of X-ray brightening and coronal dimming, suggesting a close physical relationship. The dimming appears (in movie representations) to result from outward expansion; highly structured features (multiple loops) are recognizable in the dimming regions of some events, suggesting that the soft X-ray data may be used to characterize the velocity field of the expansion. Title: Nonthermal Radio Emission from Coronal X-ray Jets Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.; Raoult, A. Bibcode: 1996ASPC...93..375K Altcode: 1996ress.conf..375K No abstract at ADS Title: Detection of Nonthermal Radio Emission from Coronal X-ray Jets Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.; Raoult, A.; Shibata, K.; Shimojo, M. Bibcode: 1996mpsa.conf..445K Altcode: 1996IAUCo.153..445K No abstract at ADS Title: YOHKOH Observations of Coronal Mass Ejections Authors: Hudson, H. Bibcode: 1996mpsa.conf...89H Altcode: 1996IAUCo.153...89H No abstract at ADS Title: X-ray Observations of an Over-the-Limb Solar Flare with Large Spectral Line Shifts Authors: Sterling, A. C.; Harra-Murnion, L. K.; Hudson, H. S.; Lemen, J. R.; Strong, K. T. Bibcode: 1996mpsa.conf..557S Altcode: 1996IAUCo.153..557S No abstract at ADS Title: Hydrodynamic Modeling of Flares Well-Observed by Yohkoh/SXT Authors: Reale, F.; Peres, G.; Hudson, H. Bibcode: 1996mpsa.conf..311R Altcode: 1996IAUCo.153..311R No abstract at ADS Title: Active Region Evolution and Flare Activity Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Hudson, H. S. Bibcode: 1996mpsa.conf..515N Altcode: 1996IAUCo.153..515N No abstract at ADS Title: Large-Scale Arcade Formation on May 15, 1992 and its Interplanetary Consequence Authors: Nakagawa, Y.; Watanabe, T.; Hudson, H.; Kojima, M. Bibcode: 1996mpsa.conf..489N Altcode: 1996IAUCo.153..489N No abstract at ADS Title: First Images of a Solar Flare at Millimeter Wavelengths Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.; de Pater, Imke; Shibasaku, K.; Hudson, Hugh S.; Kundu, Mukul R. Bibcode: 1996ADIL...AS...01S Altcode: We present the first high-spatial-resolution images of a solar flare at millimeter wavelengths. On 1994 August 17, a GOES soft X--ray class M1 flare was observed by the Berkeley--Illinois--Maryland Array (BIMA) at 86 GHz, by the Nobeyama 17 GHz array, and by the Yohkoh spacecraft. The flare displayed both a prominent impulsive phase in microwaves and a gradual phase which lasted over 30 minutes. The millimeter data were taken only during the gradual phase. The millimeter images show a source with a size of $\sim$8\arcsec, a peak brightness temperature of ~ 10^6 K, and maximum optical depth of 0.09. At both X--ray and radio wavelengths the emitting region appeared to be compact (< 20 arcseconds). In soft X--ray the images are resolved into two sources: one located at a footpoint and the other at the top of the flaring loop. The millimeter emission is consistent with the predicted free-free flux from an isothermal temperature (~ 14 MK) looptop source and a multi--temperature footpoint source with a hot (~ 22 MK) and a cold (~ 12 MK) component. Most (80%) of the millimeter flux density originates from the top of the magnetic loop, and the footpoint contribution is only 20%. Title: A Loop Flare Observed by YOHKOH on 1992 July 11 Authors: Khan, Josef I.; Hudson, Hugh S.; Sterling, Alphonse C.; Lemen, James R. Bibcode: 1996ASPC..111..162K Altcode: 1997ASPC..111..162K The authors present Yohkoh soft and hard X-ray observations of a flare. Soft X-ray morphology shows the structure of this flare to be a relatively simple loop. Nonetheless several interesting points were found including: (i) bright soft X-ray footpoints persist long after completion of the impulsive hard X-ray bursts; (ii) both legs and footpoints of the flare loop appear to move together rather than apart during the course of the flare; (iii) initially the flare loop appears to have a fairly uniform thickness but as the flare progresses the loop-top region becomes broader; (iv) 'low energy' hard X-rays appear to originate from high in the loop near the loop apex; and (v) soft X-ray spectra show strong line asymmetries suggesting the presence of upflowing plasma oriented nearly directly towards the Earth. Title: Large-Scale Active Coronal Phenomena in YOHKOH SXT Images, I Authors: Švestka, Zdeněk; Fárník, František; Hudson, Hugh S.; Uchida, Yutaka; Hick, Paul; Lemen, James R. Bibcode: 1995SoPh..161..331S Altcode: We have found several occurrences of slowly rising giant arches inYohkoh images. These are similar to the giant post-flare arches previously discovered by SMM instruments in the 80s. However, we see them now with 3-5 times better spatial resolution and can recognize well their loop-like structure. As a rule, these arches followeruptive flares with gradual soft X-ray bursts, and rise with speeds of 1.1-2.4 km s−1 which keep constant for >5 to 24 hours, reaching altitudes up to 250 000 km above the solar limb. These arches differ from post-flare loop systems by their (much higher) altitudes, (much longer) lifetimes, and (constant) speed of growth. One event appears to be a rise of a transequatorial interconnecting loop. Title: Yohkho Soft X-Ray Spectroscopic Observations of the Bright Loop-Top Kernels of Solar Flares Authors: Khan, Josef I.; Harra-Murnion, Louise K.; Hudson, Hugh S.; Lemen, James R.; Sterling, Alphonse C. Bibcode: 1995ApJ...452L.153K Altcode: Observations of solar flares by the Soft X-ray Telescope (SXT) on board Yohkoh frequently show strongly enhanced brightenings near the tops of the magnetic loops containing hot plasma. The Yohkoh Bragg Crystal Spectrometer (BCS) cannot normally make observations of these loop-top sources in the absence of contamination by the legs and the feet of the loops since it has no spatial resolution. We have overcome this limitation by using the solar limb as an occulting edge in a sequence of similar flares that occurred over an interval of ~10 hr near the west limb on 1992 November 24. The progressive occultation by the limb restricts the line of sight to higher and higher altitudes during this sequence, with the final event showing only a compact source of the type often found at loop tops. BCS observations in Fe XXV, Ca XIX, and S XV show that electron temperatures and nonthermal velocities in these compact sources are similar to those quantities determined for disk flares in previous studies. As with disk flares, the nonthermal line broadening persists late into the decay phase of the flaring isolated loop tops. Our results favor mechanisms for nonthermal-velocity generation that are either independent of height or place the source near the apex of the flaring loop. In addition, there may be a temporal relationship between the hard X-ray emission and the nonthermal velocity, which suggests a possible association between the primary energy release of the flare, the nonthermal-velocity generation mechanism, and the loop top. Title: Using Fe X 6374 Å and Fe XIV 5303 Å spectral line intensities to study the effect of line of sight integration on coronal temperature inferences Authors: Esser, R.; Brickhouse, N. S.; Habbal, S. R.; Altrock, R. C.; Hudson, H. S. Bibcode: 1995JGR...10019829E Altcode: Polar coronal holes are relatively stable structures persisting over many solar rotations. The appearance of coronal holes in remote observations, however, can change on a daily basis due to variations of the denser and hotter plasma surrounding them. We explore the effect of these denser and hotter surrounding regions on coronal hole observations, using daily intensity measurements at 1.15 RS of the green Fe XIV 5303 Å and red Fe X 6374 Å spectral lines. The observations, which were carried out at the National Solar Observatory a Sacramento Peak, New Mexico, cover at time period of about four solar rotations.

We show that the ``coronal hole'' temperatures derived using the line ratio technique, vary by more than 0.8×106 K over the time interval considered here. We also provide a short discussion of the expected accuracy of the atomic data for these two iron spectral lines. Using intensity measurements as a function of distance from the Sun, we briefly discuss how the regions surrounding the coronal holes might influence the inference of the temperature gradient in the coronal holes. The line of sight effect on the temperature gradient should be explored in more detail in the future using daily observations of the line intensities as a function of distance. These observations could be provided by ground-based coronagraphs and by instruments on board SOHO. Title: Detection of Nonthermal Radio Emission from Coronal X-Ray Jets Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.; Shimojo, M.; Shibata, K.; Raoult, A. Bibcode: 1995ApJ...447L.135K Altcode: We report the detection of a type III burst in association with a dynamic X-ray coronal jet observed by Yohkoh/SXT. The type III burst observed with the Nancay (France) multifrequency radioheliograph is spatially and temporally coincident with the X-ray jet. The radio locations at different frequencies (236.6 and 164 MHz) are aligned along the length of the jet. The observation of the type III burst in association with the X-ray jet implies the acceleration of electrons to several tens of keV, along with the heating responsible for the production of soft X-rays. This association implies the existence of open field lines in dense coronal structures identified on the Sun's disk. This is the first observation of dense coronal structures on the disk, along which type III emitting nonthermal electrons propagate. We find that this structure begins to form before the type III emission. At the time of the type III burst we estimate a density of 6--10 x 108 cm-3 for a temperature of ~5--6 MK at an altitude of 20,000 km. Title: Coordinated Observation of the Solar Corona Using the Norikura Coronagraph and the YOHKOH Soft X-Ray Telescope Authors: Ichimoto, K.; Hara, H.; Takeda, A.; Kumagai, K.; Sakurai, T.; Shimizu, T.; Hudson, H. S. Bibcode: 1995ApJ...445..978I Altcode: Spectroscopic observations of coronal emission lines were carried out at the Norikura Solar Observatory in cooperation with the soft X-ray telescope on board the Yohkoh satellite to study the plasma distributions at different temperatures. Intensity and velocity distributions in Fe XIV wavelength 5303 (green), Fe X wavelength 6374 (red), and Ca XV wavelength 5694 (yellow) lines are compared with the soft X-ray images. It is found that the soft X-ray images closely resemble those of the yellow line that represents a rather high temperature component of the corona. On the other hand the low-temperature component seen in the green and the red lines shows quite a different distribution from that of the high-temperature component; the low-temperature component consists of many thin loops or streaks, while the high-temperature component is more diffuse. We find that the active elements of the cool component, i.e., complex loop systems, rapid changes of small structures, and localized large plasma motions, all tend to be cospatial with the hot component. Title: The solar/interplanetary event of 14 April 1994 observed by Yohkoh/SXT Authors: Alexander, D.; Harvey, K. L.; Hudson, H. S.; Hoeksema, J. T.; Zhao, X. Bibcode: 1995sowi.conf...57A Altcode: The polar crown event of April 14 1994 is one of the largest scale eruptive events observed by the Yohkoh/SXT. Associated with the formation of an arcade of soft X-ray loops at the Sun was the detection of an interplanetary forward/reverse shock event by the Ulysses spacecraft some 4-7 days later. The relationship between the coronal and interplanetary signatures of these events is important if we are to address fully the initialization and consequent acceleration of interplanetary phenomena, such as CMEs and counter-streaming electrons, originating at the Sun. From detailed analysis of the energetics of the arcade formed during the eruption of April 14 1994, we find peak temperatures and emission measures of approximately 5MK and approximately 1048cm-3 respectively. The total thermal content of the arcade loop structure observed in soft X-rays is calculated to be some 5 x 1029 ergs. The development of these parameters as the event proceeds and their relationship to the dynamics of the eruption are investigated. Although spanning a longitudinal range of some 150 degrees the April 14 event displayed the typical helmet streamer structure normally associated with coronal mass ejections These helmet streamers are thought to be related to the global solar magnetic field through the heliospheric current sheet (HCS). The arcade formation, together with the eruption of material into interplanetary space, signifies a large-scale reconfiguration of the coronal magnetic field. We examine the effects of the formation of such a coronal arcade structure on the HCS and discuss the dynamics involved with the passage of a large scale disturbance through the interplanetary magnetic field. Title: Comparison of YOHKOH x-ray coronal events with ULYSSES interplanetary events Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.; Harvey, K. L.; Hoecksema, J. T.; Zhao, X.; Hudson, H. Bibcode: 1995sowi.conf...58L Altcode: The Yohkoh soft X-ray telescope (SXT) has observed several largescale eruptive events per year for the first three years of observations (Aug. 1991 - Nov. 1994) Such events are most prominent at high latitudes, but resemble long-duration flare events seen in active regions. Some of the high-latitude events have now been identified in the Ulysses/SWICS data base during the Ulysses south polar passage. There are puzzling examples of solar events with no interplanetary counterparts. A comparison of coronal and interplanetary events can lead to better models for mapping interplanetary disturbances back to their source location, especially by combining Yohkoh morphology with three-dimensional representations of the coronal magnetic field. In this paper we describe the parameters of the hot plasma seen by SXT. There is clear evidence for non radial motion in specific events. We present comparisons between the ionization temperature of the interplanetary plasma with that observed at the Sun in cases where this is possible. Title: Yohkoh/SXT soft x-ray observations of sudden mass loss from the solar corona Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Freeland, S. L.; Lemen, J. R.; Harvey, K. L. Bibcode: 1995sowi.confR..58H Altcode: Direct X-ray observations allow us to estimate the hot coronal mass before and after a flare or other disturbance of the type leading to a coronal mass ejection. The sudden disappearance of a large coronal structure (scale greater than 105 km) gives evidence that an ejection has occurred, if the time scales are much shorter than the conductive or radiative cooling times for such structures. A flare also typically adds large amounts of new material to the corona via evaporation resulting from the coronal energy release. This provides a competing mechanism that makes the estimation of the total mass loss somewhat difficult. We note that the X-ray observations have the advantage of covering the entire corona rather than the limb regions unlike the coronagraph observations. We have identified two examples of coronal mass disappearances. before and during long duration flare events on 21 Feb. 1992 (on the E limb) and 13 Nov. 1994 (near disk center). In latter case the total mass amounted to some 4 x 1014 g with a density of 3 x 108cm-3 and a temperature of 2.8 MK before its disappearance. This corresponds to a radiative cooling time of some 104 S. much longer than the observed time of disappearance. We therefore suggest that these sudden mass disappearances correspond with coronal mass ejections (CMEs), and suggest that further data analysis will be able to confirm this by comparison with optical observations of specific CMEs. Title: The solar origins of two high-latitude interplanetary disturbances Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.; Kurokawa, H.; Kahler, S.; Lemen, J. R. Bibcode: 1995sowi.confS..58H Altcode: Two extremely similar interplanetary forward/reverse shock events, with bidirectional electron streaming were detected by Ulysses in 1994. Ground-based and Yohkoh/SXT observations show two strikingly different solar events that could be associated with them: an LDE flare on 20 Feb. 1994, and a extremely large-scale eruptive event on 14 April 1994. Both events resulted in geomagnetic storms and presumably were associated with coronal mass ejections. The sharply contrasting nature of these solar events argues against an energetic causal relationship between them and the bidirectional streaming events observed by Ulysses during its S polar passage. We suggest instead that for each pair of events. a common solar trigger may have caused independent instabilities leading to the solar and interplanetary phenomena. Title: Inference of 3-dimensional structure underlying large-scale coronal events observed by YOHKOH and ULYSSES Authors: Slater, G. L.; Freeland, S. L.; Hoeksema, T.; Zhao, X.; Hudson, H. S. Bibcode: 1995sowi.confQ..63S Altcode: The Yohkoh/SXT images provide full-disk coverage of the solar corona, usually extending before and after one of the large-scale eruptive events that occur in the polar crown These produce large arcades of X-ray loops, often with a cusp-shaped coronal extension, and are known to be associated with coronal mass ejections. The Yohkoh prototype of such events occurred 12 Nov. 1991. This allows us to determine heights from the apparent rotation rates of these structures. In comparison v with magnetic-field extrapolations from Wilcox Solar Observatory. use use this tool to infer the three dimensional structure of the corona in particular cases: 24 Jan. 1992, 24 Feb. 1993, 14 Apr. 1994, and 13 Nov. 1994. The last event is a long-duration flare event. Title: Comment on ``The solar flare myth'' by J. T. Gosling Authors: Hudson, Hugh; Haisch, Bernhard; Strong, Keith T. Bibcode: 1995JGR...100.3473H Altcode: In a recent paper Gosling (1993) claims that solar flares are relatively unimportant for understanding the terrestrial consequences of solar activity, and argues that coronal mass ejections (CMEs) produce the most powerful terrestrial disturbances. This opinion conflicts with observation, as it is well known that CMEs and flares are closely associated, and we disagree with Gosling's insistence on a simplistic cause-and-effect description of the interrelated phenomena of a solar flare. In this brief response we present new Yohkoh data and review older results that demonstrate the close relationships among CMEs, flares, filament eruptions, and other forms of energy release such as particle acceleration. Title: A Solar Radio Telescope for the Future: Science Summary from the SRT Workshop Authors: Gary, D. E.; Bastian, T. S.; Hudson, H. S.; Hurford, G. J.; Klimchuk, J. A.; Petrosian, V.; White, S. M. Bibcode: 1995SPD....26..801G Altcode: 1995BAAS...27..971G No abstract at ADS Title: Yohkoh Multi-Wavelength Observations of the Bright Loop-Top Kernels in Solar Flares Authors: Sterling, A.; Khan, J.; Harra-Murnion, L.; Hudson, H.; Lemen, J. Bibcode: 1995SPD....26.1211S Altcode: 1995BAAS...27..985S No abstract at ADS Title: Yohkoh Observations of Impulse-Response Flares Authors: Hudson, H. S.; Farnik, F.; Watanabe, T. Bibcode: 1995SPD....26.1210H Altcode: 1995BAAS...27..984H No abstract at ADS Title: Correlated brightness variations in solar radiative output from the photosphere to the corona Authors: Lean, J. L.; Mariska, J. T.; Strong, K. T.; Hudson, H. S.; Acton, L. W.; Rottman, G. J.; Woods, T. N.; Willson, R. C. Bibcode: 1995GeoRL..22..655L Altcode: Correlated brightness variations are shown to occur in time series of coronal soft X-rays exclusive of prominent active regions, chromospheric ultraviolet radiation, and the photospheric total solar irradiance corrected for sunspot effects. These temporal correlations suggest that upwardly extending magnetic fields may have a large scale impact on the solar atmosphere in addition to their demonstrable role of generating localized active regions. The correlations have implications for improving and extending solar spectrum variability models. Title: Nouthermal Radio Emission From Coronal X-Ray Jets Authors: Raulin, J. P.; Kundu, M. R.; Hudson, H. S.; Nitta, N.; Raoult, A. Bibcode: 1995SPD....26.1318R Altcode: 1995BAAS...27..991R No abstract at ADS Title: A Solar Radio Telescope for the Future: Strawman Concept from the SRT Workshop Authors: Hurford, G. J.; Bastian, T. S.; Gary, D. E.; Hudson, H. S.; Klimchuk, J. A.; Petrosian, V.; White, S. M. Bibcode: 1995SPD....26..802H Altcode: 1995BAAS...27..971H No abstract at ADS Title: Fractal Pixon Image Reconstruction for Yohkoh's Hard X-Ray Telescope Authors: Metcalf, T. R.; Hudson, H. S.; Kosugi, T.; Puetter, R. C.; Piña, R. K. Bibcode: 1995SPD....26.1314M Altcode: 1995BAAS...27..990M No abstract at ADS Title: The 1991 October 24 Flare: A Challenge for Standard Models Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Hudson, H. S.; Wulser, J. -P.; Acton, L.; Kosugi, T.; Masuda, S. Bibcode: 1995ApJ...440..386D Altcode: The M9.8 solar flare of 1991 October 24 22:30 UT presents several interesting characteristics: (1) energy release starts high in the corona; (2) the primary chromospheric ribbons are initially well separated and do not move apart at an observable rate; (3) no evidence is found for an erupting filament or other driver. To explain this flare, we consider several canonical flare models, including a filament eruption, a confined filament eruption, current interruption, and interacting loops. We conclude that none of these scenarios unequivocally explains this flare. Two possibilities which cannot be ruled out are (1) the eruption of a filament unobservable in H-alpha which starts high in the corona and produces no ribbon motions smaller than our detection threshold and no perceptible expansion of the coronal X-ray source, and (2) energy release due to spontaneous, propagating reconnection which allows the system to essentially brighten in place. Title: High-Energy Particles In Solar Flares Authors: Hudson, H.; Ryan, J. Bibcode: 1995ARA&A..33..239H Altcode: Accelerated particles appear to co exist inseparably with most forms of energy release in solar flares and coronal mass ejections. We identify at least six different populations of high-energy electrons and ions. High-energy particles, accelerated efficiently in the flare in great numbers, transport a large fraction of the flare energy to other sites. This behavior makes them an integral part of the flare process. Much new data has come from two satellites launched in 1991: the Compton Gamma-Ray Observatory and Yohkoh. This review concentrates on particles in flares, mainly using X-ray and gamma-ray data rather than measurements of "escaping'' particles observed in interplanetary space. Title: Solar flares: No "myth" Authors: Hudson, H. S. Bibcode: 1995EOSTr..76..405H Altcode: No abstract at ADS Title: Stellar x-ray flares Authors: Haisch, B.; Uchida, Y.; Kosugi, T.; Hudson, H. S. Bibcode: 1995lock.reptR....H Altcode: What is the importance of stellar X-ray flares to astrophysics, or even more, to the world at large? In the case of the Sun, changes in solar activity at the two temporal extremes can have quite significant consequences. Longterm changes in solar activity, such as the Maunder Minimum, can apparently lead to non-negligible alterations of the earth's climate. The extreme short term changes are solar flares, the most energetic of which can cause communications disruptions, power outages and ionizing radiation levels amounting to medical X-ray dosages on long commercial flights and even potentially lethal exposures for unshielded astronauts. Why does the Sun exhibit such behaviour? Even if we had a detailed knowledge of the relevant physical processes on the Sun - which we may be on the way to having in hand as evidenced by these Proceedings- our understanding would remain incomplete in regard to fundamental causation so long as we could not say whether the Sun is, in this respect, unique among the stars. This current paper discusses the stellar x-ray flare detections and astronomical models (quasi-static cooling model and two-ribbon model) that are used to observe the x-ray emission. Title: Recovering the fine structures in solar images Authors: Karovska, M.; Habbal, S. R.; Golub, L.; DeLuca, E.; Hudson, H. Bibcode: 1994ESASP.373..183K Altcode: 1994soho....3..183K No abstract at ADS Title: The large scale coronal eruptive event of April 14 1994 Authors: Alexander, D.; Slater, Greg L.; Hudson, Hugh S.; McAllister, Alan H.; Harvey, Karen L. Bibcode: 1994ESASP.373..187A Altcode: 1994soho....3..187A No abstract at ADS Title: The Yohkoh context for high-energy particles in solar flares Authors: Hudson, Hugh S. Bibcode: 1994AIPC..294..151H Altcode: 1994hesp.conf..151H Yohkoh, a satellite dedicated to high-energy observations of solar flares, began observations in September, 1991. It carries (i) a soft X-ray telescope with arcsecond resolution and excellent temporal sampling; (ii) a hard X-ray imager making the first images above 30 keV; (iii) a sensitive Bragg crystal spectrometer for soft X-ray emission lines; and (iv) a set of proportional and scintillation counters. The flare observations confirm the central role of impulsive-phase electron acceleration in causing ``evaporation'' and white-light flare emission. SXT has found impulsive soft X-ray time profiles at the footpoints. It also shows compact bright structures apparently at the tops of flaring loops during the gradual phase. Large flares may show cusp-shaped structures that strongly resemble the usual picture of coronal magnetic reconnection, but otherwise do not match the details of the classical flare scenario. The data taken as a whole suggest that large-scale magnetic reconnection in the solar corona does not drive flare energy release, but rather is driven by the flare; the reconnection may have an important role in flare triggering. Title: A Gigantic Coronal Jet Ejected from a Compact Active Region in a Coronal Hole Authors: Shibata, K.; Nitta, N.; Strong, K. T.; Matsumoto, R.; Yokoyama, T.; Hirayama, T.; Hudson, H.; Ogawara, Y. Bibcode: 1994ApJ...431L..51S Altcode: A gigantic coronal jet greater than 3 x 105 km long (nearly half the solar radius) has been found with the soft X-ray telescope (SXT) on board the solar X-ray satellite, Yohkoh. The jet was ejected on 1992 January 11 from an 'anemone-type' active region (AR) appearing in a coronal hole and is one of the largest coronal X-ray jets observed so far by SXT. This gigantic jet is the best observed example of many other smaller X-ray jets, because the spatial structures of both the jet and the AR located at its base are more easily resolved. The range of apparent translational velocities of the bulk of the jet was between 90 and 240 km s-1, with the corresponding kinetic energy estimated to be of order of 1028 ergs. A detailed analysis reveals that the jet was associated with a loop brightening (a small flare) that occurred in the active region. Several features of this observation suggest and are consistent with a magnetic reconnection mechanism for the production of such a 'jet-loop-brightening' event. Title: Evidence for Both Electron Acceleration and Direct Heating in Solar Flares Authors: Dennis, B. R.; Holman, G. D.; Hudson, H. S.; Kosugi, T.; Strong, K. T.; Zarro, D. M. Bibcode: 1994kofu.symp..217D Altcode: It is well known that in many impulsive solar flares, the time profile of the total soft X-ray flux closely matches the time integral of the hard X-ray profile, the so-called Neupert Effect. We have selected several flares detected by the X-ray telescopes on Yohkoh that clearly show this effect and examined the time profiles for different spatial locations throughout the