Author name code: alexander ADS astronomy entries on 2022-09-14 author:"Alexander, David" AND (aff:"Rice" OR aff:"Lockheed") ------------------------------------------------------------------------ Title: Effects of Region 2 Field-Aligned Currents on Ionospheric Saturation and Resulting Impact on Auroral Radio Emission Authors: Tracy, Preston; Sciola, Anthony; Toffoletto, Frank; Alexander, David; Merkin, Viacheslav; Sorathia, Kareem Bibcode: 2021AGUFMSM25D2036T Altcode: We compare the effects of ionospheric saturation on auroral radio emission when driven by the differing sources of an increased ionospheric conductance and a decreased solar wind conductance through the use of the Multiscale Atmosphere Geospace Environment (MAGE) developed by the Center for Geospace Storms (CGS), which includes a coupled global magnetohydrodynamic (MHD) and inner magnetosphere model. This work is of particular interest to the detection of exoplanets, as the current method for estimating radio emission for magnetized Solar System planets, the empirically derived Radiometric Bodes Law (RBL), would predict exoplanets that produce radio emission capable of being detected with current observational methods. Because there have been no confirmed detections of exoplanetary radio emission yet, RBL is expected to be inaccurate for these extreme environments. Ionospheric saturation is a compelling explanation for the reduced radio emission of these exoplanets. While the mechanism by which ionospheric saturation occurs is still unresolved, the conditions for a saturated system are agreed to be met for a planetary system in which the ionospheric (Pedersen) conductance dominates the solar wind (Alfven) conductance. Current analytic models for estimating the effects of ionospheric saturation on auroral radio emission give a proportional relationship between the radio emission and the ratio of Alfven and Pedersen conductances. We explore the effect on radio emission via the two avenues of saturation: increasing Bz (the main mechanism of saturation during geomagnetic storms at Earth), and increasing the ionospheric conductance (the likely cause of saturation at exoplanets). We examine this relation using the MAGE model including the Magnetosphere-Ionosphere Coupler (ReMIX), which is used to compute radio emission from region 1 (high-latitude) field aligned currents (FACs) and the Rice Convection Model (RCM) which is used to compute the radio emission from region 2 (low-latitude) FACs. This work offers a unique insight into testing how region 2 currents scale with the degree of saturation. This work was supported by the Rice Space Institute and the Chancey and Evelyn Juday Endowment. Title: Simulating the X-ray and EUV Emission of Cool Exoplanet Host Stars Authors: Farrish, Alison; Barnes, Will; Alexander, David; Garcia-Sage, Katherine Bibcode: 2021AGUFM.P55D1954F Altcode: By analogy with the Earth-Sun system, the search for potentially habitable exoplanets focuses mainly on terrestrial exoplanets orbiting cool stars. Cool stars include the range of partially-convective stars of late-F, G, K, and early-M types, in addition to fully-convective late-M stars. In previous work we have employed a surface flux transport (SFT) model (Schrijver 2001, Schrijver et al. 2003) to examine the emergence and dynamics of magnetic flux on the surfaces of cool stars like the Sun and other exoplanet host stars of interest. Terrestrial exoplanets orbiting cool stars are influenced by their host stars in a variety of ways, including via interaction with the stellar magnetic field (Garaffo et al. 2016, Farrish et al. 2019) and by stellar coronal X-ray emission (e.g., Farrish et al. 2021) which may ionize planetary atmospheric gases. Exoplanet atmospheres are also influenced by stellar emission in the extreme ultraviolet (EUV) wavelength regime (~100-900 Å) through photochemical reactions and escape processes. An understanding of the high-energy emission of the central host star through its X-ray and EUV (collectively, XUV) output is therefore integral to the study of atmospheric and ionospheric evolution at the associated exoplanets. However, stellar EUV observations are historically extremely sparse (Youngblood et al. 2019). Thus, detailed modeling of the dependence of host star XUV emission on stellar magnetic activity can fill many gaps in our current understanding of exoplanet atmospheric processes. We present a study integrating our previous simulations of exoplanet host star magnetic activity with models of coronal heating and the associated XUV emission for a range of cool stars. Particular attention is paid to the relevance of this high-energy emission to atmospheric processes at the associated planets. Title: Modeling Stellar Activity-rotation Relations in Unsaturated Cool Stars Authors: Farrish, Alison O.; Alexander, David; Johns-Krull, Christopher M.; Li, Minjing Bibcode: 2021ApJ...916...99F Altcode: We apply a surface flux transport model developed for the Sun to reconstruct the stellar activity-rotation relationship, LX/Lbol versus Ro, observed for unsaturated cool stars (Rossby numbers Ro ≳ 0.1). This empirical flux transport model incorporates modulations of magnetic flux strength consistent with observed solar activity cycles, as well as surface flux dynamics consistent with observed and modeled stellar relationships. We find that for stellar flux models corresponding to a range of 0.1 ≲ (Ro/RoSun) ≲ 1.2, the LX/Lbol versus Ro relation matches well the power-law behavior observed in the unsaturated regime of cool stars. Additionally, the magnetic activity cycles captured by the stellar simulations produce a spread about the power-law relation consistent with that observed in cool star populations, indicating that the observed spread may be caused by intrinsic variations resulting from cyclic stellar behavior. The success of our flux transport modeling in reproducing the observed activity relationship across a wide range of late-F, G, K, and M stars suggests that the photospheric magnetic fields of all unsaturated cool stars exhibit similar flux emergence and surface dynamic behavior, and may hint at possible similarities in stellar dynamo action across a broad range of stellar types. Title: Incorporating Inner Magnetosphere Current-driven Electron Acceleration in Numerical Simulations of Exoplanet Radio Emission Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David; Sorathia, Kareem; Merkin, Viacheslav; Farrish, Alison Bibcode: 2021ApJ...914...60S Altcode: We present calculations of auroral radio emission for an Earth-like planet produced by field-aligned current (FAC) driven electron acceleration using a coupled global magnetohydrodynamic (MHD) and inner magnetosphere model, extending the capabilities of previous works which focus solely on the direct transmission of magnetic energy between the stellar wind and ionosphere. Magnetized exoplanets are expected to produce radio emission via interaction between the host star's stellar wind and planetary magnetosphere-ionosphere system. The empirically derived Radiometric Bode's Law (RBL) is a linear relation between the magnetic solar wind power and total emitted radio power from magnetized Solar System planets, and is often extrapolated to extreme exoplanet systems. It has been shown that the magnitudes of the FACs coupling the stellar wind to planetary ionospheres are likely to be significantly limited (often referred to as ionospheric saturation), resulting in an estimated radio power up to several orders of magnitude less than that predicted by RBL. In this paper, we demonstrate the significance of intense, sporadic FACs, driven by nightside magnetic reconnection and inner magnetosphere plasma flow, to the total radio power produced by wind-ionosphere interaction in terrestrial planets. During periods of strong stellar wind variability, the contribution from these secondary currents can be over an order of magnitude greater than the primary current systems that previous models describe. The results highlight the role of the variability of the stellar wind on the magnitude and location of the resulting emission, subsequently affecting the conditions for detectability. Title: Using the Sun-Earth Interaction to Explore Exoplanetary Systems Authors: Alexander, D.; Toffoletto, F.; Farrish, A.; Sciola, A. Bibcode: 2021BAAS...53c1025A Altcode: A crucial component in assessing the potential habitability of an exoplanet is an understanding of its interaction with the host star. As more terrestrial "habitable zone" exoplanets are discovered, the detailed characterization of the space environment of these planets raises new challenges, both from a physical and an observational perspective. The "space weather" environment of the planet is primarily governed by the level of magnetic activity of the star (XUV flux, stellar wind and high energy transients), the orbital distance of the planet, the nature and strength of the exoplanet's magnetic field and the magnetic and electromagnetic interactions of this coupled system. To address this, we take advantage of the wealth of knowledge gained about the sole existing habitable system of which we are sure, namely, the Sun-Earth system. We approach this by modeling the stellar activity, which governs much of the expected star-planet interaction and so has an important role to play on potential habitability, and the planetary response, which enables us to place constraints on the expected emission signatures of the star-planet interaction.

On the stellar side, we employ a magnetic flux transport model (SFT), devised from a full 22-yr solar magnetic cycle, to characterize the asterospheric magnetic field in systems with stars of varying levels of activity, up to 10x that of the Sun. This empirical flux transport model incorporates modulations of magnetic flux strength consistent with observed solar activity cycles, as well as surface flux dynamics consistent with observed stellar relationships. We verify the viability of the SFT model for application to stars other than the Sun by reproducing the observed stellar activity-rotation relationship across a wide range of stellar types. We find that the simulations match the activity-rotation relationship in the unsaturated regime of cool stars extremely well and that the observed spread in the observations can be reasonably explained as a result of cycle variability. From our modeling of the asterospheric field at the various levels of activity consider, we are able to detail the star-exoplanet interaction through several quantitative measures such as the ratio of open to total stellar magnetic flux and its variation with stellar latitude, the location and variability of the mean stellar Alfven surface, and the strength of interplanetary magnetic field polarity inversions, all of which have the potential to influence the magnetic environment of the exoplanet.

On the planetary side, we explore the coupling of the stellar activity to the planetary magnetic environment and determine whether or not such interactions produce potentially observable signatures. In this work, we focus on the expected signatures of auroral radio emission for Earth-like planets orbiting active stars. Magnetized exoplanets are expected to produce radio emission via interaction between the host star's stellar wind and planetary magnetosphere-ionosphere system both of which can be significantly enhanced for very active stars. Auroral radio emission is produced by field-aligned current (FAC) driven electron acceleration and this is calculated using a coupled global magnetohydrodynamic (MHD) and inner magnetosphere model, extending the capabilities of previous work. We find that intense, sporadic FACs, driven by night-side magnetic reconnection and inner magnetosphere plasma flow, contribute significantly to the total radio power produced by wind-ionosphere interaction in terrestrial planets. During periods of strong stellar wind variability, the contribution from these secondary currents can be up to several orders of magnitude greater than the primary current systems which previous models describe. This may be even more pronounced for systems in which the primary current system is strongly limited (e.g. ionospheric saturation). The results suggest that magnetized exoplanets may temporarily produce greater radio power than previously estimated increasing their likelihood of producing a detectable signature. Additionally, due to the strong beaming of the emission, the ideal observing angle is dependent on the intensity of the interaction between the stellar wind and exoplanetary magnetosphere. Such observations could provide direct information on the strength of the planetary magnetic field and consequently knowledge about planetary dynamos, planetary evolution, atmospheric escape, and the offset of magnetic and rotation axes. Title: Acceleration of Non-Maxwellian Electron Distributions and Estimates of Radio Emission Observables Authors: Sciola, A.; Toffoletto, F.; Lin, D.; Alexander, D. Bibcode: 2020AGUFMSA0270002S Altcode: Mono-energetic electron precipitation is driven by strong field-aligned currents (FACs) which require the acceleration of otherwise assumed to be Maxwellian electron distributions. In many MHD magnetosphere-ionosphere models the associated field-aligned potential drop is calculated using the MHD plasma temperature, which is also assumed to be Maxwellian. In this work, we utilize the discrete electron energy channels used by the Rice Convection Model (RCM) coupled to the 3D MHD model GAMERA, developed as part of the NASA DRIVE Science Center for Geospace Storms (CGS), to test the validity of the assumption of a Maxwellian electron distribution, and the effects on the field-aligned potential drop when the distribution deviates from the Maxwellian model. Additionally we estimate the radio emission produced by the accelerated electrons via the Electron Cyclotron Maser Instability (ECMI) as would be observable by satellite. Title: Anomalous Active Region Dynamics and Influence on Activity Cycles Authors: Farrish, A.; Alexander, D. Bibcode: 2020AGUFMSH006..05F Altcode: We present a study of the influence of anomalous active regions on solar variability, in particular on the suppression of solar activity cycles in grand minima. The study involves the application of a Solar Flux Transport (SFT) model [Schrijver 2001] which has, in this work, been modified to allow for the emergence and evolution of anomalously oriented (anti-Hale and/or anti-Joy) active regions. Such anomalous active regions have been shown preliminarily to `shut down' dynamo action in dynamo modeling efforts, potentially explaining the appearance of Grand Minima in the Sun's cycle behavior [Nagy et al. 2017].

Following on from these efforts, we test the behavior of anomalous active regions with the computationally lightweight SFT model, which tracks the emergence, evolution, and decay of photospheric magnetic flux concentrations without complex feedback to interior dynamo models. Previous work [Farrish et al. 2019, Farrish et al. 2020 (in prep.)] that the SFT model can be extended to represent the behavior of other cool stars. We aim to study the behavior and influence of anomalous active regions on magnetic activity cycles for the Sun and other stars, with an eye toward the impact of magnetic cycle variability on associated exoplanets. Title: Modeling Stellar Activity-Rotation Relations in Unsaturated Cool Stars Authors: Farrish, A.; Alexander, D.; Johns-Krull, C.; Li, M. Bibcode: 2020SPD....5120702F Altcode: We apply a surface flux transport (SFT) model developed for the Sun to reconstruct the stellar activity-rotation relationship, LX/Lbol vs. Ro, observed for "unsaturated" cool stars (Rossby numbers Ro > about 0.1). This empirical flux transport model incorporates modulations of magnetic flux strength consistent with observed stellar activity cycles, as well as surface flux dynamics consistent with observed stellar relationships. We find that for stellar flux models corresponding to a range ~0.1 < (Ro/RoSun) < 2, the LX/Lbol vs. Ro relation matches the power-law behavior observed in the unsaturated regime of cool stars extremely well. Additionally, the magnetic activity cycles captured by the stellar simulations produce a spread about the power-law relation consistent with that observed in cool star populations, indicating that the observed spread may be caused by intrinsic variations resulting from cyclic stellar behavior. The success of our flux transport modeling in reproducing the observed activity relationship across a wide range of late-F, G, K, and M stars suggests that the photospheric magnetic fields of all unsaturated cool stars exhibit similar flux emergence and surface dynamic behavior, and may hint at possible similarities in stellar dynamo action across a broad range of stellar types. Title: Modeling Luminosity-Activity Relations in Unsaturated Cool Stars Authors: Farrish, A.; Alexander, D.; Johns-Krull, C. Bibcode: 2020AAS...23620706F Altcode: We investigate whether cyclic variations in stellar activity can account for the observed spread in fractional X-ray luminosity, LX/Lbol, for cool stars in the unsaturated range of Rossby number, Ro ≳ 0.1. To address this question, we employ an empirical flux transport model of the stellar surface, incorporating modulations of magnetic flux strength consistent with observed stellar activity cycles. We find that for stellar flux models corresponding to a range ~0.1 ≲ Ro ≲ 2, the LX/Lbol vs. Ro relation matches well the power-law behavior observed in the "unsaturated" regime of cool stars. Additionally, the magnetic activity cycles incorporated into the stellar simulations produce a spread about the power-law relation consistent with the observed spread in unsaturated cool star populations. We find, therefore, that the solar-based flux transport approach employed in this work can reproduce the X-ray luminosity-magnetic activity relation observed across the range of unsaturated late-F, G, K, and M stars, providing support for the hypothesis of a universal dynamo mechanism operating in all unsaturated cool stars. We further conclude that the spread in fractional X-ray luminosity, LX/Lbol, across the unsaturated range of stellar activity corresponding to ~0.1 ≲ Ro ≲ 2 can be explained by the intrinsic variation due to stellar activity cycles. Title: Characterizing the Magnetic Environment of Exoplanet Stellar Systems Authors: Farrish, Alison O.; Alexander, David; Maruo, Mei; DeRosa, Marc; Toffoletto, Frank; Sciola, Anthony M. Bibcode: 2019ApJ...885...51F Altcode: We employ a flux transport model incorporating observed stellar activity relations to characterize stellar interplanetary fields on cycle timescales for a range of stellar activity defined by the Rossby number. This framework allows us to examine the asterospheric environments of exoplanetary systems and yields references against which exoplanetary observations can be compared. We examine several quantitative measures of star-exoplanet interaction: the ratio of open to total stellar magnetic flux, the location of the stellar Alfvén surface, and the strength of interplanetary magnetic field polarity inversions, all of which influence planetary magnetic environments. For simulations in the range of Rossby numbers considered (0.1-5 RoSun), we find that (1) the fraction of open magnetic flux available to interplanetary space increases with Rossby number, with a maximum of around 40% at stellar minimum for low-activity stars, while the open flux for very active stars (Ro ∼ 0.1-0.25 RoSun) is ∼1-5% (2) the mean Alfvén surface radius, R A, varies between 0.7 and 1.3 R A,Sun and is larger for lower stellar activity; and (3) at high activity, the asterospheric current sheet becomes more complex with stronger inversions, possibly resulting in more frequent reconnection events (e.g., magnetic storms) at the planetary magnetosphere. The simulations presented here serve to bound a range of asterospheric magnetic environments within which we can characterize the conditions impacting any exoplanets present. We relate these results to several known exoplanets and discuss how they might be affected by changes in asterospheric magnetic field topologies. Title: Planetary Magnetic Response to Young Star Stellar Wind Environment Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David Bibcode: 2019ESS.....432208S Altcode: Young stars are known to exhibit strong magnetic activity in the form of both steady and eruptive outflow. This, combined with the majority of known Earth-like planets orbiting their stars closer than Mercury orbits the Sun, results in an extreme stellar wind environment at the planet. We employ a 3D coupled MHD model to simulate the planetary response to this environment with an emphasis on the exchange of plasma between the planetary magnetosphere, ionosphere and stellar wind. We will present the results of two cases: the case of extreme, steady stellar wind and the case of successive Sun-like Coronal Mass Ejection (CME) impacts over short timescales. These results demonstrate how exoplanetary environments differ from Earth's and provide insight into how early stage star-planet interactions may impact future evolution of the planetary environment. Title: Simulating the Inner Asterospheric Magnetic Fields of Exoplanet Host Stars Authors: Farrish, Alison; Alexander, David; Maruo, Mei; Sciola, Anthony; Toffoletto, Frank; DeRosa, Marc L. Bibcode: 2019AAS...23430305F Altcode: We study magnetic and energetic activity across a range of stellar behavior via the application of an observationally-based heliophysics modeling framework. We simulate the inner asterospheric magnetic fields of host stars with the aim of better understanding and constraining the space weather environments of exoplanets, and improving our knowledge of the solar-stellar connection. As astronomy instrumentation has improved, Earth-like exoplanets are increasingly being found orbiting in the habitable zones of a variety of stars, ranging from the smallest and coolest M dwarfs to larger and more solar-like stars. We are therefore interested in characterizing a broad range of stellar magnetic activity and the resulting impacts on asterospheric environments. We will present our work simulating stellar magnetic activity on cycle timescales via the integration of modeled magnetic flux emergence, coronal field structure and related plasma emission, and stellar winds. We use this self-consistent framework of heliophysics-based models to simulate stellar and asterospheric evolution, in order to better understand the dynamic connections between host stars and potential impacts on planetary space weather and habitability. We also remark on the comparative heliophysics approach which we plan to extend to star-planet interactions via coupling to models of magnetospheric activity and dynamo-driven stellar flux emergence. Title: Magnetic Properties of Asterospheres of Exoplanet Systems Authors: Farrish, Alison; Alexander, David; Maruo, Mei; Sciola, Anthony; Toffoletto, Frank; DeRosa, Marc Bibcode: 2019shin.confE.152F Altcode: We study magnetic and energetic activity across a range of stellar behavior via the application of an observationally-based heliophysics modeling framework. We simulate the inner asterospheric magnetic fields of host stars with the aim of better understanding and constraining the space weather environments of exoplanets, and improving our knowledge of the solar-stellar connection. As astronomy instrumentation has improved, Earth-like exoplanets are increasingly being found orbiting in the habitable zones of a variety of stars, ranging from the smallest and coolest M dwarfs to larger and more solar-like stars. We are therefore interested in characterizing a broad range of stellar magnetic activity and the resulting impacts on asterospheric environments. We will present our work simulating stellar magnetic activity on cycle timescales via the integration of modeled magnetic flux emergence, coronal field structure and related plasma emission, and stellar winds. We use this self-consistent framework of heliophysics-based models to simulate stellar and asterospheric evolution, in order to better understand the dynamic connections between host stars and potential impacts on planetary space weather and habitability. We also remark on the comparative heliophysics approach which we plan to extend to star-planet interactions via coupling to models of magnetospheric activity and dynamo-driven stellar flux emergence. Title: Reconstructing Extreme Space Weather From Planet Hosting Stars Authors: Airapetian, Vladimir; Adibekyan, V.; Ansdell, M.; Alexander, D.; Barklay, T.; Bastian, T.; Boro Saikia, S.; Cohen, O.; Cuntz, M.; Danchi, W.; Davenport, J.; DeNolfo, G.; DeVore, R.; Dong, C. F.; Drake, J. J.; France, K.; Fraschetti, F.; Herbst, K.; Garcia-Sage, K.; Gillon, M.; Glocer, A.; Grenfell, J. L.; Gronoff, G.; Gopalswamy, N.; Guedel, M.; Hartnett, H.; Harutyunyan, H.; Hinkel, N. R.; Jensen, A. G.; Jin, M.; Johnstone, C.; Kahler, S.; Kalas, P.; Kane, S. R.; Kay, C.; Kitiashvili, I. N.; Kochukhov, O.; Kondrashov, D.; Lazio, J.; Leake, J.; Li, G.; Linsky, J.; Lueftinger, T.; Lynch, B.; Lyra, W.; Mandell, A. M.; Mandt, K. E.; Maehara, H.; Miesch, M. S.; Mickaelian, A. M.; Mouschou, S.; Notsu, Y.; Ofman, L.; Oman, L. D.; Osten, R. A.; Oran, R.; Petre, R.; Ramirez, R. M.; Rau, G.; Redfield, S.; Réville, V.; Rugheimer, S.; Scheucher, M.; Schlieder, J. E.; Shibata, K.; Schnittman, J. D.; Soderblom, David; Strugarek, A.; Turner, J. D.; Usmanov, A.; Van Der Holst, B.; Vidotto, A.; Vourlidas, A.; Way, M. J.; Wolk, Scott J.; Zank, G. P.; Zarka, P.; Kopparapu, R.; Babakhanova, S.; Pevtsov, A. A.; Lee, Y.; Henning, W.; Colón, K. D.; Wolf, E. T. Bibcode: 2019BAAS...51c.564A Altcode: 2019astro2020T.564A; 2019arXiv190306853A The goal of this white paper is to identify and describe promising key research goals to aid the theoretical characterization and observational detection of ionizing radiation from quiescent and flaring upper atmospheres of planet hosts as well as properties of stellar coronal mass ejections (CMEs) and stellar energetic particle (SEP) events. Title: Earth-like Exoplanet Response to Extreme Northward IMF Stellar Wind Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David Bibcode: 2019shin.confE.147S Altcode: Periods of solar wind where the interplanetary magnetic field (IMF) is pointing Northward are traditionally considered not to be geo-effective, or having little interaction with the Earth's magnetosphere and ionosphere. We simulate and present an example of extreme Northward IMF stellar wind impacting an Earth-like, closely orbiting exoplanet. In this regime magnetic reconnection occurs above the magnetic poles, allowing for stellar wind to penetrate and convect through the magnetosphere. This results in the loss of planetary plasma via reconnected field lines and charge exchange with the captured stellar wind population, which may have long-term implications on how the planetary environment evolves. We compare this scenario to that where the same stellar wind event is incident on the Earth-like planet at 1AU. Title: Active Star Space Weather and its Planetary Consequences Authors: Toffoletto, F.; Alexander, D.; Farrish, A.; Sciola, A.; Barnes, W. Bibcode: 2018AGUFM.P43H3847T Altcode: Of particular importance to understanding the space weather effects on exo-planetary systems is an understanding of the quiescent state of such systems, frequently observed around very active M-dwarf stars. We apply an empirical photospheric magnetic flux transport model, derived from solar behavior, and a magnetically-driven stellar atmosphere model to explore the range of stellar effects on the habitability of Earth-size exoplanets around range of active stars, delineated by their rotation rates. We discuss how the stellar activity influences the asterospheric magnetic field and its consequent impact on any planets present. We also apply a hydrostatic coronal loop heating model to estimate the expected EUV and X-ray emission associated with the magnetic fields of these stars. Finally, we consider the response of the planetary magnetosphere and ionosphere to the stellar activity, in the more active stars, and, in particular, the role of accurately incorporating the inner magnetosphere and enhanced ionospheric currents resulting from the significantly larger EUV and X-ray fluxes. These all play an important role in understanding the potential and rate of atmospheric loss. Title: Simulation of Exoplanet Host Star Magnetic Activity on Stellar Cycle Timescales Authors: Farrish, Alison; Maruo, M.; Barnes, W.; Alexander, D.; Bradshaw, S.; DeRosa, M. Bibcode: 2018shin.confE...4F Altcode: We apply an empirical photospheric magnetic flux transport model, derived from solar behavior, and a magnetically-driven stellar atmosphere model to explore the range of stellar effects on habitability of Earth-size exoplanets around M dwarf stars. We create detailed, dynamic simulations of stellar activity and its variability over stellar cycle timescales. In particular, we examine how the asterospheric magnetic field and related extreme ultraviolet (EUV) and X-ray emission vary in time and consider the potential impact on exoplanet habitability. Title: Predicting Radio Emission of Exoplanets in Enhanced Stellar Wind Environments Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David Bibcode: 2018shin.confE...5S Altcode: The Radiometric Bode's Law is a scaling law that predicts the power of a planet's radio emission based on the incident stellar wind power, which works for the magnetized solar system planets. This scaling law has since been used to predict the radio emission from exoplanets such as Proxima Centauri b without addressing several space weather factors that greatly alter the assumptions used in the calculation, which may result in an incorrect prediction of the actual emission power. Utilizing 3D MHD modeling coupled with inner magnetosphere and ionosphere, we provide a means of more accurately estimating the efficiency at which the incident stellar wind power penetrates into the ionosphere and is available for the production of planetary radio emission. This more representative efficiency can be taken into account when using the Radiometric Bode's Law for exoplanetary systems whose space weather environment strongly differ from that of our solar system. Title: Modeling the Space Weather environment of Proxima-b. Authors: Toffoletto, Frank; Alexander, David; Sciola, Anthony; Farrish, Alison; Sazykin, Stanislav Y. Bibcode: 2018tess.conf11901T Altcode: There has been a tremendous increase in interest in exoplanet research in recent years with it an increased emphasis on discovering potentially habitable planets around other stars. Based on our understanding of our own solar system, habitability is most feasible around a magnetically active star that has the protection provided by a substantial planetary magnetosphere. The recent discovery of a planetary system around Proxima Centauri has been particularly exciting because of it being in the closest to our solar system, which increases the possibility of detection of the magnetic interaction as well as the exploration of potential habitability. This presentation will summarize recent work that has focused on the space plasma conditions of this system via 2 related avenues: one is to get a better understanding of the environment of the host star using an empirical solar magnetic flux transport model, and the other is to a 3D coupled magnetosphere model, that was originally developed for the Earth, that has been adapted for the more extreme environments thought to represent the around Proxima-b. We also investigate the question of the how large a ring current can a planetary magnetosphere contain. In the future, we expect use the empirical flux transport model to better constrain the input stellar wind conditions for