Author name code: antiochos ADS astronomy entries on 2022-09-14 author:"Antiochos, Spiro K." ------------------------------------------------------------------------ Title: Advancing Theory and Modeling Efforts in Heliophysics Authors: Guo, Fan; Antiochos, Spiro; Cassak, Paul; Chen, Bin; Chen, Xiaohang; Dong, Chuanfei; Downs, Cooper; Giacalone, Joe; Haggerty, Colby C.; Ji, Hantao; Karpen, Judith; Klimchuk, James; Li, Wen; Li, Xiaocan; Oka, Mitsuo; Reeves, Katharine K.; Swisdak, Marc; Tu, Weichao Bibcode: 2022arXiv220903611G Altcode: Heliophysics theory and modeling build understanding from fundamental principles to motivate, interpret, and predict observations. Together with observational analysis, they constitute a comprehensive scientific program in heliophysics. As observations and data analysis become increasingly detailed, it is critical that theory and modeling develop more quantitative predictions and iterate with observations. Advanced theory and modeling can inspire and greatly improve the design of new instruments and increase their chance of success. In addition, in order to build physics-based space weather forecast models, it is important to keep developing and testing new theories, and maintaining constant communications with theory and modeling. Maintaining a sustainable effort in theory and modeling is critically important to heliophysics. We recommend that all funding agencies join forces and consider expanding current and creating new theory and modeling programs--especially, 1. NASA should restore the HTMS program to its original support level to meet the critical needs of heliophysics science; 2. a Strategic Research Model program needs to be created to support model development for next-generation basic research codes; 3. new programs must be created for addressing mission-critical theory and modeling needs; and 4. enhanced programs are urgently required for training the next generation of theorists and modelers. Title: Quantifying magnetic reconnection in the Solar corona Authors: Aslanyan, Valentin; Pontin, David; Wyper, Peter; Antiochos, Spiro; Scott, Roger; Higginson, Aleida Bibcode: 2022cosp...44.1495A Altcode: Magnetic reconnection is understood to have important effects on the dynamics of the Solar atmosphere, including those that lead to the formation of the slow Solar wind. Of particular importance is interchange reconnection between very long "open" field lines emerging from coronal holes into the heliosphere and shorter "closed" field lines between two points on the photosphere. We have used the Adaptively Refined Magnetohydrodynamic Solver to perform a number of simulations of the global corona with varying magnetic geometries, from which we subsequently determine regions where reconnection has taken place. Energy is injected into the magnetic field by plasma flows at the photosphere which transport the footpoints of field lines. We find that the total reconnected magnetic flux of numerous localized vortices representing supergranules exceeds that of a global differential rotation profile. We also find systematic differences in the interchange reconnection rates based on the length of the closed field lines involved. Our simulations show that shorter closed field lines of pseudostreamers reconnect more readily than the longer field lines of helmet streamers. Consequently, we predict smoother coronal hole boundaries in the vicinity of pseudostreamers than other coronal structures. We have identified signatures of these processes which may be detected both remotely and in-situ by spacecraft such as the Solar Dynamics Observatory, Parker Solar Probe, and Solar Orbiter. Title: Relating the variability of the middle corona to the structure of the slow solar wind Authors: Higginson, Aleida; DeVore, C. Richard; Antiochos, Spiro; Viall, Nicholeen Bibcode: 2022cosp...44.1320H Altcode: The recent revolution in heliospheric measurements, brought about by NASAʼs Parker Solar Probe and ESAʼs Solar Orbiter, has shown that processes in the middle corona can influence the structure and dynamics of the solar wind across spatial scales. Understanding the formation of the young solar wind structures currently being measured by Parker Solar Probe and Solar Orbiter is now essential. Numerical calculations have shown that magnetic field dynamics at coronal hole boundaries in the middle corona, in particular interchange reconnection driven by photospheric motions, can be responsible for the dynamic release of structured slow solar wind, including along huge separatrix-web (S-Web) arcs formed by pseudostreamers. Quantifying the plasma and magnetic variability along the heliospheric current sheet and these S-Web arcs is crucial to furthering our understanding of how coronal magnetic field dynamics can influence the slow solar wind throughout the heliosphere. Here we present fully dynamic, 3D numerical calculations of a coronal hole boundary driven continuously by realistic photospheric motions at its base. We consider our simulation results within the context of Parker Solar Probe and Solar Orbiter, and make predictions for the structure and variability of the young slow solar wind. Title: Driving of Heliospheric Structure and Dynamics by the Closed Corona Authors: Antiochos, Spiro; Schlenker, Michael; MacNeice, Peter; Mason, Emily Bibcode: 2022cosp...44.1082A Altcode: The heliosphere is observed to have structure and dynamics on a vast range of spatial and temporal scales. Much of the observed dynamics is turbulent in nature, and is believed to be generated primarily in situ by stream-stream interactions for example; but, coherent structures such as magnetic islands are also ubiquitous. These are especially common near the Heliospheric Current Sheet (HCS) where the slow wind is generally located. Since the HCS maps down to the Y-null at the top of the streamer belt, it has long been proposed that interactions between the closed and open flux are the origin of much of the HCS dynamics and, perhaps, of the slow wind, itself. We investigate thermal non-equilibrium (TNE) in the closed field as a possible driver of HCS dynamics. TNE is a process by which condensations form quasi-randomly in coronal loops as a result of spatial localization of the coronal heating. We use 2.5D MHD simulations of the corona and inner heliosphere that include a full thermodynamics treatment of the plasma. Our calculations show that plasma dynamics in the closed field couple to magnetic dynamics that end up driving reconnection in the HCS. Our conclusion is that coronal heating very low in the solar atmosphere, near the chromosphere, may well be responsible for the quasi-periodic dynamics observed far out in the heliosphere. We discuss the implications of our model for observations. This work was supported by the NASA Living With a Star, GSFC/ISFM, and DRIVE Center Programs. Title: Driving Solar Eruptions by Flux Rope Emergence Authors: Antiochos, Spiro; Linton, Mark; Leake, James Bibcode: 2022cosp...44.2405A Altcode: Solar eruptive events (SEEs) consisting of a filament ejection, fast coronal mass ejections (CME) and X-class flare are the most powerful explosions in our solar system and the primary drivers of highly destructive space weather at Earth and in interplanetary space. SEEs are known to be due to the release of the free magnetic energy stored in a filament channel; consequently, the formation of the filament field is the fundamental origin of SEEs. Flux emergence has long been observed to be a primary mechanism for filament channel formation, and the emergence into the corona of a sub-photospheric twisted flux rope, which leads to a filament channel, has been modeled by many authors. The key point of the emergence models is that they make no assumptions on the nature of the filament field, sheared arcade or twisted flux rope, the channel forms self-consistently as a result of the interaction between the corona and the convection zone. In previous work we showed that the alignment of the subsurface twisted flux rope with respect to a pre-existing coronal arcade plays the determining role in whether eruption occurs or not. In this presentation we describe the effect of the relative orientation of the coronal and subsurface fluxes on coronal evolution, and demonstrate that a range of eruptive/non-eruptive behaviors can occur depending on this parameter. We discuss the implications of our results for understanding the eruption mechanism and for interpreting observations. This work was supported by the NASA Living With a Star, GSFC/ISFM, and DRIVE Center Programs. Title: The Role of Reconnection in the Onset of Solar Eruptions Authors: Leake, James E.; Linton, Mark G.; Antiochos, Spiro K. Bibcode: 2022ApJ...934...10L Altcode: 2022arXiv220512957L Solar eruptive events such as coronal mass ejections and eruptive flares are frequently associated with the emergence of magnetic flux from the convection zone into the corona. We use three-dimensional magnetohydrodynamic numerical simulations to study the interaction of coronal magnetic fields with emerging flux and determine the conditions that lead to eruptive activity. A simple parameter study is performed, varying the relative angle between emerging magnetic flux and a preexisting coronal dipole field. We find that in all cases the emergence results in a sheared magnetic arcade that transitions to a twisted coronal flux rope via low-lying magnetic reconnection. This structure, however, is constrained by its own outer field and so is noneruptive in the absence of reconnection with the overlying coronal field. The amount of this overlying reconnection is determined by the relative angle between the emerged and preexisting fields. The reconnection between emerging and preexisting fields is necessary to generate sufficient expansion of the emerging structure so that flare-like reconnection below the coronal flux rope becomes strong enough to trigger its release. Our results imply that the relative angle is the key parameter in determining whether the resultant active regions exhibit eruptive behavior and is thus a potentially useful candidate for predicting eruptions in newly emerging active regions. More generally, our results demonstrate that the detailed interaction between the convection zone/photosphere and the corona must be calculated self-consistently in order to model solar eruptions accurately. Title: Variability of the Reconnection Guide Field in Solar Flares Authors: Dahlin, Joel T.; Antiochos, Spiro K.; Qiu, Jiong; DeVore, C. Richard Bibcode: 2022ApJ...932...94D Altcode: 2021arXiv211004132D Solar flares may be the best-known examples of the explosive conversion of magnetic energy into bulk motion, plasma heating, and particle acceleration via magnetic reconnection. The energy source for all flares is the highly sheared magnetic field of a filament channel above a polarity inversion line (PIL). During the flare, this shear field becomes the so-called reconnection guide field (i.e., the nonreconnecting component), which has been shown to play a major role in determining key properties of the reconnection, including the efficiency of particle acceleration. We present new high-resolution, three-dimensional, magnetohydrodynamics simulations that reveal the detailed evolution of the magnetic shear/guide field throughout an eruptive flare. The magnetic shear evolves in three distinct phases: shear first builds up in a narrow region about the PIL, then expands outward to form a thin vertical current sheet, and finally is transferred by flare reconnection into an arcade of sheared flare loops and an erupting flux rope. We demonstrate how the guide field may be inferred from observations of the sheared flare loops. Our results indicate that initially the guide field is larger by about a factor of 5 than the reconnecting component, but it weakens by more than an order of magnitude over the course of the flare. Instantaneously, the guide field also varies spatially over a similar range along the three-dimensional current sheet. We discuss the implications of the remarkable variability of the guide field for the timing and localization of efficient particle acceleration in flares. Title: The Dynamic Structure of Coronal Hole Boundaries Authors: Aslanyan, V.; Pontin, D. I.; Scott, R. B.; Higginson, A. K.; Wyper, P. F.; Antiochos, S. K. Bibcode: 2022ApJ...931...96A Altcode: The boundaries of solar coronal holes are difficult to uniquely define observationally but are sites of interest in part because the slow solar wind appears to originate there. The aim of this article is to explore the dynamics of interchange magnetic reconnection at different types of coronal hole boundaries-namely streamers and pseudostreamers-and their implications for the coronal structure. We describe synthetic observables derived from three-dimensional magnetohydrodynamic simulations of the atmosphere of the Sun in which coronal hole boundaries are disturbed by flows that mimic the solar supergranulation. Our analysis shows that interchange reconnection takes place much more readily at the pseudostreamer boundary of the coronal hole. As a result, the portion of the coronal hole boundary formed by the pseudostreamer remains much smoother, in contrast to the highly distorted helmet-streamer portion of the coronal hole boundary. Our results yield important new insights on coronal hole boundary regions, which are critical in coupling the corona to the heliosphere as the formation regions of the slow solar wind. Title: The Dynamic Coupling of Streamers and Pseudostreamers to the Heliosphere Authors: Aslanyan, V.; Pontin, D. I.; Higginson, A. K.; Wyper, P. F.; Scott, R. B.; Antiochos, S. K. Bibcode: 2022ApJ...929..185A Altcode: 2022arXiv220102388A The slow solar wind is generally believed to result from the interaction of open and closed coronal magnetic flux at streamers and pseudostreamers. We use three-dimensional magnetohydrodynamic simulations to determine the detailed structure and dynamics of open-closed interactions that are driven by photospheric convective flows. The photospheric magnetic field model includes a global dipole giving rise to a streamer together with a large parasitic polarity region giving rise to a pseudostreamer that separates a satellite coronal hole from the main polar hole. Our numerical domain extends out to 30R ⊙ and includes an isothermal solar wind, so that the coupling between the corona and heliosphere can be calculated rigorously. This system is driven by imposing a large set of quasi-random surface flows that capture the driving of coronal flux in the vicinity of streamer and pseudostreamer boundaries by the supergranular motions. We describe the resulting structures and dynamics. Interchange reconnection dominates the evolution at both streamer and pseudostreamer boundaries, but the details of the resulting structures are clearly different from one another. Additionally, we calculate in situ signatures of the reconnection and determine the dynamic mapping from the inner heliosphere back to the Sun for a test spacecraft orbit. We discuss the implications of our results for interpreting observations from inner heliospheric missions, such as Parker Solar Probe and Solar Orbiter, and for space weather modeling of the slow solar wind. Title: SynthIA: A Synthetic Inversion Approximation for the Stokes Vector Fusing SDO and Hinode into a Virtual Observatory Authors: Higgins, Richard E. L.; Fouhey, David F.; Antiochos, Spiro K.; Barnes, Graham; Cheung, Mark C. M.; Hoeksema, J. Todd; Leka, K. D.; Liu, Yang; Schuck, Peter W.; Gombosi, Tamas I. Bibcode: 2022ApJS..259...24H Altcode: 2021arXiv210812421H Both NASA's Solar Dynamics Observatory (SDO) and the JAXA/NASA Hinode mission include spectropolarimetric instruments designed to measure the photospheric magnetic field. SDO's Helioseismic and Magnetic Imager (HMI) emphasizes full-disk, high-cadence, and good-spatial-resolution data acquisition while Hinode's Solar Optical Telescope Spectro-Polarimeter (SOT-SP) focuses on high spatial resolution and spectral sampling at the cost of a limited field of view and slower temporal cadence. This work introduces a deep-learning system, named the Synthetic Inversion Approximation (SynthIA), that can enhance both missions by capturing the best of each instrument's characteristics. We use SynthIA to produce a new magnetogram data product, the Synthetic Hinode Pipeline (SynodeP), that mimics magnetograms from the higher-spectral-resolution Hinode/SOT-SP pipeline, but is derived from full-disk, high-cadence, and lower-spectral-resolution SDO/HMI Stokes observations. Results on held-out data show that SynodeP has good agreement with the Hinode/SOT-SP pipeline inversions, including magnetic fill fraction, which is not provided by the current SDO/HMI pipeline. SynodeP further shows a reduction in the magnitude of the 24 hr oscillations present in the SDO/HMI data. To demonstrate SynthIA's generality, we show the use of SDO/Atmospheric Imaging Assembly data and subsets of the HMI data as inputs, which enables trade-offs between fidelity to the Hinode/SOT-SP inversions, number of observations used, and temporal artifacts. We discuss possible generalizations of SynthIA and its implications for space-weather modeling. This work is part of the NASA Heliophysics DRIVE Science Center at the University of Michigan under grant NASA 80NSSC20K0600E, and will be open-sourced. Title: Correlated Spatio-temporal Evolution of Extreme-Ultraviolet Ribbons and Hard X-Rays in a Solar Flare Authors: Naus, S. J.; Qiu, J.; DeVore, C. R.; Antiochos, S. K.; Dahlin, J. T.; Drake, J. F.; Swisdak, M. Bibcode: 2022ApJ...926..218N Altcode: 2021arXiv210915314N We analyze the structure and evolution of ribbons from the M7.3 SOL2014-04-18T13 flare using ultraviolet images from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), magnetic data from the SDO/Helioseismic and Magnetic Imager, hard X-ray (HXR) images from the Reuven Ramaty High Energy Solar Spectroscopic Imager, and light curves from the Fermi/Gamma-ray Burst Monitor, in order to infer properties of coronal magnetic reconnection. As the event progresses, two flare ribbons spread away from the magnetic polarity inversion line. The width of the newly brightened front along the extension of the ribbon is highly intermittent in both space and time, presumably reflecting nonuniformities in the structure and/or dynamics of the flare current sheet. Furthermore, the ribbon width grows most rapidly in regions exhibiting concentrated nonthermal HXR emission, with sharp increases slightly preceding the HXR bursts. The light curve of the ultraviolet emission matches the HXR light curve at photon energies above 25 keV. In other regions the ribbon-width evolution and light curves do not temporally correlate with the HXR emission. This indicates that the production of nonthermal electrons is highly nonuniform within the flare current sheet. Our results suggest a strong connection between the production of nonthermal electrons and the locally enhanced perpendicular extent of flare ribbon fronts, which in turn reflects the inhomogeneous structure and/or reconnection dynamics of the current sheet. Despite this variability, the ribbon fronts remain nearly continuous, quasi-one-dimensional features. Thus, although the reconnecting coronal current sheets are highly structured, they remain quasi-two-dimensional and the magnetic energy release occurs systematically, rather than stochastically, through the volume of the reconnecting magnetic flux. Title: Driving of Reconnection in the Heliospheric Current Sheet by Thermal Nonequilibrium Authors: Antiochos, Spiro; Schlenker, Michael; MacNeice, Peter; Mason, Emily Bibcode: 2021AGUFMSH25F2155A Altcode: Observations have shown that the coupling between the corona and heliosphere is intrinsically dynamic with quasi-periodic density structures ubiquitously seen in the Heliospheric Current Sheet (HCS) that maps down to the top of the streamer belt. These structures have been identified by several authors as due to magnetic reconnection that produces magnetic islands in the HCS. Such islands have important implications for understanding the origins of heliospheric energetic particles and plasma/field variability. A key feature of the density structures is their quasi-periodicity, on time scales of one to two hours. We propose that the mechanism responsible for the periodicity is thermal nonequilibrium (TNE), a process by which solar coronal loops undergo quasi-periodic cycles of heating and cooling due to the spatial localization of coronal heating near the loop base. Since the requirement for TNE onset is that the loop length is large compared to the scale of the heating, it is most likely to occur on the largest coronal loops, those near the open-closed boundary. We use 2.5D MHD numerical simulations to investigate the effect of TNE in the corona and heliosphere of an axisymmetric helmet streamer and polar coronal holes. As in the many 1D loop studies, we find that TNE occurs in coronal loops with sufficiently large length, but in contrast to previous studies, we find that the process also drives substantial magnetic dynamics, especially near the top of the streamer where the plasma beta becomes of order unity. From the simulation results we determine predictions for spectroscopic and imaging observations of both the hot and cool helmet streamer plasma and the solar wind near to the streamer stalk. We conclude that TNE occurring in the largest closed loops in the corona may explain several puzzling observations of the corona and wind, such as the ubiquitous blue shifts observed at the edges of active regions, and the quasi-periodic solar wind blobs. This work was supported by the NASA Living With a Star Program. Title: How does photospheric driving effect helmet streamer reconnection and HCS structure? Authors: Higginson, Aleida; Antiochos, Spiro; DeVore, C. Richard Bibcode: 2021AGUFMSH25F2146H Altcode: It is now well understood that the transition from coronal helmet streamers to the corresponding current sheet in the heliosphere is a region of increased magnetic field dynamics. The boundary between the two structures is the preferred location for both interchange magnetic reconnection and helmet streamer pinch-off reconnection, both of which are believed to be important processes for the formation of the solar wind. These magnetic field dynamics can leave behind imprints in the solar wind in the form of heliospheric current sheet (HCS) blobs, which have been observed across a continuum of temporal and spatial scales in remote and in situ observations. As we begin to unravel the unprecedented data returned from missions like Parker Solar Probe (PSP) and Solar Orbiter (SO), it becomes more important than ever to understand the dynamic processes occurring in this middle corona region and to determine the subsequent effects on the solar wind. We present here simulations of a helmet streamer - HCS system with an isothermal solar wind driven by photospheric motions where we fully resolve the entire separatrix system. We quantify the effect of photospheric driving on the formation of the HCS blobs and make predictions for PSP, SO, and future observations as these resulting structures propagate out into the solar wind and heliosphere. Title: Correlated Spatio-temporal Evolution of Extreme-Ultraviolet Ribbons and Hard X-rays in a Solar Flare Authors: Naus, Stephen; Qiu, Jiong; Antiochos, Spiro; Dahlin, Joel; DeVore, C. Richard; Drake, James; Swisdak, Marc Bibcode: 2021AGUFMSH23B..05N Altcode: We analyzed the structure and evolution of flare ribbons in the solar chromosphere to infer properties of coronal magnetic reconnection. We used ultraviolet (UV) imaging observations of the M7.3 SOL2014-04-18T13 flare obtained by IRIS and SDO/AIA, magnetic data from SDO/HMI, and hard X-ray (HXR) images from RHESSI and light curves from Fermi/GBM. Two flare ribbons spread away from the magnetic polarity inversion line as the event progressed. From high-resolution IRIS observations, we measured the width of the newly brightening front along the extension of the ribbon, which maps the feet of magnetic field lines reconnecting in the corona. We find that the ribbon front is highly intermittent in both space and time, presumably reflecting non-uniformities in the structure and/or dynamics of the flare current sheet. Early in the event, the ribbon fronts form and widen to near maximum thickness, and the rate of change of reconnected magnetic flux rises to its peak value. Subsequently, the flux change rate drops steeply, the UV light curves continue to rise toward their maxima, and the HXR emissions rise rapidly to their own maxima, which are simultaneous with those in the UV. We find indirect evidence that well-resolved local peaks in the UV ribbon-front widths are cospatial and cotemporal with the UV emissions and poorly resolved HXR emissions. This result suggests that there is a strong connection between the production of non-thermal electrons and locally enhanced perpendicular extent of flare ribbon fronts, which reflect the inhomogeneous structure and/or reconnection dynamics of the flare current sheet in the corona. We discuss the implications of these results for understanding the inhomogeneous structure and dynamics of the coronal flare current sheet and the origin of the flare-accelerated particles. NASA supported our research via the SOLFER DRIVE Center at UMD, the H-GI program at MSU, and the H-ISFM program at GSFC. Title: Temporal Evolution of the Guide Field in Eruptive Flares Authors: Dahlin, Joel; Antiochos, Spiro; Jiong, Qiu; DeVore, C. Richard Bibcode: 2021AGUFMSH23B..07D Altcode: Solar flares are explosive space weather events that rapidly convert stored magnetic energy into bulk motion, plasma heating, and particle acceleration via magnetic reconnection. For all flares, the free energy source is ultimately the highly sheared magnetic field of a filament channel above a polarity inversion line. During the flare, the shear field becomes the reconnection guide field, the strength of which is widely believed to control the efficiency of reconnection-driven particle acceleration. We present new high-resolution 3D MHD simulations that calculate the evolution of the magnetic shear/guide field throughout an eruptive flare. The magnetic shear evolves in three distinct phases: shear first builds up in a narrow region about the PIL, expands outward to drive the formation of a thin current sheet, and is finally transferred by the flare reconnection into sheared post-flare loops and erupting flux rope. We show that the guide field weakens more than an order of magnitude over the course of the flare, and instantaneously varies over a similar range along the three-dimensional current sheet. We demonstrate how the guide field may be inferred from observations of sheared post-flare loops. Interestingly, we find that the number of plasmoids in the flare reconnecting current sheet increases with weakening guide field, underscoring the important role of the guide field in particle acceleration. We discuss implications for observations by IRIS, SDO/AIA, and DKIST. This work was supported by NASA via the SOLFER DRIVE Center at UMD, the H-ISFM program, and the HGI program. Title: STITCH: A Subgrid-Scale Model for Energy Buildup in the Solar Corona Authors: Dahlin, Joel T.; DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2021arXiv211200641D Altcode: The solar corona routinely exhibits explosive activity, in particular coronal mass ejections and their accompanying eruptive flares, that have global-scale consequences. These events and their smaller counterparts, coronal jets, originate in narrow, sinuous filament channels. The key processes that form and evolve the channels operate on still smaller spatial scales and much longer time scales, culminating in a vast separation of characteristic lengths and times that govern these explosive phenomena. In this article, we describe implementation and tests of an efficient subgrid-scale model for generating eruptive structures in magnetohydrodynamics (MHD) coronal simulations. STITCH -- STatistical InjecTion of Condensed Helicity -- is a physics-based, reduced representation of helicity condensation: a process wherein small-scale vortical surface convection forms ubiquitous current sheets, and pervasive reconnection across the sheets mediates an inverse cascade of magnetic helicity and free energy, thereby forming the filament channels. STITCH abstracts these complex processes into a single new term, in the MHD Ohm's law and induction equation, which directly injects tangential magnetic flux into the low corona. We show that this approach is in very good agreement with a full helicity-condensation calculation that treats all of the dynamics explicitly, while enabling substantial reductions in temporal duration especially, but also in spatial resolution. In addition, we illustrate the flexibility of STITCH at forming localized filament channels and at energizing complex surface flux distributions that have sinuous boundaries. STITCH is simple to implement and computationally efficient, making it a powerful new technique for event-based, data-driven modeling of solar eruptions. Title: The Effect of Thermal Nonequilibrium on Helmet Streamers Authors: Schlenker, Michael J.; Antiochos, Spiro K.; MacNeice, Peter J.; Mason, Emily I. Bibcode: 2021ApJ...916..115S Altcode: Solar loops in which the coronal heating scale is short compared to the loop length are known to be susceptible to thermal nonequilibrium (TNE). We investigate the effects of this process on the largest loops in the corona, those of a helmet streamer. Our numerical study uses a 2.5D MHD code that includes the full magnetic field dynamics as well as the detailed plasma thermodynamics. The simulation model is axisymmetric, consisting of an equatorial streamer belt and two polar coronal holes. As in previous 1D loop studies, we find that TNE occurs in coronal loops with sufficiently large length, but in contrast to these studies, we find that the process also drives substantial magnetic dynamics, especially near the top of the streamer where the plasma beta becomes of order unity. From the simulation results we determine predictions for spectroscopic and imaging observations of both the hot and cool helmet streamer plasma. Simulations are preformed using different scale heights for the heating and different numerical resolution in order to determine the dependence of our findings on these important parameters. We conclude that TNE in streamers may explain several puzzling observations, such as the ubiquitous blueshifts observed at the edges of active regions. We also discuss the implications of our results for the solar wind. Title: Coupled Pseudostreamer/Helmet Streamer Eruptions Authors: Wyper, P.; Antiochos, S.; DeVore, C.; Lynch, B.; Karpen, J.; Kumar, P. Bibcode: 2021AAS...23832205W Altcode: An important aspect of solar activity is the coupling between eruptions and the surrounding coronal magnetic field topology. This coupling determines the trajectory and morphology of the event and can even trigger sympathetic eruptions from multiple sources. Here we report on a numerical simulation of a new type of coupled eruption, in which a large-scale coronal jet initiated by a pseudostreamer filament eruption triggers a streamer-blowout coronal mass ejection (CME). The initial pseudostreamer in our simulation is typical of many observed pseudostreamers in that it separates an equatorial and polar coronal hole and is associated with a broad S-Web arc in the heliosphere. Our results show that the coupled eruption is a result of the enhanced breakout reconnection that occurs above the erupting filament channel as the jet is launched and progresses into the neighbouring helmet streamer. This partially launches the jet along closed helmet streamer field lines which blows out the streamer top to produce a classic bubble-shaped CME. Another key finding is that the CME is strongly deflected from the jet's initial trajectory and contains a mixture of open and closed magnetic field lines. We present the detailed dynamics of this new type of coupled eruption and discuss the implications of this work for interpreting in-situ and remote-sensing observations and for understanding CME formation and evolution in general. Title: Switch-on Shock and Nonlinear Kink Alfvén Waves in Solar Coronal-Hole Jets Authors: DeVore, C. R.; Karpen, J. T.; Antiochos, S. K.; Uritsky, V. M.; Roberts, M. A.; Pariat, E. Bibcode: 2021AAS...23821322D Altcode: It is generally accepted that solar coronal-hole jets are generated by fast magnetic reconnection in the low corona, whether driven directly by flux emergence from below or indirectly by instability onset above the photosphere. In either case, twisted flux on closed magnetic field lines reconnects with untwisted flux on neighboring open field lines. Some of that twist is inherited by the newly reconnected open flux, which rapidly relaxes due to magnetic tension forces that transmit the twist impulsively into the outer corona and heliosphere. We suggest that the transfer of twist launches switch-on MHD shock waves, which propagate parallel to the ambient coronal magnetic field ahead of the shock and convect a perpendicular component of magnetic field behind the shock. In the frame moving with the shock front, the post-shock flow is precisely Alfvénic in all three directions, whereas the pre-shock flow is super-Alfvénic along the ambient magnetic field. Consequently, there is a density enhancement across the shock front. Nonlinear kink Alfvén waves are exact solutions of the time-dependent MHD equations in the post-shock region when the ambient corona is uniform and the magnetic field is straight. We report 3D spherical simulations of coronal-hole jets driven by instability onset in the corona. The results are consistent with the generation of MHD switch-on shocks trailed predominantly by incompressible, irrotational, kink Alfvén waves. We will discuss the implications of our results for understanding solar jets and interpreting their heliospheric signatures in light of the new data on S-bends (a.k.a. switchbacks) from Parker Solar Probe. Our research is supported by NASA's H-ISFM program. Title: An Observational Study of a "Rosetta Stone" Solar Eruption Authors: Mason, E. I.; Antiochos, Spiro K.; Vourlidas, Angelos Bibcode: 2021ApJ...914L...8M Altcode: 2021arXiv210509164M This Letter reports observations of an event that connects all major classes of solar eruptions: those that erupt fully into the heliosphere versus those that fail and are confined to the Sun, and those that eject new flux into the heliosphere, in the form of a flux rope, versus those that eject only new plasma in the form of a jet. The event originated in a filament channel overlying a circular polarity inversion line and occurred on 2016 March 13 during the extended decay phase of the active region designated NOAA 12488/12501. The event was especially well observed by multiple spacecraft and exhibited the well-studied null-point topology. We analyze all aspects of the eruption using Solar Dynamics Observatory Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager, Solar-Terrestrial Relations Observatory Extreme Ultraviolet Imager, and Solar and Heliospheric Observatory Large Angle and Spectrometric Coronagraph (SOHO LASCO) imagery. One section of the filament undergoes a classic failed eruption with cool plasma subsequently draining onto the section that did not erupt, but a complex structured coronal mass ejection/jet is clearly observed by SOHO/LASCO C2 shortly after the failed filament eruption. We describe in detail the slow buildup to eruption, the lack of an obvious trigger, and the immediate reappearance of the filament after the event. The unique mixture of major eruption properties observed during this event places severe constraints on the structure of the filament channel field and, consequently, on the possible eruption mechanism. Title: From Pseudostreamer Jets to Coronal Mass Ejections: Observations of the Breakout Continuum Authors: Kumar, P.; Karpen, J.; Antiochos, S.; Wyper, P.; DeVore, C.; Lynch, B. Bibcode: 2021AAS...23832203K Altcode: The magnetic breakout model, in which reconnection in the corona leads to destabilization of a filament channel, explains numerous features of eruptive solar events, from small-scale jets to global-scale coronal mass ejections (CMEs). The underlying multipolar topology, pre-eruption activities, and sequence of magnetic-reconnection onsets (first breakout, then flare) of many observed fast CMEs/eruptive flares are fully consistent with the model. Recently, we demonstrated that most observed coronal-hole jets in fan/spine topologies also are induced by breakout reconnection at the null point above a filament channel (with or without a filament). For these two types of eruptions occurring in similar topologies, the key question is, why do some events generate jets while others form CMEs? We focused on the initiation of eruptions in large bright points/small active regions that were located in coronal holes and clearly exhibited null-point (fan/spine) topologies: such configurations are referred to as pseudostreamers. We analyzed and compared Solar Dynamics Observatory/Atmospheric Imaging Assembly, Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph Experiment, and Reuven Ramaty High Energy Solar Spectroscopic Imager observations of three events. Our analysis of the events revealed two new observable signatures of breakout reconnection prior to the explosive jet/CME outflows and flare onset: coronal dimming and the opening up of field lines above the breakout current sheet. Most key properties were similar among the selected erupting structures, thereby eliminating region size, photospheric field strength, magnetic configuration, and pre-eruptive evolution as discriminating factors between jets and CMEs. We consider the factors that contribute to the different types of dynamic behavior, and conclude that the main determining factor is the ratio of the magnetic free energy associated with the filament channel compared to the energy associated with the overlying flux inside and outside the pseudostreamer dome. Title: Mind The Gap: Observing The Jet/CME Continuum In A Hybrid Eruption Authors: Mason, E.; Antiochos, S.; Vourlidas, A. Bibcode: 2021AAS...23821316M Altcode: Coronal mass ejections, jets, prominence eruptions: solar eruptions are an active field with a broad range of accepted phenomena, and an even broader range of proposed mechanisms that cause the phenomena. This talk reports the observations of an event that connects the major eruption classes, and could provide a holistic explanation for all of them. The event originated in a filament channel overlying a circular polarity inversion line (PIL) and occurred on 2013 March 13 during the extended decay phase of the active region designated (sequentially) NOAA 12488/12501. This event was especially well-observed by multiple spacecraft and was seen to have the well-studied null-point topology. We analyze all aspects of the eruption using SDO AIA and HMI, STEREO-A, and SOHO LASCO imagery. One section of the filament undergoes a classic failed eruption with cool plasma subsequently draining onto the section that did not erupt, but a complex structured CME/jet is clearly observed by SOHO LASCO C2 shortly after the failed filament eruption. We describe in detail the long, slow buildup to eruption; the lack of an obvious trigger; and the immediate reappearance of the filament after the event. The unique mixture of major eruption properties that are observed in this event places severe constraints on the structure of the filament channel field and, consequently, on the possible eruption mechanism. Title: The 3D Dynamics of Flare Reconnection Authors: Dahlin, J.; Antiochos, S.; Qiu, J.; DeVore, C.; Wyper, P. Bibcode: 2021AAS...23812710D Altcode: Solar flares are explosive space weather events that rapidly convert stored magnetic energy into bulk motion, plasma heating, and particle acceleration. Understanding the structure and dynamics of the magnetic reconnection that powers flares is critical for predicting the energy release. In particular, the amount of energy transferred to energetic particles is thought to be highly dependent on whether the reconnection is primarily turbulent (e.g., plasmoids) or instead laminar. We present new high-resolution MHD simulations of three-dimensional reconnection in an eruptive flare and compare the results to recent data. Although flare reconnection is challenging to observe directly in the corona, highly detailed constraints on its dynamics can be obtained from observations of flare ribbons that track the chromospheric footpoints of newly reconnected field lines. The analogues of flare ribbons in our simulations are identified by tracking discontinuous changes in field-line magnetic connectivity due to the reconnection. In our highest-resolution calculations, we find that these ribbon analogues are highly structured and exhibit many 'whorl' patterns that are linked to turbulent plasmoids in the reconnecting current sheet. Such flare ribbon fine structure therefore reveals crucial information about the fundamental turbulent vs. laminar nature of the reconnection critical for understanding particle acceleration. We discuss implications for SDO, IRIS, and GST observations of explosive flare energy release. Title: The Dynamic Formation of Pseudostreamers Authors: Scott, R. B.; Pontin, D. I.; Antiochos, S. K.; DeVore, C. R.; Wyper, P. F. Bibcode: 2021AAS...23832818S Altcode: Streamers and pseudostreamers structure the corona at the largest scales, as seen in both eclipse and coronagraph white-light images. Their inverted-goblet appearance encloses broad coronal loops at the Sun and tapers to a narrow radial stalk away from the star. The streamer associated with the global solar dipole magnetic field is long-lived, predominantly contains a single arcade of nested loops within it, and separates opposite-polarity interplanetary magnetic fields with the heliospheric current sheet anchored at its apex. Pseudostreamers, on the other hand, are transient, enclose double arcades of nested loops, and separate like-polarity fields with a dense plasma sheet. We use numerical magnetohydrodynamic simulations to calculate, for the first time, the formation of pseudostreamers in response to photospheric magnetic-field evolution. Convective transport of a minority-polarity flux concentration, initially positioned under one side of a streamer, through the streamer boundary into the adjacent, pre-existing coronal hole forms the pseudostreamer. Interchange magnetic reconnection at the overlying coronal null point(s) governs the development of the pseudostreamer above - and of a new, satellite coronal hole behind - the moving minority polarity. The reconnection dynamics liberate coronal-loop plasma that can escape into the heliosphere along so-called separatrix-web ("S-Web") arcs, which reach far from the heliospheric current sheet and the solar equatorial plane, and can explain the origin of high-latitude slow solar wind. We describe the implications of our results for in-situ and remote-sensing observations of the corona and heliosphere as obtained, most recently, by Parker Solar Probe and Solar Orbiter. Title: Relating the Variability of the Middle Corona to the Structure of the Slow Solar Wind Authors: Higginson, A.; Antiochos, S.; DeVore, C. Bibcode: 2021AAS...23822905H Altcode: The recent revolution in heliospheric measurements, brought about by NASA's Parker Solar Probe and ESA's Solar Orbiter, has shown that processes in the middle corona can influence the structure and dynamics of the solar wind across spatial scales. Understanding the formation of the young solar wind structures currently being measured by Parker Solar Probe and Solar Orbiter is now essential. Numerical calculations have shown that magnetic field dynamics at coronal hole boundaries in the middle corona, in particular interchange reconnection driven by photospheric motions, can be responsible for the dynamic release of structured slow solar wind, including along huge separatrix-web (S-Web) arcs formed by pseudostreamers. Quantifying the plasma and magnetic variability along these S-Web arcs is crucial to furthering our understanding of how coronal magnetic field dynamics can influence the slow solar wind throughout the heliosphere. Here we present fully dynamic, 3D numerical calculations of a coronal hole boundary driven continuously by realistic photospheric motions at its base. We consider our simulation results within the context of Parker Solar Probe and Solar Orbiter, and make predictions for the structure and variability of the young slow solar wind. Title: How Turbulent is the Magnetically Closed Corona? Authors: Klimchuk, James A.; Antiochos, Spiro K. Bibcode: 2021FrASS...8...83K Altcode: 2021arXiv210512212K We argue that the magnetically closed corona evolves primarily quasi-statically, punctuated by many localized bursts of activity associated with magnetic reconnection at a myriad of small current sheets. The sheets form by various processes that do not involve a traditional turbulent cascade whereby energy flows losslessly through a continuum of spatial scales starting from the large scale of the photospheric driving. If such an inertial range is a defining characteristic of turbulence, then the magnetically closed corona is not a turbulent system. It nonetheless has a complex structure that bears no direct relationship to the pattern of driving. Title: The Dynamic Formation of Pseudostreamers Authors: Scott, Roger B.; Pontin, David I.; Antiochos, Spiro K.; DeVore, C. Richard; Wyper, Peter F. Bibcode: 2021ApJ...913...64S Altcode: Streamers and pseudostreamers structure the corona at the largest scales, as seen in both eclipse and coronagraph white-light images. Their inverted-goblet appearance encloses broad coronal loops at the Sun and tapers to a narrow radial stalk away from the star. The streamer associated with the global solar dipole magnetic field is long-lived, predominantly contains a single arcade of nested loops within it, and separates opposite-polarity interplanetary magnetic fields with the heliospheric current sheet (HCS) anchored at its apex. Pseudostreamers, on the other hand, are transient, enclose double arcades of nested loops, and separate like-polarity fields with a dense plasma sheet. We use numerical magnetohydrodynamic simulations to calculate, for the first time, the formation of pseudostreamers in response to photospheric magnetic-field evolution. Convective transport of a minority-polarity flux concentration, initially positioned under one side of a streamer, through the streamer boundary into the adjacent preexisting coronal hole forms the pseudostreamer. Interchange magnetic reconnection at the overlying coronal null point(s) governs the development of the pseudostreamer above—and of a new satellite coronal hole behind—the moving minority polarity. The reconnection dynamics liberate coronal-loop plasma that can escape into the heliosphere along so-called separatrix-web ("S-Web") arcs, which reach far from the HCS and the solar equatorial plane, and can explain the origin of high-latitude slow solar wind. We describe the implications of our results for in situ and remote-sensing observations of the corona and heliosphere as obtained, most recently, by Parker Solar Probe and Solar Orbiter. Title: Fast and Accurate Emulation of the SDO/HMI Stokes Inversion with Uncertainty Quantification Authors: Higgins, Richard E. L.; Fouhey, David F.; Zhang, Dichang; Antiochos, Spiro K.; Barnes, Graham; Hoeksema, J. Todd; Leka, K. D.; Liu, Yang; Schuck, Peter W.; Gombosi, Tamas I. Bibcode: 2021ApJ...911..130H Altcode: 2021arXiv210317273H The Helioseismic and Magnetic Imager (HMI) on board NASA's Solar Dynamics Observatory produces estimates of the photospheric magnetic field, which are a critical input to many space weather modeling and forecasting systems. The magnetogram products produced by HMI and its analysis pipeline are the result of a per-pixel optimization that estimates solar atmospheric parameters and minimizes disagreement between a synthesized and observed Stokes vector. In this paper, we introduce a deep-learning-based approach that can emulate the existing HMI pipeline results two orders of magnitude faster than the current pipeline algorithms. Our system is a U-Net trained on input Stokes vectors and their accompanying optimization-based Very Fast Inversion of the Stokes Vector (VFISV) inversions. We demonstrate that our system, once trained, can produce high-fidelity estimates of the magnetic field and kinematic and thermodynamic parameters while also producing meaningful confidence intervals. We additionally show that despite penalizing only per-pixel loss terms, our system is able to faithfully reproduce known systematic oscillations in full-disk statistics produced by the pipeline. This emulation system could serve as an initialization for the full Stokes inversion or as an ultrafast proxy inversion. This work is part of the NASA Heliophysics DRIVE Science Center (SOLSTICE) at the University of Michigan, under grant NASA 80NSSC20K0600E, and will be open sourced. Title: A Model for the Coupled Eruption of a Pseudostreamer and Helmet Streamer Authors: Wyper, P. F.; Antiochos, S. K.; DeVore, C. R.; Lynch, B. J.; Karpen, J. T.; Kumar, P. Bibcode: 2021ApJ...909...54W Altcode: 2021arXiv210101962W A highly important aspect of solar activity is the coupling between eruptions and the surrounding coronal magnetic field topology, which determines the trajectory and morphology of the event and can even lead to sympathetic eruptions from multiple sources. In this paper, we report on a numerical simulation of a new type of coupled eruption, in which a coronal jet initiated by a large pseudostreamer filament eruption triggers a streamer-blowout coronal mass ejection (CME) from the neighboring helmet streamer. Our configuration has a large opposite-polarity region positioned between the polar coronal hole and a small equatorial coronal hole, forming a pseudostreamer flanked by the coronal holes and the helmet streamer. Further out, the pseudostreamer stalk takes the shape of an extended arc in the heliosphere. We energize the system by applying photospheric shear along a section of the polarity inversion line within the pseudostreamer. The resulting sheared-arcade filament channel develops a flux rope that eventually erupts as a classic coronal-hole-type jet. However, the enhanced breakout reconnection above the channel as the jet is launched progresses into the neighboring helmet streamer, partially launching the jet along closed helmet streamer field lines and blowing out the streamer top to produce a classic bubble-like CME. This CME is strongly deflected from the jet's initial trajectory and contains a mixture of open and closed magnetic field lines. We present the detailed dynamics of this new type of coupled eruption, its underlying mechanisms, and the implications of this work for the interpretation of in situ and remote-sensing observations. Title: Effects of Pseudostreamer Boundary Dynamics on Heliospheric Field and Wind Authors: Aslanyan, V.; Pontin, D. I.; Wyper, P. F.; Scott, R. B.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2021ApJ...909...10A Altcode: Interchange reconnection has been proposed as a mechanism for the generation of the slow solar wind, and a key contributor to determining its characteristic qualities. In this paper we study the implications of interchange reconnection for the structure of the plasma and field in the heliosphere. We use the Adaptively Refined Magnetohydrodynamic Solver to simulate the coronal magnetic evolution in a coronal topology containing both a pseudostreamer and helmet streamer. We begin with a geometry containing a low-latitude coronal hole that is separated from the main polar coronal hole by a pseudostreamer. We drive the system by imposing rotating flows at the solar surface within and around the low-latitude coronal hole, which leads to a corrugation (at low altitudes) of the separatrix surfaces that separate open from closed magnetic flux. Interchange reconnection is induced both at the null points and separators of the pseudostreamer, and at the global helmet streamer. We demonstrate that a preferential occurrence of interchange reconnection in the "lanes" between our driving cells leads to a filamentary pattern of newly opened flux in the heliosphere. These flux bundles connect to but extend far from the separatrix-web (S-Web) arcs at the source surface. We propose that the pattern of granular and supergranular flows on the photosphere should leave an observable imprint in the heliosphere. Title: Particle acceleration in erupting 3D coronal mass ejections in the breakout model Authors: Xia, Qian; Zharkova, Valentina; Dahlin, Joel; Antiochos, Spiro Bibcode: 2021cosp...43E1005X Altcode: We examine particle energisation in CMEs generated via the breakout mechanism and explore both 2D and 3D MHD configurations. In the breakout scenario, reconnection at a breakout current sheet (CS) initiates the flux rope eruption by destabilizing the quasi-static force balance. Reconnection at the flare CS triggers the fast acceleration of the CME, which forms flare loops below and triggers particle acceleration in flares. We present test-particle studies that focus on two selected times during the impulsive and decay phases of the eruption and obtain particle energy gains and spatial distributions. We find that particles are accelerated more efficiently in the flare CS than in the breakout CS even in the presence of large magnetic islands. The maximum particle energy gain is estimated from the energization terms based on the guiding-centre approximation. Particles are first accelerated in the CSs (with or without magnetic islands) where Fermi-type acceleration dominates. Accelerated particles escape to the interplanetary space along open field lines rather than trapped in flux ropes, precipitate into the chromosphere along the flare loops, or become trapped in the flare loop top due to the magnetic mirror structure. Some trapped particles are re-accelerated, either via re-injection to the flare CS or through a local Betatron-type acceleration associated with compression of the magnetic field. The energy gains of particles result in relatively hard energy spectra during the impulsive phase. During the gradual phase, the relaxation of the shear in the magnetic field reduces the guiding magnetic field in the flare CS, which leads to a decrease in particle energization efficiency. Title: The Role of 3D Reconnection in the Escape of Impulsive SEPs Authors: Antiochos, Spiro; Masson, Sophie; DeVore, C. Richard Bibcode: 2021cosp...43E1004A Altcode: It is widely accepted that impulsive solar energetic particle (SEP) events are due to the escape into the interplanetary medium of flare-accelerated particles produced by solar eruptive events. According to the standard solar eruption model, however, particles accelerated by flare reconnection should remain trapped in the closed field lines of the flare loops and the flux rope comprising the coronal mass ejection. To resolve this paradox, we performed fully 3D high-resolution MHD simulations of a CME/eruptive flare in a coronal system that consists of a bipolar active region embedded in a background global dipole field structured by solar wind. Our simulations show that multiple magnetic reconnection episodes occur prior to and during the CME eruption and its interplanetary propagation. In addition to the episodes that build up the flux rope, reconnection between the open field and the CME couples the closed corona to the open interplanetary field. Flare-accelerated particles initially trapped in the CME thereby gain access to the open interplanetary field along a trail blazed by magnetic reconnection. A key difference between these 3D results and our previous 2.5D calculations is that the interchange reconnection allows accelerated particles to escape from deep within the CME flux rope. We estimate the spatial extent of the particle-escape channels. The relative timings between flare acceleration and release of the energetic particles through CME/open-field coupling are also determined. We discuss the implications of these results for Parker Solar Probe and Solar Orbiter observations. This work was supported by the NASA Living with a Star Program. Title: Understanding the Onset of CMEs/Eruptive Flares Authors: Antiochos, Spiro; Linton, Mark Bibcode: 2021cosp...43E2397A Altcode: The most important drivers of destructive space weather are the giant solar eruptions consisting of a filament ejection, an intense X-ray flare, and a fast coronal mass ejection (CME). These major eruptive events drive space weather such as particle radiation throughout interplanetary space, powerful geomagnetic storms, and ground-level electric-power disruptions. Understanding the physical origin of these major eruptive events is absolutely essential for developing an eventual first-principles-based predictive capability. It is well-known that solar eruptions are due to the explosive release of free magnetic energy that is slowly built up in the corona, but, the exact mechanisms are still intensely debated. The over-arching objectives of this ISWAT Team are to advance our understanding of both the pre-eruption magnetic field and of the onset mechanism. In order to decide between competing theories for these processes, we use forward modeling: first select several best-observed events for detailed study, then perform the most comprehensive calculations possible for energy build up and eruption onset using the various theories proposed for these processes, and finally compare the results with the actual events to determine which of the theories are most likely to be valid. We present results from the Team's studies, and discuss prospects for future progress. Title: From Pseudostreamer Jets to Coronal Mass Ejections: Observations of the Breakout Continuum Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.; Wyper, Peter F.; DeVore, C. Richard; Lynch, Benjamin J. Bibcode: 2021ApJ...907...41K Altcode: 2020arXiv201107029K The magnetic breakout model, in which reconnection in the corona leads to destabilization of a filament channel, explains numerous features of eruptive solar events, from small-scale jets to global-scale coronal mass ejections (CMEs). The underlying multipolar topology, pre-eruption activities, and sequence of magnetic-reconnection onsets (first breakout, then flare) of many observed fast CMEs/eruptive flares are fully consistent with the model. Recently, we demonstrated that most observed coronal-hole jets in fan/spine topologies also are induced by breakout reconnection at the null point above a filament channel (with or without a filament). For these two types of eruptions occurring in similar topologies, the key question is, why do some events generate jets while others form CMEs? We focused on the initiation of eruptions in large bright points/small active regions that were located in coronal holes and clearly exhibited null-point (fan/spine) topologies: such configurations are referred to as pseudostreamers. We analyzed and compared Solar Dynamics Observatory/Atmospheric Imaging Assembly, Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph Experiment, and Reuven Ramaty High Energy Solar Spectroscopic Imager observations of three events. Our analysis of the events revealed two new observable signatures of breakout reconnection prior to the explosive jet/CME outflows and flare onset: coronal dimming and the opening up of field lines above the breakout current sheet. Most key properties were similar among the selected erupting structures, thereby eliminating region size, photospheric field strength, magnetic configuration, and pre-eruptive evolution as discriminating factors between jets and CMEs. We consider the factors that contribute to the different types of dynamic behavior, and conclude that the main determining factor is the ratio of the magnetic free energy associated with the filament channel compared to the energy associated with the overlying flux inside and outside the pseudostreamer dome. Title: The Effect of 3D Complexity on the Flux Cancellation Model Authors: Antiochos, S. K.; Dahlin, J.; DeVore, C. R. Bibcode: 2020AGUFMSH034..07A Altcode: Explosive solar activity ranging from giant CMEs/eruptive flares to tiny coronal hole jets is all believed to be due to the fast release of the free magnetic energy stored in the highly stressed field of filament channels. Understanding the formation and resulting topology of the filament field, therefore, is absolutely essential for understanding the physical mechanisms driving solar explosions. For example, ideal eruption mechanisms such as the kink or torus instability require that the filament field have a substantial amount of large-scale magnetic twist. The most widely studied model for the formation of this twist is so-called flux cancellation, in which the magnetic flux normal to the photosphere is assumed to reconnect along a polarity inversion line resulting in a twisted flux tube in the corona. A key feature of the flux cancellation model is that the reconnection is systematic - close to 2D in nature, so that a post-reconnection flux rope with a globally coherent twist forms in the corona. The random motions of the photosphere, however, are likely to induce complex structure to the normal flux, in which case the reconnection will be far from systematic and be fully 3D. We present preliminary calculations using the ARMS MHD code of fully 3D flux cancellation. As in the previous work, we start with a 2D-like sheared arcade, but then introduce some random photospheric motions prior to the cancellation. We discuss the resulting filament channel topology and compare it to the standard models. The implications for both theories and observations of explosive solar activity are discussed.

This work was supported in part by the NASA LWS Program. Title: The Nature of Solar Flare Reconnection Authors: Dahlin, J.; Antiochos, S. K.; Jiong, Q.; DeVore, C. R.; Wyper, P. F. Bibcode: 2020AGUFMSH045..04D Altcode: Solar flares are explosive space weather events that can, in the span of only a few minutes, release well over 10^32 ergs of energy in the forms of plasma heating, energetic particles, and bulk motion. Flares are known to be driven by magnetic reconnection; consequently, determining the structure and dynamics of flare reconnection is essential for modeling and eventually predicting the energy release channels of these events. In particular, the amount of energy that ends up as energetic particles is believed to be highly dependent on whether the reconnection is primarily turbulent or primarily laminar. We use the well-proven adaptive mesh refinement capabilities of the ARMS MHD code to perform new very-high-resolution simulations of three-dimensional reconnection in an eruptive flare and compare the results to recent data. Although flare reconnection is difficult to observe directly in the corona, highly detailed constraints on its dynamics can be obtained from observations of the so-called flare ribbons that track the chromospheric footpoints of newly reconnected field lines. The analogues of flare ribbons in our simulations are identified by tracking discontinuous changes in field-line magnetic connectivity due to the reconnection. We discuss the implications of the simulated ribbon formation for the nature of flare reconnection. We also determine how the time-evolving guide field in flares affects the formation of the ribbons and, hence, the reconnection dynamics. We interpret our results through an analytical model, and discuss implications for SDO, IRIS, and GST observations of explosive flare energy release.

This work was supported in part by the SolFER DRIVE Center and by the NASA LWS Program. Title: Relating the Variability of the Middle Corona to the Structure of the Slow Solar Wind Authors: Higginson, A. K.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2020AGUFMSH0300002H Altcode: The recent revolution in heliospheric measurements, brought about by NASA's Parker Solar Probe and ESA's Solar Orbiter, has shown that processes in the middle corona can influence the structure and dynamics of the solar wind across spatial scales. Understanding the formation of the young solar wind structures currently being measured by Parker Solar Probe and Solar Orbiter is now essential. Numerical calculations have shown that magnetic field dynamics at coronal hole boundaries in the middle corona, in particular interchange reconnection driven by photospheric motions, can be responsible for the dynamic release of structured slow solar wind, including along huge separatrix-web (S-Web) arcs formed by pseudostreamers. Quantifying the plasma and magnetic variability along these S-Web arcs is crucial to furthering our understanding of how coronal magnetic field dynamics can influence the slow solar wind throughout the heliosphere. Here we present fully dynamic, 3D numerical calculations of a coronal hole boundary driven continuously by realistic photospheric motions at its base. We consider our simulation results within the context of Parker Solar Probe and Solar Orbiter, and make predictions for the structure and variability of the young slow solar wind. Title: Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.; Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos, A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2020SSRv..216..131P Altcode: 2020arXiv201010186P A clear understanding of the nature of the pre-eruptive magnetic field configurations of Coronal Mass Ejections (CMEs) is required for understanding and eventually predicting solar eruptions. Only two, but seemingly disparate, magnetic configurations are considered viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes (MFR). They can form via three physical mechanisms (flux emergence, flux cancellation, helicity condensation). Whether the CME culprit is an SMA or an MFR, however, has been strongly debated for thirty years. We formed an International Space Science Institute (ISSI) team to address and resolve this issue and report the outcome here. We review the status of the field across modeling and observations, identify the open and closed issues, compile lists of SMA and MFR observables to be tested against observations and outline research activities to close the gaps in our current understanding. We propose that the combination of multi-viewpoint multi-thermal coronal observations and multi-height vector magnetic field measurements is the optimal approach for resolving the issue conclusively. We demonstrate the approach using MHD simulations and synthetic coronal images. Title: Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena in Solar and Heliospheric Plasmas Authors: Ji, H.; Karpen, J.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Bhattacharjee, A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, B.; Chen, L. -J.; Chen, Y.; Chien, A.; Comisso, L.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hare, J.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le, A.; Lebedev, S.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.; Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn, V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.; Shay, M.; Sironi, L.; Sitnov, M.; Stanier, A.; Swisdak, M.; TenBarge, J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E. Bibcode: 2020arXiv200908779J Altcode: Magnetic reconnection underlies many explosive phenomena in the heliosphere and in laboratory plasmas. The new research capabilities in theory/simulations, observations, and laboratory experiments provide the opportunity to solve the grand scientific challenges summarized in this whitepaper. Success will require enhanced and sustained investments from relevant funding agencies, increased interagency/international partnerships, and close collaborations of the solar, heliospheric, and laboratory plasma communities. These investments will deliver transformative progress in understanding magnetic reconnection and related explosive phenomena including space weather events. Title: Trigger Shy? Flare-less Active Region Circular Prominence Eruption Authors: Mason, E.; Antiochos, S.; Vourlidas, A. Bibcode: 2020SPD....5121001M Altcode: Prominence eruptions have been studied since the days of Skylab, and generally fall into two categories based on their locations: quiet Sun and active regions. Quiescent prominences are generally slow to grow and take can days to erupt, with or without any evidence of energization prior to eruption. By contrast, active region prominences generally erupt on time scales of hours or minutes, and are often accompanied by powerful flares. This study reports on an observation of an unusual circular prominence eruption located in an active region which occurs without any evidence of flaring as a trigger. The prominence is under the dome surface of a raining null point topology, which was part of the extended decay phase of the active region designated NOAA 12488/12501. One half of the prominence undergoes a partial eruption, and the cool plasma subsequently drains onto the side which did not erupt, followed by a poorly-structured CME observed by SOHO LASCO C2 shortly after the eruption. We analyze both the failed eruption and secondary CME using SDO AIA, STEREO-A, and SOHO LASCO imagery. The location of the null-point topology raises critical questions about the role of open/closed boundaries in eruptive phenomena and CME structure. The poor structure of the outflowing CME is likely the result of the destruction of the flux rope through reconnection as it passes through the null-point structure, and possibly through additional overlying closed field. The eruption does not show a trigger, but arcades and ribbons form over the erupted half of the prominence. Taken together, the failed eruption presents eruption characteristics of both a CME and a jet, with potential evidence of a low-energy reconnection mechanism driving failed eruptions in highly decayed but still topologically compact magnetic fields. Title: Magnetic Origins of Cool Plasma in the Sun's Corona Authors: Mason, E.; Antiochos, S.; Viall, N. Bibcode: 2020AAS...23610606M Altcode: Much of solar physics research focuses on two questions: how the corona's temperature becomes hundreds of times hotter than the surface, and how the slow solar wind forms. Among the most fascinating phenomena produced by coronal heating is coronal rain, in which plasma undergoes rapid cooling (from roughly 106 to 103 K), condenses, and falls to the surface. One proposed rain origin theory, thermal nonequilibrium (TNE), posits a height restriction in coronal heating. By studying condensations, physicists hope to better understand coronal heating. Solar wind is often subdivided into fast and slow wind. The former originates in coronal hole regions; slow wind's source, however, is still under debate. One leading theory postulates that it comes from coronal hole boundaries, where magnetic field lines frequently reconnect. This research investigates the origins and dynamics of coronal rain via study of recently-discovered structures called raining null-point topologies, or RNPTs. RNPTs — the first identification and characterization of which comprise part of this work — are decaying active regions situated near coronal hole boundaries, between 50-150 Mm in height. They are host to long periods of continuous coronal rain formation, and provide insight into coronal heating, slow solar wind origins, and coronal dynamics. We focus on identifying and analyzing RNPTs' observational characteristics. We process and analyze RNPT data using both the Solar Dynamics Observatory Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager. Potential field source-surface extrapolations that model the magnetic field in the corona aid in the interpretation of the structures' topology. Results indicate that RNPTs experience two rain-forming mechanisms, TNE and interchange reconnection. The interchange reconnection is posited to power much of the early bursts of coronal rain, which constitutes a new rain-formation mechanism and allows for plasma from closed loops to escape into the slow solar wind. Observations also show evidence of partial condensations, which condense but do not fully cool. Title: High-Resolution Three-Dimensional MHD Simulations of Plasmoid Formation in Solar Flares Authors: Dahlin, Joel; Antiochos, Spiro; DeVore, C. Richard Bibcode: 2020EGUGA..2210039D Altcode: In highly conducting plasmas, reconnecting current sheets are often unstable to the generation of plasmoids, small-scale magnetic structures that play an important role in facilitating the rapid release of magnetic energy and channeling that energy into accelerated particles. There is ample evidence for plasmoids throughout the heliosphere, from in situ observations of flux ropes in the solar wind and planetary magnetospheres to remote-sensing imaging of plasma 'blobs' associated with explosive solar activity such as eruptive flares and coronal jets. Accurate models for plasmoid formation and dynamics must capture the large-scale self-organization responsible for forming the reconnecting current sheet. However, due to the computational difficulty inherent in the vast separation between the global and current sheet scales, previous numerical studies have typically explored configurations with either reduced dimensionality or pre-formed current sheets. We present new three-dimensional MHD studies of an eruptive flare in which the formation of the current sheet and subsequent reconnection and plasmoid formation are captured within a single simulation. We employ Adaptive Mesh Refinement (AMR) to selectively resolve fine-scale current sheet dynamics. Reconnection in the flare current sheet generates many plasmoids that exhibit highly complex, three-dimensional structure. We show how plasmoid formation and dynamics evolve through the course of the flare, especially in response to the weakening of the reconnection "guide field" linked to the global reduction of magnetic shear. We discuss implications of our results for particle acceleration and transport in eruptive flares as well as for observations by Parker Solar Probe and the forthcoming Solar Orbiter. Title: Acceleration of particles in different parts of erupting coronal mass ejections Authors: Zharkova, Valentina; Xia, Qian; Dahlin, Joel; Antiochos, Spiro Bibcode: 2020EGUGA..2220181Z Altcode: We examine particle energisation in CMEs generated via the breakout mechanism and explore both 2D and 3D MHD configurations. In the breakout scenario, reconnection at a breakout current sheet (CS) initiates the flux rope eruption by destabilizing the quasi-static force balance. Reconnection at the flare CS triggers the fast acceleration of the CME, which forms flare loops below and triggers particle acceleration in flares. We present test-particle studies that focus on two selected times during the impulsive and decay phases of the eruption and obtain particle energy gains and spatial distributions. We find that particles accelerated more efficiently in the flare CS than in the breakout CS even in the presence of large magnetic islands. The maximum particle energy gain is estimated from the energization terms based on the guiding-center approximation. Particles are first accelerated in the CSs (with or without magnetic islands) where Fermi-type acceleration dominates. Accelerated particles escape to the interplanetary space along open field lines rather than trapped in flux ropes, precipitate into the chromosphere along the flare loops, or become trapped in the flare loop top due to the magnetic mirror structure. Some trapped particles are re-accelerated, either via re-injection to the flare CS or through a local betatron-type acceleration associated with compression of the magnetic field. The energy gains of particles result in relatively hard energy spectra during the impulsive phase. During the gradual phase, the relaxation of the shear in magnetic field reduces the guiding magnetic field in the flare CS, which leads to a decrease in particle energization efficiency. Title: Observations and Simulations of Reconnecting Current Sheets in the Solar Corona Authors: Antiochos, Spiro; Kumar, Pankaj; Jarpen, Judy; Dahlin, Joel Bibcode: 2020EGUGA..22.5597A Altcode: Jets and mass ejections are ubiquitous features of the Sun's corona. These explosive dynamics are all believed to be driven by magnetic reconnection at two types of current sheets that form in the solar atmosphere: those that form at magnetic null points and separatrix surfaces, and those, such as the heliospheric current sheet, that form as a result of a large expansion of a bipolar magnetic field. In our breakout model, both types of current sheets are essential for the explosive release of magnetic energy. We report on the first direct observations of reconnection and island formation in a null-point current sheet associated with a large coronal jet. The topology and velocities of the islands are in excellent agreement with our numerical simulations of coronal jets. We discuss the implications of the observations and our models for understanding the energetic particles produced by these events and their release into interplanetary space, as well as the implications for observations by Solar Orbiter and the Parker Solar Probe.This work was supported by the NASA Living With a Star Program. Title: Particle Acceleration and Transport during 3D CME Eruptions Authors: Xia, Qian; Dahlin, Joel T.; Zharkova, Valentina; Antiochos, Spiro K. Bibcode: 2020ApJ...894...89X Altcode: We calculate particle acceleration during coronal mass ejection (CME) eruptions using combined magnetohydrodynamic and test-particle models. The 2.5D/3D CMEs are generated via the breakout mechanism. In this scenario a reconnection at the "breakout" current sheet (CS) above the flux rope initiates the CME eruption by destabilizing a quasi-static force balance. Reconnection at the flare CS below the erupting flux rope drives the fast acceleration of the CME, which forms flare loops below and produces the energetic particles observed in flares. For test-particle simulations, two times are selected during the impulsive and decay phases of the eruption. Particles are revealed to be accelerated more efficiently in the flare CS rather than in the breakout CS even in the presence of large magnetic islands. Particles are first accelerated in the CSs (with or without magnetic islands) by the reconnection electric field mainly through particle curvature drift. We find, as expected, that accelerated particles precipitate into the chromosphere, become trapped in the loop top by magnetic mirrors, or escape to interplanetary space along open field lines. Some trapped particles are reaccelerated, either via reinjection to the flare CS or through a local Betatron-type acceleration associated with compression of the magnetic field. The energetic particles produce relatively hard energy spectra during the impulsive phase. During the gradual phase, the relaxation of magnetic field shear reduces the guiding field in the flare CS, which leads to a decrease in particle energization efficiency. Important implications of our results for observations of particle acceleration in the solar coronal jets are also discussed. Title: Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena throughout the Universe Authors: Ji, H.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.; Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.; Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn, V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.; Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E. Bibcode: 2020arXiv200400079J Altcode: This white paper summarizes major scientific challenges and opportunities in understanding magnetic reconnection and related explosive phenomena as a fundamental plasma process. Title: Reconnection-Driven Energy Release in the Solar Corona Authors: Antiochos, Spiro Bibcode: 2020APS..DPPP10002A Altcode: The Sun's corona is characterized by bursts of energy release that are most strikingly observed as intense X-Ray solar flares. The underlying origin for this activity is that magnetic free energy builds up and is released impulsively to the plasma in the form of heating, mass motions, and/or particle acceleration. We present high-resolution observations from NASA/ESA/JAXA space missions showing that the energy buildup process appears to be similar for flaring activity ranging across orders of magnitude in scale and energy. Furthermore, the observations demonstrate conclusively that magnetic reconnection is the energy release process. We also present very recent MHD numerical simulations of solar flares that include self-consistently both the energy buildup and explosive release. Our models show that current sheet formation leading to reconnection and energy release occurs almost continuously in the corona, but explosive energy release occurs only when there is strong feedback between the reconnection and the global ideal evolution. We discuss the mechanism for flare reconnection onset and its 3D nature. Capturing accurately the multiscale feedback inherent in flare reconnection remains as the greatest challenge to understanding and eventually predicting these critically important space weather events.

This work was supported by the NASA LWS Program. Title: From Jets to CMEs: Observations of the Breakout Continuum Authors: Karpen, J. T.; Kumar, P.; Antiochos, S. K.; Wyper, P. F.; DeVore, C. R. Bibcode: 2019AGUFMSH43D3354K Altcode: The magnetic breakout model can explain a variety of eruptive solar events, from jets to coronal mass ejections (CMEs). The breakout model is consistent with many observed fast CMEs/eruptive flares, in terms of the underlying multipolar topology, pre-eruption activities, and sequence of reconnection onsets. We have also demonstrated that most observed coronal-hole jets in fan-spine topologies are produced by breakout and flare reconnection above a filament channel (with or without a filament). For eruptions occurring in such topologies, the key question is, why are some events jets while others form slow or fast CMEs? We have analyzed SDO/AIA, LASCO, and RHESSI observations focusing on the initiation of CMEs in large bright points (small active regions) in coronal holes with clear fan-spine topologies. Our analysis revealed pre-eruptive evidence for slow breakout reconnection before the onset of jets, slow CMEs, and fast CMEs from these ARs. We find that this continuum of activity is consistent with the breakout model of solar eruptions, and explore the factors contributing to the different forms of dynamic behavior. Title: First Detection of Plasmoids from Breakout Reconnection Authors: Kumar, P.; Karpen, J.; Antiochos, S. K.; Wyper, P. F.; DeVore, C. R. Bibcode: 2019AGUFMSH44A..08K Altcode: Transient collimated plasma ejections (jets) occur frequently throughout the solar corona, in active regions, quiet Sun, and coronal holes. Our previous studies demonstrated that the magnetic breakout model explains the triggering and evolution of these jets over a wide range of scales, through detailed comparisons between our numerical simulations and high-resolution observations. Here we report direct observations of breakout reconnection during a small eruptive flare accompanied by a filament eruption in the fan-spine topology of an embedded bipole. Breakout reconnection operated in two distinct phases in this event. The first narrow jet was launched by magnetic reconnection at the breakout null without significant flare reconnection or a filament eruption. In contrast, the second jet and release of cool filament plasma were triggered by explosive breakout reconnection when the leading edge of the rising flux rope formed by flare reconnection beneath the filament encountered the preexisting breakout current sheet. We observed plasma heating in the flare arcade and at the top of the flux rope during the latter episode of breakout reconnection. For the first time, we detected the formation and evolution of multiple plasmoids with bidirectional flows in the breakout current sheet originating at a deformed 3D null point. These observations provide evidence for both models: the resistive kink for the first jet, and the breakout model for the second explosive jet with filament eruption. Title: Determining the Transport of Magnetic Helicity in the Sun's Atmosphere Authors: Antiochos, S. K.; Schuck, P. W. Bibcode: 2019AGUFMSH41B..07A Altcode: A critically important factor determining solar coronal activity is the constraint of magnetic helicity conservation. Direct measurement of the magnetic helicity in the coronal volume is difficult, but its value may be estimated from measurements of the helicity transport rates through the photosphere. We examine this transport for a topologically open system such as the corona, in which the magnetic field has a nonzero normal component at the boundaries, and derive a new formula for the helicity transport rate at the boundaries. In addition, we derive new expressions for helicity transport due to flux emergence/submergence versus photospheric horizontal motions. The key new feature of our formulas is that they are manifestly gauge invariant. We discuss the physical interpretation of our results and their implications for using photospheric vector magnetic and velocity field measurements to derive the solar coronal helicity, which can then be used to constrain and drive models for coronal activity.