flaring region. We find that footpoint locations show coincident impulsive bursts in both soft and hard X-ray emissions whereas loop-top locations show more gradually varying soft X-ray emission with weaker hard X-ray emission. We interpret these observations in terms of an electric field model in which both Joule heating and electron runaway acceleration take place, with the ratio of the two dependent on how strong the field is compared to the local Dreicer field. The pre-impulsive phase emissions and the early gradually-varying soft X-ray emission can be attributed to the direct heating by the electri! c field in the coronal part of the loop and the impulsive footpoint emission can be attributed to both enhanced Joule heating and electron precipitation. Title: Metric Type III Bursts from Flaring X-ray Bright Points Authors: Kundu, M. R.; Strong, K. T.; Pick, M.; Harvey, K. T.; Kane, S. R.; White, S. M.; Hudson, H. S. Bibcode: 1994kofu.symp..343K Altcode: X-ray bright points (XBP's) are known to show variability on a number of timescales, including impulsive X-ray brightenings. The relationship between these XBP ``flares'' and normal solar flares is poorly known. A fundamental question is whether nonthermal acceleration of particles takes place in XBP flares. We address this issue by searching for nonthermal radio emission at metric wavelengths from flaring XBPs identified in Yohkoh/SXT data. Unequivocal evidence for type-III-like radio bursts, usually attributed to beams of nonthermal electrons on open field lines, is found. This suggests that XBP flares are similar to normal flares and can indeed accelerate nonthermal populations of energetic particles. Title: Hard and Soft X-ray Observations of a Super-Hot Thermal Flare of 6 February, 1992 Authors: Kosugi, T.; Sakao, T.; Masuda, S.; Hara, H.; Shimizu, T.; Hudson, H. S. Bibcode: 1994kofu.symp..127K Altcode: No abstract at ADS Title: Determining Point Spread Function of Space Observations Using BID Algorithm Authors: Karovska, M.; Hudson, H. S. Bibcode: 1994kofu.symp..327K Altcode: With the advent of improved data analysis tools and the superior image restoration capabilities of the newly developed Blind Iterative Deconvolution algorithm, the scientific return from the observations from space can be significantly improved. We present the results of the application of this algorithm to the EUV/Skylab images and to a sample of YOHKOH data. Title: A Search for ``Black-Light Flares'' Authors: Driel-Gesztelyi, L. V.; Hudson, H. S.; Anwer, B.; Hiei, E. Bibcode: 1994kofu.symp..375D Altcode: No abstract at ADS Title: Temperature Analysis of the Post-Flare Loops of June 25-26, 1992 Authors: Anwer, B.; Hiei, E.; Hudson, H. S.; Acton, L. W.; Lemen, J.; Metcalf, T. R. Bibcode: 1994kofu.symp..137A Altcode: We have performed an analysis of temperatures and emission measures of thermal plasma on a post--flare loop system following an X3.9 flare of June 25, 1992, at 20:14 UT in NOAA active region 7205 near the west limb (N09, W67). The filter ratio method was applied to the data sets taken using the Al 0.1 micron (thin Al) and Al 12 micron (thick Al) filters of the Yohkoh Soft X-ray Telescope (SXT). We found that the plasma temperature of the top of loops was in the range 5 - 8 x 10^6 K and log emission measure between 44.6 and 46.7 cm^(-3) for data sets taken from 22:56:57 UT of June 25 to 09:00 UT of June 26. Furthermore, the occurrence of a C1-class flare at the top of the flare loops increased the plasma temperature from 5.5 x 10^6 K to 6.6 x 10^6 K at 06:57:11 UT. The loops top was much brighter than the legs and footpoints, with delta_T was about 0.1 x 10^6 K. Title: Thermal Plasmas in the Solar Corona: the YOHKOH Soft X-Ray Observations Authors: Hudson, H. S. Bibcode: 1994kofu.symp....1H Altcode: The Yohkoh observatory carries two instruments whose main emphasis is the observation of soft X-radiation, principally emitted by thermal radiation processes. The other two Yohkoh instruments also observe some of these thermal sources. The combination of instruments makes Yohkoh unprecedented in its ability to observe the remarkable array of hot plasmas in the solar corona, and this has resulted in many new discoveries as well as in the sharpening of our knowledge of known (but often unexplained) phenomena. The scope of this review consists of the Yohkoh observations of thermal plasmas, and it emphasizes the new discoveries. The SXT data show many phenomena whose geometry and dynamics strongly suggest magnetic reconnection. Title: Multiwavelength Observations of a Solar Flare Authors: White, S. M.; Silva, A.; de Pater, I.; Lin, R. P.; Gary, D. E.; Hudson, H. S.; Doyle, J. G.; Hagyard, M. J.; Kundu, M. R. Bibcode: 1994kofu.symp..203W Altcode: No abstract at ADS Title: Analysis of Three YOHKOH White-Light Flares Authors: Hudson, H. S.; Driel-Gesztelyi, L. V.; Kosugi, T. Bibcode: 1994kofu.symp..397H Altcode: Three of the nine white-light flares thus far discovered in the Yohkoh white-light data were also observed at the Nobeyama Radio Observatory at its 80 GHz polarimeter. Each of the three flares was relatively close to the center of the Sun and had an H_alpha importance of 2B-3B, but the X-ray classes were quite different: the flares of 27 Oct. 1991, 15 Nov. 1991, and 14 Feb. 1992 had GOES X-ray magnitudes of X6.1, X1.5, and M7.0 respectively. We have analyzed the characteristics of these flares at white-light, X-ray and radio wavelengths in order to find clues for the emission mechanisms of the white-light and millimeter-wave continuum. Each of these three flares had both impulsive and gradual optical emissions. Using the Yohkoh soft X-ray images we associate the gradual component of white-light emission in these flares with compact high-temperature loops. We speculate that the white light itself comes from fine structures embedded in these loops, at densities greater than normal photospheric densities. Title: Preface Authors: Pap, Judit M.; Fröhlich, Claus; Hudson, Hugh S.; Tobiska, W. Kent Bibcode: 1994SoPh..152D...9P Altcode: No abstract at ADS Title: Improvement of the Photometric Sunspot Index and Changes of the Disk-Integrated Sunspot Contrast with Time Authors: Froehlich, Claus; Pap, Judit M.; Hudson, Hugh S. Bibcode: 1994SoPh..152..111F Altcode: 1994IAUCo.143..111F; 1994svs..coll..111F The photometric sunspot index (PSI) was developed to study the effects of sunspots on solar irradiance. It is calculated from the sunspot data published in theSolar-Geophysical Data catalogue. It has been shown that the formerPSI models overestimate the effect of dark sunspots on solar irradiance; furthermore results of direct sunspot photometry indicate that the contrast of spots depends on their area. An improvedPSI calculation is presented; it takes into account the area dependence of the contrast and calculates 'true' daily means for each observation using the differential rotation of the spots. Moreover, the observations are screened for outliers which improves the homogeneity of the data set substantially, at least for the period after December 1981 when NOAA started to report data from a few instead of one to two stations. A detailed description of the method is provided. The correlation between the newly calculatedPSI and total solar irradiance is studied for different phases of the solar cycles 21 and 22 using bi-variate spectral analysis. The results can be used as a `calibration' ofPSI in terms of gain, the factor by whichPSI has to be multiplied to yield the observed irradiance change. This factor changes with time from about 0.6 in 1980 to 1.1 in 1990. This unexpected result cannot be interpreted by a change of the contrast relative to the quiet Sun (as it is normally defined and determined by direct photometry) but rather as a change of the contrast between the spots and their surrounding as seen in total irradiance (integrated over the solar disk). This may partly be explained by a change in the ratio between the areas of the spots and the surrounding faculae. Title: Astronomical photometry from the moon Authors: Hudson, Hugh S. Bibcode: 1994AdSpR..14f..99H Altcode: 1994AdSpR..14...99H The Moon would be an excellent platform for photometric astronomical observations. This paper discusses such observations, emphasizing time-series photometry of oscillating stars (asteroseismology), of faint gravitating bodies (microlensing), and of the interplanetary medium. To prepare for the deployment of major new telescopes and instrumentation on the surface of the Moon, I suggest that smaller ``site-survey'' instruments be put in place as soon as possible. Each application suggested can derive great benifits from small site-survey instruments established relatively soon, and each would ultimately need extensive arrays of large instruments. Title: Electric Currents and Coronal Heating in NOAA Active Region 6952 Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey, D. L.; Wulser, J. -P.; Martens, P. C. H.; Tsuneta, S. Bibcode: 1994ApJ...428..860M Altcode: We examine the spatial and temporal relationship between coronal structures observed with the soft X-ray telescope (SXT) on board the Yohkoh spacecraft and the vertical electric current density derived from photospheric vector magnetograms obtained using the Stokes Polarimeter at the Mees Solar Observatory. We focus on a single active region: AR 6952 which we observed on 7 days during 1991 December. For 11 independent maps of the vertical electric current density co-aligned with non-flaring X-ray images, we search for a morphological relationship between sites of high vertical current density in the photosphere and enhanced X-ray emission in the overlying corona. We find no compelling spatial or temporal correlation between the sites of vertical current and the bright X-ray structures in this active region. Title: Observations of Jovian Thermal Structure at Mid-Infrared Wavelengths Authors: Fisher, B. M.; Jones, B.; Hudson, H. S. Bibcode: 1994DPS....26.1118F Altcode: 1994BAAS...26Q1103F No abstract at ADS Title: A Yohkoh search for ``black-light flares'' Authors: van Driel-Gesztelyi, Lidia; Hudson, Hugh S.; Anwar, Bachtiar; Hiei, Eijiro Bibcode: 1994SoPh..152..145V Altcode: 1994svs..coll..145V; 1994IAUCo.143..145V Calculations which predict that a phenomenon analogous to stellar negative pre-flares could also exist on the Sun were published by Hénouxet al. (1990), and Aboudarhamet al., (1990), who showed that at the beginning of a solar white-light flare (WLF) event an electron beam can cause a transient darkening before the WLF emission starts, under certain conditions. They named this event a "black light flare" (BLF). Such a BLF event should appear as diffuse dark patches lasting for about 20 seconds preceding the WLF emission, which would coincide with intense and impulsive hard X-ray bursts. The BLF location would be at (or in the vicinity of) the forthcoming bright patches. Their predicted contrast depends on the position of the flare on the solar disc and on the wavelength band of the observation. Title: The sun as a variable star: Solar and stellar irradiance variations Authors: Pap, Judit M.; Froehlich, Claus; Hudson, Hugh S.; Tobiska, W. Kent Bibcode: 1994SoPh..152.....P Altcode: 1994svs..coll.....P; 1994IAUCo.143.....P Variations in solar and stellar irradiances have long been of interest. An International Astronomical Union (IAU) colloquium reviewed such relevant subjects as observations, theoretical interpretations, and empirical and physical models, with a special emphasis on climatic impact of solar irradiance variability. Specific topics discussed included: (1) General Reviews on Observations of Solar and Stellar Irradiance Variability; (2) Observational Programs for Solar and Stellar Irradiance Variability; (3) Variability of Solar and Stellar Irradiance Related to the Network, Active Regions (Sunspots and Plages), and Large-Scale Magnetic Structures; (4) Empirical Models of Solar Total and Spectral Irradiance Variability; (5) Solar and Stellar Oscillations, Irradiance Variations and their Interpretations; and (6) The Response of the Earth's Atmosphere to Solar Irradiance Variations and Sun-Climate Connections. For individual titles, see A95-78168 through A95-78218. Title: Nonthermal Processes in Flaring X-Ray--bright Points Authors: Kundu, M. R.; Strong, K. T.; Pick, M.; White, S. M.; Hudson, H. S.; Harvey, K. L.; Kane, S. R. Bibcode: 1994ApJ...427L..59K Altcode: X-ray-bright point (XBPs) are known to show variability on a number of timescales, including impulsive X-ray brightenings. The relationship between these XBP 'flares' and normal solar flares is poorly known. A fundamental question is whether nonthermal acceleration of particles takes place in XBP flares. We address this issue by searching for nonthermal radio emission at metric wavelengths from flaring XBPs identified in Yohkoh soft x-ray telescope (SXT) data. Unequivocal evidence for type III-like radio bursts, usually attributed to beams of nonthermal electrons on open field lines, is found. This suggests that XBP flares are similar to normal flares and can indeed accelerate nonthermal populations of energetic particles. Title: Impulsive Behavior in Solar Soft X-Radiation Authors: Hudson, H. S.; Strong, K. T.; Dennis, B. R.; Zarro, D.; Inda, M.; Kosugi, T.; Sakao, T. Bibcode: 1994ApJ...422L..25H Altcode: The Yohkoh soft X-ray telescope has observed impulsive, thermal, soft X-ray emission at the footpoints of magnetic loops during solar flares. The soft X-ray (thermal) time profiles at the footpoints closely match the hard X-ray (nonthermal) time profiles, directly demonstrating the heating of the lower solar atmosphere on short timescales during the interval of nonthermal energy release. This phenomenon is the rule, rather than the exception, occurring in the majority of flares that we have examined with the Yohkoh data. We illustrate the impulsive behavior with data from the major flare of 1992 January 26. For this flare, the soft X-ray peak times matched the hard X-ray peak times within the time resolution of the soft X-ray measurements (about 10 s), and the soft and hard X-ray locations match within the resolution of the hard X-ray imager. The impulsive soft X-ray emission clearly has a thermal spectral signature, but not at the high temperature of a 'superhot' source. We conclude that the impulsive soft X-ray emission comes from material heated by precipitating electrons at loop footpoints and evaporating from the deeper atmosphere into the flaring flux tube. Title: YOHKOH Observations of Weak Events Within AR7218 Authors: Linford, G. A.; Hudson, H.; Sterling, A. Bibcode: 1994xspy.conf...49L Altcode: No abstract at ADS Title: X-ray solar physics from YOHKOH Authors: Uchida, Yutaka; Watanabe, Tetsuya; Shibata, Kazunari; Hudson, Hugh S. Bibcode: 1994xspy.conf.....U Altcode: No abstract at ADS Title: Rapid Sunspot Motion Associated with Large Solar Flares Authors: Reardon, K. W.; Canfield, R. C.; McClymont, A. N.; Hudson, H. S. Bibcode: 1994ASPC...68..336R Altcode: 1994sare.conf..336R No abstract at ADS Title: Eclipses of the solar X-ray corona by Mercury and the Moon. Authors: Hudson, H.; Freeland, S.; Lemen, J.; Kosugi, T.; Soma, M.; Watanabe, T.; Hara, H.; Shimizu, T. Bibcode: 1994BAAS...26..795H Altcode: No abstract at ADS Title: Spatial Resolution of Solar Total Irradiance Variability: The YOHKOH White-Light Observations Authors: Hudson, H. S. Bibcode: 1994svsp.coll..196H Altcode: 1994IAUCo.143P.196H No abstract at ADS Title: Two Types of Interaction Between Emerging Flux and Coronal Magnetic Field Authors: Shibata, K.; Nitta, N.; Matsumoto, R.; Tajima, T.; Yokoyama, T.; Hirayama, T.; Hudson, H. Bibcode: 1994xspy.conf...29S Altcode: No abstract at ADS Title: Coronal Eruptions Observed by YOHKOH Authors: Klimchuk, J. A.; Acton, L. W.; Harvey, K. L.; Hudson, H. S.; Kluge, K. L.; Sime, D. G.; Strong, K. T.; Watanabe, Ta. Bibcode: 1994xspy.conf..181K Altcode: No abstract at ADS Title: Interplanetary Consequences of Transient Coronal Events Authors: Watanabe, Ta.; Kojima, M.; Kozuka, Y.; Tsuneta, S.; Lemen, J. R.; Hudson, H.; Joselyn, J. A.; Klimchuk, J. A. Bibcode: 1994xspy.conf..207W Altcode: No abstract at ADS Title: Poster Proceedings from IAU Colloquium 143: The Sun as a Variable Star: Solar and Stellar Irradiance Variations Authors: Pap, J. M.; Frohlich, C.; Hudson, H. S.; Solanki, S. K. Bibcode: 1994svsp.coll.....P Altcode: 1994IAUCo.143P....P No abstract at ADS Title: Evidence for Impulsive Soft-X Bursts during Flares Authors: Strong, K.; Hudson, H.; Dennis, B. Bibcode: 1994xspy.conf...65S Altcode: No abstract at ADS Title: Diagnostics of Twisted Flux Emergence (noaa AR7260) Authors: Leka, K. D.; van Driel-Gesztelyi, L.; Anwar, B.; Canfield, R. C.; Hudson, H. S.; Metcalf, T. R.; Mickey, D. L.; Nitta, N.; Kurokawa, H. Bibcode: 1994xspy.conf...25L Altcode: No abstract at ADS Title: Non-Thermal Effects in Slow Solar Flares Authors: Hudson, H. S.; Acton, L. W.; Sterling, A. C.; Tsuneta, S.; Fishman, J.; Meegan, C.; Paciesas, W.; Wilson, R. Bibcode: 1994xspy.conf..143H Altcode: No abstract at ADS Title: A Loop-Loop Interaction Observed with YOHKOH SXT Authors: Akioka, M.; Acton, L. W.; Hudson, H. S. Bibcode: 1994xspy.conf..241A Altcode: No abstract at ADS Title: Electric Currents and Coronal Structures in NOAA Active Region 6952 Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey, D. L.; Wülser, J. -P.; Tsuneta, S. Bibcode: 1994xspy.conf...51M Altcode: No abstract at ADS Title: Morphological Evolution of the Post-Flare Loops of June 25-26, 1992 Authors: Anwar, B.; Hiei, E.; Hudson, H. S.; Acton, L. W.; Metacalf, T.; Lemen, J.; Martens, P. Bibcode: 1994xspy.conf..121A Altcode: No abstract at ADS Title: Rapid Sunspot Motion during a Major Solar Flare Authors: Anwar, B.; Acton, L. W.; Hudson, H. S.; Makita, M.; McClymont, A. N.; Tsuneta, S. Bibcode: 1993SoPh..147..287A Altcode: A major solar flare on 15 November, 1991 produced a striking perturbation in the position and shape of the sunspot related most closely to the flare. We have studied these perturbations by use of the aspect-sensor images from the Soft X-ray Telescope on board YOHKOH, and with ground-based data from the Mees Solar Observatory. The perturbation occurred during the impulsive phase of the flare, with a total displacement on the order of 1 arc sec. The apparent velocity of approximately 2 km s−1 exceeds that typically reported for sunspot proper motions even in flare events. We estimate that the magnetic energy involved in displacing the sunspot amounted to less than 4 × 1030 ergs, comparable to the radiant energy from the perturbed region. Examination of the Mees Observatory data shows that the spot continued moving at lower speed for a half-hour after the impulsive phase. The spot perturbation appears to have been a result of the coronal restructuring and flare energy release, rather than its cause. Title: The 1992 January 5 Flare at 13.3 UT: Observations from YOHKOH Authors: Doschek, G. A.; Strong, K. T.; Bentley, R. D.; Brown, C. M.; Culhane, J. L.; Fludra, A.; Hiei, E.; Lang, J.; Mariska, J. T.; Phillips, K. J. H.; Pike, C. D.; Sterling, A. C.; Watanabe, T.; Acton, L. W.; Bruner, M. E.; Hirayama, T.; Tsuneta, S.; Rolli, E.; Kosugi, T.; Yoshimori, M.; Hudson, H. S.; Metcalf, T. R.; Wuelser, J. -P.; Uchida, Y.; Ogawara, Y. Bibcode: 1993ApJ...416..845D Altcode: We discuss X-ray spectra and soft X-ray images of an M1.9 flare that occurred on 1992 January 5 near 13.3 UT. These data were obtained with instrumentation on the Japanese Yohkoh spacecraft. They cover the entire rise phase of the flare. To supplement these data we have ground-based magnetograms and Hα spectroheliograms. We calculate the electron temperature and emission measure of the flare as a function of time during the early rise phase using X-ray spectral line intensities and line ratios. Using spectral line widths, line profile asymmetries, and wavelength shifts due to the Doppler effect, we calculate the dynamical properties of the flare. The time development of the morphology of the flare, as revealed by the soft X-ray images and the Hα spectroheliograms, and the physical quantities inferred from the X-ray spectra, are compared with chromospheric evaporation models. There is an enhancement of blueshifted emission that is closely correlated with the hard X-ray bursts. Heating of one loop in the flare is consistent with a conduction-evaporation model, but heating is found in several structures that do not appear to be physically associated with each other. No standard evaporation model can adequately explain all of the observations. Title: The Confined Two-Ribbon Flare of 1991 October 24 Authors: de La Beaujardiere, J. F.; Canfield, R. C.; Hudson, H. S.; Wuelser, J. -P.; Kosugi, T.; Masuda, S.; Acton, L. W. Bibcode: 1993BAAS...25.1178D Altcode: No abstract at ADS Title: Imaging Solar Bolometric and Spectral Intensity Using Thermal Detector Arrays Authors: Deming, D.; Glenar, D.; Kostiuk, T.; Bly, V.; Forrest, K.; Nadler, D.; Hudson, H.; Lindsey, C.; Kopp, G.; Avrett, E.; Terrill, C. W. Bibcode: 1993BAAS...25R1221D Altcode: No abstract at ADS Title: Electric Currents and Coronal Structures in NOAA Active Region 6952 Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey, D. L.; Martens, P. C. H.; Tsuneta, S. Bibcode: 1993BAAS...25.1179M Altcode: No abstract at ADS Title: Survey of Loop-Footpoint Brightenings During the Impulsive Phase of Flares Authors: Strong, K.; Dennis, B.; Hudson, H.; Kosugi, T.; Sakao, T. Bibcode: 1993BAAS...25Q1187S Altcode: No abstract at ADS Title: Yohkoh-SXT Observations of AR Brightenings Authors: Linford, G. A.; Lemen, J. R.; Hudson, H. S. Bibcode: 1993BAAS...25Q1187L Altcode: No abstract at ADS Title: Millimeter, Microwave and X-Ray Morphology and Spectra of the 07Jan92 Flare Authors: Silva, A. V.; Lin, R. P.; de Pater, I.; White, S. M.; Kundu, M. R.; Gary, D. E.; Hudson, H. S. Bibcode: 1993BAAS...25Q1223S Altcode: No abstract at ADS Title: Temperatures in Flares Determined from Fe XXV Spectra, Resonance Line Ratios, and GOES X-ray Flux Authors: Sterling, A. C.; Doschek, G. A.; Pike, C. D.; Hudson, H. S.; Lemen, J. R.; Zarro, D. M. Bibcode: 1993BAAS...25.1178S Altcode: No abstract at ADS Title: Impulsive Soft X-Ray Emission in Solar Flares Authors: Hudson, H. S.; Strong, K. T.; Dennis, B. R.; Zarro, D.; Inda, M.; Kosugi, T.; Sakao, T. Bibcode: 1993BAAS...25.1177H Altcode: No abstract at ADS Title: Evidence for Both Electron Acceleration and Direct Heating in Solar Flares Authors: Dennis, B. R.; Holman, G. D.; Hudson, H. S.; Kosugi, T.; Strong, K. T.; Zarro, D. Bibcode: 1993BAAS...25Q1177D Altcode: No abstract at ADS Title: The Extended and Diffuse X-Ray Corona Observed by Yohkoh-SXT Authors: Lemen, J. R.; Slater, G. L.; Hudson, H. S.; Acton, L. W. Bibcode: 1993BAAS...25.1179L Altcode: No abstract at ADS Title: Yohkoh-SXT Observations from the Spartan and Nixt Max91 Campaign Authors: Morrison, M.; Bruner, M.; Freeland, S.; Lemen, J.; Linford, G.; Nitta, N.; Slater, G.; Strong, K.; Hara, H.; Kano, R.; Shimizu, T.; Tsuneta, S.; Hudson, H.; Ogawara, Y.; Kosugi, T.; Sakao, T.; Watanabe, T.; Takeda, A.; Acton, L. Bibcode: 1993BAAS...25.1213M Altcode: No abstract at ADS Title: Large-scale structure of the solar plasma corona. An analysis of Yohkoh SXT images. Authors: Saito, T.; Minami, Shigeyuki; Kozuka, Y.; Takahashi, T.; Hudson, H.; Tsuneta, Saku; Watanabe, T. Bibcode: 1993ppcn.conf..215S Altcode: The YOHKOH SXT images are analyzed to find the large-scale structure of the solar plasma corona. The clarified characteristics are preferred position of active regions and its possible mechanism, rules governing the coronal hole channel, a relation between the inner and outer corona, formation of the 4-sector structure, and the outer corona in pseudo-aligned phase. These characteristics are in agreement with both the rotational reversing model and the triple-dipole model on the structure and dynamics of the heliomagnetosphere. Title: Asteroseismology: the impact of solar space observations. Authors: Hudson, H. S. Bibcode: 1993ASPC...40...43H Altcode: 1993IAUCo.137...43H; 1993ist..proc...43H Observations from space relevant to solar global properties (oscillations, magnetic activity, etc.) are helpful both scientifically and technically in preparing for stellar observations. This paper summarizes the results from the main previous experiments (ACRIM, SOUP, and IPHIR), and also gives an initial technical report from the SXT instrument on board Yohkoh, launched in August 1991. The solar observations to date demonstrate the existence of several mechanisms for low-level variability: spots, faculae, the photospheric network, granulation, and p-mode oscillations. The observations of oscillations have been particularly helpful in setting limits on solar interior rotation. In addition to the solar processes, stars of other types may have different mechanisms of variability. These may include the analogues of coronal holes or solar flares, modes of oscillation not detected in the Sun, collisions with small bodies, duplicity, and probably mechanisms not invented yet but related in interesting ways to stellar convection and magnetism. Title: Soft X-ray Imaging of Impulsive Evaporation Authors: Hudson, H.; Nitta, N.; Strong, K.; Takakura, T. Bibcode: 1992AAS...181.5501H Altcode: 1992BAAS...24.1211H The time development of a solar flare can often be broken into two phases, viz. the impulsive phase and the gradual phase. For many reasons the impulsive phase is known to result from powerful electron acceleration, to energies of tens of keV (and higher), with a total energy that is a large fraction of the energy subsequently radiated in various flare emissions. A grazing-incidence soft X-ray telescopes on board the Yohkoh spacecraft now enables us to study the few-keV properties of the impulsive phase for the first time. In a representative set of impulsive solar flares, we find an excellent match between the soft X-ray time profiles at the footpoints of coronal magnetic flux tubes and the hard X-ray impulsive emission. The hard X-ray images directly show the sites of the particle precipitation. The impulsive soft X-ray emission could arise directly as non-thermal bremsstrahlung, extending to the few-keV range; or it could contain contributions from the impulsively evaporating plasma seen during the process of the flare explosion from the chromosphere. We discuss these interpretations and the physics resulting from them. Title: Correlation between X-ray Temporal Variability and Magnetic Environment in Solar Flares Authors: Nitta, N.; Harvey, K.; Hudson, H.; Ichimoto, K.; Metcalf, T.; Mickey, D.; Sakai, J. -I.; Sakao, T.; Sakurai, T.; Takahashi, M. Bibcode: 1992AAS...181.5503N Altcode: 1992BAAS...24.1211N The X-ray time history of a solar flare can reflect basic processes of heating and/or acceleration, which in turn may depend on the magnetic environment of the site. Some flares show a simple rise and fall temporal behavior, whereas others show more than one peak. Comparisons of images taken by the Soft X-ray Telescope (SXT) aboard the Yohkoh spacecraft with ground-based magnetic data (Hawaii, Kitt Peak and Mitaka) reveal that, at least for a flare-productive active region (NOAA 7260), flares with double-peaked and single-peaked time profiles occurred at systematically different locations within the region. We discuss this result in terms of theoretical models, especially those of coalescence of two current loops. Title: White-Light Flares Observed by YOHKOH Authors: Hudson, Hugh S.; Acton, Loren W.; Hirayama, Tadashi; Uchida, Yutaka Bibcode: 1992PASJ...44L..77H Altcode: The Yohkoh observatory is producing a first sample of white-light flares observed from space. We present observations of four of them, all X-class events. The Yohkoh (SXT) white-light data typically have a 12-s cadence for images with 2.''46 pixels over a field of view of 2.'62 in one of two