Author name code: krucker ADS astronomy entries on 2022-09-14 author:"Krucker, Sam" ------------------------------------------------------------------------ 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: Spatial distribution of jets in solar active regions Authors: Odermatt, J.; Barczynski, K.; Harra, L. K.; Schwanitz, C.; Krucker, S. Bibcode: 2022A&A...665A..29O Altcode: 2022arXiv220709923O Context. Solar active regions are known to have jets. These jets are associated with heating and the release of particles into the solar wind.
Aims: Our aim is to understand the spatial distribution of coronal jets within active regions to understand if there is a preferential location for them to occur.
Methods: We analysed five active regions using Solar Dynamics Observatory Atmospheric Imaging Assembly data over a period of 2-3.5 days when the active regions were close to disk centre. Each active region had a different age, magnetic field strength, and topology. We developed a methodology for determining the position and length of the jets.
Results: Jets are observed more frequently at the edges of the active regions and are more densely located around a strong leading sunspot. The number of coronal jets for our active regions is dependent on the age of the active region. The older active regions produce more jets than younger ones. Jets were observed dominantly at the edges of the active regions, and not as frequently in the centre. The number of jets is independent of the average unsigned magnetic field and total flux density in the whole active region. The jets are located around the edges of the strong leading sunspot. Title: The Lyman-$\alpha$ Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter Authors: Li, Ying; Li, Qiao; Song, De-Chao; Battaglia, Andrea Francesco; Xiao, Hualin; Krucker, Säm; Schühle, Udo; Li, Hui; Gan, Weiqun; Ding, M. D. Bibcode: 2022arXiv220806182L Altcode: The hydrogen Lyman-$\alpha$ (H {\sc i} Ly$\alpha$) emission during solar flares has rarely been studied in spatially resolved images and its physical origin has not been fully understood. In this paper, we present novel Ly$\alpha$ images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-waveband and multi-perspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Ly$\alpha$ emission has a good temporal correlation with the thermal emissions at 1--8 Å and 5--7 keV, indicating that the flaring Ly$\alpha$ is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Ly$\alpha$ at flare ribbons during the rise phase of the flare, as revealed by the hard X-ray imaging and spectral fitting. Besides originating from flare ribbons, the Ly$\alpha$ emission can come from flare loops, likely caused by plasma heating and also cooling that happen in different flare phases. It is also found that the Ly$\alpha$ emission shows fairly similar features with the He {\sc ii} 304 Å emission in light curve and spatio-temporal variation along with small differences. These observational results improve our understanding of the Ly$\alpha$ emission in solar flares and also provide some insights for investigating the Ly$\alpha$ emission in stellar flares. Title: Detection of weak ubiquitous impulsive nonthermal emissions from the solar corona Authors: Sharma, Rohit; Oberoi, Divya; Battaglia, Marina; Krucker, Sam Bibcode: 2022arXiv220807147S Altcode: A ubiquitous presence of weak energy releases is one of the most promising hypotheses to explain coronal heating, referred to as the nanoflare hypothesis. The accelerated electrons associated with such weak heating events are also expected to give rise to coherent impulsive emission via plasma instabilities in the meterwave radio band, making this a promising spectral window to look for their presence. Recently \citet{Mondal2020b} reported the presence of weak impulsive emissions from quiet Sun regions which seem to meet the requirements of being radio counterparts of the hypothesized nanoflares. Detection of such low-contrast weak emission from the quiet Sun is challenging and, given their implications, it is important to confirm their presence. In this work, using data from the Murchison Widefield Array, we explore the use of an independent robust approach for their detection by separating the dominant slowly varying component of emission from the weak impulsive one in the visibility domain. We detect milli-SFU level bursts taking place all over the Sun and characterize their brightness temperatures, distributions, morphologies, durations and association with features seen in EUV images. We also attempt to constraint the energies of the nonthermal particles using inputs from the FORWARD coronal model along with some reasonable assumptions and find them to lie in the sub-pico flare ($\sim 10^{19}-10^{21}$ ergs) range. In the process, we also discover perhaps the weakest type III radio burst and another one that shows clear signatures of weakest quasi-periodic pulsations. 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: The Spectrometer Telescope for Imaging X-rays (STIX) on Solar Orbiter Authors: Hayes, Laura A.; Musset, Sophie; üller, Daniel M; Krucker, S äm Bibcode: 2022arXiv220702079H Altcode: The Spectrometer/Telescope for Imaging X-rays (STIX) is one of the 10 instruments on-board the scientific payload of ESA's Solar Orbiter mission. STIX provides hard X-ray imaging spectroscopy in the 4-150~keV energy range, observing hard X-ray bremsstrahlung emission from the Sun. These observations provide diagnostics of the hottest thermal plasmas ($>$10~MK) and information on the non-thermal energetic electrons accelerated above 10~keV during solar flares. STIX has a spectral resolution of 1~keV, and employs the use of in-direct bi-grid Fourier imaging to spatially locate hard X-ray emission. Given that STIX provides critical information about accelerated electrons at the Sun through hard X-ray diagnostics, it is a powerful contribution to the Solar Orbiter suite and has a significant role to explore the dynamics of solar inputs to the heliosphere. This chapter describes the STIX instrument, its design, objectives, first observations and outlines the new perspectives STIX provides over the mission lifetime of Solar Orbiter. Title: A statistical analysis of fast time variations in the hard X-ray emission of solar flares with Solar Orbiter's STIX. Authors: Collier, Hannah; Krucker, Sam; Battaglia, Andrea; Hayes, Laura Bibcode: 2022cosp...44.2567C Altcode: The Spectrometer Telescope for Imaging X-rays (STIX) onboard Solar Orbiter consists of 32 energy channels which detect X-ray emission from solar flares with energies in the range of 4-150 keV. With STIX, it is possible to perform imaging spectroscopy for solar flares with a 1 keV energy resolution at an unprecedented high time cadence (0.5 s), in the hard X-ray range. This work exploits the novel capability of STIX by focusing on the investigation of fast time, oscillatory variations in the hard X-ray emission of flares. Gaining an understanding of the mechanism behind such fast time variations in solar flares is pertinent for revealing the fundamental processes involved in the large energy release of flares and ultimately for the generation of a unified solar flare model. Solar Orbiter will reach its perihelion of ~0.3 AU in March 2022. In this work a statistical overview of fast time variations in the hard X-ray emission from flares is given based on the existing catalogue including tens of M class flares observed by STIX since the launch of Solar Orbiter plus additional observations obtained during perihelion. Furthermore, imaging spectroscopy for a selection of events is presented. Title: Heating during small solar flares with Solar Orbiter STIX, Hinode and SDO. Authors: Harra, Louise K.; Reeves, Kathy; Krucker, Sam; Barczynski, Krzysztof; Battaglia, Andrea; Collier, Hannah Bibcode: 2022cosp...44.2525H Altcode: One of the key aspects of solar flares is how, where and how fast plasma is heated. Spectroscopic observations are constrained by the duration it takes to build up an image and hence the time resolution is often of the order of 5 minutes during a flare. To obtain higher cadence spectroscopic information, it is also possible to use the wide slot (260") - or 'overlappogram' data. An observing campaign (HOP361) was carried out in October using Hinode EIS, XRT and coordinating with Solar Orbiter STIX. The campaign was carried out at high time cadence and was telemetry intensive. SDO is also of key interest to these observations. On the 8th October, a B2 classfication flare took place, and was observed by all instruments. The EIS 'overlappogram' allows high time cadence observations (10 secs) of the whole active region. During this flare, it is possible to extract information on the hot plasma from Fe XXIV, STIX and XRT. There was a smaller flare which wasn't registered by GOES that followed, that provides a contrast example that doesn't show clear hot emission. Title: The relativistic solar particle event on 28 October 2021: Evidence of particle acceleration within and escape from the solar corona Authors: Klein, Karl-Ludwig; Musset, Sophie; Vilmer, Nicole; Briand, Carine; Krucker, Säm; Francesco Battaglia, Andrea; Dresing, Nina; Palmroos, Christian; Gary, Dale E. Bibcode: 2022A&A...663A.173K Altcode:
Aims: We analyse particle, radio, and X-ray observations during the first relativistic proton event of solar cycle 25 detected on Earth. The aim is to gain insight into the relationship between relativistic solar particles detected in space and the processes of acceleration and propagation in solar eruptive events.
Methods: To this end, we used ground-based neutron monitor measurements of relativistic nucleons and space-borne measurements of electrons with similar speed to determine the arrival times of the first particles at 1 AU and to infer their solar release times. We compared the release times with the time histories of non-thermal electrons in the solar atmosphere and their escape to interplanetary space, as traced by radio spectra and X-ray light curves and images.
Results: Non-thermal electrons in the corona are found to be accelerated in different regions. Some are confined in closed magnetic structures expanding during the course of the event. Three episodes of electron escape to the interplanetary space are revealed by groups of decametric-to-kilometric type III bursts. The first group appears on the low-frequency side of a type II burst produced by a coronal shock wave. The two latter groups are accompanied at higher frequencies by bursts with rapid drifts to both lower and higher frequencies (forward- or reverse-drifting bursts). They are produced by electron beams that propagate both sunward and anti-sunward. The first relativistic electrons and nucleons observed near Earth are released with the third group of type III bursts, more than ten minutes after the first signatures of non-thermal electrons and of the formation of the shock wave in the corona. Although the eruptive active region is near the central meridian, several tens of degrees east of the footpoint of the nominal Parker spiral to the Earth, the kilometric spectrum of the type III bursts and the in situ detection of Langmuir waves demonstrate a direct magnetic connection between the L1 Lagrange point and the field lines onto which the electron beams are released at the Sun.
Conclusions: We interpret the forward- and reverse-drifting radio bursts as evidence of reconnection between the closed expanding magnetic structures of an erupting flux rope and ambient open magnetic field lines. We discuss the origin of relativistic particles near the Earth across two scenarios: (1) acceleration at the CME-driven shock as it intercepts interplanetary magnetic field lines rooted in the western solar hemisphere and (2) an alternative where the relativistic particles are initially confined in the erupting magnetic fields and get access to the open field lines to the Earth through these reconnection events.

Movie is available at https://www.aanda.org Title: First Hard X-Ray Imaging Results by Solar Orbiter STIX Authors: Massa, Paolo; Battaglia, Andrea F.; Volpara, Anna; Collier, Hannah; Hurford, Gordon J.; Kuhar, Matej; Perracchione, Emma; Garbarino, Sara; Massone, Anna Maria; Benvenuto, Federico; Schuller, Frederic; Warmuth, Alexander; Dickson, Ewan C. M.; Xiao, Hualin; Maloney, Shane A.; Ryan, Daniel F.; Piana, Michele; Krucker, Säm Bibcode: 2022SoPh..297...93M Altcode: 2022arXiv220209334M The Spectrometer/Telescope for Imaging X-rays (STIX) is one of six remote sensing instruments on-board Solar Orbiter. The telescope applies an indirect imaging technique that uses the measurement of 30 visibilities, i.e., angular Fourier components of the solar flare X-ray source. Hence, the imaging problem for STIX consists of the Fourier inversion of the data measured by the instrument. In this work, we show that the visibility amplitude and phase calibration of 24 out of 30 STIX sub-collimators has reached a satisfactory level for scientific data exploitation and that a set of imaging methods is able to provide the first hard X-ray images of solar flares from Solar Orbiter. Four visibility-based image reconstruction methods and one count-based are applied to calibrated STIX observations of six events with GOES class between C4 and M4 that occurred in May 2021. The resulting reconstructions are compared to those provided by an optimization algorithm used for fitting the amplitudes of STIX visibilities. We show that the five imaging methods produce results morphologically consistent with the ones provided by the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (SDO/AIA) in UV wavelengths. The χ2 values and the parameters of the reconstructed sources are comparable between methods, thus confirming their robustness. Title: On the faintest solar coronal hard X-rays observed with FOXSI Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe, Steven; Krucker, Säm; Vievering, Juliana; Athiray, P. S.; Musset, Sophie; Davis, Lance; Courtade, Sasha; Dalton, Gregory; Turin, Paul; Turin, Zoe; Ramsey, Brian; Bongiorno, Stephen; Ryan, Daniel; Takahashi, Tadayuki; Furukawa, Kento; Watanabe, Shin; Narukage, Noriyuki; Ishikawa, Shin-nosuke; Mitsuishi, Ikuyuki; Hagino, Kouichi; Shourt, Van; Duncan, Jessie; Zhang, Yixian; Bale, Stuart D. Bibcode: 2022arXiv220504291B Altcode: Solar nanoflares are small eruptive events releasing magnetic energy in the quiet corona. If nanoflares follow the same physics as their larger counterparts, they should emit hard X-rays (HXRs) but with a rather faint intensity. A copious and continuous presence of nanoflares would deliver enormous amounts of energy into the solar corona, possibly accounting for its high temperatures. To date, there has not been any direct observation of such sustained and persistent HXRs from the quiescent Sun. However, Hannah et al. in 2010 constrained the quiet Sun HXR emission using almost 12 days of quiescent solar-off-pointing observations by RHESSI. These observations set upper limits at $3.4\times 10^{-2}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$ and $9.5\times 10^{-4}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$ for the 3-6 keV and 6-12 keV energy ranges, respectively. Observing feeble HXRs is challenging because it demands high sensitivity and dynamic range instruments in HXRs. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment excels in these two attributes. Particularly, FOXSI completed its third successful flight (FOXSI-3) on September 7th, 2018. During FOXSI-3's flight, the Sun exhibited a fairly quiet configuration, displaying only one aged non-flaring active region. Using the entire $\sim$6.5 minutes of FOXSI-3 data, we constrained the quiet Sun emission in HXRs. We found $2\sigma$ upper limits in the order of $\sim 10^{-3}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$ for the 5-10 keV energy range. FOXSI-3's upper limit is consistent with what was reported by Hannah et al., 2010, but FOXSI-3 achieved this result using $\sim$1/2640 less time than RHESSI. A possible future spacecraft using FOXSI's concept would allow enough observation time to constrain the current HXR quiet Sun limits further or perhaps even make direct detections. Title: The Coupling of an EUV Coronal Wave and Ion Acceleration in a Fermi-LAT Behind-the-Limb Solar Flare Authors: Pesce-Rollins, Melissa; Omodei, Nicola; Krucker, Säm; Di Lalla, Niccolò; Wang, Wen; Battaglia, Andrea F.; Warmuth, Alexander; Veronig, Astrid M.; Baldini, Luca Bibcode: 2022ApJ...929..172P Altcode: 2022arXiv220504760P We present the Fermi-LAT observations of the behind-the-limb (BTL) flare of 2021 July 17 and the joint detection of this flare by STIX on board the Solar Orbiter. The separation between Earth and the Solar Orbiter was 99.°2 at 05:00 UT, allowing STIX to have a front view of the flare. The location of the flare was S20E140 in Stonyhurst heliographic coordinates, making this the most distant behind-the-limb flare ever detected in >100 MeV gamma-rays. The LAT detection lasted for ~16 minutes, the peak flux was 3.6 ± 0.8 (10-5) ph cm-2 s-1 with a significance >15σ. A coronal wave was observed from both STEREO-A and SDO in extreme ultraviolet (EUV), with an onset on the visible disk in coincidence with the LAT onset. A complex type II radio burst was observed by GLOSS also in coincidence with the onset of the LAT emission, indicating the presence of a shock wave. We discuss the relation between the time derivative of the EUV wave intensity profile at 193 Å as observed by STEREO-A and the LAT flux to show that the appearance of the coronal wave at the visible disk and the acceleration of protons as traced by the observed >100 MeV gamma-ray emission are coupled. We also report how this coupling is present in the data from three other BTL flares detected by Fermi-LAT, suggesting that the protons driving the gamma-ray emission of BTL solar flares and the coronal wave share a common origin. Title: Forward-fitting STIX visibilities Authors: Volpara, Anna; Massa, Paolo; Perracchione, Emma; Battaglia, Andrea Francesco; Garbarino, Sara; Benvenuto, Federico; Massone, Anna Maria; Krucker, Sam; Piana, Michele Bibcode: 2022arXiv220414148V Altcode: Aima. To determine to what extent the problem of forward fitting visibilities measured by the Spectrometer/Telescope Imaging X-rays (STIX) on-board Solar Orbiter is more challenging with respect to the same problem in the case of previous hard X-ray solar imaging missions; to identify an effective optimization scheme for parametric imaging for STIX. Methods. This paper introduces a Particle Swarm Optimization (PSO) algorithm for forward fitting STIX visibilities and compares its effectiveness with respect to the standard simplex-based optimization algorithm used so far for the analysis of visibilities measured by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). This comparison is made by considering experimental visibilities measured by both RHESSI and STIX, and synthetic visibilities generated by accounting for the STIX signal formation model. Results. We found out that the parametric imaging approach based on PSO is as reliable as the one based on the simplex method in the case of RHESSI visibilities. However, PSO is significantly more robust when applied to STIX simulated and experimental visibilities. Conclusions. Standard deterministic optimization is not effective enough for forward-fitting the few visibilities sampled by STIX in the angular frequency plane. Therefore a more sofisticated optimization scheme must be introduced for parametric imaging in the case of the Solar Orbiter X-ray telescope. The forward-fitting routine based on PSO we introduced in this paper proved to be significantly robust and reliable, and could be considered as an effective candidate tool for parametric imaging in the STIX context. Title: Multi-instrument STIX microflare study Authors: Saqri, Jonas; Veronig, Astrid M.; Warmuth, Alexander; Dickson, Ewan C. M.; Battaglia, Andrea Francesco; Podladchikova, Tatiana; Xiao, Hualin; Battaglia, Marina; Hurford, Gordon J.; Krucker, Säm Bibcode: 2022A&A...659A..52S Altcode: 2022arXiv220100712S Context. During its commissioning phase in 2020, the Spectrometer/Telescope for Imaging X-rays (STIX) on board the Solar Orbiter spacecraft observed 69 microflares. The two most significant events from this set (of GOES class B2 and B6) were observed on-disk from the spacecraft as well as from Earth and analysed in terms of the spatial, temporal, and spectral characteristics.
Aims: We complement the observations from the STIX instrument with EUV imagery from SDO/AIA and GOES soft X-ray data by adding imaging and plasma diagnostics over different temperature ranges for a detailed microflare case study that is focussed on energy release and transport.
Methods: We used data from the GOES for SXR plasma diagnostics and SDO/AIA for carrying out high-cadence EUV imaging and reconstruction of differential emission measure (DEM) maps of the thermal flare plasma. The reconstructed DEM profiles were used to study the temporal evolution of thermal flare plasma in the kernels and loops independently. We derived the time evolution of the flare plasma parameters (EM, T) and thermal energy from STIX, GOES, and AIA observations. In particular, we studied the STIX spectra to determine the nonthermal emission from accelerated electrons.
Results: A spectral fitting of the STIX data shows clear nonthermal emission for both microflares studied here. For both events, the plasma temperature and EM derived from STIX and GOES as well as the reconstructed DEM maps differ in absolute values and timing, with AIA (which is sensitive to lower plasma temperatures) lagging behind. The deduced plasma parameters from either method roughly agree with the values in the literature for microflares as do the nonthermal fit parameters from STIX. This finding is corroborated by the Neupert effect exhibited between the time derivative of the GOES SXR emission and the STIX HXR profiles. For the B6 event, for which such an analysis was possible, the non-thermal energy deduced from STIX roughly coincides with the lower estimates of the thermal energy requirement deduced from the SXR and EUV emissions.
Conclusions: The observed Neupert effects and impulsive and gradual phases indicate that both events covered in this study are consistent with the standard chromospheric evaporation flare scenario. For the B6 event on 7 June 2020, this interpretation is further supported by the temporal evolution seen in the DEM profiles of the flare ribbons and loops. For this event, we also find that accelerated electrons can roughly account for the required thermal energy. The 6 June 2020 event demonstrates that STIX can detect nonthermal emission for GOES class B2 events that is nonetheless smaller than the background rate level. We demonstrate for the first time how detailed multi-instrument studies of solar flares can be performed with STIX. Title: Heliospheric Distributed In-Situ Constellation (HelioDISC): A Mission to Untangle the Dynamic Mesoscale Sun-Earth Connection Authors: Allen, R. C.; Leary, M.; Vievering, J.; Vines, S. K.; Jian, L.; Wilson, L.; Li, G.; Lepri, S.; Livi, S.; Raines, J. M.; Anderson, B. J.; Ho, G. C.; Mostafavi, P.; Smith, E.; Wimmer-Schweingruber, R. F.; Krucker, S.; Malaspina, D.; Borovsky, J.; Whittlesey, P.; Maruca, B.; Verscharen, D.; Lugaz, N. Bibcode: 2022heli.conf.4015A Altcode: HelioDISC is a mission concept supported by the NASA Heliophysics Mission Concept Study (HMCS) program to be considered by the National Academy of Sciences and Engineering Heliophysics Decadal Committee as a future Solar Terrestrial Probes mission. Title: Real-Time Solar Flare Predictions for Improved Flare Observations Authors: Vievering, J. T.; Athiray, P. S.; Buitrago-Casas, J. C.; Chamberlin, P.; Glesener, L.; Golub, L.; Knoer, V.; Krucker, S.; Machol, J.; Pantazides, A.; Peck, C.; Reeves, K.; Savage, S.; Schmit, D.; Smith, B.; Vigil, G.; Winebarger, A. Bibcode: 2022heli.conf.4038V Altcode: Improving near-term flare forecasts is important for observatories targeting flare physics that are restricted in FOV and/or observing time. We propose a tool that aggregates near-real-time signatures of flare onset to provide early flare predictions. Title: Solar Flare Observations with STIX in 2021 Authors: Krucker, Sam Bibcode: 2021AGUFMSH21A..04K Altcode: The hard X-ray imaging spectrometer STIX onboard Solar Orbiter has been operating almost continuously in 2021 and has recorded over 1000 solar flares in the first half of 2021 alone. In this talk we will give an overview of initial scientific results with a focus on comparing Solar Orbiters perspective with observations taken from Earth. Of particular interest are joint STIX and FERMI/GBM observations, which provide a first systematic look at hard X-ray directivity measurements from these two vantage points. Title: Evidence for Energetic Electrons Trapped and Accelerated in a Magnetic Bottle above a Solar Flare Arcade Authors: Chen, Bin; Kong, Xiangliang; Shen, Chengcai; Guo, Fan; Yu, Sijie; Glesener, Lindsay; Zhang, Yixian; Li, Xiaocan; Fleishman, Gregory; Krucker, Sam Bibcode: 2021AGUFMSH24B..01C Altcode: Recent studies have shown a local depression of magnetic field strength right above the post-reconnection solar flare arcade. This peculiar structure, referred to as a "magnetic bottle," is a natural consequence of the presence of a large-scale reconnection current sheet that drives the flare energy release. It has been suggested that the magnetic bottle may play a crucial role in confining charged particles and facilitating their repeated acceleration to high energies. However, testing this scenario requires joint observationalmodeling studies in a realistic solar flare context, which has been heretofore elusive. Here we present results by combining microwave, extreme ultraviolet, and X-ray observations of an eruptive solar flare with macroscopic magnetohydrodynamics and particle modeling in a standard flare geometry. We show that the location, morphology, and spectral properties of the observed nonthermal microwave and hard X-ray sources agree well with the synthetic emissions from energetic electrons accelerated and confined in the above-the-flare-arcade magnetic bottle. Such an agreement in the emission signatures supports the magnetic bottle as a candidate for the primary site of particle acceleration in solar flares. Title: Energy Spectrum of Solar Energetic Electron Events Over 25 Years Authors: Wang, Wen; Wang, Linghua; Liu, Zixuan; Krucker, Sam Bibcode: 2021AGUFMSH55F1902W Altcode: We investigate 427 SEEs with good energy spectrum at energies from 4.2keV to 108keV observed by WIND/3DP from 1994 Dec through 2019 Dec. The SEE energy spectrum can be classified into three types: Power-Law (PL), Ellison-Ramaty (ER) and Log-parabola (LP), respectively. Among the 335 PL cases, 253 are downward DPL cases where 2 > 1 with low energy spectral index 1 = 2.1 0.4, high energy spectral index 2 = 4.1 1.0 and a double peak distribution of break energy EB: first peak at 6.3keV and the second peak at 69 keV ; 39 cases are upward DPL where 2 < 1 with 1 = 3.1 0.6, 2 = 2.2 0.3 and average break energy EB = 7.0keV ; 43 cases are SPL where 2 1 with 1 = 2.70.5, 2 = 3.20.8. 33 cases are ER with low energy spectral index ER = 2.00.4 and average cut-off energy Ec = 47 keV; 59 cases are LP with low energy spectral index 1 = 1.7 0.5 and high energy spectral index 2 = 3.8 0.8. By comparing HXR-related events with HXR-not-related events, we find that: no differences for spectral indexes of PL but PL break energy EB and energy flux of HXR-related events are larger than that of HXR-not-related events; No differences for spectral indexes and energy flux of LP. By comparing solar cycle 23 events with solar cycle 24 events, we find that: energy flux of events in solar cycle 23 is larger than that in cycle 24 for PL, ER and LP events; No significant solar cycle differences found in spectral indexes for PL, ER and LP events. Title: Modeling Effects of Charge Sharing on the Response of the FOXSI Sounding Rockets Authors: Duncan, Jessie; Glesener, Lindsay; Athiray, P. S.; Buitrago-Casas, Juan Camilo; Christe, Steven; Krucker, Sam; Musset, Sophie; Nagasawa, Shunsaku; Takahashi, Tadayuki; Vievering, Juliana; Watanabe, Shin Bibcode: 2021AGUFMSH55B1829D Altcode: The FOXSI sounding rocket experiments are the first solar-dedicated direct-focusing hard X-ray (HXR) instruments. FOXSI sounding rockets use Wolter-1 style concentric-shell hard X-ray (HXR) optics and both silicon (Si) and cadmium telluride (CdTe) double-sided strip detectors to observe the Sun. FOXSI images of solar HXR sources are shaped by the point spread function (PSF) of the optics, the 2D segmentation of the detector into square strip intersections (with dimensions 60m^2 (CdTe) or 75m^2 (Si)), and noise in the detector readout. FOXSI images are also affected by charge-sharing in the detector, which occurs when one incident photon causes signals in multiple adjacent strips. Charge sharing is more likely to occur near strip boundaries, making it a sub-strip-position-dependent effect. In this work, a model has been developed combining the FOXSI optical and detector response. Using this model, generated sources can be convolved with the FOXSI system to simulate FOXSI data. Additionally, a corresponding deconvolution process can be used to extract a reconstructed incident source from the simulated data, and the original and reconstructed sources can be compared. The incorporation of charge sharing into the modeled detector response means that the reconstructed source approximates the original to a higher degree of spatial resolution than what would result from using strip-based position knowledge only. This FOXSI response model is useful to aid in interpreting existing FOXSI data, and also to predict possible future results from the upcoming FOXSI-4 flare campaign (March 2024). Additionally, the capacity to model charge sharing in spatially segmented semiconductor detectors has the potential to be useful in maximizing the scientific results from future space-based solar HXR imagers, as well as any other application of such a system where charge sharing occurs. Title: Hard X-ray upper limits of the quiet Sun with new FOXSI observations Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe, Steven; Krucker, Sam; Vievering, Juliana; Athiray, P. S.; Musset, Sophie; Davis, Lance; Courtade, Sasha; Dalton, Gregory; Turin, Paul; Turin, Zoe; Ramsey, Brian; Bongiorno, Stephen; Ryan, Daniel; Takahashi, Tadayuki; Furukawa, Kento; Watanabe, Shin; Narukage, Noriyuki; Ishikawa, Shin-nosuke; Mitsuishi, Ikuyuki; Hagino, Kouichi Bibcode: 2021AGUFMSH51A..04B Altcode: Solar nanoflares are small eruptive events releasing magnetic energy in the quiet corona. If nanoflares follow the same physics as their larger counterparts, they should emit hard X-rays (HXRs) but with a rather faint intensity. A copious and continuous presence of nanoflares would result in a sustained and persistent emission in HXRs, which in turn would deliver enormous amounts of energy into the solar corona, possibly accounting for its high temperatures. To date, there has not been any direct observation of such sustained and persistent HXRs from the quiescent Sun. However, Hannah et al. in 2010 constrained the quiet Sun HXR emission using almost 12 days of quiescent solar-off-pointing observations by RHESSI. These observations set $2\sigma$ upper limits at $3.4\times 10^{-2}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$ and $9.5\times 10^{-4}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$ for the 3-6 keV and 6-12 keV energy ranges, respectively. Observing feeble HXRs is challenging because it demands high sensitivity and dynamic range instruments in the HXR energy band. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment excels in these two attributes when compared with RHESSI. Particularly, FOXSI completed its third successful flight (FOXSI-3) on September 7th, 2018. During FOXSI-3s flight, the Sun exhibited a fairly quiet configuration, displaying only one aged non-flaring active region. Using the entire $\sim$6.5 minutes of FOXSI-3 data, we constrained the quiet Sun emission in HXRs. We found $2\sigma$ upper limits in the order of $\sim 10^{-3}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$ for the 5-10 keV energy range. FOXSI-3's upper limit is consistent with what was reported by Hannah et al., 2010, but FOXSI-3 achieved this result using $\sim$1/2640 less time than RHESSI. A possible future spacecraft using FOXSI's concept would allow enough observation time to constrain the current HXR quiet Sun limits further or perhaps even make direct detections. Title: The solar PolArization and Directivity X-Ray Experiment: PADRE Authors: Martinez Oliveros, Juan Carlos; Christe, Steven; Saint-Hilaire, Pascal; Krucker, Sam; Hayes, Laura; Caspi, Amir; Limousin, Olivier; Peretz, Eliad Bibcode: 2021AGUFMSH51A..03M Altcode: The solar PolArization and Directivity X-Ray Experiment (PADRE) is a 12U Cubesat observatory that will observe the Sun in hard X-ray (HXRs) from low earth orbit. PADRE will investigate the accelerated electron angular distribution in solar flares with two unique and complementary approaches (1) by making spatially-integrated spectro-polarimetric x-ray measurements (~10-100 keV) and (2) by coordinating with Solar Orbiter/STIX to make the first two point measurements of x-rays and determining their directivity. We present the PADRE observatory concept, its science objectives and design. We will discuss the type of observations that will be made by the SHARP and XDSOX instruments. Spectro-polarimetric observations will provide unique information of the electron angular distribution. Furthermore, STIX on board Solar Orbiter will perform X-ray observations of solar flares from 0.28 AU (at perihelion) to 1.2AU (at aphelion) and up to inclinations of 25 degrees at heliospheric angles significantly different from the Earth. This provides a unique opportunity to make stereoscopic X-ray observations and measure the electron anisotropy of individual flares confidently for the first time. PADRE has one science objective: determine the angular distribution of accelerated electrons from standalone and joint SolO/STIX observations. This will allow the determination of the angular distribution of flare-accelerated electrons. Title: The FOXSI-4 Sounding Rocket: High Resolution Focused X-ray Observations of the Sun Authors: Glesener, Lindsay; Buitrago-Casas, Juan Camilo; Vievering, Juliana; Pantazides, Athanasios; Musset, Sophie; Panchapakesan, Subramania Athiray; Baumgartner, Wayne; Bongiorno, Stephen; Champey, Patrick; Christe, Steven; Courtade, Sasha; Duncan, Jessie; Ishikawa, Shin-nosuke; Krucker, Sam; Martinez Oliveros, Juan Carlos; Perez-Piel, Savannah; Mitsuishi, Ikuyuki; Narukage, Noriyuki; Ryan, Daniel; Takahashi, Tadayuki; Watanabe, Shin Bibcode: 2021AGUFMSH55B1831G Altcode: It has been firmly demonstrated that direct-focusing instruments can transform the way high-energy X-rays from astronomical objects, including the Sun, are measured. The NuSTAR spacecraft has increased the sensitivity to faint astrophysical sources by 100 times as compared with previous, indirect, imagers. The first three flights of the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket established the usefulness and feasibility of a similar method optimized for the Sun, and showed that in addition to greater sensitivity, a vastly improved dynamic range can be obtained in this way. This technology stands ready to revolutionize understanding of solar flares by elucidating particle acceleration sites in the corona, studying how electrons propagate and deposit their energies, and how accelerated particles escape into interplanetary space. While the fundamental building blocks of solar hard X-ray (HXR) focusing are in place and ready for a spacecraft mission, concurrent development is required to prepare for the next generation of high-energy solar explorers, which will require higher rate capability and higher angular resolution to investigate finer-scale structure and to better complement instruments at other wavelengths. FOXSI-4 features technological advances that enable high angular resolution as well as measurement of bright sources. In the first category, we will develop high-precision mirror production methods and finely pixelated Si CMOS sensors, and will demonstrate substrip/subpixel resolution in fine-pitch CdTe sensors. Secondly, we will demonstrate rate capability of these sensors sufficient for flare measurement, and will develop novel pixelated attenuators that optimize energy coverage even at high rates. The experiment will demonstrate these technologies in NASAs first-ever solar flare campaign, flying in tandem with the Hi-C FLARE rocket and in a campaign with the SNIFS rocket. The campaign will position multiple rocket experiments awaiting an opportunistic signal and will launch the experiments for near-simultaneous observation of the flare.This campaign will allow for direct collaboration with the Parker Solar Probe (PSP) during one of its perihelia. 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: Solar Energetic Electron Events Associated with Hard X-ray Flares Authors: Wang, Wen; Wang, Linghua; Krucker, Sam; Mason, Glenn; Su, Yang; Bucik, Radoslav Bibcode: 2021AGUFMSH52B..07W Altcode: We investigate 16 solar energetic electron (SEE) events measured by WIND/3DP with a double-power-law spectrum and the associated western hard X-ray (HXR) flares measured by RHESSI with good count statistics, from 2002 February to 2016 December. In all the 16 cases, the presence of an SEE power-law spectrum extending down to 5 keV at 1 AU implies that the SEE source would be high in the corona, at heliocentric distance of >1.3 solar radii, while the footpoint or footpoint-like emissions shown in HXR images suggest that the observed HXRs are likely produced mainly by thick-target bremsstrahlung processes very low in the corona. We find that the power-law spectral index of HXR-producing electrons, estimated under the relativistic thick-target bremsstrahlung model, is significantly larger than (similar to) and positively correlated with the observed high-energy spectral index of SEEs, in 8 cases (8 cases). In addition, the estimated number of SEEs is only 10-4-10-2 of the estimated number of HXR-producing electrons at energies above 30 keV, but with a positive correlation between the two numbers. These results suggest that in these cases, SEEs are likely formed by upward-traveling electrons from an acceleration source high in the corona, while their downward-traveling counterparts may undergo a secondary acceleration before producing HXRs via thick-target bremsstrahlung processes. In addition, the associated 3He=4He ratio is positively correlated with the observed high-energy spectral index of SEEs, indicating a possible relation of the 3He ion acceleration with high-energy SEEs. Title: The Multiview Observatory for Solar Terrestrial Science (MOST) Authors: Gopalswamy, Nat; Kucera, Therese; Leake, James; MacDowall, Robert; Wilson, Lynn; Kanekal, Shrikanth; Shih, Albert; Christe, Steven; Gong, Qian; Viall, Nicholeen; Tadikonda, Sivakumar; Fung, Shing; Yashiro, Seiji; Makela, Pertti; Golub, Leon; DeLuca, Edward; Reeves, Katharine; Seaton, Daniel; Savage, Sabrina; Winebarger, Amy; DeForest, Craig; Desai, Mihir; Bastian, Tim; Lazio, Joseph; Jensen, P. E., C. S. P., Elizabeth; Manchester, Ward; Wood, Brian; Kooi, Jason; Wexler, David; Bale, Stuart; Krucker, Sam; Hurlburt, Neal; DeRosa, Marc; Pevtsov, Alexei; Tripathy, Sushanta; Jain, Kiran; Gosain, Sanjay; Petrie, Gordon; Kholikov, Shukirjon; Zhao, Junwei; Scherrer, Philip; Woods, Thomas; Chamberlin, Philip; Kenny, Megan Bibcode: 2021AGUFMSH12A..07G Altcode: The Multiview Observatory for Solar Terrestrial Science (MOST) is a comprehensive mission concept targeting the magnetic coupling between the solar interior and the heliosphere. The wide-ranging imagery and time series data from MOST will help understand the solar drivers and the heliospheric responses as a system, discerning and tracking 3D magnetic field structures, both transient and quiescent in the inner heliosphere. MOST will have seven remote-sensing and three in-situ instruments: (1) Magnetic and Doppler Imager (MaDI) to investigate surface and subsurface magnetism by exploiting the combination of helioseismic and magnetic-field measurements in the photosphere; (2) Inner Coronal Imager in EUV (ICIE) to study large-scale structures such as active regions, coronal holes and eruptive structures by capturing the magnetic connection between the photosphere and the corona to about 3 solar radii; (3) Hard X-ray Imager (HXI) to image the non-thermal flare structure; (4) White-light Coronagraph (WCOR) to seamlessly study transient and quiescent large-scale coronal structures extending from the ICIE field of view (FOV); (5) Faraday Effect Tracker of Coronal and Heliospheric structures (FETCH), a novel radio package to determine the magnetic field structure and plasma column density, and their evolution within 0.5 au; (6) Heliospheric Imager with Polarization (HIP) to track solar features beyond the WCOR FOV, study their impact on Earth, and provide important context for FETCH; (7) Radio and Plasma Wave instrument (M/WAVES) to study electron beams and shocks propagating into the heliosphere via passive radio emission; (8) Solar High-energy Ion Velocity Analyzer (SHIVA) to determine spectra of electrons, and ions from H to Fe at multiple spatial locations and use energetic particles as tracers of magnetic connectivity; (9) Solar Wind Magnetometer (MAG) to characterize magnetic structures at 1 au; (10) Solar Wind Plasma Instrument (SWPI) to characterize plasma structures at 1 au. MOST will have two large spacecraft with identical payloads deployed at L4 and L5 and two smaller spacecraft ahead of L4 and behind L5 to carry additional FETCH elements. MOST will build upon SOHO and STEREO achievements to expand the multiview observational approach into the first half of the 21st Century. Title: Multiple Electron Acceleration Instances during a Series of Solar Microflares Observed Simultaneously at X-Rays and Microwaves Authors: Battaglia, Marina; Sharma, Rohit; Luo, Yingjie; Chen, Bin; Yu, Sijie; Krucker, Säm Bibcode: 2021ApJ...922..134B Altcode: 2021arXiv210912847B Even small solar flares can display a surprising level of complexity regarding their morphology and temporal evolution. Many of their properties, such as energy release and electron acceleration can be studied using highly complementary observations at X-ray and radio wavelengths. We present X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager and radio observations from the Karl G. Jansky Very Large Array (VLA) of a series of GOES A3.4-B1.6 class flares observed on 2013 April 23. The flares, as seen in X-ray and extreme ultraviolet, originated from multiple locations within active region NOAA 11726. A veritable zoo of different radio emissions between 1 GHz and 2 GHz was observed cotemporally with the X-ray flares. In addition to broadband continuum emission, broadband short-lived bursts and narrowband spikes, indicative of accelerated electrons, were observed. However, these sources were located up to 150″ away from the flaring X-ray sources but only some of these emissions could be explained as signatures of electrons that were accelerated near the main flare site. For other sources, no obvious magnetic connection to the main flare site could be found. These emissions likely originate from secondary acceleration sites triggered by the flare, but may be due to reconnection and acceleration completely unrelated to the cotemporally observed flare. Thanks to the extremely high sensitivity of the VLA, not achieved with current X-ray instrumentation, it is shown that particle acceleration happens frequently and at multiple locations within a flaring active region. Title: Detection of weak solar radio transients using residual radio imaging Authors: Oberoi, Divya; Sharma, Rohit; Battaglia, Marina; Krucker, Sam Bibcode: 2021AGUFMSH52A..02O Altcode: Recently very weak radio bursts, down to mSFU levels, have been detected at low radio frequencies with the Murchison Widefield Array (Mondal et al., 2020, ApJ, 895, L39). This opens the tantalising possibility that these bursts are the radio counterparts of the nanoflares originally hypothesized to explain coronal heating. Here we use an alternative approach to detect and image very weak transient solar radio emissions. We estimate the slowly varying component of the solar emission using a median filter for cross-correlations (visibilities) measured for every baseline individually and subtract it. We then image the residual visibilities obtained thus at time and frequency resolutions of 0.5 s and 2 MHz. About 30 min of data from a quiet sun period in 8 spectral bands spanning the frequency range from 108 to 240 MHz are used. Median subtraction is found to reduce the amplitudes of individual visibilities by 2 to 3 orders of magnitude, enabling us to reliably probe emissions with brightness temperatures down to around 500 K and flux densities in the range of 0.5-10 mSFU (1 SFU = 10,000 Jy). The distribution of the number of reliably detected impulsive features covers the entire solar disc, including the quiet Sun regions. At the highest two frequencies there is a clear clustering of these features with locations of active regions. For the time series of 0.5 s residual maps, we computed the rms for every pixel (see figure). Both these sets of maps show smoothly varying trends across frequency. Above about 145 MHz, the entire solar surface shows very similar values of rms, implying a uniform distribution of weak impulsive features all over the quiet Sun. At the highest frequencies the observed rms is much higher at the locations in the vicinity of the active regions. At frequencies below 145 MHz, a different cluster of high rms value is seen which lies in a large coronal hole region, but does not seem to have an association with any other solar feature. These are among the weakest detections of impulsive nonthermal solar emissions. This paper will present the details of this work and its implications. Title: Imaging from STIX visibility amplitudes Authors: Massa, Paolo; Perracchione, Emma; Garbarino, Sara; Battaglia, Andrea F.; Benvenuto, Federico; Piana, Michele; Hurford, Gordon; Krucker, Säm Bibcode: 2021A&A...656A..25M Altcode: 2021arXiv210804901M
Aims: This study is aimed at providing the first demonstration of STIX Fourier-transform X-ray imaging using semi-calibrated (amplitude-only) visibility data acquired during Solar Orbiter's cruise phase.
Methods: We used a parametric imaging approach whereby STIX visibility amplitudes are fitted by means of two non-linear optimization methods: a fast meta-heuristic technique inspired by social behavior and a Bayesian Monte Carlo sampling method that, although slower, provides a better way of quantifying the uncertainties.
Results: When applied to a set of solar flare visibility amplitudes recorded by STIX on November 18, 2020, the two parametric methods provide very coherent results. The analysis also demonstrates the ability of STIX to reconstruct data at a high time resolution and, from a spectral viewpoint, it shows the reliability of a double-source scenario that is consistent with a thermal versus nonthermal interpretation.
Conclusions: In this preliminary analysis of STIX imaging based only on visibility amplitudes, we formulate the imaging problem as a non-linear parametric issue addressed by means of two high-performance optimization techniques that both show the ability to sample the parametric space in an effective fashion, thus avoiding any local minima. Title: STIX X-ray microflare observations during the Solar Orbiter commissioning phase Authors: Battaglia, Andrea Francesco; Saqri, Jonas; Massa, Paolo; Perracchione, Emma; Dickson, Ewan C. M.; Xiao, Hualin; Veronig, Astrid M.; Warmuth, Alexander; Battaglia, Marina; Hurford, Gordon J.; Meuris, Aline; Limousin, Olivier; Etesi, László; Maloney, Shane A.; Schwartz, Richard A.; Kuhar, Matej; Schuller, Frederic; Senthamizh Pavai, Valliappan; Musset, Sophie; Ryan, Daniel F.; Kleint, Lucia; Piana, Michele; Massone, Anna Maria; Benvenuto, Federico; Sylwester, Janusz; Litwicka, Michalina; Stȩślicki, Marek; Mrozek, Tomasz; Vilmer, Nicole; Fárník, František; Kašparová, Jana; Mann, Gottfried; Gallagher, Peter T.; Dennis, Brian R.; Csillaghy, André; Benz, Arnold O.; Krucker, Säm Bibcode: 2021A&A...656A...4B Altcode: 2021arXiv210610058B Context. The Spectrometer/Telescope for Imaging X-rays (STIX) is the hard X-ray instrument onboard Solar Orbiter designed to observe solar flares over a broad range of flare sizes.
Aims: We report the first STIX observations of solar microflares recorded during the instrument commissioning phase in order to investigate the STIX performance at its detection limit.
Methods: STIX uses hard X-ray imaging spectroscopy in the range between 4-150 keV to diagnose the hottest flare plasma and related nonthermal electrons. This first result paper focuses on the temporal and spectral evolution of STIX microflares occuring in the Active Region (AR) AR12765 in June 2020, and compares the STIX measurements with Earth-orbiting observatories such as the X-ray Sensor of the Geostationary Operational Environmental Satellite (GOES/XRS), the Atmospheric Imaging Assembly of the Solar Dynamics Observatory, and the X-ray Telescope of the Hinode mission.
Results: For the observed microflares of the GOES A and B class, the STIX peak time at lowest energies is located in the impulsive phase of the flares, well before the GOES peak time. Such a behavior can either be explained by the higher sensitivity of STIX to higher temperatures compared to GOES, or due to the existence of a nonthermal component reaching down to low energies. The interpretation is inconclusive due to limited counting statistics for all but the largest flare in our sample. For this largest flare, the low-energy peak time is clearly due to thermal emission, and the nonthermal component seen at higher energies occurs even earlier. This suggests that the classic thermal explanation might also be favored for the majority of the smaller flares. In combination with EUV and soft X-ray observations, STIX corroborates earlier findings that an isothermal assumption is of limited validity. Future diagnostic efforts should focus on multi-wavelength studies to derive differential emission measure distributions over a wide range of temperatures to accurately describe the energetics of solar flares.
Conclusions: Commissioning observations confirm that STIX is working as designed. As a rule of thumb, STIX detects flares as small as the GOES A class. For flares above the GOES B class, detailed spectral and imaging analyses can be performed. Title: The high-energy Sun - probing the origins of particle acceleration on our nearest star Authors: Matthews, S. A.; Reid, H. A. S.; Baker, D.; Bloomfield, D. S.; Browning, P. K.; Calcines, A.; Del Zanna, G.; Erdelyi, R.; Fletcher, L.; Hannah, I. G.; Jeffrey, N.; Klein, L.; Krucker, S.; Kontar, E.; Long, D. M.; MacKinnon, A.; Mann, G.; Mathioudakis, M.; Milligan, R.; Nakariakov, V. M.; Pesce-Rollins, M.; Shih, A. Y.; Smith, D.; Veronig, A.; Vilmer, N. Bibcode: 2021ExA...tmp..135M Altcode: As a frequent and energetic particle accelerator, our Sun provides us with an excellent astrophysical laboratory for understanding the fundamental process of particle acceleration. The exploitation of radiative diagnostics from electrons has shown that acceleration operates on sub-second time scales in a complex magnetic environment, where direct electric fields, wave turbulence, and shock waves all must contribute, although precise details are severely lacking. Ions were assumed to be accelerated in a similar manner to electrons, but γ-ray imaging confirmed that emission sources are spatially separated from X-ray sources, suggesting distinctly different acceleration mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic understanding of accelerated particle spectra and the total energy budgets are therefore poorly constrained. Additionally, the recent detection of relativistic ion signatures lasting many hours, without an electron counterpart, is an enigma. We propose a single platform to directly measure the physical conditions present in the energy release sites and the environment in which the particles propagate and deposit their energy. To address this fundamental issue, we set out a suite of dedicated instruments that will probe both electrons and ions simultaneously to observe; high (seconds) temporal resolution photon spectra (4 keV - 150 MeV) with simultaneous imaging (1 keV - 30 MeV), polarization measurements (5-1000 keV) and high spatial and temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray) regimes. These instruments will observe the broad range of radiative signatures produced in the solar atmosphere by accelerated particles. 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: The Solar Orbiter Radio and Plasma Waves (RPW) instrument (Corrigendum) Authors: Maksimovic, M.; Bale, S. D.; Chust, T.; Khotyaintsev, Y.; Krasnoselskikh, V.; Kretzschmar, M.; Plettemeier, D.; Rucker, H. O.; Souček, J.; Steller, M.; Štverák, Š.; Trávníček, P.; Vaivads, A.; Chaintreuil, S.; Dekkali, M.; Alexandrova, O.; Astier, P. -A.; Barbary, G.; Bérard, D.; Bonnin, X.; Boughedada, K.; Cecconi, B.; Chapron, F.; Chariet, M.; Collin, C.; de Conchy, Y.; Dias, D.; Guéguen, L.; Lamy, L.; Leray, V.; Lion, S.; Malac-Allain, L. R.; Matteini, L.; Nguyen, Q. N.; Pantellini, F.; Parisot, J.; Plasson, P.; Thijs, S.; Vecchio, A.; Fratter, I.; Bellouard, E.; Lorfèvre, E.; Danto, P.; Julien, S.; Guilhem, E.; Fiachetti, C.; Sanisidro, J.; Laffaye, C.; Gonzalez, F.; Pontet, B.; Quéruel, N.; Jannet, G.; Fergeau, P.; Brochot, J. -Y.; Cassam-Chenai, G.; Dudok de Wit, T.; Timofeeva, M.; Vincent, T.; Agrapart, C.; Delory, G. T.; Turin, P.; Jeandet, A.; Leroy, P.; Pellion, J. -C.; Bouzid, V.; Katra, B.; Piberne, R.; Recart, W.; Santolík, O.; Kolmašová, I.; Krupař, V.; Krupařová, O.; Píša, D.; Uhlíř, L.; Lán, R.; Baše, J.; Ahlèn, L.; André, M.; Bylander, L.; Cripps, V.; Cully, C.; Eriksson, A.; Jansson, S. -E.; Johansson, E. P. G.; Karlsson, T.; Puccio, W.; Břínek, J.; Öttacher, H.; Panchenko, M.; Berthomier, M.; Goetz, K.; Hellinger, P.; Horbury, T. S.; Issautier, K.; Kontar, E.; Krucker, S.; Le Contel, O.; Louarn, P.; Martinović, M.; Owen, C. J.; Retino, A.; Rodríguez-Pacheco, J.; Sahraoui, F.; Wimmer-Schweingruber, R. F.; Zaslavsky, A.; Zouganelis, I. Bibcode: 2021A&A...654C...2M Altcode: No abstract at ADS Title: FOXSI-4: the high resolution focusing X-ray rocket payload to observe a solar flare Authors: Buitrago-Casas, Juan Camilo; Vievering, Juliana; Musset, Sophie; Glesener, Lindsay; Athiray, P. S.; Baumgartner, Wayne; Bongiorno, Stephen; Champey, Patrick; Christe, Steven; Courtade, Sasha; Dalton, Gregory; Duncan, Jessie; Gilchrist, Kelsey; Ishikawa, Shin-nosuke; Jhabvala, Christine; Kanniainen, Hunter; Krucker, Säm.; Gregory, Kyle; Martinez Oliveros, Juan Carlos; McCracken, Jeff; Mitsuishi, Ikuyuki; Narukage, Noriyuki; Pantazides, Athanasios; Peretz, Eliad; Perez-Piel, Savannah; Ramanayaka, Aruna; Ramsey, Brian; Ryan, Danny; Savage, Sabrina; Takahashi, Tadayuki; Watanabe, Shin; Winebarger, Amy; Zhang, Yixian Bibcode: 2021SPIE11821E..0LB Altcode: The FOXSI-4 sounding rocket will fly a significantly upgraded instrument in NASA's first solar are campaign. It will deploy direct X-ray focusing optics which have revolutionized our understanding of astrophysical phenomena. For example, they have allowed NuSTAR to provide X-ray imaging and IXPE (scheduled for launch in 2021) to provide X-ray polarization observations with detectors with higher photon rate capability and greater sensitivity than their predecessors. The FOXSI sounding rocket is the first solar dedicated mission using this method and has demonstrated high sensitivity and improved imaging dynamic range with its three successful flights. Although the building blocks are already in place for a FOXSI satellite instrument, further advances are needed to equip the next generation of solar X-ray explorers. FOXSI-4 will develop and implement higher angular resolution optics/detector pairs to investigate fine spatial structures (both bright and faint) in a solar are. FOXSI-4 will use highly polished electroformed Wolter-I mirrors fabricated at the NASA/Marshall Space Flight Center (MSFC), together with finely pixelated Si CMOS sensors and fine-pitch CdTe strip detectors provided by a collaboration with institutes in Japan. FOXSI-4 will also implement a set of novel perforated attenuators that will enable both the low and high energy spectral components to be observed simultaneously in each pixel, even at the high rates expected from a medium (or large) size solar are. The campaign will take place during one of the Parker Solar Probe (PSP) perihelia, allowing coordination between this spacecraft and other instruments which observe the Sun at different wavelengths. Title: ALMA small-scale features in the quiet Sun and active regions Authors: Brajša, R.; Skokić, I.; Sudar, D.; Benz, A. O.; Krucker, S.; Ludwig, H. -G.; Saar, S. H.; Selhorst, C. L. Bibcode: 2021A&A...651A...6B Altcode: 2021arXiv210503644B
Aims: The main aim of the present analysis is to decipher (i) the small-scale bright features in solar images of the quiet Sun and active regions obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and (ii) the ALMA correspondence of various known chromospheric structures visible in the Hα images of the Sun.
Methods: Small-scale ALMA bright features in the quiet Sun region were analyzed using single-dish ALMA observations (1.21 mm, 248 GHz) and in an active region using interferometric ALMA measurements (3 mm, 100 GHz). With the single-dish observations, a full-disk solar image is produced, while interferometric measurements enable the high-resolution reconstruction of part of the solar disk, including the active region. The selected quiet Sun and active regions are compared with the Hα (core and wing sum), EUV, and soft X-ray images and with the magnetograms.
Results: In the quiet Sun region, enhanced emission seen in the ALMA is almost always associated with a strong line-of-sight magnetic field. Four coronal bright points were identified, while other small-scale ALMA bright features are most likely associated with magnetic network elements and plages. In the active region, in 14 small-scale ALMA bright features randomly selected and compared with other images, we found five good candidates for coronal bright points, two for plages, and five for fibrils. Two unclear cases remain: a fibril or a jet, and a coronal bright point or a plage. A comparison of the Hα core image and the 3 mm ALMA image of the analyzed active region showed that the sunspot appears dark in both images (with a local ALMA radiation enhancement in sunspot umbra), the four plage areas are bright in both images and dark small Hα filaments are clearly recognized as dark structures of the same shape also in ALMA. Title: Assessing quiet Sun hard X-rays using observations from the FOXSI Sounding Rockets Authors: Buitrago-Casas, J.; Glesener, L.; Christe, S.; Krucker, S.; Vievering, J.; Athiray, P.; Musset, S.; Ryan, D.; Ishikawa, S.; Narukage, N.; Bongiorno, S.; Furukawa, K.; Ramsey, B.; Courtade, S.; Dalton, G.; Turin, P.; Takahashi, T.; Watanabe, S.; Mitsuishi, I.; Hagino, K.; Duncan, J. Bibcode: 2021AAS...23810604B Altcode: In solar and helio-physics, the coronal heating problem relates to the question of identifying and explaining the mechanism(s) causing the corona's temperatures to be a few hundred times hotter than the solar surface. Among the various plausible hypotheses proposed to explain this problem, one of the strongest candidates relates to copious low energy magnetic reconnections (nanoflares) occurring throughout the solar corona. When examined thoroughly, this mechanism implies heating that happens impulsively on individual flux tubes (strands). Emission of hard X-rays (HXRs) should be a consequence of such non-thermal phenomena, or even of purely thermal transients, if hot enough. In quiescent solar corona areas, nanoflares should manifest in HXRs via very faint signatures covering vast regions. Observing feeble HXRs demands an instrument with high sensitivity and dynamic range for energies between 4 and 15 keV. FOXSI (which stands for the Focusing Optics X-ray Solar Imager) is such an instrument. As a payload of a NASA/LCAS (low-cost access to space program) sounding rocket, FOXSI has successfully completed three launches. The two most recent flights (FOXSI-2 and -3) included quiescent areas of the Sun as part of the targets. For this presentation, we will show a full assessment of the HXR flux from the quiet Sun observed with FOXSI. We begin by presenting a thorough characterization of the stray light (ghost rays) impinging into FOXSI's detectors caused by sources outside of the field of view. We then identify areas free of ghost rays where the instrument sensitivity reaches a maximum to quiet Sun HXR detections. Finally, we implement a Bayesian (known as ON/OFF analysis) to estimate an upper detectability threshold of quiet Sun HXRs and a probability distribution for quiet-Sun HXR fluxes when sources are supposed to exist. Title: Solar Energetic Electron Events Associated with Hard X-Ray Flares Authors: Wang, Wen; Wang, Linghua; Krucker, Säm; Mason, Glenn M.; Su, Yang; Bučík, Radoslav Bibcode: 2021ApJ...913...89W Altcode: We investigate 16 solar energetic electron (SEE) events measured by WIND/3DP with a double-power-law spectrum and the associated western hard X-ray (HXR) flares measured by RHESSI with good count statistics, from 2002 February to 2016 December. In all the 16 cases, the presence of an SEE power-law spectrum extending down to ≤5 keV at 1 au implies that the SEE source would be high in the corona, at a heliocentric distance of ≥1.3 solar radii, while the footpoint or footpoint-like emissions shown in HXR images suggest that the observed HXRs are likely produced mainly by HXR-producing electrons via thick-target bremsstrahlung processes very low in the corona. We find that for all the 16 cases, the estimated power-law spectral index of HXR-producing electrons is no less than the observed high-energy spectral index of SEEs, and it shows a positive correlation with the high-energy spectral index of SEEs. In addition, the estimated number of SEEs is only ∼10-4-10-2 of the estimated number of HXR-producing electrons at energies above 30 keV, but with a positive correlation between the two numbers. These results suggest that in these cases, SEEs are likely formed by upward-traveling electrons from an acceleration source high in the corona, while their downward-traveling counterparts may undergo a secondary acceleration before producing HXRs via thick-target bremsstrahlung processes. In addition, the associated 3He/4He ratio is positively correlated with the observed high-energy spectral index of SEEs, indicating a possible relation of the 3He ion acceleration with high-energy SEEs. Title: The Solar PolArization and Directivity X-Ray Experiment (PADRE) Authors: Martinez Oliveros, J.; Christe, S.; Saint-Hilaire, P.; Krucker, S.; Caspi, A.; Peretz, E.; Hayes, L.; Limousin, O.; Meuris, A. Bibcode: 2021AAS...23831309M Altcode: The solar PolArization and Directivity X-Ray Experiment (PADRE) is a 12U Cubesat observatory that will observe the Sun in hard X-ray (HXRs) from low earth orbit. PADRE will investigate the accelerated electron angular distribution in solar flares with two unique and complementary approaches (1) by making spatially-integrated spectro-polarimetric x-ray measurements (~10-100 keV) and (2) by coordinating with Solar Orbiter/STIX to make the first two point measurements of x-rays and determining their directivity. We present the PADRE observatory concept, its science objectives and design. We will discuss the type of observations that will be made by the SHARP and XDSOX instruments. Spectro-polarimetric observations will provide unique information of the electron angular distribution. Furthermore, STIX on board Solar Orbiter will perform X-ray observations of solar flares from 0.28 AU (at perihelion) to 1.2AU (at aphelion) and up to inclinations of ∼25 degrees at heliospheric angles significantly different from the Earth. This provides a unique opportunity to make stereoscopic X-ray observations and measure the electron anisotropy of individual flares confidently for the first time. PADRE has one science objective: determine the angular distribution of accelerated electrons from standalone and joint SolO/STIX observations. This will allow the determination of the angular distribution of flare-accelerated electrons. Title: High Resolution FOXSI: The Development Of FOXSI-4 Authors: Glesener, L.; Buitrago-Casas, J.; Duncan, J.; Nagasawa, S.; Pantazides, A.; Perez-Piel, S.; Zhang, Y.; Vievering, J.; Musset, S.; Panchapakesan, S.; Baumgartner, W.; Bongiorno, S.; Champey, P.; Christe, S.; Courtade, S.; Kanniainen, H.; Krucker, S.; Ishikawa, S.; Martinez Oliveros, J.; Mitsuishi, I.; Narukage, N.; Peretz, E.; Ryan, D.; Takahashi, T.; Watanabe, S.; Winebarger, A. Bibcode: 2021AAS...23831301G Altcode: It has been firmly demonstrated that direct-focusing instruments can transform the way high-energy X-rays from astronomical objects, including the Sun, are measured. The NuSTAR spacecraft has increased the sensitivity to faint astrophysical sources by 100 times as compared with previous, indirect, imagers. The first three flights of the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket established the usefulness and feasibility of a similar method optimized for the Sun, and showed that in addition to greater sensitivity, a vastly improved dynamic range can be obtained in this way. This technology stands ready to revolutionize understanding of solar flares by elucidating particle acceleration sites in the corona, studying how electrons propagate and deposit their energies, and how accelerated particles escape into interplanetary space. While the fundamental building blocks of solar hard X-ray (HXR) focusing are in place and ready for a spacecraft mission, concurrent development is required to prepare for a subsequent generation of high-energy solar explorers, which will require higher rate capability and higher angular resolution to investigate finer-scale structure and to better complement instruments at other wavelengths.The fourth flight of FOXSI (FOXSI-4) features technological advances that enable high angular resolution as well as measurement of bright sources. In the first category, we are developing high-precision mirror production methods and finely pixelated Si CMOS sensors, and will demonstrate substrip/subpixel resolution in fine-pitch CdTe sensors. Secondly, we will demonstrate a rate capability of these sensors sufficient for flare measurement, and are developing novel pixelated attenuators that optimize energy coverage even at high rates.The experiment will demonstrate these technologies in NASA's first-ever solar flare campaign, flying in tandem with the Hi-C FLARE rocket and on the same campaign as the SNIFS rocket. The campaign will position multiple rocket experiments awaiting an opportunistic signal and will launch multiple payloads for near-simultaneous observation of the flare.This campaign will allow for direct collaboration with the Parker Solar Probe (PSP) during one of its perihelia. Title: FOXSI-2 Solar Microflares. II. Hard X-ray Imaging Spectroscopy and Flare Energetics Authors: Vievering, Juliana T.; Glesener, Lindsay; Athiray, P. S.; Buitrago-Casas, Juan Camilo; Musset, Sophie; Ryan, Daniel F.; Ishikawa, Shin-nosuke; Duncan, Jessie; Christe, Steven; Krucker, Säm Bibcode: 2021ApJ...913...15V Altcode: 2020arXiv201104753V We study the nature of energy release and transfer for two sub-A class solar microflares observed during the second Focusing Optics X-ray Solar Imager (FOXSI-2) sounding rocket flight on 2014 December 11. FOXSI is the first solar-dedicated instrument to utilize focusing optics to image the Sun in the hard X-ray (HXR) regime, sensitive to energies of 4-20 keV. Through spectral analysis of the microflares using an optically thin isothermal plasma model, we find evidence for plasma heated to ∼10 MK and emission measures down to ∼1044 cm-3. Though nonthermal emission was not detected for the FOXSI-2 microflares, a study of the parameter space for possible hidden nonthermal components shows that there could be enough energy in nonthermal electrons to account for the thermal energy in microflare 1, indicating that this flare is plausibly consistent with the standard thick-target model. With a solar-optimized design and improvements in HXR focusing optics, FOXSI-2 offers approximately five times greater sensitivity at 10 keV than the Nuclear Spectroscopic Telescope Array for typical microflare observations and allows for the first direct imaging spectroscopy of solar HXRs with an angular resolution at scales relevant for microflares. Harnessing these improved capabilities to study small-scale events, we find evidence for spatial and temporal complexity during a sub-A class flare. This analysis, combined with contemporaneous observations by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, indicates that these microflares are more similar to large flares in their evolution than to the single burst of energy expected for a nanoflare. Title: Plasma Diagnostics of Microflares observed by STIX and AIA Authors: Saqri, Jonas; Veronig, Astrid; Dickson, Ewan; Krucker, Säm; Battaglia, Andrea Francesco; Battaglia, Marina; Xiao, Hualin; Warmuth, Alexander; the STIX Team Bibcode: 2021EGUGA..23.7966S Altcode: Solar flares are generally thought to be the impulsive release of magnetic energy giving rise to a wide range of solar phenomena that influence the heliosphere and in some cases even conditions of earth. Part of this liberated energy is used for particle acceleration and to heat up the solar plasma. The Spectrometer/Telescope for Imaging X-rays (STIX) instrument onboard the Solar Orbiter mission launched on February 10th 2020 promises advances in the study of solar flares of various sizes. It is capable of measuring X-ray spectra from 4 to 150 keV with 1 keV resolution binned into 32 energy bins before downlinking. With this energy range and sensitivity, STIX is capable of sampling thermal plasma with temperatures of≳10 MK, and to diagnose the nonthermal bremsstrahlung emission of flare-accelerated electrons. During the spacecraft commissioning phase in the first half of the year 2020, STIX observed 68 microflares. Of this set, 26 events could clearly be identified in at least two energy channels, all of which originated in an active region that was also visible from earth. These events provided a great opportunity to combine the STIX observations with the multi-band EUV imagery from the Atmospheric Imaging Assembly (AIA) instrument on board the earth orbiting Solar Dynamics Observatory (SDO). For the microflares that could be identified in two STIX science energy bands, it was possible to derive the temperature and emission measure (EM) of the flaring plasma assuming an isothermal source. For larger events where more detailed spectra could be derived, a more accurate analysis was performed by fitting the spectra assuming various thermal and nonthermal sources. These results are compared to the diagnostics derived from AIA images. To this aim, the Differential EmissionMeasure (DEM) was reconstructed from AIA observations to infer plasma temperatures and EM in the flaring regions. Combined with the the relative timing between the emission seen by STIX and AIA, this allows us to get deeper insight into the flare energy release and transport processes. Title: Early results for the STIX image reconstruction problem: imaging from visibility amplitudes Authors: piana, michele; massa, paolo; perracchione, emma; battaglia, andrea francesco; benvenuto, federico; massone, anna maria; hurford, gordon; krucker, sam Bibcode: 2021EGUGA..23.9422P Altcode: The Spectrometer/Telescope for Imaging X-rays (STIX) is the instrument of the Solar Orbiter mission conceived for the observation of the hard X-ray flaring emission, with the objective of providing insights on the diagnosis of thermal and non-thermal accelerated electrons at the Sun. The STIX imaging system is composed of 30 pairs of tungsten grids, each one placed in front of a four-pixel detector, and produces as many Fourier components of the angular distribution of the flaring source, via Moiré pattern modulation. Therefore, the data recorded by STIX, named visibilities, can be interpreted as a sparse sampling of the Fourier transform of the X-ray signal and the corresponding image reconstruction problem requires the inversion of the Fourier transform from limited data, usually addressed with regularization techniques. Since the current calibration status of STIX measurements still prevents the use of visibility phases for imaging purposes, here we propose a parameter identification process based on forward fitting just the amplitude of the experimental visibilities. Specifically, we have parameterized the flaring source by means of pre-assigned source shapes (e.g., circular and elliptical bi-variate Gaussian functions), and we relied on several approaches to non-linear optimization in order to estimating the shape parameters. In particular, we have implemented a forward-fit method based on deterministic chi-squared minimization, a stochastic optimization algorithm and a deep neural approach based on ensemble learning, also equipping them with an ad hoc statistical technique for uncertainty quantification. The performances of the three approaches are compared in the case of both microflares and M class events recorded by STIX during its commissioning phase and the validation of results is realized also exploiting the EUV information provided by the Atmospheric Imaging Assembly within the Solar Dynamics Observatory. Title: First results of the STIX hard X-ray telescope onboard Solar Orbiter Authors: Battaglia, Andrea Francesco; Saqri, Jonas; Dickson, Ewan; Xiao, Hualin; Veronig, Astrid; Warmuth, Alexander; Battaglia, Marina; Krucker, Säm Bibcode: 2021EGUGA..23.4390B Altcode: With the launch and commissioning of Solar Orbiter, the Spectrometer/Telescope for Imaging X-rays (STIX) is the latest hard X-ray telescope to study solar flares over a large range of flare sizes. STIX uses hard X-ray imaging spectroscopy in the range from 4 to 150 keV to diagnose the hottest temperature of solar flare plasma and the related nonthermal accelerated electrons. The unique orbit away from the Earth-Sun line in combination with the opportunity of joint observations with other Solar Orbiter instruments, STIX will provide new inputs into understanding the magnetic energy release and particle acceleration in solar flares. Commissioning observations showed that STIX is working as designed and therefore we report on the first solar microflare observations recorded on June 2020, when the spacecraft was at 0.52 AU from the Sun. STIX"s measurements are compared with Earth-orbiting observatories, such as GOES and SDO/AIA, for which we investigate and interpret the different temporal evolution. The detected early peak of the STIX profiles relative to GOES is due either by nonthermal X-ray emission of accelerated particles interacting with the dense chromosphere or the higher sensitivity of STIX toward hotter plasma. Title: Solar Energetic Electron Events Associated with Hard X-ray Flares Authors: Wang, Wen; Wang, Linghua; Krucker, Sam; Mason, Glenn M.; Su, Yang; Bucik, Radoslav Bibcode: 2021EGUGA..2310734W Altcode: We investigate 16 solar energetic electron (SEE) events measured by WIND/3DP with a double power-law spectrum and the associated western hard X-ray (HXR) flares measured by RHESSI with good count statistics, from 2002 February to 2016 December. In all 16 cases, the presence of an SEE power-law spectrum extending down to 65 keV at 1 AU implies that the SEE source would be high in the corona, at a heliocentric distance of >1.3 solar radii, while the footpoint or footpoint-like emissions shown in HXR images suggest that the observed HXRs are likely produced mainly by thick target bremsstrahlung processes very low in the corona. We find that in 8 cases (the other 8 cases), the power-law spectral index of HXR-producing electrons, estimated under the relativistic thick-target bremsstrahlung model, is significantly larger than (similar to) the observed high-energy spectral index of SEEs, with a positive correlation. In addition, the estimated number of SEEs is only ∼10-4 - 10-2 of the estimated number of HXRproducing electrons at energies above 30 keV, but also with a positive correlation. These results suggest that in these cases, SEEs are likely formed by upward-traveling electrons from an acceleration source high in the corona, while their downward-traveling counterparts may undergo a secondary acceleration before producing HXRs via thick-target bremsstrahlung processes. In addition, the associated 3He=4He ratio is positively correlated with the observed high-energy spectral index of SEEs, indicating a possible relation of the 3He ion acceleration with high-energy SEEs Title: Radio Spectral Imaging of an M8.4 Eruptive Solar Flare: Possible Evidence of a Termination Shock Authors: Luo, Yingjie; Chen, Bin; Yu, Sijie; Bastian, T. S.; Krucker, Säm Bibcode: 2021ApJ...911....4L Altcode: 2021arXiv210206259L Solar flare termination shocks have been suggested as one of the viable mechanisms for accelerating electrons and ions to high energies. Observational evidence of such shocks, however, remains rare. Using radio dynamic spectroscopic imaging of a long-duration C1.9 flare obtained by the Karl G. Jansky Very Large Array (VLA), Chen et al. suggested that a type of coherent radio bursts, referred to as "stochastic spike bursts," were radio signatures of nonthermal electrons interacting with myriad density fluctuations at the front of a flare termination shock. Here we report another stochastic spike burst event recorded during the extended energy release phase of a long-duration M8.4-class eruptive flare on 2012 March 10. VLA radio spectroscopic imaging of the spikes in 1.0-1.6 GHz shows that, similar to the case of Chen et al., the burst centroids form an extended, ∼10″-long structure in the corona. By combining extreme-ultraviolet imaging observations of the flare from two vantage points with hard X-ray and ultraviolet observations of the flare ribbon brightenings, we reconstruct the flare arcade in three dimensions. The results show that the spike source is located at ∼60 Mm above the flare arcade, where a diffuse supra-arcade fan and multitudes of plasma downflows are present. Although the flare arcade and ribbons seen during the impulsive phase do not allow us to clearly understand how the observed spike source location is connected to the flare geometry, the cooling flare arcade observed 2 hr later suggests that the spikes are located in the above-the-loop-top region, where a termination shock presumably forms. Title: Desaturated EUV Flare Ribbons in an X-class Flare Authors: Krucker, Säm; Torre, Gabriele; Schwartz, Richard A. Bibcode: 2021ApJ...909...43K Altcode: This paper studies the energy dissipation of nonthermal electrons in the chromospheric flare ribbons during the peak time of a Geostationary Operational Environmental Satellite X-class flare (SOL2011-09-06) using desaturated Solar Dynamics Observatory/Atmospheric Imaging Assembly extreme-ultraviolet (EUV) narrow-band images. The temperature distribution in emission measure, called the differential emission measure (DEM), derived from the EUV fluxes from the flare ribbons shows an increase in the emission measure up to a temperature around 9 × 106 K, followed by a steep decline at higher temperatures. In contrast, the flare loop reaches temperatures up to 27 × 106 K. This result is in agreement with previously reported single-temperature measurements using soft X-ray filter images, as well as DEM distributions reported for smaller flares obtained from EUV line observations. The main difference between small and large flares appears to be an increased emission measure in the flare ribbons, while the ribbon peak temperature is similar for all flares. This is different from the flare loop temperatures, where the hottest temperatures occur in the largest flares. However, the physically relevant quantity for energy dissipation, the energy content of the heated plasma as a function of temperature, does not need to peak at the same temperature as the DEM. The poorly constrained source thickness in radial extent of the flare ribbons has a significant impact on the shape of the differential thermal energy distribution. In particular, if the highest temperatures occur over a wide radial extent as "evaporating" plasma starts expending, the largest amount of energy could potentially be hidden above the peak temperature of the DEM. Title: Energetic Electron Distribution of the Coronal Acceleration Region: First Results from Joint Microwave and Hard X-Ray Imaging Spectroscopy Authors: Chen, Bin; Battaglia, Marina; Krucker, Säm; Reeves, Katharine K.; Glesener, Lindsay Bibcode: 2021ApJ...908L..55C Altcode: 2021arXiv210205173C Nonthermal sources located above bright flare arcades, referred to as the "above-the-loop-top" sources, have been often suggested as the primary electron acceleration site in major solar flares. The X8.2 limb flare on 2017 September 10 features such an above-the-loop-top source, which was observed in both microwaves and hard X-rays (HXRs) by the Expanded Owens Valley Solar Array and the Reuven Ramaty High Energy Solar Spectroscopic Imager, respectively. By combining the microwave and HXR imaging spectroscopy observations with multifilter extreme ultraviolet and soft X-ray imaging data, we derive the coronal magnetic field and energetic electron distribution of the source over a broad energy range from <10 keV up to ∼MeV during the early impulsive phase of the flare. The source has a strong magnetic field of over 800 G. The best-fit electron distribution consists of a thermal "core" from ∼25 MK plasma. A nonthermal power-law "tail" joins the thermal core at ∼16 keV with a spectral index of ∼3.6, which breaks down at above ∼160 keV to >6.0. Temporally resolved analysis suggests that the electron distribution above the break energy rapidly hardens with the spectral index decreasing from >20 to ∼6.0 within 20 s, or less than ∼10 Alfvén crossing times in the source. These results provide strong support for the above-the-loop-top source as the primary site where an ongoing bulk acceleration of energetic electrons is taking place very early in the flare energy release. 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: New Methods for Solar Hard X-ray Imaging Analysis with the FOXSI Sounding Rocket Experiment Authors: Vievering, J. T.; Glesener, L.; Athiray, P. S.; Buitrago-Casas, J. C.; Musset, S.; Ryan, D.; Ishikawa, S. N.; Watanabe, S.; Takahashi, T.; Duncan, J. M.; Christe, S.; Krucker, S. Bibcode: 2020AGUFMSH0480010V Altcode: Spectroscopic imaging in the hard X-ray (HXR) regime is essential for investigating the sites of energy release and transfer in solar flares, as HXRs provide insight into the acceleration of electrons and plasma heating. The Focusing Optics X-ray Solar Imager ( FOXSI ) sounding rocket experiment is the first solar-dedicated direct imager for HXRs and offers unprecedented capabilities for high-energy imaging spectroscopy of flares. Through use of grazing incidence optics, FOXSI achieves greater sensitivity and imaging dynamic range than its esteemed predecessor, the Reuven Ramaty High-Energy Solar Spectroscopic Imager ( RHESSI ), which was limited by the high background inherent to its indirect imaging technique. Additionally, FOXSI improves upon the Nuclear Spectroscopic Telescope Array ( NuSTAR ) with an angular resolution at scales relevant for studying flares and a higher detector throughput for measuring solar sources. Here we demonstrate the power of these enhanced capabilities through imaging analyses of two sub-A class microflares observed by the second FOXSI sounding rocket ( FOXSI-2 ) on December 11, 2014. We apply a custom FOXSI image deconvolution method using a maximum likelihood procedure to explore the changing morphology of these small scale-events and compare the results to features observed in contemporaneous data from the Solar Dynamics Observatory Atmospheric Imaging Assembly ( SDO /AIA). Additionally, we perform imaging spectroscopy to study the spatial complexity of energy release in these microflares and assess whether they are more similar in evolution to large flares or nanoflares. 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: Updates on the Fundamentals of Impulsive Energy Release in the Corona Explorer (FIERCE) mission concept Authors: Shih, A. Y.; Glesener, L.; Krucker, S.; Guidoni, S. E.; Christe, S.; Reeves, K.; Gburek, S.; Caspi, A.; Alaoui, M.; Allred, J. C.; Battaglia, M.; Baumgartner, W.; Dennis, B. R.; Drake, J. F.; Goetz, K.; Golub, L.; Hannah, I. G.; Hayes, L.; Holman, G.; Inglis, A.; Ireland, J.; Kerr, G. S.; Klimchuk, J. A.; McKenzie, D. E.; Moore, C. S.; Musset, S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Schwartz, R.; Seaton, D. B.; Steslicki, M.; Woods, T. N. Bibcode: 2020AGUFMSH0480012S Altcode: The Fundamentals of Impulsive Energy Release in the Corona Explorer ( FIERCE ) Medium-Class Explorer (MIDEX) mission concept addresses the following science questions:

What are the physical origins of space-weather events?

How are particles accelerated at the Sun?

How is impulsively released energy transported throughout the solar atmosphere?

How is the solar corona heated?

FIERCE achieves its science objectives through co-optimized X-ray and extreme ultraviolet (EUV) observations by the following instruments:

FOXSI, a focusing hard X-ray spectroscopic imager that is able to capture the full range of emission in flares and CMEs (e.g., faint coronal sources near bright chromospheric sources)

THADIS, a high-resolution, fast-cadence EUV imager that will not saturate for even intense flares to follow dynamic changes in the configuration of plasma structures

STC, a soft X-ray spectrometer that provides detailed thermal and elemental composition diagnostics

If selected, FIERCE will launch in 2025, near the peak of the next solar cycle, which is also well timed with perihelia of Parker Solar Probe and Solar Orbiter . We describe the status and latest updates of the mission concept since it was proposed to NASA last year. We also highlight the anticipated science return from co-observations with other observatories/instruments such as the Expanded Owens Valley Solar Array (EOVSA) or the STIX instrument on Solar Orbiter . Title: The Micro Solar Flare Apparatus (MiSolFA) Authors: Hayes, L. A.; Christe, S.; Ryan, D.; Krucker, S.; Martinez Oliveros, J. C.; Kontar, E.; Jeffrey, N.; Caspi, A.; Saint-Hilaire, P.; Limousin, O.; Meuris, A.; Battaglia, M.; Casadei, D. Bibcode: 2020AGUFMSH056..08H Altcode: During a solar flare the rapid release of magnetic energy drives extremely efficient particle acceleration through a mechanism which remains largely unknown. Hard X-ray observations are one of the most direct signatures of flare accelerated energetic electrons at and near the acceleration site, and X-ray spectra can provide key diagnostics to the physical processes occurring in flares. In particular, a measure of the electron angular distribution (the hard X-ray directivity) is a prime diagnostic of the unknown acceleration mechanism. However, to-date, observational constraints of directivity have not been clear. Looking towards the next solar cycle, stereoscopic X-ray observations of solar flares will make significant advances at measuring the hard X-ray directivity. The Micro Solar Flare Apparatus (MiSolFA) is a CubeSat Observatory that will observe solar flares at the same time as Solar Orbiter/STIX using cross-calibrated flight-spare detectors. During the rise of the next solar maximum, STIX on board Solar Orbiter will perform X-ray observations of solar flares from 0.28 AU (at perihelion) and up to inclinations of ∼25 degrees at heliospheric angles significantly different than the Earth. These two instruments working together will be the first pair of cross-calibrated X-ray spectrometers to observe solar flares from very different points of view allowing us to measure the anisotropy of the flare hard x-rays and finally confidently constrain the flare-accelerated electron directivity in individual flares for the first time. Title: The Above-the-looptop Source of the 2017 September 10 Solar Flare: Energetic Electron Distribution over a Broad Energy Range Authors: Chen, B.; Battaglia, M.; Krucker, S.; Reeves, K.; Glesener, L. Bibcode: 2020AGUFMSH0430001C Altcode: No abstract at ADS Title: Pan-Spectrum Fitting Formula for Suprathermal Particles Authors: Liu, Zixuan; Wang, Linghua; Wimmer-Schweingruber, Robert F.; Krucker, Säm.; Mason, Glenn M. Bibcode: 2020JGRA..12528702L Altcode: We propose a pan-spectrum fitting formula of suprathermal particles, J=A×E-β1[1 + (E/E0)α]β1-β2/α, where J is the particle flux (or intensity), E is the particle energy, A is the amplitude coefficient, E0 represents the spectral transition energy, α (>0) describes the sharpness and width of spectral transition around E0, and the power-law index β1 (β2) gives the spectral shape before (after) the transition. This formula incorporates many commonly used spectrum functions as special cases. When α goes to infinity (zero), this spectral formulabecomes the classical double-power-law (logarithmic-parabola) function. When both β2 and E0 approach infinity and α is equal to 1, this formula can be simplified to the Ellison-Ramaty function. Under some other specific parameter conditions, this formula can be transformed to the Kappa or Maxwellian distribution. Considering the uncertainties in both particle intensity and energy, we improve the fitting method and fit this pan-spectrum formula well to the representative energy spectra of various suprathermal particle phenomena including SEPs (electrons, protons, 3He, and heavier ions), ESPs, bow-shocked electrons, solar wind suprathermal electrons, anomalous cosmic rays, and hard X-rays. Therefore, this pan-spectrum fitting formula would help us comparatively examine the properties of energy spectrum of different suprathermal particle phenomena typically with a single energy break. Title: The 2017 September 10 loop-prominence system: A polarization and density analysis Authors: Martinez Oliveros, J. C.; Guevara Gómez, J. C.; Saint-Hilaire, P.; Krucker, S. Bibcode: 2020AGUFMSH046..05M Altcode: Classical loop-prominence systems associated with powerful solar flares have been observed since the 1930's. Since their initial observations, loop prominence systems (LPS) were a topic of particular interest for solar physicists as they seem to have a clear relationship with solar flares. The time evolution of loop prominence systems is related to the thermodynamic and radiative properties of the chromospheric plasma. From observations it is clear that these systems do not grow from lower to higher altitudes, instead their generation is the product of higher loops getting more intense, while the lower decay. Here, 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 interpret this as a classical loop-prominence system, but find it to be 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 at altitudes, where the polarization levels detected match well those expected from pure Thomson scattering by the coronal material. 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: GOES-class Estimation for Behind-the-limb Solar Flares Using MESSENGER SAX Authors: Lastufka, Erica; Krucker, Säm Bibcode: 2020ApJ...905..161L Altcode: 2020arXiv201210221L Mercury mission MESSENGER's Solar Assembly for X-rays (SAX) observed almost 700 solar flares between 2007 May 28 and 2013 August 19, as cataloged by Dennis Brian et al. The SAX instrument, part of the X-ray Spectrometer, operated at 1-10 keV, partially overlapping the energy range of the GOES X-ray spectrometers. SAX provides viewing angles different from the Earth-Sun line and can therefore be used as a GOES proxy for partially or fully occulted flares as seen from Earth. For flares with GOES classes above C2 seen on-disk for both instruments, we found an empirical relationship between the soft X-ray (SXR) fluxes measured by both SAX and GOES. Due to the different energy response of the two SXR instruments, individual events can deviate on average by about a factor of 2 from the empirical relationship, implying that predictions of the GOES class of occulted flares from SAX data are therefore accurate to within the same factor. The distinctive GOES energy response in combination with the multithermal nature of flares makes it difficult for any instrument, even other soft X-ray spectrometers, to provide a GOES proxy more accurate than a factor of 2. Title: X-ray Microflare Observations by Solar Orbiter STIX Authors: Krucker, S. Bibcode: 2020AGUFMSH038..06K Altcode: The Spectrometer Telescope for Imaging X-rays (STIX) on Solar Orbiter is a hard X-ray imaging spectrometer covering the energy range from 4 to 150 keV. Through the bremsstrahlung process, STIX provides diagnostics of the hottest (above ~8 MK) flare plasmas while simultaneously quantifying the location, spectrum, and energy content of flare-accelerated non-thermal electrons. This presentation will highlight scientific results obtained during the STIX commissioning phase in May and June 2020, when STIX observed a total of 63 solar microflares at GOES class level A and B. We will present statistical results of our initial set of observations and compare them to previously obtained X-ray microflare statistics by RHESSI and NuSTAR. Title: Solar Energetic Electron Events Associated with Hard X-ray Flares Authors: Wang, W.; Wang, L.; Krucker, S.; Mason, G. M.; Bucik, R.; Tian, H.; He, J.; Su, Y.; Bale, S. Bibcode: 2020AGUFMSH045..05W Altcode: We presented a comparison survey of 16 solar energetic electron (SEE) events measured by WIND/3DP with associated hard X-ray (HXR) flares measured by RHESSI with good count statistics, from 2002 February to 2016 December. Among the 16 good cases, all show a double-power-law shape with a downward break at ~ 68keV in the electron peak flux versus energy spectrum at ~ 5-200keV; All cases show a clear single-power-law in the HXR peak flux versus energy spectrum, and a single-power-law in the flux versus energy spectrum of HXR-producing electrons derived via bremsstrahlung mechanisms. For SEEs, their acceleration site at the Sun should occur high (at a heliocentric altitude of ≧1.3 Rs (the average solar radius)), to remain a power-law spectrum extending down to ~ 5 keV during their anti-sunward escape into the interplanetary medium (IPM). For 14 out of 16 cases, the spectral index above the energy break of SEEs observed at 1AU basically agreed with the derived spectral index of HXR-producing electrons. However, the SEE spectral index below the energy break is all smaller than the HXR-producing electron spectral index. These results suggest that SEEs and HXR-producing electrons share a common origin and an additional acceleration could exist between the SEE source and the flare region. In addition, for cases with 3He/4He>0.01, the observed 3He/4He shows a correlation with the spectral index above the energy break of SEEs, indicating a possible relation of the 3He-rich ion acceleration at high corona with SEEs. Title: The FOXSI-4 Sounding Rocket: High Resolution Focused X-ray Observations of the Sun Authors: Glesener, L.; Buitrago-Casas, J. C.; Musset, S.; Vievering, J. T.; Athiray, P. S.; Baumgartner, W.; Bongiorno, S.; Champey, P. R.; Christe, S.; Courtade, S.; Duncan, J. M.; Ishikawa, S. N.; Krucker, S.; Martinez Oliveros, J. C.; Mitsuishi, I.; Narukage, N.; Ryan, D.; Takahashi, T.; Watanabe, S.; Winebarger, A. R. Bibcode: 2020AGUFMSH0480011G Altcode: It has been firmly demonstrated that direct-focusing instruments can transform the way high-energy X-rays from astronomical objects, including the Sun, are measured. The NuSTAR spacecraft has increased the sensitivity to faint astrophysical sources by 100 times as compared with previous, indirect, imagers. The first three flights of the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket established the usefulness and feasibility of a similar method optimized for the Sun, and showed that in addition to greater sensitivity, a vastly improved dynamic range can be obtained in this way. This technology stands ready to revolutionize understanding of solar flares by elucidating particle acceleration sites in the corona, studying how electrons propagate and deposit their energies, and how accelerated particles escape into interplanetary space. While the fundamental building blocks of solar hard X-ray (HXR) focusing are in place and ready for a spacecraft mission, concurrent development is required to prepare for the next generation of high-energy solar explorers, which will require higher rate capability and higher angular resolution to investigate finer-scale structure and to better complement instruments at other wavelengths.

