Author name code: klimchuk ADS astronomy entries on 2022-09-14 author:"Klimchuk, James A." or author:"Klimchuk, Jim" ------------------------------------------------------------------------ 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: Contribution of spicules to solar coronal emission Authors: Sow Mondal, Shanwlee; Klimchuk, James A.; Sarkar, Aveek Bibcode: 2022arXiv220805240S Altcode: Recent high-resolution imaging and spectroscopic observations have generated renewed interest in spicules' role in explaining the hot corona. Some studies suggest that some spicules, often classified as type II, may provide significant mass and energy to the corona. Here we use numerical simulations to investigate whether such spicules can produce the observed coronal emission without any additional coronal heating agent. Model spicules consisting of a cold body and hot tip are injected into the base of a warm ($0.5$ MK) equilibrium loop with different tip temperatures and injection velocities. Both piston- and pressure-driven shocks are produced. We find that the hot tip cools rapidly and disappears from coronal emission lines such as Fe XII $195$ and Fe XIV $274$. Prolonged hot emission is produced by pre-existing loop material heated by the shock and by thermal conduction from the shock. However, the shapes and Doppler shifts of synthetic line profiles show significant discrepancies with observations. Furthermore, spatially and temporally averaged intensities are extremely low, suggesting that if the observed intensities from the quiet Sun and active regions were solely due to type II spicules, one to several orders of magnitude more spicules would be required than have been reported in the literature. This conclusion applies strictly to the ejected spicular material. We make no claims about emissions connected with waves or coronal currents that may be generated during the ejection process and heat the surrounding area. Title: The Coronal Veil Authors: Malanushenko, A.; Cheung, M. C. M.; DeForest, C. E.; Klimchuk, J. A.; Rempel, M. Bibcode: 2022ApJ...927....1M Altcode: 2021arXiv210614877M Coronal loops, seen in solar coronal images, are believed to represent emission from magnetic flux tubes with compact cross sections. We examine the 3D structure of plasma above an active region in a radiative magnetohydrodynamic simulation to locate volume counterparts for coronal loops. In many cases, a loop cannot be linked to an individual thin strand in the volume. While many thin loops are present in the synthetic images, the bright structures in the volume are fewer and of complex shape. We demonstrate that this complexity can form impressions of thin bright loops, even in the absence of thin bright plasma strands. We demonstrate the difficulty of discerning from observations whether a particular loop corresponds to a strand in the volume, or a projection artifact. We demonstrate how apparently isolated loops could deceive observers, even when observations from multiple viewing angles are available. While we base our analysis on a simulation, the main findings are independent from a particular simulation setup and illustrate the intrinsic complexity involved in interpreting observations resulting from line-of-sight integration in an optically thin plasma. We propose alternative interpretation for strands seen in Extreme Ultraviolet images of the corona. The "coronal veil" hypothesis is mathematically more generic, and naturally explains properties of loops that are difficult to address otherwise-such as their constant cross section and anomalously high density scale height. We challenge the paradigm of coronal loops as thin magnetic flux tubes, offering new understanding of solar corona, and by extension, of other magnetically confined bright hot plasmas. Title: Impact of 3D Structure on Magnetic Reconnection Authors: Daldorff, Lars K. S.; Leake, James E.; Klimchuk, James A. Bibcode: 2022ApJ...927..196D Altcode: 2022arXiv220204761D Results from 2.5D and 3D studies of the onset and development of the tearing instability are presented, using high-fidelity resistive MHD simulations. A limited parameter study of the strength of the reconnecting field (or shear angle) was performed. An initially simple 1D equilibrium was used, consisting of a modified force-free current sheet, with periodic boundary conditions in all directions. In all cases, the linear and nonlinear evolution led to a primary current sheet between two large flux ropes. The global reconnection rate during this later stage was analyzed in all simulations. It was found that in 2.5D the primary current sheet fragmented owing to plasmoids, and as expected, the global reconnection rate, calculated using multiple methods, increases with the strength of the reconnecting field owing to a stronger Alfvén speed. In 3D, the presence of interacting oblique modes of the tearing instability complicates the simple 2.5D picture, entangling the magnetic field of the inflow and introducing a negative effect on the reconnection rate. The two competing effects of stronger Alfvén speed and entangling, which both increase with the strength of the reconnecting field, resulted in a decrease in the reconnection rate with increasing reconnecting field. For all simulations, the 3D rates were less than in 2.5D but suggest that as one goes to weak reconnecting field (or strong guide field) the system becomes more 2.5D-like and the 2.5D and 3D rates converge. These results have relevance to situations like nanoflare heating and flare current sheets in the corona. Title: Static and dynamic solar coronal loops with cross-sectional area variations Authors: Cargill, P. J.; Bradshaw, S. J.; Klimchuk, J. A.; Barnes, W. T. Bibcode: 2022MNRAS.509.4420C Altcode: 2021arXiv211109339C; 2021MNRAS.tmp.2869C The Enthalpy Based Thermal Evolution of Loops approximate model for static and dynamic coronal loops is developed to include the effect of a loop cross-sectional area which increases from the base of the transition region (TR) to the corona. The TR is defined as the part of a loop between the top of the chromosphere and the location where thermal conduction changes from an energy loss to an energy gain. There are significant differences from constant area loops due to the manner in which the reduced volume of the TR responds to conductive and enthalpy fluxes from the corona. For static loops with modest area variation the standard picture of loop energy balance is retained, with the corona and TR being primarily a balance between heating and conductive losses in the corona, and downward conduction and radiation to space in the TR. As the area at the loop apex increases, the TR becomes thicker and the density in TR and corona larger. For large apex areas, the coronal energy balance changes to one primarily between heating and radiation, with conduction playing an increasingly unimportant role, and the TR thickness becoming a significant fraction of the loop length. Approximate scaling laws are derived that give agreement with full numerical solutions for the density, but not the temperature. For non-uniform areas, dynamic loops have a higher peak temperature and are denser in the radiative cooling phase by of order 50 per cent than the constant area case for the examples considered. They also show a final rapid cooling and draining once the temperature approaches 1 MK. Although the magnitude of the emission measure will be enhanced in the radiative phase, there is little change in the important observational diagnostic of its temperature dependence. Title: Flows in Enthalpy-based Thermal Evolution of Loops Authors: Rajhans, Abhishek; Tripathi, Durgesh; Bradshaw, Stephen J.; Kashyap, Vinay L.; Klimchuk, James A. Bibcode: 2022ApJ...924...13R Altcode: 2021arXiv211003204R Plasma-filled loop structures are common in the solar corona. Because detailed modeling of the dynamical evolution of these structures is computationally costly, an efficient method for computing approximate but quick physics-based solutions is to rely on space-integrated 0D simulations. The enthalpy-based thermal evolution of loops (EBTEL) framework is a commonly used method to study the exchange of mass and energy between the corona and transition region. EBTEL solves for density, temperature, and pressure, averaged over the coronal part of the loop, velocity at coronal base, and the instantaneous differential emission measure distribution in the transition region. The current single-fluid version of the code, EBTEL2, assumes that at all stages the flows are subsonic. However, sometimes the solutions show the presence of supersonic flows during the impulsive phase of heat input. It is thus necessary to account for this effect. Here, we upgrade EBTEL2 to EBTEL3 by including the kinetic energy term in the Navier-Stokes equation. We compare the solutions from EBTEL3 with those obtained using EBTEL2, as well as the state-of-the-art field-aligned hydrodynamics code HYDRAD. We find that the match in pressure between EBTEL3 and HYDRAD is better than that between EBTEL2 and HYDRAD. Additionally, the velocities predicted by EBTEL3 are in close agreement with those obtained with HYDRAD when the flows are subsonic. However, EBTEL3 solutions deviate substantially from HYDRAD's when the latter predicts supersonic flows. Using the mismatches in the solution, we propose a criterion to determine the conditions under which EBTEL can be used to study flows in the system. Title: Magnetic Reconnection in 3D vs. 2D and Dependence on Magnetic Shear Authors: Daldorff, Lars; Leake, James; Klimchuk, James Bibcode: 2021AGUFMSH25E2125D Altcode: Magnetic reconnection plays a central role for heating the solar coronal plasma including during flares as well as other places in the heliosphere and beyond, where each event can be characterized by its geometry and magnetic configurations, forming structures that we can categorize to be more 2D or 3D. The difference in reconnection rate between 2D and 3D simulations have been reported in multiple studies over the years. We report on a study of the reconnection rate using the resistive MHD code LaRe3D. We show that the rate depends strongly on the existence and interaction of different tearing layers (oblique tearing modes) within the current sheet. Such modes are only present with a finite guide field and a spatial dependence in this third direction. We find, as have others, that reconnection rates are artificially high in 2D simulations, and we offer an explanation. Title: Distributed Space Telescopes Enabled by Constellation of Small Satellites Authors: Kamalabadi, Farzad; Lightsey, E.; Rabin, Douglas; Daw, Adrian; D'Amico, Simone; Koenig, Adam; Chamberlin, Philip; Woods, Thomas; Gupta, Subhanshu; Ekici, Eylem; Sample, John; Park, Hyeongjun; Alexeenko, Alina; Hwang, John; Denis, Kevin; Klimchuk, James Bibcode: 2021AGUFM.A33C..03K Altcode: New pathways to high-resolution sensing and imaging for a multitude of high-priority scientific investigations are being realized by small multi-spacecraft systems. Such pathfinder mission concepts circumvent the limitations of conventional remote sensing/imaging systems by utilizing multiple baselines, synthesized apertures, diffractive optics, combined with computational imaging via interferometry, tomography, or super-resolution. Regardless of the specific scientific questions targeted, such small satellite constellation pathfinders require technological breakthroughs in precision formation flying and associated advances in guidance, navigation, and control; proximity operations and associated autonomy and robust orbit design with passive safety; innovations in sensor miniaturization; inter-satellite communication; and sophisticated computational sensing and reconstruction algorithms. We describe recent advances in such enabling technologies in the context of a scalable ultra-high-resolution spectral imaging mission for investigating the solar corona currently under development by a multi-university consortium in collaboration with NASA and under sponsorship by NSF. Title: Study of Type III Radio bursts in the Closed Corona and the Solar Wind from Small-scale Reconnection: Observations Authors: Chhabra, Sherry; Klimchuk, James; Gary, Dale Bibcode: 2021AGUFMSH24B..06C Altcode: It is widely agreed that the ubiquitous presence of reconnection events and the associated impulsive heating (nanoflares) are a strong candidate for heating the magnetically closed corona. Whether nanoflares accelerate energetic particles like fullsized flares is unknown. The lack of strong emission in hard Xrays suggests that the quantity of highly energetic particles is small. There could, however, be large numbers of mildly energetic particles (~ 10 keV). Similarly, in the context of the solar wind, these energetic particles can originate from interchange reconnection, streamer tip reconnection, or turbulence reconnection in the solar wind itself, in which case they stream away from the Sun along the open field lines. To understand whether these processes are efficient at accelerating particles, we search for the type III radio bursts that they may produce. The timelag technique that was developed to study subtle delays in light curves from different EUV channels [Viall & Klimchuk 2012] can also be used to detect subtle delays at different radio frequencies. We have modeled the expected radio emission from mildly energetic particles propagating in the closed corona and open corona/solar wind. The models were used to test and calibrate the technique. We are currently applying the technique to radio observations from VLA (Very Large Array), LOFAR (LowFrequency Array), LWA, NM (Long Wavelength Array, New Mexico), and the FIELDS experiment (encounters 1-6) to search for such signatures of type IIIs. We also plan to investigate the relationship between the bursts and activity on the Sun, such as the presence/absence of active regions, relationship with their age etc. We will report the results from our analysis. Title: Coronal Heating: A Coupled Multi-Scale Problem Authors: Klimchuk, James Bibcode: 2021AGUFMSH13A..01K Altcode: Despite substantial observational and theoretical progress, the heating of the magnetically closed corona to temperatures 1000 times hotter than the solar surface has yet to be fully explained. Like many problems in Heliophysics, it involves an enormous range of spatial scales that are coupled in fundamental ways. In this talk, I will review our current understanding of how the coronal plasma is heated and how it responds to produce the time variable spectrum of radiation that is an important driver of space weather. I will emphasize that future progress requires a more integrated approach than has typically been used. Title: Onset of Magnetic Reconnection in the Solar Corona Authors: Leake, James; Klimchuk, James; Daldorff, Lars Bibcode: 2021AGUFMSH34C..01L Altcode: Magnetic reconnection plays a vital role in the dynamics, heating, and emission in the solar corona. It also plays a role in the onset of transient events such as jets, spicules, filament eruptions, and coronal mass ejections. One of the major questions still unanswered is the nature of the onset of magnetic reconnection. How is it that the solar corona can slowly build up magnetic energy and then rapidly release it to manifest in the above phenomena? Recent theoretical developments have considered the nature of the tearing instability in dynamically thinning current sheets in terms of this rapid switch-on of reconnection. We present a study of the onset of magnetic reconnection in 3D dynamically evolving current sheets unstable to both oblique and parallel modes of the tearing instability. While it is not currently possible to realize the actual parameters in the solar corona such as Lundquist number and current sheet thickness, we are able to perform our study in a regime which has relevant ratios of tearing timescale to dynamic timescale that allow us to make relevant conclusions about the solar corona. Title: Signatures of Type III Solar Radio Bursts from Nanoflares: Modeling Authors: Chhabra, Sherry; Klimchuk, James A.; Gary, Dale E. Bibcode: 2021ApJ...922..128C Altcode: 2021arXiv210903355C There is a wide consensus that the ubiquitous presence of magnetic reconnection events and the associated impulsive heating (nanoflares) are strong candidates for solving the solar coronal heating problem. Whether nanoflares accelerate particles to high energies like full-sized flares is unknown. We investigate this question by studying the type III radio bursts that the nanoflares may produce on closed loops. The characteristic frequency drifts that type III bursts exhibit can be detected using a novel application of the time-lag technique developed by Viall & Klimchuk (2012) even when there are multiple overlapping events. We present a simple numerical model that simulates the expected radio emission from nanoflares in an active region, which we use to test and calibrate the technique. We find that in the case of closed loops the frequency spectrum of type III bursts is expected to be extremely steep such that significant emission is produced at a given frequency only for a rather narrow range of loop lengths. We also find that the signature of bursts in the time-lag signal diminishes as: (1) the variety of participating loops within that range increases; (2) the occurrence rate of bursts increases; (3) the duration of bursts increases; and (4) the brightness of bursts decreases relative to noise. In addition, our model suggests a possible origin of type I bursts as a natural consequence of type III emission in a closed-loop geometry. Title: Computing Emission Signatures from Coronal MHD Models Without a Realistic Lower Atmosphere Authors: Klimchuk, James; Knizhnik, Kalman; Uritsky, Vadim Bibcode: 2021AGUFMSH43A..08K Altcode: The emission properties of the corona depend crucially on its energetic and dynamic coupling with the lower atmosphere, involving processes like thermal conduction and radiation cooling, chromospheric evaporation, and draining/condensation. Incorporating these processes into an MHD model is extremely challenging, because of the need to spatially resolve both the transition region and current sheets involved in the heating, both of which are thin and non-stationary. (New methods lesson the demands on resolving the transition region, especially when the heating is quasi steady.) We have developed a simple technique that can be applied post facto to MHD simulations without a realistic lower atmosphere to obtain useful approximations of the nonuniform and evolving radiation spectrum. We demonstrate the technique with a simulation of an initially uniform plasma/magnetic field system that is subjected to small-scale driving at the photospheric boundary. Important results are obtained concerning the nature of both the diffuse component of the corona and bright coronal loops. Title: Signatures of Type III Radio Bursts from Small-scale Reconnection Events in the Solar Wind Authors: Chhabra, S.; Klimchuk, J.; Gary, D.; Psp/Fields Team Bibcode: 2021AAS...23812307C Altcode: We look for evidence of energetic particles in the solar wind, that could be produced by reconnection in the solar wind itself or reconnection in the lower corona, where particles escape on open field lines, from e.g. interchange reconnection.We expect reconnection to be common at the current sheets that separate the thin magnetic strands that make up the corona and solar wind, but whether it is efficient at accelerating particles is an open question. Type III radio bursts have been very frequently observed in the solar wind over the past few decades. Energetic electron beams propagating along magnetic field lines cause a bump-on-tail instability generating Langmuir waves. Produced by their interaction with other particles and waves, type III bursts exhibit a characteristic drift in frequency as they propagate through the density gradient in the field. An interesting question is whether there is a ubiquitous presence of type IIIs in the radio 'background' observed. The radio background outside of clearly identified bursts may actually be comprised of multiple overlapping events. The time-lag technique that was developed to study subtle delays in light curves from different EUV channels [Viall & Klimchuk 2012] can also be used to detect subtle delays at different frequencies in the radio background even when there is no hint in the individual light curves. The FIELDS instrument onboard the Parker Solar Probe (PSP) is utilized to investigate the solar wind for these signatures. We perform a systematic study of the observed type III storms in Encounters 1-5, to understand the signatures that can identify their presence and the different features observed using the technique. Our findings are then employed to study the times where no activity is visibly detected by the instrument. We are currently analyzing multiple periods of no visible activity and will report our findings. Title: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) Authors: Caspi, A.; Shih, A. Y.; Panchapakesan, S.; Warren, H. P.; Woods, T. N.; Cheung, M.; DeForest, C. E.; Klimchuk, J. A.; Laurent, G. T.; Mason, J. P.; Palo, S. E.; Seaton, D. B.; Steslicki, M.; Gburek, S.; Sylwester, J.; Mrozek, T.; Kowaliński, M.; Schattenburg, M.; The CubIXSS Team Bibcode: 2021AAS...23821609C Altcode: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) is a 6U CubeSat proposed to NASA H-FORT. CubIXSS is motivated by a compelling overarching science question: what are the origins of hot plasma in solar flares and active regions? Elemental abundances are a unique diagnostic of how mass and energy flow into and within the corona, and CubIXSS addresses its science question through sensitive, precise measurements of abundances of key trace ion species, whose spectral signatures reveal the chromospheric or coronal origins of heated plasma across the entire temperature range from ~1 to >30 MK. CubIXSS measurements of the coronal temperature distribution and elemental abundances directly address longstanding inconsistencies from prior studies using instruments with limited, differing temperature and composition sensitivities.

CubIXSS comprises two co-optimized and cross-calibrated instruments that fill a critical observational gap:

MOXSI, a novel diffractive spectral imager using a pinhole camera and X-ray transmission diffraction grating for spectroscopy of flares and active regions from 1 to 55 Å, with spectral and spatial resolutions of 0.28-0.37 Å and 29-39 arcsec FWHM, respectively; and

SASS, a suite of four spatially-integrated off-the-shelf spectrometers for high-cadence, high-sensitivity X-ray spectra from 0.5 to 50 keV, with spectral resolution of 0.06-0.5 keV FWHM across that range.

If selected for implementation, CubIXSS will launch in late 2023 to mid-2024 to observe intense solar flares and active regions during the rising phase and peak of the solar cycle. Its 1-year prime mission is well timed with perihelia of Parker Solar Probe and Solar Orbiter, and with the launches of complementary missions such as the PUNCH Small Explorer. CubIXSS is a pathfinder for the next generation of Explorer-class missions with improved capabilities for SXR imaging spectroscopy. We present the CubIXSS motivating science background, its suite of instruments and expected performances, and other highlights from the completed Concept Study Report, including novel analysis techniques to fully exploit the rich data set of CubIXSS spectral observations. Title: Cross Sections of Coronal Loop Flux Tubes Authors: Klimchuk, J. A.; DeForest, C. Bibcode: 2021AAS...23832808K Altcode: Coronal loops reveal crucial information about the nature of both coronal magnetic fields and coronal heating. The shape of the corresponding flux tube cross section and how it varies with position are especially important properties. They are a direct indication of the expansion of the field and of the cross-field spatial distribution of the heating. We have studied 20 loops using high spatial resolution observations from the first flight of the Hi-C rocket experiment, measuring the intensity and width as a function of position along the loop axis. We find that intensity and width tend to either be uncorrelated or to have a direct dependence, such that they increase or decrease together. This implies that the flux tube cross sections are approximately circular under the assumptions that the tubes have non-negligible twist and that the plasma emissivity is approximately uniform along the magnetic field. The shape need not be a perfect circle and the emissivity need not be uniform within the cross section, but sub-resolution patches of emission must be distributed quasi-uniformly within an envelope that has an aspect ratio of order unity. This raises questions about the suggestion that flux tubes expand with height, but primarily in the line-of-sight direction so that the corresponding (relatively noticeable) loops appear to have roughly uniform width, a long-standing puzzle. It also casts doubt on the idea that most loops correspond to simple warped sheets, although we leave open the possibility of more complex manifold structures. Title: Nonthermal Velocity in the Transition Region of Active Regions and Its Center-to-limb Variation Authors: Ghosh, Avyarthana; Tripathi, Durgesh; Klimchuk, James A. Bibcode: 2021ApJ...913..151G Altcode: 2021arXiv210315081G We derive the nonthermal velocities (NTVs) in the transition region of an active region using the Si IV 1393.78 Å line observed by the Interface Region Imaging Spectrograph and compare them with the line-of-sight photospheric magnetic fields obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. The active region consists of two strong field regions with opposite polarity, separated by a weak field corridor that widened as the active region evolved. The means of the NTV distributions in strong field regions (weak field corridors) range between ∼18-20 (16-18) km s-1, albeit the NTV maps show a much larger range. In addition, we identify a narrow lane in the middle of the corridor with significantly reduced NTV. The NTVs do not show a strong center-to-limb variation, albeit they show somewhat larger values near the disk center. The NTVs are well correlated with redshifts as well as line intensities. The results obtained here and those presented in our companion paper on Doppler shifts suggest two populations of plasma in the active region emitting in Si IV. The first population exists in the strong field regions and extends partway into the weak field corridor between them. We attribute this plasma to spicules heated to ∼0.1 MK (often called type II spicules). They have a range of inclinations relative to vertical. The second population exists in the center of the corridor, is relatively faint, and has smaller velocities, likely horizontal. These results provide further insights into the heating of the transition region. 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 Heating of the Solar Corona Authors: Viall, Nicholeen M.; De Moortel, Ineke; Downs, Cooper; Klimchuk, James A.; Parenti, Susanna; Reale, Fabio Bibcode: 2021GMS...258...35V Altcode: No abstract at ADS Title: High resolution soft X-ray spectroscopy and the quest for the hot (5-10 MK) plasma in solar active regions Authors: Del Zanna, Giulio; Andretta, Vincenzo; Cargill, Peter J.; Corso, Alain J.; Daw, Adrian N.; Golub, Leon; Klimchuk, James A.; Mason, Helen E. Bibcode: 2021FrASS...8...33D Altcode: 2021arXiv210306156D We discuss the diagnostics available to study the 5--10 MK plasma in the solar corona, which is key to understanding the heating in the cores of solar active regions. We present several simulated spectra, and show that excellent diagnostics are available in the soft X-rays, around 100 Angstroms, as six ionisation stages of Fe can simultaneously be observed, and electron densities derived, within a narrow spectral region. As this spectral range is almost unexplored, we present an analysis of available and simulated spectra, to compare the hot emission with the cooler component. We adopt recently designed multilayers to present estimates of count rates in the hot lines, with a baseline spectrometer design. Excellent count rates are found, opening up the exciting opportunity to obtain high-resolution spectroscopy of hot plasma. Title: The Fascinating Phenomenon of Thermal Nonequilibrium Authors: Klimchuk, James Bibcode: 2021cosp...43E.960K Altcode: Thermal nonequilibrium (TNE) is an intriguing, counter-intuitive situation in which the plasma contained in a coronal magnetic loop is constantly evolving even though the heating is perfectly steady. It is believed to be the explanation for prominences, coronal rain, and long-period pulsating loops on the Sun, and there may be additional applications in other astrophysical settings. In this talk, I will attempt to peal away the mystery of TNE and provide some basic physical insights. I will present two analytical formulae for predicting whether TNE will occur, including the important role of asymmetries in preventing TNE. Finally, I will explain how TNE is related to, but different from, thermal instability. Title: Why do different current sheets reconnect differently? Authors: Daldorff, L. K. S.; Leake, J. E.; Klimchuk, J. A. Bibcode: 2020AGUFMSH034..03D Altcode: What variability can we expect for heating events from magnetic reconnection in the solar corona is an important research question. Complex flows in the photosphere entangle, twist, and compress the magnetic field in the chromosphere and corona, resulting in a complex forest of evolving current sheets. These sheets undergo tearing and magnetic reconnection, the details of which depend on the properties of the sheet, including its dimensions in all three directions, as well as the shear angle of the field. The associated energy release and heating can be very different for different sheets. We discuss the possibilities with emphasis on the role played by the interaction among the different tearing modes, parallel and oblique, that are present in the system. Title: Transition Region Contribution to AIA Observations in the Context of Coronal Heating Authors: Schonfeld, S. J.; Klimchuk, J. A. Bibcode: 2020ApJ...905..115S Altcode: 2020arXiv200906759S We investigate the ratio of coronal and transition region intensity in coronal loops observed by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO). Using Enthalpy-based Thermal Evolution of Loops (EBTEL) hydrodynamic simulations, we model loops with multiple lengths and energy fluxes heated randomly by events drawn from power-law distributions with different slopes and minimum delays between events to investigate how each of these parameters influences observable loop properties. We generate AIA intensities from the corona and transition region for each realization. The variations within and between models generated with these different parameters illustrate the sensitivity of narrowband imaging to the details of coronal heating. We then analyze the transition region and coronal emission from a number of observed active regions and find broad agreement with the trends in the models. In both models and observations, the transition region brightness is significant, often greater than the coronal brightness in all six "coronal" AIA channels. We also identify an inverse relationship, consistent with heating theories, between the slope of the differential emission measure (DEM) coolward of the peak temperature and the observed ratio of coronal to transition region intensity. These results highlight the use of narrowband observations and the importance of properly considering the transition region in investigations of coronal heating. Title: Updates on the Fundamentals of Impulsive Energy Release in the Corona Explorer (FIERCE) mission concept Authors: Shih, A. Y.; Glesener, L.; Krucker, S.; Guidoni, S. E.; Christe, S.; Reeves, K.; Gburek, S.; Caspi, A.; Alaoui, M.; Allred, J. C.; Battaglia, M.; Baumgartner, W.; Dennis, B. R.; Drake, J. F.; Goetz, K.; Golub, L.; Hannah, I. G.; Hayes, L.; Holman, G.; Inglis, A.; Ireland, J.; Kerr, G. S.; Klimchuk, J. A.; McKenzie, D. E.; Moore, C. S.; Musset, S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Schwartz, R.; Seaton, D. B.; Steslicki, M.; Woods, T. N. Bibcode: 2020AGUFMSH0480012S Altcode: The Fundamentals of Impulsive Energy Release in the Corona Explorer ( FIERCE ) Medium-Class Explorer (MIDEX) mission concept addresses the following science questions:

What are the physical origins of space-weather events?

How are particles accelerated at the Sun?

How is impulsively released energy transported throughout the solar atmosphere?

How is the solar corona heated?

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

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

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

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

If selected, FIERCE will launch in 2025, near the peak of the next solar cycle, which is also well timed with perihelia of Parker Solar Probe and Solar Orbiter . We describe the status and latest updates of the mission concept since it was proposed to NASA last year. We also highlight the anticipated science return from co-observations with other observatories/instruments such as the Expanded Owens Valley Solar Array (EOVSA) or the STIX instrument on Solar Orbiter . Title: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) Authors: Caspi, A.; Shih, A. Y.; Warren, H.; Winebarger, A. R.; Woods, T. N.; Cheung, C. M. M.; DeForest, C.; Klimchuk, J. A.; Laurent, G. T.; Mason, J. P.; Palo, S. E.; Schwartz, R.; Seaton, D. B.; Steslicki, M.; Gburek, S.; Sylwester, J.; Mrozek, T.; Kowaliński, M.; Schattenburg, M. Bibcode: 2020AGUFMSH0480007C Altcode: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) is a 6U CubeSat currently in a formulation phase under the 2019 NASA H-FORT program. CubIXSS is motivated by a compelling overarching science question: what are the origins of hot plasma in solar flares and active regions? Elemental abundances are a unique diagnostic of how mass and energy flow into and within the corona, and CubIXSS addresses its science question through sensitive, precise measurements of abundances of key trace ion species, whose spectral signatures reveal the chromospheric or coronal origins of heated plasma across the entire range of coronal temperatures, from ~1 to >30 MK. CubIXSS measurements of the coronal temperature distribution and elemental abundances directly address longstanding inconsistencies from prior studies using instruments with limited, differing temperature and composition sensitivities.

CubIXSS comprises two co-optimized and cross-calibrated instruments that fill a critical observational gap:

MOXSI, a novel diffractive spectral imager using a pinhole camera and X-ray transmission diffraction grating to achieve spectroscopy of flares and active regions from 1 to 55 Å, with spectral resolution of 0.24 Å FWHM and a spatial resolution of 25 arcsec FWHM; and

SASS, a suite of four spatially-integrated off-the-shelf spectrometers for high-cadence, high-sensitivity measurements of soft and hard X-rays, from 0.5 to 50 keV, with spectral resolution from 0.06 to 0.5 keV FWHM.

If selected for implementation, CubIXSS will launch in mid-2023 to observe intense solar flares and active regions during the rising phase of the solar cycle. Its nominal 1-year mission is well timed with perihelia of Parker Solar Probe and Solar Orbiter, and with the launches of complementary missions such as the PUNCH Small Explorer. CubIXSS is also a pathfinder for the next generation of Explorer-class missions with improved capabilities for SXR imaging spectroscopy. We present the CubIXSS motivating science background, its suite of instruments and expected performances, and other highlights from the completed Concept Study Report, including novel analysis techniques to fully exploit the rich data set of CubIXSS spectral observations. Title: Signatures of Type III Solar Radio Bursts from Nanoflares: Final Results Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M. Bibcode: 2020AGUFMSH0430016C Altcode: The heating mechanisms responsible for the million degree solar corona remain one of the most intriguing problems in space science. It is widely agreed, that the ubiquitous presence o f reconnection events and the associated impulsive heating (nanoflares) are a strong candidate in solving this problem [Klimchuk J.A., 2015 and references therein].

Whether nanoflares accelerate energetic particles like full sized flares is unknown. The lack of strong emission in hard X rays suggests that the quantity of highly energetic particles is small. There could, however, be large numbers of mildly energetic particles (~ 10 keV). We investigate such particles by searching for the type III radio bursts that they may produce. If energetic electron beams propagating along magnetic field lines generate a bump on tail instability, they will produce Langmuir waves, which can then interact with other particles and waves to give rise to emission at the local plasma frequency and its first harmonic. Type III radio bursts bursts are characteristically known to exhibit high frequency drifts as the beam propagates through a density gradient. The time lag technique that was developed to study subtle delays in light curves from different EUV channels [Viall & Klimchuk 2012] can also be used to detect subtle delays at different radio frequencies. We have modeled the expected radio emission from nanoflares, which we used to test and calibrate the technique. We will present the final results of our modeling efforts along with results from application of the technique to actual radio observations from VLA (Very Large Array), MWA (Murchison Widefield Array) and seeking data from LOFAR (Low Frequency Array) as well.We are also using data from the PSP (Parker Solar Probe) to look for similar reconnection signatures in the Solar Wind. Our goal is to determine whether nanoflares accelerate energetic particles and to determine their properties. The results will have important implications for both the particle acceleration and reconnection physics. Title: Cross Sections of Coronal Loop Flux Tubes Authors: Klimchuk, J. A.; DeForest, C. Bibcode: 2020AGUFMSH0370001K Altcode: Coronal loops reveal crucial information about the nature of both coronal magnetic fields and coronal heating. The shape of the corresponding flux tube cross section and how it varies with position are especially important properties. They are a direct indication of the expansion of the field and of the cross-field spatial distribution of the heating. We have studied 20 loops using high spatial resolution observations from the first flight of the Hi-C rocket experiment, measuring the intensity and width as a function of position along the loop axis. We find that intensity and width tend to either be uncorrelated or to have a direct dependence, such that they increase or decrease together. This implies that the flux tube cross sections are approximately circular under the assumptions that the tubes have non-negligible twist and that the plasma emissivity is approximately uniform along the magnetic field. The shape need not be a perfect circle and the emissivity need not be uniform within the cross section, but sub-resolution patches of emission must be distributed quasi-uniformly within an envelope that has an aspect ratio of order unity. This raises questions about the suggestion that flux tubes expand with height, but primarily in the line-of-sight direction so that the corresponding (relatively noticeable) loops appear to have roughly uniform width, a long-standing puzzle. It also casts doubt on the idea that most loops correspond to simple warped sheets, although we leave open the possibility of more complex manifold structures. Title: Can nanoflare heating define the coronal loop size? Authors: Uritsky, V. M.; Knizhnik, K.; Klimchuk, J. A. Bibcode: 2020AGUFMSH0370002U Altcode: The idea of nanoflare heating first coined by T. Gold (1964) and further developed by E. Parker (1972) has resulted in a dramatic paradigm shift in our understanding of the Sun, by emphasizing the critical role of the microscale physics in the formation of huge coronal structures such as coronal loops. Nanoflares are small, short-living energy conversion events in which the magnetic free energy is efficiently converted into heat. While the outcome of a single nanoflare is negligibly small, their cooperative contribution to the bulk coronal heating can be enormous. In this talk, we present recent advances in the numerical modeling of the nanoflare heating process. Our simulations demonstrate that densely distributed nanoflare events occur naturally inside a topologically complex coronal loop energized by photospheric vortices. These events lead to a formation of visible loops of various sizes, with the largest loop size imposed by the photospheric convection pattern and the smaller-scale loops reflecting clusters of nanoflares. Observational implications of our findings will be discussed. Title: Spectroscopic Constraints on the Dimension of Active Region Loops Along the Line of Sight Authors: Kucera, T. A.; Young, P. R.; Klimchuk, J. A.; DeForest, C. Bibcode: 2020AGUFMSH041..05K Altcode: Understanding the cross sections of coronal loops and how they vary along the loop is important both for understanding coronal heating and how the loops are shaped by the coronal magnetic field. To better address this question we have developed a new method to constrain the dimension of loops along the line of sight by utilizing spectroscopic observations. We apply this method to a cool (5.5<logT<6.2) loop using data from the Hinode/EUV Imaging Spectrometer (EIS) with supporting data from Solar Dynamic Observatory (SDO) and the Solar TErrestrial RElations Observatory (STEREO), and discuss the results and their limitations. Our results are consistent with circular loop cross sections, but could also be consistent with aspect ratios of 2 or 3. 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: Cross Sections of Coronal Loop Flux Tubes Authors: Klimchuk, James A.; DeForest, Craig E. Bibcode: 2020ApJ...900..167K Altcode: 2020arXiv200715085K Coronal loops reveal crucial information about the nature of both coronal magnetic fields and coronal heating. The shape of the corresponding flux tube cross section and how it varies with position are especially important properties. They are a direct indication of the expansion of the field and of the cross-field spatial distribution of the heating. We have studied 20 loops using high spatial resolution observations from the first flight of the Hi-C rocket experiment, measuring the intensity and width as a function of position along the loop axis. We find that intensity and width tend to either be uncorrelated or to have a direct dependence, such that they increase or decrease together. This implies that the flux tube cross sections are approximately circular under the assumptions that the tubes have nonnegligible twist and that the plasma emissivity is approximately uniform along the magnetic field. The shape need not be a perfect circle and the emissivity need not be uniform within the cross section, but subresolution patches of emission must be distributed quasi-uniformly within an envelope that has an aspect ratio of order unity. This raises questions about the suggestion that flux tubes expand with height, but primarily in the line-of-sight direction so that the corresponding (relatively noticeable) loops appear to have roughly uniform width, a long-standing puzzle. It also casts doubt on the idea that most loops correspond to simple warped sheets, although we leave open the possibility of more complex manifold structures. Title: The Significance of the Transition Region in AIA Channels: Modeling and Observations Authors: Schonfeld, S. J.; Klimchuk, J. Bibcode: 2020SPD....5121014S Altcode: We investigate the relative contributions from the transition region and corona of coronal loops observed by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO). Using EBTEL (Enthalpy-Based Thermal Evolution of Loops) hydrodynamic simulations, we model loops with multiple lengths and energy fluxes heated randomly by events drawn from power-law distributions with different slopes and minimum event sizes to investigate how each of these parameters influences observable loop properties. We generate AIA intensities from the corona and transition region for each realization. The variations within and between models generated with these different parameters illustrate the sensitivity of narrowband imaging to the details of coronal heating. We then analyze the transition region and coronal emission from a number of observed active regions and find broad agreement with the trends in the models. We find that in both models and observations, the transition region brightness is significant, often greater than the coronal brightness in all six "coronal" AIA channels. These results highlight the use of narrowband observations and the importance of properly considering the transition region in investigations of coronal heating. Title: The Onset of 3D Magnetic Reconnection and Heating in the Solar Corona Authors: Leake, James E.; Daldorff, Lars K. S.; Klimchuk, James A. Bibcode: 2020ApJ...891...62L Altcode: 2020arXiv200102971L Magnetic reconnection, a fundamentally important process in astrophysics, is believed to be initiated by the tearing instability of an electric current sheet, a region where magnetic field abruptly changes direction. Recent studies have suggested that the amount of magnetic shear in these structures is a critical parameter for the switch-on nature of magnetic reconnection in the solar atmosphere, at large spatial scales. We present results of visco-resistive magnetohydrodynamic simulations of magnetic reconnection in 3D current sheets with conditions appropriate to the solar corona. We follow the evolution of the linear and nonlinear 3D tearing instability. We find that, depending on the parameter space, magnetic shear can play a vital role in the onset of significant energy release and plasma heating. Two regimes in our study exist, dependent on whether the current sheet is longer or shorter than the wavelength of the fastest growing mode, thus determining whether subharmonics are present in the actual system. In one parameter regime, where the fastest growing parallel mode has subharmonics, the subsequent coalescence of 3D plasmoids dominates the nonlinear evolution, with magnetic shear playing only a weak role in the amount of energy released. In the second parameter regime, where the fastest growing parallel mode has no subharmonics, only strongly sheared current sheets, where 3D effects are strong enough, show any significant energy release. We expect both regimes to exist on the Sun, and so our results have important consequences for the question of reconnection onset in various solar physics applications. 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: Volume-filling Simulations of Coronal Loops Heated by Nanoflares Authors: Plowman, J.; Barnes, W.; Bradshaw, S. J.; Caspi, A.; DeForest, C.; Klimchuk, J. A. Bibcode: 2019AGUFMSH53B3380P Altcode: We present results of a coronal simulation consisting of loop strands that fill the coronal volume in a self-consistent fashion. The simulation is heated by a fully controllable 3D distribution, which can be specified independent of the loop geometry and can include nanoflares and continuous heating. The heating is then mapped to the loop strands, and the physics of each strand are simulated using the HYDRAD field-aligned hydrodynamics code. The simulation is applied to a small example active region and used to produce synthetic AIA data, which are then processed to produce a distribution of coronal EUV brightening events. This distribution is then compared with that found in the real AIA data for the same region, and we use the results to determine if the observations are consistent with our prescribed heating distribution. 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: Combined Next-Generation X-ray and EUV Observations with the FIERCE Mission Concept Authors: Shih, A. Y.; Glesener, L.; Christe, S.; Reeves, K.; Gburek, S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis, B. R.; Drake, J. F.; Goetz, K.; Golub, L.; Guidoni, S. E.; Inglis, A.; Hannah, I. G.; Holman, G.; Hayes, L.; Ireland, J.; Kerr, G. S.; Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset, S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton, D. B.; Steslicki, M.; Woods, T. N. Bibcode: 2019AGUFMSH33A..08S Altcode: While there have been significant advances in our understanding of impulsive energy release at the Sun through the combination of RHESSI X-ray observations and SDO/AIA EUV observations, there is a clear science need for significantly improved X-ray and EUV observations. These new observations must capture the full range of emission in flares and CMEs (e.g., faint coronal sources near bright chromospheric sources), connect the intricate evolution of energy release with dynamic changes in the configuration of plasma structures, and identify the signatures of impulsive energy release in even the quiescent Sun. The Fundamentals of Impulsive Energy Release in the Corona Explorer ( FIERCE ) MIDEX mission concept makes these observations by combining the two instruments previously proposed on the FOXSI SMEX mission concept - a focusing hard X-ray spectroscopic imager and a soft X-ray spectrometer - with a high-resolution EUV imager that will not saturate for even intense flares. All instruments observe at high cadence to capture the initiation of solar transient events and the fine time structure within events. FIERCE would launch in mid-2025, near the peak of the next solar cycle, which is also well timed with perihelions of Parker Solar Probe and Solar Orbiter. Title: The Sensitivity of AIA Observations to Coronal Heating Parameters Authors: Schonfeld, S.; Klimchuk, J. A. Bibcode: 2019AGUFMSH41F3323S Altcode: We explore the effects of changing heating parameters in closed coronal loops on the intensity of Atmospheric Imaging Assembly (AIA) observations. Using EBTEL (Enthalpy-Based Thermal Evolution of Loops) hydrodynamic simulations, we produce realizations of coronal loops with a series of heating events randomly drawn from a power law distribution. We repeat this procedure for multiple loop lengths, heating intensities, and characteristic heating frequencies to investigate how each of these parameters individually and in combination influences observable loop properties. We generate AIA intensities from the corona and transition region for each realization. The variations within and between models generated with these different parameters illustrate the sensitivity of narrowband imaging to the details of coronal heating. Using insights from this analysis, we generate images of observed coronal active regions using the GX Simulator SSW IDL package and interpret the causes of discrepancies between the models and observations. Title: Study of Type III Solar Radio Bursts in Nanoflares Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M. Bibcode: 2019AGUFMSH23C3337C Altcode: The heating mechanisms responsible for the million-degree solar corona remain one of the most intriguing problems in space science. It is widely agreed, that the ubiquitous presence of reconnection events and the associated impulsive heating (nanoflares) are a strong candidate in solving this problem [Klimchuk J.A., 2015 and references therein].