the planetary magnetosphere. Title: Simulated Coronal EUV Emission from Exoplanet Host Stars Authors: Farrish, Alison; Barnes, Will; Alexander, David; Bradshaw, Stephen J.; DeRosa, Mark L. Bibcode: 2018tess.conf40649F Altcode: We apply a modified solar flux transport (SFT) model, developed by Schrijver and colleagues, to emulate the magnetic activity of the host stars of recently discovered habitable-zone planets. The magnetic flux distributions produced by the SFT simulations serve as a first-order proxy for the expected magnetic behavior of an exoplanet host star. We couple the simulated magnetic structure to a coronal heating model and simulate the expected EUV and X-ray emission of the target star. The EUV emission is a key energetic input to the exoplanetary atmosphere, governing ionospheric conductance and therefore influencing the magnetospheric response to stellar activity - a key factor in determining the overall atmospheric loss and ultimately the potential of the planet for habitation. In addition, the simulated stellar coronal emission may provide signatures for comparison with astronomical observations. We consider the simulated activity over a number of stellar cycles to explore the long-term impact of the star on the exo-planetary environment. Title: Coupled MHD and Inner Magnetosphere Modeling of Proxima-b Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David; Sazykin, Stanislav Y. Bibcode: 2018tess.conf40650S Altcode: <span color="#000000" Being our closest stellar neighbor, much effort has gone into modeling the Proxima system in order to predict the likelihood of habitabilty. In order to better understand Proxima b's magnetic response to the enhanced stellar wind of its active M dwarf host, we employ the 3D MHD model BATSRUS coupled with the Rice Convection Model (RCM) driven by an enhanced stellar wind assumed to simulate the properties of the host star. The RCM's modeling of the inner magnetosphere is important as it can be a source region of enhanced plasma population and pressure that can influence the global response of the planet's magnetosphere. It can also lead to enhanced region-2 field aligned currents in the polar caps. This detailed modeling of the magnetosphere enables us to more accurately estimate the parameters that govern habitability, such as rate of thermal ion loss, as well as potential detectability of auroral radio emission. Title: The Application of Solar Flux Transport Modeling to Exoplanet Systems Authors: Alexander, David; Farrish, Alison; Maruo, Mei; De Rosa, Marc L. Bibcode: 2018tess.conf40648A Altcode: Earth-size exoplanets are preferentially detected close-in around small stars; Proxima Centauri b, Ross 128 b, and the TRAPPIST-1 planets are newly discovered exoplanets in this class. The effects of magnetic interactions between the host star and such close-in exoplanets are still not well-constrained. We utilize an empirical solar magnetic flux transport model, first developed by Schrijver and colleagues, to explore the possible relationships between stellar properties (e.g. rotation period, radius, flux emergence rate, and meridional and differential flow rates) and the expected surface magnetic flux distributions, along with their evolution over the stellar cycle. The surface field then provides key information about the interplanetary magnetic fields, stellar winds, dynamic activity, and coronal emission, all of which influence the star-planet interactions. We present simulated magnetic flux patterns representing a range of possible exoplanet host stars, including potential observables such as starspots, active regions, and stellar cycle variations. The magnetic and energetic environment at an exoplanet and its impact on the magnetospheric-atmospheric coupling are important components of a planet's habitability that we expect to constrain further with this application of simulated stellar magnetic activity. Title: Planetary Magnetospheric Response to Increased Stellar Activity Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David Bibcode: 2017shin.confE..97S Altcode: As exemplified by the Proxima Centauri and TRAPPIST-1 systems, transit-detected Earth-like exoplanets are likely to be closely orbiting M dwarfs. The lower temperature of these stars cause their Habitable Zones to be closer in towards the star, which in turn results in the planet experiencing much stronger stellar wind conditions in the form of heightened dynamic and magnetic pressures. With the high activity and expectedly stronger stellar wind of Proxima Centauri as motivation, I present the response to these conditions of an Earth-like planet's magnetosphere using the Space Weather Modeling Framework computational model. An emphasis is placed on the ring current's role in both helping buff the magnetopause standoff distance and in storing pressure in the inner magnetosphere between storm events. This understanding of the magnetic interaction between star and planet can help determine other characteristics such as atmospheric loss rate and auroral emissions. Title: Hard X-Ray Asymmetry Limits in Solar Flare Conjugate Footpoints Authors: Daou, Antoun G.; Alexander, David Bibcode: 2016ApJ...832...63D Altcode: The transport of energetic electrons in a solar flare is modeled using a time-dependent one-dimensional Fokker-Planck code that incorporates asymmetric magnetic convergence. We derive the temporal and spectral evolution of the resulting hard X-ray (HXR) emission in the conjugate chromospheric footpoints, assuming thick target photon production, and characterize the time evolution of the numerically simulated footpoint asymmetry and its relationship to the photospheric magnetic configuration. The thick target HXR asymmetry in the conjugate footpoints is found to increase with magnetic field ratio as expected. However, we find that the footpoint HXR asymmetry saturates for conjugate footpoint magnetic field ratios ≥4. This result is borne out in a direct comparison with observations of 44 double-footpoint flares. The presence of such a limit has not been reported before, and may serve as both a theoretical and observational benchmark for testing a range of particle transport and flare morphology constraints, particularly as a means to differentiate between isotropic and anisotropic particle injection. Title: Observation of the Evolution of a Current Sheet in a Solar Flare Authors: Zhu, Chunming; Liu, Rui; Alexander, David; McAteer, James Bibcode: 2016SPD....4730205Z Altcode: We report multi-wavelength and multi-viewpoint observations of a solar eruptive event which involves loop-loop interactions. During a C2.0 flare, motions associated with inflowing and outflowing plasma provide evidence for ongoing magnetic reconnection. The flare loop top and a rising "concave-up" feature are connected by a current-sheet-like structure (CSLS). The physical properties (thickness, length, temperature, and density) of the CSLS are evaluated. In regions adjacent to the CSLS, the EUV emission (characteristic temperature at 1.6 MK) begins to increase more than ten minutes prior to the onset of the flare, and steeply decreases during the decay phase. The reduction of the emission resembles that expected from coronal dimming. The dynamics of this event imply a magnetic reconnection rate in the range 0.01--0.05. Title: Observation of the Evolution of a Current Sheet in a Solar Flare Authors: Zhu, Chunming; Liu, Rui; Alexander, David; McAteer, R. T. James Bibcode: 2016ApJ...821L..29Z Altcode: 2016arXiv160307062Z We report multi-wavelength and multi-viewpoint observations of a solar eruptive event that involves loop-loop interactions. During a C2.0 flare, motions associated with inflowing and outflowing plasma provide evidence for ongoing magnetic reconnection. The flare loop top and a rising “concave-up” feature are connected by a current-sheet-like structure (CSLS). The physical properties (thickness, length, temperature, and density) of the CSLS are evaluated. In regions adjacent to the CSLS, the EUV emission (characteristic temperature at 1.6 MK) begins to increase more than 10 minutes prior to the onset of the flare, and steeply decreases during the decay phase. The reduction of the emission resembles that expected from coronal dimming. The dynamics of this event imply a magnetic reconnection rate in the range 0.01-0.05. Title: Complex Flare Dynamics Initiated by a Filament-Filament Interaction Authors: Zhu, Chunming; Liu, Rui; Alexander, David; Sun, Xudong; McAteer, R. T. James Bibcode: 2015ApJ...813...60Z Altcode: 2015arXiv150705889Z We report on an eruption involving a relatively rare filament-filament interaction on 2013 June 21, observed by SDO and STEREO-B. The two filaments were separated in height with a “double-decker” configuration. The eruption of the lower filament began simultaneously with a descent of the upper filament, resulting in a convergence and direct interaction of the two filaments. The interaction was accompanied by the heating of surrounding plasma and an apparent crossing of a loop-like structure through the upper filament. The subsequent coalescence of the filaments drove a bright front ahead of the erupting structures. The whole process was associated with a C3.0 flare followed immediately by an M2.9 flare. Shrinking loops and descending dark voids were observed during the M2.9 flare at different locations above a C-shaped flare arcade as part of the energy release, giving us unique insight into the flare dynamics. Title: Optimal Electron Energies for Driving Chromospheric Evaporation in Solar Flares Authors: Reep, J. W.; Bradshaw, S. J.; Alexander, D. Bibcode: 2015ApJ...808..177R Altcode: 2015arXiv150608115R In the standard model of solar flares, energy deposition by a beam of electrons drives strong chromospheric evaporation leading to a significantly denser corona and much brighter emission across the spectrum. Chromospheric evaporation was examined in great detail by Fisher et al., who described a distinction between two different regimes, termed explosive and gentle evaporation. In this work, we examine the importance of electron energy and stopping depths on the two regimes and on the atmospheric response. We find that with explosive evaporation, the atmospheric response does not depend strongly on electron energy. In the case of gentle evaporation, lower energy electrons are significantly more efficient at heating the atmosphere and driving up-flows sooner than higher energy electrons. We also find that the threshold between explosive and gentle evaporation is not fixed at a given beam energy flux, but also depends strongly on the electron energy and duration of heating. Further, at low electron energies, a much weaker beam flux is required to drive explosive evaporation. Title: Complex Flare Dynamics Initiated by a Filament-Filament Interaction Authors: Zhu, Chunming; Liu, Rui; Alexander, David; Sun, Xudong; McAteer, James Bibcode: 2015TESS....120317Z Altcode: We report on a filament eruption that led to a relatively rare filament-filament interaction event. The filaments were located at different heights above the same segment of a circular polarity inversion line (PIL) around a condensed leading sunspot. The onset of the eruption of the lower of the two filaments was accompanied by a simultaneous descent of the upper filament resulting in a convergence and direct interaction of the two filaments. The interaction led to the subsequent merger of the filaments into a single magnetically complex structure that erupted to create a large solar flare and an array of complex dynamical activity. A hard X-ray coronal source and an associated enhancement of hot plasma are observed at the interface between the two interacting filaments. These phenomena are related to the production of a small C flare and the subsequent development of a much stronger M flare. Magnetic loop shrinkage and descending dark voids were observed at different locations as part of the large flare energy release giving us a unique insight into these dynamic flare phenomena. Title: The Role of Interchange Reconnection in Facilitating a Filament Eruption Authors: Zhu, C.; Alexander, D.; Sun, X.; Daou, A. Bibcode: 2014SoPh..289.4533Z Altcode: 2014SoPh..tmp..123Z We study the interaction between an erupting solar filament and a nearby coronal hole, based on multi-viewpoint observations from the Solar Dynamics Observatory and STEREO. During the early evolution of the filament eruption, it exhibits a clockwise rotation that brings its easternmost leg in contact with the oppositely aligned field at the coronal hole boundary. The interaction between the two magnetic-field systems is manifested as the development of a narrow contact layer in which we see enhanced EUV brightening and bi-directional flows, suggesting that the contact layer is a region of strong and ongoing magnetic reconnection. The coronal mass ejection (CME) resulting from this eruption is highly asymmetric, with its southern portion opening up to the upper corona, while the northern portion remains closed and connected to the Sun. We suggest that the erupting flux rope that made up the filament reconnected with both the open and closed fields at the coronal hole boundary via interchange reconnection and closed-field disconnection, respectively, which led to the observed CME configuration. Title: Eruption of a Bifurcated Solar Filament Authors: Zhu, Chunming; Alexander, David Bibcode: 2014SoPh..289..279Z Altcode: We study the partial eruption of a solar filament observed by the Solar Dynamics Observatory (SDO) and the Solar TErrestrial RElations Observatory-Ahead (STEREO-A) spacecraft on 9 May 2012. This filament was located in Active Region NOAA 11475 and consisted of two distinct branches, separated in height above the active region's primary polarity-inversion line. For two days prior to the filament eruption, several threads of filament material were observed to connect the lower branch to the upper branch with evidence of a transfer of mass along them. The eruption commenced as a slow rise of the upper branch that began at 9 May 2012 23:40 UT, with the main eruption occurring half an hour later, producing a coronal mass ejection (CME). During the eruption, the upper branch was observed to rotate approximately 120 degrees in a counter-clockwise direction. We suggest that the mass transfer events also comprised a transfer of magnetic flux that led the upper branch of the filament to lose equilibrium as a result of a helical kink instability or torus instability. Title: Eruption of a Bifurcated Solar Filament Authors: Zhu, Chunming; Alexander, D. Bibcode: 2013SPD....44...23Z Altcode: We study the partial eruption of a solar filament observed by the SDO and the STEREO-A spacecraft on 9 May 2012. This filament was located in active region NOAA 11475, and consisted of two distinct branches, separated in height above the active region's primary polarity-inversion line. For two days prior to the filament eruption, several threads of filament material were observed to connect the lower branch to the upper branch with evidence of a transfer of mass along them. The eruption commenced as a slow rise of the upper branch that began at 9 May 2012 23:40 UT, with the main eruption occurring half an hour later, producing a coronal mass ejection (CME). During the eruption, the upper branch was observed to rotate approximately 120 degrees in a counter-clockwise direction. We suggest that the mass transfer events also comprised a transfer of magnetic flux that led the upper branch of the filament to lose equilibrium as a result of a helical kink instability or torus instability. Title: Eruption of a Bifurcated Solar Filament Authors: Zhu, Chunming; Alexander, David Bibcode: 2013shin.confE.100Z Altcode: We study the partial eruption of a solar filament observed by the SDO and the STEREO-A spacecraft on 9 May 2012. This filament was located in active region NOAA 11475, and consisted of two distinct branches, separated in height above the active region's primary polarity-inversion line. For two days prior to the filament eruption, several threads of filament material were observed to connect the lower branch to the upper branch with evidence of a transfer of mass along them. The eruption commenced as a slow rise of the upper branch that began at 9 May 2012 23:40 UT, with the main eruption occurring half an hour later, producing a coronal mass ejection (CME). During the eruption, the upper branch was observed to rotate approximately 120 degrees in a counter-clockwise direction. We suggest that the mass transfer events also comprised a transfer of magnetic flux that led the upper branch of the filament to lose equilibrium as a result of a helical kink instability or torus instability. Title: Velocity Characteristics of Rotating Sunspots Authors: Zhu, C.; Alexander, D.; Tian, L. Bibcode: 2012SoPh..278..121Z Altcode: A statistical study is carried out to investigate the detailed relationship between rotating sunspots and the emergence of magnetic flux tubes. This paper presents the velocity characteristics of 132 sunspots in 95 solar active regions. The rotational characteristics of the sunspots are calculated from successive SOHO/MDI magnetograms by applying the Differential Affine Velocity Estimator (DAVE) technique (Schuck, 2006, Astrophys. J.646, 1358). Among 82 sunspots in active regions exhibiting strong flux emergence, 63 showed rotation with rotational angular velocity larger than 0.4° h−1. Among 50 sunspots in active regions without well-defined flux emergence, 14 showed rotation, and the rotation velocities tend to be slower, compared to those in emerging regions. In addition, we investigated 11 rotating sunspot groups in which both polarities show evidence for co-temporary rotation. In seven of these cases the two polarities co-rotate, while the other four are found to be counter-rotating. Plausible reasons for the observed characteristics of the rotating sunspots are discussed. Title: Multi-wavelength Observations Of The Evolution Of A Multi-filament Complex Authors: Alexander, David; Zhu, C. Bibcode: 2012AAS...22020107A Altcode: The strong and clear association between eruptive filaments and the production of solar flares and CMEs provides a natural starting point from which to explore the connections between solar conditions and eruptive events. The development of the filament in the build up to any eruption is critical to understanding how eruptions occur. We use high cadence ground-based observations from the Mauna Loa Solar Observatory (MLSO), in conjunction with complementary data from SDO, and STEREO to quantify the development of a multipolar magnetic complex comprising several distinct filaments, that formed in a decaying active region group over the course of November/December 2011. Two neighboring filaments in this complex erupt within 6 hours of each other on 2011 December, 25. In this paper we describe the evolution of this complex over the course of its development across the solar disk, as viewed from STEREO A, B and SDO, and discuss the role played by the field development, filament dynamics and twisting motions in forming the filament complex, initiating the eruptions, and controlling the subsequent reformation of the filament channels. The MLSO CHIP instrument, in particular, allows for some measure of Doppler velocity discrimination in the He I line at 10830Å providing a unique diagnostic capability for determining the strength and location of the filament dynamics, particularly the twisting and writhing motions in the erupting filaments. These data are combined with a 3D reconstruction using STEREO EUVI and SDO AIA data to better determine the 3D velocity evolution and spatial distribution of the twist. Title: Spectral Characteristics of Hard X-Ray Emission in Double Footpoint Flares Authors: Alexander, David; Lastufka, Erica; Daou, Antoun Bibcode: 2011shin.confE..90A Altcode: Utilizing RHESSI observations we examine the evolution of footpoint asymmetries in the hard X-ray emission over energy and time for 53 double-footpoint flares. Most of the flares observed displayed a low asymmetry and very little change in asymmetry with energy, as expected for comparable footpoint field strengths and isotropic particle injection. However, a number of events exhibited clear dependences of the footpoint hard X-ray asymmetry on photon energy confirming the results of a previous study by Alexander and Metcalf (2002). The presence of an energy dependence in the footpoint emission cannot be accounted for by simple particle transport with scattering due to Coulomb collisions and a converging field geometry. McClements & Alexander (2005) used a Fokker-Planck model of particle transport with an anisotropic injection of particles to reproduce the observed energy dependence in the footpoint asymmetry. The need for an anisotropic distribution of accelerated particle places constraints on the particle acceleration processes occurring in solar flares. Furthermore, the observed time development of the observed asymmetry provides further constraints on the nature of the accelerated particles injected into the flaring structure and provides insight into the 3D nature of the flaring process. Title: Velocity characteristics of rotating sunspots Authors: Zhu, Chunming; Alexander, David; Tian, Lirong Bibcode: 2011shin.confE..56Z Altcode: A statistical study is carried out to investigate the detailed relationship between rotating sunspots and the emergence of magnetic flux tubes. This study presents the velocity characteristics of 132 sunspots. The rotational characteristics of the sunspots are calculated from successive SOHO/MDI magnetograms by applying the Differential Affine Velocity Estimator (DAVE) technique (Schuck, 2006). Among 82 sunspots in active regions exhibiting strong flux emergence, which are related to flux tube emergence, or footpoint separation, 63 are rotating with rotational angular velocity larger than 0.4 deg/hr. Among 50 sunspots in active regions without well-defined flux emergence, 14 are rotating, and the rotation velocities tend to be slower, compared to those in emerging regions. In addition, we investigated 11 rotating sunspot groups in which both polarities show evidence for co-temporary rotation. In seven of these cases the two polarities co-rotate while the other four are found to be counter-rotating. Plausible reasons for the observed characteristics of the rotating sunspots are discussed. Title: Spatial and Temporal Development of Fiducial Current Systems in Solar Flares Authors: Daou, Antoun Georges; Alexander, David Bibcode: 2011shin.confE..87D Altcode: We use the unprecedented spatial and spectral resolution of the RHESSI telescope to investigate the temporal evolution of thermal and non-thermal hard X-ray emission in spatially independent substructures in 2 major flares: the 2006 December 6 X6.5 and the 2003 November 2 X8.3, one of the large Halloween 2003 flares. The hard X-ray emission in these flares is concentrated in a number of distinct sources displaying independent temporal and spectral evolution. We find that while the total flare energy content rises during the impulsive phase, the non-thermal hard X-ray fluxes in many of the identified substructures saturates after an initial rise phase. In a previous study, we investigated this non-thermal flux saturation at the peak of the hard X-ray emission in 10 flares spanning a factor of 200 (M1 to X17) in the GOES 1-8A peak flux. The indication was that the saturation was due to a transition in the dominant energy loss mechanism from Coulomb collisions to return current Ohmic dissipation as the flare peak magnitude increased. In this study, we analyze the temporal variation of the hard X-ray fluxes within two large flares, and infer the spectral properties of the emitting electron population, as the flare transits from one energy loss regime to the other. We find that the increase in the non-thermal hard X-ray flux in the substructures towards saturation is mainly due to a hardening of the spectrum, with more higher energy electrons accelerated, while the total flux of lower energy electrons remaining roughly the same. Title: On Asymmetry of Magnetic Helicity in Emerging Active Regions: High Resolution Observations Authors: Alexander, David; Tian, Lirong; Demoulin, Pascal; Zhu, Chunming Bibcode: 2011shin.confE..11A Altcode: We employ the DAVE (differential affine velocity estimator, Schuck 2005; 2006) tracking technique on a time series of MDI/1m high spatial resolution lineof- sight magnetograms to measure the photospheric flow velocity for three newly emerging bipolar active regions. We separately calculate the magnetic helicity injection rate of the leading and following polarities to confirm or refute the magnetic helicity asymmetry, found by Tian & Alexander (2009) using MDI/96m low spatial resolution magnetograms. Our results demonstrate that the magnetic helicity asymmetry is robust being present in the three active regions studied, two of which have an observed balance of the magnetic flux. The magnetic helicity injection rate measured is found to depend little on the window size selected, but does depend on the time interval used between the two successive magnetograms tracked. It is found that the measurement of the magnetic helicity injection rate performs well for a window size between 12-10 and 18-15 pixels, and at a time interval ∼t=10 minutes. Moreover, the short-lived magnetic structures, 10-60 minutes, are found to contribute 30-50% of the magnetic helicity injection rate. Comparing with the results calculated by MDI/96m data, we find that the MDI/96m data, in general, can outline the main trend of the magnetic properties, but they significantly underestimate the magnetic flux in strong field region and are not appropriate for quantitative tracking studies, so provide a poor estimate of the amount of magnetic helicity injected into the corona. Title: Velocity Characteristics Of Rotating Sunspots Authors: Zhu, Chunming; Alexander, D.; Tian, L. Bibcode: 2011SPD....42.1709Z Altcode: 2011BAAS..43S.1709Z A statistical study is carried out to investigate the detailed relationship between the rotating sunspots and the emergence of magnetic flux tubes. This paper presents the velocity characteristics of 132 sunspots in 95 solar active regions. The rotational characteristics of the sunspots are calculated from successive SOHO/MDI magnetograms by applying the Differential Affine Veloicty Estimator (DAVE) technique (Schuck, Astrophys. J. 646, 1358, 2006). Among 82 sunspots in active regions exhibiting strong flux emergence, 77% are rotating sunspots with rotational angular velocity larger than 0.4 deg/hr. Among 50 sunspots in active regions without well-defined flux emergence, 28% are rotating, and the sunspot rotation tend to be slower, even ceased, compared to those during the emergence. In addition, we investigated 11 rotating sunspot groups in which both polarities show evidence for contemporary rotation. In seven of these cases the two polarities co-rotate while the other four are found to be counter-rotating. Plausible reasons for the observed characteristics of the rotating sunspots are discussed. Title: On Asymmetry of Magnetic Helicity in Emerging Active Regions: High-resolution Observations Authors: Tian, Lirong; Démoulin, Pascal; Alexander, David; Zhu, Chunming Bibcode: 2011ApJ...727...28T Altcode: 2011arXiv1101.1068T We employ the DAVE (differential affine velocity estimator) tracking technique on a time series of Michelson Doppler Imager (MDI)/1 minute high spatial resolution line-of-sight magnetograms to measure the photospheric flow velocity for three newly emerging bipolar active regions (ARs). We separately calculate the magnetic helicity injection rate of the leading and following polarities to confirm or refute the magnetic helicity asymmetry, found by Tian & Alexander using MDI/96 minute low spatial resolution magnetograms. Our results demonstrate that the magnetic helicity asymmetry is robust, being present in the three ARs studied, two of which have an observed balance of the magnetic flux. The magnetic helicity injection rate measured is found to depend little on the window size selected, but does depend on the time interval used between the two successive magnetograms being tracked. It is found that the measurement of the magnetic helicity injection rate performs well for a window size between 12 × 10 and 18 × 15 pixels and at a time interval Δt = 10 minutes. Moreover, the short-lived magnetic structures, 10-60 minutes, are found to contribute 30%-50% of the magnetic helicity injection rate. Comparing with the results calculated by MDI/96 minute data, we find that the MDI/96 minute data, in general, can outline the main trend of the magnetic properties, but they significantly underestimate the magnetic flux in strong field regions and are not appropriate for quantitative tracking studies, so provide a poor estimate of the amount of magnetic helicity injected into the corona. Title: Comparing Spatial Distributions of Solar Prominence Mass Derived from Coronal Absorption Authors: Gilbert, Holly; Kilper, Gary; Alexander, David; Kucera, Therese Bibcode: 2011ApJ...727...25G Altcode: In a previous study, Gilbert et al. derived the column density and total mass of solar prominences using a new technique, which measures how much coronal radiation in the Fe XII (195 Å) spectral band is absorbed by prominence material, while considering the effects of both foreground and background radiation. In the present work, we apply this method to a sample of prominence observations in three different wavelength regimes: one in which only H0 is ionized (504 Å < λ < 911 Å), a second where both H0 and He0 are ionized (228 Å < λ < 504 Å), and finally at wavelengths where H0, He0, and He+ are all ionized (λ < 228 Å). This approach, first suggested by Kucera et al., permits the separation of the contributions of neutral hydrogen and helium to the total column density in prominences. Additionally, an enhancement of the technique allowed the calculation of the two-dimensional (2D) spatial distribution of the column density from the continuum absorption in each extreme-ultraviolet observation. We find the total prominence mass is consistently lower in the 625 Å observations compared to lines in the other wavelength regimes. There is a significant difference in total mass between the 625 Å and 195 Å lines, indicating the much higher opacity at 625 Å is causing a saturation of the continuum absorption and thus, a potentially large underestimation of mass. Title: Magnetic helicity injection by rotating sunspots Authors: Zhu, Chunming; Alexander, David; Tian, Lirong Bibcode: 2010shin.confE.100Z Altcode: We present the observations by SOHO/MDI on four active regions with rotating sunspots. The magnetic helicity and the angular speed of sunspot rotating are calculated and the relations with the emergence of these acitvie regions are discussed. The asymmetry of the magnetic helcity should be responsible for the asymmetry of the magnetic morphology, as shown by Tian & Alexander (2009). The observations also show that CMEs could result from the accumulation of the magnetic helicity injection. Title: Checking Asymmetry of Magnetic Helicity Using Magnetograms with High Spatial and Temporal Resolution Authors: Tian, Lirong; Zhu, C.; Alexander, D. Bibcode: 2010AAS...21640114T Altcode: 2010BAAS...41..859T In order to check if the helicity imbalance is robust between the leading and following polarities, found by Tian & Alexander, we use an improved technique, differential affline velocity estimator (DAVE), on series of MDI 1m and 96m line-of-sight magnetograms with spatial resolution of 0.6 and 2 arcsecs. respectively, to measure photospheric flow motions of an emerging active region: NOAA 10365 (S08). A better parameter of helicity density (Gθ) than GA is employed to calculate helicity injection rate of leading and following polarities.