FOXSI-4 features technological advances that enable high angular resolution as well as measurement of bright sources. In the first category, we will develop high-precision mirror production methods and finely pixelated Si CMOS sensors, and will demonstrate substrip/subpixel resolution in fine-pitch CdTe sensors. Secondly, we will demonstrate rate capability of these sensors sufficient for flare measurement, and will develop novel pixelated attenuators that optimize energy coverage even at high rates.

The experiment will demonstrate these technologies in NASA's first-ever solar flare campaign, flying in tandem with the Hi-C FLARE rocket. The campaign will position multiple rocket experiments awaiting an opportunistic signal and will launch the experiments for near-simultaneous observation of the flare.This campaign will allow for direct collaboration with the Parker Solar Probe (PSP) during one of its perihelia. Title: Limits on the X-ray emission of the quiet Sun from the FOXSI sounding rockets Authors: Buitrago-Casas, J. C.; Glesener, L.; Christe, S.; Krucker, S.; Vievering, J. T.; Athiray, P. S.; Musset, S.; Ryan, D.; Ishikawa, S. N.; Narukage, N.; Bongiorno, S.; Furukawa, K.; Ramsey, B.; Davis, L.; Courtade, S.; Dalton, G.; Turin, P.; Turin, Z.; Takahashi, T.; Watanabe, S.; Mitsuishi, I.; Hagino, K.; Duncan, J. M. Bibcode: 2020AGUFMSH0430002B Altcode: Nanoflares are a potential solution for the coronal heating problem. In the quiet Sun, nanoflares should exhibit hard X-ray (HXR) signatures manifested to an observer via either i) a single HXR kernel or ii) HXRs from many flares spread all over the Sun. In both cases, detecting nanoflare HXRs requires an instrument with superior dynamic range and sensitivity in the ~4 to 15 keV energy range over previous solar dedicated HXR telescopes, like RHESSI. The Focusing Optics X-ray Solar Imager (FOXSI) fulfills these requirements. FOXSI has successfully flown on three sounding rocket campaigns; the last two (FOXSI-2 and -3) included the observation of quiet areas of the solar disk. For FOXSI-3, several techniques were tested and developed to minimize a background effect unique to hard X-ray focusing optics, frequently referred to as ghost rays.

In this presentation, we provide, for the first time, a FOXSI sounding rocket assessment of the HXR flux from the quiet Sun. To fully characterize the sensitivity of FOXSI, we assessed ghost rays generated by sources outside of the field of view via a ray-tracing algorithm. This is particularly important for observations from FOXSI-2 when techniques to minimize ghost rays were not implemented yet. Using the ray tracing tool, we identify areas free of ghost rays that contribute to the background and therefore have maximum sensitivity to quiet Sun HXR emission. We use a bayesian approach to provide upper thresholds of quiet Sun HXR emission and probability distributions for the expected flux when a quiet-Sun HXR source is assumed to exist. We compare this result with prior constraints such as that made by RHESSI (Hannah et al. 2010). Title: Solar Orbiter: connecting remote sensing and in situ measurements Authors: Horbury, T. S.; Auchere, F.; Antonucci, E.; Berghmans, D.; Bruno, R.; Carlsson, M.; del Toro Iniesta, J. C.; Fludra, A.; Harra, L.; Hassler, D.; Heinzel, P.; Howard, R. A.; Krucker, S.; Livi, S. A.; Long, D.; Louarn, P.; Maksimovic, M.; Mueller, D.; Owen, C. J.; Peter, H.; Rochus, P. L.; Rodriguez-Pacheco, J.; Romoli, M.; Schühle, U.; Solanki, S. K.; Teriaca, L.; Wimmer-Schweingruber, R. F.; Zouganelis, Y.; Laker, R. Bibcode: 2020AGUFMSH038..10H Altcode: A key science goal of the Solar Orbiter mission is to make connections between phenomena on the Sun and their manifestations in interplanetary space. To that end, the spacecraft carries a carefully tailored payload of six remote sensing instruments and four making in situ measurements. During June 2020, while the spacecraft was around 0.5 AU from the Sun, the remote sensing instruments operated for several days. While this was primarily an engineering activity, the resulting observations provided outstanding measurements and represent the ideal first opportunity to investigate the potential for making connections between the remote sensing and in situ payloads on Solar Orbiter.

We present a preliminary analysis of the available remote sensing and in situ observations, showing how connections can be made, and discuss the potential for further, more precise mapping to be performed as the mission progresses. Title: Use of a ray-tracing simulation to characterize ghost rays in the FOXSI rocket experiment Authors: Buitrago-Casas, J. C.; Christe, S.; Glesener, L.; Krucker, S.; Ramsey, B.; Bongiorno, S.; Kilaru, K.; Athiray, P. S.; Narukage, N.; Ishikawa, S.; Dalton, G.; Courtade, S.; Musset, S.; Vievering, J.; Ryan, D.; Bale, S. Bibcode: 2020JInst..15P1032B Altcode: 2020arXiv201007049B Imaging X-rays by direct focusing offers greater sensitivity and a higher dynamic range compared to techniques based on indirect imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload that uses seven sets of nested Wolter-I figured mirrors to observe the Sun in hard X-rays through direct focusing. Characterizing the performance of these optics is critical to optimize their performance and to understand their resulting data. In this paper, we present a ray-tracing simulation we created and developed to study Wolter-I X-ray mirrors. We validated the accuracy of the ray-tracing simulation by modeling the FOXSI rocket optics. We found satisfactory agreements between the simulation predictions and laboratory data measured on the optics. We used the ray-tracing simulation to characterize a background pattern of singly reflected rays (i.e., ghost rays) generated by photons at certain incident angles reflecting on only one of a two-segment Wolter-I figure and still reaching the focal plane. We used the results of the ray-tracing simulation to understand, and to formulate a set of strategies that can be used to mitigate, the impact of ghost rays on the FOXSI optical modules. These strategies include the optimization of aperture plates placed at the entrance and exit of the smallest Wolter-I mirror used in FOXSI, a honeycomb type collimator, and a wedge absorber placed at the telescope aperture. The ray-tracing simulation proved to be a reliable set of tools to study Wolter-I X-ray optics. It can be used in many applications, including astrophysics, material sciences, and medical imaging. Title: Coordination within the remote sensing payload on the Solar Orbiter mission Authors: Auchère, F.; Andretta, V.; Antonucci, E.; Bach, N.; Battaglia, M.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Caminade, S.; Carlsson, M.; Carlyle, J.; Cerullo, J. J.; Chamberlin, P. C.; Colaninno, R. C.; Davila, J. M.; De Groof, A.; Etesi, L.; Fahmy, S.; Fineschi, S.; Fludra, A.; Gilbert, H. R.; Giunta, A.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.; Howard, R. A.; Hurford, G.; Kleint, L.; Kolleck, M.; Krucker, S.; Lagg, A.; Landini, F.; Long, D. M.; Lefort, J.; Lodiot, S.; Mampaey, B.; Maloney, S.; Marliani, F.; Martinez-Pillet, V.; McMullin, D. R.; Müller, D.; Nicolini, G.; Orozco Suarez, D.; Pacros, A.; Pancrazzi, M.; Parenti, S.; Peter, H.; Philippon, A.; Plunkett, S.; Rich, N.; Rochus, P.; Rouillard, A.; Romoli, M.; Sanchez, L.; Schühle, U.; Sidher, S.; Solanki, S. K.; Spadaro, D.; St Cyr, O. C.; Straus, T.; Tanco, I.; Teriaca, L.; Thompson, W. T.; del Toro Iniesta, J. C.; Verbeeck, C.; Vourlidas, A.; Watson, C.; Wiegelmann, T.; Williams, D.; Woch, J.; Zhukov, A. N.; Zouganelis, I. Bibcode: 2020A&A...642A...6A Altcode: Context. To meet the scientific objectives of the mission, the Solar Orbiter spacecraft carries a suite of in-situ (IS) and remote sensing (RS) instruments designed for joint operations with inter-instrument communication capabilities. Indeed, previous missions have shown that the Sun (imaged by the RS instruments) and the heliosphere (mainly sampled by the IS instruments) should be considered as an integrated system rather than separate entities. Many of the advances expected from Solar Orbiter rely on this synergistic approach between IS and RS measurements.
Aims: Many aspects of hardware development, integration, testing, and operations are common to two or more RS instruments. In this paper, we describe the coordination effort initiated from the early mission phases by the Remote Sensing Working Group. We review the scientific goals and challenges, and give an overview of the technical solutions devised to successfully operate these instruments together.
Methods: A major constraint for the RS instruments is the limited telemetry (TM) bandwidth of the Solar Orbiter deep-space mission compared to missions in Earth orbit. Hence, many of the strategies developed to maximise the scientific return from these instruments revolve around the optimisation of TM usage, relying for example on onboard autonomy for data processing, compression, and selection for downlink. The planning process itself has been optimised to alleviate the dynamic nature of the targets, and an inter-instrument communication scheme has been implemented which can be used to autonomously alter the observing modes. We also outline the plans for in-flight cross-calibration, which will be essential to the joint data reduction and analysis.
Results: The RS instrument package on Solar Orbiter will carry out comprehensive measurements from the solar interior to the inner heliosphere. Thanks to the close coordination between the instrument teams and the European Space Agency, several challenges specific to the RS suite were identified and addressed in a timely manner. Title: Models and data analysis tools for the Solar Orbiter mission Authors: Rouillard, A. P.; Pinto, R. F.; Vourlidas, A.; De Groof, A.; Thompson, W. T.; Bemporad, A.; Dolei, S.; Indurain, M.; Buchlin, E.; Sasso, C.; Spadaro, D.; Dalmasse, K.; Hirzberger, J.; Zouganelis, I.; Strugarek, A.; Brun, A. S.; Alexandre, M.; Berghmans, D.; Raouafi, N. E.; Wiegelmann, T.; Pagano, P.; Arge, C. N.; Nieves-Chinchilla, T.; Lavarra, M.; Poirier, N.; Amari, T.; Aran, A.; Andretta, V.; Antonucci, E.; Anastasiadis, A.; Auchère, F.; Bellot Rubio, L.; Nicula, B.; Bonnin, X.; Bouchemit, M.; Budnik, E.; Caminade, S.; Cecconi, B.; Carlyle, J.; Cernuda, I.; Davila, J. M.; Etesi, L.; Espinosa Lara, F.; Fedorov, A.; Fineschi, S.; Fludra, A.; Génot, V.; Georgoulis, M. K.; Gilbert, H. R.; Giunta, A.; Gomez-Herrero, R.; Guest, S.; Haberreiter, M.; Hassler, D.; Henney, C. J.; Howard, R. A.; Horbury, T. S.; Janvier, M.; Jones, S. I.; Kozarev, K.; Kraaikamp, E.; Kouloumvakos, A.; Krucker, S.; Lagg, A.; Linker, J.; Lavraud, B.; Louarn, P.; Maksimovic, M.; Maloney, S.; Mann, G.; Masson, A.; Müller, D.; Önel, H.; Osuna, P.; Orozco Suarez, D.; Owen, C. J.; Papaioannou, A.; Pérez-Suárez, D.; Rodriguez-Pacheco, J.; Parenti, S.; Pariat, E.; Peter, H.; Plunkett, S.; Pomoell, J.; Raines, J. M.; Riethmüller, T. L.; Rich, N.; Rodriguez, L.; Romoli, M.; Sanchez, L.; Solanki, S. K.; St Cyr, O. C.; Straus, T.; Susino, R.; Teriaca, L.; del Toro Iniesta, J. C.; Ventura, R.; Verbeeck, C.; Vilmer, N.; Warmuth, A.; Walsh, A. P.; Watson, C.; Williams, D.; Wu, Y.; Zhukov, A. N. Bibcode: 2020A&A...642A...2R Altcode: Context. The Solar Orbiter spacecraft will be equipped with a wide range of remote-sensing (RS) and in situ (IS) instruments to record novel and unprecedented measurements of the solar atmosphere and the inner heliosphere. To take full advantage of these new datasets, tools and techniques must be developed to ease multi-instrument and multi-spacecraft studies. In particular the currently inaccessible low solar corona below two solar radii can only be observed remotely. Furthermore techniques must be used to retrieve coronal plasma properties in time and in three dimensional (3D) space. Solar Orbiter will run complex observation campaigns that provide interesting opportunities to maximise the likelihood of linking IS data to their source region near the Sun. Several RS instruments can be directed to specific targets situated on the solar disk just days before data acquisition. To compare IS and RS, data we must improve our understanding of how heliospheric probes magnetically connect to the solar disk.
Aims: The aim of the present paper is to briefly review how the current modelling of the Sun and its atmosphere can support Solar Orbiter science. We describe the results of a community-led effort by European Space Agency's Modelling and Data Analysis Working Group (MADAWG) to develop different models, tools, and techniques deemed necessary to test different theories for the physical processes that may occur in the solar plasma. The focus here is on the large scales and little is described with regards to kinetic processes. To exploit future IS and RS data fully, many techniques have been adapted to model the evolving 3D solar magneto-plasma from the solar interior to the solar wind. A particular focus in the paper is placed on techniques that can estimate how Solar Orbiter will connect magnetically through the complex coronal magnetic fields to various photospheric and coronal features in support of spacecraft operations and future scientific studies.
Methods: Recent missions such as STEREO, provided great opportunities for RS, IS, and multi-spacecraft studies. We summarise the achievements and highlight the challenges faced during these investigations, many of which motivated the Solar Orbiter mission. We present the new tools and techniques developed by the MADAWG to support the science operations and the analysis of the data from the many instruments on Solar Orbiter.
Results: This article reviews current modelling and tool developments that ease the comparison of model results with RS and IS data made available by current and upcoming missions. It also describes the modelling strategy to support the science operations and subsequent exploitation of Solar Orbiter data in order to maximise the scientific output of the mission.
Conclusions: The on-going community effort presented in this paper has provided new models and tools necessary to support mission operations as well as the science exploitation of the Solar Orbiter data. The tools and techniques will no doubt evolve significantly as we refine our procedure and methodology during the first year of operations of this highly promising mission. Title: The Solar Orbiter Radio and Plasma Waves (RPW) instrument Authors: Maksimovic, M.; Bale, S. D.; Chust, T.; Khotyaintsev, Y.; Krasnoselskikh, V.; Kretzschmar, M.; Plettemeier, D.; Rucker, H. O.; Souček, J.; Steller, M.; Štverák, Š.; Trávníček, P.; Vaivads, A.; Chaintreuil, S.; Dekkali, M.; Alexandrova, O.; Astier, P. -A.; Barbary, G.; Bérard, D.; Bonnin, X.; Boughedada, K.; Cecconi, B.; Chapron, F.; Chariet, M.; Collin, C.; de Conchy, Y.; Dias, D.; Guéguen, L.; Lamy, L.; Leray, V.; Lion, S.; Malac-Allain, L. R.; Matteini, L.; Nguyen, Q. N.; Pantellini, F.; Parisot, J.; Plasson, P.; Thijs, S.; Vecchio, A.; Fratter, I.; Bellouard, E.; Lorfèvre, E.; Danto, P.; Julien, S.; Guilhem, E.; Fiachetti, C.; Sanisidro, J.; Laffaye, C.; Gonzalez, F.; Pontet, B.; Quéruel, N.; Jannet, G.; Fergeau, P.; Brochot, J. -Y.; Cassam-Chenai, G.; Dudok de Wit, T.; Timofeeva, M.; Vincent, T.; Agrapart, C.; Delory, G. T.; Turin, P.; Jeandet, A.; Leroy, P.; Pellion, J. -C.; Bouzid, V.; Katra, B.; Piberne, R.; Recart, W.; Santolík, O.; Kolmašová, I.; Krupař, V.; Krupařová, O.; Píša, D.; Uhlíř, L.; Lán, R.; Baše, J.; Ahlèn, L.; André, M.; Bylander, L.; Cripps, V.; Cully, C.; Eriksson, A.; Jansson, S. -E.; Johansson, E. P. G.; Karlsson, T.; Puccio, W.; Břínek, J.; Öttacher, H.; Panchenko, M.; Berthomier, M.; Goetz, K.; Hellinger, P.; Horbury, T. S.; Issautier, K.; Kontar, E.; Krucker, S.; Le Contel, O.; Louarn, P.; Martinović, M.; Owen, C. J.; Retino, A.; Rodríguez-Pacheco, J.; Sahraoui, F.; Wimmer-Schweingruber, R. F.; Zaslavsky, A.; Zouganelis, I. Bibcode: 2020A&A...642A..12M Altcode: The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter mission is described in this paper. This instrument is designed to measure in-situ magnetic and electric fields and waves from the continuous to a few hundreds of kHz. RPW will also observe solar radio emissions up to 16 MHz. The RPW instrument is of primary importance to the Solar Orbiter mission and science requirements since it is essential to answer three of the four mission overarching science objectives. In addition RPW will exchange on-board data with the other in-situ instruments in order to process algorithms for interplanetary shocks and type III langmuir waves detections. Title: The Energetic Particle Detector. Energetic particle instrument suite for the Solar Orbiter mission Authors: Rodríguez-Pacheco, J.; Wimmer-Schweingruber, R. F.; Mason, G. M.; Ho, G. C.; Sánchez-Prieto, S.; Prieto, M.; Martín, C.; Seifert, H.; Andrews, G. B.; Kulkarni, S. R.; Panitzsch, L.; Boden, S.; Böttcher, S. I.; Cernuda, I.; Elftmann, R.; Espinosa Lara, F.; Gómez-Herrero, R.; Terasa, C.; Almena, J.; Begley, S.; Böhm, E.; Blanco, J. J.; Boogaerts, W.; Carrasco, A.; Castillo, R.; da Silva Fariña, A.; de Manuel González, V.; Drews, C.; Dupont, A. R.; Eldrum, S.; Gordillo, C.; Gutiérrez, O.; Haggerty, D. K.; Hayes, J. R.; Heber, B.; Hill, M. E.; Jüngling, M.; Kerem, S.; Knierim, V.; Köhler, J.; Kolbe, S.; Kulemzin, A.; Lario, D.; Lees, W. J.; Liang, S.; Martínez Hellín, A.; Meziat, D.; Montalvo, A.; Nelson, K. S.; Parra, P.; Paspirgilis, R.; Ravanbakhsh, A.; Richards, M.; Rodríguez-Polo, O.; Russu, A.; Sánchez, I.; Schlemm, C. E.; Schuster, B.; Seimetz, L.; Steinhagen, J.; Tammen, J.; Tyagi, K.; Varela, T.; Yedla, M.; Yu, J.; Agueda, N.; Aran, A.; Horbury, T. S.; Klecker, B.; Klein, K. -L.; Kontar, E.; Krucker, S.; Maksimovic, M.; Malandraki, O.; Owen, C. J.; Pacheco, D.; Sanahuja, B.; Vainio, R.; Connell, J. J.; Dalla, S.; Dröge, W.; Gevin, O.; Gopalswamy, N.; Kartavykh, Y. Y.; Kudela, K.; Limousin, O.; Makela, P.; Mann, G.; Önel, H.; Posner, A.; Ryan, J. M.; Soucek, J.; Hofmeister, S.; Vilmer, N.; Walsh, A. P.; Wang, L.; Wiedenbeck, M. E.; Wirth, K.; Zong, Q. Bibcode: 2020A&A...642A...7R Altcode: After decades of observations of solar energetic particles from space-based observatories, relevant questions on particle injection, transport, and acceleration remain open. To address these scientific topics, accurate measurements of the particle properties in the inner heliosphere are needed. In this paper we describe the Energetic Particle Detector (EPD), an instrument suite that is part of the scientific payload aboard the Solar Orbiter mission. Solar Orbiter will approach the Sun as close as 0.28 au and will provide extra-ecliptic measurements beyond ∼30° heliographic latitude during the later stages of the mission. The EPD will measure electrons, protons, and heavy ions with high temporal resolution over a wide energy range, from suprathermal energies up to several hundreds of megaelectronvolts/nucleons. For this purpose, EPD is composed of four units: the SupraThermal Electrons and Protons (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) plus the Instrument Control Unit that serves as power and data interface with the spacecraft. The low-energy population of electrons and ions will be covered by STEP and EPT, while the high-energy range will be measured by HET. Elemental and isotopic ion composition measurements will be performed by SIS and HET, allowing full particle identification from a few kiloelectronvolts up to several hundreds of megaelectronvolts/nucleons. Angular information will be provided by the separate look directions from different sensor heads, on the ecliptic plane along the Parker spiral magnetic field both forward and backwards, and out of the ecliptic plane observing both northern and southern hemispheres. The unparalleled observations of EPD will provide key insights into long-open and crucial questions about the processes that govern energetic particles in the inner heliosphere. Title: The Spectrometer/Telescope for Imaging X-rays (STIX) Authors: Krucker, Säm; Hurford, G. J.; Grimm, O.; Kögl, S.; Gröbelbauer, H. -P.; Etesi, L.; Casadei, D.; Csillaghy, A.; Benz, A. O.; Arnold, N. G.; Molendini, F.; Orleanski, P.; Schori, D.; Xiao, H.; Kuhar, M.; Hochmuth, N.; Felix, S.; Schramka, F.; Marcin, S.; Kobler, S.; Iseli, L.; Dreier, M.; Wiehl, H. J.; Kleint, L.; Battaglia, M.; Lastufka, E.; Sathiapal, H.; Lapadula, K.; Bednarzik, M.; Birrer, G.; Stutz, St.; Wild, Ch.; Marone, F.; Skup, K. R.; Cichocki, A.; Ber, K.; Rutkowski, K.; Bujwan, W.; Juchnikowski, G.; Winkler, M.; Darmetko, M.; Michalska, M.; Seweryn, K.; Białek, A.; Osica, P.; Sylwester, J.; Kowalinski, M.; Ścisłowski, D.; Siarkowski, M.; Stęślicki, M.; Mrozek, T.; Podgórski, P.; Meuris, A.; Limousin, O.; Gevin, O.; Le Mer, I.; Brun, S.; Strugarek, A.; Vilmer, N.; Musset, S.; Maksimović, M.; Fárník, F.; Kozáček, Z.; Kašparová, J.; Mann, G.; Önel, H.; Warmuth, A.; Rendtel, J.; Anderson, J.; Bauer, S.; Dionies, F.; Paschke, J.; Plüschke, D.; Woche, M.; Schuller, F.; Veronig, A. M.; Dickson, E. C. M.; Gallagher, P. T.; Maloney, S. A.; Bloomfield, D. S.; Piana, M.; Massone, A. M.; Benvenuto, F.; Massa, P.; Schwartz, R. A.; Dennis, B. R.; van Beek, H. F.; Rodríguez-Pacheco, J.; Lin, R. P. Bibcode: 2020A&A...642A..15K Altcode:
Aims: The Spectrometer Telescope for Imaging X-rays (STIX) on Solar Orbiter is a hard X-ray imaging spectrometer, which covers the energy range from 4 to 150 keV. STIX observes hard X-ray bremsstrahlung emissions from solar flares and therefore provides diagnostics of the hottest (⪆10 MK) flare plasma while quantifying the location, spectrum, and energy content of flare-accelerated nonthermal electrons.
Methods: To accomplish this, STIX applies an indirect bigrid Fourier imaging technique using a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of 32 coarsely pixelated CdTe detectors to provide information on angular scales from 7 to 180 arcsec with 1 keV energy resolution (at 6 keV). The imaging concept of STIX has intrinsically low telemetry and it is therefore well-suited to the limited resources available to the Solar Orbiter payload. To further reduce the downlinked data volume, STIX data are binned on board into 32 selectable energy bins and dynamically-adjusted time bins with a typical duration of 1 s during flares.
Results: Through hard X-ray diagnostics, STIX provides critical information for understanding the acceleration of electrons at the Sun and their transport into interplanetary space and for determining the magnetic connection of Solar Orbiter back to the Sun. In this way, STIX serves to link Solar Orbiter's remote and in-situ measurements. Title: Coordination of the in situ payload of Solar Orbiter Authors: Walsh, A. P.; Horbury, T. S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Wimmer-Schweingruber, R. F.; Zouganelis, I.; Anekallu, C.; Bonnin, X.; Bruno, R.; Carrasco Blázquez, I.; Cernuda, I.; Chust, T.; De Groof, A.; Espinosa Lara, F.; Fazakerley, A. N.; Gilbert, H. R.; Gómez-Herrero, R.; Ho, G. C.; Krucker, S.; Lepri, S. T.; Lewis, G. R.; Livi, S.; Louarn, P.; Müller, D.; Nieves-Chinchilla, T.; O'Brien, H.; Osuna, P.; Plasson, P.; Raines, J. M.; Rouillard, A. P.; St Cyr, O. C.; Sánchez, L.; Soucek, J.; Varsani, A.; Verscharen, D.; Watson, C. J.; Watson, G.; Williams, D. R. Bibcode: 2020A&A...642A...5W Altcode: Solar Orbiter's in situ coordination working group met frequently during the development of the mission with the goal of ensuring that its in situ payload has the necessary level of coordination to maximise science return. Here we present the results of that work, namely how the design of each of the in situ instruments (EPD, MAG, RPW, SWA) was guided by the need for coordination, the importance of time synchronisation, and how science operations will be conducted in a coordinated way. We discuss the mechanisms by which instrument sampling schemes are aligned such that complementary measurements will be made simultaneously by different instruments, and how burst modes are scheduled to allow a maximum overlap of burst intervals between the four instruments (telemetry constraints mean different instruments can spend different amounts of time in burst mode). We also explain how onboard autonomy, inter-instrument communication, and selective data downlink will be used to maximise the number of transient events that will be studied using high-resolution modes of all the instruments. Finally, we briefly address coordination between Solar Orbiter's in situ payload and other missions. Title: The Solar Orbiter Science Activity Plan. Translating solar and heliospheric physics questions into action Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.; Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra, A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.; Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.; Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.; Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio, L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun, A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso, F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.; Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.; Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.; van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi, L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine, D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot, S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham, G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier, K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins, J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis, I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.; Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis, G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.; Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.; Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis, K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien, H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.; Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.; Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines, J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.; Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.; Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.; Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.; Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.; Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula, G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio, A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.; Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann, T.; Young, P. R.; Zhukov, A. N. Bibcode: 2020A&A...642A...3Z Altcode: 2020arXiv200910772Z Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, we introduce Solar Orbiter's SAP through a series of examples and the strategy being followed. Title: Understanding the origins of the heliosphere: integrating observations and measurements from Parker Solar Probe, Solar Orbiter, and other space- and ground-based observatories Authors: Velli, M.; Harra, L. K.; Vourlidas, A.; Schwadron, N.; Panasenco, O.; Liewer, P. C.; Müller, D.; Zouganelis, I.; St Cyr, O. C.; Gilbert, H.; Nieves-Chinchilla, T.; Auchère, F.; Berghmans, D.; Fludra, A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Wimmer-Schweingruber, R. F.; Bale, S.; Kasper, J.; McComas, D. J.; Raouafi, N.; Martinez-Pillet, V.; Walsh, A. P.; De Groof, A.; Williams, D. Bibcode: 2020A&A...642A...4V Altcode: Context. The launch of Parker Solar Probe (PSP) in 2018, followed by Solar Orbiter (SO) in February 2020, has opened a new window in the exploration of solar magnetic activity and the origin of the heliosphere. These missions, together with other space observatories dedicated to solar observations, such as the Solar Dynamics Observatory, Hinode, IRIS, STEREO, and SOHO, with complementary in situ observations from WIND and ACE, and ground based multi-wavelength observations including the DKIST observatory that has just seen first light, promise to revolutionize our understanding of the solar atmosphere and of solar activity, from the generation and emergence of the Sun's magnetic field to the creation of the solar wind and the acceleration of solar energetic particles.
Aims: Here we describe the scientific objectives of the PSP and SO missions, and highlight the potential for discovery arising from synergistic observations. Here we put particular emphasis on how the combined remote sensing and in situ observations of SO, that bracket the outer coronal and inner heliospheric observations by PSP, may provide a reconstruction of the solar wind and magnetic field expansion from the Sun out to beyond the orbit of Mercury in the first phases of the mission. In the later, out-of-ecliptic portions of the SO mission, the solar surface magnetic field measurements from SO and the multi-point white-light observations from both PSP and SO will shed light on the dynamic, intermittent solar wind escaping from helmet streamers, pseudo-streamers, and the confined coronal plasma, and on solar energetic particle transport.
Methods: Joint measurements during PSP-SO alignments, and magnetic connections along the same flux tube complemented by alignments with Earth, dual PSP-Earth, and SO-Earth, as well as with STEREO-A, SOHO, and BepiColumbo will allow a better understanding of the in situ evolution of solar-wind plasma flows and the full three-dimensional distribution of the solar wind from a purely observational point of view. Spectroscopic observations of the corona, and optical and radio observations, combined with direct in situ observations of the accelerating solar wind will provide a new foundation for understanding the fundamental physical processes leading to the energy transformations from solar photospheric flows and magnetic fields into the hot coronal plasma and magnetic fields and finally into the bulk kinetic energy of the solar wind and solar energetic particles.
Results: We discuss the initial PSP observations, which already provide a compelling rationale for new measurement campaigns by SO, along with ground- and space-based assets within the synergistic context described above. Title: The Solar Orbiter mission. Science overview Authors: Müller, D.; St. Cyr, O. C.; Zouganelis, I.; Gilbert, H. R.; Marsden, R.; Nieves-Chinchilla, T.; Antonucci, E.; Auchère, F.; Berghmans, D.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rochus, P.; Rodriguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Bruno, R.; Carlsson, M.; Fludra, A.; Harra, L.; Hassler, D. M.; Livi, S.; Louarn, P.; Peter, H.; Schühle, U.; Teriaca, L.; del Toro Iniesta, J. C.; Wimmer-Schweingruber, R. F.; Marsch, E.; Velli, M.; De Groof, A.; Walsh, A.; Williams, D. Bibcode: 2020A&A...642A...1M Altcode: 2020arXiv200900861M
Aims: Solar Orbiter, the first mission of ESA's Cosmic Vision 2015-2025 programme and a mission of international collaboration between ESA and NASA, will explore the Sun and heliosphere from close up and out of the ecliptic plane. It was launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address key questions of solar and heliospheric physics pertaining to how the Sun creates and controls the Heliosphere, and why solar activity changes with time. To answer these, the mission carries six remote-sensing instruments to observe the Sun and the solar corona, and four in-situ instruments to measure the solar wind, energetic particles, and electromagnetic fields. In this paper, we describe the science objectives of the mission, and how these will be addressed by the joint observations of the instruments onboard.
Methods: The paper first summarises the mission-level science objectives, followed by an overview of the spacecraft and payload. We report the observables and performance figures of each instrument, as well as the trajectory design. This is followed by a summary of the science operations concept. The paper concludes with a more detailed description of the science objectives.
Results: Solar Orbiter will combine in-situ measurements in the heliosphere with high-resolution remote-sensing observations of the Sun to address fundamental questions of solar and heliospheric physics. The performance of the Solar Orbiter payload meets the requirements derived from the mission's science objectives. Its science return will be augmented further by coordinated observations with other space missions and ground-based observatories.

ARRAY(0x207ce98) 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: 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: The Micro Solar Flare Apparatus (MiSolFA) instrument concept Authors: Lastufka, Erica; Casadei, Diego; Hurford, Gordon; Kuhar, Matej; Torre, Gabriele; Krucker, Säm Bibcode: 2020AdSpR..66...10L Altcode: The Micro Solar-Flare Apparatus (MiSolFA) is a compact X-ray imaging spectrometer designed for a small 6U micro-satellite. As a relatively inexpensive yet capable Earth-orbiting instrument, MiSolFA is designed to image the high-energy regions of solar flares from a different perspective than that of Solar Orbiter's STIX, operating from a highly elliptical heliocentric orbit. Two instruments working together in this way would provide a 3-dimensional view of X-ray emitting regions and can bypass the dynamic range limitation preventing simultaneous coronal and chromospheric imaging. Stereoscopic X-ray observations would also contain valuable information about the anisotropy of the flare-accelerated electron distribution. To perform these types of observations, MiSolFA must be capable of imaging sources with energies between 10 and 100 keV, with 10 arcsec angular resolution.

MiSolFA's Imager will be the most compact X-ray imaging spectrometer in space. Scaling down the volume by a factor of ten from previous instrument designs requires special considerations. Here we present the design principles of the MiSolFA X-ray optics, discuss the necessary compromises, and evaluate the performance of the Engineering Model. Title: The STIX Aspect System (SAS): The Optical Aspect System of the Spectrometer/Telescope for Imaging X-Rays (STIX) on Solar Orbiter Authors: Warmuth, A.; Önel, H.; Mann, G.; Rendtel, J.; Strassmeier, K. G.; Denker, C.; Hurford, G. J.; Krucker, S.; Anderson, J.; Bauer, S. -M.; Bittner, W.; Dionies, F.; Paschke, J.; Plüschke, D.; Sablowski, D. P.; Schuller, F.; Senthamizh Pavai, V.; Woche, M.; Casadei, D.; Kögl, S.; Arnold, N. G.; Gröbelbauer, H. -P.; Schori, D.; Wiehl, H. J.; Csillaghy, A.; Grimm, O.; Orleanski, P.; Skup, K. R.; Bujwan, W.; Rutkowski, K.; Ber, K. Bibcode: 2020SoPh..295...90W Altcode: The Spectrometer/Telescope for Imaging X-rays (STIX) is a remote sensing instrument on Solar Orbiter that observes the hard X-ray bremsstrahlung emission of solar flares. This paper describes the STIX Aspect System (SAS), a subunit that measures the pointing of STIX relative to the Sun with a precision of ±4″, which is required to accurately localize the reconstructed X-ray images on the Sun. The operating principle of the SAS is based on an optical lens that images the Sun onto a plate that is perforated by small apertures arranged in a cross-shaped configuration of four radial arms. The light passing through the apertures of each arm is detected by a photodiode. Variations of spacecraft pointing and of distance from the Sun cause the solar image to move over different apertures, leading to a modulation of the measured lightcurves. These signals are used by ground analysis to calculate the locations of the solar limb, and hence the pointing of the telescope. Title: Measurement of magnetic field and relativistic electrons along a solar flare current sheet Authors: Chen, B.; Shen, C.; Gary, D.; Reeves, K.; Fleishman, G.; Yu, S.; Guo, F.; Krucker, S.; Lin, J.; Nita, G.; Kong, X. Bibcode: 2020AAS...23611202C Altcode: In the standard model of solar flares, a large-scale reconnection current sheet is postulated as the central engine for powering the flare energy release and accelerating particles. However, where and how the energy release and particle acceleration occur remain unclear due to the lack of measurements for the magnetic properties of the current sheet. Here we report the first measurement of spatially-resolved magnetic field and flare-accelerated relativistic electrons along a current-sheet feature in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the current sheet above the flare loop-top, referred to as a "magnetic bottle". This spatial structure agrees with theoretical predictions and numerical modeling results. A strong reconnection electric field of ~4000 V/m is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. In contrast, these electrons concentrate at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest crucial new input to the current picture of high energy electron acceleration. Title: Microwave and Hard X-Ray Flare Observations by NoRH/NoRP and RHESSI: Peak-flux Correlations Authors: Krucker, Säm; Masuda, Satoshi; White, Stephen M. Bibcode: 2020ApJ...894..158K Altcode: This paper presents initial results from a statistical study of solar microwave and hard X-ray flares jointly observed over the past two solar cycles by the Nobeyama Radio Polarimeters, the Nobeyama Radio Heliograph, and the Reuven Ramaty High Energy Solar Spectroscopic Imager. As has been previously demonstrated, the microwave (17 GHz and 34 GHz) peak flux shows a linear correlation with the nonthermal hard X-ray bremsstrahlung peak emission seen above 50 keV. The correlation holds for the entire rise phase of each individual burst, while the decay phases tend to show more extended emission at microwaves than is generally attributed to particle trapping. While the correlation is highly significant (coefficient of 0.92) and holds over more than four orders of magnitude, individual flares can be above or below the fitted line by an average factor of about 2. By restricting the flare selection to source morphologies with the radio emission from the top of the flare loop, the correlation tightens significantly, with a correlation coefficient increasing to 0.99 and the scatter reduced to a factor of 1.3. These findings corroborate the assumption that gyrosynchrotron microwave and hard X-ray bremsstrahlung emissions are produced by the same flare-accelerated electron population. The extent of the linear correlation over four orders of magnitude suggests that magnetic field strengths within nonthermal 17 GHz sources are surprisingly similar over a wide range of flare sizes. Title: The Energy Spectrum of Solar Energetic Electron Events Authors: Wang, Linghua; Liu, Zixuan; Fu, Haobo; Krucker, Sam Bibcode: 2020EGUGA..22.1944W Altcode: Solar energetic electron events (SEEs) are one of the most common particle acceleration phenomena occurring at the Sun, and their energy spectrum likely reflects the crucial information on the acceleration. Here we present a statistical survey of the energy spectrum of 160 SEEs measured by Wind/3DP with a clear velocity dispersion at energies of ~1-200 keV from January 1995 through December 2016, utilizing a general spectrum formula proposed by Liu et al. (2000). We find that among these 160 SEEs, 144 (90%) have a power-law (or power-law-like) spectrum bending down at high energies, including 108 (67.5%) double-power-law events, 24 (15%) Ellison-Ramaty-like events and 12 (7.5%) log-parabola events, while 16 (10%) have a power-law spectrum extending to high energies. The average power-law spectral index β1 is 2.1±0.4 for double-power-law events, 1.7±0.8 for Ellison-Ramaty-like events, and 2.8±0.11 for single-power-law events. For the 108 double-power-law events, the spectral break energy E0 ranges from 2 keV to 165 keV, with an average of 71±79 keV, while the average spectral index β2 at energies above E0is 4.4±2.3. E0 shows a positive correlation with the electron peak flux at energies above ~40 keV, while β1 has a negative correlation with the electron peak flux at energies above ~15 keV. 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: FOXSI-2 Solar Microflares. I. Multi-instrument Differential Emission Measure Analysis and Thermal Energies Authors: Athiray, P. S.; Vievering, Juliana; Glesener, Lindsay; Ishikawa, Shin-nosuke; Narukage, Noriyuki; Buitrago-Casas, Juan Camilo; Musset, Sophie; Inglis, Andrew; Christe, Steven; Krucker, Säm; Ryan, Daniel Bibcode: 2020ApJ...891...78A Altcode: 2020arXiv200204200A In this paper we present the differential emission measures (DEMs) of two sub-A class microflares observed in hard X-rays (HXRs) by the FOXSI-2 sounding rocket experiment, on 2014 December 11. The second Focusing Optics X-ray Solar Imager (FOXSI) flight was coordinated with instruments X-ray Telescope (Hinode/XRT) and Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA), which provided observations in soft X-rays and Extreme Ultraviolet. This unique data set offers an unprecedented temperature coverage, useful for characterizing the plasma temperature distribution of microflares. By combining data from FOXSI-2, XRT, and AIA, we determined a well-constrained DEM for the microflares. The resulting DEMs peak around 3 MK and extend beyond 10 MK. The emission measures determined from FOXSI-2 were lower than 1026 cm-5 for temperatures higher than 5 MK; faint emission in this range is best measured in HXRs. The coordinated FOXSI-2 observations produce one of the few definitive measurements of the distribution and the amount of plasma above 5 MK in microflares. We utilize the multi-thermal DEMs to calculate the amount of thermal energy released during both the microflares as ∼5.0 × 1028 erg for Microflare 1 and ∼1.6 × 1028 erg for Microflare 2. We also show the multi-thermal DEMs provide more comprehensive thermal energy estimates than isothermal approximation, which systematically underestimates the amount of thermal energy released. Title: Focus on the Extremes: Harnessing the Power of Hard X-ray Focusing Optics to Probe Events Ranging from Solar Microflares to Stellar Superflares Authors: Vievering, J.; Glesener, L.; Grefenstette, B.; Smith, D.; Panchapakesan, S.; Buitrago-Casas, J.; Musset, S.; Ryan, D.; Inglis, A.; Christe, S.; Krucker, S. Bibcode: 2020AAS...23544203V Altcode: Sensitive measurements of solar and stellar flares in the hard X-ray regime are necessary for investigating energy release and transfer during flaring events, as hard X-rays provide insight into the acceleration of electrons and emission from hot plasmas. This research harnesses the powerful capabilities of two instruments that use focusing optics for hard X-rays, the Focusing Optics X-ray Solar Imager (FOXSI), flown on three sounding rocket campaigns, and the Nuclear Spectroscopic Telescope Array (NuSTAR). With the heightened sensitivity of these instruments, it is finally possible to probe faint events in hard X-rays that have previously been elusive, ranging from small-scale solar events to bright X-ray flares on distant stars. In this presentation, we explore the nature of energy release for flaring events of vastly different magnitudes, including solar microflares observed by FOXSI-2 and superflares on young stellar objects (YSOs) observed by NuSTAR, and ask whether these events are linked by the common thread of the standard flare model. Additionally, we investigate the complexity of these solar microflares and the impact of ionizing radiation from YSO flares on protoplanetary disks. In exploring these uncharted regimes, this work probes some of the most intriguing mysteries of the stars, from coronal heating to the formation of planetary systems. Title: Measurement of magnetic field and relativistic electrons along a solar flare current sheet Authors: Chen, Bin; Shen, Chengcai; Gary, Dale E.; Reeves, Katharine K.; Fleishman, Gregory D.; Yu, Sijie; Guo, Fan; Krucker, Säm; Lin, Jun; Nita, Gelu M.; Kong, Xiangliang Bibcode: 2020NatAs...4.1140C Altcode: 2020arXiv200512757C; 2020NatAs.tmp..150C In the standard model of solar flares, a large-scale reconnection current sheet is postulated to be the central engine for powering the flare energy release1-3 and accelerating particles4-6. However, where and how the energy release and particle acceleration occur remain unclear owing to the lack of measurements of the magnetic properties of the current sheet. Here we report the measurement of the spatially resolved magnetic field and flare-accelerated relativistic electrons along a current-sheet feature in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the current sheet above the flare loop-top, referred to as a `magnetic bottle'. This spatial structure agrees with theoretical predictions1,7 and numerical modelling results. A strong reconnection electric field of about 4,000 V m-1 is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. These electrons instead concentrate at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest that the loop-top magnetic bottle is probably the primary site for accelerating and confining the relativistic electrons. 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: FOXSI-4: Instrument Upgrades for a Proposed Fourth Focusing Optics X-Ray Solar Imager Sounding Rocket Experiment Authors: Vievering, J. T.; Glesener, L.; Buitrago-Casas, J. C.; Panchapakesan, S. A.; Musset, S.; Duncan, J. M.; Narukage, N.; Ryan, D.; Inglis, A. R.; Takahashi, T.; Watanabe, S.; Christe, S.; Krucker, S.; Turin, P.; Ramsey, B. Bibcode: 2019AGUFMSH31C3315V Altcode: Observations of the Sun in hard X-rays can provide insight into many solar phenomena, including the mechanisms behind energy release and transport in flares. The indirect imaging methods used by RHESSI , the previous state-of-the-art solar hard X-ray instrument, however, were fundamentally limited in sensitivity and imaging dynamic range . By instead using the direct imaging technique of focusing hard X-rays, the structure and evolution of faint coronal sources, including microflares, active regions, and flare acceleration sites, can be investigated in greater depth. FOXSI ( Focusing Optics X-ray Solar Imager ), a hard X-ray instrument flown on three sounding rocket campaigns to date, seeks to achieve these improved capabilities by using focusing optics for solar observations in the 4-20 keV range. A fourth FOXSI sounding rocket experiment, FOXSI-4, has been proposed for launch in 2023, and the planned instrument upgrades are presented here. One main focus of FOXSI-4 will be to improve the angular resolution for solar hard X-ray instruments through the use of high-resolution optics, with the goal of reaching a half power diameter (HPD) of ∼5". Higher resolution will allow for individual footpoints and acceleration sites to be distinguished during a flare, which would benefit studies of the chromospheric response and provide constraints on flare acceleration mechanisms. FOXSI-4 will ideally be part of a flare campaign, in which multiple solar sounding rockets would be launched one after another to demonstrate the strength of new solar technologies when utilized in conjunction for flare observations. Title: Solar flare hard X-ray and gamma-ray imaging spectro-polarimetry with GRIPS and SAPPHIRE/SHARPIE Authors: Saint-Hilaire, P.; Jeffrey, N. L. S.; Martinez Oliveros, J. C.; Shih, A. Y.; Zoglauer, A.; Caspi, A.; Lichtmacher, P.; Boggs, S.; Hurford, G. J.; Krucker, S.; Sample, J. G.; Tremsin, A. Bibcode: 2019AGUFMSH31C3310S Altcode: To date, flare hard X-ray (HXR) and gamma-ray polarimetry is a poorly exploited field of Solar Physics.

The Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) has already flown once on a high-altitude balloon payload from Antarctica in January 2016. Using a rotating monogrid eight meters in front of spatially-differentiated germanium strip detectors (3D-GeDs), GRIPS images the sun from ~40 keV to ~10 MeV, with a 12.5" resolution and ~1 degree FOV. It can also provide polarization information above ~150 keV.

The SolAr Polarimeter for Hard x-Rays (SAPPHIRE) is a spatially-integrated polarimeter module with a CubeSat form factor currently being proposed. It aims to extract polarization information from solar flare-generated HXRs (~5-100 keV). The SHARPIE (Solar HArd x-ray Polarimer/Imager Experiment) concept is an imager using several SAPPHIRE modules behind RHESSI-like bigrid subcollimators.

We shall review these instruments or instrument concepts and their capabilities and the science questions that they address, with an emphasis on the polarization measurements. Title: Hardware upgrades and science outcomes from the latest flights of the FOXSI rocket Authors: Buitrago-Casas, J. C.; Glesener, L.; Courtade, S.; Vievering, J. T.; Athiray, P. S.; Musset, S.; Ryan, D.; Dalton, G.; Ishikawa, S. N.; Narukage, N.; Bongiorno, S.; Furukawa, K.; Davis, L.; Turin, P.; Turin, Z.; Takahashi, T.; Watanabe, S.; Krucker, S.; Christe, S.; Ramsey, B. Bibcode: 2019AGUFMSH31C3316B Altcode: FOXSI (which stands for the Focusing Optics X-ray Solar Imager) is a sounding rocket payload that has completed three successful flights supported by the Low-Cost Access to Space (LCAS) program of NASA. FOXSI stands out for being the first telescope optimized to use direct focusing to perform imaging spectroscopy of solar hard X-rays (HXRs). We present the latest instrument upgrades incorporated into the rocket payload for its third flight. We highlight the way these upgrades substantially improved the telescope performance and we present the observations obtained. Particularly, we describe the strategies implemented to reduce stray-light (ghost-ray) background and the inclusion of a soft X-ray CMOS sensor as well as HXR CdTe strip detectors to widen the spectral range of the telescope.

We finalize with an overview of the science results of the FOXSI flights. We present a differential emission measure (DEM) and thermal analyses of microflares observed during the second flight. We prove the relevance of the FOXSI measurements for obtaining unprecedented constrained microflare DEMs and discuss their implications on the energetics of these small flaring events. We finish presenting a discussion on the FOXSI measurements of an aged active region and quiet-sun areas observed during its latest flight. Title: Combined Next-Generation X-ray and EUV Observations with the FIERCE Mission Concept Authors: Shih, A. Y.; Glesener, L.; Christe, S.; Reeves, K.; Gburek, S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis, B. R.; Drake, J. F.; Goetz, K.; Golub, L.; Guidoni, S. E.; Inglis, A.; Hannah, I. G.; Holman, G.; Hayes, L.; Ireland, J.; Kerr, G. S.; Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset, S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton, D. B.; Steslicki, M.; Woods, T. N. Bibcode: 2019AGUFMSH33A..08S Altcode: While there have been significant advances in our understanding of impulsive energy release at the Sun through the combination of RHESSI X-ray observations and SDO/AIA EUV observations, there is a clear science need for significantly improved X-ray and EUV observations. These new observations must capture the full range of emission in flares and CMEs (e.g., faint coronal sources near bright chromospheric sources), connect the intricate evolution of energy release with dynamic changes in the configuration of plasma structures, and identify the signatures of impulsive energy release in even the quiescent Sun. The Fundamentals of Impulsive Energy Release in the Corona Explorer ( FIERCE ) MIDEX mission concept makes these observations by combining the two instruments previously proposed on the FOXSI SMEX mission concept - a focusing hard X-ray spectroscopic imager and a soft X-ray spectrometer - with a high-resolution EUV imager that will not saturate for even intense flares. All instruments observe at high cadence to capture the initiation of solar transient events and the fine time structure within events. FIERCE would launch in mid-2025, near the peak of the next solar cycle, which is also well timed with perihelions of Parker Solar Probe and Solar Orbiter. Title: Characterization of Charge Sharing in the FOXSI Sounding Rocket Hard X-ray Detectors Using the Advanced Light Source at Berkeley Authors: Duncan, J. M.; Panchapakesan, S. A.; Musset, S.; Vievering, J. T.; MacDowell, A. A.; Glesener, L.; Davis, L.; Buitrago-Casas, J. C.; O'Brien, C.; Ishikawa, S. N.; Takahashi, T.; Watanabe, S.; Narukage, N.; Furukawa, K.; Ryan, D.; Hagino, K.; Courtade, S.; Christe, S.; Krucker, S. Bibcode: 2019AGUFMSH31C3317D Altcode: The FOXSI sounding rocket experiments represent the first ever solar-dedicated direct-focusing hard X-ray (HXR) instruments. The most recent flight (FOXSI-3) occurred in September 2018, utilizing Wolter-1 style concentric-shell optics and both silicon (Si) and cadmium telluride (CdTe) double-sided strip detectors. The angular resolution of the optics (5" FWHM, equivalent to 50um at the detector bench) is finer than the strip pitch of the Si and CdTe detectors (75um and 60um, respectively), meaning that the resolution of the instrument at sounding rocket focal lengths is detector-limited [1]. In order to further improve characterization of the spatial and spectral properties of the FOXSI-3 detectors, experiments were recently completed using Beamline 3.3.2. at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. Using this beamline, a fine (2um x 2um or 5um x 5um) monoenergetic x-ray beam was scanned across detector strips and inter-strip regions in steps much smaller than the strip pitch. Such scans were performed for both Si and CdTe detectors at a number of x-ray energies between 5.5-12 keV. These experiments were particularly motivated by the desire to measure the incidence of charge sharing, which occurs when a single incident photon causes signals to be registered in multiple adjacent strips. Precise measurement of charge sharing behavior is important to optimize characterization of detector efficiency. Additionally, with a strong understanding of charge sharing behavior in a detector, spatial resolution could be improved by sub-strip localization of photon impacts. Here, we present initial analysis of results from these ALS experiments, including investigation into charge sharing properties in both the Si and CdTe FOXSI-3 detectors. We also discuss the implications of these results for the use of these detectors in the future FOXSI-4 mission, which will utilize yet-higher-resolution HXR optics.

[1] Furukawa,K.,et al. "Development of 60um pitch CdTe Double-sided Strip Detectors for the FOXSI-3 sounding rocket experiment." NIMPR, Section A: HSTD11, 2017. Title: Spectrometer for Temperature and Composition for the FIERCE MIDEX mission concept Authors: Steslicki, M.; Mrozek, T.; Gburek, S.; Caspi, A.; Podgórski, P.; Ścisłowski, D.; Bąkała, J.; Kowaliński, M.; Shih, A. Y.; Christe, S.; Glesener, L.; Krucker, S.; Saint-Hilaire, P. Bibcode: 2019AGUFMSH31C3314S Altcode: Spectrometer for Temperature and Composition (STC) will be a small soft x-ray spectrometer and will monitor solar X-ray flux between 1 keV and 15 with 0.125 keV energy resolution. The data collected by STC will complement two other FIERCE mission instruments and additionally allowing temperature and emission measure analysis of small flares as well as a quiet Sun. The instrument will provide fast solar X-ray spectroscopy in the entire flux intensity range, variable seven order in intensity form quiet solar conditions to strongest flares. The exceptionally wide dynamic range of this instrument as well as its high spectral and temporal resolution and wide plasma temperature coverage will result in unprecedented measurements, crucial for energy release processes and space weather monitoring. Title: The magnetic structure and electrodynamics of the emerging solar wind Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.; Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; Dudok de Wit, T.; Eastwood, J. P.; Webster, J.; Farrell, W. M.; Fong, C.; Goetz, K.; Goldstein, M. L.; Goodrich, K.; Harvey, P.; Horbury, T. S.; Howes, G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck, K. E.; Krasnoselskikh, V.; Krucker, S.; Laker, R.; Larson, D. E.; MacDowall, R. J.; Maksimovic, M.; Malaspina, D.; Martinez Oliveros, J. C.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F.; Phan, T.; Pulupa, M.; Raouafi, N. E.; Salem, C. S.; Stansby, D.; Stevens, M. L.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R. Bibcode: 2019AGUFMSH11A..05B Altcode: Convection and rotation drive the solar dynamo and, ultimately, provide the mechanical energy flux required to heat the solar corona and accelerate the solar wind. However, the way in which energy is then dissipated to heat the corona and wind are not well understood. Some energization models invoke non-thermal energy flux imparted by plasma Alfvén waves, while others rely on a carpet of small nano-flares as energy input, however these models have been unconstrained by direct measurements of the solar wind near its origin. Here we use in situ measurements from the FIELDS instrument suite during the first solar encounter (E1) at 35.7 solar radii (Rs) of the NASA Parker Solar Probe (PSP) mission to reveal the magnetic structure and kinetics of slow Alfvénic solar wind emerging from a small, equatorial coronal hole. Our measurements show that, at solar minimum, the slow wind can escape from above the low-lying, complex magnetic structures of the equatorial streamer belt, carrying a magnetic field that is highly dynamic, exhibiting polarity reversals on timescales from seconds to hours. These rapidly oscillating field structures are associated with clustered radial jets of plasma in which the energy flux is dramatically enhanced and turbulence levels are higher. Time intervals between groups of jets indicate a solar wind that is steady with a mostly radial magnetic field and relatively low levels of Alfvénic turbulent fluctuations. This 'quiet' wind however shows clear signatures of plasma micro-instabilities associated with ion and electron beams and velocity-space structure. Title: The Hard X-ray Spectrometer STIX onboard Solar Orbiter Authors: Krucker, S. Bibcode: 2019AGUFMSH21D3300K Altcode: We will present the STIX hard X-ray imaging concept and highlight key science objectives of STIX. Title: A Statistical Study of Magnetic Features of White-light Flares Authors: Watanabe, K.; Tsuruda, K.; Masuda, S.; Krucker, S. Bibcode: 2019AGUFMSH13D3425W Altcode: It is believed that the origin of white-light enhancement is accelerated non-thermal electrons. Previously, we performed a statistical analysis that search for the difference between white-light flares (WLFs) and no white-light flares (NWLFs), and found that the relationship between emission measure and temperature for WLFs indicated the existence of strong magnetic field around the acceleration site (Watanabe et al., 2017). Although the field strength at the acceleration site cannot measure directly, we tried to estimate by using the turn-over frequency of gyro-synchrotron emission in microwaves (Dulk 1985). We used the Nobeyama Radio polarimeters (NoRP) for >M3 class flares during the period from January 2011 to December 2017. We found 29 events which were simultaneously observed with white-light (SDO/HMI continuum) and microwave (NoRP) which had loop-top microwave source in Nobeyama Radioheliograph (NoRH) image. However, we couldn't find any difference in turn-over frequencies for WLF and NWLF. This is because the turn-over frequency is determined by not only the magnetic field strength but also the electron density (Dulk 1985). Thus, we have to think about the variation of electron density for this study.

In order to investigate the relationship between magnetic field strength and white-light emissions, next we tried to compare field strength and temperature of white-light emission region. The former/latter can be derived from SDO/HMI magnetogram/three continuum bands of Hinode/SOT. We performed a statistical analysis for 26 Hinode white-light events, we found the field strength of white-light emission region for WLFs were correlated with the maximum counts of white-light emission. When we assume the photosperic field strength is correlated with the field strength at the flare acceleration site, these results support the existence of strong field strength for WLFs. Title: A new spectrum fitting model: asymptotic double power law Authors: Liu, Z.; Wang, L.; Fu, H.; Krucker, S.; Wimmer-Schweingruber, R. F. Bibcode: 2019AGUFMSH23C3344L Altcode: We propose a new formula of energy spectrum of energetic particles, J=A(E/E0)-β1[1+(E/E0)α](β2-β1)/α, where J is the particle intensity (e.g., flux), E is the particle energy, A is the amplitude factor,E0 likely represents the spectral break energy, β1 (β2 ) is equivalent to the power-law spectral index at energies below (above) E0, and α (>0) describes the transition shape of energy spectrum around E0. When α >> 10, this spectral formula becomes a classical double-power-law spectrum, while, when α << 10, this formula has a curved spectral transition. In addition, when α = 1, E0 and β2 tend to infinity, t his formula can be simplified to the Ellison-Ramaty equation. We can utilize this spectral formula to fit the energy spectrum of energetic electron, proton, 3He and heavier ions, HXRs, Gamma rays, etc., and obtain self-consistently all the spectral parameters by including both uncertainties in particle intensity and energy. Title: FIERCE Science: Expected Results From a High-Energy Medium-Class Explorer Authors: Glesener, L.; Shih, A. Y.; Christe, S.; Reeves, K.; Gburek, S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis, B. R.; Drake, J. F.; Golub, L.; Goetz, K.; Guidoni, S. E.; Hannah, I. G.; Hayes, L.; Holman, G.; Inglis, A.; Ireland, J.; Kerr, G. S.; Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset, S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton, D. B.; Steslicki, M.; Woods, T. N. Bibcode: 2019AGUFMSH31C3313G Altcode: A variety of individual X-ray and EUV instruments have probed high-energy aspects of the Sun over the decades, each contributing pieces to the puzzles of the energization, heating, and acceleration of solar plasma and particles. But fundamental difficulties in sensitivity and dynamic range impart big challenges in probing the details of particle acceleration sites, understanding how eruptions and flares are initiated, and tracking the intricacies of energy transfer as flares evolve. The Fundamentals of Impulsive Energy Release in the Corona Explorer ( FIERCE ) mission will make substantial leaps forward in these scientific ventures by combining a variety of instruments into one platform, each optimized to have high sensitivity and dynamic range. FIERCE is a proposed NASA Heliophysics Medium-Class Explorer that will investigate high-energy solar phenomena across a variety of spectral and spatial dimensions. It combines hard X-ray imaging spectroscopy (via focusing, for the first time for a solar-dedicated spacecraft), spatially integrated soft X-ray spectroscopy, and fast, high-resolution extreme ultraviolet imaging at coronal and flare temperatures. FIERCE uses this array of instruments to make important contributions toward probing the genesis of space weather events, the acceleration of particles, the transport of flare energy, and the heating of the corona. Here, we present some of the expected science outcomes for the FIERCE observatory, concentrating on the ways in which FIERCE can probe confined and eruptive events, particle acceleration everywhere it may occur on the Sun, and the connections of solar high-energy phenomena to the heliosphere. Title: Highly structured slow solar wind emerging from an equatorial coronal hole Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.; Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; de Wit, T. Dudok; Eastwood, J. P.; Ergun, R. E.; Farrell, W. M.; Fong, C.; Goetz, K.; Goldstein, M.; Goodrich, K. A.; Harvey, P. R.; Horbury, T. S.; Howes, G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck, K. E.; Krasnoselskikh, V. V.; Krucker, S.; Laker, R.; Larson, D. E.; MacDowall, R. J.; Maksimovic, M.; Malaspina, D. M.; Martinez-Oliveros, J.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F. S.; Phan, T. D.; Pulupa, M.; Raouafi, N. E.; Salem, C.; Stansby, D.; Stevens, M.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R. Bibcode: 2019Natur.576..237B Altcode: During the solar minimum, when the Sun is at its least active, the solar wind1,2 is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvénic rarefied stream of plasma originating from deep within coronal holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind3 of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain4; theories and observations suggest that they may originate at the tips of helmet streamers5,6, from interchange reconnection near coronal hole boundaries7,8, or within coronal holes with highly diverging magnetic fields9,10. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfvén-wave turbulence11,12, heating by reconnection in nanoflares13, ion cyclotron wave heating14 and acceleration by thermal gradients1. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe15 at 36 to 54 solar radii that show evidence of slow Alfvénic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities10,16 that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude coronal holes are a key source of the slow solar wind. Title: Joint hard X-ray observations with ASO-S/HXI and SO/STIX Authors: Krucker, Säm; Hurford, Gordon J.; Su, Yang; Gan, Wei-Qun Bibcode: 2019RAA....19..167K Altcode: 2019RAA....19..167H This paper discusses the potential of future joint hard X-ray solar flare observations between the Hard X-ray Imager (HXI) onboard the Advanced Space-based Solar Observatory (ASO-S) mission and the Spectrometer/Telescope for Imaging X-rays (STIX) on Solar Orbiter. The different viewing perspectives of the two telescopes relative to the Sun will allow us for the first time to systematically study non-thermal hard X-ray emissions stereoscopically. During the 4-years of the nominal mission of ASO-S, we expect to jointly observe about 160 flares above GOES M1 class to systematically study hard X-ray directivity. For about 16 partially limb-occulted STIX flares, we will have observations of the entire flare by HXI. Such observations will enable us to simultaneously study the all-important coronal hard X-ray sources, which are generally lost in the instrument’s individual imaging dynamic range, in combination with the chromospheric footpoint emissions. The two different detector systems used in the two telescopes make the relative calibration between the two instruments a key task that needs to be addressed before creditable science results can be published. If an accurate inter-calibration can be achieved using jointly observed flares on the disk, observations with HXI and STIX will provide new and essential key diagnostics for solar flare physics. Title: Multiwavelength Stereoscopic Observation of the 2013 May 1 Solar Flare and CME Authors: Lastufka, Erica; Krucker, Säm; Zimovets, Ivan; Nizamov, Bulat; White, Stephen; Masuda, Satoshi; Golovin, Dmitriy; Litvak, Maxim; Mitrofanov, Igor; Sanin, Anton Bibcode: 2019ApJ...886....9L Altcode: 2020arXiv201210179L A M-class behind-the-limb solar flare on 2013 May 1 (SOL2013-05-01T02:32), accompanied by a (∼400 km s-1) coronal mass ejection (CME), was observed by several space-based observatories with different viewing angles. We investigated the RHESSI-observed occulted hard X-ray (HXR) emissions that originated at least 0.1 {R}{{S}} above the flare site. Emissions below ∼10 keV revealed a hot, extended (11 MK, >60″) thermal source from the escaping CME core, with densities around 109 cm-3. In such a tenuous hot plasma, ionization times scales are several minutes, consistent with the nondetection of the hot CME core in SDO/AIA’s 131 Å filter. The nonthermal RHESSI source originated from an even larger area (∼100″) at lower densities (108 cm-3) located above the hot core, but still behind the CME front. This indicates that the observed part of the nonthermal electrons are not responsible for heating the CME core. Possibly the hot core was heated by nonthermal electrons before it became visible from Earth, meaning that the unocculted part of the nonthermal emission likely originates from a more tenuous part of the CME core, where nonthermal electrons survive long enough to became visible from Earth. Simultaneous HXR spectra from the Mars Odyssey mission, which viewed the flare on disk, indicated that the number of nonthermal electrons >20 keV within the high coronal source is ∼0.1%-0.5% compared with the number within the chromospheric flare ribbons. The detection of high coronal HXR sources in this moderate size event suggests that such sources are likely a common feature within solar eruptive events. Title: The Micro Solar Flare Apparutus (MiSolFA) Instrument Concept Authors: Lastufka, Erica; Casadei, Diego; Hurford, Gordon; Kuhar, Matej; Torre, Gabriele; Krucker, Säm Bibcode: 2019arXiv191001825L Altcode: The Micro Solar-Flare Apparatus (MiSolFA) is a compact X-ray imaging spectrometer designed for a small 6U micro-satellite. As a relatively inexpensive yet capable Earth-orbiting instrument, MiSolFA is designed to image the high-energy regions of solar flares from a different perspective than that of Solar Orbiter's STIX, operating from a highly elliptical heliocentric orbit. Two instruments working together in this way would provide a 3-dimensional view of X-ray emitting regions and can bypass the dynamic range limitation preventing simultaneous coronal and chromospheric imaging. Stereoscopic X-ray observations would also contain valuable information about the anisotropy of the flare-accelerated electron distribution. To perform these types of observations, MiSolFA must be capable of imaging sources with energies between 10 and 100 keV, with 10 arcsec angular resolution. MiSolFA's Imager will be the most compact X-ray imaging spectrometer in space. Scaling down the volume by a factor of ten from previous instrument designs requires special considerations. Here we present the design principles of the MiSolFA X-ray optics, discuss the necessary compromises, and evaluate the performance of the Engineering Model. Title: Ghost-ray reduction and early results from the third FOXSI sounding rocket flight Authors: Musset, Sophie; Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Bongiorno, Stephen; Courtade, Sasha; Athiray, P. S.; Vievering, Juliana; Ishikawa, Shin-nosuke; Narukage, Noriyuki; Furukawa, Kento; Ryan, Daniel; Dalton, Greg; Turin, Zoe; Davis, Lance; Takahashi, Tadayuki; Watanabe, Shin; Mitsuishi, Ikuyuki; Hagino, Kouichi; Kawate, Tomoko; Turin, Paul; Christe, Steven; Ramsey, Brian; Krucker, Säm. Bibcode: 2019SPIE11118E..12M Altcode: The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment demonstrates the technique of focusing hard X-ray (HXR) optics for the study of fundamental questions about the high-energy Sun. Solar HXRs provide one of the most direct diagnostics of accelerated electrons and the impulsive heating of the solar corona. Previous solar missions have been limited in sensitivity and dynamic range by the use of indirect imaging, but technological advances now make direct focusing accessible in the HXR regime, and the FOXSI rocket experiment optimizes HXR focusing telescopes for the unique scientific requirements of the Sun. FOXSI has completed three successful flights between 2012 and 2018. This paper gives a brief overview of the experiment, focusing on the third flight of the instrument on 2018 Sept. 7. We present the telescope upgrades highlighting our work to understand and reduce the effects of singly reflected X-rays and show early science results obtained during FOXSI's third flight. Title: Frequency Agile Solar Radiotelescope Authors: Bastian, Tim; Bain, H.; Bradley, R.; Chen, B.; Dahlin, J.; DeLuca, E.; Drake, J.; Fleishman, G.; Gary, D.; Glesener, L.; Guo, Fan; Hallinan, G.; Hurford, G.; Kasper, J.; Ji, Hantao; Klimchuk, J.; Kobelski, A.; Krucker, S.; Kuroda, N.; Loncope, D.; Lonsdale, C.; McTiernan, J.; Nita, G.; Qiu, J.; Reeves, K.; Saint-Hilaire, P.; Schonfeld, S.; Shen, Chengcai; Tun, S.; Wertheimer, D.; White, S. Bibcode: 2019astro2020U..56B Altcode: We describe the science objectives and technical requirements for a re-scoped Frequency Agile Solar Radiotelescope (FASR). FASR fulfills a long term community need for a ground-based, solar-dedicated, radio telescope - a next-generation radioheliograph - designed to perform ultra-broadband imaging spectropolarimetry. Title: Statistical Study of Hard X-Ray Spectral Breaks in Solar Flares Authors: Alaoui, Meriem; Krucker, Säm; Saint-Hilaire, Pascal Bibcode: 2019SoPh..294..105A Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides hard X-ray spectral observations with ≈1 keV resolution to study flare-accelerated (>10 keV) electrons through their bremsstrahlung emission. Here we report on a statistical study of RHESSI flares with emission above 150 keV, focusing on the spectral shape at the hard X-ray peak. Spectral parameters are derived by fitting the photon spectrum with a broken power law and by the standard thick-target fit. Consistent with previous studies, the most common spectral shape of the photon spectrum (52 out of 65 events) is a double power law with a downward break ("knee"), with ten events showing a single power law and three events having an upward break ("ankle"). The spectral breaks occur typically around 55 keV and the difference of the spectral index above and below the break, γ2 and γ1, is typically between 0.3 and 1. We show correlations between the downward break parameters. The most prominent correlation, with a rank order coefficient of ρ =0.92 , is between the power-law indices above and below the break: γ1=(0.74 ±0.04 )γ2+(0.34 ±0.14 ). Applying a thick target fit to the photon spectrum, a similar correlation is also found for the flare-accelerated electron spectra with δ1=(0.85 ±0.08 )δ2−(0.3 ±0.3 ) (ρ =0.67 ). Spectral breaks could be a property of the acceleration mechanism itself or they could be a secondary effect produced by particle transport or wave-particle interactions. Any theoretical models should be consistent with these correlations. In addition, we find that one upward and 23 (49%) downward breaks are consistent with nonuniform ionization within the thick target. 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: FOXSI-2 Solar Microflares: Hard X-ray Spectroscopy and Flare Energetics Authors: Vievering, Juliana; Glesener, Lindsay; Athiray Panchapakesan, Subramania; Buitrago-Casas, Juan Camilo; Ryan, Daniel; Musset, Sophie; Inglis, Andrew; Christe, Steven; Krucker, Sam Bibcode: 2019AAS...23420412V Altcode: Sensitive measurements of solar flares in the hard X-ray regime are necessary for investigating energy release and transfer during flaring events, as hard X-rays provide insight into the acceleration of electrons and emission of high-temperature plasmas. Due to use of indirect imaging, past solar-dedicated instruments in this energy range have been fundamentally limited in sensitivity and dynamic range. By instead using a direct imaging technique, the structure and evolution of even small flares and active regions can be investigated in greater depth. The Focusing Optics X-ray Solar Imager (FOXSI), a hard X-ray instrument flown on three sounding rocket campaigns, seeks to achieve these improved capabilities by using focusing optics for solar observations in the 4-20 keV range. During the second flight, which occurred on 2014 December 11, FOXSI-2 observed two microflares, estimated as GOES class A0.1 and A0.3. Here we present imaging and spectral analyses of these microflares, investigating the nature of energy release and exploring the structure and dynamics in comparison to larger flares. Through this work, we find evidence for high-temperature plasma ( 10 MK) as well as spatial and temporal complexity for microflares that are an order of magnitude fainter than those observed by previous solar-dedicated instruments in this energy range, highlighting the benefits of direct imaging. Title: The FOXSI-3 rocket: Overview and early results of its latest flight Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Courtade, Sasha; Vievering, Juliana; Athiray Panchapakesan, Subramania; Musset, Sophie; Ryan, Daniel; Dalton, Gregory; Ishikawa, Shin-Nosuke; Narukage, Noriyuki; Bongiorno, Stephen; Furukawa, Kento; Davis, Lance; Turin, Paul; Turin, Zoe; Takahashi, Tadayuki; Watanabe, Shin; Krucker, Sam; Christe, Steven; Ramsey, Brian Bibcode: 2019AAS...23412602B Altcode: Hard X-rays (HXRs) from the solar corona are closely connected to energy releases and particle transport in solar flares of all sizes. Expressly, faint solar HXR emissions are of remarkable interest in understanding solar flare structure and dynamics. This is because of their connection with, for instance, loop top emission and small flares. Mainly due to the indirect imaging methods used by past solar-dedicated HXR imagers, like RHESSI, faint HXRs observations have been limited by the imaging dynamic range and sensitivity of the instruments.

The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload is the first solar-dedicated instrument designed for performing imaging spectroscopy in the 4-20 keV range by using direct focusing optics. FOXSI has successfully flown three times from the White Sands Missile Range in New Mexico. For its latest rocket campaign (FOXSI-3), an enhanced version of the experiment, which includes optics and detector upgrades, was implemented for the launch which happened on September 7, 2018. In this talk, we present an overview of the FOXSI-3 campaign, describing in detail the improved capabilities of the telescope and how they allow for better investigation of faint coronal HXRs. We also present a preliminary analysis of the FOXSI-3 observations. Title: Radio Spectroscopic Imaging of Solar Flare Termination Shocks: Split-band Feature and A Second Possible Event Authors: Chen, Bin; Luo, Yingjie; Yu, Sijie; Krucker, Sam; Reeves, Kathy; Shen, Chengcai; Bastian, Timothy S. Bibcode: 2019AAS...23421003C Altcode: Solar termination shocks (TSs) can form above the looptop when reconnection outflows that impinge upon newly reconnected flare arcades exceed the local fast-mode magnetosonic speed. TSs have been suggested as one of the promising drivers for particle acceleration in solar flares, yet observational evidence remains rare. By utilizing radio dynamic spectral imaging of decimetric stochastic spike bursts (SSBs) observed during a C1.9 eruptive flare on 2012 March 3, Chen et al. (2015) found that the bursts were associated with a dynamic TS-like feature above the looptop. They also showed evidence for the TS as an electron accelerator. One piece of observational evidence that strongly supports the TS interpretation is the split-band feature - a phenomenon well-known in type II radio bursts associated with CME-driven shocks, one interpretation for which attributes to radio emission from both the upstream and downstream side of the shock. We perform detailed spectral imaging analysis of the split-band feature in the 2012 March 3 SSB event, and find evidence that supports the shock upstream-downstream interpretation. We also report another SSB event observed during an M8.4 eruptive flare on 2012 March 10, and show that the radio centroids of the SSBs form a similar shock-surface-shaped structure to the earlier event, located above the reconnected flare arcades and below supra-arcade plasma downflows. 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: FOXSI-2 Solar Microflares : Multi-Instrument Differential Emission Measure Analysis Authors: Athiray, P. S.; Glesener, Lindsay; Vievering, Juliana; Ishikawa, Shin-Nosuke; Inglis, Andrew; Narukage, Noriyuki; Ryan, Daniel; Buitrago-Casas, Juan Camilo; Christe, Steven; Musset, Sophie; Krucker, Sam Bibcode: 2019AAS...23422502A Altcode: The plasma temperature distribution above 5 MK during microflares is often loosely constrained due to limited high sensitivity hard X-ray measurements covering the wide range of temperatures observed. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment performs direct imaging and spectroscopy of the Sun in hard X-rays, in the energy range 4 to 20 keV. FOXSI offers better sensitivity for temperatures above 5 MK by using direct focusing grazing incidence X-ray optics, the first of its kind for dedicated solar observations. The second FOXSI flight was launched on 2014 December 11 and observed microflares, quiescent Sun and quiescent active regions. This flight was coordinated with the X-ray Telescope (XRT) onboard Hinode and the Atmospheric Imaging Assembly (AIA) onboard SDO which offers unprecedented temperature coverage for characterizing the plasma temperature distribution. We present an overview of microflares observed during the FOXSI-2 flight with concurrent brightening observed in Extreme Ultraviolet (EUV) and soft X-rays, which indicates emission beyond 10 MK. By combining data from FOXSI-2, XRT, and AIA, we determined a well-constrained DEM for the microflares. The coordinated FOXSI-2 observations produce one of the few definitive measurements of the distribution and the amount of plasma above 5 MK in microflares. 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: On the observation of a classical loop-prominence system during the 2017 September 10 flare. Authors: Martinez Oliveros, Juan Carlos; Krucker, Sam; Guevara Gomez, Juan Camilo Bibcode: 2019AAS...23420411M 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: Hot Plasma in a Quiescent Solar Active Region as Measured by RHESSI, XRT, and AIA Authors: Ishikawa, Shin-nosuke; Krucker, Säm Bibcode: 2019ApJ...876..111I Altcode: 2019arXiv190311293I This paper investigates a quiescent (nonflaring) active region observed on 2010 July 13 in extreme ultraviolet (EUV), soft X-ray (SXR), and hard X-rays to search for a hot component that is speculated to be a key signature of coronal heating. We use a combination of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) imaging and long-duration time integration (up to 40 minutes) to detect the active regions in the 3-8 keV range during apparently nonflaring times. The RHESSI imaging reveals a hot component that originates from the entire active region, as speculated for a nanoflare scenario where the entire active region is filled with a large number of unresolved small energy releases. An isothermal fit to the RHESSI data gives temperatures around ∼7 MK with an emission measure of several times 1046 cm-3. Adding EUV and SXR observations taken by AIA and the X-ray Telescope, respectively, we derive a differential emission measure (DEM) that shows a peak between 2 and 3 MK with a steeply decreasing high-temperature tail, similar to what has been previously reported. The derived DEM reveals that a wide range of temperatures contributes to the RHESSI flux (e.g., 40% of the 4 keV emission being produced by plasma below 5 MK, while emission at 7 keV is almost exclusively from plasmas above 5 MK) indicating that the RHESSI spectrum should not be fitted with an isothermal. The hot component has a rather small emission measure (∼0.1% of the total EM is above 5 MK), and the derived thermal energy content is of the order of 10% for a filling factor of unity, or potentially below 1% for smaller filling factors. Title: Penetrating particle ANalyzer (PAN) Authors: Wu, X.; Ambrosi, G.; Azzarello, P.; Bergmann, B.; Bertucci, B.; Cadoux, F.; Campbell, M.; Duranti, M.; Ionica, M.; Kole, M.; Krucker, S.; Maehlum, G.; Meier, D.; Paniccia, M.; Pinsky, L.; Plainaki, C.; Pospisil, S.; Stein, T.; Thonet, P. A.; Tomassetti, N.; Tykhonov, A. Bibcode: 2019AdSpR..63.2672W Altcode: 2019arXiv190104351W PAN is a scientific instrument suitable for deep space and interplanetary missions. It can precisely measure and monitor the flux, composition, and direction of highly penetrating particles (> ∼ 100 MeV/nucleon) in deep space, over at least one full solar cycle (11 years). The science program of PAN is multi- and cross-disciplinary, covering cosmic ray physics, solar physics, space weather and space travel. PAN will fill an observation gap of galactic cosmic rays in the GeV region, and provide precise information of the spectrum, composition and emission time of energetic particle originated from the Sun. The precise measurement and monitoring of the energetic particles is also a unique contribution to space weather studies. PAN will map the flux and composition of penetrating particles, which cannot be shielded effectively, precisely and continuously, providing valuable input for the assessment of the related health risk, and for the development of an adequate mitigation strategy. PAN has the potential to become a standard on-board instrument for deep space human travel.