Whether nanoflares accelerate energetic particles like full-sized flares is unknown. The lack of strong emission in hard X-rays suggests that the quantity of highly energetic particles is small. There could, however, be large numbers of mildly energetic particles (~ 10 keV). We investigate such particles by searching for the type III radio bursts that they may produce. If energetic electron beams propagating along magnetic field lines generate a bump-on-tail instability, they will produce Langmuir waves, which can then interact with other particles and waves to give rise to emission at the local plasma frequency and its first harmonic. Type III bursts are characteristically known to exhibit high frequency drifts as the beam propagates through a density gradient. The time-lag technique that was developed to study subtle delays in light curves from different EUV channels [Viall & Klimchuk 2012] can also be used to detect subtle delays at different radio frequencies. We have modeled the expected radio emission from nanoflares, which we used to test and calibrate the technique. We are applying the technique to actual radio observations from VLA (Very Large Array), MWA (Murchison Widefield Array) and seeking data from LOFAR (Low-Frequency Array) as well. We also plan to use data from the PSP (Parker Solar Probe) to look for similar reconnection signatures in the Solar Wind. Our goal is to determine whether nanoflares accelerate energetic particles and to determine their properties. The results will have important implications for both the particle acceleration and reconnection physics. Title: The magnetic structure and electrodynamics of the emerging solar wind Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.; Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; Dudok de Wit, T.; Eastwood, J. P.; Webster, J.; Farrell, W. M.; Fong, C.; Goetz, K.; Goldstein, M. L.; Goodrich, K.; Harvey, P.; Horbury, T. S.; Howes, G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck, K. E.; Krasnoselskikh, V.; Krucker, S.; Laker, R.; Larson, D. E.; MacDowall, R. J.; Maksimovic, M.; Malaspina, D.; Martinez Oliveros, J. C.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F.; Phan, T.; Pulupa, M.; Raouafi, N. E.; Salem, C. S.; Stansby, D.; Stevens, M. L.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R. Bibcode: 2019AGUFMSH11A..05B Altcode: Convection and rotation drive the solar dynamo and, ultimately, provide the mechanical energy flux required to heat the solar corona and accelerate the solar wind. However, the way in which energy is then dissipated to heat the corona and wind are not well understood. Some energization models invoke non-thermal energy flux imparted by plasma Alfvén waves, while others rely on a carpet of small nano-flares as energy input, however these models have been unconstrained by direct measurements of the solar wind near its origin. Here we use in situ measurements from the FIELDS instrument suite during the first solar encounter (E1) at 35.7 solar radii (Rs) of the NASA Parker Solar Probe (PSP) mission to reveal the magnetic structure and kinetics of slow Alfvénic solar wind emerging from a small, equatorial coronal hole. Our measurements show that, at solar minimum, the slow wind can escape from above the low-lying, complex magnetic structures of the equatorial streamer belt, carrying a magnetic field that is highly dynamic, exhibiting polarity reversals on timescales from seconds to hours. These rapidly oscillating field structures are associated with clustered radial jets of plasma in which the energy flux is dramatically enhanced and turbulence levels are higher. Time intervals between groups of jets indicate a solar wind that is steady with a mostly radial magnetic field and relatively low levels of Alfvénic turbulent fluctuations. This 'quiet' wind however shows clear signatures of plasma micro-instabilities associated with ion and electron beams and velocity-space structure. Title: The Onset and Development of 3D Magnetic Reconnection in the Solar Corona: New Insights Authors: Klimchuk, J. A.; Daldorff, L. K. S.; Leake, J. E.; Knizhnik, K. J. Bibcode: 2019AGUFMSH52A..08K Altcode: Magnetic reconnection, one of the most important processes in the universe, is believed to be initiated in most cases by the tearing instability of an electric current sheet. Whether and how the tearing develops nonlinearly and releases substantial amounts of magnetic free energy depends on the relative growth rates of the different normal modes that are allowed in the system. In the general case where there is a guide field—in which the field rotates across the sheet by a shear angle—there exist both parallel modes centered on the sheet and oblique modes offset from it. We have performed a series of resistive magnetohydrodynamic (MHD) simulations of weakly perturbed, triply periodic current sheets with conditions appropriate to the solar corona to study how the different modes evolve and interact. We find three primary evolutionary paths: (1) a parallel mode dominates and saturates at a level that releases only a small amount of energy, but is large enough to choke off the growth of other modes; (2) parallel and oblique modes grow at nearly the same rate and interact violently as they become nonlinear, releasing a large amount of energy; (3) subharmonics of the fastest growing parallel mode induce a coalescence of islands, also releasing a large amount of energy. Which path will be taken depends in a predictable way on the length, thickness, and shear angle of the current sheet, as well as the resistivity. A critical issue is whether the wavelength of the fastest growing mode is longer or shorter than the current sheet, i.e., whether subharmonics exist. We expect all three behaviors on the Sun. These results have important implications for the question of reconnection onset. Observed phenomena require the buildup of high levels of magnetic stress before reconnection switches on to release the stored magnetic energy. Physical details of these simulations will be discussed in a companion presentation by Daldorff et al. Title: The Onset and Development of 3D Magnetic Reconnection in the Solar Corona: Important Physical Details Authors: Daldorff, L. K. S.; Leake, J. E.; Klimchuk, J. A.; Knizhnik, K. J. Bibcode: 2019AGUFMSH53B3366D Altcode: Magnetic reconnection, one of the most important processes in the universe, is believed to be initiated in most cases by the tearing instability of an electric current sheet. Whether and how the tearing develops nonlinearly and releases substantial amounts of magnetic free energy is a crucial unanswered question. Using a series of resistive magnetohydrodynamic (MHD) simulations of weakly perturbed, triply periodic current sheets with conditions appropriate to the solar corona, we have found that the answer depends on the relative growth rates of the different normal modes that are allowed in the system. Dramatically different evolutionary paths are possible depending on the length, thickness, and shear angle of the current sheet, as well as the resistivity. The basic results are discussed in the companion presentation by Klimchuk et al. Here, we delve into the important physical details underpinning the complex behavior. Title: FIERCE Science: Expected Results From a High-Energy Medium-Class Explorer Authors: Glesener, L.; Shih, A. Y.; Christe, S.; Reeves, K.; Gburek, S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis, B. R.; Drake, J. F.; Golub, L.; Goetz, K.; Guidoni, S. E.; Hannah, I. G.; Hayes, L.; Holman, G.; Inglis, A.; Ireland, J.; Kerr, G. S.; Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset, S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton, D. B.; Steslicki, M.; Woods, T. N. Bibcode: 2019AGUFMSH31C3313G Altcode: A variety of individual X-ray and EUV instruments have probed high-energy aspects of the Sun over the decades, each contributing pieces to the puzzles of the energization, heating, and acceleration of solar plasma and particles. But fundamental difficulties in sensitivity and dynamic range impart big challenges in probing the details of particle acceleration sites, understanding how eruptions and flares are initiated, and tracking the intricacies of energy transfer as flares evolve. The Fundamentals of Impulsive Energy Release in the Corona Explorer ( FIERCE ) mission will make substantial leaps forward in these scientific ventures by combining a variety of instruments into one platform, each optimized to have high sensitivity and dynamic range. FIERCE is a proposed NASA Heliophysics Medium-Class Explorer that will investigate high-energy solar phenomena across a variety of spectral and spatial dimensions. It combines hard X-ray imaging spectroscopy (via focusing, for the first time for a solar-dedicated spacecraft), spatially integrated soft X-ray spectroscopy, and fast, high-resolution extreme ultraviolet imaging at coronal and flare temperatures. FIERCE uses this array of instruments to make important contributions toward probing the genesis of space weather events, the acceleration of particles, the transport of flare energy, and the heating of the corona. Here, we present some of the expected science outcomes for the FIERCE observatory, concentrating on the ways in which FIERCE can probe confined and eruptive events, particle acceleration everywhere it may occur on the Sun, and the connections of solar high-energy phenomena to the heliosphere. Title: The Distinction Between Thermal Nonequilibrium and Thermal Instability Authors: Klimchuk, James A. Bibcode: 2019SoPh..294..173K Altcode: 2019arXiv191111849K For some forms of steady heating, coronal loops are in a state of thermal nonequilibrium and evolve in a manner that includes accelerated cooling, often resulting in the formation of a cold condensation. This is frequently confused with thermal instability, but the two are in fact fundamentally different. We explain the distinction and discuss situations where they may be interconnected. Large-amplitude perturbations, perhaps associated with MHD waves, likely play a role in explaining phenomena that have been attributed to thermal nonequilibrium but also seem to require cross-field communication. Title: Highly structured slow solar wind emerging from an equatorial coronal hole Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.; Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; de Wit, T. Dudok; Eastwood, J. P.; Ergun, R. E.; Farrell, W. M.; Fong, C.; Goetz, K.; Goldstein, M.; Goodrich, K. A.; Harvey, P. R.; Horbury, T. S.; Howes, G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck, K. E.; Krasnoselskikh, V. V.; Krucker, S.; Laker, R.; Larson, D. E.; MacDowall, R. J.; Maksimovic, M.; Malaspina, D. M.; Martinez-Oliveros, J.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F. S.; Phan, T. D.; Pulupa, M.; Raouafi, N. E.; Salem, C.; Stansby, D.; Stevens, M.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R. Bibcode: 2019Natur.576..237B Altcode: During the solar minimum, when the Sun is at its least active, the solar wind1,2 is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvénic rarefied stream of plasma originating from deep within coronal holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind3 of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain4; theories and observations suggest that they may originate at the tips of helmet streamers5,6, from interchange reconnection near coronal hole boundaries7,8, or within coronal holes with highly diverging magnetic fields9,10. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfvén-wave turbulence11,12, heating by reconnection in nanoflares13, ion cyclotron wave heating14 and acceleration by thermal gradients1. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe15 at 36 to 54 solar radii that show evidence of slow Alfvénic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities10,16 that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude coronal holes are a key source of the slow solar wind. Title: Spectroscopic Constraints on the Cross-sectional Asymmetry and Expansion of Active Region Loops Authors: Kucera, T. A.; Young, P. R.; Klimchuk, J. A.; DeForest, C. E. Bibcode: 2019ApJ...885....7K Altcode: We explore the constraints that can be placed on the dimensions of coronal loops out of the plane of the sky by utilizing spectroscopic observations from the Hinode/EUV Imaging Spectrometer (EIS). The usual assumption is that loop cross sections are circular. Changes in intensity are assumed to be the result of changing density, filling factor, and/or point of view. In this work we instead focus on the possibility that the loop dimensions may be changing along the line of sight while the filling factor remains constant. We apply these ideas to two warm (5.5≲ {log}T({{K}})< 6.2) loops observed by EIS in Active Region 11150 on 2011 February 6 with supporting observations from Solar Dynamics Observatory's Atmospheric Imaging Assembly and the Solar TErrestrial RElations Observatory-A's Extreme Ultraviolet Imager. Our results are generally consistent with nonexpanding loops but could also allow linear expansions of up to a factor of 2.5 along a 40 Mm section of one loop and up to a factor of 3.9 in another loop, both under the assumption that the filling factor is constant along the loop. Expansions in the plane of the sky over the same sections of the loops are 1.5 or less. For a filling factor of 1, the results of the analysis are consistent with circular cross sections but also with aspect ratios of 2 or greater. Count rate statistics are an important part of the uncertainties, but the results are also significantly dependent on radiometric calibration of EIS and the selection of the loop backgrounds. Title: On Doppler Shift and Its Center-to-limb Variation in Active Regions in the Transition Region Authors: Ghosh, Avyarthana; Klimchuk, James A.; Tripathi, Durgesh Bibcode: 2019ApJ...886...46G Altcode: 2019arXiv191012033G A comprehensive understanding of the structure of Doppler motions in the transition region including the center-to-limb variation and its relationship with the magnetic field structure is vital for the understanding of mass and energy transfer in the solar atmosphere. In this paper, we have performed such a study in an active region using the Si IV 1394 Å emission line recorded by the Interface Region Imaging Spectrograph and the line-of-sight photospheric magnetic field obtained by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. The active region has two opposite polarity strong field regions separated by a weak field corridor, which widened as the active region evolved. On average, the strong field regions (corridor) show(s) redshifts of 5-10 (3-9) km s-1 (depending on the date of observation). There is, however, a narrow lane in the middle of the corridor with near-zero Doppler shifts at all disk positions, suggesting that any flows there are very slow. The Doppler velocity distributions in the corridor seem to have two components—a low velocity component centered near 0 km s-1 and a high-velocity component centered near 10 km s-1. The high-velocity component is similar to the velocity distributions in the strong field regions, which have just one component. Both exhibit a small center-to-limb variation and seem to come from the same population of flows. To explain these results, we suggest that the emission from the lower transition region comes primarily from warm type II spicules, and we introduce the idea of a “chromospheric wall”—associated with classical cold spicules—to account for a diminished center-to-limb variation. Title: The Role of Asymmetries in Thermal Nonequilibrium Authors: Klimchuk, James A.; Luna, Manuel Bibcode: 2019ApJ...884...68K Altcode: 2019arXiv190509767K Thermal nonequilibrium (TNE) is a fascinating situation that occurs in coronal magnetic flux tubes (loops) for which no solution to the steady-state fluid equations exists. The plasma is constantly evolving even though the heating that produces the hot temperatures does not. This is a promising explanation for isolated phenomena such as prominences, coronal rain, and long-period pulsating loops, but it may also have much broader relevance. As known for some time, TNE requires that the heating be both (quasi-)steady and concentrated at low coronal altitudes. Recent studies indicate that asymmetries are also important, with large enough asymmetries in the heating and/or cross-sectional area resulting in steady flow rather than TNE. Using reasonable approximations, we have derived two formulae for quantifying the conditions necessary for TNE. As a rough rule of thumb, the ratio of apex to footpoint heating rates must be less than about 0.1, and asymmetries must be less than about a factor of 3. The precise values are case-dependent. We have tested our formulae with 1D hydrodynamic loop simulations and find a very acceptable agreement. These results are important for developing physical insight about TNE and assessing how widespread it may be on the Sun. Title: Achievements of Hinode in the first eleven years Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick; Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks, David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio; Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra, Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.; Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao, Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota, Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu, Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin; Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young, Peter R. Bibcode: 2019PASJ...71R...1H Altcode: Hinode is Japan's third solar mission following Hinotori (1981-1982) and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in operation currently. Hinode carries three instruments: the Solar Optical Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These instruments were built under international collaboration with the National Aeronautics and Space Administration and the UK Science and Technology Facilities Council, and its operation has been contributed to by the European Space Agency and the Norwegian Space Center. After describing the satellite operations and giving a performance evaluation of the three instruments, reviews are presented on major scientific discoveries by Hinode in the first eleven years (one solar cycle long) of its operation. This review article concludes with future prospects for solar physics research based on the achievements of Hinode. 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: Frequency Agile Solar Radiotelescope Authors: Bastian, Tim; Bain, H.; Bradley, R.; Chen, B.; Dahlin, J.; DeLuca, E.; Drake, J.; Fleishman, G.; Gary, D.; Glesener, L.; Guo, Fan; Hallinan, G.; Hurford, G.; Kasper, J.; Ji, Hantao; Klimchuk, J.; Kobelski, A.; Krucker, S.; Kuroda, N.; Loncope, D.; Lonsdale, C.; McTiernan, J.; Nita, G.; Qiu, J.; Reeves, K.; Saint-Hilaire, P.; Schonfeld, S.; Shen, Chengcai; Tun, S.; Wertheimer, D.; White, S. Bibcode: 2019astro2020U..56B Altcode: We describe the science objectives and technical requirements for a re-scoped Frequency Agile Solar Radiotelescope (FASR). FASR fulfills a long term community need for a ground-based, solar-dedicated, radio telescope - a next-generation radioheliograph - designed to perform ultra-broadband imaging spectropolarimetry. Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener, Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Gburek, Szymon; Steslicki, Marek; Allred, Joel C.; Battaglia, Marina; Baumgartner, Wayne H.; Drake, James; Goetz, Keith; Grefenstette, Brian; Hannah, Iain; Holman, Gordon D.; Inglis, Andrew; Ireland, Jack; Klimchuk, James A.; Ishikawa, Shin-Nosuke; Kontar, Eduard; Massone, Anna-maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard A.; Woods, Thomas N.; Chen, Bin; Gary, Dale E.; Hudson, Hugh S.; Kowalski, Adam; Warmuth, Alexander; White, Stephen M.; Veronig, Astrid; Vilmer, Nicole Bibcode: 2019AAS...23422501C Altcode: The Focusing Optics X-ray Solar Imager (FOXSI), a SMEX mission concept in Phase A, is the first-ever solar-dedicated, direct-imaging, hard X-ray telescope. FOXSI provides a revolutionary new approach to viewing explosive magnetic-energy release on the Sun by detecting signatures of accelerated electrons and hot plasma directly in and near the energy-release sites of solar eruptive events (e.g., solar flares). FOXSI's primary science objective is to understand the mystery of how impulsive energy release leads to solar eruptions, the primary drivers of space weather at Earth, and how those eruptions are energized and evolve. FOXSI addresses three important science questions: (1) How are particles accelerated at the Sun? (2) How do solar plasmas get heated to high temperatures? (3) How does magnetic energy released on the Sun lead to flares and eruptions? These fundamental physics questions are key to our understanding of phenomena throughout the Universe from planetary magnetospheres to black hole accretion disks. FOXSI measures the energy distributions and spatial structure of accelerated electrons throughout solar eruptive events for the first time by directly focusing hard X-rays from the Sun. This naturally enables high imaging dynamic range, while previous instruments have typically been blinded by bright emission. FOXSI provides 20-100 times more sensitivity as well as 20 times faster imaging spectroscopy than previously available, probing physically relevant timescales (<1 second) never before accessible. FOXSI's launch in July 2022 is aligned with the peak of the 11-year solar cycle, enabling FOXSI to observe the many large solar eruptions that are expected to take place throughout its two-year mission. Title: Constraints from Hinode/EIS on the Expansion of Active Region Loops Along the Line of Sight Authors: Kucera, Therese A.; Young, Peter R.; Klimchuk, James A.; DeForest, Craig Bibcode: 2019AAS...23411706K Altcode: We explore the constraints that can be placed on the dimensions of coronal loops out of the plane of the sky by utilizing spectroscopic observations from the Hinode/EUV Imaging Spectrometer (EIS). The usual assumption is that loop cross sections are circular. Changes in intensity not constant with the measured width are assumed to be the result of changing density and/or filling factor. Here we instead focus on the possibility that the loop dimensions may be changing along the line of sight while the filling factor remains constant. We apply these ideas to two cool (5.5<logT<6.2) loops observed by EIS with supporting observations from Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA) and the Solar TErrestrial RElations Observatory-A's Extreme Ultraviolet Imager (STEREO-A/EUVI). Our results are generally consistent with non-expanding loops, but allow for line-of-sight expansion factors up to 3-4. The uncertainties are sizable and are driven by count rate statistics, radiometric calibration of EIS, and the selection of the loop backgrounds. Title: Ultrahigh-Resolution Imaging of the Solar Corona using a Distributed Diffractive Telescope Authors: Rabin, Douglas M.; Daw, Adrian N.; Denis, Kevin; Kamalabadi, Farzad; Klimchuk, James A. Bibcode: 2019AAS...23410704R Altcode: Several observational and theoretical considerations suggest that energy is often released in the solar corona on small spatial scales of order 100 km. It has been a longstanding goal of solar physics to subject this hypothesis to direct observational test. However, extreme ultraviolet (EUV) and soft x-ray (SXR) telescopes rarely approach diffraction-limited performance because conventional reflective optics of adequate size typically cannot be manufactured to the requisite figure accuracy. Diffractive optics can overcome the angular-resolution limitations of EUV/SXR mirrors. We describe a mission approach that employs diffractive optics and small satellites flying in formation to form a distributed solar telescope operating at EUV wavelengths. Title: Study of Type III Radio Bursts in Nanoflares Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M.; Gary, Dale E. Bibcode: 2019shin.confE..12C Altcode: The heating mechanisms responsible for the million-degree solar corona remain one of the most intriguing problems in space science. It is widely agreed, that the ubiquitous presence of reconnection events and the associated impulsive heating (nanoflares) are a strong candidate in solving this problem [Klimchuk J.A., 2015 and references therein].