This work was supported by the NASA LWS Program. Title: Erratum: “The Role of Magnetic Helicity in Coronal Heating” (2019, ApJ, 883, 26) Authors: Knizhnik, K. J.; Antiochos, S. K.; Klimchuk, J. A.; DeVore, C. R. Bibcode: 2019ApJ...887..270K Altcode: No abstract at ADS Title: Simulations of Thermal Nonequilibrium in Raining Null-Point Topologies Authors: Antiochos, S. K.; Mason, E. I.; Viall, N. M. Bibcode: 2019AGUFMSH53B3381A Altcode: Coronal heating and the origins of slow solar wind remain central open questions of solar physics. The recent discovery of raining null-point topologies allows study of regions that hold implications for both questions. We present observations from SDO AIA that show persistent coronal condensations in null-point topologies formed by decaying active regions located near coronal hole boundaries. Coronal rain - catastrophically-cooled plasma precipitating along flux tubes - can be used as a tracer of several physical processes to provide insight into local heating and cooling dynamics. The rain forms in two observationally-distinct ways: along the lower spine and null, and within the closed loops under the fan surface. The former is attributed to interchange reconnection, while the latter is due to thermal nonequilibrium (TNE). TNE is caused by height-dependent footpoint heating, which creates a runaway cooling affect far from the loop base and triggers condensation. Using the one-dimensional HYDrodynamic and RADiation solver code (HYDRAD, Bradshaw & Mason 2003), we model asymmetric flux tubes with a large expansion factor where the loop apex occurs near the null point. A broad parameter study of heating scale heights and heating rates show the ranges within which rain could occur, and point to highly restricted coronal heating scale heights in the decaying active regions. This study provides predictions for Parker Solar Probe and Solar Orbiter observations. Title: Numerical simulation of helical jets at active region peripheries Authors: Wyper, Peter F.; DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2019MNRAS.490.3679W Altcode: 2019MNRAS.tmp.2312W; 2019arXiv190909423W Coronal jets are observed above minority-polarity intrusions throughout the solar corona. Some of the most energetic ones occur on the periphery of active regions where the magnetic field is strongly inclined. These jets exhibit a non-radial propagation in the low corona as they follow the inclined field, and often have a broad, helical shape. We present a three-dimensional magnetohydrodynamic simulation of such an active-region-periphery helical jet. We consider an initially potential field with a bipolar flux distribution embedded in a highly inclined magnetic field, representative of the field nearby an active region. The flux of the minority polarity sits below a bald-patch separatrix initially. Surface motions are used to inject free energy into the closed field beneath the separatrix, forming a sigmoidal flux rope that eventually erupts producing a helical jet. We find that a null point replaces the bald patch early in the evolution and that the eruption results from a combination of magnetic breakout and an ideal kinking of the erupting flux rope. We discuss how the two mechanisms are coupled, and compare our results with previous simulations of coronal-hole jets. This comparison supports the hypothesis that the generic mechanism for all coronal jets is a coupling between breakout reconnection and an ideal instability. We further show that our results are in good qualitative and quantitative agreement with observations of active-region-periphery jets. Title: Estimating Coronal Helicity Injection from Photospheric Measurements Authors: Schuck, P. W.; Antiochos, S. K. Bibcode: 2019AGUFMSH43E3390S Altcode: Magnetic helicity is one of the most important factors determining solar coronal activity. Direct measurements of the helicity in the corona are difficult, but its value may be estimated from measurements of the helicity transport rates through the photosphere. However, the accurate measurements of the electric and magnetic fields necessary to compute helicity transport are usually available only over a limited field of view of the photosphere corresponding to only one boundary of a coronal volume. This adds additional complexity to computing helicity transport into the corona which generally requires a closed surface. We discuss the issues that must be addressed to accurately compute helicity transport across the photosphere using a recently developed manifestly gauge invariant helicity transport formula. We present explicit helicity transport calculations for several coronal field configurations in a Cartesian box subject to a driven boundary on the bottom face corresponding to the photosphere. We discuss the extension of these calculations to photospheric vector magnetic and velocity field measurements in spherical geometry to derive the solar coronal helicity, which can then be used to constrain and drive models for coronal activity. Title: Three-Dimensional Numerical Studies of Plasmoid Formation in Eruptive Flares Authors: Dahlin, J.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2019AGUFMSH12B..04D Altcode: Solar flares are among the most energetic phenomena in the solar system, notable in particular for generating non-thermal particles that may comprise a large fraction of the released energy. In an eruptive flare, the energy release process is generally understood to be magnetic reconnection in a current sheet beneath an erupting flux rope. In a highly conductive plasma such as the corona, current sheets are unstable to the generation of plasmoids, small-scale magnetic structures that play an important role in facilitating the rapid release of magnetic energy and channeling that energy into accelerated particles. Due to the computational difficulty inherent in the vast separation in spatial scales between the global eruption dynamics and the current sheet dissipation, previous numerical studies have largely focused on cases with either reduced dimensionality or highly idealized initial configurations. We present new numerical studies of plasmoid formation in three dimensional MHD calculations of a self-consistent eruptive flare. Using adaptive mesh refinement (AMR), our calculations simultaneously capture both the global flare evolution and the fine-scale generation and evolution of plasmoids in the thin flare current sheet. We show how the evolution of plasmoid structure of generated plasmoids in different phases of the flare and discuss observational signatures and implications for particle acceleration. Title: Modeling the onset of solar eruptions in active regions Authors: Leake, J. E.; Linton, M.; Antiochos, S. K.; Schuck, P. W. Bibcode: 2019AGUFMSH41F3320L Altcode: We present results of a numerical investigation into the initiation of solar coronal eruptions. Using numerical magnetohydrodynamic (MHD) models, we investigate the role of magnetic reconnection between emerging magnetic flux at the solar photosphere and pre-existing coronal flux in the eruptivity likelihood of emerging solar active regions. Our investigation covers a large range of spatial and temporal scales. We analyze the photospheric and coronal magnetic field to investigate how the relevant mechanisms that drive solar coronal eruptions can be detected by current and future observations. Title: Relating the Structure and Dynamics of the Corona to the Variability Ofthe Slow Solar Wind Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2019AGUFMSH11C3405H Altcode: Recent coronagraph and in situ observations have shown that the slow solar wind includes highly structured and dynamic outflow across spatial scales, most likely due to magnetic reconnection processes in the solar corona. In light of the recently launched Parker Solar Probe and anticipated Solar Orbiter missions, understanding this temporal and spatial variability has become essential. Numerical calculations have shown that magnetic field dynamics at coronal hole boundaries, in particular interchange reconnection driven by photospheric motions, can be responsible for the dynamic release of structured slow solar wind, including along huge separatrix-web (S-Web) arcs formed by pseudostreamers. Quantifying the slow solar wind variability along these S-Web arcs is crucial to furthering our understanding of how coronal magnetic field dynamics can influence the plasma and magnetic field throughout the heliosphere. Here we present for the first time, fully dynamic, 3D numerical calculations of an S-Web arc driven continuously by realistic photospheric motions at its base. We present an analysis of the resulting magnetic field dynamics and subsequent plasma release, both near and far from the heliospheric current sheet. We consider our simulation results within the context of future Parker Solar Probe and Solar Orbiter observations and make predictions for the structure and variability of the slow solar wind. Title: First Detection of Plasmoids from Breakout Reconnection on the Sun Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.; Wyper, Peter F.; DeVore, C. Richard Bibcode: 2019ApJ...885L..15K Altcode: 2019arXiv190906637K Transient collimated plasma ejections (jets) occur frequently throughout the solar corona, in active regions, quiet Sun, and coronal holes. Although magnetic reconnection is generally agreed to be the mechanism of energy release in jets, the factors that dictate the location and rate of reconnection remain unclear. Our previous studies demonstrated that the magnetic breakout model explains the triggering and evolution of most jets over a wide range of scales, through detailed comparisons between our numerical simulations and high-resolution observations. An alternative explanation, the resistive-kink model, invokes breakout reconnection without forming and explosively expelling a flux rope. Here we report direct observations of breakout reconnection and plasmoid formation during two jets in the fan-spine topology of an embedded bipole. For the first time, we observed the formation and evolution of multiple small plasmoids with bidirectional flows associated with fast reconnection in 3D breakout current sheets (BCSs) in the solar corona. The first narrow jet was launched by reconnection at the BCS originating at the deformed 3D null, without significant flare reconnection or a filament eruption. In contrast, the second jet and release of cool filament plasma were triggered by explosive breakout reconnection when the leading edge of the rising flux rope formed by flare reconnection beneath the filament encountered the preexisting BCS. These observations solidly support both reconnection-driven jet models: the resistive kink for the first jet, and the breakout model for the second explosive jet with a filament eruption. Title: Escape of Flare-accelerated Particles in Solar Eruptive Events Authors: Masson, S.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2019ApJ...884..143M Altcode: 2019arXiv190913578M Impulsive solar energetic particle events are widely believed to be due to the prompt escape into the interplanetary medium of flare-accelerated particles produced by solar eruptive events. According to the standard model for such events, however, particles accelerated by the flare reconnection should remain trapped in the flux rope comprising the coronal mass ejection. The particles should reach the Earth only much later, along with the bulk ejecta. To resolve this paradox, we have extended our previous axisymmetric model for the escape of flare-accelerated particles to fully three-dimensional (3D) geometries. We report the results of magnetohydrodynamic simulations of a coronal system that consists of a bipolar active region embedded in a background global dipole field structured by solar wind. Our simulations show that multiple magnetic reconnection episodes occur prior to and during the coronal mass ejection (CME) eruption and its interplanetary propagation. In addition to the episodes that build up the flux rope, reconnection between the open field and the CME couples the closed corona to the open interplanetary field. Flare-accelerated particles initially trapped in the CME thereby gain access to the open interplanetary field along a trail blazed by magnetic reconnection. A key difference between these 3D results and our previous calculations is that the interchange reconnection allows accelerated particles to escape from deep within the CME flux rope. We estimate the spatial extent of the particle-escape channels. The relative timings between flare acceleration and release of the energetic particles through CME/open-field coupling are also determined. All our results compare favorably with observations. Title: The Role of Magnetic Helicity in Coronal Heating Authors: Knizhnik, K. J.; Antiochos, S. K.; Klimchuk, J. A.; DeVore, C. R. Bibcode: 2019ApJ...883...26K Altcode: 2019arXiv190903768K One of the greatest challenges in solar physics is understanding the heating of the Sun’s corona. Most theories for coronal heating postulate that free energy in the form of magnetic twist/stress is injected by the photosphere into the corona where the free energy is converted into heat either through reconnection or wave dissipation. The magnetic helicity associated with the twist/stress, however, is expected to be conserved and appear in the corona. In previous works, we showed that the helicity associated with the small-scale twists undergoes an inverse cascade via stochastic reconnection in the corona and ends up as the observed large-scale shear of filament channels. Our “helicity condensation” model accounts for both the formation of filament channels and the observed smooth, laminar structure of coronal loops. In this paper, we demonstrate, using helicity- and energy-conserving numerical simulations of a coronal system driven by photospheric motions, that the model also provides a natural mechanism for heating the corona. We show that the heat generated by the reconnection responsible for the helicity condensation process is sufficient to account for the observed coronal heating. We study the role that helicity injection plays in determining coronal heating and find that, crucially, the heating rate is only weakly dependent on the net helicity preference of the photospheric driving. Our calculations demonstrate that motions with 100% helicity preference are least efficient at heating the corona; those with 0% preference are most efficient. We discuss the physical origins of this result and its implications for the observed corona. Title: Determining the Transport of Magnetic Helicity and Free Energy in the Sun’s Atmosphere Authors: Schuck, Peter W.; Antiochos, Spiro K. Bibcode: 2019ApJ...882..151S Altcode: 2019arXiv190710598S The most important factors determining solar coronal activity are believed to be the availability of magnetic free energy and the constraint of magnetic helicity conservation. Direct measurements of the helicity and magnetic free energy in the coronal volume are difficult, but their values may be estimated from measurements of the helicity and free energy transport rates through the photosphere. We examine these transport rates for a topologically open system such as the corona, in which the magnetic fields have a nonzero normal component at the boundaries, and derive a new formula for the helicity transport rate at the boundaries. In addition, we derive new expressions for helicity transport due to flux emergence/submergence versus photospheric horizontal motions. The key feature of our formulas is that they are manifestly gauge invariant. Our results are somewhat counterintuitive in that only the lamellar electric field produced by the surface potential transports helicity across boundaries, and the solenoidal electric field produced by a surface stream function does not contribute to the helicity transport. We discuss the physical interpretation of this result. Furthermore, we derive an expression for the free energy transport rate and show that a necessary condition for free energy transport across a boundary is the presence of a closed magnetic field at the surface, indicating that there are current systems within the volume. We discuss the implications of these results for using photospheric vector magnetic and velocity field measurements to derive the solar coronal helicity and magnetic free energy, which can then be used to constrain and drive models for coronal activity. Title: A Model for Energy Buildup and Eruption Onset in Coronal Mass Ejections Authors: Dahlin, J. T.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2019ApJ...879...96D Altcode: 2019arXiv190513218D Coronal mass ejections (CMEs) and eruptive flares (EFs) are the most energetic explosions in the solar system. Their underlying origin is the free energy that builds up slowly in the sheared magnetic field of a filament channel. We report the first end-to-end numerical simulation of a CME/EF, from zero-free-energy initial state through filament channel formation to violent eruption, driven solely by the magnetic-helicity condensation process. Helicity is the topological measure of linkages between magnetic flux systems, and is conserved in the corona, building up inexorably until it is ejected into interplanetary space. Numerous investigations have demonstrated that helicity injected by small-scale vortical motions, such as those observed in the photosphere, undergoes an inverse cascade from small scales to large, “condensing” at magnetic-polarity boundaries. Our new results verify that this process forms a filament channel within a compact bipolar region embedded in a background dipole field, and show for the first time that a fast CME eventually occurs via the magnetic-breakout mechanism. We further show that the trigger for explosive eruption is reconnection onset in the flare current sheet that develops above the polarity inversion line: this reconnection forms flare loops below the sheet and a CME flux rope above, and initiates high-speed outward flow of the CME. Our findings have important implications for magnetic self-organization and explosive behavior in solar and other astrophysical plasmas, as well as for understanding and predicting explosive solar activity. Title: New Insights into the 10 September 2017 Mega-Eruption Authors: Karpen, Judith T.; Kumar, Pankaj; Antiochos, Spiro K.; Gary, Dale E.; Dahlin, Joel Bibcode: 2019AAS...23431702K Altcode: The X8.2 flare on 10 September 2017 was part of a well-observed, extremely energetic solar eruption that has been intensely studied. Much attention has been devoted to the striking appearance and persistence of a current sheet behind the explosively accelerating CME. We focus here on the unusual appearance of prominent emission features on either side of the flare arcade, which were detected in microwave emissions by NJIT's EOVSA before the peak impulsive phase. Our analysis combines the results of 3D numerical simulations with observations by SDO, EOVSA, and IRIS to decipher the underlying magnetic structure of the erupting region and the initiation mechanism. The event originated in a complex active region with a large-scale quadrupolar magnetic field punctuated by many intrusions of minority polarity. We interpret the observed microwave features as evidence of electron acceleration due to breakout reconnection, and present compelling evidence for this conclusion. Title: Multiscale Helicity Condensation and Filament Channel Formation Authors: DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2019AAS...23410602D Altcode: Solar eruptive events ranging from small-scale jets to global-scale coronal mass ejections are associated with filaments and their underlying filament-channel magnetic structures. In previous work, we have demonstrated that sheared-arcade filament channels can be formed via the process of helicity condensation. Magnetic twist, representing helicity, is transported across unipolar regions in response to reconnection induced by small-scale, close-packed, surface flows (e.g., the granulation or supergranulation) that possess a vortical component of motion. The small-scale twists induced by the flows inverse-cascade to the largest scales and boundaries of the unipolar regions, i.e., to the polarity inversion lines (PILs). If the flows have a preferred sense of rotation, clockwise or counter, they inject a net helicity into the magnetic field, as well as transport it so that it condenses into filament channels at the PILs. We now have examined how the helicity condensation mechanism is modified when the small-scale flows have no preferred sense of rotation, and large-scale flows are solely responsible for introducing net helicity into the corona. On the Sun, differential rotation is well-known to be a prodigious generator of helicity. Our new simulation results show that a large-scale shear flow produces structure with large-scale magnetic twist, but this twist concentrates near the PILs to form filament channels only when small-scale vortical flows also are present. We conclude that the key role of the vortical flows is to transport the injected net helicity and condense it at the PILs. The source of the net helicity, on the other hand, can be flows at any scale. We refer to this extended concept as multiscale helicity condensation: it is a more general, hence more robust, explanation for the formation of filament channels on the Sun. Our work was supported by NASA's H-ISFM, H-SR, and LWS TR&T programs. Title: STITCH: A New Method for Generating Filament Channels and Driving Solar Eruptions Authors: Dahlin, Joel; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2019AAS...23431703D Altcode: We present a new formalism for generating eruptive magnetic structure in MHD simulations of the solar corona. STITCH (STatistical InjecTion of Condensed Helicity) derives from the helicity condensation model of Antiochos (2013). In the helicity condensation model, small-scale photospheric convection drives a reconnection-mediated inverse cascade that concentrates energy and structure to form highly sheared filament channels.Our recent 3D MHD calculations using more than 100 cyclonic surface flows have demonstrated explosive solar eruptions driven by helicity condensation. However, this manner of direct calculation of small-scale flows and the resulting reconnection is prohibitively expensive for use in data-driven event modeling or long-duration magnetofrictional studies of the global solar magnetic field (Mackay et al. 2014, 2018). Our new method, STITCH, directly injects the tangential field (shear) resulting from statistically averaged, sub-grid helicity condensation. Numerically, this represents a source term in the induction equation, consisting of the curl of the vertical field times a factor proportional to the cyclonic specific angular momentum - a single free parameter. The new approach reproduces prior calculations with small-scale flows at greatly reduced computational expense. We present a variety of simulations with complex initial flux distributions to demonstrate the flexibility of the model. STITCH is both simple to implement and computationally inexpensive, making it a useful new technique for event-based and data-driven modeling of solar eruptions. This work was supported by the NASA LWS, NPP, H-SR and ISFM programs. Title: Observations and Modelling of Condensation Formation at Coronal Hole Boundaries Authors: Mason, Emily; Antiochos, Spiro; Viall, Nicholeen; Macneice, Peter; Bradshaw, Stephen Bibcode: 2019shin.confE..40M Altcode: One of the primary mechanisms suggested for slow solar wind formation is interchange reconnection. This tool for leveraging closed-loop plasma into the heliosphere is believed to occur ubiquitously in the corona, but has few definitive observational characteristics. We present recent observations from SDO AIA and STEREO-A of frequent condensations in small null-point topologies. These structures, termed raining null-point topologies, result from decayed active regions bordering on or entirely within coronal holes. These structures may have unique S-Web fingerprints, aiding slow wind detection and prediction capability. Our observations clearly show condensation formation at the open-closed boundary, where interchange reconnection is widely believed to occur. The condensations take the form of coronal rain, catastrophically cooled plasma that is easy to track using remote observations; their formation on apparently newly-opened coronal flux tubes gives interchange reconnection a hallmark signature. We will also present 1D hydrodynamic models for how these condensations can form via interchange reconnection and thermal nonequilibrium. Due to the nature of these null-point topologies and their proximity to coronal holes, they share characteristics common to open-closed boundaries, pseudostreamers, and active regions. Coordination between Parker Solar Probe, DKIST, Solar Orbiter, etc. would provide a deeper understanding of these ideal targets, which encompass such useful signatures for slow wind investigation. Title: A New Method for Generating Filament Channels and Driving Solar Eruptions Authors: Dahlin, Joel T.; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2019shin.confE.209D Altcode: We present a new formalism for generating eruptive magnetic structure in MHD simulations of the solar corona. STITCH (STatistical InjecTion of Condensed Helicity) derives from the helicity condensation model of Antiochos (2013). In the helicity condensation model, small-scale photospheric convection drives a reconnection-mediated inverse cascade that concentrates energy and structure to form highly sheared filament channels. Our recent 3D MHD calculations using more than 100 cyclonic surface flows have demonstrated explosive solar eruptions driven by helicity condensation. However, this manner of direct calculation of small-scale flows and the resulting reconnection is prohibitively expensive for use in data-driven event modeling or long-duration magnetofrictional studies of the global solar magnetic field (Mackay et al. 2014, 2018). Our new method, STITCH, directly injects the tangential field (shear) resulting from statistically averaged, sub-grid helicity condensation. Numerically, this represents a source term in the induction equation, consisting of the curl of the vertical field times a factor proportional to the cyclonic specific angular momentum - a single free parameter. The new approach reproduces prior calculations with small-scale flows at greatly reduced computational expense. We present a variety of simulations with complex initial flux distributions to demonstrate the flexibility of the model. STITCH is both simple to implement and computationally inexpensive, making it a useful new technique for event-based and data-driven modeling of solar eruptions. This work was supported by the NASA LWS, NPP, H-SR and ISFM programs. Title: Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena throughout the Universe Authors: Ji, Hantao; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.; Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.; Longcope, D.; Louriero, N.; Lu, Q. -M.; Ma, Z. -W.; Matthaeus, W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn, V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.; Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E. Bibcode: 2019BAAS...51c...5J Altcode: 2019astro2020T...5J This is a group white paper of 100 authors (each with explicit permission via email) from 51 institutions on the topic of magnetic reconnection which is relevant to 6 thematic areas. Grand challenges and research opportunities are described in observations, numerical modeling and laboratory experiments in the upcoming decade. Title: Observations of Solar Coronal Rain in Null Point Topologies Authors: Mason, E. I.; Antiochos, Spiro K.; Viall, Nicholeen M. Bibcode: 2019ApJ...874L..33M Altcode: 2019arXiv190408982M Coronal rain is the well-known phenomenon in which hot plasma high in the Sun’s corona undergoes rapid cooling (from ∼106 to <104 K), condenses, and falls to the surface. Coronal rain appears frequently in active region coronal loops and is very common in post-flare loops. This Letter presents discovery observations, which show that coronal rain is ubiquitous in the commonly occurring coronal magnetic topology of a large (∼100 Mm scale) embedded bipole very near a coronal hole boundary. Our observed structures formed when the photospheric decay of active-region-leading-sunspots resulted in a large parasitic polarity embedded in a background unipolar region. We observe coronal rain to appear within the legs of closed loops well under the fan surface, as well as preferentially near separatrices of the resulting coronal topology: the spine lines, null point, and fan surface. We analyze three events using SDO Atmospheric Imaging Assembly observations in the 304, 171, and 211 Å channels, as well as SDO Helioseismic and Magnetic Imager magnetograms. The frequency of rain formation and the ease with which it is observed strongly suggests that this phenomenon is generally present in null point topologies of this size scale. We argue that these rain events could be explained by the classic process of thermal nonequilibrium or via interchange reconnection at the null; it is also possible that both mechanisms are present. Further studies with higher spatial resolution data and MHD simulations will be required to determine the exact mechanism(s). Title: Multiwavelength Study of Equatorial Coronal-hole Jets Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.; Wyper, Peter F.; DeVore, C. Richard; DeForest, Craig E. Bibcode: 2019ApJ...873...93K Altcode: 2019arXiv190200922K Jets (transient/collimated plasma ejections) occur frequently throughout the solar corona and contribute mass/energy to the corona and solar wind. By combining numerical simulations and high-resolution observations, we have made substantial progress recently on determining the energy buildup and release processes in these jets. Here we describe a study of 27 equatorial coronal-hole jets using Solar Dynamics Observatory/Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager observations on 2013 June 27-28 and 2014 January 8-10. Out of 27 jets, 18 (67%) are associated with mini-filament ejections; the other nine (33%) do not show mini-filament eruptions but do exhibit mini-flare arcades and other eruptive signatures. This indicates that every jet in our sample involved a filament-channel eruption. From the complete set of events, six jets (22%) are apparently associated with tiny flux-cancellation events at the polarity inversion line, and two jets (7%) are associated with sympathetic eruptions of filaments from neighboring bright points. Potential-field extrapolations of the source-region photospheric magnetic fields reveal that all jets originated in the fan-spine topology of an embedded bipole associated with an extreme ultraviolet coronal bright point. Hence, all our jets are in agreement with the breakout model of solar eruptions. We present selected examples and discuss the implications for the jet energy buildup and initiation mechanisms. Title: Magnetic Helicity Condensation and the Solar Cycle Authors: Mackay, Duncan H.; DeVore, C. Richard; Antiochos, Spiro K.; Yeates, Anthony R. Bibcode: 2018ApJ...869...62M Altcode: Solar filaments exhibit a global chirality pattern where dextral/sinistral filaments, corresponding to negative/positive magnetic helicity, are dominant in the northern/southern hemisphere. This pattern is opposite to the sign of magnetic helicity injected by differential rotation along east-west oriented polarity inversion lines, posing a major conundrum for solar physics. A resolution of this problem is offered by the magnetic helicity-condensation model of Antiochos. To investigate the global consequences of helicity condensation for the hemispheric chirality pattern, we apply a temporally and spatially averaged statistical approximation of helicity condensation. Realistic magnetic field configurations in both the rising and declining phases of the solar cycle are simulated. For the helicity-condensation process, we assume convective cells consisting of positive/negative vorticities in the northern/southern hemisphere that inject negative/positive helicity. The magnitude of the vorticity is varied as a free parameter, corresponding to different rates of helicity injection. To reproduce the observed percentages of dominant and minority filament chiralities, we find that a vorticity of magnitude 2.5 × 10-6 s-1 is required. This rate, however, is insufficient to produce the observed unimodal profile of chirality with latitude. To achieve this, a vorticity of at least 5 × 10-6 s-1 is needed. Our results place a lower limit on the small-scale helicity injection required to dominate differential rotation and reproduce the observed hemispheric pattern. Future studies should aim to establish whether the helicity injection rate due to convective flows and/or flux emergence across all latitudes of the Sun is consistent with our results. Title: The role of small-scale photospheric motions in coronal magnetic energy buildup and explosive release Authors: Dahlin, Joel; Antiochos, Spiro; DeVore, C. Richard Bibcode: 2018csc..confE..62D Altcode: CMEs/eruptive flares are spectacular examples of explosive solar activity resulting from magnetic self-organization in the corona. Recent theory and modeling studies have demonstrated a mechanism by which small-scale stochastic flows (e.g., photospheric convection) trigger an inverse cascade that concentrates coronal magnetic structure at polarity inversion lines to form highly sheared filament channels. We report on new 3D MHD simulations of an eruptive flare driven by this process of 'helicity condensation'. Energy buildup occurs in the form of a sheared arcade that explosively erupts via magnetic breakout. Interestingly, the magnetic shear above the PIL undergoes a three-phase evolution: an initial increase in response to the driving followed by a decrease as the magnetic structure expands outward, concluding with a sharp increase upon the onset of flare reconnection and fast downflows. We discuss implications of our results for SDO observations of CMEs/eruptive flares. Our simulations are especially relevant to the many SDO observations of eruptions from circular filament channels. We also discuss future opportunities for data-driven modeling of the magnetic energy build up leading to explosive solar activity, and for possible application to space weather prediction. This work was supported by the NASA LWS, H-SR and ISFM programs. Title: A Model for Coronal Hole Bright Points and Jets Due to Moving Magnetic Elements Authors: Wyper, P. F.; DeVore, C. R.; Karpen, J. T.; Antiochos, S. K.; Yeates, A. R. Bibcode: 2018ApJ...864..165W Altcode: 2018arXiv180803688W Coronal jets and bright points occur prolifically in predominantly unipolar magnetic regions, such as coronal holes (CHs), where they appear above minority-polarity intrusions. Intermittent low-level reconnection and explosive, high-energy-release reconnection above these intrusions are thought to generate bright points and jets, respectively. The magnetic field above the intrusions possesses a spine-fan topology with a coronal null point. The movement of magnetic flux by surface convection adds free energy to this field, forming current sheets and inducing reconnection. We conducted three-dimensional magnetohydrodynamic simulations of moving magnetic elements as a model for coronal jets and bright points. A single minority-polarity concentration was subjected to three different experiments: a large-scale surface flow that sheared part of the separatrix surface only, a large-scale surface flow that also sheared part of the polarity inversion line surrounding the minority flux, and the latter flow setup plus a “flyby” of a majority-polarity concentration past the moving minority-polarity element. We found that different bright-point morphologies, from simple loops to sigmoids, were created. When only the field near the separatrix was sheared, steady interchange reconnection modulated by quasi-periodic, low-intensity bursts of reconnection occurred, suggestive of a bright point with periodically varying intensity. When the field near the polarity inversion line was strongly sheared, on the other hand, filament channels repeatedly formed and erupted via the breakout mechanism, explosively increasing the interchange reconnection and generating nonhelical jets. The flyby produced even more energetic and explosive jets. Our results explain several key aspects of CH bright points and jets, and the relationships between them. Title: Observations of Coronal Rain in Null Point Topologies Authors: Mason, Emily; Antiochos, Spiro; MacNiece, Peter; Schlenker, Michael Bibcode: 2018shin.confE..17M Altcode: The most traditional example of time-dependent heating is the solar flare. These explosive events are often accompanied by cascades of coronal rain (CR), a phenomenon in which plasma at coronal temperatures undergoes rapid cooling (from roughly 10^6 to below 10^4 K), condenses, and falls to the surface. However, CR is not seen exclusively in flares; this presentation reports multiple observations of rain forming and precipitating along the legs of null point topologies (NPT) in the low corona, from the spine, null point, and within the legs. We analyze the events using SDO Atmospheric Imaging Assembly (AIA) observations in the 304, 171, and 211 Å channels, as well as SDO Helioseismic and Magnetic Imager (HMI) magnetograms; we also include 1D loop simulations to model the small-scale dynamics driving the events. The frequency of rain formation, and the ease with which these observations were identified lead us to believe that this phenomenon is very common in NPTs. These CR events could be explained via heating cutoff secondary to magnetic reconnection, or by thermal nonequilibrium; it is also possible that both mechanisms are present. Further study involving higher spatial resolution data and a greater range of loop lengths (i.e., pseudostreamers and helmet streamers) will be required to constrain the drivers. Title: Relating the Structure and Dynamics of the Corona to the Variability of the Slow Solar Wind Authors: Higginson, Aleida Katherine; Antiochos, Spiro K.; Lynch, Ben. J.; DeVore, C. Rick; Wyper, Peter F. Bibcode: 2018shin.confE..52H Altcode: Recent coronagraph and in situ observations have shown that the slow solar wind includes highly structured and dynamic outflow across spatial scales, most likely due to magnetic reconnection processes in the solar corona. As we prepare for Parker Solar Probe and Solar Orbiter, understanding this temporal and spatial variability has become essential. Numerical calculations have shown that magnetic field dynamics at coronal hole boundaries, in particular interchange reconnection driven by photospheric motions, can be responsible for the dynamic release of structured slow solar wind, including along huge separatrix-web (S-Web) arcs formed by pseudostreamers. Quantifying the slow solar wind variability along these S-Web arcs is crucial to furthering our understanding of how coronal magnetic field dynamics can influence the plasma and magnetic field throughout the heliosphere. Here we present for the first time, fully dynamic, 3D numerical calculations of an S-Web arc driven continuously by realistic photospheric motions at its base. We present an analysis of the resulting magnetic field dynamics and subsequent plasma release, with a focus on quantifying how the photospheric drivers affect the width of the separatrix arc in the heliosphere. We consider our simulation results within the context of future Parker Solar Probe and Solar Orbiter observations and make predictions for the structure and variability of the slow solar wind. Title: Measuring the Free Energy and Helicity Leading to Solar Eruptive Events Authors: Antiochos, Spiro K.; Schuck, P. W. Bibcode: 2018shin.confE.151A Altcode: The essential ingredients determining solar coronal activity are believed to be the availability of magnetic free energy and the constraint of magnetic helicity conservation. Direct measurements of the helicity and magnetic free energy in the corona are difficult, but it should be possible to infer them from measurements of the helicity and free energy transport through the photosphere. We examine the rate of change of helicity and free energy for a topological open system such as the corona in which the magnetic fields have a non-zero normal component at the boundaries and derive a new formula for the helicity transport rate through the boundaries. A key feature of this formula is that it is manifestly gauge invariant. The result is somewhat counter-intuitive in that only the irrotational electric field transports helicity across boundaries and the inductive electric field does not contribute. We discuss the physical interpretation of our result and demonstrate its application with instructive examples. Furthermore, we derive an expression for the free energy flux, and show that a necessary condition for free energy transport across a boundary is the presence of normal electric currents at the boundary. We discuss the implications of our results for using photospheric vector magnetic and velocity field measurements to derive the solar coronal helicity and magnetic free energy, which can then be used to constrain and drive space weather models for coronal activity. Title: The Effect of Thermal Nonequilibrium in Streamers Authors: Antiochos, Spiro; Schlenker, Michael; MacNeice, Peter; Mason, Emily Bibcode: 2018cosp...42E..95A Altcode: Thermal nonequilibrium (TNE) is the process in which a solar coronal loop undergoes a nonsteady cycle of condensation formation due to the spatial localization of coronal heating near the loop base. Since the requirements for TNE onset is that the loop length is large compared to the scale of the heating, we investigate the effects of TNE on the largest loops in the corona, those of a helmet streamer. Our numerical study uses a 2.5D MHD code that includes the full magnetic field dynamics as well as the detailed plasma thermodynamics. As in previous 1D loop studies, we find that TNE occurs in coronal loops with sufficiently large length, but in contrast to 1D studies, we find that the process also drives substantial magnetic dynamics, especially near the top of the streamer where the plasma beta becomes of order unity. From the simulation results we determine predictions for spectroscopic and imaging observations of both the hot and cool helmet streamer plasma. We conclude that TNE occurring in the largest closed loops in the corona may explain several puzzling observations of the corona, such as the ubiquitous blue shifts observed at the edges of active regions. We also discuss the implications of our results for the solar wind.This work was supported, in part, by the NASA Living With a Star Program. Title: A model for coronal mass ejection energy buildup and eruption onset Authors: Dahlin, Joel T.; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2018shin.confE.214D Altcode: Determining the mechanism that drives coronal mass ejections is one of the most important problems in all of space science. Understanding the trigger for eruption onset is essential for accurate prediction of major space weather events. Self-consistent modeling of the energy buildup and resulting magnetic field configuration is vital for distinguishing the role of ideal instabilities (e.g. the torus instability) versus reconnection (e.g. magnetic breakout) in the onset of CMEs. We present new 3D spherical MHD simulations in which the initial state is a minimum energy potential field and the system is driven by small-scale motions observed for photospheric convection. This simple, self-consistent model drives large-scale energy build up through an inverse cascade of magnetic helicity, forming a filament channel consistent with solar observations of the pre-eruptive magnetic field. We show that energy buildup continues until reconnection in the overlying magnetic field destabilizes the configuration resulting in the ejection of a fast CME. We conclude from these simulations that the trigger mechanism for eruption onset is magnetic reconnection in the coronal field overlying the filament. Title: Numerical Simulation of a Helical Active Region Jet Authors: Wyper, Peter Fraser; DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2018shin.confE..65W Altcode: Parker Solar Probe (PSP) promises to shed light on the origins of solar wind variability. One major contributor to this variability is expected to be coronal jets: high-speed ejections of plasma launched by impulsive interchange reconnection above parasitic polarities. Due to the higher field strengths at the edges of active regions, the most energetic jets often occur there. Such jets are associated with jet-like CMEs in coronagraphs and impulsive SEP events, making them excellent candidates for detection by PSP. Building on our previous work simulating coronal hole jets with filaments - the breakout-jet model - we present a 3D MHD model of a helical active region jet generated by the eruption of a small-scale filament channel. The jet is triggered by interchange reconnection within a current layer formed around a magnetic null point. Following a complex two-step eruption process, an extended helical jet is formed by the transfer of twist from the filament channel to open field lines. Our simulation results explain recent observations of helical jets with complex base dynamics occurring at the periphery of active regions. We gratefully acknowledge support from an RAS fellowship (PFW) and by NASA's LWS and H-SR programs (CRD and SKA). Title: Multiwavelength Study of 24 Equatorial Coronal-Hole Jets Authors: Kumar, Pankaj; Antiochos, Spiro; Karpen, Judy; DeForest, Craig; DeVore, C. Richard; Wyper, Peter Bibcode: 2018cosp...42E1863K Altcode: We studied 24 equatorial coronal-hole (ECH) jets using SDO/AIA and HMI observations on 27-28 June 2013 and 8-10 January 2014. Out of 24 jets (i) 16 jets (67%) are associated with mini-filament eruptions; (ii) 8 jets (34%) are triggered without mini-filament eruptions but with mini-flare arcades and other CME-like signatures; (iii) 5 jets (21%) are apparently associated with tiny flux-cancellation events at the polarity inversion line; (iv) 3 events are associated with sympathetic eruptions of filaments from neighboring jet source regions. The potential field extrapolations of the source regions reveal that almost all jets occurred in the fan-spine topology, and most of the events are in agreement with the breakout model of solar jets. We will present selected examples of each type, and discuss the implications for the jet energy-buildup and initiation mechanisms. Title: The Effect of Thermal Non-equilibrium on Helmet Streamers; yes Authors: Schlenker, Michael John; Antiochos, Spiro Bibcode: 2018shin.confE.256S Altcode: Thermal nonequilibrium is the well-known process in which a solar coronal loop undergoes a nonsteady cycle of heating and cooling due to the spatial localization of coronal heating near the loop base. Since the requirements for thermal nonequilibrium onset is that the loop length is large compared to the scale of the heating, we investigate the effects of this process on the largest loops in the corona, those of a helmet streamer. Our numerical study uses a 2.5D MHD code that includes the full magnetic field dynamics as well as the detailed plasma thermodynamics. The simulation model is axisymmetric and consists of the magnetic field of a dipole at Sun center, which results in a streamer belt centered about the equator and two polar coronal holes. As in previous 1D loop studies, we find that thermal nonequilibrium occurs in coronal loops with sufficiently large length, but in contrast to these studies, we find that the process also drives substantial magnetic dynamics, especially near the top of the streamer where the plasma beta becomes of order unity. From the simulation results we determine predictions for spectroscopic and imaging observations of both the hot and cool helmet streamer plasma. Simulations are preformed using different scale heights for the heating in order to determine the dependence of our findings on this key parameter. The dependence of the results on numerical resolution is also determined via a parameter study. We conclude that thermal nonequilibrium occurring in the largest closed loops in the corona may explain several puzzling observations of the corona, such as the ubiquitous blue shifts observed at the edges of active regions. We also discuss the implications of our results for the solar wind. Title: The physics of thermal nonequilibrium Authors: Karpen, Judy; Antiochos, Spiro Bibcode: 2018cosp...42E1690K Altcode: The presence of cool, dense mass in the hot, rarefied solar corona, in the form of prominences, has mystified scientists for over a century. Its more fragmentary and dynamic manifestation, coronal rain, was discovered more recently but has been equally perplexing. Several processes have been proposed to explain this phenomenon: levitation (bulk lifting of chromospheric mass into the corona), injection (bulk expulsion of chromospheric mass), and evaporation-condensation methods. This talk addresses the last category, which has received the greatest quantitative scrutiny over the past 20 years, particularly in the form of thermal nonequilibrium (TNE). Thermal nonequilibrium has specific requirements and observable signatures, which have been explored thoroughly with theoretical analyses, numerical simulations, and comparison with known characteristics of prominences and coronal rain. I will discuss the basic physical processes at the heart of TNE, the parameter studies that have established the strengths and limitations of this mechanism as applied to these solar phenomena, the latest extensions to multidimensional magnetic geometries and more realistic physics, and future research directions. Title: Relating the Structure and Dynamics of the Corona to the Variability of the Slow Solar Wind Authors: Higginson, Aleida Katherine; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2018tess.conf31705H Altcode: Recent coronagraph and in situ observations have shown that the slow solar wind includes highly structured and dynamic outflow across spatial scales, most likely due to magnetic reconnection processes in the solar corona. As we prepare for Parker Solar Probe and Solar Orbiter, understanding this temporal and spatial variability has become essential. Numerical calculations have shown that magnetic field dynamics at coronal hole boundaries, in particular interchange reconnection driven by photospheric motions, can be responsible for the dynamic release of structured slow solar wind, including along huge separatrix-web (S-Web) arcs formed by pseudostreamers. Quantifying the slow solar wind variability along these S-Web arcs is crucial to furthering our understanding of how coronal magnetic field dynamics can influence the plasma and magnetic field throughout the heliosphere. Here we present for the first time, fully dynamic, 3D numerical calculations of an S-Web arc driven continuously by realistic photospheric motions at its base. We present an analysis of the resulting magnetic field dynamics and subsequent plasma release, both near and far from the heliospheric current sheet. We consider our simulation results within the context of future Parker Solar Probe and Solar Orbiter observations and make predictions for the structure and variability of the slow solar wind. Title: Magnetic Energy Buildup and Explosive Release Authors: Antiochos, Spiro K.; Dahlin, Joel; DeVore, C. Richard Bibcode: 2018tess.conf22202A Altcode: It is now generally accepted that major solar eruptions such as CMEs and eruptive flares are due to the explosive release of magnetic free energy stored in the corona; specifically, in the highly stressed magnetic field that supports filaments and prominences. An important observational finding in recent years is that the mechanisms underlying these eruptions may be invariant over many decades in energy release. We have proposed that the formation of the filament field and, consequently, the free energy buildup, is due to an inverse cascade of magnetic helicity injected into the corona by motions and flux emergence at the photosphere. We present our latest 3D MHD numerical simulations of the self-consistent energy buildup by helicity condensation and eventual explosive energy release. The calculations are in a realistic spherical domain that extends outward to 30 solar radii. We conclude from these simulations that the onset for the eruption, the trigger mechanism, is magnetic reconnection in the coronal field overlying the filament. Our results demonstrate that solar eruptions are an amazing example on cosmic scales of self-organization leading to catastrophic dynamics.

This work was supported by the NASA LWS and HSR Programs. Title: Using Solar Wind Structures as a Rosetta Stone for Understanding Solar Wind Formation Authors: Viall, Nicholeen M.; Kepko, Larry; Antiochos, Spiro K.; Higginson, Aleida Katherine; Vourlidas, Angelos; Lepri, Susan T. Bibcode: 2018tess.conf31702V Altcode: In the inner heliosphere, the slow solar wind is often comprised of mesoscale structures: structures with timescales of hours and length scales of hundreds of mega meters. White light coronagraph data suggest that these mesoscale structures are formed and embedded in the solar wind within the first several solar radii above the solar surface, which is still below even the closest approach of Parker Solar Probe at nine solar radii. We argue that these mesoscale structures represent a 'Rosetta Stone' for using the embedded solar wind plasma signatures to understand the fundamental release and acceleration of solar wind plasma. We study events identified in data from current missions to demonstrate how mesoscale structures can link dynamics observed remotely in the lower corona with in situ observations. We discuss the observations that Parker Solar Probe will make and how to capitalize on this remote-to-in situ data connection. Title: Multilevel Numerical Simulations of Explosive Magnetic Energy Release at the Sun Authors: DeVore, C. Richard; Antiochos, Spiro K.; Karpen, Judith T. Bibcode: 2018tess.conf10417D Altcode: Reconnection onset at current sheets and the resultant magnetic energy release are important at the Sun (coronal heating, coronal mass ejections, flares, jets) and at the Earth (magnetopause flux transfer events, magnetotail substorms) and other magnetized planets. The most dramatic consequences include highly explosive releases of kinetic and thermal energy and of accelerated particles in solar eruptions. We use the Adaptively Refined Magnetohydrodynamics Solver (ARMS) to investigate self-consistent formation and reconnection of current sheets in an initially potential, axisymmetric magnetic field in which four flux systems are separated by a magnetic null line. Stressing the equatorial flux system by applying shear flows eventually leads to reconnection-driven onset of a coronal mass ejection and eruptive flare due to the breakout mechanism (see figure). We report ultrahigh-resolution simulations of this process that extend our previous work (Karpen et al. 2012) by investigating grid-resolution effects on the eruption. Each simulation conserves the injected magnetic helicity, which we calculate analytically, extremely well, and the maximum magnetic free energy stored prior to onset is essentially identical, consistent with convergence of the results versus effective Lundquist number. As expected, the number of null-point pairs created in the current sheets and the kinetic energy released by the eruption increase as the resolution improves. Somewhat counter-intuitively, eruption initiation occurs progressively earlier at higher resolution, due to the increasing aspect ratio (length to width) of the extended flare current sheet; reconnection onset there triggers the transition from slow to very fast outward expansion. We discuss the implications of our work for understanding explosive energy release in the solar atmosphere.

Our research was supported by NASA's Heliophysics SR, LWS, and ISFM programs. Title: Structure and Dynamics of Helmet Streamer Coronal Rain (or, Is this even the Right Haystack?) Authors: Mason, Emily Irene; Schlenker, Michael; Antiochos, Spiro K. Bibcode: 2018tess.conf20542M Altcode: One of the outstanding problems in heliophysics is the nature of the process that heats the solar atmosphere to temperatures more than two orders of magnitude larger than those at the Sun's surface. Physical insight and critical constraints on this process can be obtained by observing the structure and dynamics of coronal plasma. One of the most intriguing forms of dynamics is coronal rain, a phenomenon in which plasma at coronal temperatures undergoes rapid cooling (from roughly 106 to 103 K), condenses, and falls to the surface. Its origins are not thoroughly understood, but proposed theories posit mechanisms of either temporal or spatial variations in coronal heating. The goal of this work is to determine which of these models, if either, agrees with observations. Observation sources include SDO AIA, IRIS, HAO K-Coronagraph, Proba2 SWAP, and LASCO C2. This presentation will include the results of the investigation, and conclusions on the root mechanisms producing these signatures. Title: Statistical Study of 24 Equatorial Coronal-Hole Jets Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.; Fraser Wyper, Peter; DeVore, C. Richard; DeForest, Craig Bibcode: 2018tess.conf40805K Altcode: To understand the trigger mechanisms of coronal-hole jets, we analysed 24 equatorial coronal-hole (ECH) jets using SDO/AIA and HMI observations during 2013-2014. Out of 24 jets (i) 16 jets (67%) are associated with mini-filament eruptions; (ii) 8 jets (34%) are triggered without mini-filament eruptions but with mini-flare arcades and other CME-like signatures; (iii) 5 jets (21%) are apparently associated with tiny flux-cancellation events at the polarity inversion line; (iv) 3 events are associated with sympathetic eruptions of filaments from neighboring jet source regions. The potential field extrapolations of the source regions reveal that almost all jets occurred in the fan-spine topology, and most of the events are in agreement with the breakout model of solar jets. We will present selected examples of each type, and discuss the implications for the jet energy-buildup and initiation mechanisms. Title: Determining the physical mechanism for magnetic helicity injection into the Sun's corona Authors: Schuck, Peter W.; Antiochos, Spiro K. Bibcode: 2018tess.conf20340S Altcode: Magnetic helicity is widely believed to play a major role in solar activity, especially in CMEas/eruptive flares. Consequently, understanding and measuring accurately the helicity injection into the corona is critical for developing predictive models of major solar eruptions. As with magnetic flux, there are two physically distinct mechanisms for helicity injection: emergence through photosphere and mass motions at the photosphere that shear and twist pre-existing coronal magnetic field. We revisit the helicity transport equation and derive an expression for this transport that rigorously distinguishes between the two mechanisms. We discuss the application of our results for observation of helicity build up in the corona and the implications for developing predictive models of eruptive activity.

This work was supported by the NASA Living with a Star Program Title: Evidence for the Magnetic Breakout Model in an Equatorial Coronal-hole Jet Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.; Wyper, Peter F.; DeVore, C. Richard; DeForest, Craig E. Bibcode: 2018ApJ...854..155K Altcode: 2018arXiv180108582K Small, impulsive jets commonly occur throughout the solar corona, but are especially visible in coronal holes. Evidence is mounting that jets are part of a continuum of eruptions that extends to much larger coronal mass ejections and eruptive flares. Because coronal-hole jets originate in relatively simple magnetic structures, they offer an ideal testbed for theories of energy buildup and release in the full range of solar eruptions. We analyzed an equatorial coronal-hole jet observed by the Solar Dynamics Observatory (SDO)/AIA on 2014 January 9 in which the magnetic-field structure was consistent with the embedded-bipole topology that we identified and modeled previously as an origin of coronal jets. In addition, this event contained a mini-filament, which led to important insights into the energy storage and release mechanisms. SDO/HMI magnetograms revealed footpoint motions in the primary minority-polarity region at the eruption site, but show negligible flux emergence or cancellation for at least 16 hr before the eruption. Therefore, the free energy powering this jet probably came from magnetic shear concentrated at the polarity inversion line within the embedded bipole. We find that the observed activity sequence and its interpretation closely match the predictions of the breakout jet model, strongly supporting the hypothesis that the breakout model can explain solar eruptions on a wide range of scales. Title: A Breakout Model for Solar Coronal Jets with Filaments Authors: Wyper, P. F.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2018ApJ...852...98W Altcode: 2017arXiv171200134W Recent observations have revealed that many solar coronal jets involve the eruption of miniature versions of large-scale filaments. Such “mini-filaments” are observed to form along the polarity inversion lines of strong, magnetically bipolar regions embedded in open (or distantly closing) unipolar field. During the generation of the jet, the filament becomes unstable and erupts. Recently we described a model for these mini-filament jets, in which the well-known magnetic-breakout mechanism for large-scale coronal mass ejections is extended to these smaller events. In this work we use 3D magnetohydrodynamic simulations to study in detail three realizations of the model. We show that the breakout-jet generation mechanism is robust and that different realizations of the model can explain different observational features. The results are discussed in relation to recent observations and previous jet models. Title: Evidence for the Magnetic Breakout Model in AN Equatorial Coronal-Hole Jet Authors: Kumar, P.; Karpen, J.; Antiochos, S. K.; Wyper, P. F.; DeVore, C. R.; DeForest, C. E. Bibcode: 2017AGUFMSH52B..02K Altcode: We analyzed an equatorial coronal-hole jet observed by Solar Dynamic Observatory (SDO)/AtmosphericImaging Assembly (AIA). The source-region magnetic field structure is consistent withthe embedded-bipole topology that we identified and modeled previously as a source of coronal jets. Theinitial brightening was observed below a sigmoid structure about 25 min before the onset of an untwisting jet.A circular magnetic flux rope with a mini-filament rose slowly at the speed of ∼15 km/s , then accelerated(∼126 km/s) during the onset of explosive breakout reconnection. Multiple plasmoids, propagating upward(∼135 km/s) and downward (∼55 km/s ), were detected behind the rising flux rope shortly before andduring explosive breakout reconnection. The jet was triggered when the rising flux rope interacted with theoverlying magnetic structures near the outer spine. This event shows a clear evidence of reconnection not onlybelow the flux rope but also a breakout reconnection above the flux rope. During the breakout reconnection,we observed heating of the flux rope, deflection of loops near the spine, and formation of multiple ribbons.The explosive breakout reconnection destroyed the flux rope that produced an untwisting jet with a speed of∼380 km/s . HMI magnetograms reveal the shear motion at theeruption site, but do not show any significant flux emergence or cancellation during or 2 hours before theeruption. Therefore, the free energy powering this jet most likely originated in magnetic shear concentratedat the polarity inversion line within the embedded bipole-a mini-filament channel-possibly created by helicitycondensation. The result of of a statistical study of multiple jets will also be discussed. Title: First Demonstration of a Coronal Mass Ejection Driven by Helicity Condensation Authors: Dahlin, J. T.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2017AGUFMSH52B..07D Altcode: Understanding the mechanism for CMEs/eruptive flares is one of the most important problems in all space science. Two classes of theories have been proposed: ideal processes such as the torus instability, or magnetic reconnection as in the breakout model. Previous simulations of eruptions have used special assumptions, such as a particular initial condition ripe for instability and/or particular boundary conditions designed to induce eruption. We report on a simulation in which the initial state is the minimum-energy potential field, and the system is driven solely by the small-scale random motions observed for photospheric convection. The only requirement on the system is that the flows are sufficiently complex to induce pervasive and random reconnection throughout the volume, as expected for coronal heating, and a net helicity is injected into the corona, in agreement with the observed hemispheric helicity preference. We find that as a result of a turbulent-like cascade, the helicity "condenses" onto a polarity inversion line forming a filament channel, which eventually erupts explosively. We discuss the implications of this fully self-consistent eruption simulation for understanding CMEs/flares and for interpreting coronal observations. This work was supported by the NASA LWS and SR Programs. Title: Self-Organization by Stochastic Reconnection: The Mechanism Underlying CMEs/Flares Authors: Antiochos, S. K.; Knizhnik, K. J.; DeVore, C. R. Bibcode: 2017AGUFMSH14B..01A Altcode: The largest explosions in the solar system are the giant CMEs/flares that produce the most dangerous space weather at Earth, yet may also have been essential for the origin of life. The root cause of CMEs/flares is that the lowest-lying magnetic field lines in the Sun's corona undergo the continual buildup of stress and free energy that can be released only through explosive ejection. We perform the first MHD simulations of a coronal-photospheric magnetic system that is driven by random photospheric convective flows and has a realistic geometry for the coronal field. Furthermore, our simulations accurately preserve the key constraint of magnetic helicity. We find that even though small-scale stress is injected randomly throughout the corona, the net result of "stochastic" coronal reconnection is a coherent stretching of the lowest-lying field lines. This highly counter-intuitive demonstration of self-organization - magnetic stress builds up locally rather than spreading out to a minimum energy state - is the fundamental mechanism responsible for the Sun's magnetic explosions and is likely to be a mechanism that is ubiquitous throughout space and laboratory plasmas. This work was supported in part by the NASA LWS and SR Programs. Title: Is the S-Web the Secret to Observed Heliospheric Particle Distributions? Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R.; Daldorff, L. K. S.; Wyper, P. F.; Ukhorskiy, A. Y.; Sorathia, K. Bibcode: 2017AGUFMSH22B..02H Altcode: Particle transport in the heliosphere remains an unsolved problem across energy regimes. Observations of slow solar wind show that plasma escapes from the closed-field corona, but ends up far away from the heliospheric current sheet, even though the release mechanisms are expected to occur at the HCS. Similarly, some impulsive SEP events have extreme longitudinal extents of 100 degrees or more. Recent theoretical and numerical work has shown that interchange reconnection near a coronal-hole corridor can release plasma from originally closed magnetic field lines into a large swath spread across the heliosphere, forming what is known as an S-Web arc. This is a promising mechanism for explaining both the slow solar wind, with its large latitudinal extent, and impulsive SEP particles, with their large longitudinal extent. Here we compute, for the first time, the dynamics of the S-Web when the photospheric driver is applied over a large portion of the solar surface compared to the scale of the driving. We examine the time scales for the interchange reconnection and compute the angular extent of the plasma released, in the context of understanding both the slow solar wind and flare-accelerated SEPs. We will make predictions for Solar Orbiter and Parker Solar Probe and discuss how these new measurements will help to both pinpoint the source of the slow solar wind and illuminate the transport mechanisms of wide-spread impulsive SEP events. Title: The Mechanism for the Energy Buildup Driving Solar Eruptive Events Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R.; Wyper, P. F. Bibcode: 2017ApJ...851L..17K Altcode: 2017arXiv171100166K The underlying origin of solar eruptive events (SEEs), ranging from giant coronal mass ejections to small coronal-hole jets, is that the lowest-lying magnetic flux in the Sun’s corona undergoes continual buildup of stress and free energy. This magnetic stress has long been observed as the phenomenon of “filament channels:” strongly sheared magnetic field localized around photospheric polarity inversion lines. However, the mechanism for the stress buildup—the formation of filament channels—is still debated. We present magnetohydrodynamic simulations of a coronal volume that is driven by transient, cellular boundary flows designed to model the processes by which the photosphere drives the corona. The key feature of our simulations is that they accurately preserve magnetic helicity, the topological quantity that is conserved even in the presence of ubiquitous magnetic reconnection. Although small-scale random stress is injected everywhere at the photosphere, driving stochastic reconnection throughout the corona, the net result of the magnetic evolution is a coherent shearing of the lowest-lying field lines. This highly counterintuitive result—magnetic stress builds up locally rather than spreading out to attain a minimum energy state—explains the formation of filament channels and is the fundamental mechanism underlying SEEs. Furthermore, this process is likely to be relevant to other astrophysical and laboratory plasmas. Title: Combining Remote and In Situ Observations with MHD models to Understand the Formation of the Slow Solar Wind Authors: Viall, N. M.; Kepko, L.; Antiochos, S. K.; Lepri, S. T.; Vourlidas, A.; Linker, J. Bibcode: 2017AGUFMSH21C..05V Altcode: Connecting the structure and variability in the solar corona to the Heliosphere and solar wind is one of the main goals of Heliophysics and space weather research. The instrumentation and viewpoints of the Parker Solar Probe and Solar Orbiter missions will provide an unprecedented opportunity to combine remote sensing with in situ data to determine how the corona drives the Heliosphere, especially as it relates to the origin of the slow solar wind. We present analysis of STEREO coronagraph and heliospheric imager observations and of in situ ACE and Wind measurements that reveal an important connection between the dynamics of the corona and of the solar wind. We show observations of quasi-periodic release of plasma into the slow solar wind occurring throughout the corona - including regions away from the helmet streamer and heliospheric current sheet - and demonstrate that these observations place severe constraints on the origin of the slow solar wind. We build a comprehensive picture of the dynamic evolution by combining remote imaging data, in situ composition and magnetic connectivity information, and MHD models of the solar wind. Our results have critical implications for the magnetic topology involved in slow solar wind formation and magnetic reconnection dynamics. Crucially, this analysis pushes the limits of current instrument resolution and sensitivity, showing the enormous potential science to be accomplished with the Parker Solar Probe and Solar Orbiter missions. Title: The Effects of Thermal Non-Equilibrium on a Helmet Streamer Authors: Schlenker, M.; Antiochos, S. K.; MacNeice, P. J. Bibcode: 2017AGUFMSH23D2691S Altcode: We investigate the effects of localized heating on the evolution of the plasma within helmet streamers. By implementing a sufficiently small heating scale height, the process of thermal non-equilibrium triggers the formation of coronal rain within the helmet streamer. We present the comparative formation rates of coronal rain in simulations of 3 different grid resolutions. The heating scale height itself is also varied to examine its affect on the rain that is observed. Lastly, we present the evolution of plasma along particular field lines. Our model shows that the thermal physics of the plasma and the dynamical motions of the magnetic field work together to affect the creation rate of coronal rain. This finding has wider implications and suggests that the presence of coronal rain within a helmet streamer can drive the process of magnetic reconnection above the cusp of the streamer. This work was supported in part by the NASA LWS and SR Programs. Title: Filament Channel Formation, Eruption, and Jet Generation Authors: DeVore, C. Richard; Antiochos, Spiro K.; Karpen, Judith T. Bibcode: 2017SPD....4810618D Altcode: The mechanism behind filament-channel formation is a longstanding mystery, while that underlying the initiation of coronal mass ejections and jets has been studied intensively but is not yet firmly established. In previous work, we and collaborators have investigated separately the consequences of magnetic-helicity condensation (Antiochos 2013) for forming filament channels (Zhao et al. 2015; Knizhnik et al. 2015, 2017a,b) and of the embedded-bipole model (Antiochos 1996) for generating reconnection-driven jets (Pariat et al. 2009, 2010, 2015, 2016; Wyper et al. 2016, 2017). Now we have taken a first step toward synthesizing these two lines of investigation. Our recent study (Karpen et al. 2017) of coronal-hole jets with gravity and wind employed an ad hoc, large-scale shear flow at the surface to introduce magnetic free energy and form the filament channel. In this effort, we replace the shear flow with an ensemble of local rotation cells, to emulate the Sun’s ever-changing granules and supergranules. As in our previous studies, we find that reconnection between twisted flux tubes within the closed-field region concentrates magnetic shear and free energy near the polarity inversion line, forming the filament channel. Onset of reconnection between this field and the external, unsheared, open field releases stored energy to drive the impulsive jet. We discuss the results of our new simulations with implications for understanding solar activity and space weather. Title: Evidence for the Magnetic Breakout Model in an Equatorial Coronal-Hole Jet Authors: Karpen, Judith T.; Kumar, Pankaj; Antiochos, Spiro K.; Wyper, Peter; DeVore, C. Richard Bibcode: 2017SPD....4820303K Altcode: We have analyzed an equatorial coronal-hole jet observed by SDO/AIA on 09 January 2014. The source-region magnetic field structure is consistent with the embedded-bipole topology that we identified and modeled previously as a source of coronal jets (Pariat et al. 2009, 2010, 2015, 2016; Karpen et al. 2017; Wyper et al. 2016). Initial brightenings were observed below a small but distinct “mini-filament” about 25 min before jet onset. A bright circular structure, interpreted as magnetic flux rope (MFR), surrounded the mini-filament. The MFR and filament rose together slowly at first, with a speed of ∼15 km s-1. When bright footpoints and loops appeared below, analogous to flare ribbons and arcade, the MFR/mini-filament rose rapidly (∼126 km s-1), and a bright elongated feature interpreted as a current sheet appeared between the MFR and the growing arcade. Multiple plasmoids propagating upward (∼135 km s-1) and downward (∼55 km s-1) were detected in this sheet. The jet was triggered when the rising MFR interacted with the overlying magnetic structure, most likely at a stressed magnetic null distorted into a current sheet. This event thus exhibits clear evidence of “flare” reconnection below the MFR as well as breakout reconnection above it, consistent with the breakout model for a wide range of solar eruptions (Antiochos et al. 1999; Devore & Antiochos 2008; Karpen et al. 2012; Wyper et al. 2017). Breakout reconnection destroyed the MFR and enabled the entrained coronal plasma and mini-filament to escape onto open field lines, producing an untwisting jet. SDO/HMI magnetograms reveal small footpoint motions at the eruption site and its surroundings, but do not show significant flux emergence or cancellation during or 1-2 hours before the eruption. Therefore, the free energy powering this jet most likely originated in magnetic shear concentrated at the polarity inversion line within the embedded bipole - a mini-filament channel - possibly created by helicity condensation (Antiochos 2013; Knizhnik et al. 2015, 2017).This work was supported in part by a grant from the NASA H-SR program and the NASA Postdoctoral Program. Title: Current Sheet Proliferation, Turbulence, and the Heating of the Magnetically-Closed Corona Authors: Klimchuk, James A.; Antiochos, Spiro K. Bibcode: 2017SPD....4830302K Altcode: Electric current sheets in the solar corona are essential to many theories of coronal heating and activity. They can form by a number of mechanisms. The magnetic field is known to be very clumpy in the photosphere, with approximately 100,000 elemental flux tubes in a single active region. Convection causes the tubes to become twisted and tangled, with current sheets forming unavoidably at their boundaries in the corona. Partial reconnections of the tubes as well as a patchiness of the reconnection process lead to a multiplication of the number of distinct sheets. Quasi-ideal instabilities, such as kinking, multiply the numbers even more. We conclude, therefore, that there will be a proliferation of current sheets in the corona. An important question is whether large-scale (active region size) models of the corona need to take this complexity into account to successfully predict the distribution of plasma and the resulting radiation. We discuss the picture of current sheet proliferation and compare and contrast it to MHD turbulence. We also discuss the implication of our results for coronal observations. Title: Solar Jetlets and Plumes Authors: DeForest, Craig; Antiochos, Spiro K.; DeVore, C. Richard; Karpen, Judith T.; Kumar, Pankaj; Raouafi, Nour-Eddine; Roberts, Merrill; Uritsky, Vadim; Wyper, Peter Bibcode: 2017SPD....4830401D Altcode: We present results of a careful deep-field (low-noise) analysis of evolution and structure of solar plumes using multiple wavelength channels from SDO/AIA. Using new noise-reduction techniques on SDO/AIA images, we reveal myriad small, heating events that appear to be the primary basis of plume formation and sustenance. These events ("jetlets") comprise a dynamic tapestry that forms the more distributed plume itself. We identify the "jetlets" with ejecta that have been previously observed spectroscopically, and distinguish them from the quasi-periodic slow mode waves that are seen as large collective motions. We speculate that the jetlets themselves, which are consistent with multiple interchange reconnection events near the base of the plume, are the primary energy driver heating plasma in the plume envelope.Solar polar (and low-latitude) plumes have been analyzed by many authors over many years. Plumes are bright, persistent vertical structures embedded in coronal holes over quasi-unipolar magnetic flux concentrations. They are EUV-bright in the ~1MK lines, slightly cooler (by ionization fraction) than the surrounding coronal hole, persistent on short timescales of a few hours, and recurrent on timescales of a few days. Their onset has been associated with large X-ray jets, although not all plumes are formed that way. Plumes appear to comprise myriad small "threads" or "strands", and may (or may not) contribute significantly to the solar wind, though they have been associated with myriad small, frequent eruptive ejection events.Our results are new and interesting because they are the lowest-noise, time-resolved observations of polar plumes to date; and they reveal the deep association between small-scale magnetic activity and the formation of the plumes themselves. Title: Driving Solar Eruptions via Helicity Condensation Authors: Dahlin, Joel Timothy; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2017SPD....4840605D Altcode: One of the important questions in solar physics is, “How does the Sun store and release energy in coronal mass ejections"? Key to answering this question is understanding how the sun (a) stores magnetic energy in the form of a solar filament and (b) suddenly releases this energy as a coronal mass ejection. An important model for the energy release is the ‘magnetic breakout’ - a positive-feedback mechanism between filament ejection and magnetic reconnection. Recent theory and numerical calculations have demonstrated that helicity injected into the corona via photospheric driving can accumulate in the form of a filament channel of strongly sheared magnetic fields that can provide the free energy for a coronal mass ejection. We present preliminary calculations that, for the first time, incorporate helicity injection in a breakout topology to model a fully self-consistent eruption, from filament formation to ejection. Title: Implications of the S-Web Model for Impulsive SEPs Authors: Antiochos, Spiro K.; Higginson, Aleida K.; DeVore, C. Richard Bibcode: 2017SPD....4840403A Altcode: One of the most important discoveries of the STEREO mission is that impulsive Solar Energetic Particle (SEP) events frequently exhibit large longitudinal spread in the heliosphere, up to 100 degrees or more. This result is especially puzzling given the long-standing observations that impulsive SEPs originate in highly localized regions in the corona, angular extent less than one degree, and that the SEPs frequently show so-called drop-outs, effectively ruling out diffusion as a mechanism for the observed spread. We discuss the implications of the S-Web slow solar wind model for the propagation of SEPs and their distribution in the heliosphere. We present results from 3D MHD simulations demonstrating that for commonly-observed coronal magnetic topologies, the connectivity of the corona to heliosphere will be quasi-singular, with small regions near the Sun dynamically connecting to giant arcs in the heliosphere that span tens of degrees in both latitude and longitude. We show that the S-Web model can account for both SEP longitudinal spread and dropouts, and discuss implications for observations from the upcoming Solar Orbiter and Solar Probe Plus missions.This research was supported, in part, by the NASA LWS Program. Title: A Universal Model for Solar Eruptions Authors: Wyper, Peter; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2017SPD....4820302W Altcode: We present a universal model for solar eruptions that encompasses coronal mass ejections (CMEs) at one end of the scale, to coronal jets at the other. The model is a natural extension of the Magnetic Breakout model for large-scale fast CMEs. Using high-resolution adaptive mesh MHD simulations conducted with the ARMS code, we show that so-called blowout or mini-filament coronal jets can be explained as one realisation of the breakout process. We also demonstrate the robustness of this “breakout-jet” model by studying three realisations in simulations with different ambient field inclinations. We conclude that magnetic breakout supports both large-scale fast CMEs and small-scale coronal jets, and by inference eruptions at scales in between. Thus, magnetic breakout provides a unified model for solar eruptions. P.F.W was supported in this work by an award of a RAS Fellowship and an appointment to the NASA Postdoctoral Program. C.R.D and S.K.A were supported by NASA’s LWS TR&T and H-SR programs. Title: Slow Solar Wind from S-Web Arcs Authors: Higginson, Aleida K.; Antiochos, Spiro K.; DeVore, C. Richard; Wyper, Peter; Zurbuchen, Thomas H. Bibcode: 2017SPD....4830106H Altcode: A long-standing mystery posed by in-situ heliospheric observations is the large angular extent of slow solar wind about the heliospheric current sheet (HCS). Measurements of plasma composition strongly imply that much of the slow wind consists of plasma from the closed corona that escapes onto open field lines, presumably by field-line opening or by interchange reconnection. Both of these processes are expected to release closed-field plasma into the solar wind within and immediately adjacent to the HCS. The recently proposed Separatrix-Web (S-Web) Theory postulates that the observations of slow wind far from the HCS can be explained by the dynamical interaction of open and closed flux in regions of complex coronal-hole topology. We present the first high-resolution, three-dimensional numerical simulations of the dynamic S-Web. These simulations suggest that photospheric motions at coronal-hole boundaries are responsible for the release of slow solar wind plasma from the magnetically closed solar corona, specifically through prolific interchange magnetic reconnection. The location of this plasma once it is released into the solar wind depends strongly on the geometry of the coronal-hole flux. We demonstrate how the dynamics at the boundaries of narrow corridors of open flux (coronal hole corridors) can create giant S-Web arcs of slow solar wind at high latitudes in the heliosphere, far from the HCS, accounting for the long-puzzling slow-wind observations. Title: Observational Tests of Slow Wind Theories Authors: Antiochos, Spiro K.; Higginson, Aleida; DeVore, C. Richard DeVore Bibcode: 2017shin.confE..75A Altcode: Our theoretical understanding of the slow solar wind has undergone a revolution during the past decade, due to the recognition that topological complexity must be an essential feature of all slow wind models. In this scene setting presentation I will briefly review the slow wind models, emphasizing the distinguishing physics of each. A key point is the role of dynamics, especially magnetic reconnection, in the various theories. I will contrast the theories according to their dynamics and then discuss how observations may be used to differentiate between them. In particular, I will discuss how the future observations expected from Solar Probe Plus and Solar Orbiter may finally reveal the true origins of the slow solar wind.

This work was supported by the NASA LWS and HSR Programs. Title: Formation of Heliospheric Arcs of Slow Solar Wind Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R.; Wyper, P. F.; Zurbuchen, T. H. Bibcode: 2017ApJ...840L..10H Altcode: 2017arXiv170108797H A major challenge in solar and heliospheric physics is understanding the origin and nature of the so-called slow solar wind. The Sun’s atmosphere is divided into magnetically open regions, known as coronal holes, where the plasma streams out freely and fills the solar system, and closed regions, where the plasma is confined to coronal loops. The boundary between these regions extends outward as the heliospheric current sheet (HCS). Measurements of plasma composition strongly imply that much of the slow wind consists of plasma from the closed corona that escapes onto open field lines, presumably by field-line opening or by interchange reconnection. Both of these processes are expected to release closed-field plasma into the solar wind within and immediately adjacent to the HCS. Mysteriously, however, slow wind with closed-field plasma composition is often observed in situ far from the HCS. We use high-resolution, three-dimensional, magnetohydrodynamic simulations to calculate the dynamics of a coronal hole with a geometry that includes a narrow corridor flanked by closed field and is driven by supergranule-like flows at the coronal-hole boundary. These dynamics produce giant arcs of closed-field plasma that originate at the open-closed boundary in the corona, but extend far from the HCS and span tens of degrees in latitude and longitude at Earth. We conclude that such structures can account for the long-puzzling slow-wind observations. Title: A universal model for solar eruptions Authors: Wyper, Peter F.; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2017Natur.544..452W Altcode: Magnetically driven eruptions on the Sun, from stellar-scale coronal mass ejections to small-scale coronal X-ray and extreme-ultraviolet jets, have frequently been observed to involve the ejection of the highly stressed magnetic flux of a filament. Theoretically, these two phenomena have been thought to arise through very different mechanisms: coronal mass ejections from an ideal (non-dissipative) process, whereby the energy release does not require a change in the magnetic topology, as in the kink or torus instability; and coronal jets from a resistive process involving magnetic reconnection. However, it was recently concluded from new observations that all coronal jets are driven by filament ejection, just like large mass ejections. This suggests that the two phenomena have physically identical origin and hence that a single mechanism may be responsible, that is, either mass ejections arise from reconnection, or jets arise from an ideal instability. Here we report simulations of a coronal jet driven by filament ejection, whereby a region of highly sheared magnetic field near the solar surface becomes unstable and erupts. The results show that magnetic reconnection causes the energy release via ‘magnetic breakout’—a positive-feedback mechanism between filament ejection and reconnection. We conclude that if coronal mass ejections and jets are indeed of physically identical origin (although on different spatial scales) then magnetic reconnection (rather than an ideal process) must also underlie mass ejections, and that magnetic breakout is a universal model for solar eruptions. Title: Dynamics of Coronal Hole Boundaries Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R.; Wyper, P. F.; Zurbuchen, T. H. Bibcode: 2017ApJ...837..113H Altcode: 2016arXiv161104968H Remote and in situ observations strongly imply that the slow solar wind consists of plasma from the hot, closed-field corona that is released onto open magnetic field lines. The Separatrix Web theory for the slow wind proposes that photospheric motions at the scale of supergranules are responsible for generating dynamics at coronal-hole boundaries, which result in the closed plasma release. We use three-dimensional magnetohydrodynamic simulations to determine the effect of photospheric flows on the open and closed magnetic flux of a model corona with a dipole magnetic field and an isothermal solar wind. A rotational surface motion is used to approximate photospheric supergranular driving and is applied at the boundary between the coronal hole and helmet streamer. The resulting dynamics consist primarily of prolific and efficient interchange reconnection between open and closed flux. The magnetic flux near the coronal-hole boundary experiences multiple interchange events, with some flux interchanging over 50 times in one day. Additionally, we find that the interchange reconnection occurs all along the coronal-hole boundary and even produces a lasting change in magnetic-field connectivity in regions that were not driven by the applied motions. Our results show that these dynamics should be ubiquitous in the Sun and heliosphere. We discuss the implications of our simulations for understanding the observed properties of the slow solar wind, with particular focus on the global-scale consequences of interchange reconnection. Title: Coronal Jets Simulated with the Global Alfvén Wave Solar Model Authors: Szente, J.; Toth, G.; Manchester, W. B., IV; van der Holst, B.; Landi, E.; Gombosi, T. I.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2017ApJ...834..123S Altcode: This paper describes a numerical modeling study of coronal jets to understand their effects on the global corona and their contribution to the solar wind. We implement jets into a well-established three-dimensional, two-temperature magnetohydrodynamic (MHD) solar corona model employing Alfvén-wave dissipation to produce a realistic solar-wind background. The jets are produced by positioning a compact magnetic dipole under the solar surface and rotating the boundary plasma around the dipole's magnetic axis. The moving plasma drags the magnetic field lines along with it, ultimately leading to a reconnection-driven jet similar to that described by Pariat et al. We compare line-of-sight synthetic images to multiple jet observations at EUV and X-ray bands, and find very close matches in terms of physical structure, dynamics, and emission. Key contributors to this agreement are the greatly enhanced plasma density and temperature in our jets compared to previous models. These enhancements arise from the comprehensive thermodynamic model that we use and, also, our inclusion of a dense chromosphere at the base of our jet-generating regions. We further find that the large-scale corona is affected significantly by the outwardly propagating torsional Alfvén waves generated by our polar jet, across 40° in latitude and out to 24 R⊙. We estimate that polar jets contribute only a few percent to the steady-state solar-wind energy outflow. Title: The Role of Magnetic Helicity in Structuring the Solar Corona Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2017ApJ...835...85K Altcode: 2016arXiv160706756K Two of the most widely observed and striking features of the Sun's magnetic field are coronal loops, which are smooth and laminar, and prominences or filaments, which are strongly sheared. Loops are puzzling because they show little evidence of tangling or braiding, at least on the quiet Sun, despite the chaotic nature of the solar surface convection. Prominences are mysterious because the origin of their underlying magnetic structure—filament channels—is poorly understood at best. These two types of features would seem to be quite unrelated and wholly distinct. We argue that, on the contrary, they are inextricably linked and result from a single process: the injection of magnetic helicity into the corona by photospheric motions and the subsequent evolution of this helicity by coronal reconnection. In this paper, we present numerical simulations of the response of a Parker (1972) corona to photospheric driving motions that have varying degrees of helicity preference. We obtain four main conclusions: (1) in agreement with the helicity condensation model of Antiochos (2013), the inverse cascade of helicity by magnetic reconnection in the corona results in the formation of filament channels localized about polarity inversion lines; (2) this same process removes most complex fine structure from the rest of the corona, resulting in smooth and laminar coronal loops; (3) the amount of remnant tangling in coronal loops is inversely dependent on the net helicity injected by the driving motions; and (4) the structure of the solar corona depends only on the helicity preference of the driving motions and not on their detailed time dependence. We discuss the implications of our results for high-resolution observations of the corona. Title: Electron Acceleration in Contracting Magnetic Islands during Solar Flares Authors: Borovikov, D.; Tenishev, V.; Gombosi, T. I.; Guidoni, S. E.; DeVore, C. R.; Karpen, J. T.; Antiochos, S. K. Bibcode: 2017ApJ...835...48B Altcode: Electron acceleration in solar flares is well known to be efficient at generating energetic particles that produce the observed bremsstrahlung X-ray spectra. One mechanism proposed to explain the observations is electron acceleration within contracting magnetic islands formed by magnetic reconnection in the flare current sheet. In a previous study, a numerical magnetohydrodynamic simulation of an eruptive solar flare was analyzed to estimate the associated electron acceleration due to island contraction. That analysis used a simple analytical model for the island structure and assumed conservation of the adiabatic invariants of particle motion. In this paper, we perform the first-ever rigorous integration of the guiding-center orbits of electrons in a modeled flare. An initially isotropic distribution of particles is seeded in a contracting island from the simulated eruption, and the subsequent evolution of these particles is followed using guiding-center theory. We find that the distribution function becomes increasingly anisotropic over time as the electrons’ energy increases by up to a factor of five, in general agreement with the previous study. In addition, we show that the energized particles are concentrated on the Sunward side of the island, adjacent to the reconnection X-point in the flare current sheet. Furthermore, our analysis demonstrates that the electron energy gain is dominated by betatron acceleration in the compressed, strengthened magnetic field of the contracting island. Fermi acceleration by the shortened field lines of the island also contributes to the energy gain, but it is less effective than the betatron process. Title: Reconnection-Driven Coronal-Hole Jets with Gravity and Solar Wind Authors: Karpen, J. T.; DeVore, C. R.; Antiochos, S. K.; Pariat, E. Bibcode: 2017ApJ...834...62K Altcode: 2016arXiv160609201K Coronal-hole jets occur ubiquitously in the Sun's coronal holes, at EUV and X-ray bright points associated with intrusions of minority magnetic polarity. The embedded-bipole model for these jets posits that they are driven by explosive, fast reconnection between the stressed closed field of the embedded bipole and the open field of the surrounding coronal hole. Previous numerical studies in Cartesian geometry, assuming uniform ambient magnetic field and plasma while neglecting gravity and solar wind, demonstrated that the model is robust and can produce jet-like events in simple configurations. We have extended these investigations by including spherical geometry, gravity, and solar wind in a nonuniform, coronal hole-like ambient atmosphere. Our simulations confirm that the jet is initiated by the onset of a kink-like instability of the internal closed field, which induces a burst of reconnection between the closed and external open field, launching a helical jet. Our new results demonstrate that the jet propagation is sustained through the outer corona, in the form of a traveling nonlinear Alfvén wave front trailed by slower-moving plasma density enhancements that are compressed and accelerated by the wave. This finding agrees well with observations of white-light coronal-hole jets, and can explain microstreams and torsional Alfvén waves detected in situ in the solar wind. We also use our numerical results to deduce scaling relationships between properties of the coronal source region and the characteristics of the resulting jet, which can be tested against observations. Title: Turbulence, Current Sheet Proliferation, and the Heating of the Magnetically-Closed Corona Authors: Klimchuk, J. A.; Antiochos, S. K.; Dahlburg, R. B. Bibcode: 2016AGUFMSH33A..03K Altcode: Turbulence plays an important role in heating and accelerating the solar wind, and it has been proposed to also be important in heating active regions and the quiet Sun. These regions are fundamentally different from the sources of the solar wind, however, in that they are magnetically closed and have a small plasma beta. We suggest that the strong, line-tied magnetic field resists being distorted and inhibits turbulence from developing. To test this idea, we performed a 3D MHD simulation representing a solar active region being driven by slow photospheric motions. The conditions said to be necessary for turbulence are met, yet the system evolves quasi-statically up to the point where a kink instability occurs. We conclude that the magnetically-closed corona is not turbulent in the classical sense. There is no inertial range of spatial scales where energy flows without dissipation through a continuum of eddies. Rather, there is a quasi-static evolution that is interrupted by localized and temporary bursts of turbulent behavior associated with the tearing and reconnection of current sheets. Because of a proliferation of current sheets, these episodes are widespread and frequent, with many occurring at the same time within a single active region. This picture is fundamentally different from MHD turbulence, despite some similarities. In addition to the lack of an inertial range, the amount of heating is not independent of the details of the dissipation. On the contrary, it depends critically on the onset conditions for tearing and reconnection. Title: Coronal and Heliospheric Impacts of Reconnection-driven Coronal-Hole Jets, and Implications for Plume Formation Authors: Karpen, J. T.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2016AGUFMSH53A..04K Altcode: Jets from coronal holes on the Sun have been observed for decades, but the physical mechanism responsible for these events is still debated. An important clue about their origin lies in their association with small intrusions of minority polarity within the large-scale open magnetic field, strongly suggesting that these jets are powered by interchange reconnection between embedded bipoles (closed flux) and the surrounding open flux (Antiochos 1996). Through computational investigations of this embedded-bipole paradigm, we have demonstrated that energetic, collimated, Alfvénic flows can be driven by explosive reconnection between twisted closed flux of the minority polarity and the unstressed external field (e.g., Pariat et al. 2009, 2010, 2015, 2016). Our recent numerical study (Karpen et al. 2016) explored the dynamics and energetics of this process under the more realistic conditions of spherical geometry, solar gravity, and an isothermal solar wind out to 9 Rsun. We present results of an extension of this simulation to 30 Rsun, which allows us to predict observable signatures within the orbit of Solar Probe Plus (see Roberts et al. 2016, this meeting). Coronal-hole jets also have been implicated in the formation and maintenance of plumes (e.g., Raouafi & Stenborg 2014), but the physical relationship between the transient, narrow jets and the diffuse, longer-lived plumes is far from understood. To address this question, we analyze the mass density enhancements and fluctuations from the Sun to the inner heliosphere, driven by both slow and explosive reconnection in the embedded-bipole scenario and the associated nonlinear Alfvén wave. Our preliminary results indicate that a substantial ( 20%) density increase over background appears at the moving location of the wave front as far as 12 Rsun. We present the full spatial extent and temporal evolution of mass and momentum after reconnection onset, as well as synthetic coronagraph images of the perturbed corona and inner heliosphere, for comparison with AIA/SDO, LASCO/SOHO, and SECCHI/STEREO observations of jets and plumes. Our goal is to determine the contribution of individual reconnection-driven jets to a plume. This research was supported by NASA's Living With a Star Targeted Research and Technology and Heliophysics Supporting Research programs. Title: Fundamental Physics of the Slow Solar Wind - What do we Know? Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.; Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.; von Steiger, R.; Wang, Y. M. Bibcode: 2016AGUFMSH42A..01O Altcode: Fundamental physical properties of the slow solar wind (SSW), such as density, temperature, outflow speed, heavy ion abundances and charges states were obtained from in-situ measurements at 1AU in the past from WIND, ACE, and other spacecraft. Plasma and magnetic field measurement are available as close as 0.3 AU from Helios data, Spektr-R, and MESSENGER spacecraft. Remote sensing spectroscopic measurements are available in the corona and below from SOHO/UVCS, Hinode, and other missions. One of the major objectives of the Solar Orbiter and Solar Probe Plus missions is to study the sources of the SSW close to the Sun. The present state of understanding of the physics of the SSW is based on the combination of the existing observations, theoretical and numerical 3D MHD and multi-fluid models, that connect between the SSW sources in the corona and the heliosphere. Recently, hybrid models that combine fluid electrons and kinetic ions of the expanding solar wind were developed, and provide further insights of the local SSW plasma heating processes that related to turbulent magnetic fluctuations spectra and kinetic ion instabilities observed in the SSW plasma. These models produce the velocity distribution functions (VDFs) of the protons and heavier ions as well as the ion anisotropic temperatures. I will discuss the results of the above observations and models, and review the current status of our understanding of the fundamental physics of the SSW. I will review the open questions, and discuss how they could be addressed with near future observations and models. Title: Studying the thermodynamics of coronal jets through modeling- and observational diagnostics techniques Authors: Szente, J.; Manchester, W.; Landi, E.; Toth, G.; van der Holst, B.; Gombosi, T. I.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2016AGUFMSH21E2577S Altcode: We present a comprehensive study of simulated and observed coronal jets using EUV and soft X-ray narrow-band images and EUV high resolution spectra. The goal of our study is to understand the thermodynamics and time evolution of jets and their impact on the coronal plasma. We simulate jets with a full 3D MHD coronal model with separate electron and proton temperatures and heating due to Alfvén wave turbulence. Due to the fast dynamics of the small-scale eruptive reconnections at the footpoint of the jet, it is essential to undertake this effort with a model with separate electron and proton temperatures to interpret the observed signatures in EUV and soft X-ray bands. The obtained synthetic images are compared to observations done by the instrumentations of SDO, STEREO and Hinode space crafts. The turbulence in this model is ideally suited to analyze the spectroscopic signatures, such as line broadening. The 3-hour long simulation of jets interacting with the global solar corona shows plasma responses potentially being observed with the upcoming Solar Probe Plus mission. Title: On the Origin of the Slow Solar Wind: Periodic Plasma Release from Pseudostreamers Authors: Viall, N. M.; Kepko, L.; Antiochos, S. K. Bibcode: 2016AGUFMSH54A..05V Altcode: We present observations of quasi-periodic release of plasma from pseudostreamers, and demonstrate that these observations place severe constraints on the origin of both the slow solar wind and pseudostreamer dynamics. Though quasi-periodic release of slow solar wind plasma is routinely observed in remote white light images, such plasma release is often associated with the tips of helmet streamers and the heliospheric current sheet. Helmet streamers and the heliospheric current sheet are natural locations for magnetic reconnection to occur, both in the form of complete disconnections and interchange reconnection. However, pseudostreamers are not associated with the heliospheric current sheet, and are predicted by some models to have steady solar wind release. In contrast, in the S-web model of solar wind formation, pseudostreamers and their magnetic extensions into the heliosphere are also locations where slow solar wind is released sporadically through magnetic reconnection. We present the first observations demonstrating that quasi-periodic plasma release occurs in pseudostreamers as well. We build a comprehensive picture of the dynamics by combining remote-sensing data with in situ composition and magnetic connectivity information. Our results have critical implications for the magnetic topology of pseudostreamers and for their reconnection dynamics. This analysis pushes the limits of current instrument resolution and sensitivity, showing the enormous potential science to be accomplished with Solar Probe Plus and Solar Orbiter. Title: Achieving Consistent Vector Magnetic Field Measurements from SDO/HMI Authors: Schuck, P. W.; Antiochos, S. K.; Scherrer, P. H.; Hoeksema, J. T.; Leka, K. D.; Barnes, G. Bibcode: 2016AGUFMSH31B2575S Altcode: NASA's Solar Dynamics Observatory (SDO) is delivering vector magnetic field observations of the full solar disk with unprecedented temporal and spatial resolution; however, the satellite is in a highly inclined geosynchronous orbit. The relative spacecraft-Sun velocity varies by ±3 km/s over a day which introduces significant orbital artifacts in the Helioseismic Magnetic Imager (HMI) data. We have recently demonstrated that the orbital artifacts contaminate all spatial and temporal scales in the data and developed a procedure for mitigating these artifacts in the Doppler data obtained from the Milne-Eddington inversions in the HMI Pipeline. Simultaneously, we have found that the orbital artifacts may be introduced by inaccurate estimates for the free-spectral ranges (FSRs) of the optical elements in HMI. We describe our approach and attempt to minimize orbital artifacts in the hmi.V_720 Dopplergram series by adjusting the FSRs for the optical elements of HMI within their measurement uncertainties of ±1%. Title: The Formation of Filament Channels in the Corona Authors: Karpen, J.; Knizhnik, K. J.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2016AGUFMSH43C2585K Altcode: We investigate a new model for the formation of highly sheared filament channels above photospheric polarity inversion lines (PILs). The question of filament channel formation is a major problem in solar physics, its significance stemming from the propensity of filament channels to erupt in coronal mass ejections. The free energy released in these eruptions was originally stored as filament channel shear, indicating that filament channels are highly non-potential structures, containing tremendous magnetic helicity. Since magnetic helicity is conserved under magnetic reconnection in a high-Rm environment such as the solar corona, this helicity must be injected at the photospheric level. We present helicity-conserving numerical simulations that show, for the first time, the formation of such highly sheared filament channels as a result of photospheric helicity injection into a coronal magnetic field containing both a PIL and a coronal hole (CH). Remarkably, sheared filament channels form only at the PIL, leaving the rest of the corona laminar and smooth. We show that this result is in excellent agreement with observations, and follows directly from the recently proposed helicity condensation model (Antiochos, 2013). Building on initial tests of this model performed by Knizhnik, Antiochos & DeVore (2015, 2016), we show that the rate of helicity injection drastically affects the timescale of filament channel formation, and discuss the implications for observations. Title: The Dynamics of Open-Field Corridors Authors: Viall, N. M.; Antiochos, S. K.; Higginson, A. K.; DeVore, C. R. Bibcode: 2016AGUFMSH54A..06V Altcode: The source of the slow solar wind and the origins of its dynamics have long been major problems in solar/heliospheric physics. Due to its observed location in the heliosphere, its plasma composition, and its variability, the slow wind is widely believed to be due to the release of closed-field plasma onto open field lines. In the S-Web model the slow wind is postulated to result from the driving of the open-closed boundary in the corona by the quasi-random photospheric convective motions. A key feature of the model is the topological complexity of the open field regions at the Sun, in other words, the distribution and geometry of coronal holes. In particular, narrow corridors of open field and even singular topologies are required in order to account for the observed angular extent of the slow wind in the heliosphere. We present the first calculations of the dynamics of an open-field corridor driven by photospheric flows. The calculations use our high-resolution MHD code and an isothermal approximation for the coronal and solar wind plasma. We show that the corridor dynamics do, in fact, result in the release of closed field plasma far from the heliospheric current sheet, in agreement with observations and as predicted by the S-Web model. The implications of our results for understanding the corona-heliosphere connection and especially for interpreting observations from the upcoming Solar Orbiter and Solar Probe Plus missions will be discussed. This research was supported by the NASA LWS programs. Title: Slow Solar Wind: Observations and Modeling Authors: Abbo, L.; Ofman, L.; Antiochos, S. K.; Hansteen, V. H.; Harra, L.; Ko, Y. -K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.; von Steiger, R.; Wang, Y. -M. Bibcode: 2016SSRv..201...55A Altcode: 2016SSRv..tmp...34A While it is certain that the fast solar wind originates from coronal holes, where and how the slow solar wind (SSW) is formed remains an outstanding question in solar physics even in the post-SOHO era. The quest for the SSW origin forms a major objective for the planned future missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless, results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW. Advanced models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations. Title: A model for stealth coronal mass ejections Authors: Lynch, B. J.; Masson, S.; Li, Y.; DeVore, C. R.; Luhmann, J. G.; Antiochos, S. K.; Fisher, G. H. Bibcode: 2016JGRA..12110677L Altcode: 2016arXiv161208323L Stealth coronal mass ejections (CMEs) are events in which there are almost no observable signatures of the CME eruption in the low corona but often a well-resolved slow flux rope CME observed in the coronagraph data. We present results from a three-dimensional numerical magnetohydrodynamics (MHD) simulation of the 1-2 June 2008 slow streamer blowout CME that Robbrecht et al. (2009) called "the CME from nowhere." We model the global coronal structure using a 1.4 MK isothermal solar wind and a low-order potential field source surface representation of the Carrington Rotation 2070 magnetogram synoptic map. The bipolar streamer belt arcade is energized by simple shearing flows applied in the vicinity of the helmet streamer's polarity inversion line. The flows are large scale and impart a shear typical of that expected from the differential rotation. The slow expansion of the energized helmet streamer arcade results in the formation of a radial current sheet. The subsequent onset of expansion-induced flare reconnection initiates the stealth CME while gradually releasing the stored magnetic energy. We present favorable comparisons between our simulation results and the multiviewpoint SOHO-LASCO (Large Angle and Spectrometric Coronagraph) and STEREO-SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) coronagraph observations of the preeruption streamer structure and the initiation and evolution of the stealth streamer blowout CME. Title: A model for straight and helical solar jets. II. Parametric study of the plasma beta Authors: Pariat, E.; Dalmasse, K.; DeVore, C. R.; Antiochos, S. K.; Karpen, J. T. Bibcode: 2016A&A...596A..36P Altcode: 2016arXiv160908825P Context. Jets are dynamic, impulsive, well-collimated plasma events that develop at many different scales and in different layers of the solar atmosphere.
Aims: Jets are believed to be induced by magnetic reconnection, a process central to many astrophysical phenomena. Within the solar atmosphere, jet-like events develop in many different environments, e.g., in the vicinity of active regions, as well as in coronal holes, and at various scales, from small photospheric spicules to large coronal jets. In all these events, signatures of helical structure and/or twisting/rotating motions are regularly observed. We aim to establish that a single model can generally reproduce the observed properties of these jet-like events.
Methods: Using our state-of-the-art numerical solver ARMS, we present a parametric study of a numerical tridimensional magnetohydrodynamic (MHD) model of solar jet-like events. Within the MHD paradigm, we study the impact of varying the atmospheric plasma β on the generation and properties of solar-like jets.
Results: The parametric study validates our model of jets for plasma β ranging from 10-3 to 1, typical of the different layers and magnetic environments of the solar atmosphere. Our model of jets can robustly explain the generation of helical solar jet-like events at various β ≤ 1. We introduces the new result that the plasma β modifies the morphology of the helical jet, explaining the different observed shapes of jets at different scales and in different layers of the solar atmosphere.
Conclusions: Our results enable us to understand the energisation, triggering, and driving processes of jet-like events. Our model enables us to make predictions of the impulsiveness and energetics of jets as determined by the surrounding environment, as well as the morphological properties of the resulting jets. Title: Achieving Consistent Vector Magnetic Field Measurements from SDO/HMI Authors: Schuck, P. W.; Scherrer, Phil; Antiochos, Spiro; Hoeksema, Todd Bibcode: 2016usc..confE..71S Altcode: NASA's Solar Dynamics Observatory (SDO) is delivering vector magnetic field observations of the full solar disk with unprecedented temporal and spatial resolution; however, the satellite is in a highly inclined geosynchronous orbit. The relative spacecraft-Sun velocity varies by ±3 km/s over a day which introduces significant orbital artifacts in the Helioseismic Magnetic Imager (HMI) data. We have recently demonstrated that the orbital artifacts contaminate all spatial and temporal scales in the data and developed a procedure for mitigating these artifacts in the Doppler data obtained from the Milne-Eddington inversions in the HMI Pipeline. Simultaneously, we have found that the orbital artifacts may be introduced by inaccurate estimates for the free-spectral ranges (FSRs) of the optical elements in HMI. We describe our approach and attempt to minimize orbital artifacts in the hmi.V_720 Dopplergram series by adjusting the FSRs for the optical elements of HMI within their measurement uncertainties of ±1%. introduces major orbital artifacts in the Helioseismic Magnetic Imager (HMI) data. We have recently demonstrated that the orbital artifacts contaminate all spatial and temporal scales in the data and developed a procedure for mitigating these artifacts in the Doppler data obtained from the Milne-Eddington inversions in the HMI Pipeline. Simultaneously, we have found that the orbital artifacts may be introduced by inaccurate estimates for the free-spectral ranges (FSRs) of the optical elements in HMI. We describe our approach and attempt to minimize orbital artifacts in the hmi.V_720 Dopplergram series by adjusting the FSRs for the optical elements in HMI within their measurement uncertainties of ±1%. Title: The Source of the Slow Wind and the Origin of its Dynamics Authors: Antiochos, S. K.; Higginson, A. K.; DeVore, C. R. Bibcode: 2016usc..confE..18A Altcode: The origin of the slow solar wind has long been one of the major unsolved problems in solar physics. Recently, we have proposed the S-Web model in which the slow wind originates from a dense web of separatrices and quasi-separatrix layers that form the boundary between open and closed magnetic flux in the corona. The large-scale dynamics of the photosphere and corona drive this S-Web, causing closed field plasma to be released onto open field lines, which is observed in the heliosphere as the slow wind. The S-Web model, therefore, predicts that both the source and variability of the slow wind are due to the dynamics of the open-closed magnetic field boundary. We argue that two main processes drive these dynamics: photospheric motions and thermal nonequilibrium. We present simulations showing the form of the variability expected from the S-web dynamics and discuss the implications of our calculations for understanding the observed properties of the slow wind and especially for interpreting SDO observations of coronal hole evolution. This work was supported by the NASA LWS Program. Title: Coronal response to EUV jets modeled with the Alfvén Wave Solar Model Authors: Szente, Judith; Toth, Gabor; Manchester, Ward B., IV; van der Holst, Bartholomeus; Landi, Enrico; Gombosi, Tamas; DeVore, Carl R.; Antiochos, Spiro K. Bibcode: 2016usc..confE..72S Altcode: We study the thermodynamics of jet phenomena with the use of multiple wavelength SDO-AIA observations [e.g. Adams (2014) and Moore (2015)] combined with advanced numerical simulations made with AWSoM coronal model [van der Holst (2014)]. AWSoM provides a fully three-dimensional, magnetohydrodynamic description of the solar atmosphere heated by the dissipation of kinetic Alfvén waves in a self-consistent manner. In addition, the model's multi-species thermodynamics with electron heat conduction provides for the accurate construction of synthetic line-of-sight images of phenomena. We implement our jets in the solar wind with a magnetic dipole twisted about axis, resulting in EUV jets similar in topology and dynamics as being observed. We show that the coronal atmosphere responds at a large-scale as torsional Alfvén waves propagate into the outer corona (up to 24 solar radii and 40 degrees in latitude), introduced by the small-scale eruptive reconnection events at the footpoint of the jet. Title: The Breakout Model for Coronal Jets with Filaments Authors: Wyper, Peter Fraser; DeVore, C. Richard; Antiochos, Spiro Bibcode: 2016shin.confE.111W Altcode: Coronal jets are impulsive, collimated plasma outflows originating low in the solar corona. Many of these events exhibit broad, curtain-like morphologies. Recently, Sterling et al. (2015) [doi:10.1038/nature14556] reported that such jets are associated with the eruption of small filaments and, therefore, are miniature versions of corona mass ejections (CMEs). We present 3D simulations, performed with the Adaptively Refined MHD Solver (ARMS), which demonstrate how the magnetic breakout mechanism generates mini-CME-type jets in a compact bipolar region energized by simple footpoint motions. Our numerical model captures the formation of the strongly sheared pre-jet filament structure, the post-jet flare-like loops and ribbons, and the curtain-like untwisting dynamics observed higher in the corona. Similar to large-scale breakout calculations (e.g. Karpen et al. (2012) [doi:10.1088/0004-637X/760/1/81]) tearing and intermittent reconnection also plays a role in the dynamics and naturally explains the intermittent blob-like outflows observed in many jets. NASA supported this research by awards to the NASA Postdoctoral Program (P.F.W.) and the LWS TR&T and H-SR programs (C.R.D. & S.K.A.). Title: Slow Solar Wind at Mid-Latitudes Due to Photospheric Motions Authors: Higginson, Aleida Katherine; Antiochos, S. K.; DeVore, C. R.; Zurbuchen, T. H. Bibcode: 2016shin.confE..82H Altcode: In-situ measurements of charge-state compositions and elemental abundances show that slow wind plasma closely resembles the plasma in the closed corona as determined by remote observations rather than the plasma in coronal holes, which are known to be the source of the fast solar wind. The likely origin of the slow solar wind, therefore, is the release of closed field plasma onto open field lines. The S-Web model predicts that photospheric motions at coronal-hole boundaries are responsible for the transfer of plasma from closed magnetic field to open magnetic field through reconnection or the opening of field lines, or both. Our previous work showed that simple rotational motions at a coronal boundary result primarily in prolific and efficient interchange reconnection. On the Sun, when coronal-hole boundaries become sufficiently complex, they map to locations in the heliosphere far from the heliospheric current sheet. In such cases, the dynamics of reconnection and/or opening can cause plasma to be released as much as 30 degrees from the sheet. We performed a fully dynamic, 3D MHD simulation of a complex coronal-hole boundary in an isothermal solar wind. The quasi-steady open/closed boundary was disturbed by introducing rotational motions. We discuss the reconnection that takes place, the amount of closed-field plasma that is released, and the implications of our results for understanding the origin of the slow solar wind.