broad-band optical filters, and the 1991 November 15 flare produced a brightness increase of about 38% over the photospheric brightness in the 30 Angstroms \ passband filter centered at 4308 Angstroms. The white-light flare morphology in the best-observed flares displays a double ``footpoint'' character, establishing a close relationship with the compact magnetic flux tubes involved with both hard and soft X-ray emissions. The ``footpoint'' brightnesses may vary independently with time. We describe the data in the context of the soft and hard X-ray observations simultaneously carried out on board the Yohkoh satellite, emphasizing energetics and timing. Title: The Morphology of 20times 10(6) K Plasma in Large Non-Impulsive Solar Flares Authors: Acton, Loren W.; Feldman, Uri; Bruner, Marilyn E.; Doschek, George A.; Hirayama, Tadashi; Hudson, Hugh S.; Lemen, James R.; Ogawara, Yoshiaki; Strong, Keith T.; Tsuneta, Saku Bibcode: 1992PASJ...44L..71A Altcode: We have examined images of 10 flares observed by the Soft X-ray Telescope on-board the Yohkoh spacecraft. These images show that the hottest portion of the soft X-ray flare is located in compact regions that appear to be situated at the tops of loops. These compact regions form at, or shortly after, flare onset, and persist well into the decay phase of the flares. In some cases, the compact regions are only a few thousand kilometers in size and are small compared to the lengths of flaring loops. This is inconsistent with the smoother intensity distribution along the loops expected from models of chromospheric evaporation. Title: Continual Expansion of the Active-Region Corona Observed by the YOHKOH Soft X-Ray Telescope Authors: Uchida, Yutaka; McAllister, Alan; Strong, Keith T.; Ogawara, Yoshiaki; Shimizu, Toshifumi; Matsumoto, Ryoji; Hudson, Hugh S. Bibcode: 1992PASJ...44L.155U Altcode: We have found from the observations of the Yohkoh Soft X-ray Telescope (SXT) that the corona above active regions expands occasionally, and almost continually in the cases of ``active" active regions. This is contrary to the commonly accepted idea of magnetohydrostatic equilibrium of these regions. The key to this discovery has been a movie representation of the Yohkoh-SXT data, which, for the first time, provides adequate sampling and continuity for this purpose. The movies show ubiquitous expansions above the active regions, with velocities in the range of a few to a few tens km s(-1) as measured when they are on the limb. The expansion appears to preserve the overall structure of the active-region corona. We suggest that the expansion may have a physical relationship with the transient loop brightenings found within the active regions. This finding of almost continual expansion of the active-region corona may affect some of the basic ideas concerning active regions, as well as those of the mass-loss from the Sun and Sun-like stars. Title: The X Flare of 1991 November 15: Coordinated Mees/Yohkoh Observations Authors: Canfield, Richard C.; Hudson, Hugh S.; Leka, K. D.; Mickey, Donald L.; Metcalf, Thomas R.; Wuelser, Jean-Pierre; Acton, Loren W.; Strong, Keith T.; Kosugi, Takeo; Sakao, Taro; Tsuneta, Saku; Culhane, J. Leonard; Phillips, Andrew; Fludra, Andrzej Bibcode: 1992PASJ...44L.111C Altcode: This is a preliminary report on two unique new results from coordinated observations at Mees Solar Observatory and Yohkoh of the X1.5 flare of 1991 November 15, using vector magnetograms, Hα imaging spectra, X-ray images, and X-ray spectra. First, we find a close spatial relationship between Hα redshifts and X-rays from a flare loop and its footpoints at a time of large X-ray blueshifts. Second, we find that impulsive-phase hard X-rays originate in regions that are near, but not coincident with, the peaks of the vertical electrical current density distribution in AR 6919. Title: The Status of YOHKOH in Orbit: an Introduction to the Initial Scientific Results Authors: Ogawara, Yoshiaki; Acton, Loren W.; Bentley, Robert D.; Bruner, Marilyn E.; Culhane, J. Leonard; Hiei, Eijiro; Hirayama, Tadashi; Hudson, Hugh S.; Kosugi, Takeo; Lemen, James R.; Strong, Keith T.; Tsuneta, Saku; Uchida, Yutaka; Watanabe, Tetsuya; Yoshimori, Masato Bibcode: 1992PASJ...44L..41O Altcode: In this introductory article accompanying the initial scientific papers from the Yohkoh mission, we briefly summarize the design and in-orbit function of the spacecraft and its four scientific instruments. Although these initial results include mainly studies based upon individual Yohkoh experiments at this early stage, there are also analyses of combined data sets provided by several on-board and ground-based instruments in progress. The results presented here, and anticipated future results, suggest that the Yohkoh observations with their comprehensive coverage of solar high-energy phenomena will come to represent a significant milestone in the progress of solar physics. This will be true not only regarding flares, but also for fainter coronal structures and even coronal holes. Title: Hard X-Ray Imaging Observations by YOHKOH of the 1991 November 15 Solar Flare Authors: Sakao, Taro; Kosugi, Takeo; Masuda, Satoshi; Inda, Mika; Makishima, Kazuo; Canfield, Richard C.; Hudson, Hugh S.; Metcalf, Thomas R.; Wuelser, Jean-P.; Acton, Loren W.; Ogawara, Yoshiaki Bibcode: 1992PASJ...44L..83S Altcode: We report on hard X-ray imaging observations of the 1991 November 15 flare with the HXT instrument aboard {Yohkoh}. Distributions of the hard X-ray sources at various stages of the flare, together with an overlay of the white-light flare, are presented. Attention is concentrated on the behavior of hard X-ray sources during the impulsive phase. The hard X-ray source appeared initially as a single source near the magnetic neutral line, then evolved into a double-source shape with the separation increasing with time. We believe that this is evidence for a multiple loop system flaring successively with a rising energy-release site. At the minima between the individual spikes of the time profile, the hard X-rays at 20--30 keV were concentrated near the apex of the flaring loop, whereas the hard X-rays above 30 keV originated from the footpoints. These observations are compared with the existing models. Title: Observation of a Solar Flare at the Limb with the YOHKOH Soft X-Ray Telescope Authors: Tsuneta, Saku; Hara, Hirohisa; Shimizu, Toshifumi; Acton, Loren W.; Strong, Keith T.; Hudson, Hugh S.; Ogawara, Yoshiaki Bibcode: 1992PASJ...44L..63T Altcode: A long-enduring soft X-ray flare at the solar limb was well observed by the Soft X-ray Telescope aboard the Yohkoh spacecraft from its pre-flare stage through the post-flare phase. A ``helmet streamer" arch appears several hours prior to the flare, in association with a continuous expansion and restructuring of the active-region magnetic structure. This arch then starts to flare, and increases its height and footpoint separation at v = 10--30 km s(-1) . The arch has a complex temperature structure in the rising phase, whereas the outer arches have systematically higher temperatures in the decay phase. Magnetic reconnection in a neutral sheet at the loop top, created by pre-flare magnetic restructuring, would explain this type of flare. Title: Estimating Solar Chromospheric UV Fluxes from Sunspot and Solar Radio Data Authors: Donnelly, R. F.; Hudson, H.; Pap, J.; Willson, R. Bibcode: 1992sers.conf..275D Altcode: No abstract at ADS Title: Photometric Measurements of Sunspots Deficits and Facular Excesses Authors: Chapman, G. A.; Lawrence, J. K.; Hudson, H. S. Bibcode: 1992sers.conf..135C Altcode: No abstract at ADS Title: Precise ground-based solar photometry and variations of total irradiance Authors: Chapman, G. A.; Herzog, A. D.; Lawrence, J. K.; Walton, S. R.; Hudson, H. S.; Fisher, B. M. Bibcode: 1992JGR....97.8211C Altcode: Variations in the total solar irradiance measured by the active cavity radiometer irradiance monitor (ACRIM) on SMM have been correlated with measures of magnetic activity on the solar disk. Quantitative indices of magnetic activity were derived from ground-based, full-disk, photometric images of the Sun at red (6723 Å) and violet (3934-Å K line) wavelengths. The red images have been obtained on a daily basis at the San Fernando Observatory since 1985, and the K line images since 1988. Sunspot irradiance deficits are calculated directly from the red images while proxy measures of facular irradiance excesses are derived from the K line images. The images analyzed here were made during 21 days between June 20 and July 14, 1988, a period centered on the disk passage of a large sunspot group. The best two-parameter multiple correlation coefficient between the ACRIM data and the photometric data is R2=0.97 (21 data points, 18 degrees of freedom). The zero point S0=1367.27 W m-2 agrees well with the solar irradiance measured by ACRIM/SMM during the 1986 activity minimum: the residual standard deviation was 0.13 W m-2 (about 100 ppm). The multiple correlations were extended to include measures of the irradiance contribution of ``network'' magnetic fields, unassociated with active regions. NOAA 9 spacecraft observations of UV MgII lines at 2800 Å gave R2=0.99 (17 degrees of freedom) with S0=1366.68+0.08 W m-2. The index of 10.7-cm microwave flux gave R2=0.98, with S0=1366.43+0.11 W m-2. We can thus model short-term irradiance changes to within 100 ppm relative precision from ground-based data. Title: White--Light Flares Observed by YOHKOH Authors: Hudson, H. S.; Wulser, J. -P.; Acton, L.; Uchida, Y. Bibcode: 1992AAS...180.2309H Altcode: 1992BAAS...24..761H The YOHKOH observatory is producing a first sample of white-light flares observed from space. We present observations of three of these flares, 1991 Oct. 27, 1991 Nov. 15, and 1991 Dec. 3. Of these, the 1991 Nov. 15 was also well-observed with H-alpha spectroscopic imaging observations at Mees Observatory, Haleakala. The YOHKOH (SXT) white-light data typically have a 12-second cadence for images with 2.46 arc-sec pixels over a field of view of 2.62 arc min in one of two broad-band optical filters, and the Nov. 15 flare produced a brightness excess of about 25\ We describe the YOHKOH white-light observations in terms of morphology and flare energetics in the context of the X-ray observations. For the Nov. 15 flare, we find that the H-alpha emission wing spectroheliograms match closely with the continuum images. Such observations permit tests of models (``electron precipitation'') in which the non--thermal electrons responsible for the hard X-ray bremsstrahlung also excite the upper photosphere by direct heating and ionization. We find by comparison with the hard X-ray data that this mechanism is viable, but confirm the need for at least one additional mechanism for the excitation of the continuum. Title: Electric Currents and Coronal Structures in Two Flare- Productive Active Regions, AR 6850 and AR 6952 Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey, D. L.; Strong, K. T.; Tsuneta, S. Bibcode: 1992AAS...180.3004M Altcode: 1992BAAS...24R.775M In this study, we examine the spatial and temporal relationship between coronal structures observed with the Soft X-Ray Telescope (SXT) on board the YOHKOH spacecraft and vertical electric currents derived from vector magnetograms obtained at the Mees Solar Observatory, Haleakala, Hawaii. We have focused on two active regions, AR 6850 (October 1991) and AR 6952 (December 1991). In both active regions, we observed significant current structures which persisted over time scales of days. The SXR emitting coronal structures, however, changed on much shorter time scales, indicating that there is no compelling, direct spatial and temporal relationship between the non-flaring SXR structures and the long-lived electric current systems. We have seen at least one case (in AR 6952) where a SXR brightening was associated spatially with a change in the vertical electric current. In this case, the the vertical current dissipated between December 8, 00:35 UT and the next observation at 00:48 UT on December 9, leaving a bright SXR structure which was observed at 24:27 UT on December 8. Hence, although more data must be analyzed to make a compelling case, it is possible that the SXR emission is related more closely to changes in the electric current systems rather than simply to the presence of these currents. Title: SPAM: A Canned Internet-Accessible Database of Interest to Solar Flare Researchers Authors: Canfield, R. C.; Hudson, H. S.; Kiernan, E.; Metcalf, T. R.; Wulser, J. -P. Bibcode: 1992AAS...180.5103C Altcode: 1992BAAS...24..813C We have established a searchable database, called SPAM (Spectroscopy and Polarimetry at Mees), which contains logs of observations made at Mees Solar Observatory (Haleakala, Maui). Of more general interest, the database also includes the Events List and Region Report from the Space Environment Laboratory (Boulder). Logs from YOHKOH are currently being added. Hence, SPAM can be used to determine, for example, whether Mees has vector magnetograms of a certain NOAA AR or whether YOHKOH has certain types of observations in specified time ranges. As well, it can be used to search the SEL database for flares with selected attributes. Included logs (and searchable attributes, in addition to date, day of year, and time) are: Mees Solar Observatory Log (instrument, NOAA AR, data type, observing setup), SEL Event List (NOAA AR, X-ray Class), SEL Region Report (NOAA AR), YOHKOH Orbit Summary, YOHKOH SXT Quiet Mode PFI Observations (latitude, longitude, X-ray and optical image size), YOHKOH Flare Observations (latitude, longitude, specific channel counts or ratios). SPAM runs on a Sun workstation at Mees Solar Observatory, and is available over Internet. Simply access (e.g., telnet) koa.ifa.hawaii.edu (128.171.167.1) from any vt100, Sun, or xterm emulator. Log on as spam (lower case); there is no password. New users are asked to read release notes and hints. Title: YOHKOH and Compton Observations of an LDE Event: Reconnection and the Neupert Effect? Authors: Hudson, H. S.; Nitta, N.; Paciesas, W.; Fishman, G.; Meegan, C.; Wilson, R. Bibcode: 1992AAS...180.3409H Altcode: 1992BAAS...24..784H We present X-ray images and time profiles of a gradual flare (GOES magnitude M3.2) that occurred on the solar limb at 0430 UT (time of GOES maximum), Feb. 21, 1992. The soft X-ray structure of this event showed a remarkably clear loop development with complicated geometry, including a prominent nearly-vertical bar extending from near the top of the loop(s). During the presence of this bar, the large-area BATSE hard X-ray detector on the Compton Observatory showed extended but highly irregular hard X-ray emission, roughly in the proportion predicted by the ``Neupert Effect'' that associates the integral of the hard X-ray light curve with the instantaneous soft X-ray flux. The peak hard X-ray flux (about 1 ph/(cm(2) sec) above 25 keV) was at 0321 UT, and the emission extended throughout the rise phase of the soft X-ray event. Coronal magnetic reconnection provides a natural interpretation for this morphology, and the time profiles strongly suggest that we can identify the hard X-ray emission with bremsstrahlung from electrons accelerated in the coronal volume of the reconnection region (vertical bar structure). Title: Electric Currents and Hard X-ray Images in the X Class Flare of November 15, 1991 Authors: Metcalf, T. R.; Sakao, T.; Acton, L. W.; Canfield, R. C.; Hudson, H. S.; Inda, M.; Kosugi, T.; Wulser, J. P. Bibcode: 1992AAS...180.3005M Altcode: 1992BAAS...24..776M We present co-aligned observations of hard x-rays observed with the Hard X-ray Telescope (HXT) on board the YOHKOH spacecraft and vertical electric currents derived from a vector magnetogram obtained at the Mees Solar Observatory, Haleakala, Hawaii. Previous work comparing the wings of the Hα line to vertical electric currents has suggested that electron precipitation in flares occurs at the edges of these currents. The Stark wings of Hα were interpreted as a signature of non-thermal electrons penetrating the relatively dense chromosphere and used as a proxy for direct observation of the non-thermal electrons. The hard X-rays used in this study provide a direct determination of the locations of the electron energy losses. In the X class flare of November 15, 1991, we find the same relation between hard X-ray emission and vertical electric currents as was found between Hα Stark wing emission and vertical currents: the hard x-ray emission occurs predominantly at the edges of the vertical current sites, and not spatially on top of these currents. Canfield, R. C., de La Beaujardiere, J., and Leka, K. D., in ``The Physics of Solar Flares", ed. Culhane and Jordan, The Royal Society, London, 1991 Canfield, R. C., Leka, K. D., and Wulser,J. P., in ``Flare Physics in Solar Activity Maximum 22", ed. Uchida, Canfield, Watanabe, and Hiei, Springer-Verlag, Berlin, 1991 Title: Search for evidence of low energy protons in solar flares. Authors: Metcalf, Thomas R.; Wuelser, Jean-Pierre; Canfield, Richard C.; Hudson, Hugh S. Bibcode: 1992NASCP3137..536M Altcode: 1992como.work..536M We searched for linear polarization in the H alpha line using the Stokes Polarimeter at Mees Solar Observatory and present observations of a flare from NOAA active region 6659 which began at 01:30 UT on 14 Jun. 1991. Our dataset also includes H alpha spectra from the Mees charge coupled device (MCCD) imaging spectrograph as well as hard x ray observations from the Burst and Transient Source Experiment (BATSE) instrument on board the Gamma Ray Observatory (GRO). The polarimeter scanned a 40 x 40 inch field of view using 16 raster points in a 4 x 4 grid. Each scan took about 30 seconds with 2 seconds at each raster point. The polarimeter stopped 8.5 inches between raster points and each point covered a 6 inch region. This sparse sampling increased the total field of view without reducing the temporal cadence. At each raster point, an H alpha spectrum with 20 mA spectral sampling is obtained covering 2.6 A centered on H alpha line center. The preliminary conclusions from the research are presented. Title: Gamma-ray and microwave emission from 1991 June events. Authors: Enome, Shinzo; Nakajima, Hiroshi; Hudson, Hugh S.; Schwartz, Richard Bibcode: 1992NASCP3137..522E Altcode: 1992como.work..522E The Sun showed unprecedented microwave activities in Jun. 1991, which produced four major and numerous weaker bursts and gamma ray emission measured by the Gamma Ray Observatory. The 4 Jun. 1991 event shows a sharp maximum around 03:41 UT and weak emission a few minutes before the maximum in the gamma ray record of the Burst and Transient Source Experiment (BATSE), with a preliminary estimated energy of 5 MeV. Although the 80-GHz and possible 35-GHz records show more prominent emission in the pre-maximum stage. This strongly suggests the first observational evidence for gamma ray and mm-wave emission from relativistic electrons. Comparisons of the other three major events on 6 Jun. at 01:00 UT, 9 Jun. at 01:34 UT, and 11 Jun. at 01:51 UT between gamma ray and microwave emission are also in progress. Title: Variability of solar ultraviolet irradiance. Authors: Pap, J. M.; Donnelly, R. F.; Hudson, H. S.; Rottman, G. J.; Willson, R. C. Bibcode: 1991JATP...53..999P Altcode: A model of solar Lyman alpha irradiance developed by multiple linear regression analysis, including the daily values and 81-day running means of the full disk equivalent width of the Helium line at 1083 nm, predicts reasonably well both the short- and long-term variations observed in Lyman alpha. In contrast, Lyman alpha models calculated from the 10.7 cm radio flux overestimate the observed variations in the rising portion and maximum period of solar cycle, and underestimates them during solar minimum. The authors show models of Lyman alpha based on the He line equivalent width and 10.7 cm radio flux for those time intervals when no satellite observations exist, namely back to 1974 and after April 1989, when the measurements of the Solar Mesosphere Satellite were terminated. Title: Using Image Area to Control CCD Systematic Errors in Spacebourne Photometric and Astrometric Time-Series Measurements Authors: Buffington, Andrew; Booth, Corwin H.; Hudson, Hugh S. Bibcode: 1991PASP..103..685B Altcode: The effect of some systematic errors for high-precision time-series spaceborne photometry and astrometry has been investigated with a CCD as the detector. The 'pixelization' of the images causes systematic error in astrometric measurements. It is shown that this pixelization noise scales as image radius r exp -3/2. Subpixel response gradients, not correctable by the 'flat field', and in conjunction with telescope pointing jitter, introduce further photometric and astrometric errors. Subpixel gradients are modeled using observed properties of real flat fields. These errors can be controlled by having an image span enough pixels. Large images are also favored by CCD dynamic range considerations. However, magnified stellar images can overlap, thus introducing another source of systematic error. An optimum image size is therefore a compromise between these competing factors. Title: Book reviews Authors: Hudson, H. S.; Kleczek, J.; Švestka, Z. Bibcode: 1991SoPh..133..403H Altcode: No abstract at ADS Title: Solar flares, microflares, nanoflares, and coronal heating Authors: Hudson, H. S. Bibcode: 1991SoPh..133..357H Altcode: Solar flare occurrence follows a power-law distribution against total flare energy W: dN/dW ∼ W−α with an index α ∼ 1.8 as determined by several studies. This implies (a) that microflares must have a different, steeper distribution if they are energetically significant, and (b) there must be a high-energy cutoff of the observed distribution. We identify the distinct `soft' distribution needed for coronal heating, if such a distribution exists, with Parker's nanoflares. Title: The Sun's luminosity over a complete solar cycle Authors: Willson, Richard C.; Hudson, Hugh S. Bibcode: 1991Natur.351...42W Altcode: THE Active Cavity Radiometer Irradiance Monitor (ACRIM I), an instrument carried on NASA's Solar Maximum Mission satellite, measured the Sun's luminosity (total power outflow) from early 1980 to late 19891-5. Here we present the first account of the complete ACRIM I data set, and give evidence confirming our previous suggestion that solar luminosity varies with the 11-year solar cycle6. As previously reported, this slow variation closely follows statistical measures of the distribution of magnetic and photospheric features on the Sun's surface4-8. But there was an exception to this correlation in the form of a remarkable irradiance excess during 1980, at about the time of the sunspot maximum of solar cycle 21. The linkage, over a whole cycle, of luminosity variation to photospheric activity suggests the existence of an unknown physical mechanism other than the thermal diffusion model that explains luminosity deficits due to sunspots. Luminosity models connecting total irradiance to global indicators of solar activity, such as the equivalent width of the 1,083-nm helium line, are consistent with the gross features of the variability, but fail to account for the 1980 irradiance excess. Title: Report of the solar physics panel Authors: Withbroe, George L.; Fisher, Richard R.; Antiochos, Spiro; Brueckner, Guenter; Hoeksema, J. Todd; Hudson, Hugh; Moore, Ronald; Radick, Richard R.; Rottman, Gary; Scherrer, Philip Bibcode: 1991spsi....1...67W Altcode: Recent accomplishments in solar physics can be grouped by the three regions of the Sun: the solar interior, the surface, and the exterior. The future scientific problems and areas of interest involve: generation of magnetic activity cycle, energy storage and release, solar activity, solar wind and solar interaction. Finally, the report discusses a number of future space mission concepts including: High Energy Solar Physics Mission, Global Solar Mission, Space Exploration Initiative, Solar Probe Mission, Solar Variability Explorer, Janus, as well as solar physics on Space Station Freedom. Title: Differential Emission-Measure Variations and the "Neupert Effect" Authors: Hudson, H. S. Bibcode: 1991BAAS...23R1064H Altcode: No abstract at ADS Title: Total Solar Irradiance Variations Compared with Ground-Based Photometry at the SFO Authors: Chapman, G. A.; Lawrence, J. K.; Hudson, H. S. Bibcode: 1991BAAS...23.1067C Altcode: No abstract at ADS Title: Solar Gamma-Ray Spectroscopy with BATSE Authors: Hudson, H. S. Bibcode: 1991BAAS...23.1074H Altcode: No abstract at ADS Title: Total Irradiance and the Solar Cycle Authors: Hudson, H. S.; Willson, R. C. Bibcode: 1991BAAS...23.1067H Altcode: No abstract at ADS Title: Time-Series Images of Jupiter at 10 Microns Authors: Fisher, B.; Hudson, H.; Jones, B.; Piña, R.; Puetter, R. Bibcode: 1991BAAS...23..961F Altcode: No abstract at ADS Title: Ground-Based Modelling of Solar Irradiance Variations Authors: Lawrence, J. K.; Chapman, G. A.; Herzog, A. D.; Walton, S. R.; Hudson, H. S.; Fisher, B. M. Bibcode: 1991BAAS...23..960L Altcode: No abstract at ADS Title: A Space Parasol as a Countermeasure Against the Greenhouse Effect Authors: Hudson, H. S. Bibcode: 1991JBIS...44..139H Altcode: The greenhouse effect threatens to increase the temperature of the Earth substantially leading to gross changes in the quality of human life. This paper suggests a solution to this problem via the deployment of a "Space Parasol" at the Lagrangian point L1 of the Earth-Sun system, to intercept some desired fraction of the solar radiant energy. Title: Helioseismology with the ACRIM instrument on the solar maximum mission Authors: Hudson, Hugh S. Bibcode: 1991AdSpR..11d..61H Altcode: 1991AdSpR..11R..61H The Active Cavity Radiometer Irradiance Monitor (ACRIM) instrument on board SMM pioneered high-precision solar photometry from space, and provided the first detection of solar p-mode oscillations at low degree by this technique. The observations extended from February, 1980, until December, 1989, with a hiatus of low sampling rate in 1981-1984. During summer 1989, the instrument operated in a ``no-shutter'' mode with continuous viewing between the orbital gaps. This resulted in a fourfold increase of the duty cycle, and an effective increase in the Nyquist frequency from 3.815 mHz to some tens of mHz. This review discusses the initial results from this campaign along with a general review of the analyses to date of the entire ACRIM data set. Title: Capabilities and Limitations of SOLAR-A Authors: Acton, L. W.; Hudson, H. S. Bibcode: 1991LNP...387...28A Altcode: 1991fpsa.conf...28A The purpose of this paper is to present an overview useful to scientists, not familiar with Solar-A, who are interested in the capabilities of the mission for solar research. In order to keep the paper to a manageable size it will be assumed that the reader is generally familiar both with the experimental techniques of Solar-A and current research in solar high-energy physics. We do not provide detailed technical descriptions of the Solar-A instruments. We do, however, briefly discuss the capabilities of Solar-A in the context of solar activity research in the 1990's. Title: Solar Irradiance Variability from Modern Measurements Authors: Froehlich, C.; Foukal, P. V.; Hickey, J. R.; Hudson, H. S.; Willson, R. C. Bibcode: 1991suti.conf...11F Altcode: Direct measurements from satellites of the solar 'constant' (the total irradiance at mean sun-earth distance) during more than ten years show variations over time scales from minutes to years and decades. At high frequencies, solar oscillations contribute to the variance. The most important influences are related to solar activity: during the passage of active regions on the solar disk (sunspots and faculae) changes of a few 0.1 percent lasting for several days are observed. The