PAN is based on the proven detection principle of a magnetic spectrometer, but with novel layout and detection concept. It will adopt advanced particle detection technologies and industrial processes optimized for deep space application. The device will require limited mass (20 kg) and power (20 W) budget. Dipole magnet sectors built from high field permanent magnet Halbach arrays, instrumented in a modular fashion with high resolution silicon strip detectors, allow to reach an energy resolution better than 10% for nuclei from H to Fe at 1 GeV/n. The charge of the particle, from 1 (proton) to 26 (Iron), can be determined by scintillating detectors and silicon strip detectors, with readout ASICs of large dynamic range. Silicon pixel detectors used in a low power setting will maintain the detection capabilities for even the strongest solar events. A fast scintillator with silicon photomultiplier (SiPM) readout will provide timing information to determine the entering direction of the particle, as well as a high rate particle counter. Low noise, low power and high density ASIC will be developed to satisfy the stringent requirement of the position resolution and the power consumption of the tracker. 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: Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar Jet Authors: Chen, Bin; Yu, Sijie; Battaglia, Marina; Farid, Samaiyah; Savcheva, Antonia; Reeves, Katharine K.; Krucker, Säm; Bastian, T. S.; Guo, Fan; Tassev, Svetlin Bibcode: 2018ApJ...866...62C Altcode: 2018arXiv180805951C Magnetic reconnection, the central engine that powers explosive phenomena throughout the universe, is also perceived to be one of the principal mechanisms for accelerating particles to high energies. Although various signatures of magnetic reconnection have been frequently reported, observational evidence that links particle acceleration directly to the reconnection site has been rare, especially for space plasma environments currently inaccessible to in situ measurements. Here we utilize broadband radio dynamic imaging spectroscopy available from the Karl G. Jansky Very Large Array to observe decimetric type III radio bursts in a solar jet with high angular (∼20″), spectral (∼1%), and temporal resolution (50 ms). These observations allow us to derive detailed trajectories of semirelativistic (tens of keV) electron beams in the low solar corona with unprecedentedly high angular precision (<0.″65). We found that each group of electron beams, which corresponds to a cluster of type III bursts with 1-2 s duration, diverges from an extremely compact region (∼600 km2) in the low solar corona. The beam-diverging sites are located behind the erupting jet spire and above the closed arcades, coinciding with the presumed location of magnetic reconnection in the jet eruption picture supported by extreme ultraviolet/X-ray data and magnetic modeling. We interpret each beam-diverging site as a reconnection null point where multitudes of magnetic flux tubes join and reconnect. Our data suggest that the null points likely consist of a high level of density inhomogeneities possibly down to 10 km scales. These results, at least in the present case, strongly favor a reconnection-driven electron-acceleration scenario. Title: Solar radio emission as a disturbance of aeronautical radionavigation Authors: Marqué, Christophe; Klein, Karl-Ludwig; Monstein, Christian; Opgenoorth, Hermann; Pulkkinen, Antti; Buchert, Stephan; Krucker, Säm; Van Hoof, Rudiger; Thulesen, Peter Bibcode: 2018JSWSC...8A..42M Altcode: 2018arXiv180806878M On November 4th, 2015 secondary air traffic control radar was strongly disturbed in Sweden and some other European countries. The disturbances occurred when the radar antennas were pointing at the Sun. In this paper, we show that the disturbances coincided with the time of peaks of an exceptionally strong (∼105 Solar Flux Units) solar radio burst in a relatively narrow frequency range around 1 GHz. This indicates that this radio burst is the most probable space weather candidate for explaining the radar disturbances. The dynamic radio spectrum shows that the high flux densities are not due to synchrotron emission of energetic electrons, but to coherent emission processes, which produce a large variety of rapidly varying short bursts (such as pulsations, fiber bursts, and zebra patterns). The radio burst occurs outside the impulsive phase of the associated flare, about 30 min after the soft X-ray peak, and it is temporarily associated with fast evolving activity occurring in strong solar magnetic fields. While the relationship with strong magnetic fields and the coherent spectral nature of the radio burst provide hints towards the physical processes which generate such disturbances, we have so far no means to forecast them. Well-calibrated monitoring instruments of whole Sun radio fluxes covering the UHF band could at least provide a real-time identification of the origin of such disturbances, which reports in the literature show to also affect GPS signal reception. Title: Fermi-LAT Observations of the 2017 September 10 Solar Flare Authors: Omodei, Nicola; Pesce-Rollins, Melissa; Longo, Francesco; Allafort, Alice; Krucker, Säm Bibcode: 2018ApJ...865L...7O Altcode: 2018arXiv180307654O The Fermi-Large Area Telescope detection of the X8.2 GOES class solar flare of 2017 September 10 provides for the first time observations of a long-duration high-energy gamma-ray flare associated with a ground-level enhancement (GLE). The >100 MeV emission from this flare lasted for more than 12 hr covering both the impulsive and extended phases. We present the localization of the gamma-ray emission and find that it is consistent with the active region from which the flare occurred over a period lasting more than 6 hr. The temporal variation of the gamma-ray flux and of the proton index inferred from the gamma-ray data seems to suggest three phases in acceleration of the proton population. Based on timing arguments we interpret the last phase to be tied to the acceleration mechanism powering the powering the production of the GLE particles. Title: Hard X-Ray Constraints on Small-scale Coronal Heating Events Authors: Marsh, Andrew J.; Smith, David M.; Glesener, Lindsay; Klimchuk, James A.; Bradshaw, Stephen J.; Vievering, Juliana; Hannah, Iain G.; Christe, Steven; Ishikawa, Shin-nosuke; Krucker, Säm Bibcode: 2018ApJ...864....5M Altcode: 2018arXiv180802630M Much evidence suggests that the solar corona is heated impulsively, meaning that nanoflares may be ubiquitous in quiet and active regions (ARs). Hard X-ray (HXR) observations with unprecedented sensitivity >3 keV are now enabled by focusing instruments. We analyzed data from the Focusing Optics X-ray Solar Imager (FOXSI) rocket and the Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft to constrain properties of AR nanoflares simulated by the EBTEL field-line-averaged hydrodynamics code. We generated model X-ray spectra by computing differential emission measures for homogeneous nanoflare sequences with heating amplitudes H 0, durations τ, delay times between events t N , and filling factors f. The single quiescent AR observed by FOXSI-2 on 2014 December 11 is well fit by nanoflare sequences with heating amplitudes 0.02 erg cm-3 s-1 <H 0 < 13 erg cm-3 s-1 and a wide range of delay times and durations. We exclude delays between events shorter than ∼900 s at the 90% confidence level for this region. Three of five regions observed by NuSTAR on 2014 November 1 are well fit by homogeneous nanoflare models, while two regions with higher fluxes are not. Generally, the NuSTAR count spectra are well fit by nanoflare sequences with smaller heating amplitudes, shorter delays, and shorter durations than the allowed FOXSI-2 models. These apparent discrepancies are likely due to differences in spectral coverage between the two instruments and intrinsic differences among the regions. Steady heating (t N = τ) was ruled out with >99% confidence for all regions observed by either instrument. Title: Microwave and Hard X-Ray Observations of the 2017 September 10 Solar Limb Flare Authors: Gary, Dale E.; Chen, Bin; Dennis, Brian R.; Fleishman, Gregory D.; Hurford, Gordon J.; Krucker, Säm; McTiernan, James M.; Nita, Gelu M.; Shih, Albert Y.; White, Stephen M.; Yu, Sijie Bibcode: 2018ApJ...863...83G Altcode: 2018arXiv180702498G We report the first science results from the newly completed Expanded Owens Valley Solar Array (EOVSA), which obtained excellent microwave (MW) imaging spectroscopy observations of SOL2017-09-10, a classic partially occulted solar limb flare associated with an erupting flux rope. This event is also well-covered by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXRs). We present an overview of this event focusing on MW and HXR data, both associated with high-energy nonthermal electrons, and we discuss them within the context of the flare geometry and evolution revealed by extreme ultraviolet observations from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory. The EOVSA and RHESSI data reveal the evolving spatial and energy distribution of high-energy electrons throughout the entire flaring region. The results suggest that the MW and HXR sources largely arise from a common nonthermal electron population, although the MW imaging spectroscopy provides information over a much larger volume of the corona. 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: Microwave Spectral Imaging of Bi-Directional Magnetic Reconnection Outflow Region of the 2017 Sep 10 X8.2 Flare Authors: Chen, Bin; Gary, Dale E.; Fleishman, Gregory D.; Krucker, Sam; Nita, Gelu M.; Dennis, Brian R.; Yu, Sijie; Kuroda, Natsuha; Reeves, Katharine K.; Polito, Vanessa; Shih, Albert Bibcode: 2018shin.confE.211C Altcode: The newly commissioned Expanded Owens Valley Solar Array (EOVSA) obtained microwave spectral imaging of the spectacular eruptive solar flare on 2017 September 10 in 2.5-18 GHz. During the early impulsive phase of the flare ( 15:54 UT), An elongated microwave source appears to connect the top of the flare arcade to the bottom of the erupting magnetic flux rope. Multi-frequency images reveal that the source bifurcates into two parts: One is located at and above the hard X-ray looptop source, and another located behind the flux rope. They appear to follow closely with the bi-directional reconnection downflow and upflow region as inferred from the SDO/AIA EUV images. The spatially resolved spectra of this microwave source show characteristics of gyrosynchrotron radiation, suggesting the presence of high-energy (100s of keV to MeV) electrons throughout the bi-directional reconnection outflow region. We derive physical parameters of the source region, and discuss their implications in magnetic energy release and electron acceleration. Title: Identifying Typical Mg II Flare Spectra Using Machine Learning Authors: Panos, Brandon; Kleint, Lucia; Huwyler, Cedric; Krucker, Säm; Melchior, Martin; Ullmann, Denis; Voloshynovskiy, Sviatoslav Bibcode: 2018ApJ...861...62P Altcode: 2018arXiv180510494P The Interface Region Imaging Spectrograph (IRIS) performs solar observations over a large range of atmospheric heights, including the chromosphere where the majority of flare energy is dissipated. The strong Mg II h&k spectral lines are capable of providing excellent atmospheric diagnostics, but have not been fully utilized for flaring atmospheres. We aim to investigate whether the physics of the chromosphere is identical for all flare observations by analyzing if there are certain spectra that occur in all flares. To achieve this, we automatically analyze hundreds of thousands of Mg II h&k-line profiles from a set of 33 flares and use a machine learning technique, which we call supervised hierarchical k-means, to cluster all profile shapes. We identify a single peaked Mg II profile, in contrast to the double-peaked quiet Sun profiles, appearing in every flare. Additionally, we find extremely broad profiles with characteristic blueshifted central reversals appearing at the front of fast-moving flare ribbons. These profiles occur during the impulsive phase of the flare, and we present results of their temporal and spatial correlation with non-thermal hard X-ray signatures, suggesting that flare-accelerated electrons play an important role in the formation of these profiles. The ratio of the integrated Mg II h&k lines can also serve as an opacity diagnostic, and we find higher opacities during each flare maximum. Our study shows that machine learning is a powerful tool for large scale statistical solar analyses. Title: Microwave Spectroscopic Imaging of the Magnetic Reconnection Region in the 2017 September 10 Eruptive Solar Flare Authors: Chen, Bin; Gary, Dale E.; Fleishman, Gregory D.; Krucker, Sam; Nita, Gelu M.; Dennis, Brian R.; Yu, Sijie; Kuroda, Natsuha; Reeves, Katharine; Polito, Vanessa; Shih, Albert Y. Bibcode: 2018tess.conf30603C Altcode: The newly commissioned Expanded Owens Valley Solar Array (EOVSA) obtained excellent high-cadence (1 s), microwave spectroscopic imaging of the spectacular eruptive solar flare on 2017 September 10 in 2.5-18 GHz. During the early impulsive phase of the flare (~15:53-15:55 UT), EOVSA images reveal an elongated microwave source that connects the top of the cusp-shaped flare arcade to the bottom of the erupting magnetic flux rope. The spatially resolved spectra of this microwave source show characteristics of gyrosynchrotron radiation, suggesting the presence of high-energy nonthermal electrons throughout the source region that presumably encloses the magnetic reconnection site(s) and bi-directional reconnection outflows. In addition, the lower and upper portions of the source seem to have different spatial and spectral properties. We derive physical parameters of the source region, and discuss their implications in magnetic energy release and electron acceleration. 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: NuSTAR X-ray observations of tiny solar flares Authors: Hannah, Iain G.; Krucker, Sam; Grefenstette, Brian; Glesener, Lindsay; Kuhar, Matej; Miles Smith, David; Kleint, Lucia Bibcode: 2018tess.conf40801H Altcode: NuSTAR is an astrophysics X-ray telescope, with direct imaging spectroscopy providing a unique sensitivity for observing the Sun above 2.5keV. This is ideal for capturing the response of the solar atmosphere to the energy released in the smallest flares. NuSTAR has observed the Sun several times since Sep 2014 and we present some of the initial observations of tiny microflares within active regions, events down to the GOES A0.1 equivalent level. These microflares show thermal emission up to 10MK, and the possibility of non-thermal emission powering this heating. In conjunction with observations at lower energy wavelengths (Hinode/XRT, SDO/AIA and IRIS) we are able to present a more complete picture of the solar atmosphere's thermal response to these small flares. We also show tiny impulsive events outside of active regions, that are orders of magnitude smaller, and yet still emit a clear X-ray signature. Title: NGSPM Objectives Addressed by the FOXSI SMEX Mission Authors: Shih, Albert Y.; Christe, Steven; Krucker, Sam; Glesener, Lindsay; Saint-Hilaire, Pascal; Caspi, Amir Bibcode: 2018tess.conf41005S Altcode: In addition to the baseline instrument complement recommended by the NGSPM study, the study also identified a hard X-ray spectroscopic imager as the next-highest-priority instrument. While there have been significant advances in our understanding of impulsive energy release at the Sun since the advent of RHESSI X-ray 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 NGSPM study acknowledges grazing-incidence focusing X-ray optics as the way to achieve spectroscopic imaging with the combination of high dynamic range and high sensitivity.

The FOXSI Small Explorer (SMEX) mission, currently undergoing a Phase A concept study, addresses this measurement need within the envelope of a NASA SMEX mission. 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 is proposed to launch in 2022, and thus there will be synergy between its X-ray observations and the observations made at other wavelengths by the mission(s) recommended by the NGSPM study. Here we present examples with simulated observations to show how FOXSI's capabilities address NGSPM science objectives. Title: The Micro Solar-Flare Apparatus (MiSolFA) Authors: Christe, Steven; Casadei, Diego; Krucker, Sam; Lastufka, Erica Bibcode: 2018tess.conf40237C Altcode: Advances in the study of coronal electron-accelerating regions have so far been limited by a single view-point. This means that we have limited information about the three dimensional structure of sources. The micro-satellite MiSolFA (Micro Solar-Flare Apparatus) is designed both as a stand-alone X-ray imaging spectrometer and a complement to the Spectrometer/Telescope for Imaging X-rays (STIX) mission to provide stereoscopic x-ray imaging.

MiSolFA will be the most compact X-ray imaging spectrometer in space. Thanks to absorbing grids produced using a novel approach and to new CdTe photon detectors with small pixel size and excellent energy resolution, MiSolFA will perform indirect imaging between 10 and 100 keV and with 10 arcsec angular resolution, sufficient to separate most hard X-ray footpoint sources from each other. The instrument is small enough to equip a 6-units cubesat such as the GSFC-developed Dellingr platform. Title: RHESSI Observations of the 2017 Sep 10 Solar Flare Authors: Shih, Albert Y.; Krucker, Sam; Dennis, Brian R. Bibcode: 2018tess.conf31502S Altcode: We present soft X-ray and hard X-ray observations of the 2017 September 10 X-class flare by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). RHESSI observed this flare from ~6 keV to ~300 keV with an angular resolution of 7" FWHM. We use RHESSI images to determine the spatial distributions of hot plasma and non-thermal electrons and their evolution in time, including their associations with coronal loops, chromospheric footpoints, and the large-scale current sheet. We analyze RHESSI spectra to obtain the temperature of the hot plasma and the energy in non-thermal electrons over the course of the flare. Finally, we compare the RHESSI observations with observations at other wavelengths. 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: 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: Combined SDO/AIA, Hinode/XRT and FOXSI-2 microflare observations - DEM analysis and energetics Authors: Panchapakesan, S. A.; Glesener, L.; Vievering, J. T.; Ryan, D.; Christe, S.; Inglis, A. R.; Buitrago-Casas, J. C.; Musset, S.; Krucker, S. Bibcode: 2017AGUFMSH41A2745P Altcode: The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket makes directimaging and spectral observation of the Sun in hard X-rays (HXRs) using highlysensitive focusing HXR optics. The second flight of FOXSI was launchedsuccessfully on 11 December 2014 and observed significant HXR emissions duringmicroflares. Some of these flares showed heating up to severalmillion Kelvin and were visible in the Extreme Ultraviolet (EUV) with the AtmosphericImaging Assembly (SDO/AIA). Spectral observations from FOXSI suggest emission upto 10-12 MK. We utilize SDO/AIA EUV, Hinode/XRT soft X-ray, and FOXSI-2 highenergy X-ray observations to derive the differential emission measure (DEM) ofthe microflares. The AIA and XRT observations provide broad temperaturecoverage but are poorly constrained at the hotter end. We therefore use FOXSI-2to better determine the high temperature component, thus producing a moreconstrained DEM than is possible with typically available observations. We usethis more highly constrained DEM to investigate the energetics of the observedmicroflares. Title: Tracing Fast Electron Beams Emanating from the Magnetic Reconnection Site in a Solar Jet Authors: Chen, B.; Yu, S.; Battaglia, M.; Krucker, S. Bibcode: 2017AGUFMSH43C..06C Altcode: Fast electron beams propagating in the solar corona can emit radio waves commonly known as type III radio bursts. At decimetric wavelengths, these bursts are emitted from the low corona where flare energy release is thought to take place. As such, decimetric type III radio bursts can serve as an excellent tool to directly trace fast electron beams in the vicinity of the flare energy release site. Here we report observations of decimetric type III bursts during a jet event using the Jansky Very Large Array (VLA) in 1-2 GHz. Taking advantage of VLA's highly sensitive spectral imaging capability with an ultra-high cadence of 50 ms, we derive detailed trajectories of fast electron beams (with a bulk speed of at least 0.3-0.5c, or several tens of keV) and place them in the context of extreme ultraviolet and X-ray images obtained by SDO/AIA and RHESSI. Our results show that the electron beams originated in a region just below the jet and above the lower-lying small-scale flare loops, presumably where the magnetic energy release took place. We show that the electron beams appear in groups, each with a duration of only a few seconds. Each group, consisting of beams propagating along magnetic field lines at different angles, is seen to emanate from a single site trailing the jet, interpreted as the magnetic reconnection null point. Our results suggest, at least for the present case, that the fast electron beams were energized directly at the magnetic reconnection site which was highly inhomogeneous and fragmentary possibly down to kilometer scales. Title: Joint NuSTAR and IRIS observation of a microflaring active region Authors: Hannah, I. G.; Kleint, L.; Krucker, S.; Glesener, L.; Grefenstette, B. Bibcode: 2017AGUFMSH41A2743H Altcode: We present observations of a weakly microflaring active region observed in X-rays with NuSTAR, UV with IRIS and EUV with SDO/AIA. NuSTAR was pointed at this unnamed active region near the East limb between 23:27UT and 23:37UT 26-July-2016, finding mostly quiescent emission except for a small microflare about 23:35UT. The NuSTAR spectrum for the pre-microflare time (23:27UT to 23:34UT) is well fitted by a single thermal component of about 3MK and combined with SDO/AIA we can determine the differential emission measure (DEM), finding it, as expected, drops very sharply to higher temperatures. During the subsequent microflare, the increase in NuSTAR counts matches a little brightening loop observed with IRIS SJI 1400Å and SDO/AIA. Fortuitously the IRIS slit crosses this microflaring loop and we find an increased emission in Si IV 1394Å, Si IV 1403Å and O IV 1402Å but only average line widths and velocities. The NuSTAR microflare spectrum shows heating to higher temperatures and also allows us to investigate the energetics of this event. Title: Electron Beams Escaping the Sun: Hard X-ray Diagnostics of Jet-related Electron Acceleration Authors: Glesener, L.; Musset, S.; Saint-Hilaire, P.; Fleishman, G. D.; Krucker, S.; Christe, S.; Shih, A. Y. Bibcode: 2017AGUFMSH23D2704G Altcode: Coronal jets, which arise via an interaction between closed and open magnetic field, offer a convenient configuration for accelerated electrons to escape the low corona. Jets occur in all regions of the Sun, but those flare-related jets that occur in active regions are associated with bremsstrahlung hard X-rays (HXRs) from accelerated electrons. However, HXR measurement of the escaping beams themselves is elusive as it requires extremely high sensitivity. Jets are strongly correlated with Type III radio bursts in the corona and in interplanetary space. In this poster we present RHESSI observations of HXRs from flare-related jets, including multiwavelength analysis (with extreme ultraviolet and radio emission) and modeling of the emitting electron populations. We also present predicted observations of Type III-emitting electron beams by the FOXSI Small Explorer, which is currently undergoing a NASA Phase A concept study. FOXSI will measure HXRs from jets and flares in the low corona, providing quantitative diagnostics of accelerated electron beams at their origin. These same electron beams will be measured at higher altitudes by instruments aboard NASA's Parker Solar Probe and ESA's Solar Orbiter. With a planned launch in the rising phase of Solar Cycle 25, FOXSI will be ideally timed and optimized for collaborative study of electron beams escaping the Sun. Title: Investigation of Energy Release in Microflares Observed by the Second Sounding Rocket Flight of the Focusing Optics X-ray Solar Imager (FOXSI-2) Authors: Vievering, J. T.; Glesener, L.; Panchapakesan, S. A.; Ryan, D.; Krucker, S.; Christe, S.; Buitrago-Casas, J. C.; Inglis, A. R.; Musset, S. Bibcode: 2017AGUFMSH41A2744V Altcode: Observations of the Sun in hard x-rays can provide insight into many solar phenomena which are not currently well-understood, including the mechanisms behind particle acceleration in flares. RHESSI is the only solar-dedicated imager currently operating in the hard x-ray regime. Though RHESSI has greatly added to our knowledge of flare particle acceleration, the indirect imaging method of rotating collimating optics is fundamentally limited in sensitivity and dynamic range. By instead using a direct imaging technique, the structure and evolution of even small flares and active regions can be investigated in greater depth. FOXSI (Focusing Optics X-ray Solar Imager), a hard x-ray instrument flown on two sounding rocket campaigns, seeks to achieve these improved capabilities by using focusing optics for solar observations in the 4-20 keV range. During the second of the FOXSI flights, flown on December 11, 2014, two microflares were observed, estimated as GOES class A0.5 and A2.5 (upper limits). Here we present current imaging and spectral analyses of these microflares, exploring the nature of energy release and comparing to observations from other instruments. Additionally, we feature the first analysis of data from the FOXSI-2 CdTe strip detectors, which provide improved efficiency above 10 keV. Through this analysis, we investigate the capabilities of FOXSI in enhancing our knowledge of smaller-scale solar events. 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: 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: The size of coronal hard X-ray sources in solar flares: How big are they? Authors: Effenberger, F.; Krucker, S.; Rubio da Costa, F. Bibcode: 2017AGUFMSH41A2746E Altcode: Coronal hard X-ray sources are considered to be one of the key signatures of non-thermal particle acceleration and heating during the energy release in solar flares. In some cases, X-ray observations reveal multiple components spatially located near and above the loop top and even further up in the corona. Here, we combine a detailed RHESSI imaging analysis of near-limb solar flares with occulted footpoints and a multi-wavelength study of the flare loop evolution in SDO/AIA. We connect our findings to different current sheet formation and magnetic break-out scenarios and relate it to particle acceleration theory. We find that the upper and usually fainter emission regions can be underestimated in their size due to the majority of flux originating from the lower loops. 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: Detection of nanoflare-heated plasma in the solar corona by the FOXSI-2 sounding rocket Authors: Ishikawa, Shin-nosuke; Glesener, Lindsay; Krucker, Säm; Christe, Steven; Buitrago-Casas, Juan Camilo; Narukage, Noriyuki; Vievering, Juliana Bibcode: 2017NatAs...1..771I Altcode: The processes that heat the solar and stellar coronae to several million kelvins, compared with the much cooler photosphere (5,800 K for the Sun), are still not well known1. One proposed mechanism is heating via a large number of small, unresolved, impulsive heating events called nanoflares2. Each event would heat and cool quickly, and the average effect would be a broad range of temperatures including a small amount of extremely hot plasma. However, detecting these faint, hot traces in the presence of brighter, cooler emission is observationally challenging. Here we present hard X-ray data from the second flight of the Focusing Optics X-ray Solar Imager (FOXSI-2), which detected emission above 7 keV from an active region of the Sun with no obvious individual X-ray flare emission. Through differential emission measure computations, we ascribe this emission to plasma heated above 10 MK, providing evidence for the existence of solar nanoflares. The quantitative evaluation of the hot plasma strongly constrains the coronal heating models. Title: On the Nature of Off-limb Flare Continuum Sources Detected by SDO/HMI Authors: Heinzel, P.; Kleint, L.; Kašparová, J.; Krucker, S. Bibcode: 2017ApJ...847...48H Altcode: 2017arXiv170906377H The Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory has provided unique observations of off-limb flare emission. White-light continuum enhancements were detected in the “continuum” channel of the Fe 6173 Å line during the impulsive phase of the observed flares. In this paper we aim to determine which radiation mechanism is responsible for such enhancement being seen above the limb, at chromospheric heights around or below 1000 km. Using a simple analytical approach, we compare two candidate mechanisms, the hydrogen recombination continuum (Paschen) and the Thomson continuum due to scattering of disk radiation on flare electrons. Both mechanisms depend on the electron density, which is typically enhanced during the impulsive phase of a flare as the result of collisional ionization (both thermal and also non-thermal due to electron beams). We conclude that for electron densities higher than 1012 cm-3, the Paschen recombination continuum significantly dominates the Thomson scattering continuum and there is some contribution from the hydrogen free-free emission. This is further supported by detailed radiation-hydrodynamical (RHD) simulations of the flare chromosphere heated by the electron beams. We use the RHD code FLARIX to compute the temporal evolution of the flare-heating in a semi-circular loop. The synthesized continuum structure above the limb resembles the off-limb flare structures detected by HMI, namely their height above the limb, as well as the radiation intensity. These results are consistent with recent findings related to hydrogen Balmer continuum enhancements, which were clearly detected in disk flares by the IRIS near-ultraviolet spectrometer. Title: Calibration of the hard x-ray detectors for the FOXSI solar sounding rocket Authors: Athiray, P. S.; Buitrago-Casas, Juan Camilo; Bergstedt, Kendra; Vievering, Juliana; Musset, Sophie; Ishikawa, Shin-nosuke; Glesener, Lindsay; Takahashi, Tadayuki; Watanabe, Shin; Courtade, Sasha; Christe, Steven; Krucker, Säm.; Goetz, Keith; Monson, Steven Bibcode: 2017SPIE10397E..0AA Altcode: The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment conducts direct imaging and spectral observation of the Sun in hard X-rays, in the energy range 4 to 20 keV. These high-sensitivity observations are used to study particle acceleration and coronal heating. FOXSI is designed with seven grazing incidence optics modules that focus X-rays onto seven focal plane detectors kept at a 2m distance. FOXSI-1 was flown with seven Double-sided Si Strip Detectors (DSSD), and two of them were replaced with CdTe detectors for FOXSI-2. The upcoming FOXSI-3 flight will carry DSSD and CdTe detectors with upgraded optics for enhanced sensitivity. The detectors are calibrated using various radioactive sources. The detector's spectral response matrix was constructed with diagonal elements using a Gaussian approximation with a spread (sigma) that accounts for the energy resolution of the detector. Spectroscopic studies of past FOXSI flight data suggest that the inclusion of lower energy X-rays could better constrain the spectral modeling to yield a more precise temperature estimation of the hot plasma. This motivates us to carry out an improved calibration to better understand the finer-order effects on the spectral response, especially at lower energies. Here we report our improved calibration of FOXSI detectors using experiments and Monte-Carlo simulations. 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: Methods for reducing singly reflected rays on the Wolter-I focusing mirrors of the FOXSI rocket experiment Authors: Buitrago-Casas, Juan Camilo; Elsner, Ronald; Glesener, Lindsay; Christe, Steven; Ramsey, Brian; Courtade, Sasha; Ishikawa, Shin-nosuke; Narukage, Noriyuki; Turin, Paul; Vievering, Juliana; Athiray, P. S.; Musset, Sophie; Krucker, Säm. Bibcode: 2017SPIE10399E..0JB Altcode: In high energy solar astrophysics, imaging hard X-rays by direct focusing offers higher dynamic range and greater sensitivity compared to past techniques that used indirect imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload that uses seven sets of nested Wolter-I figured mirrors together with seven high-sensitivity semiconductor detectors to observe the Sun in hard X-rays through direct focusing. The FOXSI rocket has successfully flown twice and is funded to fly a third time in summer 2018. The Wolter-I geometry consists of two consecutive mirrors, one paraboloid and one hyperboloid, that reflect photons at grazing angles. Correctly focused X-rays reflect once per mirror segment. For extended sources, like the Sun, off-axis photons at certain incident angles can reflect on only one mirror and still reach the focal plane, generating a background pattern of singly reflected rays (i.e., ghost rays) that can limit the sensitivity of the observation to faint, focused sources. Understanding and mitigating the impact of the singly reflected rays on the FOXSI optical modules will maximize the instruments' sensitivity to background-limited sources. We present an analysis of the FOXSI singly reflected rays based on ray-tracing simulations and laboratory measurements, as well as the effectiveness of different physical strategies to reduce them. Title: Methods for reducing ghost rays on the Wolter-I focusing figures of the FOXSI rocket payload Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe, Steven; Ramsey, Brian; Elsner, Ronald; Courtade, Sasha; Vievering, Juliana; Subramania, Athiray; Krucker, Sam; Bale, Stuart Bibcode: 2017SPD....4830504B Altcode: In high energy solar astrophysics, imaging hard X-rays by direct focusing offers higher dynamic range and greater sensitivity compared to past techniques that used indirect imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload which uses seven sets of nested Wolter-I figured mirrors that, together with seven high-sensitive semiconductor detectors, observes the Sun in hard X-rays by direct focusing. The FOXSI rocket has successfully flown twice and is funded to fly a third time in summer 2018.The Wolter-I geometry consists of two consecutive mirrors, one paraboloid, and one hyperboloid, that reflect photons at grazing angles. Correctly focused X-rays reflect twice, once per mirror segment. For extended sources, like the Sun, off-axis photons at certain incident angles can reflect on only one mirror and still reach the focal plane, generating a pattern of single-bounce photons, or ‘ghost rays’ that can limit the sensitivity of the observation of focused X-rays. Understanding and cutting down the ghost rays on the FOXSI optics will maximize the instrument’s sensitivity of the solar faintest sources for future flights. We present an analysis of the FOXSI ghost rays based on ray-tracing simulations, as well as the effectiveness of different physical strategies to reduce them. Title: NuSTAR's X-ray observations of a microflaring active region Authors: Hannah, Iain; Kleint, Lucia; Krucker, Sam; Wright, Paul James; Glesener, Lindsay; Grefenstette, Brian Bibcode: 2017SPD....4820101H Altcode: We present observations of a weakly microflaring active region observed in X-rays with NuSTAR, UV with IRIS and EUV with SDO/AIA. NuSTAR was pointed at this unnamed active region near the East limb between 23:27UT and 23:37UT 26-July-2016, finding mostly quiescent emission except for a small microflare about 23:35UT. The NuSTAR spectrum for the pre-microflare time (23:27UT to 23:34UT) is well fitted by a single thermal of about 3MK and combined with SDO/AIA we can determine the differential emission measure (DEM), finding it, as expected, drops very sharply to higher temperatures. During the subsequent microflare, the increase in NuSTAR counts matches a little brightening loop observed with IRIS SJI 1400Å and SDO/AIA 94Å/Fe XVIII. Fortuitously the IRIS slit was on this microflaring loop and we find that the IRIS spectrum shows increased emission in Si IV 1394Å, O IV 1402Å and Si IV 1403Å but only average line widths and velocities. The NuSTAR microflare spectrum shows heating to higher temperatures and also allows us to investigate the energetics of this event. 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: Nonequilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind (Invited Review) Authors: Dudík, Jaroslav; Dzifčáková, Elena; Meyer-Vernet, Nicole; Del Zanna, Giulio; Young, Peter R.; Giunta, Alessandra; Sylwester, Barbara; Sylwester, Janusz; Oka, Mitsuo; Mason, Helen E.; Vocks, Christian; Matteini, Lorenzo; Krucker, Säm; Williams, David R.; Mackovjak, Šimon Bibcode: 2017SoPh..292..100D Altcode: 2017arXiv170603396D We review the presence and signatures of the non-equilibrium processes, both non-Maxwellian distributions and non-equilibrium ionization, in the solar transition region, corona, solar wind, and flares. Basic properties of the non-Maxwellian distributions are described together with their influence on the heat flux as well as on the rates of individual collisional processes and the resulting optically thin synthetic spectra. Constraints on the presence of high-energy electrons from observations are reviewed, including positive detection of non-Maxwellian distributions in the solar corona, transition region, flares, and wind. Occurrence of non-equilibrium ionization is reviewed as well, especially in connection to hydrodynamic and generalized collisional-radiative modeling. Predicted spectroscopic signatures of non-equilibrium ionization depending on the assumed plasma conditions are summarized. Finally, we discuss the future remote-sensing instrumentation that can be used for the detection of these non-equilibrium phenomena in various spectral ranges. 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: UV and X-ray Evolution of AR12230 as Observed with IRIS and FOXSI-II Authors: Ryan, Daniel; Christe, Steven; Glesener, Lindsay; Vievering, Julie; Krucker, Sam; Ishikawa, Shin-Nosuke Bibcode: 2017SPD....4820401R Altcode: We present a multi-spectral and spatio-temporal analysis of AR12230 using both UV and X-ray spectroscopic imaging obtained as part of a coordinated observing campaign on 11 December 2014. The campaign involved IRIS (Interface Region Imaging Spectrometer) -- which provides both UV imaging and slit spectrograph observations of optically thick chromospheric and transition region emission -- and FOXSI-II (Focusing Optics X-ray Solar Imager) -- the second in a series of sounding rocket flights which combines grazing incidence direct focusing optics to produce solar X-ray spectroscopic imaging in the range 4-15keV. The active region exhibits a prolonged compact brightening in the IRIS 1330 A and 1400 A slit-jaw channels near the center of the active region throughout the duration of the observations. In the early phase of the observations FOXSI-II shows an X-ray source approximately 20x20 arcsec centered at the same location. The X-ray spectra show the presence of hot (~8 MK) thermal plasma and is suggestive of the presence of non-thermal electrons.. Later, two additional transient, spatially extended, simultaneous brightenings are observed, one of which was captured by the IRIS slit spectrograph. We combine these observations to explore the evolution and topology of the active region. Hydrodynamic modeling of the chromosphere is used to place a limit on the amount of non-thermal electrons required to produce the observed UV emission. This result is then compared to the limit inferred from the FOXSI-II X-ray spectra. Thus, we explore the role of non-thermal electrons and hydrodynamics in the energization and evolution of plasma in active regions. Title: DEM analysis of FOXSI-2 microflare using AIA observations Authors: Athiray Panchapakesan, Subramania; Glesener, Lindsay; Vievering, Juliana; Camilo Buitrago-Casas, Juan; Christe, Steven; Inglis, Andrew; Krucker, Sam; Musset, Sophie Bibcode: 2017SPD....4810632A Altcode: The second flight of Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment was successfully completed on 11 December 2014. FOXSI makes direct imaging and spectral observation of the Sun in hard X-rays using grazing incidence optics modules which focus X-rays onto seven focal plane detectors kept at a 2m distance, in the energy range 4 to 20 keV, to study particle acceleration and coronal heating. Significant HXR emissions were observed by FOXSI during microflare events with A0.5 and A2.5 class, as classified by GOES, that occurred during FOXSI-2 flight.Spectral analysis of FOXSI data for these events indicate presence of plasma at higher temperatures (>10MK). We attempt to study the plasma content in the corona at different temperatures, characterized by the differential emission measure (DEM), over the FOXSI-2 observed flare regions using the Atmospheric Imaging Assembly (SDO/AIA) data. We utilize AIA observations in different EUV filters that are sensitive to ionized iron lines, to determine the DEM by using a regularized inversion method. This poster will show the properties of hot plasma as derived from FOXSI-2 HXR spectra with supporting DEM analysis using AIA observations. Title: Hard X-Ray Constraints on Small-Scale Coronal Heating Events Authors: Marsh, Andrew; Smith, David M.; Glesener, Lindsay; Klimchuk, James A.; Bradshaw, Stephen; Hannah, Iain; Vievering, Juliana; Ishikawa, Shin-Nosuke; Krucker, Sam; Christe, Steven Bibcode: 2017SPD....4810614M Altcode: A large body of evidence suggests that the solar corona is heated impulsively. Small-scale heating events known as nanoflares may be ubiquitous in quiet and active regions of the Sun. Hard X-ray (HXR) observations with unprecedented sensitivity >3 keV have recently been enabled through the use of focusing optics. We analyze active region spectra from the FOXSI-2 sounding rocket and the NuSTAR satellite to constrain the physical properties of nanoflares simulated with the EBTEL field-line-averaged hydrodynamics code. We model a wide range of X-ray spectra by varying the nanoflare heating amplitude, duration, delay time, and filling factor. Additional constraints on the nanoflare parameter space are determined from energy constraints and EUV/SXR data. 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: 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: On the Origin of the Flare Emission in IRIS’ SJI 2832 Filter:Balmer Continuum or Spectral Lines? Authors: Kleint, Lucia; Heinzel, Petr; Krucker, Säm Bibcode: 2017ApJ...837..160K Altcode: 2017arXiv170207167K Continuum (“white-light,” WL) emission dominates the energetics of flares. Filter-based observations, such as the IRIS SJI 2832 filter, show WL-like brightenings during flares, but it is unclear whether the emission arises from real continuum emission or enhanced spectral lines, possibly turning into emission. The difficulty in filter-based observations, contrary to spectral observations, is to determine which processes contribute to the observed brightening during flares. Here we determine the contribution of the Balmer continuum and the spectral line emission to IRIS’ SJI 2832 emission by analyzing the appropriate passband in simultaneous IRIS NUV spectra. We find that spectral line emission can contribute up to 100% to the observed slitjaw images (SJI) emission, that the relative contributions usually temporally vary, and that the highest SJI enhancements that are observed are most likely because of the Balmer continuum. We conclude that care should be taken when calling SJI 2832 a continuum filter during flares, because the influence of the lines on the emission can be significant. Title: Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles Authors: Effenberger, Frederic; Rubio da Costa, Fatima; Oka, Mitsuo; Saint-Hilaire, Pascal; Liu, Wei; Petrosian, Vahé; Glesener, Lindsay; Krucker, Säm Bibcode: 2017ApJ...835..124E Altcode: 2016arXiv161202856E Flares close to the solar limb, where the footpoints are occulted, can reveal the spectrum and structure of the coronal looptop source in X-rays. We aim at studying the properties of the corresponding energetic electrons near their acceleration site, without footpoint contamination. To this end, a statistical study of partially occulted flares observed with Reuven Ramaty High-Energy Solar Spectroscopic Imager is presented here, covering a large part of solar cycles 23 and 24. We perform detailed spectra, imaging, and light curve analyses for 116 flares and include contextual observations from SDO and STEREO when available, providing further insights into flare emission that were previously not accessible. We find that most spectra are fitted well with a thermal component plus a broken power-law, non-thermal component. A thin-target kappa distribution model gives satisfactory fits after the addition of a thermal component. X-ray imaging reveals small spatial separation between the thermal and non-thermal components, except for a few flares with a richer coronal source structure. A comprehensive light curve analysis shows a very good correlation between the derivative of the soft X-ray flux (from GOES) and the hard X-rays for a substantial number of flares, indicative of the Neupert effect. The results confirm that non-thermal particles are accelerated in the corona and estimated timescales support the validity of a thin-target scenario with similar magnitudes of thermal and non-thermal energy fluxes. Title: Fermi-LAT Observations of High-energy Behind-the-limb Solar Flares Authors: Ackermann, M.; Allafort, A.; Baldini, L.; Barbiellini, G.; Bastieri, D.; Bellazzini, R.; Bissaldi, E.; Bonino, R.; Bottacini, E.; Bregeon, J.; Bruel, P.; Buehler, R.; Cameron, R. A.; Caragiulo, M.; Caraveo, P. A.; Cavazzuti, E.; Cecchi, C.; Charles, E.; Ciprini, S.; Costanza, F.; Cutini, S.; D'Ammando, F.; de Palma, F.; Desiante, R.; Digel, S. W.; Di Lalla, N.; Di Mauro, M.; Di Venere, L.; Drell, P. S.; Favuzzi, C.; Fukazawa, Y.; Fusco, P.; Gargano, F.; Giglietto, N.; Giordano, F.; Giroletti, M.; Grenier, I. A.; Guillemot, L.; Guiriec, S.; Jogler, T.; Jóhannesson, G.; Kashapova, L.; Krucker, S.; Kuss, M.; La Mura, G.; Larsson, S.; Latronico, L.; Li, J.; Liu, W.; Longo, F.; Loparco, F.; Lubrano, P.; Magill, J. D.; Maldera, S.; Manfreda, A.; Mazziotta, M. N.; Mitthumsiri, W.; Mizuno, T.; Monzani, M. E.; Morselli, A.; Moskalenko, I. V.; Negro, M.; Nuss, E.; Ohsugi, T.; Omodei, N.; Orlando, E.; Pal'shin, V.; Paneque, D.; Perkins, J. S.; Pesce-Rollins, M.; Petrosian, V.; Piron, F.; Principe, G.; Rainò, S.; Rando, R.; Razzano, M.; Reimer, O.; Rubio da Costa, F.; Sgrò, C.; Simone, D.; Siskind, E. J.; Spada, F.; Spandre, G.; Spinelli, P.; Tajima, H.; Thayer, J. B.; Torres, D. F.; Troja, E.; Vianello, G. Bibcode: 2017ApJ...835..219A Altcode: 2017arXiv170200577A We report on the Fermi-LAT detection of high-energy emission from the behind-the-limb (BTL) solar flares that occurred on 2013 October 11, and 2014 January 6 and September 1. The Fermi-LAT observations are associated with flares from active regions originating behind both the eastern and western limbs, as determined by STEREO. All three flares are associated with very fast coronal mass ejections (CMEs) and strong solar energetic particle events. We present updated localizations of the >100 MeV photon emission, hard X-ray (HXR) and EUV images, and broadband spectra from 10 keV to 10 GeV, as well as microwave spectra. We also provide a comparison of the BTL flares detected by Fermi-LAT with three on-disk flares and present a study of some of the significant quantities of these flares as an attempt to better understand the acceleration mechanisms at work during these occulted flares. We interpret the HXR emission to be due to electron bremsstrahlung from a coronal thin-target loop top with the accelerated electron spectra steepening at semirelativistic energies. The >100 MeV gamma-rays are best described by a pion-decay model resulting from the interaction of protons (and other ions) in a thick-target photospheric source. The protons are believed to have been accelerated (to energies >10 GeV) in the CME environment and precipitate down to the photosphere from the downstream side of the CME shock and landed on the front side of the Sun, away from the original flare site and the HXR emission. Title: Fermi Large Area Telescope Observations of High-Energy Gamma-ray Emission From Behind-the-limb Solar Flares Authors: Omodei, Nicola; Pesce-Rollins, Melissa; Petrosian, Vahe; Liu, Wei; Rubio da Costa, Fatima; Golenetskii, Sergei; Kashapova, Larisa; Krucker, Sam; Palshin, Valentin; Fermi Large Area Telescope Collaboration Bibcode: 2017APS..APR.Y3005O Altcode: Fermi LAT >30 MeV observations of the active Sun have increased the number of detected solar flares by almost a factor of 10 with respect to previous space observations. Of particular interest are the recent detections of three solar flares whose position behind the limb was confirmed by the STEREO-B spacecraft. These observations sample flares from active regions originating from behind both the eastern and western limbs and include an event associated with the second ground level enhancement event (GLE) of the 24th Solar Cycle. While gamma-ray emission up to tens of MeV resulting from proton interactions has been detected before from occulted solar flares, the significance of these particular events lies in the fact that these are the first detections of >100 MeV gamma-ray emission from footpoint-occulted flares. These detections present an unique opportunity to diagnose the mechanisms of high-energy emission and particle acceleration and transport in solar flares. We will present the Fermi-LAT, RHESSI and STEREO observations of these flares and discuss the various emission scenarios for these sources. 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: Hard X-Ray Observations of Coronal Sources: Implications for Particle Acceleration Authors: Effenberger, F.; Rubio da Costa, F.; Oka, M.; Saint-Hilaire, P.; Liu, W.; Krucker, S.; Glesener, L.; Petrosian, V. Bibcode: 2016AGUFMSH51E2634E Altcode: The properties of hard X-ray emission from solar flares can provide insight into particle acceleration and transport processes. Commonly, at higher energies the bright footpoint emission from the flare loop prevents a detailed analysis of the weaker loop-top source due to the limited dynamic range. Thus, flares close to the solar limb, where the footpoints are occulted, are interesting events to study because they can reveal the coronal loop-top emission and thus the electron properties at their acceleration site. We present results of a survey study of partially occulted flares observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We found that most of the flare spectra allow a fit to a thermal plus non-thermal component, either with a broken power-law or a kappa function. The spatial separation between the thermal and non-thermal component, as derived from imaging, is usually small. The light curve analysis shows for many flares a very good correlation between the derivative of the soft X-ray flux and the hard X-rays. We discuss implications for particle acceleration models that result from our study. Title: Analysis of Microflares from the Second Sounding Rocket Flight of the Focusing Optics X-ray Solar Imager (FOXSI-2) Authors: Vievering, J. T.; Glesener, L.; Krucker, S.; Christe, S.; Buitrago-Casas, J. C.; Ishikawa, S. N.; Ramsey, B.; Takahashi, T.; Watanabe, S. Bibcode: 2016AGUFMSH11D..05V Altcode: Observations of the sun in hard x-rays can provide insight into many solar phenomena which are not currently well-understood, including the mechanisms behind particle acceleration in flares. Currently, RHESSI is the only solar-dedicated spacecraft observing in the hard x-ray regime. Though RHESSI has greatly added to our knowledge of flare particle acceleration, the method of rotation modulation collimators is limited in sensitivity and dynamic range. By instead using a direct imaging technique, the structure and evolution of even small flares and active regions can be investigated in greater depth. FOXSI (Focusing Optics X-ray Solar Imager), a hard x-ray instrument flown on two sounding rocket campaigns, seeks to achieve these improved capabilities by using focusing optics for solar observations in the 4-20 keV range. During the second of the FOXSI flights, flown on December 11, 2014, two microflares were observed, estimated as GOES class A0.5 and A2.5 (upper limits). Preliminary analysis of these two flares will be presented, including imaging spectroscopy, light curves, and photon spectra. Through this analysis, we investigate the capabilities of FOXSI in enhancing our knowledge of smaller-scale solar events. 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: VLA imaging spectroscopic observations of decimetric radio bursts in the impulsive phase of a C7.2 solar flare Authors: Yu, S.; Chen, B.; Gray, L.; Prijatelj, M.; Kouzmenkov, T.; Krucker, S. Bibcode: 2016AGUFMSH51E2632Y Altcode: Broadband radio dynamic imaging spectroscopic observations at decimetric wavelengths provide powerful diagnostics for magnetic energy release and particle acceleration processes in solar flares. On November 1st, 2014, we used the Karl G. Janskey Very Large Array (VLA) to observe a C7.2 flare in 1-2 GHz with 512 spectral channels and a 50 millisecond cadence. A variety of decimetric radio bursts were recorded during the impulsive phase of the flare, including type III radio bursts, spike bursts, and other bursts with unusual appearance in the dynamic spectrum. We use simultaneous imaging and dynamic spectroscopy to obtain the location and morphology of the radio sources as well as their evolution in time and frequency, which are used to elucidate the physical mechanisms of the radio bursts and trace energetic electrons in the flaring region. The flare was also observed by SDO/AIA in EUV and RHESSI in hard X-rays. We discuss implications of our results on the energy release and particle acceleration processes of this flare. 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 Solar injection of ten electron/3He-rich SEP events Authors: Wang, L.; Krucker, S.; Mason, G. M.; Li, G. Bibcode: 2016AGUFMSH51E2631W Altcode: We have derived the particle injections at the Sun for ten good electron/3He-rich solar energetic particle (SEP) events, using a 1.2 AU particle path length (suggested by analysis of the velocity dispersion). The inferred solar injections of high-energy ( 10 to 300 keV) electrons and of ∼MeV/nucleon ions (carbon and heavier) start with a delay of 17 ± 3 min and 75 ± 14 min, respectively, after the injection of low-energy ( 0.4 to 9 keV) electrons. The injection duration (averaged over energy) ranges from 200 to 550 min for ions, from 90 to 160 min for low-energy electrons, and from 10 to 30 min for high-energy electrons. Most of the selected events have no reported Hα flares or GOES SXR bursts, but all have type III radio bursts that typically start after the onset of a low-energy electron injection. All nine events with SOHO/LASCO coverage have a relatively fast (>570 km s-1), mostly narrow (< 30°), west-limb coronal mass ejection (CME) that launches near the start of the low-energy electron injection, and reaches an average altitude of 1.0 and 4.7 RS, respectively, at the start of the high-energy electron injection and of the ion injection. The electron energy spectra show a continuous power law extending across the transition from low to high energies, suggesting that the low-energy electron injection may provide seed electrons for the delayed high-energy electron acceleration. The delayed ion injections and high ionization states may suggest an ion acceleration along the lower altitude flanks, rather than at the nose of the CMEs. 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: Simulation of Quiet-Sun Hard X-rays Related to Solar Wind Superhalo Electrons Authors: Wang, W.; Wang, L.; Krucker, S.; Hannah, I. G. Bibcode: 2016AGUFMSH51E2635W Altcode: Abstract. In this paper, we propose that the accelerated electrons in the quiet-Sun could collide with the solar atmosphere to emit Hard X-rays (HXRs) via non-thermal bremsstrahlung, while some of these electrons would move upwards and escape into the interplanetary medium, to form a superhalo electron population measured in the solar wind. After considering the electron energy loss due to Coulomb collisions and the ambipolar electrostatic potential, we find that the sources of superhalo could only occur high in the corona (at a heliocentric altitude ≥ 1.9 Rs), to remain a power-law shape of electron spectrum as observed by STEREO at 1 AU near solar minimum (Wang et al, 2012).The modeled quiet-Sun HXRs related to the superhalo electrons fit well to a power-law spectrum, f(ɛ) ∝ ɛ-γ, with an index γ ≈ 2.0-2.3 (3.3-3.7) at 10-100 keV, for the warm/cold thick-target (thin-target) emissions produced by the downward-traveling (upward-traveling) accelerated electrons. These simulated quiet-Sun spectra are significantly harder than the observed spectra of most solar HXR flares. Assuming that the quiet-Sun sources cover 5% of the solar surface, the modeled thin-target HXRs are more than six orders of magnitude weaker than the RHESSI upper limits of quiet-Sun HXRs (Hannah et al., 2010). Using the thick-target model for the downward-traveling electrons, the RHESSI upper limits restrict the number of downward-traveling electrons to maximal ≈ 3 times the number of escaping electrons. This ratio is fundamentally different from what is observed during solar flares associated with escaping electrons where the fraction of downward-traveling electrons dominates by a factor of 100 to 1000 over the escaping population. References: 1. Hannah et al., APJ, 724, 487(2010) 2. Wang et al., APJ Letters,753,L23(2012) 3. Yang et al., RAA,Vol.15,No.3,348-362(2015) 4. Brown J.C., Solar Physics,Vol.18,Issue 3,489,502­­­(1971) Title: Quiet-sun and non-flaring active region measurements from the FOXSI-2 sounding rocket Authors: Buitrago-Casas, J. C.; Glesener, L.; Christe, S.; Ishikawa, S. N.; Narukage, N.; Krucker, S.; Bale, S. D. Bibcode: 2016AGUFMSH13A2280B Altcode: Solar hard X-ray (HXR) emissions are a cornerstone for understanding particle acceleration and energy release in the corona. These phenomena are present at different size scales and intensities, from large eruptive events down to the smallest flares. The presence of HXRs in small, unresolved flares would provide direct evidence of small reconnection events, i.e. nano-flares, that are thought to be be important for the unsolved coronal heating problem. Currently operating solar-dedicated instruments that observe HXRs from the Sun do not have the dynamic range, nor the sensitivity, crucial to observe the faintest solar HXRs. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload is a novel experiment that develops and applies direct focusing optics coupled with semiconductor detectors to observe faint HXRs from the Sun. The FOXSI rocket has successfully completed two flights, observing areas of the quiet-Sun, active regions and micro-flares. We present recent data analysis to test the presence of hot plasma in and outside of active regions observed during the two flights, focusing on the differential emission measure distribution of the non-flaring corona. Title: The FIELDS Instrument Suite for Solar Probe Plus. Measuring the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence, and Radio Signatures of Solar Transients Authors: Bale, S. D.; Goetz, K.; Harvey, P. R.; Turin, P.; Bonnell, J. W.; Dudok de Wit, T.; Ergun, R. E.; MacDowall, R. J.; Pulupa, M.; Andre, M.; Bolton, M.; Bougeret, J. -L.; Bowen, T. A.; Burgess, D.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen, C. H. K.; Choi, M. K.; Connerney, J. E.; Cranmer, S.; Diaz-Aguado, M.; Donakowski, W.; Drake, J. F.; Farrell, W. M.; Fergeau, P.; Fermin, J.; Fischer, J.; Fox, N.; Glaser, D.; Goldstein, M.; Gordon, D.; Hanson, E.; Harris, S. E.; Hayes, L. M.; Hinze, J. J.; Hollweg, J. V.; Horbury, T. S.; Howard, R. A.; Hoxie, V.; Jannet, G.; Karlsson, M.; Kasper, J. C.; Kellogg, P. J.; Kien, M.; Klimchuk, J. A.; Krasnoselskikh, V. V.; Krucker, S.; Lynch, J. J.; Maksimovic, M.; Malaspina, D. M.; Marker, S.; Martin, P.; Martinez-Oliveros, J.; McCauley, J.; McComas, D. J.; McDonald, T.; Meyer-Vernet, N.; Moncuquet, M.; Monson, S. J.; Mozer, F. S.; Murphy, S. D.; Odom, J.; Oliverson, R.; Olson, J.; Parker, E. N.; Pankow, D.; Phan, T.; Quataert, E.; Quinn, T.; Ruplin, S. W.; Salem, C.; Seitz, D.; Sheppard, D. A.; Siy, A.; Stevens, K.; Summers, D.; Szabo, A.; Timofeeva, M.; Vaivads, A.; Velli, M.; Yehle, A.; Werthimer, D.; Wygant, J. R. Bibcode: 2016SSRv..204...49B Altcode: 2016SSRv..tmp...16B NASA's Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products. Title: The FOXSI solar sounding rocket campaigns Authors: Glesener, Lindsay; Krucker, Säm.; Christe, Steven; Ishikawa, Shin-nosuke; Buitrago-Casas, Juan Camilo; Ramsey, Brian; Gubarev, Mikhail; Takahashi, Tadayuki; Watanabe, Shin; Takeda, Shin'ichiro; Courtade, Sasha; Turin, Paul; McBride, Stephen; Shourt, Van; Hoberman, Jane; Foster, Natalie; Vievering, Juliana Bibcode: 2016SPIE.9905E..0EG Altcode: The Focusing Optics X-ray Solar Imager (FOXSI) is, in its initial form, a sounding rocket experiment designed to apply the technique of focusing hard X-ray (HXR) optics to the study of fundamental questions about the high-energy Sun. Solar HXRs arise via bremsstrahlung from energetic electrons and hot plasma produced in solar flares and thus are one of the most direct diagnostics of are-accelerated electrons and the impulsive heating of the solar corona. Previous missions have always been limited in sensitivity and dynamic range by the use of indirect (Fourier) imaging due to the lack of availability of direct focusing optics, but technological advances now make direct focusing accessible in the HXR regime (as evidenced by the NuSTAR spacecraft and several suborbital missions). The FOXSI rocket experiment develops and optimizes HXR focusing telescopes for the unique scientific requirements of the Sun. To date, FOXSI has completed two successful flights on 2012 November 02 and 2014 December 11 and is funded for a third flight. This paper gives a brief overview of the experiment, which is sensitive to solar HXRs in the 4-20 keV range, describes its first two flights, and gives a preview of plans for FOXSI-3. Title: Fine-pitch CdTe detector for hard X-ray imaging and spectroscopy of the Sun with the FOXSI rocket experiment Authors: Ishikawa, Shin-nosuke; Katsuragawa, Miho; Watanabe, Shin; Uchida, Yuusuke; Takeda, Shin'ichiro; Takahashi, Tadayuki; Saito, Shinya; Glesener, Lindsay; Buitrago-Casas, Juan Camilo; Krucker, Säm.; Christe, Steven Bibcode: 2016JGRA..121.6009I Altcode: 2016arXiv160603887I We have developed a fine-pitch hard X-ray (HXR) detector using a cadmium telluride (CdTe) semiconductor for imaging and spectroscopy for the second launch of the Focusing Optics Solar X-ray Imager (FOXSI). FOXSI is a rocket experiment to perform high sensitivity HXR observations from 4 to 15 keV using the new technique of HXR focusing optics. The focal plane detector requires <100μm position resolution (to take advantage of the angular resolution of the optics) and ≈1 keV energy resolution (full width at half maximum (FWHM)) for spectroscopy down to 4 keV, with moderate cooling (>-30°C). Double-sided silicon strip detectors were used for the first FOXSI flight in 2012 to meet these criteria. To improve the detectors' efficiency (66% at 15 keV for the silicon detectors) and position resolution of 75 μm for the second launch, we fabricated double-sided CdTe strip detectors with a position resolution of 60 μm and almost 100% efficiency for the FOXSI energy range. The sensitive area is 7.67 mm × 7.67 mm, corresponding to the field of view of 791'' × 791''. An energy resolution of 1 keV (FWHM) and low-energy threshold of ≈4 keV were achieved in laboratory calibrations. The second launch of FOXSI was performed on 11 December 2014, and images from the Sun were successfully obtained with the CdTe detector. Therefore, we successfully demonstrated the detector concept and the usefulness of this technique for future HXR observations of the Sun. Title: The injection of ten electron/3He-rich SEP events Authors: Wang, Linghua; Krucker, Säm; Mason, Glenn M.; Li, Gang Bibcode: 2016shin.confE...1W Altcode: We have derived the particle injections at the Sun for ten good electron/3He-rich solar energetic particle (SEP) events, using a 1.2 AU particle path length (suggested by analysis of the velocity dispersion). The inferred solar injections of high-energy ( 10 to 300 keV) electrons and of MeV/nucleon ions (carbon and heavier) start with a delay of 17 ± 3 min and 75 ± 14 min, respectively, after the injection of low-energy ( 0.4 to 9 keV) electrons. The injection duration (averaged over energy) ranges from 200 to 550 min for ions, from ∼90 to 160 min for low-energy electrons, and from 10 to 30 min for high-energy electrons. Most of the selected events have no reported Hα flares or GOES SXR bursts, but all have type III radio bursts that typically start after the onset of a low-energy electron injection. All nine events with SOHO/LASCO coverage have a relatively fast (>570 km/s), mostly narrow (< 30°), west-limb coronal mass ejection (CME) that launches near the start of the low-energy electron injection, and reaches an average altitude of ∼1.0 and 4.7 RS, respectively, at the start of the high-energy electron injection and of the ion injection. The electron energy spectra show a continuous power law extending across the transition from low to high energies, suggesting that the low-energy electron injection may provide seed electrons for the delayed high-energy electron acceleration. The delayed ion injections and high ionization states may suggest an ion acceleration along the lower altitude flanks, rather than at the nose of the CMEs. Title: Flight production of Caliste-SO: the hard x-ray spectrometers for solar orbiter/STIX instrument Authors: Limousin, Olivier; Meuris, Aline; Gevin, Olivier; Blondel, Claire; Donati, Modeste; Dumaye, Luc; Le Mer, Isabelle; Martignac, Jérôme; Tourrette, Thierry; Vassal, Marie-Cécile; Blain, Dominique; Boussadia, Mohamed; Fiant, Nicolas; Soufflet, Fabrice; Bednarzik, Martin; Birrer, Guy; Stutz, Stefan; Wild, Christopher; Billot, Marc; Fratter, Isabelle; Grimm, Oliver; Krucker, Säm. Bibcode: 2016SPIE.9905E..0FL Altcode: Caliste-SO are CdTe hybrid detectors that will be used as spectrometer units in the Spectrometer Telescope for Imaging X-rays (STIX) on-board the Solar Orbiter space mission. Each unit is placed below one collimator of this Fourier telescope to measure one visibility of the image in the 4-150 keV energy range, with a spectral resolution of 1 keV FWHM at 6 keV. The paper presents the scientific requirements, the design, the fabrication and the tests of the Caliste- SO devices before mounting them onto printed circuits boards. Spectral response was characterized on the 98 spacegrade units for various operating parameters. The devices will equip the different instrument validation models, including 32 units for the final instrument flight model to be launched in 2018. 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 Focusing Optics X-ray Solar Imager Small Explorer Concept Mission Authors: Christe, Steven; Shih, Albert Y.; Dennis, Brian R.; Glesener, Lindsay; Krucker, Sam; Saint-Hilaire, Pascal; Gubarev, Mikhail; Ramsey, Brian Bibcode: 2016SPD....47.0802C 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 combined withpixelated solid-state detectors, allowing for direct imaging of solar X-rays. The current design being studied features multiple telescopes with a 14 meter focal length enabled by a deployable boom.FOXSI will observe the Sun in the 3-100 keV energy 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. FOXSI is a direct imaging spectrometer with high dynamic range and sensitivity and will provide a brand-new perspective on energy release on the Sun. We describe the mission and its science objectives. 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: The FOXSI sounding rocket: Latest analysis and results Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe, Steven; Krucker, Sam; Ishikawa, Shin-Nosuke; Takahashi, Tadayuki; Ramsey, Brian; Han, Raymond Bibcode: 2016SPD....4740702B Altcode: 2016SPD....4740702C Hard X-ray (HXR) observations are a linchpin for studying particle acceleration and hot thermal plasma emission in the solar corona. Current and past indirectly imaging instruments lack the sensitivity and dynamic range needed to observe faint HXR signatures, especially in the presences of brighter sources. These limitations are overcome by using HXR direct focusing optics coupled with semiconductor detectors. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment is a state of the art solar telescope that develops and applies these capabilities.The FOXSI sounding rocket has successfully flown twice, observing active regions, microflares, and areas of the quiet-Sun. Thanks to its far superior imaging dynamic range, FOXSI performs cleaner hard X-ray imaging spectroscopy than previous instruments that use indirect imaging methods like RHESSI.We present a description of the FOXSI rocket payload, paying attention to the optics and semiconductor detectors calibrations, as well as the upgrades made for the second flight. We also introduce some of the latest FOXSI data analysis, including imaging spectroscopy of microflares and active regions observed during the two flights, and the differential emission measure distribution of the nonflaring corona. 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: Simulation of Quiet-Sun Hard X-Rays Related to Solar Wind Superhalo Electrons Authors: Wang, Wen; Wang, Linghua; Krucker, Säm; Hannah, Iain Bibcode: 2016SoPh..291.1357W Altcode: 2016arXiv160506339W; 2016SoPh..tmp...75W In this paper, we propose that the accelerated electrons in the quiet Sun could collide with the solar atmosphere to emit Hard X-rays (HXRs) via non-thermal bremsstrahlung, while some of these electrons would move upwards and escape into the interplanetary medium, to form a superhalo electron population measured in the solar wind. After considering the electron energy loss due to Coulomb collisions and the ambipolar electrostatic potential, we find that the sources of the superhalo could only occur high in the corona (at a heliocentric altitude ≳1.9 R⊙ (the mean radius of the Sun)), to remain a power-law shape of electron spectrum as observed by Solar Terrestrial Relations Observatory (STEREO) at 1 AU near solar minimum (Wang et al. in Astrophys. J. Lett.753, L23, 2012). The modeled quiet-Sun HXRs related to the superhalo electrons fit well to a power-law spectrum, f ∼ε−γ in the photon energy ε , with an index γ ≈2.0 -2.3 (3.3 - 3.7) at 10 - 100 keV, for the warm/cold-thick-target (thin-target) emissions produced by the downward-traveling (upward-traveling) accelerated electrons. These simulated quiet-Sun spectra are significantly harder than the observed spectra of most solar HXR flares. Assuming that the quiet-Sun sources cover 5 % of the solar surface, the modeled thin-target HXRs are more than six orders of magnitude weaker than the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) upper limit for quiet-Sun HXRs (Hannah et al. in Astrophys. J.724, 487, 2010). Using the thick-target model for the downward-traveling electrons, the RHESSI upper limit restricts the number of downward-traveling electrons to at most ≈3 times the number of escaping electrons. This ratio is fundamentally different from what is observed during solar flares associated with escaping electrons where the fraction of downward-traveling electrons dominates by a factor of 100 to 1000 over the escaping population. 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: The dynamics and magnetism of the X1 flare on 2014-03-29 Authors: Kleint, Lucia; Heinzel, Petr; Philip, Judge; Krucker, Sam Bibcode: 2016SPD....47.0613K Altcode: The X1 flare on 2014-03-29 was observed with an unprecedented number of instruments including chromospheric polarimetry and spectroscopy from the UV to the IR. By combining data from these instruments, we can answer several open questions: Where is the observed continuum emission during flares formed and through which physical processes? How does the magnetic field structure in the photosphere and in the chromosphere change during a flare? We discuss the implications of our findings on standard flare models. 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: 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: FOXSI-2: Upgrades of the Focusing Optics X-ray Solar Imager for its Second Flight Authors: Christe, Steven; Glesener, Lindsay; Buitrago-Casas, Camilo; Ishikawa, Shin-Nosuke; Ramsey, Brian; Gubarev, Mikhail; Kilaru, Kiranmayee; Kolodziejczak, Jeffery J.; Watanabe, Shin; Takahashi, Tadayuki; Tajima, Hiroyasu; Turin, Paul; Shourt, Van; Foster, Natalie; Krucker, Sam Bibcode: 2016JAI.....540005C Altcode: The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload flew for the second time on 2014 December 11. To enable direct Hard X-Ray (HXR) imaging spectroscopy, FOXSI makes use of grazing-incidence replicated focusing optics combined with fine-pitch solid-state detectors. FOXSI’s first flight provided the first HXR focused images of the Sun. For FOXSI’s second flight several updates were made to the instrument including updating the optics and detectors as well as adding a new Solar Aspect and Alignment System (SAAS). This paper provides an overview of these updates as well as a discussion of their measured performance. 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: Continuum Enhancements in the Ultraviolet, the Visible and the Infrared during the X1 Flare on 2014 March 29 Authors: Kleint, Lucia; Heinzel, Petr; Judge, Phil; Krucker, Säm Bibcode: 2016ApJ...816...88K Altcode: 2015arXiv151104161K Enhanced continuum brightness is observed in many flares (“white light flares”), yet it is still unclear which processes contribute to the emission. To understand the transport of energy needed to account for this emission, we must first identify both the emission processes and the emission source regions. Possibilities include heating in the chromosphere causing optically thin or thick emission from free-bound transitions of Hydrogen, and heating of the photosphere causing enhanced H- continuum brightness. To investigate these possibilities, we combine observations from Interface Region Imaging Spectrograph (IRIS), SDO/Helioseismic and Magnetic Imager, and the ground-based Facility Infrared Spectrometer instrument, covering wavelengths in the far-UV, near-UV (NUV), visible, and infrared during the X1 flare SOL20140329T17:48. Fits of blackbody spectra to infrared and visible wavelengths are reasonable, yielding radiation temperatures ∼6000-6300 K. The NUV emission, formed in the upper photosphere under undisturbed conditions, exceeds these simple fits during the flare, requiring extra emission from the Balmer continuum in the chromosphere. Thus, the continuum originates from enhanced radiation from photosphere (visible-IR) and chromosphere (NUV). From the standard thick-target flare model, we calculate the energy of the nonthermal electrons observed by Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) and compare it to the energy radiated by the continuum emission. We find that the energy contained in most electrons >40 keV, or alternatively, of ∼10%-20% of electrons >20 keV is sufficient to explain the extra continuum emission of ∼(4-8) × 1010 erg s-1 cm-2. Also, from the timing of the RHESSI HXR and the IRIS observations, we conclude that the NUV continuum is emitted nearly instantaneously when HXR emission is observed with a time difference of no more than 15 s. Title: The injection of ten electron/3He-rich SEP events Authors: Wang, Linghua; Krucker, Säm; Mason, Glenn M.; Lin, Robert P.; Li, Gang Bibcode: 2016A&A...585A.119W Altcode: 2016arXiv160507882W We have derived the particle injections at the Sun for ten good electron/3He-rich solar energetic particle (SEP) events, using a 1.2 AU particle path length (suggested by analysis of the velocity dispersion). The inferred solar injections of high-energy (~10 to 300 keV) electrons and of ~MeV/nucleon ions (carbon and heavier) start with a delay of 17 ± 3 min and 75 ± 14 min, respectively, after the injection of low-energy (~0.4 to 9 keV) electrons. The injection duration (averaged over energy) ranges from ~200 to 550 min for ions, from ~90 to 160 min for low-energy electrons, and from ~10 to 30 min for high-energy electrons. Most of the selected events have no reported Hα flares or GOES SXR bursts, but all have type III radio bursts that typically start after the onset of a low-energy electron injection. All nine events with SOHO/LASCO coverage have a relatively fast (>570 km s-1), mostly narrow (≲30°), west-limb coronal mass ejection (CME) that launches near the start of the low-energy electron injection, and reaches an average altitude of ~1.0 and 4.7 RS, respectively, at the start of the high-energy electron injection and of the ion injection. The electron energy spectra show a continuous power law extending across the transition from low to high energies, suggesting that the low-energy electron injection may provide seed electrons for the delayed high-energy electron acceleration. The delayed ion injections and high ionization states may suggest an ion acceleration along the lower altitude flanks, rather than at the nose of the CMEs. 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: Hard X-ray Detectability of Small Impulsive Heating Events in the Solar Corona Authors: Glesener, L.; Klimchuk, J. A.; Bradshaw, S. J.; Marsh, A.; Krucker, S.; Christe, S. Bibcode: 2015AGUFMSH13B2440G Altcode: Impulsive heating events ("nanoflares") are a candidate to supply the solar corona with its ~2 MK temperature. These transient events can be studied using extreme ultraviolet and soft X-ray observations, among others. However, the impulsive events may occur in tenuous loops on small enough timescales that the heating is essentially not observed due to ionization timescales, and only the cooling phase is observed. Bremsstrahlung hard X-rays could serve as a more direct and prompt indicator of transient heating events. A hard X-ray spacecraft based on the direct-focusing technology pioneered by the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket could search for these direct signatures. In this work, we use the hydrodynamical EBTEL code to simulate differential emission measures produced by individual heating events and by ensembles of such events. We then directly predict hard X-ray spectra and consider their observability by a future spaceborne FOXSI, and also by the RHESSI and NuSTAR spacecraft. 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: Particle acceleration by a solar flare termination shock Authors: Chen, Bin; Bastian, Timothy S.; Shen, Chengcai; Gary, Dale E.; Krucker, Säm; Glesener, Lindsay Bibcode: 2015Sci...350.1238C Altcode: 2015arXiv151202237C Solar flares—the most powerful explosions in the solar system—are also efficient particle accelerators, capable of energizing a large number of charged particles to relativistic speeds. A termination shock is often invoked in the standard model of solar flares as a possible driver for particle acceleration, yet its existence and role have remained controversial. We present observations of a solar flare termination shock and trace its morphology and dynamics using high-cadence radio imaging spectroscopy. We show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. The observed properties of the shock are well reproduced by simulations. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares. Title: Observations and Simulations of a Termination Shock in an Eruptive Solar Flare as a Possible Particle Accelerator Authors: Chen, B.; Bastian, T.; Shen, C.; Gary, D. E.; Krucker, S.; Glesener, L. Bibcode: 2015AGUFMSH11F..05C Altcode: A termination shock has been often invoked in the standard model for eruptive solar flares as a possible driver for particle acceleration. It is hypothesized as a standing shock wave generated by super-magnetosonic reconnection outflows impinging upon dense, newly-reconnected magnetic loops during the flare energy release process. However, such shock wave has largely remained a theoretical concept inferred from model predictions due to the lack of observational evidence. Here we present observations of a termination shock in a solar flare and trace its morphology and dynamics using high-cadence radio imaging spectroscopy enabled by the upgraded Karl G. Jansky Very Large Array. The observed properties of the shock, including its location, morphology, and dynamics, are well-reproduced by magnetohydrodynamics simulations in a standard Kopp-Pneuman-type reconnection geometry for two-ribbon flares. We further show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares. Title: FOXSI-2 Observations and Coronal Heating Authors: Christe, S.; Glesener, L.; Krucker, S.; Ramsey, B.; Ishikawa, S. N.; Buitrago Casas, J. C.; Takahashi, T.; Foster, N. Bibcode: 2015AGUFMSH31D..07C Altcode: Energy release and particle acceleration on the Sun is a frequent occurrence associated with a number of different solar phenomenon including but not limited to solar flares, coronal mass ejections and nanoflares. The exact mechanism through which particles are accelerated and energy is released is still not well understood. This issue is related to the unsolved coronal heating problem, the mystery of the heating mechanism for the million degree solar corona. One prevalent theory posits the existence of a multitude of small flares, dubbed nanoflares. Recent observations of active region AR11890 by IRIS (Testa et al. 2014) are consistent with numerical simulations of heating by impulsive beams of nonthermal electrons, suggesting that nanoflares may be similar to large flares in that they accelerate particles. Furthermore, observations by the EUNIS sounding rocket (Brosius et al. 2014) of faint Fe XIX (592.2 Angstrom) emission in an active region is indicative of plasma at temperatures of at least 8.9 MK providing further evidence of nanoflare heating. One of the best ways to gain insight into accelerated particles on the Sun and the presence of hot plasma is by observing the Sun in hard X-rays (HXR). We present on observations taken during the second successful flight of the Focusing Optics X-ray Solar Imager (FOXSI-2). FOXSI flew on December 11, 2014 with upgraded optics as well as new CdTe strip detectors. FOXSI-2 observed thermal emission (4-15 keV) from at least three active regions (AR#12234, AR#12233, AR#12235) and observed regions of the Sun without active regions. We present on using FOXSI observations to test the presence of hot temperatures in and outside of active regions. 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: Second flight of the Focusing Optics X-ray Solar Imager sounding rocket [FOXSI-2] Authors: Buitrago-Casas, J. C.; Krucker, S.; Christe, S.; Glesener, L.; Ishikawa, S. N.; Ramsey, B.; Foster, N. D. Bibcode: 2015AGUFMSH43B2454B Altcode: The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket experiment that has flown twice to test a direct focusing method for measuring solar hard X-rays (HXRs). These HXRs are associated with particle acceleration mechanisms at work in powering solar flares and aid us in investigating the role of nanoflares in heating the solar corona. FOXSI-1 successfully flew for the first time on November 2, 2012. After some upgrades including the addition of extra mirrors to two optics modules and the inclusion of new fine-pitch CdTe strip detectors, in addition to the Si detectors from FOXSI-1, the FOXSI-2 payload flew successfully again on December 11, 2014. During the second flight four targets on the Sun were observed, including at least three active regions, two microflares, and ~1 minute of quiet Sun observation. This work is focused in giving an overview of the FOXSI rocket program and a detailed description of the upgrades for the second flight. In addition, we show images and spectra investigating the presence of no thermal emission for each of the flaring targets that we observed during the second flight. Title: Hard X-ray Detector Calibrations for the FOXSI Sounding Rocket Authors: Lopez, A.; Glesener, L.; Buitrago Casas, J. C.; Han, R.; Ishikawa, S. N.; Christe, S.; Krucker, S. Bibcode: 2015AGUFMSH43B2455L Altcode: In the study of high-energy solar flares, detailed X-ray images and spectra of the Sun are required. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment is used to test direct-focusing X-ray telescopes and Double-sided Silicon Strip Detectors (DSSD) for solar flare study and to further understand coronal heating. The measurement of active region differential emission measures, flare temperatures, and possible quiet-Sun emission requires a precisely calibrated spectral response. This poster describes recent updates in the calibration of FOXSI's DSSDs based on new calibration tests that were performed after the second flight. The gain for each strip was recalculated using additional radioactive sources. Additionally, the varying strip sensitivity across the detectors was investigated and based on these measurements, the flight images were flatfielded. These improvements lead to more precise X-ray data for future FOXSI flights and show promise for these new technologies in imaging the Sun. 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: Characterizing Temperatures of FOXSI-2 Microflares Using RHESSI and AIA Observations Authors: Han, R.; Glesener, L.; Buitrago Casas, J. C.; Lopez, A.; Badman, S.; Krucker, S. Bibcode: 2015AGUFMSH43B2453H Altcode: The second flight of the Focusing Optics X-ray Solar Imager sounding rocket payload (FOXSI-2) was successfully completed on December 11, 2014. FOXSI's direct imaging technology allows it to measure hard X-ray (HXR) emissions from the Sun with superior dynamic range and sensitivity relative to indirect HXR observations. During the December FOXSI-2 flight, several microflares were observed. We wish to characterize the temperature distributions of these microflares using supporting measurements in order to validate the FOXSI spectral response. The temperature distribution of solar plasma is best described by the differential emission measure (DEM), a physical quantity that characterizes the amount of material present in the corona in each temperature range. To determine the DEM, we employ multi-wavelength extreme ultraviolet observations by the Atmospheric Imaging Assembly (SDO/AIA) using a regularization method. We also perform isothermal fitting of thermal X-ray spectra from the Reuven Ramaty Solar Spectroscopic Imager (RHESSI). This poster will show the temperature distribution for each of the December 11 microflares and compare these results with those obtained from FOXSI-2. 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: The Energy Spectrum of Solar Energetic Electrons Authors: Wang, L.; Yang, L.; Krucker, S.; Wimmer-Schweingruber, R. F.; Bale, S. D. Bibcode: 2015AGUFMSH21A2385W Altcode: Here we present a statistical survey of the energy spectrum of solar energetic electron events (SEEs) observed by the WIND 3DP instrument from 1995 though 2014. For SEEs with the minimum energy below 10 keV and the maximum energy above 100 keV, ~85% (~2%) have a double-power-law energy spectrum with a steepening (hardening) above the break energy, while ~13% have a single-power-law energy spectrum at all energies. The average spectral index is ~2.4 below the energy break and is ~4.0 above the energy break. For SEEs detected only at energies <10 keV (>20 keV), they generally show a single-power-law spectrum with the average index of ~3.0 (~3.3). The spectrum of SEEs detected only below 10 keV appears to get harder with increasing solar activity, but the spectrum of SEEs with higher-energy electrons shows no clear correlation with solar activity. We will also investigate whether the observed energy spectrum of SEEs at 1 AU mainly reflects the electron acceleration at the Sun or the electron transport in the interplanetary medium. Title: A Statistical Correlation of Sunquakes Based on Their Seismic and White-Light Emission Authors: Buitrago-Casas, J. C.; Martínez Oliveros, J. C.; Lindsey, C.; Calvo-Mozo, B.; Krucker, S.; Glesener, L.; Zharkov, S. Bibcode: 2015SoPh..290.3151B Altcode: 2015arXiv150207798B; 2015SoPh..tmp..169B Several mechanisms have been proposed to explain the transient seismic emission, i.e. "sunquakes," from some solar flares. Some theories associate high-energy electrons and/or white-light emission with sunquakes. High-energy charged particles and their subsequent heating of the photosphere and/or chromosphere could induce acoustic waves in the solar interior. We carried out a correlative study of solar flares with emission in hard X-rays, enhanced continuum emission at 6173 Å, and transient seismic emission. We selected those flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) with a considerable flux above 50 keV between 1 January 2010 and 26 June 2014. We then used data from the Helioseismic and Magnetic Imager onboard the Solar Dynamic Observatory to search for excess visible-continuum emission and new sunquakes not previously reported. We found a total of 18 sunquakes out of 75 flares investigated. All of the sunquakes were associated with an enhancement of the visible continuum during the flare. Finally, we calculated a coefficient of correlation for a set of dichotomic variables related to these observations. We found a strong correlation between two of the standard helioseismic detection techniques, and between sunquakes and visible-continuum enhancements. We discuss the phenomenological connectivity between these physical quantities and the observational difficulties of detecting seismic signals and excess continuum radiation. Title: How Important Are Electron Beams in Driving Chromospheric Evaporation in the 2014 March 29 Flare? Authors: Battaglia, Marina; Kleint, Lucia; Krucker, Säm; Graham, David Bibcode: 2015ApJ...813..113B Altcode: 2015arXiv150909186B We present high spatial resolution observations of chromospheric evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging Spectrograph observations of the Fe xxi λ 1354.1 line indicate evaporating plasma at a temperature of 10 MK along the flare ribbon during the flare peak and several minutes into the decay phase with upflow velocities between 30 and 200 km s-1. Hard X-ray (HXR) footpoints were observed by the Ramaty High Energy Solar Spectroscopic Imager for two minutes during the peak of the flare. Their locations coincided with the locations of the upflows in parts of the southern flare ribbon but the HXR footpoint source preceded the observation of upflows in Fe xxi by 30-75 s. However, in other parts of the southern ribbon and in the northern ribbon, the observed upflows were not coincident with an HXR source in time or space, most prominently during the decay phase. In this case evaporation is likely caused by energy input via a conductive flux that is established between the hot (25 MK) coronal source, which is present during the whole observed time-interval, and the chromosphere. The presented observations suggest that conduction may drive evaporation not only during the decay phase but also during the flare peak. Electron beam heating may only play a role in driving evaporation during the initial phases of the flare. Title: Hard X-Ray Imaging of Individual Spectral Components in Solar Flares Authors: Caspi, Amir; Shih, Albert Y.; McTiernan, James M.; Krucker, Säm Bibcode: 2015ApJ...811L...1C Altcode: 2015arXiv150806003C We present a new analytical technique, combining Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) high-resolution imaging and spectroscopic observations, to visualize solar flare emission as a function of spectral component (e.g., isothermal temperature) rather than energy. This computationally inexpensive technique is applicable to all spatially invariant spectral forms and is useful for visualizing spectroscopically determined individual sources and placing them in context, e.g., comparing multiple isothermal sources with nonthermal emission locations. For example, while extreme ultraviolet images can usually be closely identified with narrow temperature ranges, due to the emission being primarily from spectral lines of specific ion species, X-ray images are dominated by continuum emission and therefore have a broad temperature response, making it difficult to identify sources of specific temperatures regardless of the energy band of the image. We combine RHESSI calibrated X-ray visibilities with spatially integrated spectral models including multiple isothermal components to effectively isolate the individual thermal sources from the combined emission and image them separately. We apply this technique to the 2002 July 23 X4.8 event studied in prior works, and image for the first time the super-hot and cooler thermal sources independently. The super-hot source is farther from the footpoints and more elongated throughout the impulsive phase, consistent with an in situ heating mechanism for the super-hot plasma. Title: Magnetic field line lengths inside interplanetary magnetic flux ropes Authors: Hu, Qiang; Qiu, Jiong; Krucker, Sam Bibcode: 2015JGRA..120.5266H Altcode: 2015arXiv150205284H We report on the detailed and systematic study of field line twist and length distributions within magnetic flux ropes embedded in interplanetary coronal mass ejections (ICMEs). The Grad-Shafranov reconstruction method is utilized together with a constant-twist nonlinear force-free (Gold-Hoyle) flux rope model to reveal the close relation between the field line twist and length in cylindrical flux ropes, based on in situ Wind spacecraft measurements. We show that the field line twist distributions within interplanetary flux ropes are inconsistent with the Lundquist model. In particular, we utilize the unique measurements of magnetic field line lengths within selected ICME events as provided by Kahler et al. (<link href="#jgra51898-bib-0016"/>) based on energetic electron burst observations at 1 AU and the associated type III radio emissions detected by the Wind spacecraft. These direct measurements are compared with our model calculations to help assess the flux rope interpretation of the embedded magnetic structures. By using the different flux rope models, we show that the in situ direct measurements of field line lengths are consistent with a flux rope structure with spiral field lines of constant and low twist, largely different from that of the Lundquist model, especially for relatively large-scale flux ropes. Title: The Fast Filament Eruption Leading to the X-flare on 2014 March 29 Authors: Kleint, Lucia; Battaglia, Marina; Reardon, Kevin; Sainz Dalda, Alberto; Young, Peter R.; Krucker, Säm Bibcode: 2015ApJ...806....9K Altcode: 2015arXiv150400515K We investigate the sequence of events leading to the solar X1 flare SOL2014-03-29T17:48. Because of the unprecedented joint observations of an X-flare with the ground-based Dunn Solar Telescope and the spacecraft IRIS, Hinode, RHESSI, STEREO, and the Solar Dynamics Observatory, we can sample many solar layers from the photosphere to the corona. A filament eruption was observed above a region of previous flux emergence, which possibly led to a change in magnetic field configuration, causing the X-flare. This was concluded from the timing and location of the hard X-ray emission, which started to increase slightly less than a minute after the filament accelerated. The filament showed Doppler velocities of ∼2-5 km s-1 at chromospheric temperatures for at least one hour before the flare occurred, mostly blueshifts, but also redshifts near its footpoints. Fifteen minutes before the flare, its chromospheric Doppler shifts increased to ∼6-10 km s-1 and plasma heating could be observed before it lifted off with at least 600 km s-1 as seen in IRIS data. Compared to previous studies, this acceleration (∼3-5 km s-2) is very fast, while the velocities are in the common range for coronal mass ejections. An interesting feature was a low-lying twisted second filament near the erupting filament, which did not seem to participate in the eruption. After the flare ribbons started on each of the second filament’s sides, it seems to have untangled and vanished during the flare. These observations are some of the highest resolution data of an X-class flare to date and reveal some small-scale features yet to be explained. Title: FOXSI: Properties of optics and detectors for hard-X rays Authors: Buitrago-Casas, Juan Camilo; Glesener, Lindsay; Christe, Steven; Krucker, Sam; Ishikawa, Shin-nosuke; Foster, Natalie Bibcode: 2015TESS....140305B Altcode: 2015TESS....140305C The Focusing Optics X-ray Solar Imager (FOXSI) is a state-of-the-art direct focusing X-ray telescope designed to observe the Sun. This experiment completed its second flight onboard a sounding rocket last December 11, 2014 from the White Sands Missile Range in New Mexico. The optics use a set of iridium-coated nickel/cobalt mirrors made using a replication technique based on an electroformed perfect polished surface. Since this technique creates full shells that no need to be co-aligned with other segments, an angular resolution of up to ~5 arcsec is gotten. The FOXSI focal plane consists of seven double-sided strip detectors. Five Silicon and 2 CdTe detectors were used during the second flight.We present on various properties of Wolter-I optics that are applicable to solar HXR observation, including ray-tracing simulations of the single-bounce (“ghost ray”) patterns from sources outside the field of view and angular resolution for different source angles and effective area measurements of the FOXSI optics. We also present the detectors calibration results, paying attention to energy resolution (~0.5 keV), energy thresholds (~4-15 keV for Silicon and ~4-20 keV for CdTe detectors), and spatial coherence of these values over the entire detector. Title: The Focusing Optics X-ray Solar Imager: Second Flight and Recent Results Authors: Christe, Steven; Krucker, Sam; Glesener, Lindsay; Ramsey, Brian; Ishikawa, Shin-nosuke; Buitrago Casas, Juan Camilo; Foster, Natalie; Takahashi, Tadayuki Bibcode: 2015TESS....121301C Altcode: Energy release and particle acceleration on the Sun is a frequent occurrence associated with a number of different solar phenomenon including but not limited to solar flares and coronal mass ejections. The exact mechanism through which particle are accelerated is still not well understood. One of the best ways to gain insight into accelerated particles on the Sun is by observing the Sun in hard X-rays (HXR) which provide one of the most direct diagnostics of energetic electrons. Past and current HXR observations lack the sensitivity and dynamic range necessary to observe the faint signature of accelerated electrons where they are accelerated in the solar corona. However these limitations can be overcome through the use of HXR focusing optics coupled with solid-state pixelated detectors. We present on the second successful launch of the Focusing Optics X-ray Solar Imager, a sounding rocket payload which flew on December 11, 2014. In this flight, the FOXSI optics were upgraded for better sensitivity and new CdTe strip detectors were included to provide increased detection efficiency. During this flight, FOXSI observed thermal emission from at least three active regions (AR#12234, AR#12233, AR#12235). Another observation target for FOXSI was the quiet Sun. In this presentation we summarize the flight as well as the latest observations and analysis. 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: An Investigation of Flare Footpoint DEMs using AIA Diffraction Patterns Authors: Raftery, Claire; Bain, Hazel; Schwartz, Richard; Torre, Gabriele; Krucker, Sam Bibcode: 2015TESS....130211R Altcode: The heating of flare footpoints by accelerated electrons is a well-established component of the standard flare model. However, limitations of current instruments make it challenging to obtain high cadence, high resolution observations of the brightest footpoint regions, predominantly due to low cadence, or pixel saturation.In moderate and large flares observed by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly, CCD pixels in the footpoint regions are frequently saturated despite the automatic exposure control. Using the method of Schwartz et al. (2014), we reconstruct saturated footpoint kernels in the brightest flaring regions and investigate the evolving footpoint differential emission measure at the full 12 second AIA cadence. This is compared to the changing electron fluxes observed with the Reuven Ramaty Solar Spectroscopic Imager (RHESSI) to investigate the relationship between the non-thermal electron energy flux and the footpoint thermal response.(Schwartz, R. A., Torre, G., & Piana, M. (2014), Astrophysical Journal Letters, 793, LL23 ) Title: The Miniature X-ray Spectrograph (MiXS) Authors: Martinez Oliveros, Juan Carlos; Glesener, Lindsay; Saint Hilaire, Pascal; Sundkvist, David; Hurford, Gordon; Bain, Hazel; Bale, Stuart D.; Krucker, Sam Bibcode: 2015TESS....140301M Altcode: The Miniature X-ray Spectrograph (MiXS) is an innovative, small, and fully functional solar X-ray observatory concept designed to fit within a 6U CubeSat platform. MiXS will provide the community with X-ray spectroscopy up to 100 keV of solar flares at a small fraction of the cost of a conventional mission. It includes layered Si/CdTe detectors, providing routine observations of both soft and hard X-ray emission with low background. If selected for funding, MiXS will provide hard X-ray (HXR) spectroscopy throughout the declining phase of this solar cycle allowing continuous solar observations while new generation HXR instrumentation put in orbit. MiXS is the first stage of a much ambitious cube design the Miniature Xray Imager (MiXI), which can provide to the community X-ray imaging up to 40 - 50 keV. In the next solar cycle, coordinated observations between Solar Orbiter’s STIX instrument and future MiXS or MiXI iterations will enable solar flare observation from two vantage points, while new observatories will be commissioned. This will provide new insight into the directivity of flare HXR emission and will allow detailed study of both coronal and footpoint sources within the same flare. These results may have profound implications for theories of flare acceleration processes. We describe here the MiXS concept and its usefulness to the solar and heliophysics communities. 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: Magnetic Field-line Twist and Length Distributions inside Interplanetary Magnetic Flux Ropes Authors: Hu, Qiang; Qiu, Jiong; Krucker, Sam Bibcode: 2015TESS....121204H Altcode: ​We report on the detailed and systematic study of field-line twist and length distributions within magnetic flux ropes embedded in Interplanetary Coronal Mass Ejections (ICMEs). The Grad-Shafranov reconstruction method is utilized together with a constant-twist nonlinear force-free (Gold-Hoyle) flux rope model and the commonly known Lundquist (linear force-free) model to reveal the close relation between the field-line twist and length in cylindrical flux ropes, based on in-situ spacecraft magnetic field and plasma measurements. In particular, we utilize energetic electron burst observations at 1 AU together with associated type III radio emissions detected by the Wind spacecraft to provide unique measurements of magnetic field-line lengths within selected ICME events. These direct measurements are compared with flux-rope model calculations to help assess the fidelity of different models and to provide diagnostics of internal structures. We show that our initial analysis of field-line twist indicates clear deviation from the Lundquist model, but better consistency with the Gold-Hoyle model. By using the different flux-rope models, we conclude that the in-situ direct measurements of field-line lengths are consistent with a flux-rope structure with spiral field lines of constant and low twist, largely different from that of the Lundquist model, especially for relatively large-scale flux ropes. We will also discuss the implications of our analysis of flux-rope structures on the origination and evolution processes in their corresponding solar source regions. 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: Hard X-ray imaging spectroscopy of FOXSI microflares Authors: Glesener, Lindsay; Krucker, Sam; Christe, Steven; Buitrago-Casas, Juan Camilo; Ishikawa, Shin-nosuke; Foster, Natalie Bibcode: 2015TESS....140601G Altcode: The ability to investigate particle acceleration and hot thermal plasma in solar flares relies on hard X-ray imaging spectroscopy using bremsstrahlung emission from high-energy electrons. Direct focusing of hard X-rays (HXRs) offers the ability to perform cleaner imaging spectroscopy of this emission than has previously been possible. Using direct focusing, spectra for different sources within the same field of view can be obtained easily since each detector segment (pixel or strip) measures the energy of each photon interacting within that segment. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload has successfully completed two flights, observing microflares each time. Flare images demonstrate an instrument imaging dynamic range far superior to the indirect methods of previous instruments like the RHESSI spacecraft.In this work, we present imaging spectroscopy of microflares observed by FOXSI in its two flights. Imaging spectroscopy performed on raw FOXSI images reveals the temperature structure of flaring loops, while more advanced techniques such as deconvolution of the point spread function produce even more detailed images. Title: Observation and Modeling of a Termination Shock in a Solar Eruption as a Possible Particle Accelerator Authors: Gary, Dale E.; Chen, Bin; Bastian, Timothy S.; Shen, Chengcai; Krucker, Sam Bibcode: 2015TESS....130402G Altcode: Solar eruptions and their associated solar flares are the most energetic particle accelerators in our solar system. Yet the acceleration mechanism remains uncertain. A possible candidate often invoked in the standard picture of solar eruptions is a termination shock, produced by fast reconnection outflows impinging upon dense, closed loops in a helmet-type geometry. However, the importance of termination shocks in solar particle acceleration remains controversial, mainly because there has been no direct detection of such shocks. Here we report direct imaging of the location and evolution of a termination shock during the rise phase of a solar eruption. The shock appears at radio wavelengths as a narrow surface sandwiched between multitudes of downward-moving plasma blobs and the underlying, newly-reconnected flaring loops, and evolves coherently with a loop-top hard X-ray source in the shock downstream region. The shock produces many short-lived, point-like radio sources, each interpreted as emission from a turbulence cell interacting with fast (nonthermal) electrons. These point-like radio sources clearly outline the termination shock front and their positions change in reaction to the arrival of the fast plasma blobs, which are well-reproduced by our numerical simulations based on a resistive magnetohydrodynamics reconnection model in a standard two-ribbon flare geometry. We further show that a temporary disruption of the shock coincides with a reduction of radio and hard X-ray emission associated with the energetic electron population. Our observations strongly favor a scenario in which the termination shock is responsible for accelerating electrons to high energies. Title: STEREO- Wind Radio Positioning of an Unusually Slow Drifting Event Authors: Martínez-Oliveros, J. C.; Raftery, C.; Bain, H.; Liu, Y.; Pulupa, M.; Saint-Hilaire, P.; Higgins, P.; Krupar, V.; Krucker, Säm; Bale, S. D. Bibcode: 2015SoPh..290..891M Altcode: 2014arXiv1410.3352M On 13 March 2010 an unusually long-duration event was observed by radio spectrographs onboard the STEREO-B and Wind spacecraft. The event started at about 13:00 UT and ended at approximately 06:00 UT on 14 March. The event presents itself as slow drifting, quasi-continuous emission in a very narrow frequency interval, with an apparent frequency drift from about 625 kHz to approximately 425 kHz. Using the Leblanc, Dulk, and Bougeret (Solar Phys.183, 165, 1998) interplanetary density model, we determined that the drift velocities of the radio source are ≈ 33 km s−1 and ≈ 52 km s−1 for 0.2 and 0.5 times the densities of Leblanc model, respectively, with a normalization density of 7.2 cm−3 at 1 AU and assuming harmonic emission. A joint analysis of the radio direction-finding data, coronograph white-light observations and modeling revealed that the radio sources appear to be located in interaction regions with relatively high density and slow solar wind speed. 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: Sign Singularity and Flares in Solar Active Region NOAA 11158 Authors: Sorriso-Valvo, L.; De Vita, G.; Kazachenko, M. D.; Krucker, S.; Primavera, L.; Servidio, S.; Vecchio, A.; Welsch, B. T.; Fisher, G. H.; Lepreti, F.; Carbone, V. Bibcode: 2015ApJ...801...36S Altcode: 2015arXiv150104279S Solar Active Region NOAA 11158 has hosted a number of strong flares, including one X2.2 event. The complexity of current density and current helicity are studied through cancellation analysis of their sign-singular measure, which features power-law scaling. Spectral analysis is also performed, revealing the presence of two separate scaling ranges with different spectral index. The time evolution of parameters is discussed. Sudden changes of the cancellation exponents at the time of large flares and the presence of correlation with Extreme-Ultra-Violet and X-ray flux suggest that eruption of large flares can be linked to the small-scale properties of the current structures. 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: 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: The Focusing Optics X-ray Solar Imager: Second Flight and Recent Results Authors: Christe, S.; Krucker, S.; Glesener, L.; Ishikawa, S. N.; Ramsey, B.; Buitrago Casas, J. C.; Foster, N. Bibcode: 2014AGUFMSH53B4227C Altcode: Solar flares accelerate particles up to high energies through various acceleration mechanisms which are not currently understood. Hard X-rays are the most direct diagnostic of flare-accelerated electrons. However past and current hard x-ray observation lack the sensitivity and dynamic range necessary to observe the faint signature of accelerated electrons in the acceleration region, the solar corona. These limitations can be easily overcome through the use of HXR focusing optics coupled with solid state pixelated detectors. We present on recent updates on the FOXSI sounding rocket program. During its first flight FOXSI observed imaged a microflare with simultaneous observations by RHESSI. We present recent imaging analysis of the FOXSI observations and detailed comparison with RHESSI. New detector calibration results are also presented and, time-permitting, preliminary results from the second launch of FOXSI scheduled for December 2014. Title: Constraining hot plasma in a non-flaring solar active region with FOXSI hard X-ray observations Authors: Ishikawa, Shin-nosuke; Glesener, Lindsay; Christe, Steven; Ishibashi, Kazunori; Brooks, David H.; Williams, David R.; Shimojo, Masumi; Sako, Nobuharu; Krucker, Säm Bibcode: 2014PASJ...66S..15I Altcode: 2015arXiv150905288I; 2014PASJ..tmp..102I We present new constraints on the high-temperature emission measure of a non-flaring solar active region using observations from the recently flown Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload. FOXSI has performed the first focused hard X-ray (HXR) observation of the Sun in its first successful flight on 2012 November 2. Focusing optics, combined with small strip detectors, enable high-sensitivity observations with respect to previous indirect imagers. This capability, along with the sensitivity of the HXR regime to high-temperature emission, offers the potential to better characterize high-temperature plasma in the corona as predicted by nanoflare heating models. We present a joint analysis of the differential emission measure (DEM) of active region 11602 using coordinated observations by FOXSI, Hinode/XRT, and Hinode/EIS. The Hinode-derived DEM predicts significant emission measure between 1 MK and 3 MK, with a peak in the DEM predicted at 2.0-2.5 MK. The combined XRT and EIS DEM also shows emission from a smaller population of plasma above 8 MK. This is contradicted by FOXSI observations that significantly constrain emission above 8 MK. This suggests that the Hinode DEM analysis has larger uncertainties at higher temperatures and that > 8 MK plasma above an emission measure of 3 × 1044 cm-3 is excluded in this active region. 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: MiXI: The Miniature X-ray Imager Authors: Martinez Oliveros, J. C.; Glesener, L.; Sundkvist, D. J.; Saint-Hilaire, P.; Bain, H. M.; Fivian, M. D.; Hurford, G. J.; Sample, J. G.; Bale, S. D.; Krucker, S. Bibcode: 2014AGUFMSH53B4222M Altcode: The Miniature X-ray Imager (MiXI) is an innovative, small, and fully functional solar X-ray observatory concept designed to fit within a 6U CubeSat platform. MiXI will provide the community with X-ray imaging in the energy range from ~6 to 40-50 keV and spectroscopy up to 100 keV of solar flares at a small fraction of the cost of a conventional mission. It includes rotation modulation collimators and layered Si/CdTe detectors, providing routine observations of both soft and hard X-ray emission with low background. Coordinated observations between MiXI and the STIX instrument onboard Solar Orbiter will enable solar flare observation from two vantage points, providing new insights into the directivity of flare HXR emission and will allow detailed study of both coronal and footpoint sources within the same flare. These results may have profound implications for theories of flare acceleration processes. Title: Harnessing AIA Diffraction Patterns to Determine Flare Footpoint Temperatures Authors: Bain, H. M.; Schwartz, R. A.; Torre, G.; Krucker, S.; Raftery, C. L. Bibcode: 2014AGUFMSH13B4105B Altcode: In the "Standard Flare Model" energy from accelerated electrons is deposited at the footpoints of newly reconnected flare loops, heating the surrounding plasma. Understanding the relation between the multi-thermal nature of the footpoints and the energy flux from accelerated electrons is therefore fundamental to flare physics. Extreme ultraviolet (EUV) images of bright flare kernels, obtained from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory, are often saturated despite the implementation of automatic exposure control. These kernels produce diffraction patterns often seen in AIA images during the most energetic flares. We implement an automated image reconstruction procedure, which utilizes diffraction pattern artifacts, to de-saturate AIA images and reconstruct the flare brightness in saturated pixels. Applying this technique to recover the footpoint brightness in each of the AIA EUV passbands, we investigate the footpoint temperature distribution. Using observations from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we will characterize the footpoint accelerated electron distribution of the flare. By combining these techniques, we investigate the relation between the nonthermal electron energy flux and the temperature response of the flare footpoints. Title: Coronal Signatures of Magnetic Energy Dissipation in Solar Eruptions Authors: Krucker, S. Bibcode: 2014AGUFMSM43C..01K Altcode: This talk will highlight recent observational results of magnetic energy dissipation in solar flares and coronal mass ejection with a focus on hard X-ray observations provided by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Title: High-resolution Observations of the X-flare on 2014-03-29 Authors: Kleint, L.; Battaglia, M.; Krucker, S.; Reardon, K.; Sainz Dalda, A. Bibcode: 2014AGUFMSH31C..06K Altcode: We investigate the sequence of events leading to the X1 flare SOL2014-03-29T17:48. Because of the unprecedented joint observations of an X-flare with the ground-based Dunn Solar Telescope and the spacecraft IRIS, Hinode, RHESSI, STEREO, and SDO, we can sample many solar layers from the photosphere to the corona. We find that a filament eruption, which was possibly caused by a thermal instability, was the cause of this X-flare. The filament was rising in the chromosphere for at least one hour before the flare occurred with a velocity of ∼2--5 km/s. 15 minutes before the flare, its chromospheric rise velocity increased to ∼6--10 km/s, before it lifted off with at least 600 km/s, as seen by IRIS in the transition region. Doppler velocities from H-alpha images reveal intriguing small scale flows along the filament and enable us to derive its probable shape. An unusual feature was a low-lying twisted flux rope near the filament, which did not participate in the filament eruption. After the flare ribbons started on each of its sides, the flux rope seems to have untangled and vanished during the flare. We present a comprehensive overview of the flare, including polarimetric and spectroscopic data at subarcsecond resolution. Title: Does There Exist a Relationship Between Acoustic and White-Light Emission in Hard-X ray Solar Flares? Authors: Buitrago-Casas, J. C.; Martinez Oliveros, J. C.; Glesener, L.; Krucker, S.; Calvo-Mozo, B. Bibcode: 2014AGUFMSH11D..02B Altcode: Several mechanisms have been proposed to explain the observed seismicity during some solar flares. One theory associates high-energy electrons and white-light emission with sunquakes. This relationship is based on the back-warming model, where high-energy electrons and their subsequent heating of the photosphere induce acoustic waves in the solar interior. We carried out a correlative study of solar flares with emission in hard-X rays (HXRs) above 50 keV, enhanced white light emission at 6573Å, and acoustic sources. We selected those flares observed by RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) with a considerable flux in the 50-100 and 100-300 keV bands between January 1, 2010 and June 26, 2014. Additionally, we restricted the sample to flares close to disk center where it is observationally easiest to detect a sunquake. We then used data from the Helioseismic and Magnetic Imager onboard the Solar Dynamic Observatory (SDO/HMI) to search for white-light emission and helioseismic signatures. Finally, we calculated a coefficient of correlation for this set of dichotomic observables. We discuss the phenomenological connectivity between these physical quantities and the observational difficulties of detecting seismic signals and white-light radiation with terrestrial and space-borne observations. 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: Magnetic Field-line Length and Twist Distributions within Interplanetary Flux Fopes from Wind Spacecraft Measurements Authors: Hu, Q.; Qiu, J.; Krucker, S.; Wang, L.; Wang, B.; Chen, Y.; Moestl, C. Bibcode: 2014AGUFMSH31A4104H Altcode: We report on the detailed and systematic study of field-line twist and length distributions within magnetic flux ropes embedded in Interplanetary Coronal Mass Ejections (ICMEs). In particular we will utilize energetic electron burst observations at 1 AU together with associated type III radio emissions detected by the Wind spacecraft to provide unique measurements of magnetic field-line lengths within selected ICME events. These direct measurements will be compared with flux-rope model calculations to help assess the fidelity of different models and to provide diagnostics of internal structures. The Grad-Shafranov reconstruction method will be utilized together with a constant-twist nonlinear force-free (Gold-Hoyle) flux rope model and the commonly known Lundquist (linear force-free) model to reveal the close relation between the field-line twist and length in cylindrical flux ropes, based on in-situ Wind spacecraft magnetic field and plasma measurements. We show that our initial analysis of field-line twist indicates clear deviation from the Lundquist model, but better consistency with the Gold-Hoyle model. We will also discuss the implications of our analysis of flux-rope structures on the origination and evolution processes in their corresponding solar source regions. Title: First Images from the Focusing Optics X-Ray Solar Imager Authors: Krucker, Säm; Christe, Steven; Glesener, Lindsay; Ishikawa, Shin-nosuke; Ramsey, Brian; Takahashi, Tadayuki; Watanabe, Shin; Saito, Shinya; Gubarev, Mikhail; Kilaru, Kiranmayee; Tajima, Hiroyasu; Tanaka, Takaaki; Turin, Paul; McBride, Stephen; Glaser, David; Fermin, Jose; White, Stephen; Lin, Robert Bibcode: 2014ApJ...793L..32K Altcode: The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload flew for the first time on 2012 November 2, producing the first focused images of the Sun above 5 keV. To enable hard X-ray (HXR) imaging spectroscopy via direct focusing, FOXSI makes use of grazing-incidence replicated optics combined with fine-pitch solid-state detectors. On its first flight, FOXSI observed several targets that included active regions, the quiet Sun, and a GOES-class B2.7 microflare. This Letter provides an introduction to the FOXSI instrument and presents its first solar image. These data demonstrate the superiority in sensitivity and dynamic range that is achievable with a direct HXR imager with respect to previous, indirect imaging methods, and illustrate the technological readiness for a spaceborne mission to observe HXRs from solar flares via direct focusing optics. Title: Caliste-SO: the x-ray spectrometer unit of the STIX instrument onboard the Solar Orbiter space mission Authors: Meuris, Aline; Limousin, Olivier; Gevin, Olivier; Vassal, Marie-Cécile; Soufflet, Fabrice; Fiant, Nicolas; Bednarzik, Martin; Wild, Christopher; Stutz, Stefan; Birrer, Guy; Blondel, Claire; Le Mer, Isabelle; Huynh, Duc-Dat; Donati, Modeste; Grimm, Oliver; Commichau, Volker; Hurford, Gordon; Krucker, Säm.; Gonzalez, François; Billot, Marc Bibcode: 2014SPIE.9154E..0YM Altcode: Caliste-SO is a hybrid detector integrating in a volume of 12 × 14 × 18 mm3 a 1 mm-thick CdTe pixel detector, a frontend IDeF-X HD ASIC and passive parts to perform high resolution spectroscopy in the 4-200 keV energy range with high count rate capability (104-105 photons/s/cm2). The detector hybridization concept was designed by CEA and 3DPlus to realize CdTe cameras for space astronomy missions with various pixel patterns. For the STIX instrument onboard the Solar Orbiter mission, the imaging system is made by 32 collimators that sample the visibilities of the spatial Fourier transform and doesn't require fine pitch pixels. The Al-Schottky CdTe detectors produced by Acrorad are then patterned and tested by the Paul Scherrer Institute to produce 12 pixels surrounded by a guard ring within 1 cm2. Electrical and spectroscopic performance tests of the Caliste-SO samples are performed in France at key manufacturing steps, before sending the samples to the principal investigator to mount them in the Detector Electronics Module of STIX in front of each collimator. Four samples were produced in 2013 to be part of the STIX engineering model. Best pixels show an energy resolution of 0.7 keV FWHM at 6 keV (1 keV resolution requirement for STIX) and a low-level detection threshold below 3 keV (4 keV requirement for STIX). The paper describes the design and the production of Caliste-SO and focuses on main performance tests performed so far to characterize the spectrometer unit. Title: Observing the Sun in hard X-rays using grazing incidence optics: the FOXSI and HEROES projects Authors: Christe, Steven; Glesener, Lindsay; Krucker, Sam; Shih, Albert Y.; Gaskin, Jessica; Wilson, Colleen Bibcode: 2014AAS...22412362C Altcode: Solar flares accelerate particles up to high energies through various acceleration mechanisms which are not currently understood. Hard X-rays are the most direct diagnostic of flare-accelerated electrons. However past and current hard x-ray observation lack the sensitivity and dynamic range necessary to observe the faint signature of accelerated electrons in the acceleration region, the solar corona. These limitations can be easily overcome through the use of HXR focusing optics coupled with solid state pixelated detectors. We present results from the recent flights of two sub-orbital payloads that have applied grazing incidence HXR optics to solar observations. FOXSI, short for Focusing Optics X-Ray Solar Imager, was launched on a sounding rocket in November 2012 from White Sanda and observed a solar flare. HEROES, short for High Energy Replicated Optics to Explore the Sun, observed the sun for 7 hours from a high altitude balloon on September 21, 2013. We present recent results as well as the capabilities of a possible future satellite mission Title: RHESSI and EIS observations of an above-the-looptop reconnection region Authors: Glesener, Lindsay; Hara, Hirohisa; Krucker, Sam Bibcode: 2014AAS...22410406G Altcode: A variety of solar flare observations suggest particle acceleration in the corona, at or above the flare looptop. Hard X-ray (HXR) studies, for example, occasionally reveal accelerated electrons above flare looptops, in some cases suggesting the location of the acceleration region. However, since coronal HXR sources are faint and the structure of the flare as seen in extreme ultraviolet (EUV) images is complicated, it is difficult to say where these sources lie with respect to, for example, the reconnection region.HXR and EUV observations can provide complementary information for investigating this topic. EUV imaging spectroscopy reveals bulk flows and locations of line-broadened (potentially turbulent) sources. Such observations can, for example, identify outflows from the reconnection region. HXR imaging places the flare-accelerated electrons in the context of the overall flare geometry, allowing comparison of the locations of accelerated electrons, the reconnection region, and the flare loop.In this work, data from RHESSI and Hinode/EIS are used to investigate above-the-looptop sources in the 2013 May 15 X-class flare. Above-the-looptop EIS flows and loop-top line-broadening are compared with RHESSI HXR sources in the preimpulsive phase of the flare. 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: Energetic analysis of the white light emission associated to seismically active flares in solar cycle 24 Authors: Buitrago-Casas, Juan Camilo; Martinez Oliveros, Juan Carlos; Glesener, Lindsay; Krucker, Sam Bibcode: 2014AAS...22412338B Altcode: Solar flares are explosive phenomena, thought to be driven by magnetic free energy accumulated in the solar corona. Some flares release seismic transients, "sunquakes", into the Sun's interior. Different mechanisms are being considered to explain how sunquakes are generated. We are conducting an analysis of white-light emission associated with those seismically active solar flares that have been reported by different authors within the current solar cycle. Seismic diagnostics are based upon standard time-distance techniques, including seismic holography, applied to Dopplergrams obtained by SDO/HMI and GONG. The relation between white-light emissions and seismic activity may provide important information on impulsive chromospheric heating during flares, a prospective contributor to seismic transient emission, at least in some instances. We develop a method to get an estimation of Energy associated whit white-light emission and compare those results whit values of energy needed to generate a sunquake according with holographic helioseismology techniques. 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: 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: Radio Imaging of a Type IVM Radio Burst on the 14th of August 2010 Authors: Bain, H. M.; Krucker, S.; Saint-Hilaire, P.; Raftery, C. L. Bibcode: 2014ApJ...782...43B Altcode: Propagating coronal mass ejections (CMEs) are often accompanied by burst signatures in radio spectrogram data. We present Nançay Radioheliograph observations of a moving source of broadband radio emission, commonly referred to as a type IV radio burst (type IVM), which occurred in association with a CME on the 14th of August 2010. The event was well observed at extreme ultraviolet (EUV) wavelengths by SDO/AIA and PROBA2/SWAP, and by the STEREO SECCHI and SOHO LASCO white light (WL) coronagraphs. The EUV and WL observations show the type IVM source to be cospatial with the CME core. The observed spectra is well fitted by a power law with a negative slope, which is consistent with optically thin gyrosynchrotron emission. The spectrum shows no turn over at the lowest Nançay frequencies. By comparing simulated gyrosynchrotron spectra with Nançay Radioheliograph observations, and performing a rigorous parameter search we are able to constrain several key parameters of the underlying plasma. Simulated spectra found to fit the data suggest a nonthermal electron distribution with a low energy cutoff of several tens to 100 keV, with a nonthermal electron density in the range 100-102 cm-3, in a magnetic field of a few Gauss. The nonthermal energy content of the source is found to contain 0.001%-0.1% of the sources thermal energy content. Furthermore, the energy loss timescale for this distribution equates to several hours, suggesting that the electrons could be accelerated during the CME initiation or early propagation phase and become trapped in the magnetic structure of the CME core without the need to be replenished. Title: Statistical Properties of Super-hot Solar Flares Authors: Caspi, Amir; Krucker, Säm; Lin, R. P. Bibcode: 2014ApJ...781...43C Altcode: 2013arXiv1312.0371C We use Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) high-resolution imaging and spectroscopy observations from ~6 to 100 keV to determine the statistical relationships between measured parameters (temperature, emission measure, etc.) of hot, thermal plasma in 37 intense (GOES M- and X-class) solar flares. The RHESSI data, most sensitive to the hottest flare plasmas, reveal a strong correlation between the maximum achieved temperature and the flare GOES class, such that "super-hot" temperatures >30 MK are achieved almost exclusively by X-class events; the observed correlation differs significantly from that of GOES-derived temperatures, and from previous studies. A nearly ubiquitous association with high emission measures, electron densities, and instantaneous thermal energies suggests that super-hot plasmas are physically distinct from cooler, ~10-20 MK GOES plasmas, and that they require substantially greater energy input during the flare. High thermal energy densities suggest that super-hot flares require strong coronal magnetic fields, exceeding ~100 G, and that both the plasma β and volume filling factor f cannot be much less than unity in the super-hot region. 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: Particle Densities within the Acceleration Region of a Solar Flare Authors: Krucker, Säm; Battaglia, Marina Bibcode: 2014ApJ...780..107K Altcode: The limb flare SOL2012-07-19T05:58 (M7.7) provides the best example of a non-thermal above-the-loop-top hard X-ray source with simultaneous observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. By combining the two sets of observations, we present the first direct measurement of the thermal proton density and non-thermal electron density within the above-the-loop-top source where particle acceleration occurs. We find that both densities are of the same order of magnitude of a few times 109 cm-3, about 30 times lower than the density in the underlying thermal flare loops. The equal densities indicate that the entire electron population within the above-the-loop-top source is energized. While the derived densities depend on the unknown source depth and filling factor, the ratio of these two densities does not. Within the uncertainties, the ratio is one for a low energy cutoff of the non-thermal electron spectrum between 10 and 15 keV. RHESSI observations only constrain the cutoff energy to below ~15 keV, leaving the spectral shape of the electrons within the above-the-loop-top source at lower energies unknown. Nevertheless, these robust results strongly corroborate earlier findings that the above-the-loop-top source is the acceleration region where a bulk energization process acts on all electrons. Title: Observation of Heating by Flare-accelerated Electrons in a Solar Coronal Mass Ejection Authors: Glesener, Lindsay; Krucker, Säm; Bain, Hazel M.; Lin, Robert P. Bibcode: 2013ApJ...779L..29G Altcode: We report a Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observation of flare-accelerated electrons in the core of a coronal mass ejection (CME) and examine their role in heating the CME. Previous CME observations have revealed remarkably high thermal energies that can far surpass the CME's kinetic energy. A joint observation by RHESSI and the Atmospheric Imaging Assembly of a partly occulted flare on 2010 November 3 allows us to test the hypothesis that this excess energy is collisionally deposited by flare-accelerated electrons. Extreme ultraviolet (EUV) images show an ejection forming the CME core and sheath, with isothermal multifilter analysis revealing temperatures of ~11 MK in the core. RHESSI images reveal a large (~100 × 50 arcsec2) hard X-ray (HXR) source matching the location, shape, and evolution of the EUV plasma, indicating that the emerging CME is filled with flare-accelerated electrons. The time derivative of the EUV emission matches the HXR light curve (similar to the Neupert effect observed in soft and HXR time profiles), directly linking the CME temperature increase with the nonthermal electron energy loss, while HXR spectroscopy demonstrates that the nonthermal electrons contain enough energy to heat the CME. This is the most direct observation to date of flare-accelerated electrons heating a CME, emphasizing the close relationship of the two in solar eruptive events. Title: Plasma Heating in a Post Eruption Current Sheet: A Case Study Based on Ultraviolet, Soft, and Hard X-Ray Data Authors: Susino, R.; Bemporad, A.; Krucker, Säm Bibcode: 2013ApJ...777...93S Altcode: 2013arXiv1310.2853S Off-limb observations of the solar corona after coronal mass ejections (CMEs) often show strong, compact, and persistent UV sources behind the eruption. They are primarily observed by the SOHO/UVCS instrument in the "hot" [Fe XVIII] λ974 line and are usually interpreted as a signature of plasma heating due to magnetic reconnection in the post-CME current sheet (CS). Nevertheless, the physical process itself and the altitude of the main energy release are currently not fully understood. In this work, we study the evolution of plasma heating after the CME of 2004 July 28 by comparing UV spectra acquired by UVCS with soft and hard X-ray (SXR, HXR) images of the post-flare loops taken by GOES/SXI and RHESSI. The X-ray data show a long-lasting extended source that is rising upward, toward the high-temperature source detected by UVCS. UVCS data show the presence of significant non-thermal broadening in the CS (a signature of turbulent motions) and a strong density gradient across the CS region. The thermal energy released in the HXR source is on the order of ~1032 erg, a factor ~2-5 larger than the energy required to explain the high-temperature plasma sampled by UVCS. Nevertheless, the very different time evolutions of SXR and HXR sources compared with the UV emission suggest that reconnection occurring above the post-eruption arcades is not directly responsible for the high-temperature plasma sampled higher up by UVCS. We conclude that an additional plasma heating mechanism (such as turbulent reconnection) in the CS is likely required. 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: Hard X-Ray Emission from the February 15, 2011 Solar Flare Authors: Marsh, Andrew; Smith, D. M.; Krucker, S.; Glesener, L. Bibcode: 2013SPD....44...57M Altcode: The February 15, 2011 solar flare was the first X-class flare of Solar Cycle 24, and as such has received much attention in the literature. This flare has many interesting features, including the presence of a sun-quake, a large coronal mass ejection, and a distinct “pre-impulsive” phase during which the thermal emission rises appreciably before any significant rise in non-thermal flux. It is presently unknown why certain flares exhibit pre-impulsive behavior, or what heating or particle acceleration mechanisms are responsible for these events. In addition, there has been limited analysis of the hard x-ray emission from this particular flare. We present x-ray images, spectra, and analysis of RHESSI data from this event with particular focus on the pre-impulsive phase. Additional SDO AIA and HMI images will be used to determine the context of the RHESSI observations. Looking at the early stages of this and similar flares presents excellent opportunities to constrain acceleration and heating modes and to learn more about what physical processes underlie flare and CME initiation. 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: MiXI: The Miniature X-ray Imager Authors: Martinez Oliveros, Juan Carlos; Glesener, L.; Hurford, G. J.; Sundkvist, D.; Krucker, S.; Bale, S. Bibcode: 2013SPD....44..144M Altcode: The Miniature X-ray Imager (MiXI) is an ambitious, innovative, small, and fully functional solar X-ray observatory concept designed to fit within a 6U CubeSat platform. MiXI will provide the community with X-ray imaging and spectroscopy of solar flares, but at a small fraction of the cost of a conventional mission. MiXI will observe from 3 to 50 keV. It includes rotation modulation collimators and layered Si/CdTe detectors, providing routine observations of both soft and hard X-ray emission with low background. If selected for funding, MiXI could launch in 2017 to coincide with the launch of Solar Orbiter. In the next solar cycle, coordinated observations between the STIX instrument onboard Solar Orbiter and a future version of MiXI will enable solar flare observation from two vantage points. This will provide new insight into the directivity of flare HXR emission and will allow detailed study of both coronal and footpoint sources in the same flare. These results may have profound implications for theories of flare acceleration processes. We describe here the MiXI concept and its usefulness to the solar and heliophysics communities. Title: The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket, first flight Authors: Christe, Steven; Glesener, L.; Ishikawa, S.; Ramsey, B.; Takahashi, T.; Watanabe, S.; Saito, S.; Lin, R. P.; Krucker, S. Bibcode: 2013SPD....44...88C Altcode: Understanding electron acceleration in solar flares requires X-ray studies with greater sensitivity and dynamic range than are available with current solar hard X-ray observers (i.e. the RHESSI spacecraft). RHESSI employs an indirect Fourier imaging method that is intrinsically limited in dynamic range and therefore can rarely image faint coronal flare sources in the presence of bright footpoints. With greater sensitivity and dynamic range, electron acceleration sites in the corona could be studied in great detail. Both these capabilities can be advanced by the use of direct focusing optics. The recently flown Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload demonstrates the feasibility and usefulness of hard X-ray focusing optics for observations of solar hard X-rays. FOXSI features grazing-incidence replicated nickel optics made by the NASA Marshall Space Flight Center and fine-pitch silicon strip detectors developed by the Astro-H team at JAXA/ISAS. FOXSI flew successfully on November 2, 2012, producing images and spectra of a microflare and performing a search for nonthermal emission (4-15 keV) from nanoflares in the quiet Sun. Nanoflares are a candidate for providing the required energy to heat the solar corona to its high temperature of a few million degrees. A future satellite version of FOXSI, featuring similar optics and detectors, could make detailed observations of hard X-rays from flare-accelerated electrons, identifying and characterizing particle acceleration sites and mapping out paths of energetic electrons as they leave these sites and propagate throughout the solar corona.Abstract (2,250 Maximum Characters): Understanding electron acceleration in solar flares requires X-ray studies with greater sensitivity and dynamic range than are available with current solar hard X-ray observers (i.e. the RHESSI spacecraft). RHESSI employs an indirect Fourier imaging method that is intrinsically limited in dynamic range and therefore can rarely image faint coronal flare sources in the presence of bright footpoints. With greater sensitivity and dynamic range, electron acceleration sites in the corona could be studied in great detail. Both these capabilities can be advanced by the use of direct focusing optics. The recently flown Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload demonstrates the feasibility and usefulness of hard X-ray focusing optics for observations of solar hard X-rays. FOXSI features grazing-incidence replicated nickel optics made by the NASA Marshall Space Flight Center and fine-pitch silicon strip detectors developed by the Astro-H team at JAXA/ISAS. FOXSI flew successfully on November 2, 2012, producing images and spectra of a microflare and performing a search for nonthermal emission (4-15 keV) from nanoflares in the quiet Sun. Nanoflares are a candidate for providing the required energy to heat the solar corona to its high temperature of a few million degrees. A future satellite version of FOXSI, featuring similar optics and detectors, could make detailed observations of hard X-rays from flare-accelerated electrons, identifying and characterizing particle acceleration sites and mapping out paths of energetic electrons as they leave these sites and propagate throughout the solar corona. Title: The Spectrometer Telescope for Imaging X-rays (STIX) Authors: Krucker, Sam; STIX Team Bibcode: 2013SPD....44..140K Altcode: The Spectrometer Telescope for Imaging X-rays (STIX) is one of 10 instruments on board Solar Orbiter, a confirmed M-class mission of the European Space Agency (ESA) within the Cosmic Vision program scheduled to be launched in 2017. STIX applies a Fourier-imaging technique using a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of 32 pixelized CdTe detectors to provide imaging spectroscopy of solar thermal and non-thermal hard X-ray emissions from 4 to 150 keV. The status of the instrument that will be presented at the Critical Design Review (CDR) later this year will be discussed in this poster. Title: Densities within the Acceleration Region of a Solar Flare Authors: Krucker, Sam; Battaglia, M. Bibcode: 2013SPD....4440204K Altcode: The limb flare of 2012 July 19 around 5UT provides the best example of a non-thermal above-the-loop-top hard X-ray source with simultaneous observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamic Observatory (SDO). Combining the two sets of observations we present the first direct measurement of the thermal proton density and non-thermal electron density within the above-the-loop-top source of a solar flare. We find that both densities are of the same order of magnitude of a few times 1e9 cm-3, about 30 times lower than the density in the underlying thermal flare loops. The equal densities indicate that the entire electron distribution within the above-the-loop-top source is energized. While the derived densities depend on the unknown source depth and filling factor, the ratio of these two densities does not. Within the uncertainties, the ratio is one for a low energy cut-off of the non-thermal electron spectrum between 10 and 16 keV, consistent with equal densities. RHESSI observations only constrain the cut-off energy to below 20 keV, leaving the spectral shape of the electrons within the above-the-loop-top source below 20 keV unknown. Nevertheless, these robust results strongly corroborate earlier findings that the above-the-loop-top source is the acceleration region where a bulk energization process acts on all electrons. Title: Radio imaging spectroscopy of synchrotron emission associated with a CME on the 14th of August 2010 Authors: Bain, Hazel; Krucker, S.; Saint-Hilaire, P.; Raftery, C. Bibcode: 2013SPD....4440203B Altcode: We present Nancay Radioheliograph observations of a moving type IV solar radio burst which occurred in association with a CME on the 14th of August 2010. The event was well observed at extreme ultraviolet wavelengths by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, the SWAP instrument onboard Proba2 and by the LASCO white light coronograph. The burst emission was found to be cospatial with the core of the CME. Using radio imaging spectroscopy we are able to characterize the underlying electron distribution and plasma parameters within the source. Fitted spectra reveal a clear power law component consistent with optically thin synchrotron emission from accelerated electrons trapped in the erupting flux rope. As is often observed in type IV bursts, polarization measurements show the source to be moderately polarized during the peak of the burst, before steadily increasing to around 70% as the brightness temperature of the burst decays. Title: Observations of a CME heated by flare-accelerated electrons Authors: Glesener, Lindsay; Krucker, S.; Bain, H.; Lin, R. P. Bibcode: 2013SPD....44...35G Altcode: We present an updated analysis of a RHESSI observation of flare-accelerated electrons in the core of a coronal mass ejection (CME) and examine their role in heating the CME. Previous CME observations have found remarkably high temperatures in the core. A joint observation by RHESSI and AIA of a partly occulted flare on 2010 November 3 allows us to test the hypothesis that this excess energy is collisionally deposited by flare-accelerated electrons. Extreme ultraviolet (EUV) images from AIA show an ejected plasmoid forming the CME core and sheath, with isothermal multifilter analysis revealing temperatures of &gt 11 MK in the core. RHESSI images produced via the Two-Step CLEAN algorithm reveal a large 100 x 50 arcsec2), diffuse hard X-ray source matching the location, shape, and evolution of the AIA plasma, indicating that the emerging CME core is filled with energetic electrons. Spectroscopy demonstrates that the nonthermal electrons contain enough energy to heat the CME. The time integral of the EUV emission matches the hard X-ray light curve (similar to the ``Neupert effect'' observed in soft and hard X-ray time profiles), directly linking the CME temperature increase with the nonthermal electron energy loss. This is the most direct observation to date of energetic flare electrons heating a CME, emphasizing the close relationship of the two in solar eruptive events. Title: Results from the first flight of the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket Authors: Glesener, Lindsay; Christe, S.; Ishikawa, S.; Ramsey, B.; Takahashi, T.; Watanabe, S.; Saito, S.; Lin, R. P.; Krucker, S.; FOXSI Team Bibcode: 2013SPD....4420004G Altcode: Understanding electron acceleration in solar flares requires X-ray studies with greater sensitivity and dynamic range than are available with current solar hard X-ray observers (i.e. the RHESSI spacecraft). RHESSI employs an indirect Fourier imaging method that is intrinsically limited in dynamic range and therefore can rarely image faint coronal flare sources in the presence of bright footpoints. With greater sensitivity and dynamic range, electron acceleration sites in the corona could be studied in great detail. Both these capabilities can be advanced by the use of direct focusing optics. The recently flown Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload demonstrates the feasibility and usefulness of hard X-ray focusing optics for solar study. FOXSI features grazing-incidence replicated nickel optics from the NASA Marshall Space Flight Center and fine-pitch silicon strip detectors developed by the Astro-H team at JAXA/ISAS. FOXSI flew successfully on November 2, 2012, producing images and spectra of a microflare and performing a search for nonthermal emission (4-15 keV) from nanoflares in the quiet Sun. Nanoflares are a candidate for providing the required energy to heat the solar corona to its high temperature of a few million degrees. A future satellite version of FOXSI, featuring similar optics and detectors, could make detailed observations of hard X-rays from flare-accelerated electrons, identifying and characterizing particle acceleration sites and mapping out paths of energetic electrons as they leave these sites and propagate throughout the solar corona. Title: The Focusing Optics X-ray Solar Imager (FOXSI): Instrument and First Flight Authors: Glesener, Lindsay; Christe, S.; Ishikawa, S.; Ramsey, B.; Takahashi, T.; Saito, S.; Lin, R. P.; Krucker, S.; FOXSI Team Bibcode: 2013HEAD...1312314G Altcode: Understanding electron acceleration in solar flares requires hard X-ray studies with greater sensitivity and dynamic range than are available with current solar hard X-ray observers (i.e. the RHESSI spacecraft). Both these capabilities can be advanced by the use of direct focusing optics instead of the indirect Fourier methods of current and previous generations. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload demonstrates the feasibility and usefulness of hard X-ray focusing optics for solar observation. FOXSI flew for the first time on 2012 November 2, producing images and spectra of a microflare and performing a search for nonthermal X-rays from the quiet Sun. Such measurements are important for characterizing the impact of small "nanoflares" on the solar coronal heating problem. A spaceborne solar observer featuring similar optics could make detailed observations of hard X-rays from flare-accelerated electrons, identifying and characterizing particle acceleration sites and mapping out paths of energetic electrons as they leave these sites and propagate throughout the solar corona. Solar observations from NuSTAR are also expected to be an important step in this direction. Title: Suzaku/WAM and RHESSI Observations of Non-thermal Electrons in Solar Microflares Authors: Ishikawa, Shin-nosuke; Krucker, Säm; Ohno, Masanori; Lin, Robert P. Bibcode: 2013ApJ...765..143I Altcode: We report on hard X-ray spectroscopy of solar microflares observed by the Wide-band All-sky Monitor (WAM), on board the Suzaku satellite, and by RHESSI. WAM transient data provide wide energy band (50 keV-5 MeV) spectra over a large field of view (~2π sr) with a time resolution of 1 s. WAM is attractive as a hard X-ray solar flare monitor due to its large effective area (~800 cm2 at 100 keV, ~13 times larger than that of RHESSI). In particular, this makes it possible to search for high energy emission in microflares that is well below the RHESSI background. The WAM solar flare list contains six GOES B-class microflares that were simultaneously observed by RHESSI between the launch of Suzaku in 2005 July and 2010 March. At 100 keV, the detected WAM fluxes are more than ~20 times below the typical RHESSI instrumental background count rates. The RHESSI and WAM non-thermal spectra are in good agreement with a single power law with photon spectral indices between 3.3 and 4.5. In a second step, we also searched the RHESSI microflare list for events that should be detectable by WAM, assuming that the non-thermal power-law emission seen by RHESSI extends to >50 keV. From the 12 detectable events between 2005 July and 2007 February, 11 were indeed seen by WAM. This shows that microflares, similar to regular flares, can accelerate electrons to energies up to at least 100 keV. 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: Kappa Distribution Model for Hard X-Ray Coronal Sources of Solar Flares Authors: Oka, M.; Ishikawa, S.; Saint-Hilaire, P.; Krucker, S.; Lin, R. P. Bibcode: 2013ApJ...764....6O Altcode: 2012arXiv1212.2579O Solar flares produce hard X-ray emission, the photon spectrum of which is often represented by a combination of thermal and power-law distributions. However, the estimates of the number and total energy of non-thermal electrons are sensitive to the determination of the power-law cutoff energy. Here, we revisit an "above-the-loop" coronal source observed by RHESSI on 2007 December 31 and show that a kappa distribution model can also be used to fit its spectrum. Because the kappa distribution has a Maxwellian-like core in addition to a high-energy power-law tail, the emission measure and temperature of the instantaneous electrons can be derived without assuming the cutoff energy. Moreover, the non-thermal fractions of electron number/energy densities can be uniquely estimated because they are functions of only the power-law index. With the kappa distribution model, we estimated that the total electron density of the coronal source region was ~2.4 × 1010 cm-3. We also estimated without assuming the source volume that a moderate fraction (~20%) of electrons in the source region was non-thermal and carried ~52% of the total electron energy. The temperature was 28 MK, and the power-law index δ of the electron density distribution was -4.3. These results are compared to the conventional power-law models with and without a thermal core component. 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: Observational Aspects of Particle Acceleration in Large Solar Flares Authors: Raymond, John C.; Krucker, Säm; Lin, Robert P.; Petrosian, Vahé Bibcode: 2013pacp.book..197R Altcode: No abstract at ADS Title: The Focusing Optics X-ray Solar Imager Authors: Glesener, L.; Christe, S.; Krucker, S.; Ishikawa, S.; Ramsey, B.; Takahashi, T.; Saito, S.; Lin, R. P. Bibcode: 2012AGUFMSH13A2237G Altcode: Measurements of the nonthermal energies and occurrence frequencies of nanoflares are important for understanding the overall flare contribution to coronal heating. Nanoflares have been observed to be ubiquitous in the quiet Sun in extreme ultraviolet and soft X-ray wavelengths, but so far remain undetected at nonthermal hard X-ray (HXR) energies, likely due to the insufficient sensitivity of current instruments. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload has been designed for high sensitivity in the 5-15 keV range by combining grazing-incidence HXR optics with fine-resolution silicon strip detectors. FOXSI will make the first measurement of nonthermal HXR from accelerated electrons in nanoflares, and will also measure hot (5-10 MK) components of active region temperatures. FOXSI is scheduled for a first flight in October 2012, and the first results of this flight will be presented. Title: Radio imaging of synchrotron emission associated with a CME on the 14th of August 2010 Authors: Bain, H. M.; Krucker, S.; Raftery, C. L.; Saint-Hilaire, P. Bibcode: 2012AGUFMSH51A2215B Altcode: Radio observations can be used to identify sources of electron acceleration within flares and CMEs. In a small number of events, radio imaging has revealed the presence of synchrotron emission from nonthermal electrons in the expanding loops of the CME (Bastian et al. (2001), Maia et al. (2007) and Démoulin et al. (2012)). Events in which the synchrotron emission is sufficiently bright to be identified in the presence of plasma emission from radio bursts, which are prevalent at meter wavelengths, are infrequent. Using radio images from the Nançay Radioheliograph (NRH) we present observations of synchrotron emission associated with a CME which occurred on the 14th of August 2010. Using context observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory, the SWAP instrument onboard Proba2, the LASCO coronograph onboard SOHO and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we follow the propagation of the CME out to 2-3 solar radii and characterize the associated electron distribution. We find that the synchrotron emission is cospatial with the CME core. Title: STEREO-Wind Direction Finding of a Stream Interaction Region Authors: Martinez Oliveros, J. C.; Raftery, C. L.; Liu, Y.; Krupar, V.; Higgins, P. A.; Bale, S. D.; Krucker, S. Bibcode: 2012AGUFMSH43A2137M Altcode: On 13 March 2010 a long duration event was seen by spectrographs onboard the STEREO-B and Wind spacecraft. The event started at about 13:00 UT and ended at approximately 05:00 UT on 14 March. The event presents itself as a slow drifting quasi-continuous emission in a very narrow frequency band, ranging from about 425 MHz to approximately 625 MHz. We study the propagation behavior of the radio event, which appears to be a stream interaction region (SIR). Using radio direction-finding techniques, we locate the SIR in the interplanetary space and then correlate these results with visible and in-situ data from instruments onboard STEREO and Wind. Title: The Spectrometer Telescope for Imaging X-rays STIX on Solar Orbiter Authors: Csillaghy, A.; Battaglia, M.; Krucker, S.; Hurford, G. J. Bibcode: 2012AGUFMSA11A2119C Altcode: The Spectrometer Telescope for Imaging X-rays (STIX) will provide imaging spectroscopy of solar thermal and non-thermal X-ray emissions from ~4 to 150 keV. STIX will play an important role in answering two of Solar Orbiter's main science questions: (1) How and where are energetic particles accelerated at the Sun, and how are they transported into interplanetary space? X-ray images and spectra will provide information on the location, spectrum and intensity of flare accelerated electrons near the Sun. (2) What is the magnetic connection from Solar Orbiter back to the Sun? STIX will play a key role in linking remote sensing and in-situ observations on Solar Orbiter. Radio signatures of flare accelerated electrons will be observed by the Radio and Plasma wave instrument (RPW), while the SupraThermal Electron sensor (STE) of the Energetic Particle Detector suite (EPD) will detect electrons in-situ. Thus, the magnetic structure, field line length and connectivity can be tracked. STIX is based on a Fourier-transform imaging technique similar to that used successfully by the Hard X-ray Telescope (HXT) on the Japanese Yohkoh mission, and related to that used for the Reuven Ramaty High Energy Solar Spectroscopic Imager mission. STIX has a higher sensitivity than RHESSI, with comparable image quality and spectral and spatial resolution. It will be able to observe thermal and non-thermal emission from nanoflares up to the largest X- class events. STIX consists of three main parts: 1. An X-ray window, 2. An imager with 32 subcollimators, and 3. A spectrometer with 32 Cadmium Telluride (CdTe) X-ray detectors The transmission through the grid pairs to the detectors is a very sensitive function of the direction of incidence of the X-ray flux. The relative count rates of the detectors behind the different sets of grids encode the spatial information that can be subsequently decoded on the ground to reconstruct images of the source region at different X-ray energies. Title: A Space Weather Mission to the Earth's 5th Lagrangian Point (L5) Authors: Howard, R. A.; Vourlidas, A.; Ko, Y.; Biesecker, D. A.; Krucker, S.; Murphy, N.; Bogdan, T. J.; St Cyr, O. C.; Davila, J. M.; Doschek, G. A.; Gopalswamy, N.; Korendyke, C. M.; Laming, J. M.; Liewer, P. C.; Lin, R. P.; Plunkett, S. P.; Socker, D. G.; Tomczyk, S.; Webb, D. F. Bibcode: 2012AGUFMSA13D..07H Altcode: The highly successful STEREO mission, launched by NASA in 2006, consisted of two spacecraft in heliocentric orbit, one leading and one trailing the Earth and each separating from Earth at the rate of about 22.5 degrees per year. Thus the two spacecraft have been probing different probe/Sun/Earth angles. The utility of having remote sensing and in-situ instrumentation away from the Sun-Earth line was well demonstrated by STEREO. Here we propose the concept of a mission at the 5th Lagrangian "point" in the Earth/Sun system, located behind Earth about 60 degrees to the East of the Sun-Earth line. Such a mission would enable many aspects affecting space weather to be well determined and thus improving the prediction of the conditions of the solar wind as it impinges on geospace. For example, Coronal Mass Ejections can tracked for a significant distance toward Earth, new active regions can be observed before they become visible to the Earth observer, the solar wind can be measured before it rotates to Earth. The advantages of such a mission will be discussed in this presentation. Title: A Survey of Pre-Impulsive Hard X-Ray Emission from RHESSI Flares Authors: Marsh, A.; Smith, D. M.; Krucker, S. Bibcode: 2012AGUFMSH43B2158M Altcode: We present a survey of pre-impulsive flare emission in a set of non-occulted flares observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). This emission can take the form of a gradual rise, and sometimes a distinct, brief plateau (e.g. the November 4, 2003 X28 flare). Pre-impulsive emission gives an opportunity to look for coronal acceleration sites without contamination from footpoint or thermal loop emission, even without occultation. There is also the opportunity to directly compare the pre-impulsive emission to where the footpoint and loop emission later appears, something that is not possible for occulted flares. We are preparing a systematic sample of images and spectra of all non-occulted RHESSI flares above GOES-class X1 that have good coverage of the pre-impulsive and impulsive data. While the hard x-ray images later in the flare will serve as a starting point to discuss magnetic loop and reconnection geometries, we will ultimately include EUV, magnetogram, and other data to clarify the geometry of the initial reconnection. As with all spectra from hard x-ray coronal sources, the energy spectra from these pre-impulsive measurements may allow a relatively direct measurement of the electron spectrum produced by the reconnection process. Title: Spectrometer Telescope for Imaging X-rays (STIX) Authors: Benz, A. O.; Gallagher, P.; Veronig, A.; Grimm, O.; Sylwester, J.; Orleanski, P.; Arnold, N.; Bednarzik, M.; Farnik, F.; Hurford, G.; Krucker, S.; Limousin, O.; Mann, G.; Vilmer, N. Bibcode: 2012IAUSS...6E.509B Altcode: The Solar Orbiter Mission has been confirmed within ESA's M-class Cosmic Vision plan. Launch date is January 2017 into an orbit that reaches nearly one quarter AU in the perihelion. STIX is one of the 10 instruments selected for close cooperation. STIX applies a Fourier-imaging technique using shading tungsten grids. A total of 32 pixelized CdTe detectors will permit high resolution imaging spectroscopy. The design has passed ESA's Preliminary Design Review and will be finalized by the end of 2012. The instrument specification will be presented and its scientific potential discussed. Title: Observational Aspects of Particle Acceleration in Large Solar Flares Authors: Raymond, John C.; Krucker, Säm; Lin, Robert P.; Petrosian, Vahé Bibcode: 2012SSRv..173..197R Altcode: 2012SSRv..tmp...47R Solar flares efficiently accelerate electrons to several tens of MeV and ions to 10 GeV. The acceleration is usually thought to be associated with magnetic reconnection occurring high in the corona, though a shock produced by the Coronal Mass Ejection (CME) associated with a flare can also accelerate particles. Diagnostic information comes from emission at the acceleration site, direct observations of Solar Energetic Particles (SEPs), and emission at radio wavelengths by escaping particles, but mostly from emission from the chromosphere produced when the energetic particles bombard the footpoints magnetically connected to the acceleration region. This paper provides a review of observations that bear upon the acceleration mechanism. Title: A Statistical Study of Solar Electron Events over One Solar Cycle Authors: Wang, Linghua; Lin, R. P.; Krucker, Säm; Mason, Glenn M. Bibcode: 2012ApJ...759...69W Altcode: We survey the statistical properties of 1191 solar electron events observed by the WIND 3DP instrument from <1 keV to gsim300 keV for a solar cycle (1995 through 2005). After taking into account times of high background, the corrected occurrence frequency of solar electron events versus peak flux exhibits a power-law distribution over three orders of magnitude with exponents between -1.0 and -1.6 for different years, comparable to the frequency distribution of solar proton events, microflares, and coronal mass ejections (CMEs), but significantly flatter than that of soft X-ray (SXR) flares. At 40 keV (2.8 keV), the integrated occurrence rate above ~0.29 (~330) cm-2 s-1 sr-1 keV-1 near 1 AU is ~1000 year-1 (~600 year-1) at solar maximum and ~35 year-1 (~25 year-1) at solar minimum, about an order of magnitude larger than the observed occurrence rate. We find these events typically extend over ~45° in longitude, implying the occurrence rate over the whole Sun is ~104 year-1 near solar maximum. The observed solar electron events have a 98.75% association with type III radio bursts, suggesting all type III bursts may be associated with a solar electron event. They have a close (~76%) association with the presence of low-energy (~0.02-2 MeV nucleon-1), 3He-rich (3He/4He >= 0.01) ion emissions measured by the ACE ULEIS instrument. For these electron events, only ~35% are associated with a reported GOES SXR flare, but ~60% appear to be associated with a CME, with ~50% of these CMEs being narrow. These electrons are often detected down to below 1 keV, indicating a source high in the corona. Title: Asymmetry of Hard X-Ray Emissions at Conjugate Footpoints in Solar Flares Authors: Yang, Ya-Hui; Cheng, C. Z.; Krucker, Säm; Hsieh, Min-Shiu; Chen, Nai-Hwa Bibcode: 2012ApJ...756...42Y Altcode: The chromospheric double hard X-ray (HXR) sources generally appear at the conjugate footpoints of flaring loops with asymmetric flux distributions. The behavior of such HXR footpoint asymmetry should be affected by several effects simultaneously and cannot be attributed to a single effect easily. In this study, we attempt to address the properties of photospheric magnetic fields in the areas coinciding with asymmetric HXR footpoints based on RHESSI observations during 2002-2009. A total of 172 time intervals in 22 flares closed to the solar disk center with available pre-flare MDI magnetograms are investigated. The strong HXR footpoint is found to preferentially (75%) locate at the region with weak magnetic field strength, which is qualitatively consistent with the asymmetric magnetic mirror scenario. The HXR footpoint fluxes become more asymmetric when the footpoints move to the areas with more asymmetric field strength. A feature of asymmetry reversal between different energy ranges is observed in some flares, although no significant energy dependence of footpoint asymmetry is found in our statistical results. We also investigated the possible causes of time-dependent HXR footpoint asymmetry by examining the 2004 November 4 M5.4 flare and the 2004 November 6 M3.6 flare. By comparing the estimated asymmetry quantities with the HXR light curves, the asymmetry reversal in the late period of the M5.4 flare is mainly attributed to the difference of coronal energy release or acceleration processes in different periods, while it is associated with the location changes of HXR footpoints moving to different magnetic field regions in the M3.6 flare. Title: The spectrometer telescope for imaging x-rays on board the Solar Orbiter mission Authors: Benz, A. O.; Krucker, S.; Hurford, G. J.; Arnold, N. G.; Orleanski, P.; Gröbelbauer, H. -P.; Klober, S.; Iseli, L.; Wiehl, H. J.; Csillaghy, A.; Etesi, L.; Hochmuth, N.; Battaglia, M.; Bednarzik, M.; Resanovic, R.; Grimm, O.; Viertel, G.; Commichau, V.; Meuris, A.; Limousin, O.; Brun, S.; Vilmer, N.; Skup, K. R.; Graczyk, R.; Stolarski, M.; Michalska, M.; Nowosielski, W.; Cichocki, A.; Mosdorf, M.; Seweryn, K.; Przepiórka, A.; Sylwester, J.; Kowalinski, M.; Mrozek, T.; Podgorski, P.; Mann, G.; Aurass, H.; Popow, E.; Onel, H.; Dionies, F.; Bauer, S.; Rendtel, J.; Warmuth, A.; Woche, M.; Plüschke, D.; Bittner, W.; Paschke, J.; Wolker, D.; Van Beek, H. F.; Farnik, F.; Kasparova, J.; Veronig, A. M.; Kienreich, I. W.; Gallagher, P. T.; Bloomfield, D. S.; Piana, M.; Massone, A. M.; Dennis, B. R.; Schwarz, R. A.; Lin, R. P. Bibcode: 2012SPIE.8443E..3LB Altcode: The Spectrometer Telescope for Imaging X-rays (STIX) is one of 10 instruments on board Solar Orbiter, a confirmed Mclass mission of the European Space Agency (ESA) within the Cosmic Vision program scheduled to be launched in 2017. STIX applies a Fourier-imaging technique using a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of 32 pixelized CdTe detectors to provide imaging spectroscopy of solar thermal and non-thermal hard X-ray emissions from 4 to 150 keV. The status of the instrument reviewed in this paper is based on the design that passed the Preliminary Design Review (PDR) in early 2012. Particular emphasis is given to the first light of the detector system called Caliste-SO. Title: Hard X-Ray Observations of a Jet and Accelerated Electrons in the Corona Authors: Glesener, Lindsay; Krucker, Säm; Lin, R. P. Bibcode: 2012ApJ...754....9G Altcode: We report the first hard X-ray observation of a solar jet on the limb with flare footpoints occulted, so that faint emission from accelerated electrons in the corona can be studied in detail. In this event on 2003 August 21, RHESSI observed a double coronal hard X-ray source in the pre-impulsive phase at both thermal and nonthermal energies. In the impulsive phase, the first of two hard X-ray bursts consists of a single thermal/nonthermal source coinciding with the lower of the two earlier sources, and the second burst shows an additional nonthermal, elongated source, spatially and temporally coincident with the coronal jet. Analysis of the jet hard X-ray source shows that collisional losses by accelerated electrons can deposit enough energy to generate the jet. The hard X-ray time profile above 20 keV matches that of the accompanying Type III and broadband gyrosynchrotron radio emission, indicating both accelerated electrons escaping outward along the jet path and electrons trapped in the flare loop. The double coronal hard X-ray source, the open field lines indicated by Type III bursts, and the presence of a small post-flare loop are consistent with significant electron acceleration in an interchange reconnection geometry. Title: Determination of Electromagnetic Source Direction as an Eigenvalue Problem Authors: Martínez-Oliveros, Juan C.; Lindsey, Charles; Bale, Stuart D.; Krucker, Säm Bibcode: 2012SoPh..279..153M Altcode: 2012arXiv1205.2393M; 2012SoPh..tmp...98M Low-frequency solar and interplanetary radio bursts are generated at frequencies below the ionospheric plasma cutoff and must therefore be measured in space, with deployable antenna systems. The problem of measuring both the general direction and polarization of an electromagnetic source is commonly solved by iterative fitting methods such as linear regression that deal simultaneously with both directional and polarization parameters. We have developed a scheme that separates the problem of deriving the source direction from that of determining the polarization, avoiding iteration in a multi-dimensional manifold. The crux of the method is to first determine the source direction independently of concerns as to its polarization. Once the source direction is known, its direct characterization in terms of Stokes vectors, in a single iteration if desired, is relatively simple. This study applies the source-direction determination to radio signatures of flares received by STEREO. We studied two previously analyzed radio type III bursts and found that the results of the eigenvalue decomposition technique are consistent with those obtained previously by Reiner et al. (Solar Phys.259, 255, 2009). For the type III burst observed on 7 December 2007, the difference in travel times from the derived source location to STEREO A and B is the same as the difference in the onset times of the burst profiles measured by the two spacecraft. This is consistent with emission originating from a single, relatively compact source. For the second event of 29 January 2008, the relative timing does not agree, suggesting emission from two sources separated by 0.1 AU, or perhaps from an elongated region encompassing the apparent source locations. 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: Investigating the Dependency of Footpoint Temperature on Hard X-ray Energy using AIA Dispersion Spectra and RHESSI Imaging Authors: Raftery, Claire; Bain, H. M.; Krucker, S. Bibcode: 2012AAS...22030906R Altcode: The nature of energy deposition in solar flares remains largely a mystery. There have been many studies, analyzing the energy and temperature distributions of solar flares throughout their lifetime, though few that directly characterize the temperature distribution of flaring footpoint plasma as a function of footpoint energy. Here we will use the SDO/AIA dispersion spectra technique (Raftery et al. 2011) to identify the differential emission measure distribution of individual flaring ribbons as a function of time. We will interpret the distributions with respect to the energies observed in those footpoints using RHESSI imaging and spectroscopy techniques. In doing so, we will be directly observing the changes in the temperature response of the footpoint plasma to electron beams of varying energies. Title: Radio Imaging of Shock-accelerated Electrons Associated with an Erupting Plasmoid on 2010 November 3 Authors: Bain, H. M.; Krucker, Säm; Glesener, L.; Lin, R. P. Bibcode: 2012ApJ...750...44B Altcode: We present observations of a metric type II solar radio burst that occurred on the 3rd of November 2010 in association with an erupting plasmoid. The eruption was well observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory and the Reuven Ramaty High Energy Solar Spectroscopic Imager, while the burst occurred in the frequency range of the Nançay Radioheliograph (NRH). Such events, where the type II emission occurs in the NRH frequency range, allowing us to image the burst, are infrequent. Combining these data sets, we find that the type II is located ahead of the hot (~11 MK) core of the plasmoid, which is surrounded by a well-defined envelope of cool (few MK) plasma. Using two methods, we determine the propagation velocity of the shock: (1) fitting the type II emission observed in PHOENIX and HUMAIN radio spectrogram data; (2) direct imaging of the type II source location using NRH observations. We use LASCO C2 polarized brightness images to normalize our coronal density model. However, we find that information from imaging is required in order to fine-tune this normalization. We determine a shock propagation velocity between 1900 km s-1 and 2000 km s-1. This is faster than the plasmoid observed at extreme-ultraviolet wavelengths by AIA (v = 670-1440 km s-1, where the cooler plasma propagates faster than the hot core). The positioning of the type II, ahead of the plasmoid, suggests that the electrons are accelerated in a piston-driven shock. Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Christe, Steven; Krucker, S.; Glesener, L.; Ishikawa, S.; Ramsey, B.; Takahashi, T.; Lin, R. Bibcode: 2012AAS...22041003C Altcode: Hard x-ray (HXR) observations are a powerful diagnostic tool providing quantitative measurements of nonthermal energetic (>10 keV) electrons. Energetic electrons traveling in a plasma radiate HXR emission through the well-known process of bremsstrahlung. Solar eruptive events are the most powerful particle accelerators in the solar system, accelerating electrons up to hundreds of MeV. It is thought that the energy release process and particle acceleration occur somewhere in the corona. Since bremsstrahlung emission depends on the density of the ambient medium, solar HXR emission is strongest when electron beams enter the chromosphere where they lose their energy quickly through collisions. Energetic electrons moving in the relatively tenuous corona suffer few collisions, losing little energy and producing only faint HXR emission. Present-day instruments do not have the sensitivity to see the faint HXR emission from electrons traveling in the corona, nor the dynamic range to see such faint emission in the presence of bright HXR footpoint emission. Existing observations therefore show us only where energetic electrons are stopped but not where they are accelerated, nor along what path they escape from the acceleration site. Thus, to make the next breakthrough in understanding the energy release in solar eruptive events requires HXR imaging with much higher sensitivity and dynamic range. HXR focusing optics combined with position sensitive solid state detectors can provide both. We discuss the current state of technological development in this area and the science it would make possible. 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: Radio Imaging Of Shock-accelerated Electrons Associated With An Erupting Plasmoid On The 3rd Of November 2010 Authors: Bain, Hazel; Krucker, S.; Glesener, L.; Lin, R. P. Bibcode: 2012AAS...22050804B Altcode: We present observations of a metric type II solar radio burst that occurred in association with an erupting plasmoid on the 3rd of November 2010. The event was well observed by the Nancay Radioheliograph (NRH) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). Events in which the type II emission is present in the NRH frequency range of a few hundred MHz are infrequent. From NRH images, we found the type II source location to be situated ahead of the hot ( 11 MK) core of the plasmoid, which is surrounded by an envelope of cooler (few MK) plasma. Fitting the type II emission observed in radio spectrogram data, we were able to determine that the shock which produced the type II emission, was propagating with a velocity of 1900 - 2000 km/s. Using a combination of direct imaging from NRH and polarized brightness images from LASCO C2, we were able to normalize the coronal density model used for the fit. The shock velocity was found to be significantly greater than the velocity of the hot core and cooler envelope (670 - 1440 km/s) seen at extreme ultraviolet wavelengths with AIA. The location of the burst emission ahead of the core and the relative velocities of the shock and the plasmoid are indicative of a piston-driven shock.