This work was supported by the NASA LWS Program. Title: The Variability of the S-Web Authors: Antiochos, Spiro K. Bibcode: 2016shin.confE..75A Altcode: In the S-Web model the slow wind originates from a dense web of separatrices and quasi-separatrix layers that form the boundary between open and closed flux. The dynamics of this S-Web causes closed field plasma to be released onto open field lines, which is observed in the heliosphere as the slow wind. The S-Web model, therefore, predicts that the variability of the slow wind is due to the dynamics of the open closed boundary. We argue that these dynamics are driven by two main processes: direct driving by photospheric motions and thermal nonequilibrium. we discuss the variability that would be expected from each of these processes. Title: A Model for Filament Channel Formation in a Coronal Magnetic Field Authors: Knizhnik, Kalman J.; DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2016shin.confE.136K Altcode: We investigate a new model for the formation of highly sheared filament channels above photospheric polarity inversion lines (PILs). The question of filament channel formation is a major problem in solar physics, its significance stemming from the propensity of filament channels to erupt in coronal mass ejections. The free energy released in these eruptions was originally stored as filament channel shear, indicating that filament channels are highly non-potential structures, containing tremendous magnetic helicity. Since magnetic helicity is conserved under magnetic reconnection in a high-Rm environment such as the solar corona, this helicity must be injected at the photospheric level. We present numerical simulations that show, for the first time, the formation of such highly sheared filament channels as a result of photospheric helicity injection into a coronal magnetic field containing both a PIL and a coronal hole (CH). Remarkably, sheared filament channels form only at the PIL, leaving the rest of the corona laminar and smooth. We show that this result is in excellent agreement with observations, and follows directly from the recently proposed helicity condensation model (Antiochos, 2013). Building on initial tests of this model performed by Knizhnik, Antiochos & DeVore (2015), we show that that interchange reconnection between open and closed magnetic fields drastically affects the CH boundary, and discuss the implications of this result for observations. Title: Composition of Coronal Mass Ejections Authors: Zurbuchen, T. H.; Weberg, M.; von Steiger, R.; Mewaldt, R. A.; Lepri, S. T.; Antiochos, S. K. Bibcode: 2016ApJ...826...10Z Altcode: We analyze the physical origin of plasmas that are ejected from the solar corona. To address this issue, we perform a comprehensive analysis of the elemental composition of interplanetary coronal mass ejections (ICMEs) using recently released elemental composition data for Fe, Mg, Si, S, C, N, Ne, and He as compared to O and H. We find that ICMEs exhibit a systematic abundance increase of elements with first ionization potential (FIP) < 10 eV, as well as a significant increase of Ne as compared to quasi-stationary solar wind. ICME plasmas have a stronger FIP effect than slow wind, which indicates either that an FIP process is active during the ICME ejection or that a different type of solar plasma is injected into ICMEs. The observed FIP fractionation is largest during times when the Fe ionic charge states are elevated above Q Fe > 12.0. For ICMEs with elevated charge states, the FIP effect is enhanced by 70% over that of the slow wind. We argue that the compositionally hot parts of ICMEs are active region loops that do not normally have access to the heliosphere through the processes that give rise to solar wind. We also discuss the implications of this result for solar energetic particles accelerated during solar eruptions and for the origin of the slow wind itself. Title: Streamer Blowout CME Initiation: Not Loss-of-Equilibrium, Not Flux-Cancellation, Not the Kink Instability, and Not the Torus Instability Authors: Lynch, Benjamin J.; Masson, S.; Li, Y.; DeVore, C. R.; Luhmann, J. G.; Antiochos, S. K.; Fisher, G. H. Bibcode: 2016shin.confE..49L Altcode: We present results from a three-dimensional numerical magnetohydrodynamics (MHD) simulation of the 2008 June 1-2 slow streamer blowout CME that Robbrecht et al. [2009] called 'the CME from nowhere.'

We investigate the CME initiation mechanism in detail, showing definitely the eruption is *not* caused by loss-of-equilibrium, flux-cancellation, the kink instability or the torus instability, rather, the rising sheared arcade becomes a CME in the traditional sense only when the eruptive flare reconnection occurs at the radial current sheet, forming a flux rope structure *during* the eruption.

We present favorable comparisons between our simulation results and the multi-viewpoint SOHO-LASCO and STEREO-SECCHI coronagraph observations of the pre-eruption streamer structure and the initiation and evolution of the stealth streamer blowout CME. We also present synthetic in-situ time series at r=15Rs of the plasma and field signatures of the flux rope CME and show qualitative agreement to the ICME observed by STB on 2008 June 6-7. Title: Achieving Consistent Doppler Measurements from SDO/HMI Vector Field Inversions Authors: Schuck, Peter W.; Antiochos, S. K.; Leka, K. D.; Barnes, Graham Bibcode: 2016ApJ...823..101S Altcode: 2015arXiv151106500S NASA’s Solar Dynamics Observatory is delivering vector magnetic field observations of the full solar disk with unprecedented temporal and spatial resolution; however, the satellite is in a highly inclined geosynchronous orbit. The relative spacecraft-Sun velocity varies by ±3 km s-1 over a day, which introduces major orbital artifacts in the Helioseismic Magnetic Imager (HMI) data. We demonstrate that the orbital artifacts contaminate all spatial and temporal scales in the data. We describe a newly developed three-stage procedure for mitigating these artifacts in the Doppler data obtained from the Milne-Eddington inversions in the HMI pipeline. The procedure ultimately uses 32 velocity-dependent coefficients to adjust 10 million pixels—a remarkably sparse correction model given the complexity of the orbital artifacts. This procedure was applied to full-disk images of AR 11084 to produce consistent Dopplergrams. The data adjustments reduce the power in the orbital artifacts by 31 dB. Furthermore, we analyze in detail the corrected images and show that our procedure greatly improves the temporal and spectral properties of the data without adding any new artifacts. We conclude that this new procedure makes a dramatic improvement in the consistency of the HMI data and in its usefulness for precision scientific studies. Title: Science Objectives of the FOXSI Small Explorer Mission Concept Authors: Shih, Albert Y.; Christe, Steven; Alaoui, Meriem; Allred, Joel C.; Antiochos, Spiro K.; Battaglia, Marina; Buitrago-Casas, Juan Camilo; Caspi, Amir; Dennis, Brian R.; Drake, James; Fleishman, Gregory D.; Gary, Dale E.; Glesener, Lindsay; Grefenstette, Brian; Hannah, Iain; Holman, Gordon D.; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa, Shin-Nosuke; Jeffrey, Natasha; Klimchuk, James A.; Kontar, Eduard; Krucker, Sam; Longcope, Dana; Musset, Sophie; Nita, Gelu M.; Ramsey, Brian; Ryan, Daniel; Saint-Hilaire, Pascal; Schwartz, Richard A.; Vilmer, Nicole; White, Stephen M.; Wilson-Hodge, Colleen Bibcode: 2016SPD....47.0814S Altcode: Impulsive particle acceleration and plasma heating at the Sun, from the largest solar eruptive events to the smallest flares, are related to fundamental processes throughout the Universe. While there have been significant advances in our understanding of impulsive energy release since the advent of RHESSI observations, there is a clear need for new X-ray observations that can capture the full range of emission in flares (e.g., faint coronal sources near bright chromospheric sources), follow the intricate evolution of energy release and changes in morphology, and search for the signatures of impulsive energy release in even the quiescent Sun. The FOXSI Small Explorer (SMEX) mission concept combines state-of-the-art grazing-incidence focusing optics with pixelated solid-state detectors to provide direct imaging of hard X-rays for the first time on a solar observatory. We present the science objectives of FOXSI and how its capabilities will address and resolve open questions regarding impulsive energy release at the Sun. These questions include: What are the time scales of the processes that accelerate electrons? How do flare-accelerated electrons escape into the heliosphere? What is the energy input of accelerated electrons into the chromosphere, and how is super-heated coronal plasma produced? Title: Implications of L1 observations for slow solar wind formation by solar reconnection Authors: Kepko, L.; Viall, N. M.; Antiochos, S. K.; Lepri, S. T.; Kasper, J. C.; Weberg, M. Bibcode: 2016GeoRL..43.4089K Altcode: While the source of the fast solar wind is known to be coronal holes, the source of the slow solar wind has remained a mystery. Long time scale trends in the composition and charge states show strong correlations between solar wind velocity and plasma parameters, yet these correlations have proved ineffective in determining the slow wind source. We take advantage of new high time resolution (12 min) measurements of solar wind composition and charge state abundances at L1 and previously identified 90 min quasiperiodic structures to probe the fundamental timescales of slow wind variability. The combination of new high temporal resolution composition measurements and the clearly identified boundaries of the periodic structures allows us to utilize these distinct solar wind parcels as tracers of slow wind origin and acceleration. We find that each 90 min (2000 Mm) parcel of slow wind has near-constant speed yet exhibits repeatable, systematic charge state and composition variations that span the entire range of statistically determined slow solar wind values. The classic composition-velocity correlations do not hold on short, approximately hourlong, time scales. Furthermore, the data demonstrate that these structures were created by magnetic reconnection. Our results impose severe new constraints on slow solar wind origin and provide new, compelling evidence that the slow wind results from the sporadic release of closed field plasma via magnetic reconnection at the boundary between open and closed flux in the Sun's atmosphere. Title: Switch-on Shock and Nonlinear Kink Alfvén Waves in Solar Polar Jets Authors: DeVore, C. Richard; Karpen, Judith T.; Antiochos, Spiro K.; Uritsky, Vadim Bibcode: 2016SPD....47.0309D Altcode: It is widely accepted that solar polar jets are produced by fast magnetic reconnection in the low corona, whether driven directly by flux emergence from below or indirectly by instability onset above the photosphere. In either scenario, twisted flux on closed magnetic field lines reconnects with untwisted flux on nearby open field lines. Part of the twist is inherited by the newly reconnected open flux, which rapidly relaxes due to magnetic tension forces that transmit the twist impulsively into the outer corona and heliosphere. We propose that this transfer of twist launches switch-on MHD shock waves, which propagate parallel to the ambient coronal magnetic field ahead of the shock and convect a perpendicular component of magnetic field behind the shock. In the frame moving with the shock front, the post-shock flow is precisely Alfvénic in all three directions, whereas the pre-shock flow is super-Alfvénic along the ambient magnetic field, yielding a density enhancement at the shock front. Nonlinear kink Alfvén waves are exact solutions of the time-dependent MHD equations in the post-shock region when the ambient corona is uniform and the magnetic field is straight. We have performed and analyzed 3D Cartesian and spherical simulations of polar jets driven by instability onset in the corona. The results of both simulations are consistent with the generation of MHD switch-on shocks trailed predominantly by incompressible kink Alfvén waves. It is noteworthy that the kink waves are irrotational, in sharp contrast to the vorticity-bearing torsional waves reported from previous numerical studies. We will discuss the implications of the results for understanding solar polar jets and predicting their heliospheric signatures. Our research was supported by NASA’s LWS TR&T and H-SR programs. Title: The Breakout Model for Coronal Jets with Filaments Authors: Wyper, Peter; DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2016SPD....4740203W Altcode: Coronal jets are impulsive, collimated plasma outflows originating low in the solar corona. Many of these events exhibit broad, curtain-like morphologies with helical structure and motions. Recently, Sterling et al. (2015) [doi:10.1038/nature14556] reported that such jets are associated with the eruption of small filaments and, therefore, are miniature versions of corona mass ejections (CMEs). This account differs from the traditional picture of jets, in that internal flare reconnection, rather than interchange reconnection with the external ambient magnetic field, creates the bright loops observed at the jet base. We present 3D simulations, performed with the Adaptively Refined MHD Solver (ARMS), which demonstrate how the magnetic breakout mechanism generates mini-CME-type jets in a compact bipolar region energized by simple footpoint motions. Our numerical model captures the formation of the strongly sheared pre-jet filament structure, the post-jet flare-like loops and ribbons, and the curtain-like untwisting dynamics observed higher in the corona. We will discuss the significance of our new results for understanding solar EUV and X-ray jets and CMEs in general. NASA supported this research by awards to the NASA Postdoctoral Program (P.F.W.) and the LWS TR&T and H-SR programs (C.R.D. & S.K.A.). Title: Simulations of Filament Channel Formation in a Coronal Magnetic Field Authors: Knizhnik, Kalman; DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2016SPD....4710301K Altcode: A major unanswered problem in solar physics has been explaining the presence of sheared filament channels above photospheric polarity inversion lines (PILs) and the simultaneous lack of structure in the ‘loop’ portion of the coronal magnetic field. The shear inherent in filament channels represents not only a form of magnetic energy, but also magnetic helicity. As a result, models of filament channel formation need to explain not only why helicity is observed above PILs, but also why it is apparently not observed anywhere else in the corona. Previous results (Knizhnik, Antiochos & DeVore, 2015) have suggested that any helicity injected into the coronal field inverse-cascades in scale, a process known as magnetic helicity condensation (Antiochos, 2013). In this work, we present high resolution numerical simulations of photospheric helicity injection into a coronal magnetic field that contains both a PIL and a coronal hole (CH). We show conclusively that the inverse cascade of magnetic helicity terminates at the PIL, resulting in the formation of highly sheared filament channels and a smooth, untwisted corona. We demonstrate that even though magnetic helicity is injected throughout the flux system, it accumulates only at the PIL, where it manifests itself in the form of highly sheared filament channels, while any helicity obtained by the CH is ejected out of the system. We show that the formation of filament channels is both qualitatively and quantitatively in agreement with observations and discuss the implications of our simulations for observations.This work was supported by the NASA Earth and Space Science Fellowship, LWS TR&T and H-SR Programs. Title: Achieving Consistent Doppler Measurements from SDO/HMI Vector Field Inversions Authors: Schuck, Peter W.; Antiochos, Spiro K.; Leka, K. D.; Barnes, Graham Bibcode: 2016SPD....47.1207S Altcode: NASA’s Solar Dynamics Observatory is delivering vector magnetic field observations of the full solar disk with unprecedented temporal and spatial resolution; however, the satellite is in a highly inclined geosynchronous orbit. The relative spacecraft-Sun velocity varies by ±3 km/s over a day which introduces major orbital artifacts in the Helioseismic Magnetic Imager data. We demonstrate that the orbital artifacts contaminate all spatial and temporal scales in the data. We describe a newly-developed three stage procedure for mitigating these artifacts in the Doppler data obtained from the Milne-Eddington inversions in the HMI Pipeline. The procedure ultimately uses 32 velocity dependent coefficients to adjust 10 million pixels - a remarkably sparse correction model given the complexity of the orbital artifacts. This procedure was applied to full disk images of AR11084 to produce consistent Dopplergrams. The data adjustments reduce the power in the orbital artifacts by 31dB. Furthermore, we analyze in detail the corrected images and show that our procedure greatly improves the temporal and spectral properties of the data without adding any new artifacts. We conclude that this new procedure makes a dramatic improvement in the consistency of the HMI data and in its usefulness for precision scientific studies. Title: A Model for Stealth Coronal Mass Ejections Authors: Lynch, Benjamin J.; Masson, Sophie; Li, Yan; DeVore, C. Richard; Luhmann, Janet; Antiochos, Spiro K.; Fisher, George H. Bibcode: 2016SPD....47.0616L Altcode: Stealth coronal mass ejections (CMEs) are events in which there are almost no observable signatures of the CME eruption in the low corona but often a well-resolved slow flux rope CME observed in the coronagraph data. We present results from a three-dimensional numerical magnetohydrodynamics (MHD) simulation of the 2008 June 1-2 slow streamer blowout CME that Robbrecht et al. [2009] called “the CME from nowhere.” We model the global coronal structure using a 1.4 MK isothermal solar wind and a low-order potential field source surface representation of the Carrington Rotation 2070 magnetogram synoptic map. The bipolar streamer belt arcade is energized by simple shearing flows applied in the vicinity of the helmet streamer’s polarity inversion line. The slow expansion of the energized helmet-streamer arcade results in the formation of a radial current sheet. The subsequent onset of expansion-driven flare reconnection initiates the stealth CME while gradually releasing ~1.5E+30 erg of stored magnetic energy over the 20+ hour eruption duration. We show the energy flux available for flare heating and flare emission during the eruption is approximately two orders of magnitude below the energy flux required to heat the ambient background corona, thus confirming the “stealth” character of the 2008 June 1-2 CME’s lack of observable on disk signatures. We also present favorable comparisons between our simulation results and the multi-viewpoint SOHO-LASCO and STEREO-SECCHI coronagraph observations of the pre-eruption streamer structure and the initiation and evolution of the stealth streamer blowout CME. Title: Structures in the Outer Solar Atmosphere Authors: Fletcher, L.; Cargill, P. J.; Antiochos, S. K.; Gudiksen, B. V. Bibcode: 2016mssf.book..231F Altcode: No abstract at ADS Title: Reconnection Between Twisted Flux Tubes - Implications for Coronal Heating Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R.; Klimchuk, J. A.; Wyper, P. F. Bibcode: 2015AGUFMSH13B2439K Altcode: The nature of the heating of the Sun's corona has been a long-standing unanswered problem in solar physics. Beginning with the work of Parker (1972), many authors have argued that the corona is continuously heated through numerous small-scale reconnection events known as nanoflares. In these nanoflare models, stressing of magnetic flux tubes by photospheric motions causes the field to become misaligned, producing current sheets in the corona. These current sheets then reconnect, converting the free energy stored in the magnetic field into heat. In this work, we use the Adaptively Refined MHD Solver (ARMS) to perform 3D MHD simulations that dynamically resolve regions of strong current to study the reconnection between twisted flux tubes in a plane-parallel Parker configuration. We investigate the energetics of the process, and show that the flux tubes accumulate stress gradually before undergoing impulsive reconnection. We study the motion of the individual field lines during reconnection, and demonstrate that the connectivity of the configuration becomes extremely complex, with multiple current sheets being formed, which could lead to enhanced heating. In addition, we show that there is considerable interaction between the twisted flux tubes and the surrounding untwisted field, which contributes further to the formation of current sheets. The implications for observations will be discussed. This work was funded by a NASA Earth and Space Science Fellowship, and by the NASA TR&T Program. Title: Slow Solar Wind: Observable Characteristics for Constraining Modelling Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.; Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.; von Steiger, R.; Wang, Y. M. Bibcode: 2015AGUFMSH11F..03O Altcode: The Slow Solar Wind (SSW) origin is an open issue in the post SOHO era and forms a major objective for planned future missions such as the Solar Orbiter and Solar Probe Plus.Results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in-situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW.Advances models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations.Nevertheless, there are still debated questions such as:What are the source regions of SSW? What are their contributions to the SSW?Which is the role of the magnetic topology in corona for the origin, acceleration and energy deposition of SSW?Which are the possible acceleration and heating mechanisms for the SSW?The aim of this study is to present the insights on the SSW origin and formationarisen during the discussions at the International Space Science Institute (ISSI) by the Team entitled ''Slowsolar wind sources and acceleration mechanisms in the corona'' held in Bern (Switzerland) in March2014--2015. The attached figure will be presented to summarize the different hypotheses of the SSW formation. Title: Capabilities of a FOXSI Small Explorer Authors: Inglis, A. R.; Christe, S.; Glesener, L.; Krucker, S.; Dennis, B. R.; Shih, A.; Wilson-Hodge, C.; Gubarev, M.; Hudson, H. S.; Kontar, E.; Buitrago Casas, J. C.; Drake, J. F.; Caspi, A.; Holman, G.; Allred, J. C.; Ryan, D.; Alaoui, M.; White, S. M.; Saint-Hilaire, P.; Klimchuk, J. A.; Hannah, I. G.; Antiochos, S. K.; Grefenstette, B.; Ramsey, B.; Jeffrey, N. L. S.; Reep, J. W.; Schwartz, R. A.; Ireland, J. Bibcode: 2015AGUFMSH43B2456I Altcode: We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer (SMEX) concept, a mission dedicated to studying particle acceleration and energy release on the Sun. FOXSI is designed as a 3-axis stabilized spacecraft in low-Earth orbit making use of state-of-the-art grazing incidence focusing optics, allowing for direct imaging of solar X-rays. The current design being studied features three telescope modules deployed in a low-inclination low-earth orbit (LEO). With a 15 meter focal length enabled by a deployable boom, FOXSI will observe the Sun in the 3-50 keV energe range. The FOXSI imaging concept has already been tested on two sounding rocket flights, in 2012 and 2014 and on the HEROES balloon payload flight in 2013. FOXSI will image the Sun with an angular resolution of 5'', a spectral resolution of 0.5 keV, and sub-second temporal resolution using CdTe detectors. In this presentation we investigate the science objectives and targets which can be accessed from this mission. Because of the defining characteristic of FOXSI is true imaging spectroscopy with high dynamic range and sensitivity, a brand-new perspective on energy release on the Sun is possible. Some of the science targets discussed here include; flare particle acceleration processes, electron beams, return currents, sources of solar energetic particles (SEPs), as well as understanding X-ray emission from active region structures and the quiescent corona. Title: The Dynamics of Coronal-Hole Boundaries Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R.; Wyper, P. F.; Zurbuchen, T. Bibcode: 2015AGUFMSH24A..06H Altcode: The source of the slow solar wind at the Sun is the subject of intense debate in solar and heliospheric physics. Because the majority of the solar wind observed at Earth is slow wind, understanding its origin is essential for understanding and predicting Earth's space weather environment. In-situ and remote observations show that, compared to the fast wind, the slow solar wind corresponds to higher freeze-in temperatures, as indicated by charge-state ratios, and more corona-like elemental abundances. These results indicate that the most likely source for the slow wind is the hot plasma in the closed-field corona; however, the release mechanism for the wind from the closed-field regions is far from understood. Here we present the first fully dynamic, 3D MHD simulations of a coronal-hole boundary driven by photospheric convective flows. We determine in detail the opening and closing of coronal flux due to photospheric motions at the base of a helmet streamer. These changes should lead to the release of plasma from the closed magnetic field at the edge of the streamer. Our analysis demonstrates that the bulk of the release is due to interchange reconnection. We calculate the effective of numerical Lundquist number on the dynamics and discuss the implications of our results for theories of slow-wind origin, in particular the S-Web model. We also discuss the implications of our results for observations, in particular from the upcoming Solar Orbiter and Solar Probe Plus missions. This work was supported by the NASA SR&T and TR&T Programs. Title: Determining the Energy Transport Through the Photosphere into Corona with HMI Authors: Schuck, P. W.; Antiochos, S. K. Bibcode: 2015AGUFMSH31B2422S Altcode: The underlying source of all solar activity, from the largest explosive solar eruptive event to the quasi-steady radiation from quiet regions is the transport of free magnetic energy into the chromosphere and corona through the photosphere. There are two mechanisms for this transport, the emergence of magnetic field and accompanying electric currents, and the stressing of pre-emerged chromosphere and coronal field by photospheric flows. Consequently, if we are ever to understand solar activity it is essential that the flow of energy be measured accurately. In principle, this can be estimated from the high spatial and temporal resolution vector magnetograms obtained by HMI on SDO. However, due to the artifacts introduced by the orbital motions of SDO convolved with the finite-sampling of the relevant polarizations, the data cannot be used for measuring the energy flux or helicity flux accurately. We present recent work on the first successful attempt to remove the orbital artifacts. We describe the procedure, which consists of correcting each pixel separately for its bias and gain. We show results for before and after our artifact removal procedures. The method is being prepared for community use through the SDO pipeline. We discuss the application of our methods to analyzing active regions both with and without major eruptions. This work was supported by the NASA LWS and R&A Programs. Title: The Origin and Development of Solar Eruptive Events Authors: Antiochos, S. K.; DeVore, C. R.; Karpen, J. T.; Masson, S. Bibcode: 2015AGUFMSH11A2384A Altcode: Solar eruptive events (SEE), which consist of fast coronal mass ejections and intense flares, are the largest and most energetic form of solar activity, and are the drivers of the most destructive space weather throughout interplanetary space. Understanding the physical origin of these giant magnetic explosions is absolutely essential for any first-principles based space weather forecasting and, consequently, is a core focus of the NASA LWS Program. We describe how magnetic reconnection is responsible for the energy buildup that leads to SEEs, how it drives the explosive energy release, and how it controls the propagation of the event. Reconnection turns out to be especially important for understanding the escape of high-energy particles into the heliosphere. A key issue for numerical simulation of SEEs is the effect of the resistivity model used by the simulation, because the onset and subsequent development of reconnection inherently dependent on the effective resistivity. We present the latest ultra-high numerical resolution 2.5D simulations quantifying how the reconnection dynamics scale with effective resistivity. We also present 3D simulations demonstrating the complexities introduced by reconnection in a realistic 3D system. The implications of our results for interpreting observations and for developing space weather capabilities will be described. This work was supported by the NASA LWS Strategic Capability Program. Title: Shear-Driven Reconnection in Kinetic Models Authors: Black, C.; Antiochos, S. K.; Germaschewski, K.; Karpen, J. T.; DeVore, C. R.; Bessho, N. Bibcode: 2015AGUFMSH43A2432B Altcode: The explosive energy release in solar eruptive phenomena is believed to be due to magnetic reconnection. In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field countered by a downward tension due to overlying unsheared field. Magnetic reconnection disrupts this force balance; therefore, it is critical for understanding CME/flare initiation, to model the onset of reconnection driven by the build-up of magnetic shear. In MHD simulations, the application of a magnetic-field shear is a trivial matter. However, kinetic effects are dominant in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is challenging, however, and indicates the necessity of a true multiscale model for such processes in the solar environment. The field must be sheared self-consistently and indirectly to prevent the generation of waves that destroy the desired system. Plasma instabilities can arise nonetheless. In the work presented here, we show that we can control this instability and generate a predicted out-of-plane magnetic flux. This material is based upon work supported by the National Science Foundation under Award No. AGS-1331356. Title: On Streamer-Blowout CMEs That Aren't Really CMEs: How the Corona Makes Slow Flux Rope-Like ICMEs Without an Explosive Release of Free Magnetic Energy Authors: Lynch, B. J.; Masson, S.; Li, Y.; DeVore, C. R.; Luhmann, J. G.; Antiochos, S. K. Bibcode: 2015AGUFMSH53A2459L Altcode: We present a 3D numerical MHD simulation of the 2008 Jun 2 gradual streamer blowout CME that had virtually no identifiable low coronal signatures. We energize the field by simple footpoint shearing along the source region's polarity inversion line and model the background solar wind structure using an ~2MK isothermal wind and a low-order potential field source surface representation of the CR2070 synoptic magnetogram. Our results show that the CME "initiation" is obtained by slowly disrupting the quasi-steady-state configuration of the helmet streamer, resulting in the standard eruptive flare picture that ejects the sheared/twisted fields -- very slowly and on a relatively large scale -- with virtually no decrease in the global magnetic energy. We obtain a relatively slow CME eruption of order the background solar wind speed (Vcme ~ 300 km/s by 15 Rs). We argue that these very slow, expansion-driven "eruptions" are merely the natural and gradual response of the large-scale corona to the accumulation of global-scale stress (e.g. differential rotation). We present comparisons of the CME propagation through the corona (≤15Rs) in synthetic white-light images derived from the simulation density structure with multi-spacecraft coronagraph data from STEREO/SECCHI and SOHO/LASCO. We show a favorable comparison between the simulation's ICME flux rope structure with the in situ STEREO observations. Title: Filament Channel Formation via Magnetic Helicity Condensation Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2015ApJ...809..137K Altcode: 2014arXiv1411.5396K A major unexplained feature of the solar atmosphere is the accumulation of magnetic shear in the form of filament channels at photospheric polarity inversion lines (PILs). In addition to free energy, this shear represents magnetic helicity, which is conserved under reconnection. In this paper we address the problem of filament channel formation and show how filaments acquire their shear and magnetic helicity. The results of three-dimensional (3D) simulations using the Adaptively Refined Magnetohydrodynamics Solver are presented. Our findings support the model of filament channel formation by magnetic helicity condensation that was developed by Antiochos. We consider the small-scale photospheric twisting of a quasi-potential flux system that is bounded by a PIL and contains a coronal hole (CH). The magnetic helicity injected by the small-scale photospheric motions is shown to inverse cascade up to the largest allowable scales that define the closed flux system: the PIL and the CH. This process produces field lines that are both sheared and smooth, and are sheared in opposite senses at the PIL and the CH. The accumulated helicity and shear flux are shown to be in excellent quantitative agreement with the helicity condensation model. We present a detailed analysis of the simulations, including comparisons of our analytical and numerical results, and discuss their implications for observations. Title: Understanding Magnetic Reconnection Drivers: Magnetic Field Shear in Kinetic Models Authors: Black, Carrie E.; Antiochos, Spiro K.; DeVore, C. Richard; Germaschewski, Kai; Bessho, Naoki; Karpen, Judith T. Bibcode: 2015shin.confE..25B Altcode: The explosive energy release in solar eruptive phenomena believed to be due to magnetic reconnection. In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field countered by a downward tension due to overlying unsheared field. Magnetic reconnection disrupts this force balance, therefore, it is critical for understanding CME/flare initiation, to model the onset of reconnection driven by the build-up of magnetic shear. In MHD simulations, the application of a magnetic-field shear is a trivial matter. However, kinetic effects are important in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is nontrivial, however, and indicates the necessity of a true multiscale model for such processes in the solar environment. The field must be sheared self-consistently and indirectly to prevent the generation of waves that destroy the desired system. Plasma instabilities can arise nonetheless. In the work presented here, we show that we can control this instability and generate a predicted out-of-plane magnetic flux. This material is based upon work supported by the National Science Foundation under Award No. AGS-1331356. Title: Erratum: “Numerical Simulations of Helicity Condensation in the Solar Corona” (2015, ApJ, 805, 61) Authors: Zhao, L.; DeVore, C. R.; Antiochos, S. K.; Zurbuchen, T. H. Bibcode: 2015ApJ...807..113Z Altcode: No abstract at ADS Title: Helicity Condensation During Reconnection of Twisted Flux Tubes: Implications for Coronal Heating Authors: Knizhnik, Kalman Joshua; Antiochos, Spiro K.; DeVore, C. Richard; Klimchuk, James A.; Wyper, Peter F. Bibcode: 2015shin.confE..18K Altcode: The nature of the heating of the Sun's corona has been a long-standing unanswered problem in solar physics. Beginning with the work of Parker (1972), many authors have argued that the corona is continuously heated through numerous small-scale reconnection events known as nanoflares. In these nanoflare models, braiding of magnetic flux tubes by surface motions causes the field to become misaligned. The current sheet separating the misaligned field eventually reconnects, converting the energy stored in the magnetic field into heat. A major challenge facing these models, however, is that the braiding required for this process injects magnetic helicity into the corona, and helicity is conserved under reconnection. In contrast, EUV and X-ray images of coronal loops reveal invariably smooth, laminar structures. The recently proposed helicity condensation model (Antiochos 2013) resolves this difficulty, explaining how reconnection transports helicity throughout the solar atmosphere and produces a smooth, hot corona. In this model, reconnection between adjacent flux tubes, twisted and tangled by surface convection, transports helicity to ever larger scales, where it ultimately condenses in filament channels. The reconnection that occurs throughout the solar atmosphere not only results in a smooth corona, but its net effect is to convert much of the magnetic energy injected by surface motions into heat. In this work, we use the Adaptively Refined MHD Solver (ARMS) to perform 3D MHD simulations that dynamically resolve regions of strong current to study the reconnection between multiple twisted flux tubes in a plane-parallel Parker configuration. We investigate the energetics of the process, and show that the flux tubes accumulate stress gradually before undergoing impulsive reconnection. We place constraints on the amount of heating expected from such reconnection. Finally, we study the motion of the individual field lines during the impulsive reconnection events. Title: Understanding the Effects of 3D Reconnection in a Breakout CME Authors: Antiochos, Spiro K. Bibcode: 2015shin.confE...5A Altcode: Coronal mass ejections/solar flares are the most dramatic and most energetic forms of solar energy release. Most theories for this energy release postulate magnetic reconnection as the underlying physical mechanism. In previous 2.5D calculations we showed how two different types of reconnection both trigger the eruption and produce the bulk of the energy release. The problem, however, is that reconnection in 2D is tightly constrained and, consequently, leads to magnetic topologies such as disconnected plasmoids that are not physical and not observed. Consequently, we have extended our calculations to a fully 3D topology that includes a multi-polar coronal field suitable for a breakout CME/eruptive flare near a coronal hole region. We performed high-resolution 3D MHD numerical simulations with the Adaptively Refined MHD Solver (ARMS). We show how the complex interactions between the 3D flare and breakout reconnections reproduce all the main observational features of CMEs/flares and impulsive SEPs. We also discuss the implications of our results for understanding the heliospheric signatures of CMEs and for further tests of the model.

This research was supported, in part, by the NASA SR&T and TR&T Programs. Title: Simulations of Coronal-Hole Boundary Dynamics Near Helmet Streamers Authors: Higginson, Aleida Katherine; Antiochos, S. K.; DeVore, C. R.; Zurbuchen, T. H. Bibcode: 2015shin.confE..19H Altcode: The source of the slow solar wind at the Sun is the subject of intense debate in solar and heliospheric physics. Because the majority of the solar wind observed at Earth is slow wind, understanding its origin is essential for understanding and predicting Earth's space weather environment. In-situ and remote observations show that, compared to the fast wind, the slow solar wind corresponds to higher freeze-in temperatures, as indicated by charge-state ratios, and more corona-like elemental abundances. These results indicate that the most likely source for the slow wind is the hot plasma in the closed-field corona; however, the release mechanism for the wind from the closed-field regions is far from understood. Here we present fully dynamic, 3D MHD simulations of a driven coronal-hole boundary. We determine in detail the opening and closing of coronal flux due to photospheric motions at the base of a helmet streamer. These changes should lead to the release of plasma from the closed magnetic field at the edge of the streamer. We discuss the implications of our results for theories of slow-wind origin, in particular the S-Web model. Title: Structures in the Outer Solar Atmosphere Authors: Fletcher, L.; Cargill, P. J.; Antiochos, S. K.; Gudiksen, B. V. Bibcode: 2015SSRv..188..211F Altcode: 2014SSRv..tmp...52F; 2014arXiv1412.7378F The structure and dynamics of the outer solar atmosphere are reviewed with emphasis on the role played by the magnetic field. Contemporary observations that focus on high resolution imaging over a range of temperatures, as well as UV, EUV and hard X-ray spectroscopy, demonstrate the presence of a vast range of temporal and spatial scales, mass motions, and particle energies present. By focusing on recent developments in the chromosphere, corona and solar wind, it is shown that small scale processes, in particular magnetic reconnection, play a central role in determining the large-scale structure and properties of all regions. This coupling of scales is central to understanding the atmosphere, yet poses formidable challenges for theoretical models. Title: Numerical Simulations of Helicity Condensation in the Solar Corona Authors: Zhao, L.; DeVore, C. R.; Antiochos, S. K.; Zurbuchen, T. H. Bibcode: 2015ApJ...805...61Z Altcode: The helicity condensation model has been proposed by Antiochos to explain the observed smoothness of coronal loops and the observed buildup of magnetic shear at filament channels. The basic hypothesis of the model is that magnetic reconnection in the corona causes the magnetic stress injected by photospheric motions to collect only at those special locations where prominences are observed to form. In this work we present the first detailed quantitative MHD simulations of the reconnection evolution proposed by the helicity condensation model. We use the well-known ansatz of modeling the closed corona as an initially uniform field between two horizontal photospheric plates. The system is driven by applying photospheric rotational flows that inject magnetic helicity into the corona. The flows are confined to a finite region on the photosphere so as to mimic the finite flux system of a bipolar active region, for example. The calculations demonstrate that, contrary to common belief, opposite helicity twists do not lead to significant reconnection in such a coronal system, whereas twists with the same sense of helicity do produce substantial reconnection. Furthermore, we find that for a given amount of helicity injected into the corona, the evolution of the magnetic shear is insensitive to whether the pattern of driving photospheric motions is fixed or quasi-random. In all cases, the shear propagates via reconnection to the boundary of the flow region while the total magnetic helicity is conserved, as predicted by the model. We discuss the implications of our results for solar observations and for future, more realistic simulations of the helicity condensation process. Title: Filament Channel Formation Via Magnetic Helicity Condensation Authors: Knizhnik, Kalman Joshua; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2015TESS....111305K Altcode: A major unexplained feature of the solar atmosphere is the accumulation of magnetic shear, in the form of filament channels, at photospheric polarity inversion lines (PILs). In addition to free energy, this shear also represents magnetic helicity, which is conserved under reconnection. In this work, we address the problem of filament channel formation and show how they acquire their shear and magnetic helicity. The results of 3D simulations using the Adaptively Refined Magnetohydrodynamics Solver (ARMS) are presented that support the model of filament channel formation by magnetic helicity condensation developed by Antiochos (2013). We consider the convective twisting of a quasi-potential flux system that is bounded by a PIL and contains a coronal hole (CH). The magnetic helicity injected by the small-scale photospheric motions is shown to inverse-cascade up to the largest allowable scales that defined the closed flux system: the PIL and the CH. This process produces field lines that are both sheared and smooth, and are sheared in opposite senses at the PIL and the CH. The accumulated helicity and shear flux are shown to be in excellent quantitative agreement with the helicity-condensation model. We present a detailed analysis of the simulations, including comparisons of our analytical and numerical results, and discuss their implications for observations. Our research was supported by NASA's Earth and Space Science Fellowship (K.J.K.) and Heliophysics Supporting Research (S.K.A. and C.R.D.) programs. Title: Magnetic Reconnection Onset and Energy Release at Current Sheets Authors: DeVore, C. R.; Antiochos, Spiro K. Bibcode: 2015TESS....110201D Altcode: Reconnection and energy release at current sheets are important at the Sun (coronal heating, coronal mass ejections, flares, and jets) and at the Earth (magnetopause flux transfer events and magnetotail substorms) and other magnetized planets, and occur also at the interface between the Heliosphere and the interstellar medium, the heliopause. The consequences range from relatively quiescent heating of the ambient plasma to highly explosive releases of energy and accelerated particles. We use the Adaptively Refined Magnetohydrodynamics Solver (ARMS) model to investigate the self-consistent formation and reconnection of current sheets in an initially potential 2D magnetic field containing a magnetic null point. Unequal stresses applied to the four quadrants bounded by the X-line separatrix distort the potential null into a double-Y-type current sheet. We find that this distortion eventually leads to onset of fast magnetic reconnection across the sheet, with copious production, merging, and ejection of magnetic islands due to plasmoid instability. In the absence of a mechanism for ideal instability or loss of equilibrium of the global structure, however, this reconnection leads to minimal energy release. Essentially, the current sheet oscillates about its force-free equilibrium configuration. When the structure is susceptible to a large-scale rearrangement of the magnetic field, on the other hand, the energy release becomes explosive. We identify the conditions required for reconnection to transform rapidly a large fraction of the magnetic free energy into kinetic and other forms of plasma energy, and to restructure the current sheet and its surrounding magnetic field dramatically. We discuss the implications of our results for understanding heliophysical activity, particularly eruptions, flares, and jets in the corona.Our research was supported by NASA’s Heliophysics Supporting Research and Living With a Star Targeted Research and Technology programs. Title: Understanding Magnetic Reconnection: The Physical Mechanism Driving Space Weather Authors: Black, Carrie; Antiochos, Spiro K.; Karpen, Judith T.; Germaschewski, Kai; Bessho, Naoki Bibcode: 2015TESS....111004B Altcode: The explosive energy release in solar eruptive events is believed to be due to magnetic reconnection. In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field countered by the downward tension of the overlying unsheared field. Magnetic reconnection disrupts this force balance. Therefore, to understand CME/flare initiation, it is critical to model the onset of reconnection driven by the build-up of magnetic shear. In MHD simulations, the application of a magnetic-field shear is trivial. However, kinetic effects are important in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is nontrivial, however, and indicates the necessity of a true multiscale model for such processes in the solar environment. The field must be sheared self-consistently and indirectly to prevent the generation of waves that destroy the desired system. In the work presented here, we show reconnection in an X-Point geometry due to a velocity shear driver perpendicular to the plane of reconnection.This material is based upon work supported by the National Science Foundation under Award No. AGS-1331356 and NASA's Living With a Star Targeted Research and Technology program. Title: Modeling Reconnection-driven Polar Jets from the Sun to the Heliosphere Authors: Karpen, Judith T.; DeVore, C. R.; Antiochos, Spiro K. Bibcode: 2015TESS....120303K Altcode: Jets from coronal holes on the Sun have been observed in EUV and white-light emissions since the launch of SOHO, but the physical mechanism responsible for these events remains elusive. An important clue about their origin lies in their association with small intrusions of minority polarity within the large-scale open magnetic field, strongly suggesting that these jets are powered by interchange reconnection between embedded bipoles (closed flux) and the surrounding open flux (Antiochos 1999). We have explored this model for polar jets through a series of computational investigations of the embedded-bipole paradigm. The results demonstrate that energetic, collimated, Alfvénic flows can be driven by explosive reconnection between twisted closed flux of the minority polarity and the unstressed external field (e.g., Pariat et al. 2009, 2010, 2015). Our previous studies were focused on the dynamics and energetics of this process close to the solar surface, utilizing Cartesian geometry without gravity or wind. In the present study, we compare new simulations of reconnection-driven polar jets in spherical geometry and an isothermal solar wind with Cartesian, gravity- and wind-free simulations. Our new, more realistic simulations strongly support the interchange reconnection model as the explanation for observed polar jets. We pay particular attention to identifying observable signatures and measuring the evolving mass, wave, and energy fluxes as the jet extends toward heights comparable to the perihelion of Solar Probe Plus.This research was supported by NASA's Living With a Star Targeted Research and Technology program. Title: 3D Simulations of Helmet Streamer Dynamics and Implications for the Slow Solar Wind Authors: Higginson, Aleida K.; Antiochos, Spiro K.; DeVore, C. R.; Zurbuchen, Thomas H. Bibcode: 2015TESS....110804H Altcode: The source of the slow solar wind at the Sun is still an issue of intense debate in solar and heliospheric physics. Because the majority of the solar wind observed at Earth is slow wind, understanding its origin is essential for understanding and predicting Earth’s space weather environment. In-situ and remote observations show that, when compared to the fast wind, the slow solar wind corresponds to higher freeze-in temperatures, as indicated by charge-state ratios, and more corona-like elemental abundance ratios. These results indicate that the most likely source for the slow wind is the hot plasma in the closed-field corona, but the release mechanism(s) for the wind from the closed-field regions is far from understood. We perform fully dynamic, 3D MHD simulations in order to the study the opening and closing of the Sun’s magnetic field that leads to the escape of the slow solar wind. In particular, we calculate the dynamics of helmet streamers that are driven by photospheric motions such as supergranular flows. We determine in detail the opening and closing of coronal flux, and discuss the implications of our results for theories of slow wind origin, especially the S-Web model. We also determine observational signatures for the upcoming inner heliosphere missions Solar Orbiter and Solar Probe Plus.This work was supported by the NASA SR&T and TR&T Programs. Title: Flare Particle Escape in 3D Solar Eruptive Events Authors: Antiochos, Spiro K.; Masson, Sophie; DeVore, C. R. Bibcode: 2015TESS....111404A Altcode: Among the most important, but least understood forms of space weather are the so-called Impulsive Solar Energetic Particle (SEP) events, which can be especially hazardous to deep-space astronauts. These energetic particles are generally believed to be produced by the flare reconnection that is the primary driver of solar eruptive events (SEE). A key point is that in the standard model of SEEs, the particles should remain trapped in the coronal flare loops and in the ejected plasmoid, the CME. However, flare-accelerated particles frequently reach the Earth long before the CME does. In previous 2.5D calculations we showed how the external reconnection that is an essential element of the breakout model for CME initiation could lead to the escape of flare-accelerated particles. The problem, however, is that in 2.5D this reconnection also tends to destroy the plasmoid, which disagrees with the observation that SEP events are often associated with well-defined plasmoids at 1 AU known as “magnetic clouds”. Consequently, we have extended our model to a fully 3D topology that includes a multi-polar coronal field suitable for a breakout SEE near a coronal hole region. We performed high-resolution 3D MHD numerical simulations with the Adaptively Refined MHD Solver (ARMS). Our results demonstrate that the model allows for the effective escape of energetic particles from deep within an ejecting well-defined plasmoid. We show how the complex interactions between the flare and breakout reconnection reproduce all the main observational features of SEEs and SEPs. We discuss the implications of our calculations for the upcoming Solar Orbiter and Solar Probe Plus missions, which will measure SEEs and SEPs near the Sun, thereby, mitigating propagation effects.This research was supported, in part, by the NASA SR&T and TR&T Programs. Title: A Model for the Electrically Charged Current Sheet of a Pulsar Authors: DeVore, C. R.; Antiochos, S. K.; Black, C. E.; Harding, A. K.; Kalapotharakos, C.; Kazanas, D.; Timokhin, A. N. Bibcode: 2015ApJ...801..109D Altcode: Global-scale solutions for the magnetosphere of a pulsar consist of a region of low-lying, closed magnetic field near the star, bounded by opposite-polarity regions of open magnetic field along which the pulsar wind flows into space. Separating these open-field regions is a magnetic discontinuity—an electric current sheet—consisting of generally nonneutral plasma. We have developed a self-consistent model for the internal equilibrium structure of the sheet by generalizing the charge-neutral Vlasov/Maxwell equilibria of Harris and Hoh to allow for net electric charge. The resulting equations for the electromagnetic field are solved analytically and numerically. Our results show that the internal thermal pressure needed to establish equilibrium force balance, and the associated effective current-sheet thickness and magnetization, can differ by orders of magnitude from the Harris/Hoh charge-neutral limit. The new model provides a starting point for kinetic or fluid investigations of instabilities that can cause magnetic reconnection and flaring in pulsar magnetospheres. Title: Model for straight and helical solar jets. I. Parametric studies of the magnetic field geometry Authors: Pariat, E.; Dalmasse, K.; DeVore, C. R.; Antiochos, S. K.; Karpen, J. T. Bibcode: 2015A&A...573A.130P Altcode: Context. Jets are dynamic, impulsive, well-collimated plasma events developing at many different scales and in different layers of the solar atmosphere.
Aims: Jets are believed to be induced by magnetic reconnection, a process central to many astrophysical phenomena. Studying their dynamics can help us to better understand the processes acting in larger eruptive events (e.g., flares and coronal mass ejections) as well as mass, magnetic helicity, and energy transfer at all scales in the solar atmosphere. The relative simplicity of their magnetic geometry and topology, compared with larger solar active events, makes jets ideal candidates for studying the fundamental role of reconnection in energetic events.
Methods: In this study, using our recently developed numerical solver ARMS, we present several parametric studies of a 3D numerical magneto-hydrodynamic model of solar-jet-like events. We studied the impact of the magnetic field inclination and photospheric field distribution on the generation and properties of two morphologically different types of solar jets, straight and helical, which can account for the observed so-called standard and blowout jets.
Results: Our parametric studies validate our model of jets for different geometric properties of the magnetic configuration. We find that a helical jet is always triggered for the range of parameters we tested. This demonstrates that the 3D magnetic null-point configuration is a very robust structure for the energy storage and impulsive release characteristic of helical jets. In certain regimes determined by magnetic geometry, a straight jet precedes the onset of a helical jet. We show that the reconnection occurring during the straight-jet phase influences the triggering of the helical jet.
Conclusions: Our results allow us to better understand the energization, triggering, and driving processes of straight and helical jets. Our model predicts the impulsiveness and energetics of jets in terms of the surrounding magnetic field configuration. Finally, we discuss the interpretation of the observationally defined standard and blowout jets in the context of our model, as well as the physical factors that determine which type of jet will occur. Title: Implications of the S-Web for the Corona and Inner Heliosphere Authors: Antiochos, S. K. Bibcode: 2014AGUFMSH21B4100A Altcode: Decades of satellite observations have shown that the solar wind that forms the heliosphere consists of two distinct types: the so-called fast and slow. The fast wind originates back at the Sun in long-lived open field regions observed as "coronal holes" in X-Ray images, but the source of the slow wind has long been an issue of intense debate. Due to its observed location in the heliosphere, its plasma composition, and its variability, many models for the origin of the slow wind postulate that it is a result of the release of closed-field plasma onto open field lines. In the recent S-Web model we proposed that the slow wind originates at a dynamic boundary region between open and closed flux in the solar corona. Consequently, the detailed structure of the open-closed magnetic boundary and, most important, the dynamics of this boundary are essential for determining the properties of the slow wind and of the heliosphere, in general. The three important magnetic topologies of the open-closed boundary in the corona and their consequences for the inner heliosphere will be discussed. Furthermore, recent simulations will be presented revealing the dynamics of this boundary and demonstrating that these dynamics can account for the observed properties of the slow wind. The implications of our results for understanding the corona-heliosphere connection and especially for interpreting observations from the upcoming Solar Orbiter and Solar Probe Plus missions will be discussed. This work was supported by the NASA TR&T and SR&T Programs. Title: X-Point Reconnection from Shear Driving in Kinetic Simulations Authors: Black, C.; Antiochos, S. K.; DeVore, C. R.; Germaschewski, K.; Bessho, N.; Karpen, J. T. Bibcode: 2014AGUFMSH23A4154B Altcode: The explosive energy release in solar eruptive phenomena such as CMEs/eruptive flares and coronal jets is believed to be due to magnetic reconnection. Magnetic free energy builds up slowly in the corona due to footpoint stressing by the photospheric motions. Along with the free energy, current sheets build up at coronal nulls, which eventually triggers fast reconnection and explosive energy release. This basic scenario has been modeled extensively by MHD simulations and applied to both CMEs/eruptive flares and jets, but the reconnection itself is well-known to be due to kinetic processes. Consequently, it is imperative that shear-driven X-point reconnection be modeled in a fully kinetic system so as to test and guide the MHD results. In MHD simulations, the application of a magnetic-field shear at the system boundary is a trivial matter, but this is definitely not the case for a kinetic system, because the electric currents need to be fully consistent with all the mass motions. We present the first results of reconnection in a 2D X-Point geometry due to a velocity shear driver perpendicular to the plane of reconnection. We compare the results to high-resolution MHD simulations and discuss the implications for coronal activity. Title: Vision for the Future of Lws TR&T Authors: Schwadron, N.; Mannucci, A. J.; Antiochos, S. K.; Bhattacharjee, A.; Gombosi, T. I.; Gopalswamy, N.; Kamalabadi, F.; Linker, J.; Pilewskie, P.; Pulkkinen, A. A.; Spence, H. E.; Tobiska, W. K.; Weimer, D. R.; Withers, P.; Bisi, M. M.; Kuznetsova, M. M.; Miller, K. L.; Moretto, T.; Onsager, T. G.; Roussev, I. I.; Viereck, R. A. Bibcode: 2014AGUFMSH33B..02S Altcode: The Living With a Star (LWS) program addresses acute societal needs for understanding the effects of space weather and developing scientific knowledge to support predictive capabilities. Our society's heavy reliance on technologies affected by the space environment, an enormous number of airline customers, interest in space tourism, and the developing plans for long-duration human exploration space missions are clear examples that demonstrate urgent needs for space weather models and detailed understanding of space weather effects and risks. Since its inception, the LWS program has provided a vehicle to innovate new mechanisms for conducting research, building highly effective interdisciplinary teams, and ultimately in developing the scientific understanding needed to transition research tools into operational models that support the predictive needs of our increasingly space-reliant society. The advances needed require broad-based observations that cannot be obtained by large missions alone. The Decadal Survey (HDS, 2012) outlines the nation's needs for scientific development that will build the foundation for tomorrow's space weather services. Addressing these goals, LWS must develop flexible pathways to space utilizing smaller, more diverse and rapid development of observational platforms. Expanding utilization of ground-based assets and shared launches will also significantly enhance opportunities to fulfill the growing LWS data needs. Partnerships between NASA divisions, national/international agencies, and with industry will be essential for leveraging resources to address increasing societal demand for space weather advances. Strengthened connections to user communities will enhance the quality and impact of deliverables from LWS programs. Thus, we outline the developing vision for the future of LWS, stressing the need for deeper scientific understanding to improve forecasting capabilities, for more diverse data resources, and for project deliverables that address the growing needs of user communities. Title: Simulations of Filament Channel Formation Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2014AGUFMSH23C..06K Altcode: A major unexplained feature of the solar atmosphere is the accumulation of magnetic shear, in the form of filament channels, at photospheric polarity inversion lines (PILs). In addition to free energy, this shear also represents magnetic helicity, which is conserved under reconnection. In this work, we address the problem of filament channel formation and show how they acquire their shear and magnetic helicity. Results of 3D MHD simulations using the Adaptively Refined MHD Solver (ARMS) are presented that support the magnetic helicity-condensation model of filament-channel formation described by Antiochos, 2013. We consider the supergranular twisting of a quasi-potential flux system that is bounded by a PIL and contains a coronal hole (CH). The magnetic helicity injected by the small-scale photospheric motions is shown to inverse-cascade up to the largest allowable scales that define the closed flux system: the PIL and the CH boundary. This process produces field lines that are both sheared and smooth, and are sheared in opposite senses at the PIL and the CH. The accumulated helicity and shear flux are shown to be in excellent quantitative agreement with the helicity-condensation model. We present a detailed analysis of the simulation, including comparisons of our analytical and numerical results, and discuss their implications for observations. Title: Measuring Coronal Energy and Helicity Buildup with SDO/HMI Authors: Schuck, P. W.; Antiochos, S. K.; Barnes, G.; Leka, K. D. Bibcode: 2014AGUFMSH44A..08S Altcode: Solar eruptions are driven by energy and helicity transported through the photosphere and into the corona. However, the mechanism by which energy and helicity emerge from the solar interior to form the observed coronal structures is poorly understood. SDO/HMI data are the first space-based full-disk vector field observations of the Sun with a near 100% duty cycle and, therefore, represent an unprecedented opportunity to quantify the energy end helicity fluxes through the photosphere. However, because of the SDO satellite's highly inclined geostationary orbit, the relative velocity of the instrument varies by ±3~km/s which introduces major orbital artifacts. We have developed a procedure for mitigating these artifacts and have applied this analysis to AR11084 to produce a cleaned data set. Our analysis procedure is described, in detail, and the results for AR11084 presented. We have also recast the Berger and Field (1984) helicity transport equation in manifestly gauge invariant form and derived the terms quantifying the injection of helicity into the corona by the emergence of closed field, versus helicity injection by the stressing of pre-emerged flux. The plasma velocity fields in the photosphere, necessary for computing energy and helicity fluxes are determined using an upgraded version of DAVE4VM that incorporates the spherical geometry of the solar images. We find that the bulk of the helicity into the corona is injected by twisting motions, and we discuss the implications of our results for understanding solar activity and especially for data-driven modeling of solar eruptions.This work was supported, in part, by NASA Title: How Well Does the S-Web Theory Predict In-Situ Observations of the Slow Solar Wind? Authors: Young, A. K.; Antiochos, S. K.; Linker, J.; Zurbuchen, T. Bibcode: 2014AGUFMSH33A4124Y Altcode: The S-Web theory provides a physical explanation for the origin and properties of the slow solar wind, particularly its composition. The theory proposes that magnetic reconnection along topologically complex boundaries between open and closed magnetic fields on the sun releases plasma from closed magnetic field regions into the solar wind at latitudes away from the heliospheric current sheet. Such a wind would have elevated charge states compared to the fast wind and an elemental composition resembling the closed-field corona. This theory is currently being tested using time-dependent, high-resolution, MHD simulations, however comparisons to in-situ observations play an essential role in testing and understanding slow-wind release mechanisms. In order to determine the relationship between S-Web signatures and the observed, slow solar wind, we compare plasma data from the ACE and Ulysses spacecraft to solutions from the steady-state models created at Predictive Science, Inc., which use observed magnetic field distributions on the sun as a lower boundary condition. We discuss the S-Web theory in light of our results and the significance of the S-Web for interpreting current and future solar wind observations. This work was supported, in part, by the NASA TR&T and SR&T programs. Title: Strategic Science to Address Current and Future Space Weather Needs Authors: Mannucci, A. J.; Schwadron, N.; Antiochos, S. K.; Bhattacharjee, A.; Bisi, M. M.; Gopalswamy, N.; Kamalabadi, F.; Pulkkinen, A. A.; Tobiska, W. K.; Weimer, D. R.; Withers, P. Bibcode: 2014AGUFMSM24A..09M Altcode: NASA's Living With a Star (LWS) program has contributed a wealth of scientific knowledge that is relevant to space weather and user needs. A targeted approach to science questions has resulted in leveraging new scientific knowledge to improve not only our understanding of the Heliophysics domain, but also to develop predictive capabilities in key areas of LWS science. This fascinating interplay between science and applications promises to benefit both domains. Scientists providing feedback to the LWS program are now discussing an evolution of the targeted approach that explicitly considers how new science improves, or enables, predictive capability directly. Long-term program goals are termed "Strategic Science Areas" (SSAs) that address predictive capabilities in six specific areas: geomagnetically induced currents, satellite drag, solar energetic particles, ionospheric total electron content, radio frequency scintillation induced by the ionosphere, and the radiation environment. SSAs are organized around user needs and the impacts of space weather on society. Scientists involved in the LWS program identify targeted areas of research that reference (or bear upon) societal needs. Such targeted science leads to new discoveries and is one of the valid forms of exploration. In this talk we describe the benefits of targeted science, and how addressing societal impacts in an appropriate way maintains the strong science focus of LWS, while also leading to its broader impacts. Title: A Model for the Formation of Filament Channels on the Sun Authors: Knizhnik, Kalman Joshua; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2014shin.confE..63K Altcode: A major unexplained feature of the solar atmosphere is the accumulation of magnetic shear, in the form of filament channels, at photospheric polarity inversion lines (PILs). In addition to free energy, this shear also represents magnetic helicity, which is conserved under reconnection. Consequently, the observations raise the question: Why is the magnetic shear observed to be concentrated along PILs? Results of 3D MHD simulations using the Adaptively Refined MHD Solver (ARMS) are presented that support the magnetic-helicity condensation model of filament channel formation (Antiochos 2013). In this work, we consider the supergranular twisting of a quasi potential flux system that is bounded by a PIL and contains a coronal hole (CH). The magnetic helicity injected by the small-scale photospheric motions is shown to inverse-cascade up to the largest allowable scales that define the closed flux system: the PIL and the CH boundary. This process produces field lines that are both sheared and smooth, and are sheared in opposite senses at the PIL and the CH, in agreement with Antiochos (2013). The accumulated helicity and shear flux are shown to be in excellent quantitative agreement with the helicity condensation model. We present a detailed analysis of the simulation, including comparisons of our analytical and numerical results, and discuss their implications for observations. This work was supported, in part, by the NASA TR&T and SR&T programs. Title: Kinetic Simulations of the Electrically Charged Current Sheet of a Pulsar Authors: Black, Carrie; DeVore, C. Richard; Antiochos, Spiro K.; Harding, Alice Kust; Kazanas, Demosthenes; Kalapotharakos, Constantinos; Timokhin, Andrey Bibcode: 2014AAS...22412110B Altcode: The pulsar magnetosphere is believed to comprise a volume of low-lying, closed field about the magnetic equator, bounded by polar open-field regions in which the pulsar wind flows into space. In the standard global-scale models, a magnetic discontinuity (electric current sheet) of nonneutral plasma separates these opposite-polarity open fields. We use the particle-in-cell Plasma Simulation Code, PSC, to examine the dynamics of a self-consistent model for the internal structure of this sheet, in which the charge-neutral Vlasov/Maxwell equilibria of Harris (1962) and Hoh (1966) are generalized to allow a net electric charge. PSC accommodates both Maxwell (nonrelativistic) and Jüttner/Synge (relativistic) distribution functions for the electrons and positrons. Numerical equilibrium solutions to the 1D Maxwell equations are initialized on the 2D PSC grid, supplemented by periodic boundary conditions in the direction parallel to the sheet and insulating-wall boundary conditions remote from the sheet in the perpendicular direction. As is typical in kinetic studies of pair plasmas, the particle thermal energy and the relative drift velocity driving the current are assumed to be of order the rest energy and the speed of light, respectively. In this limit, the Debye length, skin depth, and Harris/Hoh current-sheet width are all comparable to each other, rather than widely separated and arranged in order of increasing size as generally occurs in nonrelativistic plasmas. The qualitatively new feature of our pulsar simulations is the equilibrium electric field, whose strength can be comparable to that of the magnetic field in the relativistic limit. We expect its presence to have profound consequences for the linear stability and nonlinear evolution of charged pulsar current sheets, substantially modifying the tearing and reconnection of the magnetic field. Exploratory PSC simulations of magnetic reconnection in representative electrified Harris/Hoh equilibria will be presented. The derivation, solution, and analysis of the equilibrium Vlasov/Maxwell equations are discussed in a companion paper at this conference (C. R. DeVore et al. 2014).This work was supported by NASA GSFC’s Science Innovation Fund. Title: CME Initiation Driven by Velocity-Shear Kinetic Reconnection Simulations Authors: Black, Carrie; Antiochos, Spiro K.; DeVore, C. Richard; Karpen, Judith T.; Germaschewski, Kai Bibcode: 2014shin.confE..40B Altcode: In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field countered by a downward tension due to overlying unsheared field. Magnetic reconnection is widely believed to be the mechanism that disrupts this force balance, leading to explosive eruption. For understanding CME/flare initiation, therefore, it is critical to model the onset of reconnection that is driven by the build-up of magnetic shear. In MHD simulations, the application of a magnetic-field shear is a trivial matter. However, kinetic effects are important in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is nontrivial, however, and indicates the necessity of a true multiscale model for such processes in the solar environment. The field must be sheared self-consistently and indirectly to prevent the generation of waves that destroy the desired system. In the work presented here, we discuss methods for applying a velocity shear perpendicular to the plane of reconnection in a system with open boundary conditions. This material is based upon work supported by the National Science Foundation under Award No. AGS-1331356. Title: Numerical Simulation of a Slow Streamer-Blowout CME Authors: Lynch, Benjamin J.; Masson, Sophie; Li, Yan; DeVore, C. Richard; Luhmann, Janet; Antiochos, Spiro K. Bibcode: 2014AAS...22421816L Altcode: We present a 3D numerical MHD simulation of the 2008 Jun 2 gradual streamer blowout CME that had virtually no identifiable low coronal signatures. We energize the field by simple footpoint shearing along the source region's polarity inversion line and model the background solar wind structure using an ∼2MK isothermal wind and a low-order potential field source surface representation of the CR2070 synoptic magnetogram. Our results show that the CME ``initiation’’ is obtained by slowly disrupting the quasi-steady-state configuration of the helmet streamer, resulting in the standard eruptive flare picture that ejects the sheared fields, but very slowly, on a relatively large scale, and with very little magnetic energy release. We obtain a relatively slow CME eruption of order the background solar wind speed and argue that these slow streamer blowout CMEs (now also known as ``stealth CMEs’’) are simply at the lowest end of the CME energy distribution. We present comparisons of the CME propagation through the corona (≤15Rs) in synthetic white-light images derived from the simulation density structure with multi-spacecraft coronagraph data from STEREO/SECCHI and SOHO/LASCO. Title: Simulation of S-Web Corridor Dynamics Authors: Young, Aleida Katherine; Antiochos, Spiro; DeVore, C.; Zurbuchen, Thomas Bibcode: 2014shin.confE..64Y Altcode: The higher average charge-state composition and bias towards heavier elements (Zurbuchen et al. 1999) of the slow solar wind suggest that its source is the release of coronal plasma from high-temperature, closed-field regions. The S-Web (separatrix web) model for the source of the slow solar wind is based on the conclusion that the apparent multiple coronal holes observed within single-polarity regions are connected by narrow corridors at scales smaller than the spatial resolution of current measurements. Magnetic field lines from the boundary of such a corridor map to the heliospheric current sheet, while field lines from the interior of the corridor map to an arc extending to high latitudes in the heliosphere (Antiochos et al. 2011). In this work, we simulate the dynamics of an S-Web corridor using the Adaptively Refined MHD Solver (ARMS). The objective is to quantify the release of coronal plasma at high heliospheric latitudes and show that the dynamics support the S-Web model as an explanation for the source of the slow solar wind. We will present results from our efforts to simulate open-field corridor dynamics, outline plans for further work, and discuss implications for understanding the slow solar wind. This work was supported, in part, by the NASA TR&T and SR&T programs. Title: The role of CME in the escape of solar energtic particles Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R. Bibcode: 2014shin.confE..81M Altcode: Heliospheric manifestations of intense energy release linked to solar activity include the impact at the Earth of energetic particles accelerated during solar eruptions. Observationally, the magnetic configuration of active regions, where solar eruptions occur, agrees well with the standard model of eruption, consisting of a flare and a coronal mass ejection (CME). According to the standard model, particles accelerated at the flare reconnection site should remain trapped in the CME. However, flare-accelerated particles frequently reach the Earth long before the CME does.