Whether nanoflares accelerate energetic particles like full-sized flares is unknown. The lack of strong emission in hard X-rays suggests that the quantity of highly energetic particles is small. There could, however, be large numbers of mildly energetic particles ( 10 keV). We investigate such particles by searching for the type III radio bursts that they may produce. If energetic electron beams propagating along magnetic field lines generate a bump-on-tail instability, they will produce Langmuir waves, which can then interact with other particles and waves to give rise to emission at the local plasma frequency and its first harmonic. Type III bursts are characteristically known to exhibit high frequency drifts as the beam propagates through a density gradient. The time-lag technique that was developed to study subtle delays in light curves from different EUV channels [Viall & Klimchuk 2012] can also be used to detect subtle delays at different radio frequencies. We have modeled the expected radio emission from nanoflares, which we used to test and calibrate the technique. We have begun applying the technique to actual radio observations from VLA (Very Large Array) and seeking data from MWA (Murchison Widefield Array) as well. We also plan to use data from the PSP(Parker Solar Probe) to look for similar reconnection signatures in the Solar Wind. Our goal is to determine whether nanoflares accelerate energetic particles and to determine their properties. The results will have important implications for both the particle acceleration and reconnection physics." Title: Studying Coronal Heating with Data Driven Active Region Modeling Authors: Schonfeld, Samuel J.; Klimchuk, James Bibcode: 2019shin.confE.158S Altcode: Successfully forecasting coronal emission requires an accurate understanding of the mechanism(s) that heat the corona to temperatures well above a million degrees Kelvin. We explore the relevance of a range of coronal heating models by varying the physical parameters needed to simulate EUV emission from an active region. We use GX Simulator to generate synthetic images from non-linear force-free field (NLFFF) magnetic field extrapolations that are filled with plasma using EBTEL (Enthalpy-Based Thermal Evolution of Loops) hydrodynamic simulations and compare these with AIA images. Modifying the heating function, heating event recurrence time, and treatment of the transition region allows us to quantify the effects of these parameters on the observed coronal emission. We discuss the implications of these findings on the nature of heating properties in the magnetically closed corona. 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: ICSF: Intensity Conserving Spectral Fitting Authors: Klimchuk, James A.; Patsourakos, Spiros; Tripathi, Durgesh Bibcode: 2019ascl.soft03007K Altcode: ICSF (Intensity Conserving Spectral Fitting) "corrects" (x,y) data in which the ordinate represents the average of a quantity over a finite interval in the abscissa. A typical example is spectral data, where the average intensity over a wavelength bin (the measured quantity) is assigned to the center of the bin. If the profile is curved, the average will be different from the discrete value at the bin center location. ICSF, written in IDL and available separately and as part of SolarSoft (ascl:1208.013), corrects the intensity using an iterative procedure and cubic spline. The corrected intensity equals the "true" intensity at bin center, rather than the average over the bin. Unlike other methods that are restricted to a single fitting function, typically a spline, ICSF can be used with any function, such as a cubic spline or a Gaussian, with slight changes to the code. Title: Hard X-Ray Constraints on Small-scale Coronal Heating Events Authors: Marsh, Andrew J.; Smith, David M.; Glesener, Lindsay; Klimchuk, James A.; Bradshaw, Stephen J.; Vievering, Juliana; Hannah, Iain G.; Christe, Steven; Ishikawa, Shin-nosuke; Krucker, Säm Bibcode: 2018ApJ...864....5M Altcode: 2018arXiv180802630M Much evidence suggests that the solar corona is heated impulsively, meaning that nanoflares may be ubiquitous in quiet and active regions (ARs). Hard X-ray (HXR) observations with unprecedented sensitivity >3 keV are now enabled by focusing instruments. We analyzed data from the Focusing Optics X-ray Solar Imager (FOXSI) rocket and the Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft to constrain properties of AR nanoflares simulated by the EBTEL field-line-averaged hydrodynamics code. We generated model X-ray spectra by computing differential emission measures for homogeneous nanoflare sequences with heating amplitudes H 0, durations τ, delay times between events t N , and filling factors f. The single quiescent AR observed by FOXSI-2 on 2014 December 11 is well fit by nanoflare sequences with heating amplitudes 0.02 erg cm-3 s-1 <H 0 < 13 erg cm-3 s-1 and a wide range of delay times and durations. We exclude delays between events shorter than ∼900 s at the 90% confidence level for this region. Three of five regions observed by NuSTAR on 2014 November 1 are well fit by homogeneous nanoflare models, while two regions with higher fluxes are not. Generally, the NuSTAR count spectra are well fit by nanoflare sequences with smaller heating amplitudes, shorter delays, and shorter durations than the allowed FOXSI-2 models. These apparent discrepancies are likely due to differences in spectral coverage between the two instruments and intrinsic differences among the regions. Steady heating (t N = τ) was ruled out with >99% confidence for all regions observed by either instrument. Title: Shifting and broadening of coronal spectral lines by nanoflare heating Authors: López Fuentes, M.; Klimchuk, J. A. Bibcode: 2018BAAA...60..207L Altcode: The heating of the solar corona by nanoflares is one of the most succesful theories in recent times to explain the wide variety of available observations. In a recent work we developed and analyzed a model based on coronal loops formed by elemental magnetic strands that interact with each other, reconnecting and consequently heating the plasma. Each of these heating events (or cluster of linked events) is considered a nanoflare. In a later article we showed that the model reproduces the main statistical characteristics of observed coronal loop lightcurves and the typical emission measure distributions obtained from observations. One of the predictions of the model is the presence of intense plasma flows in the heated strands. The summed radiative emission of strands at different temperatures and velocities produce spectral lines of coronal ions with characteristic broadenings and Doppler shifts. In this work, we study the contribution to emission of flows with different temperatures and velocities obtained with our model and construct synthetic spectral lines that we plan to compare in the future with observational results. Title: Study of Type III Radio Bursts in Nanoflares Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M. Bibcode: 2018shin.confE..18C Altcode: The heating mechanisms responsible for the million-degree solar corona remain one of the most intriguing problems in space science. It is widely agreed, that the ubiquitous presence of reconnection events and the associated impulsive heating (nanoflares) are a strong candidate in solving this problem [Klimchuk J.A., 2015 and references therein]. Title: Power-Law Statistics of Driven Reconnection in the Magnetically Closed Corona Authors: Knizhnik, Kalman Joshua; Uritsky, Vadim M.; Klimchuk, James A.; DeVore, C. Richard Bibcode: 2018tess.conf21164K Altcode: Numerous observations have revealed that power-law distributions are ubiquitous in energetic solar processes. Hard X-rays, soft X-rays, extreme ultraviolet radiation, and radio waves all display power-law frequency distributions. Since magnetic reconnection is the driving mechanism for many energetic solar phenomena, it is likely that reconnection events themselves display such power-law distributions. In this work, we perform numerical simulations of the solar corona driven by simple convective motions at the photospheric level. Using temperature changes, current distributions, and Poynting fluxes as proxies for heating, we demonstrate that energetic events occurring in our simulation display power-law frequency distributions, with slopes in good agreement with observations. We suggest that the braiding-associated reconnection in the corona can be understood in terms of a self-organized criticality model driven by convective rotational motions similar to those observed at the photosphere. Title: The Onset of Magnetic Reconnection in the Solar Corona Authors: Leake, James Edward; Daldorff, Lars K. S.; Klimchuk, James A.; Knizhnik, Kalman Joshua Bibcode: 2018tess.conf10418L Altcode: Understanding why the release of magnetic energy on the Sun only occurs after substantial stresses have been built up is a fundamental question. We investigate the onset of magnetic reconnection in the corona, initially focusing on the linear growth and non-linear interaction of both parallel and oblique tearing modes in 3D sheared coronal current sheets. We then study the critical parameters of current sheet thickness and magnetic field angle. We also investigate the release of magnetic energy in the corona by magnetic reconnection driven by realistic surface motions. Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener, Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel C.; Chen, Bin; Battaglia, Marina; Drake, James Frederick; Gary, Dale E.; Goetz, Keith; Gburek, Szymon; Grefenstette, Brian; Hannah, Iain G.; Holman, Gordon; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa, Shin-nosuke; Klimchuk, James A.; Kontar, Eduard; Kowalski, Adam F.; Massone, Anna Maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard; Steslicki, Marek; Ryan, Daniel; Warmuth, Alexander; Veronig, Astrid; Vilmer, Nicole; White, Stephen M.; Woods, Thomas N. Bibcode: 2018tess.conf40444C Altcode: We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer (SMEX) Heliophysics mission that is currently undergoing a Phase A concept study. FOXSI will provide a revolutionary new perspective on energy release and particle acceleration on the Sun. FOXSI's primary instrument, the Direct Spectroscopic Imager (DSI), is a direct imaging X-ray spectrometer with higher dynamic range and better than 10x the sensitivity of previous instruments. Flown on a 3-axis-stabilized spacecraft in low-Earth orbit, DSI uses high-angular-resolution grazing-incidence focusing optics combined with state-of-the-art pixelated solid-state detectors to provide direct imaging of solar hard X-rays for the first time. DSI is composed of a pair of X-ray telescopes with a 14-meter focal length enabled by a deployable boom. DSI has a field of view of 9 arcminutes and an angular resolution of better than 8 arcsec FWHM; it will cover the energy range from 3 up to 50-70 keV with a spectral resolution of better than 1 keV. DSI will measure each photon individually and will be able to create useful images at a sub-second temporal resolution. FOXSI will also measure soft x-ray emission down to 0.8 keV with a 0.25 keV resolution with its secondary instrument, the Spectrometer for Temperature and Composition (STC) provided by the Polish Academy of Sciences. Making use of an attenuator-wheel and high-rate-capable detectors, FOXSI will be able to observe the largest flares without saturation while still maintaining the sensitivity to detect X-ray emission from weak flares, escaping electrons, and hot active regions. This presentation will cover the data products and software that can be expected from FOXSI and how they could be used by the community. Title: The Case for Spectroscopic Observations of Very Hot Plasmas Authors: Klimchuk, James A.; Daldorff, Lars K. S.; Liu, Yi-Hsin; Brosius, Jeffrey W.; Daw, Adrian Nigel; Leake, James Edward Bibcode: 2018tess.conf11003K Altcode: Explosive magnetic energy release is responsible for many solar phenomena, ranging from coronal heating, to jets, to CMEs and flares. Despite its importance, many of the basic details of how this works are still not well established. In order to make significant progress, we must observe the actual energy release process. That means measuring plasmas at high temperatures (> 5 MK). Most existing coronal observations are of plasma that has cooled dramatically, after vital information has been lost, or, worse yet, plasma that has evaporated from the chromosphere and is only an indirect bi-product of the energy release. Spectroscopic observations are especially valuable because of their potential for diagnosing flows, temperatures, and densities. Furthermore, only with spectroscopy can we disentangle the disparate plasmas that are invariably present along optically-thin lines-of-sight. I will discuss these points in more detail, review the pros and cons of different wavelength regimes, and present preliminary results on synthetic line profiles from a particle-in-cell (PIC) simulation of magnetic reconnection. Title: The Role of Asymmetries in Thermal Non-Equilibrium Authors: Klimchuk, James A.; Luna Bennasar, Manuel Bibcode: 2018tess.conf22205K Altcode: Thermal non-equilibrium (TNE) is a fascinating situation that applies when quasi-steady coronal heating, including impulsive heating with a high repetition frequency, is concentrated at sufficiently low altitudes in the corona. No equilibrium exists, and as the atmosphere "searches" for one, it undergoes convulsions that typically involve the formation of a cold condensation. This is the likely explanation of prominences and coronal rain. Under the right conditions, a condensation does not fully form, and it has been suggested that such conditions may be rather common. Asymmetries in heating or flux tube cross-sectional area can have a major impact on the behavior. If the asymmetries are large enough, a steady flow develops rather than TNE. We here present an analytical formula for predicting steady flow vs. TNE, and we compare it with 1D hydrodynamic simulations. Title: Magnetic Braids in Eruptions of a Spiral Structure in the Solar Atmosphere Authors: Huang, Zhenghua; Xia, Lidong; Nelson, Chris J.; Liu, Jiajia; Wiegelmann, Thomas; Tian, Hui; Klimchuk, James A.; Chen, Yao; Li, Bo Bibcode: 2018ApJ...854...80H Altcode: 2018arXiv180105967H We report on high-resolution imaging and spectral observations of eruptions of a spiral structure in the transition region, which were taken with the Interface Region Imaging Spectrograph, and the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The eruption coincided with the appearance of two series of jets, with velocities comparable to the Alfvén speeds in their footpoints. Several pieces of evidence of magnetic braiding in the eruption are revealed, including localized bright knots, multiple well-separated jet threads, transition region explosive events, and the fact that all three of these are falling into the same locations within the eruptive structures. Through analysis of the extrapolated 3D magnetic field in the region, we found that the eruptive spiral structure corresponded well to locations of twisted magnetic flux tubes with varying curl values along their lengths. The eruption occurred where strong parallel currents, high squashing factors, and large twist numbers were obtained. The electron number density of the eruptive structure is found to be ∼3 × 1012 cm-3, indicating that a significant amount of mass could be pumped into the corona by the jets. Following the eruption, the extrapolations revealed a set of seemingly relaxed loops, which were visible in the AIA 94 Å channel, indicating temperatures of around 6.3 MK. With these observations, we suggest that magnetic braiding could be part of the mechanisms explaining the formation of solar eruption and the mass and energy supplement to the corona. Title: Power-law Statistics of Driven Reconnection in the Magnetically Closed Corona Authors: Knizhnik, K. J.; Uritsky, V. M.; Klimchuk, J. A.; DeVore, C. R. Bibcode: 2018ApJ...853...82K Altcode: 2018arXiv180105245K Numerous observations have revealed that power-law distributions are ubiquitous in energetic solar processes. Hard X-rays, soft X-rays, extreme ultraviolet radiation, and radio waves all display power-law frequency distributions. Since magnetic reconnection is the driving mechanism for many energetic solar phenomena, it is likely that reconnection events themselves display such power-law distributions. In this work, we perform numerical simulations of the solar corona driven by simple convective motions at the photospheric level. Using temperature changes, current distributions, and Poynting fluxes as proxies for heating, we demonstrate that energetic events occurring in our simulation display power-law frequency distributions, with slopes in good agreement with observations. We suggest that the braiding-associated reconnection in the corona can be understood in terms of a self-organized criticality model driven by convective rotational motions similar to those observed at the photosphere. Title: Dressing the Coronal Magnetic Extrapolations of Active Regions with a Parameterized Thermal Structure Authors: Nita, Gelu M.; Viall, Nicholeen M.; Klimchuk, James A.; Loukitcheva, Maria A.; Gary, Dale E.; Kuznetsov, Alexey A.; Fleishman, Gregory D. Bibcode: 2018ApJ...853...66N Altcode: The study of time-dependent solar active region (AR) morphology and its relation to eruptive events requires analysis of imaging data obtained in multiple wavelength domains with differing spatial and time resolution, ideally in combination with 3D physical models. To facilitate this goal, we have undertaken a major enhancement of our IDL-based simulation tool, GX_Simulator, previously developed for modeling microwave and X-ray emission from flaring loops, to allow it to simulate quiescent emission from solar ARs. The framework includes new tools for building the atmospheric model and enhanced routines for calculating emission that include new wavelengths. In this paper, we use our upgraded tool to model and analyze an AR and compare the synthetic emission maps with observations. We conclude that the modeled magneto-thermal structure is a reasonably good approximation of the real one. Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.; Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Gburek, S.; Grefenstette, B.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Schwartz, R.; Steslicki, M.; Turin, P.; Ryan, D.; Warmuth, A.; Veronig, A.; Vilmer, N.; White, S. M.; Woods, T. N. Bibcode: 2017AGUFMSH44A..07C Altcode: We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer (SMEX) Heliophysics mission that is currently undergoing a Phase A concept study. FOXSI will provide a revolutionary new perspective on energy release and particle acceleration on the Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic range and better than 10x the sensitivity of previous instruments. Flown on a 3-axis-stabilized spacecraft in low-Earth orbit, FOXSI uses high-angular-resolution grazing-incidence focusing optics combined with state-of-the-art pixelated solid-state detectors to provide direct imaging of solar hard X-rays for the first time. FOXSI is composed of a pair of x-ray telescopes with a 14-meter focal length enabled by a deployable boom. Making use of a filter-wheel and high-rate-capable solid-state detectors, FOXSI will be able to observe the largest flares without saturation while still maintaining the sensitivity to detect x-ray emission from weak flares, escaping electrons, and hot active regions. This mission concept is made possible by past experience with similar instruments on two FOXSI sounding rocket flights, in 2012 and 2014, and on the HEROES balloon flight in 2013. FOXSI's hard X-ray imager has a field of view of 9 arcminutes and an angular resolution of better than 8 arcsec; it will cover the energy range from 3 up to 50-70 keV with a spectral resolution of better than 1 keV; and it will have sub-second temporal resolution. Title: Anticipated Results from the FOXSI SMEX Mission Authors: Shih, A. Y.; Christe, S.; Krucker, S.; Glesener, L.; Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.; Drake, J. F.; Gary, D. E.; Gburek, S.; Goetz, K.; Grefenstette, B.; Gubarev, M.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Ryan, D.; Schwartz, R.; Steslicki, M.; Turin, P.; Veronig, A.; Vilmer, N.; Warmuth, A.; White, S. M.; Woods, T. N. Bibcode: 2017AGUFMSH43C..03S Altcode: While there have been significant advances in our understanding of impulsive energy release at the Sun since the advent of RHESSI observations, there is a clear need for new X-ray observations that can capture the full range of emission in flares (e.g., faint coronal sources near bright chromospheric sources), follow the intricate evolution of energy release and changes in morphology, and search for the signatures of impulsive energy release in even the quiescent Sun. The FOXSI Small Explorer (SMEX) mission, currently undergoing a Phase A concept study, combines state-of-the-art grazing-incidence focusing optics with pixelated solid-state detectors to provide direct imaging of hard X-rays for the first time on a solar observatory. FOXSI's X-ray observations will provide quantitative information on (1) the non-thermal populations of accelerated electrons and (2) the thermal plasma distributions at the high temperatures inaccessible through other wavelengths. FOXSI's major science questions include: Where are electrons accelerated and on what time scales? Where do escaping flare-accelerated electrons originate? What is the energy input of accelerated electrons into the chromosphere and corona? How much do flare-like processes heat the corona above active regions? Here we present examples with simulated observations to show how FOXSI's capabilities will address and resolve these and other questions. Title: Conditions for Thermal Non-Equilibrium Authors: Klimchuk, J. A.; Luna Bennasar, M. Bibcode: 2017AGUFMSH43A2798K Altcode: Thermal non-equilibrium (TNE) is a fascinating phenomenon in which perfectly steady heating produces inherently dynamic behavior in coronal loops. Sometimes, but not always, a condensation of cold ( 10,000 K) material is produced. This is the likely explanation of coronal rain, prominences, and "pulsating loops," and has been suggested to have additional application in active regions and even helmet streamers. A primary requirement of TNE is that steady heating must be concentrated at low altitudes in the corona. Another requirement is that asymmetries in the heating and/or loop cross sectional area cannot be too great. We present theoretical formulas for predicting whether TNE will occur and whether the associated condensations will be full or incomplete. The predictions are compared against hydrodynamic simulations. Title: The Onset of Magnetic Reconnection Authors: Daldorff, L. K. S.; Klimchuk, J. A.; Leake, J. E.; Knizhnik, K. J. Bibcode: 2017AGUFMSH11B2454D Altcode: A fundamental question concerning the release of the magnetic energy on the Sun is why it occurs only after substantial stresses have built up in the field. If reconnection were to occur readily, the released energy would be insufficient to explain coronal heating, CMEs, flares, jets, spicules, etc. How can we explain this switch-on property? What is the physical nature of the onset conditions? One idea involves the ``secondary instability'' of current sheets, which switches on when the rotation of the magnetic field across a current sheet reaches a critical angle. Such conditions would occur, for example, at the boundaries of flux tubes that become tangled and twisted by turbulent photospheric convection. Other ideas involve a critical thickness for the current sheet. Our ultimate goal is to simulate the evolution of a current sheet that is driven by shear flows at the photopheric boundary and extends upward through the chromosphere and transition region and into the corona. We report here on the status of our investigation into this fundamental problem. Title: Nanoflare Heating: Observations and Theory Authors: Klimchuk, James A. Bibcode: 2017arXiv170907320K Altcode: This is a review of the observational and theoretical evidence for nanoflare heating of the magnetically-closed corona. 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: Diagnosing Coronal Heating in a Survey of Active Regions using the Time Lag Method Authors: Viall, Nicholeen; Klimchuk, James A. Bibcode: 2017SPD....4840202V Altcode: In this paper we examine 15 different active regions observed with the Solar Dynamics Observatory and analyze their nanoflare properties using the time lag method. The time lag method is a diagnostic of whether the plasma is maintained at a steady temperature, or if it is dynamic, undergoing heating and cooling cycles. An important aspect of our technique is that it analyses both observationally distinct coronal loops as well as the much more prevalent diffuse emission surrounding them. Warren et al. (2012) first studied these same 15 active regions, which are all quiescent and exhibit a broad range of characteristics, including age, total unsigned magnetic flux, area, hot emission, and emission measure distribution. We find that widespread cooling is a generic property of both loop and diffuse emission from all 15 active regions. However, the range of temperatures through which the plasma cools varies between active regions and within each active region, and only occasionally is there full cooling from above 7 MK to well below 1 MK. We find that the degree of cooling is not well correlated with slopes of the emission measure distribution measured by Warren et al. (2012). We show that these apparently contradictory observations can be reconciled with the presence of a distribution of nanoflare energies and frequencies along the line of sight, with the average delay between successive nanoflare events on a single flux tube being comparable to the plasma cooling timescale. Warren, H. P., Winebarger, A. R., & Brooks, D. H. 2012, ApJ, 759, 141 Title: Constraints on Nonuniform Expansion in Coronal Loops Authors: Kucera, Therese A.; DeForest, Craig; Klimchuk, James A.; Young, Peter R. Bibcode: 2017SPD....4810608K Altcode: We use measurements of coronal loop properties to constrain the hypothesis that coronal loops expand differently in different directions. A long standing problem in understanding coronal loops is that although the magnetic field is expected to expand with altitude and does indeed seem to do so on scales of active regions, individual loops seem to have fairly uniform diameters along the length of the loop. Malanushenko & Schrijver (2013) have suggested that loops may be expanding, but with a non-circular cross section. In this scenario a loop might have a constant width in the plane of the sky, but expand along the line of sight. Furthermore, such loops might be easier to see from the point of view that does not show expansion. We use Hinode/EIS and SDO/AIA data to measure loop intensities, electron densities, temperatures and dimensions in order to determine the extent to which loops may be expanding along the line of sight. Title: The Onset of Magnetic Reconnection: Tearing Instability in Current Sheets with a Guide Field Authors: Daldorff, Lars K. S.; Klimchuk, James A.; Leake, James E.; Knizhnik, Kalman Bibcode: 2017SPD....4810616D Altcode: Magnetic reconnection is fundamental to many solar phenomena, ranging from coronal heating, to jets, to flares and CMEs. A poorly understood yet crucial aspect of reconnection is that it does not occur until magnetic stresses have built to sufficiently high levels for significant energy release. If reconnection were to happen too soon, coronal heating would be weak and flares would be small. As part of our program to study the onset conditions for magnetic reconnection, we have investigated the instability of current sheets to tearing. Surprisingly little work has been done on this problem for sheets that include a guide field, i.e., for which the field rotates by less than 180 degrees. This is the most common situation on the Sun. We present numerical 3D resistive MHD simulations of several sheets and show how the behavior depends on the shear angle (rotation). We compare our results to the predictions of linear theory and discuss the nonlinear evolution in terms of plasmoid formation and the interaction of different oblique tearing modes. The relevance to the Sun is explained. Title: Hard X-Ray Constraints on Small-Scale Coronal Heating Events Authors: Marsh, Andrew; Smith, David M.; Glesener, Lindsay; Klimchuk, James A.; Bradshaw, Stephen; Hannah, Iain; Vievering, Juliana; Ishikawa, Shin-Nosuke; Krucker, Sam; Christe, Steven Bibcode: 2017SPD....4810614M Altcode: A large body of evidence suggests that the solar corona is heated impulsively. Small-scale heating events known as nanoflares may be ubiquitous in quiet and active regions of the Sun. Hard X-ray (HXR) observations with unprecedented sensitivity >3 keV have recently been enabled through the use of focusing optics. We analyze active region spectra from the FOXSI-2 sounding rocket and the NuSTAR satellite to constrain the physical properties of nanoflares simulated with the EBTEL field-line-averaged hydrodynamics code. We model a wide range of X-ray spectra by varying the nanoflare heating amplitude, duration, delay time, and filling factor. Additional constraints on the nanoflare parameter space are determined from energy constraints and EUV/SXR data. Title: A Survey of Nanoflare Properties in Active Regions Observed with the Solar Dynamics Observatory Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2017ApJ...842..108V Altcode: In this paper, we examine 15 different active regions (ARs) observed with the Solar Dynamics Observatory and analyze their nanoflare properties. We have recently developed a technique that systematically identifies and measures plasma temperature dynamics by computing time lags between light curves. The time lag method tests whether the plasma is maintained at a steady temperature, or if it is dynamic, undergoing heating and cooling cycles. An important aspect of our technique is that it analyzes both observationally distinct coronal loops as well as the much more prevalent diffuse emission between them. We find that the widespread cooling reported previously for NOAA AR 11082 is a generic property of all ARs. The results are consistent with impulsive nanoflare heating followed by slower cooling. Only occasionally, however, is there full cooling from above 7 MK to well below 1 MK. More often, the plasma cools to approximately 1-2 MK before being reheated by another nanoflare. These same 15 ARs were first studied by Warren et al. We find that the degree of cooling is not well correlated with the reported slopes of the emission measure distribution. We also conclude that the Fe xviii emitting plasma that they measured is mostly in a state of cooling. These results support the idea that nanoflares have a distribution of energies and frequencies, with the average delay between successive events on an individual flux tube being comparable to the plasma cooling timescale. Title: Exploring impulsive solar magnetic energy release and particle acceleration with focused hard X-ray imaging spectroscopy Authors: Christe, Steven; Krucker, Samuel; Glesener, Lindsay; Shih, Albert; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel; Battaglia, Marina; Chen, Bin; Drake, James; Dennis, Brian; Gary, Dale; Gburek, Szymon; Goetz, Keith; Grefenstette, Brian; Gubarev, Mikhail; Hannah, Iain; Holman, Gordon; Hudson, Hugh; Inglis, Andrew; Ireland, Jack; Ishikawa, Shinosuke; Klimchuk, James; Kontar, Eduard; Kowalski, Adam; Longcope, Dana; Massone, Anna-Maria; Musset, Sophie; Piana, Michele; Ramsey, Brian; Ryan, Daniel; Schwartz, Richard; Stęślicki, Marek; Turin, Paul; Warmuth, Alexander; Wilson-Hodge, Colleen; White, Stephen; Veronig, Astrid; Vilmer, Nicole; Woods, Tom Bibcode: 2017arXiv170100792C Altcode: How impulsive magnetic energy release leads to solar eruptions and how those eruptions are energized and evolve are vital unsolved problems in Heliophysics. The standard model for solar eruptions summarizes our current understanding of these events. Magnetic energy in the corona is released through drastic restructuring of the magnetic field via reconnection. Electrons and ions are then accelerated by poorly understood processes. Theories include contracting loops, merging magnetic islands, stochastic acceleration, and turbulence at shocks, among others. Although this basic model is well established, the fundamental physics is poorly understood. HXR observations using grazing-incidence focusing optics can now probe all of the key regions of the standard model. These include two above-the-looptop (ALT) sources which bookend the reconnection region and are likely the sites of particle acceleration and direct heating. The science achievable by a direct HXR imaging instrument can be summarized by the following science questions and objectives which are some of the most outstanding issues in solar physics (1) How are particles accelerated at the Sun? (1a) Where are electrons accelerated and on what time scales? (1b) What fraction of electrons is accelerated out of the ambient medium? (2) How does magnetic energy release on the Sun lead to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI) instrument, which can be built now using proven technology and at modest cost, would enable revolutionary advancements in our understanding of impulsive magnetic energy release and particle acceleration, a process which is known to occur at the Sun but also throughout the Universe. Title: Solving the Coronal Heating Problem using X-ray Microcalorimeters Authors: Christe, Steven; Bandler, Simon; DeLuca, Edward; Caspi, Amir; Golub, Leon; Smith, Randall; Allred, Joel; Brosius, Jeffrey W.; Dennis, Brian; Klimchuk, James Bibcode: 2017arXiv170100795C Altcode: Even in the absence of resolved flares, the corona is heated to several million degrees. However, despite its importance for the structure, dynamics, and evolution of the solar atmosphere, the origin of this heating remains poorly understood. Several observational and theoretical considerations suggest that the heating is driven by small, impulsive energy bursts which could be Parker-style "nanoflares" (Parker 1988) that arise via reconnection within the tangled and twisted coronal magnetic field. The classical "smoking gun" (Klimchuk 2009; Cargill et al. 2013) for impulsive heating is the direct detection of widespread hot plasma (T > 6 MK) with a low emission measure. In recent years there has been great progress in the development of Transition Edge Sensor (TES) X-ray microcalorimeters that make them more ideal for studying the Sun. When combined with grazing-incidence focusing optics, they provide direct spectroscopic imaging over a broad energy band (0.5 to 10 keV) combined with extremely impressive energy resolution in small pixels, as low as 0.7 eV (FWHM) at 1.5 keV (Lee 2015), and 1.56 eV (FWHM) at 6 keV (Smith 2012), two orders of magnitude better than the current best traditional solid state photon-counting spectrometers. Decisive observations of the hot plasma associated with nanoflare models of coronal heating can be provided by new solar microcalorimeters. These measurements will cover the most important part of the coronal spectrum for searching for the nanoflare-related hot plasma and will characterize how much nanoflares can heat the corona both in active regions and the quiet Sun. Finally, microcalorimeters will enable to study all of this as a function of time and space in each pixel simultaneously a capability never before available. Title: Unravelling the Components of a Multi-thermal Coronal Loop using Magnetohydrodynamic Seismology Authors: Krishna Prasad, S.; Jess, D. B.; Klimchuk, J. A.; Banerjee, D. Bibcode: 2017ApJ...834..103K Altcode: 2016arXiv161104011K; 2017ApJ...834..103P Coronal loops, constituting the basic building blocks of the active Sun, serve as primary targets to help understand the mechanisms responsible for maintaining multi-million Kelvin temperatures in the solar and stellar coronae. Despite significant advances in observations and theory, our knowledge on the fundamental properties of these structures is limited. Here, we present unprecedented observations of accelerating slow magnetoacoustic waves along a coronal loop that show differential propagation speeds in two distinct temperature channels, revealing the multi-stranded and multithermal nature of the loop. Utilizing the observed speeds and employing nonlinear force-free magnetic field extrapolations, we derive the actual temperature variation along the loop in both channels, and thus are able to resolve two individual components of the multithermal loop for the first time. The obtained positive temperature gradients indicate uniform heating along the loop, rather than isolated footpoint heating. Title: Hard X-ray Detectability of Small-Scale Coronal Heating Events Authors: Marsh, A.; Glesener, L.; Klimchuk, J. A.; Bradshaw, S. J.; Smith, D. M.; Hannah, I. G. Bibcode: 2016AGUFMSH11D..06M Altcode: The nanoflare heating theory predicts the ubiquitous presence of hot ( >5 MK) plasma in the solar corona, but evidence for this high-temperature component has been scarce. Current hard x-ray instruments such as RHESSI lack the sensitivity to see the trace amounts of this plasma that are predicted by theoretical models. New hard X-ray instruments that use focusing optics, such as FOXSI (the Focusing Optics X-ray Solar Imager) and NuSTAR (the Nuclear Spectroscopic Telescope Array) can extend the visible parameter space of nanoflare "storms" that create hot plasma. We compare active-region data from FOXSI and NuSTAR with a series of EBTEL hydrodynamic simulations, and constrain nanoflare properties to give good agreement with 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: Focusing Solar Hard X-rays: Expected Results from a FOXSI Spacecraft Authors: Glesener, L.; Christe, S.; Shih, A. Y.; Dennis, B. R.; Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Ryan, D.; Inglis, A. R.; Hannah, I. G.; Caspi, A.; Klimchuk, J. A.; Drake, J. F.; Kontar, E.; Holman, G.; White, S. M.; Alaoui, M.; Battaglia, M.; Vilmer, N.; Allred, J. C.; Longcope, D. W.; Gary, D. E.; Jeffrey, N. L. S.; Musset, S.; Swisdak, M. Bibcode: 2016AGUFMSH13A2282G Altcode: Over the course of two solar cycles, RHESSI has examined high-energy processes in flares via high-resolution spectroscopy and imaging of soft and hard X-rays (HXRs). The detected X-rays are the thermal and nonthermal bremsstrahlung from heated coronal plasma and from accelerated electrons, respectively, making them uniquely suited to explore the highest-energy processes that occur in the corona. RHESSI produces images using an indirect, Fourier-based method and has made giant strides in our understanding of these processes, but it has also uncovered intriguing new mysteries regarding energy release location, acceleration mechanisms, and energy propagation in flares. Focusing optics are now available for the HXR regime and stand poised to perform another revolution in the field of high-energy solar physics. With two successful sounding rocket flights completed, the Focusing Optics X-ray Solar Imager (FOXSI) program has demonstrated the feasibility and power of direct solar HXR imaging with its vastly superior sensitivity and dynamic range. Placing this mature technology aboard a spacecraft will offer a systematic way to explore high-energy aspects of the solar corona and to address scientific questions left unanswered by RHESSI. Here we present examples of such questions and show simulations of expected results from a FOXSI spaceborne instrument to demonstrate how these questions can be addressed with the focusing of hard X-rays. Title: The Onset of Magnetic Reconnection: Tearing Instability in Current Sheets with a Guide Field Authors: Daldorff, L. K. S.; Klimchuk, J. A.; Knizhnik, K. J. Bibcode: 2016AGUFMSH51B2590D Altcode: Magnetic reconnection is fundamental to many solar phenomena, ranging from coronal heating, to jets, to flares and CMEs. A poorly understood yet crucial aspect of reconnection is that it does not occur until magnetic stresses have built to sufficiently high levels for significant energy release. If reconnection were to happen too soon, coronal heating would be weak and flares would be small. As part of our program to study the onset conditions for magnetic reconnection, we have investigated the instability of current sheets to tearing. Surprisingly little work has been done on this problem for sheets that include a guide field, i.e., for which the field rotates by less than 180 degrees. This is the most common situation on the Sun. We present numerical 3D resistive MHD simulations of several sheets and show how the behaviour depends on the shear angle (rotation). We compare our results to the predictions of linear theory and discuss the nonlinear evolution in terms of plasmoid formation and the interaction of different oblique tearing modes. The relevance to the Sun is explained. Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.; Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Grefenstette, B.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Gubarev, M.; Schwartz, R. A.; Steslicki, M.; Ryan, D.; Turin, P.; Warmuth, A.; White, S. M.; Veronig, A.; Vilmer, N.; Dennis, B. R. Bibcode: 2016AGUFMSH13A2281C Altcode: We present FOXSI (Focusing Optics X-ray Solar Imager), a recently proposed Small Explorer (SMEX) mission that will provide a revolutionary new perspective on energy release and particle acceleration on the Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic range and better than 10x the sensitivity of previous instruments. Flown on a 3-axis stabilized spacecraft in low-Earth orbit, FOXSI uses high-angular-resolution grazing-incidence focusing optics combined with state-of-the-art pixelated solid-state detectors to provide direct imaging of solar hard X-rays for the first time. FOXSI is composed of two individual x-ray telescopes with a 14-meter focal length enabled by a deployable boom. Making use of a filter-wheel and high-rate-capable solid-state detectors, FOXSI will be able to observe the largest flares without saturation while still maintaining the sensitivity to detect x-ray emission from weak flares, escaping electrons, and hot active regions. This SMEX mission is made possible by past experience with similar instruments on two sounding rocket flights, in 2012 and 2014, and on the HEROES balloon flight in 2013. FOXSI will image the Sun with a field of view of 9 arcminutes and an angular resolution of better than 8 arcsec; it will cover the energy range from 3 to 100 keV with a spectral resolution of better than 1 keV; and it will have sub-second temporal resolution. Title: The FIELDS Instrument Suite for Solar Probe Plus. Measuring the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence, and Radio Signatures of Solar Transients Authors: Bale, S. D.; Goetz, K.; Harvey, P. R.; Turin, P.; Bonnell, J. W.; Dudok de Wit, T.; Ergun, R. E.; MacDowall, R. J.; Pulupa, M.; Andre, M.; Bolton, M.; Bougeret, J. -L.; Bowen, T. A.; Burgess, D.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen, C. H. K.; Choi, M. K.; Connerney, J. E.; Cranmer, S.; Diaz-Aguado, M.; Donakowski, W.; Drake, J. F.; Farrell, W. M.; Fergeau, P.; Fermin, J.; Fischer, J.; Fox, N.; Glaser, D.; Goldstein, M.; Gordon, D.; Hanson, E.; Harris, S. E.; Hayes, L. M.; Hinze, J. J.; Hollweg, J. V.; Horbury, T. S.; Howard, R. A.; Hoxie, V.; Jannet, G.; Karlsson, M.; Kasper, J. C.; Kellogg, P. J.; Kien, M.; Klimchuk, J. A.; Krasnoselskikh, V. V.; Krucker, S.; Lynch, J. J.; Maksimovic, M.; Malaspina, D. M.; Marker, S.; Martin, P.; Martinez-Oliveros, J.; McCauley, J.; McComas, D. J.; McDonald, T.; Meyer-Vernet, N.; Moncuquet, M.; Monson, S. J.; Mozer, F. S.; Murphy, S. D.; Odom, J.; Oliverson, R.; Olson, J.; Parker, E. N.; Pankow, D.; Phan, T.; Quataert, E.; Quinn, T.; Ruplin, S. W.; Salem, C.; Seitz, D.; Sheppard, D. A.; Siy, A.; Stevens, K.; Summers, D.; Szabo, A.; Timofeeva, M.; Vaivads, A.; Velli, M.; Yehle, A.; Werthimer, D.; Wygant, J. R. Bibcode: 2016SSRv..204...49B Altcode: 2016SSRv..tmp...16B NASA's Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products. Title: Towards a Physics-Based Flare Irradiance Model Authors: Hock-Mysliwiec, R. A.; Klimchuk, J. A.; Eparvier, F. G.; Woods, T. N.; Balasubramaniam, K. S. Bibcode: 2016usc..confE..46H Altcode: The Extreme UltraViolet (EUV) irradiance from solar flares is a critical driver of short term variability in the Earth's upper atmosphere. The EUV Variability Experiment (EVE) onboard NASA's Solar Dynamics Observatory (SDO) has been making moderate spectral resolution (0.1 nm), high time cadence (10 s) measurements of the solar EUV irradiance (5-105 nm) since 2010. A key observation from EVE is that flares of the same magnitude at one wavelength (e.g. GOES XRS) have different peak intensities and time profiles in other wavelengths. As it is impractical to measure the entire EUV spectrum with sufficient spectral resolution and temporal cadence to capture these differences for space weather operations, the next generation of flare irradiance models must be able to capture these variations. We have developed a framework for a physics-based flare irradiance model based on the EBTEL model. At present, this Multi-Strand Flare Irradiance Model (MS-FIM) is able to predict EUV lightcurves over a range of coronal temperatures given the lightcurves from two EVE lines as inputs. In this paper, we present an overview of the Multi-Strand Flare Irradiance Model as well as initial results showing its ability to predict the irradiances for a diverse range of flares, including EUV late phase flares. We also describe preliminary efforts to drive the model with parameters derived from images of the flaring region instead of EUV lightcurves. Title: Signatures of Steady Heating in Time Lag Analysis of Coronal Emission Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2016ApJ...828...76V Altcode: 2016arXiv160702008V Among the multitude of methods used to investigate coronal heating, the time lag method of Viall & Klimchuk is becoming increasingly prevalent as an analysis technique that is complementary to those that are traditionally used. The time lag method cross correlates light curves at a given spatial location obtained in spectral bands that sample different temperature plasmas. It has been used most extensively with data from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory. We have previously applied the time lag method to entire active regions and surrounding the quiet Sun and created maps of the results. We find that the majority of time lags are consistent with the cooling of coronal plasma that has been impulsively heated. Additionally, a significant fraction of the map area has a time lag of zero. This does not indicate a lack of variability. Rather, strong variability must be present, and it must occur in phase between the different channels. We have previously shown that these zero time lags are consistent with the transition region response to coronal nanoflares, although other explanations are possible. A common misconception is that the zero time lag indicates steady emission resulting from steady heating. Using simulated and observed light curves, we demonstrate here that highly correlated light curves at zero time lag are not compatible with equilibrium solutions. Such light curves can only be created by evolution. Title: A Nanoflare-based Cellular Automaton Model and the Observed Properties of the Coronal Plasma Authors: López Fuentes, Marcelo; Klimchuk, James A. Bibcode: 2016ApJ...828...86L Altcode: 2016arXiv160703917L We use the cellular automaton model described in López Fuentes & Klimchuk to study the evolution of coronal loop plasmas. The model, based on the idea of a critical misalignment angle in tangled magnetic fields, produces nanoflares of varying frequency with respect to the plasma cooling time. We compare the results of the model with active region (AR) observations obtained with the Hinode/XRT and SDO/AIA instruments. The comparison is based on the statistical properties of synthetic and observed loop light curves. Our results show that the model reproduces the main observational characteristics of the evolution of the plasma in AR coronal loops. The typical intensity fluctuations have amplitudes of 10%-15% both for the model and the observations. The sign of the skewness of the intensity distributions indicates the presence of cooling plasma in the loops. We also study the emission measure (EM) distribution predicted by the model and obtain slopes in log(EM) versus log(T) between 2.7 and 4.3, in agreement with published observational values. Title: The Onset of Magnetic Reconnection: Tearing Instability in Current Sheets with a Guide Field Authors: Daldorff, Lars K. S.; Klimchuk, James A. Bibcode: 2016shin.confE.110D Altcode: Magnetic reconnection is fundamental to many solar phenomena, ranging from coronal heating o jets, to flares and CMEs. A poorly understood yet crucial aspect of reconnection is that it does not occur until magnetic stresses have built to sufficiently high levels for significant energy release. If reconnection were to happen too soon, coronal heating would be weak and flares would be small. As part of our program to study the onset conditions for magnetic reconnection, we have investigated the instability of current sheets to tearing. Surprisingly little work has been done on this problem for sheets that include a guide field, i.e., for which the field rotates by less than 180 degrees. This is the most common situation on the Sun. We present numerical 3D resistive MHD simulations of several sheets and show how the behaviour depends on the shear angle (rotation). We compare our results to the predictions of linear theory and discuss the nonlinear evolution in terms of plasmoid formation and the interaction of different oblique tearing modes. The relevance to the Sun is explained. Title: Hard X-ray Detectability of Small-Scale Coronal Heating Events Authors: Marsh, Andrew; Glesener, Lindsay; Klimchuk, James A.; Bradshaw, Stephen; Smith, David; Hannah, Iain Bibcode: 2016SPD....4720204M Altcode: The nanoflare heating theory predicts the ubiquitous presence of hot (~>5 MK) plasma in the solar corona, but evidence for this high-temperature component has been scarce. Current hard x-ray instruments such as RHESSI lack the sensitivity to see the trace amounts of this plasma that are predicted by theoretical models. New hard X-ray instruments that use focusing optics, such as FOXSI (the Focusing Optics X-ray Solar Imager) and NuSTAR (the Nuclear Spectroscopic Telescope Array) can extend the visible parameter space of nanoflare “storms” that create hot plasma. We compare active-region data from FOXSI and NuSTAR with a series of EBTEL hydrodynamic simulations, and constrain nanoflare properties to give good agreement with observations. Title: The Transition Region Response to a Coronal Nanoflare: Forward Modeling and Observations in SDO/AIA Authors: Viall, Nicholeen; Klimchuk, James A. Bibcode: 2016SPD....4720202V Altcode: The corona and transition region (TR) are fundamentally coupled through the processes of thermal conduction and mass exchange. Yet the temperature-dependent emissions from the two locations behave quite differently in the aftermath of an impulsive heating event such as a coronal nanoflare. In this presentation, we use results from the EBTEL hydrodynamics code to demonstrate that after a coronal nanoflare, the TR is multithermal and the emission at all temperatures responds in unison. This is in contrast to the coronal plasma, which cools sequentially, emitting first at higher temperatures and then at lower temperatures. We apply the time lag technique of Viall & Klimchuk (2012) to the simulated Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory emission and show that coronal plasma light curves exhibit post-nanoflare cooling time lags, while TR light curves show time lags of zero, as observed. We further demonstrate that time lags of zero, regardless of physical cause, do not indicate a lack of variability. Rather, strong variability must be present, and it must occur in unison in the different channels. Lastly, we show that the 'coronal' channels in AIA can be dominated by bright TR emission. When defined in a physically meaningful way, the TR reaches a temperature of roughly 60% the peak temperature in a flux tube. The TR resulting from impulsive heating can extend to 3 MK and higher, well within the range of the 'coronal' AIA channels. 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: The Onset of Magnetic Reconnection: Tearing Instability in Current Sheets with a Guide Field Authors: Daldorff, Lars K. S.; Klimchuk, James A. Bibcode: 2016SPD....4740207D Altcode: Magnetic reconnection is fundamental to many solar phenomena, ranging from coronal heating, to jets, to flares and CMEs. A poorly understood yet crucial aspect of reconnection is that it does not occur until magnetic stresses have built to sufficiently high levels for significant energy release. If reconnection were to happen too soon, coronal heating would be weak and flares would be small. As part of our program to study the onset conditions for magnetic reconnection, we have investigated the instability of current sheets to tearing. Surprisingly little work has been done on this problem for sheets that include a guide field, i.e., for which the field rotates by less than 180 degrees. This is the most common situation on the Sun. We present numerical 3D resistive MHD simulations of several sheets and show how the behaviour depends on the shear angle (rotation). We compare our results to the predictions of linear theory and discuss the nonlinear evolution in terms of plasmoid formation and the interaction of different oblique tearing modes. The relevance to the Sun is explained. Title: Comparing Loop Cross Sections Observed with Hi-C and AIA/SDO Authors: Klimchuk, James A.; DeForest, Craig Bibcode: 2016SPD....47.0301K Altcode: Many studies have reported coronal loop widths measured with AIA/SDO, TRACE, and other data. For warm loops (T ~ 1 MK), the characteristic diameter is about 1500 km. Sub-structure is likely to exist on smaller scales, but the envelope of the "strands" has this typical size. Since 1500 km (2 arcsec) is not large compared to the spatial resolution of the observations, there remained a question about whether the loops are actually much thinner. To address this concern, we have measured the widths of several loops observed at 193 A by both AIA and the Hi-C rocket experiment. Hi-C has 3-6 times better spatial resolution, so if the loops are substantially unresolved by AIA, it should be readily apparent. We find that the measured widths are very similar. Small differences (< 25%) are explainable by uncertainties in the point spread functions. We conclude that previous measurements of loop widths made by AIA and TRACE are essentially correct. We also find little evidence for loop sub-structure at the resolution of Hi-C. The individual strands that comprise loops are therefore smaller than 200 km. These results have important implications for coronal heating. Title: Intensity Conserving Spectral Fitting Authors: Klimchuk, J. A.; Patsourakos, S.; Tripathi, D. Bibcode: 2016SoPh..291...55K Altcode: 2015SoPh..tmp..180K The detailed shapes of spectral-line profiles provide valuable information about the emitting plasma, especially when the plasma contains an unresolved mixture of velocities, temperatures, and densities. As a result of finite spectral resolution, the intensity measured by a spectrometer is the average intensity across a wavelength bin of non-zero size. It is assigned to the wavelength position at the center of the bin. However, the actual intensity at that discrete position will be different if the profile is curved, as it invariably is. Standard fitting routines (spline, Gaussian, etc.) do not account for this difference, and this can result in significant errors when making sensitive measurements. We have developed an iterative procedure that corrects for this effect. It converges rapidly and is very flexible in that it can be used with any fitting function. We present examples of cubic-spline and Gaussian fits and give special attention to measurements of blue-red asymmetries of coronal emission lines. 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: Hard X-ray Detectability of Small Impulsive Heating Events in the Solar Corona Authors: Glesener, L.; Klimchuk, J. A.; Bradshaw, S. J.; Marsh, A.; Krucker, S.; Christe, S. Bibcode: 2015AGUFMSH13B2440G Altcode: Impulsive heating events ("nanoflares") are a candidate to supply the solar corona with its ~2 MK temperature. These transient events can be studied using extreme ultraviolet and soft X-ray observations, among others. However, the impulsive events may occur in tenuous loops on small enough timescales that the heating is essentially not observed due to ionization timescales, and only the cooling phase is observed. Bremsstrahlung hard X-rays could serve as a more direct and prompt indicator of transient heating events. A hard X-ray spacecraft based on the direct-focusing technology pioneered by the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket could search for these direct signatures. In this work, we use the hydrodynamical EBTEL code to simulate differential emission measures produced by individual heating events and by ensembles of such events. We then directly predict hard X-ray spectra and consider their observability by a future spaceborne FOXSI, and also by the RHESSI and NuSTAR spacecraft. Title: Nanoflare Heating of the Quiet Sun Authors: Viall, N. M.; Klimchuk, J. A. Bibcode: 2015AGUFMSH31D..05V Altcode: How the solar corona is heated to temperatures of over 1 MK, while the photosphere below is only ~ 6000 K remains one of the outstanding problems in all of space science. Solving this problem is crucial for understanding Sun-Earth connections, and will provide new insight into universal processes such as magnetic reconnection and wave-particle interactions. We use a systematic technique to analyze the properties of coronal heating throughout the solar corona using data taken with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. Our technique computes cooling times of the coronal plasma on a pixel-by-pixel basis and has the advantage that it analyzes all of the coronal emission, including the diffuse emission surrounding distinguishable coronal features. We have already applied this technique to 15 different active regions, and find clear evidence for dynamic heating and cooling cycles that are consistent with the 'impulsive nanoflare' scenario. What about the rest of the Solar corona? Whether the quiet Sun is heated in a similar or distinct manner from active regions is a matter of great debate. Here we apply our coronal heating analysis technique to quiet Sun locations. We find areas of quiet Sun locations that also undergo dynamic heating and cooling cycles, consistent with impulsive nanoflares. However, there are important characteristics that are distinct from those of active regions. Title: The Onset of Magnetic Reconnection Authors: Daldorff, L. K. S.; Klimchuk, J. A. Bibcode: 2015AGUFMSH13B2437D Altcode: A fundamental question concerning magnetic energy release on the Sun is why the release occurs only after substantial stresses have been built up in the field. If reconnection were to occur readily, the released energy would be insufficient to explain coronal heating, CMEs, flares, jets, spicules, etc. How can we explain this switch-on property? What is the physical nature of the onset conditions? One idea involves the "secondary instability" of current sheets, which switches on when the rotation of the magnetic field across a current sheet reaches a critical angle. Such conditions would occur at the boundaries of flux tubes that become tangled and twisted by turbulent photospheric convection, for example. Other ideas involve a critical thickness for the current sheet. We report here on the preliminary results of our investigation of reconnect onset. Unlike our earlier work on the secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), we treat the coupled chromosphere-corona system. Using the BATS-R-US MHD code, we simulate a single current sheet in a sheared magnetic field that extends from the chromosphere into the corona. Driver motions are applied at the base of the model. The configuration and chromosphere are both idealized, but capture the essential physics of the problem. The advantage of this unique approach is that it resolves the current sheet to the greatest extent possible while maintaining a realistic solar atmosphere. It thus bridges the gap between"reconnection in a box" studies and studies of large-scale systems such as active regions. One question we will address is whether onset conditions are met first in the chromosphere or corona. We will report on the work done on the project. Title: The Details of Coronal Heating Matter! Authors: Klimchuk, J. A.; Daldorff, L. K. S. Bibcode: 2015AGUFMSH13B2438K Altcode: Understanding how the magnetically-closed corona is heated remains one of the most important goals in Heliophysics. It is generally believed that much or most of the heating involves the conversion of energy that is stored in stressed magnetic fields. What makes the problem so challenging is that the conversion process involves very small spatial scales. Simulations of, e.g., solar active regions cannot resolve the thin current sheets that separate the approximately 100,000 elemental magnetic flux strands that comprise a real active region. Heating in the simulations primarily takes the form of Ohmic dissipation of currents that are far less structured. Thus, the heating is only a proxy for the real heating mechanism. How good a proxy is it? Does it have the essential properties of the real mechanism? We suggest that the details of coronal heating matter. They determine not only the temporal behavior and spatial distribution of the heating, but also the total amount of energy release. We present some idealized MHD simulations that demonstrate this last point. 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: How Gas-dynamic Flare Models Powered by Petschek Reconnection Differ from Those with Ad Hoc Energy Sources Authors: Longcope, D. W.; Klimchuk, J. A. Bibcode: 2015ApJ...813..131L Altcode: 2015arXiv151005985L Aspects of solar flare dynamics, such as chromospheric evaporation and flare light curves, have long been studied using one-dimensional models of plasma dynamics inside a static flare loop, subjected to some energy input. While extremely successful at explaining the observed characteristics of flares, all such models so far have specified energy input ad hoc, rather than deriving it self-consistently. There is broad consensus that flares are powered by magnetic energy released through reconnection. Recent work has generalized Petschek’s basic reconnection scenario, topological change followed by field line retraction and shock heating, to permit its inclusion in a one-dimensional flare loop model. Here we compare the gas dynamics driven by retraction and shocking to those from more conventional static loop models energized by ad hoc source terms. We find significant differences during the first minute, when retraction leads to larger kinetic energies and produces higher densities at the loop top, while ad hoc heating tends to rarify the loop top. The loop-top density concentration is related to the slow magnetosonic shock, characteristic of Petschek’s model, but persists beyond the retraction phase occurring in the outflow jet. This offers an explanation for observed loop-top sources of X-ray and EUV emission, with advantages over that provided by ad hoc heating scenarios. The cooling phases of the two models are, however, notably similar to one another, suggesting that observations at that stage will yield little information on the nature of energy input. Title: Chromospheric Nanoflares as a Source of Coronal Plasma. II. Repeating Nanoflares Authors: Bradshaw, S. J.; Klimchuk, J. A. Bibcode: 2015ApJ...811..129B Altcode: 2016arXiv160306673B The million degree plasma of the solar corona must be supplied by the underlying layers of the atmosphere. The mechanism and location of energy release, and the precise source of coronal plasma, remain unresolved. In earlier work, we pursued the idea that warm plasma is supplied to the corona via direct heating of the chromosphere by nanoflares, contrary to the prevailing belief that the corona is heated in situ and the chromosphere is subsequently energized and ablated by thermal conduction. We found that single (low-frequency) chromospheric nanoflares could not explain the observed intensities, Doppler-shifts, and red/blue asymmetries in Fe xii and xiv emission lines. In the present work, we follow up on another suggestion that the corona could be powered by chromospheric nanoflares that repeat on a timescale substantially shorter than the cooling/draining timescale. That is, a single magnetic strand is re-supplied with coronal plasma before the existing plasma has time to cool and drain. We perform a series of hydrodynamic experiments and predict the Fe xii and xiv line intensities, Doppler-shifts, and red/blue asymmetries. We find that our predicted quantities disagree dramatically with observations and fully developed loop structures cannot be created by intermediate- or high-frequency chromospheric nanoflares. We conclude that the mechanism ultimately responsible for producing coronal plasma operates above the chromosphere, but this does not preclude the possibility of a similar mechanism powering the chromosphere, extreme examples of which may be responsible for heating chromospheric plasma to transition region temperatures (e.g., type II spicules). Title: Loop observations and the coronal heating problem Authors: López Fuentes, M. C.; Klimchuk, J. A. Bibcode: 2015BAAA...57..231L Altcode: Coronal heating continues to be one of the fundamental problems of solar physics. In recent years, instrumental advances and the availability of data from space observatories produced important progress, imposing restrictions to the models proposed. However, since the physical processes occur at spatial scales below the present instrumental resolution, definitive answers are still due. Since the corona is strongly dominated by the magnetic field, active region plasma is confined in closed structures or loops. These are the basic observable blocks of the corona, so the analysis of their structure and evolution is essential to understand the heating. In this report, mainly addressed to astronomers not necessarily familiarized with the subject, we review some of the proposed heating models and we pay special attention to the sometimes confusing and apparently contradictory observations of coronal loops. We discuss the consequences of these observations for some of the heating models proposed, in particular those based on impulsive events known as nanoflares. Title: Division II: Commission 10: Solar Activity Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk, James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul; Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi; Vršnak, Bojan; Yan, Yihua Bibcode: 2015IAUTB..28..106V Altcode: The Business Meeting of Commission 10 was held as part of the Business Meeting of Division II (Sun and Heliosphere), chaired by Valentin Martínez-Pillet, the President of the Division. The President of Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took the chair for the business meeting of C10. She summarised the activities of C10 over the triennium and the election of the incoming OC. Title: 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: How gas-dynamic flare models powered by Petschek reconnection differ from those with ad hoc energy sources Authors: Longcope, Dana; Klimchuk, Jim Bibcode: 2015shin.confE...9L Altcode: Aspects of solar flare dynamics, such as chromospheric evaporation and flare light-curves, have long been studied using one-dimensional models of plasma dynamics inside a static flare loop, subjected to some energy input. While extremely successful at explaining the observed characteristics of flares, all such models so far have specified energy input ad hoc, rather than deriving it self-consistently. There is broad consensus that flares are powered by magnetic energy released through reconnection. Recent work has generalized Petschek's basic reconnection scenario, topological change followed by field line retraction and shock heating, to permit its inclusion into a one-dimensional flare loop model. Here we compare the gas dynamics driven by retraction and shocking to those from more conventional static loop models energized by ad hoc source terms. We find significant differences during the first minute, when retraction leads to larger kinetic energies and produces higher densities at the loop top, while ad hoc heating tends to rarify the loop top. The loop-top density concentration is related to the slow magnetosonic shock, characteristic of Petschek's model, but persists beyond the retraction phase occurring in the outflow jet. This offers an explanation of observed loop-top sources of X-ray and EUV emission, with advantages over that provided by ad hoc heating scenarios. The cooling phases of the two models are, however, notably similar to one another, suggesting observations at that stage will yield little information on the nature of energy input. Title: The Myth of Long Duration Flare Emission: Slow Heating or Slow Cooling? Authors: Qiu, Jiong; Longcope, Dana; Klimchuk, James A. Bibcode: 2015TESS....130214Q Altcode: Long duration flare emissions lasting for a few hours are likely governed by magnetic reconnection that continuously heats flare plasmas in continuously formed flare loops. In this study, we confirm that this process leads to the long-duration total emission for up to four hours in a C2.9 flare on 2011 September 13. Observed by AIA, the flare exhibits an ordered spread of flare UV ribbons along the polarity inversion line, followed by the sequential formation of post-flare loops in EUV emissions. We infer heating rates of thousands of flare loops from the UV light curves at the flare foot-points, and model the flare total emission with the 0d EBTEL model, which reproduces the global evolution pattern of the long-duration flare EUV emissions as the result of superposition of continuously formed and heated flare loops. However, observations at single loop pixels also show long duration EUV emission at 10 MK, long cooling time from 10 MK to 3 MK, and later on very short duration of EUV emission at 1-2 MK. All of these signatures cannot be produced by superposition of multiple impulsive heating events. Our experiments, with both the 0d EBTEL model and a 1d hydrodynamic model, have demonstrated that a heating profile in a single loop consisting of two parts, an intense impulsive heating followed by a low-rate heating 1-2 orders of magnitude smaller that is attenuated over 20-30 minutes, is required to produce the observed time evolution signatures in a single loop. The total energy in the gradual heating phase is comparable with that in the impulsive heating phase in a flare loop. We discuss viable physical mechanisms for such two-phase heating in a post-reconnection flare loop. Title: How gas-dynamic flare models powered by Petschek reconnection differ from thosewith ad hoc energy sources Authors: Longcope, Dana; Klimchuk, James A. Bibcode: 2015TESS....130212L Altcode: Many aspects of solar flare dynamics including chromospheric evaporation have been, for more than thirty years, studied using one-dimensional models of static flaring loops. These models solve one-dimensional gas-dynamic equations for the dynamics of plasma inside a static loop, subjected to energy input through either non-thermal particles or heating. While they have been extremely successful at explaining the characteristics of emission observed in flares, none so far have been developed in which the energy input is derived self-consistently from the loop's dynamics. Instead the energy input is specified ad hoc. According to another line of theoretical investigation, flares derive their energy from magnetic energy released through fast magnetic reconnection. In the model due originally to Petschek reconnection occurring in a small diffusion region produces a bent flux tube whose retraction generates fast flows (an outflow jet) and shocks where flow energy is thermalized. In a recent line of work this scenario has been generalized so it may be incorporated into a one-dimensional loop model of the kind used so successfully in flare modeling. In this new model the flaring loop itself undergoes the retraction and shock formation, and thereby introduces the flare energy self-consistently. Here we compare the gas dynamics driven by retraction and shocking to those from more conventional static loop models. We find significant differences during the first minute, when retraction produces high densities at the loop top, while ad hoc heating tends to rarify the loop top. Title: Key Aspects of Coronal Heating Authors: Klimchuk, James A. Bibcode: 2015TESS....120308K Altcode: We highlight ten key aspects of coronal heating that must be understood before we can consider the problem to be solved. (1) All coronal heating is impulsive. (2) The details of coronal heating matter. (3) The corona is filled with elemental magnetic stands. (4) The corona is densely populated with current sheets. (5) The strands must reconnect to prevent an infinite buildup of stress. (6) Nanoflares repeat with different frequencies. (7) What is the characteristic magnitude of energy release? (8) What causes the collective behavior responsible for loops? (9) What are the onset conditions for energy release? (10) Chromospheric nanoflares are not a primary source of coronal plasma. Significant progress in solving the coronal heating problem will require a coordination of approaches: observational studies, field-aligned hydrodynamic simulations, large-scale and localized 3D MHD simulations, and possibly also kinetic simulations. There is a unique value to each of these approaches, and the community must strive to coordinate better. Title: Key aspects of coronal heating Authors: Klimchuk, James A. Bibcode: 2015RSPTA.37340256K Altcode: 2014arXiv1410.5660K We highlight 10 key aspects of coronal heating that must be understood before we can consider the problem to be solved. (1) All coronal heating is impulsive. (2) The details of coronal heating matter. (3) The corona is filled with elemental magnetic stands. (4) The corona is densely populated with current sheets. (5) The strands must reconnect to prevent an infinite build-up of stress. (6) Nanoflares repeat with different frequencies. (7) What is the characteristic magnitude of energy release? (8) What causes the collective behaviour responsible for loops? (9) What are the onset conditions for energy release? (10) Chromospheric nanoflares are not a primary source of coronal plasma. Significant progress in solving the coronal heating problem will require coordination of approaches: observational studies, field-aligned hydrodynamic simulations, large-scale and localized three-dimensional magnetohydrodynamic simulations, and possibly also kinetic simulations. There is a unique value to each of these approaches, and the community must strive to coordinate better. Title: Intensity Conserving Spline Interpolation (ICSI): A New Tool for Spectroscopic Analysis Authors: Klimchuk, James A.; Patsourakos, Spiros; Tripathi, Durgesh Bibcode: 2015TESS....120309K Altcode: 2015arXiv150608102K Spectroscopy is an extremely powerful tool for diagnosing astrophysical and other plasmas. For example, the shapes of line profiles provide valuable information on the distribution of velocities along an optically thin line-of-sight and across the finite area of a resolution element. A number of recent studies have measured the asymmetries of line profiles in order to detect faint high-speed upflows, perhaps associated with coronal nanoflares or perhaps associated with chromospheric nanoflares and type II spicules. Over most of the Sun, these asymmetries are very subtle, so great care must be taken. A common technique is to perform a spline fit of the points in the profile in order to extract information at a spectral resolution higher than that of the original data. However, a fundamental problem is that the fits do not conserve intensity. We have therefore developed an iterative procedure called Intensity Conserving Spline Interpolation that does preserve the observed intensity within each wavelength bin. It improves the measurement of line asymmetries and can also help with the determination of line blends. Title: The Onset of Magnetic Reconnection Authors: Daldorff, Lars K. S.; Klimchuk, James A.; van der Holst, Bart Bibcode: 2015TESS....110404D Altcode: A fundamental question concerning magnetic energy release on the Sun is why the release occurs only after substantial stresses have been built up in the field. If reconnection were to occur readily, the released energy would be insufficient to explain coronal heating, CMEs, flares, jets, spicules, etc. How can we explain this switch-on property? What is the physical nature of the onset conditions? One idea involves the "secondary instability" of current sheets, which switches on when the rotation of the magnetic field across a current sheet reaches a critical angle. Such conditions would occur at the boundaries of flux tubes that become tangled and twisted by turbulent photospheric convection, for example. Other ideas involve a critical thickness for the current sheet. We report here on the preliminary results of our investigation of reconnect onset. Unlike our earlier work on the secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), we treat the coupled chromosphere-corona system. Using the BATS-R-US MHD code, we simulate a single current sheet in a sheared magnetic field that extends from the chromosphere into the corona. Driver motions are applied at the base of the model. The configuration and chromosphere are both idealized, but capture the essential physics of the problem. The advantage of this unique approach is that it resolves the current sheet to the greatest extent possible while maintaining a realistic solar atmosphere. It thus bridges the gap between "reconnection in a box" studies and studies of large-scale systems such as active regions. One question we will address is whether onset conditions are met first in the chromosphere or corona. We will report on the work done on the project. Title: Nanoflare Heating of the Quiet Sun Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2015TESS....121303V Altcode: How the solar corona is heated to temperatures of over 1 MK, while the photosphere below is only ~ 6000 K remains one of the outstanding problems in all of space science. Solving this problem is crucial for understanding Sun-Earth connections, and will provide new insight into universal processes such as magnetic reconnection and wave-particle interactions. We use a new systematic technique to analyze the properties of coronal heating throughout the solar corona using data taken with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. Our technique computes cooling times of the coronal plasma on a pixel-by-pixel basis and has the advantage that it analyzes all of the coronal emission, including the diffuse emission surrounding distinguishable coronal features. We have already applied this technique to 15 different active regions, and find clear evidence for dynamic heating and cooling cycles that are consistent with the 'impulsive nanoflare' scenario. What about the rest of the Solar corona? Whether the quiet Sun is heated in a similar or distinct manner from active regions is a matter of great debate. In this paper, we apply our coronal heating analysis technique to quiet Sun locations. We find that the majority of the analyzed quiet Sun locations do undergo dynamic heating and cooling cycles, consistent with impulsive nanoflares. However, there are important characteristics that are distinct from those of active regions.This research was supported by a NASA Guest Investigator grant. Title: Synthetic 3D modeling of active regions and simulation of their multi-wavelength emission Authors: Nita, Gelu M.; Fleishman, Gregory; Kuznetsov, Alexey A.; Loukitcheva, Maria A.; Viall, Nicholeen M.; Klimchuk, James A.; Gary, Dale E. Bibcode: 2015TESS....131204N Altcode: To facilitate the study of solar active regions, we have created a synthetic modeling framework that combines 3D magnetic structures obtained from magnetic extrapolations with simplified 1D thermal models of the chromosphere, transition region, and corona. To handle, visualize, and use such synthetic data cubes to compute multi-wavelength emission maps and compare them with observations, we have undertaken a major enhancement of our simulation tools, GX_Simulator (ftp://sohoftp.nascom.nasa.gov/solarsoft/packages/gx_simulator/), developed earlier for modeling emission from flaring loops. The greatly enhanced, object-based architecture, which now runs on Windows, Mac, and UNIX platform, offers important new capabilities that include the ability to either import 3D density and temperature distribution models, or to assign to each individual voxel numerically defined coronal or chromospheric temperature and densities, or coronal Differential Emission Measure distributions. Due to these new capabilities, the GX_Simulator can now apply parametric heating models involving average properties of the magnetic field lines crossing a given voxel volume, as well as compute and investigate the spatial and spectral properties of radio (to be compared with VLA or EOVSA data), (sub-)millimeter (ALMA), EUV (AIA/SDO), and X-ray (RHESSI) emission calculated from the model. The application integrates shared-object libraries containing fast free-free, gyrosynchrotron, and gyroresonance emission codes developed in FORTRAN and C++, and soft and hard X-ray and EUV codes developed in IDL. We use this tool to model and analyze an active region and compare the synthetic emission maps obtained in different wavelengths with observations.This work was partially supported by NSF grants AGS-1250374, AGS-1262772, NASA grant NNX14AC87G, the Marie Curie International Research Staff Exchange Scheme "Radiosun" (PEOPLE-2011-IRSES-295272), RFBR grants 14-02-91157, 15-02-01089, 15-02-03717, 15-02-03835, 15-02-08028. Title: Two-dimensional Cellular Automaton Model for the Evolution of Active Region Coronal Plasmas Authors: López Fuentes, Marcelo; Klimchuk, James A. Bibcode: 2015ApJ...799..128L Altcode: 2016arXiv160703883L We study a two-dimensional cellular automaton (CA) model for the evolution of coronal loop plasmas. The model is based on the idea that coronal loops are made of elementary magnetic strands that are tangled and stressed by the displacement of their footpoints by photospheric motions. The magnetic stress accumulated between neighbor strands is released in sudden reconnection events or nanoflares that heat the plasma. We combine the CA model with the Enthalpy Based Thermal Evolution of Loops model to compute the response of the plasma to the heating events. Using the known response of the X-Ray Telescope on board Hinode, we also obtain synthetic data. The model obeys easy-to-understand scaling laws relating the output (nanoflare energy, temperature, density, intensity) to the input parameters (field strength, strand length, critical misalignment angle). The nanoflares have a power-law distribution with a universal slope of -2.5, independent of the input parameters. The repetition frequency of nanoflares, expressed in terms of the plasma cooling time, increases with strand length. We discuss the implications of our results for the problem of heating and evolution of active region coronal plasmas. Title: The Transition Region Response to a Coronal Nanoflare: Forward Modeling and Observations in SDO/AIA Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2015ApJ...799...58V Altcode: The corona and transition region (TR) are fundamentally coupled through the processes of thermal conduction and mass exchange. It is not possible to understand one without the other. Yet the temperature-dependent emissions from the two locations behave quite differently in the aftermath of an impulsive heating event such as a coronal nanoflare. Whereas the corona cools sequentially, emitting first at higher temperatures and then at lower temperatures, the TR is multithermal and the emission at all temperatures responds in unison. We have previously applied the automated time lag technique of Viall & Klimchuk to disk observations of an active region (AR) made by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory. Lines of sight passing through coronal plasma show clear evidence for post-nanoflare cooling, while lines of sight intersecting the TR footpoints of coronal strands show zero time lag. In this paper, we use the EBTEL hydrodynamics code to demonstrate that this is precisely the expected behavior when the corona is heated by nanoflares. We also apply the time lag technique for the first time to off-limb observations of an AR. Since TR emission is not present above the limb, the occurrence of zero time lags is greatly diminished, supporting the conclusion that zero time lags measured on the disk are due to TR plasma. Lastly, we show that the ''coronal'' channels in AIA can be dominated by bright TR emission. When defined in a physically meaningful way, the TR reaches a temperature of roughly 60% the peak temperature in a flux tube. The TR resulting from impulsive heating can extend to 3 MK and higher, well within the range of the ''coronal'' AIA channels. Title: Intensity Conserving Spline Interpolation (ICSI): A New Tool for Spectroscopic Analysis Authors: Klimchuk, J. A.; Patsourakos, S.; Tripathi, D. Bibcode: 2014AGUFMSH13B4109K Altcode: Spectroscopy is an extremely powerful tool for diagnosing astrophysical and other plasmas. For example, the shapes of line profiles provide valuable information on the distribution of velocities along an optically thin line-of-sight and across the finite area of a resolution element. A number of recent studies have measured the asymmetries of line profiles in order to detect faint high-speed upflows, perhaps associated with coronal nanoflares or perhaps associated with chromospheric nanoflares and type II spicules. Over most of the Sun, these asymmetries are very subtle, so great care must be taken. A common technique is to perform a spline fit of the points in the profile in order to extract information at a spectral resolution higher than that of the original data. However, a fundamental problem is that the fits do not conserve intensity. We have therefore developed an iterative procedure called Intensity Conserving Spline Interpolation that does preserve the observed intensity within each wavelength bin. It improves the measurement of line asymmetries and can also help with the determination of line blends. Title: The Onset of Magnetic Reconnection in the Solar Atmosphere Authors: Evans, R. M.; Klimchuk, J. A.; van der Holst, B. Bibcode: 2014AGUFMSH12A..02E Altcode: A fundamental question concerning magnetic energy release on the Sun is why the release occurs only after substantial stresses have been built up in the field. If reconnection were to occur readily, then the released energy would be much less than the energy required for coronal heating, CMEs, flares, jets, spicules, etc. How can we explain this switch-on property? What is the physical nature of the onset conditions? One idea involves the secondary instability of current sheets, which switches on when the rotation of the magnetic field across a current sheet reaches a critical angle. Such conditions would occur at the boundaries of flux tubes that become tangled and twisted by turbulent photospheric convection, for example. Other ideas focus on a critical thickness for the current sheet. We report here on the preliminary results of our investigation of reconnection onset. Unlike our earlier work on the secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), here we treat the coupled chromosphere-corona system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a single current sheet in a sheared magnetic field that extends from the chromosphere into the corona. Driver motions are applied at the base of the model. The configuration and chromosphere are both idealized, but capture the essential physics of the problem. The advantage of this unique approach is that it resolves the current sheet to the greatest extent possible while maintaining a realistic solar atmosphere. It thus bridges the gap between reconnection in a box studies and studies of large-scale systems such as active regions. One question we will address is whether onset conditions are met first in the chromosphere or corona. Title: Hi-C Observations and the Structure of Coronal Loops Authors: DeForest, C. E.; Klimchuk, J. A. Bibcode: 2014AGUFMSH31C..04D Altcode: Despite nearly four decades of study since the launch of Skylab, the physical structure of coronal loops remains an enigma. Loops are guided by the magnetic field and, in the common EUV emission lines, appear to be composed of stranded structures reminiscent of field lines. This stranded structure appears to have constant or nearly-constant width, at odds with naive understanding of flux tube behavior in a field gradient. Possible explanations range from physical solutions such as twisted magnetic structure or peculiar properties of separators and quasi-separators, to observation effects that invoke finite resolution or anisotropy of the field containing each strand. The uncertainty affects many aspects of basic coronal physics, because some of the possible explanations for stranded structure have strong implications for other mysteries such as the anomalously tall scale height of the EUV corona. The Hi-C EUV images are the highest resolution coronal images to date, and offer new insights into the structure of coronal loops. We present an overview of research to date, show results from a detailed analysis of several dozen well-presented loops that are visible in the Hi-C data set, and speculate on the implications for the rest of the corona. Title: Emission Measure Distribution for Diffuse Regions in Solar Active Regions Authors: Subramanian, Srividya; Tripathi, Durgesh; Klimchuk, James A.; Mason, Helen E. Bibcode: 2014ApJ...795...76S Altcode: 2014arXiv1409.1447S Our knowledge of the diffuse emission that encompasses active regions is very limited. In this paper we investigate two off-limb active regions, namely, AR 10939 and AR 10961, to probe the underlying heating mechanisms. For this purpose, we have used spectral observations from Hinode/EIS and employed the emission measure (EM) technique to obtain the thermal structure of these diffuse regions. Our results show that the characteristic EM distributions of the diffuse emission regions peak at log T = 6.25 and the coolward slopes are in the range 1.4-3.3. This suggests that both low- as well as high-frequency nanoflare heating events are at work. Our results provide additional constraints on the properties of these diffuse emission regions and their contribution to the background/foreground when active region cores are observed on-disk. Title: Are Chromospheric Nanoflares a Primary Source of Coronal Plasma? Authors: Klimchuk, J. A.; Bradshaw, S. J. Bibcode: 2014ApJ...791...60K Altcode: 2014arXiv1405.1708K It has been suggested that the hot plasma of the solar corona comes primarily from impulsive heating events, or nanoflares, that occur in the lower atmosphere, either in the upper part of the ordinary chromosphere or at the tips of type II spicules. We test this idea with a series of hydrodynamic simulations. We find that synthetic Fe XII (195) and Fe XIV (274) line profiles generated from the simulations disagree dramatically with actual observations. The integrated line intensities are much too faint; the blueshifts are much too fast; the blue-red asymmetries are much too large; and the emission is confined to low altitudes. We conclude that chromospheric nanoflares are not a primary source of hot coronal plasma. Such events may play an important role in producing the chromosphere and powering its intense radiation, but they do not, in general, raise the temperature of the plasma to coronal values. Those cases where coronal temperatures are reached must be relatively uncommon. The observed profiles of Fe XII and Fe XIV come primarily from plasma that is heated in the corona itself, either by coronal nanoflares or a quasi-steady coronal heating process. Chromospheric nanoflares might play a role in generating waves that provide this coronal heating. Title: MHD modelling of coronal loops: injection of high-speed chromospheric flows Authors: Petralia, A.; Reale, F.; Orlando, S.; Klimchuk, J. A. Bibcode: 2014A&A...567A..70P Altcode: 2014arXiv1405.2198P Context. Observations reveal a correspondence between chromospheric type II spicules and bright upward-moving fronts in the corona observed in the extreme-ultraviolet (EUV) band. However, theoretical considerations suggest that these flows are probably not the main source of heating in coronal magnetic loops.
Aims: We investigate the propagation of high-speed chromospheric flows into coronal magnetic flux tubes and the possible production of emission in the EUV band.
Methods: We simulated the propagation of a dense 104 K chromospheric jet upward along a coronal loop by means of a 2D cylindrical MHD model that includes gravity, radiative losses, thermal conduction, and magnetic induction. The jet propagates in a complete atmosphere including the chromosphere and a tenuous cool (~0.8 MK) corona, linked through a steep transition region. In our reference model, the jet initial speed is 70 km s-1, its initial density is 1011 cm-3, and the ambient uniform magnetic field is 10 G. We also explored other values of jet speed and density in 1D and different magnetic field values in 2D, as well as the jet propagation in a hotter (~1.5 MK) background loop.
Results: While the initial speed of the jet does not allow it to reach the loop apex, a hot shock-front develops ahead of it and travels to the other extreme of the loop. The shock front compresses the coronal plasma and heats it to about 106 K. As a result, a bright moving front becomes visible in the 171 Å channel of the SDO/AIA mission. This result generally applies to all the other explored cases, except for the propagation in the hotter loop.
Conclusions: For a cool, low-density initial coronal loop, the post-shock plasma ahead of upward chromospheric flows might explain at least part of the observed correspondence between type II spicules and EUV emission excess.