This work was supported in part by the RHESSI project, NASA contract NAS598033. LG was partly supported by NASA GSRP grant NNX09AM40H. RPL was partly supported by the WCU grant (No. R31-10016) funded by the Korean Ministry of Education, Science and Technology. Title: Instrument data processing unit for spectrometer/telescope for imaging x-rays (STIX) Authors: Skup, Konrad R.; Cichocki, A.; Graczyk, R.; Michalska, M.; Mosdorf, M.; Nowosielski, W.; Orleański, P.; Przepiórka, A.; Seweryn, K.; Stolarski, M.; Winkler, M.; Sylwester, J.; Kowalinski, M.; Mrozek, T.; Podgorski, P.; Benz, A. O.; Krucker, S.; Hurford, G. J.; Arnold, N. G.; Önel, H.; Meuris, A.; Limousin, O.; Grimm, O. Bibcode: 2012SPIE.8454E..0KS Altcode: The Spectrometer/Telescope for Imaging X-rays (STIX) is one of 10 instruments on board Solar Orbiter, an M-class mission of the European Space Agency (ESA) scheduled to be launch in 2017. STIX applies a Fourier-imaging technique using a set of tungsten grids in front of 32 pixelized CdTe detectors to provide imaging spectroscopy of solar thermal and non-thermal hard X-ray emissions from 4 to 150 keV. These detectors are source of data collected and analyzed in real-time by Instrument Data Processing Unit (IDPU). Besides the data processing the IDPU controls and manages other STIX's subsystems: ASICs and ADCs associated with detectors, Aspect System, Attenuator, PSU and HK. The instrument reviewed in this paper is based on the design that passed the Instrument Preliminary Design Review (IPDR) in early 2012 and Software Preliminary Design Review (SW PDR) in middle of 2012. Particular emphasis is given to the IDPU and low level software called Basic SW (BSW). 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: Solar Particle Acceleration Radiation and Kinetics (SPARK). A mission to understand the nature of particle acceleration Authors: Matthews, Sarah A.; Williams, David R.; Klein, Karl-Ludwig; Kontar, Eduard P.; Smith, David M.; Lagg, Andreas; Krucker, Sam; Hurford, Gordon J.; Vilmer, Nicole; MacKinnon, Alexander L.; Zharkova, Valentina V.; Fletcher, Lyndsay; Hannah, Iain G.; Browning, Philippa K.; Innes, Davina E.; Trottet, Gerard; Foullon, Clare; Nakariakov, Valery M.; Green, Lucie M.; Lamoureux, Herve; Forsyth, Colin; Walton, David M.; Mathioudakis, Mihalis; Gandorfer, Achim; Martinez-Pillet, Valentin; Limousin, Olivier; Verwichte, Erwin; Dalla, Silvia; Mann, Gottfried; Aurass, Henri; Neukirch, Thomas Bibcode: 2012ExA....33..237M Altcode: 2011ExA...tmp..124M Energetic particles are critical components of plasma populations found throughout the universe. In many cases particles are accelerated to relativistic energies and represent a substantial fraction of the total energy of the system, thus requiring extremely efficient acceleration processes. The production of accelerated particles also appears coupled to magnetic field evolution in astrophysical plasmas through the turbulent magnetic fields produced by diffusive shock acceleration. Particle acceleration is thus a key component in helping to understand the origin and evolution of magnetic structures in, e.g. galaxies. The proximity of the Sun and the range of high-resolution diagnostics available within the solar atmosphere offers unique opportunities to study the processes involved in particle acceleration through the use of a combination of remote sensing observations of the radiative signatures of accelerated particles, and of their plasma and magnetic environment. The SPARK concept targets the broad range of energy, spatial and temporal scales over which particle acceleration occurs in the solar atmosphere, in order to determine how and where energetic particles are accelerated. SPARK combines highly complementary imaging and spectroscopic observations of radiation from energetic electrons, protons and ions set in their plasma and magnetic context. The payload comprises focusing-optics X-ray imaging covering the range from 1 to 60 keV; indirect HXR imaging and spectroscopy from 5 to 200 keV, γ-ray spectroscopic imaging with high-resolution LaBr3 scintillators, and photometry and source localisation at far-infrared wavelengths. The plasma environment of the regions of acceleration and interaction will be probed using soft X-ray imaging of the corona and vector magnetography of the photosphere and chromosphere. SPARK is designed for solar research. However, in addition it will be able to provide exciting new insights into the origin of particle acceleration in other regimes, including terrestrial gamma-ray flashes (TGF), the origin of γ-ray bursts, and the possible existence of axions. Title: The 2010 August 1 Type II Burst: A CME-CME Interaction and its Radio and White-light Manifestations Authors: Martínez Oliveros, Juan Carlos; Raftery, Claire L.; Bain, Hazel M.; Liu, Ying; Krupar, Vratislav; Bale, Stuart; Krucker, Säm Bibcode: 2012ApJ...748...66M Altcode: 2012arXiv1202.2375M We present observational results of a type II burst associated with a CME-CME interaction observed in the radio and white-light (WL) wavelength range. We applied radio direction-finding techniques to observations from the STEREO and Wind spacecraft, the results of which were interpreted using WL coronagraphic measurements for context. The results of the multiple radio direction-finding techniques applied were found to be consistent both with each other and with those derived from the WL observations of coronal mass ejections (CMEs). The results suggest that the type II burst radio emission is causally related to the CMEs interaction. Title: Electron Acceleration Associated with Solar Jets Authors: Krucker, Säm; Kontar, E. P.; Christe, S.; Glesener, L.; Lin, R. P. Bibcode: 2011ApJ...742...82K Altcode: This paper investigates the solar source region of supra-thermal (few keV up to the MeV range) electron beams observed near Earth by combining in situ measurements of the three-dimensional Plasma and Energetic Particles experiment on the WIND spacecraft with remote-sensing hard X-ray observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager. The in situ observations are used to identify events, and the hard X-ray observations are then searched for signatures of supra-thermal electrons radiating bremsstrahlung emission in the solar atmosphere. Only prompt events detected above 50 keV with a close temporal correlation between the flare hard X-ray emission and the electrons seen near Earth are selected, limiting the number of events to 16. We show that for 7 of these 16 events, hard X-ray imaging shows three chromospheric sources: two at the footpoints of the post-flare loop and one related to an apparently open field line. The remaining events show two footpoints (seven events, four of which show elongated sources possibly hiding a third source) or are spatially unresolved (two events). Out of the 16 events, 6 have a solar source region within the field of view of the Transition Region and Corona Explorer (TRACE). All events with TRACE data show EUV jets that have the same onset as the hard X-ray emission (within the cadence of tens of seconds). After the hard X-ray burst ends, the jets decay. These results suggest that escaping prompt supra-thermal electron events observed near Earth are accelerated in flares associated with reconnection between open and closed magnetic field lines, the so-called interchange reconnection scenario. Title: Statistical Properties of Super-Hot Solar Flares Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2011AGUFMSH13B1942C Altcode: Observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have shown that "super-hot" (T > 30 MK) plasma temperatures appear to be commonly achieved by intense, GOES M- and X-class flares. Recent studies of individual X-class flares (Caspi & Lin 2010, ApJ 725, L161; Longcope et al. 2010, SolPhys 267, 107) showed that the super-hot thermal component is spectrally and spatially distinct from the ubiquitous ~10-20 MK plasma observed by GOES, suggesting that the two populations are heated by different physical mechanisms. However, how and why some flares achieve super-hot temperatures while others do not, and on what physical parameters this depends, remains unknown; consequently, the origins of super-hot plasmas remain poorly understood. We present results from a survey of 37 M- and X-class flares observed by RHESSI and GOES, which show that the maximum plasma temperature measured by RHESSI is strongly correlated with GOES class, such that super-hot temperatures are achieved almost exclusively by X-class flares. This correlation is significantly steeper than that observed for the peak GOES temperature. The maximum (instantaneous) thermal energy of the RHESSI-observed plasma, which occurs after the peak RHESSI temperature, is also correlated with GOES class, and super-hot flares are observed to be strongly associated with high number densities and (instantaneous) thermal energy densities compared to cooler flares, suggesting that strong magnetic fields are a requirement for the formation of super-hot plasma. We discuss these results and their implications for flare plasma heating, temperature distribution evolution, and energy transport. Title: Imaging Spectroscopy Using AIA Diffraction Patterns in Conjunction with RHESSI and EVE Observations Authors: Raftery, Claire L.; Krucker, Säm; Lin, Robert P. Bibcode: 2011ApJ...743L..27R Altcode: Extreme-ultraviolet (EUV) spectroscopy is a very powerful tool that can be used for probing the dynamic response of the solar corona and chromosphere during solar flares. Here we present a unique application of observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory using the artifacts of diffraction and dispersion. Using these techniques we can achieve imaging spectroscopy at the resolution of AIA (0farcs6 plate scale) and at the revolutionary cadence of the instrument (nominally 12 s) for the brightest (saturated) pixels during solar flares. Analyzing the dispersion and diffraction effects that are observed as a result of the support grids used for the instrument's front filters, we can achieve up to 0.5 Å spectral resolution across the EUV, optically thin passbands. Here we describe the technique used and present the first result of its application—the emission measure distribution for a single pixel at the top of a flaring loop. We analyze the AIA dispersion spectrum in conjunction with Extreme Ultraviolet Variability Experiment observations and spectroscopic and imaging results from the Reuven Ramaty High Energy Solar Spectroscopic Imager. 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: High Energy Solar Physics Data in Europe (HESPE): a European project for the exploitation of hard X-ray data in solar flare physics Authors: Piana, M.; Csillaghy, A.; Kontar, E. P.; Fletcher, L.; Veronig, A. M.; Vilmer, N.; Hurford, G. J.; Dennis, B. R.; Schwartz, R. A.; Massone, A.; Krucker, S.; Benvenuto, F.; Etesi, L. I.; Guo, J.; Hochmuth, N.; Reid, H. Bibcode: 2011AGUFMSH33B2068P Altcode: It has been recognized since the early days of the space program that high-energy observations play a crucial role in understanding the basic mechanisms of solar eruptions. Unfortunately, the peculiar nature of this radiation makes it so difficult to extract useful information from it that non-conventional observational techniques together with complex data analysis procedures must be adopted. HESPE is a European project funded within the seventh Framework Program, with the aim of realizing computational methods for solar high-energy data analysis and technological tools for the intelligent exploitation of science-ready products. Such products and methods are put at disposal of the solar, heliospheric and space weather communities, who will exploit them in order to build flare prediction models and to integrate the information extracted from hard X-rays and gamma rays data, with the one extracted from other wavelengths data. Title: Electron Acceleration in Solar Flares and in Impulsive Solar Energetic Particle (SEP) Events Authors: Lin, R. P.; Wang, L.; Krucker, S.; Glesener, L. Bibcode: 2011AGUFMSH44A..01L Altcode: We investigate the relationship between electron acceleration in solar flares [detected via the electron bremsstrahlung hard x-ray (HXR) emission], and in impulsive solar energetic particle (SEP) events where the escaping electrons are detected in situ near 1 AU. These are most common types of particle acceleration by the Sun, with of order ~104 events occurring per year over the whole Sun during solar maximum. Both types predominantly accelerate electrons (large electron/proton ratios) to ~1-10s of keV energies, typically with double power-law energy spectra with a downward break at a few tens of keV. Impulsive electron events are associated one-to-one with low frequency type III radio bursts, but only ~25% have reported chromospheric flares or detectable HXR emission. Wind 3DP observations show that the electrons often exhibit nearly scatter-free propagation in the interplanetary medium. Relative to the start of the type III burst at the Sun, the injection of the electrons above ~20 keV is found to be often (~80%) delayed by ~10-30 minutes. Where HXR emission is detected, RHESSI imaging often shows coronal sources that suggests magnetic reconnection between open field lines and an underlying loop (interchange reconnection), resulting in a jet. Comparison of the electron energy spectrum derived from the RHESSI hard X-ray measurements with the impulsive event electron spectrum observed by Wind for prompt (not delayed) events, suggests a model where electrons accelerated by this interchange magnetic reconnection on open field lines escape, while electrons on closed field lines are further accelerated as those fields collapse, to produce the spectrum and much larger (by a factor of >100) total number of HXR emitting electrons. Impulsive electron events also appear to be associated with fast (>~580 km/s) narrow CMEs as well as jets, suggesting that in delayed events the escaping >~20 keV electrons (and ions) might be accelerated by the jet/fast narrow CME as it travels outward through the corona. Title: Hard X-ray and Extreme Ultraviolet Correlations in Solar Flares Authors: Bain, H. M.; Raftery, C. L.; Krucker, S.; Allred, J. C. Bibcode: 2011AGUFMSH13B1936B Altcode: It is well established that there is a connection between hard X-ray and extreme ultraviolet (EUV) flare emission. In the thick target model, hard X-ray emission is produced by a beam of nonthermal electrons, which deposit their energy in the chromosphere as a result of Coulomb collisions with the denser plasma. At EUV wavelengths we observe the response of the chromosphere and corona to this deposition of energy. Using X-ray observations from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and EUV observations from the Atmospheric Imaging Assembly (AIA) and the Extreme Ultraviolet Variability Experiment (EVE) instruments onboard the Solar Dynamics Observatory (SDO) we investigate the spectral, spatial and temporal correlations between these emission mechanisms. With these new observations from SDO we can achieve this with unprecedented spectral coverage and cadence. Title: Imaging spectroscopy with AIA: using AIA diffraction patterns to probe flare DEM curves in conjunction with RHESSI and EVE spectra Authors: Raftery, C. L.; Krucker, S. Bibcode: 2011AGUFMSH13B1945R Altcode: Here we present an innovative use of AIA data, utilizing diffraction and dispersion artifacts to preform imaging spectroscopy with AIA. Using this technique, we probe the time and spatial dependence of emission measure distributions in a compact flare. This is achieved through comparison of AIA dispersion spectra to EVE and CHIANTI spectra. In this way, we can no only probe the original spectral components of the flare emission but we can achieve this with the cadence and spatial resolution of AIA. To further extend the dataset, RHESSI spectral results are included and, in a break from tradition, RHESSI images are used to place AIA spectra in context. Unlike traditional spectroscopic methods, imaging spectroscopy with AIA allows us to capture the highly dynamic nature of flares with no compromise for spatial resolution or cadence. Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Krucker, S. Bibcode: 2011AGUFMSH13B1950K Altcode: The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost Access to Space sounding rocket payload that will launch in early 2012. A larger sensitivity and dynamic range than currently available are needed in order to image faint X-rays from electron beams in the tenuous corona, particularly those near the coronal acceleration region and those that escape into interplanetary space. FOXSI combines nested, grazing-incidence replicated optics with double-sided silicon strip detectors to achieve a dynamic range of >100 and a sensitivity 100 times that of RHESSI. Advances in the fabrication and assembly of the optics at the NASA Marshall Space Flight Center provide a spatial resolution of 8 arcseconds (FWHM), while the silicon detectors, developed by the Astro-H team at ISAS/JAXA, offer an energy resolution of 0.4 keV. FOXSI's first flight will conduct a search for nonthermal electrons in the quiet Sun, possibly related to nanoflares. FOXSI will serve as a pathfinder for future space-based solar hard X-ray spectroscopic imagers, which will be able to image nonthermal electrons in flare acceleration sites and provide quantitative measurements such as energy spectra, densities, and energy content in accelerated electrons. Title: Observations of Langmuir Waves Associated with Type III Radio Bursts : Wind Observations and Further Improvements with Inner Heliospheric Missions Authors: Maksimovic, M.; Vidojevic, S.; Arnaud, Z.; Krucker, S. Bibcode: 2011AGUFMSH44B..08M Altcode: Interplanetary electron beams, produced by solar flares or CMEs, are unstable in the solar wind and generate Langmuir waves at the local plasma frequency Fp. These waves are then converted into the so-called Type III radio bursts which are freely propagating electromagnetic emissions at Fp or its harmonic. We present a statistical analysis of both in-situ Langmuir Waves, associated energetic electrons and Type III bursts recorded simultaneously by the WIND spacecraft. We discuss the relevance of these observations with respect to the latest developments of Type III generation theories. Finally we describe the possible improvements that the forthcoming inner heliospheric mission can bring to this topic. Title: Energetics And Heating In A Solar Plasma Ejection Observed By RHESSI And AIA Authors: Glesener, L.; Krucker, S.; Bain, H. M.; Lin, R. P. Bibcode: 2011AGUFMSH44A..04G Altcode: For the past nine years, hard X-ray observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have provided remarkable insight into the locations and spectra of energetic flare particles. With the advent of high-cadence, multiwavelength observations by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory, it is now more possible to study the dynamic structures among which these energetic particles move. On November 3, 2010, a C4.9 solar flare occurred just behind the eastern limb of the Sun, accompanied by a coronal mass ejection. Because the bright flare footpoints were occulted by the solar disk (by about 6 degrees), faint coronal X-ray sources can be studied in detail. Extreme ultraviolet (EUV) images from AIA show a mass of plasma ejected from the solar surface. Isothermal analysis using multiple EUV filters shows that this erupting plasma reaches a high temperature of ~11 MK. Meanwhile, RHESSI X-ray images reveal a large, diffuse hard X-ray source matching the location, shape, and evolution of the ejecting plasma, suggesting the presence of nonthermal electrons that may be magnetically trapped in the region. Here, RHESSI spectroscopy and AIA temperature analysis are combined in order to examine the relationship between the populations of thermal and nonthermal electrons in the ejected plasma. Electron spectra, locations, and temporal evolution are examined in order to test the hypothesis that nonthermal electrons collisionally heat the erupting plasma to this high temperature. Title: A diffusive description of the focused transport of solar energetic particles. Intensity- and anisotropy-time profiles as a powerful diagnostic tool for interplanetary particle transport conditions Authors: Artmann, S.; Schlickeiser, R.; Agueda, N.; Krucker, S.; Lin, R. P. Bibcode: 2011A&A...535A..92A Altcode: The transport of solar energetic charged particles along the interplanetary magnetic field in the ecliptic plane of the sun can be described roughly by a one-dimensional diffusion equation. Large-scale spatial variations of the guide magnetic field can be taken into account by adding an additional term to the diffusion equation that includes the effect of adiabatic focusing. We solve this equation analytically by assuming a point-like particle injection in time and space and a spatial power-law dependence for the focusing length and the spatial diffusion coefficient. We infer the intensity- and anisotropy-time profiles of solar energetic particles from this solution. Through these the influence of different assumptions for the diffusion parameters can be seen in a mathematically closed form. The comparison of calculated and measured intensity- and anisotropy-time profiles, which are a powerful diagnostic tool for interplanetary particle transport, gives information about the large-scale spatial dependence of the focusing length and the diffusion coefficient. For an exceptionally large solar energetic particle event, which did occur on 2001 April 15, we fit the 27 - 512 keV electron intensities and anisotropies observed by the Wind spacecraft using the theoretically derived profiles. We find a linear spatial dependence of the mean free path along the guiding magnetic field. We also find the mean free path to be energy independent, which supports the theory of "velocity-dependent diffusion". This means that the intensity profiles for the discussed energies exhibit the same shape if they are plotted against the traveled distance and not against the time. In this case the profiles differ only in their maximum values and we can determine the energy spectra of the solar flare electrons out of the scaling factor we need to fit the data. The derived spectra exhibits a power-law dependence ∝ E_kin-3 in an energy range from ~ 50 keV to ~ 500 keV.