We present a 3D model that explains how flare-accelerated particles escape into the interplanetary magnetic flux tubes during a solar eruption. Our model is based on results of large-scale 3D MHD simulations of a breakout-CME erupting into the heliosphere build by an isothermal solar wind. The simulations are performed with the Adaptively Refined Mhd Solver (ARMS). We describe the multiple reconnection episodes that occur during the evolution of the event, and show that the CME magnetic flux reconnect with the open magnetic field from the coronal hole nearby. Such a dynamic implies that the flare-accelerated particles initially trapped in the CME can now be release onto open field lines. Analyzing the dynamics of the reconnected flux during the eruption, we evaluate the spatial distribution of particles beams and find that the particle release can occur over a wide-longitudinal range scaling to the size of the CME front. We discuss the implications of results for CME/flare models and for SEPs observations.

This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: A Model for the Formation of Filament Channels on the Sun Authors: Knizhnik, Kalman J.; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2014AAS...22440802K Altcode: A major unexplained feature of the solar atmosphere is the accumulation of magnetic shear, in the form of filament channels, at photospheric polarity inversion lines (PILs). In addition to free energy, this shear also represents magnetic helicity, which is conserved under reconnection. Consequently, the observations raise the question: Why is the magnetic shear observed to be concentrated along PILs? Results of 3D MHD simulations using the Adaptively Refined MHD Solver (ARMS) are presented that support the magnetic-helicity condensation model of filament-channel formation (Antiochos 2013). In this work, we consider the supergranular twisting of a quasi-potential flux system that is bounded by a PIL and contains a coronal hole (CH). The magnetic helicity injected by the small-scale photospheric motions is shown to inverse-cascade up to the largest allowable scales that define the closed flux system: the PIL and the CH boundary. This process produces field lines that are both sheared and smooth, and are sheared in opposite senses at the PIL and the CH, in agreement with Antiochos (2013). The accumulated helicity and shear flux are shown to be in excellent quantitative agreement with the helicity-condensation model. We present a detailed analysis of the simulation, including comparisons of our analytical and numerical results, and discuss their implications for observations. This work was supported, in part, by the NASA TR&T and SR&T programs. Title: Solar Polar Jets Driven by Magnetic Reconnection, Gravity, and Wind Authors: DeVore, C. Richard; Karpen, Judith T.; Antiochos, Spiro K. Bibcode: 2014AAS...22432351D Altcode: Polar jets are dynamic, narrow, radially extended structures observed in solar EUV emission near the limb. They originate within the open field of coronal holes in “anemone” regions, which are intrusions of opposite magnetic polarity. The key topological feature is a magnetic null point atop a dome-shaped fan surface of field lines. Applied stresses readily distort the null into a current patch, eventually inducing interchange reconnection between the closed and open fields inside and outside the fan surface (Antiochos 1996). Previously, we demonstrated that magnetic free energy stored on twisted closed field lines inside the fan surface is released explosively by the onset of fast reconnection across the current patch (Pariat et al. 2009, 2010). A dense jet comprised of a nonlinear, torsional Alfvén wave is ejected into the outer corona along the newly reconnected open field lines. Now we are extending those exploratory simulations by including the effects of solar gravity, solar wind, and expanding spherical geometry. We find that the model remains robust in the resulting more complex setting, with explosive energy release and dense jet formation occurring in the low corona due to the onset of a kink-like instability, as found in the earlier Cartesian, gravity-free, static-atmosphere cases. The spherical-geometry jet including gravity and wind propagates far more rapidly into the outer corona and inner heliosphere than a comparison jet simulation that excludes those effects. We report detailed analyses of our new results, compare them with previous work, and discuss the implications for understanding remote and in-situ observations of solar polar jets.This work was supported by NASA’s LWS TR&T program. Title: A Model for the Electrically Charged Current Sheet of a Pulsar Authors: DeVore, C. Richard; Antiochos, Spiro K.; Black, Carrie; Harding, Alice Kust; Kalapotharakos, Constantinos; Kazanas, Demosthenes; Timokhin, Andrey Bibcode: 2014AAS...22412111D Altcode: Global-scale electromagnetohydrodynamic solutions for the magnetosphere of a pulsar consist of a region of low-lying, closed magnetic field near the star bounded by opposite-polarity regions of open magnetic field along which the pulsar wind flows into space. Separating these open-field regions is a magnetic discontinuity - an electric current sheet - consisting of nonneutral plasma. We have developed a self-consistent model for the internal structure of this sheet by generalizing the charge-neutral Vlasov/Maxwell equilibria of Harris (1962) and Hoh (1966) to allow a net electric charge. The resulting equations for the electromagnetic field are identical for Maxwell (nonrelativistic) and Jüttner/Synge (relativistic) distribution functions of the particles. The solutions have a single sign of net charge everywhere, with the minority population concentrated near the current sheet and the majority population completely dominant far from the sheet. As the fractional charge imbalance at the sheet increases, for fixed relative drift speed and total thermal pressure of the particles, both the electric- and magnetic-field strengths far from the sheet increase. The electrostatic force acts to disperse the charged particles from the sheet, so the magnetic force must increase proportionately, relative to the charge-neutral case, to pinch the sheet together and maintain the equilibrium. The charge imbalance in the sheet that can be accommodated has an upper bound, which increases monotonically with the relative drift speed. Implications of the model for the steady-state structure of pulsar magnetospheres will be discussed. The model also provides a rigorous starting point for investigating electromagnetohydrodynamic and kinetic instabilities that could lead to magnetic reconnection and current-sheet disruption in pulsars. Exploratory particle-in-cell simulations of representative equilibria are presented in a companion paper at this conference (C. E. Black et al. 2014).This work was supported by NASA GSFC’s Science Innovation Fund. Title: The Onset of Fast Magnetic Reconnection in Solar and Laboratory Plasmas Authors: Antiochos, Spiro K.; DeVore, C. Richard; Karpen, Judith T.; Guidoni, Silvina Bibcode: 2014AAS...22440306A Altcode: Magnetic reconnection is widely believed to be the physical process underlying explosive activity in both solar and laboratory plasmas. The question of what determines whether and when magnetic reconnection will produce explosive energy release has long been one of the most important problems in all plasma physics. We examine this problem using numerical simulations of major solar eruptions, coronal mass ejections and eruptive flares. These events are among the most energetic and the best observed examples of the onset phenomenon. Our calculations show that reconnection in the solar corona invariably exhibits two distinct phases. First, we observe an initial slow growth characterized by the appearance of an extended current sheet and magnetic islands, somewhat analogous to resistive tearing. Eventually, however, the reconnection transitions to an explosive phase characterized by well-developed jets and further island formation. We discuss how these results scale with numerical refinement level, i.e., effective Lundquist number. We conclude that, at least for the case of solar plasmas, fast reconnection onset requires an interaction between reconnection and an ideal instability. We discuss the implications of our results for observations of both solar and laboratory plasmas. This work was supported in part by the NASA TR&T and SR&T Programs. Title: The Role of CMEs in the Escape of Solar Energetic Particles Authors: Masson, Sophie; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2014AAS...22421837M Altcode: Heliospheric manifestations of intense energy release linked to solar activity include the impact at Earth of energetic particles accelerated during solar eruptions. Observationally, the magnetic configuration of active regions, where solar eruptions occur, agrees well with the standard model of eruption, consisting of a flare and a coronal mass ejection (CME). According to the standard model, particles accelerated at the flare reconnection site should remain trapped in the CME. However, flare-accelerated particles frequently reach the Earth long before the CME does. We present a 3D model that demonstrates how flare-accelerated particles escape into interplanetary magnetic flux tubes during a solar eruption. Our model is based on results of large-scale 3D MHD simulations of a breakout CME erupting into a heliospheric magnetic field that is opened by an isothermal solar wind. The simulations are performed with the Adaptively Refined MHD Solver (ARMS). We describe the multiple reconnection episodes that occur during the evolution of the event, and show how CME magnetic flux reconnects with the open field from a nearby coronal hole. This reconnection allows flare-accelerated particles initially trapped in the CME to escape onto open field lines. Analyzing the dynamics of the reconnected flux during the eruption, we determine the spatial distribution of particle beams originating in the CME flux rope. We find that particle release can occur over a wide longitudinal range, which heretofore has been a puzzling feature of SEP observations. We discuss the implications of our results for CME/flare models and for the origin and transport of SEPs.This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: CME Initiation Driven by Velocity-Shear Kinetic Reconnection Simulations Authors: Black, Carrie; Antiochos, Spiro K.; Karpen, Judith T.; DeVore, C. Richard; Germaschewski, Kai Bibcode: 2014AAS...22440303B Altcode: In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field countered by a downward tension due to overlying unsheared field. Magnetic reconnection is widely believed to be the mechanism that disrupts this force balance, leading to explosive eruption. For understanding CME/flare initiation, therefore, it is critical to model the onset of reconnection that is driven by the build-up of magnetic shear. In MHD simulations, the application of a magnetic-field shear is a trivial matter. However, kinetic effects are important in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is nontrivial, however, and indicates the necessity of a true multiscale model for such processes in the solar environment. The field must be sheared self-consistently and indirectly to prevent the generation of waves that destroy the desired system. In the work presented here, we discuss methods for applying a velocity shear perpendicular to the plane of reconnection in a system with open boundary conditions. This material is based upon work supported by the National Science Foundation under Award No. AGS-1331356. Title: Simulation of S-Web Corridor Dynamics Authors: Young, Aleida; Antiochos, Spiro K.; DeVore, C. Richard; Zurbuchen, Thomas H. Bibcode: 2014AAS...22440203Y Altcode: The higher average charge-state composition and bias towards heavier elements (Zurbuchen et al. 1999) of the slow solar wind suggest that its source is the release of coronal plasma from high-temperature, closed-field regions. The S-Web (separatrix web) model for the source of the slow solar wind is based on the conclusion that the apparent multiple coronal holes observed within single-polarity regions are connected by narrow corridors at scales smaller than the spatial resolution of current measurements. Magnetic field lines from the boundary of such a corridor map to the heliospheric current sheet, while field lines from the interior of the corridor map to an arc extending to high latitudes in the heliosphere (Antiochos et al. 2011). In this work, we simulate the dynamics of an S-Web corridor using the Adaptively Refined MHD Solver (ARMS). The objective is to quantify the release of coronal plasma at high heliospheric latitudes and show that the dynamics support the S-Web model as an explanation for the source of the slow solar wind. We will present results from our efforts to simulate open-field corridor dynamics, outline plans for further work, and discuss implications for understanding the slow solar wind. This work was supported, in part, by the NASA TR&T and SR&T programs. Title: Simulations of Emerging Magnetic Flux. II. The Formation of Unstable Coronal Flux Ropes and the Initiation of Coronal Mass Ejections Authors: Leake, James E.; Linton, Mark G.; Antiochos, Spiro K. Bibcode: 2014ApJ...787...46L Altcode: 2014arXiv1402.2645L We present results from three-dimensional magnetohydrodynamic simulations of the emergence of a twisted convection zone flux tube into a pre-existing coronal dipole field. As in previous simulations, following the partial emergence of the sub-surface flux into the corona, a combination of vortical motions and internal magnetic reconnection forms a coronal flux rope. Then, in the simulations presented here, external reconnection between the emerging field and the pre-existing dipole coronal field allows further expansion of the coronal flux rope into the corona. After sufficient expansion, internal reconnection occurs beneath the coronal flux rope axis, and the flux rope erupts up to the top boundary of the simulation domain (~36 Mm above the surface). We find that the presence of a pre-existing field, orientated in a direction to facilitate reconnection with the emerging field, is vital to the fast rise of the coronal flux rope. The simulations shown in this paper are able to self-consistently create many of the surface and coronal signatures used by coronal mass ejection (CME) models. These signatures include surface shearing and rotational motions, quadrupolar geometry above the surface, central sheared arcades reconnecting with oppositely orientated overlying dipole fields, the formation of coronal flux ropes underlying potential coronal field, and internal reconnection which resembles the classical flare reconnection scenario. This suggests that proposed mechanisms for the initiation of a CME, such as "magnetic breakout," are operating during the emergence of new active regions. Title: Global-scale Consequences of Magnetic-helicity Injection and Condensation on the Sun Authors: Mackay, Duncan H.; DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2014ApJ...784..164M Altcode: In the recent paper of Antiochos, a new concept for the injection of magnetic helicity into the solar corona by small-scale convective motions and its condensation onto polarity inversion lines (PILs) was developed. We investigate this concept through global simulations of the Sun's photospheric and coronal magnetic fields, and compare the results with the hemispheric pattern of solar filaments. Assuming that the vorticity of the cells is predominantly counterclockwise/clockwise in the northern/southern hemisphere, the convective motions inject negative/positive helicity into each hemisphere. The simulations show that: (1) on a north-south oriented PIL, both differential rotation and convective motions inject the same sign of helicity, which matches that required to reproduce the hemispheric pattern of filaments. (2) On a high-latitude east-west oriented polar crown or subpolar crown PIL, the vorticity of the cells has to be approximately 2-3 times greater than the local differential-rotation gradient in order to overcome the incorrect sign of helicity injection from differential rotation. (3) In the declining phase of the cycle, as a bipole interacts with the polar field, in some cases, helicity condensation can reverse the effect of differential rotation along the east-west lead arm but not in all cases. The results show that this newly developed concept of magnetic helicity injection and condensation, in conjunction with the mechanisms used in Yeates et al., is a viable explanation for the hemispheric pattern of filaments. Future observational studies should focus on examining the vorticity component within convective motions to determine both its magnitude and latitudinal variation relative to the differential-rotation gradient on the Sun. Title: A Model for the Electrically Charged Current Sheet of a Pulsar Authors: DeVore, C. R.; Antiochos, S. K.; Black, C. E.; Harding, A. K.; Kalapotharakos, C.; Kazanas, D.; Timokhin, A. Bibcode: 2014AAS...22315326D Altcode: Global-scale electromagnetohydrodynamic solutions for the magnetosphere of a pulsar consist of a region of low-lying, closed magnetic field near the star bounded by opposite-polarity regions of open magnetic field along which the pulsar wind flows into space. Separating these open-field regions is a magnetic discontinuity - an electric current sheet - consisting of nonneutral plasma. We have developed a self-consistent model for the internal structure of this sheet by generalizing the charge-neutral Vlasov/Maxwell equilibria of Harris (1962) and Hoh (1966) to allow a net electric charge. The resulting equations for the electromagnetic field are identical for Maxwell (nonrelativistic) and Jüttner/Synge (relativistic) distribution functions of the particles. The solutions have a single sign of net charge everywhere, with the minority population concentrated near the current sheet and the majority population completely dominant far from the sheet. As the fractional charge imbalance at the sheet increases, for fixed relative drift speed and total thermal pressure of the particles, both the electric- and magnetic-field strengths far from the sheet increase. The electrostatic force acts to disperse the charged particles from the sheet, so the magnetic force must increase proportionately, relative to the charge-neutral case, to pinch the sheet together and maintain the equilibrium. The charge imbalance in the sheet that can be accommodated has an upper bound, which increases monotonically with the relative drift speed. In the limit of maximum charge imbalance and field strength, the density of majority particles asymptotically approaches a uniform value far from the sheet, rather than falling exponentially to zero as in the charge-neutral case. This model provides a rigorous starting point for investigating electromagnetohydrodynamic and kinetic instabilities that could lead to magnetic reconnection and current-sheet disruption in pulsar magnetospheres. Exploratory particle-in-cell simulations of some representative equilibria are presented in a companion paper at this conference (C. E. Black et al. 2014). This work was supported by NASA GSFC’s Science Innovation Fund. Title: Kinetic Simulations of the Electrically Charged Current Sheet of a Pulsar Authors: Black, Carrie; Antiochos, S. K.; DeVore, C. R.; Harding, A. K.; Kalapotharakos, C.; Kazanas, D.; Timokhin, A. Bibcode: 2014AAS...22315327B Altcode: The pulsar magnetosphere is believed to comprise a volume of low-lying, closed field about the magnetic equator, bounded by polar open-field regions in which the pulsar wind flows into space. In the standard global-scale models, a magnetic discontinuity (electric current sheet) of nonneutral plasma separates open field regions of opposite polarity. We use the particle-in-cell Plasma Simulation Code, PSC, to examine the dynamics of a self-consistent model for the internal structure of this sheet, in which the charge-neutral Vlasov/Maxwell equilibria of Harris (1962) and Hoh (1966) are generalized to allow a net electric charge. PSC accommodates both Maxwell (nonrelativistic) and Jüttner/Synge (relativistic) distribution functions for the electrons and positrons. Numerical equilibrium solutions to the 1D Maxwell equations are initialized on the 2D PSC grid, supplemented by periodic boundary conditions in the direction parallel to the sheet and insulating boundary conditions remote from the sheet in the perpendicular direction. As is typical in kinetic studies of pair plasmas, the particle thermal energy and the relative drift velocity driving the current are assumed to be of order the rest energy and the speed of light, respectively. In this limit, the Debye length, skin depth, and Harris/Hoh width of the current sheet are all comparable to each other, rather than widely separated and arranged in order of increasing size as typically occurs in nonrelativistic plasmas. The qualitatively new feature of our pulsar simulations is the equilibrium electric field, whose strength can be comparable to that of the magnetic field in the relativistic limit. We expect its presence to have profound consequences for the linear stability and nonlinear evolution of charged pulsar current sheets, especially with regard to tearing and reconnection of the magnetic field. Exploratory PSC simulations of magnetic reconnection in some representative “electrified Harris/Hoh” equilibria will be presented. The derivation, solution, and analysis of the equilibrium Vlasov/Maxwell equations are discussed in a companion paper at this conference (C. R. DeVore et al. 2014). This work was supported by NASA GSFC’s Science Innovation Fund. Title: Particle escape in the interplanetary medium: Link between CME observations and MHD simulations Authors: Masson, Sophie; Antiochos, Spiro; DeVore, C. Richard Bibcode: 2014cosp...40E2031M Altcode: Among the more hazardous forms of space weather at Earth and in the heliosphere are the intense solar energetic particle (SEP) bursts associated with fast coronal mass ejections (CMEs) and eruptive flares. A fundamental question to understand the origin and the evolution of solar energetic particles is: How do solar energetic particles escape the Sun? Answering this question is critical for understanding how the corona couples dynamically to the heliosphere during explosive events, and is fundamental to developing any future forecasting capability for SEP events. The release onto open field lines of energetic particles originating in the low corona is the bridge connecting the acceleration site to the interplanetary propagation and is, therefore, the key to reconciling remote and in-situ observations of energetic particles. Recent multi-instrument studies showed that CMEs are important factors that determine whether the energetic particles escape into the heliosphere and partly define the spatial distribution of particle flux. In order to understand how and why CMEs play a crucial role in the particle escape, we must understand the dynamics of the corona disturbed by a CME ejection. The details of the dynamics can be studied through MHD simulations. To advance understanding, it is pertinent to combine observations and simulations to develop models that respect the observational constraints. Thus, first we will describe the observational results, then discuss how MHD simulations help demonstrate why CMEs are important for particle release. Title: Modeling Reconnection-Driven Solar Polar Jets with Gravity and Wind Authors: Karpen, Judith T.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2013SPD....44...43K Altcode: Solar polar jets are dynamic, narrow, radially extended structures observed in EUV emission. They have been found to originate within the open magnetic field of coronal holes in “anemone” regions, which are generally accepted to be intrusions of opposite polarity. The associated embedded-dipole topology consists of a spine line emanating from a null point atop a dome-shaped fan surface. Previous work (Pariat et al. 2009, 2010) has validated the idea that magnetic free energy stored on twisted closed field lines within the fan surface can be released explosively by the onset of fast reconnection between the highly stressed closed field inside the null and the unstressed open field outside (Antiochos 1996). The simulations showed that a dense jet comprising a nonlinear, torsional Alfven wave is ejected into the outer corona on the newly reconnected open field lines. While proving the principle of the basic model, those simulations neglected the important effects of gravity, the solar wind, and an expanding spherical geometry. We introduce those additional physical processes in new simulations of reconnection-driven jets, to determine whether the model remains robust in the resulting more realistic setting, and to begin establishing the signatures of the jets in the inner heliosphere for comparison with observations. Initial results demonstrate explosive energy release and a jet in the low corona very much like that in the earlier Cartesian, gravity-free, static-atmosphere runs. We report our analysis of the results, their comparison with previous work, and their implications for observations. This work was supported by NASA’s LWS TR&T program.Abstract (2,250 Maximum Characters): Solar polar jets are dynamic, narrow, radially extended structures observed in EUV emission. They have been found to originate within the open magnetic field of coronal holes in “anemone” regions, which are generally accepted to be intrusions of opposite polarity. The associated embedded-dipole topology consists of a spine line emanating from a null point atop a dome-shaped fan surface. Previous work (Pariat et al. 2009, 2010) has validated the idea that magnetic free energy stored on twisted closed field lines within the fan surface can be released explosively by the onset of fast reconnection between the highly stressed closed field inside the null and the unstressed open field outside (Antiochos 1996). The simulations showed that a dense jet comprising a nonlinear, torsional Alfven wave is ejected into the outer corona on the newly reconnected open field lines. While proving the principle of the basic model, those simulations neglected the important effects of gravity, the solar wind, and an expanding spherical geometry. We introduce those additional physical processes in new simulations of reconnection-driven jets, to determine whether the model remains robust in the resulting more realistic setting, and to begin establishing the signatures of the jets in the inner heliosphere for comparison with observations. Initial results demonstrate explosive energy release and a jet in the low corona very much like that in the earlier Cartesian, gravity-free, static-atmosphere runs. We report our analysis of the results, their comparison with previous work, and their implications for observations. This work was supported by NASA’s LWS TR&T program. Title: Helicity Condensation as the Origin of Coronal and Solar Wind Structure Authors: Antiochos, S. K. Bibcode: 2013ApJ...772...72A Altcode: 2012arXiv1211.4132A Three of the most important and most puzzling features of the Sun's atmosphere are the smoothness of the closed-field corona (the so-called coronal loops), the accumulation of magnetic shear at photospheric polarity inversion lines (PILs; filament channels), and the complex dynamics of the slow wind. We propose that a single process, helicity condensation, is the physical mechanism giving rise to all three features. A simplified model is presented for how helicity is injected and transported in the closed corona by magnetic reconnection. With this model, we demonstrate that magnetic shear must accumulate at PILs and coronal hole boundaries, and estimate the rate of shear growth at PILs and the loss to the wind. Our results can account for many of the observed properties of the corona and wind. Title: The Initiation of Coronal Mass Ejections (CMEs) by Dynamical Magnetic Flux Emergence Authors: Leake, James; Linton, M.; Antiochos, S. K. Bibcode: 2013SPD....44...46L Altcode: We present results from 3D numerical MHD simulations, which show how the partial emergence of twisted magnetic flux tubes from the convection zone, and their interaction with background coronal magnetic fields, leads to the formation of unstable magnetic configurations in the corona. These unstable configurations are capable of initiating the ejection of a flux rope. Our studies improve upon the traditional approach of driving the magnetically-dominated corona with kinematic boundary conditions by explicitly including the dynamic emergence of flux through the convection zone and lower, pressure-dominated, solar atmosphere. By showing that magnetic flux emergence is capable of initiating coronal ejections, we can root these dynamic events in the convection zone and hence to the source of solar activity, the solar dynamo, which is a vital step in improving our understanding of space weather.Abstract (2,250 Maximum Characters): We present results from 3D numerical MHD simulations, which show how the partial emergence of twisted magnetic flux tubes from the convection zone, and their interaction with background coronal magnetic fields, leads to the formation of unstable magnetic configurations in the corona. These unstable configurations are capable of initiating the ejection of a flux rope. Our studies improve upon the traditional approach of driving the magnetically-dominated corona with kinematic boundary conditions by explicitly including the dynamic emergence of flux through the convection zone and lower, pressure-dominated, solar atmosphere. By showing that magnetic flux emergence is capable of initiating coronal ejections, we can root these dynamic events in the convection zone and hence to the source of solar activity, the solar dynamo, which is a vital step in improving our understanding of space weather. Title: A Model for the Escape of Solar-flare-accelerated Particles Authors: Masson, S.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2013ApJ...771...82M Altcode: 2013arXiv1301.0654M We address the problem of how particles are accelerated by solar flares can escape into the heliosphere on timescales of an hour or less. Impulsive solar energetic particle (SEP) bursts are generally observed in association with so-called eruptive flares consisting of a coronal mass ejection (CME) and a flare. These fast SEPs are believed to be accelerated directly by the flare, rather than by the CME shock. However, the precise mechanism by which the particles are accelerated remains controversial. Regardless of the origin of the acceleration, the particles should remain trapped in the closed magnetic fields of the coronal flare loops and the ejected flux rope, given the magnetic geometry of the standard eruptive-flare model. In this case, the particles would reach the Earth only after a delay of many hours to a few days (coincident with the bulk ejecta arriving at Earth). We propose that the external magnetic reconnection intrinsic to the breakout model for CME initiation can naturally account for the prompt escape of flare-accelerated energetic particles onto open interplanetary magnetic flux tubes. We present detailed 2.5-dimensional magnetohydrodynamic simulations of a breakout CME/flare event with a background isothermal solar wind. Our calculations demonstrate that if the event occurs sufficiently near a coronal-hole boundary, interchange reconnection between open and closed fields can occur. This process allows particles from deep inside the ejected flux rope to access solar wind field lines soon after eruption. We compare these results to standard observations of impulsive SEPs and discuss the implications of the model on further observations and calculations. Title: The Formation and Evolution of Coronal Flux Ropes Created by Dynamical Flux Emergence Authors: Linton, Mark; Leake, J. E.; Antiochos, S. K. Bibcode: 2013SPD....44..105L Altcode: We investigate the formation and evolution of coronal flux ropes created by the dynamical emergence of convection zone magnetic fields into various pre-existing coronal magnetic fields. While we found earlier that 2D dynamical flux emergence is not effective at creating coronal flux ropes, either eruptive or stable, we find that in 3D the results are dramatically different. We show that with a dipolar coronal field in a non-reconnecting configuration, a 3D emerging flux rope can form into a stable, prominence-like twisted coronal flux rope. In contrast, we show that when the coronal dipole field is in a reconnecting configuration, a 3D emerging flux rope will reconnect with it to form a breakout quadrupolar field, and then an erupting flux rope. We therefore conclude that, while 2D flux emergence is not an effective mechanism for eruptive flux rope formation, 3D flux emergence is an effective way to simultaneously create a breakout quadrupolar field and to emerge the magnetic shear needed to drive a breakout coronal mass ejection. Title: Helicity Condensation as the Origin of Coronal and Solar Wind Structure Authors: Antiochos, Spiro K.; DeVore, C. R. Bibcode: 2013SPD....4410101A Altcode: Three of the most important and most puzzling features of the Sun’s atmosphere are the smoothness of the closed field corona (so-called coronal loops), the accumulation of magnetic shear at photospheric polarity inversion lines (filament channels), and the complex dynamics of the slow wind. We propose that a single process, helicity condensation, is the physical mechanism giving rise to all three features. A simplified model is presented for how helicity is injected and transported in the closed corona by magnetic reconnection. With this model we demonstrate that magnetic shear must accumulate at PILs and coronal hole boundaries, and estimate the rate of shear growth at PILs and the loss to the wind. Our results can account for many of the observed properties of the corona and wind. This work was supported in part by the NASA TR&T and SR&T Programs. Title: The Effects of Solar Eruption Dynamics on Particle Escape Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R. Bibcode: 2013SPD....4420304M Altcode: Magnetic reconnection in the solar atmosphere is believed to be the driver of most solar and heliospheric activity; therefore, understanding the structure and dynamics of the coronal magnetic field is central to understanding this activity. Important heliospheric manifestations of intense energy release linked to solar activity include the impact at the Earth of energetic particles accelerated during solar eruptions. Observationally, the magnetic configuration of active regions where solar eruptions occur agrees well with the standard model of eruption, consisting of a flare and a coronal mass ejection (CME). According to the standard model, particles accelerated at the flare reconnection site should remain trapped in the CME. However, flare-accelerated particles frequently reach the Earth long before the CME does. We present a 3D model that explains how flare-accelerated particles escape onto interplanetary magnetic flux tubes during a solar eruption. Our model is based on results from large-scale 3D MHD simulations of a breakout-CME erupting into a heliosphere with an isothermal solar wind. The simulations are performed with the Adaptively Refined Mhd Solver (ARMS). We describe the multiple reconnection episodes that occur during the evolution of the event, and show how they lead to the release of flare-accelerated particles onto open field lines. Analyzing the dynamics of the reconnected flux during the eruption, we evaluate the spatial distribution and the timing of the particle beams injected into the heliosphere. We discuss the implications of results for CME/flare models and for SEPs observations. This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: CME Initiation Driven by Velocity-Shear Kinetic Reconnection Simulations Authors: Black, Carrie; Antiochos, S. K.; Karpen, J. T.; Germaschewski, K.; DeVore, C. R. Bibcode: 2013SPD....44..104B Altcode: In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field balanced by a downward tension due to overlying unsheared field. Magnetic reconnection is widely believed to be the mechanism that disrupts this force balance, leading to explosive eruption. For understanding CME/flare initiation, therefore, it is critical to model the onset or reconnection that is driven by the buildup of magnetic shear. In MHD simulations, the application of a magnetic field shear is a trivial matter. However, kinetic effects are important in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is nontrivial and indicates necessity of a true multiscale model for such processes in the Solar environment. The field must be sheared self-consistently/ indirectly to prevent the generation of waves that destroy the desired system. In the work presented here, we discuss methods for applying a velocity shear perpendicular to the plane of reconnection for periodic and nonperiodic systems. Title: Simulation of S-Web Corridor Dynamics Authors: Young, Aleida Katherine; Antiochos, S. K.; Karpen, J. T.; DeVore, C. R.; Zurbuchen, T. H. Bibcode: 2013shin.confE...2Y Altcode: Unlike the fast solar wind, the slow solar wind compositionally resembles the corona. Its higher average charge state composition and bias towards heavier elements (Zurbuchen et al., 1999) suggests that the most likely source for the slow solar wind is the release of closed-field coronal plasma. The S-Web (separatrix web) model for the source of slow solar wind is based on the uniqueness conjecture, which states that only one coronal hole can exist in a single-polarity region on the Sun (Antiochos et al. 2007). The apparent multiple coronal holes observed within single-polarity regions therefore must be connected by narrow corridors at scales smaller than the spatial resolution of current measurements of the photosphere. Magnetic field lines from the boundary of such a corridor map to the heliospheric current sheet, while field lines from the interior of the corridor map to an arc extending to high latitudes in the heliosphere (Antiochos et al. 2011). Magnetic reconnection along a narrow corridor is a possible release mechanism for coronal plasma. In this work, we simulate the dynamics of an S-Web corridor using the Adaptively Refined MHD Solver (ARMS) to examine the effects of magnetic reconnection along the corridor on the opening and closing of field lines at high latitudes in the heliosphere. The objective is to quantify the release of coronal plasma due to reconnection and show that these dynamics support the S-Web model as an explanation for the source of slow solar wind. We will present results from our initial efforts to simulate open-field corridor dynamics, outline plans for further work, and discuss implications for understanding the slow solar wind. Title: The ISS Space Plasma Laboratory: A Proposed Orbital Solar Physics Simulation Lab Authors: Antiochos, Spiro K.; DeVore, C. R.; Thompson, B. J.; Bering, E. A., III; Edeen, G.; Carter, M.; Giambusso, M.; Olsen, C. S.; Squire, J.; Larson, D.; McFadden, J. P.; Longmier, B. Bibcode: 2013shin.confE.162A Altcode: We describe a proposed laboratory-experiment research program that will answer several fundamental questions concerning the dynamical opening and closing of the Sun's magnetic field - the defining property of CMEs and eruptive flares. Our experiment is specifically designed to address the key questions of the rate of reconnection in the topology of a flare or heliospheric current sheet, its burstiness, and the energy partition between thermal, kinetic, and particle. Of course, it seems completely contradictory to use a laboratory experiment to study an open magnetic system, because so far all laboratory plasmas have very solid walls. The pioneering feature of our program is that the experiments will be performed on the International Space Station (ISS). Only by going into space can we obtain the open domain that is absolutely essential for studying the opening and closing of coronal flux. Our research program will provide the instrumentation infrastructure, modeling and solar data expertise and initial scientific understanding required to develop the VASIMR® VF-200 high powered plasma source into a wall-less, orbiting ISS Space Plasma Laboratory (ISPL) national facility. For example, the VF-200 exhaust will simulate conditions in the solar corona during CMEs/eruptive flares by creating plasma jets in open magnetic field geometries. Such a facility would measure quantities in the plasma flow with the goal of measuring magnetic reconnection and transport phenomena that should be similar in nature to those occurring in the corona and solar wind. Our experiment will capture all the effects inherent in a fully 3D magnetic system and reproduce some of the physics occurring in the post initiation phase of CMEs/eruptive flares. The Aurora Plasma Diagnostics Package (APDP) will carry Langmuir probes, a retarding potential analyzer (RPA), dc magnetometer, plasma wave detectors, Faraday cups, electrostatic analyzers, solid state energetic particle telescope and Ar II and broadband imagers. Title: A Model for the Escape of Solar-Flare Accelerated Particles Authors: Masson, Sophie; Antiochos, S.; DeVore, C. R. Bibcode: 2013shin.confE.132M Altcode: Magnetic reconnection in the solar atmosphere is believed to be the driver of most solar active phenomena. Therefore, the structure and dynamics of the coronal magnetic field are central to understanding solar and heliospheric activity. Important heliospheric manifestations of intense energy release linked to solar activity include the impact at the Earth of energetic particles accelerated during solar eruptions. Observationally, the magnetic configuration of active regions, where solar eruptions occur, agrees well with the standard model of eruption, consisting of a flare and a coronal mass ejection (CME). According to the standard model, particles accelerated at the flare reconnection site should remain trapped in the CME. However, flare-accelerated particles frequently reach the Earth long before the CME does.

We present a new model that may lead to injection of energetic particles onto open interplanetary magnetic flux tubes. Our model is based on results of 2.5D MHD simulations of a large-scale coronal null-point topology with the outer spine opened to interplanetary space by an isothermal solar wind. The simulations are performed with the Adaptively Refined Mhd Solver (ARMS). We describe the multiple reconnections that occur during the evolution of the event, and show how they lead to the release of flare accelerated particles onto open field lines. We discuss the implications of our results for CME/flare models and for observations.

This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: The Magnetic Topology of Slow Wind Sources Authors: Antiochos, Spiro K. Bibcode: 2013shin.confE..19A Altcode: Due to its observed location in the heliosphere, its plasma composition, and its variability, most models for the origins of the slow wind postulate that it is a result of the release of closed-field plasma onto open field lines. In particular, in the S-Web model the slow wind originates at a dynamic boundary region between open and closed flux in the solar corona. Consequently, the detailed topology of the open-closed magnetic boundary is critically important for determining the properties of the slow wind. We discuss the possible magnetic topologies for the open-closed boundary. There are three main topologies corresponding to three types of observed coronal structures: helmet streamers, plumes/coronal jets, and plasma sheets (also known as pseudostreamers). I present models for each of these topologies and show that each is very different at the Sun. I also argue that each will have different consequences for the observed properties of the wind in the heliosphere. Title: Velocity-Shear Driven CME Initiation in Kinetic Reconnection Simulations Authors: Black, Carrie; Antiochos, Spiro K.; Karpen, Judith; DeVore, C. Richard; Germaschewski, Kai Bibcode: 2013shin.confE..79B Altcode: In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field balanced by a downward tension due to overlying unsheared field. Magnetic reconnection is widely believed to be the mechanism that disrupts this force balance, leading to explosive eruption. For understanding CME/flare initiation, therefore, it is critical to model the onset or reconnection that is driven by the buildup of magnetic shear. In MHD simulations, the application of a magnetic field shear is a trivial matter. However, kinetic effects are important in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is nontrivial and indicates necessity of a true multiscale model for such processes in the Solar environment. The field must be sheared self-consistently/ indirectly to prevent the generation of waves that destroy the desired system. In the work presented here, we discuss preliminary results from a shear driven system in a 2.5D PiC simulation. Title: Helicity transport through the photosphere Authors: Schuck, P. W.; Antiochos, S. K.; Linton, M. Bibcode: 2013AGUSMSH53C..04S Altcode: Solar eruptions are driven by energy and helicity transported through the photosphere and into the Corona. However, the mechanism by which helicity is transferred from the solar dynamo to coronal structures is pooly understood. We recast the Berger and Field (1984) helicity transport equation in manifestly gauge invariant form and examine the individual terms leading to the transport of helicity through the emergence of closed field, and twisting and tangling of potential fields. These theoretical results are applied to erupting active regions observed by SDO/HMI. The plasma velocity fields in the photosphere, necessary for computing energy and helicity fluxes are determined using an upgraded version of DAVE4VM that incorporates the spherical geometry of the solar images. We find that the bulk of the helicity into the corona is injected by twisting motions, and we discuss the implications of our results for understanding coronal activity. Title: The Structure and Dynamics of the Corona—Heliosphere Connection Authors: Antiochos, Spiro K.; Linker, Jon A.; Lionello, Roberto; Mikić, Zoran; Titov, Viacheslav; Zurbuchen, Thomas H. Bibcode: 2013mspc.book..169A Altcode: No abstract at ADS Title: Simulation of S-Web Corridor Dynamics Authors: Young, A. K.; Antiochos, S. K.; Karpen, J.; DeVore, C. R.; Zurbuchen, T. Bibcode: 2012AGUFMSH53A2267Y Altcode: The S-Web (separatrix web) model for the source of slow solar wind is based on the uniqueness conjecture, which states that only one coronal hole can exist in a single-polarity region on the Sun (Antiochos et al. 2007). The apparent multiple coronal holes observed within single-polarity regions therefore must be connected by narrow corridors at scales smaller than the spatial resolution of current measurements of the photosphere. Magnetic field lines from the boundary of such a corridor map to the heliospheric current sheet, while field lines from the interior of the corridor map to an arc extending to high latitudes in the heliosphere (Antiochos et al. 2011). In this work, we simulate the dynamics of an S-Web corridor using the Adaptively Refined MHD Solver (ARMS), to examine the effects of magnetic reconnection along the corridor on the opening and closing of field lines at high latitudes in the heliosphere. The objective is to show that these dynamics support the S-Web model as an explanation for the source of slow solar wind. We will present results from our initial efforts to simulate open-field corridor dynamics, outline plans for further work, and discuss implications for understanding the slow solar wind. Title: The Magnetic Topology of Slow Wind Sources Authors: Antiochos, S. K. Bibcode: 2012AGUFMSH52A..05A Altcode: Due to its observed location in the heliosphere, plasma composition, and variability, most models for the origins of the slow wind postulate that it is a result of the release of closed-field plasma onto open field lines. In particular, in the S-Web model the slow wind originates at a dynamic boundary region between open and closed flux in the solar corona. Consequently, the detailed topology of the open-closed magnetic boundary is critically important for determining the properties of the slow wind. We discuss the possible magnetic topologies for the open-closed boundary. There are three main topologies corresponding to three types of observed coronal structures: helmet streamers, plumes/coronal jets, and plasma sheets (also known as pseudostreamers). I present models for each of these topologies and show that each is very different at the Sun. I also argue that each will have very different consequences for the observed properties of the wind in the heliosphere. This work was funded in part by the NASA TR&T and SR&T Programs. Title: Kinetic Reconnection Simulations for CME Initiation Driven by Velocity-Shear Authors: Black, C.; Antiochos, S. K.; Karpen, J.; DeVore, C. R.; Germaschewski, K. Bibcode: 2012AGUFMSH51A2213B Altcode: In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field balanced by a downward tension due to overlying unsheared field. Magnetic reconnection is widely believed to be the mechanism that disrupts this force balance, leading to explosive eruption. For understanding CME/flare initiation, therefore, it is critical to model the onset or reconnection that is driven by the buildup of magnetic shear. In MHD simulations, the application of a magnetic field shear is a trivial matter. However, kinetic effects are important in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is nontrivial. The field must be sheared self-consistently/ indirectly to prevent the generation of waves that destroy the desired system. In the work presented here, we discuss methods for applying a velocity shear perpendicular to the plane of reconnection for a nonperiodic system. We also discuss the implementation of boundary conditions that are open to electric currents that flow through the system boundary. C.B. is supported through an appointment to the NASA Postdoctoral Program at GSFC, administered by Oak Ridge Associated Universities through a contract with NASA. Title: The Structure and Dynamics of the Corona—Heliosphere Connection Authors: Antiochos, Spiro K.; Linker, Jon A.; Lionello, Roberto; Mikić, Zoran; Titov, Viacheslav; Zurbuchen, Thomas H. Bibcode: 2012SSRv..172..169A Altcode: 2011SSRv..tmp..371A; 2011SSRv..tmp..224A; 2011SSRv..tmp..148A; 2011SSRv..tmp...79A Determining how the heliospheric magnetic field and plasma connect to the Sun's corona and photosphere is, perhaps, the central problem in solar and heliospheric physics. For much of the heliosphere, this connection appears to be well understood. It is now generally accepted that so-called coronal holes, which appear dark in X-rays and are predominantly unipolar at the photosphere, are the sources of quasi-steady wind that is generally fast, >500 km/s, but can sometimes be slow. However, the connection to the Sun of the slow, non-steady wind is far from understood and remains a major mystery. We review the existing theories for the sources of the non-steady wind and demonstrate that they have difficulty accounting for both the observed composition of the wind and its large angular extent. A new theory is described in which this wind originates from the continuous opening and closing of narrow open field corridors in the corona, which give rise to a web of separatrices (the S-Web) in the heliosphere. Note that in this theory the corona—heliosphere connection is intrinsically dynamic, at least for this type of wind. Support for the S-Web model is derived from MHD solutions for the corona and wind during the time of the August 1, 2008 eclipse. Additionally, we perform fully dynamic numerical simulations of the corona and heliosphere in order to test the S-Web model as well as the interchange model proposed by Fisk and co-workers. We discuss the implications of our simulations for the competing theories and for understanding the corona—heliosphere connection, in general. Title: The Mechanisms for the Onset and Explosive Eruption of Coronal Mass Ejections and Eruptive Flares Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2012ApJ...760...81K Altcode: We have investigated the onset and acceleration of coronal mass ejections (CMEs) and eruptive flares. To isolate the eruption physics, our study uses the breakout model, which is insensitive to the energy buildup process leading to the eruption. We performed 2.5D simulations with adaptive mesh refinement that achieved the highest overall spatial resolution to date in a CME/eruptive flare simulation. The ultra-high resolution allows us to separate clearly the timing of the various phases of the eruption. Using new computational tools, we have determined the number and evolution of all X- and O-type nulls in the system, thereby tracking both the progress and the products of reconnection throughout the computational domain. Our results show definitively that CME onset is due to the start of fast reconnection at the breakout current sheet. Once this reconnection begins, eruption is inevitable; if this is the only reconnection in the system, however, the eruption will be slow. The explosive CME acceleration is triggered by fast reconnection at the flare current sheet. Our results indicate that the explosive eruption is caused by a resistive instability, not an ideal process. Moreover, both breakout and flare reconnections begin first as a form of weak tearing characterized by slowly evolving plasmoids, but eventually transition to a fast form with well-defined Alfvénic reconnection jets and rapid flux transfer. This transition to fast reconnection is required for both CME onset and explosive acceleration. We discuss the key implications of our results for CME/flare observations and for theories of magnetic reconnection. Title: Existence of two MHD reconnection modes in a solar 3D magnetic null point topology Authors: Pariat, Etienne; Antiochos, Spiro; DeVore, C. Richard; Dalmasse, Kévin Bibcode: 2012cosp...39.1450P Altcode: 2012cosp.meet.1450P Magnetic topologies with a 3D magnetic null point are common in the solar atmosphere and occur at different spatial scales: such structures can be associated with some solar eruptions, with the so-called pseudo-streamers, and with numerous coronal jets. We have recently developed a series of numerical experiments that model magnetic reconnection in such configurations in order to study and explain the properties of jet-like features. Our model uses our state-of-the-art adaptive-mesh MHD solver ARMS. Energy is injected in the system by line-tied motion of the magnetic field lines in a corona-like configuration. We observe that, in the MHD framework, two reconnection modes eventually appear in the course of the evolution of the system. A very impulsive one, associated with a highly dynamic and fully 3D current sheet, is associated with the energetic generation of a jet. Before and after the generation of the jet, a quasi-steady reconnection mode, more similar to the standard 2D Sweet-Parker model, presents a lower global reconnection rate. We show that the geometry of the magnetic configuration influences the trigger of one or the other mode. We argue that this result carries important implications for the observed link between observational features such as solar jets, solar plumes, and the emission of coronal bright points. Title: Generation of plasma flows and waves during the development of coronal jets Authors: Pariat, Etienne; Antiochos, Spiro; DeVore, C. Richard Bibcode: 2012cosp...39.1449P Altcode: 2012cosp.meet.1449P No abstract at ADS Title: The effect of magnetic topology on the escape of flare accelerated particles Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R. Bibcode: 2012shin.confE..84M Altcode: Magnetic reconnection in the solar atmosphere is believed to be the driver of most solar active phenomena. Therefore, the structure and dynamics of the coronal magnetic field are central to understanding solar and heliospheric activity. This study investigates the dynamics of the magnetic topology when a CME interacts with the interplanetary magnetic field.Our model is based on results of 2.5D MHD simulations of a large-scale coronal null-point topology with the outer spine opened to interplanetary space by an isothermal solar wind. The simulations are performed with the Adaptively Refined Mhd Solver (ARMS). We describe the multiple reconnections that occur during the evolution of the event. These dynamics of the magnetic field during solar eruption provide a new model for the injection of flare-accelerated particles from the closed coronal field of the CME onto open interplanetary flux tubes. Wediscussthe implications of our results for CME/flare models and for observations. This work was supported, in part, by the NASA TR&T and SR&T Title: Investigating the Sources of Solar Activity via Flux Emergence Simulations Authors: Linton, Mark George; Leake, James; Schuck, Peter; Antiochos, Spiro Bibcode: 2012shin.confE..41L Altcode: Solar coronal activity, and the space weather it drives, is generatedby the dynamics and energy release of interacting coronal magneticfields. These fields are created far below the solar surface, at thebase of the convection zone. They then rise to the surface and emergethrough the photosphere and chromosphere to create the dynamic corona.However, as much of this process can not be observed, many importantquestions about how this flux reaches and populates the corona haveyet to be solved. Theory and numerical simulations are powerful toolsto address these fundamental issues, and this presentation will focuson several of these unsolved problems in flux emergence which arecurrently being investigated with these tools.Current models show that magnetic flux should neither be able torise to the solar surface, nor to emerge into the corona unless itis highly twisted, yet the majority of emerged magnetic regions areobserved to have very little twist. A related issue is that manysuccessful models of coronal mass ejection initiation requireenergized, sheared magnetic field to emerge into the corona todrive these solar eruptions, yet flux emergence simulations showthat it is primarily twisted field which emerges to form asimple arcade, rather than sheared field. This presentation willreview the evidence behind these conundrums, and presentresults from ongoing investigations which show how these issuesmay be solved.This work was supported by the ONR and by the NASA LWS and HGIprograms. Title: The effect of magnetic topology on the escape of flare-accelerated particles Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R. Bibcode: 2012shin.confE..20M Altcode: Magnetic reconnection in the solar atmosphere is believed to be the driver of most solar active phenomena. Therefore, the structure and dynamics of the coronal magnetic field are central to understanding solar and heliospheric activity. Important heliospheric manifestations of intense energy release linked to solar activity include the impact at the Earth of energetic particles accelerated during solar eruptions. Observationally, the magnetic configuration of active regions, where solar eruptions occur, agrees well with the standard model of eruption, consisting of a flare and a coronal mass ejection (CME). According to the standard model, particles accelerated at the flare reconnection site should remain trapped in the CME. However, flare-accelerated particles frequently reach the Earth long before the CME does.We present a new model that may lead to injection of energetic particles onto open interplanetary magnetic flux tubes. Our model is based on results of 2.5D MHD simulations of a large-scale coronal null-point topology with the outer spine opened to interplanetary space by an isothermal solar wind. The simulations are performed with the Adaptively Refined Mhd Solver (ARMS). We describe the multiple reconnections that occur during the evolution of the event, and show how they lead to the release of flare accelerated particles onto open field lines. We discuss the implications of our results for CME/flare models and for observations.This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: Coronal jets in an inclined coronal magnetic field : a parametric 3D MHD study Authors: Dalmasse, K.; Pariat, E.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2012EAS....55..201D Altcode: X-ray solar coronal jets are short-duration, fast, well collimated plasma brightenings occurring in the solar corona. To explain and understand the processes driving the jets, one must be able to model an explosive release of free energy. Magnetic reconnection is believed to play a key role in the generation of these energetic bursting events. The model of jets that we have been developing is based on a magnetic field constructed by embedding a vertical magnetic dipole in a uniform open magnetic field. In this study, we investigate the influence of the inclination of the open field on the properties of the jet using numerical simulations. We will show that the inclination of the open field is of critical importance for the properties of the jet such as the energy released. We conclude that the characteristics of the open field at the time of observations are a central criterion that must be taken into account and reported on in observational studies. Title: Understanding Solar Flares Authors: Antiochos, Spiro K.; Karpen, J. T.; DeVore, C. R. Bibcode: 2012AAS...22041001A Altcode: Solar flares and their associated coronal mass ejections are the most energetic explosions in the solar system. The largest events pose the greatest space weather dangers to life and civilization, and are of extreme importance to human space exploration. They also provide the best opportunity to study the universal processes of magnetic reconnection and particle acceleration that underlie most solar activity. The two great mysteries of solar flares are: how can so much energy be released so quickly, and how can such a large fraction (50% or more) end up in energetic particles. We present results from recent numerical modeling that sheds new light on these mysteries. These calculations use the highest spatial resolution yet achieved in order to resolve the flare dynamics as clearly as possible. We conclude from this work that magnetic island formation is the defining property of magnetic reconnection in the solar corona, at least, in the large-scale current sheet required for a solar flare. Furthermore, we discuss the types of future observations and modeling that will be required to solve definitively the solar flare mysteries.

This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: Comparison of Prominence Structures with Instances of Flux Rope CMEs in STEREO Data Authors: Rager, Amy; Thompson, B. J.; Antiochos, S. K.; Thernisien, A.; Thompson, W. T. Bibcode: 2012AAS...22020004R Altcode: STEREO A and B CME data have been visually searched for instances of flux ropes, signified by a concave outward cavity feature in the COR1 coronagraph. The flux rope events selected were observed by both spacecraft, and also had visible prominences in both EUVI-A and EUVI-B. The appearance of a flux rope was compared to the angle of the inferred magnetic neutral line of the CME to discover if a relationship existed. The GCS CME flux rope model was fit to the COR1 data, allowing for a clearer representation of the flux rope structure to compare with the magnetic neutral line. Title: The Effect of Magnetic Topology on the Escape of Flare-accelerated Particles Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R. Bibcode: 2012AAS...22051606M Altcode: Magnetic reconnection in the solar atmosphere is believed to be the driver of most solar active phenomena. Therefore, the structure and dynamics of the coronal magnetic field are central to understanding solar and heliospheric activity. Important heliospheric manifestations of intense energy release linked to solar activity include the impact at the Earth of energetic particles accelerated during solar eruptions. Observationally, the magnetic configuration of active regions, where solar eruptions occur, agrees well with the standard model of eruption, consisting of a flare and a coronal mass ejection (CME). According to the standard model, particles accelerated at the flare reconnection site should remain trapped in the CME. However, flare-accelerated particles frequently reach the Earth long before the CME does.

We present a new model that may lead to injection of energetic particles onto open interplanetary magnetic flux tubes. Our model is based on results of 2.5D MHD simulations of a large-scale coronal null-point topology with the outer spine opened to interplanetary space by an isothermal solar wind. The simulations are performed with the Adaptively Refined Mhd Solver (ARMS). We describe the multiple reconnections that occur during the evolution of the event, and show how they lead to the release of flare accelerated particles onto open field lines. We discuss the implications of our results for CME/flare models and for observations.

This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: Global network of slow solar wind Authors: Crooker, N. U.; Antiochos, S. K.; Zhao, X.; Neugebauer, M. Bibcode: 2012JGRA..117.4104C Altcode: 2012JGRA..11704104C The streamer belt region surrounding the heliospheric current sheet (HCS) is generally treated as the primary or sole source of the slow solar wind. Synoptic maps of solar wind speed predicted by the Wang-Sheeley-Arge model during selected periods of solar cycle 23, however, show many areas of slow wind displaced from the streamer belt. These areas commonly have the form of an arc that is connected to the streamer belt at both ends. The arcs mark the boundaries between fields emanating from different coronal holes of the same polarity and thus trace the paths of belts of pseudostreamers, i.e., unipolar streamers that form over double arcades and lack current sheets. The arc pattern is consistent with the predicted topological mapping of the narrow open corridor or singular separator line that must connect the holes and, thus, consistent with the separatrix-web model of the slow solar wind. Near solar maximum, pseudostreamer belts stray far from the HCS-associated streamer belt and, together with it, form a global-wide web of slow wind. Recognition of pseudostreamer belts as prominent sources of slow wind provides a new template for understanding solar wind stream structure, especially near solar maximum. Title: The Role of Topology in the Energetics of the Coupled Solar Atmosphere Authors: Antiochos, Spiro K. Bibcode: 2012decs.confE..43A Altcode: The defining physical property of the solar atmosphere is that the magnetic field dominates the plasma. This property implies that magnetic topology plays the central role in determining the structure and energetics of the atmosphere. For example, the formation of observed coronal loops, the explosive energy release in flares and coronal mass ejections, and the creation of the solar wind are all controlled by the topological constraints imposed on the plasma by the magnetic field. In this presentation I discuss how magnetic topology leads to the formation of the structures and dynamics observed in the solar atmosphere including the wind. Not surprisingly, the most important process for driving the dynamics is magnetic reconnection, which acts to break many of the topological constraints. Reconnection, however, preserves some of the topology, in particular, helicity. This turns out to have major implications for the coupled atmosphere. In this presentation, I will also discuss the implications of the topological constraints on observations from SDO and Hinode. This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: Forced Magnetic Reconnection at an X-point: Comparative Fluid and Fully Kinetic Studies Authors: Wang, L.; Antiochos, S. K.; Bessho, N.; Bhattacharjee, A.; Black, C.; DeVore, C. R.; Dorelli, J.; Karpen, J. T. Bibcode: 2011AGUFMSM23B2049W Altcode: We have undertaken a challenge problem of investigating current sheet formation and the resulting magnetic reconnection at an X-point of an initially potential field by a suite of MHD, Hall MHD, and fully electromagnetic PIC codes, all with the same initial conditions. Our goals are to investigate the similarities and differences between the various physical models, and to seek suitable parameterization of kinetic effects in the fluid models. We use two types of forcing: (i) shearing flows at the boundaries, and (ii) pressure perturbations imposed in two spatial domains on opposite sides of the initial separatrix. In both cases the system is driven slowly compared to the characterstic Alfven speed, and the forcing is far from the initial separatrices. While both the fluid and PIC models show current sheet formation and magnetic reconnection, the reconnection onset, the rate, and the energy released show significant differences. We will present scaling results in the fluid as well as PIC simulations, and discuss reasons for the differences between them. We also discuss possible extensions of the MHD model in order to reconcile it with the PIC model. This challenge problem is carried out under the auspices of a Focus Team in the NASA Living With a Star Targeted Research and Technology Program. Title: Magnetohydrodynamic Simulations of Current-Sheet Formation and Reconnection at a Magnetic X Line Authors: DeVore, C. R.; Antiochos, S. K.; Karpen, J. T.; Black, C. Bibcode: 2011AGUFMSH43A1923D Altcode: Phenomena ranging from the quiescent heating of the ambient plasma to the highly explosive release of energy and acceleration of particles in flares are conjectured to result from magnetic reconnection at electric current sheets in the Sun's corona. We are investigating numerically, using a macroscopic magnetohydrodynamic (MHD) model with adaptive mesh refinement, the formation and reconnection of a current sheet in an initially potential 2D magnetic field containing a null. Subjecting this simple configuration to unequal stresses in the four quadrants bounded by the X-line separatrix distorts the potential null into a double-Y-line current sheet. We find that even small distortions of the magnetic field induce the formation of a tangential discontinuity in the high-beta region around the null. A continuously applied stress eventually leads to the onset of fast magnetic reconnection across the sheet, with copious production, merging, and ejection of magnetic islands. We compare the current-sheet development and evolution for three cases: quasi-ideal MHD with numerical resistivity only; uniformly resistive MHD; and MHD with an embedded kinetic reconnection model. Analogous kinetic simulations using particle-in-cell (PIC) methods to investigate the small-scale dynamics of the system also are being pursued (C. Black et al., this meeting). Our progress toward understanding this simple system will be reported, as will the implications of our results for the dynamic activity associated with coronal current sheets and for general multiscale modeling of magnetized plasmas in the Heliosphere. Our research was supported by NASA. Title: Current-Sheet Formation and Reconnection at a Magnetic X Line in Particle-in-Cell Simulations Authors: Black, C.; Antiochos, S. K.; Hesse, M.; Karpen, J. T.; DeVore, C. R.; Zenitani, S.; Kuznetsova, M. M. Bibcode: 2011AGUFMSH43A1919B Altcode: The integration of kinetic effects into macroscopic numerical models is currently of great interest to the heliophysics community, particularly in the context of magnetic reconnection. Reconnection governs the large-scale energy release and topological rearrangement of magnetic fields in a wide variety of laboratory, heliophysical, and astrophysical systems. We are examining the formation and reconnection of current sheets in a simple, two-dimensional X-line configuration using high-resolution particle-in-cell (PIC) simulations. The initial minimum-energy, potential magnetic field is perturbed by excess thermal pressure introduced into the particle distribution function far from the X line. Subsequently, the relaxation of this added stress leads self-consistently to the development of a current sheet that reconnects for imposed stress of sufficient strength. We compare the time-dependent evolution and final state of our PIC simulations with macroscopic magnetohydrodynamic simulations assuming both uniform and localized electrical resistivities (C. R. DeVore et al., this meeting), as well as with force-free magnetic-field equilibria in which the amount of reconnection across the X line can be constrained to be zero (ideal evolution) or optimal (minimum final magnetic energy). We will discuss implications of our results for understanding magnetic-reconnection onset and cessation at kinetic scales in dynamically formed current sheets, such as those occurring in the solar corona and terrestrial magnetotail. This research was supported by NASA. Title: A Numerical Simulation for the Origins of Solar Magnetic Structure Authors: Zhao, L.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2011AGUFMSH43A1930Z Altcode: We investigate numerically a new model for the origin of the solar coronal magnetic field structure observed in filament channels and in the complex structures of the slow wind. Using the Adaptively Refined Magnetohydrodynamic Solver (ARMS), we perform a series of numerical experiments to study the evolution of magnetic helicity injected into the solar corona by photospheric motions. Our simulation domain consists of a Cartesian box with an initially uniform vertical magnetic field and a low-beta plasma with uniform pressure and density. This system is driven by imposing flow patterns at the top and bottom boundary planes corresponding to the twisting motions expected from the quasi-random photospheric motions. We consider a variety of flow patterns made up of twist arranged in regular geometric orders (e.g. four twists in a quadrilateral arrangement, seven twists in a hexagonal), which generate sets of twisted flux tubes in the interior of the simulation box, the corona. This driving twist injects both energy and helicity into the coronal field. Depending upon the sense of the applied twist, we can inject either positive or negative helicity. If helicity of the same sign is injected into each of the flux tubes (the co-helicity case), we expect that the twist magnetic-field component of neighboring flux tubes will be oppositely directed and, therefore, will reconnect; on the other hand, if helicity of opposite signs is injected into neighboring flux tubes (the counter-helicity case), reconnection will not occur. This conjecture is confirmed by our simulations. We also have found generally that in co-helicity cases the reconnection indeed occurs and leads to a state in which the twist propagates to the largest scale: essentially, the individual flux tubes merge into one large twisted tube, with the twist concentrated at its outer boundary. We discuss the implications of our results for the evolution of coronal helicity and for the formation of filament channels on the Sun and slow-wind structures in the Heliosphere. Our research was sponsored by NASA. Title: Ion-neutral Coupling in Solar Prominences Authors: Gilbert, H. R.; DeVore, C. R.; Karpen, J. T.; Kucera, T. A.; Antiochos, S. K.; Kawashima, R. Bibcode: 2011AGUFMSH13B1953G Altcode: Coupling between ions and neutrals in magnetized plasmas is fundamentally important to many aspects of heliophysics, including our ionosphere, the solar chromosphere, the solar wind interaction with planetary atmospheres, and the interface between the heliosphere and the interstellar medium. Ion-neutral coupling also plays a major role in the physics of solar prominences. By combining theory, modeling, and observations we are working toward a better understanding of the structure and dynamics of partially ionized prominence plasma. Two key questions are addressed in the present work: 1) what physical mechanism(s) sets the cross-field scale of prominence threads? 2) Are ion-neutral interactions responsible for the vertical flows and structure in prominences? We present initial results from a study investigating what role ion-neutral interactions play in prominence dynamics and structure. This research was supported by NASA. Title: The Magnetic Connectivity of the Sun to the Heliosphere Authors: Antiochos, S. K. Bibcode: 2011AGUFMSH43F..05A Altcode: A prime research focus of the upcoming Solar Probe Plus and Solar Orbiter missions will be to determine how the heliospheric magnetic field and plasma connect to the Sun's corona and photosphere. For much of the heliosphere this connection appears to be well understood. The quasi-steady fast wind emanates from so-called coronal holes, which appear dark in X-rays and are predominantly unipolar at the photosphere. However, the connection to the Sun of the slow, non-steady wind is far from understood and remains a major mystery. We review the existing theories for the sources of the non-steady wind and demonstrate that they have difficulty accounting for both the observed composition of the wind and its large angular extent. A new theory is described in which this wind originates from the continuous opening and closing of narrow open field corridors in the corona, which gives rise to a web of separatrices (the S-Web) in the heliosphere. Note that in this theory the corona - heliosphere connection is intrinsically dynamic, at least, for this type of wind. We present numerical simulations of the model and describe observational tests. We discuss the implications of our results for the competing slow wind theories and for understanding the corona - heliosphere connection, in general. This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: Consequences of the Breakout Model for Particle Acceleration in CMEs and Flares Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R. Bibcode: 2011AGUFMSH51E..01A Altcode: The largest and most efficient particle accelerators in the solar system are the giant events consisting of a fast coronal mass ejection (CME) and an intense X-class solar flare. Both flares and CMEs can produce 1032 ergs or more in nonthermal particles. Two general processes are believed to be responsible: particle acceleration at the strong shock ahead of the CME, and reconnection-driven acceleration in the flare current sheet. Although shock acceleration is relatively well understood, the mechanism by which flare reconnection produces nonthermal particles is still an issue of great debate. We address the question of CME/flare particle acceleration in the context of the breakout model using 2.5D MHD simulations with adaptive mesh refinement (AMR). The AMR capability allows us to achieve ultra-high numerical resolution and, thereby, determine the detailed structure and dynamics of the flare reconnection region. Furthermore, we employ newly developed numerical analysis tools for identifying and characterizing magnetic nulls, so that we can quantify accurately the number and location of magnetic islands during reconnection. Our calculations show that flare reconnection is dominated by the formation of magnetic islands. In agreement with many other studies, we find that the number of islands scales with the effective Lundquist number. This result supports the recent work by Drake and co-workers that postulates particle acceleration by magnetic islands. On the other hand, our calculations also show that the flare reconnection region is populated by numerous shocks and other indicators of strong turbulence, which can also accelerate particles. We discuss the implications of our calculations for the flare particle acceleration mechanism and for observational tests of the models. This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: CMEs with multiple reconnection sites : A model for energetic particle injection Authors: Masson, S.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2011AGUFMSH43A1931M Altcode: Magnetic reconnection in the solar atmosphere is believed to be the driver of most solar explosive phenomena. Therefore, the structure and dynamics of the coronal magnetic field are central to understanding solar and heliospheric activity. Important heliospheric manifestations of intense energy release linked to solar activity include the impact at the Earth of energetic particles accelerated during solar eruptions. Observationally, the magnetic configuration of active regions where solar eruptions occur, agrees well with the standard model of an eruption consisting of a flare and a coronal mass ejection (CME). According to the standard model, particles accelerated at the flare reconnection site should remain trapped in the CME. However, flare-accelerated particles frequently reach the Earth long before the CME does. We present a new model that may lead to injection of energetic particles onto open magnetic flux tubes connecting to the Earth. Our model is based on the well-known 2.5D breakout topology, which has a coronal null point (null line) and a four-flux system. A key new addition, however, is that we include an isothermal solar wind. Depending on the location of the open flux with respect to the null point, we find that the flare reconnection can consist of two distinct phases. At first, the flare reconnection involves only closed field, but if the eruption occurs close to the open field, we find a second phase involving interchange reconnection between open and closed. We argue that this second reconnection episode is responsible for the injection of flare-accelerated particles into the interplanetary medium. We will report on our recent work toward understanding how flare particles escape to the heliosphere. This work uses high-resolution 2.5D MHD numerical simulations performed with the Adaptively Refined MHD Solver (ARMS). This research was supported, in part, by the NASA SR&T and TR&T Programs. Title: Numerical Simulation of a "Stealth" CME: Why Slow and Simple is Not Mysterious Authors: Lynch, B. J.; Li, Y.; Antiochos, S. K.; DeVore, C. R.; Luhmann, J. G.; Fisher, G. H. Bibcode: 2011AGUFMSH43A1937L Altcode: The stereoscopic viewing and improvements in coronagraph observations by STEREO/SECCHI and low corona EUV and X-ray observations at multiple wavelengths by STEREO, Hinode, and SDO -- combined with this solar minimum's exceptionally low activity -- have given rise to the community's interest in so-called "stealth" CMEs. A "stealth" CME is one in which there are almost no low coronal signatures of the CME eruption but often a very well resolved slow, flux-rope like eruption seen in the coronagraph data. The fact that the in situ observations of "stealth" CMEs have shown many of the signatures of magnetic clouds (including the interplanetary flux rope structure) poses the question, "Just how different these events are from normal CMEs?" We present a 3D numerical MHD simulation of the 2008 Jun 2 gradual streamer blowout CME which had virtually no identifiable low coronal signatures. We energize the field by simple footpoint shearing along the source region's polarity inversion line (PIL) and model the background solar wind structure using an ~2MK isothermal wind and a low-order PFSS representation of the CR2070 synoptic magnetogram. Our results will show that the CME "initiation" is obtained by slowly disrupting the quasi-steady-state configuration of the helmet streamer, resulting in the standard eruptive flare picture (albeit, on a large scale) that ejects the sheared fields and lowers the magnetic energy stored in filament channel. We obtain a relatively slow CME eruption and argue that these "stealth" CMEs are no different than the standard quasi-2D picture but are simply at the low end of the CME energy distribution. We will show preliminary comparisons between the simulation results and the coronagraph observations of the low coronal evolution of the CME. Title: Erratum: "Tests of Dynamical Flux Emergence as a Mechanism for Coronal Mass Ejection Initiation" (2010, ApJ, 722, 550) Authors: Leake, James E.; Linton, Mark G.; Antiochos, Spiro K. Bibcode: 2011ApJ...741..125L Altcode: No abstract at ADS Title: Preface Authors: Lewis, W. S.; Antiochos, S. K.; Drake, J. F. Bibcode: 2011SSRv..160....1L Altcode: 2011SSRv..tmp..259L; 2011SSRv..tmp..294L; 2011SSRv..tmp..241L No abstract at ADS Title: The S-Web Hypothesis and the Slow Solar Wind Authors: Linker, Jon A.; Lionello, Roberto; Titov, Viacheslav S.; Mikic, Zoran; Antiochos, Spiro Bibcode: 2011shin.confE.160L Altcode: The origin of the slow solar wind is controversial. A successful theory must explain the plasma composition and angular extent of the slow wind, as well as its frequent asymmetry with respect to the heliospheric current sheet. Recently, a new idea has been put forward for the origin of the slow wind, dubbed the S-Web model. The name comes from high-resolution MHD calculations that have revealed that coronal hole boundaries are not smooth, but are highly corrugated with a web of separatrices and quasi-separatrix layers. These are regions that are likely to be susceptible to interchange reconnection. In this talk we describe the origin of this idea, how it may explain key features of the slow solar wind, and further calculations/observational tests that may help confirm or refute this idea.

Work supported by NASA and NSF. Title: Current Sheet Formation and Reconnection at a Magnetic X Line Authors: DeVore, C. Richard; Antiochos, S. K. Bibcode: 2011SPD....42.1820D Altcode: 2011BAAS..43S.1820D Phenomena ranging from the quiescent heating of the ambient plasma to the highly explosive release of energy and acceleration of particles in flares are conjectured to result from magnetic reconnection at electric current sheets in the Sun's corona. We are investigating numerically the formation and eventual reconnection of a current sheet in an initially potential 2D magnetic field containing a null. Subjecting this simple configuration to unequal stresses in the four quadrants bounded by the X-line separatrix distorts the potential null into a double-Y-line current sheet. Although the gas pressure is finite in our simulations, so that the plasma beta is infinite at the null, we find that even small distortions of the magnetic field induce the formation of a tangential discontinuity there. This result is well known to occur in the zero-beta, force-free limit; surprisingly, it persists into the high-beta regime where, in principle, a small plasma pressure inhomogeneity could balance all of the magnetic stress. In addition to working to understand the dynamical details of this ideal process, we are examining the effect of resistive dissipation on the development of the current sheet and are seeking to determine the critical condition for fast-reconnection onset in the sheet. Our progress on understanding these issues, and the implications for the dynamic activity associated with current sheets in the solar corona, will be reported at the conference. We gratefully acknowledge NASA sponsorship of our research. Title: CME Onset and Take-off Authors: Antiochos, Spiro K.; Karpen, J. T.; DeVore, C. R. Bibcode: 2011SPD....42.1302A Altcode: 2011BAAS..43S.1302A For understanding and eventually predicting coronal mass ejections/eruptive flares, two critical questions must be answered: What is the mechanism for eruption onset, and what is the mechanism for the rapid acceleration? We address these questions in the context of the breakout model using 2.5D MHD simulations with adaptive mesh refinement (AMR). The AMR capability allowed us to achieve ultra-high numerical resolution and, thereby, determine the influence of the effective Lundquist number on the eruption. Our calculations show that, at least, for the breakout model, the onset of reconnection external to the highly-sheared filament channel is the onset mechanism. Once this reconnection turns on, eruption is inevitable. However, as long as this is the only reconnection in the system, the eruption remains slow. We find that the eruption undergoes an abrupt "take-off" when the flare reconnection below the erupting plasmoid develops significant reconnection jets. We conclude that in fast CMEs, flare reconnection is the primary mechanism responsible for both flare heating and CME acceleration. We discuss the implications of these results for SDO observations and describe possible tests of the model.