effects of spots can be well reproduced by the projected sunspot index, whereas the influence of faculae have to be modeled from proxy data like the Ca-K plage index or the He I index. Long-term trends are detected which are connected to the 11-yr solar activity cycle. Title: Hard X-ray and gamma-ray imaging spectroscopy for the next solar maximum. Authors: Hudson, H. S.; Crannell, C. J.; Dennis, B. R.; Spicer, D. S.; Davis, J. M.; Hurford, G. J.; Lin, R. P. Bibcode: 1990SPIE.1344..492H Altcode: 1990exrg.conf..492H A single high-energy instrument based on rotating modulation collimators with germanium semiconductor spectrometers as the detectors can provide high angular resolution (<1″, high time resolution (<1 s), and high spectral resolution (about 1 keV), all in one package. Such rotating modulation-collimator optics provide excellent spatial coverage for high-contrast images in the hard X-ray domain, where there will be a large signal-to-noise ratio during even modest flares. The use of thick modulation plates will make it possible to image gamma rays with <5″angular resolution to energies in excess of 10 MeV during the more energetic flares without compromising the ability of the germanium detectors to resolve the gamma-ray lines. Energetic neutrons will also be imaged for the first time with <20″angular resolution. This combination of imaging and spectroscopy at high resolution will be a powerful tool for helping to answer central questions of solar flare physics. Title: Conference on EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy Authors: Siegmund, Oswald H. W.; Hudson, Hugh S. Bibcode: 1990SPIE.1344.....S Altcode: Various papers on EUV, X-ray, and gamma-ray instrumentation for astronomy are presented. Individual topics addressed include: performance of lithium scatterers for X-ray polarimetry, shared X-ray concentration via crystal diffraction, optimum shields for spaceborne gamma-ray spectrometers, position-sensitive high-resolution spectrometer, IPCs for stellar X-ray polarimeter, soft X-ray windows for position-sensitive proportional counters, EUV imaging telescope array on the spectrum X-G satellite, European Photon Imaging Camera for X-ray astronomy, development of a UV auroral imager, background reduction in microchannel plates, 2D delay-line anode detector for astronomical imaging, dynamic range considerations for EUV MAMA detectors, Rosat WFD imaging detectors. Also discussed are: EUV band-pass filters for the Rosat wide-field camera, calibration of the Rosat High-Resolution Imager, superconducting tunneling junction detectors, test results of a prototype dielectric microcalorimeter, novel high-speed high-resolution position readout SPAN, after emission in microchannel plate detectors, highly curved microchannel plates, soft X-ray performance of back-illuminated EEV CCDs, proton damage effects in EEV CCDs, PN-CCDs for the XMM satellite mission, intensified CCD detectors using the phosphor TPB, silicon X-ray array detector concept, multilayer telescope for soft X-ray surveys, hard X-ray and soft gamma-ray astronomy mission EXOS. Title: The Advanced Solar Observatory Authors: Walker, Arthur B. C., Jr.; Bailey, Wayne; Chupp, Edward L.; Hudson, Hugh S.; Moore, Ronald; Roberts, William; Hoover, Richard B. Bibcode: 1990OptEn..29.1306W Altcode: A conceptual plan for the development of a comprehensive long duration solar space observatory, The Advanced Solar Observatory (ASO) is described. The ASO is intended to provide solar astronomers with the observational power necessary to address fundamental problems relating to the solar convection zone and activity cycle; the thermal and nonthermal processes that control the transport of energy, mass, and magnetic flux in the solar atmosphere; the generation of the solar wind; and the dynamics of the inner heliosphere. The ASO concept encompasses three proposed Space Station-based instrument ensembles: (1) the High Resolution Telescope Cluster, which includes far ultraviolet, extreme ultraviolet, and X-ray telescopes; (2) the Pinhole/Occulter Facility, which includes Fourier transform and coded aperture hard X-ray and gamma ray telescopes and occulted ultraviolet and visible light coronagraphs; and (3) the High Energy Facility, which contains neutron, gamma ray, and low frequency radio spectrometers. Two other facilities, the Orbiting Solar Laboratory, and a package of Global Dynamics Instrumentation, will, with the Space Station ensembles, form a comprehensive capability for solar physics. The scientific program of the ASO, current instrument concepts for the Space Station based ASO instrument ensembles, and plans for their accommodation on the Space Station are described. Title: Using Image Size to Control CCD Systematic Errors in Spaceborne Photometric and Astrometric Time-Series Measurements Authors: Booth, C. H.; Hudson, H. S.; Buffington, A. Bibcode: 1990BAAS...22.1259B Altcode: No abstract at ADS Title: The ACRIM data in the context of stellar variability. Authors: Hudson, Hugh S. Bibcode: 1990NASCP3086..280H Altcode: 1990cisv.nasa..280H The ACRIM total-irradiance data from the Solar Maximum Mission have given us a first comprehensive view of solar variability in the stellar sense. Five types of solar variability have been identified thus far. These have small amplitudes, less than a few tenths of one percent, and are at levels generally not yet detectable on other stars. The possible stellar analogs are interesting physically, and in particular may help us to understand solar behavior on longer time scales. This paper describes the ACRIM data from the stellar point of view and comments on the present state of stellar time-series photometry. Title: Modeling solar Lyman alpha irradiance. Authors: Pap, J.; Hudson, H. S.; Rottman, G. J.; Willson, R. C.; Donnelly, R. F.; London, J. Bibcode: 1990NASCP3086..189P Altcode: 1990cisv.nasa..189P Solar Lyman alpha irradiance is estimated from various solar indices using linear regression analyses. Models developed with multiple linear regression analysis, including daily values and 81-day running means of solar indices, predict reasonably well both the short-and long-term variations observed in Lyman alpha. It is shown that the full disk equivalent width of the He line at 1083 nm offers the best proxy for Lyman alpha, and that the total irradiance corrected for sunspot effect also has a high correlation with Lyman alpha. Title: Advanced Solar Observatory Authors: Walker, Arthur B.; Bailey, Wayne L.; Chupp, Edward L.; Hudson, Hugh S.; Moore, Ronald L.; Roberts, William T.; Hoover, Richard B.; Wu, Shi T. Bibcode: 1990SPIE.1235..802W Altcode: No abstract at ADS Title: Solar physics from the Moon: Summary of panel discussions Authors: Hudson, Hugh Bibcode: 1990AIPC..207..109H Altcode: 1990am...proc..109H No abstract at ADS Title: A ``solar watch'' program for the Human Exploration Initiative Authors: Hudson, H.; Hildner, E. Bibcode: 1990AIPC..207..584H Altcode: 1990am...proc..584H Because of the healt hazards of solar activity, human traffic in deep space will require facilities for monitoring, predicting, and altering the astronauts to potential danger. This suggests a ``solar watch'' program consisting of a network of platforms at one A.U. from the Sun, capable of monitoring its global behavior. The instrumentation carried by these network platforms can evolve in sophistication with successive launches, in order to lead to a deep understanding of the physical mechanisms of solar activity - the best basis for maximally reliable activity forecasts. The possibility of steroscopic viewing of the solar surface and corona will confer unique advantages for the physical understanding of these physical mechanisms. Title: A Laboratory Measurement of CCD Photometric and Dimensional Variability Authors: Buffington, Andrew; Hudson, Hugh S.; Booth, Corwin H. Bibcode: 1990PASP..102..688B Altcode: The sun exhibits periodic and quasi-periodic variability in its total luminosity, which provides information about its internal structure and dynamics. Variability ranges from a few minutes to many-year time scales, with amplitudes as small as a few ppm in the milliHz band. Extension of this analysis to a large sample of outer stars would be interesting: a panoramic detector such as a CCD could record many stars at once. To meet this objective, a ppm time-series differential precision is required. Laboratory CCD photometric measurements presented here are promising for such an instrument. Normalizing the response from a portion of the CCD area removes most of the individual-frame variability. When a trend attributed to a thermal transient in the CCD dewar is removed, the individual-frame photometric precision is about 0.0001, limited by photoelectron counting statistics. The time-series power spectrum is flat within the desired frequency domain. Analysis of the dimensional stability of the CCD within the same data set indicates better than ppm performance, when first-order bulk motion and magnification changes are removed. Title: Solar Flares and "Microflaring" Authors: Hudson, H. S. Bibcode: 1990BAAS...22..898H Altcode: No abstract at ADS Title: Solar Luminosity Variability up to 10 mHz Authors: Fisher, B. M.; Hudson, H. S.; Willson, R. C. Bibcode: 1990BAAS...22..897F Altcode: No abstract at ADS Title: A Lunar Based Solar Observatory and the Human Exploration Initiative Authors: Davis, J. M.; Hudson, H. S. Bibcode: 1990BAAS...22..880D Altcode: No abstract at ADS Title: Solar Variability Measured by SMM/ACRIM Compared with Ground-Based Photometry Authors: Chapman, G. A.; Herzog, A. D.; Lawrence, J. K.; Walton, S. R.; Hudson, H. S.; Fisher, B. Bibcode: 1990BAAS...22..897C Altcode: No abstract at ADS Title: Space Astrophysics with Large Structures: CASES and P/OF Authors: Hudson, Hugh S.; Davis, J. M. Bibcode: 1990AIPC..211..134H Altcode: 1990heac.work..134H Space instruments for remote sensing, of the types used for astrophysics and solar-terrestrial physics among many disciplines, will grow to larger physical sizes in the future. The zero-g space environment does not inherently restrict such growth, because relatively lightweight structures can be used. Active servo control of the structures can greatly increase their size for a given mass. The Pinhole/Occulter Facility, a candidate Space Station attached payload, offers an example: it will achieve 0.2 arc s resolution by use of a 50-m baseline for coded-aperture telescopes for hard X-ray and γ-ray imagers. The CASES experiment (NASA Office of Applications and Space Technology)-deployable on the Space Shuttle as early as 1994-will provide an engineering and scientific demonstration of active structural control in this context. Title: Book reviews Authors: de Graaf, W.; Doom, C.; Wesselius, P. R.; de Jager, Cornelis; van der Kruit, P. C.; Achterberg, A.; de Waard, H.; van Hugo, Woerden; Kleczek, J.; Hudson, H. S.; van der Hucht, K. A. Bibcode: 1989SSRv...51..425D Altcode: No abstract at ADS Title: Book-Review - Solar Physics in the 1990'S Authors: Neidig, D. F.; Hudson, H. S. Bibcode: 1989Sci...246..246N Altcode: No abstract at ADS Title: Pinhole/Occulter Facility Authors: Hudson, Hugh S.; Davis, John M. Bibcode: 1989SPIE.1159..318H Altcode: To image X-radiation efficiently at energies above about 10 keV requires the use of 'shadow optics' techniques. The Pinhole/Occulter Facility (P/OF) represents an application of these techniques for observations in high-energy astrophysics, especially the study of solar coronal activity in hard X-rays and gamma rays. P/OF will achieve angular resolutions on the order of 0.2 arcsec for an instrument deployment length of 50 m. Because of this large structural scale, P/OF has been proposed as an attached payload for the Space Station. Meanwhile, several smaller-scale instruments are being developed. Title: Laboratory Analysis of a CCD as a Panoramic Astrometric Detector Authors: Buffington, A.; Hudson, H. S.; Booth, C. Bibcode: 1989BAAS...21Q1109B Altcode: No abstract at ADS Title: Imaging Solar Flare Gamma-Rays and Hard X-rays with The Gamma Ray Imaging Device (GRID) on a Balloon Authors: Starr, R.; Crannell, C. J.; Orwig, L. E.; Dennis, B. R.; Davis, J. M.; Johnson, W. N.; Norris, J. P.; Wood, K. S.; Green, M. E.; Vanbeek, H. F.; Hurford, G. J.; Prince, T. A.; Hudson, H. S. Bibcode: 1989BAAS...21.1151S Altcode: No abstract at ADS Title: Laboratory Analysis of a CCD as a Sensor for Time-Series Photometry Authors: Hudson, H. S.; Booth, C.; Buffington, A. Bibcode: 1989BAAS...21R1070H Altcode: No abstract at ADS Title: Report from solar physics Authors: Walker, A. B. C.; Acton, L.; Brueckner, G.; Chupp, E. L.; Hudson, H. S.; Roberts, W. Bibcode: 1989stss.work...31W Altcode: A discussion of the nature of solar physics is followed by a brief review of recent advances in the field. These advances include: the first direct experimental confirmation of the central role played by thermonuclear processes in stars; the discovery that the 5-minute oscillations of the Sun are a global seismic phenomenon that can be used as a probe of the structure and dynamical behavior of the solar interior; the discovery that the solar magnetic field is subdivided into individual flux tubes with field strength exceeding 1000 gauss. Also covered was a science strategy for pure solar physics. Brief discussions are given of solar-terrestrial physics, solar/stellar relationships, and suggested space missions. Title: Solar Flare Gamma-Ray and Hard X-ray Imaging with the GRID-on-a-Balloon Authors: Orwig, L. E.; Crannell, C. J.; Dennis, B. R.; Starr, R.; Hurford, G. J.; Prince, T. A.; Hudson, H. S.; van Beek, F.; Greene, M. E.; Johnson, W. N.; Norris, J. P.; Wood, K. S.; Davis, J. M. Bibcode: 1989BAAS...21..861O Altcode: No abstract at ADS Title: The BATSE Experiment for the GRO - Solar Flare Hard X-Ray and Gamma-Ray Capabilities Authors: Fishman, G. J.; Meegan, C. A.; Wilson, R. B.; Parnell, T. A.; Paciesas, W. S.; Pendleton, G. N.; Hudson, H. S.; Matteson, J. L.; Peterson, L. E.; Cline, T. L.; Teegarden, B. J.; Schaefer, B. E. Bibcode: 1989BAAS...21..860F Altcode: No abstract at ADS Title: Design considerations for a "Solar Mass Ejection Imager" on a rotating spacecraft. Authors: Jackson, B. V.; Hudson, H. S.; Nichols, J. D.; Gold, R. E. Bibcode: 1989GMS....54..291J Altcode: 1989opss.conf..291J; 1989sspp.conf..291J The authors describe an instrument capable of imaging the time-varying features of the entire outer corona (from near the Sun to beyond 90° elongation) via the Thomson-scattered diffuse solar light. This "all sky" imager works on a spin-stabilized spacecraft, preferably in deep space. Title: Solar flare gamma-ray and hard X ray imaging with the GRID-on-a-balloon Authors: Orwig, Larry E.; Crannell, C. J.; Dennis, Brian R.; Starr, R.; Hurford, G. J.; Hudson, H. S.; Vanbeek, F.; Greene, M. E.; Johnson, W. N.; Norris, J. P. Bibcode: 1989dots.work..143O Altcode: A primary scientific objective for solar flare research during the rapidly approaching maximum in solar activity is the imaging of gamma-ray and hard x ray sources of solar flare emissions. These goals will be pursued by the Gamma Ray Imaging Device (GRID) instrument, one of three instruments recently selected for NASA's Max '91 Solar Balloon Program. The GRID instrument is based on the technique of Fourier transform imaging and utilizes scanning modulation grid collimator optics to provide full-Sun imaging with 1.9-arcsecond resolution over the energy range from 20 to 700 keV at time resolutions from 0.1 to 2 s. The GRID telescope will employ 32 subcollimators, each composed of a matched pair of high-Z collimator grids separated by 5.2 meters and a phoswich scintillation spectrometer detector having no spatial resolution. The subcollimators and integrally-mounted fine aspect system are contained within a telescope canister which will be pointed to 0.1 degree accuracy and cyclically scanned to produce source modulation. The 32 subcollimators provide a uniform distribution of grid slit orientations and a logarithmic distribution of slit spacings corresponding to angular dimensions of 1.9 arcseconds to several arcminutes. The instrument is several orders of magnitude more sensitive than the HXIS instrument on the Solar Maximum Mission (SMM) and nearly 10 times more sensitive than any similar instrument scheduled to fly during the next solar maximum. The payload, designed for long-duration high-altitude balloon capability, is scheduled for its first science flight (8 to 14 days duration) from the Antarctic in January of 1992. Title: The BATSE experiment on the Gamma Ray Observatory: Solar flare hard X ray and gamma-ray capabilities Authors: Fishman, G. J.; Meegan, C. A.; Wilson, R. B.; Parnell, T. A.; Paciesas, W. S.; Pendleton, G. N.; Hudson, H. S.; Matteson, J. L.; Peterson, L. E.; Cline, T. L. Bibcode: 1989dots.work...96F Altcode: The Burst and Transient Source Experiment (BATSE) for the Gamma Ray Observatory (GRO) consists of eight detector modules that provide full-sky coverage for gamma-ray bursts and other transient phenomena such as solar flares. Each detector module has a thin, large-area scintillation detector (2025 sq cm) for high time-resolution studies, and a thicker spectroscopy detector (125 sq cm) to extend the energy range and provide better spectral resolution. The total energy range of the system is 15 keV to 100 MeV. These 16 detectors and the associated onboard data system should provide unprecedented capabilities for observing rapid spectral changes and gamma-ray lines from solar flares. The presence of a solar flare can be detected in real-time by BATSE; a trigger signal is sent to two other experiments on the GRO. The launch of the GRO is scheduled for June 1990, so that BATSE can be an important component of the Max '91 campaign. Title: Flare energetics. Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veck, N. J. Bibcode: 1989epos.conf..377W Altcode: The authors have sought to establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. To achieve this goal, they chose five flares in 1980 that were well observed with instruments on the Solar Maximum Mission, and with other space-borne and ground-based instruments. The events were chosen to represent various types of flares. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energetics of these flares. The authors also discuss the role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model. Finally, a critique of our current understanding of flare energetics and the methods used to determine various energetics terms is outlined, and possible future directions of research in this area are suggested. Title: Book-Review - Advances in Helio and Asteroseismology Authors: Christiansen-Dalsgaard, J.; Frandsen, S.; Hudson, H. S. Bibcode: 1989SSRv...51..431C Altcode: No abstract at ADS Title: Solar response of the BATSE instrument on the gamma-ray observatory Authors: Fishman, G. J.; Meegan, C. A.; Parnell, T. A.; Wilson, R. B.; Paciesas, W.; Cline, T.; Teegarden, B.; Schaefer, B.; Hudson, Hugh; Matteson, J. L. Bibcode: 1988fnsm.work..204F Altcode: The Burst and Transient Source Experiment (BATSE) on board the gamma ray observatory (GRO) aims at comprehensive observations of time profiles, spectra, and locations of high-energy transient sources. The mysterious cosmic gamma ray bursts provided the main motivation for the observations, but BATSE will make excellent observations of many classes of sources, and in particular solar flares. The solar response of BATSE, as inferred from its design parameters, is analyzed for two purposes: the optimization of the solar observations themselves, and the characterization of the solar effects on ordinary nonsolar observations. Title: Solar luminosity variations in solar cycle 21 Authors: Willson, Richard C.; Hudson, H. S. Bibcode: 1988Natur.332..810W Altcode: The ACRIM I experiment (Active Cavity Radiometer Irradiance Monitor) on the solar maximum Mission (SMM) satellite has provided a nearly continuous record of solar total irradiance variations since early 19801. It has detected variations on time scales ranging from minutes to SMM's lifetime. The long-term variations have revealed a downward trend during the declining phase of solar cycle 21 (ref. 2) of the sunspot cycle, a flat period between mid-1985 and mid-1987, and an upturn in late 1987 which suggests a direct correlation of luminosity and solar active region population. If the upturn continues into the activity maximum of solar cycle 22, a relation between solar activity and luminosity of possible climatological significance could have been discovered. The sense of the correlation agrees with what has been predicted from the coincidence of the 'little ice age' climate anomaly in the sixteenth and seventeenth centuries and the Maunder Minimum of solar activity3. The best-fit relationship for the variation of total irradiance S, with sunspot number Rz, and 10-cm flux F10, are S=1,366.82+7.7l×10-3 Rz and S=1,366.27+8.98×10-3 F10 (W m-2). These could be used to approximate total irradiance variations over the periods for which these indices have been compiled. Title: Solar Capability of the BATSE Experiment on GRO Authors: Hudson, H.; Matteson, J. L.; Fishman, G. J.; Meegan, C. A.; Parnell, T. A.; Paciesas, W.; Cline, T. A.; Schaefer, B.; Teegarden, B. Bibcode: 1988BAAS...20..747H Altcode: No abstract at ADS Title: Gamma-Ray and Hard X-Ray Imaging of Solar Flares Authors: Prince, T. A.; Hurford, G. J.; Hudson, H. S.; Crannell, C. J. Bibcode: 1988SoPh..118..269P Altcode: We discuss the scientific and technical aspects of high-resolution γ-ray and X-ray imaging of solar flares. The scientific necessity for imaging observations of solar flares and the implications of future observations for the study of solar flare electrons and ions are considered. Performance parameters for a future hard X-ray and γ-ray imager are then summarized. We briefly survey techniques for high-energy photon imaging including direct collimation imaging, coded apertures, and modulation collimators. We then discuss in detail the technique of Fourier-transform imaging. The basic formalism is presented, followed by a discussion of several practical aspects of the technique. We conclude our discussion of imaging techniques with a description of the options for detectors and grid fabrication. Several planned future high-energy imagers are described including the Solar-A hard X-ray imager, the balloon-borne GRID γ-ray imager, and the Pinhole/Occulter Facility. Title: An Instrument for High Angular Resolution, Hard X-Ray Observations of the Galactic Center Authors: Davis, J. M.; Weisskopf, M. C.; Hudson, H. S.; Hurford, G. J. Bibcode: 1988BAAS...20..677D Altcode: No abstract at ADS Title: Outstanding problems of solar flare research Authors: Hudson, H. S. Bibcode: 1988AdSpR...8k...7H Altcode: 1988AdSpR...8....7H This paper describes in very broad terms the current status of problems in understanding solar flares, from the personal perspective of an observer. It goes on to summarize new observational directions to help solve the problems, including support for the suggestion for stereoscopic imaging of the solar atmosphere. Title: Max '91: Flare research at the next solar maximum Authors: Dennis, Brian; Canfield, Richard; Bruner, Marilyn; Emslie, Gordon; Hildner, Ernest; Hudson, Hugh; Hurford, Gordon; Lin, Robert; Novick, Robert; Tarbell, Ted Bibcode: 1988STIN...8814919D Altcode: To address the central scientific questions surrounding solar flares, coordinated observations of electromagnetic radiation and energetic particles must be made from spacecraft, balloons, rockets, and ground-based observatories. A program to enhance capabilities in these areas in preparation for the next solar maximum in 1991 is recommended. The major scientific issues are described, and required observations and coordination of observations and analyses are detailed. A program plan and conceptual budgets are provided. Title: High-energy detector calibration and observation of non-thermal and ``superhot'' sources Authors: Hudson, Hugh S. Bibcode: 1988AdSpR...8k.229H Altcode: 1988AdSpR...8..229H No abstract at ADS Title: Modelling of total solar irradiance variability: An overview Authors: Hudson, Hugh S. Bibcode: 1988AdSpR...8g..15H Altcode: 1988AdSpR...8...15H Several components contribute to the observed variations of the total solar irradiance. There has been considerable effort expended on building empirical models for specific components, especially for sunspots and faculae. These models typically use time series of ground-based data as a means of representing the total-irradiance variability. There are several reasons to do this modeling. The models may help to identify the physical cause of a variation; the parameters of a model (e.g. the effective temperature of a sunspot) may be determinable via correlation with the total-irradiance observations; the models may be used as proxy representations for total-irradiance variability during periods of no data; finally, the models in principle can be used as a basis for ``correcting'' the total-irradiance data, as a means for better identification of additional components of variability. Title: The Nature of the Solar-Cycle Variation of Total Irradiance Authors: Hudson, H.; Willson, R. Bibcode: 1988srov.proc..318H Altcode: No abstract at ADS Title: Observed variability of the solar luminosity. Authors: Hudson, H. S. Bibcode: 1988ARA&A..26..473H Altcode: Data on solar luminosity variations are compiled in graphs and analyzed, with a focus on the SMM satellite bolometric measurements and other observations made since 1983. Consideration is given to total irradiance measurements in integrated light; emission-line variability in the visible, UV, and IR; mechanisms contributing to the solar variability (sunspots, faculae, rotation, oscillations, granulation, and active networks); the role of the solar magnetic cycle; and other observable parameters indicating variability. Also discussed are the effects of solar variations on the earth climate and the implications of the observations for theoretical models of convective-envelope processes (thermal diffusion near the surface and magnetic pressure in the interior). Title: Solar physics in the 1990s. Proceedings. Workshop XV and the COSPAR Interdisciplinary Scientific Commission E (Meeting E1) of the 27. COSPAR Plenary Meeting: Solar physics in the 1990s, Espoo (Finland), 18 - 29 Jul 1988. Authors: Neidig, D. F.; Hudson, H. S. Bibcode: 1988AdSpR...8k....N Altcode: 1988AdSpR...8.....N Papers concerning developments in solar physics are presented, focusing on scientific planning for the solar maximum and high-energy detector calibration and observation of nonthermal and superhot sources. Specific topics include solar radioastronomy, VLA observations of the sun, coronal loops, solar observation in the Phobos mission, the Solar-A mission, the Solar and Heliospheric Observatory satellite, the Global Oscillation Network Group, the relation between convection flows and magnetic structure and the solar surface, and solar flares research programs including quasi-dedicated mm-wave imaging, H-alpha, far IR, X-ray spectroscopy, and optical observations. Additional subjects include the manifestation of supergranulation structure of active regions during solar flares, post-flare loops, the relationship of peak emission measure and temperature to peak flare X-ray flux, turbulent and directed motions in solar flares, coronal temperature diagnostics from high-resolution soft W-ray spectra, the study of coronal densities from X-ray line ratios of Ne IX and Mg XI, electron densities in the solar atmosphere, the Coronal Magnetic Structures Observing Campaign, observations of a giant filament, the determination of coronal fieldline connectivity from photospheric flare observations, MHD simulation of mass injection, numerical simulation of solar atmospheric dynamics, intercalibration