Movies associated to Figs. 3, 6, 7 are available in electronic form at http://www.aanda.org Title: The Onset of Magnetic Reconnection in the Solar Atmosphere Authors: Evans, Rebekah Minnel; Klimchuk, James; van der Holst, Bart Bibcode: 2014shin.confE..65E Altcode: A fundamental question concerning magnetic energy release on the Sun is why the release occurs only after substantial stresses have been built up in the field. If reconnection were to occur readily, then the released energy would be much less than the energy required for coronal heating, CMEs, flares, jets, spicules, etc. How can we explain this switch-on property? What is the physical nature of the onset conditions? One idea involves the secondary instability of current sheets, which switches on when the rotation of the magnetic field across a current sheet reaches a critical angle. Such conditions would occur at the boundaries of flux tubes that become tangled and twisted by turbulent photospheric convection, for example. Other ideas focus on a critical thickness for the current sheet. We report here on the preliminary results of our investigation of reconnection onset. Unlike our earlier work on the secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), here we treat the coupled chromosphere-corona system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a single current sheet in a sheared magnetic field that extends from the chromosphere into the corona. Driver motions are applied at the base of the model. The configuration and chromosphere are both idealized, but capture the essential physics of the problem. The advantage of this unique approach is that it resolves the current sheet to the greatest extent possible while maintaining a realistic solar atmosphere. It thus bridges the gap between reconnection in a box studies and studies of large-scale systems such as active regions. One question we will address is whether onset conditions are met first in the chromosphere or corona. Title: Modeling the response of the lower atmosphere to flare reconnection Authors: Longcope, Dana; Qiu, Jiong; Klimchuk, James A. Bibcode: 2014AAS...22412324L Altcode: It has long been recognized that energy release in a solar flare gives rise to ablation of material from the chromosphere (more commonly called evaporation). The prevailing view is that energy is initially transformed from stored magnetic energy by the process of magnetic reconnection. In some models reconnection accelerates electrons, either directly or indirectly, and these non-thermal electrons carry energy to the chromospheric footpoints. In others the reconnection converts magnetic energy into heat in the corona and thermal conduction carries that heat to the chromosphere. While no comprehensive, self-consistent model yet exists for the conversion of magnetic energy to non-thermal electron energy, models of the conversion to heat, via slow magnetosonic shocks, have been available since Petschek's 1964 paper. We present a numerical model encompassing the conversion of magnetic energy to shocks, to heat, and then to conduction-driven evaporation. We compare its results to those of more traditional conduction-driven models where reconnection is replaced by an ad hoc plasma heating. We consider, in particular, observable signatures such as doppler shifts and formation of flare ribbons.This work was supported by the NASA SR&T program. Title: A one-dimensional solar flare model capturing reconnection energy release, evaporation, and gradually cooling post-flare loops Authors: Longcope, Dana; Qiu, Jiong; Klimchuk, Jim Bibcode: 2014shin.confE..32L Altcode: The most obvious signature of energy release in a solar flare is the large amount of chromospheric material heated to coronal temperatures through a process known (inaccurately) as chromospheric evaporation. The thousand-fold increase in X-ray luminosity we associate with a flare is due entirely to evaporation. The most successful models of flare evaporation to date have come from one-dimensional flux tube simulations. These have provided the best means of resolving the very thin pre-flare transition region, and of easily accommodating the perfect field-alignment of the energy transport by either non-thermal electrons or thermal conduction. In traditional flux tube models magnetic reconnection is represented by an ad hoc heating term. This adds energy but no momentum, and represents only crudely known models of magnetic reconnection. Here we present a new approach which captures the physics of fast Petschek reconnection in a flux tube simulation. Following its creation by localized reconnection within a current sheet, the flux tube retracts under magnetic tension, converting magnetic energy into bulk flows; this is the outflow jet. These flows form a slow magnetosonic shock which heats the coronal plasma and drives a conduction front into the chromosphere. Our one-dimensional model captures the energy release, thermalization, and the evaporation it drives. We find observable signatures of the interplay between reconnection energy release and evaporation; signatures different from those found in conventional flux tube models with ad hoc heating, but similar to actual flare observations. Title: The Onset of Magnetic Reconnection in the Solar Atmosphere Authors: Evans, Rebekah M.; Klimchuk, James A.; Van Der Holst, Bart Bibcode: 2014AAS...22432342E Altcode: A fundamental question concerning magnetic energy release on the Sun is why the release occurs only after substantial stresses have been built up in the field. If reconnection were to occur readily, then the released energy would be much less than the energy required for coronal heating, CMEs, flares, jets, spicules, etc. How can we explain this switch-on property? What is the physical nature of the onset conditions? One idea involves the "secondary instability" of current sheets, which switches on when the rotation of the magnetic field across a current sheet reaches a critical angle. Such conditions would occur at the boundaries of flux tubes that become tangled and twisted by turbulent photopheric convection, for example. Other ideas focus on a critical thickness for the current sheet. We report here on the preliminary results of our investigation of reconnection onset. Unlike our earlier work on the secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), here we treat the coupled chromosphere-corona system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a single current sheet in a sheared magnetic field that extends from the chromosphere into the corona. Driver motions are applied at the base of the model. The configuration and chromosphere are both idealized, but capture the essential physics of the problem. The advantage of this unique approach is that it resolves the current sheet to the greatest extent possible while maintaining a realistic solar atmosphere. It thus bridges the gap between "reconnection in a box" studies and studies of large-scale systems such as active regions. One question we will address is whether onset conditions are met first in the chromosphere or corona. Title: Long Duration Flare Emission by Sequential Reconnection and Heating Authors: Qiu, Jiong; Longcope, Dana; Klimchuk, James A. Bibcode: 2014AAS...22412325Q Altcode: Long duration flare emissions lasting for a few hours are likely produced by magnetic reconnection that continuously forms flare loops and heats plasma inside. In this study, we demonstrate that this process leads to the long duration emission in a C2.9 flare on 2011 September 13. Observed by AIA, the flare exhibits an organized pattern of evolution with UV brightenings in flare ribbons spreading along the polarity inversion line, followed by sequential formation of post-flare loops seen in EUV emissions. The spatially resolved observation of flare ribbons can be used to infer heating rates insequentially formed and heated flare loops, with which we synthesize flare emission in these loops with hydrodynamic models. The 0d EBTEL model (Klimchuk et al. 2008) efficiently computes meanproperties of thousands of flare loops identified from flare ribbon signatures, and the synthetic tempo-spatial evolution of the total emission is in reasonable agreement with EUV observations. The 1d model applied on a few selected loops reveals physics of the heating mechanism and along-the-loop dynamics, particularly during the impulsive heating phase. During the four hours of this event, the estimated total energy in the heating amounts to 2e30 erg, with the total reconnection flux about 1e21 Mx. Title: A Survey of Coronal Heating Properties in Solar Active Regions Authors: Viall, Nicholeen; Klimchuk, James A. Bibcode: 2014AAS...22432315V Altcode: We investigate the properties of coronal heating in solar active regions (AR) by systematically analyzing coronal light curves observed by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. Our automated technique computes time-lags (cooling times) on a pixel-by-pixel basis, and has the advantage that it allows us to analyze all of the coronal AR emission, including the so-called diffuse emission between coronal loops. We recently presented results using this time-lag analysis on NOAA AR 11082 (Viall & Klimchuk 2012) and found that the majority of the pixels contained cooling plasma along their line of sight. This result is consistent with impulsive coronal nanoflare heating of both coronal loops and the surrounding diffuse emission in the AR. Here we present the results of our time-lag technique applied to a survey of 15 AR of different magnetic complexity, total unsigned magnetic flux, size and age. We show that the post-nanoflare cooling patterns identified in NOAA AR 11082 are identified throughout all of the active regions in this survey, indicating that nanoflare heating is ubiquitous in solar active regions. However, some details of the nanoflare properties, such as the nanoflare energy, are different across these different active regions.We thank the SDO/AIA team for the use of these data, and the Coronal Heating ISSI team for helpful discussion of these topics. This research was supported by a NASA Heliophysics GI. Title: Evidence for Impulsive Coronal Heating from EUNIS 2013 Authors: Daw, Adrian N.; Brosius, Jeffrey W.; Rabin, Douglas M.; Landi, Enrico; Klimchuk, James A. Bibcode: 2014AAS...22431204D Altcode: Pervasive, faint Fe XIX 592 Å line emission was observed in active regions by the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket instrument on 23 April 2013. The broad spectral coverage (303-370 Å, 527-635 Å) and unprecedented dynamic range of the EUNIS observations includes emission lines of ionization stages from He I to Fe XX, and thus a wide temperature range of 0.03 to 10 MK. Comparison of observed line intensities with calculations demonstrates that the Fe XIX emission, formed at temperatures around 8 MK, is evidence of the faint hot emission predicted by impulsive heating models of the solar corona (such as nanoflares). Title: Chromospheric Nanoflares Authors: Klimchuk, James A.; Bradshaw, Stephen Bibcode: 2014AAS...22430206K Altcode: The traditional view is that coronal plasma results from energy release (reconnection, waves, etc.) that takes place above the chromosphere, in the corona itself. However, this need not be the case. Cool chromospheric plasma could be directly heated to high temperatures and rise upward to fill the corona. We here investigate this scenario in the context of impulsive nanoflares that occur in the upper chromosphere. Such events could represent the sudden heating of the tips of type II spicules, or they could take place in the ordinary chromosphere, completely unrelated to spicules. The results are general. We generate synthetic line profiles of Fe XIV (274) and Fe XII (195) based on a series of simulations performed with the HYDRAD 1D hydro code, which includes non-equilibrium ionization. We find that the profiles are strongly blue-shifted and/or have large asymmetries that are grossly inconsistent with actual observations. The results are in agreement with our previous analytical predictions (Klimchuk 2012). We conclude that most coronal plasma is a result of coronal energy release, not chromospheric nanoflares or type II spicules. Title: MHD modeling of coronal loops: the transition region throat Authors: Guarrasi, M.; Reale, F.; Orlando, S.; Mignone, A.; Klimchuk, J. A. Bibcode: 2014A&A...564A..48G Altcode: 2014arXiv1402.0338G Context. The expansion of coronal loops in the transition region may considerably influence the diagnostics of the plasma emission measure. The cross-sectional area of the loops is expected to depend on the temperature and pressure, and might be sensitive to the heating rate.
Aims: The approach here is to study the area response to slow changes in the coronal heating rate, and check the current interpretation in terms of steady heating models.
Methods: We study the area response with a time-dependent 2D magnetohydrodynamic (MHD) loop model, including the description of the expanding magnetic field, coronal heating and losses by thermal conduction, and radiation from optically thin plasma. We run a simulation for a loop 50 Mm long and quasi-statically heated to about 4 MK.
Results: We find that the area can change substantially with the quasi-steady heating rate, e.g., by ~40% at 0.5 MK as the loop temperature varies between 1 MK and 4 MK, and, therefore, affects the interpretation of the differential emission measure vs. temperature (DEM(T)) curves.