Appendices are available in electronic form at http://www.aanda.org 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: 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: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Krucker, Säm; Christe, Steven; Glesener, Lindsay; Ishikawa, Shin-nosuke; McBride, Stephen; Glaser, David; Turin, Paul; Lin, R. P.; Gubarev, Mikhail; Ramsey, Brian; Saito, Shinya; Tanaka, Yasuyuki; Takahashi, Tadayuki; Watanabe, Shin; Tanaka, Takaaki; Tajima, Hiroyasu; Masuda, Satoshi Bibcode: 2011SPIE.8147E..05K Altcode: 2011SPIE.8147E...4K The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket payload funded under the NASA Low Cost Access to Space program to test hard x-ray (HXR) focusing optics and position-sensitive solid state detectors for solar observations. Today's leading solar HXR instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides excellent spatial (2 arcseconds) and spectral (1 keV) resolution. Yet, due to its use of an indirect imaging system, the derived images have a low dynamic range (typically <10) and sensitivity. These limitations make it difficult to study faint x-ray sources in the solar corona which are crucial for understanding the particle acceleration processes which occur there. Grazing-incidence x-ray focusing optics combined with position-sensitive solid state detectors can overcome both of these limitations enabling the next breakthrough in understanding impulsive energy release on the Sun. The FOXSI project is led by the Space Sciences Laboratory at the University of California, Berkeley. The NASA Marshall Space Flight Center is responsible for the grazingincidence optics, while the Astro-H team at JAXA/ISAS has provided double-sided silicon strip detectors. FOXSI is a pathfinder for the next generation of solar hard x-ray spectroscopic imagers. Such observatories will be able to image the non-thermal electrons within the solar flare acceleration region, trace their paths through the corona, and provide essential quantitative measurements such as energy spectra, density, and energy content in accelerated electrons. 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: On the Relation of Above-the-loop and Footpoint Hard X-Ray Sources in Solar Flares Authors: Ishikawa, S.; Krucker, Säm; Takahashi, T.; Lin, R. P. Bibcode: 2011ApJ...737...48I Altcode: We report on the most prominent example of an above-the-loop hard X-ray source in the extensive solar flare database of RHESSI. The limb flare of 2003 October 22 around 20 UT resembles the famous Masuda flare, except that only one of the footpoint sources is visible with the other one occulted. However, even for this very prominent event, the above-the-loop source is only visible during one of the four hard X-ray peaks, highlighting the rare occurrence of above-the-loop sources that are equally bright as footpoint sources. The relative timing between the above-the-loop and footpoint sources shows that the coronal source peaks about 10 s before the footpoint source and decays during the time the footpoint source is most prominent. Furthermore, the derived number of non-thermal electrons within the above-the-loop source is large enough to provide the needed number of precipitating electrons to account for the footpoint emission over the duration of the hard X-ray peak. Hence, these observations support the simple scenario where bulk energization is accelerating all electrons within the above-the-loop source and precipitating electrons are emptying out of the above-the-loop source to produce the footpoint emissions. Title: On the Near-Earth Observation of Protons and Electrons from the Decay of Low-energy Solar Flare Neutrons Authors: Agueda, Neus; Krucker, Säm; Lin, Robert P.; Wang, Linghua Bibcode: 2011ApJ...737...53A Altcode: We investigate the near-Earth observation of interplanetary protons and electrons that result from the in-flight beta decay of low-energy (1-10 MeV) solar neutrons. We use in situ measurements throughout solar cycle 23 of 1-11 MeV protons and 50-400 keV electrons by the 3DP experiment on board the Wind spacecraft. We select a sample of isolated large (X-class) eastern hemisphere flares occurring during quiescent interplanetary conditions with the goal of discriminating neutron-decay particles from primary solar energetic particles. Unfortunately, all major flares of solar cycle 23 have to be excluded, with the largest flare in our sample being a X3.6 flare. For these relatively small event sizes, no in situ events due to the decay of solar flare neutrons are observed by Wind. From the one event with simultaneous γ-ray observations, we estimate the expected signal of neutron-decay protons in the Wind/3DP detectors. We use theoretical calculations of the spectrum of escaping neutrons at the Sun combined with an interplanetary propagation model to predict the neutron-decay proton spectrum expected near the Earth. We find that the expected spectrum is indeed well below the background intensities. However, using the estimates derived from the largest solar event of cycle 23 (2003 October 28) and assuming the flare would have occurred isolated in the eastern hemisphere, a clear signal above 5 MeV is expected to be seen. Title: Fine-Pitch Semiconductor Detector for the FOXSI Mission Authors: Ishikawa, S.; Saito, S.; Tajima, H.; Tanaka, T.; Watanabe, S.; Odaka, H.; Fukuyama, T.; Kokubun, M.; Takahashi, T.; Terada, Y.; Krucker, S.; Christe, S.; McBride, S.; Glesener, L. Bibcode: 2011ITNS...58.2039I Altcode: 2015arXiv150905717I The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA sounding rocket mission which will study particle acceleration and coronal heating on the Sun through high sensitivity observations in the hard X-ray energy band (5-15 keV). Combining high-resolution focusing X-ray optics and fine-pitch imaging sensors, FOXSI will achieve superior sensitivity; two orders of magnitude better than that of the RHESSI satellite. As the focal plane detector, a Double-sided Si Strip Detector (DSSD) with a front-end ASIC (Application Specific Integrated Circuit) will fulfill the scientific requirements of spatial and energy resolution, low energy threshold and time resolution. We have designed and fabricated a DSSD with a thickness of 500 μm and a dimension of 9.6 mm × 9.6 mm, containing 128 strips with a pitch of 75 μm, which corresponds to 8 arcsec at the focal length of 2 m. We also developed a low-noise ASIC specified to FOXSI. The detector was successfully operated in the laboratory at a temperature of -20°C and with an applied bias voltage of 300 V. Extremely good energy resolutions of 430 eV for the p-side and 1.6 keV for the n-side at a 14 keV line were achieved for the detector. We also demonstrated fine-pitch imaging successfully by obtaining a shadow image. Hence the implementation of scientific requirements was confirmed. 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: The RHESSI Microflare Height Distribution Authors: Christe, S.; Krucker, S.; Saint-Hilaire, P. Bibcode: 2011SoPh..270..493C Altcode: 2011arXiv1104.2624C; 2011SoPh..tmp..107C We present the first in-depth statistical survey of flare source heights observed by RHESSI. Flares were found using a flare-finding algorithm designed to search the 6 - 10 keV count-rate when RHESSI's full sensitivity was available in order to find the smallest events (Christe et al. in Astrophys. J.677, 1385, 2008). Between March 2002 and March 2007, a total of 25 006 events were found. Source locations were determined in the 4 - 10 keV, 10 - 15 keV, and 15 - 30 keV energy ranges for each event. In order to extract the height distribution from the observed projected source positions, a forward-fit model was developed with an assumed source height distribution where height is measured from the photosphere. We find that the best flare height distribution is given by g(h)∝exp (−h/λ) where λ=6.1±0.3 Mm is the scale height. A power-law height distribution with a negative power-law index, γ=3.1±0.1 is also consistent with the data. Interpreted as thermal loop-top sources, these heights are compared to loops generated by a potential-field model (PFSS). The measured flare heights distribution are found to be much steeper than the potential-field loop height distribution, which may be a signature of the flare energization process. Title: The Focusing Optics X-ray Solar Imager Authors: Glesener, Lindsay; Krucker, S.; Christe, S.; Ramsey, B.; Ishikawa, S.; Takahashi, T.; Saito, S. Bibcode: 2011SPD....42.1511G Altcode: 2011BAAS..43S.1511G The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost Access to Space sounding rocket payload that will launch in late 2011. A larger sensitivity and dynamic range than currently available are needed in order to image faint X-rays from electron beams in the tenuous corona, particularly those near any coronal acceleration region and those that escape into interplanetary space. FOXSI combines fast-replication, nested, grazing-incidence optics with double-sided silicon strip detectors to achieve a dynamic range of >100 and a sensitivity 100 times that of RHESSI. Advances in the fabrication and assembly of the optics at the NASA Marshall Space Flight Center provide a spatial resolution of 8 arcseconds, while the silicon detectors, developed by the Astro-H team at ISAS/JAXA, offer an energy resolution of 0.5 keV. FOXSI's first flight will be used to conduct a search for X-ray emission from nonthermal electron beams in quiet Sun nanoflares. In addition, FOXSI will serve as a pathfinder for future space-based solar hard X-ray spectroscopic imagers, which will be able to image nonthermal electrons in flare acceleration sites and provide quantitative measurements such as energy spectra, densities, and energy content in accelerated electrons. 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: Estimation of the Reconnection Electric Field in the 2003 October 29 X10 Flare Authors: Yang, Ya-Hui; Cheng, C. Z.; Krucker, Säm; Hsieh, Min-Shiu Bibcode: 2011ApJ...732...15Y Altcode: The electric field in the reconnecting current sheet of the 2003 October 29 X10 flare is estimated to be a few kV m-1 in this study, based on the rate of change in the photospheric magnetic flux in the newly brightened areas of Transition Region and Coronal Explorer (TRACE) UV ribbons. For comparison, the motion speed of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) hard X-ray (HXR) footpoints and the photospheric magnetic field strength are also used for the electric field calculation. This X10 flare event is selected due to its distinct two-phase HXR kernel motion, two arcade systems with different magnetic shear, and the high cadence and complete coverage of the TRACE 1600 Å Michelson Doppler Imager (MDI) magnetogram and RHESSI HXR observations. We pay particular attention to the electric field characteristics in different flare phases, as well as the temporal correlation with the HXR emission and its power-law spectral index and the photospheric magnetic field strength. We found that in the early impulsive phase, the reconnection electric field peaks just before the HXR emission peaks and the energy spectrum hardens. The result is consistent with the scenario that more particles are accelerated to higher energies by larger reconnection electric fields and then precipitate into the lower chromosphere to produce stronger HXR emissions. Such a particle acceleration mechanism plays its most significant role in the impulsive phase of this flare. In addition, our results provide evidence that the highly sheared magnetic field lines are mapped to the magnetic reconnection diffusion region to produce a large reconnection electric field. 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: Estimation of Reconnection Electric Field in the 2003 October 29 X10 Flare Authors: Cheng, C. Z.; Yang, Ya-Hui; Krucker, S.; Hsieh, M. Bibcode: 2011SPD....42.2235C Altcode: 2011BAAS..43S.2235C The electric field in the reconnecting current sheet is estimated from the change rate of photospheric magnetic flux in the newly brightened areas of TRACE UV ribbons. The X10 flare on 2003 October 29 is selected due to its distinct two-phase HXR footpoint motion, two arcade systems with different magnetic shear, and the high-cadence and complete coverage of TRACE 1600 Å, MDI magnetogram, and RHESSI HXR observations. Besides the strengths of reconnection electric field in different flare phases, we particularly pay attention to the temporal correlation between the reconnection electric field and the corresponding characteristics at the conjugate HXR footpoints (such as the HXR emissions, HXR power-law spectral indexes, and the photospheric magnetic field strengths). We found that in the early impulsive phase, the reconnection electric field peaks just before the HXR emission peaks and the energy spectrum hardens. The result could be consistent with the scenario that more particles are accelerated to higher energies by larger reconnection electric field and then precipitate into lower chromosphere to produce stronger HXR emissions. Moreover, such particle acceleration mechanism plays most significant role in the impulsive phase of this X10 flare. In addition, our results provide the evidence that the highly-sheared magnetic field lines are mapped to the magnetic reconnection diffusion region to produce large reconnection electric field. Title: Synchrotron Emission in Expanding CME Loops Authors: Bain, Hazel; Raftery, C.; Krucker, S. Bibcode: 2011SPD....42.2307B Altcode: 2011BAAS..43S.2307B Hard X-ray and radio observations can be used to identify source of electron acceleration within the flare-CME system. Observations at meter wavelengths using the Nancay Radioheliograph are ideal for locating sources of synchrotron and plasma emission. Bastian et al. (2001) and Maia et al. (2007) imaged synchrotron emission present in the expanding loops of two independent CME's. It is unclear how these electrons were accelerated. Previous suggestions include acceleration at the reconnection site/current sheet between the flare and CME or in the foreshock generated as the ejected material propagates outward. We present observations of an event which occurred on the 14th of August which is a new candidate for studying these events. Title: Solar Eruptive Events (SEE) Mission for the Next Solar Maximum Authors: Lin, Robert P.; Krucker, S.; Caspi, A.; Hurford, G.; Dennis, B.; Holman, G.; Christe, S.; Shih, A. Y.; Bandler, S.; Davila, J.; Milligan, R.; Kahler, S.; Weidenbeck, M.; Doschek, G.; Vourlidas, A.; Share, G.; Raymond, J.; McConnell, M.; Emslie, G. Bibcode: 2011SPD....42.2204L Altcode: 2011BAAS..43S.2204L Major solar eruptive events 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 10s-100s 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, producing the most extreme space weather in geospace, at other planets, and in the heliosphere. Thus, the understanding of the flare/CME energy release process and of the related particle acceleration processes in SEEs is a major goal in Heliophysics. Here we present a concept for a Solar Eruptive Events (SEE) mission, consisting of a comprehensive set of advanced new instruments on the single spacecraft in low Earth orbit, that focus directly on the coronal energy release and particle acceleration in flares and CMEs. SEE will provide new focussing hard X-ray imaging spectroscopy of energetic electrons in the flare acceleration region, new energetic neutral atom (ENA) imaging spectroscopy of SEPs being accelerated by the CME at altitudes above 2 solar radii, gamma-ray imaging spectroscopy of flare-accelerated energetic ions, plus detailed EUV/UV/Soft X-ray diagnostics of the plasmas density, temperature, and mass motions in the energy release and particle acceleration regions. Together with ground-based measurements of coronal magnetic fields from ATST, FASR, and COSMO, SEE will enable major breakthroughs in our understanding of the fundamental physical processes involved in major solar eruptive events. Title: Metric Type II Emission in Association with an Erupting Plasmoid Authors: Bain, Hazel; Glesener, L.; Krucker, S. Bibcode: 2011SPD....42.2319B Altcode: 2011BAAS..43S.2319B We present observations of a metric type II radio burst that occurred on the 3rd of November 2010. The burst, which occurred in the frequency range of the Nancay Radioheliograph (NRH), was observed in conjunction with an erupting plasmoid. Events occurring in this frequency range are infrequent and as such makes this an interesting event to study. Using NRH observations together with images from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory (SDO/AIA) we investigate the generation mechanism of this coronal type II burst. Title: On the Relation of Above-the-loop-top and Footpoint Hard X-ray Sources in Solar Flares Authors: Ishikawa, Shin-Nosuke; Krucker, S.; Takahashi, T.; Lin, R. P. Bibcode: 2011SPD....42.1104I Altcode: 2011BAAS..43S.1104I We report on the most prominent example of an above-the-loop-top hard X-ray source in the extensive solar flare data base of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The limb flare of 2003 October 22 around 20UT shows a coronal source in the 40-100 keV range that is significantly above the thermal flare loop top, similar as seen in the famous Masuda flare. The above-the-loop-top source is only visible during one of the four hard X-ray peaks, highlighting the rare occurrence of above-the-loop-top sources that are equally bright as footpoint sources. The chromospheric emissions for the peak with above-the-loop-top source shows a single footpoint, with the second footpoint occulted. The relative timing between the above-the-loop-top and footpoint source shows that the coronal source peaks about 10 s before the footpoint source and decays during the time the footpoint source is most prominent.

Thanks to the good energy resolution of RHESSI, this is the first time to obtain accurate spectra of both above-the-loop-top and footpoint sources. Photon indices of the above-the-loop-top and the footpoint sources are 4.8±0.4 and 3.7±0.5, respectively. These difference of indices 1.1±0.6 suggests that these emission are from the same components of electrons through thin- and thick-target bremsstrahlung.

The large intensity of the above-the-loop-top source and the absence of thermal emission in the above-the-loop-top region suggests that the electrons within above-the-loop-top source are purely non-thermal. Based on this assumption, we estimate the number of the accelerated electrons and time scale. We discuss on the relation of the above-the-loop-top and footpoint sources and a possible model to explain this event. Title: High-Resolution Imaging of Solar Flare Ribbons and its Implication on the Thick-Target Beam Model Authors: Krucker, Sam Bibcode: 2011SPD....42.2208K Altcode: 2011BAAS..43S.2208K 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 (SOT) with the hard X-rays observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). 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 0.5" and 1.8''. 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 <1.1'' compared to the G-band width of 1.8'' 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 >5x1012 erg s-1 cm-2 provided by electrons above 18 keV. This requires that the beam density of electrons above 18 keV be at least 1x1010 cm-3. Even if field lines converge towards the chromospheric footpoints, the required beam in the corona has a too high 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: Probing Flare Temperatures Using AIA Dispersion Patterns In Conjunction With EVE And RHESSI Spectra Authors: Raftery, Claire; Krucker, S. Bibcode: 2011SPD....42.2106R Altcode: 2011BAAS..43S.2106R The thermal distribution of plasma in solar flares has been studied extensively and yet, remains evasive. Using the revolutionary spatial resolution and cadence of SDO/AIA, we analyze diffraction and dispersion effects out to more than 40 orders. Comparing the dispersion patterns to spectral results from SDO/EVE and RHESSI, and synthetic spectra from CHIANTI, we identify the distribution of plasma temperatures in a compact flaring loop with 12 second cadence. Unlike traditional spectroscopy, the high cadence of the AIA observations allow us to capture the highly dynamic nature of these eruptions with no compromise for spatial resolution or cadence. Title: Energetics And Heating In A Solar Plasma Ejection Observed By RHESSI And AIA Authors: Glesener, Lindsay; Krucker, S.; Bain, H. M.; Lin, R. P. Bibcode: 2011SPD....42.0903G Altcode: 2011BAAS..43S.0903G For the past nine years, hard X-ray observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have provided remarkable insight into the locations and spectra of energetic flare particles. With the advent of high-cadence, multiwavelength observations by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory, it is now possible to study the dynamic structures among which these energetic particles move. On November 3, 2010, a C4.9 solar flare emerged from behind the eastern limb of the Sun, accompanied by a coronal mass ejection. Because the bright flare footpoints were occulted by the solar disk (by about 12 degrees), the coronal X-ray sources can be studied in detail without contamination by the footpoints. Extreme ultraviolet (EUV) images from AIA show a mass of plasma ejected from the solar surface. Isothermal analysis using multiple EUV filters shows that this erupting plasma reaches a high temperature of 11 MK. Meanwhile, RHESSI X-ray images reveal a large, diffuse hard X-ray source that matches the location and shape of the ejecting plasma, suggesting the presence of nonthermal electrons magnetically trapped in the region. Using RHESSI spectroscopy and AIA temperature analysis, we will test the hypothesis that nonthermal electrons in the erupting plasma are responsible for heating the plasma to this high temperature via collisional heating. 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: RHESSI Imaging Survey of γ-ray Bremsstrahlung Emission in Solar Flares Authors: Ishikawa, S.; Krucker, Säm; Takahashi, T.; Lin, R. P. Bibcode: 2011ApJ...728...22I Altcode: We present a high-energy (>150 keV) imaging survey of all solar γ-ray flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to study bremsstrahlung emission from relativistic electrons. Using RHESSI rear segment data, images in the energy range from 150 to 450 keV integrated over the total duration of the impulsive phase of the flare are derived. Out of the 29 γ-ray peaks in 26 RHESSI flares, we successfully obtained images for 21 γ-ray peaks in 20 flares. The remaining eight peaks have >150 keV fluences of less than a few hundred photons per cm2 and counting statistics are too poor for detailed imaging. The flux ratio of the footpoint sources is found to be similar at 50 keV and above 150 keV, indicating that relativistic electrons are present in both footpoints of the flare loop. No correlation between the footpoint separation and the fluence ratio of the 2.2 MeV line and the >300 keV photons is found. This indicates that the relative efficiency of proton to electron acceleration does not depend on loop length, as could have been expected from stochastic acceleration models. As previously reported, the three flares with the best counting statistics show not only footpoint emission, but also a coronal γ-ray bremsstrahlung source. For events with lower counting statistics, no coronal source could be identified. However, instrumental limitation could easily hide a coronal source for events with lower statistics, suggesting that coronal γ-ray bremsstrahlung sources are nevertheless a general feature of γ-ray flares. Title: Pitch-angle Distributions and Temporal Variations of 0.3-300 keV Solar Impulsive Electron Events Authors: Wang, L.; Lin, R. P.; Krucker, Säm Bibcode: 2011ApJ...727..121W Altcode: We investigate the propagation of ~0.3-300 keV electrons in five solar impulsive electron events, observed by the WIND three-dimensional Plasma and Energetic Particle instrument, that have rapid-rise and rapid-decay temporal profiles. In two events, the temporal profiles above 25 keV show a second peak of inward-traveling electrons tens of minutes after the first peak, followed by a third peak due to outward-traveling electrons minutes later—likely due to reflection/scattering first at ~0.7-1.7 AU past the Earth, and then in the inner heliosphere inside 1 AU. In the five events, below a transition energy E 0 (~10-40 keV), the pitch-angle distributions are highly anisotropic with a pitch-angle width at half-maximum (PAHM) of <15° (unresolved) through the time of the peak; the ratio Λ of the peak flux of scattered (22fdg5-90° relative to the outward direction) to field-aligned scatter-free (0°-22fdg5) electrons is lsim0.1. Above E 0, the PAHM at the flux peak increases with energy up to 85° at 300 keV, and Λ also increases with energy up to ~0.8 at 300 keV. Thus, low-energy electrons propagated essentially scatter-free through the interplanetary medium, while high-energy electrons experienced pitch-angle scattering, with scattering strength increasing with energy. The transition energy E 0 between the two populations is always such that the electron gyroradius (ρ e ) is approximately equal to the local thermal proton gyroradius (ρ Tp ), suggesting that the higher energy electrons were scattered by resonance with turbulent fluctuations at scale gsimρ Tp in the solar wind. Title: Solar energetic electron probes of magnetic cloud field line lengths Authors: Kahler, S. W.; Krucker, S.; Szabo, A. Bibcode: 2011JGRA..116.1104K Altcode: Magnetic clouds (MCs) are large interplanetary coronal mass ejections of enhanced and low-variance fields with rotations indicative of magnetic flux ropes originally connected to the Sun. The MC flux rope models require field lines with larger pitch angles and longer lengths with increasing distance from the MC axis. While the models can provide good fits to the in situ solar wind observations, there have not been definitive observational tests of the global magnetic field geometry, particularly for the field line lengths. However, impulsive solar energetic (E > 10 keV) electron events occasionally occur within an MC, and the electron onsets can be used to infer Le, the magnetic field line lengths traveled by the electrons from the Sun to the points in the MC where the electron onsets occur. We selected 8 MCs in and near which 30 solar electron events were observed by the 3DP instrument on the Wind spacecraft. We compared the corresponding Le values with calculated model field line lengths to test two MC models. Some limitations on the technique are imposed by variations of the models and uncertainly about MC boundary locations. We found generally poor correlations between the computed electron path lengths and the model field line lengths. Only one value of Le inside an MC, that of 18 October 1995, exceeded 3.2 AU, indicating an absence of the long path lengths expected in the highly wound outer regions of MC models. We briefly consider the implications for MC models. Title: AIA and RHESSI Observations of Solar Coronal Jets Authors: Krucker, S.; Lin, R. P.; Csillaghy, A. Bibcode: 2010AGUFMSH23A1826K Altcode: The broad temperature coverage and high time resolution of AIA observations provides an excellent diagnostic tool to study solar flares. We combine AIA EUV observations with RHESSI hard X-ray observations to study the geometry, time evolution, and energetics of solar coronal jets. We report on a jet event of 2010 July 20 2UT (GOES A5) that shows the standard jet geometry of interchange reconnection (e.g. Moore et al. 2010). The hard X-ray observations reveal a very compact (4"x4") thermal flare loop. Counts in the non-thermal range, however, are too low to make an image of the HXR footpoints. The jet itself is only seen at EUV wavelengths with a width of 15" and a length of 60". The EUV time evolution shows two components, one that closely follows the HXR time profile and a second one that lasts longer that represents the jet evolution. RHESSI temperature diagnostics of hot flare plasma nicely complements the three AIA filters (193A, 131A, and 94A) with temperature responses to hot plasma (>6 MK). The fit to the HXR spectrum reveals a peak temperature of 9.1 MK. For this temperature range, the filter ratio of 131A and 94A is unique and steeply increasing with temperature. The 131/94 ratio gives a peak temperatures around 9.9 MK with the same temperature trend as seen with RHESSI. This indicates that the presently available filter response curves for 131A and 94A can be used as temperature diagnostics of hot (6-18 MK) flare plasmas. Title: Electron Acceleration by Multi-Island Coalescence Authors: Oka, M.; Phan, T.; Krucker, S.; Fujimoto, M.; Shinohara, I. Bibcode: 2010AGUFMSH33A1812O Altcode: Energetic electrons of up to tens of MeV are created during explosive phenomena in the solar corona. While many theoretical models consider magnetic reconnection as a possible way of generating energetic electrons, the precise roles of magnetic reconnection during acceleration and heating of electrons still remain unclear. Here we show from 2D particle-in-cell simulations that coalescence of magnetic islands that naturally form as a consequence of tearing mode instability and associated magnetic reconnection leads to efficient energization of electrons. The key process is the secondary magnetic reconnection at the merging points, or the `anti-reconnection', which is, in a sense, driven by the converging outflows from the initial magnetic reconnection regions. By following the trajectories of the most energetic electrons, we found a variety of different acceleration mechanisms but the energization at the anti-reconnection is found to be the most important process. We discuss possible applications to the energetic electrons observed in the solar flares. We anticipate our results to be a starting point for more sophisticated models of particle acceleration during the explosive energy release phenomena. Title: itch angle distributions and temporal variations of 0.3-300 keV solar impulsive electron events Authors: Wang, L.; Lin, R. P.; Krucker, S. Bibcode: 2010AGUFMSH33A1828W Altcode: We investigate pitch angle distributions (PADs) and temporal variations for five solar impulsive electron events with a rapid rise and rapid decay, observed by the WIND 3D Plasma & Energetic Particle instrument in the energy range from ∼0.3 to 300 keV. Below a transition energy E0 ( ∼10-30 keV), the PADs are highly anisotropic with a pitch-angle width at half maximum (PAHM) of <15o (unresolved) through the peak; the ratio of the peak flux of scattered (22.5-90o) to field-aligned scatter-free (0-22.5o, relative to the outward direction) electrons is <0.1. Above E0, the PADs become broader and the PAHM at peak increases with energy up to 85o at 300 keV; the peak-flux ratio of scattered to scatter-free electrons also increases with energy up to ∼0.8 at 300 keV. Thus, in these events, electrons with energies below E0 propagated essentially scatter-free through the interplanetary medium, while electrons at higher energies experienced pitch-angle scattering, with scattering strength increasing with energy. In the five events studied, the transition energy E0 between the two populations occurs where the electron gyroradius (ρ e) is equal to the local thermal proton gyroradius (ρ Tp), suggesting that the higher energy electrons were scattered by resonance with turbulent fluctuations at scale ≳ ρ Tp in the solar wind. In two events, the temporal profiles above 25 keV also show a second peak of inward-traveling electrons tens of minutes after the first peak, possibly due to reflection/scattering at ∼0.5-1 AU past the Earth, and a third peak due to outward-traveling electrons minutes later, likely caused by reflection/scattering of those inward-traveling populations between the Sun and Earth. Title: AIA observations of a flare/CME system in conjunction with X-ray and radio data Authors: Bain, H. M.; Krucker, S. Bibcode: 2010AGUFMSH11A1600B Altcode: During solar flares and coronal mass ejections, free magnetic energy, previously stored within the magnetic field, is released and used to accelerate particles and heat the surrounding plasma. Using multiwavelength observations from AIA onboard SDO, combined with X-ray observations from RHESSI and radio observations from the Nancay Radioheliograph we investigate sources of particle acceleration and heating within the flare-CME system of an event which occurred on the 14 August 2010. This was the first proton event in more than three years. In addition, radio spectrogram data show Type III radio bursts associated with the impulsive/launch phase of the CME, indicating the location of energetic electrons as they escape into the interplanetary medium. A Type II radio burst is also observed as electrons are accelerated in a CME driven shock as the ejected material propagates away from 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: SDO and RHESSI Observations of Microflares Authors: Christe, S.; Krucker, S.; Hannah, I. G. Bibcode: 2010AGUFMSH23A1830C Altcode: We present observations of 10 microflares (B and A GOES Class) with simultaneous observations in both hard X-ray (HXR) and ultraviolet (UV/EUV) observations by RHESSI and SDO/AIA, respectively. The microflares were chosen by visually searching for time-isolated events in the available concurrent RHESSI/SDO dataset. Identifying the flare region using RHESSI imaging observations, we compare HXR (thermal and nonthermal) light curves and images with those observed by AIA in every available UV/EUV channels. Many events show complex morphologies with multiple flaring loops observed in AIA images. For the simplest event with a single flare loop, good correlation is found between the hot thermal (~10 MK) HXR emission and emission from 131 and 94 Angstrom and the filter-ratio derived temperatures agree with the HXR-derived temperatures during the decay phase. During the impulsive phase, footpoints emission was observed at all wavelengths with filter-ratios suggest the presence of multiple temperatures. Title: Multi-temperature Observations of Solar Microflares using RHESSI and SDO/AIA Authors: McTiernan, J. M.; Krucker, S. Bibcode: 2010AGUFMSH23A1831M Altcode: In this work, we estimate the Differential Emission Measure for solar microflares using data from the AIA instrument on SDO and RHESSI data. Seven of the AIA wavelength bands (94, 131, 171, 193, 304, and 335 Angstroms), which detect emission from solar plasma with temperatures ranging from less than 20,000 K to as high as 20 MK, are useful for this calculation. The RHESSI instrument observes solar flare plasma in the temperature range above 8 MK. Preliminary results for a microflare observed on 11 July 2010 show loop footpoint sources with broad emission measure distributions, with both low (<100,000 K) and high (>10 MK) plasma. There is also a loop visible in the AIA 131 and 94 Angstrom wavelength bands, which are sensitive to high temperatures. RHESSI images in the energy range between 4 and 10 keV also show the loop, which has a relatively narrow (in temperature) emission measure distribution, concentrated above 10 MK. Implications for flare heating scenarios which may result in this sort of emission measure distribution will be discussed. Title: Thermal Imaging of Multi-Temperature Flare Plasma with RHESSI Visibilities Authors: Caspi, A.; Krucker, S.; Hurford, G. J.; McTiernan, J. M. Bibcode: 2010AGUFMSH53B..08C Altcode: Solar flares contain thermal plasma at multiple temperatures. Observations with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have shown that the most intense (GOES X-class) flares commonly achieve maximum temperatures of ~30-50 MK. Recent analysis reveals that in such flares, the "super-hot" (>30 MK) plasma exists simultaneously with, but separately from, the ~10-20 MK plasma observed in nearly all flares; the X-ray spectra are well-fit by two distinct isothermal components. RHESSI images at multiple energies can show differing morphologies and/or centroid positions, as each thermal component contributes a different but varying amount to the flux in each energy band, making it difficult to separately characterize the spatial properties of the multiple emitting plasmas. We present a novel technique to visualize the individual thermal plasmas by combining RHESSI X-ray visibilities using the precise but spatially-unresolved spectra and model fits, isolating the contributions from each thermal component to effectively image as a function of temperature rather than energy. This technique is computationally inexpensive and generalizable to an arbitrary number of thermal components, e.g. a binned differential emission measure model of the temperature distribution. We apply this technique to the 2002 July 23 X4.8 event to show that the super-hot and cool plasmas are spatially distinct and to examine their evolution throughout the flare. We then discuss the implications for plasma heating in large solar flares.

FIGURE CAPTION: Image contours of the two individual thermal components in the 2002 Jul 23 X4.8 event, derived using our technique; the non-thermal contours are shown for reference. (Figure not shown in ADS abstract.) Title: Longitudinal Spread of Protons from the Decay of Solar Flare Neutrons Authors: Agueda, N.; Krucker, S.; Lin, R. P. Bibcode: 2010AGUFMSH33B1834A Altcode: Solar energetic particle events which arise from the in-flight decay of energetic neutrons emitted from the Sun were observed in three cases during solar cycle 21. These events were associated with X-class gamma-ray solar flares occurring on the eastern hemisphere of the Sun during quiescent interplanetary conditions. The beginning of solar cycle 24 presents the opportunity to observe this kind of events from spacecraft separated in longitude and distance from the Sun. We present a Monte Carlo simulation of the propagation of solar flare neutrons and its resulting decay protons in the heliosphere. We find that the longitudinal spread of solar neutron decay protons depends on the angular distribution of solar neutrons at the Sun. We conclude that the observation of neutron decay proton events by spacecraft separated in longitude could be used to infer the degree of neutron isotropy at the Sun and constrain theoretical models of high-energy processes in solar flares. Title: Probing flare temperatures using AIA dispersion effects and RHESSI imaging/spectroscopy Authors: Raftery, C. L.; Krucker, S. Bibcode: 2010AGUFMSH23A1828R Altcode: Diffraction patterns that occur as a result of very intense, often saturated emission can provide many useful tools not ordinarily available. Specifically, analyzing dispersion features of high order diffraction patterns can provide useful insight into the thermal distributions of plasma within a structure. Here, we analyze the diffraction patterns of multiple SDO/AIA wavelength bands to more than 40 orders. Combining these results with imaging and spectroscopy from RHESSI enabled us to probe temperature and emission measure distributions within saturated flare sources. Title: Study of Flare Energetics Using X-ray, Radio, and EUV Observations Authors: Glesener, L.; Krucker, S.; Bain, H. M.; Lin, R. P. Bibcode: 2010AGUFMSH23A1829G Altcode: In studying solar flare energetics, the following questions are usually considered: What is the number of accelerated electrons? What are their energies? Where are they located and where do they ultimately end up? Multiwavelength studies are useful for answering these questions. Information on the instantaneous number of electrons and their energies can be inferred from the nonthermal bremsstrahlung spectrum in the hard X-ray (HXR) regime. HXR images can also identify locations of energetic electrons. Thermal bremsstrahlung emission in soft X-rays provides an estimate of the energy that goes into heating ambient electrons, as well as temperature and density diagnostics. UV and EUV observations provide more detailed temperature estimates, show the dynamics of plasma structures, and help to locate emitting volumes. Radio emissions offer information on the energy lost to escaping electrons (which emit Type III bursts) and the energies of gyrosynchrotron-emitting electrons in flare loops. Here, we combine X-ray data from the Reuven Ramaty High Energy Solar Spectrometer (RHESSI), EUV data from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory, and radio data from various solar radio spectrographs. Several case studies of flare electron energetics will be presented. The time profiles of HXR and radio emission will be compared, along with the electron energies derived from each. EUV and UV data will be used to image flare loops and investigate temperature evolution. Title: Statistically Derived Flaring Chromospheric-Coronal Density Structure from Non-thermal X-ray Observations of the Sun Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P. Bibcode: 2010ApJ...721.1933S Altcode: 2011arXiv1111.4251S For the first time, we have used RHESSI's spatial and energy resolution to determine the combined chromospheric and coronal density profile of the flaring solar atmosphere in a statistical manner, using a data set of 838 flares observable in hard X-rays above 25 keV. Assuming the thick-target beam model, our "average flaring atmosphere" was found to have density scale heights of 131 ± 16 km at low altitudes (chromosphere, up to ≈1-1.5 Mm above photosphere), and of 5-6 Mm at high altitudes (corona, above ≈2-3 Mm). Assuming a unit step change in ionization level, modeling yields a height of 1.3 ± 0.2 Mm for the transition between fully neutral and fully ionized atmosphere. Furthermore, centroids of emission above 50 keV, produced by electrons of similar or higher energies, are located mostly in a small region ~0.5 Mm in vertical extent, where neutral densities are beyond 3 × 1013 cm-3. Title: Development of double-sided silicon strip detectors for solar hard x-ray observation Authors: Saito, Shinya; Ishikawa, Shin-Nosuke; Watanabe, Shin; Odaka, Hirokazu; Sugimoto, Soichiro; Fukuyama, Taro; Kokubun, Motohide; Takahashi, Tadayuki; Terada, Yukikatsu; Tajima, Hiroyasu; Tanaka, Takaaki; Krucker, Säm; Christe, Steven; McBride, Steve; Glesener, Lindsay Bibcode: 2010SPIE.7732E..0QS Altcode: 2010SPIE.7732E..19S The Focusing Optics X-ray Solar Imager (FOXSI) is a rocket experiment scheduled for January 2011 launch. FOXSI observes 5 - 15 keV hard X-ray emission from quiet-region solar flares in order to study the acceleration process of electrons and the mechanism of coronal heating. For observing faint hard X-ray emission, FOXSI uses focusing optics for the first time in solar hard X-ray observation, and attains 100 times higher sensitivity than RHESSI, which is the present solar hard X-ray observing satellite. Now our group is working on developments of both Double-sided Silicon Strip Detector (DSSD) and read-out analog ASIC "VATA451" used for FOXSI. Our DSSD has a very fine strip pitch of 75 μm, which has sufficient position resolution for FOXSI mirrors with angular resolution (FWHM) of 12 arcseconds. DSSD also has high spectral resolution and efficiency in the FOXSI's energy range of 5 - 15 keV, when it is read out by our 64-channel analog ASIC. In advance of the FOXSI launch, we have established and tested a setup of 75 μm pitch DSSD bonded with "VATA451" ASICs. We successfully read out from almost all the channels of the detector, and proved ability to make a shadow image of tungsten plate. We also confirmed that our DSSD has energy resolution (FWHM) of 0.5 keV, lower threshold of 5 keV, and position resolution less than 63 μm. These performance satisfy FOXSI's requirements. Title: The Focusing Optics Solar X-ray Imager (FOXSI) Authors: Christe, Steven; Glesener, L.; Krucker, S.; Ramsey, B.; Ishikawa, S.; Takahashi, T.; Tajima, H. Bibcode: 2010AAS...21641309C Altcode: The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket payload funded under the NASA Low Cost Access to Space program to test hard x-ray focusing optics and position-sensitive solid state detectors for solar observations. Today's leading solar hard x-ray instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides excellent spatial (2 arcseconds) and spectral (1 keV) resolution. Yet, due to its use of indirect imaging, the derived images have a low dynamic range (<30) and sensitivity. These limitations make it difficult to study faint x-ray sources in the solar corona which are crucial for understanding the solar flare acceleration process. Grazing-incidence x-ray focusing optics combined with position-sensitive solid state detectors can overcome both of these limitations enabling the next breakthrough in understanding particle acceleration in solar flares. The FOXSI project is led by the Space Science Laboratory at the University of California. The NASA Marshall Space Flight Center, with experience from the HERO balloon project, is responsible for the grazing-incidence optics, while the Astro H team (JAXA/ISAS) will provide double-sided silicon strip detectors. FOXSI will be a pathfinder for the next generation of solar hard x-ray spectroscopic imagers. Such observatories will be able to image the non-thermal electrons within the solar flare acceleration region, trace their paths through the corona, and provide essential quantitative measurements such as energy spectra, density, and energy content in accelerated electrons. Title: Electron Acceleration by Multi-Island Coalescence Authors: Oka, M.; Phan, T. -D.; Krucker, S.; Fujimoto, M.; Shinohara, I. Bibcode: 2010ApJ...714..915O Altcode: 2010arXiv1004.1154O Energetic electrons of up to tens of MeV are created during explosive phenomena in the solar corona. While many theoretical models consider magnetic reconnection as a possible way of generating energetic electrons, the precise roles of magnetic reconnection during acceleration and heating of electrons still remain unclear. Here, we show from two-dimensional particle-in-cell simulations that coalescence of magnetic islands that naturally form as a consequence of tearing mode instability and associated magnetic reconnection leads to efficient energization of electrons. The key process is the secondary magnetic reconnection at the merging points, or the "anti-reconnection," which is, in a sense, driven by the converging outflows from the initial magnetic reconnection regions. By following the trajectories of the most energetic electrons, we found a variety of different acceleration mechanisms but the energization at the anti-reconnection is found to be the most important process. We discuss possible applications to the energetic electrons observed in the solar flares. We anticipate our results to be a starting point for more sophisticated models of particle acceleration during the explosive energy release phenomena. 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: Solar Flare Electrons at the Sun and in the Interplanetary Space Authors: Krucker, Säm. Bibcode: 2010EGUGA..12.5954K Altcode: How solar flares accelerate electrons up to relativistic energies is one of the main unsolved problems in Heliophysics. Remote sensing at radio and hard X-ray wavelengths provide crucial diagnostics of flare-accelerated electrons in the solar corona. In-situ observations in interplanetary space give direct observations of flare-accelerated electrons escaping the Sun. In this review, recent results combining these observations are discussed, with a special emphases on future observations with Solar Orbiter and Solar Probe +. Title: Statistics Of Solar Flares Hard X-ray Spectra Using RHESSI. Authors: Alaoui, Meriem; Krucker, S.; Saint-Hilaire, P.; Lin, R. P. Bibcode: 2010AAS...21640437A Altcode: Non-thermal X-ray emission provides a key diagnostic on particle acceleration and transport during solar flares. Thermal and non-thermal emissions can be derived from the spatially integrated photon flux spectrum obtained by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The thermal radiation is usually represented by a maxwellian and the non-thermal emission is represented by a single or a double (broken) power-law. We present spectral characteristics of 65 flares observed by RHESSI and having emission to at least 150keV above the background level. The spectra are integrated over 8s at the hard x-ray peak time. Our major findings are as follows: (1)The spectra almost always break downwards i.e. the spectral indices above the breakenergy are greater than indices below the break energy (47/65). (2) The break energy is typically around 60 keV (3) There is no dependence of the break energy upon the spectral index. (4) A small dependence of the photon flux upon the break energy. 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: RHESSI and TRACE Observations of Emerging Flux Reconnection in a Solar Jet on August 21, 2003 Authors: Glesener, Lindsay; Krucker, S.; Lin, R. P. Bibcode: 2010AAS...21632006G Altcode: 2010BAAS...41R.911G Solar jets, or transient, collimated brightenings, were first observed in soft x-rays by Yohkoh in the 1990's. These observations provided confirmation for the emerging flux model predicted by Heyvaerts and Shibata to describe reconnection in the corona. This model also predicts hard x-ray (HXR) emission during jet production, and recent RHESSI and HINODE observations appear to support this prediction. Unfortunately, projection effects make it difficult to determine whether HXR sources on the solar disk correspond to flare footpoints or to the jet itself. Observation of jets near or at the solar limb can mitigate projection effects and allow the jet geometry to be resolved.

Here, we present RHESSI observations of a partially occulted HXR flare that coincided with an EUV jet observed by TRACE on August 21, 2003. Comparisons between TRACE and RHESSI images and time profiles are used to illustrate several aspects of the emerging flux reconnection model, including the locations of outflow jets, post-reconnection flare loops, and HXR emission from the jet base. Radio data is used to provide corroborating evidence for the sources and timing of particle acceleration. Title: Search for Heliospheric Electrons and Protons from the Decay of Solar Flare Neutrons over Solar Cycle 23 Authors: Agueda, Neus; Krucker, S.; Lin, R. P.; Wang, L.; Larson, D. E. Bibcode: 2010AAS...21640435A Altcode: The observation of solar neutrons has long been recognized as a probe to learn the properties of ions accelerated in solar flares. To date, few energetic particle events produced by decaying solar flare neutrons have been reported, and the information they have provided has covered the 20-200 MeV energy range of the spectrum.

We present preliminary results of a comprehensive search of in-situ measurements of 1-10 MeV protons and 0.05-1 MeV electrons related to the decay of solar flare neutrons. We select a sample of 39 isolated large eastern hemisphere flares through solar cycle 23, and characterize the interplanetary magnetic field conditions after each flare. We analyze the electron and proton in-situ intensities observed by Wind following each solar flare and discuss the challenges that poses the detection of the decay products of low energy solar flare neutrons.

This work was supported by NASA under research grants NNX08AE34G and NAS5-98033. Title: Observations of a Sun-grazing Comet's Interaction with the Solar Corona Authors: Raftery, Claire; Martinez Oliveros, J. C.; Krucker, S. Bibcode: 2010AAS...21640723R Altcode: A number of the "Kreutz” family of Sun-grazing comets were observed to fly in close proximity to the Sun on 12 March 2010. Here we present observations of one such comet during its initial approach and "impact” with the Sun. This study makes use of observations from a range of spacecraft with multiple points of view including both STEREO spacecraft and multiple instruments on board SOHO. Title: Observing Solar Hard X-rays from Heliospheric Orbits Authors: Hurford, Gordon J.; Benz, A.; Dennis, B.; Krucker, S.; Limousin, O.; Lin, R.; Vilmer, N. Bibcode: 2010AAS...21640416H Altcode: 2010BAAS...41Q.902H The coming decade provides two opportunities to acquire a different observational perspective on solar hard x-ray emission. Both ESA's Solar Orbiter and NASA's Solar Probe Plus missions will be in heliocentric orbits with perihelia of 0.28 au and 0.05 au respectively. This poster indicates the unique scientific advantages of hard x-ray imaging/spectroscopy observations from such platforms.

These advantages stem from three factors: First, in combination with other payload elements, the hard x-rays provide the ability to observationally link accelerated electrons at the Sun to radio observations of the propagating electrons and to direct observations of in situ electrons. Second, the substantial gain in sensitivity afforded by close-in vantage points enables exploration of the origin of non-flare associated SEP events to be studied and the character of quiescent active-region heating and electron acceleration to be evaluated. Third, the different observational perspectives provided by the heliocentric orbits compared to low-Earth orbits enable improved separation of coronal and footpoint sources as well as measurements of the isotropy of the x-ray emission.

Despite the limited payload resources (mass, power, telemetry) afforded by such missions, scientifically effective hard x-ray imaging spectroscopy from 5 keV to 150 keV is still feasible. The Spectrometer/Telescope for Imaging X-rays (STIX), accepted as part of the Solar Orbiter payload, combines high spectral resolution ( 1 keV FWHM at 10 keV) with spatial resolution as good as 1500 km, and can efficiently encode the data for several hundred optimized images per hour within a modest telemetry allocation and 4 kg / 4 watt budget. The X-ray Imaging Spectrometer (XIS) proposed for Solar Probe Plus, views the Sun through its thermal shield. It also features high spectral resolution from 6 to 150 keV and spatial resolution of 1500 km at perihelion. The poster describes the imaging principles and current configurations of both instruments. Title: Magnetic Connectivity of the CME Event on December 31, 2007 Authors: Martinez Oliveros, Juan Carlos; Raftery, C.; Agueda, N.; Krucker, S.; Bale, S.; Lindsey, C. Bibcode: 2010AAS...21640624M Altcode: Dynamical changes in the solar magnetic field are responsible for coronal mass ejections (CMEs), one of the most violent phenomena observed in the Sun. During a CME, mass is lifted away from the Sun and accelerated into the interplanetary medium, sometimes interacting with the Earth or other planets. Some of these CMEs produce radio emission, including type II bursts, which are generated by a strong upstream interplanetary (IP) shocks. The emission is strongest in regions where the direction of the IP shock is quasi-perpendicular to the interplanetary magnetic field. Due to small-scale shock structure, this occurs at various sites along the shock front. We study the different emissions associated with the CME observed on 2007 December 31 using data from the RHESSI, Proba-2/SWAP, and STEREO A/B spacecraft. We track and probe the CME and associated shock signatures from its origin in the lower corona through the interplanetary medium. 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: Combined STEREO/RHESSI Study of Coronal Mass Ejection Acceleration and Particle Acceleration in Solar Flares Authors: Temmer, M.; Veronig, A. M.; Kontar, E. P.; Krucker, S.; Vršnak, B. Bibcode: 2010ApJ...712.1410T Altcode: 2010arXiv1002.3080T Using the potential of two unprecedented missions, Solar Terrestrial Relations Observatory (STEREO) and Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), we study three well-observed fast coronal mass ejections (CMEs) that occurred close to the limb together with their associated high-energy flare emissions in terms of RHESSI hard X-ray (HXR) spectra and flux evolution. From STEREO/EUVI and STEREO/COR1 data, the full CME kinematics of the impulsive acceleration phase up to ~4 R sun is measured with a high time cadence of <=2.5 minutes. For deriving CME velocity and acceleration, we apply and test a new algorithm based on regularization methods. The CME maximum acceleration is achieved at heights h <= 0.4 R sun, and the peak velocity at h <= 2.1 R sun (in one case, as small as 0.5 R sun). We find that the CME acceleration profile and the flare energy release as evidenced in the RHESSI HXR flux evolve in a synchronized manner. These results support the "standard" flare/CME model which is characterized by a feedback relationship between the large-scale CME acceleration process and the energy release in the associated flare. Title: Time-resolved hard X-Ray hardness variation of solar flares observed by Suzaku Wide-band All-sky monitor Authors: Endo, Akira; Morigami, Kouichi; Tashiro, Makoto; Terada, Yukikatsu; Yamaoka, Kazutaka; Sonoda, Eri; Minoshima, Takashi; Krucker, Sam; HXD-WAM Team Bibcode: 2010ecsa.conf..194E Altcode: No abstract at ADS Title: The Focusing Optics X-Ray Solar Imager Authors: Glesener, L.; Krucker, S.; Christe, S. Bibcode: 2010LPICo1534...30G Altcode: No abstract at ADS 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: Evidence for extended acceleration of solar flare ions from 1-8 MeV solar neutrons detected with the MESSENGER Neutron Spectrometer Authors: Feldman, William C.; Lawrence, David J.; Goldsten, John O.; Gold, Robert E.; Baker, Daniel N.; Haggerty, Dennis K.; Ho, George C.; Krucker, SäM.; Lin, Robert P.; Mewaldt, Richard A.; Murphy, Ronald J.; Nittler, Larry R.; Rhodes, Edgar A.; Slavin, James A.; Solomon, Sean C.; Starr, Richard D.; Vilas, Faith; Vourlidas, Angelos Bibcode: 2010JGRA..115.1102F Altcode: 2010JGRA..11501102F Neutrons produced on the Sun during the M2 flare on 31 December 2007 were observed at 0.48 AU by the MESSENGER Neutron Spectrometer. These observations are the first detection of solar neutrons inside 1 AU. This flare contained multiple acceleration episodes as seen in type III radio bursts. After these bursts ended, both the energetic particle and neutron fluxes decayed smoothly to background with an e-folding decay time of 2.84 h, spanning a 9 h time period. This time is considerably longer than the mean lifetime of a neutron, which indicates that either the observed neutrons were generated in the spacecraft by solar energetic particle protons, or they originated on the Sun. If most of the neutrons came from the Sun, as our simulations of neutron production on the spacecraft show, they must have been continuously produced. A likely explanation of their long duration is that energetic ions were accelerated over an extended time period onto closed magnetic arcades above the corona and then slowly pitch angle-scattered by coronal turbulence into their chromospheric loss cones. Because of their relatively low energy loss in the Neutron Spectrometer (0.5-7.5 MeV), most of these neutrons beta decay to energetic protons and electrons close to the Sun, thereby forming an extended seed population available for further acceleration by subsequent shocks driven by coronal mass ejections in interplanetary space. 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: A Statistical Study of Spectral Hardening in Solar Flares and Related Solar Energetic Particle Events Authors: Grayson, James A.; Krucker, Säm; Lin, R. P. Bibcode: 2009ApJ...707.1588G Altcode: Using hard X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we investigate the reliability of spectral hardening during solar flares as an indicator of related solar energetic particle (SEP) events at Earth. All RHESSI data are analyzed, from 2002 February through the end of Solar Cycle 23, thereby expanding upon recent work on a smaller sample of flares. Previous investigations have found very high success when associating soft-hard-harder (SHH) spectral behavior with energetic proton events, and confirmation of this link would suggest a correlation between electron acceleration in solar flares and SEPs seen in interplanetary space. In agreement with these past findings, we find that of 37 magnetically well-connected flares (W30-W90), 12 of 18 flares with SHH behavior produced SEP events and none of 19 flares without SHH behavior produced SEPs. This demonstrates a statistically significant dependence of SHH and SEP observations, a link that is unexplained in the standard scenario of SEP acceleration at the shock front of coronal mass ejections and encourages further investigation of the mechanisms which could be responsible. Title: STATISTICAL STUDY of HARD X-RAY SPECTRAL CHARACTERISTICS OF SOLAR FLARES Authors: Alaoui, M.; Krucker, S.; Saint-Hilaire, P.; Lin, R. P. Bibcode: 2009AGUFMSH23A1519A Altcode: We investigate the spectral characteristics of 75 solar flares at the hard X-ray peak time observed by RHESSI (Ramaty High Energy Solar Spectroscopic Imager) in the energy range 12-150keV. At energies above 40keV, the Hard X-ray emission is mostly produced by bremsstrahlung of suprathermal electrons as they interact with the ambient plasma in the chromosphere. The observed photon spectra therefore provide diagnostics of electron acceleration processes in Solar flares. We will present statistical results of spectral fitting using two models: a broken power law plus a thermal component which is a direct fit of the photon spectrum and a thick target model plus a thermal component which is a fit of the photon spectra with assumptions on the electrons emitting bremsstrahlung in the thick target approximation. Title: RHESSI and TRACE Observations of Emerging Flux Reconnection in a Solar Jet on August 21, 2003 Authors: Glesener, L.; Krucker, S.; Lin, R. P. Bibcode: 2009AGUFMSH21C..02G Altcode: Solar jets, or transient, collimated brightenings, were first observed in soft x-rays by Yohkoh in the 1990’s. These observations led to the establishment of the Shibata-Heyvaerts emerging flux model for reconnection associated with jets. This model also predicts hard x-ray (HXR) emission during jet production, and recent RHESSI and HINODE observations appear to support this prediction. Unfortunately, projection effects make it difficult to determine whether HXR sources on the solar disk correspond to flare footpoints or the jet itself. Observation of jets near or at the solar limb can lessen this type of source confusion. Here, we present RHESSI observations of a partially occulted HXR flare that coincides with an EUV jet observed by TRACE on August 21, 2003. A double coronal HXR source is observed shortly before the emission of the jet, indicating the location of reconnection outflows. Thick- and thin-target models are used to estimate the electron energies and densities in the HXR sources, and allow for comparison with the emerging flux model. 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: A Statistical Study of Spectral Hardening in Solar Flares and Related Solar Energetic Particle Events Authors: Grayson, J.; Krucker, S.; Lin, R. P. Bibcode: 2009AGUFMSH33A1480G Altcode: Using hard X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we investigate the reliability of spectral hardening during solar flares as an indicator of related solar energetic particle (SEP) events at Earth. All RHESSI data are analyzed, from February 2002 through the end of Solar Cycle 23, thereby expanding upon recent work on a smaller sample of flares. Previous investigations have found very high success when associating soft-hard-harder (SHH) spectral behavior with energetic proton events, and confirmation of this link would suggest a correlation between electron acceleration in solar flares and SEPs seen in interplanetary space. In agreement with these past findings, we find that of 37 magnetically well-connected flares (W30-W90), 12 of 18 flares with SHH behavior produced SEP events and none of 19 flares without SHH behavior produced SEPs. This demonstrates a statistically significant dependence of SHH and SEP observations, a link that is unexplained in the standard scenario of SEP acceleration at the shock front of coronal mass ejections, and encourages further investigation of the mechanisms which could be responsible. Title: Solar flare electrons at the Sun and near the Earth. Insights from interplanetary transport simulations Authors: Agueda, N.; Krucker, S.; Lin, R. P.; Vainio, R. O.; Lario, D. D.; Sanahuja, B. Bibcode: 2009AGUFMSH33A1478A Altcode: We study 15 prompt 3He-rich electron events observed near 1 AU by Wind/3DP above 50 keV. The events are associated with magnetically well-connected flares (between W10 and W85) with rather short soft X-ray duration (~10 min) and HXR sources seen by RHESSI on the visible solar disk. We compare the Wind/3DP observations with the results of interplanetary transport simulations, to infer the properties of the propagation and constrain the injection of solar energetic electrons in the interplanetary medium. We discuss the correlation between the inferred total number of injected electrons with the number of electrons required to produce the observed X-ray flux. Title: Energetic Electron Probes of Magnetic Cloud Topology Authors: Kahler, S. W.; Krucker, S.; Szabo, A. Bibcode: 2009AGUFMSH41A1643K Altcode: Magnetic clouds (MCs) are large interplanetary coronal mass ejections (ICMEs) of enhanced fields with rotations indicative of magnetic flux ropes originally connected to the Sun. The MC models require field lines with larger pitch angles and longer lengths with increasing distance from the MC axis. While the models can provide good fits to the observations, the global magnetic field geometry has not been tested, particularly for the field-line lengths. Impulsive solar energetic (E > 30 keV) electron events occurring within MCs can be used to infer the propagation distance and magnetic field-line length from the Sun at the point in the MC where the electron onset occurs. We surveyed the 18 MCs listed on the Wind/MFI MC list in or near which solar electron events were observed by the SSL/Berkeley 3DP instrument on the Wind spacecraft. By plotting the electron onset times as functions of their speeds for each event and taking their solar injections to coincide with observed type III radio bursts in the Wind/WAVES detector, the travel distances of the electrons along the field lines of the MCs were computed. Those electron travel distances were compared with the field-line lengths computed from two different MC model fits to test the models. Title: The Focusing Optics Solar X-ray Imager (FOXSI) Authors: Christe, S.; Glesener, L.; Krucker, S.; Ramsey, B.; Ishikawa, S.; Takahashi, T. Bibcode: 2009AGUFMSH33B1496C Altcode: The Focusing Optics x-ray Solar Imager is a sounding rocket payload funded under the NASA Low Cost Access to Space program to test hard x-ray focusing optics and position-sensitive solid state detectors for solar observations. Today's leading solar hard x-ray instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager provides excellent spatial (2 arcseconds) and spectral (1~keV) resolution. Yet, due to its use of indirect imaging, the derived images have a low dynamic range (<30) and sensitivity. These limitations make it difficult to study faint x-ray sources in the solar corona which are crucial for understanding the solar flare acceleration process. Grazing-incidence x-ray focusing optics combined with position-sensitive solid state detectors can overcome both of these limitations enabling the next breakthrough in understanding particle acceleration in solar flares. The foxsi project is led by the Space Science Laboratory at the University of California. The NASA Marshall Space Flight Center, with experience from the HERO balloon project, is responsible for the grazing-incidence optics, while the Astro H team (JAXA/ISAS) will provide double-sided silicon strip detectors. FOXSI will be a pathfinder for the next generation of solar hard x-ray spectroscopic imagers. Such observatories will be able to image the non-thermal electrons within the solar flare acceleration region, trace their paths through the corona, and provide essential quantitative measurements such as energy spectra, density, and energy content in accelerated electrons. Title: Submillimeter and X-ray observations of an X class flare Authors: Giménez de Castro, C. G.; Trottet, G.; Silva-Valio, A.; Krucker, S.; Costa, J. E. R.; Kaufmann, P.; Correia, E.; Levato, H. Bibcode: 2009A&A...507..433G Altcode: 2009arXiv0908.2339G The GOES X1.5 class flare that occurred on August 30, 2002 at 1327:30 UT is one of the few events detected so far at submillimeter wavelengths. We present a detailed analysis of this flare combining radio observations from 1.5 to 212 GHz (an upper limit of the flux is also provided at 405 GHz) and X-ray. Although the observations of radio emission up to 212 GHz indicates that relativistic electrons with energies of a few MeV were accelerated, no significant hard X-ray emission was detected by RHESSI above ~250 keV. Images at 12-20 and 50-100 keV reveal a very compact, but resolved, source of about ~10 arcsec×10 arcsec. EUV TRACE images show a multi-kernel structure suggesting a complex (multipolar) magnetic topology. During the peak time the radio spectrum shows an extended flatness from ~7 to 35 GHz. Modeling the optically thin part of the radio spectrum as gyrosynchrotron emission we obtained the electron spectrum (spectral index δ, instantaneous number of emitting electrons). It is shown that in order to keep the expected X-ray emission from the same emitting electrons below the RHESSI background at 250 keV, a magnetic field above 500 G is necessary. On the other hand, the electron spectrum deduced from radio observations ≥50 GHz is harder than that deduced from ~70-250 keV X-ray data, meaning that there must exist a breaking energy around a few hundred keV. During the decay of the impulsive phase, a hardening of the X-ray spectrum is observed which is interpreted as a hardening of the electron distribution spectrum produced by the diffusion due to Coulomb collisions of the trapped electrons in a medium with an electron density of ne ~ 3-5 × 1010 cm-3. Title: Physics of ion acceleration in the solar flare on 2005 September 7 determines γ-ray and neutron production Authors: Watanabe, K.; Lin, R. P.; Krucker, S.; Murphy, R. J.; Share, G. H.; Harris, M. J.; Gros, M.; Muraki, Y.; Sako, T.; Matsubara, Y.; Sakai, T.; Shibata, S.; Valdés-Galicia, J. F.; González, L. X.; Hurtado, A.; Musalem, O.; Miranda, P.; Martinic, N.; Ticona, R.; Velarde, A.; Kakimoto, F.; Tsunesada, Y.; Tokuno, H.; Ogio, S. Bibcode: 2009AdSpR..44..789W Altcode: Relativistic neutrons were observed by the neutron monitors at Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at Chacaltaya and Mt. Sierra Negra in association with an X17.0 flare on 2005 September 7. The neutron signal continued for more than 20 min with high statistical significance. Intense emissions of γ-rays were also registered by INTEGRAL, and during the decay phase by RHESSI. We analyzed these data using the solar-flare magnetic-loop transport and interaction model of Hua et al. [Hua, X.-M., Kozlovsky, B., Lingenfelter, R.E. et al. Angular and energy-dependent neutron emission from solar flare magnetic loops, Astrophys. J. Suppl. Ser. 140, 563-579, 2002], and found that the model could successfully fit the data with intermediate values of loop magnetic convergence and pitch-angle scattering parameters. These results indicate that solar neutrons were produced at the same time as the γ-ray line emission and that ions were continuously accelerated at the emission site. Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Krucker, Sam; Christe, Steven; Glesener, Lindsay; McBride, Steve; Turin, Paul; Glaser, David; Saint-Hilaire, Pascal; Delory, Gregory; Lin, R. P.; Gubarev, Mikhail; Ramsey, Brian; Terada, Yukikatsu; Ishikawa, Shin-Nosuke; Kokubun, Motohide; Saito, Shinya; Takahashi, Tadayuki; Watanabe, Shin; Nakazawa, Kazuhiro; Tajima, Hiroyasu; Masuda, Satoshi; Minoshima, Takashi; Shomojo, Masumi Bibcode: 2009SPIE.7437E..05K Altcode: 2009SPIE.7437E...4K The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket payload funded under the NASA Low Cost Access to Space program to test hard x-ray focusing optics and position-sensitive solid state detectors for solar observations. Today's leading solar hard x-ray instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides excellent spatial (2 arcseconds) and spectral (1 keV) resolution. Yet, due to its use of indirect imaging, the derived images have a low dynamic range (<30) and sensitivity. These limitations make it difficult to study faint x-ray sources in the solar corona which are crucial for understanding the solar flare acceleration process. Grazing-incidence x-ray focusing optics combined with position-sensitive solid state detectors can overcome both of these limitations enabling the next breakthrough in understanding particle acceleration in solar flares. The FOXSI project is led by the Space Science Laboratory at the University of California. The NASA Marshall Space Flight Center, with experience from the HERO balloon project, is responsible for the grazing-incidence optics, while the Astro H team (JAXA/ISAS) will provide double-sided silicon strip detectors. FOXSI will be a pathfinder for the next generation of solar hard x-ray spectroscopic imagers. Such observatories will be able to image the non-thermal electrons within the solar flare acceleration region, trace their paths through the corona, and provide essential quantitative measurements such as energy spectra, density, and energy content in accelerated electrons. Title: Acceleration of Relativistic Protons During the 20 January 2005 Flare and CME Authors: Masson, S.; Klein, K. -L.; Bütikofer, R.; Flückiger, E.; Kurt, V.; Yushkov, B.; Krucker, S. Bibcode: 2009SoPh..257..305M Altcode: 2009arXiv0905.1816M The origin of relativistic solar protons during large flare/CME events has not been uniquely identified so far. We perform a detailed comparative analysis of the time profiles of relativistic protons detected by the worldwide network of neutron monitors at Earth with electromagnetic signatures of particle acceleration in the solar corona during the large particle event of 20 January 2005. The intensity - time profile of the relativistic protons derived from the neutron monitor data indicates two successive peaks. We show that microwave, hard X-ray, and γ-ray emissions display several episodes of particle acceleration within the impulsive flare phase. The first relativistic protons detected at Earth are accelerated together with relativistic electrons and with protons that produce pion-decay γ rays during the second episode. The second peak in the relativistic proton profile at Earth is accompanied by new signatures of particle acceleration in the corona within ≈1R⊙ above the photosphere, revealed by hard X-ray and microwave emissions of low intensity and by the renewed radio emission of electron beams and of a coronal shock wave. We discuss the observations in terms of different scenarios of particle acceleration in the corona. Title: X-ray Emission from the Base of a Current Sheet in the Wake of a Coronal Mass Ejection Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P. Bibcode: 2009ApJ...699..245S Altcode: 2011arXiv1111.4248S Following a coronal mass ejection (CME) which started on 2002 November 26, RHESSI observed for 12 hr an X-ray source above the solar limb, at altitudes between 0.1 and 0.3 RS above the photosphere. The Geostationary Operational Environmental Satellite baseline was remarkably high throughout this event. The X-ray source's temperature peaked around 10-11 MK, and its emission measure increased throughout this time interval. Higher up, at 0.7 RS , hot (initially >8 MK) plasma has been observed by Ultraviolet Coronograph Spectrometer on Solar and Heliospheric Observatory for 2.3 days. This hot plasma was interpreted as the signature of a current sheet (CS) trailing the CME. The thermal energy content of the X-ray source is more than an order of magnitude larger than in the CS. Hence, it could be the source of the hot plasma in the CS, although CS heating by magnetic reconnection within it cannot be discounted. To better characterize the X-ray spectrum, we have used novel techniques (back-projection-based and visibility-based) for long-integration (several hours) imaging spectroscopy. There is no observed nonthermal hard X-ray bremsstrahlung emission, leading to the conclusion that there is either very little particle acceleration occurring in the vicinity of this postflare X-ray source, or that either the photon spectral index would have had to be uncharacteristically (in flare parlance) high (γ gsim 8) and/or the low-energy cutoff very low (Ec lsim 6 keV). Title: Energy Deposition and Hard X-Ray Source Motions in the 2002 July 23 γ-Ray Flare Authors: Fivian, M. D.; Krucker, S.; Lin, R. P. Bibcode: 2009ApJ...698L...6F Altcode: Reconnection models of solar flares predict a systematic motion of hard X-ray (HXR) footpoints as magnetic energy is released and electrons are accelerated. While the correlation of the HXR flux with the apparent motion of the footpoints has previously been investigated, we derive and investigate for the first time the correlation between cumulative deposited energy at the footpoints and their separation. Providing excellent statistics, data from the Reuven Ramaty High Energy Solar Spectroscopic Imager of the 2002 July 23 flare are re-analyzed. The data show an excellent correlation for most of the time intervals. However, despite the good correlation, for some time ranges the derived amount of released magnetic energy is far too small to account for the energy in HXR-producing electrons. Title: Hard X-ray Emission From Partially Occulted Solar Flares Authors: Glesener, Lindsay; Krucker, S.; Lin, R. P. Bibcode: 2009SPD....40.1914G Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is optimized for fine energy and spatial resolution observations of hard x-ray (HXR) emission from solar flares. In many RHESSI flares, most of this emission originates from the bright flare loop footpoints. Fainter sources located higher in the corona are often not visible due to RHESSI's limited dynamic range. However, flares which are partially occulted (i.e. have footpoints obscured by the solar disk) allow for detailed observations of faint coronal HXR sources.

We have determined occultation heights for 55 RHESSI flares from 2002-2004 and compared the occultation heights with nonthermal flux. Compared with statistical results of on-disk flares, it is estimated that the coronal emission is 10-100 times fainter than the expected footpoint emission, confirming that these coronal sources would not have been visible in the presence of footpoints.

Occulted flares can be used to investigate the role of flare-accelerated electrons in heating ambient coronal plasma. With this aim, we compare RHESSI images of nonthermal HXR from energetic electrons with TRACE images made using the 195-angstrom filter, which has a sensitivity maximum at 1.5 MK. The locations and time profiles of the nonthermal and thermal sources are compared to investigate whether the heating seen by TRACE is due to the same electrons that emit the HXR seen by RHESSI. Title: The X-ray Detectability of Electron Beams Escaping from the Sun Authors: Saint-Hilaire, Pascal; Krucker, Säm; Christe, Steven; Lin, Robert P. Bibcode: 2009ApJ...696..941S Altcode: 2011arXiv1111.4250S We study the detectability and characterization of electron beams as they leave their acceleration site in the low corona toward interplanetary space through their nonthermal X-ray bremsstrahlung emission. We demonstrate that the largest interplanetary electron beams (gsim1035 electrons above 10 keV) can be detected in X-rays with current and future instrumentation, such as RHESSI or the X-Ray Telescope (XRT) onboard Hinode. We make a list of optimal observing conditions and beam characteristics. Amongst others, good imaging (as opposed to mere localization or detection in spatially integrated data) is required for proper characterization, putting the requirement on the number of escaping electrons (above 10 keV) to gsim3 × 1036 for RHESSI, gsim3 × 1035 for Hinode/XRT, and gsim1033 electrons for the FOXSI sounding rocket scheduled to fly in 2011. Moreover, we have found that simple modeling hints at the possibility that coronal soft X-ray jets could be the result of local heating by propagating electron beams. Title: Radio Emission from RHESSI Microflares observed by the Nobeyama Radio Heliograph Authors: Christe, Steven; White, S.; Krucker, S. Bibcode: 2009SPD....40.1923C Altcode: We present a statistical survey of RHESSI microflares observed in hard X-rays (HXR) with simultaneous observations by the Nobeyama Radio Heliograph at 17 GHz and 34 GHz. These microflares 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. Between March 2002 and March 2007, a total of 25000 events are identified, of which 8500 are also observed by Nobeyama at 17 and 34 GHz. We compare HXR and radio fluxes and analyze the relationship statistically. When the events are detected at both wavelengths, the relationship between the lightcurves falls into several classes. Radio and HXR images of a small selection of events are used to investigate the relative locations of the two emissions, and we discuss the physical conditions that affect the relationship between the HXR and radio emission. Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Christe, Steven; Glesener, L.; Krucker, S.; Ramsey, B.; Takahashi, T.; Lin, R. Bibcode: 2009SPD....40.3303C Altcode: The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost Access to Space sounding rocket payload scheduled for launch late 2010. FOXSI will provide imaging spectroscopy with high sensitivity ( 50 times RHESSI) and high dynamic range ( 100) in hard X-rays (HXR) up to 15 keV. For the first time, it will be possible to search for nonthermal emission of thermal network flares occurring in the quiet corona in order to determine whether they are similar to active region flares. Additionally, FOXSI will extend the active-region flare distribution to events two orders of magnitude smaller than previously observed and determine their contribution to coronal heating. FOXSI is able to achieve this unprecendeted advance in solar HXR observations through the combination of nested HXR optics developped by the Marshall Space Flight Center and novel silicon strip detectors provided by ISAS Japan. The FOXSI mission will provide HXR spectroscopic imaging with an angular resolution of 12" (FWHM) and 1 keV energy resolution. FOXSI will be a pathfinder for the future generation of solar HXR spectroscopic imagers. 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: White Light Emission from RHESSI flares Authors: Christe, Steven; Watanabe, K.; Krucker, S. Bibcode: 2009SPD....40.1922C Altcode: In this study we investigate the relationship between RHESSI flares and white light emission as observed by Hinode/SOT at 3968.6 angstroms (Ca II H line), 4306.4 angstroms (G band), and 4505.1 angstroms(blue continuum) images. The emission mechanism for white light emission in flares is not yet understood though it is believed that emission at these wavelengths originates from the chromosphere, photosphere, and deep photosphere respectively. Using a combination of the official RHESSI flare list and a microflare-finding algorithm, we investigate all RHESSI flares with simultaneous observations by RHESSI and Hinode/SOT (a total of 200 events). Single event studies suggest that microflares are frequently observed in Ca II H line. Photospheric emission is searched for using the location of the Ca II H line chromospheric emission. We discuss the physical conditions necessary that may affect the relationship between HXR and white light emission. Title: Statistical Study of Solar Flare Hard X-ray Spectra Authors: Alaoui, Meriem; Krucker, S.; Saint-Hilaire, P.; Lin, R. P. Bibcode: 2009SPD....40.1901A Altcode: We present a statistical study of solar flare hard X-ray spectra above 40 keV observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). At these energies, the hard X-ray emission is mostly produced by bremsstrahlung of suptrathermal electrons as the interact with the ambient plasma in the chromosphere. The observed photon spectra therefore provide diagnostics of electron acceleration processes in solar flare. We will present statistical results of broken power law fits of almost 100 flares with emission seen above 100 keV. The derived results will be compared to statistical results of electron spectra observed in interplanetary space. Title: A Statistical Study of Spectral Hardening in Solar Flares and Related Solar Energetic Proton Events Authors: Grayson, James; Krucker, S.; Lin, R. P. Bibcode: 2009SPD....40.1902G Altcode: We investigate the reliability of hard x-ray spectral hardening during solar flares as an indicator of related energetic proton events at Earth, using high-resolution observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Previous work by Kiplinger (1995) obtained as high as 96% success when associating hardening behavior with energetic proton events, suggesting a correlation between electron acceleration in solar flares and solar energetic protons seen in interplanetary space. Here we further this study, and more recent work with smaller sets of RHESSI observations, by investigating all observed large flares during RHESSI's lifetime. Our data set includes over 400 M- and X-class flares with at least partial coverage by RHESSI, approximately half of which allow us to determine spectral behavior accurately. Title: Multiwavelength Observation of Electron Acceleration in the 2006 December 13 Flare Authors: Minoshima, T.; Imada, S.; Morimoto, T.; Kawate, T.; Koshiishi, H.; Kubo, M.; Inoue, S.; Isobe, H.; Masuda, S.; Krucker, S.; Yokoyama, T. Bibcode: 2009ApJ...697..843M Altcode: 2009arXiv0903.1144M We present a multiwavelength observation of a solar flare occurring on 2006 December 13 with Hinode, RHESSI, and the Nobeyama Radio Observatory to study the electron acceleration site and mechanism. The Solar Optical Telescope (SOT) on board Hinode observed elongated flare ribbons, and RHESSI observed double-footpoint hard X-ray (HXR) sources appearing in part of the ribbons. A photospheric vector magnetogram obtained from SOT reveals that the HXR sources are located at the region where horizontal magnetic fields change direction. The region is interpreted as the footpoint of magnetic separatrix. Microwave images taken with the Nobeyama Radioheliograph show a loop structure connecting the HXR sources. The brighter parts of the microwave intensity are located between the top and footpoints of the loop. We consider these observations as evidence of electron acceleration near the magnetic separatrix and injection parallel to the field line. 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: 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: Imaging Spectroscopy on Preflare Coronal Nonthermal Sources Associated with the 2002 July 23 Flare Authors: Asai, Ayumi; Nakajima, Hiroshi; Shimojo, Masumi; Yokoyama, Takaaki; Masuda, Satoshi; Krucker, Säm Bibcode: 2009ApJ...695.1623A Altcode: 2009arXiv0901.3591A We present a detailed examination on the coronal nonthermal emissions during the preflare phase of the X4.8 flare that occurred on 2002 July 23. The microwave (17 GHz and 34 GHz) data obtained with Nobeyama Radioheliograph, at Nobeyama Solar Radio Observatory and the hard X-ray (HXR) data taken with RHESSI obviously showed nonthermal sources that are located above the flare loops during the preflare phase. We performed imaging spectroscopic analyses on the nonthermal emission sources both in microwaves and in HXRs, and confirmed that electrons are accelerated from several tens of keV to more than 1 MeV even in this phase. If we assume the thin-target model for the HXR emission source, the derived electron spectral indices (~4.7) is the same value as that from microwaves (~4.7) within the observational uncertainties, which implies that the distribution of the accelerated electrons follows a single power law. The number density of the microwave-emitting electrons is, however, larger than that of the HXR-emitting electrons, unless we assume low-ambient plasma density of about 1.0 × 109 cm-3 for the HXR-emitting region. If we adopt the thick-target model for the HXR emission source, on the other hand, the electron spectral index (~6.7) is much different, while the gap of the number density of the accelerated electrons is somewhat reduced. Title: Diagnostics of interplanetary and flaring plasmas in impulsive solar energetic particle events Authors: Kartavykh, Y. Y.; Dröge, W.; Klecker, B.; Mason, G. M.; Möbius, E.; Popecki, M.; Krucker, S. Bibcode: 2009BRASP..73..291K Altcode: No abstract at ADS Title: Sub-terahertz, Microwaves and High Energy Emissions During the 6 December 2006 Flare, at 18:40 UT Authors: Kaufmann, Pierre; Trottet, Gérard; Giménez de Castro, C. Guillermo; Raulin, Jean-Pierre; Krucker, Säm; Shih, Albert Y.; Levato, Hugo Bibcode: 2009SoPh..255..131K Altcode: 2008arXiv0811.3488K The presence of a solar burst spectral component with flux density increasing with frequency in the sub-terahertz range, spectrally separated from the well-known microwave spectral component, bring new possibilities to explore the flaring physical processes, both observational and theoretical. The solar event of 6 December 2006, starting at about 18:30 UT, exhibited a particularly well-defined double spectral structure, with the sub-THz spectral component detected at 212 and 405 GHz by the Solar Submilimeter Telescope (SST) and microwaves (1 - 18 GHz) observed by the Owens Valley Solar Array (OVSA). Emissions obtained by instruments onboard satellites are discussed with emphasis to ultra-violet (UV) obtained by the Transition Region And Coronal Explorer (TRACE), soft X-rays from the Geostationary Operational Environmental Satellites (GOES) and X- and γ-rays from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The sub-THz impulsive component had its closer temporal counterparts only in the higher energy X- and γ-rays ranges. The spatial positions of the centers of emission at 212 GHz for the first flux enhancement were clearly displaced by more than one arc-minute from positions at the following phases. The observed sub-THz fluxes and burst source plasma parameters were difficult to be reconciled with a purely thermal emission component. We discuss possible mechanisms to explain the double spectral components at microwaves and in the THz ranges. Title: A Statistical Study of Hard X-Ray Footpoint Motions in Large Solar Flares Authors: Yang, Ya-Hui; Cheng, C. Z.; Krucker, Säm; Lin, R. P.; Ip, W. H. Bibcode: 2009ApJ...693..132Y Altcode: A statistical analysis of the temporal evolution of hard X-ray (HXR) footpoint motions in 27 M- and X-class solar flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager is presented. Extreme UV images from TRACE and SOHO/EIT, Hα images, and magnetograms from SOHO/MDI are used to put the HXR footpoint motions in context of flare ribbons and the magnetic neutral line. Footpoint motions are often found to be complex making a statistical analysis difficult. In an attempt to simplify the analysis, each event was searched for motions predominantly parallel and predominantly perpendicular to the neutral line or flare ribbons. Four kinds of complex motions are described and their relationships to the possible magnetic reconnection processes are discussed. In the soft X-ray (SXR) rise phase, motions along the neutral line or flare ribbons are most common (20 out of 27) and only two events show perpendicular motions (for the remaining five events a simple classification was not possible). However, at later times around the SXR peak, perpendicular motion is more frequently observed (~40%) than motions along the neutral line or ribbons ~27%). The preference of HXR kernels appearing at the footpoints of highly-sheared magnetic loops at the start of the SXR rise phase is consistent with the magnetic reconnection theory that the reconnection occurring at sheared magnetic arcade field lines produces most HXR energy release in the impulsive phase of large flares. Title: The Focusing Optics X-ray Solar Imager Authors: Glesener, Lindsay; Krucker, S.; Christe, S.; Turin, P.; McBride, S. Bibcode: 2009AAS...21347508G Altcode: 2009BAAS...41..436G The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost Access to Space sounding rocket payload scheduled to fly in late 2010 to observe hard X-ray emission (HXR) from the quiet Sun. Particle acceleration in small "nanoflares" in the quiet Sun is thought to play an important role in the heating of the corona to millions of degrees Kelvin. FOXSI HXR observations of these flares will provide first estimates of the non-thermal energy content in small flares from the quiet Sun.

Imaging nanoflares requires high energy sensitivity and a large dynamic range. To date, the most sensitive HXR images are made using a rotating modulating collimator aboard the Reuven Ramaty High Energy Spectroscopic Imager satellite (RHESSI). However, the rotating modulation technique is intrinsically limited in sensitivity and dynamic range.