of hard X-ray spectrometers, the influence of the energy calibration of broad-band X-ray detectors on the determination of plasma parameters, and space experiments measuring solar X-rays. Title: Solar variability and oscillations. Authors: Hudson, Hugh S. Bibcode: 1987RvGeo..25..651H Altcode: 1986RvGSP..25..651H Within this decade, observations of total solar irradiance have become good enough to permit us to study directly the solar luminosity variations on a wide range of time scales, up to several years. At the same time there has been considerable improvement in understanding the classical indicators of solar activity, such as UV and visible chromospheric lines, soft X-rays, and radio fluxes. The observed variations include the effects of sunspots and plages (which induce "rotational modulation"), solar-cycle effects, and signatures of global oscillations and convection. In addition new characteristic time-scales (154 days and possibly 320 days) have been discovered but not explained. This review covers these developments and comments briefly on the subject of helioseismology, the study of the solar global oscillations. Title: Recent advances in the understanding of solar flares : 2 : 1987 Authors: Hudson, H.; Kai, K. Bibcode: 1987SoPh..113.....H Altcode: Papers are presented on solar flare discovery, solar flares and magnetic topology, impulsive and hot thermal solar flares, and plasma dynamics at the very initial phases of flares. Also considered are a magnetodynamic mechanism for loop flares, the propagation and confinement of energetic electrons in solar flares, nuclear processes and accelerated particles in solar flares, and optical observations of solar flares. Other topics include delta spots and great flares, chromospheric downflow velocities as a diagnostic in solar flares, and subphotospheric current systems and flares. Papers are also presented on a long-duration balloon payload for hard X-ray and gamma-ray observations of the sun, the derivation of vector magnetic fields from Stokes profiles, and the Solar-A mission. Title: Subphotospheric current systems and flares Authors: Hudson, Hugh S. Bibcode: 1987SoPh..113..315H Altcode: 1982SoPh..113..315H Subphotospheric current systems inferred from recent vector magnetograph observations (e.g. Gary et al., 1987) imply the existence of electric currents penetrating the photosphere and thus flowing deep in the solar convection zone. These currents presumably originate in an internal dynamo that supplies the observed photospheric magnetic fields through the buoyant motions of the initially deeply-buried flux tubes. The coronal fields resulting from this process therefore must carry slowly-varying currents driven by emf's remote from the surface. These currents may then drive solar-flare energy release. This paper discusses the consequences of such a deep origin of the coronal parallel currents. Simple estimates for a large active region suggest a mean current-closure depth ≥ 10,000 km, with a subphotospheric inductance ≥ 100 H and a subphotospheric stored energy ≥ 1033 ergs. Title: Solar flare discovery Authors: Hudson, Hugh S. Bibcode: 1987SoPh..113....1H Altcode: 1982SoPh..113....1H This paper considers the discoveries that have appreciably changed our understanding of the physics of solar flares. I identify a total of 42 discoveries from all disciplines, ranging from Galileo's initial observation of faculae to the recent discovery of strong limb brightening in 10-MeV γ-radiation. The rate of discovery increased dramatically over the past four decades as new observational tools became available. My assessment of significance suggests that recent discoveries - though more numerous - are individually less significant; perhaps this is because the minor early discoveries tend to be taken for granted. In spite of the many facets of flare physics that have been explained or at least well-described, many fundamental questions remain unresolved. Title: Flare energetics Authors: Wu, S. T.; De Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J. Bibcode: 1986epos.conf..5.1W Altcode: 1986epos.confE...1W In this investigation of flare energetics, researchers sought to establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. To achieve this goal, they chose five flares in 1980 that were well observed with instruments on the Solar Maximum Mission, and with other space-borne and ground-based instruments. The events were chosen to represent various types of flares. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energics of these flares. The role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model is discussed. Finally, a critique of the current understanding of flare energetics and the methods used to determine various energetics terms is outlined, and possible future directions of research in this area are suggested. Title: Characterization of the Total Flare Energy Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.41W Altcode: 1986epos.confE..41W No abstract at ADS Title: Energetics of the Impulsive Phase Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf..5.5W Altcode: 1986epos.confE...5W No abstract at ADS Title: Energetics of the Gradual Phase Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.20W Altcode: 1986epos.confE..20W No abstract at ADS Title: Review of Impulsive Phase Phenomena Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.60W Altcode: 1986epos.confE..60W No abstract at ADS Title: Flares Chosen for Energetics Study Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.47W Altcode: 1986epos.confE..47W No abstract at ADS Title: Relationships among the Phases Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.39W Altcode: 1986epos.confE..39W No abstract at ADS Title: Long-Term Downward Trend in Total Solar Irradiance Authors: Willson, R. C.; Hudson, H. S.; Frohlich, C.; Brusa, R. W. Bibcode: 1986Sci...234.1114W Altcode: The first 5 years (from 1980 to 1985) of total solar irradiance observations by the first Active Cavity Radiometer Irradiance Monitor (ACRIM I) experiment on board the Solar Maximum Mission spacecraft show a clearly defined downward trend of -0.019% per year. The existence of this trend has been confirmed by the internal self-calibrations of ACRIM I, by independent measurements from sounding rockets and balloons, and by observations from the Nimbus-7 spacecraft. The trend appears to be due to unpredicted variations of solar luminosity on time scales of years, and it may be related to solar cycle magnetic activity. Title: Components of the Variability of Solar Luminosity Authors: Hudson, H. S. Bibcode: 1986BAAS...18..981H Altcode: No abstract at ADS Title: Book-Review - Solar-Space Observations and Stellar Prospects Authors: Harvey, J. W.; Stenflo, J. O.; Hudson, H. S.; Noyes, R. W.; Kotrc, P. Bibcode: 1986BAICz..37..252H Altcode: No abstract at ADS Title: The concept of the Pinhole/Occulter Facility. Authors: Hudson, H. S. Bibcode: 1986sfcp.nasa....5H Altcode: The Pinhole/Occulter Facility is based upon a simple idea for obtaining high angular resolution in astronomical X-ray observations, for example for solar flares at energies >10 keV. The scheme uses a coded aperture device (multiple pinhole camera) with a large separation between the aperture encoder and the detector. Such an imaging device can have an angular resolution much better than 1 arc s if desired. A large structure would also make it possible to have a large external occulter, which would have powerful applications, notably for high-sensitivity observations of the corona in EUV and white light. This capability leads to the definition of the Pinhole/Occulter Facility, which combines both X-ray and coronal observations. The present concept is based upon a 35-m deployable boom, erected in the cargo bay of the Space Shuttle and pointed in the solar direction by the Instrument Pointing System of Spacelab. Title: Solar and stellar activity. Proceedings of Symposium 11 and of the Topical Meeting of the COSPAR Interdisciplinary Scientific Commission E (Meeting E2) of the COSPAR Twenty-sixth Plenary Meeting held in Toulouse, France, 30th June - 11th July 1986. Authors: Praderie, Francoise; Hudson, H. S. Bibcode: 1986AdSpR...6h....P Altcode: 1986AdSpR...6.....P Stellar and solar activity research concerned with the relevance of the theory of magnetic field production and tracing the manifestations of the magnetic field in the sun and stars is examined. Topics discussed include the generation of magnetic flux and energetics; the sun as a star; coronal activity along the HR diagram; activity and stellar evolution; and cycles of activity in solar-type stars. Attention is also given to data obtained from solar investigation from Spacelab 2. Title: Flare energetics. Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veck, N. J. Bibcode: 1986NASCP2439....5W Altcode: In this investigation of flare energetics, the authors establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. They chose five flares in 1980 that were well observed with instruments on the SMM, and with other space-borne and ground-based instruments. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energetics of these flares. The authors also discuss the role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model. Title: The concept of the Pinhole/Occulter Facility. Authors: Hudson, H. S. Bibcode: 1986NASCP2421....5H Altcode: The Pinhole/Occulter Facility is based upon a simple idea for obtaining high angular resolution in astronomical X-ray observations, for example for solar flares at energies >10 keV. The scheme uses a coded aperture device (multiple pinhole camera) with a large separation between the aperture encoder and the detector. Such an imaging device can have an angular resolution much better than 1 arc s if desired. A large structure would also make it possible to have a large external occulter, which would have powerful applications, notably for high-sensitivity observations of the corona in EUV and white light. This capability leads to the definition of the Pinhole/Occulter Facility, which combines both X-ray and coronal observations. The present concept is based upon a 35-m deployable boom, erected in the cargo bay of the Space Shuttle and pointed in the solar direction by the Instrument Pointing System of Spacelab. Title: Energy balance in solar active regions: The dip of April, 1985 Authors: Hudson, H. S. Bibcode: 1986AdSpR...6h..81H Altcode: 1986AdSpR...6...81H The presence of a solar active region affects the luminosity of the sun. Sunspots directly produce ``dips'' in the total solar irradiance approximately proportionally to their projected area, while faculae produce excess energy. These effects were discovered during the solar maximum period of 1980, and in this paper we examine the sunspot effect during solar minimum. We examine the ``dip'' due to an active region in April, 1985, as observed in the total solar irradiance by the ACRIM instrument on the Solar Maximum Mission. These data (obtained after the spacecraft repair in May, 1984) have simple variations, relative to those observed in 1980, because of the reduced level of activity approaching solar minimum. We find that the PSI index of projected sunspot area as defined in 1980 appears to describe this ``dip'' satisfactorily. Title: The Explorer program for astronomy and astrophysics Authors: Savage, Blair D.; Becklin, Eric E.; Cassinelli, Joseph P.; Dupree, Andrea K.; Elliot, James L.; Hoffmann, William F.; Hudson, Hugh S.; Jura, Michael; Kurfess, James; Murray, Stephen S. Bibcode: 1986STIN...9014159S Altcode: This report was prepared to provide NASA with a strategy for proceeding with Explorer-class programs for research in space astronomy and astrophysics. The role of Explorers in astronomy and astrophysics and their past accomplishments are discussed, as are current and future astronomy and astrophysics Explorers. Specific cost needs for an effective Explorer program are considered. Title: The over-the-limb hard X-ray events Authors: Hudson, H. S. Bibcode: 1986AdSpR...6f.199H Altcode: 1986AdSpR...6Q.199H The over-the-limb hard X-ray events offer us a uniquely direct view of the hard X-ray emission from the solar corona during a major flare. Limb occultation at angles greater than about 10 degrees (an arbitrary definition of this class of events) excludes any confusion with brighter chromospheric sources. I review the observations of the group of seven such over-the-limb events now in the literature, beginning with the prototype 1969 March 30 flare. The hard X-ray spectra appear to fall into two classes: hard events, with power-law index of about 2.0; and soft events, with power-law index about 5.4. This tendency towards bimodality is only significant at the 90% confidence level due to the smallness of the number of events observed to date. If borne out by future data, the bimodality would suggest the existence of two different acceleration mechanisms. Title: Book-Review - Solar / Space Observations and Stellar Prospects Authors: Harvey, J. W.; Hudson, H. S.; Noyes, R. W.; Zirker, J. B. Bibcode: 1985Sci...230..660H Altcode: No abstract at ADS Title: Energetic ions in solar γ-ray flares Authors: Hudson, H. S. Bibcode: 1985SoPh..100..515H Altcode: Solar flares emit line and continuum γ-radiation as well as neutrons and charged particles. These high-energy emissions require the presence of energetic ions within the magnetic structures of the flare proper. We have already learned a great deal about the location and mode of particle acceleration. The observations have now become extensive enough so that we can begin to study the dynamics of the energetic ions within the flare structures themselves. This paper reviews the γ-ray and neutron observations and the theory of their emission, and discusses on this basis the presence of energetic ions deep within the flaring atmosphere. Title: Book-Review - Solar Space Observations and Stellar Prospects Authors: Harvey, J. W.; Hudson, H. S.; Noyes, R. W. Bibcode: 1985Natur.317...91H Altcode: No abstract at ADS Title: Book-Review - Solar / Space Observations and Stellar Prospects Authors: Harvey, J. W.; Hudson, H. S.; Noyes, R. W. Bibcode: 1985Sci...229..787H Altcode: No abstract at ADS Title: Ion Energy Storage for Post-Flare Loops Authors: Hudson, H. S. Bibcode: 1985ICRC....4...58H Altcode: 1985ICRC...19d..58H Low-energy non-thermal protons may have long lifetimes in coronal loops with low density and high temperature. If energy were stored in such protons in the initial phases of a solar flare, it could be released slowly during the later phases. Within the present observational limits for post-flare loops, this mechanism should be considered in addition to a field-line reconnection theory of the Kopp and Pneuman type. The thin-target gamma ray emission from the trapped protons is below present limits, but more sensitive observations can test the hypothesis. Title: A Extended "superhot" Solar Flare X-Ray Source Authors: Hudson, H. S.; Ohki, K. I.; Tsuneta, S. Bibcode: 1985ICRC....4...50H Altcode: 1985ICRC...19d..50H A superhot hard X-ray source in a solar flare occulted by the solar limb was identified. Its hard X-ray image was found to show great horizontal extent but little vertical extent. An H alpha brightening at the same limb position about an hour later suggests a multi-component loop prominence system, so that it appears that a superhot source can evolve in the same manner as a normal solar soft X-ray source. The assignment of plausiable values to physical parameters in the source suggests (from the simplest form of classical thermal-conduction theory) that either new physics will be required to suppress conduction, or else that gradual energy release well after the impulsive phase of the flare must occur. In this respect too, the superhot source appears to resemble ordinary soft X-ray sources, except of course that its temperature is higher. Title: Detectability of extrasolar planetary transits Authors: Borucki, W. J.; Scargle, J. D.; Hudson, H. S. Bibcode: 1985ApJ...291..852B Altcode: Precise stellar photometry can be used to detect other planetary systems. However, the intrinsic variability of stellar luminosity imposes a fundamental limit on the sensitivity of this method. Based on recent precise solar observations made from the Solar Maximum Mission satellite, it appears that the detection of earth-sized planets will be marginal during periods of high stellar activity. However, with a suitable photometer larger planets should be readily detectable even in the presence of stellar activity equal to that of the sun at the peak of its sunspot cycle. The high precision, multiple-star photometric system required to detect planets in other stellar systems could be used to monitor flares, starspots, and global oscillations. Title: Post-flare Loop Heating by Trapped Superthermal Protons Authors: Hudson, H. S. Bibcode: 1985BAAS...17..628H Altcode: No abstract at ADS Title: The Brightness of the Interplanetary Medium in Thomson Scattering from 0.3 to 1.0 AU: Comparison with a View from Helios B Authors: Venkataraman, A.; Hudson, H. S.; Jackson, B. V. Bibcode: 1985BAAS...17..638V Altcode: No abstract at ADS Title: Problems of Energy Transport in Solar Active Regions and Flares Authors: Hudson, H. S. Bibcode: 1985spit.conf..348H Altcode: No abstract at ADS Title: A survey for photometric variability from space Authors: Hudson, H. S. Bibcode: 1984NASCP2350...43H Altcode: 1984itp..work...43H A survey for photometric variability in a wide variety of astronomical objects would produce much new information about their interiors and dynamics. Reasons for such a survey are given, as well as an example of the solar-constant variations that can be used as a guide to what may be expected from main-sequence stars. A concept for a satellite dedicated to a survey of photometric variability is proposed. Title: Ionospheric Detection of X-ray Pulsars Authors: Hudson, H. S.; Te Kolste, R. Bibcode: 1984BAAS...16..982H Altcode: No abstract at ADS Title: Total irradiance observations of low degree p-modes Authors: Woodard, M.; Hudson, H. Bibcode: 1984MmSAI..55...67W Altcode: A spectral analysis of 10 months of brightness data from the Active Cavity Radiometer Irradiance Monitor on the SMM satellite has revealed new information about the low-degree (l = 0,1, and 2) p-mode oscillations of the Sun. The mean frequencies, irradiance amplitudes, and line widths of the largest amplitude modes have been determined with previously unattained accuracy. Title: The Inconstant Solar Constant Authors: Willson, R. C.; Hudson, H.; Woodard, M. Bibcode: 1984S&T....67..501W Altcode: The Active Cavity Radiometer Irradiance Monitor (ACRIM) of the Solar Maximum Mission satellite measures the radiant power emitted by the sun in the direction of the earth and has worked flawlessly since 1980. The main motivation for ACRIM's use to measure the solar constant is the determination of the extent to which this quantity's variations affect earth weather and climate. Data from the solar minimum of 1986-1987 is eagerly anticipated, with a view to the possible presence of a solar cycle variation in addition to that caused directly by sunspots. Title: Facular limb-darkening functions for irradiance modeling. Authors: Hirayama, T.; Okamoto, T.; Hudson, H. S. Bibcode: 1984NASCP2310...59H Altcode: 1984siva.work...59H The limb-darkening function of faculae is an important factor in estimating facular contributions to solar irradiance variations. The authors review the existing photometric data and generate a synthetic limb-darkening function for faculae, which is then compared with the limb-darkening functions currently in use for irradiance modeling. It is found that the excess facular flux ranges from 0.017 to 0.0349 of the solar photospheric flux for the various representations. The present limitation appears to be the lack of comprehensive photometric data. Title: A Global Irradiance Program Authors: Hudson, H. S.; Chapman, G. A.; LaBonte, B. J. Bibcode: 1984NASCP2310..313H Altcode: 1984siva.work..311H Basic requirements for a long-term program of ground-based measurements of the solar brightness are outlined. Title: Solar Irradiance Variations on Active Region Time Scales Authors: Labonte, B. J.; Chapman, G. A.; Hudson, H. S.; Willson, R. C. Bibcode: 1984NASCP2310.....L Altcode: 1984QB531.S576.....; 1984siva.work.....L No abstract at ADS Title: Drift Scan Photometry and Astrometry Authors: Hudson, H. S. Bibcode: 1984NASCP2310..297H Altcode: 1984siva.work..297H No abstract at ADS Title: High Energy Aspects of Solar Flares Authors: Hudson, Hugh S. Bibcode: 1984BAAS...16..480H Altcode: No abstract at ADS Title: Hα and Hard X-Ray Development in Two-Ribbon Flares Authors: Dwivedi, B. N.; Hudson, H. S.; Kane, S. R.; Svestka, Z. Bibcode: 1984SoPh...90..331D Altcode: Morphological features of two-ribbon flares have been studied, using simultaneous ISEE-3 hard X-ray records and high-resolution Big Bear Hα movies for more than 20 events. Long-lasting and complex hard X-ray bursts are almost invariably found associated with flares of the two-ribbon type. We find at least three events, namely March 31, 1979, April 10, 1980, and July 1, 1980, where the occurrence of individual spikes in hard X-ray radiation coincides with suddenly enhanced Hα emission covering the sunspot penumbra. There definitely exist important (≥ 1B) two-ribbon Hα flares without significant hard X-ray emission. Title: Solar irradiance variations on active region time scales. Proceedingsof a workshop held at the California Institute of Technology, Pasadena, California, June 20 - 21, 1983. Authors: Labonte, B. J.; Chapman, G. A.; Hudson, H. S.; Willson, R. C.; Newkirk, G. A., Jr.; Bruning, D. H. Bibcode: 1984sivo.book.....L Altcode: No abstract at ADS Title: A survey for photometric variability from space Authors: Hudson, H. S. Bibcode: 1984AdSpR...4h.169H Altcode: 1984AdSpR...4..169H A survey for photometric variability in a wide variety of astronomical objects would produce much new information about their interiors and dynamics. This paper discusses reasons for such a survey, showing the example of the recent precise SMM observations of total solar irradiance variations as a guide to what might be expected from main-sequence stars, and proposes a concept for a satellite dedicated to a survey of photometric variability. Title: Preface Authors: Harvey, J. W.; Hudson, H. S.; Noyes, R. W. Bibcode: 1984AdSpR...4d...1H Altcode: 1984AdSpR...4....1H No abstract at ADS Title: Solar-space observations and stellar prospects. Proceedings of the Topical Meeting of the COSPAR Interdisciplinary Scientific Commission E (Meetings E1, E2, and E6) of the COSPAR Twenty-fifth Plenary Meeting held in Graz, Austria, 25th June - 7th July 1984. Authors: Harvey, J. W.; Hudson, H. S.; Noyes, R. W. Bibcode: 1984AdSpR...4h....H Altcode: 1984AdSpR...4..180H; 1984AdSpR...4.....H Selected topics pertaining to solar-space observations and stellar prospects are discussed. Papers are presented on the fine-scale structure of solar magnetic fields, increasing solar chromosphere line intensities with solar activity, and ulraviolet spectroscopy of the chromosphere and transition zone at high spatial and temporal resolution. Consideration is also given to solar coronal studies using normal-incidence X-ray optics, immediate and long-term prospects for helioseismology, and a compact Dopplergraph/Magnetograph suitable for space-based measurements of solar oscillations and magnetic fields. Title: Solar total irradiance and sunspot area in 1981 Authors: Hudson, H. S. Bibcode: 1984AdSpR...4h.113H Altcode: 1984AdSpR...4..113H Precise observations of the total solar irradiance in 1980 from the Solar Maximum Mission showed a strong correlation with the projected total sunspot area. This correlation ``explained'' about half of the variance (r ~ 0.75) in the total-irradiance data, leaving the other half for other solar phenomena and errors in the data. We have now begun the analysis of the intervening three years of ``spin-mode'' data, which have reduced coverage. We find that the correlation persists at about the same qualitative level, but with increased scatter that can be attributed to the smaller amount of data. The flatness of the distribution of areas of sunspot groups makes it possible to estimate PSI approximately from only the large groups. Title: An All-Sky Photometric Explorer Authors: Hudson, H. S. Bibcode: 1984srps.conf..197H Altcode: These notes give ideas for an "All-sky Photometric Explorer", which would survey the sky from space for visual photometric variability of as many objects as possible. The observations would be as precise as possible and would have the maximum duty cycle possible for each object. These conflicting requirements present many difficulties for optimization, and of course the basic objective - that of studying variability on scales much finer than those of Earth-based photometry - also presents many difficulties. Several technical routes may be available; the author is basing the discussion upon a particular concept sketched in this paper. Title: Solar Flux Variations and Concepts for a Stellar Photometric Satellite Authors: Hudson, Hugh S. Bibcode: 1984LNP...193..173H Altcode: 1984csss....3..173H No abstract at ADS Title: Sunspot areas and solar irradiance variations during 1980 Authors: Hoyt, D. V.; Eddy, J. A.; Hudson, H. S. Bibcode: 1983ApJ...275..878H Altcode: The realibility of daily measurements of sunspot areas made at the Space Environment Laboratories (SEL, Boulder, Colorado) is tested by comparison with measurements from other observatories, and they are found to be typically consistent to within 9 percent. The calculated visibility loss of sunspot due to poor seeing is no more than 13 percent of the projected sunspot areas. The resulting systematic error in derived solar constant values (S) will be less than 0.007 percent, although random measurement errors for sunspot areas can lead to corresponding random errors in the solar constant of as much as + or - 0.05 percent on 5 percnt of the days. The dominant contributor to short-term variations in S is the direct effect of sunspot blocking. A Goddard Space Flight Center interpretation of solar constant variations has used simulated sunspot areas that are systematically higher than the SEL measurements by 46 percent, which is well outside the expected range of measurement error or underestimation due to visibility losses. Title: Frequencies, amplitudes and linewidths of solar oscillations from total irradiance observations Authors: Woodard, M.; Hudson, H. S. Bibcode: 1983Natur.305..589W Altcode: Ten months of solar total irradiance data from the Solar Maximum Mission satellite have generated accurate frequencies, amplitudes and linewidths for individual ~5-min solar p-mode ocillations of low degree. The modes can be described as independent and chaotically excited oscillators, and provide no evidence for the fine structure taken to imply rapid internal rotation of the Sun. Title: Variations of Total Solar Irradiance During Rapid Sunspot Growth Authors: Hudson, H. S.; Jones, H.; McIntosh, P. Bibcode: 1983BAAS...15Q.950H Altcode: No abstract at ADS Title: Upper Limits on the Internal Rotation Rate of the Sun Authors: Woodard, M. F.; Hudson, H. S. Bibcode: 1983BAAS...15..951W Altcode: No abstract at ADS Title: Observation of AN Absorption Feature in Gamma Ray Burst Spectrum Authors: Hueter, G. J.; Hudson, H. S.; Matteson, J. L.; Rothschild, R. E.; Peterson, L. E. Bibcode: 1983ICRC....1...95H Altcode: 1983ICRC...18a..95H A gamma ray burst was detected on March 25, 1978 by the High Energy X-ray and Low Energy Gamma Ray Experiment on HEAO-1. The burst spectrum shows an absorption feature at 55 + or - 5 keV with an equivalent width of 13 + or - 3 keV, values commensurate with those of similar features observed by the KONUS experiment. The burst spectrum also is characterized by a hard component extending from about 0.25-6 MeV. This component can be interpreted in terms of a fireball model for gamma ray bursts, which places the distance to the source at 1 kpc. The integrated fluence of the burst between 0.025 and 6 MeV is 1.5 x 10 to the -5th ergs/sq cm. The burst source has been localized to within a degree of RA = 237.5 deg and Dec = 76.2 deg. Title: Location of X-Ray and Microwave Sources Authors: Hudson, H. Bibcode: 1983SoPh...86..444H Altcode: No abstract at ADS Title: Open discussion of controversial points Authors: Hudson, H. S.; Uchida, Y. Bibcode: 1983SoPh...86..435H Altcode: No abstract at ADS Title: Upper Limits on the Total Radiant Energy of Solar Flares Authors: Hudson, H. S.; Willson, R. C. Bibcode: 1983SoPh...86..123H Altcode: We establish limits on the total radiant energy of solar flares during the period 1980 February - November, using the solar-constant monitor (ACRIM) on board the Solar Maximum Mission. Typical limits amount to 6 × 1029 erg/s for a 32-second integration time, with 5σ statistical significance, for an impulsive emission; for a gradual component, about 4 × 1032 ergs total radiant energy. The limits lie about an order of magnitude higher than the total radiant energy estimated from the various known emission components, suggesting that no heretofore unknown dominant component of flare radiation exists. Title: Recent advances in the understanding of solar flares : proceedings of the U.S.