This work was supported, in part, by the NASA TR&T and SR&T Programs. Title: Constraints on Coronal Mass Ejection Evolution from in Situ Observations of Ionic Charge States Authors: Gruesbeck, Jacob R.; Lepri, Susan T.; Zurbuchen, Thomas H.; Antiochos, Spiro K. Bibcode: 2011ApJ...730..103G Altcode: We present a novel procedure for deriving the physical properties of coronal mass ejections (CMEs) in the corona. Our methodology uses in situ measurements of ionic charge states of C, O, Si, and Fe in the heliosphere and interprets them in the context of a model for the early evolution of interplanetary CME (ICME) plasma, between 2 and 5 R sun. We find that the data are best fit by an evolution that consists of an initial heating of the plasma, followed by an expansion that ultimately results in cooling. The heating profile is consistent with a compression of coronal plasma due to flare reconnection jets and an expansion cooling due to the ejection, as expected from the standard CME/flare model. The observed frozen-in ionic charge states reflect this time history and, therefore, provide important constraints for the heating and expansion timescales, as well as the maximum temperature the CME plasma is heated to during its eruption. Furthermore, our analysis places severe limits on the possible density of CME plasma in the corona. We discuss the implications of our results for CME models and for future analysis of ICME plasma composition. Title: Magnetic Topology of Coronal Hole Linkages Authors: Titov, V. S.; Mikić, Z.; Linker, J. A.; Lionello, R.; Antiochos, S. K. Bibcode: 2011ApJ...731..111T Altcode: 2010arXiv1011.0009T In recent work, Antiochos and coworkers argued that the boundary between the open and closed field regions on the Sun can be extremely complex with narrow corridors of open flux connecting seemingly disconnected coronal holes from the main polar holes and that these corridors may be the sources of the slow solar wind. We examine, in detail, the topology of such magnetic configurations using an analytical source surface model that allows for analysis of the field with arbitrary resolution. Our analysis reveals three new important results. First, a coronal hole boundary can join stably to the separatrix boundary of a parasitic polarity region. Second, a single parasitic polarity region can produce multiple null points in the corona and, more important, separator lines connecting these points. It is known that such topologies are extremely favorable for magnetic reconnection, because they allow this process to occur over the entire length of the separators rather than being confined to a small region around the nulls. Finally, the coronal holes are not connected by an open-field corridor of finite width, but instead are linked by a singular line that coincides with the separatrix footprint of the parasitic polarity. We investigate how the topological features described above evolve in response to the motion of the parasitic polarity region. The implications of our results for the sources of the slow solar wind and for coronal and heliospheric observations are discussed. Title: A Model for the Sources of the Slow Solar Wind Authors: Antiochos, S. K.; Mikić, Z.; Titov, V. S.; Lionello, R.; Linker, J. A. Bibcode: 2011ApJ...731..112A Altcode: 2011arXiv1102.3704A Models for the origin of the slow solar wind must account for two seemingly contradictory observations: the slow wind has the composition of the closed-field corona, implying that it originates from the continuous opening and closing of flux at the boundary between open and closed field. On the other hand, the slow wind also has large angular width, up to ~60°, suggesting that its source extends far from the open-closed boundary. We propose a model that can explain both observations. The key idea is that the source of the slow wind at the Sun is a network of narrow (possibly singular) open-field corridors that map to a web of separatrices and quasi-separatrix layers in the heliosphere. We compute analytically the topology of an open-field corridor and show that it produces a quasi-separatrix layer in the heliosphere that extends to angles far from the heliospheric current sheet. We then use an MHD code and MDI/SOHO observations of the photospheric magnetic field to calculate numerically, with high spatial resolution, the quasi-steady solar wind, and magnetic field for a time period preceding the 2008 August 1 total solar eclipse. Our numerical results imply that, at least for this time period, a web of separatrices (which we term an S-web) forms with sufficient density and extent in the heliosphere to account for the observed properties of the slow wind. We discuss the implications of our S-web model for the structure and dynamics of the corona and heliosphere and propose further tests of the model. Title: The Evolution of Open Magnetic Flux Driven by Photospheric Dynamics Authors: Linker, Jon A.; Lionello, Roberto; Mikić, Zoran; Titov, Viacheslav S.; Antiochos, Spiro K. Bibcode: 2011ApJ...731..110L Altcode: The coronal magnetic field is of paramount importance in solar and heliospheric physics. Two profoundly different views of the coronal magnetic field have emerged. In quasi-steady models, the predominant source of open magnetic field is in coronal holes. In contrast, in the interchange model, the open magnetic flux is conserved, and the coronal magnetic field can only respond to the photospheric evolution via interchange reconnection. In this view, the open magnetic flux diffuses through the closed, streamer belt fields, and substantial open flux is present in the streamer belt during solar minimum. However, Antiochos and coworkers, in the form of a conjecture, argued that truly isolated open flux cannot exist in a configuration with one heliospheric current sheet—it will connect via narrow corridors to the polar coronal hole of the same polarity. This contradicts the requirements of the interchange model. We have performed an MHD simulation of the solar corona up to 20 R sun to test both the interchange model and the Antiochos conjecture. We use a synoptic map for Carrington rotation 1913 as the boundary condition for the model, with two small bipoles introduced into the region where a positive polarity extended coronal hole forms. We introduce flows at the photospheric boundary surface to see if open flux associated with the bipoles can be moved into the closed-field region. Interchange reconnection does occur in response to these motions. However, we find that the open magnetic flux cannot be simply injected into closed-field regions—the flux eventually closes down and disconnected flux is created. Flux either opens or closes, as required, to maintain topologically distinct open- and closed-field regions, with no indiscriminate mixing of the two. The early evolution conforms to the Antiochos conjecture in that a narrow corridor of open flux connects the portion of the coronal hole that is nearly detached by one of the bipoles. In the later evolution, a detached coronal hole forms, in apparent violation of the Antiochos conjecture. Further investigation reveals that this detached coronal hole is actually linked to the extended coronal hole by a separatrix footprint on the photosphere of zero width. Therefore, the essential idea of the conjecture is preserved, if we modify it to state that coronal holes in the same polarity region are always linked, either by finite width corridors or separatrix footprints. The implications of these results for interchange reconnection and the sources of the slow solar wind are briefly discussed. Title: On tether-cutting reconnection in sheared coronal arcades Authors: Lynch, B. J.; Li, Y.; Antiochos, S. K.; DeVore, C. R.; Fisher, G. H. Bibcode: 2010AGUFMSH31A1791L Altcode: We present preliminary numerical magnetohydrodynamic (MHD) simulation results of 3-dimensional bipolar sheared arcades and their susceptibility to eruption in a spherical geometry. The MHD simulations are run using the Adaptively Refined MHD Solver (ARMS) developed at the Naval Research Laboratory. The initially potential magnetic field is energized via highly concentrated shearing flows parallel to the polarity inversion line (PIL) of an idealized, elongated decayed active region. The tether-cutting reconnection deep in the sheared field core is generated by applying converging flows towards the active region PIL that compress magnetic fields with oppositely directed r-components together setting up a moderate guide-field reconnection scenario at the lower boundary. We present three cases that test the amount of tether-cutting reconnection required to cause the sheared field to go unstable and erupt, parameterized as the relative length of the converging flow profiles to the active region PIL. Our hypothesis is that, due to the overlying background field of the bipolar arcade, the sheared field will remain stable for a relatively small amount of tether-cutting reconnection and when the converging flows cover a substantial portion of the active region PIL, the bipolar arcade could experience a sort of eruption. We will discuss observational consequences of the eruption process and discuss future work, e.g. comparing these results with multipolar magnetic breakout eruptions. Title: High-Resolution Numerical Simulations of Breakout Coronal Mass Ejections Authors: DeVore, C. R.; Karpen, J. T.; Antiochos, S. K. Bibcode: 2010AGUFMSM31B1875D Altcode: We have conducted high-resolution numerical simulations of the gradual energization, initiation of eruption, and expansion into the inner heliosphere of coronal mass ejections. The critical triggering process underlying the eruption is the onset of magnetic reconnection. Reconnection at the deformed null point high in the corona (at the ‘breakout’ current sheet) reconfigures the restraining field overlying the eruptive core, accelerating the rise of the magnetic structure; that between the nearly vertical legs of the field above the polarity inversion lines (at the ‘flare’ current sheet) partially detaches flux from the Sun and provides a further impulse to the outward motion of the ejecta. To investigate these processes in detail, we assumed an axisymmetric (2.5D) spherical geometry and exploited the adaptive mesh refinement capabilities of our Adaptively Refined MHD Solver (ARMS) simulation model to achieve unprecedentedly high resolution of all current structures as they develop dynamically. As the maximum refinement level increases, the current sheets exhibit increasingly fine-scaled structure, with ever greater numbers of magnetic islands forming, dividing, recombining, and streaming along the sheets to their termini. The macroscopic properties of the ejecta, such as the kinetic energy and radial velocity of the CME, on the other hand, depend only weakly on the grid refinement level and the resultant numerical resistivity. This demonstrates convergence of the results toward the high-conductivity regime of the solar corona. In addition to describing these findings, we will report our progress on adding a kinetic-scale resistivity model to the global simulations. This work has been supported by the NASA HTP, SR&T, and LWS programs. Title: Multiscale Modeling of Solar Coronal Magnetic Reconnection Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R. Bibcode: 2010AGUFMSM31B1873A Altcode: Magnetic reconnection is widely believed to be the primary process by which the magnetic field releases energy to plasma in the Sun's corona. For example, in the breakout model for the initiation of coronal mass ejections/eruptive flares, reconnection is responsible for the catastrophic destabilizing of magnetic force balance in the corona, leading to explosive energy release. A critical requirement for the reconnection is that it have a "switch-on' nature in that the reconnection stays off until a large store of magnetic free energy has built up, and then it turn on abruptly and stay on until most of this free energy has been released. We discuss the implications of this requirement for reconnection in the context of the breakout model for CMEs/flares. We argue that it imposes stringent constraints on the properties of the flux breaking mechanism, which is expected to operate in the corona on kinetic scales. We present numerical simulations demonstrating how the reconnection and the eruption depend on the effective resistivity, i.e., the effective Lundquist number, and propose a model for incorporating kinetic flux-breaking mechanisms into MHD calculation of CMEs/flares. This work has been supported by the NASA HTP, SR&T, and LWS programs. High-resolution simulation of a breakout CME showing details of the reconnection region (Karpen et al 2010). Title: Constraints on CME evolution from in situ observations of ionic charge states Authors: Gruesbeck, J. R.; Lepri, S. T.; Zurbuchen, T.; Antiochos, S. K. Bibcode: 2010AGUFMSH23B1837G Altcode: We present a novel analysis of the expansion properties of Coronal Mass Ejections (CMEs) in the inner corona. This methodology uses measurements of ionic charge states of C, O, Si, and Fe from the Advanced Composition Explorer (ACE) and interprets them in the context of a quantitative ionization model. All observed charge state distributions exhibit signatures of substantial heating, yet to varying degrees, reflecting peculiar properties of the frozen-in ions despite a common expansion profile. For example, the vast majority of ICMEs exhibit bi-modal Fe charge state distributions. Using a plasma heating and expansion model, in-situ charge states are translated into constraints on the heating and expansion profiles of the CME plasma. We find that CMEs are first heated up to ~3 MK near the Sun and with a maximum electron density between ~5e9 and 8.5e9 cm^-3 then followed by rapid expansion and cooling, These CMEs exhibit frozen-in charge states qualitatively and quantitatively consistent with our observations. Title: Tests of Dynamical Flux Emergence as a Mechanism for Coronal Mass Ejection Initiation Authors: Leake, James E.; Linton, Mark G.; Antiochos, Spiro K. Bibcode: 2010ApJ...722..550L Altcode: 2010arXiv1007.5484L Current coronal mass ejection (CME) models set their lower boundary to be in the lower corona. They do not calculate accurately the transfer of free magnetic energy from the convection zone to the magnetically dominated corona because they model the effects of flux emergence using kinematic boundary conditions or simply assume the appearance of flux at these heights. We test the importance of including dynamical flux emergence in CME modeling by simulating, in 2.5D, the emergence of sub-surface flux tubes into different coronal magnetic field configurations. We investigate how much free magnetic energy, in the form of shear magnetic field, is transported from the convection zone to the corona, and whether dynamical flux emergence can drive CMEs. We find that multiple coronal flux ropes can be formed during flux emergence, and although they carry some shear field into the corona, the majority of shear field is confined to the lower atmosphere. Less than 10% of the magnetic energy in the corona is in the shear field, and this, combined with the fact that the coronal flux ropes bring up significant dense material, means that they do not erupt. Our results have significant implications for all CME models which rely on the transfer of free magnetic energy from the lower atmosphere into the corona but which do not explicitly model this transfer. Such studies of flux emergence and CMEs are timely, as we have new capabilities to observe this with Hinode and the Solar Dynamics Observatory, and therefore to test the models against observations. Title: Formation and Reconnection of Three-dimensional Current Sheets in the Solar Corona Authors: Edmondson, J. K.; Antiochos, S. K.; DeVore, C. R.; Zurbuchen, T. H. Bibcode: 2010ApJ...718...72E Altcode: Current-sheet formation and magnetic reconnection are believed to be the basic physical processes responsible for much of the activity observed in astrophysical plasmas, such as the Sun's corona. We investigate these processes for a magnetic configuration consisting of a uniform background field and an embedded line dipole, a topology that is expected to be ubiquitous in the corona. This magnetic system is driven by a uniform horizontal flow applied at the line-tied photosphere. Although both the initial field and the driver are translationally symmetric, the resulting evolution is calculated using a fully three-dimensional (3D) magnetohydrodynamic simulation with adaptive mesh refinement that resolves the current sheet and reconnection dynamics in detail. The advantage of our approach is that it allows us to directly apply the vast body of knowledge gained from the many studies of two-dimensional (2D) reconnection to the fully 3D case. We find that a current sheet forms in close analogy to the classic Syrovatskii 2D mechanism, but the resulting evolution is different than expected. The current sheet is globally stable, showing no evidence for a disruption or a secondary instability even for aspect ratios as high as 80:1. The global evolution generally follows the standard Sweet-Parker 2D reconnection model except for an accelerated reconnection rate at a very thin current sheet, due to the tearing instability and the formation of magnetic islands. An interesting conclusion is that despite the formation of fully 3D structures at small scales, the system remains close to 2D at global scales. We discuss the implications of our results for observations of the solar corona. Title: Magnetic Topology of Coronal Hole Linkages Authors: Titov, Viacheslav S.; Mikic, Zoran; Linker, Jon A.; Lionello, Roberto; Antiochos, Spiro Bibcode: 2010shin.confE.120T Altcode: In recent work, Antiochos et al. (2007) argued that the boundary between the open and closed field regions on the Sun can be extremely complex with narrow corridors of open flux connecting seemingly disconnected coronal holes from the main polar holes, and that these corridors may be the sources of the slow solar wind. We examine, in detail, the topology of such magnetic configurations using an analytical source surface model that allows for analysis of the field with arbitrary resolution. Our analysis reveals three important new results: First, a coronal hole boundary can include the separatrix boundary of a parasitic polarity region. Second, a single parasitic polarity region can produce multiple null points in the corona and, more important, separator lines connecting these points. Such topologies are extremely favorable for magnetic reconnection, because it can now occur over the entire length of the separators rather than being confined to a small region around the nulls. Finally, the coronal holes are not connected by an open-field corridor of finite width, but instead are linked by a singular line that coincides with the separatrix footprint of the parasitic polarity. We investigate how the topological features described above evolve in response to motion of the parasitic polarity region. The implications of our results for the sources of the slow wind and for coronal and heliospheric observations are discussed. Title: A numerical simulation for the origins of solar magnetic structure Authors: Zhao, Liang; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 2010shin.confE.117Z Altcode: We investigate numerically a new model for the origin of the solar coronal magnetic field structure observed in filament channels and in the complex structures of the slow wind. Using the adaptively Refined Magnetohydrodynamic Solver (ARMS), we perform a series of numerical experiments to study the evolution of magnetic helicity injected into the solar corona by photospheric motions. Our simulation domain consists of a Cartesian box with an initially uniform vertical magnetic field and a low-beta plasma with uniform pressure and density. This system is driven by imposing a flow pattern at the top and bottom boundary planes corresponding to the twisting motions expected from the quasi-random photospheric motions. We consider a variety of flow patterns made up of twist arranged in regular geometric orders, (i.e. four twists in a quadrilateral arrangement, 7 twists in a hexagonal), that generate a set of twisted flux tubes in the interior of the simulation box, the corona. Note that this driving twist injects both energy and helicity into the coronal field. Depending on the sense of the applied twist, we can inject either positive or negative helicity. If helicity of the same sign is injected into each of the flux tubes (co-helicity case), we expect that the twist magnetic-field component of neighboring flux tubes will be oppositely directed and, therefore, will reconnect, but if opposite sign helicity is injected into neighboring flux tubes, reconnection will not occur. We tested this conjecture with our simulations and found that for the co-helicity case reconnection did occur and led to a state in which the twist propagated to the largest scale. Essentially the tubes merged into one large twisted flux tube. For the opposite helicity case, on the other hand, we found that reconnection did not occur, and the tubes remained as distinct structures. We discuss the implications of our results for the evolution of coronal helicity and for the formation of magnetic structures in the Sun. Title: Formation and Reconnection of Three-Dimensional Current Sheets in the Solar Corona Authors: Edmondson, Justin K.; Antiochos, S. K.; DeVore, C. R.; Zurbuchen, T. H. Bibcode: 2010shin.confE.111E Altcode: Current-sheet formation and magnetic reconnection are believed to be the basic physical processes responsible for much of the activity observed in astrophysical plasmas, such as interchange reconnection at the boundaries between coronal holes and helmet streamers in the Sun's corona. We investigate these processes for a magnetic configuration consisting of a uniform background field and an embedded line dipole, a topology that is expected to be ubiquitous in the corona. This magnetic system is driven by a uniform horizontal flow applied at the line-tied photosphere. Although both the initial field and the driver are translationally symmetric, the resulting evolution is calculated using a fully three-dimensional magnetohydrodynamic (3D MHD) simulation with adaptive mesh refinement that resolves the current sheet and reconnection dynamics in detail. The advantage of our approach is that it allows us to apply directly the vast body of knowledge gained from the many studies of 2D reconnection to the fully 3D case. We find that a current sheet forms in close analogy to the classic Syrovatskii 2D mechanism, but the resulting evolution is different than expected. The current sheet is globally stable, showing no evidence for a disruption or a secondary instability even for aspect ratios as high as 80:1. The global evolution generally follows the standard Sweet-Parker 2D reconnection model except for an accelerated reconnection rate at a very thin current sheet, due to the tearing instability and the formation of magnetic islands. An interesting conclusion is that despite the formation of fully 3D structures at small scales, the system remains close to 2D at global scales. We discuss the implications of our results for observations of the solar corona. Title: Symmetric Coronal Jets: A Reconnection-controlled Study Authors: Rachmeler, L. A.; Pariat, E.; DeForest, C. E.; Antiochos, S.; Török, T. Bibcode: 2010ApJ...715.1556R Altcode: Current models and observations imply that reconnection is a key mechanism for destabilization and initiation of coronal jets. We evolve a system described by the theoretical symmetric jet formation model using two different numerical codes with the goal of studying the role of reconnection in this system. One of the codes is the Eulerian adaptive mesh code ARMS, which simulates magnetic reconnection through numerical diffusion. The quasi-Lagrangian FLUX code, on the other hand, is ideal and able to evolve the system without reconnection. The ideal nature of FLUX allows us to provide a control case of evolution without reconnection. We find that during the initial symmetric and ideal phase of evolution, both codes produce very similar morphologies and energy growth. The symmetry is then broken by a kink-like motion of the axis of rotation, after which the two systems diverge. In ARMS, current sheets formed and reconnection rapidly released the stored magnetic energy. In FLUX, the closed field remained approximately constant in height while expanding in width and did not release any magnetic energy. We find that the symmetry threshold is an ideal property of the system, but the lack of energy release implies that the observed kink is not an instability. Because of the confined nature of the FLUX system, we conclude that reconnection is indeed necessary for jet formation in symmetric jet models in a uniform coronal background field. Title: Interpreting Small-Scale Structure from High Resolution Global MHD Simulations Authors: Mikic, Zoran; Titov, V. S.; Linker, J. A.; Lionello, R.; Riley, P.; Antiochos, S. Bibcode: 2010AAS...21640503M Altcode: 2010BAAS...41..889M High resolution 3D MHD simulations of the solar corona are beginning to reveal how small-scale structures in the magnetic field interact with the global structure of the corona and solar wind. In particular, it has become evident that the detailed characteristics of coronal holes, especially their equatorial extensions, may be related to the source of the slow solar wind. Using structural analysis based on the squashing factor Q (Titov et al. 2002, 2008; Titov 2007) we show how small-scale structure in the magnetic field is related to the structure of the streamer belt. These results have led to a new interpretation of the source of the slow solar wind.

Research supported by NASA's Heliospheric Theory and Living With a Star Programs, and NSF/CISM. Title: The Existence and Origin of Turbulence in Solar Active Regions Authors: Klimchuk, James A.; Nigro, G.; Dahlburg, R. B.; Antiochos, S. K. Bibcode: 2010AAS...21630205K Altcode: It has been suggested that turbulence plays a fundamental role in the heating of solar active regions, with its intermittent behavior being the explanation of impulsive energy release (nanoflares). We know that episodes of turbulence are produced in the final nonlinear stage of the secondary instability of electric current sheets. However, these current sheets must exist prior to the turbulence. Whether turbulence can dynamically produce current sheets that would not otherwise be present is a different and important question.

Turbulence occurs freely in the solar wind and in other situations where the magnetic field does not dominate. However, the magnetic field strongly resists being distorted in line-tied, low-beta environments such as active regions. Can turbulence develop naturally in these environments without being driven by an instability? To answer this question, we have performed a time-dependent MHD simulation of a slowly driven system that does not contain current sheets and is stable to applied perturbations. We find no evidence for bursty energy release, steep spatial gradients, or power-law energy spectra that are the typical signatures of turbulence. We conclude that the turbulence which occurs in active regions is an important yet secondary process and not the primary cause of heating. Title: Formation and Reconnection of Three-Dimensional Current Sheets in the Solar Corona Authors: Edmondson, Justin K.; Antiochos, S. K.; DeVore, C.; Velli, M.; Zurbuchen, T. H. Bibcode: 2010AAS...21640701E Altcode: 2010BAAS...41..859E Current-sheet formation and magnetic reconnection are believed to be the basic physical processes responsible for much of the activity observed in astrophysical plasmas, such as interchange reconnection at the boundaries between coronal holes and helmet streamers in the Sun's corona. We investigate these processes for a magnetic configuration consisting of a uniform background field and an embedded line dipole, a topology that is expected to be ubiquitous in the corona. This magnetic system is driven by a uniform horizontal flow applied at the line-tied photosphere. Although both the initial field and the driver are translationally symmetric, the resulting evolution is calculated using a fully three-dimensional magnetohydrodynamic (3D MHD) simulation with adaptive mesh refinement that resolves the current sheet and reconnection dynamics in detail. The advantage of our approach is that it allows us to apply directly the vast body of knowledge gained from the many studies of 2D reconnection to the fully 3D case. We find that a current sheet forms in close analogy to the classic Syrovatskii 2D mechanism, but the resulting evolution is different than expected. The current sheet is globally stable, showing no evidence for a disruption or a secondary instability even for aspect ratios as high as 80:1. The global evolution generally follows the standard Sweet-Parker 2D reconnection model except for an accelerated reconnection rate at a very thin current sheet, due to the tearing instability and the formation of magnetic islands. An interesting conclusion is that despite the formation of fully 3D structures at small scales, the system remains close to 2D at global scales. We discuss the implications of our results for observations of the solar corona. Title: Sympathetic Coronal Mass Ejections Originating in Complex Source Regions Authors: DeVore, C. Richard; Antiochos, S. K. Bibcode: 2010AAS...21640613D Altcode: 2010BAAS...41..882D We are investigating numerically the initiation of coronal mass ejections (CMEs) in scenarios in which two bipolar active regions, side by side, spawn solar eruptions. If the overall topology is more complex than bipolar, with a magnetic null and separatrices dividing the coronal field into two or more flux systems, then the configuration is susceptible to magnetic-breakout eruptions. Previously, we found that stressing the polarity inversion line (PIL) separating the two active regions eventually initiates an eruption that breaks open the central arcade of the combined structure, while stressing the interior PIL of either active region initiates an eruption in a side arcade of the configuration. Stressing both interior PILs of the active regions together raises the possibility of initiating paired sympathetic eruptions, either simultaneous or delayed, in the remote side arcades. We will report our progress on simulating and understanding these more complex scenarios for CME initiation and their observational implications. Our research is supported by NASA. Title: Three-dimensional Modeling of Quasi-homologous Solar Jets Authors: Pariat, E.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2010ApJ...714.1762P Altcode: Recent solar observations (e.g., obtained with Hinode and STEREO) have revealed that coronal jets are a more frequent phenomenon than previously believed. This higher frequency results, in part, from the fact that jets exhibit a homologous behavior: successive jets recur at the same location with similar morphological features. We present the results of three-dimensional (3D) numerical simulations of our model for coronal jets. This study demonstrates the ability of the model to generate recurrent 3D untwisting quasi-homologous jets when a stress is constantly applied at the photospheric boundary. The homology results from the property of the 3D null-point system to relax to a state topologically similar to its initial configuration. In addition, we find two distinct regimes of reconnection in the simulations: an impulsive 3D mode involving a helical rotating current sheet that generates the jet and a quasi-steady mode that occurs in a 2D-like current sheet located along the fan between the sheared spines. We argue that these different regimes can explain the observed link between jets and plumes. Title: A Model for the Sources of the Slow Solar Wind Authors: Antiochos, Spiro K.; Mikic, Z.; Lionello, R.; Titov, V.; Linker, J. Bibcode: 2010AAS...21640521A Altcode: 2010BAAS...41..892A Models for the origin of the slow solar wind must account for two seemingly contradictory observations: The slow wind has the composition of the closed-field corona, implying that it originates at the open-closed field boundary layer, but it also has large angular width, up to 40 degrees. We propose a model that can explain both observations. The key idea is that the source of the slow wind at the Sun is a network of narrow (possibly singular) open-field corridors that map to a web of separatrices and quasi-separatrix layers in the heliosphere. We calculate with high numerical resolution, the quasi-steady solar wind and magnetic field for a Carrington rotation centered about the August 1, 2008 total solar eclipse. Our numerical results demonstrate that, at least for this time period, a web of separatrices (S-web) forms with sufficient density and extent in the heliosphere to account for the observed properties of the slow wind. We discuss the implications of our S-web model for the structure and dynamics of the corona and heliosphere, and propose further tests of the model.

This work was supported, in part, by the NASA HTP, TR&T and SR&T programs. Title: Reconnection Onset in the Breakout Model for CME Initiation Authors: Karpen, Judith T.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2010AAS...21640605K Altcode: 2010BAAS...41..880K Fast coronal mass ejections (CMEs) are the most massive explosions in the heliosphere, and the primary drivers of geoeffective space weather. Although it is generally agreed that magnetic reconnection is the key to fast CME initiation, different models incorporate reconnection in different ways. One promising model --- the breakout scenario --- involves reconnection in two distinct yet interconnected locations: breakout reconnection ahead of the CME, and flare reconnection behind it. We will discuss what we have learned about the early evolution of breakout and flare reconnection from recent high-resolution 2.5D adaptively refined MHD simulations of CME initiation, including the evolving properties of the breakout and flare current sheets, the conditions that trigger reconnection onset in each sheet, the ensuing positive feedback between breakout and flare reconnections, and implications for electron acceleration in flares. Title: Tests of Dynamical Flux Emergence as a Mechanism for CME Initiation Authors: Leake, James E.; Linton, M.; Antiochos, S. Bibcode: 2010AAS...21631402L Altcode: 2010BAAS...41..893L We present an investigation of whether the emergence of sheared flux into the corona can initiate coronal mass ejections (CMEs) in two dimensional geometries. Observations indicate that both flux emergence and sheared magnetic fields are correlated with CME eruptions.

Numerical models of CME eruptions can recreate certain observational features of CMEs using simple two dimensional geometries. These models simply assume the appearance of flux in the corona and do not self-consistently calculate a process for the flux emergence. These simulations do not, therefore, address the significant question of whether and how newly emerged, sheared magnetic flux can rise from its origins in the high beta convection zone to the low corona where it is required to drive CME models.

We study this problem by simulating the dynamical emergence of twisted magnetic flux tubes from a high beta convection zone into the low beta corona in CME eruptive configurations. We focus here on the breakout CME model, which injects sheared flux at a low beta lower boundary. The presence of this shear energy can drive eruptions in quadrupolar coronal configurations. We simulate, in 2D, the emergence of flux tubes with a range of twist and shear profiles into field-free, dipolar and quadrupolar coronas. In all cases we find that the dense plasma which is entrained in the emerging flux ropes and the inability of shear energy to reach sufficient heights in the corona prevent eruptions. CME models becomes significantly more difficult when the self-consistent dynamics of flux emergence are taken into account. These results present strong difficulties for all CME models. We suggest mechanisms for the removal of dense matter from emerging flux tubes which will allow the eruption of coronal magnetic field structures. Title: Interchange Reconnection and Coronal Hole Dynamics Authors: Edmondson, J. K.; Antiochos, S. K.; DeVore, C. R.; Lynch, B. J.; Zurbuchen, T. H. Bibcode: 2010ApJ...714..517E Altcode: We investigate the effect of magnetic reconnection between open and closed fields, often referred to as "interchange" reconnection, on the dynamics and topology of coronal hole boundaries. The most important and most prevalent three-dimensional topology of the interchange process is that of a small-scale bipolar magnetic field interacting with a large-scale background field. We determine the evolution of such a magnetic topology by numerical solution of the fully three-dimensional MHD equations in spherical coordinates. First, we calculate the evolution of a small-scale bipole that initially is completely inside an open field region and then is driven across a coronal hole boundary by photospheric motions. Next the reverse situation is calculated in which the bipole is initially inside the closed region and driven toward the coronal hole boundary. In both cases, we find that the stress imparted by the photospheric motions results in deformation of the separatrix surface between the closed field of the bipole and the background field, leading to rapid current sheet formation and to efficient reconnection. When the bipole is inside the open field region, the reconnection is of the interchange type in that it exchanges open and closed fields. We examine, in detail, the topology of the field as the bipole moves across the coronal hole boundary and find that the field remains well connected throughout this process. Our results, therefore, provide essential support for the quasi-steady models of the open field, because in these models the open and closed flux are assumed to remain topologically distinct as the photosphere evolves. Our results also support the uniqueness hypothesis for open field regions as postulated by Antiochos et al. On the other hand, the results argue against models in which open flux is assumed to diffusively penetrate deeply inside the closed field region under a helmet streamer. We discuss the implications of this work for coronal observations. Title: Can Thermal Nonequilibrium Explain Coronal Loops? Authors: Klimchuk, James A.; Karpen, Judy T.; Antiochos, Spiro K. Bibcode: 2010ApJ...714.1239K Altcode: 2009arXiv0912.0953K Any successful model of coronal loops must explain a number of observed properties. For warm (~1 MK) loops, these include (1) excess density, (2) flat temperature profile, (3) super-hydrostatic scale height, (4) unstructured intensity profile, and (5) 1000-5000 s lifetime. We examine whether thermal nonequilibrium can reproduce the observations by performing hydrodynamic simulations based on steady coronal heating that decreases exponentially with height. We consider both monolithic and multi-stranded loops. The simulations successfully reproduce certain aspects of the observations, including the excess density, but each of them fails in at least one critical way. Monolithic models have far too much intensity structure, while multi-strand models are either too structured or too long-lived. Our results appear to rule out the widespread existence of heating that is both highly concentrated low in the corona and steady or quasi-steady (slowly varying or impulsive with a rapid cadence). Active regions would have a very different appearance if the dominant heating mechanism had these properties. Thermal nonequilibrium may nonetheless play an important role in prominences and catastrophic cooling events (e.g., coronal rain) that occupy a small fraction of the coronal volume. However, apparent inconsistencies between the models and observations of cooling events have yet to be understood. Title: A Numerical Investigation of Unsheared Flux Cancelation Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R.; Linton, M. G. Bibcode: 2010ASSP...19..518K Altcode: 2010mcia.conf..518K Cancelation of magnetic flux in the solar photosphere and chromosphere has been linked observationally and theoretically to a broad range of solar activity phenomena, from filament channel formation to CME initiation. Because cancelation is typically measured at only a single layer in the atmosphere and only in the radial (line of sight) component of the magnetic field, the actual processes behind its observational signature are not fully understood. We have used our 3D MHD code with adaptive mesh refinement, ARMS, to investigate numerically the physics of flux cancelation, beginning with the simplest possible configuration: a subphotospheric Lundquist flux tube surrounded by a potential field in a gravitationally stratified atmosphere. Cancelation is driven by a two-cell circulation pattern imposed in the convection zone, in which the flows converge and form a downdraft at the polarity inversion line (PIL). We present and compare the results of 2D and 3D simulations of cancelation of initially unsheared flux - to our knowledge, these are the first such calculations in which the computational domain extends below the photosphere. The 2D simulation produces a flattened flux rope (plasmoid) whose axis remains centered along the PIL about 1650km above the photosphere, without rising higher into the corona by the end of the run (10,000 s). Our calculations also show that 3D cancelation in an arcade geometry does not produce a fully disconnected flux tube in the corona, in contrast to the 2D results. Rather, most of the reconnected field stays rooted in the photosphere and is gradually submerged by the downdrafts at the PIL. An interchange-like instability develops above the region where the converging flows are driven, breaking the horizontal symmetry along the PIL. This generates an alternating pattern of magnetic shear (magnetic field component aligned with the PIL), which ultimately produces systematic footpoint shuffling through reconnection across the folds of the convoluted PIL. These simulations demonstrate the importance of considering the effects of submergence, as well as the full 3D configuration of the magnetic field and atmosphere, in determining the physical processes behind flux cancelation on the Sun. A paper describing this work has been submitted to the Astrophysical Journal (January 2009). Title: Reconnection-Driven Dynamics of Coronal-Hole Boundaries Authors: Edmondson, J. K.; Lynch, B. J.; Antiochos, S. K.; De Vore, C. R.; Zurbuchen, T. H. Bibcode: 2009ApJ...707.1427E Altcode: We investigate the effect of magnetic reconnection on the boundary between open and closed magnetic field in the solar corona. The magnetic topology for our numerical study consists of a global dipole that gives rise to polar coronal holes and an equatorial streamer belt, and a smaller active-region bipole embedded inside the closed-field streamer belt. The initially potential magnetic field is energized by a rotational motion at the photosphere that slowly twists the embedded-bipole flux. Due to the applied stress, the bipole field expands outward and reconnects with the surrounding closed flux, eventually tunneling through the streamer boundary and encountering the open flux of the coronal hole. The resulting interchange reconnection between closed and open field releases the magnetic twist and free energy trapped inside the bipole onto open field lines, where they freely escape into the heliosphere along with the entrained closed-field plasma. Thereafter, the bipole field relaxes and reconnects back down into the interior of the streamer belt. Our simulation shows that the detailed properties of magnetic reconnection can be crucial to the coronal magnetic topology, which implies that both potential-field source-surface and quasi-steady magnetohydrodynamic models may often be an inadequate description of the corona and solar wind. We discuss the implications of our results for understanding the dynamics of the boundary between open and closed field on the Sun and the origins of the slow wind. Title: The Existence and Origin of Turbulence in Solar Active Regions Authors: Klimchuk, J. A.; Nigro, G.; Dahlburg, R. B.; Antiochos, S. K. Bibcode: 2009AGUFMSM42B..03K Altcode: It has been suggested that turbulence plays a fundamental role in the heating of solar active regions, with its intermittent behavior being the explanation of impulsive energy release (nanoflares). We know that episodes of turbulence are produced in the final nonlinear stage of the secondary instability of electric current sheets. However, these current sheets must exist prior to the turbulence. Whether turbulence can dynamically produce current sheets that would not otherwise be present is a different and important question. Turbulence occurs freely in the solar wind and in other situations where the magnetic field does not dominate. However, the magnetic field strongly resists being distorted in line-tied, low-beta environments such as active regions. Can turbulence develop naturally in these environments without being driven by an instability? To answer this question, we have performed a time-dependent MHD simulation of a slowly driven system that does not contain current sheets and is stable to applied perturbations. We find no evidence for bursty energy release, steep spatial gradients, or power-law power spectra that are the typical signatures of turbulence. We conclude that the turbulence which occurs in active regions is an important yet secondary process and not the primary cause of heating. Title: Investigating the Topology of the “Disconnection” of Coronal Holes Authors: Titov, V. S.; Mikic, Z.; Linker, J. A.; Antiochos, S. K.; Lionello, R. Bibcode: 2009AGUFMSH41B1665T Altcode: Using a potential-field-source-surface approximation, we construct an exact analytical model to describe the intrusion of a magnetic flux spot from the closed-field region into the polar coronal hole (CH). The spot, which has an opposite polarity compared to the surrounding field, moves across a local bulge in the CH, eventually detaching it into a separate minor CH. We show that the formation of a magnetic minimum point, its subsequent degeneration into a null point, and its bifurcation into a pair of nulls, plays a key role in this process. The separatrix field lines that emanate from the nulls form an interface between the open and closed field structures. This implies that the corresponding MHD evolution must involve magnetic reconnection to accommodate the redistribution of their magnetic fluxes. We anticipate that the reconnection outflows along the open part of the separatrix field lines may serve as a source of slow solar wind. Work supported by NASA and the Center for Integrated Space Weather Modeling (an NSF Science and Technology Center). Topological skeleton of the magnetic field in the neighborhood of a detached minor coronal hole; the skeleton consists of separatrix field lines emanating from two magnetic null points. The gray-shaded photospheric distribution of the squashing factor depicts the corresponding footprints of separatrix surfaces and quasi-separatrix layers. Title: Implications of the Deep Minimum for Slow Solar Wind Origin Authors: Antiochos, S. K.; Mikic, Z.; Lionello, R.; Titov, V. S.; Linker, J. A. Bibcode: 2009AGUFMSH11A1502A Altcode: The origin of the slow solar wind has long been one of the most important problems in solar/heliospheric physics. Two observational constraints make this problem especially challenging. First, the slow wind has the composition of the closed-field corona, unlike the fast wind that originates on open field lines. Second, the slow wind has substantial angular extent, of order 30 degrees, which is much larger than the widths observed for streamer stalks or the widths expected theoretically for a dynamic heliospheric current sheet. We propose that the slow wind originates from an intricate network of narrow (possibly singular) open-field corridors that emanate from the polar coronal hole regions. Using topological arguments, we show that these corridors must be ubiquitous in the solar corona. The total solar eclipse in August 2008, near the lowest point of the Deep Minimum, affords an ideal opportunity to test this theory by using the ultra-high resolution Predictive Science's (PSI) eclipse model for the corona and wind. Analysis of the PSI eclipse model demonstrates that the extent and scales of the open-field corridors can account for both the angular width of the slow wind and its closed-field composition. We discuss the implications of our slow wind theory for the structure of the corona and heliosphere at the Deep Minimum and describe further observational and theoretical tests. This work has been supported by the NASA HTP, SR&T, and LWS programs. Title: Fluxon modeling of breakout CMEs Authors: Rachmeler, L. A.; Deforest, C. E.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2009AGUFMSH41B1675R Altcode: The pivotal element of the classic breakout model of CME initiation is reconnection that occurs above inner magnetic field sheared by rotation. We research this model with the FLUX code both with and without reconnection. Without reconnection an eruption occurs after several turns have been injected into the active region. The resultant expansion or eruption is more like a kink-unstable flux rope than a classic breakout CME. By varying whether and where reconnection is allowed, we determine the location of magnetic free energy release in the breakout model. Title: Impulsive Reconnection in the Sun's Atmosphere Authors: Antiochos, Spiro Bibcode: 2009APS..DPPJM9001A Altcode: Recent high-resolution observations from the Hinode mission show dramatically that the Sun's atmosphere is filled with explosive activity ranging from chromospheric explosions that reach heights of Mm, to coronal jets that can extend to solar radii, to giant coronal mass ejections (CME) that reach the edge of the heliosphere. The driver for all this activity is believed to be 3D magnetic reconnection. From the large variation observed in the temporal behavior of solar activity, it is clear that reconnection in the corona must take on a variety of distinct forms. The explosive nature of jets and CMEs requires that the reconnection be impulsive in that it stays off until a substantial store of free energy has been accumulated, but then turns on abruptly and stays on until much of this free energy is released. The key question, therefore, is what determines whether the reconnection is impulsive or not. We present some of the latest observations and numerical models of explosive and non-explosive solar activity. We argue that, in order for the reconnection to be impulsive, it must be driven by a quasi-ideal instability. We discuss the generality of our results for understanding 3D reconnection in other contexts. Title: Observational Signatures of Reconnection-Driven Changes in the Open-Closed Magnetic Field Boundary Authors: Lynch, Benjamin J.; Edmondson, J. K.; Li, Y.; Luhmann, J. G.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2009shin.confE.134L Altcode: We will present some recent MHD modeling results of small scale transient phenomena in the solar corona. We start with a multipolar magnetic topology which is the simplest non-trivial field distribution in 3D, resulting from essentially a pair of magnetic dipole sources, and has sufficient complexity to give rise to a wide variety of transient phenomena for different specific boundary flows and flux distributions. We slowly energize the fields associated with a bipole near the boundary of the coronal helmet streamer belt and observe the interaction between the flux systems and the open and closed field boundary. During the resulting interchange reconnection magnetic fieldline connectivity can change drastically with very little associated energy release. We present these results in the context of the Antiochos et al (2007) conjecture of narrow channels of open fields, and show that in our MHD simulations, opening some amount of field associated with the active region flux system necessarily opens such a channel defined by the separatrix boundary. Additionally, we examine some of consequences and potentially observable signatures of these small scale transient openings and compare them to well known coronal signatures such as Title: Current Sheet Formation, Stability, and Reconnection Dynamics in MHD Authors: Edmondson, Justin K.; Antiochos, Spiro K.; DeVore, C. Richard; Zurbuchen, Thomas H. Bibcode: 2009shin.confE.188E Altcode: Current sheet formation is a necessary consequence of the evolution of the multi-polar magnetic field topologies that are ubiquitous throughout the solar corona. We present a very high-resolution study of 3D MHD current sheet formation and the resulting reconnection dynamics in an environment appropriate for the corona. The initial field consists of a translationally invariant, potential field with a null-point topology (i.e., 4-flux systems) and a low-beta plasma. A finite-extent, 3D Syrovatskii-type current sheet forms as a result of stressing of this system by a uniform, incompressible flow applied at the line-tied photospheric boundary. The system is assumed to be ideal, except for the presence of numerical resistivity. The fully 3-D evolution is calculated with very high resolution (9x refinement across the full extent of the current sheet) using the Adaptively Refined MHD Solver (ARMS). The initial evolution of this computationally-intensive simulation results in a current sheet with a nearly 40-to-1 aspect ratio, a significant fraction of the system characteristic length, that unexpectedly appears to be stable. In addition, up to this point in the evolution any magnetic reconnection that we observe is of the slow Sweet-Parker type. We expect, however, that as we continue stressing the field, the current sheet will become unstable and develop explosive dynamics. Title: Do Closed Field Regions Contribute Plasma to the Slow Solar Wind? Authors: Linker, Jon A.; Lionello, Roberto; Mikic, Zoran; Titov, Viacheslav S.; Antiochos, Spiro Bibcode: 2009shin.confE.140L Altcode: Composition differences between the fast and slow solar wind suggest that the slow solar wind plasma has a different origin than the fast wind. A natural way that a bifurcation in the plasma properties could arise is if the slow wind plasma originates from previously closed field regions in the corona. I this talk I will discuss arguments both for and against this idea, and I will illustrate mechanisms by which the streamer belt can be opened as part of the slow evolution of the corona. Title: Small Bipoles Interacting with a Coronal Hole: MHD Simulations Authors: Lionello, Roberto; Linker, Jon A.; Mikic, Zoran; Titov, Viacheslav S.; Antiochos, Spiro Bibcode: 2009shin.confE.128L Altcode: Coronal holes are known to be the source of the fast wind and are also believed to play a key role in the formation of the slow wind; consequently, their evolution is critical for understanding how the heliospheric magnetic field connects to the Sun. In the context of field reversal, the Fisk model postulates that open flux can be transported out of coronal holes into the closed field region through interchange reconnection with small loops associated with parasitic polarities. This scenario is supported by in-situ observations, which seem to favor interchange reconnection as the only mechanism responsible for field reversal. However, it is hard to reconcile with theoretical results on the topology of coronal holes. To determine the feasibility of this mechanism, we have used our 3D MHD algorithm in spherical coordinates to study the interaction of the magnetic field of two bipoles with a coronal hole. The model uses a polytropic treatment for the energy equation and includes a self-consistent solar wind. We have prescribed as magnetic flux distribution at the lower boundary, a smoothed Kitt Peak magnetogram for Carrington Rotation 1913 (late August 1996), to which we have added two small bipoles. After reaching a relaxed state with well-defined coronal holes and a close field region inside a helmet streamer, we have introduced surface flows, which evolve the magnetic flux distribution at the boundary. We have investigated the reconfiguration of the coronal fields in response to these motions; in particular we show what happens to the open flux in the system as the bipoles move from the coronal holes into the closed field region. We have found no evidence that open flux can be injected into closed-field regions. Portions of coronal holes that may appear to have been detached are actually still connected to the main coronal hole through zero-width corridors. We conclude that interchange reconnection, by itself, does not produce the open-closed field mixture postulated by the Fisk model. On the other hand, the magnetic topology of the coronal hole boundary becomes so complex that some of the essential features of the model, in particular the open field diffusion, may prove to be an effective approximation for capturing the magnetic dynamics. Title: Rotation of Coronal Mass Ejections during Eruption Authors: Lynch, B. J.; Antiochos, S. K.; Li, Y.; Luhmann, J. G.; DeVore, C. R. Bibcode: 2009ApJ...697.1918L Altcode: Understanding the connection between coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs) is one of the most important problems in solar-terrestrial physics. We calculate the rotation of erupting field structures predicted by numerical simulations of CME initiation via the magnetic breakout model. In this model, the initial potential magnetic field has a multipolar topology and the system is driven by imposing a shear flow at the photospheric boundary. Our results yield insight on how to connect solar observations of the orientation of the filament or polarity inversion line (PIL) in the CME source region, the orientation of the CME axis as inferred from coronagraph images, and the ICME flux rope orientation obtained from in situ measurements. We present the results of two numerical simulations that differ only in the direction of the applied shearing motions (i.e., the handedness of the sheared-arcade systems and their resulting CME fields). In both simulations, eruptive flare reconnection occurs underneath the rapidly expanding sheared fields transforming the ejecta fields into three-dimensional flux rope structures. As the erupting flux ropes propagate through the low corona (from 2 to 4 R sun) the right-handed breakout flux rope rotates clockwise and the left-handed breakout flux rope rotates counterclockwise, in agreement with recent observations of the rotation of erupting filaments. We find that by 3.5 R sun the average rotation angle between the flux rope axes and the active region PIL is approximately 50°. We discuss the implications of these results for predicting, from the observed chirality of the pre-eruption filament and/or other properties of the CME source region, the direction and amount of rotation that magnetic flux rope structures will experience during eruption. We also discuss the implications of our results for CME initiation models. Title: Current Sheet Formation and Reconnection Dynamics in the Solar Corona Authors: Edmondson, Justin K.; Antiochos, S. K.; DeVore, C.; Zurbuchen, T. H. Bibcode: 2009SPD....40.1305E Altcode: Current sheet formation is a necessary consequence of the evolution of the multi-polar magnetic field topologies that are ubiquitous throughout the solar corona. We present a very high-resolution study of 3D MHD current sheet formation and the resulting reconnection dynamics in an environment appropriate for the corona. The initial field consists of a translationally invariant, potential field with a null-point topology (i.e., 4-flux systems) and a low-beta plasma. A finite-extent, 3D Syrovatskii-type current sheet forms as a result of stressing of this system by a uniform, incompressible flow applied at the line-tied photospheric boundary. The system is assumed to be ideal, except for the presence of numerical resistivity. The fully 3-D evolution is calculated with very high resolution (9x and 10x refinement across the full extent of the current sheet) using the Adaptively Refined MHD Solver (ARMS). The initial evolution of this computationally-intensive simulation results in a current sheet with a nearly 30-to-1 aspect ratio, a significant fraction of the system characteristic length, that unexpectedly appears to be stable. In addition, up to this point in the evolution any magnetic reconnection that we observe is of the slow Sweet-Parker type. We expect, however, that as we continue stressing the field, the current sheet will become unstable and develop explosive dynamics. We discuss the implications of our results on coronal structure and activity, such as heating and eruptions.

This work has been supported, in part, by the NASA HTP and SR&T programs. Title: Generation of Homologous Coronal Jets Authors: Pariat, Etienne; Antiochos, S. K.; DeVore, C. R. Bibcode: 2009SPD....40.3201P Altcode: Recent solar observations (e.g. Hinode & STEREO) have revealed that coronal jets are a more frequent phenomenon than previously believed. This higher frequency results, in part, from the fact that jets exhibit a homologous behavior; successive jets re-occur at the same location.

We present the results of 3D numerical simulations of our model for coronal jets. The simulations were performed with our state-of-art adaptive mesh MHD solver ARMS. The basic idea of the model is that a jet is due to the release of twist as a closed field region undergoes interchange reconnection with surrounding open field. If a stress is constantly applied at the photospheric boundary we demonstrate that our model of jets is able to reproduce the observed homologous property. In addition, we find that two regimes of reconnection can occur in the simulations. This result has important implications for the observed link between jets and plumes.

This work was supported by the NASA Theory and SR&T Programs. Title: Current Sheet Formation and Reconnection Dynamics in the Solar Corona Authors: Edmondson, Justin K.; Antiochos, S. K.; DeVore, C.; Zurbuchen, T. H. Bibcode: 2009SPD....40.1401E Altcode: Current sheet formation is a necessary consequence of the evolution of the multi-polar magnetic field topologies that are ubiquitous throughout the solar corona. We present a very high-resolution study of 3D MHD current sheet formation and the resulting reconnection dynamics in an environment appropriate for the corona. The initial field consists of a translationally invariant, potential field with a null-point topology (i.e., 4-flux systems) and a low-beta plasma. A finite-extent, 3D Syrovatskii-type current sheet forms as a result of stressing of this system by a uniform, incompressible flow applied at the line-tied photospheric boundary. The system is assumed to be ideal, except for the presence of numerical resistivity. The fully 3-D evolution is calculated with very high resolution (9x and 10x refinement across the full extent of the current sheet) using the Adaptively Refined MHD Solver (ARMS). The initial evolution of this computationally-intensive simulation results in a current sheet with a nearly 30-to-1 aspect ratio, a significant fraction of the system characteristic length, that unexpectedly appears to be stable. In addition, up to this point in the evolution any magnetic reconnection that we observe is of the slow Sweet-Parker type. We expect, however, that as we continue stressing the field, the current sheet will become unstable and develop explosive dynamics. We discuss the implications of our results on coronal structure and activity, such as heating and eruptions.

This work has been supported, in part, by the NASA HTP and SR&T programs. Title: Simulations of Flare Reconnection in Breakout Coronal Mass Ejections Authors: DeVore, C. Richard; Karpen, J. T.; Antiochos, S. K. Bibcode: 2009SPD....40.2007D Altcode: We report 3D MHD simulations of the flare reconnection in the corona below breakout coronal mass ejections (CMEs). The initial setup is a single bipolar active region imbedded in the global-scale background dipolar field of the Sun, forming a quadrupolar magnetic configuration with a coronal null point. Rotational motions applied to the active-region polarities at the base of the atmosphere introduce shear across the polarity inversion line (PIL). Eventually, the magnetic stress and energy reach the critical threshold for runaway breakout reconnection, at which point the sheared core field erupts outward at high speed. The vertical current sheet formed by the stretching of the departing sheared field suffers reconnection that reforms the initial low-lying arcade across the PIL, i.e., creates the flare loops. Our simulation model, the Adaptively Refined MHD Solver, exploits local grid refinement to resolve the detailed structure and evolution of the highly dynamic current sheet. We are analyzing the numerical experiments to identify and interpret observable signatures of the flare reconnection associated with CMEs, e.g., the flare loops and ribbons, coronal jets and shock waves, and possible origins of solar energetic particles.

This research was supported by NASA and ONR. Title: 2D and 3D Numerical Simulations of Flux Cancellation Authors: Karpen, Judith T.; DeVore, C.; Antiochos, S. K.; Linton, M. G. Bibcode: 2009SPD....40.0902K Altcode: Cancellation of magnetic flux in the solar photosphere and chromosphere has been linked observationally and theoretically to a broad range of solar activity, from filament channel formation to CME initiation. Because this phenomenon is typically measured at only a single layer in the atmosphere, in the radial (line of sight) component of the magnetic field, the actual processes behind this observational signature are ambiguous. It is clear that reconnection is involved in some way, but the location of the reconnection sites and associated connectivity changes remain uncertain in most cases. We are using numerical modeling to demystify flux cancellation, beginning with the simplest possible configuration: a subphotospheric Lundquist flux tube surrounded by a potential field, immersed in a gravitationally stratified atmosphere, spanning many orders of magnitude in plasma beta. In this system, cancellation is driven slowly by a 2-cell circulation pattern imposed in the convection zone, such that the tops of the cells are located around the beta=1 level (i.e., the photosphere) and the flows converge and form a downdraft at the polarity inversion line; note however that no flow is imposed along the neutral line. We will present the results of 2D and 3D MHD-AMR simulations of flux cancellation, in which the flux at the photosphere begins in either an unsheared or sheared state. In all cases, a low-lying flux rope is formed by reconnection at the polarity inversion line within a few thousand seconds. The flux rope remains stable and does not rise, however, in contrast to models which do not include the presence of significant mass loading. Title: On The 3D Structure of the Pre- and After CME Coronal Streamer Belt Authors: Kramar, Maxim; Davila, J.; Xie, H.; Antiochos, S. Bibcode: 2009SPD....40.2211K Altcode: We select several CME events and reconstruct the 3D coronal streamer belt configurations for the periods of time before and after the corresponded CMEs. The reconstructions are based on STEREO COR1 observations and made by using a regularized tomography technique (Kramar et al. 2009). For some CME we found noticeable changes in the streamer belt structure. Particularly, for a slow CME on June 1, 2008 (Robbrecht et al. 2009) we found that for a longitudinal range of about 10 degrees in Carrington longitude centered at the CME location, the height of the overlying streamer belt was significantly reduced. This reduction in height persisted for at least 14 days. The reconstruction of the streamer belt before and after the CME allows direct estimate of the mass lost. Also it was found that positions of this and some others CME correspond to regions in the streamer belt where the latter has a double structure (i.e. splitted into two parts). Title: The Origins of Magnetic Structure in the Corona and Wind Authors: Antiochos, Spiro K. Bibcode: 2009SPD....40.3104A Altcode: One of the most important and most puzzling features of the coronal magnetic field is that it appears to have smooth magnetic structure with little evidence for non-potentiality except at two special locations: photospheric polarity inversions lines, (non-potentiality observed as a filament channel) and coronal hole boundaries, (observed as the slow solar wind). This characteristic feature of the closed-field corona is highly unexpected given that its magnetic field is continuously tangled by photospheric motions. Although reconnection can eliminate some of the injected structure, it cannot destroy the helicity, which should build up to produce observable complexity. I propose that an inverse cascade process transports the injected helicity from the interior of closed flux regions to their boundaries, inversion lines and coronal holes, creating both filament channels and the slow wind. We describe how the helicity is injected and transported and calculate the relevant rates. I argue that one process, helicity transport, can explain both the observed lack and presence of structure in the coronal magnetic field.

This work has been supported by the NASA HTP, SR&T, and LWS programs. Title: A Model for Solar Polar Jets Authors: Pariat, E.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2009ApJ...691...61P Altcode: We propose a model for the jetting activity that is commonly observed in the Sun's corona, especially in the open-field regions of polar coronal holes. Magnetic reconnection is the process driving the jets and a relevant magnetic configuration is the well known null-point and fan-separatrix topology. The primary challenge in explaining the observations is that reconnection must occur in a short-duration energetic burst, rather than quasi-continuously as is implied by the observations of long-lived structures in coronal holes, such as polar plumes. The key idea underlying our model for jets is that reconnection is forbidden for an axisymmetrical null-point topology. Consequently, by imposing a twisting motion that maintains the axisymmetry, magnetic stress can be built up to high levels until an ideal instability breaks the symmetry and leads to an explosive release of energy via reconnection. Using three-dimensional magnetohydrodynamic simulations, we demonstrate that this mechanism does produce massive, high-speed jets driven by nonlinear Alfvén waves. We discuss the implications of our results for observations of the solar corona. Title: Reconnection-Driven Changes of the Open-Closed Coronal Magnetic Field Boundary Authors: Lynch, B. J.; Edmondson, J. K.; Li, Y.; Luhmann, J. G.; Antiochos, S. K.; DeVore, C. R.; Zurbuchen, T. H. Bibcode: 2008AGUFMSH51A1598L Altcode: We present recent 3D MHD simulation results with the ARMS code that show the dynamic evolution of the coronal helmet streamer belt boundary via magnetic reconnection processes. We start with an initial multipolar PFSS configuration with a coronal null point and associated topological features located under the streamer belt, well within the closed-field region. As this configuration is slowly energized via rotational shearing flows, volumetric currents form along the separatrix boundary and during the subsequent field evolution are compressed to a thin current sheet until the numerical resistivity mimics the onset of magnetic reconnection. The reconnection scenario is analogous to the coronal breakout-reconnection in CME modeling, but here, once the closed streamer belt flux has been transferred out of the way there is interchange reconnection that opens the outer-spine fieldline and creates a narrow channel of open field surrounding the AR sepratrix boundary. This topological evolution is best understood in the context of the Antiochos (2007) conjecture about the existence of "coronal hole canals". In addition, we will discuss the implication of the MHD simulation results for creating slow, unstructured, streamer blob-like transients and as one of the potential mechanisms operating at the open-closed magnetic field boundary that could lead to the formation of the slow solar wind. This work is supported, in part, by NSF ATM-0621725 and NASA NNX08AJ04G. Title: Topological Origins of the Slow Solar Wind Authors: Antiochos, S. Bibcode: 2008AGUFMSH43B..07A Altcode: Although the slow solar wind has been studied for decades with both in situ and remote sensing observations, its origin is still a matter of intense debate. In the standard quasi-steady model, the slow wind is postulated to originate near coronal hole boundaries that define topologically well-behaved separatrices between open and closed field regions. In the interchange model, on the other hand, the slow wind is postulated to originate on open flux that is dynamically diffusing throughout the seemingly closed-field corona. We argue in favor of the quasi-steady scenario and propose that the slow wind is due to two effects: First, the open-closed boundary is highly complex due to the complexity of the photospheric flux distribution. Second, this boundary is continuously driven by the transport of magnetic helicity from the closed field region into the open. The implications of this model for the structure and dynamics of the corona and slow wind are discussed, and observational tests of the model are presented. This work has been supported, in part, by the NASA LWS, HTP, and SR&T programs. Title: Dynamic Instability Leading to Increased Interchange Reconnection Rates Authors: Edmondson, J. K.; Antiochos, S. K.; Zurbuchen, T. H. Bibcode: 2008AGUFMSH51B1605E Altcode: Interchange reconnection is widely believed to play an important role in coronal magnetic field dynamics. In this investigation we investigate the 3D dynamics of interchange reconnection by extending the concept of a magnetic null-point to a null-volume, the so-called "acute-cusp field" configuration. The acute-cusp field geometry is characterized by high-beta plasma confined with favorable curvature, surrounded by a low-beta environment. First, we construct an initial translationally-symmetric potential field configuration. This configuration contains the required topological characteristics of four separate flux systems in the perpendicular plane. We then drive the system by a slow, incompressible, uniform flow at the boundary. The resulting evolution is calculated by solving numerically the MHD equations in full 3D Cartesian coordinates using the Adaptively Refined MHD Solver developed at the U.S. Naval Research Laboratory. Field shearing along the topological boundaries changes the shape of the acute-cusp field surface separating the high and low plasma beta regions. An extended, 2D current sheet is generated by the photospheric driving. We discuss the effect of 3D perturbations on the current sheet dynamics and on the rate of the resulting interchange reconnection. Finally, we discuss the implications of our simulations for coronal observations. This work has been supported, in part, by the NASA HTP and SR&T programs. Title: Topologically driven coronal dynamics - a mechanism for coronal hole jets Authors: Müller, D. A. N.; Antiochos, S. K. Bibcode: 2008AnGeo..26.2967M Altcode: Bald patches are magnetic topologies in which the magnetic field is concave up over part of a photospheric polarity inversion line. A bald patch topology is believed to be the essential ingredient for filament channels and is often found in extrapolations of the observed photospheric field. Using an analytic source-surface model to calculate the magnetic topology of a small bipolar region embedded in a global magnetic dipole field, we demonstrate that although common in closed-field regions close to the solar equator, bald patches are unlikely to occur in the open-field topology of a coronal hole. Our results give rise to the following question: What happens to a bald patch topology when the surrounding field lines open up? This would be the case when a bald patch moves into a coronal hole, or when a coronal hole forms in an area that encompasses a bald patch. Our magnetostatic models show that, in this case, the bald patch topology almost invariably transforms into a null point topology with a spine and a fan. We argue that the time-dependent evolution of this scenario will be very dynamic since the change from a bald patch to null point topology cannot occur via a simple ideal evolution in the corona. We discuss the implications of these findings for recent Hinode XRT observations of coronal hole jets and give an outline of planned time-dependent 3-D MHD simulations to fully assess this scenario. Title: 3D Numerical Simulation of a New Model for Coronal Jets Authors: Pariat, E.; Antiochos, S.; DeVore, C. R.; Patsourakos, S. Bibcode: 2008ESPM...12.3.28P Altcode: Recent solar observations with STEREO and HINODE have revealed evidence of twisting motions during the evolution of coronal jets. Furthermore, the observations indicate that some jets achieve near-Alfvenic velocities. Most models of jet are not capable of explaining these new observational features. In addition, the impulsiveness of jets, manifested as a brief, violent energy release phase in contrast to a slow, quasi-static energy storage phase storage, is an issue not easily addressed.

We will present the results of 3D numerical simulations of our model for coronal jets. The simulations were performed with our state-of-art adaptive mesh MHD solver ARMS. The basic idea of the model is that a jet is due to the release of magnetic twist when a closed field region undergoes interchange reconnection with surrounding open field. The fast reconnection between open and closed field results in the generation of nonlinear Alfven waves that propagate along the open field, accelerating plasma upward. We will show how the new stereoscopically-observed features of jets can be explained by the results of our numerical simulations Title: Topological Evolution of a Fast Magnetic Breakout CME in Three Dimensions Authors: Lynch, B. J.; Antiochos, S. K.; DeVore, C. R.; Luhmann, J. G.; Zurbuchen, T. H. Bibcode: 2008ApJ...683.1192L Altcode: We present the extension of the magnetic breakout model for CME initiation to a fully three-dimensional, spherical geometry. Given the increased complexity of the dynamic magnetic field interactions in three dimensions, we first present a summary of the well known axisymmetric breakout scenario in terms of the topological evolution associated with the various phases of the eruptive process. In this context, we discuss the analogous topological evolution during the magnetic breakout CME initiation process in the simplest three-dimensional multipolar system. We show that an extended bipolar active region embedded in an oppositely directed background dipole field has all the necessary topological features required for magnetic breakout, i.e., a fan separatrix surface between the two distinct flux systems, a pair of spine field lines, and a true three-dimensional coronal null point at their intersection. We then present the results of a numerical MHD simulation of this three-dimensional system where boundary shearing flows introduce free magnetic energy, eventually leading to a fast magnetic breakout CME. The eruptive flare reconnection facilitates the rapid conversion of this stored free magnetic energy into kinetic energy and the associated acceleration causes the erupting field and plasma structure to reach an asymptotic eruption velocity of gtrsim1100 km s-1 over an ~15 minute time period. The simulation results are discussed using the topological insight developed to interpret the various phases of the eruption and the complex, dynamic, and interacting magnetic field structures. Title: STEREO SECCHI Stereoscopic Observations Constraining the Initiation of Polar Coronal Jets Authors: Patsourakos, S.; Pariat, E.; Vourlidas, A.; Antiochos, S. K.; Wuelser, J. P. Bibcode: 2008ApJ...680L..73P Altcode: 2008arXiv0804.4862P We report on the first stereoscopic observations of polar coronal jets made by the EUVI/SECCHI imagers on board the twin STEREO spacecraft. The significantly separated viewpoints (~11°) allowed us to infer the 3D dynamics and morphology of a well-defined EUV coronal jet for the first time. Triangulations of the jet's location in simultaneous image pairs led to the true 3D position and thereby its kinematics. Initially the jet ascends slowly at ≈10-20 km s-1 and then, after an apparent "jump" takes place, it accelerates impulsively to velocities exceeding 300 km s-1 with accelerations exceeding the solar gravity. Helical structure is the most important geometrical feature of the jet which shows evidence of untwisting. The jet structure appears strikingly different from each of the two STEREO viewpoints: face-on in one viewpoint and edge-on in the other. This provides conclusive evidence that the observed helical structure is real and does not result from possible projection effects of single-viewpoint observations. The clear demonstration of twisted structure in polar jets compares favorably with synthetic images from a recent MHD simulation of jets invoking magnetic untwisting as their driving mechanism. Therefore, the latter can be considered as a viable mechanism for the initiation of polar jets. Title: Homologous Confined Filament Eruptions via Magnetic Breakout Authors: DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2008ApJ...680..740D Altcode: We describe magnetohydrodynamic simulations of a bipolar active region embedded in the Sun's global background field and subjected to twisting footpoint displacements concentrated near its polarity inversion lines to produce strong magnetic shear. The dipole moments of the active region and background field are antiparallel, so that the initially potential magnetic field contains a coronal null. This configuration supports magnetic breakout eruptions in our simulations that exhibit three novel features. First, the eruptions are multiple and homologous: the flare reconnection following each eruption reforms the magnetic null, setting the stage for a subsequent episode of breakout reconnection and eruption driven by the ongoing footpoint motions. Second, the eruptions are confined; that is, their rapidly rising, moderately sheared field lines do not escape the Sun but instead come to rest in the outer corona, comprising a large coronal loop formed by reconnection during the rise phase. Third, the most strongly sheared field lines of the active region are quite flat prior to eruption, expand upward sharply during the event, and lose most of their shear through reconnection with overlying flux, while lower lying field lines survive the eruption and recover their flat configuration within a few hours. These behaviors are consistent with filament disappearance followed by reformation in place. We also find that the upward motion of the erupting sheared flux exhibits a distinct three-phase acceleration profile. All of these features of our simulations—homology, confinement, reformation, and multiphase acceleration—are well established aspects of solar eruptions. Title: Coronal Heating and Structure Authors: Antiochos, S. K. Bibcode: 2008AGUSMSP33A..03A Altcode: The existence of the Sun's million-degree corona is one of the oldest and most challenging problems in all space physics. It is generally accepted that the solar magnetic field is responsible for both the heating and the structure of coronal plasma, but the physical mechanisms are still not clearly understood. Gene Parker has made many seminal contributions to solving the coronal heating problem, in particular, his widely-used nano-flare model. Parker argued that in closed field regions the complex motions of the photosphere must lead to the formation of fine-scale electric currents in the corona and, consequently, to continual bursts of magnetic reconnection. We discuss the implications of these ideas for understanding the observed features of the corona. We show that the type of reconnection proposed by Parker may well account for all the well-known observations of both the closed and open field corona, and we discuss the implications of our results for upcoming NASA missions. This work was supported by the NASA HTP and TR&T programs. Title: On the Rotation of "Flux Rope" CMEs During Eruption Authors: Lynch, B. J.; Li, Y.; Luhmann, J. G.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2008AGUSMSP24A..04L Altcode: We present an analysis of the dynamics of 3-dimensional sheared arcade magnetic breakout eruptions that become flux rope CMEs during the eruptive flare reconnection process. We compare the results of two otherwise identical numerical simulations, differing only in the direction of the applied shearing motions (i.e. the handedness of the sheared arcade systems and their resulting CME fields). The right-handed flux rope rotates clockwise and the left-handed flux rope rotates counter-clockwise as they propagate through the low-corona from 2-4 Rs. We characterize the average rotation angle of their respective axes relative to their active region neutral lines and discuss the forces acting on the erupting structures. The simulation results are then placed in the context of recent observational work linking the CME source region neutral line orientation, the overlying helmet streamer belt orientation, flux-rope orientation inferred from coronagraph halo-CME observations, and 1 AU measurements of the associated magnetic cloud orientation. BJL acknowledges NSF SHINE grant ATM-0621725. Title: 3D Numerical Simulation and Stereoscopic Observations of Coronal Jets Authors: Pariat, E.; Antiochos, S. K.; Patsourakos, S.; DeVore, C. R. Bibcode: 2008AGUSMSP53A..05P Altcode: Recent solar observations have revealed that coronal jets are a more frequent phenomenon than previously believed. It is widely accepted that magnetic reconnection is the fundamental mechanism that gives rise to the jets. The improved spatial and temporal resolution of the STEREO observations in combination with stereoscopy yields new insights into the origins of coronal jets, and provides detailed data that can be used to test and refine models. We present the results of 3D numerical simulations of our model for coronal jets. The simulations were performed with our state-of-art adaptive mesh MHD solver ARMS. The basic idea of the model is that a jet is due to the release of twist as a closed field region undergoes interchange reconnection with surrounding open field. The photospheric driven evolution of the structure results in the generation of a non linear Alfven wave along the open fields. Using stereoscopic EUVI images, we reveal the presence of such twisted structure in a coronal jet event. This work was supported, in part, by NASA and ONR. Title: Understanding the Initiation of Polar Coronal Jets with STEREO/SECCHI Stereoscopic Observations Authors: Vourlidas, A.; Patsourakos, S.; Pariat, E.; Antiochos, S. Bibcode: 2008AGUSMSH23A..02V Altcode: Polar coronal jets are collimated transient ejections of plasma occurring in polar coronal holes. The kinematics and mostly the 3D morphology of jets place strong constraints on the physical mechanism(s) responsible for their initiation, and were not accessible before the STEREO mission. We report on the first stereoscopic observations of polar coronal jets made by the EUVI/SECCHI imagers on-board the twin STEREO spacecraft at spacecraft separations of ~ 11° and ~ 45°. Triangulations of the jet locations in simultaneous image pairs led to the true 3D position and thereby their kinematics. The most important geometrical feature of the observed jets is helical structures showing evidence of untwisting. The jet structure appear strikingly different from each of the two STEREO viewpoints: face-on in the one viewpoint and edge-on in the other. This provides solid evidence that the observed helical structure is real and not resulting from possible projection effects of single viewpoint observations. The clear demonstration of twisted structure in polar jets compares favorably with synthetic images from a recent MHD simulation of jets invoking magnetic untwisting as their driving mechanism. Title: Simulated Coronal Mass Ejections Originating in Complex Source Regions Authors: DeVore, C.; Antiochos, S. K. Bibcode: 2008AGUSMSP24A..03D Altcode: Previously, we have investigated numerically the initiation of coronal mass ejections (CMEs) in the simplest possible solar sources: a single bipolar active region embedded in the Sun's global background field. If the overall topology is more complex than bipolar, with a magnetic null and separatrices dividing the coronal field into two or more flux systems, then the configuration is susceptible to magnetic-breakout eruptions. Breakout reconnection across the null allows the overlying field to be pushed aside, after which the highly stressed field below undergoes a fast, free, ideal expansion into the outer corona and heliosphere. We are now beginning to address more complex scenarios in which two bipolar active regions, side by side, spawn breakout CMEs. A CME originating at the polarity inversion line (PIL) separating the two active regions breaks open the central arcade of the combined configuration, and is analogous to an eruption occurring at the interior PIL of a single active region embedded in a background field. We also are investigating single CMEs originating at the interior PIL of either active region in the new scenario, i.e., in either side arcade of the configuration. Furthermore, stressing both interior PILs of the active regions contemporaneously gives rise to the possibility of paired sympathetic eruptions, either simultaneous or delayed, originating in the two distant side arcades. We will report our progress on simulating and understanding these more complex scenarios for CME initiation. NASA and ONR support our research. Title: Modeling Coronal Jets with FLUX Authors: Rachmeler, L. A.; Pariat, E.; Antiochos, S. K.; Deforest, C. E. Bibcode: 2008AGUSMSP43B..01R Altcode: We report on a comparative study of coronal jet formation with and without reconnection using two different simulation strategies. Coronal jets are features on the solar surface that appear to have some properties in common with coronal mass ejections, but are less energetic, massive, and broad. Magnetic free energy is built up over time and then suddenly released, which accelerates plasma outward in the form of a coronal jet. We compare results from the ARMS adaptive mesh and FLUX reconnection-less codes to study the role of reconnection in this system. This is the first direct comparison between FLUX and a numerical model with a 3D spatial grid. Title: A Mechanism for Coronal Hole Jets Authors: Mueller, D. A. N.; Antiochos, S. K. Bibcode: 2008arXiv0804.3995M Altcode: Bald patches are magnetic topologies in which the magnetic field is concave up over part of a photospheric polarity inversion line. A bald patch topology is believed to be the essential ingredient for filament channels and is often found in extrapolations of the observed photospheric field. Using an analytic source-surface model to calculate the magnetic topology of a small bipolar region embedded in a global magnetic dipole field, we demonstrate that although common in closed-field regions close to the solar equator, bald patches are unlikely to occur in the open-field topology of a coronal hole. Our results give rise to the following question: What happens to a bald patch topology when the surrounding field lines open up? This would be the case when a bald patch moves into a coronal hole, or when a coronal hole forms in an area that encompasses a bald patch. Our magnetostatic models show that, in this case, the bald patch topology almost invariably transforms into a null point topology with a spine and a fan. We argue that the time-dependent evolution of this scenario will be very dynamic since the change from a bald patch to null point topology cannot occur via a simple ideal evolution in the corona. We discuss the implications of these findings for recent Hinode XRT observations of coronal hole jets and give an outline of planned time-dependent 3D MHD simulations to fully assess this scenario. Title: Condensation Formation by Impulsive Heating in Prominences Authors: Karpen, J. T.; Antiochos, S. K. Bibcode: 2008ApJ...676..658K Altcode: Our thermal nonequilibrium model for prominence formation provides an explanation for the well-observed presence of predominantly dynamic, cool, dense material suspended in the corona above filament channels. According to this model, condensations form readily along long, low-lying magnetic field lines when heating is localized near the chromosphere. Often this process yields a dynamic cycle in which condensations repeatedly form, stream along the field, and ultimately disappear by falling onto the nearest footpoint. Our previous studies employed only steady heating, as is consistent with some coronal observations, but many coronal heating models predict transient episodes of localized energy release (e.g., nanoflares). Here we present the results of a numerical investigation of impulsive heating in a model prominence flux tube and compare the outcome with previous steady-heating simulations. We find that condensations form readily when the average interval between heating events is less than the coronal radiative cooling time (~2000 s). As the average interval between pulses decreases, the plasma evolution more closely resembles the steady-heating case. The heating scale and presence or absence of background heating also determine whether or not condensations form and how they evolve. Our results place important constraints on coronal heating in filament channels and strengthen the case for thermal nonequilibrium as the process responsible for the plasma structure in prominences. Title: Comparison of Heliospheric In Situ Data with the Quasi-steady Solar Wind Models Authors: Lepri, S. T.; Antiochos, S. K.; Riley, P.; Zhao, L.; Zurbuchen, T. H. Bibcode: 2008ApJ...674.1158L Altcode: The standard theory for the solar-heliospheric magnetic field is the so-called quasi-steady model in which the field is determined by the observed magnetic flux at the photosphere and the balance between magnetic and plasma forces in the corona. In this model, the solar magnetic flux that opens to the heliosphere can increase or decrease as the photospheric flux evolves. The most sophisticated implementation of the quasi-steady theory is the SAIC model, which solves the fully time-dependent 3D MHD equations for the corona and wind until a steady state is achieved. In order to test the quasi-steady theory, we compare the 3D MHD model with observations of the heliospheric flux using multipoint measurements from the VHM instrument on the Ulysses spacecraft from 1991 to 2005 and from magnetic field measurements from various spacecraft at L1 compiled into the OMNI data set from 1976 through 2005. We also compare the observations to the predictions of the potential-field source-surface model, an older and simpler implementation of the quasi-steady theory. During solar maximum, ICMEs significantly disturb the heliospheric magnetic field, making our comparisons difficult. We find that the MHD model compares well with the general trends of the observed heliospheric fluxes. Variations on short timescales, presumably due to local effects, are missed by the model, but the long-term evolution is well matched. The model disagrees with observations most when Ulysses is in slow wind or ICME-related flows. The model underestimates the flux at solar maximum; however, this is to be expected, given the large number of ICMEs in the heliosphere at this time. We discuss the possible sources of discrepancy between the observations and the quasi-steady models. Title: The role of magnetic reconnection in solar activity Authors: Antiochos, Spiro Bibcode: 2008cosp...37..102A Altcode: 2008cosp.meet..102A The central challenge in solar/heliospheric physics is to understand how the emergence and transport of magnetic flux at the photosphere drives the structure and dynamics that we observe in the corona and heliosphere. This presentation focuses on the role of magnetic reconnection in determining solar/heliospheric activity. We demonstrate that two generic properties of the photospheric magnetic and velocity fields are responsible for the ubiquitous reconnection in the corona. First, the photospheric velocities are complex, which leads to the injection of energy and helicity into the coronal magnetic fields and to the efficient formation of small-scale structure. Second, the flux distribution at the photosphere is multi-polar, which implies that topological discontinuities and, consequently, current sheets, must be present in the coronal magnetic field. We present numerical simulations showing that photospherically-driven reconnection is responsible for the heating and dynamics of coronal plasma, and for the topology of the coronal/heliospheric magnetic field. The work was supported by the NASA HTP, SR&T, and TR&T Programs. Title: 3D numerical simulation and stereoscopic observations of coronal jets. Authors: Pariat, Etienne; Antiochos, Spiro; Patsourakos, Spiro; DeVore, C. R. Bibcode: 2008cosp...37.2354P Altcode: 2008cosp.meet.2354P Recent solar observations have revealed that coronal jets are a more frequent phenomenon than previously believed. It is widely accepted that magnetic reconnection is the fundamental mechanism that gives rise to the jets. The improved spatial and temporal resolution of the STEREO observations in combination with stereoscopy yields new insights into the origins of coronal jets, and provides detailed data that can be used to test and refine models. We present the results of a 3D numerical simulation of our model for coronal jets. The simulations were performed with our state-of-art adaptive mesh MHD solver ARMS. The basic idea of the model is that a jet is due to the release of twist as a closed field region undergoes interchange reconnection with surrounding open field. The photospheric driven evolution of the structure results in the generation of nonlinear Alfven waves propagating along the open field, which drive the jet flows. Using stereoscopic EUVI images, we reveal the presence of such twisted structure in a coronal jet event. This work was supported, in part, by NASA and ONR. Title: Breakout coronal mass ejections from solar active regions Authors: DeVore, C. Richard; Lynch, Benjamin; MacNeice, Peter; Olson, Kevin; Antiochos, Spiro Bibcode: 2008cosp...37..706D Altcode: 2008cosp.meet..706D We are performing magnetohydrodynamic simulations of single bipolar active regions (ARs) embedded in the Sun's global background field and of pairs of ARs interacting with each other. The magnetic flux near the polarity inversion lines (PILs) of the ARs is subjected to twisting footpoint displacements that introduce strong magnetic shear between the two polarities and gradually inflate the coronal volume occupied by the AR fields. If the initially current-free coronal field contains a magnetic null, then it is vulnerable to eruptions triggered by magnetic breakout, which reconnects aside the previously restraining field lines overhead. The sheared core flux promptly expands outward at the Alfven speed, opening the magnetic field in the vicinity of the PIL. Flare reconnection below the ejecta, across the vertical current sheet thus established, thereafter reforms the magnetic-null configuration above the AR. This reformation sets the stage for subsequent homologous episodes of breakout reconnection and eruption, if the energizing footpoint motions are sustained. The magnetic flux and energy of an isolated AR, relative to those of the background field, determine whether the eruption is confined or ejective, as the sheared flux either comes to rest in the corona or escapes the Sun to interplanetary space, respectively. In the latter case, the field lines accompanying the coronal mass ejection can comprise a weakly twisted "magnetic bottle" as readily as a strongly twisted flux rope, both of which are observed routinely in situ. The latest developments in this research will be reported. In particular, we will emphasize the observational signatures inferred from the simulations that could be sought in STEREO data, such as multiple three-dimensional views, EUV brightenings at reconnection sites, and coronal dimmings in regions of strong expansion. Our research is sponsored by NASA and ONR. Title: In-situ and Numerical Modeling Prospects for Multipoint ICME Observations Featuring the 22 May 2007 STEREO Event Authors: Lynch, B. J.; Li, Y.; Huttunen, K. E.; Antiochos, S. K.; DeVore, C. R.; Luhmann, J. G. Bibcode: 2007AGUFMSH51B..04L Altcode: We will present recent 3D MHD simulation results of eruptive flux-rope formation via the magnetic breakout mechanism to provide a theoretical/modeling context for multipoint in-situ observations. Many generic observational CME properties are reproduced by this idealized eruption as it propagates through the low corona (~10 R\odot). Recently, STEREO observed a "classic" flux-rope ICME on 22 May 2007 with different in- situ field and plasma signatures seen at each spacecraft. The same event was also seen by WIND and ACE, arriving at L1 arriving a few hours before either STEREO A or B. We will describe a preliminary analysis of the large-scale heliospheric structure of the ICME flux-rope utilizing the linear, force-free cylinder model fits to the in- situ magnetic field rotations in order to determine the ICME's relative simplicity and/or consistency with the standard Magnetic Cloud cartoon picture. A comparison between the data, the derived in-situ cylinder orientations at three spacecraft, and the CME large-scale magnetic structure inferred from the MHD simulation results in the low corona show a reasonable qualitative agreement. BJL would like to acknowledge support from NSF ATM-0621725 as a SHINE Postdoc, and thank the STEREO, WIND and ACE teams for making their data readily available. Title: Comparison of 3D Numerical Simulations with STEREO Observations of Coronal Jets Authors: Pariat, E.; Patsourakos, S.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2007AGUFMSH41B..03P Altcode: Recent solar observations have revealed that coronal jets are a more frequent phenomenon than previously believed. It is widely accepted that magnetic reconnection is the fundamental mechanism that gives rise to the jets. The improved spatial and temporal resolution of the STEREO observations in combination with stereoscopy yields new insights into the origins of coronal jets, and provides detailed data that can be used to test and refine models. We present the results of a 3D numerical simulation of our model for coronal jets. The simulations were performed with our state-of-art adaptive mesh MHD solver ARMS. The basic idea of the model is that a jet is due to the release of twist as a closed field region undergoes interchange reconnection with surrounding open field. We compare the structure and dynamics of the simulated jet with actual EUVI observations, focusing on how the reconfiguration of the 3D magnetic field explains observed properties of the jet. We also discuss possible signatures for STEREO of twisted structures within jets. Finally, we discuss the implications of our simulations for future stereoscopic observations with STEREO. This work was supported, in part, by NASA and ONR. Title: Solar Reconnection Authors: Antiochos, S. K. Bibcode: 2007AGUFMSH41C..01A Altcode: High spatial and temporal resolution observations from SOHO, TRACE, Hinode, and STEREO prove dramatically that the photosphere is never simple and the corona is never quiet. The photosphere exhibits a constantly evolving, multipolar flux distribution on scales ranging from the magnetic carpet to active region complexes. The corona exhibits brightenings and jetting on a vast range of temporal and spatial scales: from small transient spicules, to long-lived coronal loops, to giant coronal mass ejections. We present theoretical and numerical results demonstrating that magnetic reconnection is the physical process underlying all of this activity. These results also show that the topology of the solar field is the key to understanding why solar activity exhibits such an apparently wide variety of forms. Conservation of magnetic helicity turns out to be the critical condition that distinguishes between the different types of reconnection in the solar corona. We discuss the implications of our results for interpreting the latest observations from Hinode and STEREO. This work was supported, in part, by NASA, ONR, and the NSF. Title: A Model for Coronal Hole Jets Authors: Antiochos, S. K.; Pariat, E.; DeVore, C. Bibcode: 2007AGUFMSH21B..01A Altcode: The recent observations from XRT on Hinode show dramatically that coronal hole are populated with intense X-ray jets that can reach heights of solar radii. These jets appear to originate from closed magnetic-field regions inside the holes; consequently, a natural explanation for these jets is that they are due to interchange reconnection between the open field of the hole and the closed field of an embedded bipole. This type of interchange reconnection has long been postulated as the driver, not only of coronal jets, but also for the solar wind itself. We argue, however, that the explosive nature of the jets imposes severe requirements on the reconnection that are not easily satisfied by realistic 3D models. In particular, the reconnection must have a "switch-on" nature in that it stays off until a substantial store of free energy has been accumulated, but then turns on abruptly and stays on until much of this free energy is released. We discuss the possible magnetic topologies of an embedded bipole in an open field region and present recent 3D simulations of a model in which interchange reconnection does, indeed, yield a large burst of energy release. We also discuss the implications of these results for the Hinode observations. This work was supported, in part, by NASA, ONR, and the NSF. Title: Comparison of Heliospheric In-Situ Data with the Quasi-Steady Solar Wind Models Authors: Lepri, S. T.; Antiochos, S. K.; Riley, P.; Zhao, L.; Zurbuchen, T. H. Bibcode: 2007AGUFMSH21A0296L Altcode: The standard theory for the solar-heliospheric magnetic field is the so-called quasi-steady model in which the field is determined by the observed magnetic flux at the photosphere and the balance between magnetic and plasma forces in the corona. In this model, the solar magnetic flux that opens to the heliosphere can increase or decrease as the photospheric flux evolves. One of the most sophisticated implementations of the quasi-steady theory is the SAIC model, which solves the fully time-dependent 3D MHD equations for the corona and wind until a steady state is achieved. In order to test the quasi-steady theory, we compare the 3-D MHD model with observations of the heliospheric flux using multi-point measurements from the VHM instrument on the Ulysses spacecraft from 1991 through 2005 and from magnetic field measurements from various spacecraft at L1 compiled into the OMNI data set from 1976 through 2005. We also compare the observations to the predictions of the potential-field source-surface model, an older and simpler implementation of the quasi-steady theory. During solar maximum, ICMEs significantly disturb the heliospheric magnetic field, making our comparisons difficult. We find that the MHD model compares well with the general trends of the observed heliospheric fluxes. Variations on short timescales, presumably due to local effects, are missed by the model, but the long-term evolution is well matched. The model disagrees with observations most when Ulysses is in slow wind or ICME- related flows. The model underestimates the flux at solar maximum; however, this is to be expected, given the large number of ICMEs in the heliosphere at this time. We discuss the possible sources of discrepancy between the observations and the quasi-steady models. Title: Structure and Dynamics of the Sun's Open Magnetic Field Authors: Antiochos, S. K.; DeVore, C. R.; Karpen, J. T.; Mikić, Z. Bibcode: 2007ApJ...671..936A Altcode: 2007arXiv0705.4430A The solar magnetic field is the primary agent that drives solar activity and couples the Sun to the heliosphere. Although the details of this coupling depend on the quantitative properties of the field, many important aspects of the corona-solar wind connection can be understood by considering only the general topological properties of those regions on the Sun where the field extends from the photosphere out to interplanetary space, the so-called open field regions that are usually observed as coronal holes. From the simple assumptions that underlie the standard quasi-steady corona-wind theoretical models, and that are likely to hold for the Sun as well, we derive two conjectures as to the possible structure and dynamics of coronal holes: (1) coronal holes are unique in that every unipolar region on the photosphere can contain at most one coronal hole, and (2) coronal holes of nested polarity regions must themselves be nested. Magnetic reconnection plays the central role in enforcing these constraints on the field topology. From these conjectures we derive additional properties for the topology of open field regions, and propose several observational predictions for both the slowly varying and transient corona/solar wind. Title: Homologous Confined Filament Eruptions via Magnetic Breakout Authors: DeVore, C. R.; Antiochos, S. K. Bibcode: 2007AAS...210.2901D Altcode: 2007BAAS...39..137D We have performed numerical simulations of a bipolar active region embedded in a dipolar background field and subjected to twisting footpoint displacements concentrated near its polarity inversion lines. These displacements preserve the initial radial flux distribution at the inner surface of our spherical domain while introducing strong magnetic shear between the region’s two polarity concentrations. The dipole moments of the active region and the background field are antiparallel and aligned with the Sun’s polar axis, so that the initially potential magnetic field has a null point in the corona above the equator. This configuration is vulnerable to magnetic breakout eruptions, which occur in our MHD simulations and exhibit three novel features not previously found in our studies of coronal mass ejection initiation. First, the eruptions are multiple and homologous, i.e., as a consequence of the flare reconnection that follows each eruption, the coronal null point reforms above the equator, setting the stage for the subsequent onset of a new episode of breakout reconnection and eruption driven by the ongoing footpoint motions. Second, the eruptions are confined, that is, the highest lying, rapidly rising, strongly sheared field lines of the active region do not escape the Sun but instead come to rest in the outer corona well above the reforming null point, forming a large transequatorial loop. Third, the lowest lying, strongly sheared field lines of the active region are very flat prior to the eruption and, after expanding upward sharply during the event, return to their flat configuration within just a few hours, consistent with filament disappearance and prompt reformation. All of these features of our simulations - homology, confinement, and reformation - are commonly observed aspects of the Sun’s eruptive activity. NASA and ONR sponsored this research. Title: A Mechanism for Coronal Hole Jets Authors: Mueller, Daniel; Antiochos, S. K. Bibcode: 2007AAS...210.9117M Altcode: 2007BAAS...39..206M Bald patches are magnetic topologies in which the magnetic field is concave up over part of a photospheric polarity inversion line. A bald patch topology is believed to be the essential ingredient for filament channels and is often found in extrapolations of the observed photospheric field. We demonstrate that although common in closed field regions, bald patches are unlikely to occur in the open field topology of a coronal hole. We use an analytic source-surface model to calculate the magnetic topology of a small "active region" dipole embedded in a central magnetic dipole field. While bald patches readily occur in closed-field regions, we show that there is only a highly limited parameter range for them to form in open-field

regions. Furthermore, the inclusion of a finite gas pressure and solar wind is likely to destroy even this limited parameter range for the existence of bald patches in coronal holes. Our results give rise to the following question: What happens to a bald patch topology when the surrounding field lines open up? This would be the case when a bald patch moves into a coronal hole, or when a coronal hole forms in an area that encompasses a bald patch. Our magnetostatic models show that, in this case, the bald patch topology almost invariably transforms into a null point topology with a spine and a fan. We argue that the time-dependent evolution of this scenario will be very dynamic since the change from a bald patch to

null point topology cannot occur via a simple ideal evolution in the corona. We discuss the implications of these findings for recent Hinode XRT observations of coronal hole jets and give an outline of planned time-dependent 3D MHD simulations to fully assess this scenario.