Our results display that the helicity injection rate of using MDI/1m data is 2 times larger than that of using MDI/ 96m data. The helicity injection rate is little affected by the size of apodizing window selected and the noise level (20 Gauss). However, it is improved so much due to decreasing time difference (up to Δt=10 mines) of two images tracked. The helicity injection rate of two polarities of the active region developed as roughly same step with flux emergence,and maintain its imbalance with more amount in the negative (leading) polarity over tracking period of three days, which is a similar development tendency no matter using MDI/1m data or MDI/96m data.

These results reflect that the time difference of two tracking images is the most important factor affecting amount of helicity injection rate, while there is little relation with spatial resolution of data, the size of apodizing window, and the noise level. Therefore, it should be reliable to study the development of helicity injection rate and imbalanced relationship of two polarities when using MDI/96m data, though the amount calculated is two times smaller.

Further test for MDI/96m data of ARs 8214 and 0656 confirm that the helicity imbalance indeed exists between the leading and following polarities. Title: On the Origin of the Asymmetric Helicity Injection in Emerging Active Regions Authors: Fan, Y.; Alexander, D.; Tian, L. Bibcode: 2009ApJ...707..604F Altcode: To explore the possible causes of the observed asymmetric helicity flux in emerging active regions between the leading and following polarities reported in a recent study by Tian & Alexander, we examine the subsurface evolution of buoyantly rising Ω-shaped flux tubes using three-dimensional, spherical-shell anelastic MHD simulations. We find that due to the asymmetric stretching of the Ω-shaped tube by the Coriolis force, the leading side of the emerging tube has a greater field strength, is more buoyant, and remains more cohesive compared to the following side. As a result, the magnetic field lines in the leading leg show more coherent values of local twist α ≡ (∇ × B) · B/B 2, whereas the values in the following leg show large fluctuations and are of mixed sign. On average, however, the field lines in the leading leg do not show a systematically greater mean twist compared to the following leg. Due to the higher rise velocity of the leading leg, the upward helicity flux through a horizontal cross section at each depth in the upper half of the convection zone is significantly greater in the leading polarity region than that in the following leg. This may contribute to the observed asymmetric helicity flux in emerging active regions. Furthermore, based on a simplified model of active region flux emergence into the corona by Longcope & Welsch, we show that a stronger field strength in the leading tube can result in a faster rotation of the leading polarity sunspot driven by torsional Alfvén waves during flux emergence into the corona, contributing to a greater helicity injection rate in the leading polarity of an emerging active region. Title: Sunspot activity in the build-up to the extended solar minimum Authors: Alexander, D.; Martus, C. Bibcode: 2009AGUFMSH11A1497A Altcode: Comparison between the recent, extended, solar minimum (between Cycles 23 and 24) with the previous one (between Cycles 22 and 23) shows that one of the key distinctive features is the high degree of low latitude structure present in most, if not all, measures of solar activity. In particular, the surface magnetic configuration is significantly more complex in the more recent minimum than in the previous one and this has an impact throughout the heliosphere, leading to many of the observed discriminators used in the comparison of the minima (e.g. low latitude coronal holes, recurring fast solar wind streams). In this paper we categorize the emergence, magnetic structure, evolution and dispersion of sunspots in the years leading up to the two minima and investigate the role that these have in determining the nature of the subsequent solar minimum. Title: The Evolution of Prominence Mass and its Relation to CMEs Authors: Gilbert, H. R.; Xie, H.; St. Cyr, C.; Alexander, D. Bibcode: 2009AGUFMSH41B1672G Altcode: Prominences have long been associated with coronal mass ejections (CMEs) and provide an important component in our understanding of the Sun-Earth system as a whole. Although some prominences are long-lived and others erupt or disappear on short time scales, all types of prominences are dynamical and many show mass loss in observations through draining. Such mass loss often occurs without an eruption, while sometimes occurring prior to and/or during an eruption. We explore the relationship between the nature of a prominence eruption (i.e., full, partial, failed) and the associated CME properties (e.g., speed, mass, and energy). Title: Implications of Temporal Development of Localized Ultraviolet and Hard X-ray Emission for Large Solar Flares Authors: Coyner, Aaron J.; Alexander, David Bibcode: 2009ApJ...705..554C Altcode: Ultraviolet and hard X-ray (HXR) emissions in solar flares provide observational signatures of the interaction of flare-accelerated particles with the chromospheric plasma. Earlier studies have shown clear evidence of temporal and spatial relationships between UV and HXR emission, suggesting a common physical origin. However, more recent spatially resolved case studies suggest significant variations in the spatial distributions of the emission signatures, implying that the large-scale magnetic topology of the flaring region must play a crucial role in the spatial and temporal development of the localized UV and HXR emission. We present here an analysis of spatially resolved HXR emission and localized UV emission sources from 11 M and X class flares with observations from RHESSI and high-cadence 1600 Å observations from TRACE. Within each flare we address the overall temporal development of individual UV sources and relate them to associated impulsive bursts within the HXR flare profile. We find, for these large flares, that in the initial impulsive bursts of flare activity, the majority of the temporally correlated emission evolves in a series of localized co-spatial sources along the UV ribbons consistent with previous two-dimensional reconnection pictures. However, observations of impulses late in flares, show significant departures from the traditional co-spatial/co-temporal picture. The new results include extended UV ribbons with no corresponding HXR emission and marked spatial separations between temporally correlated sources of UV and HXR emission. In seven of the multi-burst events, we observe the development of independent sets of UV and HXR sources corresponding to the individual impulses seen in the temporal profile. The observed separations and the spatial development of co-temporal emission in multi-burst events emphasize the importance of a complex and time varying magnetic topology in shaping the observed emission distributions in both wavelengths. In four of the events, we observe late developing UV sources which show no relationship with the HXR emission. In these events, the emission sources show a strong relationship with lower energy, more spatially extended X-ray emission believed to be of thermal origin. This suggests that, in the later phase of these complex flares, emission from thermal processes comes to dominate non-thermal processes in the production of the UV emission. Title: Comparison of STEREO/EUVI Loops with Potential Magnetic Field Models Authors: Sandman, A. W.; Aschwanden, M. J.; DeRosa, M. L.; Wülser, J. P.; Alexander, D. Bibcode: 2009SoPh..259....1S Altcode: The Solar Terrestrial Relations Observatory (STEREO) provides the first opportunity to triangulate the three-dimensional coordinates of active region loops simultaneously from two different vantage points in space. Three-dimensional coordinates of the coronal magnetic field have been calculated with theoretical magnetic field models for decades, but it is only with the recent availability of STEREO data that a rigorous, quantitative comparison between observed loop geometries and theoretical magnetic field models can be performed. Such a comparison provides a valuable opportunity to assess the validity of theoretical magnetic field models. Here we measure the misalignment angles between model magnetic fields and observed coronal loops in three active regions, as observed with the Extreme Ultraviolet Imager (EUVI) on STEREO on 30 April, 9 May, and 19 May 2007. We perform stereoscopic triangulation of some 100 - 200 EUVI loops in each active region and compute extrapolated magnetic field lines using magnetogram information from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). We examine two different magnetic extrapolation methods: (1) a potential field and (2) a radially stretched potential field that conserves the magnetic divergence. We find considerable disagreement between each theoretical model and the observed loop geometries, with an average misalignment angle on the order of 20° - 40°. We conclude that there is a need for either more suitable (coronal rather than photospheric) magnetic field measurements or more realistic field extrapolation models. Title: Mass Composition in Pre-eruption Quiet Sun Filaments Authors: Kilper, Gary; Gilbert, Holly; Alexander, David Bibcode: 2009ApJ...704..522K Altcode: Filament eruptions are extremely important phenomena due to their association with coronal mass ejections and their effects on space weather. Little is known about the filament mass and composition in the eruption process, since most of the related research has concentrated on the evolution and disruption of the magnetic field. Following up on our previous work, we present here an analysis of nineteen quiet Sun filament eruptions observed by Mauna Loa Solar Observatory in Hα and He I 10830 Å that has identified a compositional precursor common to all of these eruptions. There is a combined trend of an apparent increase in the homogenization of the filament mass composition, with concurrent increases in absorption in Hα and He I and in the level of activity, all starting at least one day prior to eruption. This finding suggests that a prolonged period of mass motions, compositional mixing, and possibly even extensive mass loading is occurring during the build up of these eruptions. Title: Solar Cycle Variations of the Multipolar Field and White Light Corona Authors: Young, Daniel Edward; Alexander, David; St. Cyr, O. C. Bibcode: 2009shin.confE.186Y Altcode: We measure clear cyclic variances in the solar magnetic field and white light corona that we can therefore use to predict upcoming iterations of the solar cycle. Title: Can We Use STEREO/EUVI to Improve Boundary Conditions for Magnetic Modeling? Authors: Sandman, Anne; Aschwanden, Markus J.; Alexander, David Bibcode: 2009shin.confE..19S Altcode: The STEREO mission provides a unique opportunity to observe active region loops simultaneously from two different vantage points in space. Using the stereoscopic data, we can triangulate 3D coordinates for active region loops and use them to perform quantitative comparisons between observed loop geometries and theoretical magnetic field models. These comparisons provide some insight into the validity of magnetic field models, but recent work (DeRosa et al. 2009, Sandman et al. 2009) has shown that potential and non-potential models yield equally poor agreement with observed loop structures. In order to improve the results of magnetic modeling we must resolve the discrepancy between the typically force-free domain (the corona) and non-force-free boundary condition (the photosphere or chromosphere). We seek to address this discrepancy by 'correcting' the boundary condition using observational constraints. The magnetic field in the low corona cannot currently be mapped directly, but using STEREO 3D loop coordinates we can constrain the orientation of the magnetic field in the low corona. In a previous study we measured the misalignment angles between model magnetic fields and observed coronal loops in three active regions, as observed with STEREO/EUVI on April 30, May 9, and May 19, 2007. We now attempt to use these misalignment angles to modify the magnetogram input to the magnetic field model such that the modified model has minimal misalignment with the observed coronal loops near the base of the corona. We present the results of a test case exploring the potential and limitations of this technique. Title: Hard X-ray Emission in Kinking Filaments Authors: Liu, Rui; Alexander, David Bibcode: 2009ApJ...697..999L Altcode: We present an observational study on the impact of the dynamic evolution of kinking filaments on the production of hard X-ray (HXR) emission. The investigation of two kinking-filament events in this paper, occurring on 2003 June 12 and 2004 November 10, respectively, combined with our earlier study on the failed filament eruption of 2002 May 27, suggests that two distinct processes take place during the kink evolution, leading to HXR emission with different morphological connections to the overall magnetic configuration. The first phase of the evolution (Phase I) is characterized by compact HXR footpoint sources at the endpoints of the filament, and the second phase (Phase II) by a ribbon-like footpoint emission extending along the endpoints of the filament. The HXR emission in both the 2002 May 27 and 2004 November 10 events shows a transition from Phase I to Phase II. In the 2002 May 27 event, coronal emission was observed to be associated with EUV brightening sheaths aligned along two filament legs in Phase I, while in Phase II, it was located near the projected crossing point of the kink. The coronal emission in the 2004 November 10 event does not exhibit a clear morphological transition as in the 2002 May 27 event, probably due to the filament's relatively small size. The 2003 June 12 event mostly features a Phase I emission, with a compact footpoint emission located at one end of the filament, and an elongated coronal source oriented along the same filament leg. We propose the following scenarios to explain the different flare morphology: magnetic reconnection in Phase I occurs as a result of the interactions of the two writhing filament legs; reconnection in Phase II occurs at an X-type magnetic topology beneath the filament arch when the filament ascends and expands. Title: Implosion in a Coronal Eruption Authors: Liu, Rui; Wang, Haimin; Alexander, David Bibcode: 2009ApJ...696..121L Altcode: We present the observations of the contraction of the extreme-ultraviolet coronal loops overlying the flaring region during the preheating as well as the early impulsive phase of a GOES class C8.9 flare. During the relatively long, 6 minutes, preheating phase, hard X-ray (HXR) count rates at lower energies (below 25 keV) as well as soft X-ray fluxes increase gradually and the flare emission is dominated by a thermal looptop source with the temperature of 20-30 MK. After the onset of impulsive HXR bursts, the flare spectrum is composed of a thermal component of 17-20 MK, corresponding to the looptop emission, and a nonthermal component with the spectral index γ = 3.5-4.5, corresponding to a pair of conjugate footpoints. The contraction of the overlying coronal loops is associated with the converging motion of the conjugate footpoints and the downward motion of the looptop source. The expansion of the coronal loops following the contraction is associated with the enhancement in Hα emission in the flaring region, and the heating of an eruptive filament whose northern end is located close to the flaring region. The expansion eventually leads to the eruption of the whole magnetic structure and a fast coronal mass ejection. It is the first time that such a large scale contraction of the coronal loops overlying the flaring region has been documented, which is sustained for about 10 minutes at an average speed of ~5 km s-1. Assuming that explosive chromospheric evaporation plays a significant role in compensating for the reduction of the magnetic pressure in the flaring region, we suggest that a prolonged preheating phase dominated by coronal thermal emission is a necessary condition for the observation of coronal implosion. The dense plasma accumulated in the corona during the preheating phase may effectively suppress explosive chromospheric evaporation, which explains the continuation of the observed implosion up to ~7 minutes into the impulsive phase. Title: Using Prominence Mass Inferences in Different Coronal Lines to Obtain the He/H Abundance Authors: Gilbert, Holly; Kilper, G.; Alexander, D.; Kucera, T. Bibcode: 2009SPD....40.1011G Altcode: In a previous study we developed a new technique for deriving prominence mass by observing how much coronal radiation in the Fe XII (λ195) spectral line is absorbed by prominence material. In the present work we apply this method, which allows us to consider the effects of both foreground and background radiation in our calculations, to a sample of prominences absorbing in a coronal line that ionizes both H and He (λ < 504 Å), and a line that ionizes only H (504 Å < λ < 911 Å). This approach, first suggested by Kucera et al. (1998), permits the determination of the abundance ratio of neutral helium and hydrogen in the prominence. This ratio should depend on how the prominence is formed, on its current thermodynamic state, and on its dynamical evolution. Thus, it may provide useful insights into the formation and evolution of prominences. Title: Mass Composition in Pre-Eruption Quiet Sun Filaments Authors: Kilper, Gary; Gilbert, H.; Alexander, D. Bibcode: 2009SPD....40.1012K Altcode: Filament eruptions are extremely important phenomena due to their association with coronal mass ejections and their effects on space weather. Little is known about the filament mass and composition in the eruption process, since most of the related research has concentrated on the evolution and disruption of the magnetic field. Following up on our previous work, we are presenting an analysis of nineteen quiet Sun filament eruptions observed by MLSO in H-alpha and He I 10830 A that has identified a compositional precursor common to all of these eruptions. A general trend of increasing homogenization of the filament mass composition with concurrent increases in absorption in H-alpha and He I and the level of activity, all starting at least one day prior to eruption, suggests the possibility that an extended period of mass loading is occurring during the build-up to these eruptions. Funding was provided by a NASA GSRP training grant from GSFC. Title: Implosion in a Coronal Eruption Authors: Liu, Rui; Wang, H.; Alexander, D. Bibcode: 2009SPD....40.1918L Altcode: We present the observations of the contraction of the EUV coronal loops overlying the flaring region during the pre-heating as well as the early impulsive phase of a C8.9 flare. During the relatively long, 6 minutes, pre-heating phase, the flare emission is dominated by a thermal looptop source with the temperature of 20 - 30 MK. After the onset of impulsive hard X-ray bursts, the flare spectrum is composed of a thermal component of 17 - 20 MK, corresponding to the looptop emission, and a nonthermal component with the spectral index (3.5 - 4.5), corresponding to a pair of conjugate footpoints. The contraction of the overlying coronal loops is associated with the converging motion of the conjugate footpoints and the downward motion of the looptop source. The expansion of the coronal loops following the contraction is associated with the enhancement in H-alpha emission in the flaring region, and the heating of an eruptive filament whose northern end is located close to the flaring region. The expansion eventually leads to the eruption of the whole magnetic structure and a fast coronal mass ejection. It is the first time that such a large scale contraction of the coronal loops overlying the flaring region has been documented, which is sustained for about 10 min at an average speed of 5 km/s. Assuming that explosive chromospheric evaporation plays a significant role in compensating for the reduction of the magnetic pressure in the flaring region, we suggest that a prolonged pre-heating phase dominated by coronal thermal emission is a necessary condition for the observation of coronal implosion. The dense plasma accumulated in the corona during the pre-heating phase may effectively suppress explosive chromospheric evaporation, which explains the continuation of the observed implosion up to 7 minutes into the impulsive phase. Title: Can We Use STEREO/EUVI to Improve Boundary Conditions for Magnetic Modeling? Authors: Sandman, Anne; Aschwanden, M. J.; Alexander, D. Bibcode: 2009SPD....40.1220S Altcode: The STEREO mission provides a unique opportunity to observe active region loops simultaneously from two different vantage points in space. Using the stereoscopic data, we can triangulate 3D coordinates for active region loops and use them to perform quantitative comparisons between observed loop geometries and theoretical magnetic field models. These comparisons provide some insight into the validity of magnetic field models, but recent work (DeRosa et al. 2009, Sandman et al. 2009) has shown that potential and non-potential models yield equally poor agreement with observed loop structures. In order to improve the results of magnetic modeling we must resolve the discrepancy between the typically force-free domain (the corona) and non-force-free boundary condition (the photosphere or chromosphere). We seek to address this discrepancy by "correcting” the boundary condition using observational constraints. The magnetic field in the low corona cannot currently be mapped directly, but using STEREO 3D loop coordinates we can constrain the orientation of the magnetic field in the low corona. In a previous study we measured the misalignment angles between model magnetic fields and observed coronal loops in three active regions, as observed with STEREO/EUVI on April 30, May 9, and May 19, 2007. We now use these misalignment angles to modify the magnetogram input to the magnetic field model. The resulting model field has minimal misalignment with the observed coronal loops near the base of the corona, allowing for a more realistic field extrapolation. Title: Effect of Magnetic Flux Imbalance on the Magnetic Helicity Imbalance in Emerging Active Regions Authors: Zhu, Chunming; Tian, L.; Alexander, D. Bibcode: 2009SPD....40.0911Z Altcode: Tian & Alexander (2009) studied 15 emerging active regions and found that magnetic helicity flux injected into the corona by the leading polarity is generally several times larger than that injected by the following polarity. They thought that the asymmetry of the magnetic helicity should be responsible for the asymmetry of the magnetic morphology, i.e, the leading magnetic field of bipolar active regions is often spatially more compact, while more dispersed and fragmented in following polarity. Though they take into account and "remove" possible effect from magnetic flux imbalance, however, we still don't know how much it affects. In this paper, we will calculate the magnetic flux and helicity flux of two emerging active regions observed by SOHO/MDI and Hinode, find their difference, and study possible origins resulting in the helicity flux imbalance. Title: Asymmetry of Helicity Injection Flux in Emerging Active Regions Authors: Tian, Lirong; Alexander, David Bibcode: 2009ApJ...695.1012T Altcode: Observational and modeling results indicate that typically the leading magnetic field of bipolar active regions (ARs) is often spatially more compact, while more dispersed and fragmented in following polarity. In this paper, we address the origin of this morphological asymmetry, which is not well understood. Although it may be assumed that, in an emerging Ω-shaped flux tube, those portions of the flux tube in which the magnetic field has a higher twist may maintain its coherence more readily, this has not been tested observationally. To assess this possibility, it is important to characterize the nature of the fragmentation and asymmetry in solar ARs and this provides the motivation for this paper. We separately calculate the distribution of the helicity flux injected in the leading and following polarities of 15 emerging bipolar ARs, using the Michelson Doppler Image 96 minute line-of-sight magnetograms and a local correlation tracking technique. We find from this statistical study that the leading (compact) polarity injects several times more helicity flux than the following (fragmented) one (typically 3-10 times). This result suggests that the leading polarity of the Ω-shaped flux tube possesses a much larger amount of twist than the following field prior to emergence. We argue that the helicity asymmetry between the leading and following magnetic field for the ARs studied here results in the observed magnetic field asymmetry of the two polarities due to an imbalance in the magnetic tension of the emerging flux tube. We suggest that the observed imbalance in the helicity distribution results from a difference in the speed of emergence between the leading and following legs of an inclined Ω-shaped flux tube. In addition, there is also the effect of magnetic flux imbalance between the two polarities with the fragmented following polarity displaying spatial fluctuation in both the magnitude and sign of helicity measured. Title: POLAR investigation of the Sun—POLARIS Authors: Appourchaux, T.; Liewer, P.; Watt, M.; Alexander, D.; Andretta, V.; Auchère, F.; D'Arrigo, P.; Ayon, J.; Corbard, T.; Fineschi, S.; Finsterle, W.; Floyd, L.; Garbe, G.; Gizon, L.; Hassler, D.; Harra, L.; Kosovichev, A.; Leibacher, J.; Leipold, M.; Murphy, N.; Maksimovic, M.; Martinez-Pillet, V.; Matthews, B. S. A.; Mewaldt, R.; Moses, D.; Newmark, J.; Régnier, S.; Schmutz, W.; Socker, D.; Spadaro, D.; Stuttard, M.; Trosseille, C.; Ulrich, R.; Velli, M.; Vourlidas, A.; Wimmer-Schweingruber, C. R.; Zurbuchen, T. Bibcode: 2009ExA....23.1079A Altcode: 2008ExA...tmp...40A; 2008arXiv0805.4389A The POLAR Investigation of the Sun (POLARIS) mission uses a combination of a gravity assist and solar sail propulsion to place a spacecraft in a 0.48 AU circular orbit around the Sun with an inclination of 75° with respect to solar equator. This challenging orbit is made possible by the challenging development of solar sail propulsion. This first extended view of the high-latitude regions of the Sun will enable crucial observations not possible from the ecliptic viewpoint or from Solar Orbiter. While Solar Orbiter would give the first glimpse of the high latitude magnetic field and flows to probe the solar dynamo, it does not have sufficient viewing of the polar regions to achieve POLARIS’s primary objective: determining the relation between the magnetism and dynamics of the Sun’s polar regions and the solar cycle. Title: Asymmetric Eruptive Filaments Authors: Liu, Rui; Alexander, David; Gilbert, Holly R. Bibcode: 2009ApJ...691.1079L Altcode: Filaments are often observed to erupt asymmetrically, during which one leg is fixed to the photosphere (referred to as the anchored leg) while the other undertakes most of the dynamic motions (referred to as the active leg) during the eruptive process. In this paper, we present observations of a group of asymmetric eruptive filaments, in which two types of eruptions are identified: whipping-like, where the active leg whips upward, and hard X-ray sources shift toward the end of the anchored leg; and zipping-like, where the visible end of the active leg moves along the neutral line like the unfastening of a zipper as the filament arch rises and expands. During a zipping-like eruption, hard X-ray sources shift away from where the eruption initiates toward where the visible end of the active leg eventually stops moving. Both types of asymmetric eruptions can be understood in terms of how the highly sheared filament channel field, traced by filament material, responds to an external asymmetric magnetic confinement, where force imbalance occurs in the neighborhood of the visible end of the active leg. The dynamic motions of the active leg have a distinct impact on how hard X-ray sources shift, as observed by RHESSI. Title: Using STEREO/EUVI to Study Active Region Magnetic Fields Authors: Sandman, A.; Aschwanden, M.; Wuelser, J.; De Rosa, M.; Alexander, D. Bibcode: 2008AGUFMSH13B1523S Altcode: We examine the effect of linear transformations on the misalignment between model magnetic fields and coronal loops in active regions, as observed with STEREO/EUVI on three separate occasions between April 30 and May 19, 2007. We perform stereoscopic triangulation of some 100 EUVI loops in each active region, and identify the tangent vectors along every loop. Using magnetogram information from SOHO/MDI we compute a 3D potential field and interpolate the magnetic field vector at every position along the EUVI loops. The angle between the loop tangent vector and the magnetic field vector provides a measure of the misalignment angle between the observed field configuration and the model. We then transform the field in a way that preserves the divergence-free condition while injecting electric currents into the system. With this modified field we repeat our calculation of the misalignment angles between the magnetic field vectors and the EUV loop tangent vectors, quantifying the improvement of the transformed magnetic field model. Results of this type of magnetic modeling are presented for three active regions. Title: Temporal and Spatial Evolution of Localized UV and Hard X-ray Emission in Solar Flares Authors: Coyner, A. J.; Alexander, D. Bibcode: 2008AGUFMSH13A1512C Altcode: Localized ultraviolet and hard X-ray emission sources provide observational diagnostics of flare-accelerated particles and energy deposition within the chromosphere. The evolution of these emission sources in space and intensity provide insight into the evolving magnetic structure and energy release dynamics of the flaring region. Initial studies of UV and hard X-ray emission indicated a strong co-temporal relationship between the two emission suggesting a common energetic origin often attributed to accelerated particle production resulting from magnetic reconnection in the corona. While confirming the overall temporal correlation, recent spatially-resolved observations have determined that localized correlated UV and HXR sources can occur in spatially distinct locations emphasizing the importance of a complex evolving magnetic topology in governing the spatial emission distributions observed. We present here an analysis of M and X class flares observed in TRACE 1600 Å images and RHESSI 25-100 keV X-rays at high cadence. While observations of the initial impulses show strong co-spatial emission in both wavelengths; however, as the flare evolves, we find significant