-Japan seminar held at Komaba, Tokyo, 5-8 October 1982 Authors: Kane, S. R.; Uchida, Y.; Tanaka, K.; Hudson, H. S. Bibcode: 1983SoPh...86.....K Altcode: Papers are presented on recent research concerning the understanding of solar flares, including the general characteristics of flares; energy transport, and chromospheric heating and evaporation; emission processes and source structure; and high energy photons, nuclear processes, and particle acceleration. Topics addressed include magnetic theories of solar flares, nonthermal and nonequilibrium effects in soft X-ray flare spectra, the thermal evolution of flare plasmas, upper limits on the total radiant energy of solar flares, energetic electrons as an energy transport mechanism in solar flares, the spatial characteristics of microwave bursts, and the relation between hard X-ray spectra and electron energy spectra. Also examined are the pre-flare and post-flare X-ray variations in active regions, the imaging of impulsive solar flare phenomena, the vertical structure of hard X-ray flares, the spatial structure of high energy photon sources, gamma-ray lines and neutrons from solar flares, and a dynamical interpretation of the very hot region appearing at the top of a loop. For individual items see A83-47659 to A83-47701 Title: Foreword Authors: Kane, S. R.; Uchida, Y.; Tanaka, K.; Hudson, H. S. Bibcode: 1983SoPh...86D...9K Altcode: No abstract at ADS Title: The Pinhole/Occulter Facility. Authors: Tandberg-Hanssen, E. A.; Hudson, H. S.; Dabbs, J. R.; Baity, W. A. Bibcode: 1983pof..rept.....T Altcode: The Pinhole/Occulter is designed to study the nonthermal phenomena of plasma dynamics in the solar corona. It consists of a long boom, erected from the Shuttle bay or other space platform, separating an X-ray mask/occulting disk from X-ray detectors and coronagraphs. The long separation permits very high angular resolution. Title: First-order Fermi acceleration in solar flares as a mechanism for the second-step acceleration of prompt protons and relativistic electrons Authors: Bai, T.; Hudson, H. S.; Pelling, R. M.; Lin, R. P.; Schwartz, R. A.; von Rosenvinge, T. T. Bibcode: 1983ApJ...267..433B Altcode: Solar flare data from June 27, 1980 balloon-based observations were studied in terms of the hard X ray component. A temporal delay of 3 sec was observed for the X ray emissions above 235 keV. The delay occurred relative to the low-energy electrons and indicated a second acceleration stage. An estimation of the acceleration rate of the first-order Fermi process operating in a closed flare loop was found to be in agreement with the resulting data, including the acceleration of both protons and relativistic electrons. Additional support for the first-order Fermi process is noted in the fact that flares generally occur in magnetic loops, a condition which allows energetic particles to continually interact with the upward moving shock fronts. A correlation has also been observed between the delay times and the H-alpha areas, encouraging the interpretation that the delay times are the shock transit times. Title: Gamma-ray imaging with a rotating modulator Authors: Durouchoux, P.; Hudson, H.; Matteson, J.; Hurford, G.; Hurley, K.; Orsal, E. Bibcode: 1983A&A...120..150D Altcode: A gamma-ray imaging system, called a rotating modulator, is described, which allows a large area of the sky (radius approximately 27 deg) to be imaged with a limiting resolution approximately equal to or less than 1 deg for intense sources. It operates in the 30 keV -10 MeV energy range, and does not rely on position sensitive detection devices. It possesses a multiplex advantage which allows sky surveys to be completed in a small fraction of the time needed by a conventional collimation system. In addition, it is relatively insensitive to background fluctuations, which makes it ideally suited to satellite applications. The concept is illustrated by the results from a study of a gamma-ray spectroscopy experiment. Title: Solar Surface Granulation and Variations of Total Irradiance Authors: Hudson, H. S.; Woodard, M. F. Bibcode: 1983BAAS...15R.715H Altcode: No abstract at ADS Title: The Soft Thermal Component of Flare Hard X-ray Sources Authors: Hudson, H.; Ohki, K. Bibcode: 1983BAAS...15Q.715H Altcode: No abstract at ADS Title: Solar coronal non-thermal processes (Solar Maximum Mission) Authors: Hudson, H. S. Bibcode: 1983ucsd.reptQ....H Altcode: The Solar Maximum Mission was used to study solar coronal phenomena in hard X-radiation, since its instrument complement included the first solar hard X-ray telescope. Phenomena related to those discovered from OSO-5 and OSO-7 observations were emphasized. Title: Studies with the Pinhole/Occulter Facility Authors: Tandberg-Hanssen, E.; Dabbs, J.; Hudson, H.; Greene, M. Bibcode: 1983aiaa.meetQ....T Altcode: The scientific justifications for high-resolution hard X-ray astronomy are reviewed, and a scheme for making such observations from the Space Shuttle payload bay is presented. High-resolution X-ray observations at photon energies above 10 keV are important for the understanding of the physics of solar flares, coronal disturbances related to flares, and corona fine structure, as well as nonsolar X-ray sources. In order to study these phenomena, concepts have been developed for the Shuttle Pinhole/Occulter Facility (P/OF), an instrument based on the principles of the pinhole camera which will have an angular resolution of 0.2 arc sec. The proposed P/OF configuration consists of four separate telescopes or position-sensitive counters mounted on a detector plane and looking toward the target through separate portions of an aperture mask (occulter plane). The two planes are separated by a self-deployable 50-m boom, which is to be made essentially rigid by direct control of its lowest normal modes. Title: Variations of the Solar Radiation Input Authors: Hudson, H. S. Bibcode: 1983wcrs.proc...31H Altcode: Observations from spacecraft have now provided the first data on the variations of the total solar irradiance or 'solar constant'. The active cavity radiometer on the Solar Maximum Mission in particular produced copious data during its prime interval of about 10 mo during 1980. These data show that the total irradiance may vary by as much as 0.2 percent, with the largest excursions (deficits) due to the presence of sunspots. Other identified sources of variability include faculae and probably surface structure such as granulation. The 5-min global oscillations of the sun produce very-small-amplitude variations in the solar constant. The slower variations establish that the luminosity of the sun varies with the growth and decay of active regions; in other words, that the active regions cause appreciable storage of energy in the solar convection zone. A study of the Greenwich sunspot data by Hoyt and Eddy (1982) has shown that this sunspot effect may cause a modulation of order 0.1 percent on solar-cycle time scales. Title: Solar Oscillations Observed in the Total Irradiance Authors: Woodard, M.; Hudson, H. Bibcode: 1983SoPh...82...67W Altcode: 1983IAUCo..66...67W The total solar irradiance measurements obtained by the active-cavity radiometer on board the Solar Maximum Mission have been analyzed for evidence of global oscillations. We find that the most energetic low-degree p-mode oscillations in the five-minute band have amplitudes of a few parts per million of the total irradiance, and we positively detect modes with l = 0, 1, and 2. The distribution in l differs from that of the velocity spectrum, with relatively more power at lower l values. The individual modes have narrow line widths, corresponding to values of Q greater than a few thousand, or lifetimes of at least a week. We do not detect the 160-min oscillation in the power spectrum, and place an upper limit of 5 parts per million (99.9% confidence) on its amplitude. Title: Achievements in space astrophysics : proceedings of the topical meeting of the COSPAR Interdisciplinary Scientific Commission E of the COSPAR twenty-fourth plenary meeting held in Ottawa, Canada, 16th May-2nd June 1982 Authors: Hudson, Hugh S.; Dupree, Andrea K.; Linsky, Jeffrey L. Bibcode: 1983asap.book.....H Altcode: 1983QB495.A38v2n9.. No abstract at ADS Title: Chromospheric evaporation in a well-observed compact flare Authors: Acton, L. W.; Leibacher, J. W.; Canfield, R. C.; Gunkler, T. A.; Hudson, H. S.; Kiplinger, A. L. Bibcode: 1982ApJ...263..409A Altcode: Hudson and Ohki (1972) pointed out that the increase of the soft X-ray emission measure during flares might be accounted for in two different ways, either by 'coronal condensation', or by what they termed 'chromospheric rarefaction', now more commonly called 'chromospheric evaporation'. They ruled out coronal condensation on the basis of cornal mass content arguments. Moore et al. (1980) found it highly probable that the bulk of the mass of the soft X-ray emitting plasma is supplied during the rise phase by chromospheric evaporation from the feet of the soft X-ray loops. On the other hand, Cheng et al. (1981) argued that chromospheric evaporation is not important as a source of soft X-ray plasma. The present investigation is concerned with an event in which direct chromospheric observations contradict the conclusions reached by Cheng et al. Up to now chromospheric evaporation has always been an inference, without compelling positive evidence. In the current investigation, observations are considered which constitute such evidence. Title: The Pinhole/Occulter facility Authors: Dabbs, J. R.; Tandberg-Hanssen, E. A.; Hudson, H. S. Bibcode: 1982pof..rept.....D Altcode: The outer solar atmosphere exhibits a great variety of dynamic and energetic plasma phenomena, from the catastrophic energy release of solar flares to the steady acceleration of the solar wind. The Pinhole/Occulter Facility contains the instruments necessary for broadband X-ray imaging, combined with simultaneous ultraviolet and white light spectroscopy and imaging. Title: What are Solar Irradiance Observations of Global Oscillations Telling Us? Authors: Woodard, M. F.; Hudson, H. S. Bibcode: 1982BAAS...14..864W Altcode: No abstract at ADS Title: Sensitive Upper Limits on the Total Luminosity of Solar Flares Authors: Hudson, H. S. Bibcode: 1982BAAS...14..899H Altcode: No abstract at ADS Title: Solar analysis of solar-constant monitoring package (SMM) Authors: Hudson, H. S. Bibcode: 1982ucsd.rept.....H Altcode: The activity cavity radiometer irradiance monitor is supplying the first high precision data on solar total irradiance at the Earth. Thee classes of variability were discovered: large variations of amplitudes up to 0.2%; small high frequency variations in the form of a continuum in the periodogram, extending up to the Nyquist frequency; and sharp spikes at frequencies corresponding to the individual p modes already known from radial velocity measurements. The observed variations (up to 0.3%, on time scales of several days) were identified with sunspot darkness. The data analysis is expected to give information about the solar interior, as well as about the solar input to the terrestrial climate. Title: Detection of Gamma-Ray Bursts with the Hard X-Ray and Low Energy Gamma-Ray Experiment on HEAO-1 Authors: Hueter, G. J.; Hudson, H. S.; Matteson, J. L. Bibcode: 1982BAAS...14..619H Altcode: No abstract at ADS Title: The Effects of Sunspots on Solar Irradiance Authors: Hudson, H. S.; Silva, S.; Woodard, M.; Willson, R. C. Bibcode: 1982SoPh...76..211H Altcode: Sunspots have an obvious direct effect upon the visible radiant energy falling upon the Earth. We show how to estimate this effect and compare it quantitatively with recent observations of the solar total irradiance (Willson et al., 1981). The sunspots explain about half of the total observed variance of one-day averages. Since the sunspot effect on irradiance produces an asymmetry of the solar radiation, rather than (necessarily) a variation of the total luminosity, we have also estimated the sunspot population on the invisible hemisphere. This extrapolation allows us to estimate the true luminosity deficit produced by sunspots, in a manner that tends toward the correct long-term average value. We find no evidence for instantaneous global re-emission to compensate for the sunspot flux deficit. Title: Gamma Radiation and Photospheric White-Light Flare Continuum Authors: Hudson, H. S.; Dwivedi, B. N. Bibcode: 1982SoPh...76...45H Altcode: Recent gamma-ray observations of solar flares have provided a better means for estimating the heating of the solar atmosphere by energetic protons. Such heating has been suggested as the explanation of the continuum emission of the white-light flare. We have analyzed the effects on the photosphere of high-energy particles capable of producing the intense gamma-ray emission observed in the 1978 July 11 flare. Using a simple energy-balance argument and taking into account hydrogen ionization, we have obtained the following conclusions: Heating near τ5000 = 1 in the input HSRA model atmosphere is negligible, even for very high fluxes of energetic particles. Title: Gamma ray transients and related astrophysical phenomena Authors: Lingenfelter, R. E.; Hudson, H. S.; Worrall, D. M. Bibcode: 1982AIPC...77.....L Altcode: 1982grtr.work.....L The papers deal with observational and theoretical studies of cosmic gamma-ray bursts and line transients, X-ray burst sources, and solar flares, as well as with instrument concepts for X-ray and gamma-ray burst observations. General characteristics of gamma-ray bursts and transients are reviewed, along with burst positions and distributions, energy spectra, emission processes, and source models. HEAO-3 observations of gamma-ray bursts are discussed, as are ISEE-3 searches for time variations in the 511-keV cosmic gamma-ray line flux, gamma-ray burst spectra, neutron-star and stellar-flare models of gamma-ray burst sources, satellite observations and models of X-ray burst sources, solar energetic transients, and solar-flare energetics. The instruments described include a burst and transient source detector for the Gamma-ray Observatory, a modulated multislit camera for improved localization of gamma-ray bursts, and a space-balloon ion-chamber gamma-ray burst detector. Title: A third-generation small spectroscopy experiment for hard transient events Authors: Klebesadel, R. W.; Evans, W. D.; Laros, J. G.; Nakano, G. H.; Datlowe, D. W.; Imhof, W. L.; Hudson, H. S. Bibcode: 1982AIPC...77..469K Altcode: 1982grtr.work..469K We describe an experiment for monitoring high-energy transient events (X-ray and γ-ray bursts from both solar and stellar sources) as proposed for the OPEN (Origin of Plasmas in the Earth's Magnetosphere) mission. This experiment contains Si(Li) detectors, a high-purity germanium detector, and a bismuth germanate scintillation counter for high-energy response. Cooling for the solid-state detectors is provided by a passive radiator. The instrument gives broad spectral response with high energy resolution; its mass of 15 kg and telemetry requirement of 200 bps impose only modest demands upon spacecraft resources. The use of a passive cooler does also place a constraint upon the orbit of the satellite; deep space would be the preferred location but other orbits may also be suitable. The ideas embodied in the design may be of interest in the design of other spectroscopy measurements from deep space, as for example for participants in a future triangulation nework for transient events. Title: Second-Stage Acceleration in a Limb-Occulted Flare Authors: Hudson, H. S.; Lin, R. P.; Stewart, R. T. Bibcode: 1982SoPh...75..245H Altcode: We analyze hard and soft X-ray, microwave and meter wave radio, interplanetary particle, and optical data for the complex energetic solar event of 22 July 1972. The flare responsible for the observed phenomena most likely occurred ∼20° beyond the NW limb of the Sun, corresponding to an occultation height of 45 000 km. A group of type III radio bursts at meter wavelengths appeared to mark the impulsive phase of the flare, but no impulsive hard X-ray or microwave burst was observed. These impulsive-phase phenomena were apparently occulted by the solar disk as was the soft X-ray source that invariably accompanies an Hα flare. Nevertheless essentially all of the characteristic phenomena associated with second-stage acceleration in flares - type II radio burst, gradual second stage hard X-ray burst, meter wave flare continuum (FC II), extended microwave continuum, energetic electrons and ions in the interplanetary medium - were observed. The spectrum of the escaping electrons observed near Earth was approximately the same as that of the solar population and extended to well above 1 MeV. Title: High-energy observations of stellar flares: comparison with the sun Authors: Hudson, H. S. Bibcode: 1982AIPC...77..383H Altcode: 1982grtr.work..383H This paper reviews recent observations of flaring activity on flare stars of the UV Ceti type, concentrating on X-ray and radio data in comparison with possible solar analogs. Although detailed differences exist, we conclude that similar mechanisms may work for both cases. Extending the analogy, we estimate the hard X-ray and γ-ray fluxes expected from typical stellar flares. In the solar case, these radiations give information about accelerated particles in the flare region. Hard X-ray observations of stellar flares may be possible, eventually, but the predicted γ-ray fluxes are prohibitively weak. Title: The pinhole/occulter facility Authors: Hudson, H. S. Bibcode: 1982AdSpR...2d.307H Altcode: 1982AdSpR...2..307H The Pinhole/Occulter Facility concept uses a remote occulting mask to provide high resolution observations of the solar corona and of astronomical X-ray sources. With coded-aperture and Fourier-transform techniques, the Pinhole/Occulter makes images at a resolution of 0.2 arc sec for 2 - 120 keV X-rays, using a 50-m boom erected from the payload bay of the Space Shuttle or mounted on a free-flying platform. The remote occulter also creates a large shadow area for solar coronal observations; the Pinhole/Occulter concept includes separate optical and ultraviolet telescopes with 50-cm apertures. These large telescopes will provide a new order of resolution and sensitivity for diagnostic observations of faint structures in the solar corona. The Pinhole/Occulter is a powerful and versatile tool for general-purpose X-ray astronomy, with excellent performance in a broad spectral band complementary to that accessible with AXAF. The large collecting area of 1.5 m2 results in a 5σ detection threshold of about 0.02 μJy for the 2 - 10 keV band, or about 10-5 ph(cm2sec keV)-1 at 20 keV. Title: Variability of Solar Total Irradiance Authors: Woodward, M.; Hudson, H.; Willson, R. Bibcode: 1982pccv.conf..152W Altcode: No abstract at ADS Title: Direct evidence for chromospheric evaporation in a well-observed compact flare Authors: Canfield, R. C.; Gunkler, T. A.; Hudson, H. S.; Acton, L. W.; Leibacher, J. W.; Kiplinger, A. L. Bibcode: 1982AdSpR...2k.145C Altcode: 1982AdSpR...2..145C Observations of the solar flare of May 7, 1980 using several Solar Maximum Mission instruments are presented as an investigation of the phenomenon of chromospheric evaporation. The total amount of plasma at temperatures greater than 2 x 10 to the 6th K were determined from the X-ray data, and the amount of plasma that was evaporated from the chromosphere was determined from the H-alpha data. The H-alpha profiles indicate that for the flare as a whole, at the time of peak soft X-ray emission measure, the number of atoms evaporated from the chromosphere was 7 x 10 to the 37th. The soft X-ray emission measure of 1 x 10 to the 49th/cu cm, coupled with the flare volume estimate of 10 to the 26th cu cm, indicates that there were 3 x 10 to the 37th electrons in the soft X-ray plasma with temperatures greater than 2 x 10 to the 6th K. These results indicate that enough material had been evaporated from the chromosphere to account for the X-ray plasma. Taken together, the H-alpha, soft X-ray, and hard X-ray images indicate that chromospheric evaporation is driven both by flare-accelerated electrons during the impulsive phase and by conduction during the thermal phase. Title: Achievements in space astrophysics; Proceedings of the Topical Meeting, Ottawa, Canada, May 16-June 2, 1982 Authors: Hudson, H. S.; Dupree, A. K.; Linsky, J. Bibcode: 1982AdSpR...2i....H Altcode: 1982AdSpR...2.....H Progress in astrophysics resulting from IUE and Einstein Observatory studies is reviewed. The topics considered include: the solar corona as a testing found for plasma astrophysics; seismological studies of the sun and other stars; magnetic reconnection; IUE and Einstein observations of cool stars, high-luminosity X-ray binaries, supernovae, cataclysmic variables, active galactic nuclei, globular clusters and blue horizontal branch stars, supernova remnants, UV spectra of H II regions and galaxies, and galactic clusters. Also discussed are: highly variable X-ray emitting objects in the Rho Oph dark cloud; X-ray diagnostics of globular clusters; stellar chromospheres and coronae of solar and late-type dwarfs, active stars and systems, F-, G-, and K-type stars, hot stars, and cool luminous stars; coronal heating mechanisms; magnetic flux expulsion as an acceleration mechanism for stellar winds; and energy balance of the outer atmospheres of solar like stars. For individual items see A83-33582 to A83-33609 Title: The Pinhole/Occulter Facility. Executive summary. Authors: Dabbs, J. R.; Tandberg-Hanssen, E. A.; Hudson, H. S. Bibcode: 1982NASTP2089.....D Altcode: No abstract at ADS Title: High-Energy Observations of Stellar Flares - Comparison with the Sun Authors: Hudson, H. S. Bibcode: 1982AIPC...77..382H Altcode: 1982grtr.work..382H No abstract at ADS Title: An Einstein search for X-ray emission from comet Bradfield (1979l) Authors: Hudson, H. S.; Ip, W. -H.; Mendis, D. A. Bibcode: 1981P&SS...29.1373H Altcode: X-radiation may result from active plasma phenomena in the interactions of comets with the solar wind. We have carried out a limited but sensitive search for soft X-radiation from Comet Bradfield (1979 l), on 1980 Feb. 5. No X-radiation was detected at a level (3σ) of 1.7 × 10 -13 erg(cm 2sec keV) -1 in the 0.2 - 4.0 kev range. This corresponds to a limit on the power dissipated in the comet by non-thermal electrons of approximately 10 19 ergs sec -1, averaged over the 2568-sec exposure to the comet. This energy deposition is near the magnitude suggested by simple theoretical ideas, and further searches of appropriate comets both in soft X-radiation and at radio wavelengths seem warranted. Title: Interpretation of the 'Second-step' Acceleration in the Impulsive Phase of a Solar Flare Authors: Bai, T.; Hudson, H. S. Bibcode: 1981BAAS...13..912B Altcode: No abstract at ADS Title: The Pinhole/Occulter Facility Authors: Hudson, H. S.; Kohl, J. L.; Lin, R. P.; MacQueen, R. M.; Tandberg-Hanssen, E.; Pabbs, J. R. Bibcode: 1981pof..rept.....H Altcode: A large occulting system in space can be used for high resolution X-ray observations and for large aperture coronagraphic observations in visible and UV light. The X-ray observations can combine high angular resolution in hand (10 keV) X-radiation with the high sensitivity of a multiple pinhole camera, and can permit sensitive observations of bremsstrahlung from nonthermal particles in the corona. The large aperture coronagraphs have two major advantages: high angular resolution and good photon collection. This will permit observations of small scale structures in the corona for the first time and will give sufficient counting rates above the coronal background rates for sensitive diagnostic analysis of intensities and line profiles for coronal structures in the solar wind acceleration region. The technical basis for performing observations with a large occulting system in these three wavelength ranges is described as well as a pinhole/occulter facility presently being considered for Spacelab. Some indications about future developments are included. Title: Direct Evidence for Chromospheric Evaporation in a Well-Observed Compact Flare Authors: Canfield, R. C.; Acton, L. W.; Gunkler, T. A.; Hudson, H. S.; Kiplinger, A. L.; Leibacher, J. W. Bibcode: 1981BAAS...13R.819C Altcode: No abstract at ADS Title: The Effects of Solar Activity on the Total Solar Irradiance Authors: Hudson, H. S.; Woodward, M.; Willson, R. C. Bibcode: 1981BAAS...13R.877H Altcode: No abstract at ADS Title: Observations of p-mode Oscillations in the Total Solar Irradiance Authors: Woodard, M.; Hudson, H.; Willson, R. C. Bibcode: 1981BAAS...13..858W Altcode: No abstract at ADS Title: Variations of solar irradiance Authors: Willson, R. C.; Hudson, H. S. Bibcode: 1981ApJ...244L.185W Altcode: The active cavity radiometer experiment on the Solar Maximum Mission is providing sensitive measurements of time variations of the total solar irradiance with greater accuracy and precision than previously achieved. The mean 1 AU irradiance for the first 45 days' operation is 1368.64 W/sq m with an absolute uncertainty of less than + or 0.5%. Variations about this mean have been observed on time scales of hours to days with amplitudes up to + or - 0.04%, resolved with a statistical uncertainty as low as 0.001%. Variations within a single orbit with amplitudes as large as + or - 0.5% have been resolved with 0.005% or smaller statistical uncertainty. Although these variations do not display a systematic relationship to conventional solar activity indices over the period, correlative behavior cannot be ruled out on the basis of the present limited data set. Title: Observations of Solar Irradiance Variability Authors: Willson, R. C.; Gulkis, S.; Janssen, M.; Hudson, H. S.; Chapman, G. A. Bibcode: 1981Sci...211..700W Altcode: High-precision measurements of total solar irradiance, made by the active cavity radiometer irradiance monitor on the Solar Maximum Mission satellite, show the irradiance to have been variable throughout the first 153 days of observations. The corrected data resolve orbit-to-orbit variations with uncertainties as small as 0.001 percent. Irradiance fluctuations are typical of a band-limited noise spectrum with high-frequency cutoff near 0.15 day-1; their amplitudes about the mean value of 1368.31 watts per square meter approach ± 0.05 percent. Two large decreases in