This work was supported in part by NASA and ONR. Title: An MHD Simulation of an Emerging Bipole in the Presence of the Solar Wind Authors: Allred, J. C.; MacNeice, P. J.; Antiochos, S. K. Bibcode: 2006AGUFMSH33B0409A Altcode: We report on an MHD simulation of a bipole emerging from the solar surface in the presence of the solar wind. The initial field configuration is obtained from a source surface model, with the field at the solar surface defined by a dipole and a bipolar region near the polar axis. We use a 2.5 dimensional MHD code to model the evolution of the bipole in the presence of the solar wind and show that the emerging bipole creates an embedded coronal hole at the location of the spine of the patch of opposite polarity of the emerged bipole. This result is in contrast to source surface models which predict an X-point along the spine. We study the size and properties of the new coronal hole. Title: Constraints on the Sun-Heliosphere Magnetic Connection Authors: Antiochos, S. K. Bibcode: 2006AGUFMSH21B..04A Altcode: The solar magnetic field is the primary agent that drives solar activity and couples the Sun to the Heliosphere. Although the details of this coupling depend on the quantitative properties of the field, many important aspects of the corona - solar wind connection can be understood by considering only the topological properties of those regions on the Sun where the field extends from the photosphere out to interplanetary space, the so-called open field regions that are often observed as "coronal holes". From the well-known assumptions that underlie the standard quasi-steady corona-wind theoretical models, and that are likely to hold for the Sun, as well, we derive several constraints on the possible topology and dynamics of coronal open and closed field regions. We show how magnetic reconnection plays the central role in establishing these constraints. We discuss the implications of our results on observations, and make a number of predictions for the upcoming LWS missions. This work was supported by the LWS TR&T Program and is part of the research by the Focus Team on Connecting the Sun to the Heliosphere. Title: Energetics and Dynamics of Bipolar and Multipolar CME Source Regions Authors: Lynch, B. J.; Antiochos, S. K.; DeVore, C. R.; Luhmann, J. G. Bibcode: 2006AGUFMSH31B..04L Altcode: We present results of a numerical experiment which tests the Aly-Sturrock limit in a fully 3-dimensional, spherical geometry. We compare two common magnetic configurations corresponding to bipolar and multipolar "active region" arcades with identical photospheric normal field distributions and applied shearing flows. The bipolar response is a smooth expansion of the stressed fields, void of any explosive behavior, whereas the multipolar configuration results in the rapid expulsion of the low-lying sheared field via the magnetic breakout mechanism for CME initiation. The critical nature of the oppositely-directed overlying field and its topological consequences is discussed in the context of the breakout model. Title: Solar Prominence Merging Authors: Aulanier, Guillaume; DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2006ApJ...646.1349A Altcode: In a recent paper, we described MHD simulations of the interaction between a pair of distinct prominences formed by the photospheric line-tied shearing of two separated dipoles. One case was typical of solar observations of prominence merging, in which the prominences have the same axial field direction and sign of magnetic helicity. For that configuration, we reported the formation of linkages between the prominences due to magnetic reconnection of their sheared fields. In this paper, we analyze the evolution of the plasma-supporting magnetic dips in this configuration. As the photospheric flux is being progressively sheared, dip-related chromospheric fibrils and high-altitude threads form and develop into the two prominences, which undergo internal oscillations. As the prominences are stretched farther along their axes, they come into contact and their sheared fluxes pass each other, and new dips form in the interaction region. The distribution of these dips increasingly fills the volume between the prominences, so that the two progenitors gradually merge into a single prominence. Our model reproduces typical observational properties reported from both high-cadence and daily observations at various wavelengths. We identify the multistep mechanism, consisting of a complex coupling between photospheric shear, coronal magnetic reconnection without null points, and formation of quasi bald patches, that is responsible for the prominence merging through dip creation. The resulting magnetic topology differs significantly from that of a twisted flux tube. Title: Modeling Free Energy & Reconnection in the Corona Authors: Welsch, Brian; DeVore, C.; Antiochos, S. K. Bibcode: 2006SPD....37.0908W Altcode: 2006BAAS...38..237W The injection and storage of magnetic free energy into the coronal magnetic field is an essential component of the widely accepted ``storage & release'' paradigm of solar flares and CMEs.A central role in many models of both the storage and release phases is played by magnetic reconnection --- which can form an unstable structure (e.g., by flux cancellation) and/or reduce confinement allowing a metastable structure to erupt (as in the breakout model).To investigate changes in magnetic energy and field line connectivity in the presence of shearing, convergence, and flux cancellation, we have used the ARMS code, a 3-D, flux-corrected transport MHD code with adaptive mesh refinement, to simulate the evolution of coronal magnetic fields driven by prescribed photospheric motions.Here, we present the preliminary results of our investigations, and outline directions for future studies.This work was supported by ONR, NASA's SEC Theory program, and by a grant of computer time from the DOD High Performance Computing Modernization Program. Title: Sympathetic Breakout Coronal Mass Ejections Authors: DeVore, C. R.; Antiochos, S. K. Bibcode: 2006SPD....37.0906D Altcode: 2006BAAS...38..236D Several instances of multiple, apparently coordinated solar eruptive events have been reported, in which the close temporal association of the eruptions suggests a possible causal link between them. Variously called "global" or "sympathetic" coronal mass ejections (CMEs), the low-coronal structures where the eruptions originate in some cases neighbor each other, while in others they are more remote with no obvious magnetic connections joining them.In the breakout model for CMEs, eruption occurs due to the onset of reconnection across a coronal magnetic null shared by multiple flux systems. The resultant reconfiguration of the field overlying the participating structures loosens the restraining forces on one or more of them, initiating the accelerating outward expansion that becomes the CME. In a simple three-arcade quadrupolar geometry, an eruption in either side lobe depletes the overlying fields of both side lobes as they reconnect with each other. This raises the possibility of either an immediate or prompt second CME in the far side lobe; the two cases are distinguished according to whether the second CME precedes or follows the reconnection of a significant amount of its restraining flux. In the aftermath of a solitary eruption in either side lobe, flare reconnection closes the opened field and reforms the lobe, eventually also depleting the middle lobe of its own overlying field. This process can produce a delayed second CME in the middle lobe of the configuration.We are concluding an analysis of the magnetic free energies available to power these scenarios - immediate, prompt, and delayed - for sympathetic CMEs in simple breakout geometries. The results will be presented. We also are beginning a simulation study of susceptible configurations to verify and understand the dynamics of sympathetic eruptive events. Our progress will be reported.This research was supported by NASA and ONR. Title: Impulsive Heating And Thermal Nonequilibrium In Prominences Authors: Karpen, Judith T.; Antiochos, S. K. Bibcode: 2006SPD....37.0203K Altcode: 2006BAAS...38Q.221K Prominences are among the most spectacular manifestations of both quiescent and eruptive solar activity, yet the origins of their magnetic-field and plasma structures remain poorly understood. We have made steady progress toward a comprehensive model of prominence formation and evolution with our sheared 3D arcade model for the magnetic field and our thermal nonequilibrium model for the cool, dense material suspended in the corona. According to the thermal nonequilibrium model, condensations form readily in long, low-lying magnetic flux tubes if the heating is localized near the chromosphere. Our previous studies established the effects of steady heating in flux tubes of different geometries. In some cases this process yields a dynamic cycle in which condensations repetitively form, stream along the field line, and ultimately disappear by falling onto the nearest footpoint; in others, static condensations grow as long as the heating continues. Here we will discuss the effects of impulsive heating, as indicated by many coronal-heating models, on the formation and evolution of prominence plasmas.This work was supported by NASA and ONR. Title: A Transient Heating Model for the Structure and Dynamics of the Solar Transition Region Authors: Spadaro, D.; Lanza, A. F.; Karpen, J. T.; Antiochos, S. K. Bibcode: 2006ApJ...642..579S Altcode: Understanding the structure and dynamics of the Sun's transition region has been a major challenge to scientists since the Skylab era. In particular, the characteristic shape of the emission measure distribution and the Doppler shifts observed in EUV emission lines have thus far resisted all theoretical and modeling efforts to explain their origin. Recent observational advances have revealed a wealth of dynamic fine-scale structure at transition-region temperatures, validating earlier theories about the existence of such cool structure and explaining in part why static models focusing solely on hot, large-scale loops could not match observed conditions. In response to this newly confirmed picture, we have investigated numerically the hydrodynamic behavior of small, cool magnetic loops undergoing transient heating spatially localized near the chromospheric footpoints. For the first time we have successfully reproduced both the observed emission measure distribution over the entire range logT=4.7-6.1 and the observed temperature dependence of the persistent redshifts. The closest agreement between simulations and observations is obtained with heating timescales of the order of 20 s every 100 s, a length scale of the order of 1 Mm, and energy deposition within the typical range of nanoflares. We conclude that small, cool structures can indeed produce most of the quiet solar EUV output at temperatures below 1 MK. Title: A numerical simulation of an asymmetric breakout model for CMEs Authors: MacNeice, P.; Gao, J.; Antiochos, S. Bibcode: 2006AGUSMSH52A..04M Altcode: We made a high resolution numerical simulation on an asymmetric 'magnetic breakout' model for CMEs. A fast CME was generated. The dynamics of the system is controlled by two reconnection events, i.e. the breakout reconnection at the outer x-point and the flare reconnection at the newly formed inner x-point. The CME is triggered by the former event and is accelerated by the latter event. Title: The Dynamics of Magnetic Reconnection in the Solar Atmosphere Authors: Antiochos, Spiro Bibcode: 2006APS..APR.E3001A Altcode: Magnetic reconnection is widely believed to play the central role in the interaction between matter and magnetic field in the Sun's corona and, therefore, to underlie most solar activity. Direct plasma heating due to reconnection has been proposed as the process that produces both the quasi-steady and the flare hot corona. Reconnection-driven flows have been proposed as the explanation for transient dynamic phenomena ranging from the smallest spicule to giant surges and sprays. Reconnection has also been proposed as the origin of the electron beams in flares, and numerous authors have argued that it is the mechanism responsible for the origin of coronal mass ejections and prominence/filament eruptions. In fact, it is difficult to find a solar phenomenon that has not been blamed on magnetic reconnection! On the other hand, there is surprisingly scarce direct evidence for reconnection in coronal observations. In this talk, I will present both the latest observations and 3D models for reconnection-driven dynamics in the corona and attempt to reconcile the data with theory. Title: CME Theory and Models Authors: Forbes, T. G.; Linker, J. A.; Chen, J.; Cid, C.; Kóta, J.; Lee, M. A.; Mann, G.; Mikić, Z.; Potgieter, M. S.; Schmidt, J. M.; Siscoe, G. L.; Vainio, R.; Antiochos, S. K.; Riley, P. Bibcode: 2006SSRv..123..251F Altcode: 2006SSRv..tmp...59F This chapter provides an overview of current efforts in the theory and modeling of CMEs. Five key areas are discussed: (1) CME initiation; (2) CME evolution and propagation; (3) the structure of interplanetary CMEs derived from flux rope modeling; (4) CME shock formation in the inner corona; and (5) particle acceleration and transport at CME driven shocks. In the section on CME initiation three contemporary models are highlighted. Two of these focus on how energy stored in the coronal magnetic field can be released violently to drive CMEs. The third model assumes that CMEs can be directly driven by currents from below the photosphere. CMEs evolve considerably as they expand from the magnetically dominated lower corona into the advectively dominated solar wind. The section on evolution and propagation presents two approaches to the problem. One is primarily analytical and focuses on the key physical processes involved. The other is primarily numerical and illustrates the complexity of possible interactions between the CME and the ambient medium. The section on flux rope fitting reviews the accuracy and reliability of various methods. The section on shock formation considers the effect of the rapid decrease in the magnetic field and plasma density with height. Finally, in the section on particle acceleration and transport, some recent developments in the theory of diffusive particle acceleration at CME shocks are discussed. These include efforts to combine self-consistently the process of particle acceleration in the vicinity of the shock with the subsequent escape and transport of particles to distant regions. Title: DC coronal heating and the nonlinear evolution of current sheets Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K. Bibcode: 2006AdSpR..37.1342D Altcode: Recent theoretical developments have re-awakened interest in the role of electric current sheets in DC coronal heating [Parker. Astrophys. J. 330, 474, 1988; Priest et al. Astrophys. J. 576, 522, 2002]. Dahlburg et al. [Dahlburg et al. Adv. Space Res. 32, 1029, 2003; Dahlburg et al. Astrophys. J. 622, 1191, 2005] reported the existence of a "secondary instability" that could explain the required "switch-on" effect required for adequate energy storage. This ideal, three-dimensional instability also provided a straightforward explanation for the subsequent fast release of energy, as the rapid growth of the mode eventually results in a state of turbulent magnetic reconnection. Earlier studies of the secondary instability were limited to systems with relatively simple perturbations, viz., resistive stability eigenmodes. A current sheet in the Sun is likely to be subject to much more complex perturbations involving a waves of various wavelengths and amplitudes. We describe the evolution of three-dimensional electric current sheets disturbed by random 3D perturbations. We find that the significant characteristics of secondary instability are also observed in this case. The numerical results are compared to solar observations. Title: The Origin of High-Speed Motions and Threads in Prominences Authors: Karpen, J. T.; Antiochos, S. K.; Klimchuk, J. A. Bibcode: 2006ApJ...637..531K Altcode: Prominences are among the most spectacular manifestations of both quiescent and eruptive solar activity, yet the origins of their magnetic-field and plasma structures remain poorly understood. We have made steady progress toward a comprehensive model of prominence formation and evolution with our sheared three-dimensional arcade model for the magnetic field and our thermal nonequilibrium model for the cool, dense material suspended in the corona. According to the thermal nonequilibrium model, condensations form readily along long, low-lying magnetic field lines when the heating is localized near the chromosphere. In most cases this process yields a dynamic cycle in which condensations repetitively form, stream along the field, and ultimately disappear by falling onto the nearest footpoint. Two key observed features were not adequately explained by our earlier simulations of thermal nonequilibrium, however: the threadlike (i.e., elongated) horizontal structure and high-speed motions of many condensations. In this paper we discuss how simple modifications to the radiative loss function, the heating scale, and the geometry of our model largely eliminate these discrepancies. In particular, condensations in nearly horizontal flux tubes are most likely to develop both transient high-speed motions and elongated threads. These results strengthen the case for thermal nonequilibrium as the origin of prominence condensations and support low-twist models of prominence magnetic structure. Title: CME Theory and Models Authors: Forbes, T. G.; Linker, J. A.; Chen, J.; Cid, C.; Kóta, J.; Lee, M. A.; Mann, G.; Mikić, Z.; Potgieter, M. S.; Schmidt, J. M.; Siscoe, G. L.; Vainio, R.; Antiochos, S. K.; Riley, P. Bibcode: 2006cme..book..251F Altcode: This chapter provides an overview of current efforts in the theory and modeling of CMEs. Five key areas are discussed: (1) CME initiation; (2) CME evolution and propagation; (3) the structure of interplanetary CMEs derived from flux rope modeling; (4) CME shock formation in the inner corona; and (5) particle acceleration and transport at CME driven shocks. In the section on CME initiation three contemporary models are highlighted. Two of these focus on how energy stored in the coronal magnetic field can be released violently to drive CMEs. The third model assumes that CMEs can be directly driven by currents from below the photosphere. CMEs evolve considerably as they expand from the magnetically dominated lower corona into the advectively dominated solar wind. The section on evolution and propagation presents two approaches to the problem. One is primarily analytical and focuses on the key physical processes involved. The other is primarily numerical and illustrates the complexity of possible interactions between the CME and the ambient medium. The section on flux rope fitting reviews the accuracy and reliability of various methods. The section on shock formation considers the effect of the rapid decrease in the magnetic field and plasma density with height. Finally, in the section on particle acceleration and transport, some recent developments in the theory of diffusive particle acceleration at CME shocks are discussed. These include efforts to combine self-consistently the process of particle acceleration in the vicinity of the shock with the subsequent escape and transport of particles to distant regions. Title: A Study of the Global Heliospheric Magnetic Flux Using In-Situ Data and the SAIC MHD Model Authors: Lepri, S. T.; Antiochos, S. K.; Riley, P. Bibcode: 2005AGUFMSH13B..08L Altcode: There has been considerable controversy in recent years over the slow evolution of the Sun's open field, which extends out to become the heliospheric magnetic field. In the standard solar model (e.g., Wang and Sheeley [1993]) the open flux can increase or decrease in response to the emergence or cancellation of magnetic flux at the photosphere in relation to coronal holes. In the Fisk et al. [1999a, 1999b] model, on the other hand, the open flux is conserved and evolves primarily via interchange reconnection with closed fields. This model predicts no long-term variations in the amount of heliospheric flux. We compare and test these theories by measuring the behavior of the open magnetic flux in the global heliospheric magnetic field. Using multi-point measurements from the VHM instrument on the Ulysses spacecraft and from the MAG instrument on the ACE spacecraft, we analyze in-situ radial magnetic field data and compare the behavior to that predicted by the updated SAIC MHD model. During solar maximum, ICMEs significantly disturb the heliospheric magnetic field, making our comparisons difficult. We examine the radial component of the magnetic field in data from 1991 though the present in order to determine the variability of the open flux and hence the evolution of corona holes. Title: Magnetic Reconnection Models of Prominence Formation Authors: Welsch, B. T.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2005ApJ...634.1395W Altcode: To investigate the hypothesis that prominences form by magnetic reconnection between initially distinct flux systems in the solar corona, we simulate coronal magnetic field evolution when two flux systems are driven together by boundary motions. In particular, we focus on configurations similar to those in the quiescent prominence formation model of Martens & Zwaan. We find that reconnection proceeds very weakly, if at all, in configurations driven with global shear flows (i.e., differential rotation); reconnection proceeds much more efficiently in similar configurations that are driven to collide directly, with converging motions along the neutral line that lead to flux cancellation; reconnected fields from this process can exhibit sheared, dipped field lines along the neutral line, consistent with prominence observations. Our field configurations do not possess the ``breakout'' topology, and eruptions are not observed, even though a substantial amount of flux is canceled in some runs. Title: Prominence Formation by Thermal Nonequilibrium in the Sheared-Arcade Model Authors: Karpen, J. T.; Tanner, S. E. M.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2005ApJ...635.1319K Altcode: The existence of solar prominences-cool, dense, filamented plasma suspended in the corona above magnetic neutral lines-has long been an outstanding problem in solar physics. In earlier numerical studies we identified a mechanism, thermal nonequilibrium, by which cool condensations can form in long coronal flux tubes heated locally above their footpoints. To understand the physics of this process, we began by modeling idealized symmetric flux tubes with uniform cross-sectional area and a simplified radiative-loss function. The present work demonstrates that condensations also form under more realistic conditions, in a typical flux tube taken from our three-dimensional MHD simulation of prominence magnetic structure produced by the sheared arcade mechanism. We compare these results with simulations of an otherwise identical flux tube with uniform cross-sectional area, to determine the influence of the overall three-dimensional magnetic configuration on the condensation process. We also show that updating the optically thin radiative loss function yields more rapidly varying, dynamic behavior in better agreement with the latest prominence observations than our earlier studies. These developments bring us substantially closer to a fully self-consistent, three-dimensional model of both magnetic field and plasma in prominences. Title: Observational Implications of 3D Breakout Authors: Lynch, B. J.; Antiochos, S. K.; DeVore, C. R.; Zurbuchen, T. H. Bibcode: 2005AGUFMSH12A..02L Altcode: We present the latest numerical MHD simulations of the breakout model for coronal mass ejections in 3-dimensions. We will emphasize features of the model and simulation results that are uniquely associated with "magnetic breakout", and briefly review the more generic features that are common to almost all CME initiation models. Specifically, we will focus on the multi-polar topology and the breakout reconnection at the distorted null-point high in the corona, as the process responsible for the eruption and will attempt to draw some conclusions about possible observational signatures. We will also quantify the energetics of the eruption and show the restraining overlying field is critical in order to build up sufficient energy for a catastrophic, fast eruption of low-lying sheared flux. This work was supported by NASA and ONR. BJL acknowledges NASA GSRP NGT5-05453. Title: The Topology of Magnetic Reconnection in the Sun's Corona Authors: Antiochos, S. K. Bibcode: 2005AGUFMSM12A..07A Altcode: It has long been recognized that magnetic topology plays the critical role in reconnection at the magnetopause. The essential requirement for reconnection is that the system has a multi-flux topology, and the location of the reconnection is at the boundaries between the different flux systems, specifically separator lines and null points. On the other hand, it is commonly believed that reconnection driven by photospheric motions can occur more-or-less anywhere in the Sun's corona. We present both theoretical and 3D numerical simulation results arguing that, in fact, the magnetosphere and corona are physically similar in that topology strongly restricts how and where coronal reconnection can occur. We also discuss important differences between the reconnection in the corona and magnetosphere. This work was supported, in part, by ONR and NASA. Title: A Mechanism for the Emergence of Magnetic U-Loops and Flux Cancellation on the Sun Authors: Magara, T.; Antiochos, S. K.; DeVore, C. R.; Linton, M. G. Bibcode: 2005ESASP.596E..74M Altcode: 2005ccmf.confE..74M No abstract at ADS Title: The Breakout Model for CME Initiation in 3-Dimensions Authors: Lynch, B. J.; Antiochos, S. K.; de Vore, C. R.; Zurbuchen, T. H. Bibcode: 2005ESASP.592..297L Altcode: 2005soho...16E..44L; 2005ESASP.592E..44L No abstract at ADS Title: Magnetic Free Energies of Breakout Coronal Mass Ejections Authors: DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2005ApJ...628.1031D Altcode: A critical issue in understanding and eventually predicting coronal mass ejections (CMEs) is determining the magnetic free energy that can drive the explosive eruption. We present calculations of this free energy for the breakout CME model, which postulates that the preeruption magnetic topology is a multipolar field with a null point in the corona. Using analytical and numerical methods, we determine the free energies for two broad families of photospheric flux distributions, parameterized by the radius of the coronal null and the degree to which flux is concentrated near the poles and equator. The available CME energy attains a broad maximum for distributions whose potential null resides between about 1.25 and 1.75 solar radii, and falls off toward zero as the null approaches the surface or moves out to infinity. These results may explain the wide range of energies observed for CMEs and their associated flares. We find that concentrating the surface flux to narrower latitude bands near the poles and equator, on the other hand, has little effect on the available energy. Our mathematical approach currently is restricted to spherically axisymmetric systems. Its generalization to fully three-dimensional fields might provide the foundation of a first-principles forecasting technique for solar eruptions. Title: Solar Prominence Interactions Authors: DeVore, C. Richard; Antiochos, Spiro K.; Aulanier, Guillaume Bibcode: 2005ApJ...629.1122D Altcode: We report numerical simulations of the formation, interaction, and magnetic reconnection between pairs of solar prominences within the sheared-arcade model. Our experiments consider the four possible basic combinations of chiralities (identical or opposite) and axial magnetic fields (aligned or opposed) between the participating prominences. When the topology of the global flux system comprising the prominences and arcades is bipolar, so that a single polarity inversion line is shared by the two structures, then identical chiralities necessarily imply aligned axial fields, while opposite chiralities imply opposed axial fields. In the former case, external magnetic reconnections forming field lines linking the two prominences occur; in the latter, such reconnections are disfavored, and no linkage takes place. These results concur with empirical rules for prominence interactions. When the topology instead is quadrupolar, so that a second polarity inversion line crossing the first lies between the prominences, then the converse relation holds between chirality and axial-field alignment. External reconnections forming linking field lines now occur between prominences with opposite chiralities; they also occur, but result only in footpoint exchanges, between prominences with identical chiralities. These findings conflict with the accepted empirical rules but may not have been tested in observations to date. All of our model prominences, especially those that undergo linking reconnections, contain substantial magnetic shear and twist. Nevertheless, none exhibits any sign of onset of instability or loss of equilibrium that might culminate in an eruption. Title: Solar cycle-dependent helicity transport by magnetic clouds Authors: Lynch, B. J.; Gruesbeck, J. R.; Zurbuchen, T. H.; Antiochos, S. K. Bibcode: 2005JGRA..110.8107L Altcode: 2005JGRA..11008107L Magnetic clouds observed with the Wind and ACE spacecraft are fit with the static, linear force-free cylinder model to obtain estimates of the chirality, fluxes, and magnetic helicity of each event. The fastest magnetic clouds (MCs) are shown to carry the most flux and helicity. We calculate the net cumulative helicity which measures the difference in right- and left-handed helicity contained in MCs over time. The net cumulative helicity does not average to zero; rather, a strong left-handed helicity bias develops over the solar cycle, dominated by the largest events of cycle 23: Bastille Day 2000 and 28 October 2003. The majority of MCs ("slow" events, <Vr> < 500 km/s) have a net cumulative helicity profile that appears to be modulated by the solar activity cycle. This is far less evident for "fast" MC events (<Vr> ≥ 500 km/s), which were disproportionately left-handed over our data set. A brief discussion about the various solar sources of CME helicity and their implication for dynamo processes is included. Title: A Mechanism for the Flux Cancellation Caused by Emerging Magnetic U-Loops in the Sun Authors: Magara, T.; Antiochos, S. K.; DeVore, C. R.; Linton, M. G. Bibcode: 2005AGUSMSH51C..01M Altcode: We used three-dimensional MHD simulation to study the evolution of U-shaped magnetic field lines (U-loops) in a flux cancellation region on the Sun. Emergence of U-loops is thought to be a process for causing flux cancellation at the solar surface, although the physical mechanism for this process is not obvious because the mass tends to accumulate at the dipped part of U-loops thereby reducing the buoyancy. Our flux emergence simulation reveals that a temporary siphon flow plays a key role in enhancing the buoyancy of the dipped part of U-loops and helps it emerge into the solar atmosphere against the gravity. By applying a model of emerging U-loops to an observed flux cancellation region, we study a possible configuration of magnetic field lines related to flux cancellation. Title: 3D Numerical Simulations of the Breakout Model Authors: Choe, G. S.; Cheng, C. Z.; Lee, J.; Lynch, B. J.; Antiochos, S. K.; DeVore, C. R.; Zurbuchen, T. H. Bibcode: 2005AGUSMSP43C..02C Altcode: We present the continuing progress of the numerical simulations of the breakout model for coronal mass ejection initiation. To validate the 3D spherical ARMS code we have run the 2.5D breakout problem and compare the eruption to the published 2D results. The ARMS 2.5D CME also forms a large magnetic island ahead of the erupting plasmoid due to the code's excellent maintenance of equatorial symmetry. Progress on the fully 3D breakout problem is also discussed. To build up enough magnetic free energy for an eruption the active region field must be strong with a steep gradient near the polarity inversion line and the shear must be highly concentrated there. This requires adaptive griding techniques. In the current simulation, the active region to background field ratio is 20-to-1 and the neutral line is long compared to the active region width. We present the evolution of this topology under Br-conserving shearing flow and discuss implications for a 3D eruption. This work is supported by NASA and ONR. BJL is supported by NASA GSRP grant NGT5-50453. Title: Magnetic Flux Tube Reconnection: Tunneling Versus Slingshot Authors: Linton, M. G.; Antiochos, S. K. Bibcode: 2005ApJ...625..506L Altcode: 2005astro.ph..1473L The discrete nature of the solar magnetic field as it emerges into the corona through the photosphere indicates that it exists as isolated flux tubes in the convection zone and will remain as discrete flux tubes in the corona until it collides and reconnects with other coronal fields. Collisions of these flux tubes will in general be three-dimensional and will often lead to reconnection, both rearranging the magnetic field topology in fundamental ways and releasing magnetic energy. With the goal of better understanding these dynamics, we carry out a set of numerical experiments exploring fundamental characteristics of three-dimensional magnetic flux tube reconnection. We first show that reconnecting flux tubes at opposite extremes of twist behave very differently: in some configurations, low twist tubes slingshot while high twist tubes tunnel. We then discuss a theory explaining these differences: by assuming helicity conservation during the reconnection one can show that at high twist, tunneled tubes reach a lower magnetic energy state than slingshot tubes, whereas at low twist the opposite holds. We test three predictions made by this theory. (1) We find that the level of twist at which the transition from slingshot to tunnel occurs is about 2-3 times higher than predicted on the basis of energetics and helicity conservation alone, probably because the dynamics of the reconnection play a large role as well. (2) We find that the tunnel occurs at all flux tube collision angles predicted by the theory. (3) We find that the amount of magnetic energy a slingshot or a tunnel reconnection releases agrees reasonably well with the theory, although at the high resistivities we have to use for numerical stability, a significant amount of magnetic energy is lost to diffusion, independent of reconnection. Title: The Role of Magnetic Helicity in Coronal Mass Ejections Authors: Phillips, A. D.; MacNeice, P. J.; Antiochos, S. K. Bibcode: 2005ApJ...624L.129P Altcode: We investigate the factors responsible for initiating coronal mass ejections (CMEs), specifically, the role of magnetic helicity. Using numerical simulations of the breakout model for CMEs, we show that eruption occurs at a fixed magnitude of free energy in the corona, independent of the value of helicity. Almost identical eruptions are obtained for both large and zero-helicity cases. Furthermore, the eruption can actually lead to an increase in the helicity remaining in the corona. These results argue strongly against recent models that postulate a critical helicity buildup and shedding as the determining factors for CME initiation. Title: The Origin of High-Speed Motions and Threads in Solar Prominences Authors: Karpen, J.; Antiochos, S.; Klimchuk, J. Bibcode: 2005AGUSMSP21B..02K Altcode: Prominences are among the most spectacular manifestations of both quiescent and eruptive solar activity, yet the origins of their magnetic-field and plasma structures remain poorly understood. We have made steady progress toward a comprehensive model of prominence formation and evolution with our sheared 3D arcade model for the magnetic field and our thermal nonequilibrium model for the cool, dense material suspended in the corona. According to the thermal nonequilibrium model, condensations form readily along long, low-lying magnetic field lines if the heating is localized near the chromosphere. In most cases this process yields a dynamic cycle in which condensations repetitively form, stream along the field line, and ultimately disappear by falling onto the nearest footpoint. Two key observed features were not adequately explained by our earlier simulations of thermal nonequilibrium, however: the thread-like (i.e., elongated) horizontal structure and high-speed motions of many condensations. Here we discuss how simple modifications to our model largely eliminate these discrepancies, strengthening the case for thermal nonequilibrium as the origin of prominence condensations and for low-twist models of prominence magnetic structure. This work was supported by NASA and ONR. Title: Coronal Mass Ejections: the Most Powerful Drivers of the Sun-Earth System Authors: Antiochos, S. K. Bibcode: 2005AAS...206.2001A Altcode: 2005BAAS...37..461A A large Coronal Mass Ejection (CME) can consist of billions of tonnes of matter, along with entangled magnetic field, erupting from the Sun at speeds well over 1,000 km/s. These giant disruptions of the solar atmosphere drive the most destructive space weather at Earth and throughout the solar system. Furthermore, CMEs are the most dramatic example of how slowly-evolving processes on the Sun can conspire to produce explosive activity. Understanding their origin has long been a central objective for astrophysical research. This talk will present some of the latest observations and theories for CMEs and discuss the outstanding challenges to modeling and predicting their initiation.

This work was supported in part by NASA and ONR. Title: New Frontiers/Hale Prize Lecture: Coronal Mass Ejections, the Most Powerful Drivers of the Sun-Earth System Authors: Antiochos, S. K. Bibcode: 2005AGUSM.U15A..01A Altcode: A large Coronal Mass Ejection (CME) can consist of billions of tonnes of matter, along with entangled magnetic field, erupting from the Sun at speeds well over 1,000 km/s. These giant disruptions of the solar atmosphere drive the most destructive space weather at Earth and throughout the solar system. Furthermore, CMEs are the most dramatic example of how slowly-evolving processes on the Sun can conspire to produce explosive activity. Understanding their origin has long been a central objective for space physics research. This talk will present some of the latest observations and theories for CMEs and discuss the outstanding challenges to modeling and predicting their initiation. This work was supported in part by NASA and ONR. Title: An Explanation for the ``Switch-On'' Nature of Magnetic Energy Release and Its Application to Coronal Heating Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K. Bibcode: 2005ApJ...622.1191D Altcode: A large class of coronal heating theories postulate that the random mixing of magnetic footpoints by photospheric motions leads to the formation of current sheets in the corona and, consequently, to energy release there via magnetic reconnection. Parker pointed out that in order for this process to supply the observed energy flux into the corona, the stress in the coronal magnetic field must have a fairly specific value at the time that the energy is released. In particular, he argued that the misalignment between reconnecting flux tubes must be roughly 30° in order to match the observed heating. No physical origin for this number was given, however. In this paper we propose that secondary instability is the mechanism that ``switches on'' the energy release when the misalignment angle in the corona reaches the correct value. We calculate both the three-dimensional linear and fully nonlinear development of the instability in current sheets corresponding to various misalignment angles. We find that no secondary instability occurs for angles less than about 45°, but for larger angles the instability grows at a rapid rate, and there is an explosive release of energy. We compare our results with the observed properties of the corona and discuss the implications for future observations. Title: Prominence Formation Processes Authors: Welsch, B. T.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2005HiA....13..127W Altcode: Martens and Zwaan (ApJ v. 558 872) have proposed a prominence/ filament formation model in which differential rotation drives reconnection between two initially unconnected active regions to form helical field lines that support mass and are held down by overlying field. Using an MHD solver with adaptive refinement we simulated this process by imposing a shear flow meant to mimic differential rotation on two bipolar flux distributions meant to mimic distinct active regions. In some runs the flux systems are initially potential while in others they have been twisted by footpoint rotation to inject helicity prior to imposing the shear flow. The resulting structures are studied to understand the role of helicity in the formation of prominence-like structures. Title: 3D Breakout: Preliminary Results Authors: Lynch, B. J.; Antiochos, S. K.; DeVore, C. R.; Zurbuchen, T. H. Bibcode: 2004AGUFMSH21B0400L Altcode: We present preliminary results of the breakout model for solar coronal mass ejections in a global-scale 3D topology. Starting with a background dipole field, we use a series of point dipole sources create a latitudinally extended ( ∼ 100o) delta-spot active region configuration with a null point high in the corona. This is the natural 3D extension of our very successful 2.5D case. Magnetic free energy is added to the initial field configuration via two compressible vortex flows that preserve Br at the surface and concentrate the shear near the central neutral line of the AR flux system. Reconnection at the null point removes the restraining overlying flux allowing rapid, unstable expansion of the innermost sheared field. We examine the evolution of the system and discuss its implications for a fully 3D breakout eruption. We also discuss observational tests of the breakout model that can be performed with the unique viewing capabilities of STEREO. This work is supported in part by NASA and ONR. Title: Simulating The Breakout Model In An Asymmetric Configuration Authors: Gao, J.; MacNeice, P.; Antiochos, S. Bibcode: 2004AGUFMSH13A1151G Altcode: In a recent paper, MacNeice et al (Ap.J. v614) presented the first complete MHD simulation of the `Breakout' model for CME initation. They used an idealized magnetic topology in 2.5D, with an initial symmetry plane at the equator. We have modified this configuration to test the breakout model in an initial topology without this symmetry. We examine the differences in the two simulations, experiment with applying the driving shear to the different flux systems in this complex configuration, and study the implications for the evolution of magnetic helicity. Title: The Effects of Topology on Magnetic Reconnection Authors: Antiochos, S. K.; Devore, R.; Karpen, J. T. Bibcode: 2004AGUFMSM43B..06A Altcode: Magnetic reconnection is widely believed to be the dominant process by which plasma and magnetic field exchange energy in the cosmos. Although certain aspects of reconnection are universal, the nature of the process depends strongly on the particular topology of the reconnecting system. In the Earth's magnetosphere, the topology is fixed -- a four flux system with a pair of nulls and separators. In the Sun's corona, on the other hand, the topology can vary greatly depending on the complexity of the active region. We argue that the usual coronal topology is a two-flux system with an isolated 3D null, but four flux systems that are topologically equivalent to the magnetosphere are possible. We contrast and compare the dynamics of reconnection for these two topologies. We present both theoretical models and fully 3D simulations using ARMS, the NRL adaptively-refined MHD solver. The implications of the results for observations will be discussed. This work was supported in part by NASA and ONR. Title: Variability of the Heliospheric Magnetic Flux Authors: Lepri, S. T.; Antiochos, S. K.; Zurbuchen, T. H. Bibcode: 2004AGUFMSH31A1164L Altcode: There has been considerable controversy in recent years over the slow evolution of the Sun's open field, which extends out to become the heliospheric magnetic field. In the standard solar model [e.g., Wang and Sheeley, 1993] the open flux can increase or decrease in response to the emergence or cancellation of magnetic flux at the photosphere. In particular, the appearance of a new active region can lead to the formation of a new coronal hole, or the growth of an old one, which should be detectable as a long-term increase in the radial component of the magnetic field throughout the heliosphere. In the Fisk et al. [1999a, 1999b] model, on the other hand, the open flux is conserved and evolves primarily via interchange reconnection with closed field. This model predicts no long-term variations in the amount of heliospheric flux. To compare and test these competing theories, we measure the behavior of the open magnetic flux in the global heliospheric magnetic field. Using multi-point measurements from both the MAG instrument on the Advanced Composition Explorer (ACE) and the VHM instrument on the Ulysses spacecraft, we analyze in-situ magnetic field data. In particular, we examine the radial component of the magnetic field in data from 1998 through 2004 in order to determine the variablity of the open flux and its relation to variations in the area of coronal holes. We describe the implications of our results for the two theories. This work has been supported in part by NSF and NASA. Title: Observable Properties of the Breakout Model for Coronal Mass Ejections Authors: Lynch, B. J.; Antiochos, S. K.; MacNeice, P. J.; Zurbuchen, T. H.; Fisk, L. A. Bibcode: 2004ApJ...617..589L Altcode: We compare the ``magnetic breakout'' model for coronal mass ejections (CMEs) with observed general properties of CMEs by analyzing in detail recent high-resolution MHD simulations of a complete breakout CME. The model produces an eruption with a three-part plasma density structure that shows a bright circular rim outlining a dark central cavity in synthetic coronagraphic images of total brightness. The model also yields height-time profiles similar to most three-part CMEs, but the eruption speed by 2.5 Rsolar is of order the Alfvén speed, indicative of a fast CME. We show that the evolution of the posteruptive flare loop and chromospheric ribbons determined from the model are in agreement with observations of long-duration flares, and we propose an explanation for the long-standing observation that flares have an impulsive and gradual phase. A helical magnetic flux rope is generated during eruption and is consistent with a large class of interplanetary CME observations. The magnetic fields in this flux rope are well approximated by the Lundquist solution when the ejecta are at 15 Rsolar and beyond. Furthermore, the interior density structure of the magnetic flux rope appears to have some of the basic features of an ``average'' magnetic cloud profile at 1 AU. Future simulation improvements and more stringent observational tests are discussed. Title: A Numerical Study of the Breakout Model for Coronal Mass Ejection Initiation Authors: MacNeice, P.; Antiochos, S. K.; Phillips, A.; Spicer, D. S.; DeVore, C. R.; Olson, K. Bibcode: 2004ApJ...614.1028M Altcode: A leading theory for the initiation of coronal mass ejections (CMEs) is the breakout model, in which magnetic reconnection above a filament channel is responsible for disrupting the coronal magnetic field. We present the first simulations of the complete breakout process including the initiation, the plasmoid formation and ejection, and the eventual relaxation of the coronal field to a more potential state. These simulations were performed using a new numerical code that solves the numerical cavitation problems that prevented previous simulations from calculating a complete ejection. Furthermore, the position of the outer boundary in the new simulations is increased out to 30 Rsolar, which enables determination of the full structure and dynamics of the ejected plasmoid. Our results show that the ejection occurs at a speed on the order of the coronal Alfvén speed and hence that the breakout model can produce fast CMEs. Another key result is that the ejection speed is not sensitive to the refinement level of the grid used in the calculations, which implies that, at least for the numerical resistivity of these simulations, the speed is not sensitive to the Lundquist number. We also calculate, in detail, the helicity of the system and show that the helicity is well conserved during the breakout process. Most of the helicity is ejected from the Sun with the escaping plasmoid, but a significant fraction (of order 10%) remains in the corona. The implications of these results for observation and prediction of CMEs and eruptive flares is discussed. Title: A Model for Bright Extreme-Ultraviolet Knots in Solar Flare Loops Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A. Bibcode: 2004ApJ...614.1022P Altcode: EUV observations often indicate the presence of bright knots in flare loops. The temperature of the knot plasma is of the order of 1 MK, and the knots themselves are usually localized somewhere near the loop tops. We propose a model in which the formation of EUV knots is due to the spatial structure of the nonflare active region heating. We present the results of a series of one-dimensional hydrodynamic, flare-loop simulations, which include both an impulsive flare heating and a background, active region heating. The simulations demonstrate that the formation of the observed knots depends critically on the spatial distribution of the background heating during the decay phase. In particular, the heating must be localized far from the loop apex and have a magnitude comparable to the local radiative losses of the cooling loop. Our results, therefore, provide strong constraints on both coronal heating and postflare conditions. Title: Thermal and Nonthermal Emission in Solar Flares Authors: Warren, Harry P.; Antiochos, Spiro K. Bibcode: 2004ApJ...611L..49W Altcode: The observation that in many flares there is a linear correlation between the peak soft X-ray emission and the time-integrated nonthermal emission-the Neupert effect-indicates a strong link between particle acceleration and chromospheric evaporation. In this Letter we consider the hydrodynamic response of impulsively heated flare loops. We find that the peak soft X-ray flux should scale approximately as E1.75/V0.75L0.25, where E is the total input energy, V is the flare volume, and L is the loop length. This scaling is not consistent with the linear relationship implied by the Neupert effect unless there are additional correlations between the input energy and the other parameters of the flare. Title: Bright EUV Knots in Solar Flare Loops: Constraints on Coronal Heating Authors: Patsourakos, S.; Antiochos, S.; Klimchuk, J. Bibcode: 2004AAS...204.8705P Altcode: 2004BAAS...36Q.819P EUV observations often indicate the presence of bright knots in flare loops. The temperature of the knot plasma is of order 1MK, and the knots themselves are usually localized somewhere near the loop tops. We propose a model in which the formation of EUV knots is due to the spatial structure of the non-flare active region heating. We present the results of a series of 1D hydrodynamic, flare-loop simulations, which include both an impulsive flare heating and a background, active region heating. The simulations demonstrate that the formation of the observed knots depends critically on the spatial distribution of the background heating during the decay phase. In particular, the heating must: (1) be localized, (2) be situated far from the loop apex and (3) have a magnitude comparable with the local radiative losses of the cooling loop. Our results, therefore, provide strong constraints on both coronal heating and post-flare conditions.

Research supported by NASA and ONR. Title: Flux Collision Models of Prominence Formation, or Breaking Up is Hard to Do Authors: Welsch, B. T.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2004AAS...204.5505W Altcode: 2004BAAS...36..761W To investigate the hypothesis that the prominences form by magnetic reconnection between initially distinct flux systems above the solar photosphere, we employ the ARMS code, a 3D, flux-corrected transport MHD code with adaptive mesh refinement, to simulate magnetic field evolution when two flux systems are driven to collide by photospheric boundary motions. In particular, we focus on driving configurations similar to the prominence model of Martens and Zwaan (2001).

We find that: 1) reconnection proceeds only weakly, if at all, in typical active region configurations driven with differential-rotation-like shear, which leads to glancing collisions; 2) reconnection proceeds efficiently in configurations that are driven to collide directly, with converging motions along the neutral line; and 3) reconnected fields from this process can exhibit sheared, dipped field lines along the neutral line, consistent with prominence observations.

As our field configurations do not posses the ``breakout'' topology, eruptions are not observed.

This work was supported by ONR, NASA's SEC Theory program, and by a grant of computer time from the DOD High Performance Computing Modernization Program at the ERDC MSRC. Title: The Sheared-Arcade Model for Solar Prominences Authors: DeVore, C. R.; Antiochos, S. K. Bibcode: 2004AAS...204.5504D Altcode: 2004BAAS...36..761D The structure and stability of the magnetic field play critical roles in the formation, evolution, and eventual eruption of solar prominences. We have shown previously that a three-dimensional coronal arcade with strong localized shear exhibits several characteristic properties measured or inferred from prominence observations. These include alignment with the polarity inversion line of the photospheric field, inverse magnetic polarity in the body of the prominence, the necessary restraining overlying arcade field, formation of helical fields at high shear, and linkage of formerly distinct prominences where they come into contact and their magnetic fields reconnect. Our studies also suggested that the resulting structures are very stable, showing no tendency to erupt violently as solar prominences frequently do.

We now are extending these investigations by including in the model the corona's expanding spherical geometry and its gravitationally stratified mass density and thermal pressure. Our expectation is that topologically bipolar prominence structures will be found to rise to greater heights than in our previous cartesian studies, but still will be unable to attain the free energy needed to open the field. A multipolar structure in a breakout configuration, on the other hand, in principle could approach its free-energy threshold and then erupt once the breakout reconnection commences. This outcome would be qualitatively different from our prior results. Progress on these fronts and the implications for our understanding of prominences will be reported.

This research was supported by NASA and ONR. Title: The Topology Of Solar Eruptions Authors: Antiochos, S. K.; DeVore, C. R.; MacNeice, P. J. Bibcode: 2004AAS...204.2706A Altcode: 2004BAAS...36..694A Understanding the physical mechanisms responsible for coronal mass ejections (CME)/eruptive flares is essential for advancing both our knowledge of major solar activity and our capability for forecasting space weather. The topology of the coronal magnetic field plays the essential role in most theories for these major solar eruptions. We discuss the role of 3D effects in the `breakout model' for CMEs. We argue that the 3D topology of the magnetic field is critical for understanding the magnetic reconnection that leads to eruption and understanding the subsequent development of the ejected plasmoid. Both theoretical and numerical results on breakout simulations will be presented. From these results, we derive predictions that can test the validity of the model with the observations expected from the upcoming STEREO and SOLAR-B missions.

This work was supported in part by NASA and ONR. Title: Magnetic Cloud Net Cumulative Helicity During Solar Cycle 23 Authors: Lynch, B. J.; Gruesbeck, J. R.; Zurbuchen, T. H.; Antiochos, S. K. Bibcode: 2004AAS...204.3803L Altcode: 2004BAAS...36..712L Nine years of magnetic clouds (MCs) from the WIND and ACE spacecraft (1995-2003) are analyzed using the static, linear, force-free cylinder model. The net cumulative helicity is defined as the difference between the running totals of right- and left-handed MC helicities calculated from the model. This net helicity is a time-dependent quantity that appears to be approximately sinusoidal with a solar cycle like period. There is a right-handed bias in the helicity carried by MCs during solar minimum, and much stronger left-handed bias in the helicity transported during the rising phase and solar maximum, even though there are not drastic differences in the overall number of right- and left-handed clouds. The Bastille Day 2000 MC event is the largest contributor to this left-handed transition.

Monte Carlo simulations of random magnetic cloud sequences from the observed helicity distribution show that the magnitude of the observed left-handed net helicity bias during solar maximum is only expected 2-4 % of the time if right- and left-handed events are equally likely. Possible sources of CME helicity are discussed.

This work is supported by NASA, ONR, and NSF. BJL is supported by a NASA GSRP fellowship NGT5-50453. Title: Prominence formation through thermal nonequilibrium in a sheared arcade Authors: Karpen, J. T.; Tanner, S. E. M.; Antiochos, S. K.; DeVore, C. R. Bibcode: 2004AAS...204.5502K Altcode: 2004BAAS...36R.760K We have shown, over the past few years, that both static and dynamic prominence condensations can be formed through steady but unequal localized heating in long coronal loops (Antiochos et al. 1999, 2000; Karpen et al. 2001, 2003). Theoretical analyses and numerical simulations with ARGOS, our 1D hydrodynamic code with adaptive mesh refinement, have revealed the behavior of this thermal nonequilibrium mechanism under a wide range of solar conditions. Previously we identified several key parameters governing the existence and characteristics of the condensations: the ratio of loop length to heating scale, the loop apex height, the heating imbalance, and (for dipped fieldlines only) the dip slopes. These earlier calculations assumed a constant cross-sectional area throughout the flux tube, but on the Sun we expect the areas to be highly nonuniform.

To test this condensation process under more realistic conditions, we used our sheared 3D arcade model of the prominence magnetic field (DeVore & Antiochos 2000) to define the geometry of the model flux tube in a set of calculations with ARGOS. We selected representative field lines capable of supporting condensations from the DeVore & Antiochos 3D MHD simulation, measured the flux tube area at intervals along these lines, and derived 5th order polynomial fits to the height and area that were easily recomputed upon regridding. For comparison, constant cross-section ``control" loops also were set up with the same height variations. These field lines were subjected to localized heating near the footpoints, as before, and subsequent developments were monitored. We have explored the effects of uniform vs. nonuniform area, changing the heating imbalance, and altering the radiative loss function. Results from this study will be compared with our previous work and with prominence observations.

This work was supported by NASA and ONR. Title: DC coronal heating and the nonlinear evolution of current sheets Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K. Bibcode: 2004cosp...35.2721D Altcode: 2004cosp.meet.2721D Recent theoretical developments have re-awakened interest in the role of electric current sheets in DC coronal heating (Parker 1988; Priest et al. 2002). Dahlburg et al. (2003; 2004) reported the existence of a ``secondary instability'' that could explain the required ``switch-on'' effect required for adequate energy storage. This ideal, three-dimensional instability also provided a straightforward explanation for the subsequent fast release of energy, as the rapid growth of the mode eventually results in a state of turbulent magnetic reconnection. Earlier studies of the secondary instability were limited to systems with relatively simple perturbations, viz., resistive stability eigenmodes. A current sheet in the Sun is likely to be subject to much more complex perturbations involving a waves of various wavelengths and amplitudes. We describe the evolution of three-dimensional electric current sheets disturbed by random 3D perturbations. We find that the significant characteristics of secondary instability are also observed in this case. The relative importance of subharmonic interactions, i.e., coalescence instability, will also be discussed. R. B. Dahlburg, J. A. Klimchuk and S. K. Antiochos Adv. Space Phys. 32, 1029 (2003). R. B. Dahlburg, J. A. Klimchuk and S. K. Antiochos Astrophys. J.. submitted, (2004). E. N. Parker Astrophys. J. 330, 474 (1988). E. R. Priest, J. F. Heyvaerts and A. M. TItle, Astrophys. J. 576, 533 (2002). Title: Reconnection in solar flares and coronal mass ejections Authors: Antiochos, S. Bibcode: 2004cosp...35.2381A Altcode: 2004cosp.meet.2381A Magnetic reconnection is widely believed to be the physical process underlying much of solar activity. We argue that it plays two critical roles in eruptive flares/coronal mass ejections (CME). Reconnection is the trigger mechanism that initiates the eruption, and reconnection is the relaxation process by which the post-eruption field relaxes down to a quasi-potential closed state. We discuss the distinguishing physical features of these two types of reconnection and their implications for theory. Some of the latest results of both observations and numerical simulations of reconnection in CMEs/eruptive flares will be presented. This work is supported in part by NASA and ONR. Title: The role of flux emergence as a driver of coronal mass ejections Authors: Magara, T.; Antiochos, S. K.; Luhmann, J. G. Bibcode: 2003AGUFMSH42B0512M Altcode: Recently it has been suggested that coronal mass ejections (CMEs) are closely related to the interaction between different magnetic domains formed in the corona. For example, a so-called breakout model of CMEs shows that a core domain field which is enhanced by shearing motions in the photosphere interacts with the overlying field, and this weakens the confining effect of the overlying field and eventually enables the core domain field to erupt outwards. In this study, we take the subphotospheric dynamics into this model and see how flux emergence affects the breakout process. Our work is based on 3-dimensional resistive MHD simulations in which we initially set a potential bipolar field above the photosphere and place a magnetic flux tube below the photosphere. The flux tube then emerges into the photosphere and starts to interact with the bipolar field. As the flux tube expands into the corona, a current layer develops around the interface between emerging magnetic field and preexisting coronal field. We focus on its structure and evolution because that current layer plays a crucial role in a breakout of emerging magnetic field. To see this, we apply a locally enhanced resistivity around the current layer and study the magnetic reconnection between emerging field and preexisting field. Title: The Coronal Magnetic Field Predicted by the Breakout Model Authors: Antiochos, S. K. Bibcode: 2003AGUFMSH41A..02A Altcode: Although there has been intense theoretical work in the past decade on coronal mass ejections/eruptive flares, the mechanism for their initiation is far from accepted. The problem is that the triggering of these events is believed to be magnetically-driven and to take place in the corona, but until very recently, the field there has not been observed directly. We discuss how the new developments in coronal field instrumentation will allow us to determine the mechanism for CME/eruptive flare initiation. We focus, in particular, on the so-called breakout model in which reconnection in the corona is postulated to be the triggering process. First, we present the latest numerical results on breakout. Then, we determine the predictions of the model for coronal magnetic observations, and discuss definitive tests of breakout that will be possible with the proposed new instrumental capabilities. This work was supported in part by ONR and NASA. Title: Comparison of the Breakout Model With Flare-Loop and Ionic Composition Data Authors: Lynch, B. J.; MacNeice, P. J.; Antiochos, S. K.; Zurbuchen, T. H. Bibcode: 2003AGUFMSH22B..08L Altcode: We discuss our ongoing analysis of the observational properties of Breakout coronal mass ejections. Presented are quantitative analyses of the post-eruption evolution that produces the commonly observed flare-loop arcades at the limb and the spreading flare ribbons at disk center. The simulation dynamics are compared to observational results of long-duration flares. We also present a novel application of the ionic charge freeze-in analyses to the numerical simulation output. This post-processing allows us to 'predict' the heavy ion charge states (O7+/O6+) from the simulation density, temperature, and velocities. Due to limitations in the energy accounting and initial conditions of the MHD simulation, we do not obtain the observed O7+/O6+ values, but we emphasize the potential of this method and show how it can be used to study relative variation in charge state composition throughout the ejecta volume. Title: Coronal energy release via ideal three-dimensional instability three-dimensional instability Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K. Bibcode: 2003AdSpR..32.1029D Altcode: It is widely believed that most coronal phenomena involve the release of free energy that is stored within stressed magnetic field configurations. The availability of sufficient free energy to explain everything from coronal heating to flares and coronal mass ejections is well established. How this energy is released remains a major puzzle. Observations reveal that an important property of the energy release mechanism is its "switch on" character. The mechanism must remain dormant for long periods of time to allow the magnetic stresses to build, then it must operate very vigorously once it finally turns on. We discuss a mechanism called the "secondary instability" which exhibits this behavior. It is essentially an ideal instability of the thin twisted magnetic flux tubes that form from the resistive tearing of current sheets. We relate the mechanism to the coronal heating idea of Parker in which the coronal magnetic field becomes tangled by random motions of the photospheric footpoints. Global energy balance considerations imply that magnetic energy dissipation occurs at a particular angle in the field, and the secondary instability offers the first quantitative explanation for why this should be. It thus places Parker's popular idea on a much firmer physical footing. Title: Constraints on the Magnetic Field Geometry in Prominences Authors: Karpen, J. T.; Antiochos, S. K.; Klimchuk, J. A.; MacNeice, P. J. Bibcode: 2003ApJ...593.1187K Altcode: This paper discusses constraints on the magnetic field geometry of solar prominences derived from one-dimensional modeling and analytic theory of the formation and support of cool coronal condensations. In earlier numerical studies we identified a mechanism-thermal nonequilibrium-by which cool condensations can form on field lines heated at their footpoints. We also identified a broad range of field line shapes that can support condensations with the observed sizes and lifetimes: shallowly dipped to moderately arched field lines longer than several times the heating scale. Here we demonstrate that condensations formed on deeply dipped field lines, as would occur in all but the near-axial regions of twisted flux ropes, behave significantly differently than those on shallowly dipped field lines. Our modeling results yield a crucial observational test capable of discriminating between two competing scenarios for prominence magnetic field structure: the flux rope and sheared-arcade models. Title: Constraints on Active Region Coronal Heating Authors: Antiochos, S. K.; Karpen, J. T.; DeLuca, E. E.; Golub, L.; Hamilton, P. Bibcode: 2003ApJ...590..547A Altcode: We derive constraints on the time variability of coronal heating from observations of the so-called active region moss by the Transition Region and Coronal Explorer (TRACE). The moss is believed to be due to million-degree emission from the transition regions at the footpoints of coronal loops whose maximum temperatures are several million degrees. The two key results from the TRACE observations discussed in this paper are that in the moss regions one generally sees only moss, not million-degree loops, and that the moss emission exhibits only weak intensity variations, ~10% over periods of hours. TRACE movies showing these results are presented. We demonstrate, using both analytic and numerical calculations, that the lack of observable million-degree loops in the moss regions places severe constraints on the possible time variability of coronal heating in the loops overlying the moss. In particular, the heating in the hot moss loops cannot be truly flarelike with a sharp cutoff, but instead must be quasi-steady to an excellent approximation. Furthermore, the lack of significant variations in the moss intensity implies that the heating magnitude is only weakly varying. The implications of these conclusions for coronal heating models will be discussed. Title: Internal structure of magnetic clouds: Plasma and composition Authors: Lynch, B. J.; Zurbuchen, T. H.; Fisk, L. A.; Antiochos, S. K. Bibcode: 2003JGRA..108.1239L Altcode: A comprehensive analysis of magnetic clouds observed by the Advanced Composition Explorer (ACE) spacecraft from February 1998 to July 2001 is presented. The magnetic field data from the MAG instrument is fit with the cylindrically symmetric, linear force-free model and the fit parameter distributions are examined. This magnetic field model enables us to map plasma data from the SWEPAM and SWICS instruments to a position within the model cylinder. A superposed epoch analysis of all our magnetic cloud events is used to construct diameter cuts through an "average" cloud profile in any desired plasma, elemental composition, or charge state quantity. These diameter cuts are found to have nontrivial structure and there appears to be significant composition and structural differences between clouds of different speeds. The slow magnetic clouds (<Vrad> < 500 km/s) have an almost constant proton density profile whereas the fast magnetic cloud (<Vrad> ≥ 500 km/s) profile is depleted throughout with symmetric dips and a local maximum at the cloud center. The fast magnetic clouds have a slightly higher Nα/Np ratio than the slow clouds. Both the fast and slow events have enhanced oxygen and iron charge states compared to the slow solar wind. The fast events have a slightly increased O7+/O6+ average profile and a much stronger Fe≥16+/Fetotal profile than the slow events. We briefly discuss the implications for physical conditions at the Sun, the role these coronal mass ejections (CMEs) may play in transporting magnetic flux, and the application of our structure results to the current flux rope CME modeling effort. Title: Coronal Hole Topology Authors: Antiochos, S. K. Bibcode: 2003SPD....34.0102A Altcode: 2003BAAS...35..805A A key problem for Solar/Heliospheric physics is understanding the structure and dynamics of the open magnetic field regions on the Sun, the so-called coronal holes. There has been considerable debate in recent years on the nature of coronal hole evolution, in particular, on how closed and open field regions interact. We use the source surface model to investigate in detail the topology of the solar coronal field in cases where closed-field active regions interact with the boundaries of coronal holes. These studies lead us to the conjecture that in any continuously connected polarity region on the photosphere, there can be at most one coronal hole, but this open field region is likely to be highly complex with narrow corridors of open flux connecting apparently isolated coronal holes. We discuss both the observational and theoretical implications of our conjecture for the Sun - Heliosphere connection.

This work was supported in part by NASA and ONR. Title: Coronal Mass Ejection Breakout with Adaptive Mesh Refinement Authors: Lynch, B. J.; MacNeice, P. J.; Antiochos, S. K.; Zurbuchen, T. H. Bibcode: 2003SPD....34.0513L Altcode: 2003BAAS...35S.816L We compare the magnetic breakout model for solar coronal mass ejections (CMEs) of Antiochos, DeVore, & Klimchuck [1] to coronagraph and in-situ observations. We study the effect of adaptive mesh refinement (AMR) on the density structures and coronagraph dynamics predicted by the model. Synthetic coronagraph images are shown to reproduce the dark cavity and bright rim features of 3-part CME observations. Height-time plots are generated and compared with LASCO observations. The breakout model creates a flux-rope like structure during eruption that shares many properties with interplanetary magnetic cloud observations, e.g. internal fields that can be well described by a linear, force-free cylinder. This work is supported by ONR and NASA.

[1] Antiochos, S. K., C. R. DeVore, J. A. Klimchuck, 1999, ApJ 510, pp. 485-493. Title: Effects of nonuniform flux tube area on prominence formation through thermal nonequilibrium Authors: Karpen, J. T.; Tanner, S. E. M.; Antiochos, S. K. Bibcode: 2003SPD....34.0414K Altcode: 2003BAAS...35R.812K We have developed a dynamic model of prominence formation in which steady but unequal footpoint heating causes a dynamic cycle of chromospheric evaporation, condensation, motion, and destruction (Antiochos et al. 1999a, 2000; Karpen et al. 2001, 2002). We have performed 1D hydrodynamic simulations with varying geometries and other properties to determine the limits of this mechanism under solar conditions. In previous studies we identified several key parameters that dictate the existence and characteristics of this cyclic process: the ratio of loop length to heating scale height, the loop apex height, the heating asymmetry, and dip depth. For those idealized calculations, the cross-sectional area of the flux tube was assumed to be constant. On the Sun, however, we expect the flux tube areas to be highly nonuniform, narrowing where the flux is constrained by stronger adjacent fields and expanding where neighboring fields are weaker.

To determine the effects of varying cross-sectional area on the evaporation and condensation processes at the core of our prominence formation model, we performed a set of 1D calculations with ARGOS, our 1D hydrodynamic code with adaptive mesh refinement. Representative field lines capable of supporting prominence condensations were selected from the 3D sheared-arcade model of the prominence magnetic field (DeVore & Antiochos 2000); the flux tube area was measured at intervals along these field lines and fit by a smooth analytic function suited for our computational approach. For comparison, ``control" loops also were set up with the same 1D loop geometry but with constant cross-section. As in our earlier calculations, these field lines were subjected to steady, localized heating at the footpoints and subsequent developments were monitored. Results from this study will be presented in the context of our previous studies and compared with prominence observations, as a critical test of our model.

This work was supported by NASA and ONR. Title: Coronal Energy Release via Explosive Three-Dimensional Instability Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K. Bibcode: 2003SPD....34.0107D Altcode: 2003BAAS...35..806D It is widely believed that most coronal phenomena involve the release of magnetic free energy that is stored within stressed magnetic field configurations. The availability of sufficient free energy to explain everything from coronal heating to flares and coronal mass ejections is well established, but how this energy is released remains a major puzzle. Observations reveal that an important property of the energy release mechanism is its ``switch on" character. The mechanism must remain dormant for long periods of time to allow the magnetic stresses build, then it must operate very vigorously once it finally turns on.

We discuss a mechanism called the ``secondary instability" which exhibits this behavior. It is essentially the ideal kinking of thin twisted magnetic flux tubes that form from the resistive instability of current sheets. We relate the mechanism to the coronal heating idea of Parker in which the coronal magnetic field becomes tangled by random motions of the photospheric footpoints. Global energy balance considerations imply that magnetic energy dissipation occurs at a particular angle in the field, and the secondary instability offers the first quantitative explanation for why this should be. It thus places Parker's popular idea on a much firmer physical footing.

This research was funded by NASA. Title: The Energetics of Breakout Coronal Mass Ejections Authors: DeVore, C. R.; Antiochos, S. K. Bibcode: 2003SPD....34.0517D Altcode: 2003BAAS...35..817D A key obstacle to understanding fast coronal mass ejections lies in slowly accumulating sufficient magnetic energy to power an explosive eruption of the structure, rather than a gradual expansion, and then rapidly releasing the stored energy when a threshold is crossed and the event is triggered. In the breakout model, a low-lying stressed field is restrained by an overlying coronal field containing an embedded null. The abrupt transition to explosive behavior occurs when reconnection at the null lowers the energy required to open the remnant restraining flux below the free energy stored in the stressed field.

We investigate the energetics of opening the coronal magnetic field under two possible extremes of evolution: (1) complete reconnection at the null produces a "maximally closed" final state, whose free energy is the lower bound for opening the structure; (2) zero reconnection at the null produces a "maximally open" final state, whose free energy is an upper bound. We calculate the energies of these states for coronal fields that are potential everywhere except at discrete current sheets, where the magnetic stresses are continuous and the field is force-free. Varying the relative amounts of flux in the inner arcade and the overlying field shifts the location of the potential null. In axisymmetric spherical geometries, the free energies of the "maximally closed" states can be less than 2.5% of the energy of the initial configuration, and as small as 125% of the initial energy of the inner arcade, with the null positioned at 1.4 and 1.6 Rs, respectively. These findings change little as the surface distributions of arcade and overlying flux are increasingly concentrated, and all energy-minimizing states have a net amount of open overlying flux. Results for fully three-dimensional, nonaxisymmetric breakout configurations will be presented.

This research was supported in part by NASA and ONR. Title: A Model for Prominence Formation Authors: Welsch, B. T.; DeVore, C. R.; Antiochos, S. K.; Linton, M. G. Bibcode: 2003SPD....34.0413W Altcode: 2003BAAS...35..812W The essential features of a model prominence configuration include: dipped or helical field lines that are capable of supporting mass against gravity; sheared field lines that run nearly parallel to the photospheric polarity inversion line (PIL); and overlying field lines that restrain the sheared field lines.

To determine how reconnection might generate such field configurations, we have used the ARMS code, an MHD solver with adaptive refinement, to model the interaction of two active regions subjected to a shear flow, meant to approximate differential rotation.

We present preliminary results from two cases, one in which the model active region fields were potential prior to shearing, and one in which the active region fields were ``spun up'' prior to shearing, to approximate active region fields possessing twist at their emergence.