spatial separations between the temporally correlated emission sources requiring that a time varying complex magnetic structure must be present to transport particles and produce the varied spatial distributions. In addition, we find, for 6 events showing multiple impulses in their X-ray time profiles, that each X-ray impulse corresponds to a distinct distribution of X-ray emission emission sources suggesting that each impulse may correspond to additional magnetic reconnection events is distinct locations within the corona. Finally, we find UV sources which deviate from the expected hard X-ray temporal correlation and instead appear more strongly associated with lower energy thermal X-ray emission indicating thermal processes or the thermal particle acceleration must contribute significantly in the flare energy release process. Title: Using Prominence Mass Inferences in Different Coronal Lines to Obtain the He/H Abundance Authors: Gilbert, H.; Kilper, G.; Alexander, D.; Kucera, T. Bibcode: 2008AGUFMSH13A1509G Altcode: In a previous study we developed a new technique for deriving prominence mass by observing how much coronal radiation in the Fe XII (19.5 nm) spectral line is absorbed by prominence material. In the present work we apply this method, which allows us to consider the effects of both foreground and background radiation in our calculations, to a sample of prominences absorbing in a coronal line that ionizes both H and He (lambda < 50.4 nm), and a line that ionizes only H (50.4 nm < lambda < 91.1 nm). This approach, first suggested by Kucera et al. (1998), permits the determination of the abundance ratio of neutral helium and hydrogen in the prominence. This ratio should depend on how the prominence is formed, on its current thermodynamic state, and on its dynamical evolution. Thus, it may provide useful insights into the formation and evolution of prominences. Title: Origins of Coronal Energy and Helicity in NOAA 10030 Authors: Tian, Lirong; Alexander, David; Nightingale, Richard Bibcode: 2008ApJ...684..747T Altcode: Exploring the origins of coronal helicity and energy, as well as determining the mechanisms that lead to coronal energy release, is a fundamental topic in solar physics. Using MDI 96 minute line-of-sight and HSOS vector magnetograms in conjunction with TRACE white-light and UV (1600 Å) images and BBSO Hα and SOHO EIT (195 Å) images, we find in active region NOAA 10030 that a large positive polarity sunspot, located in the center of the region, exhibited significant counterclockwise rotation, which continued for 6 days during the period 2002 July 12-18. This rotating sunspot was related to the formation of inverse-\textsf{S}-shaped filaments, left-handed twist of the vector magnetic fields, and the production of strong negative vertical current, but exhibited little emergence of magnetic flux. In all, five M-class and two X-class flares were produced around this rotating sunspot over the 6 day period. The observed characteristics of the strongly rotating sunspot suggest that sunspots can undergo strong intrinsic rotation, the source of which may originate below the photosphere and can play a significant role in helicity production and injection and energy buildup in the corona. A sunspot with negative magnetic polarity showed fast and significant emergence in the eastern portion of the active region, and moved northeastward over several days, but exhibited little rotation. The moving sunspot also exhibited the formation of inverse-\textsf{S}-shaped filaments, left-handed twist of vector magnetic fields and coronal structure, and the production of stronger positive current. The observed characteristics of the emerging sunspot suggest that significant emergence of twisted magnetic fields may not always result in the rotation of the associated sunspots, but they do play a very important role in the coronal helicity accumulation and free-energy build-up. Title: The Filament-Moreton Wave Interaction of 2006 December 6 Authors: Gilbert, Holly R.; Daou, Antoun G.; Young, Daniel; Tripathi, Durgesh; Alexander, David Bibcode: 2008ApJ...685..629G Altcode: We utilize chromospheric observations obtained at MLSO of the 2006 December 6 Moreton wave, which exhibits two distinct fronts, and subsequent filament activation to conduct a comprehensive analysis of the wave-filament interaction. By determining the period, amplitude, and evolution of the oscillations in the activated filament, we make certain inferences regarding the physical properties of both the wave and the filament. The large-amplitude oscillations induced in the filament by the wave passage last on the order of 180 minutes and demonstrate a complicated mixture of transverse and perpendicular motion with respect to the filament spine. These oscillations are predominantly along the filament axis, with a period of ~29 minutes and maximum line-of-sight velocity amplitude of ~41 km s-1. A careful examination of the complex oscillatory response of the filament elucidates some of the fundamental characteristics of the related Moreton wave. Specifically, we infer the maximum total kinetic energy involved in the interaction, the structure and topology of the passing wave, and discuss implications for the topology of the responding magnetic structure supporting the filament. The results of this observational study equip us with a better understanding of how filaments become activated and the nature of their responses to large propagating disturbances. Title: The Effect of Magnetic Reconnection and Writhing in a Partial Filament Eruption Authors: Liu, Rui; Gilbert, Holly R.; Alexander, David; Su, Yingna Bibcode: 2008ApJ...680.1508L Altcode: We present observations from 2007 March 2 of a partial filament eruption characterized by two distinct phases of writhing motions: a quasi-static, slowly evolving phase followed by a rapid kinking phase showing a bifurcation of the filament. The quasi-static kinking motions are observed before there is any heating or flaring evident in EUV or soft X-ray (SXR) observations. As the writhe of the filament develops, a sigmoid becomes sharply defined in the SXR. Prior to eruption onset, the sigmoid in the EUV appears to be composed of two separate looplike structures, which are discontinuous at the projected location where the sigmoid crosses the filament. Coincident with the onset of the eruption and the production of a GOES class B2 flare, the original "two-loop" EUV sigmoid is now observed as a single continuous structure lying above the filament, signifying the presence of magnetic reconnection and the associated dissipative heating of field lines above the filament. During the eruption, the escaping portion of the filament rotates quickly and erupts together with the expanding arched sigmoid. The portion of the filament that is left behind develops into an inverse S-shaped configuration. The separation of the filament, the EUV brightening at the separation location, and the surviving sigmoidal structure are all signatures of magnetic reconnection occurring within the body of the original filament. Other features of the same event reported by Sterling and coworkers, e.g., the flux cancellation at the polarity inversion line prior to the eruption and the SXR compact loop formed underneath the erupting sigmoid during the eruption, indicate that magnetic reconnection also occurred in the sheared core field beneath the filament. These results suggest that a combination of the kinking motions and internal tether-cutting are responsible for the initiation of the eruption. Title: Temporal Development of Localized UV and Hard X-ray Emissions in Large Solar Flares Authors: Coyner, A. J.; Alexander, D. Bibcode: 2008AGUSMSP51C..10C Altcode: Ultraviolet and hard X-ray (HXR) emissions in solar flares provide signatures of flare-accelerated particles with the chromosphere. Initial studies of flare emission at these wavelengths have indicated a strong co-temporal relationship during the impulsive phase of solar flares, suggesting they arise from a common origin, often attributed to the accelerated particle production via coronal magnetic reconnection. While spatially-unresolved studies have shown a strong temporal correlation, recent spatially-resolved observations indicate that co- temporal UV and HXR emission can originate from distinct locations, raising the question as to how the global magnetic topology and energy release mechanisms relate to the spatial and temporal development of the localized UV and HXR emission. The spatial resolution provided by TRACE and RHESSI imaging presents an opportunity to address these global relationships on the level of individually distinguishable sources. We present an analysis and comparison of spatially-resolved hard X-ray and localized UV emission sources for a number of M and X class flares with observations from RHESSI and high-cadence 1600 Å observations from TRACE. Within each flare we investigate the overall temporal development of individual UV sources and relate them to individual impulsive hard X-ray bursts. We find that for these large flares, the bulk of the temporally correlated UV and HXR emission evolves in a series of localized co-spatial sources along the UV ribbons consistent with previous 2-D reconnection pictures, however, extended UV ribbon emission, the evolution of multiple burst events, and the existence of UV sources developing late in the flare evolution indicate the need for a 3-D magnetic topology scenario along with the suggestion that multiple physical mechanisms appear necessary to reproduce the emission in these flares. Title: Association of magnetic topology and radiative emissions in solar flares Authors: Alexander, D.; Tian, L.; Coyner, A. Bibcode: 2008AGUSMSP51C..14A Altcode: We relate the energetic process in flares to the detailed topology, dynamics and evolution of magnetic field for NOAA active region 10720, which proved to be the most flare productive region of Cycle 23. We obtain the locations of quasi-separatrix layers (QSLs) from photospheric vector magnetic field observations based on the squashing degree, Q of Titov et al. (2002)and compare these to the distribution of vertical current, magnetic field evolution, and the chromospheric sites of UV and hard X-ray flare emissions. We find a close relationship between the distribution of the QSLs and the flare emission sources. Moreover, opposite vertical current is found to concentrate near the QSLs and that several large flares occur in the vicinity of the current concentrations and QSLs. We argue that the magnetic emergence and fast evolution of the sunspot magnetic fields alter the topology of the field in the corona resulting in the production of large solar flares and the release of the free energy associated with the currents. Title: Using Prominence Mass Inferences in Different Coronal Lines to Obtain the He/H Abundance Authors: Gilbert, H. R.; Kilper, G.; Kucera, T.; Alexander, D. Bibcode: 2008AGUSMSP43B..05G Altcode: In a previous study we developed a new technique for deriving prominence mass by observing how much coronal radiation in the Fe XII (λ195) spectral line is absorbed by prominence material. In the present work we apply this method, which allows us to consider the effects of both foreground and background radiation in our calculations, to a sample of prominences absorbing in a coronal line that ionizes both H and He (λ < 504 Å), and a line that ionizes only H (504 Å < λ < 911 Å). This approach, first suggested by Kucera et al. (1998), permits the determination of the abundance ratio of neutral helium and hydrogen in the prominence. This ratio should depend on how the prominence is formed, on its current thermodynamic state, and on its dynamical evolution. Thus, it may provide useful insights into the formation and evolution of prominences. Title: Spatial and Temporal Development of Fiducial Current Systems in Solar Flares Authors: Daou, A. G.; Alexander, D. Bibcode: 2008AGUSMSP51C..16D Altcode: We use the unprecedented spatial and spectral resolution of the RHESSI telescope to investigate the temporal evolution of thermal and non-thermal hard X-ray emission in spatially independent substructures in 2 major flares: the 2006 December 6 X6.5 and the 2003 November 2 X 8.3, one of the large Halloween '03 flares. The hard X-ray emission in these flares is concentrated in a number of distinct sources displaying independent temporal and spectral evolution. We find that while the total flare energy content rises during the impulsive phase, the non-thermal hard X-ray fluxes in many of the identified substructures saturates after an initial rise phase. In a previous study, we have investigated this non-thermal flux saturation at the peak of the hard X-ray emission in 10 flares spanning a factor of ~200 (M1 to X17) in the GOES 1-8Å~peak flux. The indication was that the saturation was due to a transition in the dominant energy loss mechanism from Coulomb collisions to return current ohmic dissipation as the flare magnitude increased. In this study, we analyze the temporal variation of the hard X-ray fluxes within two large flares, and infer the spectral properties of the emitting electron population, as the flare transits from from one energy loss regime to the other. We find that the increase in the non-thermal hard X-ray flux in the substructures towards saturation is mainly due to a hardening of the spectrum rather than an increase in the normalized flux in the electron beams. Title: Theoretical modeling for the stereo mission Authors: Aschwanden, Markus J.; Burlaga, L. F.; Kaiser, M. L.; Ng, C. K.; Reames, D. V.; Reiner, M. J.; Gombosi, T. I.; Lugaz, N.; Manchester, W.; Roussev, I. I.; Zurbuchen, T. H.; Farrugia, C. J.; Galvin, A. B.; Lee, M. A.; Linker, J. A.; Mikić, Z.; Riley, P.; Alexander, D.; Sandman, A. W.; Cook, J. W.; Howard, R. A.; Odstrčil, D.; Pizzo, V. J.; Kóta, J.; Liewer, P. C.; Luhmann, J. G.; Inhester, B.; Schwenn, R. W.; Solanki, S. K.; Vasyliunas, V. M.; Wiegelmann, T.; Blush, L.; Bochsler, P.; Cairns, I. H.; Robinson, P. A.; Bothmer, V.; Kecskemety, K.; Llebaria, A.; Maksimovic, M.; Scholer, M.; Wimmer-Schweingruber, R. F. Bibcode: 2008SSRv..136..565A Altcode: 2006SSRv..tmp...75A We summarize the theory and modeling efforts for the STEREO mission, which will be used to interpret the data of both the remote-sensing (SECCHI, SWAVES) and in-situ instruments (IMPACT, PLASTIC). The modeling includes the coronal plasma, in both open and closed magnetic structures, and the solar wind and its expansion outwards from the Sun, which defines the heliosphere. Particular emphasis is given to modeling of dynamic phenomena associated with the initiation and propagation of coronal mass ejections (CMEs). The modeling of the CME initiation includes magnetic shearing, kink instability, filament eruption, and magnetic reconnection in the flaring lower corona. The modeling of CME propagation entails interplanetary shocks, interplanetary particle beams, solar energetic particles (SEPs), geoeffective connections, and space weather. This review describes mostly existing models of groups that have committed their work to the STEREO mission, but is by no means exhaustive or comprehensive regarding alternative theoretical approaches. Title: On the Origin of Magnetic Helicity in the Solar Corona Authors: Tian, Lirong; Alexander, David Bibcode: 2008ApJ...673..532T Altcode: Twenty-three active regions associated with pronounced sigmoidal structure in Yohkoh soft X-ray observations are selected to investigate the origin of magnetic helicity in the solar corona. We calculate the radial magnetic flux of each polarity, the rate of magnetic helicity injection, and total flux of the helicity injection (Δ HLCT) over 4-5 days using MDI 96 minute line-of-sight magnetograms and a local correlation tracking technique. We also estimate the contribution from differential rotation to the overall helicity budget (Δ Hrot). It is found that of the seven active regions for which the flux emergence exceeds 1.0 × 1022 Mx, six exhibited a helicity flux injection exceeding 1.0 × 1043 Mx2 (i.e., Δ H = Δ HLCT - Δ Hrot). Moreover, the rate of helicity injection and the total helicity flux are larger (smaller) during periods of more (less) increase of magnetic flux. Of the remaining 16 active regions, with flux emergence less than 1022 Mx, only 4 had significant injection of helicity, exceeding 1043 Mx2. Typical contributions from differential rotation over the same period were 2-3 times smaller than that of the strong magnetic field emergence. These statistical results signify that the strong emergence of magnetic field is the most important origin of the coronal helicity, while horizontal motions and differential rotation are insufficient to explain the measured helicity injection flux. Furthermore, the study of the helicity injection in nineteen newly emerging active regions confirms the result on the important role played by strong magnetic flux emergence in controlling the injection of magnetic helicity into the solar corona. Title: Observational Evidence Supporting Cross-field Diffusion of Neutral Material in Solar Filaments Authors: Gilbert, Holly; Kilper, Gary; Alexander, David Bibcode: 2007ApJ...671..978G Altcode: We investigate the temporal and spatial variation of the relative abundance of He to H in a sample of solar filaments by comparing cotemporal observations of Hα and He I λ10830 obtained at MLSO. Motivated by indications that cross-field diffusion of neutral filament material is an important mechanism in mass loss, the present study offers results that provide a convincing test of the mechanisms proposed in Gilbert and coworkers. Specifically, when observed across an entire disk passage, we find a majority of stable, quiescent filaments show a relative helium deficit in the upper portions of their structure coupled with a relative helium surplus in the lower regions, a consequence of the large loss timescale for neutral helium compared to neutral hydrogen. Moreover, we find that the variation of the relative He/H ratio is uniform across filament barbs and footpoints on both short and long timescales. Title: 3D Magnetic Modeling of Active Regions Using STEREO/EUVI Authors: Sandman, A.; Aschwanden, M. J.; Alexander, D.; Wuelser, J. Bibcode: 2007AGUFMSH32A0770S Altcode: With the recent availability of stereoscopic data from the Extreme Ultraviolet Imager (EUVI) on the Solar Terrestrial Relations Observatory (STEREO) we have an unprecedented opportunity to investigate the accuracy of 3D magnetic field models. These data will be put to best use by modeling techniques that make no assumptions about the nature of the field (that it is potential, force-free, etc.). The Gary-Alexander radial stretching method use a series of transformations to map a simple potential field to a more complicated target field, and compare the transformed field lines with observed coronal structures in the EUV. Unlike many other simulation techniques, this approach requires only that the field remain divergence-free and continuous at the photosphere. Here we apply this transformation method to STEREO/EUVI data. We obtain a 3D potential field extrapolation using an MDI magnetogram, and utilize stereoscopy to derive the 3D field line coordinates from pairs of EUV images at 171Å. By comparing the 3D coordinates of the transformed model field lines with those of the real field lines as seen by EUVI, we can place constraints on the distribution of magnetic field and current in an active region. Title: Filament Kinking and Its Implications for Eruption and Re-formation Authors: Gilbert, Holly R.; Alexander, David; Liu, Rui Bibcode: 2007SoPh..245..287G Altcode: Solar filaments exhibit a range of eruptive-like dynamic activity from the full, or partial, eruption of the filament mass and surrounding magnetic structure, as a CME, to a fully confined dynamic evolution or "failed" eruption, sometimes producing a flare but no CME. Additionally, observations of erupting filaments often show a clear helical structure, indicating the presence of a magnetic flux rope. Dynamic helical structures, in addition to being twisted, frequently show evidence of being kinked, with the axis of the flux rope exhibiting a large-scale writhe. Motivated by the fact that kinking motions are also detected in filaments that fail to erupt, we investigate the possible relationship between the kinking of a filament and its success or failure to erupt. We present an analysis of kinking in filaments and its implications for other filament phenomena such as the nature of the eruption, eruptive acceleration, and post-eruptive re-formation. We elucidate the relationship between kinking and the various filament phenomena via a simple physical picture of the forces involved in kinking together with specific definitions of the types of filament eruption. The present study offers results directly applicable to observations, allowing a thorough exploration of the implications of the observational relationship between kinking and filament phenomena and provides new insight for modelers of CME initiation. Title: Saturation of Nonthermal Hard X-Ray Emission in Solar Flares Authors: Alexander, David; Daou, Antoun G. Bibcode: 2007ApJ...666.1268A Altcode: We consider a number of flares spanning a range of magnitudes over two decades in GOES soft X-ray luminosity and demonstrate an observational confirmation of the saturation of the nonthermal hard X-ray photon flux in large solar flares. Using the unprecedented spectral and spatial resolution of the RHESSI telescope, we find that the integrated photon flux above 20 keV asymptotically approaches a limiting value with increasing flare intensity, suggesting a saturation of the nonthermal photon production for high electron-beam fluxes in flares. This result is strengthened by considering each of the large flares as an aggregation of multiple fiducial energy release events identified using the high spatial resolution of RHESSI. We argue that the saturation of the photon production sets in as a result of the transition from Coulomb collisions to return current ohmic dissipation dominating the energy losses as the flare magnitudes increase. Title: Kink-induced Catastrophe in a Coronal Eruption Authors: Liu, Rui; Alexander, David; Gilbert, Holly R. Bibcode: 2007ApJ...661.1260L Altcode: We investigate the kinking motion and its role in the eruption of a filament/cavity system that occurred on 2002 October 31. The evolution of the eruptive filament consists of four distinct phases. After an initial slow upward acceleration, the filament experiences a quasi-static phase exhibiting kinking motions of the filament axis. The kinking phase is followed by a sudden jump, coincident with the onset of the unkinking of the filament. The loss of equilibrium initiates a gradual relaxation phase at the end of which the filament reattains a similar unkinked configuration as its initial state. The filament/cavity structure, evident in the white-light observations, interacts with a large-scale coronal helmet streamer to the north, and material is observed to eject outward, aligned with a preexisting, low-density, dark channel that originally separated the northern helmet streamer from the southern streamer, where the dark cavity resides. The bulk of the filament, however, remains confined in the lower corona throughout the eruption along the channel. This suggests a partial eruption of the filament/cavity structure. The observations presented here manifest a catastrophic loss of equilibrium in response to the evolution of kinking motions in the filament activation. Title: Kinking and Solar Filament Eruptions Authors: Liu, Rui; Alexander, D.; Gilbert, H. R. Bibcode: 2007AAS...210.9502L Altcode: 2007BAAS...39..223L We conduct observational studies to explore the role of kinking in coronal eruptions. Our recent work has shown evidence supporting the role of writhe of the filament axis as a possible driver and regulator of filament eruptions. We identify coronal X-ray source emission occurring beneath the apex of a writhed filament with reconnection driven by kinking motions. We indicate that quasi-static kinking motions induce a catastrophe in a filament/cavity eruption. We find brightening soft X-ray and EUV features propagating along filament legs down to the solar surface during filament eruptions, suggestive of heating in the current sheet formed at the interface of writhed filaments and ambient field. We propose that weakening of magnetic tethers that anchor either leg of a twisted and/or writhed flux rope which supports the filament result in whipping and/or rotating motions observed in “zipper-like” filament eruptions. Title: Observational Evidence Supporting Cross-field Diffusion of Neutral Prominence Material Authors: Gilbert, Holly; Kilper, G.; Alexander, D. Bibcode: 2007AAS...210.9505G Altcode: 2007BAAS...39..224G Prominences over all lifetime scales are dynamical, and many exhibit mass loss in observations through draining. Draining occurs along magnetic field lines but it can also occur across field lines since prominences are partially ionized. In a careful analysis of cross-field diffusion of neutral material in a partially ionized hydrogen-helium plasma, Gilbert et al. (2002) found the loss time scales of hydrogen to be much longer than that of helium. One way to evaluate the importance of this mechanism is to compare observations of filaments in Hα and He I (1083 nm) to see if there is a relative helium deficit. We present the results of such an analysis on a large sample of filaments, which show an obvious relative helium deficit in the edges of filaments residing higher in the solar atmosphere than the lower portions. Title: Relationship Of Sunspot Rotation And Emergence Of Twisted Magnetic Fields? Authors: Tian, Lirong; Alexander, D.; Nightingale, R. Bibcode: 2007AAS...210.2403T Altcode: 2007BAAS...39..128T Is it the sunspot rotation that twists the magnetic fields? Or does the emergence of twisted magnetic fields create the sunspot rotation viewed in the photosphere due to a geometric effect resulting from the emergence of twisted magnetic fields? Using TRACE white light and 1600A images, BBSO/H_a and SOHO/EIT/195A observations with MDI/96m line-of-sight and HSOS vector magnetograms, we find in active region NOAA 10030 that a large positive polarity sunspot, located in the center of the region, exhibited significant counter-clockwise rotation, which continued for six days during July 12-18, 2002. This rotating sunspot was related to the formation of inverse-S-shaped filaments, left-handed twist of the vector magnetic fields, and the production of strong negative vertical current, but exhibiting little emergence of magnetic flux. All five M-class and two X-class flares were produced around this rotating sunspot over the six days period. A sunspot with negative magnetic polarity showed fast and significant emergence in the eastern portion of the active region, and moved north-eastward over several days, but exhibited no rotation. The moving sunspot also exhibited the formation of inverse-S-shaped filaments, left-handed twist of vector magnetic fields and coronal structure, and the production of stronger positive current. The observed characteristics of the emerging sunspot suggest that the significant emergence of twisted magnetic fields play an important role in coronal helicity injection and free-energy buildup, but does not have to result in a sunspot rotation viewed in the photosphere, due to a geometric effect resulting from the emergence of twisted magnetic fields. While from the observed characteristics of the strong rotating sunspot, we suggest that physical rotation of sunspots originates from the solar interior, and plays a remarkable role in coronal helicity and free-energy accumulation, but does not have to result from the geometric effect of the emergence of twisted magnetic fields. Title: Laboratory Exploration of Solar Energetic Phenomena Authors: Alexander, David Bibcode: 2007Ap&SS.307..197A Altcode: 2006Ap&SS.tmp..535A The solar atmosphere displays a wide variety of dynamic phenomena driven by the interaction of magnetic fields and plasma. In particular, plasma jets in the solar chromosphere and corona, coronal heating, solar flares and coronal mass ejections all point to the presence of magnetic phenomena such as reconnection, flux cancellation, the formation of magnetic islands, and plasmoids. While we can observe the signatures and gross features of such phenomena we cannot probe the essential physics driving them, given the spatial resolution of current instrumentation. Flexible and well-controlled laboratory experiments, scaled to solar parameters, open unique opportunities to reproduce the relevant unsteady phenomena under various simulated solar conditions. The ability to carefully control these parameters in the laboratory allows one to diagnose the dynamical processes which occur and to apply the knowledge gained to the understanding of similar processes on the Sun, in addition directing future solar observations and models. This talk introduces the solar phenomena and reviews the contributions made by laboratory experimentation. Title: Hard X-Ray Production in a Failed Filament Eruption Authors: Alexander, David; Liu, Rui; Gilbert, Holly R. Bibcode: 2006ApJ...653..719A Altcode: We revisit the ``failed'' filament eruption of 2002 May 27, first studied in detail by Ji et al. We investigate the temporal and spatial relationship between the filament dynamics and the production of hard X-ray emission using spatially resolved high-cadence data from TRACE and RHESSI. We confirm the presence of a hard X-ray source in the corona above the filament prior to the main activation phase and identify a second coronal hard X-ray source, not considered by earlier studies, that occurs under the apex of the filament during the erupting phase when the filament is clearly strongly kinked. We argue that this second source of coronal hard X-ray emission implies ongoing magnetic reconnection in a current sheet formed via a kink instability resulting from the interaction of the two adjacent legs underneath the writhing filament, in agreement with simulation results. The presence of this second energy release site has important implications for models of solar eruptions. Title: Getting the Word Out: Undergradute Space Physics at Rice University Authors: Reiff, P. H.; Alexander, D. Bibcode: 2006AGUFMED52B..04R Altcode: At Rice University we emphasize space physics in our non-major Physics and Astronomy undergraduate classes in addition to our graduate and majors program. In "ASTR 202" (solar system exploration for non- majors), we typically use a textbook which includes magnetospheric and auroral topics in it (many do not). In recent years, we have also created two new courses for undergraduates which highlight space physics. In spring 2005 we began PHYS 401, The Physics of Ham Radio, which includes a significant portion on the