irradiance of up to 0.2 percent lasting about 1 week are highly correlated with the development of sunspot groups. The magnitude and time scale of the irradiance variability suggest that considerable energy storage occurs within the convection zone in solar active regions. Title: Solar maximum mission experiment: Initial observations by the active cavity radiometer Authors: Willson, R. C.; Hudson, H. S. Bibcode: 1981AdSpR...1m.285W Altcode: 1981AdSpR...1..285W The Active Cavity Radiometer on board the SMM is providing high-quality measurements of the solar irradiance. After correction for the solar distance, the orbital displacement of the satellite, and the relativistic shift of irradiance due to the satellite motion, the observed standard deviation is in the range 10-15 parts per million in a 96-minute integration. Measurable solar variations occur on time scales of a few minutes to a few days. The total amplitude of the variations in the daily averages from February 16 to March 31, 1980, was 0.10% based upon 96-minute averages. Title: Solar Maximum Mission experiment: initial observations by the Active Cavity Radiometer. Authors: Willson, R. C.; Hudson, H. S. Bibcode: 1981hea..conf..285W Altcode: No abstract at ADS Title: X-ray and gamma-ray observations of a white-light flare Authors: Hudson, H. S. Bibcode: 1981AdSpR...1m.247H Altcode: 1981AdSpR...1..247H HEAO-1 observed hard radiations (X- and gamma-rays) from a major solar flare on 11 July 1978. The observations showed gamma-ray line and continuum emission extending to the highest energy observed. The lines are identified with the 2.2 MeV line of deuterium formation and the 4.4 MeV line of inelastic scattering on 12C, both previously observed in the flares of August 1972 [1]. The 11 July flare was identified as a white-light flare by observations at Debrecen [2]. It thus provides the first opportunity for a detailed examination of white-light flare theories that depend upon proton heating of the photosphere. The line strength over a four-minute integration at 2.2 MeV was 1.00 +/- 0.29 ph(cm2 sec)-1, and the gamma-ray emission (excluding the 2.2 MeV line which was appreciably delayed) lagged by less than 20 sec approximately after the hard X-ray and microwave fluxes. We conclude that the ``second-stage'' acceleration of high-energy solar particles must commence promptly after the impulsive phase. Title: High-energy astrophysics : proceedings of symposium 5 and the topical meeting of the COSPAR Interdisciplinary Scientific Commission E of the COSPAR twenty-third plenary meeting held in Budapest, Hungary, 2-14 June 1980 Authors: Hudson, H. S. Bibcode: 1981AdSpR...1m....H Altcode: 1981QB495.A38v1n13.; 1981AdSpR...1.....H Papers are presented on such topics as the X-ray background from evolving quasars, high-energy neutrinos from powerful radio galaxies, spectra of accreting X-ray pulsars, cyclotron radiation of magnetized degenerate dwarfs and neutron stars, and expanding shells of young pulsars as sources of high-energy neutrinos. Also considered are low-energy observation of Cygnus X-2 by Ariel VI, evidence from a cyclotron line at 70 keV from 4U053+21, observations of 1-30 MeV gamma rays from the galactic center, and results from the SMM experiment. Title: High-energy astrophysics. Proceedings of Symposium 5 and the Topical Meeting of the COSPAR Interdisciplinary Scientific Commission E of the COSPAR Twenty-third Plenary Meeting held in Budapest, Hungary, 2 - 14 June 1980. Authors: Hudson, H. S. Bibcode: 1981AdSpR...1m...7H Altcode: 1981AdSpR...1....7H No abstract at ADS Title: Sunspots and solar variability Authors: Hudson, H. S.; Willson, R. C. Bibcode: 1981phss.conf..434H Altcode: The analyses of Willson et al. (1981) and Hudson et al. (1981) are extended in correlating the total solar irradiance monitor (ACRIM) data with the routine synoptic sunspot data. At the simplest level, this correlation reveals clearly that the so-called 'missing flux' is truly missing in the sense that large young active regions do produce at net diminution of the irradiance when their spots cross the central meridian. It is pointed out that the irradiance deficit must of course be made up, either promptly or on intermediate time scales; this is because the surface effects cannot perturb the energy generation processes in the interior. In the approach taken here, simple models of the reemission are constructed, the total reemission is scaled to the estimated sunspot deficit, and an attempt is made to measure the parameters of the models by a statistical comparison with ACRIM data. Title: X-ray and gamma-ray observations of a white-light flare. Authors: Hudson, H. S. Bibcode: 1981hea..conf..247H Altcode: No abstract at ADS Title: Variability of Total Solar Irradiance: the Flux Deficit of Sunspots Authors: Hudson, H. S.; Woodard, M.; Willson, R. C. Bibcode: 1980BAAS...12Q.898H Altcode: No abstract at ADS Title: Hα and Hard X-Ray Development in Two-Ribbon Flares Authors: Dwivedi, B. N.; Hudson, H. S.; Kane, S. R.; Svestka, Z. Bibcode: 1980BAAS...12..905D Altcode: No abstract at ADS Title: HEAO 1 observations of gamma-ray lines from a solar flare Authors: Hudson, H. S.; Bai, T.; Gruber, D. E.; Matteson, J. L.; Nolan, P. L.; Peterson, L. E. Bibcode: 1980ApJ...236L..91H Altcode: HEAO 1 observed gamma radiation, including the 2.223 MeV line of deuterium and the 4.43 MeV line of C-12, from a white-light solar flare of 1978 July 11. Line strengths over a 4 minute integration were 1.00 + or - 0.29 and 0.18 + or - 0.07 photons/sq cm s, respectively, and the continuum in the 1-5 MeV range fitted a spectrum 10 E to the -3rd photons/sq cm s keV. The 2.2 MeV line lagged 94 + or - 30 s behind the gamma-ray continuum, which itself was delayed about 20 s from the hard X-ray (not less than 40 keV) and microwave fluxes. This is the second flare for which both MeV-range lines and continuum have been observed, and the first for which simultaneous white-light observations exist. The prompt gamma-ray line (4.43 MeV) can be directly interpreted as an energy deposition of not greater than 7 x 10 to the 26th ergs per sec by energetic protons at photospheric depths. This is insufficient to maintain the white-light continuum by normal photospheric emission mechanisms. Title: The Necessary Conditions for White-light Flaring from Proton Bombardment Authors: Hudson, H. S.; Dwivedi, B. N. Bibcode: 1980BAAS...12..480H Altcode: No abstract at ADS Title: Impulsive phase of solar flares Authors: Kane, S. R.; Crannell, C. J.; Datlowe, D.; Feldman, U.; Gabriel, A.; Hudson, H. S.; Kundu, M. R.; Maetzler, C.; Neidig, D.; Petrosian, V. Bibcode: 1980sfsl.work..187K Altcode: 1980sofl.symp..187K The present understanding of the impulsive phase of a solar flare, characterized by short-duration bursts of impulsive hard X-ray, EUV, optical and radio emission indicating the release of energetic electrons is reviewed. Observations of the spectral distribution of impulsive hard X-ray bursts and of Type III and radio continuum bursts are presented and interpreted in terms of energetic electron distributions, and impulsive EUV, XUV, soft X-ray and optical observations, which provide a lower limit to total energy release during the impulsive phase, are discussed. The role of energetic electrons in exciting the hard X-ray, EUV and microwave emissions is considered, and thin-target, thick-target, partial-precipitation and thermal models of impulsive phase electron acceleration are evaluated in light of the observations. It is noted that available data do not allow discrimination between a thermal or a nonthermal electron distribution, on which depends the proportion of flare energy supplied by the energetic electrons, and that data favors models which permit at least partial electron precipitation. Future observational and theoretical work is indicated. Title: Solar particle fluxes and the ancient sun Authors: Lingenfelter, R. E.; Hudson, H. S. Bibcode: 1980asfr.symp...69L Altcode: The implications of the statistical data on solar flare particle fluxes, for the present and the ancient sun, considering modern data from the past two solar cycles, C-14 data from the past 7000 years, and Al-26 and Mn-53 data in lunar samples for the last 10-million years are reviewed. All of these records suggest that there is a maximum proton fluence (greater than 10 MeV) from a solar flare on the order of 10 to the 10th p/sq cm, above which the size-frequency distribution steepens sharply. From this it is extremely unlikely that energetic particles from solar flares could have contributed to extinction catastrophes in the fossil record. Title: Analysis of HEAO-1 Solar Gamma-Ray Spectra Authors: Bai, T.; Hudson, H. S.; Lingfelter, R. E. Bibcode: 1979BAAS...11R.658B Altcode: No abstract at ADS Title: Ground-based observations of sources in the AFGL infrared sky survey. Authors: Gosnell, T. R.; Hudson, H.; Puetter, R. C. Bibcode: 1979AJ.....84..538G Altcode: A preliminary report is presented on ground-based observations of sources discovered by the AFGL infrared sky survey. This paper contains photometric data on 104 sources and spectrophotometry of 14 sources identified in 6.0 x 4.5-arcmin raster scans around the positions reported by the AFGL. The sources originally identified in the catalog with known objects are mainly hot bright stars. Two unusual sources discovered in this program, GL 2636 and GL 4029, appear to be multiple sources. Both these objects exhibit the 3.3-micron emission feature, and GL 4029 shows several other emission features previously found in NGC 7027. Title: Infrared continuum observations of the solar atmosphere Authors: Hudson, H.; Levan, P.; Lindsey, C. Bibcode: 1979ucsd.rept.....H Altcode: The far-infrared wavelengths (10 microns to 1 mm) were used to study the spatial and temporal structure of the solar atmosphere. Observational results were obtained on flares, faculae, sunspots, and on the center-to-limb intensity distribution, as well as on time variability within these regions. A program of precise monitoring of slow variations in the integrated solar luminosity was shown to be feasible, and initial steps to implement observations were completed. Title: Observations of particle accelerator in solar flares Authors: Hudson, H. S. Bibcode: 1979AIPC...56..115H Altcode: 1979pama.work..115H Solar flares provide several examples of non-thermal particle acceleration. This paper reviews the information gained about these processes via X-ray and γ-ray astronomy, which can presently distinguish among three separate particle-acceleration processes at the sun: An impulsive acceleration of >~20 keV electrons, a gradual accelerator of >~20 keV electrons, and a gradual accelerator of >~10 MeV ions. The acceleration energy efficiency (total particle energy divided by total flare energy) of any of these mechanisms cannot be less than about 0.1%, although the gradual acceleration does not occur in every flare. The observational material suggests that both the impulsive and gradual accelerations take place preferentially in closed magnetic-field structures, but that the electrons decay in these traps before they can escape. The ions escape very efficiently, on the other hand. Title: Indirect estimation of energy disposition by non-thermal electrons in solar flares. Authors: Hudson, H. S.; Canfield, R. C.; Kane, S. R. Bibcode: 1978SoPh...60..137H Altcode: The broad-band EUV and microwave fluxes correlate strongly with hard X-ray fluxes in the impulsive phase of a solar flare. This note presents numerical aids for the estimation of the non-thermal electron fluxes from these correlations, using the SFD (sudden frequency deviation) ionospheric data to measure the EUV flux. Title: Hard X-ray imaging from the solar probe Authors: Hudson, H. S. Bibcode: 1978clus.nasa..132H Altcode: The solar probe offers a platform with particular advantages for studying solar nonthermal plasma processes via the observations of hard X-radiation from energetic electrons in the chromosphere and corona, these include (1) high sensitivity, (2) a second line of sign (in addition to the earth's) that can aid in three dimensional reconstruction of the source distribution, and, (3) the possibility of correlation with direct measurements of the nonthermal particles from the probe itself. Title: New perspectives for solar observations Authors: Hudson, H. S. Bibcode: 1978clus.nasa...81H Altcode: The merits and demerits of an approach to the Sun (more closely than about one AU) are examined. High resolution imaging (approximately 0.1 arc sec) to be obtained with the shuttle-borne solar optical telescope, will permit conclusive observations relating to the structure of the quiet solar atmosphere, sunspots, spicules, oscillations, and many other problems of solar astrophysics. Beyond this limit important unresolved structure will exist, especially in optically thin regions or in regions with strong magnetic fields. Ambiguity will remain in solar imagery because a single line of sight cannot suffice completely to untangle the vertical dimension from the two horizontal dimensions. A solar probe with a complement of solar telescopes would provide two lines of sight for solar viewing and increase knowledge of the three dimensional structure of the solar atmosphere. Title: A purely coronal hard X-ray event. Authors: Hudson, H. S. Bibcode: 1978ApJ...224..235H Altcode: OSO 7 observations of a hard X-ray event of coronal origin are described. This event had a duration of more than 42 min as well as an abnormally large hard/soft ratio and apparently occurred after the disappearance of a bright coronal streamer. This gradual hard X-ray event is tentatively associated with open field lines extending well above the closed loop structures that participated in the originating flare. It is noted that a gradual hardening of the hard X-ray spectral distribution occurred during the event. Physical conditions in the source are considered, and the results are compared with observations of other hard X-ray events associated with flares. Title: Large scale telescopes for high resolution X-ray and gamma-ray astronomy. Authors: Hudson, H. S.; Lin, R. P. Bibcode: 1978SSI.....4..101H Altcode: This paper shows that angular-resolution, energy-range, and structural constraints on image-modulated X-ray telescopes are not fundamental and that the limits on angular resolution can be overcome by constructing such telescopes on a very large spatial scale. It is proposed that widely separated satellites be used for the modulating mask and detector array. Implementation of this concept is discussed in terms of a simple system consisting of a pinhole camera (i.e., a hole in an opaque mask on one subsatellite and a detector array on another). Advantages and problems of such systems are briefly discussed, and a solar X-ray telescope intended for deployment from a Shuttle orbiter is described. It is noted that such large-scale telescopes can be constructed to image gamma rays and even energetic neutrons as well. Title: HEAO-1 Response to Solar Gamma Rays. Authors: Hudson, H. S.; Baity, W. A.; Gruber, D. E.; Knight, F. K.; Nolan, P. L.; Matteson, J. L.; Peterson, L. E. Bibcode: 1978BAAS...10..516H Altcode: No abstract at ADS Title: Threshold effect in second-stage acceleration. Authors: Hudson, H. S. Bibcode: 1978SoPh...57..237H Altcode: Proton fluxes observed at Earth (van Hollebeke et al., 1975) have a much flatter size distribution than do other parameters of solar flares more representative of total energy. Peak proton flux varies at least as rapidly as the fourth power of total flare energy. An absolute threshold may exist, in view of the flatness of the proton distribution. Title: OSO-7 Observations of Coronal Hard X-Ray Sources. Authors: Hudson, H. S. Bibcode: 1978BAAS...10..454H Altcode: No abstract at ADS Title: A study of the planetary nebulae Abell 30 and Abell 78. Authors: Cohen, M.; Hudson, H. S.; O'Dell, S. L.; Stein, W. A. Bibcode: 1978IAUS...76..356C Altcode: No abstract at ADS Title: A Grid Telescope for Imaging Hard X-Rays Authors: Hudson, H. S.; Hurford, G. J.; Peterson, L. E.; van Beek, H. F. Bibcode: 1978nisa.symp..297H Altcode: 1978nisa.conf..297H No abstract at ADS Title: A study of the planetary nebulae Abell 30 and Abell 78. Authors: Cohen, M.; Hudson, H. S.; O'Dell, S. L.; Stein, W. A. Bibcode: 1977MNRAS.181..233C Altcode: The central regions of the planetary nebulae A 30 and A 78 have been studied by UBVRI photometry, optical spectroscopy, and near-infrared photometry. The spectra contain high-excitation emission lines and strongly resemble those of Wolf-Rayet stars of the carbon sequence. Stellar temperatures in excess of 50,000 K are inferred. The observed 3.5-micron flux of each nebula exceeds reasonable extrapolations of both the stellar flux and any possible free-free emission. The color temperature of this excess between 2.28 and 3.5 microns is of the order of 1000 K. For each nebula, the aperture dependence of the excess emission suggests an extended (about 10 arcsec radius) region centered on the nucleus. Thermal radiation from a distribution of dust that is concentrated near the nuclei seems the most plausible explanation for the excess, but no theory of dust formation or heating seems totally adequate at present. Title: Do All Flares Have Impulsive Phases? Authors: Datlowe, D. W.; Elcan, M. J.; Hudson, H. S.; Peterson, L. E. Bibcode: 1977BAAS....9..569D Altcode: No abstract at ADS Title: Effects of Electrons versus Protons in the Solar Atmosphere (Proceedings of the Meeting `How Can Flares be Understood?', held during the 16th General Assembly of the IAU in Grenoble, France, on 27 August, 1976.) Authors: Hudson, H. S. Bibcode: 1977SoPh...53..295H Altcode: No abstract at ADS Title: Surface photometry of the SB0 galaxy NGC 1023. Authors: Gallagher, J. S.; Hudson, H. S. Bibcode: 1976PASP...88..824G Altcode: Beam-switched photoelectric surface photometry has been obtained for the SB0 galaxy NGC 1023. Initially the disk surface brightness distribution is approximately exponential with a more rapid fall-off in surface brightness occurring for R 5 arc minutes ( 15 kpc). No color gradient is found in (V- i). Key words: galaxies - photometry Title: New Upper Limits on Jovian X Rays Authors: Peterson, L. E.; Hudson, H. S.; Tsikoudi, V. Bibcode: 1976Icar...29..419P Altcode: The UCSD X-ray telescope on OSO-3 scanned Jupiter for 33 days during February and March 1968. We have searched the data for a steady Jovian flux, and for a burst component at times of decametric radio bursts. Neither component was detected at a sensitivity of ∼0.1 photon (cm 2sec) -1 for hv > 7.7 keV. At 4.4AU, the 3σ upper limits correspond to X-ray luminosities of 7.4 × 10 19 ergs sec -1 for the steady component, and 2 × 10 20 ergs sec -1 for the burst component. The observations occurred during a period of high solar activity, during which three sudden-commencement magnetic storms were observed at Earth. We compare the upper limits with several different calculations of the expected flux levels, and conclude that major improvements in X-ray detection techniques will be required before Jovian X rays can be detected with near-Earth observations. Title: Hard X-ray imaging facility for space shuttle: A scientific and conceptual engineering study Authors: Peterson, L. E.; Hudson, H. S.; Hurford, G.; Schneible, D. Bibcode: 1976STIN...7822016P Altcode: A shuttle-accommodated instrument for imaging hard X-rays in the study of nonthermal particles and high temperature particles in various solar and cosmic phenomena was defined and its feasibility demonstrated. The imaging system configuration is described as well as the electronics, aspect systems, mechanical and thermal properties and the ground support equipment. Title: Surface photometry of the spiral galaxy IC 2233 and the existence of massive halos. Authors: Gallagher, J. S.; Hudson, H. S. Bibcode: 1976ApJ...209..389G Altcode: Using beam-switching to cancel the sky background, deep BVi photometric measurements have been obtained near the edge-on Scd galaxy IC 2233. No halo component is detected to a level of 1 percent of the disk surface brightness. For a concentrated model (half-intensity isophote smaller than the disk radius), the halo M/L must exceed 100 if it is to meet minimally the Ostriker and Peebles criterion for stabilizing a cold disk of stars. Subject headings: galaxies: individual-galaxies: photometry - galaxies: structure Title: Non-thermal processes in large solar flares. Authors: Lin, R. P.; Hudson, H. S. Bibcode: 1976SoPh...50..153L Altcode: We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that: The ∼10-102 keV electrons accelerated during the flash phase constitute the bulk of the total flare energy. Title: Far-infrared observations of IRC +10216. Authors: Campbell, M. F.; Elias, J. H.; Gezari, D. Y.; Harvey, P. M.; Hoffmann, W. F.; Hudson, H. S.; Neugebauer, G.; Soifer, B. T.; Werner, M. W.; Westbrook, W. E. Bibcode: 1976ApJ...208..396C Altcode: Broadband photometric observations of IRC + 10216 in five wavelength intervals from 50 to 1000 microns are reported. The observed radiation is interpreted as thermal emission from dust in the extended molecular cloud heated by the compact 2-20-micron source at the cloud core. The shape of the 50-1000-micron spectrum suggests that the emissivity of the dust particles varies approximately as the inverse wavelength over this spectral interval. The mass of dust inferred from the far-infrared emission is comparable with the mass of heavy molecules in the cloud. Title: Submillimeter observations of NGC 2024, OMC-2, and Mon R-2. Authors: Hudson, H. S.; Soifer, B. T. Bibcode: 1976ApJ...206..100H Altcode: The paper reports submillimeter (effective wavelength about 400 microns) observations of three dense molecular clouds: NGC 2024 (Orion B), OMC-2 (in Orion A), and Mon R-2. These objects strongly resemble the far-infrared source in the Kleinmann-Low nebula in Orion A. An extensive map of NGC 2024 shows the peak of submillimeter brightness to coincide with the peak locations of far-infrared continuum and HCN molecular-line emissions. The submillimeter and far-infrared brightness distributions differ in spatial detail, suggesting that the submillimeter emission comes from a cool region with temperature of about 25 K (inferred from the surface brightness of the optically thick (C-12)(O-16) line). Arguments based on the column density derived from optically thin molecular lines yield an effective mass-absorption coefficient of 17 sq cm/g for the continuum opacity at 400 microns; however, this estimate is subject to large uncertainties. It is suggested that the star 2024 No. 2 is imbedded in the molecular cloud and supplies the energy of the submillimeter emission. Title: Solar limb brightening in submillimeter wavelengths. Authors: Lindsey, C.; Hudson, H. S. Bibcode: 1976ApJ...203..753L Altcode: Differential two-beam scans of the sun in submillimeter wavelengths (350 microns to 1 millimeter) indicate limb brightening approaching 1 percent when the cosine of the angle from the normal equals 0.60. The observations also show considerable chromospheric structure, both in active and quiet regions, but with less relative amplitude than at millimeter and centimeter wavelengths. The limited angular resolution of the observing system, together with photometric errors due to fluctuating atmospheric transparency, make the brightness profile of the extreme limb uncertain. The observed degree of limb brightening is considerably less than that consistent with spherically symmetric model atmospheres based on continuum brightness-temperature measurements. The suppression of limb brightening suggests the existence of irregular granular structure with both horizontal and vertical characteristic sizes of the order of 1500 km. High-resolution images in the wings of the K-line show granular structure of about this horizontal scale. Title: The Solar-Flare Infrared Continuum: Observational Techniques and Upper Limits Authors: Hudson, H. S. Bibcode: 1975SoPh...45...69H Altcode: Exploratory observations at 20 μ and 350 μ have determined detection thresholds for solar flares in these wavelengths. In the 20 μ range solar atmospheric fluctuations (the `temperature field') set the basic limits on flare detectability at ∼5K; at 350 μ the extinction in the Earth's atmosphere provides the basic limitation of ∼30K. These thresholds are low enough for the successful detection of several infrared-emitting components of large flares. Limited observing time and lack of solar activity have prevented observations of large flares up to the present, but the techniques promise to be extremely useful in the future. Title: Upper limits on stellar flare X-ray emission from OSO-3. Authors: Tsikoudi, V.; Hudson, H. Bibcode: 1975A&A....44..273T Altcode: The OSO-3 X-ray telescope scanned 82 optically-observed flares on UV Cet, EV Lac, YZ CMi, and AD Leo as well as four radio flares on UV Cet. These data are examined individually and by superposed-epoch analysis. Upper limits of about 1000 and about 10 million are determined for the X-ray/optical and X-ray/radio luminosity ratios, respectively. The upper limits conflict with theories of stellar flaring which directly utilize nonthermal particles to excite the main part of the flare optical emission, but agree with observations of solar flares. Moderate improvement of the X-ray observations should make possible a direct test of the solar-flare analogy in the near future. The integrated X-ray luminosity of stellar flares may contribute significantly to the low-energy diffuse X-ray background in the plane of the Galaxy. Title: The Solar-flare Infrared Continuum Authors: Ohki, K.; Hudson, H. S. Bibcode: 1975SoPh...43..405O Altcode: We consider potential sources of infrared (1 μ to 1 mm) continuum in solar flares. Several mechanisms should produce detectable fluxes: in the 350 μ window for ground-based observations, impulsive emission will arise in synchrotron radiation from 1-10 MeV electrons, and possibly thermal (free-free) continuum from the source of the white-light flare; the hot flare plasma responsible for soft X-ray emission will also emit detectable fluxes of free-free continuum in the largest flares. At shorter wavelengths the dominant infrared emission will come from the Hα flare itself. Observations in the infrared wavelengths will help to complete our picture of flare structure in both the impulsive and gradual phases. Title: Submillimeter Observations of NGC 2024, OMC-2, and Mon R-2 Authors: Hudson, H. S.; Soifer, B. T. Bibcode: 1975BAAS....7..465H Altcode: No abstract at ADS Title: Beam-Switched Surface Photometry of Galaxies Authors: Gallagher, J. S.; Hudson, H. S. Bibcode: 1975BAAS....7..396G Altcode: No abstract at ADS Title: Solar Hard X-Ray Burst Phenomena. Authors: Peterson, L. E.; Datlowe, D. W.; Elcan, M. J.; Hudson, H. S. Bibcode: 1975BAAS....7..399P Altcode: No abstract at ADS Title: Center to Limb Variations in Solar Hard X-Ray Spectra Authors: Datlowe, D. W.; Elcan, M. J.; Hudson, H. S.; Peterson, L. E. Bibcode: 1975BAAS....7..354D Altcode: No abstract at ADS Title: The Shape of the Hard Solar X-Ray Spectrum Authors: Elcan, M. J.; Datlowe, D. W.; Hudson, H. S.; Peterson, L. E. Bibcode: 1975BAAS....7..354E Altcode: No abstract at ADS Title: Solar Limb Brightening at Submillimeter wavelengths Authors: Lindsey, C.; Hudson, H. S. Bibcode: 1975BAAS....7..360L Altcode: No abstract at ADS Title: The peculiar object HD 44179 ("The red rectangle"). Authors: Cohen, M.; Anderson, C. M.; Cowley, A.; Coyne, G. V.; Fawley, W.; Gull, T. R.; Harlan, E. A.; Herbig, G. H.; Holden, F.; Hudson, H. S.; Jakoubek, R. O.; Johnson, H. M.; Merrill, K. M.; Schiffer, F. H.; Soifer, B. T.; Zuckerman, B. Bibcode: 1975ApJ...196..179C Altcode: A strong infrared source detected in the AFCRL sky survey is confirmed, and is identified with the binary star HD 44179, embedded in