This work was supported in part by NASA, DoD's HPCMP, and AFOSR's Solar-MURI program. Title: A Transient Heating Model for Coronal Structure and Dynamics Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.; Antiochos, S. K.; Klimchuk, J. A.; MacNeice, P. J. Bibcode: 2003ApJ...582..486S Altcode: A wealth of observational evidence for flows and intensity variations in nonflaring coronal loops leads to the conclusion that coronal heating is intrinsically unsteady and concentrated near the chromosphere. We have investigated the hydrodynamic behavior of coronal loops undergoing transient heating with one-dimensional numerical simulations in which the timescale assumed for the heating variations (3000 s) is comparable to the coronal radiative cooling time and the assumed heating location and scale height (10 Mm) are consistent with the values derived from TRACE studies. The model loops represent typical active region loops: 40-80 Mm in length, reaching peak temperatures up to 6 MK. We use ARGOS, our state-of-the-art numerical code with adaptive mesh refinement, in order to resolve adequately the dynamic chromospheric-coronal transition region sections of the loop. The major new results from our work are the following: (1) During much of the cooling phase, the loops exhibit densities significantly larger than those predicted by the well-known loop scaling laws, thus potentially explaining recent TRACE observations of overdense loops. (2) Throughout the transient heating interval, downflows appear in the lower transition region (T~0.1 MK) whose key signature would be persistent, redshifted UV and EUV line emission, as have long been observed. (3) Strongly unequal heating in the two legs of the loop drives siphon flows from the more strongly heated footpoint to the other end, thus explaining the substantial bulk flows in loops recently observed by the Coronal Diagnostic Spectrometer and the Solar Ultraviolet Measurement of Emission Radiation instrument. We discuss the implications of our studies for the physical origins of coronal heating and related dynamic phenomena. Title: Helicity Generation and Evolution in Coronal Mass Ejections Authors: Spicer, Daniel S.; MacNiece, Peter; Antiochos, Spiro; Finn, John M. Bibcode: 2003IAUJD...3E...5S Altcode: A key issue in current modeling of coronal mass ejections is the role of helicity. A number of authors have argued that CMEs are somehow the result of the accumulation of too much helicity in the solar corona -- the so-called helicity charging concept. We present results demonstrating that this concept is incorrect and that the only essential ingredient for eruption is magnetic free energy. We also show that the Taylor conjecture is not valid for the pre-eruption corona. On the other hand A Taylor-like evolution does seem to be applicable to the post-eruption flux rope in the far outer corona. Results from numerical simulations of the CME breakout model will be presented that include magnetic helicity evolution. Other diagnostic information such as the total magnetic energy total kinetic energy and local magnetic energy density are also tracked and compared with global and local helicity. Boundary conditions that allow for finite and zero helicity injection into the computational domain will also be presented and compared Title: Theoretical Energy Analysis of Reconnecting Twisted Magnetic Flux Tubes Authors: Linton, M. G.; Antiochos, S. K. Bibcode: 2002ApJ...581..703L Altcode: It has been shown that twisted magnetic flux tubes reconnect in a number of different ways: either bouncing, tunneling, slingshotting, or merging. Here we present an analytical theory to predict under what conditions the bounce, tunnel, and slingshot will occur. This theory calculates the energy of the reconnected state relative to that of the initial state, subject to the restriction that helicity is conserved during the interaction. Comparison of this energy change for each of these interactions then indicates which is most energetically favorable. In addition to providing potentially important predictive capabilities, for example, for solar or magnetospheric flux-tube reconnection, this also provides an intuitive explanation for why the tunnel interaction occurs. Title: Hydrodynamics of coronal loops undergoing transient heating Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.; Antiochos, S. K.; MacNeice, P. J. Bibcode: 2002ASPC..277..597S Altcode: 2002sccx.conf..597S No abstract at ADS Title: Bright Knots in EUV Post-flare Loops : TRACE Observations and 1D Hydrodynamic Modeling Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A. Bibcode: 2002AGUFMSH21C..04P Altcode: EUV post-flare loops often possess bright knots along them. Some examples of such post-flare loops seen by TRACE will be shown, along with a brief outline of their properties. We will then present the results of a series of 1D hydrodynamic simulations of flaring loops, which employ different heating functions for the impulsive and decay phase of the simulated flares. It will be demonstrated that the creation of these knots depends crucially on the spatio-temporal distribution of the heating during the decay phase. This provides strong constraints on both post-flaring conditions and AR loop heating. We will finally briefly outline how SDO instrumentation could improve our knowledge of this topic. Research supported in part by NASA and ONR. Title: Coronal Canals Authors: Antiochos, S. K. Bibcode: 2002AGUFMSH21C..03A Altcode: As the first mission of LWS, one of SDO's most important goals will be to determine how the plasma and magnetic structures observed on the Sun connect to those observed in the heliosphere. In this paper, we show that the topology of the coronal open field, (and consequently the geometry of the heliospheric current sheet), is likely to be much more complex than previously believed. Using the source-surface model, we calculate the response of a polar coronal hole to the slow emergence of a high-latitude bipolar active region. We show that at a critical point in the active region growth, the coronal hole boundary must jump discontinuously to form a narrow channel of open field that encircles the trailing polarity spot. Contrary to a common misconception, the open field region does not form a new coronal hole that is disconnected from the polar one, but remains as one continuously connected region. The boundary of the open field region, however, acquires enormous structure, with canals that extend down to low latitudes. We argue that these open-field canals are the explanation for the wide-spread belief that open field emanates from closed field regions. We also discuss how magnetic reconnection would, in fact, lead to the type of structure produced by the source-surface model,and how SDO would observe the canals on the Sun. This work was supported in part by NASA and ONR. Title: Fuzzy hot post-flare loops versus sharp cool post-flare loops Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A. Bibcode: 2002ESASP.505..207P Altcode: 2002solm.conf..207P; 2002IAUCo.188..207P By using high spatial resolution TRACE EUV observations we show that hot (≍2 MK) post-flare loops are fuzzier than the cooler (≍1 MK) ones. A simple 0d model of a cooling loop arcade, where different loops in the arcade start to cool down at slightly different initial conditions, is sufficient to reproduce qualitatively the observed behavior of the EUV post-flare loops. Title: Hydrodynamic models of transiently heated coronal loops Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.; Antiochos, S. K.; Klimchuk, J. A.; MacNeice, P. J. Bibcode: 2002ESASP.505..583S Altcode: 2002solm.conf..583S; 2002IAUCo.188..583S We investigate the hydrodynamic behaviour of coronal loops undergoing transient heating. We adopt a 1-D loop model with space- and time-dependent heating, concentrated near the chromospheric footpoints. The timescale of heating variations is comparable with the radiative cooling time of the coronal plasma (~103s). We use a new numerical code that has a fully adaptive grid, in order to properly resolve the chromospheric-coronal transition region sections of the loop. We simulate here the hydrodynamics of a loop with different effective gravity (i.e., loop geometry) and heating terms. We describe the temporal behaviour of the various physical quantities along the loop (plasma density, temperature, flow velocity), showing that the increase in heating produces a chromospheric evaporation, or a siphon flow if the loop heating is taken to be significantly different at the two footpoints, followed by long-lasting downflows with velocities of a few km s-1 during the quiescent phases in between the episodic heatings. Moreover, in the case of considerable increase in heating, a catastrophic cooling of the loop plasma can occur, giving rise to downflows of several tens of km s-1. Title: Sheared magnetic fields and eruptive phenomena: prominences, flares, and coronal mass ejections Authors: Antiochos, Spiro K. Bibcode: 2002ESASP.505..219A Altcode: 2002solm.conf..219A; 2002IAUCo.188..219A The underlying cause of all the giant manifestations of solar activity - CMEs, eruptive flares and filament ejections - is the disruption of a force balance between the upward pressure of the strongly sheared field of a filament channel and the downward tension of overlying field that is quasi-potential. A key point is that the upward pressure cannot increase rapidly, because the magnetic shear/twist is produced by the slow photospheric evolution (shear flows and/or flux emergence). Therefore, explosive events such as flares and CMEs must be due fundamentally to the catastrophic removal of the downward magnetic tension. Recent theory and simulation have focused on magnetic reconnection as the mechanism for the removal of the magnetic tension. This paper discusses critically the recent models for magnetic disruptions in the Sun's corona. Title: Advances and prospects in solar theory Authors: Antiochos, Spiro K. Bibcode: 2002ESASP.505Q....A Altcode: 2002solm.confQ....A; 2002IAUCo.188Q....A Theoretical solar physics has undergone a revolution during the last decade, transitioning from a field in which the bulk of the work consisted of developing semi-ideal models of highly simplified systems, to fully 3D models using large-scale simulations and actual observations as input. We are now close to developing comprehensive theories for many of the outstanding problems in solar physics. Based on results presented at the Santorini Conference on Magnetic Coupling of the Solar Atmosphere, this paper highlights some of the recent solar theory progress and discusses the likely opportunities for new advances. Of course, it is not possible to describe even the gist of these results in this brief review, therefore, I merely attempt to organize some of them into a coherent structure and provide a personal perspective. The natural organizing structure is to consider the major theoretical problems that are now preventing us from developing a comprehensive model for how the magnetic field couples the interior to the atmosphere and, thereby, produces solar activity. Title: Coronal Magnetic Field Relaxation by Null-Point Reconnection Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R. Bibcode: 2002ApJ...575..578A Altcode: We derive the minimum energy state resulting from complete magnetic reconnection in a translationally or axisymmetric MHD system, in the limit of a low plasma beta and high magnetic Reynolds number-conditions appropriate to the solar corona. The results are necessary for determining the amount of energy that can be liberated by reconnection and, hence, are important for understanding coronal heating and other forms of solar activity. The key difference between our approach and previous work is that because of line tying at the high-beta photosphere, reconnection is limited to occur only at magnetic null points initially present in the system. We find that under these circumstances the minimum energy state is not the usual linear force-free field but a state in which the nonpotential component of the field is distributed uniformly on equal flux surfaces. We discuss the implications of our results for the Sun's corona and for laboratory plasmas. Title: Hydrodynamic simulations of coronal loops subject to transient heating Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.; MacNeice, P. J.; Antiochos, S. K.; Klimchuk, J. A. Bibcode: 2002ESASP.508..331S Altcode: 2002soho...11..331S We investigate the hydrodynamic behaviour of coronal loops undergoing transient heating. We adopt a 1-D loop model with space- and time-dependent heating, concentrated near the chromospheric footpoints. The timescale of heating variations is comparable with the radiative cooling time of the coronal plasma (~103s). We use a new numerical code that has a fully adaptive grid, in order to properly resolve the chromospheric-coronal transition region sections of the loop. We simulate here the hydrodynamics of a loop with different effective gravity (i.e., loop geometry) and heating terms. We describe the temporal behaviour of the various physical quantities along the loop (plasma density,temperature, flow velocity), showing that the increase in heating produces a chromospheric evaporation, or a siphon flow if the loop heating is taken to be significantly different at the two footpoints, followed by long-lasting downflows with velocities of a few km s-1 during the quiescent phases in between the episodic heatings. Moreover, in the case of considerable increase in heating, a thermal instability can occur during the cooling phase of the loop plasma, giving rise to downflows of several tens of km s-1. Title: Reconnection of Twisted Magnetic Flux Tubes Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K. Bibcode: 2002AAS...200.8802L Altcode: 2002BAAS...34..789L We present 3D MHD simulations of the collision and reconnection of pairs of twisted, isolated magnetic flux tubes at various collision angles. Such reconnection is likely to be an important source of energy release in solar flares, and could play a role in coronal mass ejection dynamics. We show that the dynamics of the reconnection depends strongly on the collision angle between the tube axes and on the relative sign of twist of the tubes. The most energetic interaction is a slingshot interaction, analogous to the reconnection often seen in 2D simulations. But, depending on the configuration, the tubes can also bounce without reconnecting, merge into a single tube, or tunnel through each other. We will discuss these various interactions, the topological changes they bring about, and the magnetic energy released. In addition we will present an analytical model which explains some of the results, in particular the tunnel and slingshot interactions, in terms of a simple energy calculation based on helicity conservation. This work was supported by NASA and ONR grants, an ITP-NSF grant, and a grant of computer time from the DoD/HPC Program. Title: Hot versus cool coronal loops Authors: Patsourakos, S.; Klimchuk, J. A.; Antiochos, S. K. Bibcode: 2002AAS...200.0209P Altcode: 2002BAAS...34..640P EUV and SXR observations show respectively that cool (1 MK) loops are finer and maybe more dynamic than hotter (2 MK) ones. Whether this reflects a fundamental difference in the properties of the heating mechanism in action in each loop class is not yet clear. We will address some aspects of this issue by combining EUV and SXR observations of such loops with eventually hydrodynamic simulations of a nano-flare heated corona. Research supported in part by ONR and NASA. Title: An Explanation for the ``Switch On" Character of Magnetic Energy Release Authors: Klimchuk, J. A.; Dahlburg, R. B.; Antiochos, S. K. Bibcode: 2002AAS...200.1607K Altcode: 2002BAAS...34..668K It is widely believed that most coronal phenomena involve the release of magnetic free energy that is stored in stressed magnetic field configurations. The availability of sufficient free energy to explain everything from coronal heating to flares and coronal mass ejections is well established, but how this energy is released remains a major puzzle. Observations reveal that an important property of the energy release mechanism is its ``switch on" character. The mechanism must remain dormant for long periods of time to allow the magnetic stresses to build, then it must operate very vigorously once it finally turns on. We discuss a mechanism called the ``secondary instability" which exhibits this behavior. It is essentially the ideal kinking of thin twisted magnetic flux tubes that form from the restive tearing of current sheets. We relate the mechanism to the coronal heating idea of Parker in which the coronal magnetic field becomes tangled by random motions of the photospheric footpoints. Global energy balance considerations imply that magnetic energy dissipation occurs at a particular angle in the field, and the secondary instability offers the first quantitative explanation for why this should be. It thus places Parker's popular idea on a much firmer physical footing. Title: Constraints placed by thermal nonequilibrium on the topology of prominence magnetic fields Authors: Karpen, J.; Antiochos, S. K.; MacNeice, P. Bibcode: 2002AAS...200.3719K Altcode: 2002BAAS...34..698K We have developed a dynamic model of prominence formation in which steady but unequal footpoint heating causes a dynamic cycle of chromospheric evaporation, condensation, motion, and destruction [Antiochos et al. 1999a, 2000, ApJ; Karpen et al. 2001, ApJ]. We have performed 1D hydrodynamic simulations with varying geometries and other properties to determine the limits of this mechanism under solar conditions. In previous studies we identified three key parameters that dictate the existence and characteristics of this cyclic process: the ratio of loop length to heating scale height, the loop apex height, and the heating asymmetry. Here we discuss our latest calculations, in which we studied the role of the depth of field-line dips -- a feature common to most magnetic-field configurations proposed for prominences. In long fluxtubes with dips deeper than roughly f * Hg, where f measures the heating imbalance between footpoints and Hg is the gravitational scale height, condensations form, quickly fall to the bottom of the dip, and remain there while steadily accreting mass. Therefore, strongly dipped loops are not capable of supporting the observed counterstreaming flows along prominence spines. This places stringent limitations on flux rope models [e.g., Rust & Kumar 1994, SolPhys], as only the least twisted field lines close to the axis pass this test. For our shear-based model of prominence fields [Antiochos et al. 1999b, ApJ], a larger subset of field lines can support prominences formed by thermal nonequilibrium: for the case shown (f=0.25), fluxtubes longer than ~80 Mm, lower than ~100 Mm at the apex, or less deeply dipped than ~25 Mm meet the requirements. This work was supported by NASA and ONR. Title: Active Region Loop Heating Authors: Antiochos, S. K.; Karpen, J. T.; DeLuca, E. E.; Golub, L.; Hamilton, P. Bibcode: 2002AAS...200.1606A Altcode: 2002BAAS...34..668A A long-standing unresolved question in solar physics is whether the heating in coronal loops is steady or impulsive. X-ray observations of high-temperature loops (T > 2 x 106 K) tend to show quasi-steady structures, (evolution slow compared to cooling time scales), whereas theoretical models strongly favor impulsive heating. We present simulations of impulsively heated loops using our adaptive-mesh-refinement code ARGOS, and compare the results with TRACE observations of the transition regions of high-temperature active region loops. From this comparison, we deduce that the heating in the core of active regions is quasi-steady rather than impulsive. These results pose a formidable challenge to developing theoretical models for the heating. This work was supported in part by NASA and ONR. Title: ACE Magnetic Clouds - Distributions and Statistics Authors: Lynch, B. J.; Zurbuchen, T. H.; Fisk, L. A.; Antiochos, S. K. Bibcode: 2002AGUSMSH21A..05L Altcode: We present a comprehensive analysis of magnetic cloud events observed by the Advanced Composition Explorer spacecraft. We fit the standard cylindrically symmetric force-free constant alpha magnetic field model to each event and examine the model parameter distributions. In general, our parameter distributions agree with results of previous studies. Based on significant differences in many plasma and composition quantities we distinguish between fast (< Vrad > >= 500 km/s) and slow (< Vrad > < 500 km/s) magnetic clouds and note the differences in their size and axial field strength distributions. In addition, we examine some physical properties of these clouds such as average Fe charge state, axial current, and magnetic flux. The cloud orientation evolution during solar cycle 23 is considered and compared to the corresponding time period of solar cycle 21. We also compare our magnetic cloud frequency to the variation in sunspot number and the variation in the dipole and quadrupole moments of the photospheric magnetic field. Title: Simulations of Interactions and Magnetic Reconnection Between Solar Filaments Authors: DeVore, C. R.; Antiochos, S. K.; Aulanier, G. Bibcode: 2002AAS...200.3720D Altcode: 2002BAAS...34..698D It has long been known that pairs of filaments near each other on the Sun's disk sometimes come into contact and interact. Under favorable conditions, the two structures apparently link up to form a single, larger filament. When conditions are unfavorable, on the other hand, the filaments appear to avoid each other and retain their distinct identities. Recent ground-based observational studies have shown that a key requirement for linkage to occur is that the two filaments possess the same chirality, or handedness. We have performed detailed numerical experiments of pairs of interacting filaments within the sheared-arcade model. In this model, the filament plasma resides in the magnetic hammock formed in a strongly sheared field held down by an overlying arcade. We considered four cases: like or unlike chirality of the two filaments, and like or unlike polarity of the vertical magnetic fields at their approaching ends. Only the case of like chirality and unlike polarity produces any significant reconfiguration. The magnetic structure is substantially modified, with reconnected field lines extending over the entire combined length of the filaments. Low, closed arcade fields form in the reconnection zone, forcing the newly linked filament fields above them to rise and form a magnetic 'aneurysm.' Our simple, bipolar configuration relaxes to a new equilibrium, consistent with those cases in which the linked structure is observed to persist stably after the interaction has passed. In the much more complex magnetic environment of the solar corona, on the other hand, newly linked filaments with such aneurysms sometimes are observed to erupt promptly and violently. The removal of the restraining arcade fields, by reconnection with the external field of the corona, is likely necessary for eruption to occur. This research was supported by NASA and ONR. Title: Magnetic Reconnection in Solar Flares and Coronal Mass Ejections Authors: Antiochos, Spiro Bibcode: 2002APS..APRK12003A Altcode: Magnetic reconnection is, perhaps, the most important process in space plasmas for transferring energy from magnetic fields to matter. In particular, reconnection is believed to be the underlying driver of the giant explosive releases of magnetic energy in the Sun's atmosphere that are observed as a solar flare and/or coronal mass ejection (CME) event. First, I will present compelling evidence for reconnection in flares and CMEs from the spectacular observations by TRACE and SOHO. Next the recent advances in reconnection theory will be reviewed, and their implications for flare/cme models will be discussed. Simulations will be presented of the recently developed "breakout" model for eruptive flares/CMEs, in which reconnection plays the key role in initiating the event. This work was supported in part by NASA and ONR. Title: Prominence Magnetic Dips in Three-Dimensional Sheared Arcades Authors: Aulanier, G.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2002ApJ...567L..97A Altcode: We calculate the distribution of field-line dips in the three-dimensional sheared arcade model for prominence/filament magnetic fields. We consider both moderately and highly sheared configurations computed by fully time-dependent three-dimensional MHD simulations in which the field was relaxed to a static equilibrium end state. In agreement with previous low spatial resolution measurements of the magnetic field inside prominences, we find that for all configurations, the field in the great majority of the calculated dips exhibits inverse polarity. But for each configuration we also find well-defined narrow regions with stable dips of normal polarity. These tend to be located on the edges of the filament ends and at the top of the central part of the prominence. This distinctive mixture of normal/inverse polarity dips that we find in sheared arcades is not likely to be present in twisted flux rope prominence models. Therefore, our results provide a rigorous and unique observational test that can distinguish between the two classes of models, as well as new predictions for future high spatial resolution spectropolarimetric observations of filaments and prominences. Title: CME/Flare energy release Authors: Antiochos, S. Bibcode: 2002cosp...34E.772A Altcode: 2002cosp.meetE.772A The most spectacular and most energetic manifestations of solar activity are the giant disruptions of the Sun's magnetic field that give rise to CMEs and eruptive flares. These phenomena are important both for their impact on space weather and their implications for basic space physics. The SOHO and TRACE missions have given us new insights into the physical mechanisms that give rise to eruptive flares/CMEs and their associated filament ejections. The current theories and modeling of CMEs/flares will be reviewed. In particular, we will describe a recently developed model, "magnetic breakout", which postulates that the interaction of neighboring flux systems via magnetic reconnection leads to the sudden release of magnetic stress stored in strong fields lying near the bottom of the solar atmosphere. Both 2.5D and 3D numerical simulations of breakout will be presented. The model proposes a general mechanism for explosive energy release, which should be applicable to many astrophysical plasmas. This work was supported, in part, by NASA and ONR. Title: Importance of topology Authors: Antiochos, Spiro Bibcode: 2002ocnd.confE...1A Altcode: No abstract at ADS Title: Coronal energy release via explosive magnetic reconnection Authors: Dahlburg, R.; Klimchuk, J.; Antiochos, S. Bibcode: 2002cosp...34E1264D Altcode: 2002cosp.meetE1264D It is widely believed that most coronal phenomena involve the release of magnetic free energy that is stored within stressed magnetic field configurations. The availability of sufficient free energy to explain everything from coronal heating to flares and coronal mass ejections is well established, but how this energy is released remains a major puzzle. Observations reveal that an important property of the energy release mechanism is its "switch on" character. The mechanism must remain dormant for long periods of time to allow the magnetic stresses to build, then it must operate very vigorously once it finally turns on. We discuss a mechanism called the "secondary instability" which exhibits this behavior. It is essentially the ideal kinking of thin twisted magnetic flux tubes that form from the resistive tearing of current sheets. We relate the mechanism to the coronal heating idea of Parker in which the coronal magnetic field becomes tangled by random motions of the photospheric footpoints. Global energy balance considerations imply that magnetic energy dissipation occurs at a particular angle in the field, and the secondary instability offers the first quantitative explanation for why this should be. It thus places Parker's popular idea on a much firmer physical footing. Title: Numerical Simulation and Analytical Prediction of the Reconnection of Twisted Flux Tubes Authors: Linton, M. G.; Antiochos, S. K.; Dahlburg, R. B. Bibcode: 2001AGUFMSH11C0725L Altcode: Observations of the solar corona show that some flares appear to be caused by the collision and reconnection of magnetic flux tubes. We investigate this possibility by studying, both numerically and analytically, the interaction and reconnection of colliding pairs of twisted flux tubes. In particular, we present an analytical model based on various parameters of flux tube collisions, such as tube twist, length and collision angle, which predicts whether a flux tube pair will reconnect once and slingshot or reconnect twice and tunnel (see Linton et al. ApJ 2001 533, 905). We then test these predictions against numerical simulations of the same flux tube interactions, and find good agreement between the two. This analytical model, therefore, should be a useful tool in the analysis and prediction of solar flare events due to flux tube collisions. This work was supported by NASA and ONR grants, and a grant of computer time from the DoD/HPC Program. Title: On the Time Variability of Coronal Heating Authors: Antiochos, S. K.; Karpen, J. T.; DeLuca, E. E.; Golub, L.; Hamilton, P. Bibcode: 2001AGUFMSH11A0690A Altcode: We derive constraints on the time variability of coronal heating from observations of the so-called active-region moss by the Transition and Coronal Explorer (TRACE). The moss is believed to be due to million-degree emission from the transition regions at the footpoints of coronal loops whose maximum temperatures are several million degrees. The key point of the TRACE observations is that in the moss regions one generally sees only moss, and not million degree loops. TRACE movies showing this result will be presented. We will demonstrate using both analytic and numerical calculations, that the lack of observable million-degree loops in the moss regions places severe constraints on the possible time variability of coronal heating in the loops overlying the moss. In particular, the heating in the hot moss loops cannot be truly flare-like with a sharp cutoff, but instead, must be quasi-steady to an excellent approximation. The implications of this result for coronal heating models will be discussed. This work was supported in part by NASA and ONR Title: Hydrodynamics of coronal loops subject to transient heating Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.; MacNeice, P. J.; Antiochos, S. K. Bibcode: 2001ESASP.493..367S Altcode: 2001sefs.work..367S No abstract at ADS Title: The Magnetic Origins of Coronal Mass Ejections Authors: Antiochos, S. Bibcode: 2001hell.confE..14A Altcode: The most spectacular and most energetic manifestations of solar activity are the giant disruptions of the Sun's magnetic field that give rise to coronal mass ejections (CME)/ eruptive flares. These events are also the main drivers of geoeffective space weather, producing disturbances ranging from intense particle storms to electric power disruptions. CMEs/eruptive flares are also, perhaps, the most interesting form of solar activity from the viewpoint of basic MHD physics and present a great challenge to theory. Recent observations by the SOHO and TRACE missions of CMEs/flares and their associated prominence eruptions have given us new insights into the physical mechanism for CME/eruptive flare initiation. We will first review some of the latest observations and theories. Then, we will describe a recently developed model, "magnetic breakout", which appears to explain many of the important features of CME/eruptive flares. The basic idea underlying breakout is that the interaction of neighbouring flux systems in the Sun's corona leads to a positive feedback between magnetic reconnection and outward expansion of the coronal magnetic field, and thereby, an explosive energy release. Both 2.5D and 3D numerical simulations of breakout will be presented. The model describes a general mechanism for explosive eruptions, which should be applicable to many astrophysical plasmas. Title: Reconnection of Twisted Flux Tubes as a Function of Contact Angle Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K. Bibcode: 2001ApJ...553..905L Altcode: The collision and reconnection of magnetic flux tubes in the solar corona has been proposed as a mechanism for solar flares and in some cases as a model for coronal mass ejections. We study this process by simulating the collision of pairs of twisted flux tubes with a massively parallel, collocation, viscoresistive, magnetohydrodynamic code using up to 256×256×256 Fourier modes. Our aim is to investigate the energy release and possible global topological changes that can occur in flux-tube reconnection. We have performed a number of simulations for different angles between the colliding flux tubes and for either co- or counterhelicity flux tubes. We find the following four classes of interaction: (1) bounce (no appreciable reconnection), (2) merge, (3) slingshot (the most efficient reconnection), and (4) tunnel (a double reconnection). We will describe these four classes of flux-tube reconnection and discuss in what range of parameter space each class occurs and the implications our results have for models of flares and coronal mass ejections. Title: Reconnection of Twisted Magnetic Flux Tubes as a Solar Flare Mechanism Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K. Bibcode: 2001AGUSM..SP42A11L Altcode: The collision and reconnection of magnetic flux tubes in the solar corona is often proposed as a mechanism for solar flares. We study this process by simulating the collision of pairs of twisted flux tubes with a massively parallel collocation viscoresistive MHD code using up to 256 x 256 x 256 Fourier modes. Our aim is to investigate the energy release and the possible global topological changes which occur in flux tube reconnection. We have performed a number of simulations for different angles between the colliding flux tubes and for either co- or counter-helicity flux tubes. We find the following four classes of reconnection can occur: 1) bounce (no appreciable reconnection), 2) merge, 3) slingshot (the classical reconnection picture), and 4) tunnel (a double reconnection). We will describe these four classes of flux tube reconnection, discuss in what range of parameter space each class occurs, and discuss the implications our results have for models of flares. This work was supported by an NRC/NRL Postdoctoral Fellowship, the NASA SECTP, and a grant of computer time from the DoD/HPC Program. Title: Origin and Evolution of Coronal Condensations Authors: Karpen, J. T.; Antiochos, S. K.; MacNeice, P. J. Bibcode: 2001AGUSM..SP61A02K Altcode: The existence of cool plasma high in the solar corona was first established a century ago. In addition to the well-studied phenomenon of prominences, persistent knots and episodic downflows of cool plasma commonly denoted `coronal rain' have been observed in Hα , EUV, and UV spectral lines. Our recent 1D hydrodynamic simulations of localized, steady heating near the footpoints of long coronal loops produce dynamic condensations which form, flow, and fall onto the nearest chromosphere over the course of tens of hours (Antiochos et al. 2000, Karpen et al. 2001). In low-lying loops, this process yields condensed knots with dimensions and velocities consistent with high-resolution observations of counterstreaming flows along prominence spines (Zirker et al. 1998). Similar condensations develop even in high ( ~100,000 km) model loops, although they are small, short-lived, and form at irregular intervals. In order to explain the broader phenomenon of coronal condensations beyond prominences, however, we must investigate the effects of temporally varying, localized footpoint heating on the plasma dynamics in a range of active-region and quiet-Sun loops. We will discuss the results of a series of 1D numerical simulations with spatially and temporally variable heating, their observable signatures, and how well they reproduce observations by SOHO and TRACE of `coronal rain' and coronal condensations (e.g., Brekke 1999; Schrijver 2001). Title: The Role of Magnetic Flux Ropes in CMEs Authors: Antiochos, S. K. Bibcode: 2001AGUSM..SH41C05A Altcode: A commonly-held belief is that prominences/filaments are magnetic flux ropes, consisting of a globally twisted structure with a number of turns. These ropes are presumed to form either by emerging pre-made from the photosphere, or via reconnection at the bottom boundary of the corona. The claim is that filament eruption and coronal mass ejections occur because the twist/helicity of the flux rope becomes too large. We will argue that this so-called paradigm is merely an unfortunate misconception. The key point is that flux ropes do not produce CMEs but, instead, CMEs produce flux ropes. We will present both observational and theoretical results, which demonstrate that the pre-eruption topology is that of a differentially sheared arcade and that the flux ropes observed in the upper corona by LASCO, for example, are only a by-product of the eruption. We will discuss the implications of these results for the STEREO and SOLAR-B missions. This work supported in part by NASA and ONR. Title: Are Magnetic Dips Necessary for Prominence Formation? Authors: Karpen, J. T.; Antiochos, S. K.; Hohensee, M.; Klimchuk, J. A.; MacNeice, P. J. Bibcode: 2001ApJ...553L..85K Altcode: The short answer: No. Title: Structure and Stability of Multipolar Prominence Magnetic Fields Authors: DeVore, C. R.; Antiochos, S. K. Bibcode: 2001AGUSM..SH41C09D Altcode: We have previously proposed and simulated a differential shear model for the formation of prominence magnetic fields. For very strong shears and highly stressed fields, reconnections between the prominence and the overlying arcade produce helical structures resembling flux ropes. The resulting configurations relaxed to evidently stable equilibria, showing no sign of imminent eruption, in the simple bipolar geometries we considered. Recently, we have turned our attention to more magnetically complex situations involving multipolar magnetic fields. These include an interacting two-prominence scenario, in which the ends of a pair of bipolar prominences come into contact and reconnect; and a single prominence in a 'breakout' topology, in which the overlying arcade can reconnect with an exterior field, thereby loosening the restraining forces holding down the prominence. Results from these simulations will be presented, and their implications for our understanding of prominence stability and eruption will be discussed. Title: Evidence for Magnetic Reconnection in CMEs and Eruptive Flares Authors: Antiochos, S. K. Bibcode: 2001AGUSM..SM22A01A Altcode: Magnetic reconnection is believed to be the most important process in the Sun's corona for transferring energy from magnetic field to plasma; hence, reconnection plays the central role in theories for all types of solar activity ranging from the smallest spicules to giant CMEs. In particular, reconnection has long been thought to be the mechanism for the heating and formation of closed flare loops and the ejected plasmoid/flux rope. Observational and theoretical results will be presented, which will make a compelling case for this type of reconnection. Reconnection has also been proposed as the initiation process for CMEs and eruptive flares. Theoretical models for this type of reconnection will be discussed. The observational evidence in this case is less definitive. I will demonstrate, however, that flare observations by TRACE also give strong support for the type of reconnection expected in the ``breakout'' model for CME initiation. This work was supported in part by NASA and ONR. Title: Extreme-Ultraviolet Transition-Region Line Emission during the Dynamic Formation of Prominence Condensations Authors: Lanza, Antonino F.; Spadaro, Daniele; Lanzafame, Alessandro C.; Antiochos, Spiro K.; MacNeice, Peter J.; Spicer, Daniel S.; O'Mullane, Martin G. Bibcode: 2001ApJ...547.1116L Altcode: We calculated the emission expected in EUV transition-region lines during the process of dynamic formation of prominence condensations in coronal loops, as predicted by the thermal nonequilibrium model of Antiochos et al. We selected some lines emitted by ions of carbon and oxygen because they are among the most intense and representative in the temperature range corresponding to the solar transition region. We present and discuss the principal characteristics of the line intensities and profiles synthesized from the hydrodynamic model at different times during the loop evolution. The ionization balance is computed in detail and the deviations from the ionization equilibrium caused by plasma flows and variations of temperature and density are accounted for. The atomic physics is treated using the latest atomic coefficients and the collisional-radiative theory approach. The synthesized carbon and oxygen lines exhibit a behavior significantly dependent on the variations of the plasma parameters inside the magnetic flux tube and therefore are suitable observational signatures of the processes giving rise to prominence condensations. In particular, a sizeable increase of line intensity as well as small blueshifts are expected from the loop footpoints during the first part of the evaporation phase that fills the loop with the material which subsequently condenses into the prominence. Once the condensation appears, line intensities decrease in the footpoints and simultaneously increase at the transition regions between the cool plasma of the condensation and the coronal portion of the loop. Line shifts are quite small in our symmetric model, and during most of the condensation's lifetime, the nonthermal widths are relatively small. These results can be compared with detailed ultraviolet observations of filament/prominence regions obtained by recent space missions in order to test the model proposed for the formation of solar prominences. Title: EUV line emission during the dynamic formation of prominence condensations Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Antiochos, S. K.; O'Mullane, M. G. Bibcode: 2001MmSAI..72..591S Altcode: This contribution is a short summary of a paper recently submitted to Astrophysical Journal. We calculated the emission expected in EUV transition region lines during the process of dynamic formation of prominence condensations in coronal loops, as predicted by the thermal non-equilibrium model proposed by Antiochos et al. We present and discuss the principal characteristics of the line intensities and profiles synthesized from the hydrodynamic model at different times during the loop evolution. Title: Determination of Flare Heating and Cooling Using the Transition Region and Coronal Explorer Authors: Antiochos, S. K.; DeLuca, E. E.; Golub, L.; McMullen, R. A. Bibcode: 2000ApJ...542L.151A Altcode: We describe how the Transition Region and Coronal Explorer 171 Å observations can be used to determine the properties of flare-loop heating. The key point is that the evolution of a loop transition region (TR) is much easier to measure quantitatively than the bulk flare plasma because the TR emission originates from an unobscured source with simple geometry. We derive general analytic expressions for the evolution of a flare-loop TR that, in principle, permit a determination of the heating function from the observations. These results are compared with observations of the 1998 September 20 flare. We find that the observed evolution of the flare ribbons is in good agreement with our model for the evaporative cooling of flare loops and that the heating in these loops is incompatible with the assumption of spatial uniformity. Title: Twisted Coronal Magnetic Loops Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D. Bibcode: 2000ApJ...542..504K Altcode: Observed coronal loops have a surprisingly uniform thickness that cannot be easily understood in terms of standard coronal magnetic field models. We investigate the possibility that the uniform thickness can be explained by locally enhanced twist in the field, so that observed loops correspond to twisted coronal flux tubes. Our approach is to construct numerical models of fully three-dimensional force-free magnetic fields. To resolve the internal structure of an individual loop embedded within a much larger dipole configuration, we use a nonuniform numerical grid of size 609×513×593, the largest ever applied to a solar problem to our knowledge. Our models show that twist promotes circular cross sections in loops. Such cross sections are typically assumed, and have recently been verified from observations, but their physical cause has been heretofore unexplained. Title: The Topology and Evolution of the Bastille Day Flare Authors: Aulanier, G.; DeLuca, E. E.; Antiochos, S. K.; McMullen, R. A.; Golub, L. Bibcode: 2000ApJ...540.1126A Altcode: On 1998 July 14, a class M3 flare occurred at 12:55 UT in AR 8270 near disk center. Kitt Peak line-of-sight magnetograms show that the flare occurred in a δ spot. Mees vector magnetograms show a strong shear localized near a portion of the closed neutral line around the parasitic polarity of the δ spot. Observations of the flare in 171, 195, and 1600 Å have been obtained by TRACE, with ~=40 s temporal and 0.5" spatial resolutions. They reveal that small-scale preflare loops above the sheared region expanded and disappeared for more than 1 hr before flare maximum. During the flare, bright loops anchored in bright ribbons form and grow. This occurs while large-scale dimmings, associated with large expanding loops, develop on both sides of the active region. This suggests that the flare was eruptive and was accompanied by a coronal mass ejection (CME). Magnetic field extrapolations reveal the presence of a null point in the corona, with its associated ``spine'' field line, and its ``fan'' surface surrounding the parasitic polarity. We show that while the whole event occurs, the intersections of the ``fan'' and the ``spine'' with the photosphere brighten and move continuously. The interpretation of the event shows that the magnetic evolution of the eruptive flare is strongly coupled with its surrounding complex topology. We discuss evidence supporting a ``magnetic breakout'' process for triggering this eruptive flare. We finally conclude that multipolar fields cannot be neglected in the study and modeling of the origin of CMEs in the corona. Title: Dynamical Formation and Stability of Helical Prominence Magnetic Fields Authors: DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2000ApJ...539..954D Altcode: We numerically simulated an initially bipolar magnetic field subjected to shear motions concentrated near and parallel to the photospheric polarity inversion line. The simulations yield three principal results: (1) For footpoint displacements comparable to the bipole's depth, the sheared core field acquires a dipped geometry that can support cool prominence material against gravity. This confirms previous force-free equilibrium models for forming dipped prominence fields by differential shear and extends them to much larger applied shears and time-dependent dynamics with dissipation. (2) At larger shears, we discover a new mechanism for forming the helical magnetic fields of prominences. It entails a two-step process of magnetic reconnection in the corona. First, flux in the sheared core reconnects with flux in the unsheared, restraining arcade, producing new pairs of interlinked field lines. Second, as these interlinked fields continue to be sheared, they are brought together and reconnect again, producing helical field threading and enveloping the body of the prominence. This mechanism can account for the twist that is often observed in both quiescent and erupting prominences. (3) Even for very large shears, the dipped, helical structure settles into an apparently stable equilibrium, despite the substantial amount of reconnection and twist in the magnetic field. We conclude that neither a kink instability of the helical core field, nor a tether-cutting instability of the restraining arcade, is operating in our low-lying model prominence. This concurs with both observations and a theoretical model for prominence stability. Title: The Thermal Nonequilibrium of Prominences Authors: Antiochos, S. K.; MacNeice, P. J.; Spicer, D. S. Bibcode: 2000ApJ...536..494A Altcode: We present numerical simulations and analytic theory for the thermal nonequilibrium of solar coronal flux tubes that have a stretched-out, dipped geometry, appropriate for a prominence/filament. Our simulations indicate that if the heating in such a flux tube is localized near the chromosphere, then condensations appear which undergo a continuous cycle of formation, motion, and destruction, even though the heating and all other imposed conditions on the loop are purely time independent. We show how this nonsteady evolution can be understood in terms of simple scaling-law theory. The implications of thermal nonequilibrium for observations of the solar corona are discussed. We argue that the model can explain both the formation of prominence condensations and recent observations of their dynamics. Title: Are dipped field lines required for prominence formation? Authors: Karpen, J. T.; Antiochos, S. K.; MacNeice, P. J. Bibcode: 2000BAAS...32Q.809K Altcode: No abstract at ADS Title: Reconnection of orthogonal magnetic flux tubes. Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K. Bibcode: 2000BAAS...32R.810L Altcode: No abstract at ADS Title: Observation and theory of coronal loop structure. Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.; Watko, J. A. Bibcode: 2000BAAS...32R.809K Altcode: No abstract at ADS Title: Formation and Stability of Helical Prominence Magnetic Fields Authors: DeVore, C. R.; Antiochos, S. K. Bibcode: 2000SPD....31.1401D Altcode: 2000BAAS...32R.846D We have numerically simulated the evolution of initially bipolar magnetic fields, subjected to shear motions concentrated near and directed parallel to the polarity inversion line at the photosphere. The simulations yield three principal results: (1) Footpoint displacements comparable to the bipole's depth produce a dipped geometry in the sheared core flux, capable of supporting condensed prominence material against gravity. This confirms and extends the previous results of force-free field models. (2) For much larger displacements, a new mechanism for helical field formation ensues. A two-step reconnection process acts first to reconnect the sheared core flux with flux from the overlying arcade, and then to reconnect pairs of the newly formed field lines with each other. The resultant helical field threads and envelops the body of the prominence. (3) Even for very large displacements, the dipped, helical structure finds a stable equilibrium. Despite the substantial amounts of reconnection and twist in the field, it shows no sign of eruption due to kink instability or tether-cutting. The results suggest that even helical prominence configurations in simple, bipolar topologies are immune to eruption, and do not lead to coronal mass ejections; a multipolar, break-out topology may be essential. This research was supported by NASA and ONR. Title: Observation and Theory of Coronal Loop Structure Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.; Watko, J. A. Bibcode: 2000SPD....31.0144K Altcode: We have carefully examined 43 soft X-ray loops observed by Yohkoh and 24 EUV loops observed by TRACE and find that the large majority have a nearly uniform thickness. This implies that: 1. the magnetic field in these loops expands with height much less than standard coronal models would predict; and 2. the shape of the loop cross section is approximately circular. We have investigated whether these surprising results can be explained by locally enhanced twist in the field, so that observed loops correspond to twisted coronal flux tubes. Our approach is to construct numerical models of fully three-dimensional force-free magnetic fields. To resolve the internal structure of an individual loop embedded within a much larger dipole configuration, we use a nonuniform numerical grid of size 609 x 513 x 593, the largest ever applied to a solar problem, to our knowledge. Our models indicate that twist does indeed promote circular cross sections in the corona, even when the footpoint cross section is irregular. However, twist does not seem to be a likely explanation for the observed minimal expansion with height. This work was supported by the NASA Sun-Earth Connection Theory and Guest Investigator Programs. Title: Reconnection of Orthogonal Magnetic Flux Tubes Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K. Bibcode: 2000SPD....31.0150L Altcode: The interaction and reconnection of magnetic flux tubes in the solar atmosphere may be the underlying energy release mechanism in some types of flares and coronal mass ejections. To study this mechanism, we present 3-D MHD simulations of reconnection due to the collision of two orthogonal, twisted flux tubes. For identical tubes there exist four possible interaction configurations. For the optimal reconnection configuration, where the field lines in the reconnection region are initially antiparallel, the flux tubes tunnel. For the intermediate cases where these field lines are initially orthogonal, the tubes reconnect to form two new flux tubes. Finally for the case where these field lines are initially parallel, the two tubes bounce and do not reconnect. We discuss the implications of these results for flux tube interactions in the solar atmosphere, in particular the implications for coronal mass ejection initiation, and for compact flares. This work is supported by an NRC/NRL postdoctoral grant and by a grant of time on the DoD HPC program. Title: Magnetic Energy Relaxation by Null-Point Reconnection Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R. Bibcode: 2000SPD....31.0149A Altcode: 2000BAAS...32..809A We derive the minimum energy state resulting from complete magnetic reconnection in a 2.5D MHD system, in the limit of low plasma beta and high magnetic Reynold's number --- appropriate, in particular, to the solar corona. The results are useful for determining the amount of energy that can be liberated by reconnection and, hence, are important for understanding coronal heating and other forms of solar activity. The key difference between our approach and previous work is that reconnection is assumed to occur only at magnetic null points initially present in the system. We find that the minimum energy state is not the usual linear force-free field, but a state in which magnetic stress is distributed uniformly on equal flux surfaces. Our results are especially important for physical systems such as the solar corona in which the field is line-tied at the high-beta photosphere and the volume of the system is infinite, but the results are also valid for general configurations with flux surfaces as boundaries. We discuss the implications of this work for the Sun's corona and for laboratory plasmas. This work was funded in part by ONR and NASA. Title: Are Dipped Field Lines Required for Prominence Formation? Authors: Karpen, J. T.; Antiochos, S. K.; MacNeice, P. J. Bibcode: 2000SPD....31.0147K Altcode: 2000BAAS...32..809K Previous studies of prominence formation have been focussed exclusively on flux systems with dipped geometries (e.g., Antiochos and Klimchuk 1991; Antiochos et al. 1999), under the assumption that long-lived prominences must contain locations where the cool, dense plasma can be collected and suspended well above the photosphere. Within one such configuration, localized asymmetric heating at the base of long dipped field lines has been shown to yield a continual cycle of formation, motion, and destruction of cool, dense plasma (Antiochos, MacNeice, and Spicer 2000), thus reproducing the counterstreaming flows recently observed along prominence ``spines" (Zirker, Engvold, and Martin 1998). In view of this discovery of the dynamical nature of prominences, however, we speculate that the presence of dips may not be necessary. Rather, thermal non-equilibrium in flux tubes with flat or modestly peaked topologies might be able to produce the same cycle of condensation and destruction that occurs along dipped field lines. We have tested this hypothesis by performing a series of 1D hydrodynamic simulations with ARGOS, an adaptively refined high-order Godunov solver (see Antiochos et al. 1999). The results, comparison with observations, and their implications for prominence formation and lifecycle will be discussed. This work has been supported in part by NASA and ONR. Title: The Topology and Evolution of the Bastille Day Flare Observed by TRACE Authors: Aulanier, G.; Antiochos, S. K.; DeLuca, E. E.; McMullen, R. A.; Golub, L. Bibcode: 2000SPD....31.1402A Altcode: 2000BAAS...32..846A On July 14, 1998, a class M3 flare occurred at 12:55 UT in AR 8270 near disc center. Kitt Peak line-of-sight magnetograms show that the flare occurred in a δ -spot. Mees vector magnetograms show a strong shear localized near a portion of the closed neutral line around the parasitic polarity of the δ -spot. Observations of the flare in 171 Angstroms, 195 Angstroms and 1600 Angstroms have been obtained by TRACE, with ~= 40 s temporal and 0.5 arcsec spatial resolutions. They reveal that small-scale pre-flare loops above the sheared region expanded and disappeared for more than one hour before flare maximum. During the flare, bright loops anchored in bright ribbons form and grow. This occurs while large-scale dimmings, associated with large expanding loops, develop on both sides of the AR. This suggests that the flare was eruptive, and was accompanied by a coronal mass ejection (CME). Magnetic field extrapolations reveal the presence of a null point in the corona, with its associated ``spine'' field line, and its ``fan'' surface surrounding the parasitic polarity. We show that while the whole event occurs, the intersections of the ``fan'' and the ``spine'' with the photosphere brighten and move continuously. The interpretation of the event shows that the magnetic evolution of the eruptive flare is strongly coupled with its surrounding complex topology. We discuss evidence supporting a ``magnetic breakout'' process for triggering this eruptive flare. We finally conclude that multipolar fields cannot be neglected in the study and modeling of the origin of CMEs in the corona. This work is supported, at SAO by a NASA contract to Lockheed-Martin, and at NRL by NASA and ONR. Title: Theory of solar prominences Authors: Antiochos, S. K. Bibcode: 2000ssls.work...73A Altcode: We discuss the physical mechanisms for the formation, the gravitational support, and the eruption of solar prominences and filaments. We show that the formation of cool mass in the corona is due to the thermal non-equilibrium of coronal loops that are heated preferentially near their footpoints. The gravitational support can be understood as a direct consequence of the differential shearing of a three-dimensional magnetic field. Finally, the eruption can be explained by the "breakout" model, in which magnetic reconnection leads to the explosive expansion of the sheared prominence field. We present results from numerical simulations which confirm each of these three fundamental mechanisms. Based on this work, we conclude that we are now approaching a true understanding of the basic physics of solar prominences. Title: The Dynamics of Prominence Condensations Authors: MacNeice, P.; Spicer, D. S.; Antiochos, S. K. Bibcode: 1999ESASP.448..459M Altcode: 1999mfsp.conf..459M; 1999ESPM....9..459M No abstract at ADS Title: A Model for Prominence Mass and Dynamics Authors: MacNeice, P.; Spicer, D. S.; Antiochos, S. Bibcode: 1999ESASP.446..457M Altcode: 1999soho....8..457M Solar prominences and filaments are observed to consist of a collection of small H-alpha condensations or knots. We address two key issues concerning these condensations: (1) the mechanism for their formation, and (2) the origin of their observed motions. Recently, Zirker, Martin and co-workers have found that filament condensations appear to move approximately horizontally with velocities of order 5 - 10 km/s. These observations provide important constraints on the magnetic structure of prominences. We propose that the condensations form due to a lack of thermal equilibrium in the magnetic flux tubes threading through the prominence corona. We use LOOPAMR a new 1D high-order Godunov code that includes both thermal conduction and radiation with PARAMESH, a fully adaptive mesh refinement tool, to simulate for the first time the complete formation process of a prominence condensation. We show that both the origin and the observed velocities are explained by our model. In addition we discuss the implication of our model for coronal heating and for the global structure of prominences. Title: Variation of Thermal Structure with Height of a Solar Active Region Derived from SOHO CDS and YOHKOH BCS Observations Authors: Sterling, Alphonse C.; Pike, C. D.; Mason, Helen E.; Watanabe, Tetsuya; Antiochos, Spiro K. Bibcode: 1999ApJ...524.1096S Altcode: We present observations of NOAA solar Active Region 7999 when it was near the west solar limb on 1996 December 2 and 3, using data from the Coronal Diagnostic Spectrometer (CDS) experiment on the SOHO satellite. Ratios of intensities of 2 MK material (as observed in CDS Fe XVI images) to 1 MK material (from CDS Mg IX images) indicate that there is a drop in the ratio of the hotter to the cooler material with height in the region, up to an altitude of about 105 km. At low altitudes the relative amount of 2 MK emission measure to 1 MK emission measure ranges from 8 to 10, while the ratio is minimum near 105 km, ranging from 1.3 to 3.5. The decrease with height of the CDS ratio qualitatively resembles the decrease in S XV election temperature with height (measurable up to ~85,000 km) in the same active region obtained from the Bragg crystal spectrometer instrument on Yohkoh. The CDS images indicate that the highest S XV temperatures and largest CDS ratios correspond to regions of microflares, and somewhat lower S XV temperatures and CDS ratios correspond to diffuse regions. Above 105 km, the trend of the CDS ratios changes, either increasing or remaining approximately constant with height. At these altitudes the CDS images show faint, large-scale diffuse structures. Title: Working Group 5: Prominences and Coronal Mass Ejections Authors: Kucera, T.; Antiochos, S. K. Bibcode: 1999ESASP.446...97K Altcode: 1999soho....8...97K No abstract at ADS Title: An Eruptive Flare Observed by TRACE as a Test for the Magnetic Authors: Aulaneir, G.; Deluca, E. E.; Golub, L.; McMullen, R. A.; Karpen, J. T.; Antiochos, S. K. Bibcode: 1999ESASP.446..135A Altcode: 1999soho....8..135A No abstract at ADS Title: Shear-driven Reconnection in Chromospheric Eruptions: 3D Numerical Simulations Authors: Karpen, J. T.; DeVore, C. R.; Antiochos, S. K. Bibcode: 1999AAS...194.3108K Altcode: 1999BAAS...31..869K Magnetic reconnection has been implicated in nearly all forms of solar activity. As observations continue to reveal such activity at ever smaller scales, the chromospheric transients known as explosive events and microjets have emerged as perhaps the most likely examples of reconnection at work on the Sun. Although reconnection has been studied for decades, only recently has it become feasible to explore the behavior of interacting flux systems under conditions even remotely approximating the solar environment. In particular, most analytic and numerical treatments to date have been two-dimensional and highly idealized in terms of assumed symmetries and boundary conditions. Our earlier 2.5D calculations of reconnecting arcades driven by footpoint motions demonstrated that reconnection can account for the key features of (and differences between) explosive events and microjets, most notably the characteristic jets and bidirectional flows. Encouraged by these results, we have begun to investigate three-dimensional reconnection at chromospheric/transition region heights between adjacent bipoles with a new, fully 3D, FCT-based code developed for massively parallel supercomputers under the NASA HPCC program. The dynamic and energetic consequences of shear-driven 3D reconnection between paired bipoles, and comparisons between our simulation results and SOHO and TRACE observations of chromospheric eruptions, will be presented. (*) This work is supported by NASA and ONR. Title: The Structure of Solar Prominences Authors: Antiochos, S. K.; DeVore, C. R.; Klimchuk, J. A. Bibcode: 1999AAS...194.3102A Altcode: 1999BAAS...31Q.868A With the advent of new high-spatial and high-temporal resolution observations from SOHO and TRACE, prominences/filaments have once again become a major focus of study for solar physics. Prominences/filaments are also important for their role in space weather. They yield key information on the type of magnetic structure that leads to eruptive flares and coronal mass ejections. We present results from our calculations of the 3D magnetic structure of prominences and the origin of the prominence mass. We show that many of the well-know features of their global structure, such as the prominence legs and barbs, the inverse polarity, and the sinistral-dextral property, can be easily understood as due to the geometry of a sheared bipolar field. Both fully time-dependent 3D MHD simulations and 3D force-free field equilibrium calculations demonstrate this conclusion. Furthermore, we discuss results showing that the magnetic structure of a sheared 3D bipole leads naturally to the formation of cool condensations and to their observed motions. (*) This work is supported by NASA and ONR. Title: The Dynamic Formation of Prominence Condensations Authors: Antiochos, S. K.; MacNeice, P. J.; Spicer, D. S.; Klimchuk, J. A. Bibcode: 1999ApJ...512..985A Altcode: 1998astro.ph..8199A We present simulations of a model for the formation of a prominence condensation in a coronal loop. The key idea behind the model is that the spatial localization of loop heating near the chromosphere leads to a catastrophic cooling in the corona. Using a new adaptive grid code, we simulate the complete growth of a condensation and find that after ~5000 s it reaches a quasi-steady state. We show that the size and growth time of the condensation are in good agreement with data and discuss the implications of the model for coronal heating and for observations of prominences and the surrounding corona. Title: The role of magnetic reconnection in solar activity Authors: Antiochos, S. K.; DeVore, C. R. Bibcode: 1999GMS...199..113A Altcode: 1998astro.ph..9161A We argue that magnetic reconnection plays the determining role in many of the various manifestations of solar activity. In particular, it is the trigger mechanism for the most energetic of solar events, coronal mass ejections and eruptive flares. We propose that in order to obtain explosive eruptions, magnetic reconnection in the corona must have an ``on-off'' nature, and show that reconnection in a sheared multi-polar field configuration does have this property. Numerical simulation results which support this model are presented, and implications for coronal mass ejections/eruptive flare prediction are discussed. Title: The Role of Helicity in Magnetic Reconnection: 3D Numerical Simulations Authors: Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 1999GMS...111..187A Altcode: 1999astro.ph..1039A; 1999mhsl.conf..187A We demonstrate that conservation of global helicity plays only a minor role in determining the nature and consequences of magnetic reconnection in the solar atmosphere. First, we show that observations of the solar coronal magnetic field are in direct conflict with Taylor's theory. Next, we present results from three-dimensional MHD simulations of the shearing of bipolar and multi-polar coronal magnetic fields by photospheric footpoint motions, and discuss the implications of these results for Taylor's theory and for models of solar activity. The key conclusion of this work is that significant magnetic reconnection occurs only at very specific locations and, hence, the Sun's magnetic field cannot relax completely down to the minimum energy state predicted by conservation of global helicity. Title: A Model for Solar Coronal Mass Ejections Authors: Antiochos, S. K.; DeVore, C. R.; Klimchuk, J. A. Bibcode: 1999ApJ...510..485A Altcode: 1998astro.ph..7220A We propose a new model for the initiation of a solar coronal mass ejection (CME). The model agrees with two properties of CMEs and eruptive flares that have proved to be very difficult to explain with previous models: (1) very low-lying magnetic field lines, down to the photospheric neutral line, can open toward infinity during an eruption; and (2) the eruption is driven solely by magnetic free energy stored in a closed, sheared arcade. Consequently, the magnetic energy of the closed state is well above that of the posteruption open state. The key new feature of our model is that CMEs occur in multipolar topologies in which reconnection between a sheared arcade and neighboring flux systems triggers the eruption. In this ``magnetic breakout'' model, reconnection removes the unsheared field above the low-lying, sheared core flux near the neutral line, thereby allowing this core flux to burst open. We present numerical simulations that demonstrate our model can account for the energy requirements for CMEs. We discuss the implication of the model for CME/flare prediction. Title: The Magnetic Topology of Solar Activity Authors: Antiochos, Spiro K. Bibcode: 1998APS..DPP.B1M05A Altcode: Solar activity can appear in many forms, ranging from small spicules with size scales < 1000 km and energies < 10^24 ergs, to giant coronal mass ejections and eruptive flares with scales of order the solar radius and energies up to 10^33 ergs. The underlying driver for all this dynamics is the Sun's magnetic field. The high-beta plasma below the photosphere generates and stresses magnetic field which emerges through the surface and, in turn, energizes the plasma in the low-beta corona. We describe the recent observations of violent solar activity and present a general theoretical framework for understanding them. We argue that strong activity requires two key features: the magnetic field must have a multi-polar topology, and magnetic reconnection in the solar corona must have a bursty nature. We present results from numerical simulations that show how these features lead to the observed dynamics. (This work was supported in part by NASA and ONR.) Title: The Magnetic Topology of Solar Eruptions Authors: Antiochos, S. K. Bibcode: 1998ApJ...502L.181A Altcode: 1998astro.ph..6030A We present an explanation for the well-known observation that complexity of the solar magnetic field is a necessary ingredient for strong activity such as large eruptive flares. Our model starts with the standard picture for the energy buildup--a highly sheared, newly emerged magnetic field near the photospheric neutral line held down by an overlying unsheared field. Previously, we proposed the key new idea that magnetic reconnection between the unsheared field and neighboring flux systems decreases the amount of the overlying field and, thereby, allows the low-lying sheared flux to ``break out.'' In this Letter, we show that a bipolar active region does not have the necessary complexity for this process to occur, but a delta sunspot has the right topology for magnetic breakout. We discuss the implications of these results for observations from SOHO and TRACE. Title: LOOPREF: A Fluid Code for the Simulation of Coronal Loops Authors: Defainchtein, Rosalinda; Antiochos, Spiro; Spicer, Daniel Bibcode: 1998nasa.reptW....D Altcode: This report documents the code LOOPREF. LOOPREF is a semi-one dimensional finite element code that is especially well suited to simulate coronal-loop phenomena. It has a full implementation of adaptive mesh refinement (AMR), which is crucial for this type of simulation. The AMR routines are an improved version of AMR1D. LOOPREF's versatility makes is suitable to simulate a wide variety of problems. In addition to efficiently providing very high resolution in rapidly changing regions of the domain, it is equipped to treat loops of variable cross section, any non-linear form of heat conduction, shocks, gravitational effects, and radiative loss. Title: LOOPREF: A Fluid Code for the Simulation of Coronal Loops Authors: Defainchtein, Rosalinda; Antiochos, Spiro; Spicer, Daniel Bibcode: 1998nasa.reptU....D Altcode: This report documents the code LOOPREF.LOOPREF is a semi-one dimensional finite element code that is especially well suited to simulate coronal-loop phenomena. It has a full implementation of adaptive mesh refinement (AMR), which is crucial for this type of simulation. The AMR routines are an improved version of AMR1D, an AMR code that is posted on the World Wide Web, and documented in NASA's Contractor Report. LOOPREF's versatility makes is suitable to simulate a wide variety of problems. In addition to efficiently providing very high resolution in rapidly changing regions of the domain, it is equipped to treat loops of variable cross section, any non-linear form of heat conduction, shocks, gravitational effects, and radiative loss. Title: Dynamic Responses to Magnetic Reconnection in Solar Arcades Authors: Karpen, Judith T.; Antiochos, Spiro K.; Richard DeVore, C.; Golub, Leon Bibcode: 1998ApJ...495..491K Altcode: We present a numerical simulation of the interaction between two line dipoles through magnetic reconnection in the lower solar atmosphere, a process believed to be the origin of many manifestations of solar activity. This work differs from previous studies in that the field is sheared asymmetrically and that the dipoles have markedly unequal field strengths. This calculation already yielded one key discovery, denoted reconnection driven current filamentation, as described in a previous Astrophysical Journal letter. In this paper we focus on the chromospheric and coronal dynamics resulting from the shear-driven reconnection of unequal dipoles, discuss the important implications for chromospheric eruptions, compare our calculation with high-resolution Normal Incidence X-Ray Telescope observations of a surge, and contrast our results with the predictions of ``fast reconnection'' models. Title: Prominence Formation by Localized Heating Authors: Dahlburg, Russell B.; Antiochos, Spiro K.; Klimchuk, James A. Bibcode: 1998ApJ...495..485D Altcode: We describe a model for the formation of the cool condensed material that comprises a coronal filament or prominence. Numerical calculations are presented which demonstrate that large condensations form in a coronal loop if the loop satisfies two key requirements: (1) the loop heating must be localized near the chromospheric footpoints, and (2) the loop must have a dipped geometry in order to support the prominence condensation against gravity. We calculate one-dimensional equilibrium solutions for the equations of force and energy balance assuming optically thin radiative losses and a parameterized form for the coronal heating. This physical situation is modeled as a boundary value problem, which we solve numerically using a B-spline collocation scheme. The relation of our solutions to the well-known loop scaling laws is discussed, and the implications of our model for active region and quiescent prominences are discussed. Title: An analysis of the unresolved fine structure model for the solar transition region Authors: Lanza, A. F.; Spadaro, D.; Antiochos, S. K. Bibcode: 1998MmSAI..69..695L Altcode: No abstract at ADS Title: Magnetic flux tube tunneling Authors: Dahlburg, R. B.; Antiochos, S. K.; Norton, D. Bibcode: 1997PhRvE..56.2094D Altcode: We present numerical simulations of the collision and subsequent interaction of orthogonal magnetic flux tubes. The simulations were carried out using a parallelized spectral algorithm for compressible magnetohydrodynamics. It is found that, under a wide range of conditions, the flux tubes can ``tunnel'' through each other, a behavior not previously seen in studies of either vortex tube or magnetic flux tube interactions. Two conditions must be satisfied for tunneling to occur: the magnetic field must be highly twisted with a field line pitch >>1, and the Lundquist number must be somewhat large, >=2880. An examination of magnetic field lines suggests that tunneling is due to a double-reconnection mechanism. Initially orthogonal field lines reconnect at two specific locations, exchange interacting sections, and ``pass'' through each other. The implications of these results for solar and space plasmas are discussed. Title: The Implications of 3D for Solar MHD Modelling Authors: Antiochos, S. K.; Dahlburg, R. B. Bibcode: 1997SoPh..174....5A Altcode: The effects of three-dimensionality on the modelling of solar magnetic fields are described. We focus on two processes that are believed to play an important role in coronal heating - the braiding of field lines by photospheric motions and the reconnection of colliding flux tubes. First, it is shown that a proper treatment of boundary conditions at the photosphere in 3D entails qualitatively new physical processes that are not present in 2D. The numerical resolution of even simple boundary velocity patterns in 3D leads to obstacles which have no counterpart in the 2D case. We conclude that adaptive mesh refinement is necessary for capturing the essential 3D physics of a braiding motion at the photosphere. Next, the effects of 3D on magnetic reconnection are discussed. Reconnection in 3D can lead to an evolution of interacting flux tubes, magnetic tunneling, that is not only impossible in lower dimensionality, but is strikingly counterintuitive. The implications of these results for the structure of the solar magnetic field and for coronal heating are described. Title: The Solar-B Mission Authors: Antiochos, Spiro; Acton, Loren; Canfield, Richard; Davila, Joseph; Davis, John; Dere, Kenneth; Doschek, George; Golub, Leon; Harvey, John; Hathaway, David; Hudson, Hugh; Moore, Ronald; Lites, Bruce; Rust, David; Strong, Keith; Title, Alan Bibcode: 1997STIN...9721329A Altcode: Solar-B, the next ISAS mission (with major NASA participation), is designed to address the fundamental question of how magnetic fields interact with plasma to produce solar variability. The mission has a number of unique capabilities that will enable it to answer the outstanding questions of solar magnetism. First, by escaping atmospheric seeing, it will deliver continuous observations of the solar surface with unprecedented spatial resolution. Second, Solar-B will deliver the first accurate measurements of all three components of the photospheric magnetic field. Solar-B will measure both the magnetic energy driving the photosphere and simultaneously its effects in the corona. Solar-B offers unique programmatic opportunities to NASA. It will continue an effective collaboration with our most reliable international partner. It will deliver images and data that will have strong public outreach potential. Finally, the science of Solar-B is clearly related to the themes of origins and plasma astrophysics, and contributes directly to the national space weather and global change programs. Title: The Solar-B Mission Authors: Antiochos, Spiro K. Bibcode: 1997SPD....28.1102A Altcode: 1997BAAS...29..915A Solar-B is the ISAS mission follow-on to the highly successful Japan/US/UK Yohkoh (Solar-A) collaboration. The overall science goal of Solar-B is to determine the magnetic origins of the solar variability that Yohkoh and SOHO have been observing, and that drive the Sun-Earth connection. The mission complement consists of an optical vector magnetograph and spectrograph, an X-ray telescope, and an XUV spectroheliograph. This coordinated instrument package will answer many of the outstanding questions in solar physics. We know that processes hidden deep within the Sun generate surface activity in an 11-year cycle. Emerging magnetic field topology can reveal the workings of this dynamo process. Solar-B will observe the emerging magnetic field and, for the first time, its twist in detail over large regions of the Sun. Measurement of the solar``constant" shows the Sun to be less luminous at sunspot cycle minima. Extremely small scale features in the solar photosphere cause the solar cycle changes in the luminosity. Solar-B will make the first observations with spatial resolution, wavelength coverage, and sampling adequate to determine the role of these features in the long-term solar luminosity changes. The solar UV and X-radiation originates in the chromosphere and corona, where temperatures rise to over one million degrees and where the plasma is highly dynamic, often erupting into the heliosphere. Solar-B will open a new window on the underlying causes of coronal heating and eruptions by providing the first accurate measurements of the Sun's magnetic field and electric currents, simultaneous with detailed observations of the coronal dynamics. Programmatically, Solar-B represents a unique opportunity for NASA to participate with a highly reliable partner in a frontier-probe class mission at the cost of a MIDEX. Solar-B also represents an ideal opportunity for continuing the Solar-Terrestrial Probe Line of the Sun-Earth Connections Theme. Title: Magnetic fluxtube reconnection Authors: Dahlburg, R. B.; Antiochos, S. K. Bibcode: 1997AdSpR..19.1781D Altcode: We present the results of 3D numerical simulations of initially discrete magnetic fluxtubes interacting via magnetic reconnection. The initial topology consists of two orthogonal fluxtubes. Each fluxtube has a uniform twist, force-free magnetic field specified by the Gold-Hoyle model. The fluxtubes are then forced together by an initial flow configuration consisting of two superimposed stagnation point flows. We observe three distinct types of interaction, which depend on the twist and on the Lundquist numbers, between the fluxtubes. For low twist the fluxtubes experience an elastic collision. For a higher twist complete reconnection is observed. If the Lundquist numbers are raised fluxtube tunneling occurs. Title: A Model for Coronal Mass Ejections Authors: Antiochos, S. K. Bibcode: 1996AAS...189.5603A Altcode: 1996BAAS...28R1346A Coronal mass ejections (CME) consist of huge eruptions of solar coronal plasma and magnetic field, and are now known to be the main drivers of geomagnetic disturbances. The energy source for CMEs must be magnetic since the plasma beta in the corona is observed to be low, and the gravitational energy can only increase as a result of eruption. Although there has been a great deal of work on coronal mass ejections in recent years, we argue that none of the previous models is able to satisfy the observational constraints, in particular, the result that the eruption begins at very low heights in the corona. For the case of the most energetic events which are usually accompanied by large eruptive flares, the magnetic field must blow open all the way down to the chromosphere. In this paper we propose a new model for CMEs. The key feature of our model is that magnetic reconnection occurs above the erupting flux, rather than below as in all previous models. We present theoretical arguments and numerical simulations demonstrating that our model can explain the observed opening of field lines down to the chromosphere. This work was supported in part by NASA and ONR. Title: STEREO: a solar terrestrial event observer mission concept Authors: Socker, Dennis G.; Antiochos, S. K.; Brueckner, Guenter E.; Cook, John W.; Dere, Kenneth P.; Howard, Russell A.; Karpen, J. T.; Klimchuk, J. A.; Korendyke, Clarence M.; Michels, Donald J.; Moses, J. Daniel; Prinz, Dianne K.; Sheely, N. R.; Wu, Shi T.; Buffington, Andrew; Jackson, Bernard V.; Labonte, Barry; Lamy, Philippe L.; Rosenbauer, H.; Schwenn, Rainer; Burlaga, L.; Davila, Joseph M.; Davis, John M.; Goldstein, Barry; Harris, H.; Liewer, Paulett C.; Neugebauer, Marcia; Hildner, E.; Pizzo, Victor J.; Moulton, Norman E.; Linker, J. A.; Mikic, Z. Bibcode: 1996SPIE.2804...50S Altcode: A STEREO mission concept requiring only a single new spacecraft has been proposed. The mission would place the new spacecraft in a heliocentric orbit and well off the Sun- Earth line, where it can simultaneously view both the solar source of heliospheric disturbances and their propagation through the heliosphere all the way to the earth. Joint observations, utilizing the new spacecraft and existing solar spacecraft in earth orbit or L1 orbit would provide a stereographic data set. The new and unique aspect of this mission lies in the vantage point of the new spacecraft, which is far enough from Sun-Earth line to allow an entirely new way of studying the structure of the solar corona, the heliosphere and solar-terrestrial interactions. The mission science objectives have been selected to take maximum advantage of this new vantage point. They fall into two classes: those possible with the new spacecraft alone and those possible with joint measurements using the new and existing spacecraft. The instrument complement on the new spacecraft supporting the mission science objectives includes a soft x-ray imager, a coronagraph and a sun-earth imager. Telemetry rate appears to be the main performance determinant. The spacecraft could be launched with the new Med-Lite system. Title: A Study of the Unresolved Fine-Structure Model for the Solar Transition Region Authors: Spadaro, D.; Lanza, A. F.; Antiochos, S. K. Bibcode: 1996ApJ...462.1011S Altcode: The unresolved fine-structure (UFS) model for the lower transition region was proposed by Feldman as an explanation for a number of puzzling observational results: specifically, the small filling factor of this region, the inability of the observations to resolve the temperature structure, and the observation of persistent redshifted UV emission lines even near the solar limb. It was hypothesized that opacity effects may be able to explain the redshift observations. We consider a simple model for the UFS consisting of a plasma sphere undergoing expansion and contraction due to a time-varying heating. We calculate in detail the line profile of the well-observed C IV 1548 Å line. Our calculations include the effects of both nonequilibrium ionization and radiative transfer. We find that although the model can reproduce some of the features of the observations, such as the line widths, the effect of finite optical depth is to produce a blueshifted peak for the emission line, contrary to observations. The physical origins of this blueshift are discussed. We conclude that unless the pressures of the UFS are significantly higher than the typical pressures assumed for the lower transition region, opacity effects are unlikely to explain the observations. Title: Reconnection in the Solar Corona: the Effects of Slow Footpoint Motions Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R. Bibcode: 1996AAS...188.8605K Altcode: 1996BAAS...28Q.964K Previous simulations of magnetic reconnection in both symmetric and asymmetric topologies (Karpen, Antiochos, & DeVore 1995, ApJ, 450, 422; ApJL, 460, L73) demonstrated that shear-driven reconnection can reproduce several fundamental features of chromospheric eruptions (e.g., spicules, surges, and the HRTS explosive events). In asymmetric systems, moreover, the random nature of the reconnection yields numerous current sheets over a large but well-defined volume resembling a coronal loop in profile, a phenomemon which we denoted Reconnection Driven Current Filamentation. However, in these calculations the field was subjected to footpoint shearing much stronger than typical photospheric motions. In this talk we will discuss the response of the symmetric topology to footpoint motions approximately an order of magnitude slower, commensurate with typical photospheric flow speeds. The finite-difference simulation was performed with a new, parallelized version of our 2.5-dimensional FCT-based code (MAG25D), which solves the ideal compressible MHD equations with complex boundary conditions; numerical diffusivity alone provided localized reconnection. We find that reconnection proceeds in an uneven manner, as the field around the initial X point oscillates aperiodically between vertical and horizontal current sheet formations, while the larger-scale surrounding field rises and falls. The greater separation between the driver and the characteristic plasma (e.g., the Alfven transit) time scales reveals a variety of behaviors ranging from rapid bursts of reconnection to the slow ``breathing" of the large-scale stressed field. In addition, we will explore the implications of these results for the applicability of fast (Petschek) reconnection models to the solar atmosphere. Title: Reconnection Between Open and Closed Fields in the Solar Corona Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R. Bibcode: 1996AAS...188.8606A Altcode: 1996BAAS...28..964A The effects of shear-driven magnetic reconnection in a 2.5D quadrupolar magnetic topology have been calculated previously (Karpen, Antiochos & DeVore 1995, ApJL, 460, L73). A quadrupolar topology in the solar corona corresponds to the interaction of two sheared closed magnetic arcades of opposite polarity. The key result of the previous work is that the reconnection proceeds by the creation of a long current sheet and the sporadic formation of magnetic islands along that sheet. This leads to the creation of numerous current sheets over a large volume of the post-reconnection field. Magnetic reconnection has also been frequently proposed, however, as the origin of the heating and acceleration in coronal hole regions. The relevant topology in this case must be that of a closed magnetic arcade and an open flux system. In addition, reconnection of open field should be more appropriate for phenomena such as eruptive flares and coronal mass ejections. Consequently, we have simulated numerically the interaction of a closed bipolar arcade and an open field flux system. A major difference between this simulation and the previous case is that the boundary conditions at the top of the simulation box can play an important role in the evolution of the reconnection region. We present the results of our simulations, and contrast them the results for the quadrupole case. In addition, we discuss the physical reason for the creation of long current sheets during the reconnection process. Title: Reconnection Driven Current Filament in Solar Arcades Authors: Karpen, Judith T.; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 1996ApJ...460L..73K Altcode: We present numerical simulations of the interaction between two bipoles through magnetic reconnection in the lower solar atmosphere, a process believed to be the origin of many manifestations of solar activity. The present work differs from previous studies in that the field is sheared asymmetrically and that the bipoles have markedly unequal field strengths. Our key discovery is that, under such common circumstances, reconnection leads to an apparently random distribution of shear in the magnetic field, resulting in numerous current sheets throughout the volume occupied by the reconnected field lines. To our knowledge, this is the first example of a numerical simulation yielding current sheets over a large but well-defined volume of the corona, resembling a coronal loop in profile. In this Letter, we demonstrate this process of reconnection-driven current filamentation and discuss ramifications for coronal heating and structure. Title: The Nature of Magnetic Reconnection in the Corona Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R. Bibcode: 1996ASPC..111...79A Altcode: 1997ASPC..111...79A No abstract at ADS Title: Solar Drivers of Space Weather Authors: Antiochos, Spiro K. Bibcode: 1996ASPC...95....1A Altcode: 1996sdit.conf....1A No abstract at ADS Title: Reconnection of antiparallel magnetic flux tubes Authors: Dahlburg, R. B.; Antiochos, S. K. Bibcode: 1995JGR...10016991D Altcode: Many examples of solar activity, such as large two-ribbon flares and prominence eruptions, are widely believed to involve the fast reconnection of magnetic flux tubes. Because of the difficulties associated with calculating the evolution of three-dimensional (3-D) flux tubes, however, the details of the energy-release process are poorly understood. In this paper we describe our first attempts to shed light on this important process. We describe the results of 3-D numerical simulations of initially distinct magnetic flux tubes interacting via magnetic reconnection. As a typical case, we consider an initial magnetic field given by a compact support function distribution so that the initial topology consists of two antiparallel flux tubes. We then impose an initial velocity field on this system which causes the flux tubes to move toward each other. As a result of this initial velocity, the tubes first flatten against each other and an electric current sheet begins to develop at the interface between them. After approximately 10 Alfven times we observe a burst of reconnection. The turbulent kinetic energy rises dramatically as two reconnection jets form, which are aligned parallel to the initial field. The reconnection phase lasts for approximately 20 Alfven times, by which time the central region of the initial tubes has been completely dissipated so that the system now consists of four tubes that are relatively widely separated and hence stop interacting. We find that the excitation of small-scale spatial structure in the flow field depends critically on the value of the Lundquist numbers. Compressible effects are insignificant for this particular case of flux tube reconnection. The numerical simulations are carried out using a three-dimensional explicit Fourier collocation algorithm for solving the viscoresistive equations of compressible magnetohydrodynamics. We also report on the performance of a new parallelized version of the code. Title: The Role of Magnetic Reconnection in Chromospheric Eruptions Authors: Karpen, Judith T.; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 1995ApJ...450..422K Altcode: We investigate the hypothesis that all chromospheric eruptions are manifestations of a common magnetohydrodynamic phenomenon occurring on different scales: the acceleration of chromospheric plasma driven by localized magnetic reconnection. Our approach is to perform 2.5-dimensional numerical simulations of shear-induced reconnection in a potential magnetic field with a central X-point above the photosphere, embedded in a model chromosphere with solar gravity and numerical resistivity. Calculations with two values of the footpoint displacement were performed by applying a localized body-force duration twice as long in one case as in the other; after the shearing was discontinued, the system was allowed to relax for an additional interval. For the stronger shear, the initial X-point lengthens upward into a current sheet which reconnects gradually for a while but then begins to undergo multiple tearing. Thereafter, several