The focusing optics of FOXSI will achieve a sensitivity 100 times better than that of RHESSI at energies around 10 keV. FOXSI uses nested-shell, grazing-angle optics and silicon strip detectors to achieve an angular resolution of 12" (FWHM) and 1 keV energy resolution. FOXSI will observe the quiet Sun in the 4 to 15 keV range for 5 minutes. The focusing optics technique developed by FOXSI will prove useful to future solar HXR observing missions, especially those interested in imaging faint HXR emission from particle acceleration regions in the corona. Title: The Solar Flare: A Strongly Turbulent Particle Accelerator Authors: Vlahos, L.; Krucker, S.; Cargill, P. Bibcode: 2009LNP...778..157V Altcode: The topics of explosive magnetic energy release on a large scale (a solar flare) and particle acceleration during such an event are rarely discussed together in the same article. Many discussions of magnetohydrodynamic (MHD) mod- eling of solar flares and/or CMEs have appeared (see [143] and references therein) and usually address large-scale destabilization of the coronal mag- netic field. Particle acceleration in solar flares has also been discussed exten- sively [74, 164, 116, 166, 87, 168, 95, 122, 35] with the main emphasis being on the actual mechanisms for acceleration (e.g., shocks, turbulence, DC electric fields) rather than the global magnetic context in which the acceleration takes place. Title: Spectra of Solar Impulsive Electron Events Observed Near Earth Authors: Krucker, Säm; Oakley, P. H.; Lin, R. P. Bibcode: 2009ApJ...691..806K Altcode: A statistical survey of the spectral shapes of 62 solar impulsive electron events detected in the ~ 1 to 300 keV range near Earth by the three-dimensional Plasma and Energetic Particles experiment on the WIND spacecraft is presented. The electron peak flux spectra generally show a broken power-law dependence with a steepening above ~ 60 keV. The break in the spectrum is pronounced with averaged power-law indices below and above the break of δlow = 1.9 ± 0.3 and δhigh = 3.6 ± 0.7, respectively, and an averaged ratio δlow/δhigh of 0.54 with a standard deviation of 0.09. Two correlations are found: (1) δlow and δhigh are linearly correlated (correlation coefficient of 0.61), (2) The peak fluxes around the break energy and δlow are anticorrelated (coefficient 0.74), with larger events having flatter spectra below the break. Whether the observed spectral breaks are direct signatures of the solar acceleration processes or whether they are due to transport effects from the Sun to Earth is currently not understood. 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: Observational Study of Particle Acceleration in the 2006 December 13 Flare Authors: Minoshima, T.; Morimoto, T.; Kawate, T.; Imada, S.; Koshiishi, H.; Masuda, S.; Kubo, M.; Inoue, S.; Isobe, H.; Krucker, S.; Yokoyama, T. Bibcode: 2008AGUFMSH41B1619M Altcode: We study the particle acceleration in a flare on 2006 December 13, by using the Hinode, RHESSI, Nobeyama Radio Polarimeters (NoRP) and Nobeyama Radioheliograph (NoRH) observations. For technical reasons, both RHESSI and NoRH have a problem in imaging in this flare. Since we have succeeded in solving the problem, it is now possible to discuss the particle acceleration mechanism from an image analysis. This flare shows very long-lasting (1 hour) non-thermal emissions, consisting of many spikes. We focus on the second major spike at 02:29 UT, because the RHESSI image is available only in this period. The RHESSI 35-100 keV HXR image shows double sources located at the footpoints of the western soft X-ray (SXR) loop seen by the Hinode/XRT. The non-linear force-free (NLFF) modeling based on a magnetogram data by Inoue et al. shows the NLFF to potential magnetic transition of the loop, which would induce the electric field and then accelerate particles. Overlaying the HXR image on the photospheric three-dimensional magnetic field map taken by the Hinode Spectro-Polarimeter, we find that the HXR sources are located at the region where the horizontal magnetic fields invert. The NoRH 34 GHz microwave images show the loop structure connecting the HXR sources. The microwave peaks do not located at the top of the loop but between the loop top and the footpoints. The NoRP microwave spectrum shows the soft-hard-soft pattern in the period, same as the HXR spectrum (Ning 2008). From these observational results we suggest that the electrons were accelerated parallel to the magnetic field line near the magnetic separatrix. Title: The Microflare Height Distribution Authors: Christe, S. D.; Krucker, S. Bibcode: 2008AGUFMSH13A1510C Altcode: We present an in-depth statistical survey of flare heights of all X-ray microflares as observed by RHESSI between March 2002 and March 2007, a total of >25,000 events, an order of magnitude larger then previous studies. The microflares 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. The flare centroid position are found at the peak time and as a function of energy (3-6, 6-12, 12-25 keV). Flares are found to occur only in active regions, not in the "quiet" Sun. Flare heights are found using two independent methods including that of Matsushita (1992). The distribution of flares at the limb are fitted with a Monte Carlo simulations and the flare height deduced. We find that the 6-12 keV flare heights are consistent with a flare height of ~3 arcseconds above the photosphere with small separation between the thermal and nonthermal sources. The Matsushita method confirm these values. Title: A RHESSI search for chromospheric evaporation in super-hot flares Authors: Caspi, A.; Krucker, S.; Lin, R. P.; McTiernan, J. M. Bibcode: 2008AGUFMSH51C..04C Altcode: Chromospheric evaporation (CE) - thought to occur when downward-accelerated coronal electrons impact the denser chromosphere, heating the ambient material which then rises to fill the flaring loop - has often been suggested as the primary source of hot thermal looptop plasma. Evidence for CE is given by crystal spectrometer observations of blueshifted spectral lines from footpoints. RHESSI observations show that peak flare temperatures generally occur soon after the hard X-ray (HXR; >20~keV) peak, when the emission measure is still only ~20% of its later peak value. Imaging places the thermal plasma at the looptop even for large (GOES X-class) flares that exceed super-hot (Te > 30~MK) temperatures. In such flares, non-thermal energy deposition can exceed ~1029~erg/s, which for typical electron energies, chromospheric densities, and footpoint areas should yield CE temperatures of ~10-20~MK with emission measures of ~1050~cm-3. Such bright thermal emission has never been observed from footpoints, suggesting that while CE may contribute the bulk loop material, heating occurs primarily in the corona. We present an analysis of selected RHESSI M- and X-class flares, employing imaging spectroscopy to determine the temperature and emission measure (or limits thereof) of spatially-separated footpoint and looptop sources. We compare the time evolution of the thermal footpoint signatures to HXR lightcurves and to the looptop thermal component. We calculate the energy contained in non-thermal electrons, the thermal energies at the looptop and footpoints, and discuss the implications for heating by non-thermal electrons and the contribution of CE to the thermal flare plasma. Title: Beam speeds and Langmuir wave growth in interplanetary type III bursts Authors: Bale, S. D.; Krucker, S. Bibcode: 2008AGUFMSH24A..08B Altcode: We use measurements from the Wind spacecraft to study Langmuir wave growth in the source region of four separate in situ interplanetary type III bursts. Velocity distributions of energetic electron are used to compute the advection beam speed and growth rate (in the cold beam approximation) and these beam speeds and growth rates are compared with Langmuir wave activity and detailed features of the Langmuir wave spectra and waveforms. Knowledge of the electron beam speed lets us compute the resonant wavenumber and the expected spacecraft frequency of both daughter Langmuir waves and ion acoustic waves required for the parametric decay process. We find no strong evidence for decay products nor carrier splitting that is consistent with parametric decay. On the other hand, many of the Langmuir waveforms show 'polarization' signatures that may be consistent with linear mode conversion or trapped eigenmode interpretations of type III wave processes. Title: Hard X-ray Emission From Partially Occulted Solar Flares Authors: Glesener, L.; Krucker, S.; Lin, R. P. Bibcode: 2008AGUFMSH13B1540G Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is optimized for fine energy and spatial resolution of hard X-ray (HXR) emission from solar flares. Most RHESSI flares exhibit strong HXR emission from bright flare loop footpoints, drowning out faint emission from higher in the corona. Partial occultation of flares by the solar disk obscures these bright footpoints, allowing for detailed observations of faint coronal HXR sources. It is therefore necessary to identify and characterize flares with high occultation heights. Here we present a technique for determining flare occultation heights by extrapolating the paths of active flare regions across the solar disk. A statistical study of 55 flares compares occultation heights to nonthermal X-ray emission. Compared with statistical results of on-disk flares, it is estimated that this coronal emission is at least 20 times fainter than the expected footpoint emission, confirming that partial occultation is crucial for the study of faint coronal sources. HXR images made by RHESSI are also compared with images at longer wavelengths from the TRACE, EIT, and STEREO instruments. Title: Evidence for the Magnetic Trapping of Solar-Flare Ions from 1-8-MeV Solar Neutrons Detected with the MESSENGER Neutron Spectrometer Authors: Feldman, W. C.; Lawrence, D. J.; Goldsten, J. O.; Gold, R. E.; Baker, D. N.; Haggerty, D. K.; Krucker, S.; Lin, R. P.; Murphy, R. J.; Nittler, L. R.; Slavin, J. A.; Solomon, S. C.; Starr, R. D.; Vilas, F.; Vourlidas, A. Bibcode: 2008AGUFM.U12A..02F Altcode: Neutrons produced on the Sun during the M2 flare on 31 December 2007 were observed throughout an approximately 10-hour period at 0.48 AU by the MESSENGER Neutron Spectrometer. This flare contained multiple acceleration episodes as seen in Type III radio bursts and fine structure in the energetic-particle environment at MESSENGER. After these bursts ended, both the energetic-particle and neutron fluxes decayed smoothly to background with an e-folding decay time of 2.84 hours. The fact that this time is considerably longer than the mean life of a neutron (886 s) indicates that neutrons at the Sun must have been continuously produced. A likely explanation is that a considerable fraction of the energetic ions accelerated during the flare remained trapped on closed magnetic arcades high in the corona and were slowly pitch-angle scattered by coronal turbulence into their chromospheric loss cones. Subsequent interactions with chromospheric ions generated neutrons that scatter to form a population of upward-going albedo neutrons. Title: Magnetic flux cancellation associated with a recurring solar jet observed with Hinode, RHESSI, and STEREO/EUVI Authors: Chifor, C.; Isobe, H.; Mason, H. E.; Hannah, I. G.; Young, P. R.; Del Zanna, G.; Krucker, S.; Ichimoto, K.; Katsukawa, Y.; Yokoyama, T. Bibcode: 2008A&A...491..279C Altcode: Aims: We study the physical properties of a recurring solar active region jet observed in X-rays and extreme-ultraviolet (EUV).
Methods: Multi-wavelength data from all three instruments on board Hinode were analysed. X-ray imaging and spectroscopy of the microflaring emission associated with the jets was performed with the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI). Associated EUV jets were observed with the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)/Extreme Ultraviolet Imager (EUVI) on board STEREO.
Results: We found a correlation between recurring magnetic flux cancellation close to a pore, the X-ray jet emission, and associated Ca II H ribbon brightenings. We estimated the lower limit for the decrease in magnetic energy associated with the X-ray jet emission at 3 × 1029 erg. The recurring plasma ejection was observed simultaneously at EUV and X-ray temperatures, associated with type III radio bursts and microflaring activity at the jet footpoint.
Conclusions: The recurring jet (EUV and X-ray) emissions can be attributed to chromospheric evaporation flows due to recurring coronal magnetic reconnection. In this process, the estimated minimum loss in the magnetic energy is sufficient to account for the total energy required to launch the jet.

Movie of Fig. 3 is only available in electronic form via http://www.aanda.org 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: STEREO and RHESSI Observations of Electron Acceleration in a Partially Disk-Occulted Solar Flare Authors: Krucker, S.; Wuelser, J. -P.; Vourlidas, A.; Davila, J.; Thompson, W. T.; White, S.; Lin, R. P. Bibcode: 2008ESPM...12.2.84K Altcode: RHESSI hard X-ray observations of partially-disk occulted solar flares provide crucial information on faint coronal hard X-ray sources in the absence of generally much brighter emissions from footpoints of flare loops. Coronal hard X-ray sources can differ fundamentally from the classical footpoint sources of the flare impulsive phase and provide unique information about the supra-thermal electrons closest to the site in the corona where their acceleration is believed to occur. The different view-angles provided by the STEREO spacecraft allow us to put the partially occulted hard X-ray sources observed by RHESSI in context with the EUV flare ribbons and the EUV emission from CME observed by STEREO/EUVI.

In this presentation we report on the GOES C8 flare observed on December 31, 2007 peaking around 01:11UT. From Earth-view (RHESSI), the flare occurs about 12 degrees behind the eastern limb giving an occultation height of 16 Mm. From STEREO B, the flare ribbons are seen on the disk (about 10 degrees from the limb), while the flare is highly occulted (130 Mm) for STEREO A observations so that emissions related to the associated CME are seen. Despite the occultation, RHESSI observes strong non-thermal emissions up to 100 keV that entirely originate from the corona. Initially, the coronal hard X-ray emission is seen from above the EUV flare ribbons similar to what is reported in the Masuda flare. Later on, emissions from a radially extended (approximately 20 Mm) source is seen. The radial extension is in the same direction as the current sheet of the outward moving CME suggesting that the HXR emission might be a direct signature of electrons accelerated in the reconnection process. 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: Open magnetic flux tubes in the corona and the transport of solar energetic particles Authors: Klein, K. -L.; Krucker, S.; Lointier, G.; Kerdraon, A. Bibcode: 2008A&A...486..589K Altcode: We investigate how magnetic fields guide energetic particles through the corona into interplanetary space and eventually to a spacecraft near the Earth. A set of seven simple particle events is studied, where energetic electrons (30-500 keV; Wind spacecraft) or protons (5-55 MeV; SoHO) were released together with low-energy electron beams producing metric-to-kilometric type III emission. Imaging of the coronal (metre-wave) part of this emission with the Nançay Radioheliograph is used to identify the open flux tubes that guide these electrons - and by inference all particles detected at 1 AU. Open coronal field lines are also computed using potential magnetic field extrapolations, constrained by a source surface and by SoHO/MDI measurements in the photosphere (code by Schrijver and DeRosa). We find that in all events the type III radio sources lie in open flux tubes in the potential magnetic field extrapolations. The open flux tubes are rooted in small parts of the parent active region, covering a heliocentric angle of a few degrees in the photosphere. But they expand rapidly above the neighbouring closed magnetic structures and cover several tens of degrees in longitude on the source surface. Some of these open field lines are found to connect the parent active region to the footpoint of the nominal Parker spiral on the source surface, within the uncertainty of about ±10° inherent to the evaluation of its connection longitude. This is so even when the parent active region is as far as 50° away. In two cases where the coronal flux tubes point to high heliolatitudes, the detection of Langmuir waves at the Wind spacecraft in the ecliptic plane suggests that the interplanetary field lines curve down to the ecliptic before reaching 1 AU. We conclude that non-radial open flux tubes in the corona can transport particles over several tens of degrees in longitude even in simple impulsive particle events. In all events we studied, potential magnetic field models give an adequate description of these structures. Title: A Statistical Survey of Hard X-ray Spectral Characteristics of Solar Flares with Two Footpoints Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P. Bibcode: 2008SoPh..250...53S Altcode: 2008SoPh..tmp...93S; 2011arXiv1111.4247S Using RHESSI data, we have analyzed 172 hard X-ray (HXR) peaks during 53 solar flares that exhibited a double-footpoint structure. Fitting both footpoints with power laws, we find that spectral index differences Δγ range mostly between 0 to 0.6, and only rarely go beyond. Asymmetries between footpoints were not observed to be significantly dependent on their mean heliographic position, their relative position with respect to each other, nor their orientation with respect to the solar equator. By assuming a symmetric acceleration process, it is also clear that differences in footpoint spectral indices and footpoint flux ratios can seldom be attributed to a difference in column densities between the two legs of a coronal loop. Our results corroborate better the magnetic mirror trap scenario. Moreover, footpoint asymmetries are more marked during times of peak HXR flux than when averaging over the whole HXR burst, suggesting that the magnetic configuration evolves during individual HXR bursts. We also observed a linear correlation between the peak 50-keV flux and the peak GOES 1 - 8 Å channel flux and that HXR burst duration seem correlated with loop length. Title: Coronal Hard X-Ray Emission Associated with Radio Type III Bursts Authors: Krucker, Säm; Saint-Hilaire, P.; Christe, S.; White, S. M.; Chavier, A. D.; Bale, S. D.; Lin, R. P. Bibcode: 2008ApJ...681..644K Altcode: We report on a purely coronal hard X-ray source detected in a partially disk-occulted solar flare by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) that is associated with radio type III bursts and a suprathermal electron event detected near 1 AU by the WIND 3-D Plasma and Energetic Particle (3DP) instrument. Several observational characteristics suggest that the coronal hard X-ray source is thin target bremsstrahlung emission from the escaping electrons that produce the radio type III bursts. The hard X-ray emission correlates in time with the radio type III bursts and originates from a radially elongated source in the corona with a length (~65 Mm) similar to typical coronal density scale heights. Furthermore, the difference between the hard X-ray photon spectral index (γ = 4.1 +/- 0.4) and the electron spectral index of the in situ observed event (δin situ = 2.9 +/- 0.3) is around 1, consistent with the thin target interpretation. A further test for the thin target scenario is to compare the number of electrons needed to produce the observed hard X-ray emission with the number of in situ observed electrons. However, the number of escaping electrons derived from the single-spacecraft WIND measurement is in the best case an order of magnitude estimate and could easily underestimate the actual number of escaping electrons. Using the WIND observations, the estimated number of escaping electrons is about an order of magnitude too low. Thus, the thin target interpretation only holds if the WIND measurements are significantly underestimating the actual number of escaping electrons. Future multispacecraft observations with STEREO, Solar Orbiter, and Sentinels will resolve this uncertainty. Title: Hard X-Rays Associated with Type III Radio Bursts Authors: Christe, S.; Krucker, S.; Lin, R. P. Bibcode: 2008ApJ...680L.149C Altcode: During a period of 12 minutes on 2002 July 19 14:23-14:35 UT, the WAVES instrument on WIND observed six interplanetary type III radio bursts, one approximately every 2 minutes, and each was accompanied by a weak hard X-ray (HXR) burst (12-15 keV) observed by RHESSI. The radio bursts are observed up to 150 MHz with some up to 600 MHz. Simultaneous observations by TRACE show jetlike eruptions emanating from the region of HXR emission. The observed HXRs are inconsistent with emission from the escaping type III-producing nonthermal electrons. We suggest that the type III acceleration process may be associated with an explosive release of lsim5 × 1026 ergs in the form of a "superhot" (26 MK) thermal plasma in the corona, an energy comparable to that associated with the type III-producing electrons. Title: Coronal γ-Ray Bremsstrahlung from Solar Flare-accelerated Electrons Authors: Krucker, Säm; Hurford, G. J.; MacKinnon, A. L.; Shih, A. Y.; Lin, R. P. Bibcode: 2008ApJ...678L..63K Altcode: The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) provides for the first time imaging spectroscopy of solar flares up to the γ-ray range. The three RHESSI flares with best counting statistics are analyzed in the 200-800 keV range revealing γ-ray emission produced by electron bremsstrahlung from footpoints of flare loops, but also from the corona. Footpoint emission dominates during the γ-ray peak, but as the γ-ray emission decreases the coronal source becomes more and more prominent. Furthermore, the coronal source shows a much harder spectrum (with power-law indices γ between 1.5 and 2) than the footpoints (with γ between 3 and 4). These observations suggest that flare-accelerated high-energy (~MeV) electrons stay long enough in the corona to lose their energy by collisions producing γ-ray emission, while lower energetic electrons precipitate more rapidly to the footpoints. Title: STEREO and RHESSI Observations of a Partially Disk-Occulted Solar Flare Authors: Krucker, S.; Wuelser, J.; Lin, R. R. Bibcode: 2008AGUSMSP44A..02K Altcode: RHESSI hard X-ray observations of partially-disk occulted solar flares provide crucial information on faint coronal hard X-ray sources in the absence of generally much brighter emissions from footpoints of flare loops. Coronal hard X-ray sources can differ fundamentally from the classical footpoint sources of the flare impulsive phase and provide unique information about the supra-thermal electrons closest to the site in the corona where their acceleration is believed to occur. The different view-angles provided by the STEREO spacecraft allow us to put the partially occulted hard X-ray sources observed by RHESSI in context with the EUV flare ribbons and the EUV emission from CMEs observed by STEREO/EUVI. In this talk we report on the GOES C8 flare observed on December 31, 2007 peaking around 01:11UT. From Earth-view (RHESSI), the flare occurs about 12 degrees behind the eastern limb giving an occultation height of 16 Mm. From STEREO B, the flare ribbons are seen on the disk (about 10 degrees from the limb), while the flare is highly occulted (130 Mm) for STEREO A observations so that emissions related to the associated CME are seen. Despite the occultation, RHESSI observes strong non-thermal emissions up to 100 keV that entirely originate from the corona. Initially, the coronal hard X-ray emission is seen from above the EUV flare ribbons, while later on, emissions from a radially extended (~20 Mm) source located about 20 Mm to the south of the flare ribbon are seen. Preliminary results suggest that the elongated hard X-ray source is possibly related to the CME. Title: Radio Submillimeter and γ-Ray Observations of the 2003 October 28 Solar Flare Authors: Trottet, G.; Krucker, Säm; Lüthi, T.; Magun, A. Bibcode: 2008ApJ...678..509T Altcode: Radio observations at 210 GHz taken by the Bernese Multibeam Radiometer for KOSMA (BEMRAK) are combined with hard X-ray and γ-ray observations from the SONG instrument on board CORONA-F and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to investigate high-energy particle acceleration during the energetic solar flare of 2003 October 28. Two distinct components at submillimeter wavelengths are found. The first is a gradual, long-lasting (>30 minutes) component with large apparent source sizes (~60''). Its spectrum below ~200 GHz is consistent with synchrotron emission from flare-accelerated electrons producing hard X-ray and γ-ray bremsstrahlung assuming a magnetic field strength of >=200 G in the radio source and a confinement time of the radio-emitting electrons in the source of less than 30 s. The other component is impulsive and starts simultaneously with high-energy (>200 MeV nucleon-1) proton acceleration and the production of pions. The derived radio source size is compact (<=10''), and the emission is cospatial with the location of precipitating flare-accelerated >30 MeV protons as seen in γ-ray imaging. The close correlation in time and space of radio emission with the production of pions suggests that synchrotron emission of positrons produced in charged-pion decay might be responsible for the observed compact radio source. However, order-of-magnitude approximations rather suggest that the derived numbers of positrons from charged-pion decay are probably too small to account for the observed radio emission. Synchrotron emission from energetic electrons therefore appears as the most likely emission mechanism for the compact radio source seen in the impulsive phase, although it does not account for its close correlation, in time and space, with pion production. Title: RHESSI Hard X-ray Microflare Statistics Authors: Christe, S. D.; Hannah, I.; Krucker, S.; Hurford, G.; McTiernan, J.; Lin, R. P. Bibcode: 2008AGUSMSP51C..09C Altcode: We present the first in-depth statistical survey of all X-ray microflares observed by RHESSI between March 2002 and March 2007, a total of 25,705 events. These microflares were found using a new flare-finding algorithm designed to search the 6--12~keV count rate when RHESSIs full sensitivity was available in order to find the smallest events. These microflares are small flares, from low GOES C Class to below A Class (background subtracted) and associated with active regions. Each microflare is automatically analyzed at the peak time of the 6--12~keV emission. Spectral parameters are determined both by considering ratios between energy channels and by forward fitting a thermal plus non-thermal model. Flare images are created using back-projection and centroids and by forward fitting the complex visibilities. The combination of imaging and spectral parameters allow for the first time analysis of the thermal and non-thermal energy at the peak time of these microflares. Title: Electron beam velocities derived from Wind Type III radio bursts. Authors: Saint-Hilaire, P.; Krucker, S.; Bale, S. Bibcode: 2008AGUSMSP51C..13S Altcode: Using a novel approach based on the Radon Transform to identify radio Type III bursts observed by the Waves instrument on board the Wind spacecraft, we find and characterize all interplanetary Type III bursts observed by Wind since its launch. We will be interested in those whose exciter also struck the spacecraft, allowing us to make a statistical study of exciter velocities spanning more that a solar cycle, for the first time. Title: Can chromospheric evaporation explain superhot flare plasmas? Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2008AGUSMSP44A..03C Altcode: Most flares show strong soft X-ray (SXR; <20 keV) emission from a hot looptop source and hard X-ray (HXR; >20 keV) emission from footpoint sources. The latter is generally interpreted as non-thermal bremsstrahlung from downward-accelerated coronal electrons impacting the denser chromosphere, where they deposit most of their kinetic energy as heat. Heated chromospheric material then rises and fills the flare loop - a process known as chromospheric evaporation (CE). Evidence for CE is given by crystal spectrometer observations of blueshifted spectral line emission from footpoints, suggesting upward motion of hot evaporated plasma. CE has often been thought to be the primary source of thermal looptop plasma. In large (GOES X-class) flares, non-thermal energy deposition can be as much as ~1e29~erg/s. For typical non-thermal electron energies, chromospheric densities, and footpoint areas, this input power should yield ~5-20~MK CE material at the footpoints, but with an emission measure calculated at ~1e50~cm- 3. Such bright thermal emission should be clearly visible in RHESSI imaging spectra of large flares, but no such emission has yet been observed. This suggests that while CE may indeed be the primary source of thermal loop plasma, heating by non-thermal electrons may be less efficient than once believed. A heating efficiency of only ~10-50% would result in a CE emission measure of only ~1e48~cm-3, which would be more consistent with observed footpoint fluxes. We present an analysis of selected RHESSI flares, ranging in GOES class from ~M1 to ~X17. We employ imaging spectroscopy to obtain spectra of spatially-separated footpoints and looptop sources to determine the strength of the thermal components in each. The time evolution of the thermal footpoint signatures is compared to that of the HXR emission as well as to the looptop thermal emission and temperature. We calculate the energy contained in non-thermal electrons, the thermal energies at the footpoints and the looptop, and discuss the implications for the efficiency of heating by non-thermal electrons and the contribution of chromospheric evaporation to the flare thermal plasma. Title: S/WAVES: The Radio and Plasma Wave Investigation on the STEREO Mission Authors: Bougeret, J. L.; Goetz, K.; Kaiser, M. L.; Bale, S. D.; Kellogg, P. J.; Maksimovic, M.; Monge, N.; Monson, S. J.; Astier, P. L.; Davy, S.; Dekkali, M.; Hinze, J. J.; Manning, R. E.; Aguilar-Rodriguez, E.; Bonnin, X.; Briand, C.; Cairns, I. H.; Cattell, C. A.; Cecconi, B.; Eastwood, J.; Ergun, R. E.; Fainberg, J.; Hoang, S.; Huttunen, K. E. J.; Krucker, S.; Lecacheux, A.; MacDowall, R. J.; Macher, W.; Mangeney, A.; Meetre, C. A.; Moussas, X.; Nguyen, Q. N.; Oswald, T. H.; Pulupa, M.; Reiner, M. J.; Robinson, P. A.; Rucker, H.; Salem, C.; Santolik, O.; Silvis, J. M.; Ullrich, R.; Zarka, P.; Zouganelis, I. Bibcode: 2008SSRv..136..487B Altcode: 2008SSRv..tmp....9B This paper introduces and describes the radio and plasma wave investigation on the STEREO Mission: STEREO/WAVES or S/WAVES. The S/WAVES instrument includes a suite of state-of-the-art experiments that provide comprehensive measurements of the three components of the fluctuating electric field from a fraction of a hertz up to 16 MHz, plus a single frequency channel near 30 MHz. The instrument has a direction finding or goniopolarimetry capability to perform 3D localization and tracking of radio emissions associated with streams of energetic electrons and shock waves associated with Coronal Mass Ejections (CMEs). The scientific objectives include: (i) remote observation and measurement of radio waves excited by energetic particles throughout the 3D heliosphere that are associated with the CMEs and with solar flare phenomena, and (ii) in-situ measurement of the properties of CMEs and interplanetary shocks, such as their electron density and temperature and the associated plasma waves near 1 Astronomical Unit (AU). Two companion papers provide details on specific aspects of the S/WAVES instrument, namely the electric antenna system (Bale et al., Space Sci. Rev., 2007) and the direction finding technique (Cecconi et al., Space Sci. Rev., 2007). 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: RHESSI Microflare Statistics. I. Flare-Finding and Frequency Distributions Authors: Christe, S.; Hannah, I. G.; Krucker, S.; McTiernan, J.; Lin, R. P. Bibcode: 2008ApJ...677.1385C Altcode: We present the first in-depth statistical survey of all X-ray microflares observed by RHESSI between 2002 March and 2007 March, a total of 25,705 events, an order of magnitude larger then previous studies. These microflares 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. The peak and total count rate are automatically obtained along with count spectra at the peak and the microflare centroid position. Our microflare magnitudes are below GOES C class, on average GOES A class (background subtracted). They are found to occur only in active regions, not in the "quiet" Sun, and are similar to large flares. The monthly average microflaring rate is found to vary with the solar cycle and ranges from 90 to 5 flares a day during active and quiet times, respectively. Most flares are found to be impulsive (74%), with rise times shorter than decay times. The mean flare duration is ~6 minutes with a 1 minute minimum set by the flare-finding algorithm. The frequency distributions of the peak count rate in the energy bands, 3-6, 6-12, and 12-25 keV, can be represented by power-law distributions with a negative power-law index of 1.50 +/- 0.03, 1.51 +/- 0.03, and 1.58 +/- 0.02, respectively. We find that these power-law indices are constant as a function of time. The X-ray photon spectra for individual events can be approximated with a power-law spectrum [dJ/d(hν) ~ (hν)-γ]. Using the ratio of photon fluxes between 10-15 and 15-20 keV, we find 4 < γ < 12, with an average of 7.4. Based on these values, the nonthermal power is calculated. The microflare occurrence frequency varies with the rate of energy release consistent with a power law with an exponent of -1.7 +/- 0.1. We estimate the total energy flux deposited in active regions by microflare-associated accelerated electrons (>10 keV) over the five years of observations to be, on average, below 1026 erg s-1. Title: Coronal Jet Observed by Hinode as the Source of a3He-rich Solar Energetic Particle Event Authors: Nitta, Nariaki V.; Mason, Glenn M.; Wiedenbeck, Mark E.; Cohen, Christina M. S.; Krucker, Säm; Hannah, Iain G.; Shimojo, Masumi; Shibata, Kazunari Bibcode: 2008ApJ...675L.125N Altcode: We study the solar source of the 3He-rich solar energetic particle (SEP) event observed on 2006 November 18. The SEP event showed a clear velocity dispersion at energies below 1 MeV nucleon-1, indicating its solar origin. We associate the SEP event with a coronal jet in an active region at heliographic longitude of W50°, as observed in soft X-rays. This jet was the only noticeable activity in full-disk X-ray images around the estimated release time of the ions. It was temporally correlated with a series of type III radio bursts detected in metric and longer wavelength ranges and was followed by a nonrelativistic electron event. The jet may be explained in terms of the model of an expanding loop reconnecting with a large-scale magnetic field, which is open to interplanetary space for the particles to be observed at 1 AU. The open field lines appear to be anchored at the boundary between the umbra and penumbra of the leading sunspot, where a brightening is observed in both soft and hard X-rays during the jet activity. Other flares in the same region possibly associated with 3He-rich SEP events were not accompanied by a jet, indicative of different origins of this type of SEP event. Title: Hard X-Ray Spectral Evolution and Production of Solar Energetic Particle Events during the January 2005 X-Class Flares Authors: Saldanha, R.; Krucker, Säm; Lin, R. P. Bibcode: 2008ApJ...673.1169S Altcode: High-resolution hard X-ray observations provided by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are used to study the spectral evolution of ~50-200 keV nonthermal electron bremsstrahlung emissions of five X-class flares observed during the January 2005 solar storm events. Four of these flares show progressive spectral hardening during at least some hard X-ray peaks, while only one event shows the otherwise more commonly observed soft-hard-soft behavior. Imaging observations reveal that ~50-100 keV nonthermal electron bremsstrahlung emissions originate from footpoints of flare loops at all times, including during times of progressive spectral hardening, indicating that the spectral hardening component is produced by precipitating electrons, and not by electrons trapped in the corona. The four flares with progressive spectral hardening are all related to solar energetic particle (SEP) events, while the only X-class flare with soft-hard-soft behavior is not. This finding is consistent with earlier studies, suggesting that electron acceleration and transport in flares is somehow linked to the production of SEPs escaping into interplanetary space. Title: Hard X-Ray Emissions from Partially Occulted Solar Flares Authors: Krucker, Säm; Lin, R. P. Bibcode: 2008ApJ...673.1181K Altcode: Observations of solar flares partially occulted by the solar limb provide diagnostics of coronal hard X-ray (HXR) emissions in the absence of generally much brighter emissions from footpoints of flare loops. In this paper, a statistical survey of 55 partially occulted flares observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) is presented, revealing the existence of two different components of coronal HXR emissions. Below ~15 keV thermal HXR emission with a gradual time profile is generally dominant, while at higher energies an additional component is seen in 50 out of 55 events. This additional component shows faster time variations in the order of tens of seconds and is most prominent during the rise of the thermal emission. A comparison of the centroid positions of these two emissions shows that they are most often cospatial within ~2000 km, although for a few events clear separations are observed as well. The spectra of the high-energy component show a rather steep (soft) power law with indices mostly between ~4 and ~6. Thin target emission in the corona from flare-accelerated electrons is discussed as a possible origin of the fast time variation component. Title: Ion acceleration and neutral emission mechanisms for 2005 September 7 flare Authors: Watanabe, K.; Murphy, R. J.; Share, G. H.; Hurford, G. J.; Krucker, S.; Lin, R. P.; Harris, M. J.; Gros, M.; Sako, T.; Muraki, Y.; Matsubara, Y.; Shibata, S.; Sakai, T.; Valdés-Galicia, J. F.; González, L. X.; Hurtado, A.; Musalem, O.; Velarde, A.; Ticona, R.; Martinic, N.; Miranda, P. P.; Kakimoto, F.; Tsunesada, Y.; Tokuno, H.; Ogio, S. Bibcode: 2008ICRC....1...45W Altcode: 2008ICRC...30a..45W In association with an X17.0 flare on 2005 September 7, strong neutral emissions were detected both in space and on the ground. In space, intense emissions of gamma-rays were registered by INTEGRAL and by RHESSI during the decay phase. Gamma-ray lines at 0.511, 2.2, 4.4, and 6.1 MeV were observed and there was evidence for pion-decay radiation. On the ground, relativistic neutrons were observed by the neutron monitors at Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at Chacaltaya and Mt. Sierra Negra. The neutron signal continued for more than 20 minutes with high statistical significance. The long decay of the signals suggests that ions were continuously accelerated or trapped in the emission site. We also find that gamma-rays were emitted over a corresponding extended period. Only when we in cooperate the high-energy gamma-ray emission time history can we explain the long-lasting neutron emission. We also use the Hua et al. (2002) solar-flare magnetic loop transport and interaction model to find the best model to explain the data. Title: Physics of ion acceleration in the solar flare on 2005 September 7 determines gamma-ray and neutron production Authors: Watanabe, Kyoko; Krucker, Sam; Lin, Robert; Murphy, Ronald; Share, Gerald; Harris, Michael; Gros, Maurice Bibcode: 2008cosp...37.3429W Altcode: 2008cosp.meet.3429W Relativistic neutrons were observed by the neutron monitors at Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at Chacaltaya and Mt. Sierra Negra in association with an X17.0 flare on 2005 September 7. The neutron signal continued for more than 20 minutes with high statistical significance. Intense emissions of gamma-rays were also registered by INTEGRAL, and by RHESSI during the decay phase. We analyzed these data using the solar-flare magnetic-loop transport and interaction model of Hua et al. (2002), and found that the model could successfully fit the data with intermediate values of loop magnetic convergence and pitch angle scattering parameters. These results indicate that solar neutrons were produced at the same time as the gamma-ray line emission and that ions were continuously accelerated at the emission site. Title: Evidence of a Two-Temperature Source Region in the 3He-Rich Solar Energetic Particle Event of 2000 May 1 Authors: Kartavykh, Y. Y.; Dröge, W.; Klecker, B.; Mason, G. M.; Möbius, E.; Popecki, M.; Krucker, S. Bibcode: 2007ApJ...671..947K Altcode: Using instruments on the ACE and Wind spacecraft, we investigate the temporal evolution, spectra, and ionization states of solar energetic particle (SEP) Fe in the impulsive event of 2000 May 1. Proton and electron intensities and anisotropies were used to help constrain the characteristics of the interplanetary propagation, taking into account focusing, pitch-angle scattering, adiabatic deceleration, and convection. We find that the event was nearly scatter-free, with an interplanetary scattering mean free path larger than 1 AU. The Fe spectrum spectral form is consistent with stochastic acceleration, but the observed increase of the ionization state of Fe between 200-600 keV nucleon-1 is larger than can be explained using a single temperature source even after including the effect of adiabatic deceleration in the solar wind. A two-temperature source region is required to fit the observed range of Fe charge states, with the bulk (>80%) of the particles coming from a T~106 K region, and the remainder from a region with T~1.6×107 K. 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: RHESSI and Hinode X-Ray Observations of a Partially Occulted Solar Flare Authors: Krucker, Säm; Hannah, I. G.; Lin, R. P. Bibcode: 2007ApJ...671L.193K Altcode: This Letter presents X-ray observations of a partially limb-occulted solar flare taken by the Reuven Ramaty High-Energy Solar Spectroscopic Imager and the X-ray telescope on board Hinode. Thermal emission originates from a simple loop at the western limb that rises slowly (~7 km s-1) until the flare peak time. Above 18 keV, faint nonthermal emission with a hard/flat spectrum (γ~4) and fast time variations (of the order of tens of seconds) is seen that comes from a loop slightly above (~2000 km) the thermal loop, if compared at the same time. However, the nonthermal loop agrees well in altitude with the thermal flare loop seen later, at the soft X-ray peak time. This is consistent with simple flare models where nonthermal electrons in a flare loop produce thin-target hard X-ray emission in the corona as they travel to the loop footpoints. There they lose all their energy and heat chromospheric plasma that fills the loop earlier seen in nonthermal hard X-rays. This suggests that electron acceleration in solar flares occurs in the corona. Title: HINODE and RHESSI Observations of Partially Occulted Solar Flares Authors: Krucker, S.; Hannah, I. G.; Lin, R. P. Bibcode: 2007AGUFMSH52C..04K Altcode: We present X-ray imaging and spectral observations of a series of partially occulted solar flares observed by HINODE/XRT and RHESSI on November 21, 2006. Despite that the footpoints of the flare loops are occulted, non-thermal hard X-ray emissions from flare-accelerated electrons are observed in all of these flares. Although the non-thermal emissions are faint, their spectra are rather flat/hard with power law indices between ~4 and ~5. All events look similar with a simple thermal loop seen at the limb that rises slowly (~10 km/s) during the impulsive phase. The non-thermal emission can only be imaged when averaged in time over the impulsive phase, but it originate from a similarly sized loop. Compared at the same time, the non-thermal loop is about 1500 km higher up than the thermal loop. However, the location of the non-thermal loop seen in the impulsive phase is about at the same altitude as the thermal loop seen at the soft X-ray peak time. These observations are consistent with the standard flare model where flare-accelerated electrons produce faint non-thermal hard X-ray emissions in flare loops (thin target emission) and then lose their energy in the footpoints of these loops and heat chromospheric plasma. Evaporation of heated chromospheric plasma then fills up the flare loops earlier seen in non-thermal hard X-rays and makes it visible in thermal X-ray emission. Title: RHESSI Hard X-ray Microflare Statistics Authors: Christe, S.; Hannah, I.; Krucker, S.; Hurford, G.; McTiernan, J.; Lin, R. Bibcode: 2007AGUFMSH22A0849C Altcode: We present the first in-depth statistical survey of all X-ray microflares observed by RHESSI between March 2002 and March 2007, a total of 25,705 events. These microflares were found using a new flare-finding algorithm designed to search the 6--12~keV count rate when RHESSIs full sensitivity was available in order to find the smallest events. These microflares are small flares, from low GOES C Class to below A Class (background subtracted) and associated with active regions. Each microflare is automatically analyzed at the peak time of the 6--12~keV emission. Spectral parameters are determined both by considering ratios between energy channels and by forward fitting a thermal plus non-thermal model. Flare images are created using back-projection and centroids and by forward fitting the complex visibilities. The combination of imaging and spectral parameters allow for the first time analysis of the thermal and non-thermal energy at the peak time of these microflares. Title: Statistics of Hot Plasmas in M/X Flares Using RHESSI Fe & Fe/Ni Line and Continuum Observations Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2007AGUFMSH41A0309C Altcode: Observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have shown that "super-hot" (T > ~30~MK) thermal plasmas are a somewhat common feature of (large) flares of GOES class M or X. However, the origins and evolution of such super-hot plasmas are still poorly understood. RHESSI observes solar photons >3 keV with a spectral resolution of ~1~keV FWHM, and is especially sensitive to flare plasmas hotter than ~10~MK. RHESSI's rich data set allows an unprecedented level of analysis of thermal flare plasma through imaging and spectroscopic observations of the thermal bremsstrahlung continuum and the Fe and Fe/Ni line complexes at ~6.7 and ~8~keV. Accurately characterizing the thermal plasma provides information about flare heating and cooling mechanisms and improves our understanding of flare energy transport and release. We present early results of a statistical survey of hot thermal plasmas in ~300 RHESSI-observed M/X-class flares (2002-2005), utilizing an analytical method to compare the thermal continuum and line emission (Caspi & Lin 2007). We present flare energetics, size, and temperature distributions and discuss the implications for the origins of super-hot thermal plasmas. Title: Ion acceleration and neutral emission mechanisms for 2005 September 7 flare Authors: Watanabe, K.; Lin, R. P.; Krucker, S.; Murphy, R. J.; Share, G. H.; Harris, M. J.; Gros, M.; Sako, T.; Muraki, Y. Bibcode: 2007AGUFMSH52A0908W Altcode: In association with an X17.0 flare on 2005 September 7, strong neutral emissions were detected both in space and on the ground. In space, intense emissions of gamma-rays were registered by INTEGRAL and by RHESSI during the decay phase. Gamma-ray lines at 0.511, 2.2, 4.4, and 6.1 MeV were observed and there was evidence for pion-decay radiation. On the ground, relativistic neutrons were observed by the neutron monitors at Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at Chacaltaya and Mt. Sierra Negra. The neutron signal continued for more than 20 minutes with high statistical significance. The long decay of the signals suggests that ions were continuously accelerated or trapped in the emission site. We also find that gamma-rays were emitted over a corresponding extended period. Only when we in cooperate the high-energy gamma-ray emission time history can we explain the long-lasting neutron emission. We also use the Hua et al. (2002) solar- flare magnetic loop transport and interaction model to find the best model to explain the data. Title: Comparing flare temperature distributions from RHESSI and from Hinode Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P. Bibcode: 2007AGUFMSH53A1057S Altcode: Using flares that were observed by both RHESSI and Hinode (XRT and EIS), as well as available data from STEREO, TRACE and SOHO's EIT, we compare the spatial and temporal temperature distributions of flaring plasma in the X-ray and EUV regimes. 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: Solar Flare Hard X-Ray Emission from the High Corona Authors: Krucker, Säm; White, S. M.; Lin, R. P. Bibcode: 2007ApJ...669L..49K Altcode: One of the largest solar hard X-ray (HXR) flares and solar energetic particle (SEP) events recorded by the Mars Odyssey mission while orbiting Mars occurred on 2002 October 27 and is related to a very fast (~2300 km s-1) coronal mass ejection (CME). From the Earth, the flare site is 40.4° +/- 3.5° behind the solar limb and only emissions from the high corona at least 1.5 × 105 km radially above the main flare site can be seen. Nevertheless, the Earth-orbiting Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed HXR emission up to 60 keV with a relatively flat, nonthermal spectrum (γ between 3 and 3.5) that has an onset simultaneous with the main HXR emission observed above 60 keV by the Gamma-Ray Spectrometer (GRS) orbiting Mars. While GRS records several smaller enhancements after the main peak, the high coronal source observed by RHESSI shows a long exponential decay (τ = 135 +/- 5 s) with progressive spectral hardening. The emissions from the high corona originate from an extended source (~1.5 × 105 km in diameter) that expands (390 +/- 70 km s-1) and moves upwards (750 +/- 80 km s-1) in the same direction as the CME. These observations reveal the existence of energetic electrons in the high corona in closed magnetic structures related to the CME that are accelerated at the same time as the main energy release in the flare. Although the number of energetic electrons in the high corona is only a small fraction of the total accelerated electrons, about 10% of all electrons in the high coronal source are nonthermal (>10 keV). Title: Solar Flare Electron Spectra at the Sun and near the Earth Authors: Krucker, Säm; Kontar, E. P.; Christe, S.; Lin, R. P. Bibcode: 2007ApJ...663L.109K Altcode: We compare hard X-ray (HXR) photon spectra observed by the RHESSI with the spectra of the electrons in the associated solar impulsive particle events observed near 1 AU by the WIND 3D Plasma and Energetic Particle (3DP) instrument. For prompt events, where the inferred injection time at the Sun coincides with the HXR burst, the HXR photon power-law spectral index γ and the in situ observed electron spectral index δ measured above 50 keV show a good linear fit, δ=γ+0.1(+/-0.1), with correlation coefficient of 0.83, while for delayed events (inferred injection >10 minutes after the HXR burst) only a weak correlation with a coefficient of 0.43 is seen. The observed relationship for prompt events is inconsistent, however, with both the thin target case, where the escaping electrons come from the X-ray-producing electron population, and the thick target case where some of the accelerated source population escapes to 1 AU and the rest produce the HXRs while losing all their energy to collisions. Furthermore, the derived total number of escaping electrons correlates with the number of electrons required to produce observed X-ray flux but is only about ~0.2% of the number of HXR-producing electrons. Title: First Results of the S/WAVES Experiment on the STEREO Mission. Authors: Bonnin, X.; Maksimovic, M.; Bougeret, J. -L.; Goetz, K.; Bale, S. D.; Kaiser, M. L.; Reiner, M. J.; Cecconi, B.; Briand, C.; Krucker, S.; S/Waves Team Bibcode: 2007sf2a.conf..582B Altcode: We present the first results of the STEREO/Waves (S/Waves) investigation on the STEREO Mission. The S/Waves instrument includes a suite of state-of-the-art sub-instruments that provide comprehensive measurements of the three components of the electric field from a fraction of a Hertz up to 16 MHz, plus a single frequency channel near 30 MHz. The instrument has a direction finding or goniopolarimetry capability, used to perform 3-D localization and tracking of streams of energetic electrons and of shock waves associated with Coronal Mass Ejections (CMEs). The scientific objectives include (i) remote observation and measurement of energetic phenomena throughout the 3-D heliosphere that are associated with the CMEs and with solar flare phenomena, and (ii) in-situ measurement of the properties of CMEs, such as their electron density and temperature and the associated plasma waves near 1 Astronomical Unit. Title: RHESSI Microflare Statistics: X-ray Imaging And Spectral Analysis Authors: Hannah, Iain; Christe, S.; Krucker, S.; Hurford, G.; Lin, R. Bibcode: 2007AAS...210.9315H Altcode: 2007BAAS...39R.213H We present x-ray imaging and spectral analysis of all microflares the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed between March 2002 and March 2007, a total of 24,799 events. These microflares are small flares, below C Class, and were found by searching 6-12 keV during periods when the full sensitivity RHESSI’s detectors were available. Each microflare is automatically analysed: 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 non-thermal spectral model. This wealth of information allows us to investigate many aspects of these microflares and in particular the thermal and non-thermal energy distributions of these events. Title: Statistical Properties of Hot Thermal Plasmas in M/X Flares Using RHESSI Fe & Fe/Ni Line and Continuum Observations Authors: Caspi, Amir; Krucker, S.; Lin, R. P. Bibcode: 2007AAS...210.9318C Altcode: 2007BAAS...39..214C Observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have shown that "super-hot" (T > ∼30 MK) thermal plasmas are a common feature of (large) flares of GOES class M or X. However, the origins and evolution of such super-hot plasmas are still poorly understood. RHESSI observes solar photons >3 keV with a spectral resolution of ∼1 keV FWHM, and is especially sensitive to emission from flare plasmas hotter than ∼10 MK. RHESSI's rich data set allows an unprecedented level of analysis of thermal flare plasma through imaging and spectroscopic observations of the thermal bremsstrahlung X-ray continuum above ∼3 keV and the Fe and Fe/Ni line complexes at ∼6.7 and ∼8 keV. Accurately characterizing the thermal plasma provides information about flare heating and cooling mechanisms and improves our understanding of flare energy transport and release. We present first results of a statistical survey of hot thermal plasmas in RHESSI-observed M/X-class flares, utilizing a newly-developed analytical method to compare the thermal continuum and Fe & Fe/Ni line emission (Caspi & Lin 2007). We present flare energetics and temperature distributions and discuss the implications for the origins of super-hot thermal plasmas. Title: Hard X-ray Spectral Characteristics Of Solar Flares With Two Footpoints: a survey Authors: Saint-Hilaire, Pascal; Krucker, S.; Lin, R. P. Bibcode: 2007AAS...210.9317S Altcode: 2007BAAS...39..214S Using RHESSI data, we have analyzed the spectral characteristics of some 172 hard X-ray peaks during 53 solar flares which exhibited a double-footpoint structure. Spectral index differences between footpoints do not always exist, even in large flares. It is also clear that differences in footpoint spectral indices cannot always be attributed to a difference in column densities between the two legs of a coronal loop. Moreover, footpoint asymmetries seem to be more marked during times of peak HXR flux.

This work has been supported by SNSF grant PBEZ2-108928, and NASA HGI grant NNX07AH74G. Title: Properties Of A Type III Storm Associated With Active Region 10923 Observed By The STEREO/WAVES Radio Receiver Authors: Eastwood, Jonathan; Bale, S. D.; Krucker, S.; Lin, R. P.; Bougeret, J. L.; Maksimovic, M.; Goetz, K.; Kaiser, M. L.; SWAVES Team Bibcode: 2007AAS...210.2808E Altcode: 2007BAAS...39..136E We analyze a Type III storm associated with Active Region 10923 observed by the Stereo spacecraft between November 11 - 17 2006. This active region developed on the central disk and provides an unprecedented opportunity to study how a Type III storm develops at kilometric wavelengths. Bursts are first observed at low frequency (high coronal altitude). We investigate the power law behavior in the brightness distribution of individual type III bursts. Time series of brightness at specific frequencies also appear to follow power law behavior over a restricted frequency range, indicating specific constraints on the structure of the source region. We discuss the implications these results have for the magnetic topology of the corona above active regions. Title: RHESSI Microflare Statistics: Flare Finding and Frequency Distribution Authors: Christe, Steven; Hannah, I.; Krucker, S.; McTiernan, J.; Lin, R. P. Bibcode: 2007AAS...210.9310C Altcode: 2007BAAS...39..212C The Reuven Ramaty High Energy Spectroscopic Solar Imager (RHESSI) is uniquely suited to observe solar microflares due to its sensitivity in the 3-15 keV energy range (up to 100 times better than previous solar instruments). As such, it provides new information on these low level transients. In this study, we present, for the first time, initial results from a new flare-finding algorithm which searches the 6 to 12 keV energy band in order to find the smallest flares. Over the lifetime of RHESSI (5 years), we have found a total of 24 799 microflares. We describe the flare-finding algorithm and present microflare statistics including the frequency distribution. This work was supported by NASA contract NAS5-98033. Title: STEREO measurements of shocks and in situ interplanetary type III radio bursts: early results from the S/WAVES and IMPACT experiments Authors: Bale, S. D.; Krucker, S.; Bougeret, J.; Goetz, K.; Kaiser, M. L.; Kellogg, P. J.; Larson, D. E.; Lin, R. P.; Luhmann, J. G.; Maksimovic, M.; Russell, C. T. Bibcode: 2007AGUSMSH33A..05B Altcode: We present early measurements by the STEREO/WAVES and IMPACT instruments of bow shocks, interplanetary shocks, and in situ interplanetary type III bursts. S/WAVES measures spectral density and 'burst' electric waveforms on three orthogonal antennas. IMPACT burst mode samples the thermal and suprathermal electrons and magnetic field at high cadence for several 10 minute intervals each day. The S/WAVES and IMPACT bursts systems have triggered together on several occasions at shocks and foreshock electron beams. Here we present results of these high time resolution bursts. Early in the mission, several in situ IP type III bursts were observed at 1 AU by STEREO and Wind. The similar instrumentation on the three spacecraft (with separations greater than 100 Re) allows us to limit the spatial extent of the type III radio emission region. Title: Coronal Gamma-ray Sources in Solar Flares Authors: Krucker, Sam; Hurford, G. J.; Shih, A. Y.; Lin, R. P. Bibcode: 2007AAS...210.9314K Altcode: 2007BAAS...39..213K The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) provides for the first time imaging spectroscopy of solar flares up to the gamma-ray range. The three RHESSI flares with best counting statistics are analyzed in the 200-800 keV range revealing gamma-ray emission produced by electron bremsstrahlung from footpoints of flare loops, but also from the corona.Footpoint emission dominates during the gamma-ray peak, but as the gamma-ray emission decreases the coronal source becomes more and more prominent. Furthermore, the coronal source shows a much harder spectrum (with power law indices between 1.5 and 2) than the footpoints (with indices between 3 and 4). These observations suggest that flare-accelerated high energy electrons ( MeV) are staying long enough in the corona and lose their energy by collisions producing gamma-ray emission, while lower energetic electrons precipitate to the footpoints without losing significant energy in the corona. Title: Radio and White-Light Coronal Signatures Associated with the RHESSI Hard X-Ray Event of 2002 July 23 Authors: Reiner, M. J.; Krucker, S.; Gary, D. E.; Dougherty, B. L.; Kaiser, M. L.; Bougeret, J. -L. Bibcode: 2007ApJ...657.1107R Altcode: Simultaneous radio, white-light, and hard X-ray (HXR) observations for the 2002 July 23 γ-ray flare event are used to establish the relationship of a complex type III-like burst to the corresponding coronal mass ejection (CME) and the coronal electron acceleration signatures observed in the decimeter/microwave (dm-cm) emissions and X-rays. We find that the onset of the type III-like emissions for this event is coincident with the impulsive RHESSI HXR event, the dm-cm radio emissions and with the linearly extrapolated liftoff time of the CME. The overall intensity-time characteristics of the complex type III-like burst resembles that of both the dm-cm flux and the HXR light curve that correspond to an electron acceleration event deep in the corona. Furthermore, the complex radiation characteristics of the type III-like emissions are found to be directly related to the CME kinematics, which is directly related to the frequency drift of the associated low-frequency (kilometric) type II emissions. The frequency-drift characteristics of the high-frequency (metric) type II emissions observed for this event, on the other hand, are not clearly related to the kilometric type II emissions and therefore to the observed CME height-time characteristics, indicating that these emissions may correspond to an independent coronal shock wave. 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: Solar Flares With two Hard X-ray Footpoints: a Statistical Survey Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P. Bibcode: 2006AGUFMSH23A0331S Altcode: Using RHESSI data and its unprecedented spectral and spatial coverage, we have done a statistical survey of the spectral and spatial characteristics of flares displaying two footpoints in hard X-rays (HXR). We have manually selected 53 of RHESSI's best observed flares, where both HXR footpoints could be imaged above 50 keV. Numerous correlations could thus be investigated, providing additional constraints to coronal particle acceleration models. Assuming both footpoints are fed by the same electron injections, we will in particular address the question as to whether differences in HXR footpoint spectral indices and flux ratios can be attributed to a difference in column density between the acceleration site and both chromospheric footpoints. Title: A Study of Pitch Angle Distributions for Six Solar Impulsive Electron Events Authors: Wang, L.; Lin, R. P.; Li, G.; Krucker, S. Bibcode: 2006AGUFMSH43B1530W Altcode: The pitch angle distributions (PADs) of solar impulsive electron events measured at 1 AU provide important information on the solar release of the electrons and their interplanetary transport. Using WIND/3DP electron data, we investigate the energy and time dependence of the PADs for six impulsive electron events with short time durations, low pre-event background, good count statistics and the electron energy range from ~0.5 keV to ~200 keV. The PADs of the pre-event background are anisotropic below ~2 keV due to halo electrons continuously streaming out from the Sun at these energies and isotropic above. In the preliminary study of two events (1998 Aug 29 and 2002 Oct 20), we remove such pre-event background. We find that the PAD width at half maximum is roughly constant from ~0.5 keV to 10-13 keV. It remains below ~30 degrees and changes slightly through the rapid-rise, rapid-decay phase, and it increases up to ~50-60 degrees at the slow-decay phase. We also find that the PAD width above ~13-20 keV increases with both energy and time. At the peak phase the width increases from ~40-50 degrees at 30 keV to ~60-70 degrees at 200 keV. At 200 keV it increases from ~50-60 degrees at the rapid-rise phase to ~80 degrees at the slow-decay phase. These properties suggest that, for these solar impulsive electron events, the low-energy electrons propagate nearly scatter-free in the interplanetary medium, while the high energy electrons experience some energy-dependent scattering. Furthermore, at a fixed energy, after electrons are scattered to different pitch angles, they travel to 1 AU at different speeds parallel to the magnetic field line. We analyze the pitch-angle dispersion and estimate the distance between the scattering site and 1 AU. We find that for high-energy electrons the average is ~0.03±0.06 AU. Title: Interplanetary Type II Intensity Emission as a Function of the Heliospheric Position and Coronal Mass Ejection Speed Authors: Aguilar-Rodriguez, E.; Bale, S. D.; Krucker, S. Bibcode: 2006AGUFMSH51A1464A Altcode: Interplanetary Type II radio burst radiation results from the excitation of plasma waves in the ambient medium by a coronal mass ejection (CME) driven shock, propagating outward from the Sun. However, not all disturbances are able to produce Type II emission. Factors such as the speed of the disturbance and the properties of the medium through which the disturbance propagates are fundamental to produce Type II emission. Shock formation requires that the velocity of the disturbance excedes the local fast magnetosonic speed. Some studies have evaluated the radial behaviour of the fast magnetosonic speed as well as the Alfven speed above active regions. By combining both white light (SOHO/LASCO) and radio (Wind/WAVES) observations, we present preliminary results on the Type II intensity emission as a function of heliospheric position and CME speed. This might reveal the average Alfven speed profile in the inner heliosphere which, in turn, would tell us about the location of the solar wind acceleration. This work is supported by the University of California Institute for Mexico and the United States (UC MEXUS). Title: RHESSI Microflare Statistics: Energy Budget Authors: Hannah, I. G.; Christe, S. D.; Krucker, S.; Lin, R. P. Bibcode: 2006AGUFMSH23A0348H Altcode: We present analysis of all RHESSI microflares, covering GOES class A through C , encompassing over 4 years of observations. Well over 10,000 microflares were found using our own flare finding algorithm, that scans the 6-12 keV energy range, finding considerably more than the standard RHESSI flare list. Each microflare is automatically analysed: the source size is found by forward fitting the complex visibilities and the spectral parameters (temperature, emission measure, power-law index) are found by forward fitting a thermal plus non-thermal spectral model. The combination of these allows us to compute the thermal and non-thermal energy in each of the microflares. We present the thermal and non-thermal energy distributions of these microflares and how the ratio of these two energy components changes for different characteristics of the microflares. This work is funded by NASA grant NAS5-98033. Title: A Radio Burst and Its Associated CME on March 17, 2002 Authors: Yan, Y.; Pick, M.; Wang, M.; Krucker, S.; Vourlidas, A. Bibcode: 2006SoPh..239..277Y Altcode: 2006SoPh..tmp...78Y In this study, we present a detailed analysis, based on multiwavelength observations and magnetic field extrapolation, of a radio and X-ray event observed on March 17, 2002. This event was accompanied by a Coronal Mass Ejection (CME) observed by the Large-Angle Spectrometric Coronagraph (LASCO) aboard SOHO. During the main event, the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) mission observed a hard X-ray emission correlated in time with the development of a type III burst group. The CME development, the hard X-ray emission, and the type III burst group appear to be closely associated. The multifrequency Nançay Radioheliograph (NRH) shows that the type III bursts are produced at a distance from the active region that progressively increases with time. Their emitting sources are distributed along the western edge of the CME. We conclude the type III electron beams propagate in the interface region between the ascending CME and the neighboring open field lines. Due to the development of the CME, this region becomes progressively highly compressed. By measuring, at each frequency, the shift versus time of the type III positions, we estimate that the electron density in this compression region increased roughly by a factor of 10 over a few minutes. Another signature of this compression region is a narrow white light feature interpreted as a coronal shock driven by the CME lateral expansion. 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: Pre-Impulsive Hard X-Ray Emission from Coronal Sources in X-Class Flares Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2006AGUFMSH43B1528C Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed significant non- thermal emission up to ~70~keV from coronal sources prior to the impulsive phase in some flares, which has interesting implications for particle trapping and flare energy release. Images and spectra during the pre- impulsive phase of the GOES-class X4.8 event on 23~July~2002 suggests that the coronal hard X-ray (HXR) emission is non-thermal, as analysis of the Fe (~6.7~keV) and Fe/Ni (~8~keV) line complexes constrains the thermal component to have little or no thermal continuum emission above ~15~keV. The over-the-limb X3.1 event on 24~Aug~2002 displays similar spectral characteristics, and occultation of the footpoints places the non-thermal HXR emission unambiguously in the corona. RHESSI images of this event show an extended source below ~35~keV and a compact looptop source above ~35~keV, possibly indicating trapping of energetic electrons. Spatially-integrated spectra are well-fit by a broken power-law and the spectral indices above and below the break differ by ~2, suggesting an observation of the transition between thin- and thick-target bremsstrahlung. The collisional lifetime of non-thermal electrons is <10~sec at the inferred densities of ~1011~cm-3 and the rising HXR flux thus suggests continuous particle injection. We apply the technique of Johns &Lin (1992) to invert the pre-impulsive phase photon spectra and recover the original electron energy spectra as a function of time, both for spatially-integrated spectra and source-isolated spectra using imaging spectroscopy. We then compare the inverted spectra with a forward-modeling analysis of the photon spectra using the iron line analysis technique of Caspi &Lin (2006) to constrain the thermal model. We compare the results for both 24~Aug~2002 and 23~Jul~2002, and discuss the implications for electron acceleration and flare plasma heating in the corona during this period. 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: Observations of a soft X-ray rising loop associated with a type II burst and a coronal mass ejection in the 03 November 2003 X-ray flare Authors: Dauphin, C.; Vilmer, N.; Krucker, S. Bibcode: 2006A&A...455..339D Altcode: Context: .We report observations of a type II burst - the signature of a shock wave - starting at the unusual high frequency of 650 MHz during the 03 November 2003 flare. This flare is associated with the propagation of a soft X-ray coronal loop and with a coronal mass ejection (CME).
Aims: .We study in this paper the origin of the shock wave in the low corona and present a kinematics analysis of the soft X-ray coronal loop and of the CME observed a few tens of minutes later.
Methods: .We study in this paper the spatial and temporal relation between the soft X-ray rising loop observed by the GOES soft X-ray Imager (GOES/SXI), the type II sources observed by the Nançay Radio Heliograph (NRH) and the CME observed by LASCO (Large Angle and Spectroscopic Coronograph).
Results: .This analysis shows that the type II burst observed during this flare is driven by the X-ray rising loop. Furthermore, the kinematics analysis of the X-ray coronal loop and CME shows that the two structures are related.
Conclusions: .The direct comparison of the type II sources with the GOES/SXI observations clearly shows that the type II burst is ignited by the shock wave created ahead of the rising X-ray loop. Finally, we propose to interpret these different observations in the framework of an ascending magnetic structure with a reconnecting process operating in the tail of this ascending structure. Title: Statistical Study of Hard X-ray Footpoint Motions in Solar Flares Authors: Yang, Ya-Hui; Krucker, S.; Lin, R. P.; Ip, W. H. Bibcode: 2006SPD....37.1312Y Altcode: 2006BAAS...38..242Y Hard X-ray (HXR) source locations in solar flares are thought to be the sites where flare-produced energetic electrons precipitate along reconnected magnetic loops and then lose their energy via collisions with ambient plasma in the lower atmosphere. According to a standard reconnection model, HXR footpoints should separate due to the successive reconnections. However, several previous studies showed that the footpoints move along ribbons or move approach to each other. To understand the characteristics of footpoint motions more systematically, a statistical survey of HXR footpoint motions in M-class and X-class flares is done using RHESSI observations. Such investigation provides a good way to test flare models. Title: Gamma-Ray Imaging of the 2003 October/November Solar Flares Authors: Hurford, G. J.; Krucker, S.; Lin, R. P.; Schwartz, R. A.; Share, G. H.; Smith, D. M. Bibcode: 2006ApJ...644L..93H Altcode: We present RHESSI imaging of three flares (2003 October 28 and 29 and November 2) in the 2.223 MeV neutron-capture gamma-ray line with angular resolution as high as 35". Comparisons of imaged and spatially integrated fluences show that in all cases most, if not all, of the emission was confined to compact sources with size scales of tens of arcseconds or smaller that are located within the flare active region. Thus, the gamma-ray-producing ions appear to be accelerated by the flare process and not by a widespread shock driven by a fast coronal mass ejection. The 28 October event yielded the first such image to show double-footpoint gamma-ray line sources. These footpoint sources straddled the flaring loop arcade but were displaced from the corresponding 0.2-0.3 MeV electron-bremsstrahlung emission footpoints by 14" and 17" +/- 5". As with the previously studied 2002 July 23 event, this implies spatial differences in acceleration and/or propagation between the flare-accelerated ions and electrons. Title: A Statistical Study of Solar Energetic Electron Events over one Solar Cycle Authors: Wang, Linghua; Lin, R. P.; Krucker, S.; Mason, G. M. Bibcode: 2006SPD....37.2903W Altcode: 2006BAAS...38Q.256W Impulsive solar electron events are the most common impulsive solar particle acceleration phenomenon, often observed at 1-100 keV. Most of these events are associated with small SXR bursts and type III radio bursts, while unaccompanied by HXR bursts. We survey the WIND/3DP electron data ( 1-400 keV) from 1995 through 2005 and identify 1100 solar energetic electron events over one solar cycle. The number of electron events shows a time variation similar to sunspots and SXR flares. Taking into account the effect of instrumental background, we find a quite power-law relationship between the event number N and the event peak flux J: dN/dJ=A×J^(-γ). The integration over the electron flux estimates that >= 10^4 events/year occur the whole Sun near solar maximum. Meanwhile, we survey ion data ( 0.02-10 MeV/nuc) observed by the ACE/ULEIS from 1998 to 2005. We find a good correlation: 80 % of impulsive electron events are accompanied by MeV/nuc, 3He-rich (3He/4He >= 1%) ion emissions; >93% of 3He-rich ion events with a clear velocity dispersion appear to have an impulsive electron event. Finally, We present the correlation between the peak flux of impulsive electrons and the count of 3He and 4He ions. Title: Spectral Evolution of Coronal Hard X-ray Sources during Solar Flares Authors: Krucker, Sam; Lin, R. P. Bibcode: 2006SPD....37.2706K Altcode: 2006BAAS...38..254K Hard X-ray (HXR) emissions during solar flares are most prominent at chromospheric footpoints of flare loops which reveal where flare-accelerated electrons lose their energy by collision. The lower density in the corona makes it much more difficult to detect coronal HXR emissions, but coronal HXR sources directly reveal insights into the acceleration region (e.g. Masuda et al. 1994). Observations with Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) give for the first time detailed spatial and spectral observations in the HXR range. Initial results of a few events reveal at least two different spectral behavior, possibly indicating two different acceleration mechanisms: (1) Coronal HXR sources with a 'soft-hard-soft' behavior (Battaglia & Benz 2006), and (2) sources that show spectral hardening in time, i.e. a 'soft-hard-harder' behavior (Krucker et al. 2005). After a short review of recent RHESSI observations, we will present statistical results on the spectral evolution of coronal HXR sources of 50 partly occulted limb flares seen by RHESSI. Title: Gamma-ray And Hard X-ray Imaging Of The 20 January 2005 Solar Flare Authors: Hurford, Gordon J.; Krucker, S.; Lin, R. P.; Schwartz, R. A.; Share, G. H.; Smith, D. M. Bibcode: 2006SPD....37.2804H Altcode: 2006BAAS...38..255H Gamma-ray imaging provides a direct indication of the spatial properties of accelerated ions near the Sun. We present the results of RHESSI gamma-ray imaging of the 20 January 2005 solar flare. While maps in the electron-bremsstrahlung dominated >250-500 keV band show two comparable sources coinciding with footpoints seen with TRACE, corresponding 35 arcsecond resolution images in the neutron-capture line at 2.223 MeV show the ion-generated source to be preferentially associated with just one of these two footpoints. This result is supported by imaging with 183 arcseconds resolution in four gamma-ray continuum bands between 600 keV and 8 MeV. The relative locations of their centroids show relative displacements that correlate with the relative contribution of electrons and ions at different energies. Comparison of the intensity of the imaged neutron-capture line image with the spatially integrated line intensity obtained spectroscopically is consistent with all of the neutron-capture line flux originating in a compact source (diameter <20 arcseconds). While shock-acceleration high in the corona might be expected to produce a large, diffuse source, the observed compact source indicates that the gamma-ray producing ions were flare-accelerated. These flare-integrated results are presented in the context of a hard X-ray movie showing the movement and time-evolution of the lectron-associated footpoints. This work has been supported by NASA under NAS5-98033. Title: Measuring the Temperature of Hot Flare Plasma Using RHESSI Fe and Fe/Ni Line Observations Authors: Caspi, Amir; Krucker, S.; Lin, R. P. Bibcode: 2006SPD....37.2704C Altcode: 2006BAAS...38..253C Solar flares of GOES class M or X can produce "super-hot" (temperatures of 30 MK or higher) thermal plasmas. Continuum emission from such flare plasmas can dominate the X-ray spectrum up to 20-50 keV, making it difficult to separate from the non-thermal bremsstrahlung from accelerated, tens of keV electrons. Observations suggest that flare-accelerated electrons contribute significantly to heating of the thermal plasma (manifesting as the empirical "Neupert effect"), although recent studies also suggest that other heating mechanisms may be equally significant. Accurately characterizing the thermal plasma provides information about the heating and cooling mechanisms involved, and also improves our understanding of the non-thermal emission. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes solar photons with energies >3 keV, with a spectral resolution of 1 keV FWHM, and is especially sensitive to flare plasmas above 10 MK. RHESSI observes both the thermal continuum and the Fe and Fe/Ni line complexes at 6.7 and 8 keV. The fluxes and equivalent widths of the line complexes are strongly temperature-dependent and are useful as probes of the thermal plasma. We present an analytical method using the ratio of the line fluxes (Fe to Fe/Ni) as a temperature diagnostic to compare with and constrain the continuum temperature measurements. We discuss results of this analysis on two X-class flares (23/Jul/2002 and 02/Nov/2003) and compare our method to the equivalent width method of Phillips et al. (2005). We also present an application of this analysis by examining the "pre-impulsive" phase of two flares (23/Jul/2002 and 24/Aug/2002) which both appear to show strong non-thermal emission from a coronal source. Title: Hard X-rays Associated With Solar Type III Radio Bursts Authors: Christe, Steven; Krucker, S.; Lin, R. P. Bibcode: 2006SPD....37.2904C Altcode: 2006BAAS...38..256C During a period of 12 minutes on 19 July 2002 14:23-14:35 UT, the WAVES instrument on the Wind spacecraft observed interplanetary type III radio bursts approximately every 2 minutes, and each was accompanied by a hard X-ray burst (12-15 keV) observed by RHESSI. We investigate the emission mechanism for the observed hard X-rays for each event. We find that the four smallest X-ray events are inconsistent with emission from a thermal plasma or a nonthermal beam suggesting that the observed emission is related to the acceleration mechanism of type IIIs. Title: A study of the solar injection for nine scatter-free impulsive electron events Authors: Wang, L.; Lin, R. P.; Krucker, S. Bibcode: 2006AGUSMSH41A..03W Altcode: Using WIND/3DP electron data from 1996 through 2005, we investigate the injection at the Sun of nine scatter- free impulsive electron events with good count statistics, the electron energy range from ~1keV to above 60 keV and the simple association with the solar and interplanetary type III radio burst observed by WIND/WAVES. Taking into account the effects of interplanetary propagation and instrumental response, we find that the observed time profiles of electron fluxes at all energies fit quite well to triangular injections at the Sun with equal rise and fall times. We find double injections: One for <~ 1 to 6-9 keV electrons begins 4.6± 2.6 min before the start of type III burst and lasts for an averaged duration of 90 min, while a second for electrons above 9-13 keV starts 6.9±0.7 min after the start of the type III burst and lasts for an average of 24 min. Electrons of low-energy injection are likely to be the source of the type III burst. The majority of events might have a B or C class GOES SXR flare related to the start of low-energy electron injection, three might have a B or C class SXR flare related to the start of high-energy injection, and five might have a west- limb CME with the velocity <~600km/s. Title: Imaging of a radio type II burst relative to the CME shock Authors: Krucker, S.; Dauphin, C.; Vilmer, N. R. Bibcode: 2006AGUSMSH32A..03K Altcode: Radio type II emission is thought to be produced by energetic electrons that are accelerated at shocks in the solar corona and in interplanetary space. Simultaneous imaging observations of both, the radio emission and the shock, are rare, especially in the lower corona. Here, we present imaging observations in radio waves and soft X-rays of a type II burst that occurred on November 3, 2003 during a GOES X3 flare. The radio type II burst starts at an unusally high frequency (600 MHz) and can therefore be imaged with the Nancay radio heliograph. Simultaneous soft X-ray observations provided by GOES SXI show a faint, fast moving (~800 km/s) loop-like emission in the lower corona (0.1-0.3 solar radius above the photosphere) that spatially and temporally correlates with the Coronal Mass Ejection (CME) later seen in white-light with SOHO/LASCO. Therefore, the observed SXR front is most likely emitted when the CME shock is in the lower corona. The radio type II emission is observed to occur infront of the SXR emission and is only seen from a single location, but not all along the shock front. Title: Solar HXR Emission and Energetic Electron events seen at 1~AU: A temporal and spectral comparison Authors: Krucker, S.; Christe, S. D.; Kontar, E. P.; Lin, R. P. Bibcode: 2006AGUSMSH41A..04K Altcode: Onset time analysis of impulsive electron events at 1 AU suggest that there are two classes of events: (1) events with a solar release time in close temporal agreement with solar hard X-ray (HXR) flares, and (2) events with a delayed solar release time relative to the HXR emission that seem to be accelerated later possibly at coronal shocks. Simple onset time analysis, however, are criticized for not taking propagation effect properly into account producing falsely delayed release times. Here we present a further test of the existence of two classes of events by comparing the in-situ observed electron spectra with the solar HXR photon spectra remotely observed by RHESSI. For events with a good temporal agreement, a clear correlation between the spectra has been reported indicating a common acceleration mechanism. Delayed events have not yet been analyzed. However, if indeed a second acceleration mechanism independent of the HXR flare exists that release energetic electron into interplanetary space, no correlation between the in-situ observed electron spectra and the HXR photon spectra is expected to be found. Title: Evidence for double injections in scatter-free solar impulsive electron events Authors: Wang, L.; Lin, R. P.; Krucker, S.; Gosling, J. T. Bibcode: 2006GeoRL..33.3106W Altcode: 2006GeoRL..3303106W We investigate the injection at the Sun for three scatter-free impulsive electron events (7 August 1999, 28 June and 22 August 2000) observed from ~0.4 to 300 keV by the WIND 3DP instrument. Taking into account the interplanetary scatter-free propagation and instrumental effects, we find that the observed time profiles of electron fluxes at all energies fit well to triangular injections at the Sun with equal rise and fall times. We find two distinct injections: that of ~0.4 to 6-9 keV electrons begins 9.1 +/- 4.7 min before the start of the type III radio burst and lasts for 50-300 min, while that of ~13 to 300 keV electrons starts 7.6 +/- 1.3 min after the start of the type III burst and lasts for a factor of 5-10 times shorter. Electrons of the low-energy injection are likely to be the source of the type III radio bursts, and the delayed high-energy injection occurs when the associated CME passes altitudes of ~1-6 R$\odot$. The observed electron energy spectra fit to double power-law with a downward break at ~40-50 keV, but exhibit a smooth power-law across the transition (~6-13 keV) between the two injections, suggesting the low-energy injection may provide the seed electrons for the high-energy injection. Title: Investigations of spatially resolved 1.28 GHz radio activity associated with solar X-Ray micro flares Authors: Sawant, H. S.; Krucker, S.; Madsen, F. R. H.; Kane, S.; Karlicky, M.; Ananthakrishnan, S.; Subramanian, P. Bibcode: 2006cosp...36.3365S Altcode: 2006cosp.meet.3365S On 20 and 22 November 2005 Solar observations at 1280 MHz were carried out from 04 00 to 11 30UT at the Giant Metrewave Radio Telescope -- GMRT - with time resolution of 512 ms and spatial resolution of sim 5 arc sec During GMRT observation time on both days micro flares have been observed by RHESSI satellites in the range of 3 -- 10 keV Detailed investigation of the X-ray emission shows the presence of both thermal and non-thermal components Imaging analysis of one of the micro flares in x ray shows the RHESSI data can produce image with resolution of 7 Here we report an investigation of simultaneous 1 28 GHz radio activity and 3-10 keV X-ray observations of solar X-ray micro flares as observed on 20 November during the interval 08 00-08 30 UT Title: Radio and magnetic field constraints on SEP propagation from the Sun to Earth Authors: Klein, K. -L.; Krucker, S.; Hoang, S.; Lointier, G. Bibcode: 2006cosp...36.1787K Altcode: 2006cosp.meet.1787K Energetic particles accelerated in the solar corona can be detected in space provided an appropriate magnetic connection exists The interplanetary magnetic field is usually described by an Archimedian spiral Parker spiral starting at a source surface sim 2 5 R odot from the Sun s centre While statistically speaking impulsive solar energetic particle events SEP events are associated with flares at longitudes close to the root of the Parker spiral in the western solar hemisphere there is some scatter sometimes deviations of more than 10 heliocentric degrees We present a sample of impulsive electron events where we identify coronal and interplanetary paths of the particles through their type III radio emission We identify flux tubes into which electrons are injected at the Sun by imaging their radio emission with the Nan c c ay Radioheliograph below 0 5 R footnotesize odot above the photosphere The arrival of the electrons at Earth is traced by kilometric radio emission and Langmuir waves detected by the WAVES radio receiver aboard the it Wind mission and by the 3DP particle detector We compare the propagation paths of the electrons with the nominal Parker spiral and with the magnetic field below the source surface inferred from the potential field and source surface model developed by Schrijver DeRosa 2003 whose results are available within the SolarSoft package First results suggest that the type III emission is a reliable tracer of the magnetic field topology below the source surface even in cases when the parent flare Title: The magnetic field configuration for 17-3-2002 radio bursts and associated CME Authors: Yan, Y.; Pick, M.; Wang, M.; Krucker, S. Bibcode: 2006cosp...36.3130Y Altcode: 2006cosp.meet.3130Y The coronal magnetic field configuration is important for understanding the energy storage and release processes that account for flares and or CMEs Using a model which is based on the work for potential magnetic field problems that only applies the condition at infinity with the boundary condition on the solar surface specified we have reconstructed the global coronal mangetic field structure for 17 March 2002 radio bursts and CME event It is found that the March 17 2002 event occurred in a complex multipolarity configuration of the magnetic field due to the localization of AR 9871 inside an old remnant region and to the local inclusion of small interacting loops around the leading negative polarity of this AR and open field lines from negative polarities The small interacting loops can be inferred from RHESSI SXR HXR observations The origin and evolution of the radio sources during the CME flare processes on 17 March 2002 in AR 9871 can be explained under such reconstructed magnetic field configurations The successive type III bursts are along the extrapolated open field lines and the continuum sources are confined in closed loops The west leg of the CME is approximately in the direction of the open fields suggesting the progressive interactions with these open field lines The close coupling among the CME the hard X-ray emission and the group of type III bursts and more particularly the close temporal relationship between the hard X-ray and type III emissions may be due to the dynamic processes between bipolar loops and the surrounding Title: Properties of acceleration sites in active regions as derived from heavy ion charge states Authors: Kartavykh, Y.; Dröge, W.; Klecker, B.; Möbius, E.; Popecki, M.; Mason, G.; Krucker, S. Bibcode: 2006cosp...36.1248K Altcode: 2006cosp.meet.1248K Charge states of heavy ions in solar energetic particle SEP events are determined by both the plasma conditions in the acceleration region and propagation effects The steep increase of the ionic charge of heavy ions as observed in all 3He- and Fe-rich SEP events suggests that stripping in a dense environment in the low corona is important in all these events The observed charge states and energy spectra of iron ions are used to infer the plasma conditions in the acceleration region by modelling the observations with a combined acceleration and propagation model that includes charge stripping acceleration coulomb losses and recombination in the corona and interplanetary propagation The interplanetary propagation includes anisotropic pitch-angle scattering on magnetic irregularities as well as magnetic focusing convection and adiabatic deceleration in the expanding solar wind To accurately derive the value of the scattering mean free path of particles the intensity profiles and anisotropy data from ACE and Wind spacecraft were used The comparison of the deduced parameters of the acceleration region with coronal density profiles shows that the acceleration of these ions takes place in closed magnetic structures in the low corona Title: A study of the solar injection for nine scatter-free impulsive electron events Authors: Wang, L.; Lin, R. P.; Krucker, S. Bibcode: 2006cosp...36.1406W Altcode: 2006cosp.meet.1406W Using WIND 3DP electron data from 1996 through 2005 we investigate the injection at the Sun of nine scatter-free impulsive electron events with good count statistics the electron energy range from sim 1keV to above 60 keV and the simple association with the solar and interplanetary type III radio burst observed by WIND WAVES Taking into account the effects of interplanetary propagation and instrumental response we find that the observed time profiles of electron fluxes at all energies fit quite well to triangular injections at the Sun with equal rise and fall times We find double injections One for lesssim 1 to 6-9 keV electrons begins 4 6 pm 2 6 min before the start of type III burst and lasts for an averaged duration of 90 min while a second for electrons above 9-13 keV starts 6 9 pm 0 7 min after the start of the type III burst and lasts for an average of 24 min Electrons of low-energy injection are likely to be the source of the type III burst The majority of events might have a B or C class GOES SXR flare related to the start of low-energy electron injection three might have a B or C class SXR flare related to the start of high-energy injection and five might have a west-limb CME with the velocity lesssim 600km s Title: A statistical study of solar energetic electron events over one solar cycle Authors: Wang, L.; Lin, R. P.; Krucker, S.; Mason, G. M. Bibcode: 2006cosp...36.1404W Altcode: 2006cosp.meet.1404W Using WIND 3DP electron data from 1995 through 2005 we study the number distribution of solar energetic electron events over one solar cycle We find a quite power-law relationship between the event number N and the event peak flux J dN dJ A cdot J - gamma Taking into account the effect of instrumental background we estimate the yearly number of electron events from the integration using the minimal peak flux observed at 3 and 40 keV We find that both the observed and estimated numbers of electron events show a time variation similar to the sunspot numbers We also present the statistical study of electron events accompanied by low-energy sim MeV nucleon and highly 3 He-rich ion emissions and estimate the effects of electron scattering in the interplanetary medium from in situ observations at 1 AU Title: Solar flare imaging in X-rays and g-rays Authors: Krucker, S. Bibcode: 2006ilws.conf...49K Altcode: Recent results of solar flare imaging at X-ray and γ-ray energies are briefly summarized using observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) Small Explorer mission. Title: Stereoscopic observations of Coronal Hard X-ray flare emissions with Ulysses and RHESSI Authors: Krucker, S.; Trottet, G.; McTiernan, J. M.; Lin, R. P. Bibcode: 2005AGUFMSH13A0289K Altcode: One of the major findings of the Yohkoh mission is the existence of coronal hard X-ray (HXR) sources above hot flare loops. The emission mechanism that produces these coronal sources is not understood. Coronal HXR sources are rather rarely observed, possible because of limited dynamic range of our observations that makes it difficult to see faint coronal sources next to bright footpoint emissions. For flares occurring just behind the solar limb, the footpoint emission can be occulted and coronal sources can therefore be observed independently. In this work, we were using Ulysses and RHESSI hard X-ray observations that often provides largely different flare view angles to study coronal HXR sources in partly occulted flares. Out of 112 flares seen by Ulysses, we find 5 flares that are partly occulted (5 to 15 degrees) from the RHESSI point of view, but are fully seen (i.e. on disk) by Ulysses. The flare with the best statistics (April 4, 2002, 16UT) reveals the existence of a coronal HXR source that is about 12±2 times fainter than the total HXR emission seen by Ulysses. The coronal source seen with RHESSI shows a relatively steep, progressively hardening power law spectra clearly favoring a non-thermal interpretation. The power law index decreases from ~4.5 at the beginning of the main peak to around 3.0 at the end with a value of ~3.5±0.1 at the peak. Reliable Ulysses spectra can only be derived during the peak of the event showing a power spectra slightly stepper around 3.0±0.2. The relatively small difference in the power law indices clearly excludes a simple explanation of thick target emission in footpoints and thin target emission in the corona. The observed spectral hardening can be partly explained by trapping of electrons in the corona. Title: Gamma-ray Imaging of the 2005 January 20 Solar Flare Authors: Hurford, G. J.; Krucker, S.; Lin, R. P.; Schwartz, R. A.; Smith, D. M. Bibcode: 2005AGUFMSH23A0313H Altcode: Solar gamma-ray emission provides the only available observational perspective on flare-accelerated ions near the Sun. Inelastic collisions between accelerated ions and the ambient solar atmosphere produces prompt line emission, a Doppler-broadened gamma-ray continuum as well as narrow lines due to positron annihilation (511 keV) and neutron capture (2.2 MeV). In addition to this ion-related emission, there is a significant electron-bremsstrahlung contribution from electrons accelerated to energies up to 10 MeV and beyond. The 2002 January 20 event provided one of the hardest ion and electron spectra observed to date. In this paper, we report on the results of gamma-ray imaging from ~100 keV through 7 MeV, including the gamma-ray continuum and neutron-capture line. Emphasis will be on the spatial distribution of the emission as a function of energy. 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: HXR spectral evolution and SEP production in the January 2005 X-class flares Authors: Saldanha, R.; Krucker, S.; Lin, R. P. Bibcode: 2005AGUFMSH23A0312S Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was used to study non-thermal hard X-ray spectra of four solar X-class flares of January 2005 (January 15th, 17th, 19th and 20th). Excluding the January 19th event, the other three flares were found to be associated with Solar Energetic Particles (SEPs). The three SEP related flares show progressive spectral hardening over individual flux peaks (a soft-hard-harder spectral evolution) with spectral power-law indices ranging from 4.0 to 2.0, while the January 19th flare displayed a more typical soft-hard-soft spectral evolution in the main peak (i.e. spectral power law indices are anti-correlated with the HXR flux). These findings are consistent with earlier results of Kiplinger (1995). Title: Hard X-ray Emission From Coronal Electron Beams Associated With Type III Radio Bursts Authors: Saint-Hilaire, P.; Krucker, S.; Lin, R. P. Bibcode: 2005AGUFMSH31A..03S Altcode: For the first time, a concerted effort to find X-ray emission from Type III-generating electron beams propagating in the solar corona has been undertaken. Spatially-integrated and imaging spectroscopy of partial thick-target hard X-ray bremsstrahlung emission from electron beams propagating outwards in the solar corona have been numerically modeled, and folded through the RHESSI (and GOES) instrumental response. The extensive RHESSI simulation software has been used to evaluate the possibility of detection of such X-rays. Under the assumption that Type III radio bursts are produced by the same kind of electron beams that generate the usually-observed flare hard X-rays (in our case, we are interested in beams going outwards rather than precipitating in the chromosphere), it is clear that they should be observable in X-rays with RHESSI, at least when the flare is occulted (footpoints behind the solar limb). I will present and discuss observations made with RHESSI and radio observatories. Title: RHESSI X-ray and Gamma-ray observations of the January 20, 2005 event Authors: Krucker, S.; Hurford, G. J.; Lin, R. P. Bibcode: 2005AGUFMSH21A..01K Altcode: Spacecraft and ground-based solar observations of the January 20, 2005 flare are summarized with a focus on X-ray and Gamma-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). X-ray and Gamma-ray observations are excellent diagnostics of accelerated particles: Accelerated electrons colliding with the ambient solar atmosphere produce bursts of bremsstrahlung hard X-rays; and collisions of accelerated ions with the atmosphere result in a complex spectrum of narrow and broad gamma-ray lines. In the January 20 flare, HXR emission is observed from footpoints of the flare loop. The statistics in the 2.2 MeV line emission is rather poor and a detailed spatial comparison with HXR images is inconclusive. The spectral evolution in HXRs during the main peak shows hardening during the event (so-called 'soft-hard-harder' behavior) as often observed from flares related to solar energetic particle events (Kiplinger 1995). The flare accelerated proton spectrum derived from the Gamma-ray observations is flat with a power law index of 2.3±0.3 and is surprisingly similar to the insitu observed proton spectrum near 1~AU (2.1±0.1). This suggest that the insitu observed protons are possibly flare associated and not necessarily shock accelerated. We will discuss this possibility also considering the relative timing between flare emissions, the coronal mass ejection, and the onset of solar energetic particles observed at 1 AU. Title: Marnetic and Energy Characteristics in Two-Ribbon X-Class Flares Authors: Yang, Y.; Ip, W.; Krucker, S.; Lin, R. Bibcode: 2005AGUFMSH13A0271Y Altcode: Several two-ribbon X-class flares which occurred near the solar disk center are investigated in this study. Some specific sub-regions within an active region are selected to estimate the change in magnetic flux from pre-flare to post-flare states. The high-cadence (1-min averaged) MDI magnetogram provides comprehensive photospheric magnetic measurement during these flares. The extreme ultraviolet observations in the wavelengths of 171, 195, and 1600 nm obtained by TRACE are used to search for the correlation of magnetic changes in the corona and transition region. We also used the RHESSI data to identify the hard X-ray source regions and compare them with the magnetic observations. We found that the magnetic fluxes within or in the vicinity of ribbons can change permanently after a flare eruption. Transient magnetic changes are also found in sub-regions. The characteristics of these X-class flares will be discussed in detail in this presentation. Title: Solar Impulsive Electron Events and RHESSI Hard X-ray Emission Authors: Christe, S.; Krucker, S.; Wang, L.; Lin, R. P. Bibcode: 2005AGUFMSH31A..04C Altcode: Solar impulsive electron events (SIEEs) are often related to radio type III bursts and are thought to be accelerated in the solar corona. It is therefore expected that they should produce X-rays through bremmsstrahlung whose flux is dependent on the height of the accelerator. Previous studies have shown that low energy SEIIs appear to be the source of type III radio bursts. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides unique sensitivity in the 3-15 keV energy range, with an effective area ~100 times larger than similar past instruments. RHESSI is therefore uniquely suited to study small events. The 3-D Plasma and Energetic Particle Instrument (3DP) on the WIND spacecraft measures in-situ energetic particles with energies from ~0.4 to 300 keV. WIND has observed a total of 547 SIEEs since the launch of RHESSI. A total of 303 of these events have simultaneous RHESSI observations and 444 events show an associated type III radio burst. We concentrate on low energy events as observed by 3DP (SEIIs energy < 20 keV) with an associated radio type III and simulatenous RHESSI observations. We find a total of 68 such events. We compare the X-ray producing and SEII particle populations and the expected to observed X-ray flux based on the acceleration height derived from the starting frequency of the metric type III observations. This work was supported by GSRP Grant NNM-05ZA12H and NASA contract NAS5-98033. Title: Pre-Impulsive Hard X-Ray Emission from Coronal Sources in X-Class Flares Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2005AGUFMSH13A0285C Altcode: The GOES-class X4.8 event on 23 July 2002 exhibits significant hard X-ray emission prior to the impulsive phase of the flare. Images from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) show that most of the emission appears to originate from a coronal source. Analysis of the Fe (~6.7 keV) and Fe/Ni (~8 keV) line complexes seen in RHESSI spectra suggests that there is little or no thermal continuum emission above ~15 keV, thus implying that non-thermal electron bremsstrahlung dominates the spectrum even to low energies. The X3.1 event on 24 Aug 2002 displays similar characteristics, and its position at the west limb occults the footpoints and shows that the bulk of the HXR emission unambiguously originates in the corona. Preliminary analysis shows that the pre-impulsive spectra are well-fit by a broken power-law characteristic of non-thermal emission. The spectral indices above and below the break energy differ by ~2, suggesting that we may be observing the transition between thin- and thick-target bremsstrahlung. The break energy increases with time, which suggests an average electron pitch angle of ~45°, assuming an increasing column density derived from the thermal emission measure. We analyze and compare the RHESSI X-ray spectra for the pre-impulsive phases of both 24 Aug 2002 and 23 July 2002, using the Fe and Fe/Ni line complexes to constrain the thermal parameters. We derive the time-varying characteristics of the electron populations and discuss the implications for acceleration and heating of electrons in the corona during this period. Title: A Study of the Solar Injection for Eleven Impulsive Electron/3He-Rich Sep Events Authors: Wang, L.; Lin, R. P.; Krucker, S.; Mason, G. M. Bibcode: 2005ESASP.592..457W Altcode: 2005soho...16E..81W; 2005ESASP.592E..81W No abstract at ADS Title: The Relationship between Synchrotron and Bremsstrahlung Emission of Nonthermal Electrons during a Solar Flare. A detailed study during the August 30, 2002 X1.5 event Authors: Giménez de Castro, C. G.; Silva, A. V. R.; Trottet, G.; Krucker, S.; Costa, J. E. R.; Kaufmann, P.; Correia, E.; Lüthi, T.; Magun, A.; Levato, H. Bibcode: 2005AIPC..784..566G Altcode: Synchrotron emission from nonthermal electrons has a strong dependence on the magnetic field of the medium. On the contrary, Bremsstrahlung emission does not depend on the magnetic field. The simultaneous observations of both forms of radiation may give us clues about local magnetic field configuration. In this report we use the optically thin part of the radio spectrum during the microwave maximum of the flare occurred on August 30, 2002, at 1328 UT to determine different mean magnetic field intensities and nonthermal electron density distributions compatible with the observed data. Assuming that the same electrons emit by coulomb interactions, the obtained distributions are used to compute the photon spectrum of the X-Ray emission by Bremsstrahlung and the spectra are compared with observations obtained by instruments on board the RHESSI satellite. We discuss the effects of the trapping on the Bremsstrahlung emitted radiation, giving constraints on both magnetic field intensity and trapping time. Title: Solar energetic electrons related to the 28 October 2003 flare Authors: Klassen, A.; Krucker, S.; Kunow, H.; Müller-Mellin, R.; Wimmer-Schweingruber, R.; Mann, G.; Posner, A. Bibcode: 2005JGRA..110.9S04K Altcode: 2005JGRA..11009S04K We investigate the solar origin of near-relativistic electrons and protons during the X17.2/4B flare as observed by the Comprehensive Suprathermal and Energetic Particle Analyser (COSTEP) and Three-Dimensional Plasma (3DP) analyzer experiments on board the SOHO and Wind spacecraft. These observations are combined with ground- and space-based spectral radio data obtained by the Potsdam spectrograph and the Wind/Waves instrument. Additionally, we use measurements of relativistic protons (ground-level event (GLE)) by neutron monitors (Kiel and Moscow). Timing and electron energy spectrum analysis suggest that there are three separate stages of electron injection into interplanetary space: (1) An injection of radio type III-producing electrons is observed first; (2) an impulsive injection with an almost symmetric time profile with a short duration (∼18 min) is released ∼11 min later, followed by (3) a gradual, long (>1 hour) lasting injection, with an onset ∼25 min after the first type III burst. While the first escaping type III-producing electrons are more likely related to the reconnection processes during the impulsive flare phase, the association of the two delayed electron injections with solar events is not well understood. Title: Radio and X-Ray Signatures of Magnetic Reconnection behind an Ejected Flux Rope Authors: Pick, M.; Démoulin, P.; Krucker, S.; Malandraki, O.; Maia, D. Bibcode: 2005ApJ...625.1019P Altcode: We present a detailed study of a complex solar event observed on 2002 June 2. Joint imaging EUV, X-ray, and multiwavelength radio observations allow us to trace the development of the magnetic structure involved in this solar event up to a radial distance of the order of 2 Rsolar. The event involves type II, III, and IV bursts. The type IV burst is formed by two sources: a fast-moving one (M) and a ``quasi-stationary'' one (S). The time coincidence in the flux peaks of these radio sources and the underlying hard X-ray sources implies a causal link. In the first part of our paper we provide a summary of the observations without reference to any coronal mass ejection (CME) model. The experimental results impose strong constraints on the physical processes. In the second part of our paper, we find that a model with an erupting twisted flux rope, with the formation of a current sheet behind, best relates the different observations in a coherent physical evolution (even if there is no direct evidence of the twisted flux rope). Our results show that multiwavelength radio imaging represents a powerful tool to trace the dynamical evolution of the reconnecting current sheet behind ejected flux ropes (in between sources M and S) and over an altitude range not accessible by X-ray observations. Title: Relationship of Solar Flare Accelerated Particles to Solar Energetic Particles (SEPs) Observed in the Interplanetary Medium Authors: Lin, R. P.; Shih, A. Y.; Krucker, S.; Smith, D. M.; Murphy, R.; Share, G.; Mewaldt, R.; Cohen, C. M.; Looper, M. D.; Mason, G. M.; Haggerty, D. K. Bibcode: 2005AGUSMSH33A..02L Altcode: Observations of hard X-ray/gamma-ray continuum and gamma-ray lines produced by energetic electrons and ions, respectively, colliding with the solar atmosphere, have shown that large solar flares can accelerate ions up to many GeV and electrons up to hundreds of MeV. Solar energetic particles (SEPs) are observed by spacecraft near 1 AU and by ground-based instrumentation to extend up to similar energies in large SEP events, but it is believed that a different acceleration process associated with fast Coronal mass Ejections (CMEs) is responsible. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) mission provides high-resolution spectroscopy and imaging of flare hard X-rays and gamma-rays. Such observations can provide information on the location, energy spectra, and composition of the flare accelerated energetic particles at the Sun. We compare the RHESSI observations of the 2003 October 28, 2003 November 2, and 2005 January 20 flares with both energetic electron and ion observations near 1 AU from ACE, SAMPEX, Wind,and GOES spacecraft, and discuss the implications for the particle acceleration and escape processes Title: Microflare Statistics and Frequency Distribution Authors: Christe, S. D.; Hannah, I.; Rauscher, E.; Krucker, S.; McTiernan, J.; Lin, R. P. Bibcode: 2005AGUSMSP42A..04C Altcode: RHESSI is uniquely suited to observe solar microflares due to its unique sensitivity in the 3-15 keV energy range (up to ~100 times better than previous solar instruments). As such, it provides new information on these low level transients. Initial results (Krucker et al 2002, Benz & Grigis 2002) suggest that microflares are different from larger flares. They are more often associated with steep nonthermal spectra (power law index -5 to -7). In this study, we present microflare statistics from times of low activity. A list of microflares was created by applying the standard RHESSI flare-finding algorithm to the lower 6-12 keV energy range (~10,000 events). Imaging was used in order to obtain positions of solar events and reject non-solar events. These solar events were then each spectrally analyzed. We present microflare statistics, including active region productivity, and the microflare frequency distribution. This work was supported by NASA contract NAS5-98033. Title: RHESSI Observations of Hard X-ray Footpoint Motions in Solar Flares Authors: Fivian, M. D.; Krucker, S.; Lin, R. P. Bibcode: 2005AGUSMSH31A..02F Altcode: Solar HXR bremsstrahlung from energetic electrons accelerated in the impulsive phase of a flare is observed to be primarily from the footpoints of magnetic loops. Standard magnetic reconnection models predict increasing separation of the footpoints during the flare as longer and larger loops are produced. If the reconnection process results in accelerated electrons, the HXR footpoints should show this motion.The motion is only apparent; it is due to the HXR emission shifting to footpoints of neighboring newly reconnected field lines. The speed of footpoint separation reflects the rate of magnetic reconnection and should be roughly proportional to the total energy deposition in the footpoints. We studied footpoint motions in several large flares with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and compared the motion with the energy deposition rate in the footpoints. A correlation between the motion and the energy deposition rate is found during some periods in these flares, but not always. Title: Coronal X-ray Emission in Long-Duration Occulted Flares Authors: Balciunaite, P.; Christe, S. D.; Krucker, S.; Lin, R. P. Bibcode: 2005AGUSMSP52A..06B Altcode: To investigate coronal X-ray (≥ 6 keV) emission using RHESSI, we select long-duration (≥ 3 h) flares with completely or partially occulted footpoints. Flare footpoint sources dominate and thus obstruct the observation of coronal emission; short durations do not allow enough time for observation of various changes. We examine the location, morphology, size, height, and spectral characteristics in 20 long-duration flares seen with RHESSI whose footpoints are occulted by the solar limb to at most 30°. In 4 cases we see hard X-ray (≥ 20 keV) coronal sources; in the remaining cases hard X-ray emission comes from partially occulted footpoints only or the coronal emission is thermal. Title: Comparison between impulsive 3He-rich events and energetic electron events Authors: Wang, L.; Lin, R. P.; Krucker, S.; Mason, G. M. Bibcode: 2005AGUSMSH33A..01W Altcode: Impulsive solar energetic particle (SEP) events with large enrichments of 3He are associated with ~2-100 keV impulsive electrons. Electron observations with the energy range of ~3 eV - 500 keV by the WIND 3-D Plasma and Energetic Particle experiment (3DP) and ion measurements with the energy range of ~ 0.02 - 10 MeV/nucleon by the ACE Ultra-Low Energy Isotopic Spectrometer (ULEIS) provide the first possibility of an accurate timing comparison of between impulsive 3He-rich events and energetic electron events. We select eleven solar impulsive events with enhanced 3He/4He ratios (~0.1 - 1.5) and a clear velocity dispersion of both ion and electron events over a wide energy range. We remove the contaminations of higher energy electrons in Solid State Telescopes (SST) on WIND, determine the interplanetary path length from peak times of WIND electron data observed in situ, and obtain the electron injection profiles at the Sun from triangular fits to in situ observations. The onsets and peaks of the injection of 3He-rich ion events at the Sun are derived from those of ACE ion data observed in situ by taking into account the travel time along the path length comparable to electron events. The comparison study shows a systematic delay of the injection of 3He-rich ions events with respect to the injection of electron events. Nine of ten events have a fast (> 570 km/s) west CME observed by SOHO/LASCO with the onset of electron injection close to the origin of the CME, and with the onset of ion injection corresponding to a median height ~ 5 Rs of CME. Title: Solar Hard X-ray Emissions in the Decay Phase of Gradual X-class Flares Authors: Krucker, S.; Lin, R. P. Bibcode: 2005AGUSMSH31A..01K Altcode: Solar hard X-ray emissions are most often observed during the impulsive phase of solar flares and are produced by non-thermal electrons that lose their energy in the chromosphere by collisions. During large gradual flares, however, HXR emissions can last well into the decay phase showing progressively hardening X-ray spectra (e.g. Cliver et al. 1986). Here we present X-ray observations of such events obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). RHESSI provides for the first time high resolution (1 keV) spectral observations with simultaneous imaging down to 2". In about half of all X-class flares seen by RHESSI, enhanced HXR emission lasting well into decay phase (up to two hours after the flare onset) is observed. These emissions are generally ~10 times fainter and show much less time variations than HXR bursts occurring during the impulsive phase. The X-ray spectra are found to be hard and can generally be fitted with broken power law functions. First results from one of the most prominent events (2005 January 19, 8UT) show clear spectral hardening. Over 15 minutes, the spectral power law index hardens below the break (from 2.6 to 2.1) as well as above it (from 3.3 to 2.6). Furthermore, the break energy increases from 40 to 60 keV. Imaging reveals that the emission come from footpoints of post flare loops suggesting that it is produced in a thick target. No HXR emission is seen to originate from the post flare loops. We are presently investigating the importance of trapping of energetic electrons in these events. 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: Coronal phenomena at the release of solar energetic electron events Authors: Klein, K. -L.; Krucker, S.; Trottet, G.; Hoang, S. Bibcode: 2005A&A...431.1047K Altcode: We investigate dynamical processes in the solar corona at the release of electrons (~30-500 keV) detected by the Wind/3DP experiment, with the aim to clarify the relationship between coronal acceleration and the escape of electrons to interplanetary space. Energetic electrons and plasma in the corona are traced using radio, EUV and X-ray observations. 40 events were identified where the release time of the electrons could be determined within an uncertainty of a few minutes and occurred during the observing hours of the Nançay Radioheliograph. All were accompanied by decametric-to-kilometric type III bursts (Wind/WAVES), and 30 by metric radio emission in the western hemisphere. The main findings from these 30 events are: (i) Electrons detected at Wind are released at the time of distinct episodes of electron acceleration in the corona signalled by radio emission. The release may occur at the start of the radio event or up to an hour later. (ii) The most conspicuous examples of delayed electron release occur in events associated with complex, long lasting (>10 min, up to seveal hours) radio emission. Radio observations suggest that in these cases the earlier accelerated electrons remain confined in the corona or are injected into flux tubes which are not connected to the spacecraft. (iii) Type II bursts revealing shock waves in the corona accompany about a third of the events. But the shock waves occur in general together with type IV radio signatures due to long lasting acceleration not related to the shock. With a few exceptions these type IV emissions have a clearer timing relationship with the electron release to space than the type II bursts. We conclude that the combination of time-extended acceleration at heights ≲0.5 R_⊙ above the photosphere with the injection of electrons into a variety of closed and open magnetic field structures explains the broad variety of timing shown by the radio observations and the in situ measurements.