a peculiar nebula. UBVRI and broad-band photometry between 2.2 and 27 microns are combined with blue, red, and near-infrared spectra, polarimetry and spectrophotometry of the star, and a range of direct and image-tube photographs of the nebula, to suggest a composite model of the system. In this model, the infrared radiation derives from thermal emission by dust grains contained in a disklike geometry about the central object, which appears to be of spectral type B9-A0 III and which may be in pre-main-sequence evolution. Two infrared emission features are found, peaking at 8.7 and 11.3 microns, the latter corresponding to the feature seen in the spectrum of the planetary nebula NGC 7027. The complex nebular structure is discussed on the basis of photographs through narrow-band continuum and emission-line filters. The polarization data support the suggestion of a disk containing some large particles. No radio continuum emission is detected. Title: Relationship Between Hard and Soft Solar X-Ray Sources Observed by OSO 7 Authors: Datlowe, D. W.; Hudson, H. S. Bibcode: 1975IAUS...68..209D Altcode: No abstract at ADS Title: Nonthermal Processes in Large Solar Flares Authors: Hudson, H. S.; Jones, T. W.; Lin, R. P. Bibcode: 1975IAUS...68..425H Altcode: No abstract at ADS Title: The solar X-ray continuum below 10 keV Authors: Hudson, H. S. Bibcode: 1975xris.conf..233H Altcode: Information obtained from soft X-ray (below 10 keV) continuum and broadband photometric observations of the sun is summarized. The quasi-thermal origin of the soft X-ray continuum is discussed, techniques are described for analyzing the spectrum of the continuum radiation, and OSO-7 data is used to describe the characteristics of flare-associated soft X-ray bursts. Results are reviewed for broadband observations of solar flares, thermal emission parameters are determined for the hot regions in flares, and models are considered for the temporal evolution of soft X-ray bursts from hot flare regions. It is shown that these bursts require the injection of additional material into the corona during the growth phase and that conductive cooling is the dominant cooling mechanism in hot flare regions. Soft X-ray emission from active regions is discussed, and radiative losses are shown to be the dominant cooling mechanism of these regions. It is concluded that soft solar X-radiation represents the characteristic emission of hot regions in the solar atmosphere. Title: Relationship between hard and soft X-ray sources observed by OSO-7. Authors: Datlowe, D. W.; Hudson, H. S. Bibcode: 1975IAUS...68R.209D Altcode: No abstract at ADS Title: Relationship between hard and soft solar X-ray sources observed by OSO-7. Authors: Datlowe, D. W.; Hudson, H. S. Bibcode: 1975IAUS...68Q.209D Altcode: No abstract at ADS Title: Submillimeter Observations of Planets Authors: Hudson, H. S.; Lindsey, C. A.; Soifer, B. T. Bibcode: 1974Icar...23..374H Altcode: A new program of ground-based observations at submillimeter (≈400 μm) wavelengths has yielded observations of Mercury, Venus, Mars, Jupiter, and Saturn. We report here observations near planetary conjunctions, which have minimal corrections for atmospheric extinction: Mercury, 361±65 K; Venus, 231±35 K; and Saturn, 205±15 K (based upon the area of the planetary disk), using Jupiter (150 K) and Mars (220 K) as photometric standards. The Mercury observations show that the brightness temperature does not decrease at the submillimeter wavelengths, relative to observations at 3 mm; for Venus, however, the brightness temperature appears appreciably lower than at millimeter wavelengths. The results for Saturn indicate a strong and possibly optically thick contribution from the rings. We also gave a description of our instrumentation and observational techniques, with special emphasis upon the effect of extinction by atmospheric water vapor. Title: OSO-7 observations of solar x-rays in the energy range 10 100 keV Authors: Datlowe, D. W.; Elcan, M. J.; Hudson, H. S. Bibcode: 1974SoPh...39..155D Altcode: The solar X-ray experiment on the satellite OSO-7 has provided extensive observations of hard and soft X-ray bursts. We give a general description of the hard X-ray data here, in parallel with the description of the soft X-ray data already published (Datlowe et al., 1974). The data for this study consist of 123 hard X-ray bursts which occurred between 10 October 1971 and 6 June 1972. We examine the behavior of a typical event in terms of its spectral and flux variations. For the whole data sample, we find that 2/3 of the soft X-ray bursts have detectable hard X-ray emission. We present the distributions of frequency of occurrence of peak flux, spectral index, collisional energy loss, burst duration and the duration at half maximum of the flux profile. No correlation was found between the flux and the spectral slope of an individual data sample, nor was there a correlation between the peak flux and the full width at half maximum of a burst. Title: Submillimeter Observations of the Orion Nebula and NGC 2024 Authors: Soifer, B. T.; Hudson, H. S. Bibcode: 1974ApJ...191L..83S Altcode: We report here new observations at submillimeter wavelengths ( 4O0 ) of the Orion Nebula and NGC 2024. These observations provide a fairly detailed (FWHM 1 6) map of the Orion Nebula at these wavelengths. We attribute the Orion Nebula emission to optically thin thermal radiation from dust grains in the molecular cloud with peak emission centered on the KL source. A comparison of our data with those of Harvey et a'. at longer wavelengths shows that the emissivity must vary as . We find evidence that the dust temperature varies with position, with the center having the higher temperature. If optical constants determined from lunar silicate material are used, then 0.05A).10 g of small particles would be required to produce the observed submillimeter emission, but this suggests a far larger mass ( 5 X 102 M0 in the central 1 6) in the Orion molecular cloud than is normally assumed. Alternatively, the data require an opacity at 100 greatly exceeding that of the lunar silicates. Subject headings: infrared - Orion Nebula - photometry Title: Search of OSO-3 data for X-ray emission from stellar flares. Authors: Tsikoudi, V.; Hudson, H. Bibcode: 1974BAAS....6..264T Altcode: No abstract at ADS Title: Infrared Continuum Observations of 300-sec Oscillations Authors: Hudson, H. S. Bibcode: 1974BAAS....6R.289H Altcode: No abstract at ADS Title: Non-thermal Processes in Large Solar Flares Authors: Lin, R. P.; Hudson, H. S.; Jones, T. W. Bibcode: 1974BAAS....6R.290L Altcode: No abstract at ADS Title: Observations of solar X-ray bursts in the energy range 5 15 keV Authors: Datlowe, D. W.; Hudson, H. S.; Peterson, L. E. Bibcode: 1974SoPh...35..193D Altcode: Bursts of solar X-rays in the energy range 5-15 keV are associated with flares and are due to thermal emission from a hot coronal plasma. In this paper we present the results of the first study of a large sample of separate bursts, 197 events associated with subflares and a few importance 1 events. The observations were made by a proportional counter on the satellite OSO-7 from October 1971 to June 1972. In most cases the temperature characterizing the X-ray spectrum rises impulsively at the onset of the burst and then declines slowly throughout the remainder of the burst. The emission measure rises exponentially with a time scale of 30-100 s and then declines slowly, on a time scale of the order of 103 s. From these observations we show that the growth of the thermal energy in the flare plasma throughout the burst can be due to the heating of new cool material. Title: Observations of Solar X-Ray Bursts in the Energy Range 5-15 keV Authors: Datlowe, D.; Hudson, H. Bibcode: 1974BAAS....6..285D Altcode: No abstract at ADS Title: Absence of Cosmic Gamma-Ray Bursts in Association with Normal Stellar Flares Authors: Hudson, H.; Tsikoudi, V. Bibcode: 1974tcgx.conf...93H Altcode: No abstract at ADS Title: Direct Observation of Temperature Amplitude of Solar 300-SECOND Oscillations Authors: Hudson, H. S.; Lindsey, C. A. Bibcode: 1974ApJ...187L..35H Altcode: The 300-second oscillations form the dominant source of variability of the solar infrared continuum. We have observed them at 20 with an amplitude AT, = 3.0 K over an area with an effective diameter of 33". This new mode of observation of the 300-s oscillations should make possible a fundamental improvement in our knowledge of their nature and origin. Subject headings: Infrared solar atmospheric motions Title: Absence of Cosmic γ-ray Bursts in Association with Normal Stellar Flares Authors: Hudson, H.; Tsikoudi, V. Bibcode: 1973NPhS..245...88H Altcode: 1973Natur.245...88H The OSO-3 X-ray telescope provided extensive observations of cosmic X-ray sources between March 1967 and June 1968 (refs 1 and 2). We report here a search of OSO-3 data during times of known optical stellar flares for γ-ray bursts similar to those reported by Klebesadel et al.3 Table 1 shows the number of flares and the range of observation times for each flare star. Title: Far Infrared Observations of Solar Flares Authors: Hudson, H. S. Bibcode: 1973BAAS....5..274H Altcode: No abstract at ADS Title: Nonthermal X-rays and Related Processes Authors: Hudson, H. S. Bibcode: 1973NASSP.342..207H Altcode: 1973heps.conf..207H No abstract at ADS Title: Search for Weak White-Light Flares by Time-Wise Photographic Cancellation Authors: Uchida, Yutaka; Hudson, Hugh Bibcode: 1972SoPh...26..414U Altcode: This study proposes as a working hypothesis that small white-light flares accompany all major (proton) flare events and suggests a new method for systematically finding these `patches' of white-light emission. The new technique consists of the time-wise application of the photographic cancellation method to detect small time-varying features around the time of the impulsive phase of a flare. Title: Infrared Emission from Solar Flares Authors: Hudson, H. S.; Ohki, K. Bibcode: 1972BAAS....4R.385H Altcode: No abstract at ADS Title: Thick-Target Processes and White-Light Flares Authors: Hudson, H. S. Bibcode: 1972SoPh...24..414H Altcode: Observations indicate that fast electrons in solar flares, which cause the hard X-ray burst and the impulsive microwave burst, lose energy predominantly by collisional processes. This requires a thick-target theory of the emission, for which the electron spectrum inferred from the X-ray spectrum becomes 1.5 powers steeper than in the usual thin-target theory. Title: Long-Term Temporal Variations of the Hard X-Ray Flux from the Centaurus Region Authors: Schwartz, Daniel A.; Peterson, Laurence E.; Hudson, Hugh S. Bibcode: 1972ApJ...174..549S Altcode: The University of California at San Diego (UCSD) X-ray telescope aboard the third Orbiting Solar Observatory (OSO-IlI) observed the Centaurus region daily from 1967 October to 1968 February, and also for five days in 1968 June. For this period, we derive a stable minimum flux of 0.33 + 0.03 photons (cm2 s) I between 7.7 and 38 keV from a persistent hard X-ray source around 1 = 305 . Several single days show enhanced fluxes; and two extensive flaring episodes, one with a soft and the other with a very hard spectrum, last at least 10 days. Title: Upper Limits to the X-Ray Luminosities of Five Supernovae Authors: Ulmer, M.; Grace, V.; Hudson, H. S.; Schwartz, D. A. Bibcode: 1972ApJ...173..205U Altcode: We have examined data from the 050-Ill X-ray telescope for evidence of X-ray emission from five optically detected extragalactic supernovae during the period 1967 March-1968 June. Upper limits to the X-ray emission in the range 7.7-113 keV near optical maximum (within 30 days) fall in the range 10- - 10-10 ergo (cm2 s) 2 Reasonable estimates of the distances to these supernovae lead to upper limits on the total energies of from 1010 to 1051 ergs. Title: Soft X-Ray and Microwave Observations of Hot Regions in Solar Flares Authors: Hudson, H. S.; Ohki, K. Bibcode: 1972SoPh...23..155H Altcode: Hot regions in solar flares produce X-radiation and microwaves by thermal processes. Recent X-ray data make it possible to specify the temperature and emission measure of the soft X-ray source, by using, for instance, a combination of the 1-8 Å (peak response at about 2 keV) and the 0.5-3 Å (peak response at about 5 keV) broad-band photometers. The temperatures and emission measures thus derived satisfactorily explain the radio fluxes, within systematic errors of about a factor of 3. Comparison of 15 events with differing parameters shows that a hot solar flare region has an approximately isothermal temperature distribution. The time evolution of the correlation in a single event shows that the hot material originates in the chromosphere, rather than the corona. The density must lie between 1010 and 2 × 1011 cm−3. For an Importance 1 flare, this implies a stored energy of roughly 2 x 1030-1029 ergs. A refinement of the data will enable us to choose between conductive and radiative cooling models. Title: Non-thermal electron population in solar flares. Authors: Hudson, H. S. Bibcode: 1972eucr.conf..127H Altcode: No abstract at ADS Title: Upper limits to the X-ray luminosity from five supernovae. Authors: Ulmer, M.; Grace, V.; Hudson, H.; Schwartz, D. Bibcode: 1971PASP...83..608U Altcode: No abstract at ADS Title: Type III Solar Radio Bursts Accompanied by Soft X-Radiation in the Absence of Hα Flares Authors: Teske, Richard G.; Soyumer, Tevfik; Hudson, Hugh S. Bibcode: 1971ApJ...165..615T Altcode: Several Type iii solar radio bursts which occurred in the absence of reportable flares were observed to be accompanied by weak X-radiation detectable at photon energies of between 1.0 and 12.5 keV. The X-rays almost invariably preceded the Type iii burst, an observation which suggests that a thermal event precedes the Type iii instability. Title: 10 100 keV electron acceleration and emission from solar flares Authors: Lin, R. P.; Hudson, H. S. Bibcode: 1971SoPh...17..412L Altcode: We present an analysis of spacecraft observations of non-thermal X-rays and escaping electrons for 5 selected small solar flares in 1967. OSO-3 multi-channel energetic X-ray measurements during the non-thermal component of the solar flare X-ray bursts are used to derive the parent electron spectrum and emission measure. IMP-4 and Explorer-35 observations of > 22 keV and > 45 keV electrons in the interplanetary medium after the flares provide a measure of the total number and spectrum of the escaping particles. The ratio of electron energy loss due to collisions with the ambient solar flare gas to the energy loss due to bremsstrahlung is derived. The total energy loss due to collisions is then computed from the integrated bremsstrahlung energy loss during the non-thermal X-ray burst. For > 22 keV flare electrons the total energy loss due to collisions is found to be ∼ 104 times greater than the bremsstrahlung energy loss and ∼ 102 times greater than the energy loss due to escaping electrons. Therefore the escape of electrons into the interplanetary medium is a negligible energetic electron loss mechanism and cannot be a substantial factor in the observed decay of the non-thermal X-ray burst for these solar flares. Title: Observations of Unresolved Galactic X-ray Sources Authors: Hudson, H. S.; Peterson, L. E.; Schwartz, D. A. Bibcode: 1971Natur.230..177H Altcode: WE present in this article new data obtained by the OSO III satellite1 about an apparently diffuse X-ray emission from the galactic plane. Previous observations2 have established the existence of this component at a few keV and at about 100 MeV (ref. 3), but the observations reported here cover the energy range 7.7-115 keV. Theoretical speculation after the first observations has suggested several emission mechanisms-Compton scattering by cosmic ray electrons of far-infrared radiation4-7 or of visible photons8 and the presence of numerous weak discrete sources (Setti, G., and Woltjer, L., personal communication, and Ryter, C., personal communication). Because the new data cover an energy range between the two earlier sets of observations at a few keV and 100 MeV, they impose important new constraints on the theories. Title: The Spectrum of Diffuse Cosmic X-Rays 7.7-113 keV Authors: Schwartz, Daniel A.; Hudson, Hugh S.; Peterson, Laurence E. Bibcode: 1970ApJ...162..431S Altcode: An X-ray telescope aboard the OSO-III satellite provided data from which we derive the intensity of diffuse cosmic X-rays in five energy bands between 77 and 113 keV. A ngle power law cannot represent our data in this energy range The 1 year of satellite operation has allowed a determination of systematic effects, so that the four lowest energy points have an accuracy of about 3 percent They fit a powemlaw spectrum of index 1.7 + 0.1 which "breaks" at 42 I 4 keV to become 3.0 I 0.3. The normalization constant gives an intensity of 15.2 photons (cm2 sec sterad keV)-' at 1 keV. A more complicated spectral shape suggested by Brecher and Morrison fits the present data very well and provides a better extrapolation to data at keV and a few MeV than does the power law. Title: Re-Interpretation of OSO-III Scintillation Counter Measurements of Hard Solar X-Ray Spectra Authors: Kane, S. R.; Hudson, H. S. Bibcode: 1970SoPh...14..414K Altcode: Laboratory tests show that pulse pile-up requires a significant correction to the response of scintillation counters to solar X-ray bursts with steep energy spectra. The interpretation of the OSO-III and other satellite data in view of these corrections indicates that steep solar X-ray spectra with varying emission measure produce an effect similar to that produced by relatively hard spectra with a constant emission measure. Title: The Time Variability of X-Ray Emission from Sco X-1 Authors: Hudson, H. S.; Pelling, R. M.; Peterson, L. E.; Schwartz, D. A. Bibcode: 1970BAAS....2T.200H Altcode: No abstract at ADS Title: Simultaneous X-Ray and Optical Observations of SCO X-1 Flares Authors: Hudson, Hugh S.; Peterson, Laurence E.; Schwartz, Daniel A. Bibcode: 1970ApJ...159L..51H Altcode: The X-ray telescope on the Orbiting Solar Observatory III has made extensive observations of Sco X-1 during 1967 May and June. Two occasions of X-ray flaring, at o7h26 and O9ht(y U.T. on 1967 June 3, happened to coindde with optical observations of the same phenomena, thus proving the connection between optical and X-ray flares on Sco X-1. The X-ray enhancements amounted to about a factor of 2 over the quiescent emission in the energy range 7.7-12.S keV. Title: De-Occultation X-Ray Events of 2 December, 1967 Authors: Zirin, Harold; Ingham, William; Hudson, Hugh; McKenzie, David Bibcode: 1969SoPh....9..269Z Altcode: A flare rising from behind the solar limb was recorded simultaneously by the UCSD X-ray detector on OSO-III (7.7-200 keV) and the Caltech photoheliograph on Robinson Laboratory roof (Hα). The de-occultation gives excellent spatial resolution of the X-ray source. Spectra suggest that the material was already heated to 27 000 000° and that the increase in flux was due to the de-occultation. The flux rise to maximum was proportional to the apparent area. The uniformity of this rise shows that there was no special kernel of emission. Comparison of the deduced volume with the bremsstrahlung formula gives a density of about 1010 for the 27 000 000° component of the flare; this is confirmed by consideration of the maximum possible coulomb braking. The actual decay is more likely by escape rather than coulomb braking. Title: The Hard Solar X-Ray Spectrum Observed from the Third Orbiting Solar Observatory Authors: Hudson, Hugh S.; Peterson, Laurence E.; Schwartz, Daniel A. Bibcode: 1969ApJ...157..389H Altcode: The hard solar X-ray scintillation-counter telescope on the OSO-Ill satellite covers the energy range 7.7-210 keV with 15-sec time resolution, and six logarithmically spaced energy channels. Approximately 55 per cent time coverage has been obtained for the period following the date of launching, March 8, 1967, until failure of the on-board tape recorder June 28, 1968 This paper is based mainly on solar X-ray events observed during the first two weeks of data accumulation. Approximately ten bursts per day were de- tected during the interval March 9-March 23, 1967, above the threshold sensitivity of 10-8 erg (cm2 sec)-1 for 7.7 «= liv «= 12.5 keY. About once per day a burst of peak energy flux greater than 1.6 X 1O~ erg (cm2 sec)' was observed. Although many variations were observed, the typical event had an e-folding rise time of 86 sec and a decay time of 458 sec. The bursts occurred in correlation with almost all listed flares and subflares (88 per cent), microwave bursts (92 per cent), and SID's (100 per cent). Numerous bursts were also detected without these accompanying phenomena. The correlation with type III radio bursts, although still positive, is not as good (31 per cent), a fact which suggests that coronal disturbances are not an inevitable consequence of the process which produces X-ray emission. The X-ray spectrum is appreciably non-thermal in the initial phase of the burst and thermal in the decay phase, with an effective temperature often exceeding 50 X 106 °K. The average peak temperature of subfiares exceeds 10 X 106° K, while that of importance 1 or greater exceeds 14 X 1060 K. The emission measure flefliV has a constant value of about 1.4 X 1O~ cm3 both during an X-ray burst and from burst to burst. The solar X-ray bursts therefore differ mainly in the maximum temperature attained. A less detailed examination of later data shows that the phenomena we describe are not peculiar characteristics of a single active re- gion Title: De-occultation x-Ray Event of 2 December 1967 Authors: Hudson, Hugh; McKenzie, David; Zirin, Harold; Ingham, William Bibcode: 1969BAAS....1T.280H Altcode: No abstract at ADS Title: Simultaneous x-Ray and Electron Emission from the Sun Authors: Hudson, H. S.; Lin, R. P. Bibcode: 1969BAAS....1S.280H Altcode: No abstract at ADS Title: Solar and Cosmic X-Rays above 7.7 keV Authors: Hudson, Hugh S.; Peterson, Laurence E.; Schwartz, Daniel A. Bibcode: 1969SoPh....6..205H Altcode: Since its launch on March 8, 1967, the OSO-III has continuously observed solar and cosmic X-rays over the 7.7-210 keV range. The sun emits many impulsive X-ray bursts having fluxes several orders of magnitude above the background level of 8 × 10−9 ergs(cm2-sec)−1 at 7.7 keV and characteristic times on the order of 5 min. Ninety-five such events having fluxes >3 × 10−5 ergs(cm2-sec)−1 were detected in the period from March 8 to June 15, 1967. The cosmic X-ray source Lupus XR-1 has been observed to have a power law spectral form and no significant time variations over a 40-day period. Upper limits have been obtained on the hard X-ray flux of the peculiar galaxy M 87. Title: The time structure of solar X-ray bursts above 7.7 keV Authors: Hudson, H. S.; Peterson, L. E.; Schwartz, D. A. Bibcode: 1969sfsr.conf..113H Altcode: No abstract at ADS Title: Hard Solar X-Ray Bursts Observed by OSO-III. Authors: Hudson, H. S.; Schwartz, D. A.; Peterson, L. E. Bibcode: 1968AJS....73R..64H Altcode: OSO-III has observed the solar x-ray spectrum above 7.7 keV since 9 March 1967. Solar x-ray bursts are very frequent (98 were observed in the first week of operation of the satellite) and are partially correlated with flares and radio-wave emission. The occurrence of solar x-ray bursts is well correlated with the occurrence of 2800 MHz solar radio bursts, but the detailed time structures of the events are usually different. 65% of the x-ray bursts coincide with flares; on the other hand, about 85% of flares of all importances produce detectable x-ray bursts. The peak x-ray flux for a correlated burst does not depend in a simple way on flare area or brightness. Most solar x-ray bursts exhibit a simple fast-rise, slow-decay time structure; the median rise time is about 65 sec. and the median fall time is about 340 sec. The rise and fall times are widely variable, and the peak energy flux ranges from 10-8 erg (cm2sec)-1 to more than 10-~ erg (cm2 sec)-1 above 7.7 keV. Assuming the X-ray production mechanism to be free-free transitions, the OSO data establish that the onset phase of the x-ray burst is due to a nonthermal electron distribution. The decaying phase of the burst is essentially thermal in nature. These conclusions are drawn from the strong dependence of the x-ray spectrum (or the effective temperature of the emitting region) on the magnitude of the x-ray flux. The thermal bremsstrahlung model allows a determination of neN, where N is the number of electrons radiating and ne the electron density. The distribution of values of this quantity for different bursts is strongly peaked at about 1.4X 10~~ cm-3. It is a striking fact that this value obtains for the entire observed range of sizes of solar x-ray bursts. Title: Observations of the Isotropic Component of Cosmic X Rays at Balloon Altitude Authors: Hudson, H. S.; Prinbsch, J. H.; Anderson, K. A. Bibcode: 1966JGR....71.5665H Altcode: The isotropic component of cosmic X-ray flux between 20 and 70 kev has been determined from measurements made with balloon-borne X-ray telescopes. In this energy range the spectrum is characterized by γ = 2.7+1.7-0.6 in the distribution N(hv) = k(hv). The measured flux at 40 kev was 0.014-0.003+0.008 photons (cm2 sec ster kev)-1. The observations by Ghielmetti, Becerra, Godel, Heredia, and Roederer of an X-ray influx over the South Atlantic geomagnetic anomaly can probably be explained by the cosmic primary X rays. Title: Energy spectrum of auroral zone electron precipitation Authors: Hudson, Hugh S. Bibcode: 1966PhDT.......116H Altcode: No abstract at ADS Title: Determinations of the Auroral-Zone X-Ray Spectrum Authors: Hudson, H. S.; Parks, G. K.; Milton, D. W.; Anderson, K. A. Bibcode: 1965JGR....70.4979H Altcode: A portion of the bremsstrahlung spectrum of electrons precipitating into the auroral-zone atmosphere is accessible to precise spectrum measurements from balloon-borne scintillation detectors. Such measurements were made near Flin Flon, Manitoba, Canada (geomagnetic latitude 64°N), during local daytime on a magnetically disturbed day (September 8, 1964; Ap = 23). The design of the detector embodied several features that made possible a reliable extrapolation from the spectrum observed at balloon altitude to the true spectrum at the stopping level of the electrons: 1. Measurements were made at 3.3 g cm-2 atmospheric depth, whereas the Compton interaction length is greater than 5.8 g cm-2. This served to minimize the contribution of photons that had been Compton-scattered and hence degraded in energy. That scattered photons did not contribute appreciably to the counting rates was directly confirmed by directional measurements made from the same balloon payload [Parks et al., 1965]. Absorption of the bremsstrahlung through the photoelectric process increases steeply toward lower energies; this effectively set the lower limit of the measurements at about 15 kev. Title: Spatial Asymmetry and Periodic Time Variations of X-Ray Microbursts in the Auroral Zone Authors: Parks, G. K.; Hudson, H. S.; Milton, D. W.; Anderson, K. A. Bibcode: 1965JGR....70.4976P Altcode: Describing the properties of short time constant impulsive electron precipitation in the auroral zone, Anderson and Milton [1964] found that the time interval between microbursts was nonrandom although power spectrum analysis revealed only a weak tendency toward periodicity at 0.6 second. It was thought that intrinsically this phenomenon might exhibit stronger periodicities but that they might not have appeared, owing to limitations in the experimental methods. A suspected difficulty was the use of omnidirectional detectors. Thus if several independent source regions were present differing in periodic behavior the peaks in the power spectrum could be washed out.