Sun, ionosphere, radio propagation, and space storms. It is a fun hands-on way to learn about circuits, electrical theory, antennas, and the effects of space weather, while creating a new hobby at the same time. The students are required to attempt the FCC "Technician" exam as their midterm exam, and all of the class members passed. This course is taken both by undergraduates and by local teachers in the Master of Science Teaching program (the teacher tuition is partially supported by CISM), and is offered every other year (it will be offered again in Spring 2007). In fall 2005 one of us (Alexander) started a new course, ASTR 243 "Exploring the Sun-Earth Connection", which focuses entirely on solar and space weather topics. It required the students to perform several projects over the course of the semester, and used many online resources. The feedback from the first session was very favorable, so it also will likely be offered every other year. Two of the students extended their experience by participating in summer research, one at an REU at the National Solar Observatory working on helioseismology data, and one at an international summer school in the U.K. where she focused on coronal heating. Thus with two courses in an every-other-year rotation, each academic year one undergraduate course in space physics is available at Rice. Furthermore, all senior majors are required to perform research, and each year several students choose a solar or space physics topic for their senior research, and often go on to graduate study at schools around the nation. Sun-Earth course page: http://www.owlnet.rice.edu/~astr243/ Ham radio course page: http://space.rice.edu/PHYS401/ Title: Investigating the Temporal Behavior of Localized UV and Hard X-ray Emission in Large Solar Flares Authors: Coyner, Aaron J.; Alexander, D. Bibcode: 2006SPD....37.1302C Altcode: 2006BAAS...38..241C While many studies have related the timing of localized UV emission tospatially unresolved hard X-ray lightcurves, few have compared the timeprofiles of spatially localized UV and hard X-ray sources. In this workwe present a study of large two ribbon flares observed with RHESSI andTRACE in which multiple hard X-ray sources develop along the ribbonsduring the course of the flare. Our preliminary findings indicate thatlate in these multi-burst events, the UV emission onset precedes theX-ray emission with delays on scales of seconds. These observations havepotential implications for particle production and transport mechanismswithin complex flares. Title: A Brief History of CME Science Authors: Alexander, David; Richardson, Ian G.; Zurbuchen, Thomas H. Bibcode: 2006SSRv..123....3A Altcode: 2006SSRv..tmp...54A We present here a brief summary of the rich heritage of observational and theoretical research leading to the development of our current understanding of the initiation, structure, and evolution of Coronal Mass Ejections. Title: The Pre-CME Sun Authors: Gopalswamy, N.; Mikić, Z.; Maia, D.; Alexander, D.; Cremades, H.; Kaufmann, P.; Tripathi, D.; Wang, Y. -M. Bibcode: 2006SSRv..123..303G Altcode: 2006SSRv..tmp...77G The coronal mass ejection (CME) phenomenon occurs in closed magnetic field regions on the Sun such as active regions, filament regions, transequatorial interconnection regions, and complexes involving a combination of these. This chapter describes the current knowledge on these closed field structures and how they lead to CMEs. After describing the specific magnetic structures observed in the CME source region, we compare the substructures of CMEs to what is observed before eruption. Evolution of the closed magnetic structures in response to various photospheric motions over different time scales (convection, differential rotation, meridional circulation) somehow leads to the eruption. We describe this pre-eruption evolution and attempt to link them to the observed features of CMEs. Small-scale energetic signatures in the form of electron acceleration (signified by nonthermal radio bursts at metric wavelengths) and plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs. We survey these pre-eruptive energy releases using observations taken before and during the eruption of several CMEs. Finally, we discuss how the observations can be converted into useful inputs to numerical models that can describe the CME initiation. Title: Coronal Observations of CMEs. Report of Working Group A Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.; Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.; Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.; Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J. Bibcode: 2006SSRv..123..127S Altcode: 2006SSRv..tmp...58S CMEs have been observed for over 30 years with a wide variety of instruments. It is now possible to derive detailed and quantitative information on CME morphology, velocity, acceleration and mass. Flares associated with CMEs are observed in X-rays, and several different radio signatures are also seen. Optical and UV spectra of CMEs both on the disk and at the limb provide velocities along the line of sight and diagnostics for temperature, density and composition. From the vast quantity of data we attempt to synthesize the current state of knowledge of the properties of CMEs, along with some specific observed characteristics that illuminate the physical processes occurring during CME eruption. These include the common three-part structures of CMEs, which is generally attributed to compressed material at the leading edge, a low-density magnetic bubble and dense prominence gas. Signatures of shock waves are seen, but the location of these shocks relative to the other structures and the occurrence rate at the heights where Solar Energetic Particles are produced remains controversial. The relationships among CMEs, Moreton waves, EIT waves, and EUV dimming are also cloudy. The close connection between CMEs and flares suggests that magnetic reconnection plays an important role in CME eruption and evolution. We discuss the evidence for reconnection in current sheets from white-light, X-ray, radio and UV observations. Finally, we summarize the requirements for future instrumentation that might answer the outstanding questions and the opportunities that new space-based and ground-based observatories will provide in the future. Title: Temporal and Spatial Relationships between Ultraviolet and Hard X-Ray Emission in Solar Flares Authors: Alexander, David; Coyner, Aaron J. Bibcode: 2006ApJ...640..505A Altcode: Hard X-ray and ultraviolet emissions in solar flares provide evidence for the interaction of particles accelerated in the solar corona with the ambient plasma in the chromosphere. Previous studies have shown that these emissions are temporally correlated indicating a common source, presumably magnetic reconnection, governing the flare energy release. We use the high spatial resolution of the RHESSI and TRACE telescopes to explore the temporal connections of spatially resolved hard X-ray and UV sources in a flare occurring on 2002 July 16. While exhibiting a strong temporal correlation, there are distinct differences in the spatial distribution of the UV and hard X-ray emissions. The UV emission is found to be distributed in two well-defined ribbons spanning 50"-100" and separated by some 50"-70", while the hard X-ray emission is localized to a compact distribution at one end of the southernmost ribbon. We argue that the temporally correlated, but spatially well-separated, emissions result from the interaction of an evolving multipolar magnetic flux system with the flare energy release predominantly occurring along a separator marking the intersection of separate topological domains defined by an emerging bipolar system and the preexisting active region magnetic field. Title: An Introduction to the Pre-CME Corona Authors: Alexander, David Bibcode: 2006SSRv..123...81A Altcode: 2006SSRv..tmp....4A Coronal mass ejections provide a gateway to understanding the physics of energy release and conversion in the solar corona. While it is generally accepted that the energy required to power a CME is contained in the pre-eruption coronal magnetic field, the pre-CME state of that field and the conditions leading up to the release of the magnetic energy are still not entirely clear. Recent studies point to various phenomena which are common to many, if not all, CME events, suggesting that there may be identifiable characteristics of the pre-CME corona which signal the impending eruption. However, determining whether these phenomena are necessary or even sufficient has yet to be achieved. In this paper we attempt to summarize the state of the solar corona and its evolution in the build up to a CME. Title: A Brief History of CME Science Authors: Alexander, David; Richardson, Ian G.; Zurbuchen, Thomas H. Bibcode: 2006cme..book....3A Altcode: We present here a brief summary of the rich heritage of observational and theoretical research leading to the development of our current understanding of the initiation, structure, and evolution of Coronal Mass Ejections. Title: An Introduction to the pre-CME Corona Authors: Alexander, David Bibcode: 2006cme..book...81A Altcode: Coronal mass ejections provide a gateway to understanding the physics of energy release and conversion in the solar corona. While it is generally accepted that the energy required to power a CME is contained in the pre-eruption coronal magnetic field, the pre-CME state of that field and the conditions leading up to the release of the magnetic energy are still not entirely clear. Recent studies point to various phenomena which are common to many, if not all, CME events, suggesting that there may be identifiable characteristics of the pre-CME corona which signal the impending eruption. However, determining whether these phenomena are necessary or even sufficient has yet to be achieved. In this paper we attempt to summarize the state of the solar corona and its evolution in the build up to a CME. Title: Role of Sunspot and Sunspot-Group Rotation in Driving Sigmoidal Active Region Eruptions Authors: Tian, Lirong; Alexander, David Bibcode: 2006SoPh..233...29T Altcode: We study active region NOAA 9684 (N06L285) which produced an X1.0/3B flare on November 4, 2001 associated with a fast CME (1810 km s−1) and the largest proton event (31 700 pfu) in cycle 23. SOHO/MDI continuum image data show that a large leading sunspot rotated counter-clockwise around its umbral center for at least 4 days prior to the flare. Moreover, it is found from SOHO/MDI 96 m line-of-sight magnetograms that the systematic tilt angle of the bipolar active region, a proxy for writhe of magnetic fluxtubes, changed from a positive value to a negative one. This signifies a counter-clockwise rotation of the spot-group as a whole. Using vector magnetograms from Huairou Solar Observing Station (HSOS), we find that the twist of the active region magnetic fields is dominantly left handed (αbest = −0.03), and that the vertical current and current helicity are predominantly negative, and mostly distributed within the positive rotating sunspot. The active region exhibits a narrow inverse S-shaped Hα filament and soft X-ray sigmoid distributed along the magnetic neutral line. The portion of the filament which is most closely associated with the rotating sunspot disappeared on November 4, and the corresponding portion of the sigmoid was observed to erupt, producing the flare and initiating the fast CME and proton event. These results imply that the sunspot rotation is a primary driver of helicity production and injection into the corona. We suggest that the observed active region dynamics and subsequent filament and sigmoid eruption are driven by a kink instability which occurred due to a large amount of the helicity injection. Title: Coronal Observations of CMEs Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.; Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.; Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.; Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J. Bibcode: 2006cme..book..127S Altcode: CMEs have been observed for over 30 years with a wide variety of instruments. It is now possible to derive detailed and quantitative information on CME morphology, velocity, acceleration and mass. Flares associated with CMEs are observed in X-rays, and several different radio signatures are also seen. Optical and UV spectra of CMEs both on the disk and at the limb provide velocities along the line of sight and diagnostics for temperature, density and composition. From the vast quantity of data we attempt to synthesize the current state of knowledge of the properties of CMEs, along with some specific observed characteristics that illuminate the physical processes occurring during CME eruption. These include the common three-part structures of CMEs, which is generally attributed to compressed material at the leading edge, a low-density magnetic bubble and dense prominence gas. Signatures of shock waves are seen, but the location of these shocks relative to the other structures and the occurrence rate at the heights where Solar Energetic Particles are produced remains controversial. The relationships among CMEs, Moreton waves, EIT waves, and EUV dimming are also cloudy. The close connection between CMEs and flares suggests that magnetic reconnection plays an important role in CME eruption and evolution. We discuss the evidence for reconnection in current sheets from white-light, X-ray, radio and UV observations. Finally, we summarize the requirements for future instrumentation that might answer the outstanding questions and the opportunities that new space-based and ground-based observatories will provide in the future. Title: The Pre-CME Sun Authors: Gopalswamy, N.; Mikić, Z.; Maia, D.; Alexander, D.; Cremades, H.; Kaufmann, P.; Tripathi, D.; Wang, Y. -M. Bibcode: 2006cme..book..303G Altcode: The coronal mass ejection (CME) phenomenon occurs in closed magnetic field regions on the Sun such as active regions, filament regions, transequatorial interconnection regions, and complexes involving a combination of these. This chapter describes the current knowledge on these closed field structures and how they lead to CMEs. After describing the specific magnetic structures observed in the CME source region, we compare the substructures of CMEs to what is observed before eruption. Evolution of the closed magnetic structures in response to various photospheric motions over different time scales (convection, differential rotation, meridional circulation) somehow leads to the eruption. We describe this pre-eruption evolution and attempt to link them to the observed features of CMEs. Small-scale energetic signatures in the form of electron acceleration (signified by nonthermal radio bursts at metric wavelengths) and plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs. We survey these pre-eruptive energy releases using observations taken before and during the eruption of several CMEs. Finally, we discuss how the observations can be converted into useful inputs to numerical models that can describe the CME initiation. Title: High Spectral Resolution Cryogenic Imaging Detectors for Solar Physics Authors: Stern, R.; Martinez-Galarce, D.; Rausch, A.; Shing, L.; Deiker, S.; Boerner, P.; Metcalf, T.; Cabrera, B.; Leman, S.; Brink, P.; Irwin, K.; Ullom, J.; Alexander, D. Bibcode: 2005AGUFMSH41B1130S Altcode: X-ray microcalorimeters have the potential to substantially improve our understanding of magnetic reconnection in the early phases of flares or during microflare events. Operating in the sub-Kelvin temperature range, they provide high non-dispersive energy resolution at ~ keV energies (e.g, E/Δ E ~ 1500 at 6 keV), coupled with high (msec or better) time resolution. Coupled with grazing-incidence X-ray optics, microcalorimeter arrays or position sensitive readout schemes will form the basis for a new generation of solar imaging spectrometers. An Explorer-class solar mission within the next 5-10 years, based upon these detectors, is rapidly becoming technically feasible. LMSAL currently has both internally and NASA funded laboratory research programs to investigate TES (Transition Edge Sensor) microcalorimeters. In this presentation, we will discuss the current status of these programs and their applicability to future Explorer missions and Roadmap missions such as RAM. Title: MTRAP: the magnetic transition region probe Authors: Davis, J. M.; West, E. A.; Moore, R. L.; Gary, G. A.; Kobayashi, K.; Oberright, J. E.; Evans, D. C.; Wood, H. J.; Saba, J. L. R.; Alexander, D. Bibcode: 2005SPIE.5901..273D Altcode: The Magnetic Transition Region Probe is a space telescope designed to measure the magnetic field at several heights and temperatures in the solar atmosphere, providing observations spanning the chromospheric region where the field is expected to become force free. The primary goal is to provide an early warning system (hours to days) for solar energetic particle events that pose a serious hazard to astronauts in deep space and to understand the source regions of these particles. The required magnetic field data consist of simultaneous circular and linear polarization measurements in several spectral lines over the wavelength range from 150 to 855 nm. Because the observations are photon limited an optical telescope with a large (>18m2) collecting area is required. To keep the heat dissipation problem manageable we have chosen to implement MTRAP with six separate Gregorian telescopes, each with ~ 3 m2 collecting area, that are brought to a common focus. The necessary large field of view (5 × 5 arcmin2) and high angular resolution (0.025 arcsec pixels) require large detector arrays and, because of the requirements on signal to noise (103), pixels with large full well depths to reduce the readout time and improve the temporal resolution. The optical and engineering considerations that have gone into the development of a concept that meets MTRAP's requirements are described. Title: The Role of the Kink Instability of a Long-Lived Active Region AR 9604 Authors: Tian, Lirong; Liu, Yang; Yang, Jing; Alexander, David Bibcode: 2005SoPh..229..237T Altcode: We have traced the long-term evolution of a non-Hale active region composed of NOAA 9604-9632-9672-9704-9738, which displayed strong transient activity with associated geomagnetic effects from September to December, 2001. By studying the development of spot-group and line-of-sight magnetic field together with the evolution of Hα filaments, the EUV and X-ray corona (TRACE 171 Å, Yohkoh/SXT), we have found that the magnetic structure of the active region exhibited a continuous clockwise rotation throughout its entire life. Vector magnetic data obtained from Huairou Solar Observing Station (HSOS) and full-disk line-of-sight magnetograms from SOHO/MDI allowed the determination of the best-fit force-free parameter (proxy of twist), αbest, and the systematic tilt angle (proxy of writhe) which were both found to take positive values. Soft X-ray coronal loops from Yohkoh/SXT displayed a pronounced forward-sigmoid structure in period of NOAA 9704. These observations imply that the magnetic flux tube (loops) with the same handedness (right) of the writhe and the twist rotated clockwise in the solar atmosphere for a long time. We argue that the continuous clockwise rotation of the long-lived active region may be a manifestation that a highly right-hand twisted and kinked flux tube was emerging through the photosphere and chromosphere into the corona. Title: Magnetic Twist and Writhe of δ Active Regions Authors: Tian, Lirong; Alexander, David; Liu, Yang; Yang, Jing Bibcode: 2005SoPh..229...63T Altcode: We have selected 104 active regions with a δ magnetic configuration from 1996 to 2002 to study how important a role the kink instability plays in such active regions. In this study, we employ the systematic tilt angle of each active region as a proxy for the writhe of a flux tube and the force-free parameter, αbest, as a proxy for the magnetic field twist in the flux tube. It is found that 65-67% of the active regions have the same sign of twist and writhe. About 34% (32%) of the active regions violate (follow) the Hale-Nicholson and Joy's Laws (HNJL) but follow (violate) the hemispheric helicity rule (HHR). Sixty-one (61) of the 104 active regions studied each produced more than five large flares. Active regions violating HNJL, but following HHR, have a much stronger tendency to produce X-class flares and/or strong proton events. Comparing with previous studies for active regions with well-defined (simpler) bipolar magnetic configuration, it is found that the numbers following both HNJL and HHR are significantly lower in the δ-configuration case, while numbers violating one of the laws and the rule significantly increase with the increase of the magnetic complexity of the active regions. These results support the prediction for the presence of a kink instability, that the twist and writhe of the magnetic fields exhibit the same sign for δ active regions (Linton et al., Astrophys. J.507, 40, 1998, Astrophys. J.522, 1205, 1999; Fan et al., Astrophys. J.521, 460, 1999). Finally, we analyze possible origins of the twist and writhe of the magnetic fields for the active regions studied. Title: Cryogenic 3-D Detectors for Solar Physics Authors: Stern, R. A.; Martinez-Galarce, D.; Rausch, A.; Shing, L.; Deiker, S.; Boerner, P.; Metcalf, T.; Cabrera, B.; Leman, S. W.; Brink, P.; Irwin, K.; Alexander, D. Bibcode: 2005AGUSMSP12A..02S Altcode: Cryogenic microcalorimeters operating in the sub-Kelvin temperature range provide non-dispersive energy resolution at optical through gamma ray energies (e.g, E/Δ E ~ 1500 at 6 keV). Microcalorimeters also have high time resolution (msec or better), and can be made into imaging arrays through SQUID multiplexing of individual pixels or employing position sensitive detector structures. The application of such "3-D" detector technology to solar physics will lead to significant advances in our understanding of magnetic reconnection in the Sun, including X-ray jet phenomena, and active region heating and dynamics. An Explorer-class solar mission within the next 5-10 years, based upon these detectors, is rapidly becoming technically feasible. LMSAL currently has an internally funded laboratory research program to investigate TES (Transition Edge Sensor) microcalorimeters; we recently saw our first X-ray photons using TES detectors supplied by NIST. In addition, we have recently been funded by NASA to begin work with NIST on position-sensitive X-ray strip detectors for solar physics applications. Finally, we are collaborating with with Stanford and NIST on a solar sounding rocket. In this presentation, we will discuss the current status of these programs and their applicability to future Explorer missions and Roadmap missions such as RAM. Title: The Characteristics of Hard X-ray Production in Flares Driven by Filament Eruptions Authors: Liu, R.; Alexander, D. Bibcode: 2005AGUSMSP23B..04L Altcode: We investigate the temporal and spatial relationship between filament eruptions and the production of hard X-ray emission using spatially resolved high cadence data from TRACE and RHESSI. In particular, we focus on comparing the characteristics of the hard X-ray production in `successful' and `failed' filament eruption cases. Our preliminary findings indicate even failed eruption events can generate significant energy release and hard X-ray emission with the hard X-ray production apparently correlated to the rate of expansion of the filament. The spatial distribution of the hard X-ray emission, while depending upon the overall strength of the event, also depends on the evolutionary behavior of the filament as it erupts, e.g. loop-like versus "zipper"-like. Title: Photon-Flux Saturation in Large Solar Flares Authors: Daou, A. G.; Alexander, D. Bibcode: 2005AGUSMSP21A..06D Altcode: We use the spectral and spatial resolution of RHESSI to explore the behavior of electron fluxes and their associated currents in large solar flares, including the Halloween events of October-November 2003. The incident electron spectra at the flaring footpoints are derived from the RHESSI photon spectra. Spectral images are used to determine an upper limit as well as a spectrally-averaged estimation for the footpoint area. We find that, over a wide range of flare X-ray magnitudes, the integrated fluxes above 20 keV asymptotically approach a limiting value, suggesting some form of saturation in the particle production in flares. We discuss the implications of these results in terms of the assumed model of a stable non-thermal electron beam. Title: Magnetic Twist and Writhe of Delta Active Regions Authors: Tian, L.; Alexander, D.; Liu, Y.; Yang, J. Bibcode: 2005AGUSMSH54B..03T Altcode: Active regions with a δ magnetic configuration from 1996 to 2002 were selected to study how important a role the kink instability plays in such active regions. We employ the systematic tilt angle of each active region as a proxy for the writhe of a fluxtube and the force-free parameter, αbest, as a proxy for the magnetic field twist in the fluxtube. It is found that 65-67% of the 104 active regions have the same sign of twist and writhe, which violate the Hale-Nicholson and Joy's Laws (HNJL) or the hemispheric helicity rule (HHR). 68% (46/68) of these active regions produced more than five large flares. Active regions violating HNJL, but following HHR, have a much stronger tendency to produce X-class flares and/or strong proton events. Continuously clockwise rotation of magnetic configuration of a long-lived active region (AR 9604-9632-9672-9704-9738) which produced major flares, fast CMEs and many strong proton events shows that a kink instability would play very important role in such active regions. These results support the prediction for the presence of a kink instability, that the twist and writhe of the magnetic fields exhibit the same sign for δ active regions (Linton et al, 1998, 1999, and Fan et al., 1999). Finally, we analyze possible origins of the twist and writhe of the magnetic fields for the active regions with different relations between the twist and writhe. Title: Active region evolution in the build-up to large solar energetic particle events Authors: Alexander, D.; Coyner, A.; Daou, A.; Liu, R. Bibcode: 2005AGUSMSH53A..02A Altcode: The SEP signatures of the solar flares occurring on 21 Apr 2002 and 24 Aug 2002 show marked differences in their compositions above 10 MeV/nucleon, yet at the Sun the events themselves display very similar characteristics in the chromospheric (hard X-ray) emission and their coronal (X-ray, EUV) signatures. We consider the prior evolution of the parent active regions to look for possible signatures in the ambient corona, magnetic connectivities (within the active region, large-scale closed field, and open field distributions), and flare/activity history which may differentiate the solar conditions leading to the observed disparate particle signatures at 1 AU. Title: Temporal Evolution of Hard X-ray and UV Emission Sources in Solar Flares Authors: Coyner, A. J.; Alexander, D. Bibcode: 2005AGUSMSP21A..03C Altcode: In this work, we investigate, for a number of solar flares, the evolution of both UV hard x-ray emission as observed by TRACE and RHESSI, respectively. The spatial resolution of these two instruments provides an unprecedented capability to study the individual source regions producing the observed time profiles in these events. Confirming earlier results, it is found that individual bursts in the hard X-ray time profiles are correlated with the unresolved UV emission. However, the temporal evolution of individual sources provides a deeper insight into the flare energization process. We report a number of results: 1) the UV-to-UV temporal correlation analysis allows us to identify sources which exhibit similar time profiles and therefore may indicate conjugate footpoints of a large coronal loop, 2) the UV emission is distributed over a wider area than the hard X-ray emission, 3) the bulk of the hard X-ray emission would seem to be related to a different structure within the flaring volume but sharing a common footpoint with the UV emission, and 4) the temporal and spatial behavior observed would seem to indicate an interacting loop scenario for the flares considered. Title: Multialtitude Observations of a Coronal Jet during the Third Whole Sun Month Campaign Authors: Ko, Y. -K.; Raymond, J. C.; Gibson, S. E.; Alexander, D.; Strachan, L.; Holzer, T.; Gilbert, H.; Cyr, O. C. St.; Thompson, B. J.; Pike, C. D.; Mason, H. E.; Burkepile, J.; Thompson, W.; Fletcher, L. Bibcode: 2005ApJ...623..519K Altcode: On 1999 August 26, a coronal jet occurred at the northwest limb near a sigmoid active region (AR 8668) that was the target for a joint observation plan (SOHO joint observing program 106) during the third Whole Sun Month Campaign. This jet was observed by several instruments at the limb (SOHO/CDS, SOHO/EIT, TRACE, and Mauna Loa Solar Observatory CHIP and PICS) and at 1.64 Rsolar (SOHO/UVCS). At the limb, this jet event displayed both low- and high-temperature components. Both high- and low-temperature components were evident during the early phase (first 20 minutes) of the event. However, the low-temperature component is maintained for ~1 hr after the higher temperature component is gone. There is a second brightening (a possible second jet) seen by EIT and TRACE about 50 minutes after the onset of the first jet. The line-of-sight motion at the limb began with a 300 km s-1 redshift and evolved to a 200 km s-1 blueshift. At 1.64 Rsolar, the intensities of Lyα and Lyβ in the jet increased by a factor of several hundred compared with the background corona. The C III λ977 line also brightened significantly. This indicates low-temperature [~(1-2)×105 K] emission in the jet, while the intensities of O VI λ1032 and O VI λ1037 increased by as much as a factor of 8. The UVCS data show evidence of heating at the early phase of the event. The Doppler shift in the lines indicates that the line-of-sight (LOS) velocity in the jet started from ~150 km s-1 in blueshift and ended at ~100 km s-1 in redshift. This LOS motion seen at 1.64 Rsolar was apparently opposite to what was observed when the jet emerged from the limb. The Doppler dimming analysis indicates that the radial outflow speed correlates with the magnitude of the LOS speed. Interestingly, UVCS observations at 2.33 and 2.66 Rsolar show no trace of the jet and SOHO/LASCO observations also yield no firm detection. We find that a simple ballistic model can explain most of the dynamical properties of this jet, while the morphology and the thermal properties agree well with reconnection-driven X-ray jet models. Title: Fokker-Planck Modeling of Asymmetric Footpoint Hard X-Ray Emission in Solar Flares Authors: McClements, K. G.; Alexander, D. Bibcode: 2005ApJ...619.1153M Altcode: Chromospheric hard X-ray emission