The movie associated to Fig. 4 is available in electronic form at http://www.aanda.org Title: Core and Wing Densities of Asymmetric Coronal Spectral Profiles: Implications for the Mass Supply of the Solar Corona Authors: Patsourakos, S.; Klimchuk, J. A.; Young, P. R. Bibcode: 2014ApJ...781...58P Altcode: 2013arXiv1312.4842P Recent solar spectroscopic observations have shown that coronal spectral lines can exhibit asymmetric profiles, with enhanced emissions at their blue wings. These asymmetries correspond to rapidly upflowing plasmas at speeds exceeding ≈50 km s-1. Here, we perform a study of the density of the rapidly upflowing material and compare it with that of the line core that corresponds to the bulk of the plasma. For this task, we use spectroscopic observations of several active regions taken by the Extreme Ultraviolet Imaging Spectrometer of the Hinode mission. The density sensitive ratio of the Fe XIV lines at 264.78 and 274.20 Å is used to determine wing and core densities. We compute the ratio of the blue wing density to the core density and find that most values are of order unity. This is consistent with the predictions for coronal nanoflares if most of the observed coronal mass is supplied by chromospheric evaporation driven by the nanoflares. However, much larger blue wing-to-core density ratios are predicted if most of the coronal mass is supplied by heated material ejected with type II spicules. Our measurements do not rule out a spicule origin for the blue wing emission, but they argue against spicules being a primary source of the hot plasma in the corona. We note that only about 40% of the pixels where line blends could be safely ignored have blue wing asymmetries in both Fe XIV lines. Anticipated sub-arcsecond spatial resolution spectroscopic observations in future missions could shed more light on the origin of blue, red, and mixed asymmetries. Title: Cross-Sectional Properties of Coronal Loops Authors: West, Matthew; Zhukov, Andrei; Klimchuk, James Bibcode: 2014cosp...40E3620W Altcode: In this work we assess if coronal loop cross sections observed in EUV images are symmetrical or asymmetrical in nature. To do this, we identified individual loop structures observed in EUV images taken with the EUVI instruments on the STEREO satellites. To image loops from two unique angles, we chose loops clearly discernible in both EUVI imagers during the period when the satellites were separated by approximately 90 degrees, allowing us to make observations of individual loops from two unique vantage points. Preference was given to loops which could be clearly identified in both satellites, especially those which were not crossed by other bright structures or loops, so reasonable background subtractions could be made. Once identified, the images were co-aligned and straightened, using a spline routine, for comparison. In total we identified 11 clearly discernible loops and derived the standard deviation and widths for both perspectives of the loop. It was found that within instrumental errors the loops can be considered circular in nature. Title: EUV emission along observed coronal loops Authors: Lopez Fuentes, Marcelo; Klimchuk, James Bibcode: 2014cosp...40E1872L Altcode: Theories based on steady heating concentrated at the footpoints of coronal loops predict a state of thermal nonequilibrium which produce, according to numerical studies, highly asymmetric intensity profiles. In this work we study a series of coronal loop observations obtained with the Transition Region and Coronal Explorer (TRACE) and the Atmospheric Imager Assembly (AIA) on board the Solar Dynamics Observatory (SDO) to explore how the intensity varies with position along the loops. As part of the data processing we apply a careful procedure to subtract the background contribution to the loop intensity. We find that the obtained intrinsic intensities have strong variations over short distances, due in part to a residual effect of the intense nonuniform background. Although it is not easy to separate the background contribution from the proper loop intensity fluctuations, our results show that in the majority of the cases the intensity structure is not consistent with the predicted profiles. Title: Asymmetries in Coronal Spectral Lines and Emission Measure Distribution Authors: Tripathi, Durgesh; Klimchuk, James A. Bibcode: 2013ApJ...779....1T Altcode: 2013arXiv1310.0168T It has previously been argued that (1) spicules do not provide enough pre-heated plasma to fill the corona, and (2) even if they did, additional heating would be required to keep the plasma hot as it expands upward. Here we address whether spicules play an important role by injecting plasma at cooler temperatures (<2 MK), which then gets heated to coronal values at higher altitudes. We measure red-blue asymmetries in line profiles formed over a wide range of temperatures in the bright moss areas of two active regions. We derive emission measure distributions from the excess wing emission. We find that the asymmetries and emission measures are small and conclude that spicules do not inject an important (dominant) mass flux into the cores of active regions at temperatures >0.6 MK (log T > 5.8). These conclusions apply not only to spicules but also to any process that suddenly heats and accelerates chromospheric plasma (e.g., a chromospheric nanoflare). The traditional picture of coronal heating and chromospheric evaporation appears to remain the most likely explanation of the active region corona. Title: Can the Differential Emission Measure Constrain the Timescale of Energy Deposition in the Corona? Authors: Guennou, C.; Auchère, F.; Klimchuk, J. A.; Bocchialini, K.; Parenti, S. Bibcode: 2013ApJ...774...31G Altcode: 2013arXiv1306.3114G In this paper, the ability of the Hinode/EIS instrument to detect radiative signatures of coronal heating is investigated. Recent observational studies of active region cores suggest that both the low and high frequency heating mechanisms are consistent with observations. Distinguishing between these possibilities is important for identifying the physical mechanism(s) of the heating. The differential emission measure (DEM) tool is one diagnostic that allows us to make this distinction, through the amplitude of the DEM slope coolward of the coronal peak. It is therefore crucial to understand the uncertainties associated with these measurements. Using proper estimations of the uncertainties involved in the problem of DEM inversion, we derive confidence levels on the observed DEM slope. Results show that the uncertainty in the slope reconstruction strongly depends on the number of lines constraining the slope. Typical uncertainty is estimated to be about ±1.0 in the more favorable cases. Title: Ultraviolet and Extreme-ultraviolet Emissions at the Flare Footpoints Observed by Atmosphere Imaging Assembly Authors: Qiu, Jiong; Sturrock, Zoe; Longcope, Dana W.; Klimchuk, James A.; Liu, Wen-Juan Bibcode: 2013ApJ...774...14Q Altcode: 2013arXiv1305.6899Q A solar flare is composed of impulsive energy release events by magnetic reconnection, which forms and heats flare loops. Recent studies have revealed a two-phase evolution pattern of UV 1600 Å emission at the feet of these loops: a rapid pulse lasting for a few seconds to a few minutes, followed by a gradual decay on timescales of a few tens of minutes. Multiple band EUV observations by the Atmosphere Imaging Assembly further reveal very similar signatures. These two phases represent different but related signatures of an impulsive energy release in the corona. The rapid pulse is an immediate response of the lower atmosphere to an intense thermal conduction flux resulting from the sudden heating of the corona to high temperatures (we rule out energetic particles due to a lack of significant hard X-ray emission). The gradual phase is associated with the cooling of hot plasma that has been evaporated into the corona. The observed footpoint emission is again powered by thermal conduction (and enthalpy), but now during a period when approximate steady-state conditions are established in the loop. UV and EUV light curves of individual pixels may therefore be separated into contributions from two distinct physical mechanisms to shed light on the nature of energy transport in a flare. We demonstrate this technique using coordinated, spatially resolved observations of UV and EUV emissions from the footpoints of a C3.2 thermal flare. Title: Structure of solar coronal loops: from miniature to large-scale Authors: Peter, H.; Bingert, S.; Klimchuk, J. A.; de Forest, C.; Cirtain, J. W.; Golub, L.; Winebarger, A. R.; Kobayashi, K.; Korreck, K. E. Bibcode: 2013A&A...556A.104P Altcode: 2013arXiv1306.4685P
Aims: We use new data from the High-resolution Coronal Imager (Hi-C) with its unprecedented spatial resolution of the solar corona to investigate the structure of coronal loops down to 0.2''.
Methods: During a rocket flight, Hi-C provided images of the solar corona in a wavelength band around 193 Å that is dominated by emission from Fe xii showing plasma at temperatures around 1.5 MK. We analyze part of the Hi-C field-of-view to study the smallest coronal loops observed so far and search for the possible substructuring of larger loops.
Results: We find tiny 1.5 MK loop-like structures that we interpret as miniature coronal loops. Their coronal segments above the chromosphere have a length of only about 1 Mm and a thickness of less than 200 km. They could be interpreted as the coronal signature of small flux tubes breaking through the photosphere with a footpoint distance corresponding to the diameter of a cell of granulation. We find that loops that are longer than 50 Mm have diameters of about 2'' or 1.5 Mm, which is consistent with previous observations. However, Hi-C really resolves these loops with some 20 pixels across the loop. Even at this greatly improved spatial resolution, the large loops seem to have no visible substructure. Instead they show a smooth variation in cross-section.
Conclusions: That the large coronal loops do not show a substructure on the spatial scale of 0.1'' per pixel implies that either the densities and temperatures are smoothly varying across these loops or it places an upper limit on the diameter of the strands the loops might be composed of. We estimate that strands that compose the 2'' thick loop would have to be thinner than 15 km. The miniature loops we find for the first time pose a challenge to be properly understood through modeling.

Appendices are available in electronic form at http://www.aanda.org Title: Modeling the Line-of-sight Integrated Emission in the Corona: Implications for Coronal Heating Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2013ApJ...771..115V Altcode: 2013arXiv1304.5439V One of the outstanding problems in all of space science is uncovering how the solar corona is heated to temperatures greater than 1 MK. Though studied for decades, one of the major difficulties in solving this problem has been unraveling the line-of-sight (LOS) effects in the observations. The corona is optically thin, so a single pixel measures counts from an indeterminate number (perhaps tens of thousands) of independently heated flux tubes, all along that pixel's LOS. In this paper we model the emission in individual pixels imaging the active region corona in the extreme ultraviolet. If LOS effects are not properly taken into account, erroneous conclusions regarding both coronal heating and coronal dynamics may be reached. We model the corona as an LOS integration of many thousands of completely independently heated flux tubes. We demonstrate that despite the superposition of randomly heated flux tubes, nanoflares leave distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those data taken with the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. These signatures are readily detected with the time-lag analysis technique of Viall & Klimchuk in 2012. Steady coronal heating leaves a different and equally distinct signature that is also revealed by the technique. Title: Where is Coronal Plasma Heated? Authors: Klimchuk, James A.; Bradshaw, S.; Patsourakos, S.; Tripathi, D. Bibcode: 2013SPD....4420006K Altcode: The coupling between the chromosphere and corona is a question of great current interest. It has long been understood that coronal mass originates in the chromosphere and that the energy which powers the corona flows up through the chromosphere. However, the details of how this happens are now being questioned. In the traditional view, “mechanical” energy flows into the corona in the form of waves or gradually increasing magnetic stresses. The waves and stresses dissipate and heat the plasma. The resulting downward thermal conduction flux causes material to evaporate from the chromosphere and fill the corona. If the heating is steady, an equilibrium is established whereby radiation and thermal conduction balance the energy input. If the heating is impulsive (a nanoflare), the evaporated plasma cools and drains, only to reappear during the next event. In either case, the heating occurs in the corona. A new idea is that the heating occurs instead in the chromosphere. Cold plasma is directly heated to coronal temperatures and then flows upward due to expansion and perhaps also an ejection process. The hot tips of type II spicules are one example, though spicules need not be involved. I will discuss these two fundamentally different scenarios and the observational predictions that they make. A comparison with actual observations leads to the conclusion that only a small fraction of the hot plasma in the corona comes from chromospheric heating. Most coronal plasma is a consequence of heating that occurs in the corona itself. Title: UV and EUV Emissions at the Flare Foot-points Observed by AIA Authors: Qiu, Jiong; Sturrock, Z.; Longcope, D.; Klimchuk, J. A.; Liu, W. Bibcode: 2013SPD....44...53Q Altcode: A solar flare is composed of impulsive energy release events by magnetic reconnection, which forms and heats flare loops. Recent studies have revealed a two-phase evolution pattern of UV 1600A emission at the feet of these loops: a rapid pulse lasting for a few seconds to a few minutes, followed by a gradual decay on timescales of a few tens of minutes. Multiple band EUV observations by AIA further reveal very similar signatures. These two phases represent different but related signatures of an impulsive energy release in the corona. The rapid pulse is an immediate response of the lower atmosphere to an intense thermal conduction flux resulting from the sudden heating of the corona to high temperatures (we rule out energetic particles due to a lack of significant hard X-ray emission). The gradual phase is associated with the cooling of hot plasma that has been evaporated into the corona. The observed footpoint emission is again powered by thermal conduction (and enthalpy), but now during a period when approximate steady state conditions are established in the loop. UV and EUV light curves of individual pixels may therefore be separated into contributions from two distinct physical mechanisms to shed light on the nature of energy transport in a flare. We demonstrate this technique using coordinated, spatially resolved observations of UV and EUV emission from the footpoints of a C3.2 thermal flare. Title: A Survey of Nanoflare Properties in Solar Active Regions Authors: Viall, Nicholeen; Klimchuk, J. A. Bibcode: 2013SPD....44...16V Altcode: We investigate the characteristics of coronal heating using a systematic technique that analyzes the properties of nanoflares in active regions (AR). Our technique computes cooling times, or time lags, using SDO/AIA light curves of all of the coronal AR emission, including the so-called diffuse emission. We recently presented results using this time-lag analysis on NOAA AR 11082 (Viall & Klimchuk 2012). We found that the majority of the pixels had cooling plasma along their line of sight, consistent with impulsive coronal nanoflare heating. Additionally, our results using the AIA 94 channel data showed that the nanoflare energy is stronger in the AR core and weaker in the AR periphery. Are these results representative of the nanoflare characteristics exhibited in the majority of active regions, or is AR 11082 unique? Here we present the time-lag results for a survey of active regions and determine whether these nanoflare patterns are born out in other active regions as well. This research was supported by the NASA Heliophysics Guest Investigator program. Title: Understanding Coronal Heating by Comparing SDO/AIA Observations with Modeled Light Curves Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2013enss.confE..18V Altcode: An important signature of nanoflare heated coronal plasma is the sudden appearance of the plasma at hot temperatures, followed by a comparatively slow cooling and draining phase. This is due to the impulsive nature of nanoflare heating and the heat conduction and mass exchange between the corona and chromosphere. Identifying such nanoflare signatures is complicated by the fact that the solar corona is optically thin: many thousands of flux tubes which are heated completely independently are contributing to the total emission along a given line of sight. One approach has been to analyze isolated features such as coronal loops; however the diffuse emission between and around isolated features contribute as much, if not more to the EUV coronal emission, and therefore is crucial to the understanding of coronal heating. In this study we move beyond isolated features and analyze all of the emission in an entire active region and quiet Sun area. We investigate SDO/AIA light curves, systematically identifying nanoflare signatures. We compare the observations with a model of the corona as a line-of-sight integration of many thousands of completely independently heated flux tubes. We consider that the emission from these flux tubes may be due exclusively to impulsive nanoflare bursts, quasi-steady heating, or a mix of both, depending on the cadence of heat release. We demonstrate that despite the superposition of randomly heated flux tubes, different distributions of nanoflare cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from SDO/AIA. We find that much of the solar corona is heated through impulsive nanoflares. Title: Can the Differential Emission Measure diagnostic be used to constrain the timescale of energy deposition in the corona? Authors: Guennou, C.; Auchère, F.; Klimchuk, J. A.; Bocchialini, K.; Parenti, S. Bibcode: 2013enss.confE..34G Altcode: Differential emission measure (DEM) analysis is a widespread tool used to diagnose the thermal properties of coronal plasmas. The slope of the DEM distribution coolward of the coronal peak (near 3-4MK in active regions) can be used to diagnose the timescale for the energy deposition repeating on a given magnetic strand. Recent AR studies suggest that some active region cores are consistent with low frequency heating mechanisms, where the plasma cools completely before being reheated, while other show consistency with high frequency energy deposition, where rapid reheating causes the temperature to fluctuate about a particular value. Distinguishing between these possibilities is important for identifying the physical mechanism of the heating. It is therefore crucial to understand the uncertainties in measurements of observed DEM slopes. In this work, based on a probabilistic approach and Monte Carlo simulations, we carefully assess the errors in the slopes determined from EIS data. We consider both the random errors due to photon counting statistics, and the systematic errors associated with uncertainties in atomic physics and instrument calibration. The technique developed provides all the solutions consistent with the data and their associated probabilities. We demonstrate how the quality and the accuracy of the inversion are affected by the presence of noises and systematic errors, and we characterise the quality of the DEM inversion and its statistical properties. From these results, estimation of the uncertainties in the reconstructed slopes can be derived, thereby allowing a proper interpretation of the degree of agreement between observations and heating model predictions. Title: Where is Coronal Plasma Heated? Authors: Klimchuk, James A. Bibcode: 2013enss.confE.105K Altcode: The coupling between the chromosphere and corona is a question of great interest at the moment. It has long been understood that coronal mass originates in the chromosphere and that the energy which powers the corona flows up through the chromosphere. However, the details of how this happens are now being questioned. In the traditional view, "mechanical" energy flows into the corona in the form of waves or gradually increasing magnetic stresses. The waves or stresses dissipate and heat the plasma. The resulting downward thermal conduction flux causes material to evaporate from the chromosphere and fill the corona. If the heating is steady, an equilibrium is established whereby radiation and thermal conduction balance the energy input. If the heating is impulsive (a nanoflare), the evaporated plasma cools and drains, only to reappear during the next event. In either case, the heating occurs in the corona. A new idea is that the heating occurs instead in the chromosphere. Cold plasma is directly heated to coronal temperatures and then flows upward due to expansion and perhaps also an ejection process. The hot tips of type II spicules are one example, though spicules need not be involved. I will discuss these two fundamentally different scenarios and the observational predictions that they make. A comparison with actual observations leads to the conclusion that only a small fraction of the hot plasma in the corona comes from chromospheric heating. Most coronal plasma is a consequence of heating that occurs in the corona itself. Title: Diagnosing the Time Dependence of Active Region Core Heating from the Emission Measure. II. Nanoflare Trains Authors: Reep, J. W.; Bradshaw, S. J.; Klimchuk, J. A. Bibcode: 2013ApJ...764..193R Altcode: 2013arXiv1303.4466R The time dependence of heating in solar active regions can be studied by analyzing the slope of the emission measure distribution coolward of the peak. In a previous study we showed that low-frequency heating can account for 0% to 77% of active region core emission measures. We now turn our attention to heating by a finite succession of impulsive events for which the timescale between events on a single magnetic strand is shorter than the cooling timescale. We refer to this scenario as a "nanoflare train" and explore a parameter space of heating and coronal loop properties with a hydrodynamic model. Our conclusions are (1) nanoflare trains are consistent with 86% to 100% of observed active region cores when uncertainties in the atomic data are properly accounted for; (2) steeper slopes are found for larger values of the ratio of the train duration Δ H to the post-train cooling and draining timescale Δ C , where Δ H depends on the number of heating events, the event duration and the time interval between successive events (τ C ); (3) τ C may be diagnosed from the width of the hot component of the emission measure provided that the temperature bins are much smaller than 0.1 dex (4) the slope of the emission measure alone is not sufficient to provide information about any timescale associated with heating—the length and density of the heated structure must be measured for Δ H to be uniquely extracted from the ratio Δ H /Δ C . Title: Study of the EUV intensity variation along observed coronal loops Authors: Lopez Fuentes, M.; Klimchuk, J. A. Bibcode: 2013BAAA...56..399L Altcode: The variation of intensity along coronal loops observed in X-rays and EUV can be used to constrain different coronal heating theories. For instance; thermal nonequilibrium that results from heating located at the footpoints of the loops predicts an intensity structure that is highly non uniform and asymmetric. We study the intensity variation along coronal loops observed with the Transition Region and Coronal Explorer (TRACE) in the 171 channel and find that loops present pronounced spatial fluctuations. Although during the data processing we carefully subtract the background contribution; part of the observed fluctuations is due to a residual effect of this contribution. Even allowing for the imperfect background subtraction; we conclude that most of the analyzed observations are inconsistent with the intensity profiles predicted by some numerical models. Title: The role of type II spicules in the upper solar atmosphere Authors: Klimchuk, J. A. Bibcode: 2012JGRA..11712102K Altcode: 2012arXiv1207.7048K We examine the suggestion that most of the hot plasma in the Sun's corona comes from type II spicule material that is heated as it is ejected from the chromosphere. This contrasts with the traditional view that the corona is filled via chromospheric evaporation that results from coronal heating. We explore the observational consequences of a hypothetical spicule dominated corona and conclude from the large discrepancy between predicted and actual observations that only a small fraction of the hot plasma can be supplied by spicules (<2% in active regions, <5% in the quiet Sun, and <8% in coronal holes). The red-blue asymmetries of EUV spectral lines and the ratio of lower transition region (LTR;T ≤ 0.1 MK) to coronal emission measures are both predicted to be 2 orders of magnitude larger than observed. Furthermore, hot spicule material would cool dramatically by adiabatic expansion as it rises into the corona, so substantial coronal heating would be needed to maintain the high temperatures that are seen at all altitudes. We suggest that the corona contains a mixture of thin strands, some of which are populated by spicule injections, but most of which are not. A majority of the observed hot emission originates in non-spicule strands and is explained by traditional coronal heating models. However, since these models predict far too little emission from the LTR, most of this emission comes from the bulk of the spicule material that is only weakly heated and visible in He II (304 Å) as it falls back to the surface. Title: The Role of Type II Spicules in the Upper Solar Atmosphere Authors: Klimchuk, J. A. Bibcode: 2012AGUFMSH31B..07K Altcode: We examine the suggestion that most of the hot plasma in the Sun's corona comes from type II spicule material that is heated as it is ejected from the chromosphere. This contrasts with the traditional view that the corona is filled via chromospheric evaporation that results from coronal heating. We explore the observational consequences of a hypothetical spicule dominated corona and conclude from the large discrepancy between predicted and actual observations that only a small fraction of the hot plasma can be supplied by spicules (<2% in active regions and <5% in the quiet Sun). The red-blue asymmetries of EUV spectral lines and the ratio of lower transition region (LTR; T<0.1 MK) to coronal emission measures are both predicted to be 2 orders of magnitude larger than observed. Furthermore, hot spicule material would cool dramatically by adiabatic expansion as it rises into the corona, so coronal heating would likely be required to maintain the high temperatures that are seen at all altitudes. The necessity of coronal heating seems inescapable. Traditional coronal heating models predict far too little emission from the LTR, and we suggest that this emission comes primarily from the bulk of the spicule material that is heated to <0.1 MK and is visible in He II (304 A) as it falls back to the surface. Title: The Role of Spicules in Explaining the Corona and Transition Region Authors: Klimchuk, J. Bibcode: 2012IAUSS...6E.107K Altcode: A portion of the material in the newly-discovered type II spicules is heated to coronal temperatures and contributes to the hot emission that we observe. What fraction of the coronal plasma can be attributed to spicules and what fraction must be explained by ordinary coronal heating? Is the as yet unexplained bright emission from the lower transition region (T < 0.1 MK) due to spicules? I will address these questions and offer suggestions about future high-resolution observations, both space and ground-based, that can provide important information about this fascinating phenomenon. Title: Diagnosing the Time-Dependence of Active Region Core Heating Using Emission Measures Authors: Klimchuk, J. A.; Bradshaw, S. J.; Reep, J. W. Bibcode: 2012AGUFMSH42A..01K Altcode: It is widely believed that the cross-field spatial scale of coronal heating is small, so that the fundamental plasma structures (loop strands) are spatially unresolved. We therefore must appeal to diagnostic techniques that are not strongly affected by spatial averaging. One valuable observable is the emission measure distribution, EM(T), which indicates how much material is present at each temperature. The slope of the distribution coolward of its peak is related to the frequency of the presumed impulsive energy release. Nanoflares that have a long delay before repeating on the same loop strand give rise to shallow slopes, while nanoflares that repeat with a timescale shorter than a cooling time (or truly steady heating) give rise to steep slopes. Comparing recent Hinode observations with hydrodynamic loop simulations, we find that about 36% of active region cores are consistent with low-frequency nanoflares. The observational uncertainties are large, however, so as many as 77% or as few as none are consistent with low-frequency nanoflares when the uncertainties are taken into account. Constraining the time dependence of the heating is important for identifying the physical mechanism. Title: Nanoflare Heating of the Solar Corona: Comparing SDO/AIA Observations with Modeled Light Curves Authors: Viall, N. M.; Klimchuk, J. A. Bibcode: 2012AGUFMSH42A..03V Altcode: A significant outstanding issue in current solar and astrophysical research is that of the heating of the solar corona. Coronal plasma is typically measured to be at temperatures near ~1-3 MK. Is the majority of the coronal plasma maintained at these temperatures through a form of quasi-steady heating, or is this simply a measure of the average temperature of widely varying, impulsively heated coronal plasma? Addressing even this basic question is complicated by the fact that the corona is optically thin: many thousands of flux tubes which are heated completely independently are contributing to the total emission along a given line of sight. There is a large body of work focused on the heating of isolated features - coronal loops in active regions- which are impulsively heated, however understanding of the diffuse emission between loops and the emission from the quiet Sun are also crucial. Therefore in this study we move beyond isolated features and analyze all of the emission in an entire active region and quiet Sun area from all contributing flux tubes. We investigate light curves systematically using SDO/AIA observations. We also model the corona as a line-of-sight integration of many thousands of completely independently heated flux tubes. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release. We demonstrate that despite the superposition of randomly heated flux tubes, different distributions of nanoflare cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from SDO/AIA. We discuss the quiet Sun and active region emission in the context of these predicted nanoflare signatures. Title: Exploring Small Spatial Scales in the Transition Region and Solar Corona with the Very High Angular Resolution Imaging Spectrometer (VERIS) Authors: Chua, D. H.; Korendyke, C. M.; Vourlidas, A.; Brown, C. M.; Tun-Beltran, S.; Klimchuk, J. A.; Landi, E.; Seely, J.; Davila, J. M.; Hagood, R.; Roberts, D.; Shepler, E.; Feldman, R.; Moser, J.; Shea, J. Bibcode: 2012AGUFMSH33A2217C Altcode: Theoretical and experimental investigations of the transition region and coronal loops point to the importance of processes occurring on small spatial scales in governing the strong dynamics and impulsive energy release in these regions. As a consequence, high spatial, temporal, and temperature resolution over a broad temperature range, and accuracy in velocity and density determinations are all critical observational parameters. Current instruments lack one or more of these properties. These observational deficiencies have created a wide array of opposing descriptions of coronal loop heating and questions such as whether or not the plasma within coronal loops is multi-thermal or isothermal. High spectral and spatial resolution spectroscopic data are absolutely required to resolve these controversies and to advance our understanding of the dynamics within the solar atmosphere. We will achieve this with the Very High Angular Resolution Imaging Spectrometer (VERIS) sounding rocket payload. VERIS consists of an off-axis paraboloid telescope feeding a very high angular resolution, extreme ultraviolet (EUV) imaging spectrometer that will provide the first ever, simultaneous sub-arcsecond (0.16 arcsecond/pixel) spectra in bright lines needed to study plasma structures in the transition region, quiet corona, and active region core. It will do so with a spectral resolution of >5000 to allow Doppler velocity determinations to better than 3 km/s. VERIS uses a novel two-element, normal incidence optical design with highly reflective, broad wavelength coverage EUV coatings to access a spectral range with broad temperature coverage (0.03-15 MK) and density-sensitive line ratios. Combined with Hinode Solar Optical Telescope (SOT) and ground based observatories, VERIS will deliver simultaneous observations of the entire solar atmosphere from the photosphere to the multi-million degree corona at sub-arcsecond resolution for the first time ever, allowing us to understand the missing link between chromospheric structures and the corona. VERIS will be launched from White Sands Missile Range in early 2013. This paper presents a progress report on the VERIS payload and a summary of observations planned to further our understanding of the fine-scale structure of individual coronal loops and the heating mechanisms operating within them. Title: Enthalpy-based Thermal Evolution of Loops. III. Comparison of Zero-dimensional Models Authors: Cargill, P. J.; Bradshaw, S. J.; Klimchuk, J. A. Bibcode: 2012ApJ...758....5C Altcode: Zero-dimensional (0D) hydrodynamic models provide a simple and quick way to study the thermal evolution of coronal loops subjected to time-dependent heating. This paper presents a comparison of a number of 0D models that have been published in the past and is intended to provide a guide for those interested in either using the old models or developing new ones. The principal difference between the models is the way the exchange of mass and energy between corona, transition region, and chromosphere is treated, as plasma cycles into and out of a loop during a heating-cooling cycle. It is shown that models based on the principles of mass and energy conservation can give satisfactory results at some or, in the case of the Enthalpy-based Thermal Evolution of Loops model, all stages of the loop evolution. Empirical models can have significant difficulties in obtaining accurate behavior due to invocation of assumptions incompatible with the correct exchange of mass and energy between corona, transition region, and chromosphere. Title: Diagnosing the Time-dependence of Active Region Core Heating from the Emission Measure. I. Low-frequency Nanoflares Authors: Bradshaw, S. J.; Klimchuk, J. A.; Reep, J. W. Bibcode: 2012ApJ...758...53B Altcode: 2012arXiv1209.0737B Observational measurements of active region emission measures contain clues to the time dependence of the underlying heating mechanism. A strongly nonlinear scaling of the emission measure with temperature indicates a large amount of hot plasma relative to warm plasma. A weakly nonlinear (or linear) scaling of the emission measure indicates a relatively large amount of warm plasma, suggesting that the hot active region plasma is allowed to cool and so the heating is impulsive with a long repeat time. This case is called low-frequency nanoflare heating, and we investigate its feasibility as an active region heating scenario here. We explore a parameter space of heating and coronal loop properties with a hydrodynamic model. For each model run, we calculate the slope α of the emission measure distribution EM(T)vpropT α. Our conclusions are: (1) low-frequency nanoflare heating is consistent with about 36% of observed active region cores when uncertainties in the atomic data are not accounted for; (2) proper consideration of uncertainties yields a range in which as many as 77% of observed active regions are consistent with low-frequency nanoflare heating and as few as zero; (3) low-frequency nanoflare heating cannot explain observed slopes greater than 3; (4) the upper limit to the volumetric energy release is in the region of 50 erg cm-3 to avoid unphysical magnetic field strengths; (5) the heating timescale may be short for loops of total length less than 40 Mm to be consistent with the observed range of slopes; (6) predicted slopes are consistently steeper for longer loops. Title: Active Region Moss: Doppler Shifts from Hinode/Extreme-ultraviolet Imaging Spectrometer Observations Authors: Tripathi, Durgesh; Mason, Helen E.; Klimchuk, James A. Bibcode: 2012ApJ...753...37T Altcode: 2012arXiv1204.6550T Studying the Doppler shifts and the temperature dependence of Doppler shifts in moss regions can help us understand the heating processes in the core of the active regions. In this paper, we have used an active region observation recorded by the Extreme-ultraviolet Imaging Spectrometer (EIS) on board Hinode on 2007 December 12 to measure the Doppler shifts in the moss regions. We have distinguished the moss regions from the rest of the active region by defining a low-density cutoff as derived by Tripathi et al. in 2010. We have carried out a very careful analysis of the EIS wavelength calibration based on the method described by Young et al. in 2012. For spectral lines having maximum sensitivity between log T = 5.85 and log T = 6.25 K, we find that the velocity distribution peaks at around 0 km s-1 with an estimated error of 4-5 km s-1. The width of the distribution decreases with temperature. The mean of the distribution shows a blueshift which increases with increasing temperature and the distribution also shows asymmetries toward blueshift. Comparing these results with observables predicted from different coronal heating models, we find that these results are consistent with both steady and impulsive heating scenarios. However, the fact that there are a significant number of pixels showing velocity amplitudes that exceed the uncertainty of 5 km s-1 is suggestive of impulsive heating. Clearly, further observational constraints are needed to distinguish between these two heating scenarios. Title: Evidence for Widespread Cooling in an Active Region Observed with the SDO Atmospheric Imaging Assembly Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2012ApJ...753...35V Altcode: 2012arXiv1202.4001V A well-known behavior of EUV light curves of discrete coronal loops is that the peak intensities of cooler channels or spectral lines are reached at progressively later times than hotter channels. This time lag is understood to be the result of hot coronal loop plasma cooling through these lower respective temperatures. However, loops typically comprise only a minority of the total emission in active regions (ARs). Is this cooling pattern a common property of AR coronal plasma, or does it only occur in unique circumstances, locations, and times? The new Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) data provide a wonderful opportunity to answer this question systematically for an entire AR. We measure the time lag between pairs of SDO/AIA EUV channels using 24 hr of images of AR 11082 observed on 2010 June 19. We find that there is a time-lag signal consistent with cooling plasma, just as is usually found for loops, throughout the AR including the diffuse emission between loops for the entire 24 hr duration. The pattern persists consistently for all channel pairs and choice of window length within the 24 hr time period, giving us confidence that the plasma is cooling from temperatures of greater than 3 MK, and sometimes exceeding 7 MK, down to temperatures lower than ~0.8 MK. This suggests that the bulk of the emitting coronal plasma in this AR is not steady; rather, it is dynamic and constantly evolving. These measurements provide crucial constraints on any model which seeks to describe coronal heating. Title: A cellular automaton model for coronal heating Authors: López Fuentes, M. C.; Klimchuk, J. A. Bibcode: 2012IAUS..286..433L Altcode: We present a simple coronal heating model based on a cellular automaton approach. Following Parker's suggestion (1988), we consider the corona to be made up of elemental magnetic strands that accumulate magnetic stress due to the photospheric displacements of their footpoints. Magnetic energy is eventually released in small scale reconnection events. The model consists of a 2D grid in which strand footpoints travel with random displacements simulating convective motions. Each time two strands interact, a critical condition is tested (as in self-organized critical models), and if the condition is fulfilled, the strands reconnect and energy is released. We model the plasma response to the heating events and obtain synthetic observations. We compare the output of the model with real observations from Hinode/XRT and discuss the implications of our results for coronal heating. Title: Enthalpy-based Thermal Evolution of Loops. II. Improvements to the Model Authors: Cargill, P. J.; Bradshaw, S. J.; Klimchuk, J. A. Bibcode: 2012ApJ...752..161C Altcode: 2012arXiv1204.5960C This paper develops the zero-dimensional (0D) hydrodynamic coronal loop model "Enthalpy-based Thermal Evolution of Loops" (EBTEL) proposed by Klimchuk et al., which studies the plasma response to evolving coronal heating, especially impulsive heating events. The basis of EBTEL is the modeling of mass exchange between the corona and transition region (TR) and chromosphere in response to heating variations, with the key parameter being the ratio of the TR to coronal radiation. We develop new models for this parameter that now include gravitational stratification and a physically motivated approach to radiative cooling. A number of examples are presented, including nanoflares in short and long loops, and a small flare. The new features in EBTEL are important for accurate tracking of, in particular, the density. The 0D results are compared to a 1D hydro code (Hydrad) with generally good agreement. EBTEL is suitable for general use as a tool for (1) quick-look results of loop evolution in response to a given heating function, (2) extensive parameter surveys, and (3) situations where the modeling of hundreds or thousands of elemental loops is needed. A single run takes a few seconds on a contemporary laptop. Title: Nanoflare Evidence from Analysis of the X-Ray Variability of an Active Region Observed with Hinode/XRT Authors: Terzo, S.; Reale, F.; Miceli, M.; Kano, R.; Tsuneta, S.; Klimchuk, J. A. Bibcode: 2012ASPC..455..245T Altcode: 2012arXiv1201.5482T The heating of the solar corona is one of the big questions in astrophysics. Rapid pulses called nanoflares are among the best candidate mechanisms. The analysis of the time variability of coronal X-ray emission is potentially a very useful tool to detect impulsive events. We analyze the small-scale variability of a solar active region in a high cadence Hinode/XRT observation. The dataset allows us to detect very small deviations of emission fluctuations from the distribution expected for a constant rate. We discuss the deviations in the light of the pulsed-heating scenario. Title: Nanoflare Properties throughout Active Regions: Comparing SDO/AIA Observations with Modeled Active Region Light Curves Authors: Viall, Nicholeen; Klimchuk, J. Bibcode: 2012AAS...22030904V Altcode: Coronal plasma in active regions is typically measured to be at temperatures near 1-3 MK. Is the majority of the coronal plasma in hydrostatic equilibrium, maintained at these temperatures through a form of quasi-steady heating, or is this simply a measure of the average temperature of widely varying, impulsively heated coronal plasma? Addressing this question is complicated by the fact that the corona is optically thin: many thousands of flux tubes which are heated completely independently are contributing to the total emission along a given line of sight. There is a large body of work focused on the heating of isolated features - coronal loops - which are impulsively heated, however it is the diffuse emission between loops which often comprises the majority of active region emission. Therefore in this study we move beyond isolated features and analyze all of the emission in an entire active region from all contributing flux tubes. We investigate light curves systematically using SDO/AIA observations. We also model the active region corona as a line-of-sight integration of many thousands of completely independently heated flux tubes. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release. We demonstrate that despite the superposition of randomly heated flux tubes, different distributions of nanoflare cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from SDO/AIA. We conclude that the majority of the active region plasma is not maintained in hydrostatic equilibrium, rather it is undergoing dynamic heating and cooling cycles. The observed emission is consistent with heating through impulsive nanoflares, whose energy is a function of location within the active region.