Appendix A is only available in electronic form at http://www.edpsciences.org Title: New insingts into solar physics from RHESSI Authors: Krucker, S.; Lin, R. P. Bibcode: 2005ESASP.560..101K Altcode: 2005csss...13..101K No abstract at ADS Title: Hard X-ray footpoint motions in solar flares: Comparing magnetic reconnection models with observations Authors: Krucker, Säm; Fivian, M. D.; Lin, R. P. Bibcode: 2005AdSpR..35.1707K Altcode: Hard X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) of the October 29, 2003 GOES X10 two-ribbon flare are used together with magnetic field observations from the Michelson Doppler Imager (MDI) onboard SoHO to compare footpoint motions with predictions from magnetic reconnection models. The temporal variations of the velocity v of the hard X-ray footpoint motions and the photospheric magnetic field strength B in footpoints are investigated. The underlying photospheric magnetic field strength is generally higher ( B ∼ 700-1200 G) in the slower moving ( v ∼ 20-50 km s -1) western footpoint than in the faster ( v ∼ 20-100 km s -1) moving eastern source (∼100-600 G). Furthermore, a rough temporal correlation between the HXR flux and the product vB2 is observed. Title: Modulations of broad-band radio continua and X-ray emissions in the large X-ray flare on 03 November 2003 Authors: Dauphin, C.; Vilmer, N.; Lüthi, T.; Trottet, G.; Krucker, S.; Magun, A. Bibcode: 2005AdSpR..35.1805D Altcode: The GOES X3.9 flare on 03 November 2003 at ∼09:45 UT was observed from metric to millimetric wavelengths by the Nançay Radioheliograph (NRH), the Radio Solar Telescope Network (RSTN) and by radio instruments operated by the Institute of Applied Physics (University of Bern). This flare was simultaneously observed and imaged up to several 100 keV by the RHESSI experiment. The time profile of the X-ray emission above 100 keV and of the radio emissions shows two main parts, impulsive emission lasting about 3 min and long duration emission (partially observed by RHESSI) separated in time by 4 min. We shall focus here on the modulations of the broad-band radio continua and of the X-ray emissions observed in the second part of the flare. The observations suggest that gyrosynchrotron emission is the prevailing emission mechanism even at decimetric wavelengths for the broad-band radio emission. Following this interpretation, we deduce the density and the magnetic field of the decimetric sources and briefly comment on possible interpretations of the modulations. Title: CME Interaction and the Intensity of Solar Energetic Particle Events Authors: Gopalswamy, N.; Yashiro, S.; Krucker, S.; Howard, R. A. Bibcode: 2005IAUS..226..367G Altcode: Large Solar Energetic Particles (SEPs) are closely associated with coronal mass ejections (CMEs). The significant correlation observed between SEP intensity and CME speed has been considered as the evidence for such a close connection. The recent finding that SEP events with preceding wide CMEs are likely to have higher intensities compared to those without was attributed to the interaction of the CME-driven shocks with the preceding CMEs or with their aftermath. It is also possible that the intensity of SEPs may also be affected by the properties of the solar source region. In this study, we found that the active region area has no relation with the SEP intensity and CME speed, thus supporting the importance of CME interaction. However, there is a significant correlation between flare size and the active region area, which probably reflects the spatial scale of the flare phenomenon as compared to that of the CME-driven shock. 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: Intensity variation of large solar energetic particle events associated with coronal mass ejections Authors: Gopalswamy, N.; Yashiro, S.; Krucker, S.; Stenborg, G.; Howard, R. A. Bibcode: 2004JGRA..10912105G Altcode: We studied the coronal mass ejections (CMEs) and flares associated with large solar energetic particle (SEP) events of solar cycle 23 (1996-2002) in order to determine what property of the solar eruptions might order the SEP intensity. The SEP events were divided into three groups: (1) events in which the primary CME was preceded by one or more wide CMEs from the same solar source, (2) events with no such preceding CMEs, and (3) events in which the primary CME might have interacted with a streamer or with a nearby halo CME. The SEP intensities are distinct for groups 1 and 2 although the CME properties were nearly identical. Group 3 was similar to group 1. The primary findings of this study are as follows: (1) Higher SEP intensity results whenever a CME is preceded by another wide CME from the same source region. (2) The average flare size was also larger for high-intensity SEP events. (3) The intensity of SEP events with preceding CMEs showed a tighter correlation with CME speed. The extent of scatter in the CME speed versus SEP intensity plots was reduced when various subgroups were considered separately. (4) The intensities of energetic electrons were better correlated with flare size than with CME speed. (5) The SEP intensity showed poor correlation with the flare size, except for group 3 events. Since only a third of the events did not have preceding CMEs, we conclude that the majority of SEP producing CMEs propagate through the near-Sun interplanetary medium severely disturbed and distorted by the preceding CMEs. Furthermore, the preceding CMEs are faster and wider on the average, so they may provide seed particles for CME-driven shocks that follow. Therefore we conclude that the differing intensities of SEP events in the two groups may not have resulted due to the inherent properties of the CMEs. The presence of preceding CMEs seems to be the discriminating characteristic of the high- and low-intensity SEP events. Title: RHESSI Observations of Hard X-ray Footpoint Motions in Solar Flares Authors: Fivian, M. D.; Krucker, S.; Lin, R. P. Bibcode: 2004AGUFMSH13A1132F Altcode: Solar HXR bremsstrahlung from energetic electrons accelerated in the impulsive phase of a flare is observed to be primarily from the footpoints of magnetic loops. Standard magnetic reconnection models predict increasing separation of the footpoints during the flare as longer and larger loops are produced. If the reconnection process results in accelerated electrons, the HXR footpoints should show this motion. The motion is only apparent; it is due to the HXR emission shifting to footpoints of neighboring newly reconnected field lines. The speed of footpoint separation reflects the rate of

magnetic reconnection and should be roughly proportional to the total energy deposition in the footpoints. We studied footpoint motions in several large flares with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and compared the motion with the energy deposition rate in the footpoints. A correlation between the motion and the energy deposition rate is found during some periods in these flares, but not always. Title: Coronal Hard X-ray Emission in Occulted Flares Authors: Balciunaite, P.; Krucker, S.; Lin, R. P. Bibcode: 2004AGUFMSH13A1131B Altcode: We use RHESSI to investigate coronal sources in six behind-the-limb flares with occulted or partially occulted footpoints. The major findings are as follows: (1) Coronal hard X-ray (> 20 keV) emission is observed above thermal soft X-ray (< 12 keV) and EUV (195 Å) emission (which confirms Masuda's above-the-loop-top source); (2) Coronal HXR sources show impulsive (on a scale of tens of seconds) variation in their HXR time profiles; (3) The data can be fitted with a power law component with an index of 5 to 7 or with several thermal components with very high temperatures up to 100 MK. Presently, using the derived physical parameters, we test various heating models to determine whether the coronal HXR emission is thermal or non-thermal. Title: Nonthermal X-ray Microflares Authors: Christe, S.; Rauscher, E.; Krucker, S.; Lin, R. P. Bibcode: 2004AGUFMSH24A..03C Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides unique sensitivity in the 3-15 keV energy range, with an effective area ∼100 times larger than similar past instruments. Along with its high spectral resolution (1 keV) RHESSI is uniquely suited to study small events. Microflares have been observed by Benz & Grigis (2002) and Krucker et al. (2002) to have anomalously steep spectra ( spectral index between -5 and -8) extending down to ∼ 7 keV. Thermal emission is found to dominate below ∼ 7 keV. In many other respects, microflares show properties similar to larger flares. We present single event studies of different types of x-ray microflares. RHESSI observations during quiet times (04-May 10-14; GOES level low B class) reveal a set 5 microflares (>=A Class). These microflares show power law spectra (spectral index of ∼4-8) with little or no thermal emission in the 3- ∼7 keV energy range above the nonthermal part of the spectrum. Other microflares in the same GOES class range, however, have been found which show extremely hard spectra with emission up to 50 keV (power law index ∼2). At lower energies, emission is dominated by a hot thermal component (20 MK). This work was supported by NASA contract NAS5-98033. 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: Solar X-rays and Energetic Electrons Escaping from the Sun Authors: Krucker, S.; Kontar, E. P.; Lin, R. P. Bibcode: 2004AGUFMSH13A1129K Altcode: The Sun frequently accelerates electrons in solar flares and type III radio bursts. Some of the accelerated electrons lose their energy by collisions in the denser, lower solar atmosphere producing hard X-ray (HXR) emissions and heat the corona, while others escape into interplanetary space. Whether the HXR producing and the escaping electrons are accelerated by the same mechanism is not known. Combining RHESSI X-ray observations with in-situ observations of energetic electrons from the WIND spacecraft allows for the first time a detailed temporal, spatial, and spectral study. Statistical results of 16 events with a close temporal agreement between the HXR and the in-situ detected electrons (taking the time of flight of the escaping electrons into account) show a correlation between the HXR photon spectral index and the electron spectral index observed in-situ thus indicating a common acceleration mechanism. Furthermore, the solar X-ray source structure of these events look similar showing hot loops with HXR footpoints plus an additional HXR source separated from the loop by typically ~15". This source structure can be explained by a simple magnetic reconnection model with newly emerging flux tubes that reconnect with previously open field lines, so-called interchange reconnection. Events with a delayed timing between the HXRs and the solar release of escaping electrons (Krucker et al. 1999, Haggerty & Roelof 2002) are presently investigated. If these delayed events are indeed accelerated later in the event by shocks, no correlation between the HXR photon spectrum and the in-situ observed electron spectrum is expect to be found. Title: Spectra of Solar Energetic Electrons in Flares and near Earth Authors: Kontar, E. P.; Krucker, S.; Lin, R. P. Bibcode: 2004AGUFMSH13A1130K Altcode: Successful operation of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) allows us to observe hard X-ray spectrum of many solar flares with unprecendent energy resolution. X-ray spectra provide us with vital information about the spectral properties of highly energetic electrons at the Sun. The mean electron spectrum of the solar flare can be recovered through a newly developed constraint regularization analysis of the bremsstrahlung photon spectra that energetic electrons produce. We emphasize the use of non-square inversion techniques combined with the correction of the observed spectrum for the effect of Compton photon back-scatter to achieve the most meaningful solution to the problem. For 16 solar flares the electron spectrum of temporally related solar energetic electron events has been measured using WIND/3DP allowing us simultaneous analysis of electron flux spectral properties at the Sun and near the Earth. Electron spectra from solar flares show strong correlations with the spectrum of solar energetic electrons, though vary with energy. Weaker correlation at lower energies can be viewed as a propogation and/or escape effect. Results suggest that the commonly used model of a collisional transport (thick-target) for flare electrons plus free streaming for interplanetary particles cannot explain the observed spectra. 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: Comparison between impulsive energetic electron events and 3He-rich events Authors: Wang, L.; Mason, G.; Lin, R. P.; Krucker, S. Bibcode: 2004AGUFMSH31A1153W Altcode: We present a comparison between electron data by WIND/3DP and 3He data by ACE/ULEIS. Our study shows that impulsive energetic electron events occur more often than impulsive 3He-rich events. For a set of 41 3He-rich events from 1997 November to 2003 April, WIND/3DP has good measurements during 29 events. An injection analysis shows that among the 29 3He-rich events, 13 events with the 3He/4He ratio ( ∼0.002-0.34) have an electron event with an injection time difference < 1 hour, 6 events with the 3He/4He ratio ( ∼0.081-6.2) have an electron event with an injection time difference of 1-2 hours, and the other 8 events with the 3He/4He ratio ( ∼0.0054-0.25) have an electron event with a injection difference > 2 hour or have no electron events. Further analysis including type III bursts and energy spectra may provide important information about the association between 3He-rich events and electron events. Title: Comparing Solar Hard X-ray Emissions and Impulsive Electron Events seen at 1AU Authors: Krucker, S.; Lin, R. P.; Kontar, E. P. Bibcode: 2004AGUSMSH22A..02K Altcode: The Sun frequently accelerates electrons in solar flares and type III radio bursts. Some of the accelerated electrons lose their energy by collisions in the denser, lower solar atmosphere producing hard X-ray (HXR) emissions, while others escape into interplanetary space. Whether the HXR producing and the escaping electrons are accelerated by the same mechanism is not known. We present a combined study of RHESSI X-ray observations and WIND/3dp in situ electron observations taken near 1~AU. Electron events with a solar release time in close temporal agreement with the HXR peak time are selected. For these events, the electron spectrum measured at 1~AU is compared with the electron spectrum derived from the HXR observations. We find the derived and the observed electron spectrum do not agree with a simple model of electron acceleration high in the corona with downward moving electrons producing HXRs in the lower, denser corona (thick target model) and upwards moving electrons escaping into interplanetary space without energy changes; the observed electron spectrum at 1 AU would predict a much harder HXR spectrum than what is observed. More complicated models including the effects of how particle escape from the acceleration cite are need. That a high coronal acceleration can be excluded makes it hard to explain how the low energy electrons, down to a few hundred eV, can escape to ~1~AU. This suggests that two different mechanisms may be accelerating electrons and that the HXR emission is not related to the electrons seen at 1~AU despite the close temporal correlation. Title: Above the Loop-top Non-thermal Hard X-ray Source in a Partially Occulted Flare Authors: Balciunaite, P.; Krucker, S.; Lin, R. P. Bibcode: 2004AAS...204.5407B Altcode: 2004BAAS...36..758B We present RHESSI hard X-ray imaging, spectroscopy, and source motion analysis of a long-duration GOES M 4.5 class flare (18 Nov 2003 0923-1101 UT), whose footpoints were occulted by the east limb of the Sun. During the rise phase of the flare a non-thermal hard X-ray source is seen above 20 keV with a spectral power-law index around 6. The source is initially detected 50 arcseconds above the limb and seen to rise with a velocity of 12 km/s. Its time profile varies on a scale of tens of seconds. At lower energies, a thermal hard X-ray source with a temperature of up to 17 MK is detected 10 arcseconds below the non-thermal source, and seen to be rising with a lower velocity of 7 km/s. Its time variation is slow and gradual, similar to that of the GOES time profile. Both sources are seen above GOES SXI soft X-ray and SOHO EIT EUV loops. We also discuss the energetics of the non-thermal above-the-loop-top source. Title: Characterizing thermal and non-thermal electron populations in solar flares using RHESSI Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2004AAS...204.8704C Altcode: 2004BAAS...36..819C Solar flare plasmas contain both non-thermal and thermal electron populations, with the latter often reaching temperatures above ∼10 MK. These high-temperature plasmas emit a characteristic line and continuum X-ray spectrum, including emission in the Fe/Ni line complexes at ∼6.7 and ∼8 keV. The fluxes and equivalent widths of these line complexes are strongly temperature-dependent, and thus they are a useful probe of thermal flare plasmas. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes photons with energies above 3 keV, with a spectral resolution of ∼1 keV FWHM, and is especially sensitive to flare plasmas above ∼10 MK. In many flares, RHESSI detects emission in both of the Fe/Ni line complexes, as well as in the thermal continuum. Recent results using RHESSI for full-Sun spectroscopy show that the relationship between the 6.7- to 8-keV line flux ratio and the continuum temperature does not agree with theoretical values, and further, varies from flare to flare. RHESSI provides imaging spectroscopy, and hence the individual spectra of spatially-separated sources can be obtained and and the source morphology can be studied at different energies. We perform such an analysis on a variety of flares, including the X4.8 event on 23 July 2002 and the X1.5 event on 21 April 2002, to determine the time-varying characteristics of the multiple thermal and non-thermal electron populations. We estimate and compare the energy contained in these electron populations, and discuss the implications for heating and energy transport in solar flares. Title: Scatter-free Impulsive Electron Event from the October 28, 2003 X17 Flare Authors: Krucker, S.; Lin, R. P. Bibcode: 2004AAS...204.0206K Altcode: 2004BAAS...36..668K The October 28, 11 UT X-17 flare produced an impulsive electron event detected near the Earth by the WIND 3D Plasma & Energetic Particles instrument from ∼200 keV down to ∼1 keV, even though the flare was located around E08 and therefore unlikely to be magnetically connected to Earth. This impulsive event exhibited a fast rise and decay ( ∼10 min each), and was followed by a second, much slower rising event that lasted for many hours. The peak times of the impulsive event show clear velocity dispersion consistent with a path length of 1.27±0.1 AU, indicating that the electron propagation is essentially scatter-free. Onset time analysis reveal that the impulsive event electrons left the Sun at 11:14±3 UT, just after the end of a group of type III bursts seen at 14 MHz (11:03-11:11 UT) and when the type II burst reaches 14 MHz. Radio pulsations at 0.6-1.3 GHz were seen between 11:14:30 and 11:22:00 UT. The velocity dispersion seen in the peak times suggests that the peak of the injection occurred around 11:25±3 UT roughly consistent with the end of the decimeter radio emission. This impulsive electron event also was observed near Mars at energies from ∼1 to 20 keV by the Electron Reflectometer instrument on Mars Global Surveyor spacecraft. Title: Solar Origin of Interplanetary Impulsive Electron Events Authors: Pick, M.; Maia, D. J.; Malandraki, O.; Krucker, S.; Demoulin, P. Bibcode: 2004AGUSMSH22A..03P Altcode: Many solar impulsive electron events have been traditionally associated with type III radio emissions. Several recent studies however showed that, in the majority of the events, the solar release of electrons at high energies can present delays of up-to-half an hour with respect of the onset of type III bursts. We have revisited the origin of a large number of events using multiwave-length observations. For each event, we investigated the coronal restructuring using EUV, white-light, radio imaging and spectral observations in a wide frequency range that allows us to follow the evolution of the corona from a few tenths above the solar limb up to a few solar radii. Radio observations revealed direct energetic electron signatures, close in time with the electron release. The release time for the delayed events always coincides with the onset or major changes in the complex of radio emissions. This close association indicates that the coronal processes involved in the radio emissions are at the origin of the electron acceleration. We illustrate our results by presenting, more particularly, one recent event for which the observations were also coupled with imaging spectroscopy measurements obtained by the RHESSI mission (from 3 keV to 17 MeV). RHESSI observed a hard X-ray emission, which lasted for more than fifteen minutes. This emission was closely associated in time and space with the radio emission imaged by the Nançay Radioheliograph. The results suggest that, for this event, both electrons detected in the corona and those injected in the interplanetary medium are due to a similar process involving coronal magnetic field interactions. Their respective sites of acceleration/injection are however distinct in space and time. The energetic electrons detected in the interplanetary medium are not released during the X-ray burst. Title: RHESSI Gamma-ray Imaging of the Flares of October/November 2003 Authors: Hurford, G. J.; Krucker, S.; Lin, R. P.; Schwartz, R. A.; Share, G. H.; Smith, D. M. Bibcode: 2004AAS...204.0202H Altcode: 2004BAAS...36..667H Imaging of gamma-ray line emission from solar flares provides direct information on the spatial characteristics of accelerated ions near the Sun. To date this has been possible for only one event (July 23, 2002) whose neutron-capture line image at 2.2 MeV implied a significant spatial difference between high energy ions and electrons in acceleration and/or transport.

Observations of the X17, X10 and X8 events on October 28, October 29 and November 2, 2003 by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provided a ten-fold increase in the detected nuclear-line photons available for imaging. This paper presents the results of imaging of these flares in the 2.2 MeV neutron capture line, including source size, location and motion estimates and comparisons with simultaneous gamma-ray and x-ray emission in the electron-bremsstrahlung continuum.

This work is supported by NASA contract NAS5-98033. Title: The Microflare Frequency Distribution observed by RHESSI Authors: Christe, S.; Krucker, S.; Lin, R. P.; Hannah, I. Bibcode: 2004AAS...204.8703C Altcode: 2004BAAS...36R.818C The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides uniquely high sensitivity in the 3-15 keV energy range with an effective area from 14 to 130 times larger then previous solar instruments. We present a microflare frequency distribution derived from RHESSI observations. Times were chosen such that activity is very low (GOES B Class background) and includes a 24 hour period when no active regions were present on the Sun. Microflares were found through searching for 5σ increases in count rate, summing over all detectors, between two adjacent time bins in the 3-10 keV energy band. Count rates were binned from 4 seconds to 3 minutes, increasing by 4 seconds increments, and the search repeated in order to detect flares of various time scales. Each microflare was individually checked in order to reject non-solar events. Few microflares are seen to occur below the time scale of minutes. The observed occurence rate is one microflare every ∼ 8 minutes above a threshold flux of ∼ 0.4 cm-2 s-1 keV-1 over the 5-10 keV energy range. Microflares were also found when no active regions were present on the Solar disk. This work was supported by NASA contract NAS5-98033. 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: High-frequency slowly drifting structures and X-ray sources observed by RHESSI Authors: Karlický, M.; Fárník, F.; Krucker, S. Bibcode: 2004A&A...419..365K Altcode: Three solar flares (April 4, 2002, May 17, 2002, and August 30, 2002) with the 0.4-2.0 GHz slowly drifting structures were selected and analyzed together with RHESSI X-ray observations. Two events (April 4, 2002 and May 17, 2002) were observed above and one event (August 30, 2002) close to the solar limb. While in April 4, 2002 and August 30, 2002 the radio drifting structures with relatively high frequency drifts (-32- -25 MHz s-1) were recorded at times of the start of a motion of the X-ray flare source, in May 17, 2002 event a splitting of the X-ray source into two sources was observed before observation of the 0.8-1.8 GHz radio structure drifting with very slow frequency drift (-0.4 MHz s-1). The X-ray source of the May 17, 2002 was much softer (<40 keV) than those in April 4, 2002 and August 30, 2002 (>100 keV). Velocities of the X-ray sources in the image plane were estimated as 12 km s-1 for April 4, 2002 and 10 km s-1 for August 30, 2002. Analyzing GOES data and X-ray RHESSI spectra of the May 17, 2002 flare the plasma thermal and non-thermal electron densities in the X-ray sources were determined. For two cases (April 4, 2002 and May 17, 2002) it was found that the plasma density in the coronal X-ray source is higher than maximum one derived from the radio drifting structure. The cross-correlation of the radio drifting structure and hard X-ray flux for the August 30, 2002 event reveals that the hard X-ray emission is delayed 0.5-0.7 s after the radio and it is partly correlated with an enhanced background of the drifting structure. All these results are discussed and interpreted considering the flare model with the plasmoid ejection. Title: RHESSI Observations of Hard X-ray Microflares with Interplanetary Type III Radio Bursts Authors: Rauscher, E.; Christe, S.; Krucker, S.; Lin, R. P. Bibcode: 2004AAS...204.5408R Altcode: 2004BAAS...36..758R RHESSI's increased sensitivity compared to previous instruments (100 times at 10 keV) and its ability to create high resolution images makes it ideal to study microflares. During the period of 2003 May 11 through 2003 May 13, RHESSI observed a series of microflares, eight of which occurred with temporally coincident interplanetary type III radio bursts, as observed by WIND (1-14 MHz). Statistically we would have expected one coincident event. These flares range from GOES class A1 to B1 above an average background of B1. Imaging shows that many of these flares originate from the same active region. For three of the flares TRACE 1600Å movies show similar geometric structures, with two bright spots spatially coincident with X-ray emission (3-15 keV) and jets leaving the flare site. A similar set of events was analyzed by Christe et al (SPD 2003). We present an investigation of the structure of these flares in order to determine why they are associated with type III bursts. Title: Title Requested Authors: Krucker, S.; RHESSI Team Bibcode: 2004AAS...204.4704K Altcode: 2004BAAS...36R.737K The high solar activity during the last weeks of October and beginning of November 2003 with 11 X-class flares provides a great opportunity to study particle acceleration in flares. Recent results of multi-spacecraft and ground-based observations are summarized with a focus on X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Title: Triggering of the Two X-class Flares of 28 and 29 October 2003 Authors: Choudhary, D. P.; Moore, R. L.; Falconer, D.; Pojoga, S.; Tian-Sen, H.; Krucker, S.; Uddin, W. Bibcode: 2004AAS...204.0225C Altcode: 2004BAAS...36..982C From H-alpha movies from Aryabhatta Research Institute of Observational Sciences and from Prairie View Solar Observatory, hard X-ray movies from RHESSI, line-of-sight magnetogram movies from SOHO/MDI, and vector magnetograms from Marshal Space Flight Center, we examine the magnetic structure and evolution of the large delta-sunspot active region NOAA 10486 in relation to the onset and development of the two X-class flares that occurred in this active region on 28 and 29 October 2003. We find evidence that each of these flares was triggered by strongly sheared magnetic field via ``tether-cutting" reconnection with adjacent/overlying strongly sheared field. In the first flare, the initial brightening in H-alpha (1) was partly rooted in emerging sheared magnetic field along the edge of the large positive-polarity flux domain of the delta sunspot, and (2) consisted of four flare kernels, two in negative magnetic flux and two in positive magnetic flux. In the second flare, the brightening started in the core of a Z-shaped sigmoidal sheared magnetic field and the inner two of four H-alpha kernels were visible in 30-50 Kev hard x-ray image from RHESSI. Each flare spread from the initial quadrupolar brightening to develop into a much larger two-ribbon flare straddling a much more extensive swath of strongly sheared field along the edge of the large positive-flux domain of the delta sunspot, the first flare on the leading side and the second flare on the trailing side of this domain. Thus, localized internal reconnection triggered the explosion of these extensive sheared magnetic fields.

This research was supported by NASA's Office of Space Science through the Solar and Heliospheric Physics SR&T Program, and was done during Dr. Choudhary's tenure at MSFC/NSSTC as an NRC Senior Resident Research Associate. Title: Scatter-free impulsive electron event from the October 28, 2003 X17 flare Authors: Krucker, S.; Lin, R. P.; Mitchell, D. Bibcode: 2004AGUSMSH51A..10K Altcode: The October 28, 11 UT X-17 flare produced an impulsive electron event detected near the Earth by the WIND 3D Plasma & Energetic Particles instrument from ~200 keV down to ~1~keV, even though the flare was located around E08 and therefore unlikely to be magnetically connected to Earth. This impulsive event exhibited a fast rise and decay (~10 min each), and was followed by a second, much slower rising event that lasted for many hours. The peak times of the impulsive event show clear velocity dispersion consistent with a path length of 1.27±0.1 AU, indicating that the electron propagation is essentially scatter-free. Onset time analysis reveal that the impulsive event electrons left the Sun at 11:14±3 UT, just after the end of a group of type III bursts seen at 14~MHz (11:03-11:11~UT) and when the type II burst reaches 14~MHz. Radio pulsations at 0.6-1.3~GHz were seen between 11:14:30 and 11:22:00~UT. The velocity dispersion seen in the peak times suggests that the peak of the injection occurred around 11:25±3~UT roughly consistent with the end of the decimeter radio emission. This impulsive electron event also was observed near Mars at energies from ~1 to 20~keV by the Electron Reflectometer instrument on Mars Global Surveyor spacecraft. Title: Magnetic Field, Hα, and RHESSI Observations of the 2002 July 23 Gamma-Ray Flare Authors: Yurchyshyn, Vasyl; Wang, Haimin; Abramenko, Valentyna; Spirock, Thomas J.; Krucker, Säm Bibcode: 2004ApJ...605..546Y Altcode: In this paper we examine two aspects of the 2002 July 23 gamma-ray flare by using multiwavelength observations. First, the data suggest that the interaction of the erupted field with an overlying large-scale coronal field can explain the offset between the gamma-ray and the hard X-ray sources observed in this event. Second, we pay attention to rapid and permanent changes in the photospheric magnetic field associated with the flare. MDI and BBSO magnetograms show that the following magnetic flux had rapidly decreased by 1×1020 Mx immediately after the flare, while the leading polarity was gradually increasing for several hours after the flare. Our study also suggests that the changes were most probably associated with the emergence of new flux and the reorientation of the magnetic field lines. We interpret the magnetograph and spectral data for this event in terms of the tether-cutting model. Title: Studies of Microflares in RHESSI Hard X-Ray, Big Bear Solar Observatory Hα, and Michelson Doppler Imager Magnetograms Authors: Liu, Chang; Qiu, Jiong; Gary, Dale E.; Krucker, Säm; Wang, Haimin Bibcode: 2004ApJ...604..442L Altcode: In this paper, we present a study of the morphology of 12 microflares jointly observed by RHESSI in the energy range from 3 to 15 keV and by Big Bear Solar Observatory (BBSO) at the Hα line. They are A2-B3 events in GOES classification. From their time profiles, we find that all of these microflares are seen in soft X-ray, hard X-ray, and Hα wavelengths, and their temporal evolution resembles that of large flares. Co-aligned hard X-ray, Hα, and magnetic field observations show that the events all occurred in active regions and were located near magnetic neutral lines. In almost all of the events, the hard X-ray sources are elongated structures connecting two Hα bright kernels in opposite magnetic fields. These results suggest that, similar to large flares, the X-ray sources of the microflares represent emission from small magnetic loops and that the Hα bright kernels indicate emission at footpoints of these flare loops in the lower atmosphere. Among the 12 microflares, we include five events that are clearly associated with type III radio bursts as observed by the radio spectrometer on board Wind. Spectral fitting results indicate the nonthermal origin of the X-ray emission at over ~10 keV during the impulsive phase of all the events, and the photon spectra of the microflares associated with type III bursts are generally harder than those without type III bursts. TRACE observations at EUV wavelengths are available for five events in our list, and in two of these, coincident EUV jets are clearly identified to be spatially associated with the microflares. Such findings suggest that some microflares are produced by magnetic reconnection, which results in closed compact loops and open field lines. Electrons accelerated during the flare escape along the open field lines to interplanetary space. Title: Nobeyama Radioheliograph and RHESSI Observations of the X1.5 Flare of 2002 April 21 Authors: Kundu, M. R.; Garaimov, V. I.; White, S. M.; Krucker, S. Bibcode: 2004ApJ...600.1052K Altcode: We present an overview of the radio observations of the X1.5 flare of 2002 April 21 and complementary data from other wavelengths. This flare was fairly well observed by the Ramaty High Energy Spectroscopic Imager (RHESSI) spacecraft and fully observed by the Nobeyama Radioheliograph (NoRH) at 17 and 34 GHz. This long-duration event lasted more than 2 hr and featured a beautiful arcade of rising loops on the limb visible at X-ray, EUV, and radio wavelengths. The main flare was preceded by a small event 90 minutes earlier showing a long EUV loop connecting well-separated radio and hard X-ray sources. The main flare itself starts with a compact radio and hard X-ray source at the eastern end of the region that develops into emission close to the solar surface (and well inside the solar limb) over a large region to the northwest. As the flare proceeds, a large set of loops is seen to rise well above the solar limb. Distinct regions of radio emission with very different time behavior can be identified in the radio images, and, in particular, a peculiar nonthermal source seen in radio and hard X-rays low in the corona at the base of the arcade is seen to turn on 30 minutes after the start of the impulsive phase. At about the same time, an extremely intense burst of coherent radio emission is seen from 500 to 2000 MHz; we speculate that this lower-frequency burst is produced by electrons that are accelerated in the nonthermal source at the base of the arcade and injected into the loop system where they radiate plasma emission in the 1010 cm-3 density plasma at the top of the arcade of loops. This event is striking as a demonstration of the many ways in which a flare can produce radio emission, and the combined data at different wavelengths reveal a diversity of energy release and nonthermal acceleration sites. Title: Characterizing thermal and non-thermal electron populations in solar flares using RHESSI Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2004cosp...35.3582C Altcode: 2004cosp.meet.3582C Solar flare plasmas contain both non-thermal and thermal electron populations, with the latter often reaching temperatures above ∼10 MK. These high-temperature plasmas emit a characteristic line and continuum X-ray spectrum, including emission in the Fe/Ni line complexes at ∼6.7 and ∼8 keV. The fluxes and equivalent widths of these line complexes are strongly temperature-dependent, and thus they are a useful probe of thermal flare plasmas. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes photons with energies above 3 keV, with a spectral resolution of ∼1 keV FWHM, and is especially sensitive to flare plasmas above ∼10 MK. In many flares, RHESSI detects emission in both of the Fe/Ni line complexes, as well as in the thermal continuum. Recent results using RHESSI for full-Sun spectroscopy show that the relationship between the 6.7- to 8-keV line flux ratio and the continuum temperature does not agree with theoretical values, and further, varies from flare to flare. RHESSI provides imaging spectroscopy, and hence the individual spectra of spatially-separated sources can be obtained and and the source morphology can be studied at different energies. We perform such an analysis on a variety of flares, including the X4.8 event on 23 July 2002 and the X1.5 event on 21 April 2002, to determine the time-varying characteristics of the multiple thermal and non-thermal electron populations. We estimate and compare the energy contained in these electron populations, and discuss the implications for heating and energy transport in solar flares. Title: RHESSI Hard X-ray Observations during the October/November 2003 Events Authors: Krucker, S.; Fivian, M. D.; Lin, R. P. Bibcode: 2004cosp...35.1789K Altcode: 2004cosp.meet.1789K The high solar activity during October-November 2003 with 11 X-class flares provides a great opportunity to study particle acceleration in flares. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) coverage was best for the October 29, ∼20UT (GOES X-11) event, missing only the later decay phase. This event shows a two ribbon structure with non-thermal hard X-ray emission from the ribbons and thermal emission from the loops connecting the ribbons. The ribbons move apart with averaged speeds of ∼35 km s-1 and ∼15 km s-1, respectively. The faster moving ribbon travels through a region with lower magnetic field strength (∼400-800 G) whereas the slower moving ribbons goes through a stronger field region (∼1000-1700 G). This is roughly consistent with the simplest models of magnetic reconnection theory, predicting B v ≈ const. We are analyzing the relation between the footpoint motions, the magnetic field strength and the total released energy derived from the HXR observations. If magnetic reconnection is indeed responsible for the observed energy release, the temporal evolution of the footpoint speeds and the magnetic field strength at the footpoints should be correlated with the energy deposited in the HXR footpoints. Title: X-ray and radio observations of energetic electrons produced in the 3 November 2003 solar flare at ~09:5000 UT Authors: Dauphin, C.; Vilmer, N.; Lüthi, T.; Magun, A.; Krucker, S.; Schwartz, R.; Trottet, G. Bibcode: 2004cosp...35.2506D Altcode: 2004cosp.meet.2506D Hard X-ray and radio observations provide complementary observations of energetic electrons produced in solar flares. The GOES X4 flare on 03 November 2003 at ∼ 09:50 UT was observed and imaged up to several 100 keV by the RHESSI experiment. It was simultaneously observed at metric/decimetric wavelengths by the Nançay Radioheliograph (NRH) and at centimetric/millimetric wavelengths by radio instruments operated by the Institute of Applied Physics (University of Bern). We present in this contribution an analysis of these radio and X-ray data. The time profiles of the X-ray emission above 50 keV and of the centimetric/millimetric emissions show two main parts (impulsive emission lasting about three minutes) and a long duration emission (partially observed by RHESSI) separated in time by four minutes. At metric/decimetric wavelengths a type II burst with an unusually high frequency is observed between the impulsive emissions and the long duration radio continuum. Combined analysis of RHESSI sources at energies above a few hundred keV and of metric/decimetric sources observed by the NRH shows the extension in space of both X-ray and radio sources traced by energetic electrons between the impulsive part of the event and the late energetic X-ray phase associated with the strong radio continuum. Spectral analysis of the high energy X-ray continuum and of the centimetric/millimetric will be performed to infer the characteristics of energetic electrons in both parts of the events and to further investigate in this event the relationship between centimetric-millimetric emitting electrons and HXR/GR bremsstrahlung emitting ones. Title: On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy Authors: Aschwanden, Markus J.; Metcalf, Thomas R.; Krucker, Säm; Sato, Jun; Conway, Andrew J.; Hurford, G. J.; Schmahl, Edward J. Bibcode: 2004SoPh..219..149A Altcode: 2003astro.ph..9499A We compare the photometric accuracy of spectra and images in flares observed with the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft. We test the accuracy of the photometry by comparing the photon fluxes obtained in different energy ranges from the spectral-fitting software SPEX with those fluxes contained in the images reconstructed with the Clean, MEM, MEM-Vis, Pixon, and Forward-fit algorithms. We quantify also the background fluxes, the fidelity of source geometries, and spatial spectra reconstructed with the five image reconstruction algorithms. We investigate the effects of grid selection, pixel size, field of view, and time intervals on the quality of image reconstruction. The detailed parameters and statistics are provided in an accompanying CD-ROM and web page. We find that Forward-fit, Pixon, and Clean have a robust convergence behavior and a photometric accuracy in the order of a few percent, while MEM does not converge optimally for large degrees of freedom (for large field of view and/or small pixel sizes), and MEM-Vis suffers in the case of time-variable sources. This comparative study documents the current status of the RHESSI spectral and imaging software, one year after launch. Title: The Microflare Frequency Distribution observed by RHESSI Authors: Christe, S.; Krucker, S.; Lin, R. P. Bibcode: 2004cosp...35.3164C Altcode: 2004cosp.meet.3164C The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides uniquely high sensitivity in the 3-15 keV energy range with an effective area from 14 to 130 times larger then previous solar instruments. We present a microflare frequency distribution derived from RHESSI observations. Times were chosen such that activity is very low (GOES B Class background) and includes a 24 hour period when no active regions were present on the Sun. Microflares were found through searching for 5σ increases in count rate, summing over all detectors, between two adjacent time bins in the 3-10 keV energy band. Count rates were binned from 4 seconds to 3 minutes, increasing by 4 seconds increments, and the search repeated in order to detect flares of various time scales. Each microflare was individually checked in order to reject non-solar events. Few microflares are seen to occur below the time scale of minutes. The observed occurence rate is one microflare every ∼ 8 minutes above a threshold flux of ∼ 0.4 cm-2 s-1 keV-1 over the 5-10 keV energy range. Microflares were also found when no active regions were present on the Solar disk. This work was supported by NASA contract NAS5-98033. Title: Gamma-ray imaging of the X-class flares of October/November 2003 Authors: Hurford, G.; Krucker, S.; Lin, R.; Schwartz, R.; Share, G.; Smith, D. Bibcode: 2004cosp...35.2402H Altcode: 2004cosp.meet.2402H Imaging of gamma-ray line emission from solar flares provides direct information on the spatial characteristics of accelerated ions near the Sun. To date this has been possible for only one event (July 23, 2002) whose neutron-capture line image at 2.2 MeV implied a significant spatial difference between high-energy ions and electrons in acceleration and/or transport. Observations of the X17, X10 and X8 events on October 28, October 29 and November 2, 2003 by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provided a ten-fold increase in the detected nuclear-line photons available for imaging. This paper presents the results of imaging of these flares in the 2.2 MeV neutron capture line, including source size, location and motion estimates and comparisons with simultaneous gamma-ray and X-ray emission in the electron-bremsstrahlung continuum. This work is supported by NASA contract NAS5-98033. Title: Variations in Solar X-ray Flux and its Relevance to the Coronal Heating Problem Authors: Benz, A. O.; Grigis, P. C.; Krucker, S. Bibcode: 2004cosp...35..343B Altcode: 2004cosp.meet..343B The thermal X-ray and EUV emissions of the solar corona are known to fluctuate in time and space. The fluctuations seem to be of a similar nature as the flares and thus are termed microflares or nanoflares. Several forms of such small events have been observed in the solar corona by SoHO, TRACE, Yohkoh, and RHESSI. The reported nanoevents in the quiet regions are about 3 orders of magnitude smaller than microevents reported in active regions and are radio-poor. Magnetic energy dissipation by small flares is one of the scenarios for coronal heating. These micro-events obviously increase the energy in the corona and are signatures of coronal heating, the question is whether they dominate coronal heating and are the cause of the existence of the corona. The main uncertainty in determining the role of flare heating is the flare energetics, in particular the forms of energy into which the magnetic energy is dissipated. A large fraction becomes observable in electrons having energies of some tens of keV. Another less known fraction is thought to be associated with the reconnection jets and MHD waves. The heating process(es) must be able to account for the coronae of the quiet sun including coronal holes, active regions, as well as more active stars showing coronal emissions at levels of more than 3 orders of magnitude higher than the Sun. Title: Impulsive Electron Events seen at 1 AU and their Solar Origin: a Review of Recent Results Authors: Krucker, S. Bibcode: 2004cosp...35.1773K Altcode: 2004cosp.meet.1773K Thanks to new spacecraft missions like WIND and ACE, new insights into the solar origin of impulsive electron events seen at 1 AU are revealed. Onset time studies show that there are likely at least two different mechanisms that release electrons into interplanetary space: 1) electron events that are related to radio type III bursts 2) and a later released, second population at higher energies (∼100 keV) that is possibly related to a shock, or might be related to coronal process seen in radio occurring after the shock already left the corona. I will review the most recent results including a discuss of the controversial points and I will outline how we can use the upcoming STEREO mission to test the existence of these two classes of events. Title: Comparision of HXR Footpoint Motions of solar Flares with Reconnection Model Authors: Fivian, M. D.; Krucker, S.; Lin, R. P. Bibcode: 2004cosp...35.3248F Altcode: 2004cosp.meet.3248F In a reconnection model of solar flares, the motion of the footpoints and the magnetic field along the footpoints suppose to be correlated with the deposited energy at the footpoints produced by the non-thermal electrons. RHESSI hard X-ray imaging is used to measure the location of the footpoints and RHESSI imaging spectroscopy is used to determine the deposited energy in the footpoints. While a correlation can be found for some events (e.g. July 23, 2002) the footpoint motion seems to be completely uncorrelated in others (e.g. linear motion with 15 km/sec with clearly modulated HXR flux in the Mar-18, 2003 event). We present a statistical analysis of these correlations as function of various parameters. Title: The X-ray source region of 3He-rich solar energetic particle events Authors: Krucker, S.; Lin, R. P.; Kontar, E. P.; Mason, G. M.; Wiedenbeck, M. E. Bibcode: 2003AGUFMSH11D1130K Altcode: The Reuven Ramaty High Energy Spectroscopic Imager RHESSI allows for the first time to study simultaneously the spatial and spectral characteristics of solar hard X-ray emission in detail. In this paper, RHESSI X-ray imaging spectroscopy is used to investigate the source regions of 3He-rich solar energetic particles observed at 1 AU. A series of very large 3He-rich events that occurred between August 19-21, 2002 are investigated. The timing of the simultaneously observed electron events suggests that these particle events are related to a series of GOES M class flares. All events originated from the same active region AR0069 and show a similar behavior: Next to the main flaring loops, TRACE observations additionally show an EUV jet that appears to escape from the Sun with a speed of up to 500 km s-1. X-ray emission is seen from the main flaring loops as well; however, the most prominent source in >30 keV hard X-rays appears to be displaced from the flaring loops, at the footpoint of the field lines along which the jet is moving outward. We investigate the possibility that the HXR footpoint is produced by the downward moving part of the same electron population that escapes to 1 AU, by comparing the derived electron spectrum from the HXR footpoint source with the in situ observed electron spectrum. Title: RHESSI Microflares Statistics Authors: Rauscher, E.; Christe, S.; Hannah, I.; Krucker, S.; Lin, R. P. Bibcode: 2003AGUFMSH21B0167R Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides unique sensitivity in the 3-15 keV energy range with an effective area ∼100 times larger then similar past instruments. Microflares have been observed with little emission above 15 keV due to extremely steep spectra (spectral index of ~8 with a low energy cutoff near 8 keV). Since the energy in non-thermal electrons is very sensitive to the value of the power law and the low energy cutoff, observations by RHESSI will give a better estimate of the total energy input into the corona. We present the first statistical analysis of RHESSI microflares during times with activity below GOES C Class and without solar type III radio storms. Currently, we have analyzed June 2002 and May 2003. Microflares are found through searching for peaks 3σ above background in the 6-12 keV data range. The fluxes in the 3-6 keV 12-25 keV, and 50-100 keV bands are also recorded. Statistics on the following flare characteristics are presented; max counts, total counts, duration, GOES level. We have also analyzed the equivalent WIND data set in search of correlated solar type III radio bursts. Initial results show that only a small number of RHESSI microflares are associated with interstellar type III radio bursts. This work was supported by NASA contract NAS5-98033. Title: RHESSI Observations of High-Temperature Plasmas in Solar Flares Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2003AGUFMSH22A0170C Altcode: Solar flare plasmas are multi-thermal, and in particular may contain high-temperature components above ∼20 MK. While other solar instruments (e.g. TRACE, GOES SXI) are sensitive to low-temperature plasmas below ∼20 MK, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes photons with energies above 3 keV, and thus is especially sensitive to high-temperature plasmas above ∼10 MK. High-temperature plasma emission includes the Fe/Ni line complexes at ∼6.7 and ∼8 keV, and both the equivalent widths and the fluxes of these line complexes are strongly temperature-dependent. In many flares, RHESSI detects emission in both of these line complexes, as well as emission in the thermal continuum, with spectral resolution of ∼1 keV FWHM. Through imaging and spectroscopy with RHESSI, and by comparison with other solar instruments, we can thus constrain the temperatures and emission measures over the entire range of thermal plasmas. We present a spectroscopic analysis of a variety of flares, including the X1.5 event on 21 April 2002, to determine the time-varying characteristics of the thermal electron populations. We estimate the energy contained in thermal electrons and compare with the energy contained in the time-varying non-thermal electron population. Finally, we discuss the implications for heating and energy transport in flares with high-temperature components. Title: First Detection of Hard X-ray Emission From Solar Type III Radio Bursts Authors: Christe, S.; Krucker, S.; Lin, R. P.; Arzner, K.; Benz, A. O. Bibcode: 2003AGUFMSH11D1131C Altcode: We present the first detection of non-flare related hard X-ray emission from type III radio bursts as observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). During a period of 15 minutes on 19 July 2002 14:23-14:35 UT, the WAVES instrument on the Wind spacecraft observed interplanetary type III radio bursts approximately every 2 minutes and each was accompanied by a 12-15 keV X-ray brightening observed by RHESSI. The radio and X-rays fluxes were found to be strongly correlated. No flares were reported in the SEC solar event reports during this time and only the strongest brightening is associated with a detectable enhancement in the GOES levels (A3 above a B8 background). Phoenix-2, a ground-based radio spectrometer, observed each interplanetary type III to extend down to 300 MHz (0.1 Rsun) The strongest type III was also accompagnied by a cluster of decimetric radio emission in the frequency range 1 to 2 GHz. A close correlation is found between X-ray fluxes and the decimetric fluxes. X-ray spectra show non-thermal emission (9-30 keV) with an electron spectral power-law index of ∼4, from the footpoint of a TRACE loop observed in FeXII (195 Å). Subsequently, jets are seen to originate from the RHESSI footpoint emission travelling with apparent speeds of ∼ 100 km s-1. The observed RHESSI hard X-ray fluxes require ∼1033 electrons above 10 keV. This work was supported by NASA contract NAS5-98033. Title: Temporal Variability of Gamma-Ray Lines from the X-Class Solar Flare of 2002 July 23 Authors: Shih, A. Y.; Smith, D. M.; Lin, R. P.; Hurford, G. J.; Krucker, S.; Schwartz, R. A.; Share, G. H.; Murphy, R. J. Bibcode: 2003AGUFMSH11D1132S Altcode: On 2002 July 23, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) detected a GOES X4.8 solar flare, and analysis of the data has produced the first spectrally resolved measurement of nuclear de-excitation gamma-ray lines in a solar flare. These narrow lines result from accelerated ions colliding with the ambient medium (which includes Fe, Mg, Ne, Si, C, and O nuclei), and thus their relative fluxes probe the composition of the solar atmosphere. We investigate the temporal variability of the flux of these lines by analyzing the observed spectrum for two separate time intervals of the flare (00:27:20-00:32:56 UT and 00:32:56-00:43:20 UT). The relative fluxes in the gamma-ray lines change dramatically between these two intervals, suggesting that the circumstances of ion acceleration change as the flare progresses. We consider the possibility of variations in the nuclear abundances, as well as other possible causes such as a varying alpha/proton ratio or a varying energy spectrum for the accelerated ions. Finally, we compare these variations with the source locations as determined by RHESSI's gamma-ray imaging. The work at the University of California, Berkeley, was supported by NASA contract NAS 5-98033. Title: A study of interplanetary propagation of solar impulsive ~1-100keV electron events Authors: Wang, L.; Lin, R. P.; Krucker, S.; Larson, D. E. Bibcode: 2003AGUFMSH21B0153W Altcode: We study solar impulsive ~1-100keV electron events that exhibit clear velocity dispersion, as observed by the 3D Plasma and Energetic Particle (3DP) instrument on the WIND spacecraft. At the onset of those impulsive electron events, the outward travelling electrons are observed first, then the backward travelling electrons are detected, mostly in high energy range (>20keV) and sometimes in low energy range (<20keV). The backward population must be produced by scattering and/or magnetic mirroring in the interplanetary medium beyond 1 AU. For one impulsive event observed on December 5,1997, we determine the delay between the onset of the outward travelling electrons and the onset of the backward travelling electrons in different pitch angle ranges. We find that the delays for 27, 40 and 66 keV electrons, respectively, are 10, 5.8 and 7.3 minutes at ~10° pitch angle; 1.7,1.8 and 3 minutes at ~30° pitch angle; 10.5, 6 and 7.2 minutes at ~55° pitch angle; 6.2, 3.2 and 4.2 minutes at ~80° pitch angle.Those time differences are not inversly proportional to parallel velocity along the magnetic field B, so the simple magnetic mirror is unlikely to be an explanation. We investigate pitch angle scattering by magnetic field fluctuations as a possible mechanism. Title: Non-thermal Coronal Hard X-ray Emission Observed During a Partially Occulted Flare Authors: Balciunaite, P.; Krucker, S.; Caspi, A.; Lin, R. P. Bibcode: 2003AGUFMSH22A0172B Altcode: We will present an analysis of RHESSI hard X-ray imaging and spectroscopy of a GOES M1.5 class flare which occurred on 1 June 2003 0210-0700 UT in NOAA active region 0375 and was located 9 degrees behind the east limb of the Sun. Flare footpoints were thus occulted from view by ~9000 km, allowing for a relatively unobstructed detection of emission from the associated coronal sources. Throughout the duration of the flare, a thermal hard X-ray source can be seen rising with a velocity of ~7 km/s above GOES SXI soft X-ray and EIT ultraviolet (304A) loops. During the impulsive phase of the flare (0237-0310 UT), a non-thermal source is additionally seen above 15 keV with a relatively soft power law spectrum with an index around -5. This non-thermal source is displaced from the thermal source by up to 10,000 km and shows greater variations on a time scale of tens of seconds. We will present total energy estimates and a detailed analysis of the source motions during the flare. We will also discuss implications for models that postulate above-the-loop-top sources. Title: Hard X-Ray Source Motions in the 2002 July 23 Gamma-Ray Flare Authors: Krucker, Säm; Hurford, G. J.; Lin, R. P. Bibcode: 2003ApJ...595L.103K Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is used to study the hard X-ray (HXR) source motions of the 2002 July 23 γ-ray flare. Above 30 keV, at least three HXR sources are observed during the impulsive phase that can be identified with footpoints of coronal magnetic loops that form an arcade. On the northern ribbon of this arcade, a source is seen that moves systematically along the ribbon for more than 10 minutes. On the other ribbon, at least two sources are seen that do not seem to move systematically for more than half a minute, with different sources dominating at different times. The northern source motions are fast during times of strong HXR flux but almost absent during periods with low HXR emission. This is consistent with magnetic reconnection if a higher rate of reconnection of field lines (resulting in a higher footpoint speed) produces more energetic electrons per unit time and therefore more HXR emission. The absence of footpoint motion in one ribbon is inconsistent with simple reconnection models but can be explained if the magnetic configuration there is more complex. Title: Radio and Hard X-Ray Images of High-Energy Electrons in an X-Class Solar Flare Authors: White, S. M.; Krucker, S.; Shibasaki, K.; Yokoyama, T.; Shimojo, M.; Kundu, M. R. Bibcode: 2003ApJ...595L.111W Altcode: We present the first comparison between radio images of high-energy electrons accelerated by a solar flare and images of hard X-rays produced by the same electrons at photon energies above 100 keV. The images indicate that the high-energy X-rays originate at the footpoints of the loops dominating the radio emission. The radio and hard X-ray light curves match each other well and are quantitatively consistent with an origin in a single population of nonthermal electrons with a power-law index of around 4.5-5. The high-frequency radio spectral index suggests a flatter energy spectrum, but this is ruled out by the X-ray spectrum up to 8 MeV. The preflare radio images show a large hot long-lived loop not visible at other wavelengths. Flare radio brightness temperatures exceed 109 K, and the peak in the radio spectrum is as high as 35 GHz: both these two features and the hard X-ray data require very high densities of nonthermal electrons, possibly as high as 1010 cm-3 above 20 keV at the peak of the flare. Title: RHESSI Hard X-Ray Imaging Spectroscopy of the Large Gamma-Ray Flare of 2002 July 23 Authors: Emslie, A. Gordon; Kontar, Eduard P.; Krucker, Säm; Lin, Robert P. Bibcode: 2003ApJ...595L.107E Altcode: We present Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) hard X-ray images in different energy bands for the large X-class flare of 2002 July 23; these images are used to construct spatially resolved hard X-ray spectra for each of four prominent features: a bright, soft source high in the corona, two localized, hard footpoints in opposite polarity magnetic regions that show highly correlated flux and spectral variations in time, and a third footpoint bounded by the other three sources. The power-law spectral indices of the two correlated footpoints differ by ~0.3-0.4, which may be the result of differing column densities from the electron source. Title: First Gamma-Ray Images of a Solar Flare Authors: Hurford, G. J.; Schwartz, R. A.; Krucker, S.; Lin, R. P.; Smith, D. M.; Vilmer, N. Bibcode: 2003ApJ...595L..77H Altcode: Imaging of gamma-ray lines, produced by nuclear collisions of energetic ions with the solar atmosphere, provides the only direct indication of the spatial properties of accelerated ions near the Sun. We present the first gamma-ray images of a solar flare, obtained with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) for the X4.8 flare of 2002 July 23. Two rotating modulation collimators (with 35" and 183" resolution) were used to obtain images for the same time interval in four energy bands: the narrow deuterium line at 2.223 MeV formed by the thermalization and capture of neutrons produced in the collisions; the 3.25-6.5 MeV band that includes the prompt de-excitation lines of C and O; and the 0.3-0.5 and 0.7-1.4 MeV bands that are dominated by electron bremsstrahlung. The centroid of the 2.223 MeV image was found to be displaced by 20''+/-6'' from that of the 0.3-0.5 MeV image, implying a difference in acceleration and/or propagation between the accelerated electron and ion populations near the Sun. 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: Properties of high heliolatitude solar energetic particle events and constraints on models of acceleration and propagation Authors: Dalla, S.; Balogh, A.; Krucker, S.; Posner, A.; Müller-Mellin, R.; Anglin, J. D.; Hofer, M. Y.; Marsden, R. G.; Sanderson, T. R.; Tranquille, C.; Heber, B.; Zhang, M.; McKibben, R. B. Bibcode: 2003GeoRL..30.8035D Altcode: 2003GeoRL..30sULY9D We analyse 9 large solar energetic particle (SEP) events detected by the Ulysses spacecraft at high heliolatitudes during the recent solar maximum polar passes. Properties of time intensity profiles from the Ulysses/COSPIN instrument are compared with those measured by SOHO/COSTEP and Wind/3DP near Earth. We find that onset times and times to maximum at high latitude are delayed compared to in-ecliptic values. We show that the parameter which best orders these characteristics of time profiles is the difference in latitude between the associated flare and the spacecraft. We find that the presence of a shock is not necessary for the establishing of near equal intensities at Ulysses and in the ecliptic during the decay phase. The model of SEP acceleration by coronal mass ejection driven shocks does not appear to account for our observations, which would more easily be explained by particle diffusion across the interplanetary magnetic field. Title: Characterization of SEP events at high heliographic latitudes Authors: dalla, S.; Balogh, A.; Krucker, S.; Posner, A.; Müller-Mellin, R.; Anglin, J. D.; Hofer, M. Y.; Marsden, R. G.; Sanderson, T. R.; Heber, B.; Zhang, M.; McKibben, R. B. Bibcode: 2003AIPC..679..656D Altcode: Between February 2000 and May 2002, the Ulysses spacecraft made the first ever measurements of solar energetic particles (SEPs) at high heliographic latitudes. Nine large gradual SEP events were detected at latitudes greater than 45°, their signatures being clearest at high particle energies, i.e. protons >30 MeV and electrons >0.1 MeV. In this paper we measure the onset times of Ulysses high latitude events in several energy channels, and plot them versus inverse particle speed. We repeat the procedure for near Earth observations by Wind and SOHO. Velocity dispersion is observed in all the events near Earth and in most of them at Ulysses. The plots of onset times versus inverse speed allow to derive an experimental path length and time of release from the solar atmosphere. We find that the derived path lengths at Ulysses are longer than the length of a Parker spiral magnetic field line connecting it to the Sun, by a factor between 1.2-2.7. The time of particle release from the Sun is typically between 100 and 200 mins later than the relase time derived from in-ecliptic measurements. Unlike near Earth observations, Ulysses measurements are therefore not compatible with scatter-free propagation from the Sun to the spacecraft. Title: A multiwavelength analysis of the august 30, 2002 flare Authors: Gimenez de Castro, C. G.; Silva, A. V. R.; Trottet, G.; Costa, J. E. R.; Kaufman, P.; Correia, E.; Krucker, S.; Martinez, G.; Rovira, M.; Lüthi, T.; Magun, A.; Levato, H. Bibcode: 2003BASBr..23...23G Altcode: We present preliminary results of a multiwavelength analysis of the flare classified as X1.5 which occurred on Aug 30 2002 at about 1328 UT in NOAA region 0095. The event was observed by the Ramaty High Energy Solar Spectrometer Imager (RHESSI) up to 200-300 keV and by the new Solar Submillimeter Telescope (SST) at 212 GHz. Radio image observations at long decimetric-metric waves obtained with the Nançay Radioheliograph are also being included. Our analysis utilizes radio flux from different telescopes to cover an extended range from 160 MHz up to 212 GHz. HxR images show two sources above 30-40 keV, 5 and 3 arc sec in diameter separated by about 7 arc sec, and a third source at low energies. Both the total photon spectra and individual source spectra, were fitted by a double power-law. We compare HxR with Ha images obtained with the HASTA telescope. During peak time the microwave spectrum is flat between 10 to 35 GHz. We discuss a flare model with a homogeneous source which includes both gyrosynchrotron and free-free emission/absorption to explain the observed flatness. Title: RHESSI Discovery of a Coronal Non-Thermal Hard X-Ray Source in the 23 July 2002 Gamma-Ray Line Flare Authors: Lin, R. P.; Krucker, S.; Holman, G. D.; Sui, L.; Hurford, G. J.; Schwartz, R. A. Bibcode: 2003ICRC....6.3207L Altcode: 2003ICRC...28.3207L The Reuven Ramaty High Energy Solar spectroscopic Imager (RHESSI) observations of the gamma-ray line flare of 2002 July 23, show that in the ∼8 minute rise prior to the impulsive phase, the hard X-ray emission comes from a coronal source which has no counterpart in the simultaneous TRACE EUV images or in the Hα images. The spectrum above ∼10 keV fits to a double-powerlaw shape with break energies at ∼20-40 keV and exponents of ∼5 below and ∼7 above, with no obvious thermal emission above ∼10 keV. This coronal nonthermal source implies that substantial energy release and electron acceleration occurs before the impulsive phase. Title: First Gamma-Ray Images of a Solar Flare Authors: Hurford, G. J.; Schwartz, R. A.; Krucker, S.; Lin, R. P.; Smith, D. M.; Vilmer, N. Bibcode: 2003ICRC....6.3203H Altcode: 2003ICRC...28.3203H; 2003ICRC....6.3203L We present the first gamma-ray images of a solar flare, obtained with the Reuven Ramaty High Energy Solar spectroscopic Imager (RHESSI) for the X4.8 flare of 2002 July 23. Two rotating modulation collimators (35 & 180 resolution) provided images of the narrow deuterium line at 2.223 MeV formed by thermalization and capture of neutrons produced in energetic ion collisions, the 3.25-6.5 MeV band that includes the prompt de-excitation lines of C and O, and the 0.30.5 and 0.7-1.4 MeV bands that are dominated by electron-bremsstrahlung. The centroid of the 2.223 MeV image was found to be displaced by ∼20(±6) arcsec from that of the 0.3-0.5 MeV band, implying a difference in acceleration and/or propagation between the accelerated electron and ion populations near the Sun. Title: Onsets and Release Times in Solar Particle Events Authors: Tylka, A. J.; Cohen, C. M. S.; Dietrich, W. F.; Krucker, S.; McGuire, R. E.; Mewaldt, R. A.; Ng, C. K.; Reames, D. V.; Share, G. H. Bibcode: 2003ICRC....6.3305T Altcode: 2003ICRC...28.3305T The time at which solar energetic particles (SEPs) are first released into interplanetary space, and its relation to CMEs and various photon emissions, are important clues to the site and nature of the SEP acceleration mechanism [15,7,8,10,11]. We examine velocity disp ersion among onsets in electrons and ions from Wind, ACE, and IMP8, as well as available neutron monitors, to determine the solar release time. We present results for two large impulsive events (1 May 2000 and 14 April 2001) and three western ground level events (GLEs; 6 November 1997, 6 May 1998, and 15 April 2001). In the impulsive events, the particle release coincides with hard x-ray emission. But the large GLEs show delayed release with respect to γ -ray emission, consistent with acceleration by the CME-driven shock. Title: Delay in solar energetic particle onsets at high heliographic latitudes Authors: dalla, S.; Balogh, A.; Krucker, S.; Posner, A.; Müller-Mellin, R.; Anglin, J. D.; Hofer, M. Y.; Marsden, R. G.; Sanderson, T. R.; Heber, B.; Zhang, M.; McKibben, R. B. Bibcode: 2003AnGeo..21.1367D Altcode: Ulysses observations have shown that solar energetic particles (SEPs) can easily reach high heliographic latitudes. To obtain information on the release and propagation of SEPs prior to their arrival at Ulysses, we analyse the onsets of nine large high-latitude particle events. We measure the onset times in several energy channels, and plot them versus inverse particle speed. This allows us to derive an experimental path length and time of release from the solar atmosphere. We repeat the procedure for near-Earth observations by Wind and SOHO. We find that the derived path lengths at Ulysses are 1.06 to 2.45 times the length of a Parker spiral magnetic field line connecting the spacecraft to the Sun. The time of particle release from the Sun is between 100 and 350 min later than the release time derived from in-ecliptic measurements. We find no evidence of correlation between the delay in release and the inverse of the speed of the CME associated with the event, or the inverse of the speed of the corresponding interplanetary shock. The main parameter determining the magnitude of the delay appears to be the difference in latitude between the flare and the footpoint of the spacecraft. Title: RHESSI Observations of Two Occulted Solar Flares Authors: Balciunaite, P.; Krucker, S.; Christe, S.; Lin, R. P. Bibcode: 2003SPD....34.1810B Altcode: 2003BAAS...35..841B We present RHESSI (Reuven Ramaty Solar Spectroscopic Imager) observations of two limb-occulted solar flares (GOES C2 and C5 class), which occurred on 13 May 2002 10:50-22:20 UT and 14 October 2002 14:00 UT-15 October 2002 16:00 UT, respectively. The observations of such flares allow the investigation of coronal hard X-ray sources without the interference of the generally brighter footpoint sources. The aforementioned events are inferred to originate 34 and 44 degrees behind the east limb by tracking the most probable originating active regions (NOAA 9957 and 0162, respectively). The 8-15 keV source centroid position as a function of time initially shows a constant position just above the solar limb for approximately 2 hours. Then the source appears to rise above the limb, in a manner that could not be due solely to solar rotation. Presently we are investigating spectra for both events to derive the temperature and emission measures and search for a possible non-thermal contribution. Title: RHESSI Hard X-Ray Imaging Spectroscopy of the July 23, 2002 Solar Flare Authors: Emslie, A. G.; Kontar, E. P.; Krucker, S.; Lin, R. P. Bibcode: 2003SPD....34.2208E Altcode: 2003BAAS...35..851E We present hard X-ray images in different energy bands, as obtained by the unique combination of rotating modulation collimators and high-resolution germanium spectrometers on the RHESSI satellite, for the large X-class flare of July 23, 2002. These data are then used to construct spatially-resolved hard X-ray spectra for each of four prominent features evident in these images. These four main features are a bright, low-energy (soft spectrum) source high in the corona, two localized high-energy (hard spectrum) footpoints and a bright feature bounded by the other three, which could either be an additional footpoint or a source near the top of a magnetic structure connecting the other two. The temporal evolution of the spectrum of each feature is described, with allowances for the dynamic range of the RHESSI instrument, which obscures weak sources at a given photon energy when one or more much brighter sources at this photon energy are also present. A comparison with the temporal evolution of the spatially-integrated hard X-ray spectrum is also provided.

This work was supported by NASA's Office of Space Science and by a PPARC Award. Title: RHESSI Hard X-ray observations of the March 18, 2003 X-class Flare Authors: Fivian, M. D.; Krucker, S.; Lin, R. P. Bibcode: 2003SPD....34.1814F Altcode: 2003BAAS...35..842F Hard X-ray observations of the GOES X1.5 flare on March 18, 2003, taken by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are presented. This long duration event (GOES duration slightly longer than 1 hour) occurred at S15W46. Enhanced hard X-ray emission (>30 keV) is seen for more than 25 minutes with the strongest HXR bursts detected almost 10 minutes after the peak emission seen in GOES soft X-ray. Signatures of escaping energetic electrons, such as radio type III bursts seen with WIND/WAVES in the interplanetary space and a large electron event detected by WIND/3DP at 1 AU, are only observed with the later HXR burst but not during the rise phase of the SXR. In order to understand this, the HXR sources and their motions are investigated and compared to TRACE (171 /AA) EUV images and MDI magnetograms.