magnetic islands develop in sequence, move toward the ends of the sheet, and disappear through reconnection with the overlying or underlying field. During the relaxation stage, a new quasi-equilibrium state arises with a central magnetic island. We also performed a reference calculation with the stronger shear but with greatly reduced numerical resistivity along the boundary where the X-point and subsequent current sheet are located. This simulation confirmed our expectations for the system evolution in the ideal limit: the current sheet becomes much longer, without significant reconnection. For the weaker shear, a much shorter sheet forms initially which then shrinks smoothly through reconnection to yield an X-point relocated above its original position, quite distinct from the final state of the strong-shear case. After reviewing the dynamics and plasma properties as well as the evolving magnetic topology, we conclude that geometry, shear strength, and local resistivity must determine the dynamic signatures of chromospheric eruptions. Our model reproduces such fundamental observed features as intermittency and large velocities, as well as the approximately concurrent appearance of oppositely directed flows. We also find that reconnection in the vertical current sheet is more consistent with Sweet-Parker reconnection theory, while the rapid interaction between the magnetic islands and the background field better approximates the Petschek process. Title: Asymptotic Analysis of Force-free Magnetic Fields of Cylindrical Symmetry Authors: Sturrock, P. A.; Antiochos, S. K.; Roumeliotis, G. Bibcode: 1995ApJ...443..804S Altcode: It is known from computer calculations that if a force-free magnetic-field configuration is stressed progressively by footpoint displacements, the configuration expands and approaches the open configuration with the same surface flux distribution, and, in the process, the energy of the field increases progressively. Analysis of a simple model of force-free fields of cylindrical symmetry leads to simple asymptotic expressions for the extent and energy of such a configuration. The analysis is carried through for both spherical and planar source surfaces. According to this model, the field evolves in a well-behaved manner with no indication of instability or loss of equilibrium. Title: The Magnetic Field of Solar Prominences Authors: Antiochos, S. K.; Klimchuk, J. A.; Dahlburg, R. B. Bibcode: 1995SPD....26..717A Altcode: 1995BAAS...27..969A No abstract at ADS Title: Chromospheric Reconnection in Asymmetric Topologies Authors: Karpen, J. T.; de Vore, C. R.; Antiochos, S. K. Bibcode: 1995SPD....26..507K Altcode: 1995BAAS...27..958K No abstract at ADS Title: Parallel Computation of Fluxtube Reconnection Authors: Norton, D.; Dahlburg, R. B.; Antiochos, S. K. Bibcode: 1995SPD....26.1006N Altcode: 1995BAAS...27..978N No abstract at ADS Title: Cooling of Solar Flare Plasmas. I. Theoretical Considerations Authors: Cargill, Peter J.; Mariska, John T.; Antiochos, Spiro K. Bibcode: 1995ApJ...439.1034C Altcode: Theoretical models of the cooling of flare plasma are reexamined. By assuming that the cooling occurs in two separate phase where conduction and radiation, respectively, dominate, a simple analytic formula for the cooling time of a flare plasma is derived. Unlike earlier order-of-magnitude scalings, this result accounts for the effect of the evolution of the loop plasma parameters on the cooling time. When the conductive cooling leads to an 'evaporation' of chromospheric material, the cooling time scales L5/6/p1/6, where the coronal phase (defined as the time maximum temperature). When the conductive cooling is static, the cooling time scales as L3/4n1/4. In deriving these results, use was made of an important scaling law (T proportional to n2) during the radiative cooling phase that was forst noted in one-dimensional hydrodynamic numerical simulations (Serio et al. 1991; Jakimiec et al. 1992). Our own simulations show that this result is restricted to approximately the radiative loss function of Rosner, Tucker, & Vaiana (1978). for different radiative loss functions, other scaling result, with T and n scaling almost linearly when the radiative loss falls off as T-2. It is shown that these scaling laws are part of a class of analytic solutions developed by Antiocos (1980). Title: Mass flows in coronal loops Authors: Antiochos, Spiro K. Bibcode: 1994SSRv...70..143A Altcode: Although static loop models are often used to describe the structure of coronal loops, it is evident on both observational and theoretical grounds that mass motions play a crucial role in the physics of the corona and transition region. First we review the observations of emission-line broadening and wavelength shifts, which imply the presence of random motions and systematic downflows in coronal loops. Some discrepancies in the observations are discussed. It is argued that velocities due to gas pressure gradients are the most likely explanation for the observed flows. A number of models that have been proposed for these motions are reviewed. The implications of the various models on observations of the corona and transition region by SOHO are discussed. Title: Observational evidence for non-equilibrium ionization in the solar corona Authors: Spadaro, D.; Leto, P.; Antiochos, S. K. Bibcode: 1994SSRv...70..207S Altcode: We investigate whether temperature sensitive EUV line ratios can be used as observational signatures for the presence of non-equilibrium ionization in transition region plasma. We compute the total intensity of some EUV lines of carbon and oxygen expected from coronal loop models with a steady-state flow and which are known to have significant departures from ionization equilibrium, selecting lines whose intensity ratios are useful for deducing the electron temperature in the coronal plasma. We calculate the intensity ratios with and without the approximation of ionization equilibrium, in order to determine the effects of any deviations from equilibrium on the numerical values of the line ratios examined. Title: Asymptotic Forms for the Energy of Force-free Magnetic Field Configurations of Translational Symmetry Authors: Sturrock, P. A.; Antiochos, S. K.; Klimchuk, J. A.; Roumeliotis, G. Bibcode: 1994ApJ...431..870S Altcode: It is known from computer calculations that if a force-free magnetic field configuration is stressed progressively by footpoint displacements, the configuration expands and approaches the open configuration with the same surface flux distribution and the energy of the field increases progressively. For configurations of translational symmetry, it has been found empirically that the energy tends asymptotically to a certain functional form. It is here shown that analysis of a simple model of the asymptotic form of force-free fields of translational symmetry leads to and therefore justifies this functional form. According to this model, the field evolves in a well-behaved manner with no indication of instability or loss of equilibrium. Title: Coronal Structures Observed in X-rays (NIXT) and H_alpha Surges Authors: Schmieder, B.; Mouradian, Z.; Golub, L.; Antiochos, S. Bibcode: 1994kofu.symp..317S Altcode: Ground-based coordinated observations with the Multichannel subtractive double pass spectrograph (MSDP) and the heliograph in Meudon allowed us to portray the chromospheric intensity and velocity fields below coronal structures observed with the Normal Incidence X-ray Telescope (NIXT). On July 11, 1991 (eclipse day) we have identified in AR 6713 (N38 W 42) the X-ray signatures of the network, subflares, filaments and surges. The largest H_alpha surge has only weak emission in X-ray, while a weak H_alpha feature corresponds to a very bright x-ray subflare. We calculate the emission measures of these events and give some constraints on the triggering mechanisms of surges. Title: Observational Tests for Nonequilibrium Ionization in the Solar Corona Authors: Spadaro, D.; Leto, P.; Antiochos, S. K. Bibcode: 1994ApJ...427..453S Altcode: Nonequilibrium ionization may be produced by a variety of processes in the solar corona, for example, by mass flows through the large temperature gradients of the transition region or by impulsive heating and cooling. Any deviation from equilibrium ionization would have a strong effect on the radiation from the corona and on the interpretation of solar observations; hence, it is important to determine observational signatures of nonequilibrium. The temperature-sensitive line ratios can be used as such signatures. We examine the line ratios: C IV I(1548.2 A)/I(312.4 A), O IV I(789.4 A)/I(554.4 A), O V I(629.7 A)/I(172.2 A), O VI I(1031.9 A)/I(173.0 A) and O VI I(1031.9 A)/I(150.1 A). These line ratios are calculated for four coronal loop models that have a steady flow and that are known to have significant departures from equilibrium ionization. Our results indicate that, in general, nonequilibrium causes a considerable reduction in the line ratios, more than an order of magnitude in the downflowing leg of the loop model with the largest mass flows. We find that the C IV line ratio is the most sensitive to nonequilibrium. We discuss the implications of our results for observations, specifically, the observations expected from the upcoming SOHO mission. Title: Comparison between Cool and Hot Plasma Behaviors of Surges Authors: Schmieder, B.; Golub, L.; Antiochos, S. K. Bibcode: 1994ApJ...425..326S Altcode: Ground-based coordinated observations with the Multichannel Subtractive Double Pass spectrograph (MSDP) allowed us to obtain chromospheric intensity and velocity field maps below coronal structures during the launch of the NIXT payload on 1991 July 11 (eclipse day). A large H-alpha ejection in AR 6713 (N38 W40) was detected during the NIXT flight. However, only a low level of X-ray emission was associated with this event. In contrast, bright X-ray emission associated with a subflare was observed in a nearby active region, but with only a weak associated ejection in H-alpha. A discussion of both of these events gives strong constraints on the triggering mechanisms of surges. Title: The physics of coronal closed-field structures Authors: Antiochos, Spiro K. Bibcode: 1994AdSpR..14d.139A Altcode: 1994AdSpR..14Q.139A The properties of closed coronal loops are reviewed. First we discuss the main features of the static, hot loop models. In these models thermal conduction plays the dominant role in determining the temperature and density structure. Next the cool loop models and their implications for solar observations are discussed. Finally, some new theoretical results on coronal abundances are presented. It is argued that chromospheric evaporation, which is a basic feature of the hot models, can account for the observed anomalies in the coronal element abundances. Title: A Numerical Study of the Sudden Eruption of Sheared Magnetic Fields Authors: Roumeliotis, George; Sturrock, Peter A.; Antiochos, Spiro K. Bibcode: 1994ApJ...423..847R Altcode: We investigate the quasi-static evolution of an idealized magnetic configuration in the solar corona that is subjected to photospheric shearing motions. The initial, unsheared field in our calculations is a magnetic dipole located at the center of the Sun. The assumed photospheric shearing motions are latitude-dependent and antisymmetric about the equator. The quasi-static evolution of the coronal field is calculated using the magneto-frictional method. A key difference between our study and previous work is that the outer computational boundary is placed exceedingly far from the solar surface where the shearing motions are applied. This is achieved by writing the basic equations of the magneto-frictional method in terms of the logarithm of radial distance. We find that initially, the coronal magnetic field expands steadily as the footpoint displacement is increased. However, when the footpoint displacement exceeds a certain critical amount, the qualitative behavior of the evolving field suddenly changes, so that the outward expansion of the field lines becomes a much more rapidly increasing function of the footpoint displacement. We propose that this sudden transition to a regime with very sensitive dependence on boundary conditions plays an important role in the onset of eruptive phenomena in the solar atmosphere. Title: Sleuthing the Dynamo: HST/FOS Observations of UV Emissions of Solar-Type Stars in Young Clusters Authors: Ayres, T.; Basri, G.; Simon, T.; Stauffer, J.; Stern, R.; Antiochos, S.; Bookbinder, J.; Brown, A.; Doschek, G.; Linsky, J.; Ramsey, L.; Walter, F. Bibcode: 1994ASPC...64...53A Altcode: 1994csss....8...53A No abstract at ADS Title: A Far-Ultraviolet Flare on a Pleiades G Dwarf Authors: Ayres, T. R.; Stauffer, J. R.; Simon, Theodore; Stern, R. A.; Antiochos, S. K.; Basri, G. S.; Bookbinder, J. A.; Brown, A.; Doschek, G. A.; Linsky, J. L.; Ramsey, L. W.; Walter, F. M. Bibcode: 1994ApJ...420L..33A Altcode: The Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) recorded a remarkable transient brightening in the C IV lambda lambda 1548,50 emissions of the rapidly rotating Pleiades G dwarf H II 314. On the one hand the 'flare' might be a rare event luckily observed; on the other hand it might be a bellwether of the coronal heating in very young solar-mass stars. If the latter, flaring provides a natural spin-down mechanism through associated sporadic magnetospheric mass loss. Title: The Magnetic Field of Solar Prominences Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J. A. Bibcode: 1994ApJ...420L..41A Altcode: A model is presented which accounts for the formation of coronal magnetic field lines with the appropriate 'dipped' structure to support prominences. The critical ingredients of the model are that the prominence magnetic field is a truly three-dimensional structure with significant variation along the prominence length, and the magnetic field is strongly sheared near the photospheric neutral line. Numerical calculations are presented which demonstrate that these two features lead to dip formation. In addition our model is able to account for the long-puzzling observation of inverse polarity in quiescent prominences. Title: The Asymptotic Behavior of Force-Free Magnetic-Field Configurations Authors: Sturrock, P. A.; Klimchuk, J. A.; Roumeliotis, G.; Antiochos, S. K. Bibcode: 1994ASPC...68..219S Altcode: 1994sare.conf..219S No abstract at ADS Title: Coronal Structures Observed in X-Rays (NIXT) and Hα Surges Authors: Schmieder, B.; Mouradian, Z.; Golub, L.; Antiochos, S. Bibcode: 1994emsp.conf..159S Altcode: No abstract at ADS Title: Current Sheet Formation in Complex Solar Coronal Fields Authors: Benka, Steve G.; Antiochos, Spiro K. Bibcode: 1993AAS...183.5903B Altcode: 1993BAAS...25.1386B We discuss the formation of current singularities and reconnection in magnetic fields with complex 3D topology. First, we argue that since the photospheric field is observed to consist of a complicated mixture of positive and negative polarity regions, the coronal magnetic field must, in general, contain a large number of separatrix surfaces and null points. Using numerical simulations, we calculate the effect of photospheric stressing on such a field. As initial conditions in the numerical model, we assume a cylindrically-symmetric potential field consisting of a small dipole field imbedded in a background larger dipole; hence, there are three polarity regions on the photosphere. In the corona the field has a hemisperical separatrix surface with a null point at the apex of this surface. The initial field is then stressed by footpoint motions at the photosphere that have the form of a vortical flow of finite width. Results are discussed for two different photospheric locations of this flow, one in which the flow is centered on the symmetry axis so that the system retains its cylindrical symmetry, and one in which the flow is offset from the symmetry axis. The results of these simulations are discussed, in particular, the nature of reconnection in a true 3D geometry. We find that in both cases current sheets form at the separatrix. We argue that this mechanism for current sheet formation may play a central role in coronal heating. Title: The Structure of Prominence Magnetic Fields Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J. A. Bibcode: 1993BAAS...25.1206A Altcode: No abstract at ADS Title: Coronal Current Sheet Formation: 3D Simulations Authors: Benka, S. G.; Antiochos, S. K.; Zalesak, S. T.; Spicer, D. S. Bibcode: 1993BAAS...25.1207B Altcode: No abstract at ADS Title: Reconnection of Magnetic Flux Tubes Authors: Dahlburg, R. B.; Antiochos, S. K. Bibcode: 1993BAAS...25.1199D Altcode: No abstract at ADS Title: Asymptotic Forms for the Energy of Force Free Magnetic-Field Configurations Authors: Sturrock, P. A.; Roumeliotis, G.; Antiochos, S. Bibcode: 1993BAAS...25.1218S Altcode: No abstract at ADS Title: The Kelvin-Helmholtz Instability in Photospheric Flows: Effects on Coronal Heating and Structure Authors: Karpen, Judith T.; Antiochos, Spiro K.; Dahlburg, Russell B.; Spicer, Daniel S. Bibcode: 1993ApJ...403..769K Altcode: A series of hydrodynamic numerical simulations has been used to investigate the nonlinear evolution of driven, subsonic velocity shears under a range of typical photospheric conditions. These calculations show that typical photospheric flows are susceptible to the Kelvin-Helmholtz instability (KHI), with rapid nonlinear growth times that are approximately half of a typical granule lifetime. The KHI produces vortical structures in intergranule lanes comparable to a typical fluxule radius; this is precisely the correct scale for maximum power transfer to the corona. Title: Three-Dimensional Magnetic Reconnection in a Coronal Neutral Sheet Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, T. A. Bibcode: 1993ASSL..183..611D Altcode: 1993pssc.symp..611D No abstract at ADS Title: Secondary instability in three-dimensional magnetic reconnection Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, T. A. Bibcode: 1992PhFlB...4.3902D Altcode: We consider the transition to turbulence in three-dimensional reconnection of a magnetic neutral sheet. We find that the transition can occur via a three-step process. First, the sheet undergoes the usual tearing instability. Second, the tearing mode saturates to form a two-dimensional quasi-steady state. Third, this secondary equilibrium is itself unstable when it is perturbed by three-dimensional disturbances. Most of this paper is devoted to the analysis and simulation of the three-dimensional linear stability properties of the two-dimensional saturated tearing layer. The numerical simulations are performed with a semi-implicit, pseudospectral-Fourier collocation algorithm. We identify a three-dimensional secondary linear stability which grows on the ideal timescale. An examination of the modal energetics reveals that the largest energy transfer is from the mean field to the three-dimensional field, with the two-dimensional field acting as a catalyst. Title: The physics of solar prominences. Authors: Antiochos, Spiro K. Bibcode: 1992ESASP.348..201A Altcode: 1992cscl.work..201A The outstanding questions on the formation of quiescent prominences are discussed. One key issue is identified to be the formation of dips in coronal magnetic field lines. A model is presented which can account for such dipped field lines. The critical ingredients of the model are that (a) the prominence magnetic field is a truly three dimensional structure with significant variation along the prominence length, and (b) the magnetic field has strong shear concentrated at the photospheric neutral line. Simulations are presented which demonstrate that these two features lead to dip formation, and that the geometry of the dips are such that inverse polarity prominences can be explained. Another key issue is identified to be the formation of prominence condensations on dipped field lines. It is argued that a spatially-varying coronal heating rate which is maximum near the chromosphere can explain these condensations. Title: A Model for the Magnetic Fields of Solar Prominences Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J. Bibcode: 1992AAS...180.1205A Altcode: 1992BAAS...24..748A No abstract at ADS Title: Secondary Instability in 3d Neutral Sheets Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, A. Bibcode: 1992AAS...180.5504D Altcode: 1992BAAS...24..819D No abstract at ADS Title: Dynamics of Solar Coronal Magnetic Fields Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, T. A. Bibcode: 1991ApJ...383..420D Altcode: A 3D time-dependent numerical simulation of the foot-point stressing of coronal magnetic field was developed in order to relate coronal activity with the stressing of the coronal magnetic field by foot-point motions at the photosphere. The results of the simulation did not reveal magnetic reconnection, kinking, or the formation of concave-up magnetic field lines suitable for prominence formation. It is concluded that, contrary to many models, photospheric twisting of a single arcade does not lead to the type of processes required to explain solar activity. Title: Coronal Current-Sheet Formation: The Effect of Asymmetric and Symmetric Shears Authors: Karpen, Judith T.; Antiochos, Spiro K.; DeVore, C. R. Bibcode: 1991ApJ...382..327K Altcode: A 2.5D numerical code is used to investigate the results of an asymmetric shear imposed on a potential quadrupolar magnetic field under two sets of atmospheric boundary conditions - a low-beta plasma with line tying at the base, similar to the line-tied analytic model, and a hydrostatic-equilibrium atmosphere with solar gravity, typical of the observed photosphere-chromosphere interface. The low-beta simulation confirms the crucial role of the line-tying assumption in producting current sheets. The effects of a symmetric shear on the same hydrostatic-equilibrium atmosphere is examined, using more grid points to improve the resolution of the current structures which form along the flux surfaces. It is found that true current sheets do not form in the corona when a more realistic model is considered. The amount of Ohmic dissipation in the thick currents is estimated to be two to four orders of magnitude below that required to heat the corona. It is concluded that magnetic topologies of the type examined here do not contribute significantly to coronal heating. Title: Nonequilibrium Ionization Effects in Asymmetrically Heated Loops Authors: Spadaro, D.; Antiochos, Spiro K.; Mariska, J. T. Bibcode: 1991ApJ...382..338S Altcode: The effects of nonequilibrium ionization on magnetic loop models with a steady siphon flow that is driven by a nonuniform heating rate are investigated. The model developed by Mariska (1988) to explain the observed redshifts of transition region emission lines is examined, and the number densities of the ions of carbon and oxygen along the loop are computed, with and without the approximation of ionization equilibrium. Considerable deviations from equilibrium were found. In order to determine the consequences of these nonequilibrium effects on the characteristics of the EUV emission from the loop plasma, the profiles and wavelength positions of all the important emission lines due to carbon and oxygen were calculated. The calculations are in broad agreement with Mariska's conclusions, although they show a significant diminution of the Doppler shifts, as well as modifications to the line widths. It is concluded that the inclusion of nonequilibrium effects make it more difficult to reproduce the observed characteristics of the solar transition region by means of the asymmetric-heating models. Title: Magnetic Reconnection in Three Dimensions Authors: Dahlberg, R. B.; Antiochos, S. K.; Zang, T. A. Bibcode: 1991BAAS...23.1467D Altcode: No abstract at ADS Title: A Model for the Formation of Solar Prominences Authors: Antiochos, S. K.; Klimchuk, J. A. Bibcode: 1991ApJ...378..372A Altcode: A model for the formation of prominence condensations in hot coronal loops is proposed. Previous studies have concentrated on cooling the hot plasma by decreasing the coronal heating rate. The difficulty with such models is that when the heating decreases, most of the loop mass is lost by draining onto the chromosphere. It is argued that a prominence condensation is likely to be due to an increase in the heating. The key idea of the model is that the heating increase is spatially dependent so that it is localized nearer to the chromospheric footpoints than to the loop midpoint. Results are presented of numerical simulations of hot loops that are initially heated uniformly, and then undergo heating increases that are concentreated away from the loop midpoint. The temperature at the midpoint first increases, but eventually it collapses to chromospheric values as a result of chromospheric evaporation. Hence, a curious result is obtained, that increasing the heating causes cooling. The resulting densities and time scales agree well with observations. The implications of this model for coronal heating and prominence structure are discussed. Title: Report of the solar physics panel Authors: Withbroe, George L.; Fisher, Richard R.; Antiochos, Spiro; Brueckner, Guenter; Hoeksema, J. Todd; Hudson, Hugh; Moore, Ronald; Radick, Richard R.; Rottman, Gary; Scherrer, Philip Bibcode: 1991spsi....1...67W Altcode: Recent accomplishments in solar physics can be grouped by the three regions of the Sun: the solar interior, the surface, and the exterior. The future scientific problems and areas of interest involve: generation of magnetic activity cycle, energy storage and release, solar activity, solar wind and solar interaction. Finally, the report discusses a number of future space mission concepts including: High Energy Solar Physics Mission, Global Solar Mission, Space Exploration Initiative, Solar Probe Mission, Solar Variability Explorer, Janus, as well as solar physics on Space Station Freedom. Title: The Effects of the Kelvin-Helmholtz Instability on Photospheric Flows Authors: Karpen, J. T.; Antiochos, S. K.; Dahlburg, R. B.; Spicer, D. S. Bibcode: 1991BAAS...23.1059K Altcode: No abstract at ADS Title: The Effects of Strong Shear on Solar Coronal Magnetic Fields Authors: DeVore, C. R.; Antiochos, S. K. Bibcode: 1991BAAS...23.1058D Altcode: No abstract at ADS Title: A Model for the Anomalous Elemental Abundances of the Solar Corona and Wind Authors: Antiochos, S. K. Bibcode: 1991BAAS...23.1046A Altcode: No abstract at ADS Title: Solar astronomy Authors: Rosner, Robert; Noyes, Robert; Antiochos, Spiro K.; Canfield, Richard C.; Chupp, Edward L.; Deming, Drake; Doschek, George A.; Dulk, George A.; Foukal, Peter V.; Gilliland, Ronald L. Bibcode: 1991aap..reptR....R Altcode: An overview is given of modern solar physics. Topics covered include the solar interior, the solar surface, the solar atmosphere, the Large Earth-based Solar Telescope (LEST), the Orbiting Solar Laboratory, the High Energy Solar Physics mission, the Space Exploration Initiative, solar-terrestrial physics, and adaptive optics. Policy and related programmatic recommendations are given for university research and education, facilitating solar research, and integrated support for solar research. Title: The Effects of Nonequilibrium Ionization on the Radiative Losses of the Solar Corona Authors: Spadaro, D.; Zappala, R. A.; Antiochos, S. K.; Lanzafame, G.; Noci, G. Bibcode: 1990ApJ...362..370S Altcode: 1990ApJ...362R.370S The emissivity of the ions of carbon and oxygen has been recalculated for a set of solar coronal loop models with a steady state siphon flow. The ion densities were calculated from the plasma velocities, temperatures, and densities of the models, and large departures from equilibrium were found. For purposes of comparison, the emissivity was calculated with and without the approximation of ionization equilibrium. Considerable differences in the radiative loss function Lambda(T) curve between equilibrium and nonequilibrium conditions were found. The nonequilibrium Lambda(T) function was then used to solve again the steady state flow equations of the loop models. The differences in the structure of these models with respect to the models calculated adopting the Lambda(T) curve in equilibrium are discussed. Title: Numerical simulation of solar coronal magnetic fields Authors: Dahlburg, Russell B.; Antiochos, Spiro K.; Zang, T. A. Bibcode: 1990nasa.rept.....D Altcode: Many aspects of solar activity are believed to be due to the stressing of the coronal magnetic field by footpoint motions at the photosphere. The results are presented of a fully spectral numerical simulation which is the first 3-D time dependent simulation of footpoint stressing in a geometry appropriate for the corona. An arcade is considered that is initially current-free and impose a smooth footpoint motion that produces a twist in the field of approx 2 pi. The footprints were fixed and the evolution was followed until the field relaxes to another current-free state. No evidence was seen for any instability, either ideal or resistive and no evidence for current sheet formation. The most striking feature of the evolution is that in response to photospheric motions, the field expands rapidly upward to minimize the stress. The expansion has two important effects. First, it suppresses the development of dips in the field that could support dense, cool material. For the motions assumed, the magnetic field does not develop a geometry suitable for prominence formation. Second, the expansion inhibits ideal instabilities such as kinking. The results indicate that simple stearing of a single arcade is unlikely to lead to solar activity such as flares or prominences. Effects are discussed that might possibly lead to such activity. Title: On the Formation of Current Sheets in the Solar Corona Authors: Karpen, Judith T.; Antiochos, Spiro K.; DeVore, C. Richard Bibcode: 1990ApJ...356L..67K Altcode: Several theoretical studies have proposed that, in response to photospheric footpoint motions, current sheets can be generated in the solar corona without the presence of a null point in the initial potential magnetic field. A fundamental assumption in these analyses, commonly referred to as the line-tying assumption, is that all coronal field lines are anchored to a boundary surface representing the top of the dense, gas pressure-dominated photosphere. It is shown here that line-typing cannot be applied indiscriminately to dipped coronal fields, and that the conclusions of the line-tied models are incorrect. To support the theoretical arguments, the response of a dipped potential magnetic field in a hydrostatic-equilibrium atmosphere to shearing motions of the footpoints is studied, using a 2.5-dimensional MHD code. The results show that, in the absence of artificial line-tying conditions, a current sheet indeed does not form at the location of the dip. Rather, the dipped magnetic field rises, causing upflows of photospheric and chromospheric plasma. Title: Episodic Coronal Heating Authors: Sturrock, P. A.; Dixon, W. W.; Klimchuk, J. A.; Antiochos, S. K. Bibcode: 1990ApJ...356L..31S Altcode: A study is made of the observational consequences of the hypothesis that there is no steady coronal heating, the solar corona instead being heated episodically, such that each short burst of heating is followed by a long period of radiative cooling. The form of the resulting contribution to the differential emission measure (DEM), and to a convenient related function (the differential energy flux, DEF) is calculated. Observational data for the quiet solar atmosphere indicate that the upper branch of the DEM, corresponding to temperatures above 100,000 K, can be interpreted in terms of episodic energy injection at coronal temperatures. Title: The Effect of Nonequilibrium Ionization on Ultraviolet Line Shifts in the Solar Transition Region Authors: Spadaro, D.; Noci, G.; Zappala, R. A.; Antiochos, S. K. Bibcode: 1990ApJ...355..342S Altcode: The line profiles and wavelength positions of all the important emission lines due to carbon were computed for a variety of steady state siphon flow loop models. For the lines from the lower ionization states (C II-C IV) a preponderance of blueshifts was found, contrary to the observations. The lines from the higher ionization states can show either a net red- or blueshift, depending on the position of the loop on the solar disk. Similar results are expected for oxygen. It is concluded that the observed redshifts cannot be explained by the models proposed here. Title: The Formation of Current Sheets in the Solar Corona: Asymmetric Shears Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R. Bibcode: 1990BAAS...22..869K Altcode: No abstract at ADS Title: The Structure and Dynamics of Solar Coronal Magnetic Fields Authors: Antiochos, S. K.; Dahlburg, R. B.; Zang, T. Bibcode: 1990BAAS...22..869A Altcode: No abstract at ADS Title: Solar Coronal Magnetic Field Evolution Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, T. A. Bibcode: 1990BAAS...22..851D Altcode: No abstract at ADS Title: Mass Flows and the Ionization States of Coronal Loops: Erratum Authors: Noci, G.; Spadaro, D.; Zappala, R. A.; Antiochos, S. K. Bibcode: 1990ApJ...349..678N Altcode: No abstract at ADS Title: Structures and flows in coronal loops Authors: Antiochos, Spiro K. Bibcode: 1990GMS....58..203A Altcode: In the present consideration of the plasma flows in solar loops, the field is approximated as completely rigid, since the plasma beta in the corona is low and observed motion time-scales are much longer than Alfven time scales. It is noted that the widely-used static models are less than valid, since any asymmetry in loop geometry, coronal heating, or chromospheric boundary condition will lead to a 'siphon' flow along the loop. Attention given to whether such flows can account for the observed redshifts, as well as to the possible importance of nonequilibrium ionization in these models. Impulsive heating may be able to generate the observed redshifts. Title: Heating of the corona by magnetic singularities Authors: Antiochos, Spiro K. Bibcode: 1990MmSAI..61..369A Altcode: Theoretical models of current-sheet formation and magnetic heating in the solar corona are examined analytically. The role of photospheric connectivity in determining the topology of the coronal magnetic field and its equilibrium properties is explored; nonequilibrium models of current-sheet formation (assuming an initially well connected field) are described; and particular attention is given to models with discontinuous connectivity, where magnetic singularities arise from smooth footpoint motions. It is shown that current sheets arise from connectivities in which the photospheric flux structure is complex, with three or more polarity regions and a magnetic null point within the corona. Title: The Evolution of a Sheared Potential Magnetic Field in the Solar Corona Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R. Bibcode: 1990IAUS..142..309K Altcode: No abstract at ADS Title: Episodic Coronal Heating and the Solar Differential Emission Measure Authors: Sturrock, P. A.; Klimchuk, J. A.; Antiochos, S. K. Bibcode: 1989BAAS...21R1186S Altcode: No abstract at ADS Title: Evolution of Twisted Flux-Tubes in the Solar Corona Authors: Dahlburg, R. B.; Antiochos, S. K.; Picone, J. M. Bibcode: 1989BAAS...21.1111D Altcode: No abstract at ADS Title: The Formation of Solar Prominences Authors: Antiochos, S. K.; Klimchuk, J. A. Bibcode: 1989BAAS...21.1185A Altcode: No abstract at ADS Title: Effect of Coronal Elemental Abundances on the Radiative Loss Function Authors: Cook, J. W.; Cheng, C. -C.; Jacobs, V. L.; Antiochos, S. K. Bibcode: 1989ApJ...338.1176C Altcode: The solar photosphere and corona abundances tabulated by Meyer (1985) and the chromospheric abundances given by Murphy (1985) are used here to recalculate radiative loss functions for equilibrium, low-density, optically thin plasmas. Results from a representative standard photospheric abundance set and from coronal and chromospheric abundance sets showing depletions of up to a factor of four in certain elemental abundances are compared. A significant difference is found for both the coronal and chromospheric abundance sets, with the peak of the radiative loss curve shifted closer to 10 to the 6th K than to the standard 2 x 10 to the 5th K found from photospheric abundances. Consequences of these new calculations, in particular for the cool loop model of Antiochos and Noci (1986), are discussed. Title: An Episodic Model of Coronal Heating Authors: Sturrock, P. A.; Antiochos, S. K. Bibcode: 1989BAAS...21R.829S Altcode: No abstract at ADS Title: Nonlinear Thermal Instability in Magnetized Solar Plasmas Authors: Karpen, Judith T.; Antiochos, Spiro K.; Picone, J. Michael; Dahlburg, Russell B. Bibcode: 1989ApJ...338..493K Altcode: The radiation-driven thermal instability might explain the formation and maintenance of cool dense regions embedded in a hotter more rarefied plasma. Structures of this type often are observed in astrophysical environments such as the solar corona or the interstellar medium. In the present work, the response of a magnetized solar transition-region plasma to a spatially random magnetic-field perturbation is simulated, where the magnetic field is perpendicular to the computational plane. It is found that the presence of the magnetic field, the value of the plasma beta, and the heating process significantly influence the number and size of the condensations as well as the evolutionary time scale. Title: A Model for the Heating of the Transition Region Authors: Antiochos, S. K.; Dere, K. P. Bibcode: 1989BAAS...21..841A Altcode: No abstract at ADS Title: Mass Flows and the Ionization State of Coronal Loops Authors: Noci, G.; Spadaro, D.; Zappala, R. A.; Antiochos, S. K. Bibcode: 1989ApJ...338.1131N Altcode: A basic assumption in the analysis of EUV and X-ray solar emission is that the plasma is in ionization equilibrium. The effects of mass flows on the ionization state of solar plasma have been investigated in order to check the validity of ionization equilibrium. Solar coronal loop models with a steady state flow as described by Antiochos (1984) are considered. The number densities of carbon ions have been determined for four loop models that cover a range of densities and flow velocities. The results show evidence of nonequilibrium ionization effects even for velocities of only a few km/s at the loop top and 10 times less at the base, with densities ranging from 10 to the 8th to 10 to the 10th/cu cm between the top and the footpoints. The importance of these results for the analysis of EUV and X-ray solar emission is discussed. Title: The evolution of a sheared potential magnetic field in a gravity stratified atmosphere. Authors: Karpen, J. T.; Antiochos, S. K. Bibcode: 1989BAAS...21R1027K Altcode: No abstract at ADS Title: Thermal instability in magnetized solar plasma. Authors: Karpen, J. T.; Antiochos, S. K.; Picone, J. M.; Dahlburg, R. B. Bibcode: 1989GMS....54...99K Altcode: 1989sspp.conf...99K In astrophysical plasmas such as the solar corona or the interstellar medium, the radiation-driven thermal instability might explain the formation of cool, dense regions embedded in a hotter, more rarefied medium. In the present work, the authors extend their previous investigation of this phenomenon by simulating the response of a magnetized solar transition-region plasma to a spatially random magnetic-field perturbation, where the magnetic field is perpendicular to the computational plane. This investigation has determined the effects of varying the plasma β and the heating mechanism. Title: Spectral simulation of coronal processes. Authors: Dahlburg, R. B.; Antiochos, S. K.; Picone, J. M.; Zang, T. A. Bibcode: 1989BAAS...21.1028D Altcode: No abstract at ADS Title: Chromospheric explosions. Authors: Doschek, G. A.; Antiochos, S. K.; Antonucci, E.; Cheng, C. -C.; Culhane, J. L.; Fisher, G. H.; Jordan, C.; Leibacher, J. W.; MacNiece, P.; McWhirter, R. W. P.; Moore, R. L.; Rabin, D. M.; Rust, D. M.; Shine, R. A. Bibcode: 1989epos.conf..303D Altcode: The work of this team addressed the question of the response and relationship of the flare chromosphere and transition region to the hot coronal loops that reach temperatures of about 107K and higher. Flare related phenomena such as surges and sprays were also discussed. The team members debate three main topics: 1) whether the blue-shifted components of X-ray spectral lines are signatures of "chromospheric evaporation"; 2) whether the excess line broadening of UV and X-ray lines is accounted for by "convective velocity distribution" in evaporation; and 3) whether most chromospheric heating is driven by electron beams. These debates illustrated the strengths and weaknesses of our current observations and theories. Title: Magnetic topology and current sheet formation. Authors: Antiochos, S. K. Bibcode: 1989sasf.confP.277A Altcode: 1988sasf.conf..277A; 1989IAUCo.104P.277A The author describes a mechanism for coronal heating. The basic idea is that since the photospheric flux is observed to consist of a complex pattern of positive and negative polarity regions, the topology of the coronal magnetic field (in particular the connectivity) must be discontinuous over a complex network of surfaces and magnetic null points in the corona. Consequently, photospheric motions of the field line footpoints, even if arbitrarily smooth, result in discontinuous stressing of the field. This produces coronal current sheets, reconnection at the null points, and rapid heating. Title: Sub-sonic mass flows and ionization state in coronal loops Authors: Noci, G.; Spadaro, D.; Zappala, R. A.; Antiochos, S. K. Bibcode: 1989MmSAI..60...55N Altcode: The effects of subsonic mass flows on the ionization state of the solar plasma inside magnetic loops are studied. Motions along the magnetic field lines from one footpoint of the loop to the other are considered in order to investigate the effects of the motion through positive and negative temperature gradients. The number densities of carbon ions are determined for some loop models that cover a range of densities and flow velocities. The results show that deviations from ionization equilibrium can occur in coronal loops with a steady-state subsonic flow from one footpoint to the other. The deviations depend on the electron density and flow velocity. The importance of these results for the analysis of EUV and X-ray solar emission is discussed. Title: LASCO: A wide-field white light and spectrometric coronagraph for SOHO Authors: Michels, D. J.; Schwenn, R.; Howard, R. A.; Bartoe, J. -D. F.; Antiochos, S. K.; Brueckner, G. E.; Cheng, C. -C.; Dere, K. P.; Doschek, G. A.; Mariska, J. T. Bibcode: 1988sohi.rept...55M Altcode: The scientific objectives of the LASCO (light and spectrometric coronagraph) project in the SOHO (solar and heliospheric observatory) mission are described. These include investigation of mechanisms for heating of the corona and acceleration of the solar wind, causes of coronal transients, and their role in development of large scale coronal patterns and interplanetary disturbances. The distribution and properties of dust particles, including those released from sun-grazing comets are investigated. Interactions of coronal plasma with the dust are studied. The corona is analyzed spectroscopically by a high-resolution scanning, imaging interferometer. The spectral profiles of three emission lines and one Fraunhofer line are measured for each picture point, giving temperatures, velocities, turbulent motions and volume densities. Polarization analysis yields the direction of coronal magnetic fields. Title: On the Formation of Coronal Current Sheets Without Null Points Authors: Antiochos, S. K.; Karpen, J. T. Bibcode: 1988BAAS...20.1029A Altcode: No abstract at ADS Title: The Effects of Magnetic Topology on Coronal Heating Authors: Antiochos, S. K. Bibcode: 1988BAAS...20Q.681A Altcode: No abstract at ADS Title: A Numerical Study of the Nonlinear Thermal Stability of Solar Loops Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T. Bibcode: 1987ApJ...320..409K Altcode: A time-dependent numerical model is used to investigate the nonlinear thermal stability of static loops of various heights. Simulations show that the instability of a hot state with loop heights of less than about 1000 km is physically significant, with an initially hot atmosphere in low-lying compact loops evolving to an extended atmosphere with temperatures far below 100,000 K. Results also show that high-lying loops are stable to all reasonable perturbations, including those of large initial amplitude and long wavelength. The simulation results suggest that low-lying compact loops should not be common to the sun, and that cool loops with temperatures near 100,000 K must be formed in the cool state initially and cannot evolve from preexisiting loops. Title: A numerical study of the thermal stability of solar loops. Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T. Bibcode: 1987NASCP2483..113K Altcode: 1987tphr.conf..113K An important property of all loops is their thermal stability. If low lying hot loops were thermally unstable, for example, a great majority of the low loops on the Sun might be expected to be cool. How small perturbations evolve in low lying, linearly unstable hot loops was determined and how high lying, linearly stable hot loops respond to large amplitude disturbances such as might be expected on the Sun were examined. Only general descriptions and results are given. Title: Effect on the Radiative Loss Function of Coronal Elemental Abundances Authors: Cook, J. W.; Cheng, C. -C.; Antiochos, S. K. Bibcode: 1987BAAS...19..931C Altcode: No abstract at ADS Title: The Topology of Force-free Magnetic Fields and Its Implications for Coronal Activity Authors: Antiochos, Spiro K. Bibcode: 1987ApJ...312..886A Altcode: The topological constraints on coronal magnetic fields are considered. For a field that is initially well-behaved and undergoes deformation by well-behaved ideal MHD motions, it is shown that the topology of the field lines in the corona can be determined at all times solely from the footpoint positions on the photospheric boundary. This result implies that the topology and, consequently, the history of the footpoint motions impose no further constraints on the field beyond those already included in the connectivity boundary conditions, so that there is no reason to expect a lack of equilibrium for fields that are initially well-behaved and evolve by ideal MHD. On the other hand, nonideal processes such as reconnection are bound to occur in the solar corona, and these may lead to magnetic topologies that have no well-bahaved Euler potentials. Hence Parker's hypothesis that footpoint motions lead to the formation of current sheets is still likely to be correct, but only if nonideal processes are included. The effects of reconnection on magnetic topology and the implications for coronal activity are discussed. Title: Theory of Cool Loops and the Dividing Line (Invited review) Authors: Antiochos, Spiro K. Bibcode: 1987LNP...291..283A Altcode: 1987csss....5..283A Static models for coronal loops have been widely used to interpret observations of the coronae of cool stars. Although these models have been successful in explaining several features of the observations; they have been unsuccessful in accounting for two key features: (a) in dwarf stars they do not agree with the observed form of the differential emission measure at low temperatures, T < 105 K; and (b) in certain giant stars they do not agree with the lack of emission at high temperatures, T > 105 K (the so-called dividing line). It appears that in high gravity stars there is more cool material than the standard models of the transition region predict; whereas in low gravity stars there is less hot material than the loop models predict. Title: Chromospheric explosions Authors: Doschek, G. A.; Antiochos, S. K.; Antonucci, E.; Cheng, C. -C.; Culhane, J. L.; Fisher, G. H.; Jordan, C.; Leibacher, J. W.; MacNeice, P.; McWhirter, R. W. P. Bibcode: 1986epos.conf..4.1D Altcode: 1986epos.confD...1D Three issues relative to chromospheric explosions were debated. (1) Resolved: The blue-shifted components of x-ray spectral lines are signatures of chromospheric evaporation. It was concluded that the plasma rising with the corona is indeed the primary source of thermal plasma observed in the corona during flares. (2) Resolved: The excess line broading of UV and X-ray lines is accounted for by a convective velocity distribution in evaporation. It is concluded that the hypothesis that convective evaporation produces the observed X-ray line widths in flares is no more than a hypothesis. It is not supported by any self-consistent physical theory. (3) Resolved: Most chromospheric heating is driven by electron beams. Although it is possible to cast doubt on many lines of evidence for electron beams in the chromosphere, a balanced view that debaters on both sides of the question might agree to is that electron beams probably heat the low corona and upper chromosphere, but their direct impact on evaporating the chromosphere is energetically unimportant when compared to conduction. This represents a major departure from the thick-target flare models that were popular before the Workshop. Title: Topological constraints and the existence of force-free fields. Authors: Antiochos, S. K. Bibcode: 1986NASCP2442..419A Altcode: 1986copp.nasa..419A A fundamental problem in plasma theory is the question of the existence of MHD equilibria. The issue of topological constraints is of crucial importance for the problem of the existence of equilibria. Heuristic methods are used to discuss the coronal wrapping pattern. It is concluded that for a given set of footpoint positions the wrapping pattern in the corona is completely fixed. The topological constraints are included in the boundary conditions on the Euler potentials and impost no additional restrictions on possible equilibria. Although this does not prove that equilibria always exist, it does show that the force-free problem is not overdetermined and that existence of equilibria is still an open question. Title: A numerical study of the thermal stability of low-lying coronal loops. Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T. Bibcode: 1986NASCP2442..389K Altcode: 1986copp.nasa..389K The nonlinear evolution of loops that are subjected to a variety of small but finite perturbations was studied. Only the low-lying loops are considered. The analysis was performed numerically using a one-dimensional hydrodynamical model developed at the Naval Research Laboratory. The computer codes solve the time-dependent equations for mass, momentum, and energy transport. The primary interest is the active region filaments, hence a geometry appropriate to those structures was considered. The static solutions were subjected to a moderate sized perturbation and allowed to evolve. The results suggest that both hot and cool loops of the geometry considered are thermally stable against amplitude perturbations of all kinds. Title: Force-free Magnetic Fields: The Magneto-frictional Method Authors: Yang, W. H.; Sturrock, P. A.; Antiochos, S. K. Bibcode: 1986ApJ...309..383Y Altcode: The problem under discussion is that of calculating magnetic field configurations in which the Lorentz force j x B is everywhere zero, subject to specified boundary conditions. We choose to represent the magnetic field in terms of Clebsch variables in the form B = grad alpha x grad beta. These variables are constant on any field line so that each field line is labeled by the corresponding values of alpha and beta. When the field is described in this way, the most appropriate choice of boundary conditions is to specify the values of alpha and beta on the bounding surface. We show that such field configurations may be calculated by a magneto-frictional method. We imagine that the field lines move through a stationary medium, and that each element of magnetic field is subject to a frictional force parallel to and opposing the velocity of the field line. This concept leads to an iteration procedure for modifying the variables alpha and beta, that tends asymptotically towards the force-free state. We apply the method first to a simple problem in two rectangular dimensions, and then to a problem of cylindrical symmetry that was previously discussed by Barnes and Sturrock (1972). In one important respect, our new results differ from the earlier results of Barnes and Sturrock, and we conclude that the earlier article was in error. Title: The Differential Emission Measure of Transiently Heated Coronal Loops Authors: Antiochos, S. K.; Sturrock, P. A. Bibcode: 1986BAAS...18..901A Altcode: No abstract at ADS Title: On the Dividing Line for Stellar Coronae Authors: Antiochos, S. K.; Haisch, B. M.; Stern, R. A. Bibcode: 1986ApJ...307L..55A Altcode: The authors describe a possible explanation for the observation that late-type stars falling in a certain region of the H-R diagram exhibit no X-ray emission and, hence, appear not to have coronae. The basic idea of the authors' model is that due to the low surface gravity that characterizes the stars without X-ray emission, a hot (T > 106K) corona is thermally unstable and spontaneously cools down to chromospheric temperatures. The key parameter that determines the outer atmospheric structure is shown to be the ratio of the gravitational scale height of plasma at T = 105K to the maximum height of closed magnetic field lines in the corona. Title: Modeling of Coronal X-Ray Emission from Active Cool Stars. I. Hyades Cluster Authors: Stern, R. A.; Antiochos, S. K.; Harnden, F. R., Jr. Bibcode: 1986ApJ...305..417S Altcode: X-ray pulse height spectra of the most active cool stars in the Hyades cluster obtained with the Einstein IPC cannot be satisfactorily fitted using isothermal thin plasma emission models. Addition of a second isothermal component provides acceptable fits. However, a more physically meaningful set of coronal parameters is provided by models which consist of an ensemble of loops wih a single maximum temperature, but with the temperature distribution within the loop determined by loop physics. Such models have been successfully fitted to the IPC pulse height spectra. Constraints on loop parameters are discussed for the F-G dwarfs BD + 14 deg 693, BD + 14 deg 690, BD + 15 deg 640, and 71 Tau. Models with a large variation of loop cross section from base to top do not fit the data. A consistent physical description is an ensemble of small high-pressure loops of similar maximum temperature which dominate the coronal X-ray spectrum. Title: A Numerical Study of the Stability of Low-Lying Solar Loops Authors: Mariska, J. T.; Klimchuk, J. A.; Antiochos, S. K. Bibcode: 1986BAAS...18Q.708M Altcode: No abstract at ADS Title: The Structure of the Static Corona and Transition Region Authors: Antiochos, S. K.; Noci, G. Bibcode: 1986ApJ...301..440A Altcode: Static models of coronal loops are investigated. For loops that are low-lying with heights above the chromosphere below about 5000 km, it is shown that a new type of solution appears to the static equations, in addition to the well-known coronal loop solution. The new solution is characterized by a maximum plasma temperature less than about 100,000 K. The structure and properties of these cool solutions are discussed. The differential emission measure Q(T) expected for a magnetic arcade, which must naturally contain both hot and cool loops, is calculated. It is shown that the cool loops have a dramatic effect on the form of Q(T) in the lower transition region. In particular, they can account for the observed rise in Q at low T, which has long been thought to be incompatible with the static-loop model. Finally, the implications of the cool loops on other observations of both the solar and stellar coronae and transition regions are discussed. Title: Chromospheric explosions. Authors: Doschek, G. A.; Antiochos, S. K.; Antonucci, E.; Cheng, C. -C.; Culhane, J. L.; Fisher, G. H.; Jordan, C.; Leibacher, J. W.; MacNiece, P.; McWhirter, R. W. P.; Moore, R. L.; Rabin, D. M.; Rust, D. M.; Shine, R. A. Bibcode: 1986NASCP2439....4D Altcode: The work of this team addressed the question of the response and relationship of the flare chromosphere and transition region to the hot coronal loops that reach temperatures of about 107K and higher. Flare related phenomena such as surges and sprays are also discussed. The team members debated three main topics: 1. whether the blue-shifted components of X-ray spectral lines are signatures of "chromospheric evaporation"; 2. whether the excess line broadening of UV and X-ray lines is accounted for by "convective velocity distribution" in evaporation; and 3. whether most chromospheric heating is driven by electron beams. Title: Modeling of Coronal X-Ray Emission from Active Cool Stars Authors: Stern, R. A.; Antiochos, S. K.; Harnden, F. R., Jr. Bibcode: 1986LNP...254..216S Altcode: 1986csss....4..216S X-ray pulse-height spectra of the most active cool stars in the Hyades cluster obtained with the Einstein IPC cannot be modeled using isothermal thin plasma emission. Addition of a second isothermal component provides acceptable fits. However, a more physically meaningful set of coronal parameters is provided by models which consist of an ensemble of loops with a single maximum temperature, but with the temperature distribution within the loop determined by loop physics. Such models have been successfully fit to the IPC pulse-height spectra. Constraints on loop parameters are discussed for four F-G dwarfs in the Hyades. Title: On the topology of force-free magnetic fields. Authors: Antiochos, S. K. Bibcode: 1986BAAS...18..853A Altcode: No abstract at ADS Title: Thermal stability of static coronal loops. I - Effects of boundary conditions Authors: Antiochos, S. K.; Shoub, E. C.; An, C. -H.; Emslie, A. G. Bibcode: 1985ApJ...298..876A Altcode: 1985STIN...8522330A The linear stability of static coronal-loop models undergoing thermal perturbations was investigated. The effect of conditions at the loop base on the stability properties of the models was considered in detail. The question of appropriate boundary conditions at the loop base was considered and it was concluded that the most physical assumptions are that the temperature and density (or pressure) perturbations vanish there. However, if the base is taken to be sufficiently deep in the chromosphere, either several chromospheric scale heights or several coronal loop lengths in depth, then the effect of the boundary conditions on loop stability becomes negligible so that all physically acceptable conditions are equally appropriate. For example, one could as well assume that the velocity vanishes at the base. The growth rates and eigenmodes of static models in which gravity is neglected and in which the coronal heating is a relatively simple function, either constant per-unit mass or per-unit volume were calculated. It was found that all such models are unstable with a growth rate of the order of the coronal cooling time. The physical implications of these results for the solar corona and transition region are discussed. Title: The Effect of Gravity on the X-Ray and UV Emission of Cool Stars Authors: Antiochos, S. K. Bibcode: 1985BAAS...17..570A Altcode: No abstract at ADS Title: Summary proceedings of the Standford Workshop on Solar Flare Prediction held in Paris on 28 February - 1 March 1985 Authors: Antiochos, S. K.; Bai, T.; Sturrock, P. A. Bibcode: 1985STIN...8623543A Altcode: A workshop on The Prediction of Solar Activity was held at Meudon Observatory in France in June 1984. During that meeting, a number of participants from the United States expressed interest in meeting together to discuss this topic with a view to exploring what actions might be taken to improve our predictive capability. This document contains abstracts of presentations made at the meeting. Title: The Structure of Transition Region Loops Authors: Antiochos, S. K. Bibcode: 1985BAAS...17..631A Altcode: No abstract at ADS Title: The Differential Emission Measure in the Lower Transition Region Authors: Antiochos, S. K. Bibcode: 1984BAAS...16..928A Altcode: No abstract at ADS Title: Magnetic flare model of γ-ray bursts Authors: Liang, E. P.; Antiochos, S. K. Bibcode: 1984Natur.310..121L Altcode: The thermal synchrotron (TS) interpretation of γ-ray burst continuum spectra1 has recently gained support from the analysis of an expanding database2-5. However, this interpretation requires an emission region which is hot (kT~0.2-1mc2), optically very thin (nh <~ 1021 cm-2) with a highly super-Eddington flux F ~ 1030 erg cm-2 s-1 (»FEdd = 1025 M/Msolar) for a 10-km neutron star. This picture is similar to that first proposed for the 5 March 1979 event6-8. In addition there are hints that the emission layer is very dense (ne <= 1024-1026 cm-3) and thin (h <= 10-3 cm). For example, events which show simultaneous redshifted 511-keV annihilation lines and low energy self-absorption (see Fig. 1) allow us to estimate a lower limit in the range 1023-1026 cm-1 (ref. 9) to the pair density, provided that the annihilation region coincides with the synchrotron source. To maintain the pair population close to maxwellian, the collision excitation rate into higher Landau levels9 (that is, the pitch-angle scattering rate) must exceed the cyclotron decay rates. Coulomb scattering between pairs and protons is too slow. Even if collective processes whose rates are close to the electron plasma frequency, or scattering by heavy ions (for example, Z = 26, rates ~ Z2), are invoked, a particle density n >= 1025 cm-3 is still needed. Both arguments point towards a dense but thin emitting sheet with h <~ 10-3-10-5 cm. The severe energetics and persistence of such a hot, dense, thin emitting layer prompted us to consider a picture in which the emission regions lie at the foot points of reconnecting magnetic loops, powered by downward impinging electromagnetic waves. We now examine the structure of the emitting sheet, and the generation, propagation and coupling of the electromagnetic energy fluxes to the surface layer, and show that a flare-like model can account for most of the general features of γ-ray bursts. Title: A dynamic model for the solar transition region Authors: Antiochos, S. K. Bibcode: 1984ApJ...280..416A Altcode: A model is developed for the lower transition region that can account for the persistent and ubiquitous redshifts that are observed in the UV emission lines formed at these temperatures. It is shown that these shifts are not likely to be due either to falling spicular material or to steady-state siphon flows. The model consists of two key ingredients. The redshifted radiation originates from a minority of flux tubes which have higher gas pressures than their surroundings, and consequently have their transition regions situated below the transition regions of their surroundings. The coronal heating in these loops is impulsive in nature, and this is responsible for the transient mass flows. The studies, therefore, favor theories for coronal heating which involve flare-like magnetic-energy release. Previously announced in STAR as N83-29163 Title: The Effects of Gravity on the Stability of Coronal Loops Authors: Antiochos, S. K. Bibcode: 1984BAAS...16..404A Altcode: No abstract at ADS Title: Erratum - a Giant X-Ray Flare in the Hyades Authors: Stern, R. A.; Underwood, J. H.; Antiochos, S. K. Bibcode: 1983ApJ...275L..25S Altcode: No abstract at ADS Title: Coordinated Einstein and IUE observations of a disparitions brusques type flare event and quiescent emission from Proxima Centauri. Authors: Haisch, B. M.; Linsky, J. L.; Bornmann, P. L.; Stencel, R. E.; Antiochos, S. K.; Golub, L.; Vaiana, G. S. Bibcode: 1983ApJ...267..280H Altcode: The Einstein Imaging Particle Counter observed a major X-ray flare in its entirety during a 5-hr period of simultaneous observations, with the IUE, of the dM5e flare star Proxima Centauri in August, 1980. The detailed X-ray light curve, temperature determinations during various intervals, and UV line fluxes obtained before, during, and after the flare indirectly indicate a 'two-ribbon flare' prominence eruption. The calculated ratio of coronal to bolometric luminosity for the event is about 100 times the solar ratio. The Proxima Cen corona is analyzed in the context of static loop models, in light of which it is concluded that less than 6% of the stellar surface seems to be covered by X-ray emitting active regions. Title: On the Thermal Stability of Coronal Loops Authors: Antiochos, S. K. Bibcode: 1983BAAS...15..704A Altcode: No abstract at ADS Title: A giant X-ray flare in the Hyades. Authors: Stern, R. A.; Underwood, J. H.; Antiochos, S. K. Bibcode: 1983ApJ...264L..55S Altcode: The authors have observed a giant stellar flare in the Hyades binary HD 27130 = VB 22 = BD +16°577 with the Einstein Observatory. The peak X-ray luminosity of the flare is greater than 1031erg s-1, at least several thousand times brighter than the most intense solar flares. The ratio of flare peak to quiescent X-ray luminosity is ≡35. HD 27130, first detected as an X-ray source in the central Hyades survey of Stern et al., recently has been determined to be a double-lined eclipsing binary with a period of 5.6 days. The primary is a G dwarf, and the secondary is a K dwarf. The temperature estimated for the flare (≡4×107K) and the form of the flare decay suggest that it is solar-like. It is suggested that giant flares may be typical of young or rapidly rotating systems. Title: A dynamic model for the transition region Authors: Antiochos, S. K. Bibcode: 1982STIN...8329163A Altcode: We develop a model for the lower transition region that can account for the persistent and ubiquitous redshifts that are observed in the UV emission lines formed at these temperatures. We show that these shifts are not likely to be due either to falling spicular material or to steady-state siphon flows. Our model consists of two key ingredients. The redshifted radiation originates from a minority of flux tubes which have higher gas pressures than their surroundings, and consequently have their transition regions situated below the transition regions of their surroundings. The coronal heating in these loops is impulsive in nature, and this is responsible for the transient mass flows. Our studies, therefore, favor theories for coronal heating which involve flare-like magnetic-energy release. Title: Implications of Solar Flare Observations on Stellar X-Ray Flares Authors: Antiochos, S. K.; Haisch, B. M.; Stern, R. A. Bibcode: 1982BAAS...14..864A Altcode: No abstract at ADS Title: On the thermal stability of coronal loop plasma Authors: Antiochos, S. K.; Emslie, A. G.; Shoub, E. C.; An, C. H. Bibcode: 1982STIN...8234327A Altcode: The stability to thermal perturbation of static models of coronal loops is considered including the effects of cool, radiatively stable material at the loop base. The linear stability turns out to be sensitive only to the boundary conditions assumed on the velocity at the loop base. The question of the appropriate boundary conditions is discussed, and it is concluded that the free surface condition (the pressure perturbation vanishes), rather than the rigid wall (the velocity vanishes), is relevant to the solar case. The static models are found to be thermally unstable, with a growth time of the order of the coronal cooking time. The physical implications of these results for the solar corona and transition region are examined. Title: International Ultraviolet Explorer observations of hyades stars. Authors: Zolcinski, M. C. S.; Antiochos, S. K.; Stern, R. A.; Walker, A. B. C. Bibcode: 1982ApJ...258..177Z Altcode: A description is presented of International Ultraviolet Explorer (IUE) satellite observations of transition region and chromospheric emission from a group of Hyades dwarfs which are strong X-ray emitters as seen in a survey conducted by Stern et al. (1981). Short-wavelength spectra (1175-2000 A) and long-wavelength spectra (1900-3200 A) have been obtained. Although the IUE sensitivity limit did not make it possible to detect emission lines in three stars, the presence of chromospheres and transition regions could be confirmed in BD +15 deg 640, 70 Tau, BD +14 deg 693, and BD +16 deg 592. The differential emission measure has been plotted as a function of temperature for the four considered stars. Title: Erratum - Stellar Coronae in the Hyades - a Soft X-Ray Survey with the Einstein Observatory Authors: Stern, R. A.; Zolcinski, M. C.; Antiochos, S. K.; Underwood, J. H. Bibcode: 1982ApJ...258..904S Altcode: No abstract at ADS Title: Are Coronal Loops Stable? Authors: Antiochos, S. K. Bibcode: 1982BAAS...14..623A Altcode: No abstract at ADS Title: The cooling and condensation of flare coronal plasma Authors: Antiochos, S. K.; Sturrock, P. A. Bibcode: 1982ApJ...254..343A Altcode: A model is investigated for the decay of flare heated coronal loops in which rapid radiative cooling at the loop base creates strong pressure gradients which, in turn, generate large (supersonic) downward flows. The important features of this model which distinguish it from previous models of flare cooling are: (1) Most of the thermal energy of the coronal plasma may be lost by mass motion rather than by conduction or coronal radiation. (2) Flare loops are not isobaric during their decay phase, and large downward velocities are present near the footpoints. (3) The differential emission measure has a strong temperature dependence. These results can account for recent observations of compact flare loops that are not consistent with the previous cooling models. Title: An X-ray flare in the Hyades binary HD 27130. Authors: Stern, R. A.; Underwood, J. H.; Antiochos, S. K. Bibcode: 1982SAOSR.392B.101S Altcode: 1982csss....2..101S No abstract at ADS Title: The differential emission measure of dynamic coronal loops. Authors: Antiochos, S. K. Bibcode: 1982SAOSR.392B.115A Altcode: 1982csss....2..115A No abstract at ADS Title: Progress report of an IUE survey of the Hyades star cluster. Authors: Zolcinski, M. C.; Kay, L.; Antiochos, S.; Stern, R.; Walker, A. B. C. Bibcode: 1982NASCP2238..239Z Altcode: 1982auva.nasa..239Z; 1982NASCP2338..239Z; 1982IUE82......239Z To date 11 of the brightest X-Ray stars (F-K dwarfs) in the Hyades have been observed with the IUE satellite with the short wavelength spectrograph. The IUE results and the X-Ray observations from the Hyades survey with the Einstein Observatory were combined. The differential emission measure function was estimated for each of the 7 stars which showed evidence of emission lines. Constraints on stellar atmospheric parameters (chromospheric pressure, coronal temperature and filling factor were derived. The implications of these results in the context of loop models for the corona and transition region (TR) of these stars are discussed. Title: The differential emission measure of dynamic coronal loops Authors: Antiochos, S. K. Bibcode: 1981STIN...8220087A Altcode: The effects of time dependent phenomena, such as flare energization and decay, on the temperature and density structure of the transition region and, in particular, on the form of the differential emission measure are studied. It is found that unlike the case of the static models, the form of the differential emission measure can be used to determine the important physical mechanisms in the dynamic models. Title: Stellar coronae in the hyades : a soft X-ray survey with the EinsteinObservatory. Authors: Stern, R. A.; Zolcinski, M. Ch.; Antiochos, S. K.; Underwood, J. H. Bibcode: 1981ApJ...249..647S Altcode: An X-ray survey of the central region of the Hyades cluster demonstrates that soft X-ray emission is a common property of the stars in the cluster. Half of the 85 stars surveyed are detected above a sensitivity threshold of 10 to the 28.5th ergs/s at the Hyades distance of 45 pc. The high incidence of X-ray emission and range of observed X-ray luminosities indicate that stellar coronas produce the observed X-ray emission, with a typical X-ray luminosity for solar-type Hyades of 10 to the 29th ergs/s. The use of coronal scaling laws is found to yield reasonable values of maximum coronal temperatures and the fraction of stellar surface covered for the Hyades coronas. Title: The cooling and condensation of flare coronal plasma Authors: Antiochos, S. K.; Sturrock, P. A. Bibcode: 1981STIN...8127029A Altcode: A model is investigated for the decay of flare heated coronal loops in which rapid radiative cooling at the loop base creates strong pressure gradients which, in turn, generate large (supersonic) downward flows. The coronal material cools and 'condenses' onto the flare chromosphere. The features which distinguish this model from previous models of flare cooling are: (1) most of the thermal energy of the coronal plasma may be lost by mass motion rather than by conduction or coronal radiation; (2) flare loops are not isobaric during their decay phase, and large downward velocities are present near the footpoints; (3) the differential emission measure q has a strong temperature dependence. Title: On the Thermal Stability of Coronal Loop Plasma Authors: Antiochos, S. K.; Emslie, A. G. Bibcode: 1981BAAS...13..555A Altcode: No abstract at ADS Title: The Structure of the Lower Transition Region Authors: Antiochos, S. K. Bibcode: 1981BAAS...13..835A Altcode: No abstract at ADS Title: The Structure of a Force-Free Coronal Loop Authors: Wear, K. A.; Antiochos, S. K.; Emslie, A. G.; Sturrock, P. A. Bibcode: 1981BAAS...13..542W Altcode: No abstract at ADS Title: X-Ray Flare in HD 27130 Authors: Stern, R. A.; Underwood, J. H.; Antiochos, S. K.; McClure, R. Bibcode: 1981IAUC.3585....2S Altcode: R. A. Stern and J. H. Underwood, Jet Propulsion Laboratory; and S. K. Antiochos, Stanford University, guest observers with the Einstein Observatory, write: "The 0.3-6.0-nm x-ray flux from the Hyades binary system HD 27130 underwent a 40-fold increase to 10**24 J/s shortly before 1980 Sept. 20d06h13m UT. This flare decayed with an e-foldlng time of ~ 2500 s. HD 27130 has been determined recently to be a double-lined eclipsing binary with a period of 5.6 days. The primary is a main-sequence G star, while the secondary is probably a K dwarf (R. McClure, private communication). Monitoring of this system for evidence of optical flaring or unusual spectral characteristics would be valuable." Title: The Structure of a Force Free Magnetic Flux Tube Authors: Wear, K. A.; Antiochos, S. K.; Sturrock, P. A. Bibcode: 1981BAAS...13..915W Altcode: No abstract at ADS Title: Numerical studies of the energy balance in coronal loops. Authors: Underwood, J. H.; Antiochos, S. K.; Vesecky, J. F. Bibcode: 1981ASIC...68..227U Altcode: 1981spss.conf..227U A numerical method is applied to treat the energy balance of quasi-static solar coronal loops, which have been observed to persist for periods much greater than the radiative cooling time. The quasi-static loop model employed takes into account gravity, density-, temperature- or position-dependent energy input, an accurate form of the radiative losses and variable loop cross-sectional area, under assumptions of energy input by coronal heating balanced by radiative and conductive losses, an optically thin plasma, energy conduction along the field lines only and hydrostatic equilibrium. Computations of an emission measure function for various distributions of the energy input and loop geometries are then presented which show that little information on the location of the energy input may be gained from spectral line intensity measurements integrated over a single loop. Title: Results from the central Hyades survey. Authors: Stern, R. A.; Underwood, J. H.; Zolcinski, M. C.; Antiochos, S. K. Bibcode: 1981ASIC...68..137S Altcode: 1981spss.conf..137S Results of a soft X-ray survey of the central 5 deg of the Hyades star cluster made with the Einstein Observatory Imaging Proportional Counter are presented. Virtually all of the late F and early G stars in the cluster were detected at an X-ray luminosity of greater than 10 to the 28.5 erg/sec, although only about 50% of the stars in the cluster as a whole were detected. Plots of X-ray luminosity against B-V index indicate that the typical solar-type star in the Hyades is emitting soft X-rays at a level approximately 30 times that of the active sun. Histograms of the X-ray to bolometric luminosity ratio reveal a gradual distinction between late A-early F stars, which are expected to possess little or no convective envelope, and solar-type (F8-G5) stars, with convective outer atmospheres. Results confirm the dependence of stellar coronal activity on rotation, and establish the prevalence of stellar coronae throughout the main sequence and the giant regions of the H-R diagram. Title: The evolution of active region loop plasma Authors: Krall, K. R.; Antiochos, S. K. Bibcode: 1980ApJ...242..374K Altcode: The adjustment of coronal active-region loops to changes in their heating rate is investigated numerically. The one-dimensional hydrodynamic equations are solved subject to boundary conditions in which heat flux-induced mass exchange between coronal and chromospheric components is allowed. The calculated evolution of physical parameters suggests that (1) mass supplied during chromospheric evaporation is much more effective in moderating coronal temperature excursions than when downward heat flux is dissipated by a static chromosphere, and (2) the method by which the chromosphere responds to changing coronal conditions can significantly influence coronal readjustment time scales. Observations are cited which illustrate the range of possible fluctuations in the heating rates. Title: Radiative-dominated cooling of the flare corona and transition region. Authors: Antiochos, S. K. Bibcode: 1980ApJ...241..385A Altcode: Recent observations of some compact flares indicate that the differential emission measure, q, of flare coronal and transition region plasma has a much steeper dependence on temperature than in nonflare regions. It is noted that this result is not compatible with models for a flare loop in which conduction to the chromosphere dominates the cooling, even in the case where the loop has a large divergence in its cross-sectional area. Only by a combination of many loops is it possible to reproduce the observations. Hence, models in which radiation dominates the evolution are investigated. It is found that the radiative models predict that q varies as T to the power (l + 1) where l measures the dependence of the radiative loss coefficient on temperature. It is concluded that the radiative models are also incapable of explaining the observations (unless, again, a combination of many loops is postulated) and it is suggested that large mass motions with velocities of the order of the sound speed may be required. Title: Study of the Chromospheres, Coronae, and Transitions Regions of Main Sequence Stars in the Hyades Authors: Zolcinski, M. -C.; Antiochos, S. K.; Walker, A. B. C.; Stern, R. A.; Underwood, J. H. Bibcode: 1980BAAS...12..872Z Altcode: No abstract at ADS Title: On the Differential Emission Measure of Coronal Loops Authors: Antiochos, S. K.; Underwood, J. H.; Vesecky, J. F. Bibcode: 1980BAAS...12..792A Altcode: No abstract at ADS Title: Stellar Coronae in the Hyades Authors: Stern, R.; Underwood, J.; Zolcinski, M.; Antiochos, S. Bibcode: 1980BAAS...12..801S Altcode: No abstract at ADS Title: The minimum flux corona; theory or concept Authors: Underwood, J. H.; Antiochos, S. K. Bibcode: 1980STIN...8034328U Altcode: The reply to the criticisms of the minimum flux theory is discussed. These criticisms are correct in substance, as well as in detail. Counter arguments that the minimum flux corona theory is untenable, because of errors in its formulation, are presented. Title: On the Thermal Stability of Coronal Loop Plasma Authors: Antiochos, S. K.; Emslie, A. G. Bibcode: 1980BAAS...12..519A Altcode: No abstract at ADS Title: A model of active prominences Authors: Antiochos, S. K. Bibcode: 1980ApJ...236..270A Altcode: A one-dimensional numerical model of active loop prominences is investigated. The model includes the effects of gravity, the geometry of the magnetic field, and conduction losses to the chromosphere. Calculations indicate that, as originally proposed by Goldsmith, the thermal instability mechanism is, by itself, sufficient to account for the appearance of bright H-alpha knots in postflare loops. Under certain conditions, initial perturbations in the loop temperature and density profiles of small, but finite, amplitude (approximately 5%) and large size scale (greater than or approximately equal to 10 to the 9th cm) can grow into condensations with temperature and density differences of over an order of magnitude and size scales of less than 10 to the 8th cm. In agreement with observations, the conditions that must be satisfied are such that loop prominence systems are likely to occur only in large flares and such that knots preferentially form at the tops of loops. The velocities, densities, and lifetimes calculated for the loop material are also in agreement with observations. It is concluded that in order for H-alpha knots to occur, heating of some form must continue into the late cooling phase of a flare loop, and that this heating is more intense near the loop base than near the apex. Title: The Einstein Central Hyades Survey - a Progress Report Authors: Stern, R.; Underwood, J. H.; Zolcinski, M. C.; Antiochos, S. Bibcode: 1980SAOSR.389..127S Altcode: 1980csss....1..127S No abstract at ADS Title: The thermal X-ray flare plasma Authors: Moore, R.; McKenzie, D. L.; Svestka, Z.; Widing, K. G.; Dere, K. P.; Antiochos, S. K.; Dodson-Prince, H. W.; Hiei, E.; Krall, K. R.; Krieger, A. S. Bibcode: 1980sfsl.work..341M Altcode: 1980sofl.symp..341M Following a review of current observational and theoretical knowledge of the approximately 10 to the 7th K plasma emitting the thermal soft X-ray bursts accompanying every H alpha solar flare, the fundamental physical problem of the plasma, namely the formation and evolution of the observed X-ray arches, is examined. Extensive Skylab observations of the thermal X-ray plasmas in two large flares, a large subflare and several compact subflares are analyzed to determine plasma physical properties, deduce the dominant physical processes governing the plasma and compare large and small flare characteristics. Results indicate the density of the thermal X-ray plasma to be higher than previously thought (from 10 to the 10th to 10 to the 12th/cu cm for large to small flares), cooling to occur radiatively as much as conductively, heating to continue into the decay phase of large flares, and the mass of the thermal X-ray plasma to be supplied primarily through chromospheric evaporation. Implications of the results for the basic flare mechanism are indicated. Title: Steady State Condensation of Coronal Flare Plasma Authors: Antiochos, S. K.; Sturrock, P. A. Bibcode: 1979BAAS...11..697A Altcode: No abstract at ADS Title: Numerical modeling of quasi-static coronal loops. I. Uniform energy input. Authors: Vesecky, J. F.; Antiochos, S. K.; Underwood, J. H. Bibcode: 1979ApJ...233..987V Altcode: A quasi-static numerical model for coronal loops is considered for the case of a uniform energy input per unit volume into the loops. A line dipole model is used to represent the loop magnetic field, and the variations in loop cross section observed in X-ray photographs are parameterized by the ratio between the cross-sectional areas at the loop apex and base. The results of numerical modeling indicate that for an area ratio greater than unity, increases in the area ratio of a loop with a given length and apex area cause a general rise in electron density and a fall in the temperature gradient, leading to large increases in the differential emission factor at high temperatures. The differential function obtained is significantly different from that predicted by analytical models; however, analytical predictions for the temperature-electron density relations are comparable to numerical results. It is also concluded that even a symmetrical loop may have a maximum temperature away from the apex. Title: The EINSTEIN Central Hyades Survey: A Progress Report. Authors: Stern, R.; Underwood, J. H.; Zolcinski, M.; Antiochos, S. Bibcode: 1979BAAS...11..781S Altcode: No abstract at ADS Title: The stability of solar coronal loops. Authors: Antiochos, S. K. Bibcode: 1979ApJ...232L.125A Altcode: The stability of the 'quasi-static' models of coronal loops was examined. It was found that all models in which the heat flux at the base of the loop is assumed to vanish are unstable to the growth of thermal perturbations. The growth rates and the profiles of the unstable modes indicate that the instability involves primarily the low-temperature, transition-region plasma. The models can be made stable only by assuming that the heat flux at the base of the loop is large, of the order of 13% of the maximum flux in the loop. The results imply that the transition region must be intrinsically dynamic. Title: Radiative dominated cooling of the flare corona and transition region Authors: Antiochos, S. K. Bibcode: 1979STIN...7932145A Altcode: Models in which radiation dominates cooling flare loops are investigated. The radiative models are found to predict a differential emission measure (Q) proportional to T to the (l+1) power, where l measures the dependence of the radiative loss coefficient on temperature, lamda (T) approximately T to the (-l) power. It is concluded that the radiative models are incapable of explaining the observed temperature dependence of Q for flare coronal and transitional plasma. The models suggest that large mass motions (velocities of the order of the sound speed) may be required. Title: The evolution of soft X-ray emitting flare loops. Authors: Antiochos, S. K.; Krall, K. R. Bibcode: 1979ApJ...229..788A Altcode: We have constructed a numerical model for a cooling flare loop in which the complete set of single-fluid equations in a one-dimensional geometry (i.e., parallel to the magnetic field) is solved. Both evaporative and static boundary conditions for the chromosphere-corona interface have been developed. This model is used to investigate the effects of initial temperature and density, loop geometry, and boundary conditions on the form of the plasma evolution and the soft X-ray emission. The results are then compared with Skylab S-056 observations of the 1973 August 9 flare. For this comparison, and under the present assumptions, we conclude that even highly compact flares must have a multiloop structure similar to large flares, and that both radiative and conductive cooling are necessary to explain the observations. The data appear to be consistent with the predicted emission from a combination of evaporative cooling loops. Title: The analysis of high spatial resolution UV and X-ray images by computational modeling Authors: Vesecky, J. F.; Antiochos, S. K.; Underwood, J. H. Bibcode: 1978clus.nasa..118V Altcode: Very high resolution stereoscopic images of high temperature loop structures observed at UV and X-ray wavelengths in the solar corona can be used to understand physical processes in the corona. An existing computational model is described and sample results are given to demonstrate that computational modeling of coronal structures can indeed take advantage of very high resolution images. The sample results include the run of temperature and number density along a typical loop and the variation of the differential emission measure with temperature. The integration of the differential emission measure with temperature along a column commensurate with an instrument's spatial resolution is the relevant parameter obtained from UV and X-ray observations. The effects of loop geometry and energy input are examined. Title: Evolution of the coronal and transition-zone plasma in a compact flare: the event of 1973 August 9. Authors: Underwood, J. H.; Antiochos, S. K.; Feldman, U.; Dere, K. P. Bibcode: 1978ApJ...224.1017U Altcode: X-ray and extreme ultraviolet observations of a compact flare were analyzed to determine the relative importance of radiation, thermal conduction, and 'evaporation' in the evolution of the temperature and density structure of the plasma. In the event studied (1973 August 9), the electron density was relatively high (5 x 10 to the eleventh to 1 x 10 to the twelfth) and radiation was evidently an important energy-loss and cooling mechanism. The light curves of ultraviolet lines formed at temperatures between 10 to the fifth to 10 to the seventh K indicate a time-varying emission measure gradient, and hence temperature gradient, during the flare. Radiative instability evidently played an important role in determining the steepness of these gradients during the rise and fall phases, and caused strong downward motions of material during the cooling phase. Toward the end of the event, the coronal electron density decreased and the temperature gradient relaxed toward that expected from a conduction-dominated plasma. For this flare, evaporative cooling did not appear to be a significant factor. Title: Comments on the "minimum flux corona" concept. Authors: Antiochos, S. K.; Underwood, J. H. Bibcode: 1978A&A....68L..19A Altcode: Hearn's (1975) models of the energy balance and mass loss of stellar coronae, based on a 'minimum flux corona' concept, are critically examined. First, it is shown that the neglect of the relevant length scales for coronal temperature variation leads to an inconsistent computation of the total energy flux F. The stability arguments upon which the minimum flux concept is based are shown to be fallacious. Errors in the computation of the stellar wind contribution to the energy budget are identified. Finally we criticize Hearn's (1977) suggestion that the model, with a value of the thermal conductivity modified by the magnetic field, can explain the difference between solar coronal holes and quiet coronal regions. Title: Models of Stellar Coronae. Authors: Antiochos, S. K.; Underwood, J. H.; Vesecky, J. F. Bibcode: 1978BAAS...10..510A Altcode: No abstract at ADS Title: Numerical Simulations of the Decay Phase of Compact Flares. Authors: Krall, K. R.; Antiochos, S. K. Bibcode: 1978BAAS...10..442K Altcode: No abstract at ADS Title: Evaporative cooling of flare plasma. Authors: Antiochos, S. K.; Sturrock, P. A. Bibcode: 1978ApJ...220.1137A Altcode: We investigate a one-dimensional loop model for the evaporative cooling of the coronal flare plasma. The important assumptions are that conductive losses dominate radiative cooling and that the evaporative velocities are small compared with the sound speed. We calculate the profile and evolution of the temperature and verify the accuracy of our assumptions for plasma parameters typical of flare regions. The model is in agreement with soft X-ray observations on the evolution of flare temperatures and emission measures. The effect of evaporation is to greatly reduce the conductive heat flux into the chromosphere and to enhance the EUV emission from the coronal flare plasma. Title: Thermal Instability in Post-Flare Plasmas. Authors: Antiochos, S. K. Bibcode: 1977PhDT.........2A Altcode: The cooling of post flare plasmas was investigated and a one dimensional model developed for active loop prominences, taking into consideration motion and heat fluxes parallel to the magnetic field. Included in the model are the effects of gravity, the geometry of the field, and conduction losses to the chromosphere. Calculations using the computer code developed and a two-step time differencing scheme indicate that the non-uniform cooling of the post-flare corona can be understood as a direct consequence of the temperature and density dependence of the radiative losses from a high-temperature solar plasma. Title: Observations of a Radiatively Cooling Subflare. Authors: Antiochos, S. K.; Underwood, J. H.; Feldman, U. Bibcode: 1977BAAS....9..329A Altcode: No abstract at ADS Title: Thermal instability in post-flare plasmas Authors: Antiochos, Spiro Kosta Bibcode: 1977PhDT.......104A Altcode: No abstract at ADS Title: Thermal instability in post-flare plasmas Authors: Antiochos, S. K. Bibcode: 1976STIN...7716975A Altcode: The cooling of post-flare plasmas is discussed and the formation of loop prominences is explained as due to a thermal instability. A one-dimensional model was developed for active loop prominences. Only the motion and heat fluxes parallel to the existing magnetic fields are considered. The relevant size scales and time scales are such that single-fluid MHD equations are valid. The effects of gravity, the geometry of the field and conduction losses to the chromosphere are included. A computer code was constructed to solve the model equations. Basically, the system is treated as an initial value problem (with certain boundary conditions at the chromosphere-corona transition region), and a two-step time differencing scheme is used. Title: Evaporative cooling of flare plasma Authors: Antiochos, S. K.; Sturrock, P. A. Bibcode: 1976STIN...7714971A Altcode: A one-dimensional loop model for the evaporative cooling of the coronal flare plasma was investigated. Conductive losses dominated radiative cooling, and the evaporative velocities were small compared to the sound speed. The profile and evolution of the temperature were calculated. The model was in agreement with soft X-ray observations on the evolution of flare temperatures and emission measures. The effect of evaporation was to greatly reduce the conductive heat flux into the chromosphere and to enhance the EUV emission from the coronal flare plasma. Title: An Evaporative Model of Flare Loops. Authors: Antiochos, S. K.; Sturrock, P. A. Bibcode: 1976BAAS....8R.555A Altcode: No abstract at ADS Title: The Dynamics of Active Loop Prominences. Authors: Antiochos, S. K. Bibcode: 1976BAAS....8..502A Altcode: No abstract at ADS Title: Influence of magnetic field structure on the conduction cooling of flare loops. Authors: Antiochos, S. K.; Sturrock, P. A. Bibcode: 1976SoPh...49..359A Altcode: A simple model facilitates calculation of the influence of magnetic field configuration on the conduction cooling rate of a hot post-flare coronal plasma. The magnetic field is taken to be that produced by a line dipole or point dipole at an arbitrary depth below the chromosphere. For the high temperatures (T ≳ 107 K) produced by flares, the plasma may remain static and isobaric. The influence of the field is such as to increase the heat flux (per unit area) into the chromosphere, but to decrease the total conduction cooling of the flare plasma. This leads to a significant enhancement of the total energy radiated by the flare plasma. Title: Thermal Instability in Loop Prominence Systems Authors: Antiochos, S. K.; Sturrock, P. A. Bibcode: 1975BAAS....7..472A Altcode: No abstract at ADS Title: Periodicity in the Radiofrequency Spectrum of the Pulsar CP 0328 Authors: Sturrock, P. A.; Antiochos, S.; Switzer, P.; Vallée, J. Bibcode: 1972ApJ...171L..27S Altcode: Long-term averaging of a sequence of wide-band radiofrequency spectra of CP 0328 reveals a periodicity not apparent in the original spectra. This may be caused by a mechanism intrinsic to the source, or by a propagation mechanism distinct from ordinary scintillation.