This research was supported by an appointment to the NASA Postdoctoral Program at GSFC/NASA. Title: Understanding Coronal Heating with Emission Measure Distributions Authors: Klimchuk, James A.; Tripathi, D.; Bradshaw, S. J.; Mason, H. E. Bibcode: 2012AAS...22042302K Altcode: It is widely believed that the cross-field spatial scale of coronal heating is small, so that the fundamental plasma structures (loop strands) are spatially unresolved. We therefore must appeal to diagnostic techniques that are not strongly affected by spatial averaging. One valuable observable is the emission measure distribution, EM(T), which indicates how much material is present at each temperature. Using data from the Extreme-ultraviolet Imaging Spectrograph on the Hinode mission, we have determined emission measure distributions in the cores of two active regions. The distributions have power law slopes of approximately 2.4 coolward of the peak. We compare these slopes, as well as the amount of emission measure at very high temperature, with the predictions of a series of models. The models assume impulsive heating (nanoflares) in unresolved strands and take full account of nonequilibrium ionization. Title: Division II: Sun and Heliosphere Authors: Martínez Pillet, Valentín; Klimchuk, James A.; Melrose, Donald B.; Cauzzi, Gianna; van Driel-Gesztelyi, Lidia; Gopalswamy, Natchimuthuk; Kosovichev, Alexander; Mann, Ingrid; Schrijver, Carolus J. Bibcode: 2012IAUTA..28...61M Altcode: 2012IAUTA..28...61P The solar activity cycle entered a prolonged quiet phase that started in 2008 and ended in 2010. This minimum lasted for a year longer than expected and all activity proxies, as measured from Earth and from Space, reached minimum values never observed before (de Toma, 2012). The number of spotless days from 2006 to 2009 totals 800, the largest ever recorded in modern times. Solar irradiance was at historic minimums. The interplanetary magnetic field was measured at values as low as 2.9 nT and the cosmic rays were observed at records-high. While rumors spread that the Sun could be entering a grand minimum quiet phase (such as the Maunder minimum of the XVII century), activity took over in 2010 and we are now well into Solar Cycle 24 (albeit, probably, a low intensity cycle), approaching towards a maximum due by mid 2013. In addition to bringing us the possibility to observe a quiet state of the Sun and of the Heliosphere that was previously not recorded with modern instruments, the Sun has also shown us how little we know about the dynamo mechanism that drives its activity as all solar cycle predictions failed to see this extended minimum coming. Title: Commission 10: Solar Activity Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk, James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul; Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi; Vršnak, Bojan; Yan, Yihua Bibcode: 2012IAUTA..28...69V Altcode: Commission 10 of the International Astronomical Union has more than 650 members who study a wide range of activity phenomena produced by our nearest star, the Sun. Solar activity is intrinsically related to solar magnetic fields and encompasses events from the smallest energy releases (nano- or even picoflares) to the largest eruptions in the Solar System, coronal mass ejections (CMEs), which propagate into the Heliosphere reaching the Earth and beyond. Solar activity is manifested in the appearance of sunspot groups or active regions, which are the principal sources of activity phenomena from the emergence of their magnetic flux through their dispersion and decay. The period 2008-2009 saw an unanticipated extended solar cycle minimum and unprecedentedly weak polar-cap and heliospheric field. Associated with that was the 2009 historical maximum in galactic cosmic rays flux since measurements begun in the middle of the 20th Century. Since then Cycle 24 has re-started solar activity producing some spectacular eruptions observed with a fleet of spacecraft and ground-based facilities. In the last triennium major advances in our knowledge and understanding of solar activity were due to continuing success of space missions as SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched by the breathtaking images of the solar atmosphere produced by the Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the framework of NASA's Living with a Star program. In August 2012, at the time of the IAU General Assembly in Beijing when the mandate of this Commission ends, we will be in the unique position to have for the first time a full 3-D view of the Sun and solar activity phenomena provided by the twin STEREO missions about 120 degrees behind and ahead of Earth and other spacecraft around the Earth and ground-based observatories. These new observational insights are continuously posing new questions, inspiring and advancing theoretical analysis and modelling, improving our understanding of the physics underlying magnetic activity phenomena. Commission 10 reports on a vigorously evolving field of research produced by a large community. The number of refereed publications containing `Sun', `heliosphere', or a synonym in their abstracts continued the steady growth seen over the preceding decades, reaching about 2000 in the years 2008-2010, with a total of close to 4000 unique authors. This report, however, has its limitations and it is inherently incomplete, as it was prepared jointly by the members of the Organising Committee of Commission 10 (see the names of the primary contributors to the sections indicated in parentheses) reflecting their fields of expertise and interest. Nevertheless, we believe that it is a representative sample of significant new results obtained during the last triennium in the field of solar activity. Title: Spectroscopic Diagnostics and Heating of Active Region Cores Authors: Tripathi, D.; Mason, H. E.; Klimchuk, J. A. Bibcode: 2012decs.confE..92T Altcode: It is widely believed that we are still far from spatially resolving the fundamental plasma structures in solar corona. Therefore, we must use spectroscopic diagnostic techniques such as emission measure distribution (EM(T)) and Doppler shifts that are not affected by spatial averaging. Using observations recorded by the Extreme ultraviolet Imaging Spectrometer we have studies emission measure (EM) distribution and Doppler shift in the moss and inter-moss regions. The EM distributions obtained for moss regions cab be reproduced by considering strong coronal condensation scenario suggesting bulk downflow of the plasma. Doppler shift measurements for the moss regions show that almost all the moss regions are red-shifted with velocities up to 15km/s with mean velocity of 5 km/s. However, the uncertainty on the Doppler shift was large. The EM distributions obtained for inter-moss regions have power law slopes of approximately 2.4 coolward of the peak. We compare the EM for inter-moss region with that obtained from nanoflare model using EBTEL (Enthalpy-Based Thermal Evolution of Loops). Our results suggest that the EM distribution for both the moss as well as inter-moss regions and Doppler shift in the moss regions can be explained by nanoflare heating. IRIS will provide a better account of the Doppler shift in the moss regions, which will dramatically enhance our understanding of the heating of active region core. Title: EBTEL: Enthalpy-Based Thermal Evolution of Loops Authors: Klimchuk, J. A.; Patsourakos, S.; Cargill, P. J. Bibcode: 2012ascl.soft03007K Altcode: Observational and theoretical evidence suggests that coronal heating is impulsive and occurs on very small cross-field spatial scales. A single coronal loop could contain a hundred or more individual strands that are heated quasi-independently by nanoflares. It is therefore an enormous undertaking to model an entire active region or the global corona. Three-dimensional MHD codes have inadequate spatial resolution, and 1D hydro codes are too slow to simulate the many thousands of elemental strands that must be treated in a reasonable representation. Fortunately, thermal conduction and flows tend to smooth out plasma gradients along the magnetic field, so "0D models" are an acceptable alternative. We have developed a highly efficient model called Enthalpy-Based Thermal Evolution of Loops (EBTEL) that accurately describes the evolution of the average temperature, pressure, and density along a coronal strand. It improves significantly upon earlier models of this type-in accuracy, flexibility, and capability. It treats both slowly varying and highly impulsive coronal heating; it provides the differential emission measure distribution, DEM(T), at the transition region footpoints; and there are options for heat flux saturation and nonthermal electron beam heating. EBTEL gives excellent agreement with far more sophisticated 1D hydro simulations despite using four orders of magnitude less computing time. It promises to be a powerful new tool for solar and stellar studies. Title: The Pros and Cons of 1D vs. 3D Modeling Authors: Klimchuk, James A. Bibcode: 2012decs.confE..25K Altcode: Advances in computing capability have led to tremendous improvements in 3D modeling. Entire active regions are being simulated in what might be described as a first principles way, in which plasma heating is treated self consistently rather than through the specification of heating functions. There are limitations to this approach, however, as actual heating mechanisms on the Sun involve spatial scales orders of magnitude smaller than what these simulations can resolve. Other simulations begin to resolve these scales, but they only treat a tiny volume and do not include the all important coupling with larger scales or with other parts of the atmosphere, and so cannot be readily compared with observations. Finally, 1D hydrodynamic models capture the field-aligned evolution of the plasma extremely well and are ideally suited for data comparison, but they treat the heating in a totally ad hoc manner. All of these approaches have important contributions to make, but we must be aware of their limitations. I will highlight some of the strengths and weaknesses of each. Title: Determining the Typical Nanoflare Cadence in Active Regions: Comparing SDO/AIA Observations with Modeled Active Region Light Curves Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2012decs.confE..40V Altcode: Coronal plasma in active regions is typically measured to be at temperatures near 1-3 MK. Is the majority of the coronal plasma in hydrostatic equilibrium, maintained at these temperatures through a form of quasi-steady heating, or is this simply a measure of the average temperature of widely varying, impulsively heated coronal plasma which is continually undergoing heating and cooling cycles? Addressing this question is complicated by the fact that the corona is optically thin: many thousands of strands which are heated completely independently are contributing to the total emission along a given line of sight. There is a large body of work focused on the heating of coronal loops, which are impulsively heated, however it is the diffuse emission between loops which often comprises the majority of active region emission. Therefore, a different and necessary approach to analyzing active region heating is to analyze all of the emission in an active region, and account for emission along the line of sight from all of the contributing strands. We investigate light curves systematically in an entire active region using SDO/AIA observations. We also model the active region corona as a line-of-sight integration of many thousands of completely independently heated strands. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release on each strand. We examine a full range of heat cadences from effectively steady (heat pulse repeat time << plasma cooling time) to fully impulsive (heat pulse repeat time >> plasma cooling time) and model the resulting emission when superposing strands undergoing these differing heat cycles. We demonstrate that despite the superposition of randomly heated strands, different distributions of heat cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from the AIA telescopes on SDO. Using these model predictions in conjunction with SDO/AIA observations, we evaluate the typical cadence of heat release in different active regions and patterns therein, which is a crucial constraint on coronal heating mechanisms. Title: The Origin of the EUV Late Phase: A Case Study of the C8.8 Flare on 2010 May 5 Authors: Hock, R. A.; Woods, T. N.; Klimchuk, J. A.; Eparvier, F. G.; Jones, A. R. Bibcode: 2012arXiv1202.4819H Altcode: Since the launch of NASA's Solar Dynamics Observatory on 2010 February 11, the Extreme ultraviolet Variability Experiment (EVE) has observed numerous flares. One interesting feature observed by EVE is that a subset of flares exhibit an additional enhancement of the 2-3 million K emission several hours after the flare's soft X-ray emission. From the Atmospheric Imaging Assembly (AIA) images, we observe that this secondary emission, dubbed the EUV late phase, occurs in the same active region as the flare but not in the same coronal loops. Here, we examine the C8.8 flare that occurred on 2010 May 5 as a case study of EUV late phase flares. In addition to presenting detailed observations from both AIA and EVE, we develop a physical model of this flare and test it using the Enthalpy Based Thermal Evolution of Loops (EBTEL) model. Title: Evidence of nanoflare heating in coronal loops observed with Hinode/XRT and SDO/AIA Authors: López Fuentes, M. C.; Klimchuk, J. A. Bibcode: 2012BAAA...55..103L Altcode: We study a series of coronal loop lightcurves from X-ray and EUV observations. In search for signatures of nanoflare heating, we analyze the statistical properties of the observed lightcurves and compare them with synthetic cases obtained with a 2D cellular-automaton model based on nanoflare heating driven by photospheric motions. Our analysis shows that the observed and the model lightcurves have similar statistical properties. The asymmetries observed in the distribution of the intensity fluctuations indicate the possible presence of widespread cooling processes in sub-resolution magnetic strands. Title: Understanding Coronal Heating with Emission Measure Distributions Authors: Klimchuk, J. A.; Tripathi, D.; Bradshaw, S. J.; Mason, H. E. Bibcode: 2011AGUFMSH43F..03K Altcode: It is widely believed that the cross-field spatial scale of coronal heating is small, so that the fundamental plasma structures (loop strands) are spatially unresolved. We therefore must appeal to diagnostic techniques that are not strongly affected by spatial averaging. One valuable observable is the emission measure distribution, EM(T), which indicates how much material is present at each temperature. Using data from the Extreme-ultraviolet Imaging Spectrograph on the Hinode mission, we have determined emission measure distributions in the cores of two active regions. The distributions have power law slopes of approximately 2.4 coolward of the peak. We compare these slopes, as well as the amount of emission measure at very high temperature, with the predictions of a series of models. The models assume impulsive heating (nanoflares) in unresolved strands and take full account of nonequilibrium ionization. A variety of nanoflare properties and initial conditions are considered. We also comment on the selection of spectral lines for upcoming missions like Solar Orbiter. Title: Determining the Typical Nanoflare Cadence in Active Regions: Modeling Light Curves of Active Regions Authors: Viall, N. M.; Klimchuk, J. A. Bibcode: 2011AGUFMSH33B2057V Altcode: Active region coronal loops visible at 1MK are likely composed of many unresolved strands, heated by storms of impulsive nanoflares. Though well-studied, these loops often contribute only a fraction of the total emission in an active region; the degree to which the entire active region is heated in the same manner as loops are is highly debated. Is the majority of coronal active region plasma heated impulsively, or is the majority of the heating quasi-steady? Addressing this question is complicated by the fact that the corona is optically thin: many thousands of strands which are heated completely independently are contributing to the total emission along a given line of sight. Furthermore, certain geometries preclude even the best background subtraction methods from fully isolating the emission from even a single coronal loop. Therefore, a different and necessary approach to analyzing active region heating is to account for emission along the line of sight from all of the contributing strands. We model the active region corona as a line-of-sight integration of many thousands of completely independently heated strands. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release on each strand. We examine a full range of heat cadences from effectively steady (heat pulse repeat time << plasma cooling time) to fully impulsive (heat pulse repeat time >> plasma cooling time) and model the resulting emission when superposing strands undergoing these differing heat cycles. We demonstrate that despite the superposition of randomly heated strands, different distributions of heat cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from the AIA telescopes on SDO. For this reason, high time cadence spectral information for lines sensitive to the 1-10 MK range will be especially useful in future missions. Using these model predictions, we evaluate the typical cadence of heat release in different active regions and patterns therein, which is a crucial constraint on coronal heating mechanisms. Title: New Solar Extreme-ultraviolet Irradiance Observations during Flares Authors: Woods, Thomas N.; Hock, Rachel; Eparvier, Frank; Jones, Andrew R.; Chamberlin, Phillip C.; Klimchuk, James A.; Didkovsky, Leonid; Judge, Darrell; Mariska, John; Warren, Harry; Schrijver, Carolus J.; Webb, David F.; Bailey, Scott; Tobiska, W. Kent Bibcode: 2011ApJ...739...59W Altcode: New solar extreme-ultraviolet (EUV) irradiance observations from the NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment provide full coverage in the EUV range from 0.1 to 106 nm and continuously at a cadence of 10 s for spectra at 0.1 nm resolution and even faster, 0.25 s, for six EUV bands. These observations can be decomposed into four distinct characteristics during flares. First, the emissions that dominate during the flare's impulsive phase are the transition region emissions, such as the He II 30.4 nm. Second, the hot coronal emissions above 5 MK dominate during the gradual phase and are highly correlated with the GOES X-ray. A third flare characteristic in the EUV is coronal dimming, seen best in the cool corona, such as the Fe IX 17.1 nm. As the post-flare loops reconnect and cool, many of the EUV coronal emissions peak a few minutes after the GOES X-ray peak. One interesting variation of the post-eruptive loop reconnection is that warm coronal emissions (e.g., Fe XVI 33.5 nm) sometimes exhibit a second large peak separated from the primary flare event by many minutes to hours, with EUV emission originating not from the original flare site and its immediate vicinity, but rather from a volume of higher loops. We refer to this second peak as the EUV late phase. The characterization of many flares during the SDO mission is provided, including quantification of the spectral irradiance from the EUV late phase that cannot be inferred from GOES X-ray diagnostics. Title: Emission Measure Distribution and Heating of Two Active Region Cores Authors: Tripathi, Durgesh; Klimchuk, James A.; Mason, Helen E. Bibcode: 2011ApJ...740..111T Altcode: 2011arXiv1107.4480T Using data from the Extreme-ultraviolet Imaging Spectrometer aboard Hinode, we have studied the coronal plasma in the core of two active regions. Concentrating on the area between opposite polarity moss, we found emission measure distributions having an approximate power-law form EMvpropT 2.4 from log T = 5.5 up to a peak at log T = 6.55. We show that the observations compare very favorably with a simple model of nanoflare-heated loop strands. They also appear to be consistent with more sophisticated nanoflare models. However, in the absence of additional constraints, steady heating is also a viable explanation. Title: Patterns of Nanoflare Storm Heating Exhibited by an Active Region Observed with Solar Dynamics Observatory/Atmospheric Imaging Assembly Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2011ApJ...738...24V Altcode: 2011arXiv1106.4196V It is largely agreed that many coronal loops—those observed at a temperature of about 1 MK—are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Are these regions also heated impulsively, or is the heating quasi-steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in six different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function the nanoflare storm properties. We show that while some sets of light curves exhibit clear evidence of cooling after nanoflare storms, other cases are less straightforward to interpret. Complications arise because of line-of-sight integration through many different structures, the broadband nature of the AIA channels, and because physical properties can change substantially depending on the magnitude of the energy release. Nevertheless, the light curves exhibit predictable and understandable patterns consistent with impulsive nanoflare heating. Title: Widespread Nanoflare Variability Detected with Hinode/X-Ray Telescope in a Solar Active Region Authors: Terzo, Sergio; Reale, Fabio; Miceli, Marco; Klimchuk, James A.; Kano, Ryouhei; Tsuneta, Saku Bibcode: 2011ApJ...736..111T Altcode: 2011arXiv1105.2506T It is generally agreed that small impulsive energy bursts called nanoflares are responsible for at least some of the Sun's hot corona, but whether they are the explanation for most of the multimillion-degree plasma has been a matter of ongoing debate. We present here evidence that nanoflares are widespread in an active region observed by the X-Ray Telescope on board the Hinode mission. The distributions of intensity fluctuations have small but important asymmetries, whether taken from individual pixels, multipixel subregions, or the entire active region. Negative fluctuations (corresponding to reduced intensity) are greater in number but weaker in amplitude, so that the median fluctuation is negative compared to a mean of zero. Using Monte Carlo simulations, we show that only part of this asymmetry can be explained by Poisson photon statistics. The remainder is explainable through a tendency for exponentially decreasing intensity, such as would be expected from a cooling plasma produced from a nanoflare. We suggest that nanoflares are a universal heating process within active regions. Title: Heating of Active Regions by Impulsive Nanoflares Authors: Viall, Nicholeen Mary; Klimchuk, James A. Bibcode: 2011shin.confE..57V Altcode: It has been proposed that plasma on active regions may be contributing plasma to the slow solar wind. If this is the case, then understanding the heating and dynamics of active regions adds vital knowledge to our understanding of the heating and acceleration of the slow solar wind. It seems largely agreed that many coronal loops--those observed at a temperature of about 1 MK--are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Are these regions also heated impulsively, or is the heating quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function of the nanoflare storm properties. These orderings, or time lags, are clearly exhibited in loop observations in all channels. What is especially exciting is that we identify these time lag patterns in observations of the seemingly steady diffuse corona as well. We model the diffuse corona as a line-of-sight integration of many thousands of completely independent, impulsively heated strands. The time lags of the simulated and actual observations are in excellent agreement. Our results suggest that impulsive nanoflare heating is ubiquitous within active regions. Title: What Dominates the Coronal Emission Spectrum During the Cycle of Impulsive Heating and Cooling? Authors: Bradshaw, S. J.; Klimchuk, J. A. Bibcode: 2011ApJS..194...26B Altcode: The "smoking gun" of small-scale, impulsive events heating the solar corona is expected to be the presence of hot (>5 MK) plasma. Evidence for this has been scarce, but has gradually begun to accumulate due to recent studies designed to constrain the high-temperature part of the emission measure distribution. However, the detected hot component is often weaker than models predict and this is due in part to the common modeling assumption that the ionization balance remains in equilibrium. The launch of the latest generation of space-based observing instrumentation on board Hinode and the Solar Dynamics Observatory (SDO) has brought the matter of the ionization state of the plasma firmly to the forefront. It is timely to consider exactly what emission current instruments would detect when observing a corona heated impulsively on small scales by nanoflares. Only after we understand the full effects of nonequilibrium ionization can we draw meaningful conclusions about the plasma that is (or is not) present. We have therefore performed a series of hydrodynamic simulations for a variety of different nanoflare properties and initial conditions. Our study has led to several key conclusions. (1) Deviations from equilibrium are greatest for short-duration nanoflares at low initial coronal densities. (2) Hot emission lines are the most affected and are suppressed sometimes to the point of being invisible. (3) For the many scenarios we have considered, the emission detected in several of the SDO-AIA channels (131, 193, and 211 Å) would be dominated by warm, overdense, cooling plasma. (4) It is difficult not to create coronal loops that emit strongly at 1.5 MK and in the range 2-6 MK, which are the most commonly observed kind, for a broad range of nanoflare scenarios. (5) The Fe XV (284.16 Å) emission in most of our models is about 10 times brighter than the Ca XVII (192.82 Å) emission, consistent with observations. Our overarching conclusion is that small-scale, impulsive heating inducing a nonequilibrium ionization state leads to predictions for observable quantities that are entirely consistent with what is actually observed. Title: Patterns of Nanoflare Heating Exhibited by Active Regions Observed with SDO/AIA Authors: Viall, Nicholeen; Klimchuk, J. Bibcode: 2011SPD....42.2103V Altcode: 2011BAAS..43S.2103V It seems largely agreed that many coronal loops---those observed at a temperature of about 1 MK---are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Are these regions also heated impulsively, or is the heating quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function of the nanoflare storm properties. These orderings, or time lags, are clearly exhibited in loop observations in all channels. What is especially exciting is that we identify these time lag patterns in observations of the seemingly steady diffuse corona as well. We model the diffuse corona as a line-of-sight integration of many thousands of completely independent, impulsively heated strands. The time lags of the simulated and actual observations are in excellent agreement. Our results suggest that impulsive nanoflare heating is ubiquitous within active regions.

This research was supported through an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. Title: Radiative Signatures of the Coronal Heating and Cooling Cycle Authors: Bradshaw, Stephen; Klimchuk, J. Bibcode: 2011SPD....42.0503B Altcode: 2011BAAS..43S.0503B The 'smoking gun' of small-scale, impulsive heating in the non-flaring solar corona is expected to be emission from plasma at temperatures greater than 5 MK. Recent studies, designed to strongly constrain the high temperature part of the emission measure distribution, have begun to provide evidence for such a hot component to the emission spectrum. However, it is significantly weaker than predicted by numerical models. We propose that the discrepancy can be resolved by dropping the common modeling assumption of ionization equilibrium.

The launches of Hinode and the Solar Dynamics Observatory (SDO) have brought the matter of the ionization state of the plasma, which lies at the interface between models and observations, firmly to the forefront.

We present detailed, quantitative predictions for the spectral emission that the instruments EIS and AIA would actually detect, for a broad range of impulsive heating scenarios, derived from a combination of numerical hydrodynamic and forward modeling.