in a solar flare generally occurs in two magnetically connected ``footpoint'' regions. Recent spatially resolved hard X-ray observations carried out using the RHESSI spacecraft have shown that the ratio of total X-ray fluxes from the two footpoints is time dependent and demonstrates a weak but detectable photon energy dependence. A Fokker-Planck code is used to identify possible scenarios that could reproduce the observed dependence of footpoint asymmetry on time and energy. The code, which is benchmarked against analytical results in the limit of collisionless precipitation from a symmetric flaring loop, includes collisional friction and pitch-angle scattering, asymmetric magnetic mirroring, and a source term that can be prescribed arbitrarily. This model is used to demonstrate that the observed dependence of hard X-ray asymmetry on photon energy can be attributed to an energetic electron source that is isotropic at low energy (presumed to be due to Coulomb collisions) and at high energy (presumed to be due to resonant wave-particle scattering) and strongly anisotropic at intermediate energies. Title: STEREO/SECCHI Simulations of CMEs and Flares using TRACE Images Authors: Aschwanden, M. J.; Lemen, J.; Nitta, N.; Metcalf, T.; Wuelser, J.; Alexander, D. Bibcode: 2004AGUFMSH22A..02A Altcode: We simulate 3D models of EUV images of flare and CME events, using TRACE EUV movies. TRACE movies show 2D images in projection along a particular line-of-sight. We simulate 3D models of erupting filaments, flare loops, and postflare loops using: (1) a ``finger printing'' technique to trace linear structures in 2D images; (2) geometric 3D models based on force-free fields and curvature radius maximization of flare loop and flux rope structures; (3) conservation of velocity and acceleration parameters; (4) multi-temperature plasma filling according to hydrodyamic scaling laws; and (5) 2D projections from secondary line-of-sights that correspond to viewpoints of the secondary STEREO spacecraft. From such simulations we envision to illustrat 3D time-dependent models, what would be observed at the two STEREO spacecraft positions as well as from a near-Earth spacecraft such as SoHO. These simulations are used to test STEREO analysis software and to investigate what physical parameters and geometric 3D reconstructions can be retrieved from STEREO/SECCHI data. Title: ``Transition-Edge Sensors for Solar X-ray Spectral Observations - An Update'' Authors: Martinez-Galarce, D. S.; Stern, R.; Cabrera, B.; Deiker, S.; Alexander, D.; Metcalf, T.; Irwin, K.; Brink, P.; Boerner, P. F.; Morse, K.; Leman, S.; Shing, L.; Rausch, A.; Nichols, T. D.; Chakraborty, S. Bibcode: 2004AGUFMSH13A1147M Altcode: ABSTRACT: The advent of cryogenic microcalorimeters (operating at temperatures of ∼ 0.1 K) in ground-based and space-based astronomy promises a revolution of new discoveries. Particularly, Transition-Edge Sensors (TES) have demonstrated high-energy resolution measurements of soft X-rays of up to E/Δ E ∼ 1500 (at 6keV) and with high temporal resolution of a msec or less in photon pulse detections. Fabricated into multiplexed arrays of single detectors, or position sensing macropixels, and placed at the focus of a Wolter optic would further yield high spatial resolution capability of 2 arcsec or less, thus producing unprecedented "3-D" solar observations. We report herein, on progress to date in the development of these detectors and particularly, with on-going work with the sounding rocket payload, the Advanced Technology Solar Spectroscopic Imager, which will debut a TES instrument operating in the 300eV - 1.5keV range to study active region magnetic reconnection. Furthermore, as part of our larger effort, we discuss also current technical developments and plans at the Lockheed Martin Solar & Astrophysics Laboratory to design a TES instrument (3 - 8keV range) for realization into a dedicated Explorer-class solar observatory in the next 5 - 10 years. Title: Tomographic 3D-Modeling of the Solar Corona with FASR Authors: Aschwanden, Markus J.; Alexander, David; de Rosa, Marc L. Bibcode: 2004ASSL..314..243A Altcode: 2003astro.ph..9501A The Frequency-Agile Solar Radiotelescope (FASR) literally opens up a new dimension, in addition to the 3D Euclidian geometry—the frequency dimension. The 3D geometry is degenerated to 2D in all images from astronomical telescopes, but the additional frequency dimension allows us to retrieve the missing third dimension by means of physical modeling. We call this type of 3D reconstruction Frequency Tomography. In this study we simulate a realistic 3D model of an active region, composed of 500 coronal loops with the 3D geometry [x(s), y(s), z(s)] constrained by magnetic field extrapolations and the physical parameters of the density ne(s) and temperature Te(s) given by hydrostatic solutions. We simulate a series of 20 radio images in a frequency range of ν=0.1-10 GHz, anticipating the capabilities of FASR, and investigate what physical information can be retrieved from such a dataset. We discuss also forward-modeling of the chromospheric and Quiet Sun density and temperature structure, another primary goal of future FASR science. Title: Hard X-ray Footpoint Asymmetries in Solar Flares Authors: Chollet, E.; Alexander, D.; Metcalf, T. R.; Pollack, L. Bibcode: 2004AAS...204.5406C Altcode: 2004BAAS...36R.758C RHESSI observations of a M4.0 solar flare on 17 March, 2002 are presented. This flare exhibits footpoint asymmetries in hard X-ray emission, allowing us to study the energy and time dependence of particle transport in this flare. The data, originally presented in Alexander and Metcalf (2002), is reanalyzed with new calibrations, shorter integration times and better spatial resolution, allowing for more accurate determination of the time and energy dependence of the asymmetries. While the previous work suggested that this was a simple flare with two footpoints, the new analysis suggests a more complex structure with three or more footpoints, confirmed by force free magnetic field extrapolations. The time development of the evolving flare indicates a transition from one flaring structue to another which significantly complicates the interpretation of the measured asymmetry. The implications of this new interpretation for the previously reported energy dependence will be discussed. We gratefully acknowledge support from NASA (NAS5-02048). Title: Spatial and temporal correlations between UV continuum and hard x-ray emissions in solar flares Authors: Coyner, A. J.; Alexander, D. Bibcode: 2004AAS...204.5405C Altcode: 2004BAAS...36Q.758C Combined UV and hard x-ray observations provide a means to better understanding the temporal and spatial relationship between the energization of the solar corona in flares and the response of the solar chromosphere. In this work, we investigate the evolution of an X1.1 solar flare occurring on 2003 Oct 19 at both UV and hard x-ray wavelengths. This is accomplished through analysis of data from the TRACE and RHESSI spacecraft, respectively. Uncertainties in the TRACE pointing were corrected by coaligning TRACE images with EIT images of the same wavelength for the same time. Coincident RHESSI hard X-ray images were generated in 5keV steps over the range from 10-80 keV. We compare the spatial and temporal evolution of the UV and hard X-ray emissions, for each hard X-ray energy bin, to determine the relationship, if any, between the hard x-ray flare footpoints and the UV continuum emission. We gratefully acknowledge the support of NASA under contract NAS5-02048. Title: Multi-Altitude Observations of a Coronal Jet Authors: Ko, Y. -K.; Raymond, J. C.; Gibson, S. E.; Alexander, D.; Strachan, L.; Holzer, T.; Gilbert, H.; St. Cyr, O. C.; Thompson, B. J.; Pike, C. D.; Burkepile, J.; Thompson, W.; Fletcher, L. Bibcode: 2004AAS...204.5413K Altcode: 2004BAAS...36..759K A coronal jet occurred on August 26, 1999 at the NW limb near a sigmoid active region (AR8668). This jet was observed by several instruments at the limb (SOHO/CDS, SOHO/EIT, TRACE, MLSO/CHIP, MLSO/PICS) and at 1.64 Ro (SOHO/UVCS). At the limb, this jet event has both low and high temperature components. The high temperature component appeared at the early phase (first 20 minutes) of the event along with the low temperature component while the latter seems to last long ( ∼ 1 hour) after the higher temperature component was gone. The line-of-sight motion at the limb started with red-shifted (by as much as 300 km/s) and turned blue-shifted (by as much as 200 km/s). At 1.64 Ro, the intensities of Lyα , Lyβ in the jet increased by a factor of several hundreds compared with the background corona. C III λ 977 line also brightened significantly. This indicates low temperature ( ∼ 1-2× 105 K) emission in the jet, while the intensities of O VI λ 1032 and O VI λ 1037 increased by a factor of as large as 8. Both UVCS and CDS data show evidence of heating at the early phase of the event. The line-of-sight velocity seen at 1.64 Ro started with ∼ 150 km/sec in blue shift and ended at ∼ 100 km/sec in red shift. This is apparently opposite to what were observed when the jet emerged from the limb. The Doppler dimming analysis indicates that the radial outflow speed correlates with the magnitude of the line-of-sight speed. Interestingly, UVCS observations at 2.33 and 2.66 Ro show no trace of the jet and LASCO observations also yield no firm sight of the jet. In this paper, we present the observations by these instruments and discuss the dynamical structure and physical properties of this jet. Y.-K. Ko acknowledges the support by NASA grant NAG5-12865. Title: Non-Relativistic Electron Beam Stability in Solar Flares Authors: Daou, A. G.; Alexander, D.; Metcalf, T. R. Bibcode: 2004AAS...204.0203D Altcode: 2004BAAS...36..667D The thick-target electron beam model has been used for decades as a viable description for the production of solar flare hard X-ray emission. The required very rapid transport of energy to the footpoints during solar flares is achieved in this model by fast electrons traversing the loop to deposit their energy in the dense chromosphere. For some of the largest flares the currents (up to 1017 Amps or 1036 electrons/sec) inferred can significantly exceed the Alfven-Lawson limit suggesting that the assumed electron beam is inherently unstable. In this paper, we use the spectral and spatial resolution of RHESSI to explore whether the conditions for a stable non-thermal electron beam exist in large solar flares. The incident electron spectra at flaring footpoints are derived from the RHESSI photon spectra while an upper limit to the footpoint area is detremined form the hard X-ray images. We determine the electron beam density needed to produce the hard X-ray emission for two large flares, July 17, 2002 (M8.5) and October 28, 2003 (X17.2) and determine whether such beams are viable in these cases.

We gratefully acknowledge support from NASA (NAS5-02048). Title: Fokker Planck Modelling of Asymmetric Footpoint Hard X-ray Emission in Solar Flares Authors: Alexander, D.; McClements, K. G. Bibcode: 2004AAS...204.5404A Altcode: 2004BAAS...36..758A Chromospheric hard X-ray emission in a solar flare generally occurs in two magnetically connected 'footpoint' regions. Recent spatially-resolved hard X-ray observations carried out using the RHESSI spacecraft have shown that the ratio of total X-ray fluxes from two such footpoints is time-dependent and demonstrates a weak but detectable energy-dependence. We use a Fokker-Planck code to identify possible scenarios that could reproduce the observed dependence of footpoint asymmetry on time and energy. The code, which we have benchmarked against analytical results in the limit of collisionless precipitation from a symmetric flaring loop, includes collisional friction and pitch angle scattering, asymmetric magnetic mirroring, and a source term that can be prescribed arbitrarily. Using this model, we examine in particular whether the observed imbalance in hard X-ray emission from the two footpoints in a flare can be attributed primarily to asymmetry in the loop magnetic structure or asymmetry in the acceleration process.

This work was funded partly by NASA (NAS5-02048) and by the United Kingdom Engineering and Physical Sciences Research Council. Title: Solar Week: Learning from Experience Authors: Alexander, D.; Hauck, K. Bibcode: 2003AGUFMED51C1209A Altcode: Solar Week is a week-long set of games and activities allowing students to interact directly with solar science and solar scientists. Solar Week was developed as a spin-off of the highly successful Yohkoh Public Outreach Project (YPOP). While YPOP provided access to solar images, movies and activities, the main goal of Solar Week was to enhance the participation of women, who are under-represented in the physical sciences. Solar Week achieves this by providing young women, primarily in grades 6-8, with access to role models in the sciences. The scientists participating in Solar Week are women from a variety of backgrounds and with a variety of scientific expertise. In this paper, our aim is to provide some insight into developing activity-based space science for the web and to discuss the lessons-learned from tailoring to a specific group of participants. Title: Cryogenic 3-D Detectors for Solar Physics Using Position Sensitive Transition-Edge Sensors Authors: Stern, R. A.; Martinez-Galarce, D.; Metcalf, T.; Lemen, J.; Cabrera, B.; Brink, P.; Leman, S.; Deiker, S.; Irwin, K.; Alexander, D. Bibcode: 2003AAS...203.1805S Altcode: 2003BAAS...35Q1237S Space and ground-based astronomy is currently undergoing a revolution in detector technology with the advent of cryogenic sensors operating in the sub-Kelvin temperature range. These detectors provide non-dispersive energy resolution at optical through gamma ray energies (e.g, E/Δ E ∼ 1500 at 6 keV), high time resolution (msec or better), and can be made into imaging arrays through SQUID multiplexing of individual pixels or employing macropixels with position sensing capability. The application of such ``3-D'' detector technology to solar physics will lead to significant advances in our understanding of magnetic reconnection phenomena in the Sun, including flares and microflares, X-ray jets, and active region dynamics. An Explorer-class solar mission based upon these detectors is a distinct possibility within the next 5-10 years. In this poster, we will describe some of the recent advances in cryogenic detector technology with particular applicability to solar physics, and future technical developments required to make such a mission a reality.

This work was supported in part by the Lockheed Martin Independent Research Program. Title: TRACE and Yohkoh Observations of a White-Light Flare Authors: Metcalf, Thomas R.; Alexander, David; Hudson, Hugh S.; Longcope, Dana W. Bibcode: 2003ApJ...595..483M Altcode: We present observations of a large solar white-light flare observed on 2001 August 25, using data from the Transition Region and Coronal Explorer (TRACE) white-light channel and Yohkoh/HXT. These emissions are consistent with the classic type I white-light flare mechanism, and we find that the enhanced white-light emission observed by TRACE originates in the chromosphere and temperature minimum regions via nonequilibrium hydrogen ionization induced by direct collisions with the electron beam and by back-warming of the lower atmosphere. The three flare kernels observed in hard X-rays and white light are spatially associated with magnetic separatrices, and one of the kernels is observed to move along a magnetic separatrix at 400 km s-1. This is evidence in favor of particle acceleration models, which energize the electrons via magnetic reconnection at magnetic separators. Title: Observations of Rotating Sunspots from TRACE Authors: Brown, D. S.; Nightingale, R. W.; Alexander, D.; Schrijver, C. J.; Metcalf, T. R.; Shine, R. A.; Title, A. M.; Wolfson, C. J. Bibcode: 2003SoPh..216...79B Altcode: Recent observations from TRACE in the photospheric white-light channel have shown sunspots that rotate up to 200° about their umbral centre over a period of 3-5 days. The corresponding loops in the coronal fan are often seen to twist and can erupt as flares. In an ongoing study, seven cases of rotating sunspots have been identified, two of which can be associated with sigmoid structures appearing in Yohkoh/SXT and six with events seen by GOES. This paper analyzes the rotation rates of the sunspots using TRACE white-light data. Observations from AR 9114 are presented in detail in the main text and a summary of the results for the remaining six sunspots is presented in Appendixes A-F. Discussion of the key results, particularly common features, are presented, as well as possible mechanisms for sunspot rotation. Title: TRACE, SOHO/EIT, and SOHO/MDI Observations of AR0030, Including Rotating Sunspots and the July 15, 2002 X3.0 Flare in Ultraviolet and Extreme Ultraviolet Authors: Nightingale, R. W.; Shine, R. A.; Alexander, D.; Freeland, S. L.; Frank, Z. A.; Brown, D. S. Bibcode: 2002AGUFMSH52A0467N Altcode: On July 15, 2002 TRACE and several SOHO instruments observed an X3.0 flare in AR0030 near 2000 UT. During this period TRACE was primarily observing in its 1600Å ultraviolet (UV) channel (most sensitive to temperatures around 100,000 K in the flare). The 195Å extreme ultraviolet (EUV) channel of SOHO/EIT (which is most sensitive to about 1.6 MK) will be utilized in this poster, in addition to the magnetic field measurements of SOHO/MDI during this event period. TRACE followed the active region for over 10 days, starting about 4 days before the flare. Broadband white light TRACE images of the photosphere indicate that one or more of the sunspots were rotating, a possible precursor to the flare. Images and movies of AR0030 in the various wavelengths will be shown. The flare region was so intense in the TRACE UV that it is very difficult to show both the quiescent and flaring regions, so the UV movie will focus on the flaring plasma with its 2 eruptions. In the EIT EUV, more coronal structure away from the flare can be seen. Analysis of the rotational rates of the sunspots will be given along with their possible coupling to the flare. This work was supported by NASA under contract NAS5-38099. Title: Energy dependence of electron trapping in a solar flare Authors: Alexander, David; Metcalf, Thomas R. Bibcode: 2002SoPh..210..323A Altcode: Observations of an energy-dependent asymmetry in footpoint hard X-ray emission by RHESSI for the M4.0 solar flare of 17 March 2002 allows us to probe the dynamics of particle transport with energy and time. The presence of such an asymmetry is most readily explained by the effects of a converging magnetic field with different rates of convergence at the different footpoints, as would be expected from realistic surface field distributions. Such a geometry has been discussed in the context of a trap-plus-precipitation model where the transport of energetic particles in the flare is governed by the precipitation out of the coronal trap via collisions, wave-particle interactions or some other scattering process, into the high-density chromosphere. Comparison of RHESSI observations with a trap-plus-precipitation model allows us to use the energy dependence of the asymmetry and the observed ratio of footpoint to coronal emission at the different energies to assess the role of the trapping in the transport of energetic electrons and to probe the nature of the particle precipitation process inside the loss cone. Title: Complete Models of Axisymmetric Sunspots: Magnetoconvection with Coronal Heating Authors: Hurlburt, Neal E.; Alexander, David; Rucklidge, Alastair M. Bibcode: 2002ApJ...577..993H Altcode: We present detailed results of numerical experiments into the nature of complete sunspots. The models remain highly idealized but include fully nonlinear compressible magnetoconvection in an axisymmetric layer that drives energy into an overlying, low-β plasma. We survey a range of parameters in which the resulting magnetoconvection displays the formation of pore- and sunspot-like behavior and assess the coronal signatures resulting from the energy generated by the magnetoconvection. The coronal heating is assumed to be a result of the dissipation by an unspecified means of a fraction of the Poynting flux entering the corona. The expected signatures in the EUV and soft X-ray bandpasses of the Transition Region and Coronal Explorer and Yohkoh/SXT, respectively, are examined. This ad hoc coupling of the corona to the subphotospheric region results in a dynamical behavior that is consistent with recent observational results. This agreement demonstrates that even simple coupled modeling can lead to diagnostics for investigations of both subphotospheric sunspot structures and coronal heating mechanisms. Title: The Structure and Evolution of a Sigmoidal Active Region Authors: Gibson, S. E.; Fletcher, L.; Del Zanna, G.; Pike, C. D.; Mason, H. E.; Mandrini, C. H.; Démoulin, P.; Gilbert, H.; Burkepile, J.; Holzer, T.; Alexander, D.; Liu, Y.; Nitta, N.; Qiu, J.; Schmieder, B.; Thompson, B. J. Bibcode: 2002ApJ...574.1021G Altcode: Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections. Sigmoids, or S-shaped structures, may be indicators of twisted or helical magnetic structures, having an increased likelihood of eruption. We present here an analysis of a sigmoidal region's three-dimensional structure and how it evolves in relation to its eruptive dynamics. We use data taken during a recent study of a sigmoidal active region passing across the solar disk (an element of the third Whole Sun Month campaign). While S-shaped structures are generally observed in soft X-ray (SXR) emission, the observations that we present demonstrate their visibility at a range of wavelengths including those showing an associated sigmoidal filament. We examine the relationship between the S-shaped structures seen in SXR and those seen in cooler lines in order to probe the sigmoidal region's three-dimensional density and temperature structure. We also consider magnetic field observations and extrapolations in relation to these coronal structures. We present an interpretation of the disk passage of the sigmoidal region, in terms of a twisted magnetic flux rope that emerges into and equilibrates with overlying coronal magnetic field structures, which explains many of the key observed aspects of the region's structure and evolution. In particular, the evolving flux rope interpretation provides insight into why and how the region moves between active and quiescent phases, how the region's sigmoidicity is maintained during its evolution, and under what circumstances sigmoidal structures are apparent at a range of wavelengths. Title: Numerical Simulations of Supergranular Magnetoconvection Authors: De Rosa, M. L.; Hurlburt, N. E.; Alexander, D.; Rucklidge, A. M. Bibcode: 2002AAS...200.0418D Altcode: 2002BAAS...34..646D The complex interactions between the turbulent fluid motions within the solar convection zone and the related processes of emergence, evolution, and cancellation of magnetic field at the photosphere have received much recent attention. It is likely that such interactions depend on the relative magnitudes of the field and of the flows, but the details of this coupling are not well understood. To further investigate the magnetohydrodynamics within such turbulent convection, we have constructed several idealized simulations of fully compressible MHD fluids, each contained within a curved, spherical segment that approximates a localized volume of subphotospheric convection on the sun. In some cases, the horizontal extent of the computational volume spans 30 heliographic degrees in both latitude and longitude, thereby enabling the dynamics within a large field containing approximately 100 supergranular-sized cells to be studied. By varying the amount of total (unsigned) flux permeating the domain, we are able to investigate analogs to patches of subsurface convection that generally resemble either quiet-sun or active regions when viewed from above. In addition, simplified potential-field extrapolations into the volume above the computational domain are used to illustrate how the coronal field topology might behave in response to the continually evolving magnetic field within the convecting layers. This work was supported by NASA through grant NAG 5-3077 to Stanford University and by Lockheed Martin Independent Research and Development funds. Title: Helicity driven sigmoid evolution Authors: Alexander, D.; Nightingale, R.; Metcalf, T. R.; Brown, D. Bibcode: 2002AAS...200.3608A Altcode: 2002BAAS...34..694A Recent observations of rotating sunspots in TRACE white light images and their apparent association with soft X-ray sigmoids have led to the intriguing possibility that the sunspot rotation serves as the driver for both sigmoid formation and their potential eruption. We discuss the energization of the corona resulting from currents generated by the vortex motions of the rotating sunspot. We will present data from events for which we have good white light coverage of the sunspot, an evolving sigmoid and an associated CME (in those cases where the sigmoid erupts). We investigate the relationship between the sunspot rotation and the evolution of the sigmoid structure and attempt to determine the key physical conditions which result in a sigmoid destabilizing and ultimately producing a CME. Title: Summer Fun in the Sun Authors: Alexander, D.; Noldon, D. Bibcode: 2002AAS...200.4803A Altcode: 2002BAAS...34..720A We report here on the development of a program to incorporate a math/science component, emphasizing space science and solar physics, into an existing set of summer activities sponsored by the National Youth Sports Program (NYSP). NYSP provides summer sports and classroom training components to youth whose families fall within federal poverty guidelines. Recently, a partnership between Lockheed Martin Solar and Astrophysics Lab. and Chabot Community College received NASA IDEAS funding to develop a summer curriculum in math and science to augment the already successful program. This provides an opportunity to significantly enhance the experience of the participating students by giving them access to the latest in space data and direct interaction with space scientists. This paper discusses our goals, our approach and the current status of our curricular materials. We would like to acknowledge funding by the National Youth Sports Program and NASA IDEAS. Title: GeoSail: A novel solar sail mission concept for geospace Authors: Alexander, David; McInnes, Colin R.; Angelopoulos, Vassilis; Sandman, Anne W.; MacDonald, Malcolm Bibcode: 2002AIPC..608..305A Altcode: 2002stai.conf..305A GeoSail is a mission designed to utilize the versatility of solar sail propulsion in the exploration of magnetic reconnection and electron dynamics in the Earth's magnetotail. The GeoSail mission uses a low-performance solar sail to artificially precess the apse-line of a long elliptical Earth-centered orbit to match the annual rotation of the geomagnetic tail. This unique use of a solar sail will enable the GeoSail science payload to maintain a nearly constant presence in the geotail providing an exciting opportunity to probe the rapid dynamical evolution of energetic particle distributions in this critical region of geospace. The level of solar sail performance required for GeoSail is typical of that currently being considered for near-term technology demonstration missions, i.e., a 40m×40m sail with characteristic acceleration of 0.1-0.15 mm/s2. GeoSail is therefore capable of providing both technology validation and a unique science return from a first solar sail mission. . Title: The Yohkoh Public Outreach Project: A Space Science Resource for Formal and Informal Education Authors: Lemen, J. R.; Alexander, D.; Metcalf, T. R.; Freeland, S. L.; Acton, L. W.; Larson, M.; McKenzie, D.; Slater, T. Bibcode: 2001AGUFMED12A0160L Altcode: The Yohkoh Public Outreach Project (YPOP) is a NASA-funded web site maintained by scientists and educators at Lockheed Martin Solar and Astrophysics Lab. and Montana State University. YPOP includes a range of activities for youngsters, parents, teachers and anyone interested in learning more about the Sun. YPOP utilizes a number of approaches to the dissemination of solar data which incorporates elements of both formaleducation, via a number of lesson plans and classroom activities, and informal education, via access to the latest solar images, a solar tour, and updated movies. This combination has proved extremely effective in providing quality access to scientific data for a broad audience with a wide range of interests. The Yohkoh Public Outreach Project can be found at http://www.LMSAL.com/YPOP. Title: Energization of Rotating Sunspots, Twisted Coronal Fans, Sigmoid Structures, and Coronal Mass Ejections Authors: Nightingale, R. W.; Alexander, D.; Brown, D. S.; Metcalf, T. R. Bibcode: 2001AGUFMSH11C0724N Altcode: In an on-going study, several sunspots, rotating about their umbral centers, have been identified in TRACE photospheric white light (WL) images with accompanying twisting of coronal fans connected to penumbral magnetic field lines in the corresponding EUV (171, 195 Å) images. These observations can also be temporally and spatially associated with S or inverse-S shaped regions (sigmoid structures) appearing in Yohkoh SXT images and with concurrent coronal mass ejections (CMEs) and/or flares. We have determined the rotational speed of the sunspot in AR9114 over 8-10 August 2000, established the inverse-S shape observed in the SXT data, and viewed a rapid flash of possible reconnection in a TRACE EUV movie. A CME was observed in AR8667 during the 15-18 August 1999 event, which also included an inverse-S shaped region in the SXT data, and a rotating sunspot and twisting coronal fans in the TRACE data. The large Bastille Day CME event of 14 July 2000 in AR9077 was accompanied by one or more rotating sunspots as observed in TRACE WL and by an inverse-S shaped region as seen in a difference SXT image. We will utilize these data to estimate the energization of the twisted coronal fans resulting from the sunspot rotation and compare this with the temporal evolution of the sigmoid structures. We will investigate the possibility of a direct role of the observed sunspot rotation and the potential for a sigmoid to erupt as a CME. This work was supported by NASA under contract NAS5-38099. Title: Coupled modeling of photospheric and coronal dynamics Authors: Alexander, D.; Hurlburt, N. E.; Rucklidge, A. M.; De Rosa, M. Bibcode: 2001AGUFMSH11C0718A Altcode: The coupling of the motions within and below te photosphere to the chromosphere and corona is one of the fundamental issues in solar physics. We have developed a model coupling the simulated dynamics of sunspots to the simulated heating of coronal loops. In this paper