This work is funded by NASA grant NAS5-98033-05/03. Title: RHESSI Observations of Solar Hard X-ray Flares with and without Interplanetary Type III Radio Bursts Authors: Rauscher, E.; Krucker, S.; Lin, R. P. Bibcode: 2003SPD....34.1813R Altcode: 2003BAAS...35..841R The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) allows more detailed imaging of hard X-ray flares than has been available before. The hard X-ray sources seen in solar flares are believed to be the result of accelerated electrons streaming down the legs of magnetic loops and colliding with the denser plasma at the footpoints, releasing hard X-rays through bremsstrahlung. Type III interplanetary radio bursts are evidence of accelerated electrons leaving the Sun. Previous studies have indicated a correlation between hard X-ray flares and type III interplanetary radio bursts for some events. We are now studying the structure of hard X-ray flares using RHESSI and TRACE (EUV) movies to investigate possible differences between flares that have corresponding radio bursts (as detected by WIND/WAVES) and ones that do not. We have collected over 600 events from the first year of RHESSI. From that group, we have chosen a half dozen events with good coverage from both instruments for more detailed study. Title: Type III Radio Bursts and Microflares Authors: Christe, S.; Krucker, S.; Arzner, K.; Lin, R. P. Bibcode: 2003SPD....34.1501C Altcode: 2003BAAS...35..830C We present recent observations of microflares observed simultaneously in EUV (TRACE), radio (Nancay, Phoenix-2), and X-rays (RHESSI). During a period of 15 min on 19 July 2002 14:23-14:35 UT, RHESSI observed microflares approximately every 2 minutes. Each microflare was accompagnied by a radio Type III burst. The largest flare (14:29:25 UT) was also accompagnied by a cluster of decimetric radio spikes in the frequency range 1 to 2 GHz. In addition, FeXII (195 Å) images provided by TRACE show two jets-like emissions originating from a complex double arche structure. The centroid of the jets were found to travel at apparent speeds of ∼ 100 km s-1, consistent with observations by Shimojo et al. (1996). X-ray images show non-thermal emission (9-30 keV) from the footpoints of the TRACE arches. Strong correlation in flux amplitude is found between emissions in the radio ( ∼1340 MHz) and non-thermal X-ray (9-30 keV integrated). The event is interpreted as an anemone-jet in the model by Shibata et al. (1994). This research is supported by NASA contract NAS 5-98033. Title: Statistical Properties of Solar Impulsive Electron Events Authors: Oakley, P.; Krucker, S.; Lin, R. P. Bibcode: 2003SPD....34.1607O Altcode: 2003BAAS...35..833O This is a statistical study with the WIND 3DP instrument of 509 solar impulsive electron events from 1994 to 2001. We analyze and compare the peak flux spectra, the presence of Langmuir waves, the frequency distribution of events and the time delay between the injection of electrons and the emission of the related radio type III bursts. Our most interesting finding is that electron events occurring during a radio type II burst show significantly harder spectra, indicating that shocks with type II bursts might be more efficient at accelerating higher energy particles.

We also observed that the number of events seen only at low energies increases after the peak of the solar cycle has passed, while the number of events seen only at higher energies peaks with the solar cycle. Solar wind density is not observed to play any role in determining at what energies a solar event is visible. Title: First Gamma-Ray Images of a Solar Flare: 2002 July 23 Authors: Hurford, G. J.; Schwartz, R. A.; Krucker, S.; Lin, R. P.; Smith, D. M.; Vilmer, N. Bibcode: 2003SPD....34.1409H Altcode: 2003BAAS...35..830H Imaging of gamma-ray lines, produced by nuclear collisions of energetic ions with the solar atmosphere, provides the only direct indication of the spatial properties of accelerated ions near the Sun. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was used to make the first gamma-ray line images of a solar flare. The X4.8 flare of 2002 July 23, was imaged in the 2218-2228 band containing the narrow neutron-capture deuteriom line at 2223 keV, and in the 3250-6500 keV band which includes the prompt de-excitation lines of C and O. These are compared with images at 300-500 and 700-1400 keV, which are dominated by electron-bremsstrahlung. The centroids of 35-arcsecond resolution images made at these two lower-energy bands were displaced by 20 arcseconds from the 2223 keV image of the neutron capture line. Such an offset indicates a difference in acceleration and/or propagation between the accelerated electron and ion populations near the Sun.

This work was supported by NASA grant NAS5-98033-05/03. Title: Temporal Variability of Gamma-Ray Lines from the X-Class Solar Flare of 2002 July 23 Authors: Shih, A. Y.; Smith, D. M.; Lin, R. P.; Krucker, S.; Schwartz, R. A.; Share, G. H.; Murphy, R. J. Bibcode: 2003SPD....34.1801S Altcode: 2003BAAS...35..839S On 2002 July 23, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) detected a GOES X4.8 solar flare, and analysis of the data has produced the first spectrally resolved measurement of nuclear de-excitation lines in a solar flare. We investigate the temporal variability of the flux of these lines by analyzing the observed spectrum for distinct time intervals of the flare. After the first major burst of the flare, the gamma-ray lines produced by iron, carbon, and oxygen show statistically significant changes in their fluxes relative to the neon line, suggesting that the circumstances of ion acceleration change as the flare progresses. We consider possible causes such as a varying alpha/proton ratio or a varying energy spectrum for the ions. Finally, we correlate these variations with other observed variations of the flare.

The work at the University of California, Berkeley, was supported by NASA contract NAS 5-98033. Title: Energy estimates for solar flares using RHESSI and GOES SXI Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2003SPD....34.2204C Altcode: 2003BAAS...35..850C We examine energy release and transport in a variety of midsize-to-large flares (GOES X-ray class C5 and above) using the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the GOES Solar X-ray Imager (SXI). Previous observations suggest that high-energy electrons accelerated during the flare lose their energy primarily through Coulomb collisions with the solar atmosphere, heating it to produce the soft X-ray emitting plasma. This results in the so-called Neupert effect, where the soft X-ray time profile corresponds to the time-integrated hard X-ray profile from non-thermal bremsstrahlung of the electrons. RHESSI can, for the first time, obtain both images and spectra over the full range of 3 keV to 17 MeV with excellent resolution spatially, temporally, and energetically.

We calculate the thermal energy vs. time for each flare by using temperature and emission measure values derived from GOES data, using SXI images to estimate source volumes. Using RHESSI imaging and spectroscopic data, we calculate the energies in non-thermal energetic electrons and in thermal plasmas at temperatures above the peak response of SXI/GOES. We compare the RHESSI- and SXI-obtained energies and discuss the implications for energy release and transport in flares by energetic electrons. Title: Hard X-ray Sources and Motions in the July 23, 2002 Gamma-Ray Flare Authors: Krucker, Sam; Hurford, G. H.; Lin, R. P. Bibcode: 2003SPD....34.1406K Altcode: 2003BAAS...35..829K The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is used to study the Hard X-ray (HXR) emission of the July 23, 2002 gamma-ray flare. In the inital rise (00:21-00:26 UT), a probably non-thermal coronal HXR source dominates with weaker emission from foopoints in two ribbons visible in H-alpha and EUV. During the main impulsive phase (00:27-00:42 UT), HXR footpoint emission dominates above 30 keV, whereas at lower energies a gradual coronal HXR source is seen. The footpoint emission originates from several sources close to a magnetic neutral line. The time profiles of two footpoints apparently in opposite polarity regions are similar, suggesting magnetic connection between them. The relative separation of these two footpoints increases in time. On the positive polarity side, a source moves roughly parallel to the neutral line with a speed of up to ∼100 km s-1; on the negative polarity side with slightly weaker magnetic fluxes, several sources are seen that do not seem to move systematically, with different sources dominating at different times. The coronal source shows an elongated structure, possibly containing more than one source, that moves with a similar speed in almost the same direction as the moving footpoint. The speed of both the footpoint and the coronal source motions are roughly correlated with the total HXR flux of the footpoints. The source motions are fast during times of strong HXR flux, but almost absent during periods with low HXR emission. These observations are consistent with magnetic reconnection models in which a higher rate of reconnection of field lines produces more energetic electrons per unit time and therefore more HXR emission, and also results in a higher apparent speed of the HXR sources at the footpoints of newly reconnected loops. Title: Studies of Microflares with RHESSI, Hard X-ray in BBSO Hα and MDI Magnetograms Authors: Liu, C.; Qiu, J.; Gary, D.; Krucker, S.; Wang, H. Bibcode: 2003SPD....34.2203L Altcode: 2003BAAS...35..850L Microflares are among the most interesting subjects because its accumulated energy may be a substantial part to heat the upper solar atmosphere. The unique high sensitivity in the energy range from ∼3 to 15 keV and excellent spectral and spatial resolution provided by the Ramaty High Energy Solar Spectroscopic Imager mission (RHESSI) allow for the first time the detailed study of the locations and the spectra property of solar microflares. We studied a number of microflares in the energy range from 3 to 15 keV which occurred in several active regions, in July 2002, and we found: (1) from the co-alignment results between RHESSI and MDI, we confirm that almost all the microflares are located in magnetic neutral lines; (2) from the co-alignment results between RHESSI and Hα images, we find that the hard X-ray source tends to connect two Hα footpoints and the RHESSI peaks always occur ∼1 minute earlier than the observable Hα brightening, which indicates the possible loop-top source before the Hα footpoint brightening; (3) We also notice that some microflares are associated with type III radio bursts as observed by the radio spectrometer on-board WIND. Title: Rapid Changes in the Longitudinal Magnetic Field Associated with the July 23, 2002 γ -ray Flare Authors: Yurchyshyn, V. B.; Wang, H.; Abramenko, V. I.; Spirock, T. J.; Krucker, S. Bibcode: 2003SPD....34.1508Y Altcode: 2003BAAS...35Q.832Y In this paper we analyze and discuss rapid changes of the magnetic field associated with the July 23, 2002 γ -ray flare. MDI magnetic flux profiles and BBSO vector magnetograms showed that immediately after the flare the leading polarity had increased by 2*E20Mx, while the following polarity decreased only by 1*E20Mx. The observed changes were permanent and not caused by variations in seeing or changes in the line profile, which we used to measure the magnetic field. In this active region we distinguish two separate locations, which show the most dramatic changes in the magnetic field. The first location showed an increase in the magnetic field strength and a new penumbra area and it was associated with emergence of new magnetic flux. At the second location the position of the neutral line had changed and it coincided with the footpoints of a rapidly growing post-flare loop system. Linear force-free field simulations showed that the re-orientation of the magnetic field during the flare was capable of producing the observed changes in the total magnetic flux. We also discuss a possible magnetic configuration responsible for the flare. This work was supported in part by NSF ATM-0086999 and ATM-0205157 and under NASA NAG5-10910 NAG5-12782 grants. Title: Why was there no Solar Energetic Particle Event Associated with the Gamma-ray-line Flare of 2002 July 23? Authors: Gopalswamy, N.; Dennis, B. R.; Kaiser, M. L.; Krucker, S.; Lin, R. P.; Vourlidas, A. Bibcode: 2003SPD....34.2202G Altcode: 2003BAAS...35..850G We investigated the coronal and interplanetary (IP) events associated with two X-class flares on 2002 July 20 and 23. Both flares were associated with ultra-fast (>2000 km s-1) coronal mass ejections (CMEs) and IP shocks. We use white-light, EUV, hard X-ray and radio observations to trace the origin of the CMEs to active region 0039 located close to the east limb. The July 20 flare was partly occulted by the east limb, yet it resulted in a major solar energetic particle event with intensity ∼ 20 pfu in the >10 MeV channel (1 pfu = 1 particle per (cm2 s sr MeV)). The July 23 event was the first gamma-ray-line flare detected by RHESSI, but it did not show any enhancement in SEPs above the elevated background from the July 20 event. We identified two distinguishing factors between the July 2 and July 23 CMEs: (1) The July 20 CME had a higher kinetic energy, and (2) The July 20 CME was interacting with another fast CME (1350 km s-1) that preceded by less than an hour from the same region; there were also two other CMEs on July 19 from the same region. Thus the coronal and IP environment of the July 20 event was highly disturbed due to preceding CMEs (as compared to the July 23 event). We suggest that the different coronal/IP environments may be responsible for the lack of SEP event associated with the July 23 event. Title: Nobeyama Radio Heliograph and RHESSI Observations of the X1.5 Flare of April 21, 2002 Authors: Kundu, M. R.; Garaimov, V. I.; White, S. M.; Krucker, S. Bibcode: 2003SPD....34.1812K Altcode: 2003BAAS...35..841K We present radio observations of the X1.5 flare of April 21, 2002, and complementary data from other wavelengths. This flare was fairly well observed by the spacecraft RHESSI and fully observed by the Nobeyama Radio Heliograph at 17 and 34 GHz. This long-duration event lasted more than 2 hours and features a beautful arcade of rising loops on the limb visible at X-ray, EUV and radio wavelengths. The main flare was preceded by a small event 90 minutes earlier showing a long EUV loop connecting well-separated radio and hard X-ray sources. The main flare itself starts with a compact radio and hard X-ray source at the eastern end of the region that is followed by emission close to the solar surface (well inside the solar limb) over a large region to the northwest. As the flare proceeds a large set of loops is seen to rise well above the solar limb. Distinct regions of radio emission with very different time behaviour can be identified in the radio images, and in particular a peculiar nonthermal 17 GHz source low in the corona at the base of the arcade is seen to turn on 30 minutes after the start of the impulsive phase. At about the same time an extremely intense burst of coherent radio emission is seen from 500 to 2000 MHz: we speculate that this lower-frequency burst is located at the top of the arcade of loops. Title: On the Origin of Solar Energetic Particle Events Authors: Krucker, S. Bibcode: 2003LNP...612..179K Altcode: 2003ecpa.conf..179K The 3-D Plasma and Energetic Particles (3DP) instruments on the WIND spacecraft are providing new insights on the origin of solar energetic particles. In this paper, recent results on the solar origin of 1-300 keV electrons and 0.1-6 MeV protons are reviewed. The main findings are that one class of electron events escaping into interplanetary space are related to the impulsive phase of the flare (radio type III burst related events), but there is also a second class of events released after the impulsive phase of the flare that is possibly related to coronal shocks. In two thirds of all proton events, low energy protons (<6 MeV) are released roughly an hour later than the electrons and are most likely accelerated at the shock fronts of coronal mass ejections. The remaining third of proton events show a puzzling velocity dispersion that could be explained by a non-simultaneous release of protons at different energy. Title: Hard X-ray and metric/decimetric spatially resolved observations of the 10 April 2002 solar flare Authors: Vilmer, N.; Krucker, S.; Trottet, G.; Lin, R. P. Bibcode: 2003AdSpR..32.2509V Altcode: The GOES M8.2 flare on 10 April 2002 at ∼ 1230 UT was observed at X-ray wavelengths by RHESSI and atmetric/decimetric wavelengths by the Nançay Radioheliograph (NRH). We discuss the temporal evolution of X-ray sources together with the evolution of the radio emission sites observed at different coronal heights by the NRH. While the first strong HXR peak at energies above 50 keV arises from energy release in compact magnetic structures (with spatial scales of a few 10 4 km) and is not associated with strong radio emission, the second one leads to energy release in magnetic structures with scales larger than 10 5 km and is associated with intense decimetric/metric and dekametric emissions. We discuss these observations in the context of the acceleration sites of energetic electrons interacting at the Sun and of escaping ones. Title: WIND/3DP and Nancay radioheliograph observations of solar energetic electron events Authors: Klein, K. -L.; Krucker, S.; Trottet, G. Bibcode: 2003AdSpR..32.2521K Altcode: Initial results of a combined study of electron events using the 3DP experiment on the WIND spacecraftand the Nançay Radioheliograph (NRH) are presented. A total of 57 electron events whose solar release time could be inferred from WIND/3DP observations occurred during NRH observing times. In 40 of them a distinct signature was detected in maps at decimetric and metric wavelengths (dm-m-λ) taken by the NRH. These events are equally distributed among two categories: (1) Electron release together with dm-m-λ bursts of a few minutes duration: these events are also accompanied by decametric-hectometric type III bursts seen by WAVES/WIND. They correspond to the well-known impulsive electron events. (2) Electron release during long duration (several tens of minutes) dm-m-λ emission: the electrons are most often released more than ten minutes after the start of the radio event. In the majority of cases the dm-m-λ radio source changes position, size, and/or intensity near the time of electron release. Title: Hard X-ray Microflares down to 3~keV Authors: Christe, S.; Krucker, S.; Lin, R.; Hurford, G.; Schwartz, R. A. Bibcode: 2002AGUFMSH52A0484C Altcode: The excellent sensitivity, spectral and spatial resolution, and energy coverage down to 3~keV provided by the Reuven Ramaty Solar Spectroscopic Imager mission (RHESSI) allows for the first time the detailed study of the locations and the spectra of solar microflares down to 3~keV. During a one hour quiet interval (GOES soft X-ray level around B6) on May~2, 1:40-2:40UT, at least 7 microflares occurred with the largest peaking at A6 GOES level. The microflares are found to come from 4 different active regions including one behind the west limb. At 7" resolution, some events show elongated sources, while others are unresolved point sources. In the impulsive phase of the microflares, the spectra can generally be fitted best with a thermal model plus power-law above ~6-7~keV. The decay phase sometimes can be fit with a thermal only, but in some events, power-law emission is detected late in the event indicating particle acceleration after the thermal peak of the event. The power-law fits extend to below 7~keV with exponents between -5 and -8, and imply a total non-thermal electron energy content between 1026-1027 ergs. Since the total energy in non-thermal electrons is very sensitive to the value of the power-law and the energy cutoff, these observations will give us better estimates of the total energy input into the corona. Presently, microflare observations on June~26 02:20-06:10UT are being analyzed and are expected to corroborate these results. Title: RHESSI Hard X-ray imaging spectroscopy of the 2002 July 23 Solar Gamma-ray Flare Authors: Krucker, S.; Hurford, G. J.; Schwartz, R. A.; Lin, R. P.; Smith, D. M. Bibcode: 2002AGUFMSH52A0481K Altcode: The GOES X4.8 flare of 23 July 2002 is the first gamma-ray flare seen by the recently launched Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). In this paper, first results of X-ray (>3~keV) imaging spectroscopy of this flare are discussed and related to images seen in EUV (TRACE, EIT), Hα (BBSO), and radio waves (Nobeyama). In particular, solar flare bremsstrahlung emission above 300 keV is imaged for the first time. During the first minutes of the impulsive phase of the flare, three main hard X-ray (>30~keV) sources are seen. Two of the sources show similar spectra with a break around 60~keV and exponents around -1.7 below the break and -3.0 above. The spectrum of the third source (that is observed in-between the two other sources, but slightly displaced to the side) is best fitted with a single power law with an exponent of around -3.0. Later in the impulsive phase, only the two sources with similar spectra are seen. The spatial separation of these sources is linearly increasing with a velocity of ~36 km/s over more than 5 minutes. This separation is comparable to the footpoint separation reported by Sakao et al. (1994). More detailed analysis on the temporal evolution of the spectra and a comparison with imaging observations at other wavelengths will be presented. Title: Statistical Study of Solar Impulsive Electron Events Authors: Oakley, P.; Krucker, S.; Lin, R. P. Bibcode: 2002AGUFMSH61A0426O Altcode: This study with the WIND 3DP instrument of 380 solar impulsive electron events shows many different temporal delays with respect to the correlated radio type III bursts. The majority of events have a delayed electron injection time with respect to the radio type III bursts of about 14 minutes. This is similar to other published findings, but with more events and better statistics. To try and determine why some events showed on-time arrivals of electrons while most were delayed we analyzed their peak flux spectra and the frequency distribution of events. The frequency distribution for all events is a power law with a slope of 1.35+/-.05 for energies 30 keV to 300 keV. The peak flux spectra for individual events usually shows a double power law dependence with a steepening at 50 keV. The two slopes have average indices of -3.5 and -2, and are slightly correlated. However there was no difference in these distributions between on-time events and delayed events. It is also found that there is a strong tendency for events with in-situ detected Langmuir waves at 1 AU to be delayed less (12 minutes on average) than those without visible waves (17 minutes on average). An interesting finding is that electron events occurring during a radio type II burst show significantly harder spectra, indicating that shocks with type II bursts might be more efficient at accelerating higher energy particles. Title: RHESSI Observations of Solar Hard X-ray Flares and the Relative Timing of Interplanetary Type III Radio Bursts Authors: Rauscher, E.; Krucker, S.; Lin, R. P. Bibcode: 2002AGUFMSH52A0482R Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) allows more detailed imaging of solar hard X-ray flares than has been available before. The hard X-ray sources seen in solar flares are believed to be the result of accelerated electrons streaming down the legs of a magnetic loop and colliding with the denser plasma at the footpoints, releasing hard X-rays through bremstrahlung. Interplanetary type III radio bursts are evidence of accelerated electrons leaving the Sun. We have begun a study of the temporal correlation of hard X-ray flares and interplanetary type III radio bursts to investigate the acceleration processes behind these events. The RHESSI data from February 12 through July 23 have been temporally compared with WIND/WAVES data to find correlation between hard X-ray events and interplanetary type III radio bursts. Relatively simple events with and without type III bursts have been chosen for spatial investigation. This involves making RHESSI hard X-ray (>20~keV) images and overlaying these onto corresponding EUV images from EIT and TRACE and images from MDI. Title: RHESSI Observation of an Occulted Hard X-ray Flare Authors: Balciunaite, P.; Christe, S.; Krucker, S.; Lin, R. P. Bibcode: 2002AGUFMSH52A0480B Altcode: We present an observation of an occulted GOES C2 class flare which occurred on 13 May 2002 10:50-22:20 UT taken by the recently launched Reuven Ramaty Solar Spectroscopic Imager mission (RHESSI) in open-shutter mode. The flare is inferred to originate 34 degrees behind the east limb by tracking the most probable originating active region (NOAA Active Region 9957). This relatively large occultation could explain why no impulsive flare phase was observed. The hard X-ray (8-15 keV) source centroid position as a function of time initially shows a constant position just above the solar limb for approximately 2 hours. Then the source appears to rise above the limb, in a manner that could not be due solely to solar rotation. RHESSI CLEANed images at 7" resolution demonstrate coronal source motion as a function of time. Presently we are investigating spectra with resolution down to 1 keV and energy coverage down to 3 keV and the derived temperature and emission measure. A limb flare observed on 21 April 2002 00:30-04:00 UT is presented as a comparison. Title: Correlation of RHESSI and TRACE Observations of the Rise Phase of the 21 April 2002 X1.5 Flare Authors: Caspi, A.; Krucker, S.; Lin, R. P. Bibcode: 2002AGUFMSH52A0465C Altcode: The X1.5 flare of 21 April 2002 was observed simultaneously by RHESSI and TRACE, and thus provides a great opportunity to correlate RHESSI findings with independent observations from another instrument. We begin by analyzing the thermal and non-thermal parts of the flare spectrum observed by RHESSI. We then use the temperature and volume emission measure obtained from fitting to predict the TRACE instrument response. We correlate these predictions with the observed TRACE flux at the flare site. Gallagher et al. (2002) state that TRACE showed EUV (195 Å) brightening some 4 minutes after RHESSI, but preliminary results indicate a small initial brightening in the 195 Å passband around 00:40 UT, at the same time as RHESSI, with the primary flux increase occurring around 00:44 UT. This roughly matches predictions of TRACE response based on RHESSI spectral data. Title: RHESSI Gamma-Ray Line Spectroscopy of the X-class Flare of July 23, 2002 Authors: Smith, D. M.; Schwartz, R. A.; Lin, R. P.; Share, G. H.; Murphy, R. J.; Hurford, G.; Krucker, S.; Fivian, M.; Hyatt, L.; Dennis, B. R.; Vilmer, N. Bibcode: 2002AGUFMSH52A0461S Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is the first spacecraft with high-spectral-resolution germanium gamma-ray detectors to be dedicated to solar flare studies. We will report on RHESSI's first observation of a major gamma-ray line flare, the X4.8 limb flare of July 23, 2002. RHESSI observed nuclear de-excitation lines of Mg, Si, Ne, O, and C from this flare, plus the neutron-capture line at 2.223 MeV. There is evidence for redshifts and Doppler broadening of the de-excitation lines, as well as temporal variations of the line spectrum. We will discuss the implications of these observations for the spectrum, time variation, angular distribution, and composition of the accelerated ions and for the composition of the interaction region. Title: Gamma-ray Imaging of the 2002 July 23 Solar Flare Authors: Hurford, G. J.; Schwartz, R. A.; Krucker, S.; Lin, R. P.; Smith, D. M. Bibcode: 2002AGUFMSH52A0462H Altcode: Gamma-ray spectroscopy of nuclear line emission has provided the primary observational tool for the study of accelerated nuclei near the Sun. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), launched in February 2002, is an imaging-spectrometer which not only does high-resolution spectroscopy from 3 keV to 17 MeV, but also images gamma-rays with angular resolution as high as 35 arcseconds. This enables the emission sites of the 2.2 MeV capture line (from thermalized neutrons), of the Doppler-broadened, prompt gamma-ray continuum and of the hard x-ray continuum (from accelerated electrons) to be located within the flaring active region. Comparison of these locations can provide a new perspective on acceleration and transport processes. The X4.8 limb flare of 23 July 2002 provided the first opportunity to observe solar gamma-ray lines with RHESSI. This paper presents the first solar gamma-ray images of the 2.2 MeV line and of the 4 to 7 MeV gamma-ray range which is dominated by excitation lines of C and O. The locations of these gamma-ray sources will be compared to those of the hard x-ray sources. Title: Hard X-ray Microflares down to 3 keV Authors: Krucker, Säm; Christe, Steven; Lin, R. P.; Hurford, Gordon J.; Schwartz, Richard A. Bibcode: 2002SoPh..210..445K Altcode: The excellent sensitivity, spectral and spatial resolution, and energy coverage down to 3 keV provided by the Reuven Ramaty High-Energy Solar Spectroscopic Imager mission (RHESSI) allows for the first time the detailed study of the locations and the spectra of solar microflares down to 3 keV. During a one-hour quiet interval (GOES soft X-ray level around B6) on 2 May, 1:40-2:40 UT, at least 7 microflares occurred with the largest peaking at A6 GOES level. The microflares are found to come from 4 different active regions including one behind the west limb. At 7'' resolution, some events show elongated sources, while others are unresolved point sources. In the impulsive phase of the microflares, the spectra can generally be fitted better with a thermal model plus power law above ∼ 6-7 keV than with a thermal only. The decay phase sometimes can be fitted with a thermal only, but in some events, power-law emission is detected late in the event indicating particle acceleration after the thermal peak of the event. The behind-the-limb microflare shows thermal emissions only, suggesting that the non-thermal power law emission originates lower, in footpoints that are occulted. The power-law fits extend to below 7 keV with exponents between −5 and −8, and imply a total non-thermal electron energy content between 1026-1027 erg. Except for the fact that the power-law indices are steeper than what is generally found in regular flares, the investigated microflares show characteristics similar to large flares. Since the total energy in non-thermal electrons is very sensitive to the value of the power law and the energy cutoff, these observations will give us better estimates of the total energy input into the corona. (Note that color versions of figures are on the accompanying CD-ROM.) Title: Hard x-ray and Metric/Decimetric Radio Observations of the 20 February 2002 Solar Flare Authors: Vilmer, N.; Krucker, S.; Lin, R. P.; The Rhessi Team Bibcode: 2002SoPh..210..261V Altcode: The GOES C7.5 flare on 20 February 2002 at 11:07 UT is one of the first solar flares observed by RHESSI at X-ray wavelengths. It was simultaneously observed at metric/decimetric wavelengths by the Nançay radioheliograph (NRH) which provided images of the flare between 450 and 150 MHz. We present a first comparison of the hard X-ray images observed with RHESSI and of the radio emission sites observed by the NRH. This first analysis shows that: (1) there is a close occurrence between the production of the HXR-radiating most energetic electrons and the injection of radio-emitting non-thermal electrons at all heights in the corona, (2) modifications with time in the pattern of the HXR sources above 25 keV and of the decimetric radio sources at 410 MHz are observed occurring on similar time periods, (3) in the late phase of the most energetic HXR peak, a weak radio source is observed at high frequencies, overlying the EUV magnetic loops seen in the vicinity of the X-ray flaring sites above 12 keV. These preliminary results illustrate the potential of combining RHESSI and NRH images for the study of electron acceleration and transport in flares. Title: The RHESSI Imaging Concept Authors: Hurford, G. J.; Schmahl, E. J.; Schwartz, R. A.; Conway, A. J.; Aschwanden, M. J.; Csillaghy, A.; Dennis, B. R.; Johns-Krull, C.; Krucker, S.; Lin, R. P.; McTiernan, J.; Metcalf, T. R.; Sato, J.; Smith, D. M. Bibcode: 2002SoPh..210...61H Altcode: The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observes solar hard X-rays and gamma-rays from 3 keV to 17 MeV with spatial resolution as high as 2.3 arc sec. Instead of focusing optics, imaging is based on nine rotating modulation collimators that time-modulate the incident flux as the spacecraft rotates. Starting from the arrival time of individual photons, ground-based software then uses the modulated signals to reconstruct images of the source. The purpose of this paper is to convey both an intuitive feel and the mathematical basis for this imaging process. Following a review of the relevant hardware, the imaging principles and the basic back-projection method are described, along with their relation to Fourier transforms. Several specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable to RHESSI imaging are briefly described. The characteristic strengths and weaknesses of this type of imaging are summarized. Title: Relative Timing and Spectra of Solar Flare Hard X-ray Sources Authors: Krucker, Säm; Lin, R. P. Bibcode: 2002SoPh..210..229K Altcode: Hard X-ray lightcurves, spectrograms, images, and spectra of three medium-sized flares observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) are presented. Imaging spectroscopy of the 20 February 2002, 11:06 UT flare at 10'' spatial resolution, comparable to the best previous hard X-ray imaging from Yohkoh, shows two footpoints with an ∼ 8 s delay of peak emission between footpoints. Subsequent imaging at le4'' shows three sources consistent with two separate loops and simultaneous brightening in connected footpoints. Imaging for the simple two footpoint flare of 2 June 2002 also shows simultaneous footpoint brightening. The more complex 17 March 2002 flare shows at least four different sources during the main peak of the event, and it is difficult to clearly demonstrate simultaneous brightening of connected footpoints. Non-thermal power laws are observed down to ∼ 12-13 keV without flattening in all these events, indicating the energy content in energetic electrons may be significantly greater than previously estimated from assumed 25 keV low energy cutoff. Simultaneously brightening footpoints show similar spectra, at least in the three flares investigated. Double-power-law spectra with a relatively sharp break are often observed. Title: Modeling Images and Spectra of a Solar Flare Observed by RHESSI on 20 February 2002 Authors: Sui, Linhui; Holman, Gordon D.; Dennis, Brian R.; Krucker, Säm; Schwartz, Richard A.; Tolbert, Kim Bibcode: 2002SoPh..210..245S Altcode: We have analyzed a C7.5 limb flare observed by RHESSI on 20 February 2002. The RHESSI images appear to show two footpoints and a loop-top source. Our goal was to determine if the data are consistent with a simple steady-state model in which high-energy electrons are continuously injected at the top of a semicircular flare loop. A comparison of the RHESSI images with simulated images from the model has made it possible for us to identify spurious sources and fluxes in the RHESSI images. We find that the RHESSI results are in many aspects consistent with the model if a thermal source is included between the loop footpoints, but there is a problem with the spectral index of the loop-top source. The thermal source between the footpoints is likely to be a low-lying loop interacting with the northern footpoint of a higher loop containing the loop-top source. Title: Rhessi and Trace Observations of the 21 April 2002 x1.5 Flare Authors: Gallagher, Peter T.; Dennis, Brian R.; Krucker, Säm; Schwartz, Richard A.; Tolbert, A. Kimberley Bibcode: 2002SoPh..210..341G Altcode: Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer (TRACE). The major findings are as follows: (1) The 3-25 keV X-rays started < 4 min before the EUV (195 Å) emission suggesting that the initial energy release heated plasma directly to ≳20 MK, well above the 1.6 MK needed to produce the Fe xii (195 Å) line. (2) Using coaligned 12-25 keV RHESSI and TRACE images, further evidence is found for the existence of hot (15-20 MK) plasma in the 195 Å passband. This hot, diffuse emission is attributed to the presence of the Fe xxiv (192 Å) line within the TRACE 195 Å passband. (3) The 12-25 keV source centroid moves away from the limb with an apparent velocity of ∼ 9.9 km s−1, slowing to ∼ 1.7 km s−1 after 3 hours, its final altitude being ∼ 140 Mm after ∼ 12 hours. This suggests that the energy release site moves to higher altitudes in agreement with classical flare models. (4) The 50-100 keV emission correlates well with EUV flare ribbons, suggesting thick-target interactions at the footpoints of the magnetic arcade. The 50-100 keV time profile matches the time derivative of the GOES light curve (Neupert effect), which suggests that the same electrons that produced the thick-target hard X-ray emission also heat the plasma seen in soft X-rays. (5) X-ray footpoint emission has an E−3 spectrum down to ∼ 10 keV suggesting a lower electron cutoff energy than previously thought. (6) The hard X-ray (25-200 keV) peaks have FWHM durations of ∼ 1 min suggesting a more gradual energy release process than expected. (7) The TRACE images reveal a bright symmetric front propagating away from the main flare site at speeds of ≥ 120 km s−1. This may be associated with the fast CME observed several minutes later by LASCO. (8) Dark sinuous lanes are observed in the TRACE images that extend almost radially from the post-flare loop system. This `fan of spines' becomes visible well into the decay phase of the flare and shows evidence for both lateral and downward motions. 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: 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: RHESSI Observations of Continuous Solar X-Ray Emission from 3 to 10 keV Authors: Hurford, G. J.; Krucker, Sam; RHESSI Team Bibcode: 2002AAS...200.7604H Altcode: 2002BAAS...34..776H The RHESSI spacecraft, launched 5 February 2002, provides imaging spectroscopy of solar x-ray emission from 3 keV to 15 MeV. In addition to its high spatial and spectral resolution (2.3 arcsec and ~ 1 keV respectively), RHESSI also has exceptional sensitivity at low energies. This sensitivity has enabled it to observe continuous x-ray emission from the non-flaring Sun at energies from 3 to ~ 10 keV. This paper presents preliminary observations of the spatial and spectral characteristics of this emission. Title: Modeling Images and Spectra of Solar Flares Observed by RHESSI Authors: Sui, L.; Holman, G. D.; Krucker, S.; Dennis, B. R. Bibcode: 2002AAS...200.6908S Altcode: 2002BAAS...34R.758S The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was successfully launched on February 5, 2002. RHESSI is now obtaining high resolution spectra and images of solar flares in x-rays and gamma-rays. In order to interpret these images and spectra, we have been developing flare models incorporating thermal and nonthermal bremsstrahlung emission. In our models electrons with a power-law energy distribution are continuously injected into a flare loop, with Coulomb scattering and magnetic mirroring included. We use the EFLUXPROG Fokker-Planck code ( http://hesperia.gsfc.nasa.gov/hessi/modelware.htm ) to obtain the steady-state electron distribution throughout the loop. We then compute the spectral and spatial distribution of the bremsstrahlung hard x-ray flux. We have compared these model results with results obtained by RHESSI during the impulsive phase of several moderate size flare. We will present some of these early RHESSI observations and the comparison with our flare models. This work is supported in part by the NASA Sun-Earth Connection Program and the RHESSI Project. Title: X-ray and radio observations of the 20 February 2002 solar flare Authors: Vilmer, N. R.; Krucker, S.; Lin, R. P.; Schwartz, R. A.; Klassen, A.; RHESSI Team Bibcode: 2002AAS...200.7606V Altcode: 2002BAAS...34..776V The GOES C7.5 flare on 20 February 2002 at 11:06 UT was simultaneously observed at X-ray wavelengths by RHESSI and at metric/decimetric wavelengths by the Nançay radioheliograph (NRH). We shall present a first comparison of the time evolution of X-ray images in different energy bands observed with RHESSI and of the time evolution of images of the radio emission sites at 5 frequencies in the 150-450 MHz range observed by the NRH. The identification of the different radio spectral features is provided by the Tremsdorf radiospectrograph. We shall also investigate whether the evolution in space and time of the X-ray sources are related to any spectral evolution. 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: X-ray Signatures related to Energetic Electrons Escaping from the Sun: First HESSI results Authors: Krucker, S.; Lin, R. P.; Klein, K. Bibcode: 2002AGUSMSH21D..01K Altcode: Solar energetic electrons are observed to escape from the Sun into interplanetary space. However, the release machanism(s) and the involved magnetic field geometry are not well understood. In particular, it is not well understood why some solar events release energetic electrons into interplanetary and others are not. Here, radio emissions (type III bursts) and insitu observations of escaping electron beams (impulsive electron events) are compared with X-ray signatures produced by electrons moving towards the Sun. The Nancay Radio Heliograph and the WAVES experiment on WIND provide the radio observations, WIND/3DP the insitu 1-300~keV electron observations. The X-ray observations are taken by the recently launched High Energy Solar Spectroscopic Imager (HESSI). First results of this study are presented. Title: Solar energetic electron events and coronal shocks Authors: Klassen, A.; Bothmer, V.; Mann, G.; Reiner, M. J.; Krucker, S.; Vourlidas, A.; Kunow, H. Bibcode: 2002A&A...385.1078K Altcode: Mildly relativistic electrons appear during the solar energetic particle events. A detailed investigation on the origin of such electrons is presented for four particlular solar events. The mildly relativistic electrons have been detected at energies of 0.25-0.7 MeV by COSTEP/SOHO and below 0.392 MeV by Wind 3-DP experiments. Coronal shocks associated with these electron events are identified from the metric-to-decametric solar type II radio bursts. All selected events were associated with solar activity at western longitudes, so that the magnetic footpoints connecting the spacecraft with the Sun were close to the flare/shock/CME site. The associated type II bursts were accompanied by so-called shock accelerated (SA) type III bursts appearing to be emerging from the type II emission site. We found: (1) that all of the 0.25-0.7 MeV electron events were released during or after, but never simultaneously with the onset of type II bursts and CMEs. The time delay between the type II burst onset and the release of the mildly relativistic electrons is in the range of 11.5-45 min; (2) that the mildly relativistic electrons were released rather at the end of SA type III bursts or somewhat later; (3) that the mildly relativistic electrons were released when the associated type II burst and the CME reached a certain height (h ~ 1-4 R_s) above the photosphere. For the four events studied, it is concluded that mildly relativistic electrons at 0.25-0.7 MeV energies measured in the interplanetary medium from solar energetic particle events are accelerated by coronal shock waves, commonly in association with white-light CMEs. Title: Energy Distribution of Microevents in the Quiet Solar Corona Authors: Benz, Arnold O.; Krucker, Säm Bibcode: 2002ApJ...568..413B Altcode: 2001astro.ph..9027B Recent imaging observations of EUV line emissions have shown evidence for frequent flarelike events in a majority of the pixels in quiet regions of the solar corona. The changes in coronal emission measure indicate impulsive heating of new material to coronal temperatures. These heating or evaporation events are candidate signatures of ``nanoflares'' or ``microflares'' proposed to interpret the high temperature as well as the very existence of the corona. The energy distribution of these microevents reported in the literature differ widely, and so do the estimates of their total energy input into the corona. Here we analyze the assumptions of the different methods, compare them by using the same data set, and discuss their results. We also estimate the different forms of energy input and output, keeping in mind that the observed brightenings are most likely secondary phenomena. A rough estimate of the energy input observed by EIT on the SOHO satellite is of the order of 10% of the total radiative output in the same region. It is considerably smaller for the two reported TRACE observations. The discrepancy can be explained by flare selection and different thresholds for flare detection. There is agreement on the slope and the absolute value of the distribution if the same methods are used and a numerical error is corrected. The extrapolation of the power law to unobserved energies that are many orders of magnitude smaller remains questionable. Nevertheless, these microevents and unresolved smaller events are currently the best source of information on the heating process of the corona. Title: Energetic Particle Observations during the 2000 July 14 Solar Event Authors: Bieber, John W.; Dröge, Wolfgang; Evenson, Paul A.; Pyle, Roger; Ruffolo, David; Pinsook, Udomsilp; Tooprakai, Paisan; Rujiwarodom, Manit; Khumlumlert, Thiranee; Krucker, Säm Bibcode: 2002ApJ...567..622B Altcode: Data from nine high-latitude neutron monitors are used to deduce the intensity-time and anisotropy-time profiles and pitch-angle distributions of energetic protons near Earth during the major solar event on 2000 July 14 (also known as the Bastille Day event). In addition, particle and magnetic field measurements from Wind, the Advanced Composition Explorer, and the Solar and Heliospheric Observatory (SOHO) are used in the analysis. The observations are fitted with good agreement between two independent numerical models of interplanetary transport. The rapid decrease of anisotropy from a high initial value cannot be explained by a simple model of interplanetary transport. Hence, we invoke a barrier or magnetic bottleneck consistent with an observed magnetic disturbance from an earlier coronal mass ejection that was located ~0.3 AU beyond Earth's orbit at the time of the Bastille Day event. This work includes the first treatment of focused transport through a magnetic bottleneck. We conclude that the bottleneck reflected a major fraction (~85%) of the relativistic solar protons back toward Earth. 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: WIND/3DP and Nançay Radioheliograph observations of solar energetic electron events Authors: Klein, K.; Krucker, S.; Trottet, G. Bibcode: 2002cosp...34E2239K Altcode: 2002cosp.meetE2239K Initial results of a combined study of electron events with the 3DP experiment on the WIND satellite and the Nançay Radioheliograph (NRH) are presented. A total of 60 electron events whose solar release time could be inferred from WIND/3DP observations occurred during NRH observing times. In 25 of them a distinct radio signature was detected in dm-m- maps of the NRH. These events are about equally distributed among two categories: (1) Electron release together with dm-m- bursts of a few minutes duration: these events are also accompanied by decametric-to-kilometric type III bursts seen by WAVES/WIND. They correspond to the well-known impulsive electron events. (2) Electron release during long duration (several tens of minutes) dmm- emission: the electrons are most often released more than ten minutes after the start of the radio event, frequently without simultaneous decametric type III bursts. In the majority of cases the dm-m- radio source changes position, size, and/or intensity near the time of electron release. The findings suggest that a spectrum of solar causes can be responsible for energetic electron events at 1 AU. We show examples of the different categories of events and discuss them with respect to different scenarios of acceleration and transport in the corona. Title: Hard X-ray and metric/decimetric radio observations of the 10 April 2002 solar flare Authors: Vilmer, N.; Trottet, G.; Krucker, S.; Lin, R. Bibcode: 2002cosp...34E2370V Altcode: 2002cosp.meetE2370V The GOES M8.2 flare on 10 April 2002 at1230 UT was observed at Xray- wavelengths by RHESSI and at metric/decimetric wavelengths by the Nançay Radioheliograph (NRH). We shall present some comparisons of the time evolutions of the X-ray images in different energy bands observed by RHESSI and of the time evolution of the radio emission sites at 5 frequencies in the 150-450 MHz range observed by the NRH. We shall investigate whether the evolution in space and time of X-ray and radio sources are related to any spectral evolution of the energetic particles. We shall also investigate the magnetic structures at different spatial scales in which accelerated electrons propagate from the flare region as well as the existence of coronal acceleration regions remote from the flare site. Title: Nanoflare heating in the quiet corona: evidence and problems Authors: Krucker, S. Bibcode: 2002AdSpR..30..493K Altcode: The content of coronal material in the quiet Sun is not constant, as soft X-ray (SXR) and high-temperature EUV line observations have shown. The observed temporal variations in SXR and EUV flux are interpreted as heating events showing many analogies to flares occurring in active regions. Compared to a large flare in an active region, the thermal energy input of a heating event in the quiet corona is 6 or more orders of magnitude smaller. Heating events in the quiet corona are therefore often called microflares or nanoflares. The total heating rate of nanoflares in the quiet corona has been investigated in several works by many different authors: Do nanoflares provide enough energy to heat the quiet corona? The results of the different studies are controversial. In this review, recent observational studies on nanoflare heating in the quiet corona and their different results are compared and discussed. Title: Micro-events in the Quiet Solar Corona Authors: Benz, A.; Krucker, S. Bibcode: 2002cosp...34E1929B Altcode: 2002cosp.meetE1929B The content of coronal material in the quiet Sun is not constant as soft X-ray and high-temperature EUV line observations have shown. New material, probably heated and evaporated from the chromosphere is occasionally injected even in the faintest parts above the magnetic network cell interiors. We discuss the characteristics of the largest of these events, based on simultaneous transition region observations (in EUV and radio) and the observed analogies to flares. Assuming that the smaller events follow the same pattern, we estimate the total mass input and compare it to the requirements observed in the solar wind. A rough estimate of the energy input observed by EIT on the SoHO satellite is of the order of 10% of the total radiative output in the same region. The simulation indicates that the extrapolation to smaller events is problematic and that smaller events may play an even more decisive role than previously assumed. The hypothesis of nanoflare heating is consistent with these observations if the lower corona is not just heated, but continuously replenished by chromospheric material heated to coronal temperatures. These micro-events are currently the best source of information on the heating process of the corona. Title: Understanding of Particle Acceleration from in situ Electron Measurements Authors: Krucker, S. Bibcode: 2002cosp...34E2888K Altcode: 2002cosp.meetE2888K Solar particle acceleration can be studied by investigating particles escaping from the Sun into interplanetary space. Next to the quasi-steady solar wind streaming away from the Sun, there are also large, sudden increases in particle flux observed. These events detected from GeV down to keV energies are called solar energetic particle (SEP) events. In this talk, new results obtained by the 3-D Plasma and Energetic Particle Instruments on the WIND spacecraft are reviewed. The discussion on the identification of solar events possibly involved in the acceleration of SEP is emphasized. Additonally, in situ electron observations are compared to RHESSI X-ray obseravations. Title: The Frequency Distribution and Other Statistical Properties of Solar Impulsive Electron Events Authors: Oakley, P. H.; Krucker, S.; Lin, R. P. Bibcode: 2001AGUFMSH42A0770O Altcode: A statistical survey of 391 impulsive electron events (1-300~keV) observed at 1 AU by the 3-D Plasma and Energetic Particles experiment on the WIND spacecraft is presented. The data was taken during the first 7 years of operations of the WIND spacecraft covering the last solar minimum and maximum. Two main results are presented: (a) The event frequency distribution as a function of peak flux is found to be a power law with an index around -1.4 over 3 orders of magnitude. Assuming the peak flux is proportional to the total energy released in an event, this result suggests that the few big events are energetically most important. (b) In most events, the peak flux spectra in the energy range of 1-300 keV can be fitted with a double power law with a turnover energy around 50~keV. The averaged slope at lower energies is around -1.8 and it steepens at higher energies to an average value of -3.3 Title: On the Velocity Dispersion in Solar Impulsive Electron Events Authors: Krucker, S.; Lin, R. P.; Larson, D. E.; Bale, S. D. Bibcode: 2001AGUFMSH31C..08K Altcode: A statistical survey of 26 solar impulsive electron events in the 1 to 300 keV range is presented, as observed by the 3D Plasma and Energetic Particle experiment on the Wind spacecraft. This study was triggered by results of ACE/EPAM observations (Roelof et al.) reporting the absence of velocity dispersion in solar impulsive electrons events in the energy range of 40-300~keV. Here we find that the majority of events show clear velocity dispersion. Events with high peak flux had to be first corrected for spurious counts of electrons depositing only a fraction of their energy before they are scattered out of the detector. Without this correction the onset times wrongly appeared to be almost without velocity dispersion. For the large events, the reduced distribution functions show positive slopes at high energies (around 100 keV). The calculated linear growth rates are large, but no in-situ waves are observed by the WIND/WAVES instrument. Title: Solar Origin of a Series of Well-collimated Electron Events Authors: Maia, Dalmiro; Pick, Monique; Hawkins, S. Edward, III; Krucker, Sam Bibcode: 2001ApJ...560.1058M Altcode: We investigate the solar origin and propagation of a series of well-collimated energetic electron events on 1997 November 28, measured in situ by the Electron, Proton, and Alpha Monitor (EPAM) experiment on the Advanced Composition Explorer (ACE) spacecraft. EPAM measures electrons in the energy range from 40 to 300 keV over a wide range of look directions and with better than 1 minute time resolution. During the events in our study, the particles are strongly collimated along the magnetic field. As such, these near-relativistic (beta=0.4-0.7) particles tend to be scatter free; their observed arrival at ACE provides a good estimate of the release time back to the Sun. EPAM results are extended to 20 keV using energetic electron data from the Three-Dimensional Plasma Analyzer (3DP) experiment on the Wind spacecraft. We combine these observations with fast imaging of the solar corona in the meter wave domain, provided by the Nançay radioheliograph, and dynamic spectral information from the WAVES experiment on the Wind spacecraft. Together, this complement of observations of solar energetic particle events provides insight into the onset times and sites of particle acceleration in the low corona. The imaging observations at metric radio frequencies show three series of similar events off the west limb of the Sun that extend into interplanetary type III bursts. The third event corresponds only to a strong in situ particle event. During this time period, the coronagraphic observations from the Large Angle and Spectrometric Coronagraph experiment on the Solar and Heliospheric Observatory reveal a dimming and some morphological changes in the vicinity of the injection site, suggesting the opening of the magnetic field. We find that for the strongest event, the onset times for the energetic particles measured at the spacecraft reveal a dispersion in the inferred release times back at the Sun versus energy. The onset times measured for electrons with energies above 60 keV are not compatible with the release time back at the Sun inferred from particles of lower energies. This dispersion can be successfully explained by the radio data: the radio observations reveal two successive energetic electron injections separated by a few minutes. The observations thus show that there is a later release of the particles detected in situ at higher energies and that the origin of the phenomenon is in the low corona. Title: The Source Regions of Impulsive Solar Electron Events Authors: Benz, Arnold O.; Lin, Robert P.; Sheiner, Olga A.; Krucker, Säm; Fainberg, Joe Bibcode: 2001SoPh..203..131B Altcode: Low-energy (2-19 keV) impulsive electron events observed in interplanetary space have been traced back to the Sun, using their interplanetary type III radiation and metric/decimetric radio-spectrograms. For the first time we are able to study the highest frequencies and thus the radio signatures closest to the source region. All the selected impulsive solar electron events have been found to be associated with an interplanetary type III burst. This allows to time the particle events at the 2 MHz plasma level and identify the associated coronal radio emissions. Except for 5 out of 27 cases, the electron events were found to be associated with a coronal type III burst in the metric wavelength range. The start frequency yields a lower limit to the density in the acceleration region. We also search for narrow-band spikes at the start of the type III bursts. In about half of the observed cases we find metric spikes or enhancements of type I bursts associated with the start of the electron event. If interpreted as the plasma emission of the acceleration process, the observed average frequency of spikes suggests a source density of the order of 3×108 cm−3 consistent with the energy cut-off observed. Title: High energy particle acceleration by solar flares and fast coronal mass ejections Authors: Lin, R. P.; Hurford, G.; Krucker, S.; Bale, S. Bibcode: 2001ESASP.493..275L Altcode: 2001sefs.work..275L No abstract at ADS Title: Solar Impulsive Electron Events with Unusual Velocity Dispersions Authors: Krucker, S.; Bale, S. D.; Lin, R. P. Bibcode: 2001ICRC....8.3235K Altcode: 2001ICRC...27.3235K A statistical survey of 26 solar impulsive electron events in the ∼1 to 300 keV range is presented, as observed by the 3D Plasma and Energetic Particle experiment on the Wind spacecraft. This study was triggered by results of ACE/EPAM observations (Roelof et al.) reporting the absence of velocity dispersion in solar energetic electrons events in the energy range of 40-300 keV. Earlier studies also reported the absence of dispersion in some events, whereas Roelof et al. claim to see no velocity dispersion in most of the events. The presented survey in this paper shows that besides normal dispersive events (14 out of 26), there are indeed events with unusual velocity dispersion (12 out of 26) showing nearly simultaneous onsets at 1 AU above 100 keV. A strong correlation between the peak flux and the absence of velocity dispersion is found: Events with high peak flux (≥0.2 cm-2 s-1 ster-1 eV-1 at 27 keV) show unusual velocity dispersion. This explains that earlier WIND/3DP surveys taken during solar minimum where events were smaller did not show events with unusual velocity dispersion. Title: Are Heating Events in the Quiet Solar Corona Small Flares? Authors: Krucker, S.; Benz, A. O. Bibcode: 2001AGUSM..SP52B03K Altcode: Temporary enhancements of the coronal emission measure in a quiet region have been shown to constitute a significant energy input. Here some relatively large events (Yohkoh, EIT Observations) are discussed and tested for characteristics known from full-sized impulsive flares in active regions. The differences to active region flares seem to be mainly quantitative, and the analyzed heating events may in principle be considered as microflares or large nanoflares, thus small versions of regular flares. In this presentation we focus on the following questions: (1) Are there related events seen in transient region lines (CDS/SUMER observations)? (2) What are the relative contributions of the thermal, potential, and expansion energy to the total energy released in these events. (3) Are the reported flare temperatures in quiet regions of 1-2 MK possibly higher (3-5 MK)? Title: Solar Impulsive Electron Events with Unusual Velocity Dispersions Authors: Krucker, S.; Bale, S. D.; Lin, R. P. Bibcode: 2001AGUSM..SH62B10K Altcode: Solar impulsive electron events observed at 1 AU occasionally do not show the expected velocity dispersion normally produced by the difference in time of flight of electrons at different energies. These unusual velocity dispersions were brought to attention by Roelof et al. investigating ACE EPAM data (40-300 keV). They explain the absence of velocity dispersion by wave-particle interaction slowing down the electrons. Here we present WIND/3DP observations of those events investigating the following questions: 1) Are the onset times simultaneously at all energies? In particularly, what are the onset times at low energies (1-40 keV)? 2) Are the spectra flat at the beginning of the events as predicted by wave-particle interaction? 3) Is there an enhanced level of insitu waves at the time when 40-300 keV electrons arrive at 1 AU as a result of the wave-particle interaction? Title: Heating the Quiet Corona by Nanoflares: Evidence and Problems Authors: Benz, A. O.; Krucker, S. Bibcode: 2001IAUS..203..471B Altcode: 2000astro.ph.12106B The content of coronal material in the quiet Sun is not constant as soft X-ray and high-temperature EUV line observations have shown. New material, probably heated and evaporated from the chromosphere is occasionaly injected even in the faintest parts above the magnetic network cell interiors. We discuss the characteristics of the largest of these events, based on simultaneous transition region observations (in EUV and radio) and the observed analogies to flares. Assuming that the smaller events follow the same pattern, we estimate the total energy input. Various recent analyses are compared and briefly discussed. Finally we present the results of a simulation, extrapolating the observed range of microflares to smaller energies. The simultation indicates that the extrapolation to smaller events is problematic and that smaller events may play an even more decisive role than previously assumed. The hypothesis of nanoflare heating is consistent with these observations if the lower corona is not just heated, but continuously replenished by chromospheric material heated to coronal temperatures. Title: Mechanisms for Coronal Mass Supply by Evaporative Micro-Events Authors: Brown, J. C.; Krucker, S.; Güdel, M.; Benz, A. O. Bibcode: 2001IAUS..203..498B Altcode: There is extensive evidence from SoHO and other data that ``micro-events'' play an important role in sustaining at least some components of the solar corona. These are often termed coronal micro-``heating events'' though a major part of their role is feeding coronal loops through chromospheric evaporation. We consider what can be learnt from these data concerning the energy release and transport mechanisms driving the evaporation, including thermal conduction and fast particles, and what model constraints are available from other data (such as hard X-rays and radio events). We conclude, from one large event and the statistics of many small ones, that conductive evaporation alone does not fit observations and that fast particles or some other nonthermal driver must be involved. As well as the problem of single loop events, we consider the global implications for supply of the corona and wind. Title: Relativistic solar protons on Bastille Day 2000 Authors: Bieber, J. W.; Droege, W.; Evenson, P.; Pyle, R.; Ruffolo, D.; Pinsook, U.; Tooprakai, P.; Rujiwarodom, M.; Khumlumlert, T.; Krucker, S. Bibcode: 2001ICRC...27Q....B Altcode: Relativistic protons emitted by the Sun on 2000 July 14 (Bastille Day) caused the count rate of high-latitude neutron monitors to increase by 25-40% relative to the preexisting background of galactic cosmic rays. We use a 9monitor network to show that an initial large anisotropy decreased very rapidly to a small value. We model the event with numerical codes based upon the Boltzmann equation, and conclude that solar particle transport was significantly influenced by a reflecting boundary or magnetic bottleneck from an earlier CME that was located ∼ 0.3 AU beyond Earth at the time of the Bastille Day event. We also model Wind low-energy electrons to determine the parallel mean free path over a rigidity range spanning 4 orders of magnitude. Title: Two Classes of Solar Proton Events Derived from Onset Time Analysis Authors: Krucker, Säm; Lin, R. P. Bibcode: 2000ApJ...542L..61K Altcode: We analyze onset times for 26 solar energetic (from 30 keV to 6 MeV) proton events that exhibit clear velocity dispersion, as observed by the three-dimensional Plasma and Energetic Particles experiment on the Wind spacecraft. Assuming that the particles are injected simultaneously at all energies and travel the same path length, we find two classes of proton events: (1) for the 18 class 1 events, the derived path lengths are between 1.1 and 1.3 AU, indicating that the first arriving protons travel essentially scatter-free and (2) the eight class 2 events show longer path lengths around 2 AU. For all proton events, the observed temporally related electron events all have electron path lengths around 1.1-1.3 AU. Relative to the electron injection time at the Sun, the protons of the first class are injected ~0.5-2 hr later. Assuming these particles are accelerated by the associated coronal mass ejection (CME) shock, the protons at all energies between 0.03 and 6 MeV appear to be accelerated (or released) simultaneously high in the corona, roughly ~1-10 Rsolar above the electrons. The pitch-angle distributions are observed to be similar for both classes of events, making it unlikely that propagation effects are responsible for the longer path length of class 2 events. The late proton onset times at 1 AU of class 2 are therefore more likely explained by a successively later solar release (or escape) of protons at successive lower energies. Assuming again acceleration at the CME shock, the release (or escape) of the protons of class 2 events appears to depend on energy and occur at a higher altitude for lower energies, with the most energetic protons possibly released simultaneously with the electrons. Title: On the solar release of Energetic Particles detected at 1 AU Authors: Krucker, Säm; Lin, Robert P. Bibcode: 2000AIPC..528...87K Altcode: 2000atep.conf...87K The 3-D Plasma and Energetic Particles experiment on the WIND spacecraft was designed to provide high sensitivity measurements of both suprathermal ions and electrons down to solar wind energies. A statistical survey of 26 solar proton events has been investigated. For all these proton events, a temporally related electron event is observed. The presented results focus on the properties of protons released near the Sun which show a velocity dispersion when detected at 1 AU. The particle flux onset times observed at 1 AU in the energy range between 30 keV and 6 MeV suggest that there are two classes of proton events: (1) For one class (70% of the events), the first arriving protons are traveling almost scatterfree as indicated by the derived path lengths between 1.1 and 1.3 AU, (2) whereas the events of the second class show significantly larger path lengths of around 2 AU. Relative to the electron release time at the Sun, the almost scatterfree traveling protons of the first class of events are release delayed by 0.5 to 2 hours. For the events of the second class, protons and electrons seemed to be released simultaneously within the accuracy of 20 minutes. . Title: Mechanisms for dynamic coronal mass supply via evaporative solar ``micro-events'' Authors: Brown, J. C.; Krucker, S.; Güdel, M.; Benz, A. O. Bibcode: 2000A&A...359.1185B Altcode: The idea that the corona is at least in part supplied by chromospheric evaporation in loop ``micro-events" is quantified in terms of the power requirements of evaporation mechanisms, using recent analyses of data on such events in high temperature EUV lines from the SoHO EIT instrument. Estimates are derived for the pre-event and event values of loop density and temperature and it is shown, using the conductive scaling law, that the event emission measure enhancements are too large to be accounted for solely by enhanced conductive flux from coronal heating. That is, observations demand that supply of coronal mass by evaporation events need a mechanism which enhances upper chromospheric heating and not just conductively driven evaporation. Thus coronal mass supply in transients is inextricably linked to direct chromospheric heating processes. Using parametric models of a chromospheric heating function and of the pre-event chromosphere, an estimate is made of the extra power required to yield the emission measure enhancement of a large event evaporatively. The dependence of the result on just how the EUV solar images are interpreted is emphasised and observational tests are discussed for the case of heating by fast particles. Implications of the results in terms of the global supply of the hot corona and wind mass loss are briefly mentioned. Title: Coronal Replenishment and Heating Authors: Benz, A. O.; Krucker, S.; Mitra Kraev, U. Bibcode: 2000SPD....31.0217B Altcode: 2000BAAS...32..814B The content of coronal material in the quiet Sun is not constant as soft X-ray and high-temperature EUV line observations have shown. New material, probably heated and evaporated from the chromosphere is occasionaly injected even in the faintest parts above the magnetic network cell interiors. We discuss the characteristics of the largest of these events, based on simultaneous transition region observations (in EUV and radio) and the observed analogies to flares. Assuming that the smaller events follow the same pattern, we estimate the total mass input and compare it to the requirements observed in the solar wind. Various recent analyses are compared and briefly discussed. Finally we present the results of a simulation, extrapolating the observed range of microflares to smaller energies. The simultation indicates that the extrapolation to smaller events is problematic and that smaller events may play an even more decisive role than previously assumed. The hypothesis of nanoflare heating is consistent with these observations if the lower corona is not just heated, but continuously replenished by chromospheric material heated to coronal temperatures. Title: Two Classes of Solar Proton Events derived from Onset Time Analysis Authors: Krucker, S.; Lin, R. P. Bibcode: 2000SPD....31.0287K Altcode: 2000BAAS...32R.826K The 3-D Plasma and Energetic Particles experiment on the WIND spacecraft was designed to provide high sensitivity measurements of both suprathermal ions and electrons down to solar wind energies. A statistical survey of 26 solar proton events has been investigated. For all these proton events, a temporally related electron event is observed. The presented results focus on the properties of protons released near the Sun which show a velocity dispersion when detected at 1 AU. The particle flux onset times observed at 1 AU in the energy range between 30keV and 6 MeV suggest that there are two classes of proton events: (1) For one class (70% of the events), the first arriving protons are traveling almost scatterfree as indicated by the derived path lengths between 1.1 and 1.3 AU, (2) whereas the events of the second class show significantly larger path lengths around 2 AU. Relative to the electron release time at the Sun, the almost scatterfree traveling protons of the first class of events are release delayed by 0.5 to 2 hours. For the events of the second class, protons and electrons seemed to be released simultaneously within the accuracy of 20 minutes. Events of the first class are most likely shock accelerated and the late release times suggest an acceleration region at several solar radii away from the Sun. The puzzeling events of the second class could as well be explained by pitch angle scattering or by a succesive later release time at lower energies as by an actual longer path length. Title: Are Heating Events in the Quiet Solar Corona Small Flares? Multiwavelength Observations of Individual Events Authors: Krucker, Säm; Benz, Arnold O. Bibcode: 2000SoPh..191..341K Altcode: 1999astro.ph.12501K Temporary enhancements of the coronal emission measure in a quiet region have been shown to constitute a significant energy input. Here some relatively large events are studied for simultaneous brightenings in transition region lines and in radio emission. Associated emissions are discussed and tested for characteristics known from full-sized impulsive flares in active regions. Heating events and flares are found to have many properties in common, including (i) associated polarized radio emission, which usually precedes the emission measure peak (Neupert effect) and sometimes has a non-thermal spectrum, and (ii) associated and often preceding peaks in O v and He i emission. On the other hand, heating events also differ from impulsive flares: (i) In half of the cases, their radio emission at centimeter waves shows a spectrum consistent with thermal radiation, (ii) the ratio of the gyro-synchrotron emission to the estimated thermal soft X-ray emission is smaller than in flares, and (iii) the associated emission in the O v transition region line shows red shifts and blue shifts, indicating upflows in the rise phase and downflows in the decay phase, respectively. Nevertheless, the differences seem to be mainly quantitative, and the analyzed heating events with thermal energies around 1026 erg may in principle be considered as microflares or large nanoflares, thus small versions of regular flares. Title: WIND Observations of Energetic Solar Proton Events Down to keV Energies: Onset Time Analysis Authors: Krucker, S.; Lin, R. P. Bibcode: 2000ASPC..206..187K Altcode: 2000hesp.conf..187K No abstract at ADS Title: Heating Events Observed in the Quiet Corona Authors: Benz, A. O.; Krucker, S. Bibcode: 1999ESASP.448..547B Altcode: 1999ESPM....9..547B; 1999mfsp.conf..547B No abstract at ADS Title: Heating Events in the Quiet Solar Corona Authors: Krucker, S.; Benz, A. O. Bibcode: 1999spro.proc...25K Altcode: Sensitive observations of the quiet Sun provided by (1) the SXT on board the Yohkoh satellite, (2) the EIT on board the SoHO spacecraft in high-temperature iron line emission, and (3) the Very Large Array (VLA) in the centimeter radio range are investigated in view of the coronal heating problem. The observed enhancements in coronal emission measure are interpreted as heating events (microflares) bringing chromospheric material to coronal temperatures, whereas the radio observations show the existence of non-thermal emission related to some of these heating events. Assuming an effective height of 5000~km, the thermal energy inputs by such microflares have been found in the range from 8× 1024 erg to 1.6× 1026 erg, and the total energy input amounts to about 16% of the average radiated power of the coronal plasma in the quiet corona. The frequency distribution of microflares is an approximate power-law of the form f(E) = f0 E-δ with a power-law index δ between 2.3 and 2.6. As the low-energy cutoff is due to sensitivity limitations and the power-law index is steeper than 2, these observations demonstrate the possibility that microflares dominate the energy input into the quiet corona. Title: On the Origin of Impulsive Electron Events Observed at 1 AU Authors: Krucker, Säm; Larson, Davin E.; Lin, Robert P.; Thompson, Barbara J. Bibcode: 1999ApJ...519..864K Altcode: A statistical survey of 12 impulsive electron events detected at energies down below 1 keV and 58 events detected above 25 keV observed at 1 AU by the 3-D Plasma and Energetic Particles experiment on the Wind spacecraft is presented. Timing analysis of the velocity dispersion reveals two different kinds of electron events: (1) events released from the Sun at the onset of a radio type III burst, which suggest that these electrons are part of the population producing the type III radio emission; and (2) events in which the electrons are released up to half an hour later than the onset of the type III burst. These electrons therefore may be produced by a different acceleration mechanism than the population producing the radio emission. Both types of behavior can be observed during the same impulsive electron event at different energies, but most events show the same timing at all energies. At lower energies (<25 keV), type III-related impulsive electron events are more often observed (nine of 12 events), whereas at higher energies (>25 keV), events not related to type III bursts are more numerous (41 of 58). However, events of both classes are observed below 1 keV. Impulsive electron events not related to type III radio bursts are observed to be proton rich, with an order-of-magnitude lower electron-to-proton ratio than events related to type III bursts.

For roughly 3/4 of the events not related to type III bursts, large-scale coronal transient waves, also called EIT waves or coronal Moreton waves, are observed by the Extreme Ultraviolet Imaging Telescope (EIT) on board SOHO. Temporal and spatial correlations together with hydromagnetic simulations show that at least some impulsive electron events are more likely related to the propagating Moreton wave than to the flare phenomenon itself. Title: The source region of an interplanetary type II radio burst Authors: Bale, S. D.; Reiner, M. J.; Bougeret, J. -L.; Kaiser, M. L.; Krucker, S.; Larson, D. E.; Lin, R. P. Bibcode: 1999GeoRL..26.1573B Altcode: We present the first observation of the source region of an interplanetary type II radio burst, using instruments on the Wind spacecraft. Type II radio emission tracks the motion of a CME-driven interplanetary (IP) shock which encounters the spacecraft. Upstream of the IP shock backstreaming electrons are observed, first antiparallel to the interplanetary magnetic field (IMF), and then later parallel as well. Langmuir waves are observed concomitant with the shock-accelerated electrons. The electron energy spectrum and Langmuir wave amplitudes are very similar to those observed in the terrestrial electron foreshock. From the connection times to the shock, we infer the existence and characteristic size of large scale structure on the shock front. The type II radio emission seems to be generated in a small bay upstream of the shock, and this may account for some splitting structure observed in the frequency spectrum of many type II bursts. Title: Quantitative results on heating events in the quiet corona Authors: Benz, Arnold O.; Krucker, Säm Bibcode: 1999AIPC..471...67B Altcode: 1999sowi.conf...67B The emission measure of the quiet corona, defined by the plasma hotter than one million degrees, is fluctuating in nearly every 1900 km×1900 km pixel observed by EIT on SoHO. In the average, the larger the emission measure in a pixel, the more it fluctuates. Increases in emission measure constitute a major energy input into the corona, suggesting that the lower corona is not just heated, but continuously replenished by chromospheric material heated to coronal temperatures. Title: Heating events in the quiet solar corona: multiwavelength correlations Authors: Benz, Arnold O.; Krucker, Säm Bibcode: 1999A&A...341..286B Altcode: Coronal, transition region and chromospheric lines and centimeter radio emission of the quiet Sun have been simultaneously observed by SoHO and the VLA. The corona above the magnetic network has a higher pressure and is more variable than above the interior of supergranular cells. The Fourier transform in time is found to have steeper spectra in the corona and upper chromosphere than in the transition region. The temporal sequence of brightenings has been determined by cross-correlations of identical picture elements in different emissions. The method allows to study statistically the faintest fluctuations in the corona and relate them to the layers below. The cross-correlations yield that (i) the first emissions to peak in time are O V and He I originating in the transition region and the upper chromosphere, respectively. (ii) The coronal line of Fe XII lags by about 5 minutes and Fe IX/X peaks a further half a minute later in the average, latest of all emissions. The interpretation of these lags follows readily from analogy with regular flares in active regions, where O V and He I correlate closely with hard X-rays emitted by beam electrons impinging on the chromosphere. The coronal iron lines are then emitted by the evaporating plasma expanding into the corona and cooling by conducting part of the energy to increase the emission in Fe IX. (iii) The radio emission peaks before the coronal emission measure, similar to the Neupert effect in flares, but shows considerable variation relative to O V. It is proposed that there are two emission processes at work radiating both thermal emission and non-thermal gyrosynchrotron emission at various fluxes. These statistical results show that the coronal heating events follow the properties of regular solar flares and thus may be interpreted as microflares or nanoflares. Title: Heating Events in the Quiet Solar Corona Authors: Benz, Arnold O.; Krucker, Säm Bibcode: 1998SoPh..182..349B Altcode: Sensitive observations of the quiet Sun observed by EIT on the SOHO satellite in high-temperature iron-line emission originating in the corona are presented. The thermal radiation of the quiet corona is found to fluctutate significantly, even on the shortest time scale of 2 min and in the faintest pixels. The power spectrum of the emission measure time variations is approximately a power law with an exponent of 1.79±0.08 for the brightest pixels and 1.69±0.08 for the average and the faintest pixels. The more prominent enhancements are identified with previously reported X-ray network flares (Krucker et al., 1997) above the magnetic network of the quiet chromosphere. In coronal EUV iron lines they are amenable to detailed analysis suggesting that the brightenings are caused by additional plasma injected from below and heated to slightly higher temperature than the preexisting corona. Statistical investigations are consistent with the hypothesis that the weaker emission measure enhancements originate from the same parent population. The power input derived from the impulsive brightenings is linearly proportional to the radiative loss in the observed part of the corona. The absolute amount of impulsive input is model-dependent. It cannot be excluded that it can satisfy the total requirement for heating. These observations give strong evidence that a significant fraction of the heating in quiet coronal regions is impulsive. Title: Energy Distribution of Heating Processes in the Quiet Solar Corona Authors: Krucker, Säm; Benz, Arnold O. Bibcode: 1998ApJ...501L.213K Altcode: We have determined the variations in the emission measure of the solar corona using EUV Imaging Telescope/Solar and Heliospheric Observatory observations of iron lines in a quiet region of the Sun. The emission measure is found to vary significantly in at least 85% of all the pixels within 42 minutes. The variations are interpreted as heating events that bring chromospheric material above the one million degree threshold of the observed lines and that cool the coronal plasma below that limit. A method to assess heating events has been developed. The thermal energy input by such microflares is calculated from the observed increases in emission measure and the derived temperature. Heating events have been found in the range from 8×1024 to 1.6×1026 ergs. The energy input by >=3 σ events of the emission measure increase the amounts to about 16% of the average radiated power of the coronal plasma in the quiet corona. The frequency distribution of microflares is an approximate power law of the form f(E)=f0E-δ, with a power-law index δ between 2.3 and 2.6. Since the low-energy cutoff is due to sensitivity limitations and the power-law index is steeper than 2, these observations demonstrate the possibility that microflares dominate the energy input into the quiet corona. The observed power law would have to continue to about 3×1023 ergs in order to match the observed minimum heating requirement. Title: Radio Wave and Soft X-ray Diagnostics of Heating Events in the Quiet Solar Corona Authors: Krucker, S.; Benz, A. O. Bibcode: 1998cee..workE...7K Altcode: Sensitive observations of the quiet Sun observed by (1) SXT on the Yohkoh satellite, (2) EIT on the SoHO satellite in high-temperature iron line emission, (3) CDS on the SoHO satellite in the He I and O V line emission, and (4) the Very Large Array (VLA) in the centimeter radio range are investigate in the view of the coronal heating problem. The observed enhancements are interpreted as heating events (mircoflares) bringing chromospheric material to coronal temperatures. The thermal energy inputs by such microflares have been found in the range from 8 times 1024erg to 1.6 times 1026erg, and the total energy input amounts to about 16% of the average radiated power of the coronal plasma in the quiet corona. The frequency distribution of microflares is an approximate power-law of the form f(E) = f_0 E^{-delta} with a power-law index delta between 2.3 and 2.6. As the low-energy cutoff is due to sensitivity limitations and the power-law index is steeper than 2, these observations demonstrate the possibility that microflares dominate the energy input into the quiet corona. Title: X-Ray Network Flares of the Quiet Sun Authors: Krucker, Säm; Benz, Arnold O.; Bastian, T. S.; Acton, Loren W. Bibcode: 1997ApJ...488..499K Altcode: Temporal variations in the soft X-ray (SXR) emission and the radio emission above the solar magnetic network of the quiet corona are investigated using Yohkoh SXR images with deep exposure and VLA observations in the centimeter radio range. The SXR data show several brightenings, with an extrapolated occurrence probability of one brightening per 3 seconds on the total solar surface. During the roughly 10 minutes of enhanced flux, total radiative losses of the observed plasma are around 1025 ergs per event. These events are more than an order of magnitude smaller than previously reported X-ray bright points or active region transient brightenings. For all of the four SXR events with simultaneous radio observations, a corresponding radio source correlating in space and time can be found. There are several similarities between solar flares and the SXR/radio events presented in this paper. (1) Variations in temperature and emission measure during the SXR enhancements are consistent with evaporation of cooler material from the transition region and the chromosphere. (2) The ratio of the total energies radiated in SXR and radio frequencies is similar to that observed in flares. (3) At least one radio event shows a degree of polarization as high as 35%. (4) In three out of four substructures the centimeter radio emission peaks several tens of seconds earlier than in the SXR emission. (5) The associated radio emission tends to be more structured and to have faster rise times. These events thus appear to be flare-like and are called network flares. Title: Chromospheric Events in the Quiet Network Authors: Keller, C.; Bastian, T.; Benz, A.; Krucker, S. Bibcode: 1997SPD....28.1304K Altcode: 1997BAAS...29..917K Time sequences of a quiet network region close to disk center have been simultaneously recorded with the VLA, various instruments on SOHO, and the solar telescopes on Kitt Peak. The analysis of the Hα spectra obtained at the McMath-Pierce telescope revealed down-flows with apparent velocities of more than 2.5 km/s associated with magnetic field structures in the quiet network. During such events, the Hα spectra show a pronounced asymmetry. The photospheric magnetic field was determined from rapid scans in three iron lines with the Zurich Imaging Stokes Polarimeter. Up- and down-flow velocity excursions outside of magnetic field regions are compatible with chromospheric waves. We describe the properties of these events as seen in the observations of the visible part of the spectrum and their signatures at radio and UV wavelengths. The final goal of this study is the construction of a time-dependent 3-D picture of the quiet solar atmosphere and the understanding of the dynamical coupling of photospheric magnetic fields with the chromosphere and the corona. Title: Fine structure of the X-ray and radio emissions of the quiet solar corona. Authors: Benz, A. O.; Krucker, S.; Acton, L. W.; Bastian, T. S. Bibcode: 1997A&A...320..993B Altcode: Two deep soft X-ray exposures of a quiet region on the Sun were made with the SXT telescope on board the Yohkoh satellite on 20 Feb 1995. We report on the spatial X-ray fine structure. Regions of enhanced X-ray emission, more than two orders of magnitude fainter than previously reported X-ray bright points, are loosely associated with bipolar regions in the magnetic network. The power spectrum of quiet X-ray images at small spatial scales is similar to that of active regions, but exhibits a kink at a scale of =~25,000km, possibly connected to the supergranular structure. The spatial X-ray structures in the time averaged image amount to an rms amplitude which is 6% of the mean value. The X-ray structures correlate with contemporaneous radio maps obtained by the VLA at wavelengths of 1.3, 2.0, and 3.6cm. The amplitude of the brightness variations in the images increases with radio wavelength, i.e., with increasing height. The cross-correlation coefficient with the absolute magnetic field strength, however, generally decreases with height, consistent with the idea of bipolar regions in the network and of the magnetic field deviating from vertical in the upper chromosphere. The X-ray observations require an enhanced pressure in the corona above the magnetic network, but suggest similar temperatures. Model calculations show that, under a constant temperature, an rms density increase (relative to that in the cell interior) ranging from about 20% in the chromosphere to 60% in the low corona is sufficient to explain the observed standard deviations due to the spatial structures in radio waves and soft X-rays, respectively. Title: Coronal EUV and Radio Variability and Heating Authors: Krucker, S.; Benz, A. O.; Delaboudinière, J. -P. Bibcode: 1997ESASP.404..465K Altcode: 1997cswn.conf..465K No abstract at ADS Title: YOHKOH observation of the source regions of solar narrowband, millisecond spike events. Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J. Bibcode: 1997A&A...317..569K Altcode: The source regions of metric spike events are investigated on Yohkoh soft X-ray (SXR) maps. The spikes are identified by the spectrometer Phoenix between 300MHz and 360MHz and are associated with groups of type III bursts at lower frequencies reaching also the decametric range. The Very Large Array (VLA) provides simultaneously spatial information at 333MHz, 1446MHz and 4866MHz. Similar to the previous VLA observation of a metric spike event, the new data are consistent with a high altitude of the spike sources of about 5x10^10^cm above the photosphere. The additionally available SXR data for one of the presented events give the following new informations: (i) The spike sources occur near open field lines and near regions of a slightly enhanced SXR flux relative to the ambient plasma. (ii) Contrary to SXR observations of type III bursts without metric spike activity, no SXR jet is observed. (iii) At low altitude, a weak SXR enhancement occurs, peaking about 60 s after the spike event. The SXR source and the spike sources can be connected by potential field lines. The observations corroborate a model in which a metric spike event is attributed to an energy release region at high altitude, while upwards propagating electrons produce type III bursts and downward moving electrons are responsible for SXR emission of heated plasma. Title: X-Ray/Radio Network Flares of the Quiet Sun Authors: Benz, Arnold O.; Krucker, Sam; Acton, Loren W.; Bastian, T. S. Bibcode: 1997IAUJD..19E...1B Altcode: The temporal variations in the soft X-ray (SXR) emission and the radio emission above the solar magnetic network of the quiet corona have been investigated using Yohkoh SXR images with deep exposure and VLA observations in the centimetric radio range. The SXR data show several brightenings with an extrapolated occurrence probability of one brightening per 3 seconds on the total solar surface. During the roughly 10 minutes of enhanced flux, the total radiative losses of the observed plasma are betwee () n 0.6 and 2.4 cdot 1026 erg per event. These events are more than an order of magnitude smaller than previously reported X-ray bright points or active region transient brightenings. For all of the four SXR events with simultaneous radio observations, a corresponding radio source correlating in space and time can be found. There are several similarities between these SXR/radio events and regular solar flares. These events thus appear to be flare-like and are called network flares. We will report also on very recent work using SOHO's EIT and CDS experiments combined with VLA and Kitt Peak observations. Title: Soft X-Ray (Yohkoh) and Radio (VLA) Observations of Solar Narrowband, Millisecond Spike Events Authors: Krucker, Sam; Benz, Arnold O.; Aschwanden, Markus J. Bibcode: 1996ASPC..111..129K Altcode: 1997ASPC..111..129K The source regions of metric spike events are investigated on Yohkoh soft X-ray (SXR) maps. The spikes are identified by the spectrometer Phoenix between 300 MHz and 360 MHz and are associated with groups of type III bursts at lower frequencies reaching also the decametric range. The Very Large Array (VLA) provides simultaneously spatial information at 333 MHz and 1445 MHz. Similar to the previous VLA observation of a metric spike event, the new data are consistent with a high altitude of the spike sources of about 5×1010cm above the photosphere. The additionally available SXR data for one of the presented events give the following new information: (i) The spike sources occur near open field lines and near regions of a slightly enhanced SXR flux relative to the ambient plasma. (ii) Contrary to SXR observations of type III bursts without metric spike activity, no SXR jet is observed. (iii) At low altitude, a weak SXR enhancement occurs, peaking about 60 s after the spike event. The SXR source and the spike sources can be connected by potential field lines. The observations corroborate a model in which a metric spike event is attributed to an energy release region at high altitude, while upwards propagating electrons produce type III bursts and downward moving electrons are responsible for SXR emission of heated plasma. Title: Small solar flares in radio and x-rays: microflares and radio bursts Authors: Krucker, Säm Bibcode: 1996PhDT.......127K Altcode: No abstract at ADS Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft X-ray Maps Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S. Bibcode: 1996mpsa.conf..441K Altcode: 1996IAUCo.153..441K No abstract at ADS Title: First VLA observation of a solar narrowband, millisecond spike event. Authors: Krucker, S.; Aschwanden, M. J.; Bastian, T. S.; Benz, A. O. Bibcode: 1995A&A...302..551K Altcode: The first spatially resolved observation of solar, narrowband spikes in two dimensions is presented. The 'metric' spikes around 333MHz are classified by the broadband spectrometer Phoenix (ETH Zuerich), whereas the simultaneously observing Very Large Array (VLA) provides high angular resolution images of the solar disc. At lower frequencies, a group of associated type III bursts is detected. The spikes occur at high altitude (=~4.5x10^10^cm above the photosphere), and at least 3 separated locations of emission can be identified. The different spike sources are separated by up to 130" and show different degrees of polarization. Spikes and type III bursts have the same sense of circular polarization, and according to the extrapolated potential field lines, the polarization of the different spike sources is in x-mode. With a delay of 42s, a thermal source appears on the same extrapolated potential field lines as the spikes at the second frequency of the VLA (1446MHz). The location of the energy release relative to the spikes source is discussed. A scenario is proposed where the energy is released in or near the spike source, and in which the spikes, the type III bursts and the thermal source originate from the same energy release. Hot electrons expanding along the field lines generate a type III burst (upward direction) and heat the underlying dense plasma (thermal source). Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft X-Ray Maps Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S. Bibcode: 1995SoPh..160..151K Altcode: A solar type I noise storm was observed on 30 July, 1992 with the radio spectrometer Phoenix of ETH Zürich, the Very Large Array (VLA) and the soft X-ray (SXR) telescope on board theYohkoh satellite. The spectrogram was used to identify the type I noise storm. In the VLA images at 333 MHz a fully left circular polarized (100% LCP) continuum source and several highly polarized (70% to 100% LCP) burst sources have been located. The continuum and the bursts are spatially separated by about 100″ and apparently lie on different loops as outlined by the SXR. Continuum and bursts are separated in the perpendicular direction to the magnetic field configuration. Between the periods of strong burst activities, burst-like emissions are also superimposed on the continuum source. There is no obvious correlation between the flux density of the continuum and the bursts. The burst sources have no systematic motion, whereas the the continuum source shows a small drift of ≈ 0.2″ min−1 along the X-ray loop in the long-time evolution. The VLA maps at higher frequency (1446 MHz) show no source corresponding to the type I event. The soft X-ray emission measure and temperature were calculated. The type I continuum source is located (in projection) in a region with enhanced SXR emission, a loop having a mean density of «ne» = (1.5 ± 0.4) × 109 cm−3 and a temperature ofT = (2.1 ± 0.1) × 106 K. The centroid positions of the left and right circularly polarized components of the burst sources are separated by 15″-50″ and seem to be on different loops. These observations contradict the predictions of existing type I theories. Title: Multiple frequency spike emission during solar flares Authors: Krucker, S.; Benz, A. O. Bibcode: 1994SSRv...68..247K Altcode: No abstract at ADS Title: The frequency ratio of bands of microwave spikes during solar flares Authors: Krucker, S.; Benz, A. O. Bibcode: 1994A&A...285.1038K Altcode: The frequency ratio of narrowband millisecond spikes during 17 solar flares has been measured and analyzed. The observations have been made in the frequency range 0.3-3 GHz with the ETH Zuerich spectrometers. The events have been selected from spectrograms for their harmonic structure. Auto-correlation in frequency yields the following major results: (i) The ratio of the harmonic spike groups is not integer, and therefore the term 'harmonic' is strictly speaking inappropriate. (ii) The ratios are independent of frequency. (iii) The ratios range from 1.06 to 1.54 with a prominent peak at 1.39+/-0.01. The cross-correlation of the time profile of harmonic groups peaks at zero lag, demonstrating a tight relation between individual spikes in associated groups. The results require an emission mechanism that can produce harmonic emission at a ratio 5:7 or 1:1.4 and vary significantly from this value. In several cases 3, and in one case 4 harmonic bands have been observed. Previously proposed interpretations of harmonic spike emission are discussed in view of the new observations.