We demonstrate that a nonequilibrium ionization state, as induced by small-scale, impulsive heating, leads to predictions for observable quantities that are consistent with what is actually observed. Title: Are Spicules the Primary Source of Hot Coronal Plasma? Authors: Klimchuk, James A. Bibcode: 2011SPD....42.1801K Altcode: 2011BAAS..43S.1801K The recent discovery of Type II spicules has generated considerable excitement. It has even been suggested that these ejections can account for a majority of the hot plasma observed in the corona, thus obviating the need for "coronal” heating. If this is the case, however, then there should be observational consequences. We have begun to examine some of these consequences and find reason to question the idea that spicules are the primary source of hot coronal plasma. Title: SDO/AIA Light Curves and Implications for Coronal Heating: Observations Authors: Viall, N. M.; Klimchuk, J. A. Bibcode: 2010AGUFMSH41E..02V Altcode: It seems largely agreed that many coronal loops---those observed at a temperature of about 1 MK---are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Is it also impulsive or is it quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function the nanoflare storm properties. We show that while some sets of light curves exhibit clear evidence of cooling after nanoflare storms, other cases are less straightforward to interpret. Complications arise because of line-of-sight integration through many different structures, the broadband nature of the AIA channels, and because physical properties can change substantially depending on the magnitude of the energy release. Nevertheless, the light curves exhibit predictable and understandable patterns. This presentation emphasizes the observational aspects of our study. A companion presentation emphasizes the models. Title: Modeling the Secondary Flare Irradiance Measured by Solar Dynamic Observatory (SDO) Extreme ultraviolet Variability Experiment (EVE) Authors: Hock, R. A.; Woods, T. N.; Klimchuk, J. A.; Eparvier, F. G. Bibcode: 2010AGUFMSH13A..05H Altcode: NASA’s Solar Dynamic Observatory (SDO) launched on 11 February 2010 and normal operations for all three instruments began 1 May 2010. Since then numerous small and moderate (C- and M-class) flares have been observed. One interesting feature observed by the Extreme ultraviolet Variability Experiment (EVE) is the enhancement of 2-3 million K emission several hours after the flare’s soft x-ray emission. From the Atmospheric Imaging Assembly (AIA) images, we can tell that these secondary emissions occur in the same active region as the flare but not in same coronal loops. Here, we examine the C8.8 flare that occurred on 5 May 2010. The flare occurred in Active Region 11069, a small magnetically complex region near the western limb of the Sun. The gradual phase of the flare is clearly seen in both GOES soft X-rays and the hot coronal emissions (>2 million K) measured by EVE. The secondary flare emission starts 30 minutes after the peak in gradual phase and slowly increases over an hour before decaying to the pre-flare levels. It is most strongly seen in Fe XV and Fe XVI (2-3 million K). Using the Enthalpy-Based Thermal Evolution of Loops (EBTEL) model, we are able calculate the EUV irradiance of a set of coronal loops for a given heating function. This allows us to determine the best-fit heating profile as a function of time for the C8.8 flare. The heating profile for this event clearly shows that there are two separate phases of heating. The first phase involves traditional post-flare loops. The field reconnects after erupting, and the energy released during the reconnection heats the plasma to very high temperatures. As the loops cool, emissions are seen in progressively cooler lines from 10 to 1 million K. The second phase is very different. A large number of coronal loops are heated only modestly-. The plasma in each loop, instead of reaching 10 million K, reaches 3 million K. The heating is also spread out over an hour generating the long secondary flare emission profile. We discuss the nature of this secondary flare emission, which seems to be an important component of many events. Title: SDO/AIA Light Curves and Implications for Coronal Heating: Model Predictions Authors: Klimchuk, J. A.; Viall, N. M. Bibcode: 2010AGUFMSH41E..03K Altcode: It seems largely agreed that many coronal loops---those observed at a temperature of about 1 MK---are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Is it also impulsive or is it quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function the nanoflare storm properties. We show that while some sets of light curves exhibit clear evidence of cooling after nanoflare storms, other cases are less straightforward to interpret. Complications arise because of line-of-sight integration through many different structures, the broadband nature of the AIA channels, and because physical properties can change substantially depending on the magnitude of the energy release. Nevertheless, the light curves exhibit predictable and understandable patterns. This presentation emphasizes the modeling aspects of our study. A companion presentation emphasizes the observations. Title: Science Objectives for an X-Ray Microcalorimeter Observing the Sun Authors: Laming, J. Martin; Adams, J.; Alexander, D.; Aschwanden, M; Bailey, C.; Bandler, S.; Bookbinder, J.; Bradshaw, S.; Brickhouse, N.; Chervenak, J.; Christe, S.; Cirtain, J.; Cranmer, S.; Deiker, S.; DeLuca, E.; Del Zanna, G.; Dennis, B.; Doschek, G.; Eckart, M.; Fludra, A.; Finkbeiner, F.; Grigis, P.; Harrison, R.; Ji, L.; Kankelborg, C.; Kashyap, V.; Kelly, D.; Kelley, R.; Kilbourne, C.; Klimchuk, J.; Ko, Y. -K.; Landi, E.; Linton, M.; Longcope, D.; Lukin, V.; Mariska, J.; Martinez-Galarce, D.; Mason, H.; McKenzie, D.; Osten, R.; Peres, G.; Pevtsov, A.; Porter, K. Phillips F. S.; Rabin, D.; Rakowski, C.; Raymond, J.; Reale, F.; Reeves, K.; Sadleir, J.; Savin, D.; Schmelz, J.; Smith, R. K.; Smith, S.; Stern, R.; Sylwester, J.; Tripathi, D.; Ugarte-Urra, I.; Young, P.; Warren, H.; Wood, B. Bibcode: 2010arXiv1011.4052L Altcode: We present the science case for a broadband X-ray imager with high-resolution spectroscopy, including simulations of X-ray spectral diagnostics of both active regions and solar flares. This is part of a trilogy of white papers discussing science, instrument (Bandler et al. 2010), and missions (Bookbinder et al. 2010) to exploit major advances recently made in transition-edge sensor (TES) detector technology that enable resolution better than 2 eV in an array that can handle high count rates. Combined with a modest X-ray mirror, this instrument would combine arcsecondscale imaging with high-resolution spectra over a field of view sufficiently large for the study of active regions and flares, enabling a wide range of studies such as the detection of microheating in active regions, ion-resolved velocity flows, and the presence of non-thermal electrons in hot plasmas. It would also enable more direct comparisons between solar and stellar soft X-ray spectra, a waveband in which (unusually) we currently have much better stellar data than we do of the Sun. Title: On the Isothermality of Solar Plasmas Authors: Landi, E.; Klimchuk, J. A. Bibcode: 2010ApJ...723..320L Altcode: Recent measurements have shown that the quiet unstructured solar corona observed at the solar limb is close to isothermal, at a temperature that does not appear to change over wide areas or with time. Some individual active region loop structures have also been found to be nearly isothermal both along their axis and across their cross section. Even a complex active region observed at the solar limb has been found to be composed of three distinct isothermal plasmas. If confirmed, these results would pose formidable challenges to the current theoretical understanding of the thermal structure and heating of the solar corona. For example, no current theoretical model can explain the excess densities and lifetimes of many observed loops if the loops are in fact isothermal. All of these measurements are based on the so-called emission measure (EM) diagnostic technique that is applied to a set of optically thin lines under the assumption of isothermal plasma. It provides simultaneous measurement of both the temperature and EM. In this work, we develop a new method to quantify the uncertainties in the technique and to rigorously assess its ability to discriminate between isothermal and multithermal plasmas. We define a formal measure of the uncertainty in the EM diagnostic technique that can easily be applied to real data. We here apply it to synthetic data based on a variety of assumed plasma thermal distributions and develop a method to quantitatively assess the degree of multithermality of a plasma. Title: Evidence of Impulsive Heating in Active Region Core Loops Authors: Tripathi, Durgesh; Mason, Helen E.; Klimchuk, James A. Bibcode: 2010ApJ...723..713T Altcode: 2010arXiv1009.0663T Using a full spectral scan of an active region from the Extreme-Ultraviolet Imaging Spectrometer (EIS) we have obtained emission measure EM(T) distributions in two different moss regions within the same active region. We have compared these with theoretical transition region EMs derived for three limiting cases, namely, static equilibrium, strong condensation, and strong evaporation from Klimchuk et al. The EM distributions in both the moss regions are strikingly similar and show a monotonically increasing trend from log T[K] = 5.15-6.3. Using photospheric abundances, we obtain a consistent EM distribution for all ions. Comparing the observed and theoretical EM distributions, we find that the observed EM distribution is best explained by the strong condensation case (EMcon), suggesting that a downward enthalpy flux plays an important and possibly dominant role in powering the transition region moss emission. The downflows could be due to unresolved coronal plasma that is cooling and draining after having been impulsively heated. This supports the idea that the hot loops (with temperatures of 3-5 MK) seen in the core of active regions are heated by nanoflares. Title: A Simple Model for the Evolution of Multi-stranded Coronal Loops Authors: López Fuentes, M. C.; Klimchuk, J. A. Bibcode: 2010ApJ...719..591L Altcode: 2010arXiv1004.2061L We develop and analyze a simple cellular automaton model that reproduces the main properties of the evolution of soft X-ray coronal loops. We are motivated by the observation that these loops evolve in three distinguishable phases that suggest the development, maintenance, and decay of a self-organized system. The model is based on the idea that loops are made of elemental strands that are heated by the relaxation of magnetic stress in the form of nanoflares. In this vision, usually called the "Parker conjecture," the origin of stress is the displacement of the strand footpoints due to photospheric convective motions. Modeling the response and evolution of the plasma we obtain synthetic light curves that have the same characteristic properties (intensity, fluctuations, and timescales) as the observed cases. We study the dependence of these properties on the model parameters and find scaling laws that can be used as observational predictions of the model. We discuss the implications of our results for the interpretation of recent loop observations in different wavelengths. Title: Determining the Temperature Structure of Solar Coronal Loops using their Temporal Evolution Authors: Mulu, Fana; Winebarger, A. R.; Warren, H. P.; Aschwanden, M. J.; Klimchuk, J. A. Bibcode: 2010AAS...21630001M Altcode: Despite much progress toward understanding the dynamics of the corona, the physical properties of coronal loops are not yet fully understood. Recent investigations and observations from different instruments have yielded contradictory results about the true physical properties of coronal loops, specifically as to whether the observed loops are isothermal structures or the convolution of several multi-thermal strands. In this talk, we introduce a new technique to determine if an observed loop is isothermal or multi-thermal. We determine the evolution of ten selected loops in multiple filter images from the Transition Region and Coronal Explorer (TRACE). Our new technique calculates the delay, calculates a cooling time, and determines if that cooling time is consistent with the observed lifetime. If the observational lifetime of the loop agrees with the calculated lifetime, then we can conclude that the loop is a single "monolithic” structure that heats and cools as a homogeneous unit, with isothermal temperature over the cross-section. If not, the loop must be a bundle of multiple multi-thermal strands, all being heated and cooling independently. In the second part of the talk, we utilize the concept of nanoflare storms to understand the reason behind the extended lifetimes. By simulating the observed light curves of the loops using EBTEL (Enthalpy-Based Thermal Evolution of Loops), we find that the longer observed lifetimes can be reproduced by using a set of small-scale impulsively heated strands. 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: Division II: Sun and Heliosphere Authors: Melrose, Donald B.; Martinez Pillet, Valentin; Webb, David F.; Bougeret, Jean-Louis; Klimchuk, James A.; Kosovichev, Alexander; van Driel-Gesztelyi, Lidia; von Steiger, Rudolf Bibcode: 2010IAUTB..27..146M Altcode: This report is on activities of the Division at the General Assembly in Rio de Janeiro. Summaries of scientific activities over the past triennium have been published in Transactions A, see Melrose et al. (2008), Klimchuk et al. (2008), Martinez Pillet et al. (2008) and Bougeret et al. (2008). The business meeting of the three Commissions were incorporated into the business meeting of the Division. This report is based in part on minutes of the business meeting, provided by the Secretary of the Division, Lidia van Driel-Gesztelyi, and it also includes reports provided by the Presidents of the Commissions (C10, C12, C49) and of the Working Groups (WGs) in the Division. 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: Nanoflare heating of solar and stellar coronae Authors: Klimchuk, James Bibcode: 2010cosp...38.2897K Altcode: 2010cosp.meet.2897K A combination of observational and theoretical evidence suggests that much, and perhaps most, of the Sun's corona is heated by small unresolved bursts of energy called nanoflares. It seems likely that stellar coronae are heated in a similar fashion. Nanoflares are here taken to mean any impulsive heating that occurs within a magnetic flux strand. Many mechanisms have this property, including waves, but we prefer Parker's picture of tangled magnetic fields. The tangling is caused by turbulent convection at the stellar surface, and magnetic energy is released when the stresses reach a critical level. We suggest that the mechanism of energy release is the "secondary instability" of electric current sheets that are present at the boundaries between misaligned strands. I will discuss the collective evidence for solar and stellar nanoflares and hopefully present new results from the Solar Dynamics Observatory that was just launched. Title: Nanoflares, spicules, and other small-scale dynamic phenomena on the sun Authors: Klimchuk, James Bibcode: 2010cosp...38.2831K Altcode: 2010cosp.meet.2831K There is abundant evidence of highly dynamic phenomena occurring on very small scales in the solar atmosphere. For example, the observed properties of many coronal loops can only be ex-plained if the loops are bundles of unresolved strands that are heated impulsively by nanoflares. Type II spicules recently discovered by Hinode are an example of small-scale impulsive events occurring in the chromosphere. The existence of these and other small-scale phenomena is not surprising given the highly structured nature of the magnetic field that is revealed by photo-spheric observations. Dynamic phenomena also occur on much larger scales, including coronal jets, flares, and CMEs. It is tempting to suggest that these different phenomena are all closely related and represent a continuous distribution of sizes and energies. However, this is a danger-ous over simplification in my opinion. While it is true that the phenomena all involve "magnetic reconnection" (the changing of field line connectivity) in some form, how this occurs depends strongly on the magnetic geometry. A nanoflare resulting from the interaction of tangled mag-netic strands within a confined coronal loop is much different from a major flare occurring at the current sheet formed when a CME rips open an active region. I will review the evidence for ubiquitous small-scale dynamic phenomena on the Sun and discuss why different phenomena are not all fundamentally the same. Title: A cellular automaton nanoflare model of coronal loops Authors: Lopez Fuentes, Marcelo; Klimchuk, James Bibcode: 2010cosp...38.2833L Altcode: 2010cosp.meet.2833L In Lopez-Fuentes, Klimchuk and Mandrini (2007) we found that the evolution of soft X-ray loops can be separated into three main phases that suggest the development, maintainance, and decay of a self-organized system. Here, we present a cellular automaton model that reproduces the main features of the observed evolution. The model is based on the idea that loops are made of multiple unresolved strands which have footpoints that are displaced by random photospheric motions (Parker 1988). In this scenario, there is a continuous increase of the magnetic stress between neighboring strands until a critical stress is reached and the accumulated energy is suddenly released by reconnection. Each of these reconnection "events" is associated with a nanoflare. Using the EBTEL hydrodynamic code (Klimchuk, Patsourakos and Cargill 2008) to model the plasma response we construct synthetic light curves that we compare with the observations. We also study how the properties of the light curves scale with the different parameters of the model. Finally, we discuss how the present model can be used to explain loop observations in different wavelengths. Title: XRT Detection of Hot Plasma in Active Regions and Nanoflare Heating Authors: Reale, F.; Klimchuk, J. A.; Parenti, S.; Testa, P. Bibcode: 2009ASPC..415..256R Altcode: Nanoflares occurring in sub-resolution strands have been long invoked as strong candidates for the heating of active region (AR) coronal loops. However, the frequent occurrence of nanoflares requires the steady presence of flare-hot plasma in the active region, which has been difficult to detect so far. We report on the analysis of multi-filter Hinode/XRT observations of an active region, which may show the widespread presence of 10 MK plasma. Title: Coronal Loop Models and Those Annoying Observations! (Keynote) Authors: Klimchuk, J. A. Bibcode: 2009ASPC..415..221K Altcode: 2009arXiv0904.1391K It was once thought that all coronal loops are in static equilibrium, but observational and modeling developments over the past decade have shown that this is clearly not the case. It is now established that warm (∼ 1 MK) loops observed in the EUV are explainable as bundles of unresolved strands that are heated impulsively by storms of nanoflares. A raging debate concerning the multi-thermal versus isothermal nature of the loops can be reconciled in terms of the duration of the storm. We show that short and long storms produce narrow and broad thermal distributions, respectively. We also examine the possibility that warm loops can be explained with thermal nonequilibrium, a process by which steady heating produces dynamic behavior whenever the heating is highly concentrated near the loop footpoints. We conclude that this is not a viable explanation for monolithic loops under the conditions we have considered, but that it may have application to multi-stranded loops. Serious questions remain, however. 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: Explosive Instability and Coronal Heating Authors: Dahlburg, R. B.; Liu, J. -H.; Klimchuk, J. A.; Nigro, G. Bibcode: 2009ApJ...704.1059D Altcode: The observed energy-loss rate from the solar corona implies that the coronal magnetic field has a critical angle at which energy is released. It has been hypothesized that at this critical angle an "explosive instability" would occur, leading to an enhanced conversion of magnetic energy into heat. In earlier investigations, we have shown that a shear-dependent magnetohydrodynamic process called "secondary instability" has many of the distinctive features of the hypothetical "explosive instability." In this paper, we give the first demonstration that this "secondary instability" occurs in a system with line-tied magnetic fields and boundary shearing—basically the situation described by Parker. We also show that, as the disturbance due to secondary instability attains finite amplitude, there is a transition to turbulence which leads to enhanced dissipation of magnetic and kinetic energy. These results are obtained from numerical simulations performed with a new parallelized, viscoresistive, three-dimensional code that solves the cold plasma equations. The code employs a Fourier collocation—finite difference spatial discretization, and uses a third-order Runge-Kutta temporal discretization. Title: Evidence of Widespread Hot Plasma in a Nonflaring Coronal Active Region from Hinode/X-Ray Telescope Authors: Reale, Fabio; Testa, Paola; Klimchuk, James A.; Parenti, Susanna Bibcode: 2009ApJ...698..756R Altcode: 2009arXiv0904.0878R Nanoflares, short and intense heat pulses within spatially unresolved magnetic strands, are now considered a leading candidate to solve the coronal heating problem. However, the frequent occurrence of nanoflares requires that flare-hot plasma be present in the corona at all times. Its detection has proved elusive until now, in part because the intensities are predicted to be very faint. Here, we report on the analysis of an active region observed with five filters by Hinode/X-Ray Telescope (XRT) in 2006 November. We have used the filter ratio method to derive maps of temperature and emission measure (EM) both in soft and hard ratios. These maps are approximate in that the plasma is assumed to be isothermal along each line of sight. Nonetheless, the hardest available ratio reveals the clear presence of plasma around 10 MK. To obtain more detailed information about the plasma properties, we have performed Monte Carlo simulations assuming a variety of nonisothermal EM distributions along the lines of sight. We find that the observed filter ratios imply bi-modal distributions consisting of a strong cool (log T ~ 6.3 - 6.5) component and a weaker (few percent) and hotter (6.6 < log T < 7.2) component. The data are consistent with bi-modal distributions along all lines of sight, i.e., throughout the active region. We also find that the isothermal temperature inferred from a filter ratio depends sensitively on the precise temperature of the cool component. A slight shift of this component can cause the hot component to be obscured in a hard ratio measurement. Consequently, temperature maps made in hard and soft ratios tend to be anti-correlated. We conclude that this observation supports the presence of widespread nanoflaring activity in the active region. Title: Spectroscopic Observations of Hot Lines Constraining Coronal Heating in Solar Active Regions Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2009ApJ...696..760P Altcode: 2009arXiv0903.3880P Extreme-ultraviolet observations of warm coronal loops suggest that they are bundles of unresolved strands that are heated impulsively to high temperatures by nanoflares. The plasma would then have the observed properties (e.g., excess density compared with static equilibrium) when it cools into the 1-2MK range. If this interpretation is correct, then very hot emission should be present outside of proper flares. It is predicted to be very faint, however. A critical element for proving or refuting this hypothesis is the existence of hot, yet faint plasmas which should be at amounts predicted by impulsive heating models. We report on the first comprehensive spectroscopic study of hot plasmas in active regions (ARs). Data from the Extreme-ultraviolet Imaging Spectrometer on Hinode were used to construct emission measure (EM) distributions in quiescent ARs in the 1-5 MK temperature range. The distributions are flat or slowly increasing up to approximately 3 MK and then fall off rapidly at higher temperatures. We show that AR models based on impulsive heating can reproduce the observed EM distributions relatively well. Our results provide strong new evidence that coronal heating is impulsive in nature. Title: Spectroscopic Observations of Hot Lines Constraining Coronal Heating in Solar Active Regions Authors: Patsourakos, Spiros; Klimchuk, J. A. Bibcode: 2009SPD....40.1211P Altcode: EUV observations of warm coronal loops suggest that they are bundles of unresolved strands that are heated impulsively to high temperatures by nanoflares. The plasma would then have the observed properties (e.g., excess density compared to static equilibrium) when it cools into the 1-2 MK range. If this interpretation is correct, then very hot emission should be present outside of proper flares. It is predicted to be vey faint, however. A critical element for proving or refuting this hypothesis is the existence of hot, very faint plasmas which should be at amounts predicted by impulsive heating. We report on the first comprehensive spectroscopic study of hot plasmas in active regions. Data from the EIS spectrometer on Hinode were used to construct emission measure distributions in quiescent active regions in the 1-5 MK temperature range. The distributions are flat or slowly increasing up to approximately 3 MK and then fall off rapidly at higher temperatures. We show that active region models based on impulsive heating can reproduce the observed EM distributions relatively well. Our results provide strong new evidence that coronal heating is impulsive in nature. Title: Observations of Nanoflare Produced Hot ( 10 Mk) Plasma Authors: Klimchuk, James A.; Reale, F.; Testa, P.; Parenti, S. Bibcode: 2009SPD....40.1214K Altcode: Indirect observational evidence suggests that some or most of the corona is heated impulsively on sub-resolution scales by nanoflares. Theoretical studies of possible heating mechanisms also support this picture. However, the most direct evidence of nanoflares---plasma hotter than 5 MK---has been difficult to obtain because the emission is expected to be very faint. The reason is two-fold: first, hot plasma cools very rapidly by thermal conduction; and second, densities are small because chromospheric evaporation has not had time to fill the corona. Recent observations from several instruments have now provided strong evidence of hot plasma. We report here on the detection of 10 MK plasma by the X-Ray Telescope (XRT) on Hinode. We show that the intensity of the emission is consistent with nanoflare models, but is extremely difficult to explain with steady heating. Title: Models of Impulsively Heated Solar Active Regions Authors: Airapetian, Vladimir; Klimchuk, J. Bibcode: 2009SPD....40.1202A Altcode: A number of attempts to model solar active regions with steady coronal heating have been modestly successful at reproducing the observed soft X-ray emission, but they fail dramatically at explaining EUV observations. Since impulsive heating (nanoflare) models can reproduce individual EUV loops, it seems reasonable to consider that entire active regions are impulsively heated. However, nanoflares are characterized by many parameters, such as magnitude, duration, and time delay between successive events, and these parameters may depend on the strength of the magnetic field or the length of field lines, for example, so a wide range of active region models must be examined. We have recently begun such a study. Each model begins with a magnetic "skeleton” obtained by extrapolating an observed photospheric magnetogram into the corona. Field lines are populated with plasma using our highly efficient hydro code called Enthalpy Based Thermal Evolution of Loops (EBTEL). We then produce synthetic images corresponding to emission line or broad-band observations. By determining which set of nanoflare parameters best reproduces actual observations, we hope to constrain the properties of the heating and ultimately to reveal the physical mechanism. We here report on the initial progress of our study. Title: Hinode X-Ray Telescope Detection of Hot Emission from Quiescent Active Regions: A Nanoflare Signature? Authors: Schmelz, J. T.; Saar, S. H.; DeLuca, E. E.; Golub, L.; Kashyap, V. L.; Weber, M. A.; Klimchuk, J. A. Bibcode: 2009ApJ...693L.131S Altcode: 2009arXiv0901.3122S The X-Ray Telescope (XRT) on the Japanese/USA/UK Hinode (Solar-B) spacecraft has detected emission from a quiescent active region core that is consistent with nanoflare heating. The fluxes from 10 broadband X-ray filters and filter combinations were used to construct differential emission measure (DEM) curves. In addition to the expected active region peak at log T = 6.3-6.5, we find a high-temperature component with significant emission measure at log T > 7.0. This emission measure is weak compared to the main peak—the DEM is down by almost three orders of magnitude—which accounts of the fact that it has not been observed with earlier instruments. It is also consistent with spectra of quiescent active regions: no Fe XIX lines are observed in a CHIANTI synthetic spectrum generated using the XRT DEM distribution. The DEM result is successfully reproduced with a simple two-component nanoflare model. Title: Multi-wavelength observations and modelling of a canonical solar flare Authors: Raftery, C. L.; Gallagher, P. T.; Milligan, R. O.; Klimchuk, J. A. Bibcode: 2009A&A...494.1127R Altcode: 2008arXiv0812.0311R Aims: We investigate the temporal evolution of temperature, emission measure, energy loss, and velocity in a C-class solar flare from both observational and theoretical perspectives.
Methods: The properties of the flare were derived by following the systematic cooling of the plasma through the response functions of a number of instruments - the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI; >5 MK), GOES-12 (5-30 MK), the Transition Region and Coronal Explorer (TRACE 171 Å; 1 MK), and the Coronal Diagnostic Spectrometer (CDS; ~0.03-8 MK). These measurements were studied in combination with simulations from the 0-D enthalpy based thermal evolution of loops (EBTEL) model.
Results: At the flare onset, upflows of ~90 km s-1 and low-level emission were observed in Fe XIX, consistent with pre-flare heating and gentle chromospheric evaporation. During the impulsive phase, upflows of ~80 km s-1 in Fe XIX and simultaneous downflows of ~20 km s-1 in He I and O V were observed, indicating explosive chromospheric evaporation. The plasma was subsequently found to reach a peak temperature of ⪆13 MK in approximately 10 min. Using EBTEL, conduction was found to be the dominant loss mechanism during the initial ~300 s of the decay phase. It was also found to be responsible for driving gentle chromospheric evaporation during this period. As the temperature fell below ~8 MK, and for the next ~4000 s, radiative losses were determined to dominate over conductive losses. The radiative loss phase was accompanied by significant downflows of ≤40 km s-1 in O V.
Conclusions: This is the first extensive study of the evolution of a canonical solar flare using both spectroscopic and broad-band instruments in conjunction with a 0-D hydrodynamic model. While our results are in broad agreement with the standard flare model, the simulations suggest that both conductive and non-thermal beam heating play important roles in heating the flare plasma during the impulsive phase of at least this event. Title: Division II: Sun and Heliosphere Authors: Melrose, Donald B.; Martínez Pillet, Valentin; Webb, David F.; van Driel-Gesztelyi, Lidia; Bougeret, Jean-Louis; Klimchuk, James A.; Kosovichev, Alexander; von Steiger, Rudolf Bibcode: 2009IAUTA..27...73M Altcode: Division II of the IAU provides a forum for astronomers and astrophysicists studying a wide range of phenomena related to the structure, radiation and activity of the Sun, and its interaction with the Earth and the rest of the solar system. Division II encompasses three Commissions, 10, 12 and 49, and four Working Groups. Title: Commission 10: Solar Activity Authors: Klimchuk, James A.; van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Melrose, Donald B.; Fletcher, Lyndsay; Gopalswamy, Natchimuthuk; Harrison, Richard A.; Mandrini, Cristina H.; Peter, Hardi; Tsuneta, Saku; Vršnak, Bojan; Wang, Jing-Xiu Bibcode: 2009IAUTA..27...79K Altcode: 2008arXiv0809.1444K Commission 10 deals with solar activity in all of its forms, ranging from the smallest nanoflares to the largest coronal mass ejections. This report reviews scientific progress over the roughly two-year period ending in the middle of 2008. This has been an exciting time in solar physics, highlighted by the launches of the Hinode and STEREO missions late in 2006. The report is reasonably comprehensive, though it is far from exhaustive. Limited space prevents the inclusion of many significant results. The report is divided into the following sections: Photosphere and chromosphere; Transition region; Corona and coronal heating; Coronal jets; flares; Coronal mass ejection initiation; Global coronal waves and shocks; Coronal dimming; The link between low coronal CME signatures and magnetic clouds; Coronal mass ejections in the heliosphere; and Coronal mass ejections and space weather. Primary authorship is indicated at the beginning of each section. Title: Static and Impulsive Models of Solar Active Regions Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2008ApJ...689.1406P Altcode: 2008arXiv0808.2745P The physical modeling of active regions (ARs) and of the global corona is receiving increasing interest lately. Recent attempts to model ARs using static equilibrium models were quite successful in reproducing AR images of hot soft X-ray (SXR) loops. They however failed to predict the bright extreme-ultraviolet (EUV) warm loops permeating ARs: the synthetic images were dominated by intense footpoint emission. We demonstrate that this failure is due to the very weak dependence of loop temperature on loop length which cannot simultaneously account for both hot and warm loops in the same AR. We then consider time-dependent AR models based on nanoflare heating. We demonstrate that such models can simultaneously reproduce EUV and SXR loops in ARs. Moreover, they predict radial intensity variations consistent with the localized core and extended emissions in SXR and EUV AR observations, respectively. We finally show how the AR morphology can be used as a gauge of the properties (duration, energy, spatial dependence, and repetition time) of the impulsive heating. Title: Highly Efficient Modeling of Dynamic Coronal Loops Authors: Klimchuk, J. A.; Patsourakos, S.; Cargill, P. J. Bibcode: 2008ApJ...682.1351K Altcode: 2007arXiv0710.0185K Observational and theoretical evidence suggests that coronal heating is impulsive and occurs on very small cross-field spatial scales. A single coronal loop could contain a hundred or more individual strands that are heated quasi-independently by nanoflares. It is therefore an enormous undertaking to model an entire active region or the global corona. Three-dimensional MHD codes have inadequate spatial resolution, and one-dimensional (1D) hydrodynamic codes are too slow to simulate the many thousands of elemental strands that must be treated in a reasonable representation. Fortunately, thermal conduction and flows tend to smooth out plasma gradients along the magnetic field, so zero-dimensional (0D) models are an acceptable alternative. We have developed a highly efficient model called "enthalpy-based thermal evolution of loops" (EBTEL), which accurately describes the evolution of the average temperature, pressure, and density along a coronal strand. It improves significantly on earlier models of this type—in accuracy, flexibility, and capability. It treats both slowly varying and highly impulsive coronal heating; it provides the time-dependent differential emission measure distribution, DEM(T), at the transition region footpoints; and there are options for heat flux saturation and nonthermal electron beam heating. EBTEL gives excellent agreement with far more sophisticated 1D hydrodynamic simulations despite using 4 orders of magnitude less computing time. It promises to be a powerful new tool for solar and stellar studies. Title: Hot Spectral Emissions in Quiescent Active Regions and Nanoflare Heating Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2008AGUSMSP43C..02P Altcode: A leading candidate for the heating of active region (AR) coronal loops is the nanoflare model. This model treats coronal loops as collections of impulsively heated sub-resolution strands and explains several key observational aspects of warm (1-2 MK) coronal loops. However, the basic requirement of this model is that the strands initially reach very high temperatures of several MK before they cool down to canonical coronal temperatures. Therefore, the detection of hot plasmas in AR loops represents a stringest test of the nanoflare model. Previous work has shown that the best way to observe the postulated hot plasmas is by the means of spectroscopic observations in hot lines (T > 3 MK). The emission is predicted to be quite faint, but the EIS spectrometer onboard Hinode has sufficient sensitivity to allow us to perform such a test for the first time. We will present an analysis of the emission characteristics of quiescent coronal loops in a number of hot lines spanning approximately 3-12 MK (Ni XVII, Ca XV, Fe XVII, Ca XVII, Fe XXIII). We will show that hot plasmas are ubiquitous over entire active regions, and we will compare the measured intensities of both hot and warm lines with predictions of nanoflare models. Title: Are Constant Loop Widths an Artifact of the Background and the Spatial Resolution? Authors: López Fuentes, M. C.; Démoulin, P.; Klimchuk, J. A. Bibcode: 2008ApJ...673..586L Altcode: 2007arXiv0704.0637L We study the effect of the coronal background in the determination of the diameter of EUV loops, and we analyze the suitability of the procedure followed in a previous paper for characterizing their expansion properties. For the analysis we create different synthetic loops, and we place them on real backgrounds from data obtained with the Transition Region and Coronal Explorer (TRACE). We apply to these loops the same procedure followed in our previous works, and we compare the results with real loop observations. We demonstrate that the procedure allows us to distinguish constant width loops from loops that expand appreciably with height, as predicted by simple force-free field models. This holds even for loops near the resolution limit. The procedure can easily determine when loops are below the resolution limit and therefore not reliably measured. We find that small-scale variations in the measured loop width are likely due to imperfections in the background subtraction. The greatest errors occur in especially narrow loops and in places where the background is especially bright relative to the loop. We stress, however, that these effects do not impact the ability to measure large-scale variations. The result that observed loops do not expand systematically with height is robust. Title: Understanding Warm Coronal Loops Authors: Klimchuk, J. A.; Karpen, J. T.; Patsourakos, S. Bibcode: 2007AGUFMSH51C..05K Altcode: One of the great mysteries of coronal physics that has come to light in the last few years is the discovery that warm (~ 1 MK) coronal loops are much denser than expected for quasi-static equilibrium. It has been shown that the excess density can be explained if loops are bundles of unresolved strands that are heated impulsively and quasi-randomly to very high temperatures. This picture of nanoflare heating predicts that neighboring strands of different temperature should coexist and therefore that loops should have multi-thermal cross sections. In particular, emission should be produced at temperatures hotter than 2 MK. Such emission is sometimes but not always seen, however. We offer two possible explanations for the existence of over-dense warm loops without corresponding hot emission: (1) loops are bundles of nanoflare heated strands, but a significant fraction of the nanoflare energy takes the form of a nonthermal electron beam rather then direct heating; (2) loops are bundles of strands that undergo thermal nonequilibrium that results when steady heating is sufficiently concentrated near the footpoints. We verify these possibilities with numerical hydro simulations. Time permitting, we will show FeXVII line profile observations from EIS/Hinode that support the existence of nanoflare heating. Work supported by NASA and ONR. Title: Division II: Sun and Heliosphere Authors: Webb, David F.; Melrose, Donald B.; Benz, Arnold O.; Bogdan, Thomas J.; Bougeret, Jean-Louis; Klimchuk, James A.; Martinez-Pillet, Valentin Bibcode: 2007IAUTB..26..101W Altcode: Division II provides a forum for astronomers studying a wide range of problems related to the structure, radiation and activity of the Sun, and its interaction with the Earth and the rest of the solar system. Title: Explosive Instability and Coronal Heating Authors: Dahlburg, R. B.; Liu, J.; Klimchuk, J. A.; Nigro, G. Bibcode: 2007AGUFMSH44A1726D Altcode: The observed energy loss rate from the solar corona implies that the coronal magnetic field has a critical angle at which energy is released. It has been hypothesized that at this critical angle an "explosive instability" would occur, leading to an enhanced conversion of magnetic energy into heat. In earlier investigations we have shown that a shear-dependent process called "secondary instability" could account for many of the distinctive features of the hypothetical "explosive instability." Here we show that this "secondary instability" can occur in a system with line-tied magnetic fields and boundary shearing. We also show that, as the disturbance due to secondary instability attains finite amplitude, there is a transition to turbulence which leads to enhanced dissipation of magnetic and kinetic energy. Furthermore, after each dissipative burst, the system is able to reform itself so that a subsequent burst can occur. These results are obtained from numerical simulations performed with a new parallelized, viscoresistive, three-dimensional code that solves the cold plasma equations. The code employs a Fourier collocation -- finite difference spatial discretization, and uses a third-order Runge-Kutta temporal discretization. Title: The Cross-Field Thermal Structure of Coronal Loops from Triple-Filter TRACE Observations Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2007ApJ...667..591P Altcode: The highly suppressed thermal transport across the magnetic field in the solar corona makes the determination of the cross-field thermal distribution within coronal loops a powerful diagnostic of the properties of the heating process itself. The cross-field thermal structure is currently being strongly debated. Spectroscopic observations with high temperature fidelity but low spatial resolution indicate that some observed loops are multithermal, whereas imaging observations with high spatial resolution but low temperature fidelity indicate more isothermal conditions. We report here on triple filter observations of coronal loops made by the Transition Region and Coronal Explorer (TRACE), which has the best spatial resolution currently available. We tested the isothermal hypothesis using the emission measure loci technique and found that the loops are consistent with an isothermal plasma near 1.5 MK only if a generous estimate of the photometric uncertainties is used. A more restrictive estimate based on discussions with the TRACE experimenters rules out the isothermal hypothesis. The observations are much better explained by a multithermal plasma with significant emission measure throughout the range 1-3 MK. The details of the emission measure distribution are not well defined, however. Future subarcsecond spectroscopic observations covering a wide range of temperatures are the most promising means of unlocking the thermal structure of the corona. Title: Modeling Active Regions with Steady and Impulsive Heating Authors: Patsourakos, Spiros; Klimchuk, J. Bibcode: 2007AAS...210.9124P Altcode: 2007BAAS...39..208P There has been considerable recent interest in constructing physical models of active regions (ARs) and the global coronal. Models based on static equilibrium theory are quite successful at reproducing soft X-ray (SXR) images of active regions. They however fail to predict the warm ( 1 MK) loops that are seen to permeate ARs in the EUV. Instead, the synthetic EUV images are dominated by intense footpoint emission. We demonstrate that the failure of static models to predict EUV loops is associated with the very weak dependence of loop temperature on loop length in models that are based on a single heating mechanism and that match the SXR observations. The models predict either SXR loops or EUV loops, but not both. We therefore consider time-dependent AR models based on nanoflare heating. We demonstrate that such models can simultaneously reproduce both SXR and EUV loops. Moreover, they explain the general tendency for SXR emission to dominate in the cores of ARs and EUV emission to dominate in the periphery. We finally show how the properties of nanoflares (energy, duration, spatial dependence, repetition time) can affect the AR morphology.

Research supported by NASA and ONR. Title: Coronal Loops Really Do Have Constant Cross Sections! Authors: Klimchuk, James A.; Lopez Fuentes, M.; Demoulin, P. Bibcode: 2007AAS...210.9111K Altcode: 2007BAAS...39..205K The observation that coronal loops do not expand systematically with height has been one of the more intriguing puzzles in solar physics. Simple force-free magnetic field models based on extrapolated magnetograms predict a much larger expansion than is observed. It has been suggested that the cross section uniformity is an artifact of inadequate spatial resolution, complex background emission, or both. For example, loops that are everywhere thinner than the instrument point spread function (PSF) would be seen to have a nearly constant thickness even if they actually expand. We have argued previously that actual loops are wide enough to rule out this possibility. Our present work also rules out the background emission as a possible explanation. We have simulated TRACE observations in the following manner. We constructed synthetic loops with both uniform and expanding cross sections, convolved them with the PSF, and placed them on actual TRACE images. We then measured the widths of the loops using the same technique used in our earlier studies of real observations. We find that expanding loops can be readily distinguished from loops with a constant cross section. Thus, the enigma remains! We tentatively suggest that constant cross sections are a consequence of the complex internal structure of loops (e.g., loops as bundles of tangled elemental strands). We are confident that this can explain the observed symmetry of loops, but whether it can also explain the lack of systematic expansion with height is not at all clear. Title: Energy Release in Tangled Magnetic Fields Authors: Klimchuk, James A.; DeVore, C. R. Bibcode: 2007AAS...210.5303K Altcode: 2007BAAS...39..164K A highly promising picture of coronal heating, first advocated by G. Parker, involves elemental magnetic flux tubes that become tangled by the shuffling motions of photospheric convection. The tubes must “reconnect” in the corona in order to avoid a monotonic increase of magnetic stresses. The associated release of energy heats the plasma. In earlier work, we showed that a mechanism called the secondary instability is the likely mechanism of energy release (Dahlburg, Klimchuk, and Antiochos, 2005). The instability occurs within the electric current sheets that separate misaligned tubes and switches on only after the misalignment angle reaches a critical value. It is significant that this angle matches the angle implied by the observed heating requirements of the corona. Though very encouraging, our initial MHD simulations were highly idealized. We have therefore performed new simulations that relax several assumptions by including the effects of finite current sheet size, photospheric line-tying, and time-dependent shearing. These fully 3D simulations of interacting flux tubes provide further evidence for the fundamental role of the secondary instability in coronal heating.