we present an extension of our earlier work to the inclusion of (a) fully three dimensional magnetoconvection, (b) new analytical representations of hydrostatic loops with spatially-dependent heating rates and (c) fully time-dependent hydrodynamic coronal modeling. The dynamic loop model uses the same numerical scheme as the magnetoconvective model used to simulate the photospheric behavior in this sunspot system, making it possible to more fully integrate the two regimes. We present the first results of a hybrid model utilizing a time-dependent coronal model and a fully three-dimensional magnetoconvective model. Title: Flare Plasma Cooling from 30 MK down to 1 MK modeled from Yohkoh, GOES, and TRACE Observations during the Bastille-Day Event (14 July 2000) Authors: Aschwanden, M. J.; Alexander, D. Bibcode: 2001AGUFMSH32B..05A Altcode: We present an analysis of the evolution of the thermal flare plasma during the 2000-Jul-14, 10 UT, Bastille-Day flare event, using spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and TRACE. The spatial structure of this double-ribbon flare consists of a curved arcade with some 100 post-flare loops which brighten up in a sequential manner from highly-sheared low-lying to less-sheared higher-lying bipolar loops. We reconstruct an instrument-combined, average differential emission measure distribution dEM(T)/dT that ranges from T=1 MK to 40 MK and peaks at T0=10.9 MK. We find that the time profiles of the different instrument fluxes peak sequentially over 7 minutes with decreasing temperatures from T≈ 30 MK to 1 MK, indicating the systematic cooling of the flare plasma. From these temperature-dependent relative peak times tpeak(T) we reconstruct the average plasma cooling function T(t) for loops observed near the flare peak time, and find that their temperature decrease is initially controlled by conductive cooling during the first 188 s, T(t) ~ [1+(t/τcond)]-2/7, and then by radiative cooling during the next 592 s, T(t) ~ [1-(t/τrad)]3/5. From the radiative cooling phase we infer an average electron density of ne=4.2x 1011 cm-3, which implies a filling factor near 100% for the brightest observed 23 loops with diameters of ≈ 1.8 Mm that appear simultaneously over the flare peak time and are fully resolved with TRACE. We reproduce the time delays and fluxes of the observed time profiles near the flare peak self-consistently with a forward-fitting method of a fully analytical model. The total integrated thermal energy of this flare amounts to Ethermal=2.6 x 1031 erg. >http://www.lmsal.com/~aschwand/publications/publ.html</a> Title: Hard X-ray Observations of the August 25, 2001 X Flare Authors: Metcalf, T. R.; Alexander, D.; Hudson, H. H. Bibcode: 2001AGUFMSH42A0776M Altcode: The X5.3 flare which occurred about 16:31 UT on 2001 August 25 was well observed by the Yohkoh spacecraft. The flare showed gamma-ray emission, nuclear lines and was a dramatic white light flare. A preliminary analysis of the hard X-ray images from the Yohkoh/HXT instrument show two clear footpoints to this very energetic flare, one nearly stationary and the other apparently moving as the flare progresses. We will discuss the hard X-ray and soft X-ray structure of this flare and compare the hard X-ray images to the TRACE and MDI white light data both spatially and temporally. Title: Flare Plasma Cooling from 30 MK down to 1 MK modeled from Yohkoh, GOES, and TRACE observations during the Bastille Day Event (14 July 2000) Authors: Aschwanden, Markus J.; Alexander, David Bibcode: 2001SoPh..204...91A Altcode: We present an analysis of the evolution of the thermal flare plasma during the 14 July 2000, 10 UT, Bastille Day flare event, using spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and TRACE. The spatial structure of this double-ribbon flare consists of a curved arcade with some 100 post-flare loops which brighten up in a sequential manner from highly-sheared low-lying to less-sheared higher-lying bipolar loops. We reconstruct an instrument-combined, average differential emission measure distribution dEM(T)/dT that ranges from T=1 MK to 40 MK and peaks at T0=10.9 MK. We find that the time profiles of the different instrument fluxes peak sequentially over 7 minutes with decreasing temperatures from T≈30 MK to 1 MK, indicating the systematic cooling of the flare plasma. From these temperature-dependent relative peak times tpeak(T) we reconstruct the average plasma cooling function T(t) for loops observed near the flare peak time, and find that their temperature decrease is initially controlled by conductive cooling during the first 188 s, T(t)∼[1+(t/τcond)]−2/7, and then by radiative cooling during the next 592 s, T(t)∼[1−(t/τrad)]3/5. From the radiative cooling phase we infer an average electron density of ne=4.2×1011 cm−3, which implies a filling factor near 100% for the brightest observed 23 loops with diameters of ∼1.8 Mm that appear simultaneously over the flare peak time and are fully resolved with TRACE. We reproduce the time delays and fluxes of the observed time profiles near the flare peak self-consistently with a forward-fitting method of a fully analytical model. The total integrated thermal energy of this flare amounts to Ethermal=2.6×1031 erg. Title: Evidence for the Flare Trigger Site and Three-Dimensional Reconnection in Multiwavelength Observations of a Solar Flare Authors: Fletcher, L.; Metcalf, T. R.; Alexander, D.; Brown, D. S.; Ryder, L. A. Bibcode: 2001ApJ...554..451F Altcode: Based on a multiwavelength data set and a topological model for the magnetic field, we argue that a M1.9 flare which occurred on 1993 May shows evidence of three-dimensional coronal reconnection in a spine-fan configuration. Images from the Transition Region and Coronal Explorer allow the detailed examination of the structures involved in the flare and preflare in the 171 Å (1 MK) EUV passband and the Lyα (10,000-20,000 K) passband. Yohkoh Hard X-ray Telescope maps the position of nonthermal electron precipitation and the Soft X-ray Telescope reveals preflare and flare heating on large and small scales. While the flare appears to be driven by changes in small-scale field close to the photosphere, near the interface between strong opposite magnetic polarities, the result is the disruption of large-scale field. We demonstrate how this observed activity on large and small scales, along with many other aspects of the flare, suggests a qualitative explanation in the three-dimensional reconfiguration of coronal magnetic field, following a small-scale flux cancellation at the photosphere. Title: Chromospheric Heating in the Late Phase of Two-Ribbon Flares Authors: Czaykowska, A.; Alexander, D.; De Pontieu, B. Bibcode: 2001ApJ...552..849C Altcode: Fast upflows observed in the late gradual phase of an M6.8 two-ribbon flare by the Solar and Heliospheric Observatory/Coronal Diagnostic Spectrometer have provided evidence for the presence of chromospheric evaporation more than an hour after the impulsive phase of the flare. The chromospheric heating necessary to generate these upflows requires the continued injection and deposition of energy, which we presume to be provided by magnetic reconnection in the flaring corona. We investigate the nature of the transport of this energy from the reconnection site to the chromosphere by comparing the observed upflow velocities with those expected from different chromospheric heating models. A nonthermal beam of energetic electrons (>~15 keV) that is capable of generating the observed velocities would also generate significant hard X-ray emission that is not observed at this stage of the flare. We conclude, therefore, that the most likely energy transport mechanism is thermal conduction. Title: Observational Diagnostics of Reconnection in Solar Flares Authors: Alexander, D. Bibcode: 2001AGUSM..SM22A10A Altcode: Magnetic reconnection is believed to be the prime means of releasing magnetic energy in a solar flare. The process of reconnection occurs on spatial scales significantly smaller than the best observationally resolvable scales.Consequently, we must rely on observational signatures which, indirectly, imply the presence of magnetic reconfiguration in the form of reconnection. Recent, high-resolution, multi-wavelength flare observations are allowing us to use the observed dynamical behavior to probe the connection between the small-scale localized trigger and the large-scale coronal field which ultimately participates in the solar flare. We will discuss some new results which indicate strong evidence for the presence of 3D coronal reconnection and the role it plays in the dynamics of the flare plasma. Title: Coronal Heating and the dynamics of subphotospheric magnetic fields Authors: Hurlburt, N.; Alexander, D. Bibcode: 2001AGUSM..SH31D05H Altcode: Simultaneous observations of sunspots in the photosphere and in the coronal regions above them reveal a close coupling between the dynamics of the photospheric motion and structure and the heating of coronal loops. We investigate this relationship through detailed three-dimensional simulations of dynamic, small-scale structures in sunspot penumbra and umbra in conjunction with models of coronal excitation and emission. The numerical models incorporate a fully three-dimensional magnetoconvection calculation, potential field extrapolations from the sunspot model boundary conditions, steady-state and dynamic coronal loops powered by the convective motions at the surface, EUV and X-ray instrument response functions, and a fieldline rendering. The result is a simulated dynamical active region in three dimensions which can be compared directly with observations and enables us to explore coronal heating and its relationship to the dynamics of the photosphere and convection zone. We present results of recent calculations exploring the dynamics of penumbra grains and their possible influence on the overlying corona. Title: Solar Week: An experiment of the Yohkoh Public Outreach Project Authors: Alexander, D. Bibcode: 2001AGUSM..ED41A08A Altcode: Solar Week is a week-long collection of content, games, and activities centered on the latest data from and discoveries about the Sun. Targetted at middle-school girls, the key feature of Solar Week is the "Ask a Scientist" section enabling direct interaction between participating students and volunteer scientists. All of the contributing scientists are women, serving as experts in their field and providing role models to whom the students can relate. Solar Week has completed two sessions, one in October 2000 and one in April 2001, with a total of some 80 edcuators and 8,000+ students in over 25 states. A major success of the Solar Week program has been the ability of the students to learn more about the scientists as people, through online biographies, and to discuss a variety of topics ranging from science, to careers and common hobbies. Title: Analysis of X-ray counterparts for Fast and Slow Halo CMEs Authors: Alexander, D.; Metcalf, T. R.; Nitta, N. Bibcode: 2001AGUSM..SH22B05A Altcode: In many cases, coronal mass ejections exhibit a strong counterpart in the X-ray corona, particularly in flare-related events. Observations by the Soft X-ray Telescope on Yohkoh have exhibited a number of manifestations in association with CME eruptions. These include the well-known dimming events, post-event arcades and expanding loop systems. We examine the X-ray signatures of a number of fast and slow halo CME events for evidence of two-classes of CME eruption in accord with the observed velocity profiles determined from white-light data. Flare-associated CMEs, which have a tendency to exhibit constant velocity, necessarily undergo a rapid acceleration in the low corona and should exhibit enhanced heating of the ambient corona to X-ray emitting temperatures. Slow CMEs, on the other hand, are expected to display a more subtle, if any, effect in the hot corona. We examine the Yohkoh database for evidence of a dichotomy in the X-ray signatures of halo CMEs. Title: Modeling of Coronal EUV Loops Observed with TRACE. I. Hydrostatic Solutions with Nonuniform Heating Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Alexander, David Bibcode: 2001ApJ...550.1036A Altcode: Recent observations of coronal loops in EUV wavelengths with the Transition Region and Coronal Explorer (TRACE) and the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) demonstrated three new results that cannot be explained by most of the existing loop models: (1) EUV loops are near-isothermal along their coronal segments, (2) they show an overpressure or overdensity compared with the requirements of steady state loops with uniform heating, and (3) the brightest EUV loops exhibit extended scale heights up to 4 times the hydrostatic scale height. These observations cannot be reconciled with the classical RTV (Rosner, Tucker, & Vaiana) model, they do not support models with uniform heating, and they even partially violate the requirements of hydrostatic equilibrium. In this study we are fitting for the first time steady state solutions of the hydrodynamic equations to observed intensity profiles, permitting a detailed consistency test of the observed temperature T(s) and density profiles ne(s) with steady state models, which was not possible in previous studies based on scaling laws. We calculate some 500 hydrostatic solutions, which cover a large parameter space of loop lengths (L~4-300 Mm), of nonuniform heating functions (with heating scale heights in the range of λH~1-300 Mm), approaching also the limit of uniform heating (λH>>L). The parameter space can be subdivided into three regimes, which contain (1) solutions of stably stratified loops, (2) solutions of unstably stratified loops (in the case of short heating scale heights, λH,Mm~LMm), and (3) a regime in which we find no numerical solutions (when λH,Mm<~LMm). Fitting the hydrostatic solutions to 41 EUV loops observed with TRACE (selected by the criterion of detectability over their entire length), we find that only 30% of the loops are consistent with hydrostatic steady state solutions. None of the observed EUV loops is consistent with a uniform heating function while in quasi-steady state. Those loops compatible with a steady state are found to be heated near the footpoints, with a heating scale height of λH=12+/-5 Mm, covering a fraction λH/L=0.2+/-0.1 of the loop length. These results support coronal heating mechanisms operating in or near the chromosphere and transition region. Title: Solar Week 2000: Using role models to encourage an interest in science Authors: Alexander, D. Bibcode: 2000AAS...19712009A Altcode: 2000BAAS...32.1607A Solar Week 2000 is a week-long set of games and activities allowing students to interact directly with solar science and solar scientists. The main goal of Solar Week was to provide young women, primarily in grades 6-8, with access to role models in the sciences. The scientists participating in Solar Week are women from a variety of backgrounds and with a variety of scientific expertise. An online bulletin board was used to foster discussion between the students and the scientists about both science and career issues. In this presentation I will discuss the successes and failures of the first run of Solar Week which occurred on 9-13 October 2000. Our aim is to provide some insight into doing activity-based space science on the web and to discuss the lessons-learned from tailoring to a specific group of participants. Title: Evidence for Nonuniform Heating of Coronal Loops Inferred from Multithread Modeling of TRACE Data Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Alexander, David Bibcode: 2000ApJ...541.1059A Altcode: The temperature Te(s) and density structure ne(s) of active region loops in EUV observed with TRACE is modeled with a multithread model, synthesized from the summed emission of many loop threads that have a distribution of maximum temperatures and that satisfy the steady state Rosner-Tucker-Vaiana (RTV) scaling law, modified by Serio et al. for gravitational stratification (called RTVSp in the following). In a recent Letter, Reale & Peres demonstrated that this method can explain the almost isothermal appearance of TRACE loops (observed by Lenz et al.) as derived from the filter-ratio method. From model-fitting of the 171 and 195 Å fluxes of 41 loops, which have loop half-lengths in the range of L=4-320 Mm, we find that (1) the EUV loops consist of near-isothermal loop threads with substantially smaller temperature gradients than are predicted by the RTVSp model; (2) the loop base pressure, p0~0.3+/-0.1 dynes cm-2, is independent of the loop length L, and it agrees with the RTVSp model for the shortest loops but exceeds the RTVSp model up to a factor of 35 for the largest loops; and (3) the pressure scale height is consistent with hydrostatic equilibrium for the shortest loops but exceeds the temperature scale height up to a factor of ~3 for the largest loops. The data indicate that cool EUV loops in the temperature range of Te~0.8-1.6 MK cannot be explained with the static steady state RTVSp model in terms of uniform heating but are fully consistent with Serio's model in the case of nonuniform heating (RTVSph), with heating scale heights in the range of sH=17+/-6 Mm. This heating function provides almost uniform heating for small loops (L<~20 Mm), but restricts heating to the footpoints of large loops (L~50-300 Mm). Title: SOHO/UVCS Observations of a Coronal Jet During the Third Whole Sun Month Campaign Authors: Ko, Y. -K.; Raymond, J.; Gibson, S.; Strachan, L.; Alexander, D.; Fletcher, L.; Holzer, T.; Gilbert, H.; Burkepile, J.; St. Cyr, C.; Thompson, B. Bibcode: 2000SPD....31.0271K Altcode: 2000BAAS...32R.823K On August 26 1999, a coronal jet occurred at the north west limb near a sigmoid active region which has been the target for a joint observation plan during the third Whole Sun Month Campaign. This jet was observed by several instruments at the limb (SOHO/CDS, SOHO/EIT, TRACE, MLSO/CHIP, MLSO/PICS), at 1.7 Ro (SOHO/UVCS), and at the outer corona (SOHO/LASCO). At 1.7 Ro, the intensities of Lyman alpha, Lyman beta in the jet increased by as large a factor of 100 compared with the background corona, while those for O VI 1032 and O VI 1037 increased by a factor of 2. C III 977 line also brightened significantly. The line shift in the lines indicates that the line-of-sight velocity in the jet started from 150 km/sec blue shift and ended at 120 km/sec red shift. This line-of-sight motion seen at 1.7 Ro apparently was opposite that observed when the jet emerged from the limb. In this paper, we present the observation by SOHO/UVCS and discuss the dynamic structure and physical properties of this jet as it passed through 1.7 Ro. Comparisons will be shown with the observations from other instruments. This work is supported by NASA Grant number NAG5-7822. Title: Evidence for continued reconnection in solar flares Authors: Alexander, D.; Czaykowska, A. Bibcode: 2000SPD....31.0270A Altcode: 2000BAAS...32..823A Fast upflows observed in the late gradual phase of an M6.8 two-ribbon flare by SOHO/CDS have provided evidence for the presence of chromospheric evaporation more than an hour after the main impulsive phase of the flare. The chromospheric heating necessary to generate these upflows requires the continued injection and deposition of energy, which we presume to be provided by magnetic reconnection in the flaring corona. We investigate the nature of the transport of this energy from the reconnection site to the chromosphere by comparing the observed velocities with those expected from different chromospheric heating models. A non-thermal electron-beam model capable of generating the observed velocities would also generate significant hard X-ray emission which is not observed at this stage of the flare. We, therefore, conclude that the most likely energy transport mechanism is thermal conduction. This work is supported by the Yohkoh/SXT project at LMSAL (contract NAS8-40801). Title: Solar Sail Mission Concepts Authors: Winter, H. D.; Alexander, D.; McInnes, C. R. Bibcode: 2000SPD....3102102W Altcode: 2000BAAS...32..829W Solar sail technology is fast becoming a viable option for spacecraft propulsion and appears as an enabling technology in many plans for future space physics missions. We have developed a number of novel mission concepts which utilize the full potential of solar sail propulsion. These mission concepts include enhancements to existing ideas, some of which appear in the most recent SEC Roadmap, in addition to a number of new mission ideas. Each mission considered incorporates a range of sail performance levels which allow an examination of the potential, both near-term and far-term, of attaining the specified mission goals. Two main issues arose from this work: 1) High energy orbits can be readily attained from a relatively small launch vehicle even for low performance sails, significantly reducing costs; 2) Improving technology can enhance a specific mission by either decreasing the travel times and increasing maneuverability, or by increasing the payload mass fraction. This work was supported by an internal research and development contract from Lockheed Martin Advanced Technology Center. Title: Structure of a Large low-Latitude Coronal Hole Authors: Bromage, B. J. J.; Alexander, D.; Breen, A.; Clegg, J. R.; Del Zanna, G.; DeForest, C.; Dobrzycka, D.; Gopalswamy, N.; Thompson, B.; Browning, P. K. Bibcode: 2000SoPh..193..181B Altcode: Coronal holes on the Sun are the source of high-speed solar wind streams that produce magnetic disturbances at the Earth. A series of multi-wavelength, multi-instrument observations obtained during the 1996 `Whole Sun Month' campaign examined a large coronal hole in greater detail than ever before. It appeared on the Sun in August, and extended from the north pole to a large active region in the southern hemisphere. Its physical and magnetic structure and subsequent evolution are described. Title: Three-dimensional Stereoscopic Analysis of Solar Active Region Loops. II. SOHO/EIT Observations at Temperatures of 1.5-2.5 MK Authors: Aschwanden, Markus J.; Alexander, David; Hurlburt, Neal; Newmark, Jeffrey S.; Neupert, Werner M.; Klimchuk, J. A.; Gary, G. Allen Bibcode: 2000ApJ...531.1129A Altcode: In this paper we study the three-dimensional structure of hot (Te~1.5-2.5 MK) loops in solar active region NOAA 7986, observed on 1996 August 30 with the Extreme-ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO). This complements a first study (Paper I) on cooler (Te~1.0-1.5 MK) loops of the same active region, using the same method of Dynamic Stereoscopy to reconstruct the three-dimensional geometry. We reconstruct the three-dimensional coordinates x(s), y(s), z(s), the density ne(s), and temperature profile Te(s) of 35 individual loop segments (as a function of the loop coordinate s) using EIT 195 and 284 Å images. The major findings are as follows. (1) All loops are found to be in hydrostatic equilibrium, in the entire temperature regime of Te=1.0-2.5 MK. (2) The analyzed loops have a height of 2-3 scale heights, and thus only segments extending over about one vertical scale height have sufficient emission measure contrast for detection. (3) The temperature gradient over the lowest scale height is of order dT/ds~1-10 K km-1. (4) The radiative loss rate is found to exceed the conductive loss rate by about two orders or magnitude in the coronal loop segments, implying that the loops cannot be in quasi-static equilibrium, since standard steady-state loop models show that radiative and conductive losses are comparable. (5) A steady state could only be maintained if the heating rate EH matches exactly the radiative loss rate in hydrostatic equilibrium, requiring a heat deposition length λH of the half density scale height λ. (6) We find a correlation of p~L-1 between loop base pressure and loop length, which is not consistent with the scaling law predicted from steady-state models of large-scale loops. All observational findings indicate consistently that the energy balance of the observed EUV loops cannot be described by steady-state models. Title: Coronal Trapping of Energetic Flare Particles: Yohkoh/HXT Observations Authors: Metcalf, Thomas R.; Alexander, David Bibcode: 2000ASPC..206..233M Altcode: 2000hesp.conf..233M No abstract at ADS Title: High-resolution Observations of Plasma Jets in the Solar Corona Authors: Alexander, David; Fletcher, Lyndsay Bibcode: 1999SoPh..190..167A Altcode: We present recent observations of coronal jets, made by TRACE and Yohkoh/SXT on 28 May and 19 August 1998. The high spatial resolution of TRACE enables us to see in detail the process of material ejection; in the line of Fe ix (one million degrees) we see both bright emitting material and dark absorbing/scattering material being ejected, i.e., both hot and cold material, highly collimated and apparently ejected along the direction of the overlying field lines. Bright ejecta are seen simultaneously in Lyman α for one event and Yohkoh/SXT in the other. The jets on the two days are different in that the 19 August jet displays the morphology typical of a one-sided anemone jet while the 28 May jet exhibits a two-sided jet morphology. The 19 August jet shows evidence for rotation and an interesting bifurcation at large distances from the energy release site. We study the physical properties and energetics of these jetting events, and conclude that existing theoretical models capture the essential physics of the jet phenomena. Title: A Method for Characterizing Rotation Rates in the Soft X-Ray Corona Authors: Weber, M. A.; Acton, L. W.; Alexander, D.; Kubo, S.; Hara, H. Bibcode: 1999SoPh..189..271W Altcode: Differential rotation rates of soft X-ray features in the solar corona are quantified by a method of harmonic filtering using the Lomb-Scargle periodogram. This approach leads reasonably to a quantitative discrimination between uncertainty estimates and spectral leakage of the fundamental rotation frequency due to the presence of multiple rotating tracers. Mean rotation rates as a function of latitude and year are calculated for the years 1992-1997 (roughly the declining phase of the last solar activity cycle). The corona is found to have a small but measurable latitudinal gradient in rotation rate. The presence of multiple features places a lower bound of 1-2% on the relative uncertainties with which a `mean' rotation rate can be measured. The results are compared with autocorrelation estimates and found to agree within 1σ. Title: Heating The Atmosphere Above Sunspots Authors: Alexander, David; Hurlburt, Neal E.; Rucklidge, Alastair Bibcode: 1999ESASP.446..117A Altcode: 1999soho....8..117A We present our results of a hybrid model of sunspots and their overlying corona. The two-layer model considers both the nonlinear, compressible magnetoconvection beneath the photosphere and potential, or linear force-free, models of the coronal fields. Heating of the plasma along the field lines is then consider using quasi-static and steady-state model with the heating rate being specified by the dynamics of the magnetoconvection. Two distinct magnetoconvection scenarios are considered. The first describes magnetoconvection in a 2D axisymmetric geometry and considers the time development of the overlying coronal field. The second describes a 3D cylindrical geometry with a static coronal field configuration. Both scenarios diverge from the standard practice of assuming constant temperature and vertical magnetic field conditions at the top surface. Instead a radiative linear force-free field condition is adopted. Extrapolation of the top surface boundary conditions results in a coronal field configuration which is assumed to be filled with plasma heated to coronal temperatures. The heating rate and thermodynamic behavior of the plasma is related to the sub-surface model by assuming that individual fluxtubes are heated uniformly with the necessary energy being generated from the dissipation of the Poynting flux entering the coronal volume. Radiation and conductive losses are included. The combination of a sunspot model, whereby the surface field is completely specified, with a coronal heating model, in which the plasma parameters are specified for a given energy input allows us to explore a broad class of heating paradigms. Title: Chromospheric Evaporation In The Gradual Flare Phase Authors: Czaykowska, A.; de Pontieu, B.; Alexander, D.; Rank, G. Bibcode: 1999ESASP.446..257C Altcode: 1999soho....8..257C SOHO/CDS observations are used to determine relative line-of-sight velocities and their temporal evolution in the late gradual flare phase. The observations started about 1 hour after the M6.8 two-ribbon flare in NOAA 8210 on April 29, 1998. Velocity maps in O V 629 A (Tmax = 0.25 MK), Fe XVI 360 A (Tmax = 2 MK), and Fe XIX 592 A (Tmax = 6.3 MK), covering temperatures from the transition region to the corona show strong gradients at the position of the Hα ribbons. Downflows are observed in the footpoint regions of the post-flare loops whereas the velocities observed further away from the magnetic neutral line are interpreted as upflows due to chromospheric evaporation. Loops are filled with hot plasma and their footpoints become visible later on at the former evaporation site. At the same time the Hα ribbon is slowly moving outward together with the location of the velocity gradient. Our observations strongly support models in which chromospheric evaporation driven by magnetic reconnection is responsible for the continuous formation of loops, which are visible for several hours after the flare's maximum in EUV and soft X-ray radiation. Title: Coronal Trapping of Energetic Flare Particles:Yohkoh/HXT Observations Authors: Metcalf, Thomas R.; Alexander, David Bibcode: 1999ApJ...522.1108M Altcode: We examine spectroscopic data from the Yohkoh Hard X-Ray Telescope in a search for spectral evidence of the coronal trapping of energetic particles during solar flares. Two distinct particle populations with significantly different spectral properties are found to be present in three of the six flares studied; the first population is trapped in the corona, where it encounters a ``thick-thin'' target, while the second population precipitates directly to the footpoints. In the remaining three events, a single population of energetic particles appear to be responsible for the observed hard X-ray emission, either via thermal bremsstrahlung (one case) or nonthermal thin-target emission (two cases). For the three events in which a trapped population is observed, the spectroscopic observations imply first that there is likely to be a single acceleration mechanism for both the trapped and the precipitating populations and second that the magnetic field geometry in these flares is conducive to trapping in a confined region high in the corona, above the soft X-ray loops. Both conditions are consistent with magnetic reconnection models of flares in which energetic particles are trapped between MHD slow-mode shocks attached to the reconnection region and a fast-mode shock formed by the reconnection outflow jet. Title: Evidence for Chromospheric Evaporation in the Late Gradual Flare Phase from SOHO/CDS Observations Authors: Czaykowska, A.; De Pontieu, B.; Alexander, D.; Rank, G. Bibcode: 1999ApJ...521L..75C Altcode: Using extreme-ultraviolet (EUV) spectroheliograms from the first intentional postflare observations with the Coronal Diagnostic Spectrometer (CDS) on board SOHO, we determine relative line-of-sight velocities and their temporal evolution during the gradual flare phase of an M6.8 two-ribbon flare that occurred on 1998 April 29. Dopplergrams in lines of O V, Fe XVI, and Fe XIX, with formation temperatures Tmax of, respectively, 0.25, 2.0, and 8.0 MK show strong velocity gradients coincident with the Hα ribbons, visible in Big Bear Solar Observatory (BBSO) images. These gradients are perpendicular to and moving with the Hα ribbons. Bright downflowing plasma seems to be prevalent in the regions, between the ribbons and the magnetic neutral line, that coincide with the ends of postflare loops seen with the Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. The plasma on the outer side of the ribbons is less bright in the EUV but shows strong relative blueshifts. This pattern of upflows and downflows demonstrates, for the first time in transition region and coronal lines, the existence of chromospheric evaporation during the late gradual phase of a flare and provides evidence for ongoing reconnection. Title: Coronal Loop Oscillations Observed with the Transition Region and Coronal Explorer Authors: Aschwanden, Markus J.; Fletcher, Lyndsay; Schrijver, Carolus J.; Alexander, David Bibcode: 1999ApJ...520..880A Altcode: We report here, for the first time, on spatial oscillations of coronal loops, which were detected in extreme-ultraviolet wavelengths (171 Å) with the Transition Region and Coronal Explorer, in the temperature range of Te~1.0-1.5 MK. The observed loop oscillations occurred during a flare that began at 1998 July 14, 12:55 UT and are most prominent during the first 20 minutes. The oscillating loops connect the penumbra of the leading sunspot to the flare site in the trailing portion. We identified five oscillating loops with an average length of L=130,000+/-30,000 km. The transverse amplitude of the oscillations is A=4100+/-1300 km, and the mean period is T=280+/-30 s. The oscillation mode appears to be a standing wave mode (with fixed nodes at the footpoints). We investigate different MHD wave modes and find that the fast kink mode with a period τ=205(L/1010 cm)(ne/109 cm-3)1/2(B/10 G)-1 s provides the best agreement with the observed period. We propose that the onset of loop oscillations in distant locations is triggered by a signal or disturbance that propagates from the central flare site with a radial speed of ~700 km s-1. Because the observed loop oscillation periods are comparable to photospheric 5 minute oscillations, a resonant coupling between the two systems is possible. We further find evidence for global extreme-UV dimming in the entire active region possibly associated with a coronal mass ejection. Title: Modeling CMEs in three dimensions using an analytic MHD model Authors: Gibson, Sarah E.; Alexander, David; Biesecker, Doug; Fisher, Richard; Guhathakurta, Madhulika; Hudson, Hugh; Thompson, B. J. Bibcode: 1999AIPC..471..645G Altcode: 1999sowi.conf..645G Because coronal mass ejections (CMEs) are viewed in projection, it is difficult to determine their three-dimensional nature. We use an analytic model of CMEs as an example of a fully three-dimensional magnetic field structure in MHD force balance with an emerging CME. We present the CME magnetic field and its associated density structure, seen projected at the limb from two viewing angles perpendicular to the plane of the sky, and emerging from disk center representing ``earth-directed'' CME events. The range of CME structures thus produced compares well to existing CME white-light coronagraph and full disk EUV and X-ray observations. In particular, we find that both 3-part ``front-cavity-core'' and ``U-shaped'' white light CMEs, as well as the twin dimmings (also referred to as transient coronal holes) observed in X-ray and EUV, can successfully be reproduced by the CME model. All of these structures are a direct consequence of a single three-dimensional magnetic field topology, viewed from different directions. Title: Cylindrical Compressible Magnetoconvection and Model Sunspots Authors: Hurlburt, N.; Alexander, D.; Rucklidge, A. Bibcode: 1999AAS...194.5502H Altcode: 1999BAAS...31..910H We present results of hybrid models of sunspots and pores which encompasses both the nonlinear, compressible magnetoconvection beneath the photosphere, potential models of the coronal fields and includes quasistatic coronal heating models. We solve the equations that describe compressible magnetoconvection in 2D axisymmetric and 3D cylindrical geometries using compact finite difference scheme. The convecting layer consists of electrically conducting gas which experiences a uniform gravitational acceleration directed downwards. The gas possesses a shear viscosity, a thermal conductivity, a magnetic diffusivity, and a magnetic permeability which are all assumed to be constant. We assume that the fluid satisfies the equation of state for a perfect monatomic gas with constant heat capacities. At the bottom of the cylinder, we impose a constant temperature and vertical magnetic field. On the top surface apply instead a radiative, and linear force-free field condition. The outer boundary is insolating and perfectly conducting. The magnetic fields above the computational domain are then extrapolated and heated using a quasistatic model. The heating problem is solved in an empirical way by assuming that individual fluxtubes are heated in a manner that is proportional to one or more of the parameters defining the fluxtube, e.g. pressure, length, field strength, current density etc. The combination of a sunspot model, whereby the surface field is completely specified, with a coronal heating model, in which the plasma parameters are specified for a given energy input allows us to explore a broad class of heating paradigms. We present result of 2D simulations with no net magnetic flux which display phenomena similar to that observed in sunspot moats, and 3D simulations which develop penumbral-like structure. This work was supported by NASA contract NAG5-7376. Title: The role of the large-scale corona in the production of solar flares Authors: Alexander, D.; Simnett, G. M. Bibcode: 1999AAS...194.5503A Altcode: 1999BAAS...31..910A We investigate the spatial location of large solar flares (>M5) occurring in the GOES record from 1-JAN-1986 to 1-JAN-1999. It is found, with very few exceptions, that a single active region dominates the flare production over any given time. In other words, before a flare can occur in an active region, flare production must have completely ceased in any other active regions present. This suggests some sort of communication between the active regions on the solar disk and that the large-scale corona plays an important role in the production of these large solar flares. One intriguing possibility is the treatment of the global solar corona as a multi-flux system with each part physically related to all others. In this scenario, the transfer of flux between active regions, caused by a solar flare in one of them, suppresses the ability of the other to erupt. This has important implications for flare studies and flare prediction in general. Title: Making YOHKOH SXT Images Available to the Public: The YOHKOH Public Outreach Project Authors: Larson, M. B.; McKenzie, D.; Slater, T.; Acton, L.; Alexander, D.; Freeland, S.; Lemen, J.; Metcalf, T. Bibcode: 1999AAS...194.7024L Altcode: 1999BAAS...31..941L The NASA funded Yohkoh Public Outreach Project (YPOP) provides public access to high quality Yohkoh SXT data via the World Wide Web. The products of this effort are available to the scientific research community, K-12 schools, and informal education centers including planetaria, museums, and libraries. The project utilizes the intrinsic excitement of the SXT data, and in particular the SXT movies, to develop science learning tools and classroom activities. The WWW site at URL: http://solar.physics.montana.edu/YPOP/ uses a movie theater theme to highlight available Yohkoh movies in a format that is entertaining and inviting to non-scientists. The site features informational tours of the Sun as a star, the solar magnetic field, the internal structure and the Sun's general features. The on-line Solar Classroom has proven very popular, showcasing hand-on activities about image filtering, the solar cycle, satellite orbits, image processing, construction of a model Yohkoh satellite, solar rotation, measuring sunspots and building a portable sundial. The YPOP Guestbook has been helpful in evaluating the usefulness of the site with over 300 detailed comments to date. Title: Constructing the Coronal Magnetic Field By Correlating Parameterized Magnetic Field Lines With Observed Coronal Plasma Structures Authors: Gary, G. Allen; Alexander, David Bibcode: 1999SoPh..186..123G Altcode: A method is presented for constructing the coronal magnetic field from photospheric magnetograms and observed coronal loops. A set of magnetic field lines generated from magnetogram data is parameterized and then deformed by varying the parameterized values. The coronal flux tubes associated with this field are adjusted until the correlation between the field lines and the observed coronal loops is maximized. A mathematical formulation is described which ensures that (i) the normal component of the photospheric field remains unchanged, (ii) the field is given in the entire corona over an active region, (iii) the field remains divergence-free, and (iv) electric currents are introduced into the field. It is demonstrated that a parameterization of a potential field, comprising a radial stretching of the field, can provide a match for a simple bipolar active region, AR 7999, which crossed the central meridian on 1996 November 26. The result is a non-force-free magnetic field with the Lorentz force being of the order of 10−5.5 g cm s−2 resulting from an electric current density of 0.079 μA m−2. Calculations show that the plasma beta becomes larger than unity at a relatively low height of ∼0.25 r supporting the non-force-free conclusion. The presence of such strong non-radial currents requires large transverse pressure gradients to maintain a magnetostatic atmosphere, required by the relatively persistent nature of the coronal structures observed in AR 7999. This scheme is an important tool in generating a magnetic field solution consistent with the coronal flux tube observations and the observed photospheric magnetic field. Title: Utilizing solar sails for solar physics Authors: Strong, K. T.; Alexander, D.; McInnes, C. R.; Lemen, J. R. Bibcode: 1999AAS...194.6508S Altcode: 1999BAAS...31..928S Recently, there has been much interest in the use of solar sail technology for advanced space mission concepts. We present here some trajectories and orbits for a number of potential solar missions. These missions utilize the power of solar sails in a number of different ways, providing unique capabilities in the study of the Sun. The first mission concept is a solar polar mission using a "parking orbit" above one of the solar poles, the second is a multi-spacecraft mission designed to map out the three-dimensional solar atmosphere using identical spacecraft at different heliographic latitudes, and the third is a STEREO slowdown mission designed to extend the lifetime of the STEREO mission using small sails attached to the STEREO payloads to slow down the rate of drift ahead and behind the Earth. Here, the first two payloads are assumed to be inert masses with possible instrument and spacecraft packages to be defined by the specific goals of any mission. Title: Relative Timing of Soft X-Ray Nonthermal Line Broadening and Hard X-Ray Emission in Solar Flares Authors: Alexander, David; Harra-Murnion, Louise K.; Khan, Josef I.; Matthews, Sarah A. Bibcode: 1998ApJ...494L.235A Altcode: The time development of both hard X-ray emission and soft X-ray nonthermal line widths is important for an understanding of energy transport in the flaring solar corona. In this Letter, we investigate the relationship between the temporal behavior of these two phenomena for a number of flares detected by instruments on the Yohkoh spacecraft. We examine 10 flares, all occurring within 30° of the limb, using data from the Bragg Crystal Spectrometer (BCS) and the hard X-ray telescope (HXT). We find that the nonthermal velocity either (1) exhibits a maximum prior to the first significant burst of hard X-rays or (2) is already decaying from an earlier unobserved maximum at the time of the first significant burst of hard X-rays. The decay of the nonthermal velocity as it proceeds from its observed maximum shows little evidence for a direct association with individual hard X-ray bursts. These observations suggest that the nonthermal broadening may be a direct consequence of the flare energy release process rather than a by-product of the energy deposition. In addition, the attainment of a maximum in the nonthermal line width very early in the flare is more indicative that plasma turbulence is the source of the observed broadening rather than hydrodynamic flows, such as chromospheric evaporation. Title: A Spectral Analysis of the Masuda Flare Using Yohkoh Hard X-Ray Telescope Pixon Reconstruction Authors: Alexander, David; Metcalf, Thomas R. Bibcode: 1997ApJ...489..442A Altcode: Masuda's discovery of a compact hard X-ray impulsive source at the apex of a flaring coronal loop has received a great deal of recent attention in the solar physics community. The Masuda flare, which occurred on 1992 January 13, exhibited evidence of energy deposition in a compact region some distance above the soft X-ray loop, suggesting, to some authors, a flare process similar to the classical model for two-ribbon flares proposed by Shibata et al. These conclusions were made on the basis of a maximum entropy method (MEM) reconstruction of the Yohkoh Hard X-Ray Telescope (HXT) observations. Recently, a new approach has been developed for reconstructing the spatial information from the HXT: that of pixon reconstruction, proposed by Metcalf et al.

In this paper, we apply the pixon reconstruction technique to the event of 1992 January 13 and determine the temporal and spectral characteristics of the loop-top source. While our emphasis here is on the spectral properties of the Masuda flare, we also provide a brief comparison between the pixon reconstruction and that of MEM for the hard X-ray loop top. In carrying out the comparison between the methods, we have applied recent improvements to the instrument response functions and reconstruction algorithms. We have also identified a previously unknown effect of weak source suppression that was inherent in previous analyses and that significantly compromised the ability to study weak sources of hard X-ray emission in the presence of strong sources. The improved response functions and the better flux estimation used in this paper reduce (but do not eliminate) the effects of this suppression, and consequently, it should be noted that the MEM analysis presented in this paper is quite distinct from any that have been carried out previously.

Our conclusions are that (a) a compact loop-top hard X-ray source exists with an impulsive temporal profile spanning the peak of the flare; (b) the loop-top source is nonthermal in nature at the peak of the flare; (c) there is a distinct dearth of HXT LO channel emission, relative to the higher energy channels, from the loop-top region, indicating either a very hard spectrum or the presence of a low-energy cutoff in the energetic electron spectrum; (d) the footpoint and loop-top emission during the impulsive phase of the flare are produced by two distinct particle populations; (e) following the main phase of this flare, the loop top is clearly thermal in nature with a peak temperature of ~40 MK that decreases with time as the event proceeds; and (f) the disparity between the present pixon results and previous MEM results is primarily due to the intrinsically better photometry achieved by the pixon method and the avoidance of suppression effects in the present analysis. These conclusions therefore support, in part, those made in previous works, confirming the existence of an impulsive source of hard X-rays in the corona above a flaring loop. Our analysis does, however, allow for a more comprehensive understanding of the temporal and spectral development of this event in the context of an alternative reconstruction technique. Title: A Comparison of the MEM and Pixon Algorithms for HXT Image Reconstruction Authors: Metcalf, Thomas R.; Alexander, David; Nitta, Nariaki; Kosugi, Takeo Bibcode: 1997SPD....28.0217M Altcode: 1997BAAS...29Q.896M Recently a workshop was held in Palo Alto, CA to discuss image reconstruction for the Hard X-ray Telescope (HXT) on the Yohkoh satellite. At the workshop, the participants concluded that a detailed comparison of the primary reconstruction algorithms should be undertaken. We will report on the results of a comparison of the Maximum Entropy and Pixon algorithms using pseudo data. The comparison will check photometric accuracy, speed, and image quality using a number of test images. The test images utilized in the comparison will examine a broad range of reconstruction problems, including the ability of the algorithms to accurately reconstruct single sources, multiple sources and loop-like features, as well as the ability to reconstruct weak sources in the presence of spatially distinct bright sources. Title: Using the WWW to Make YOHKOH SXT Images Available to the Public: The YOHKOH Public Outreach Project Authors: Larson, M.; McKenzie, D.; Slater, T.; Acton, L.; Alexander, D.; Freeland, S.; Lemen, J.; Metcalf, T. Bibcode: 1997SPD....28.0231L Altcode: 1997BAAS...29..898L The Yohkoh Public Outreach Project (YPOP) is funded by NASA as one of the Information Infrastructure Technology and Applications Cooperative Agreement Teams to create public access to high quality Yohkoh SXT data via the World Wide Web. These products are being made available to the scientific research community, K-12 schools, and informal education centers including planetaria, museums, and libraries. The project aims to utilize the intrinsic excitement of the SXT data, and in particular the SXT movies, to develop science learning tools and classroom activities. The WWW site at URL: http://www.space.lockheed.com/YPOP/ uses a movie theater theme to highlight available Yohkoh movies in a non-intimidating and entertaining format for non-scientists. The site features lesson plans, 'solar' activities, slide shows and, of course, a variety of movies about the Sun. Classroom activities are currently undergoing development with a team of scientists and K-12 teachers for distribution in late 1997. We will display the products currently online, which include a solar classroom with activities for teachers, background resources, and a virtual tour of our Sun. Title: Automated Identification of Soft X-ray Coronal Loops Authors: Alexander, David; Metcalf, Thomas R. Bibcode: 1997SPD....28.0138A Altcode: 1997BAAS...29..886A The presence of loop-like structures in the solar corona is clearly evident in soft X-ray images such as those from the Soft X-ray Telescope on board the Yohkoh satellite. These structures were first discovered in Skylab data and are thought to represent the enhanced heating of the coronal plasma confined in magnetic fluxtubes. In many cases the heating is not confined to a single well defined fluxtube nor is it always strong enough to dilineate the structure sharply against the diffuse coronal background, which is itself presumably composed of loop structures. In these cases it is often extremely difficult to identify the structures involved in coronal activity. We have developed a technique which uses a Pixon reconstruction of the soft X-ray images to search specifically for loop-like structures. This allows us to pick out faint loops against a strong background and to identify multiple loop structures in bright regions. We are, therefore, better able to address the heating of the solar corona both in the diffuse and active regions. Title: Multi-Spectral Imaging of Coronal Activity. Authors: Bagenal, Fran; Darnell, Tony; Burkepile, Joan; Hundhausen, Art; Alexander, David Bibcode: 1997SPD....28.0146B Altcode: 1997BAAS...29Q.887B By combining white light coronameter, Yohkoh soft x-ray and H-alpha images from time intervals that encompass coronal mass ejections (CMEs), we are able to examine the related evolution of structures in the solar atmosphere. For example, what is the role of prominences in pre-CME evolution of the corona; how well does the Pneumann & Kopp model describe post-CME re-formation of coronal loops. On the west limb, Yohkoh data show the structure of the corona before the CME while prominences/filaments are revealed by H-alpha images. On the east limb,the same data show x-ray-emitting loops that brighten and expand after the CME. We have chosen examples of events from 1994 and 1995 that illustrate the use of such composite images. Composite white light, Yohkoh and H-alpha images are also shown for the Whole Sun Month and we discuss the possibilities of adding SOHO data as they become available. Title: Solar identification of solar-wind disturbances observed at Ulysses Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.; Harvey, K. L.; Hoeksema, J. T.; Zhao, X.; Hudson, H. S. Bibcode: 1996AIPC..382...92L Altcode: The Ulysses polar passages are producing a unique set of observations of solar-wind disturbances at high heliographic latitudes. In this paper we use the Yohkoh soft X-ray telescope (SXT) to locate some of these events, as defined by the Ulysses/SWICS data, in the solar corona. Of 8 events, we identify two with flares, three with front-side large arcade events, two with far-side events, and one was not seen in the Ulysses data. The arcade events generally resemble long-duration flares seen in active regions, but are larger, slower, and cooler. We present Yohkoh images of each of these events. In the large arcade events (see Alexander et al., 1996, for a detailed look at one of them) the magnetic morphology at the location of the Yohkoh arcade is generally consistent with the development of a large system of loops. Some of the identifications are ambiguous, and we summarize the reasons for this. From the SWICS data we have obtained ionization temperatures for several events, and find that they have no obvious pattern in relation to the X-ray temperatures; this may be expected on the basis that the interplanetary plasma cloud is physically distinct from the plasma trapped in the corona. Soft X-ray observations of the solar corona show occasional occurrences of large-scale brightenings in the form of arcades of loops. Such structures have been known since Skylab (e.g., Sturrock, 1980), and have a clear relationship with coronal mass ejections (e.g., Kahler, 1977). We now may study this phenomenon statistically with the much more comprehensive Yohkoh observations; with Yohkoh movies we can also begin to extend our knowledge to the three-dimensional development of the structures. At the same time Ulysses has sampled the latitude dependence of the interplanetary effects. With this paper we introduce this subject and provide a preliminary listing of events from the passage of Ulysses through high heliographic latitudes. The starting point of the present survey is a list of interplanetary plasma clouds (IPC's) derived from Ulysses/SWICS data. These are essentially the same as the events termed CMEs by Gosling et al. (1994a, 1994b). For this identification the presence of bidirectional streaming in the suprathermal electron distribution is one of the main criteria. We note that there are no direct coronagraph observations, however. The Yohkoh observations were examined at the apparent time of origin of each Ulysses event, resulting in some clear and some less-certain identifications. We also studied the ionization temperatures of the IPC material as a beginning step to give the identifications a physical basis. There has been little study thus far of the Yohkoh soft X-ray observations in relationship to CMEs, which we believe to be closely related to the interplanetary disturbances. Hiei et al. (1993) reported the only Yohkoh event yet studied in conjunction with white-light coronagraph observations. However Klimchuk et al. (1994) showed that X-ray eruptive phenomena with parameters similar to those of CMEs occur frequently at the limb, and there have been several studies of individual eruptive events (e.g., Watanabe et al., 1992). Presently there is no systematic knowledge of the X-ray coronal counterparts of CMEs, and the survey represented here is part of the effort to rectify this situation. Title: Yohkoh/SXT soft x-ray observations of sudden mass loss from the solar corona Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Freeland, S. L.; Lemen, J. R.; Harvey, K. L. Bibcode: 1996AIPC..382...88H Altcode: With soft X-ray imaging we can study the entire coronal volume, except for cold inclusions such as prominences, as a function of time. This should allow us to observe the origins of coronal mass ejections. We report here an initial survey of the Yohkoh/SXT observations at the times of reported or apparent mass ejections: three LDE flare events and two large-scale arcade formations. For each of the events we can easily detect sudden coronal dimming, which we interpret as the launch interval of a CME. In one of the flare events we have found a well-defined plasma cloud, apparently formed from a set of loop structures, which rises and disappears during the growth phase of the flare emission. Its mass amounted to some 4×1014 g with a density of 3×108 cm-3 and a temperature of 2.8 MK before its disappearance. Title: The solar origins of two high-latitude interplanetary disturbances Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.; Kahler, S. W.; Kurokawa, H.; Lemen, J. R. Bibcode: 1996AIPC..382...84H Altcode: Two extremely similar interplanetary forward/reverse shock events, with bidirectional electron streaming, were detected by Ulysses in 1994 [Gosling et al., 1994]. Both events resulted in geomagnetic storms and presumably were associated with coronal mass ejections. In this paper we use the Yohkoh soft X-ray observations to characterize the conditions in the lower corona at the times appropriate for the launching of these two events. We find two strikingly different solar events to be the likeliest candidates: an LDE flare on 20 Feb. 1994, and a extremely large-scale arcade event on 14 April 1994. Title: Comparison of YOHKOH x-ray coronal events with ULYSSES interplanetary events Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.; Harvey, K. L.; Hoecksema, J. T.; Zhao, X.; Hudson, H. Bibcode: 1995sowi.conf...58L Altcode: The Yohkoh soft X-ray telescope (SXT) has observed several largescale eruptive events per year for the first three years of observations (Aug. 1991 - Nov. 1994) Such events are most prominent at high latitudes, but resemble long-duration flare events seen in active regions. Some of the high-latitude events have now been identified in the Ulysses/SWICS data base during the Ulysses south polar passage. There are puzzling examples of solar events with no interplanetary counterparts. A comparison of coronal and interplanetary events can lead to better models for mapping interplanetary disturbances back to their source location, especially by combining Yohkoh morphology with three-dimensional representations of the coronal magnetic field. In this paper we describe the parameters of the hot plasma seen by SXT. There is clear evidence for non radial motion in specific events. We present comparisons between the ionization temperature of the interplanetary plasma with that observed at the Sun in cases where this is possible. Title: Yohkoh/SXT soft x-ray observations of sudden mass loss from the solar corona Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Freeland, S. L.; Lemen, J. R.; Harvey, K. L. Bibcode: 1995sowi.confR..58H Altcode: Direct X-ray observations allow us to estimate the hot coronal mass before and after a flare or other disturbance of the type leading to a coronal mass ejection. The sudden disappearance of a large coronal structure (scale greater than 105 km) gives evidence that an ejection has occurred, if the time scales are much shorter than the conductive or radiative cooling times for such structures. A flare also typically adds large amounts of new material to the corona via evaporation resulting from the coronal energy release. This provides a competing mechanism that makes the estimation of the total mass loss somewhat difficult. We note that the X-ray observations have the advantage of covering the entire corona rather than the limb regions unlike the coronagraph observations. We have identified two examples of coronal mass disappearances. before and during long duration flare events on 21 Feb. 1992 (on the E limb) and 13 Nov. 1994 (near disk center). In latter case the total mass amounted to some 4 x 1014 g with a density of 3 x 108cm-3 and a temperature of 2.8 MK before its disappearance. This corresponds to a radiative cooling time of some 104 S. much longer than the observed time of disappearance. We therefore suggest that these sudden mass disappearances correspond with coronal mass ejections (CMEs), and suggest that further data analysis will be able to confirm this by comparison with optical observations of specific CMEs. Title: The solar origins of two high-latitude interplanetary disturbances Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.; Kurokawa, H.; Kahler, S.; Lemen, J. R. Bibcode: 1995sowi.confS..58H Altcode: Two extremely similar interplanetary forward/reverse shock events, with bidirectional electron streaming were detected by Ulysses in 1994. Ground-based and Yohkoh/SXT observations show two strikingly different solar events that could be associated with them: an LDE flare on 20 Feb. 1994, and a extremely large-scale eruptive event on 14 April 1994. Both events resulted in geomagnetic storms and presumably were associated with coronal mass ejections. The sharply contrasting nature of these solar events argues against an energetic causal relationship between them and the bidirectional streaming events observed by Ulysses during its S polar passage. We suggest instead that for each pair of events. a common solar trigger may have caused independent instabilities leading to the solar and interplanetary phenomena.