Work supported by NASA and ONR. Title: The Temporal Evolution of Coronal Loops Observed by GOES SXI Authors: López Fuentes, M. C.; Klimchuk, J. A.; Mandrini, C. H. Bibcode: 2007ApJ...657.1127L Altcode: 2006astro.ph.11338L We study the temporal evolution of coronal loops using data from the Solar X-Ray Imager (SXI) on board the Geosynchronous Operational Environmental Satellite 12 (GOES-12). This instrument provides continuous soft X-ray observations of the solar corona at a high temporal cadence permitting detailed study of the full lifetime of each of several coronal loops. The observed light curves suggest three evolutionary phases: rise, main, and decay. The durations and characteristic timescales of these phases [I/(dI/dt), where I is the loop intensity] are much longer than a cooling time and indicate that the loop-averaged heating rate increases slowly, reaches a maintenance level, and then decreases slowly. This suggests that a single heating mechanism operates for the entire lifetime of the loop. For monolithic (uniformly filled) loops, the loop-averaged heating rate is the intrinsic energy release rate of the heating mechanism. For loops that are bundles of impulsively heated strands, it relates to the frequency of occurrence of individual heating events, or nanoflares. We show that the timescale of the loop-averaged heating rate is proportional to the timescale of the observed intensity variation, with a constant of proportionality of approximately 1.5 for monolithic loops and 1.0 for multistranded loops. The ratios of the radiative to conductive cooling times in the loops are somewhat less than 1, putting them intermediate between the values measured previously for hotter and cooler loops. This provides further support for a trend suggesting that all loops are heated in a similar way. Title: Division II: Sun and Heliosphere Authors: Webb, David F.; Melrose, Donald B.; Benz, Arnold O.; Bogdan, Thomas J.; Bougeret, Jean-Louis; Klimchuk, James A.; Martinez Pillet, Valentin Bibcode: 2007IAUTA..26...69W Altcode: Division II of the IAU provides a forum for astronomers studying a wide range of phenomena related to the structure, radiation and activity of the Sun, and its interaction with the Earth and the rest of the solar system. Division II encompasses three Commissions, 10, 12 and 49, and four working groups. During the last triennia the activities of the division involved some reorganization of the division and its working groups, developing new procedures for election of division and commission officers, promoting annual meetings from within the division and evaluating all the proposed meetings, evaluating the division's representatives for the IAU to international scientific organizations, and participating in general IAU business. Title: Commission 10: Solar Activity Authors: Melrose, Donald B.; Klimchuk, James A.; Benz, A. O.; Craig, I. J. D.; Gopalswamy, N.; Harrison, R. A.; Kozlovsky, B. Z.; Poletto, G.; Schrijver, K. J.; van Driel-Gesztelyi, L.; Wang, J. -X. Bibcode: 2007IAUTA..26...75M Altcode: Commission 10 aims at the study of various forms of solar activity, including networks, plages, pores, spots, fibrils, surges, jets, filaments/prominences, coronal loops, flares, coronal mass ejections (CMEs), solar cycle, microflares, nanoflares, coronal heating etc., which are all manifestation of the interplay of magnetic fields and solar plasma. Increasingly important is the study of solar activities as sources of various disturbances in the interplanetary space and near-Earth "space weather".Over the past three years a major component of research on the active Sun has involved data from the RHESSI spacecraft. This review starts with an update on current and planned solar observations from spacecraft. The discussion of solar flares gives emphasis to new results from RHESSI, along with updates on other aspects of flares. Recent progress on two theoretical concepts, magnetic reconnection and magnetic helicity is then summarized, followed by discussions of coronal loops and heating, the magnetic carpet and filaments. The final topic discussed is coronal mass ejections and space weather.The discussions on each topic is relatively brief, and intended as an outline to put the extensive list of references in context.The review was prepared jointly by the members of the Organizing Committee, and the names of the primary contributors to the various sections are indicated in parentheses. Title: Coronal Heating Authors: Klimchuk, J. A.; López Fuentes, M. C. Bibcode: 2006AIPC..848...55K Altcode: Coronal heating is one of the most challenging and important problems in astrophysics. We here review some of the critical aspects of the problem and suggest that the most likely explanation is Parker's long-standing idea of nanoflares occurring in magnetic fields that become tangled by photospheric convection. An exciting new development is the identification of the ``secondary instability'' as the likely mechanism of energy release. We also present some preliminary new results indicating that the rise, main, and decay phases of the soft X-ray light curves of observed coronal loops might be associated with the development, maintenance, and destruction of self-organized critical systems. Title: Nonthermal Spectral Line Broadening and the Nanoflare Model Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2006ApJ...647.1452P Altcode: A number of theoretical and observational considerations suggest that coronal loops are bundles of unresolved, impulsively heated strands. This ``nanoflare'' model, as it is sometimes called, predicts high-speed evaporative upflows, which might be revealed as nonthermal broadening of spectral line profiles. We have therefore generated synthetic line profile observations based on one-dimensional hydrodynamic simulations for comparison with actual observations. The predicted profiles for Ne VIII (770.4 Å), a transition region line, and Mg X (624.9 Å), a warm coronal line, have modest broadening that agrees well with existing observations. The predicted profiles for Fe XVII (254.87 Å), a hot line that will be observed by the Extreme Ultraviolet Imaging Spectrometer (EIS) on the Solar-B mission, are somewhat broader and are also consistent with the limited number of hot line observations that are currently available. Moreover, depending on the properties of the assumed nanoflare and other parameters of the simulation, the Fe XVII profile can have distinctive enhancements in the line wing. This indicates a powerful diagnostic capability that can be exploited once Solar-B is launched. Title: Summary of JD3: Solar Active Regions and 3D Magnetic Structure Authors: Klimchuk, J. A. Bibcode: 2006IAUJD...3E..57K Altcode: In this summary, I will attempt to synthesize many of the individual results presented in JD3 into a more coherent picture of solar active regions and 3D magnetic structure. I will offer an assessment of our current state of understanding and suggest how we might best proceed to make further progress on this important topic. Title: Heating of the Magnetically Closed Corona Authors: Klimchuk, J. A.; López Fuentes, M. C.; DeVore, C. R. Bibcode: 2006ESASP.617E...8K Altcode: 2006soho...17E...8K No abstract at ADS Title: Testing Nanoflare Heating in Coronal Loops With Observations From the Extreme Ultraviolet Imaging Spectrometer On-board the SOLAR-B Mission Authors: Patsourakos, Spiros; Klimchuk, J. A. Bibcode: 2006SPD....37.0124P Altcode: 2006BAAS...38..219P A number of theoretical and observational considerations suggest that coronal loops are bundles of unresolved, impulsively heated strands. This "nanoflare" model, as it is sometimes called, predicts high-speed evaporative upflows, which might be revealed as non-thermal broadening of spectral line profiles. We have therefore generated synthetic line profile observations based on 1D hydrodynamic simulations of nanoflare heated loop bundles.We will show that hot lines (T>5MK) hold the imprints of the heating process via the development of distinct enhancements in the line wings. These signatures do not appear in the profiles of cooler lines, which is fully consistent with existing observations. We will demonstrate how the spectra of hot lines from the Extreme Ultraviolet Imaging Spectrometer (EIS) on-board the upcoming SOLAR-B mission can be used to test the basic nanoflare picture and perhaps even to pinpoint the properties of the nanoflares, such as their energy content, duration, and spatial dependence. We will present sample observing programs for studying nanoflare heating in coronal loops that utilize EIS and other instrumentation on-board SOLAR-B and STEREO.Research supported by NASA and ONR. Title: Why Are Coronal Loops So Symmetric? Authors: Klimchuk, James A.; Lopez Fuentes, M. C.; Demoulin, P. Bibcode: 2006SPD....37.1706K Altcode: 2006BAAS...38..246K Coronal loops are observed to be very symmetric in the sense that the two legs have a comparable thickness. Magnetic flux tubes in magnetic field extrapolation models are typically much less symmetric. We have quantified these differences using 171 A images from TRACE and magnetograms from MDI/SOHO. For a sample of 20 different loops, we found the linear force-free field that best matches the observed loop. We then measured the plane-of-the-sky widths of the loops and corresponding flux tubes and computed footpoint-to-footpoint expansion factors (i.e., asymmetry ratios). The mean expansion factor of the flux tubes is 2.62, whereas the mean expansion factor of the loops is only 1.35. Note that these expansion factors are different from the footpoint-to-midpoint expansion factors that we have presented previously.Evidence suggests that the coronal magnetic field is comprised elemental flux strands that are tangled by turbulent convection. These strands are so small that many tens of them are contained within a single TRACE loop. We suggest that this fine structure is a critical missing ingredient of the extrapolation models and that a combination of footpoint shuffling and coronal reconnection can explain the observed loop symmetry. This has important implications for coronal heating.Research supported by NASA and the Office of Naval Research. Title: On the Temperature-Emission Measure Distribution in Stellar Coronae Authors: Cargill, Peter J.; Klimchuk, James A. Bibcode: 2006ApJ...643..438C Altcode: Strong peaks in the emission measure-temperature (EM-T ) distributions in the coronae of some binary stars are associated with the presence of hot (107 K), dense (up to 1013 cm -3) plasma. These peaks are very reminiscent of those predicted to arise in an impulsively heated solar corona. A coronal model comprised of many impulsively heated strands is adapted to stellar parameters. It is shown that the properties of the EM-T distribution can be accounted for in general terms provided the emission comes from many very small loops (length under 103 km) with intense magnetic fields (1 kG) distributed across part of the surface of the star. The heating requires events that generally dissipate between 1026 and 10 28 ergs, which is in the range of solar microflares. This implies that such stars must be capable of generating regions of localized intense magnetic fields. Title: The Magnetic Structure of Coronal Loops Observed by TRACE Authors: López Fuentes, M. C.; Klimchuk, J. A.; Démoulin, P. Bibcode: 2006ApJ...639..459L Altcode: 2006ApJ...639..459F; 2005astro.ph..7462L Previous studies have found that coronal loops have a nearly uniform thickness, which seems to disagree with the characteristic expansion of active region magnetic fields. This is one of the most intriguing enigmas in solar physics. We here report on the first comprehensive one-to-one comparison of observed loops with corresponding magnetic flux tubes obtained from cotemporal magnetic field extrapolation models. We use EUV images from TRACE, magnetograms from the MDI instrument on SOHO, and linear force-free field extrapolations satisfying b.nabla XB=αB, with α equal to a constant. For each loop, we find the particular value of α that best matches the observed loop axis and then construct flux tubes using different assumed cross sections at one footpoint (circle and ellipses with different orientations). We find that the flux tubes expand with height by typically twice as much as the corresponding loops. We also find that many flux tubes are much wider at one footpoint than the other, whereas the corresponding loops are far more symmetric. It is clear that the actual coronal magnetic field is more complex than the models we have considered. We suggest that the observed symmetry of loops is related to the tangling of elemental magnetic flux strands produced by photospheric convection. Title: On Solving the Coronal Heating Problem Authors: Klimchuk, James A. Bibcode: 2006SoPh..234...41K Altcode: 2005astro.ph.11841K The question of what heats the solar corona remains one of the most important problems in astrophysics. Finding a definitive solution involves a number of challenging steps, beginning with an identification of the energy source and ending with a prediction of observable quantities that can be compared directly with actual observations. Critical intermediate steps include realistic modeling of both the energy release process (the conversion of magnetic stress energy or wave energy into heat) and the response of the plasma to the heating. A variety of difficult issues must be addressed: highly disparate spatial scales, physical connections between the corona and lower atmosphere, complex microphysics, and variability and dynamics. Nearly all of the coronal heating mechanisms that have been proposed produce heating that is impulsive from the perspective of elemental magnetic flux strands. It is this perspective that must be adopted to understand how the plasma responds and radiates. In our opinion, the most promising explanation offered so far is Parker's idea of nanoflares occurring in magnetic fields that become tangled by turbulent convection. Exciting new developments include the identification of the "secondary instability" as the likely mechanism of energy release and the demonstration that impulsive heating in sub-resolution strands can explain certain observed properties of coronal loops that are otherwise very difficult to understand. Whatever the detailed mechanism of energy release, it is clear that some form of magnetic reconnection must be occurring at significant altitudes in the corona (above the magnetic carpet), so that the tangling does not increase indefinitely. This article outlines the key elements of a comprehensive strategy for solving the coronal heating problem and warns of obstacles that must be overcome along the way. 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: Coronal loops as self-organized critical systems Authors: López Fuentes, M. C.; Klimchuk, J. A. Bibcode: 2006BAAA...49..108L Altcode: We developed a numerical model that explains the evolution of coronal loops observed with GOES-SXI (see Lopez Fuentes, Klimchuk and Mandrini, ApJ, 2006, in press) in terms of Self-organized Critical Systems (SOC). We are inspired by the idea originally proposed by Parker (1988, ApJ, 330, 474), that coronal loops are made of elemental magnetic strands that wrap around each other due to photospheric convection. In our code the magnetic strength between neighbor strands increase until a threshold is reach and strands reconnect, releasing energy and heating the plasma. The number and intensity of these release events increase and a critical steady-state is reached. At some point, the photospheric dispersion makes the ``feeding'' mechanism inefficient and the loop decays. We model the plasma response and we obtain synthetic light curves that qualitatively reproduce the observed loop evolution. Title: Magnetic structure and observed width of coronal loops Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.; Demoulin, P. Bibcode: 2006cosp...36.2575L Altcode: 2006cosp.meet.2575L Previous studies have found that coronal loops have a nearly uniform thickness which seems to disagree with the characteristic expansion of active region magnetic fields This is one of the most intriguing enigmas in solar physics We here report on the first comprehensive one-to-one comparison of observed loops with corresponding magnetic flux tubes obtained from cotemporal magnetic field extrapolation models We use EUV images from TRACE magnetograms from the MDI instrument on SOHO and linear force-free field extrapolations For each loop we find the particular value of the force-free parameter alpha that best matches the observed loop axis and then construct flux tubes using different assumed cross sections at one footpoint circle and ellipses with different orientations We find that the flux tubes expand with height by typically twice as much as the corresponding loops We also find that many flux tubes are much wider at one footpoint than the other whereas the corresponding loops are far more symmetric It is clear that the actual coronal magnetic field is more complex than the models we have considered We suggest that the observed symmetry of loops is related to the tangling of elemental magnetic flux strands produced by photospheric convection 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: Coronal heating and the need for high-resolution observations Authors: Klimchuk, J. Bibcode: 2006cosp...36.2524K Altcode: 2006cosp.meet.2524K Despite excellent progress in recent years in understanding coronal heating there remain many crucial questions that are still unanswered Limitations in the observations are one important reason Both theoretical and observational considerations point to the importance of small spatial scales impulsive energy release strong dynamics and extreme plasma nonuniformity As a consequence high spatial resolution broad temperature coverage high temperature fidelity and sensitivity to velocities and densities are all critical observational parameters Current instruments lack one or more of these properties and this has led to considerable ambiguity and confusion In this talk I will discuss recent ideas about coronal heating and emphasize that high spatial resolution observations especially spectroscopic observations are needed to make major progress on this important problem Title: Study of coronal loops observed by GOES-SXI Authors: Lopez-Fuentes, M. C.; Mandrini, C. H.; Klimchuk, J. A. Bibcode: 2006cosp...36.2549L Altcode: 2006cosp.meet.2549L We study the temporal evolution of coronal loops using data from the Solar X-ray Imager SXI on board the Geosynchronous Operational Environmental Satellite 12 GOES-12 This instrument has the advantage of providing continuous soft X-ray observations of the solar corona at a high temporal cadence which allows us to follow the full lifetime of a set of coronal loops from their brightening to their decay From the observed light curves we can divide the evolution of the loops in three phases rise main and decay For each of these phases we compute the corresponding evolutionary timescales and since we have full time coverage the real loop lifetime Using data in different filters we derive temperature and density averages The values found place SXI loops halfway between the typical ranges of physical parameters for loops observed by the Soft X-ray Telescope Yohkoh SXT and those for loops observed by the Transition Region and Coronal Explorer TRACE We compute radiative and conductive cooling times which turn out to be much shorter than the evolutionary timescales of the loops These results confirm previous findings Porter and Klimchuk 1995 based on observations covering partially the loop temporal evolution Our results can be interpreted in terms of two alternative coronal heating scenarios quasi-static heating of monolithic uniform loop structures or impulsive heating nanoflaring of multiple-stranded loops We present arguments based on recent observations and loop modelling that support the idea Title: Evolution of coronal loops Authors: López Fuentes, M. C.; Mandrini, C. H.; Klimchuk, J. A. Bibcode: 2006BAAA...49..107L Altcode: We study the temporal evolution of coronal loops using data from the Solar X-ray Imager (SXI) on board the Geosynchronous Operational Environmental Satellite 12 (GOES-12). The observed light curves show three distinct evolutionary phases: rise, main, and decay. The durations and characteristic timescales of these phases are much longer than the cooling times, suggesting two possible scenarios: (1) loops are monolithic and evolve quasi-statically; or (2) they are made of unresolved strands that are impulsively heated. We show that the timescale of the loop-averaged heating rate is proportional to the timescale of the observed intensity variation. The constant of proportionality is approximately 1.5 for case (1) and 1.0 for case (2). The ratios of the radiative to conductive cooling times place these loops intermediate between previously measured hotter (Yohkoh-SXT) and cooler (TRACE) loops. Our results help to complete a trend that seems to support the impulsive heating hypothesis. Title: Observing the Solar atmosphere with the Extreme Ultraviolet Imaging Spectrometer on Solar B Authors: Korendyke, C. M.; Brown, C.; Dere, K.; Doschek, G.; Klimchuk, J.; Landi, E.; Mariska, J.; Warren, H.; Lang, J. Bibcode: 2005AGUFMSH41B1124K Altcode: The Extreme Ultraviolet Imaging Spectrometer (EIS) is part of the instrument complement on the Solar B satellite, scheduled for launch in the summer of 2006. The instrument has been calibrated and is presently mounted on the spacecraft. EIS is the most sensitive EUV solar spectrometer to be flown. The instrument is the first of a new generation of two optical element, solar spectrographs. Preliminary results from the laboratory focussing and calibration of the instrument will be shown. The instrument wavelength coverage includes reasonably bright spectral lines emitted by plasmas from 0.1 to 20 MK in temperature. The wavelength range also provides coronal density diagnostics. Temperature, density and velocity diagnostics will be discussed. An example observing program for exploring active region evolution and dynamics will be discussed. Title: Why We Need Imaging Spectroscopy Authors: Klimchuk, J. A. Bibcode: 2005AGUFMSH44A..01K Altcode: Despite excellent progress in recent years in understand fundamental solar physics problems such as coronal heating, solar wind acceleration, flares, and coronal mass ejections, there remain many crucial questions that are still unanswered. Limitations in the observations are one important reason. Both theoretical and observational considerations point to the importance of small spatial scales, impulsive energy release, strong dynamics, and extreme plasma nonuniformity. As a consequence, high spatial resolution, broad temperature coverage, high temperature fidelity, and sensitivity to velocities and densities are all critical observational parameters. Current instruments lack one or more of these properties, and this has led to considerable ambiguity and confusion. In this talk, I will discuss some of the ways that spectroscopic information is vital to further progress, giving particular emphasis to the coronal heating problem. I will argue that a high-speed imaging spectrometer must be a top priority instrument for the future. Title: Coronal Loop Heating by Nanoflares: The Impact of the Field-aligned Distribution of the Heating on Loop Observations Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2005ApJ...628.1023P Altcode: Nanoflares occurring at subresolution strands with repetition times longer than the coronal cooling time are a promising candidate for coronal loop heating. To investigate the impact of the spatial distribution of the nanoflare heating on loop observables, we compute hydrodynamic simulations with several different spatial distributions (uniform, loop top, randomly localized, and footpoint). The outputs of the simulations are then used to calculate density and temperature diagnostics from synthetic TRACE and SXT observations. We find that the diagnostics depend only weakly on the spatial distribution of the heating and therefore are not especially useful for distinguishing among the different possibilities. Observations of the very high temperature plasmas that are present only in the earliest stages of nanoflares can shed more light on the field-aligned distribution of the heating. Title: Are Coronal Loops Self-organized Critical Systems? Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.; Mandrini, C. H. Bibcode: 2005AGUSMSP14A..06L Altcode: In Lopez-Fuentes et al. 2004 we studied a set of loops observed by GOES/SXI and found that loop evolution can be separated in three phases (rise, main and decay). We found that the time scales of all three phases are long compared to a cooling time. In this work we explore whether this evolution is consistent with the development of a self-organized critical (SOC) system. We compare the observed soft X-ray light curves with light curves predicted by a simple SOC model, and we examine how the rise and decay times are related to the properties of the system driver. The physical picture we have in mind is the shuffling of elemental flux tubes by photospheric motions, as first advocated by Parker. Work funded by NASA and ONR. Lopez Fuentes, M.C., Mandrini, C.H., & Klimchuk, J.A., 2004, American Astronomical Society Meeting Abstracts, 204, 5602L Title: Coronal Loop Heating by Nanoflares: Non-thermal Velocities Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2005AGUSMSP41A..06P Altcode: Spectroscopic observations show non-negligible non-thermal velocities under coronal conditions. These motions place tight constraints on any coronal heating mechanism that should be able to reproduce them. We calculate the non-thermal velocities predicted by the nanoflare model. We perform 1D time-dependent hydrodynamic simulations of nanoflares occurring at sub-resolution strands, that make up the observed coronal loops and calculate profiles for representative spectral lines. We show that: (1) the calculated non-thermal velocities compare favorably with observations of cool and warm spectral lines and (2) the profiles of hot lines, that would be available in observations from the Extreme Ultraviolet Imaging Spectrometer (EIS) spectrometer onboard the SOLAR-B mission, can exhibit significant blue-wing asymmetries which can be used as a monitor of nanoflare properties. Research supported by NASA and ONR. 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 Loop Heating by Nanoflares: The Influence of the Field-aligned Distribution of the Heating on Observables Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2005AGUSMSP41A..05P Altcode: We investigate the effect of the spatial distribution of nanoflare heating on loop observables. We perform 1D time-dependent hydrodynamic simulations of nanoflares occurring at sub-resolution strands, that make up the observed coronal loops. The simulations use different spatial forms for the nanoflare heating (randomly localized, footpoint, uniform loop top). The outputs of the simulations are then used to calculate diagnostics from synthetic TRACE and SXT observations. We find that the diagnostics depend only weakly on the spatial distribution of the heating, and therefore are not especially useful for distinguishing among the different possibilities. We propose that the best way to study the field-aligned spatial distribution of nanoflare heating is to observe the very high temperature plasmas that are present only in the earliest stages of an event. Research supported by NASA and ONR. Title: Highly Efficient Modeling of Dynamic Coronal Loops Authors: Klimchuk, J. A.; Patsourakos, S.; Cargill, P. J. Bibcode: 2005AGUSMSP14A..03K Altcode: It now seems clear that many coronal loops, especially those observed by TRACE and EIT, are inherently dynamic and composed of large numbers of impulsively-heated strands. Modeling these loops in full detail is extremely challenging, and modeling entire active regions or the whole Sun is completely out of the question unless approximate techniques are used. We have developed a simplified set of equations that is remarkably accurate at describing the evolution of the thermodynamic variables (T, P, n, v) averaged along the magnetic field of an individual strand. The equations can be solved ten thousand times more quickly than the full 1D hydro equations. This "0D" model relaxes two key assumptions of Cargill's (1994) nanoflare model: (1) the heating can have any time-dependent profile and need not be instantaneous; and (2) thermal conduction cooling and radiation cooling occur together at all times, in varying proportions. We here describe the essential features of the model and show examples of how well it works. 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: The Effect of the Spatial Distribution of Nanoflare Heating on Loop Observables Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2004ESASP.575..297P Altcode: 2004soho...15..297P No abstract at ADS Title: Coronal Seismology and the Propagation of Acoustic Waves along Coronal Loops Authors: Klimchuk, J. A.; Tanner, S. E. M.; De Moortel, I. Bibcode: 2004ApJ...616.1232K Altcode: 2004astro.ph.12085K We use a combination of analytical theory, numerical simulation, and data analysis to study the propagation of acoustic waves along coronal loops. We show that the intensity perturbation of a wave depends on a number of factors, including dissipation of the wave energy, pressure and temperature gradients in the loop atmosphere, work action between the wave and a flow, and the sensitivity properties of the observing instrument. In particular, the scale length of the intensity perturbation varies directly with the dissipation scale length (i.e., damping length) and the scale lengths of pressure, temperature, and velocity. We simulate wave propagation in three different equilibrium loop models and find that dissipation and pressure and temperature stratification are the most important effects in the low corona where the waves are most easily detected. Velocity effects are small and cross-sectional area variations play no direct role for lines of sight that are normal to the loop axis. The intensity perturbation scale lengths in our simulations agree very well with the scale lengths we measure in a sample of loops observed by TRACE. The median observed value is 4.35×109 cm. In some cases the intensity perturbation increases with height, which is likely an indication of a temperature inversion in the loop (i.e., temperature that decreases with height). Our most important conclusion is that thermal conduction, the primary damping mechanism, is accurately described by classical transport theory. There is no need to invoke anomalous processes to explain the observations. Title: Coronal Seismology and the Propagation of Acoustic Waves Along Coronal Loops Authors: Klimchuk, J. A.; Tanner, S. E.; De Moortel, I. Bibcode: 2004AGUFMSH24A..06K Altcode: We use a combination of analytical theory, numerical simulation, and data analysis to study the propagation of acoustic waves along coronal loops. We show that the intensity perturbation of a wave depends on a number of factors, including dissipation of the wave energy, pressure and temperature gradients in the loop atmosphere, work action between the wave and a flow, and the sensitivity properties of the observing instrument. In particular, the scale length of the intensity perturbation varies directly with the dissipation scale length (i.e., damping length) and the scale lengths of pressure, temperature, and velocity. We simulate wave propagation in three different equilibrium loop models and find that dissipation and pressure and temperature stratification are the most important effects in the low corona where the waves are most easily detected. Velocity effects are small, and cross-sectional area variations play no direct role for lines-of-sight that are normal to the loop axis. The intensity perturbation scale lengths in our simulations agree very well with the scale lengths we measure in a sample of loops observed by TRACE. The median observed value is 4.35×109 cm. In some cases the intensity perturbation increases with height, which is likely an indication of a temperature inversion in the loop (i.e., temperature that decreases with height). Our most important conclusion is that thermal conduction, the primary damping mechanism, is accurately described by classical transport theory. There is no need to invoke anomalous processes to explain the observations. 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: 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: Evolution of Coronal Loops Observed by GOES-SXI Authors: Lopez Fuentes, M. C.; Mandrini, C. H.; Klimchuk, J. A. Bibcode: 2004AAS...204.5602L Altcode: 2004BAAS...36R.761L Several years ago, Porter and Klimchuk (PK, 1995) studied a collection of coronal loops observed by the SXT instrument on Yohkoh and found that they have evolutionary timescales of typically 104 to 105 s. These very long timescales are roughly one or two orders of magnitude greater than the corresponding cooling times and therefore have important implications for loop heating. However, PK observed each loop for only a single spacecraft orbit ( 1 hr), and it has remained a question whether loops actually persist for as long as the ''instantaneous'' evolutionary timescales suggest. Perhaps loops turn on suddenly, remain nearly constant for a modest period, then decay on a cooling timescale.

We have begun to address the general issue of loop evolution using images from the Soft X-ray Imager (SXI) on board the GOES-12 satellite. This instrument has a coarser spatial resolution than TRACE or SXT, but has the advantage of continuous temporal coverage. Since GOES-12 is in geosynchronous orbit, there is no spacecraft night. We present here the light curves of several loops and compare them with the radiative cooling times inferred from the observed temperature and emission measure. We discuss heating rate requirements related to the observed turn on and maintenance of the loops, and we consider the implications for nanoflare versus steady heating scenarios.

This work is funded by NASA and the Office of Naval Research.

Porter, L. J., and Klimchuk, J. A. 1995, ApJ, 454, 499 Title: Acoustic Wave Interpretation of Propagating Intensity Disturbances in Coronal Loops Authors: Klimchuk, J. A.; Tanner, S. E. M.; De Moortel, I. Bibcode: 2004AAS...204.9503K Altcode: 2004BAAS...36..826K Intensity disturbances have been observed by TRACE and EIT to propagate upward along the legs of long active region coronal loops. The periodic nature and speed of these disturbances suggest that they are traveling acoustic waves. It is being debated, however, whether the damping of the perturbations is consistent with the acoustic wave interpretation. We here examine this issue in detail with a combination of numerical simulation, analytical theory, and improved analysis of the observations. Using our state-of-the-art 1D hydro code, we simulate the propagation of waves generated at the base of model coronal loops. We consider static equilibrium loops having constant and expanding cross-section, and an equilibrium loop with steady flow. We show that the amplitude of the intensity perturbation is affected by a number of factors: wave dissipation (direct plasma heating), work done by the wave on the flow, pressure stratification, nonuniform temperature, and temperature-dependent sensitivity of the observing instrument. We compare our theoretical results with intensity scale lengths measured in a sample of loops observed by TRACE.

Research supported by NASA and ONR. Title: Comments on `Possible Role of MHD Waves in Heating the Solar Corona' by Dwivedi and Pandey Authors: Klimchuk, J. A.; Porter, L. J.; Sturrock, P. A. Bibcode: 2004SoPh..221...47K Altcode: We comment on the recent paper by Dwivedi and Pandey (Solar Physics216, 59, 2003). Parts of that paper closely reproduce, without reference, material that we had published previously, while other parts that deviate from our earlier analysis contain several critical flaws. We show that magnetoacoustic waves are capable of heating the corona with a modest enhancement in the coefficient of compressive viscosity. Title: Nanoflare Heating of the Corona Revisited Authors: Cargill, Peter J.; Klimchuk, James A. Bibcode: 2004ApJ...605..911C Altcode: The radiative signatures of the nanoflare model for coronal heating are investigated. If an observed coronal loop is assumed to consist of many small strands that cannot be distinguished spatially by EUV or X-ray observations, we are able to calculate differential emission-measure profiles and filling factors for a range of heating models. In this picture the strands undergo continual heating and cooling, leading to a corona comprising strands with a broad range of temperatures and densities. Thus, observations over a range of temperatures will show a multithermal coronal structure. The cyclical heating-cooling leads inevitably to loops that are underdense and overdense at high and low temperatures, respectively, compared to what would be expected from static equilibrium models, and in addition, we show that differential emission-measure profiles with shallow slopes can be obtained, as reported in recent observations. The differences between filling factors that can be seen by broadband and narrowband instruments are explored. Loops with broadband filling factors near unity can still have small narrowband factors, and the narrowband factor is shown to be a strong function of the local temperature. Nanoflare energy distributions that are constant, flat, or power laws are considered. Power laws lead to wide distributions of temperatures and densities in the corona, and steep power laws lead to larger filling factors. Title: The Inability of Steady-Flow Models to Explain the Extreme-Ultraviolet Coronal Loops Authors: Patsourakos, S.; Klimchuk, J. A.; MacNeice, P. J. Bibcode: 2004ApJ...603..322P Altcode: Recent observations from the Transition Region and Coronal Explorer (TRACE) and the EUV Imaging Telescope (EIT) show that warm (T~1-1.5 MK) EUV coronal loops in active regions generally have enhanced densities, enhanced pressure scale heights, and flat filter ratio (temperature) profiles in comparison with the predictions of static-equilibrium theory. It has been suggested that mass flows may explain these discrepancies. We investigate this conjecture using one-dimensional hydrodynamic simulations of steady flows in coronal loops. The flows are driven by asymmetric heating that decreases exponentially along the loop from one footpoint to the other. We find that a sufficiently large heating asymmetry can produce density enhancements consistent with a sizable fraction of the observed loops, but that the pressure scale heights are smaller than the corresponding gravitational scale heights, and that the filter ratio profiles are highly structured, in stark contrast to the observations. We conclude that most warm EUV loops cannot be explained by steady flows. It is thus likely that the heating in these loops is time dependent. Title: An Observational Test for Coronal Heating Models Authors: van Driel-Gesztelyi, L.; Démoulin, P.; Mandrini, C. H.; Harra, L. K.; Klimchuk, J. A. Bibcode: 2004IAUS..219..473V Altcode: 2003IAUS..219E..97V We correlate the evolution of the mean X-ray flux emission measure and temperature (Yohkoh SXT & BCS) with the magnetic flux density (SOHO/MDI) in active region NOAA 7978 from its birth throughout its decay for five solar rotations. We show that these plasma parameters together with other quantities deduced from them such as the density and the pressure follow power-law relationships with the mean magnetic flux density (bar{B}). We derive the dependence of the mean coronal heating rate on the magnetic flux density. We use the obtained scaling laws of coronal loops in thermal equilibrium to derive observational estimates of the scaling of the coronal heating with bar{B}. These results are used to test the validity of coronal heating models. We find that models invoking stochastic buildup of energy current layers and MHD turbulence are in best agreement with the observations. This narrows down the range of possible models retained by previous results obtained for individual coronal loops as well as for the global coronal emission of the Sun and cool stars. Title: Coronal Loop Heating by Nanoflares: Some Observational Implications Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2004hell.conf...35P Altcode: No abstract at ADS 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: Linear Force Free Field Models of Observed Coronal Loops Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A. Bibcode: 2003AGUFMSH42B0515L Altcode: Although active region magnetic fields have an overall expansion with height, soft X-ray and EUV loops are observed to have nearly constant cross sections. To investigate this apparent discrepancy, we have compared coronal loops observed by TRACE in the 171 A band with corresponding magnetic flux tubes obtained from linear force-free extrapolations (Demoulin et al. 1997) of nearly concurrent MDI magnetograms. The flux tubes were determined using a procedure that varies the force-free parameter alpha and searches for the field line that most closely coincides with the observed loop axis. Once the axis field line is identified, we construct flux tubes from the model field using a variety of assumed footpoint shapes and orientations. Our detailed comparison confirms the mystery of constant loop cross sections. Although the expansion of the extrapolated flux tube as seen projected onto the plane-of-the-sky varies depending on the footpoint shape and orientation, it is always considerably greater than the expansion of the corresponding TRACE loop. Furthermore, the extrapolated flux tubes are often highly asymmetric (with one leg much wider than the other), in stark contast to the TRACE loops. These results imply that the magnetic structure and possibly also the heating of coronal loops are more complex than we currently understand. Extrapolations of observed photospheric fields are very useful, but direct measurement of the coronal field may be necessary to make fundamental progress on this important problem. This work was funded by NASA and the Office of Naval Research. Title: The Non-flare Emisson Measure Above 5 MK Observed By RHESSI and SXI Authors: McTiernan, J. M.; Klimchuk, J. A. Bibcode: 2003AGUFMSH21B0162M Altcode: Since RHESSI was launched in February 2002, it has observed thousands of solar flares. It also observes solar emission above 3 keV when there are no observeable flares present. In this work we present measurements of the non-flare Temperature and Emission Measure for the period from October 2002 through July 2003. The temperature is relatively stable in the 6 - 8 MK range (this is not surprising considering that RHESSI cannot reliably measure temperatures less than about 5 MK). The Emission Measure varies in the range from approximately 1.0e49 to 1.0e50, with higher values associated with periods of more solar activity. Since RHESSI is an imaging spectrometer, we locate the source of the emission when possible. Preliminary results show that the source can be associated with active regions. We also present comparisons with SXI data, and measure the Differential Emission Measure for the range from 1 MK to 10 MK. 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: Hydrodynamic Simulations of Longitudinal Intensity Oscillations Observed in Coronal Loops by TRACE Authors: Tanner, S. E.; Klimchuk, J. A.; Hood, A. W.; De Moortel, I. Bibcode: 2003SPD....34.0406T Altcode: 2003BAAS...35..811T Propagating intensity disturbances are often observed by TRACE in large coronal loops located at the perimeters of active regions (e.g., De Moortel et al., 2002, Solar Phys., 209, 61). On average, the disturbances have periods of 280 s, propagation speeds of 120 km s-1, intensity amplitudes of 4%, and surprisingly small damping (detection) lengths of 9000 km. In addition, there is a positive correlation between damping length and period. The preliminary interpretation of these disturbances is that they are rapidly dissipating slow magneto-acoustic waves.

To investigate this interpretation more rigorously, we have performed a series of detailed coronal loop simulations using our 1D hydrodynamic code, ARGOS. We generate waves in the loop by imposing a spatially localized oscillating force at the loop footpoint, using a range of different oscillation periods. We here report on the results of our study and, in particular, whether the damping lengths have the properties observed by TRACE.

This work was supported by NASA and ONR. Title: Linear force free field models of observed coronal loops Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A. Bibcode: 2003SPD....34.0105L Altcode: 2003BAAS...35R.805L Although active region magnetic fields have an overall expansion with height, soft X-ray and EUV loops are observed to have nearly constant cross sections. To investigate this apparent discrepancy, we have compared coronal loops observed by TRACE in the 171 A band with corresponding magnetic flux tubes obtained from linear force-free extrapolations (Demoulin et al. 1997) of nearly concurrent MDI magnetograms. The flux tubes were determined using a procedure that varies the force-free parameter alpha and searches for the field line that most closely coincides with the observed loop axis. Once the axis field line is identified, we construct flux tubes from the model field using a variety of assumed footpoint shapes and orientations.

Our detailed comparison confirms the mystery of constant loop cross sections. Although the expansion of the extrapolated flux tube as seen projected onto the plane-of-the-sky varies depending on the footpoint shape and orientation, it is always considerably greater than the expansion of the corresponding TRACE loop. This result implies that the magnetic structure and possibly also the heating of coronal loops are more complex than we currently understand.

This work was funded by NASA and the Office of Naval Research. Title: The Non-flare Solar Temperature and Emission Measure Observed by RHESSI Authors: McTiernan, J. M.; Klimchuk, J. A. Bibcode: 2003SPD....34.1808M Altcode: 2003BAAS...35..841M Since RHESSI was launched in February 2002, it has observed thousands of solar flares. It also observes solar emission above 3 keV when there are no observeable flares present. In this work we present measurements of the non-flare Temperature and Emission Measure for the period from October 2002 through May 2003. Preliminary results indicate that the temperature is relatively stable in the 6 - 8 MK range (this is not surprising considering that RHESSI cannot reliably measure temperatures less than about 5 MK). The emission measure varies in the range from approximately 1.0e49 to 1.0e50, with higher values associated with periods of more solar activity.

Since RHESSI is an imaging spectrometer, we will locate the source of the emission when possible. We also will discuss comparisons with TRACE and SXI data, and the possiblilty of measuring the Differential Emission Measure for the range from 1 MK to 10 MK, using the multiple data sets. Title: Are All Coronal Loops Heated by Nanoflares? Authors: Klimchuk, J. A.; Patsourakos, S.; Winebarger, A. R. Bibcode: 2003SPD....34.1006K Altcode: 2003BAAS...35R.825K Observations from TRACE, SOHO, and Yohkoh have revealed new details of coronal loops that make them more mysterious than ever. One of the biggest puzzles concerns the loop density. Hot (> 2 MK) loops observed by Yohkoh tend to be under dense compared to the predictions of equilibrium theory, while warm ( 1 MK) loops observed by TRACE and EIT tend to be over dense. Some over dense loops can be explained by steady heating that is concentrated near one or both of the loop legs, but a majority of these loops cannot.

We here consider the possibility that observed loops are comprised of large numbers of unresolved strands that are heated impulsively and randomly by nanoflares. The loops appear quasi-steady even though the individual sub-strands are highly time dependent. When the strands are hot, they cool primarily by thermal conduction and are under dense, but when they are warm, they cool primarily by radiation and are over dense. Since Yohkoh and TRACE are sensitive to different strands, we might expect them to observe the under and over densities that they do.

To evaluate the feasibility of this universal model of coronal loops, we have performed 1D hydrodynamic simulations of impulsively heated strands and compared them with observations from Yohkoh and TRACE. The results are encouraging in many respects, but difficulties remain. In this presentation, we discuss the successes and failures of the 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: Can Steady-state Mass Flows Explain the Non-hydrostatic Cool EUV Coronal Loops in Active Regions? Authors: Patsourakos, S.; Klimchuk, J. A. Bibcode: 2003SPD....34.1009P Altcode: 2003BAAS...35..826P Recent EIT/TRACE observations of cool (≈ 1-1.5 MK) EUV coronal loops in active regions showed that these loops are very often characterized by greatly enhanced pressure scale-heights and densities compared to the predictions of static equilibrium theory. It has been suggested that mass flows may explain these over-dense and over-pressure loops. We investigate this conjecture by the means of 1D hydrodynamic simulations of steady-state mass flows in coronal loops. The mass flows in our calculations are driven by asymmetric heating that decreases exponentially along the loop from one footpoint all way to the other. By considering several representative cases for the magnitude and the length scale of the applied asymmetric heating, we determine how steady-state mass flows affect the thermodynamic structure of coronal loops and assess whether they can lead to enhanced pressure scale-heights and densities. Research supported in part by NASA and ONR. Title: The Long-Term Evolution of AR 7978: The Scalings of the Coronal Plasma Parameters with the Mean Photospheric Magnetic Field Authors: van Driel-Gesztelyi, L.; Démoulin, P.; Mandrini, C. H.; Harra, L.; Klimchuk, J. A. Bibcode: 2003ApJ...586..579V Altcode: We analyze the evolution of the fluxes observed in X-rays and correlate them with the magnetic flux density in active region (AR) NOAA 7978 from its birth throughout its decay, for five solar rotations. We use Solar and Heliospheric Observatory Michelson Doppler Imager (MDI) data, together with Yohkoh Soft X-Ray Telescope (SXT) and Yohkoh Bragg Crystal Spectrometer (BCS) data, to determine the global evolution of the temperature and the emission measure of the coronal plasma at times when no significant brightenings were observed. We show that the mean X-ray flux and derived parameters, temperature and emission measure (together with other quantities deduced from them, such as the density and the pressure), of the plasma in the AR follow power-law relationships with the mean magnetic flux density (B). The exponents (b) of these power-law functions (aBb) are derived using two different statistical methods, a classical least-squares method in log-log plots and a nonparametric method, which takes into account the fact that errors in the data may not be normally distributed. Both methods give similar exponents, within error bars, for the mean temperature and for both instruments (SXT and BCS); in particular, b stays in the range [0.27, 0.31] and [0.24, 0.57] for full-resolution SXT images and BCS data, respectively. For the emission measure, the exponent b lies in the range [0.85, 1.35] and [0.45, 1.96] for SXT and BCS, respectively. The determination of such power-law relations, when combined with the results from coronal heating models, can provide us with powerful tools for determining the mechanism responsible for the existence of the high-temperature corona. Title: The Long-Term Evolution of AR 7978: Testing Coronal Heating Models Authors: Démoulin, P.; van Driel-Gesztelyi, L.; Mandrini, C. H.; Klimchuk, J. A.; Harra, L. Bibcode: 2003ApJ...586..592D Altcode: We derive the dependence of the mean coronal heating rate on the magnetic flux density. Our results are based on a previous study of the plasma parameters and the magnetic flux density (B) in the active region NOAA 7978 from its birth to its decay, throughout five solar rotations using the Solar and Heliospheric Observatory Michelson Doppler Imager, Yohkoh Soft X-Ray Telescope (SXT), and Yohkoh Bragg Crystal Spectrometer (BCS). We use the scaling laws of coronal loops in thermal equilibrium to derive four observational estimates of the scaling of the coronal heating with B (two from SXT and two from BCS observations). These results are used to test the validity of coronal heating models. We find that models based on the dissipation of stressed, current-carrying magnetic fields are in better agreement with the observations than models that attribute coronal heating to the dissipation of MHD waves injected at the base of the corona. This confirms, with smaller error bars, previous results obtained for individual coronal loops, as well as for the global coronal emission of the Sun and cool stars. Taking into account that the photospheric field is concentrated in thin magnetic flux tubes, both SXT and BCS data are in best agreement with models invoking a stochastic buildup of energy, current layers, and MHD turbulence. Title: Riding the solar wind Authors: Klimchuk, James A. Bibcode: 2003Natur.421..894K Altcode: No abstract at ADS Title: How to test coronal heating models? Authors: Mandrini, C. H.; Démoulin, P.; van Driel-Gesztelyi, L.; Klimchuk, J. A.; Harra, L. K. Bibcode: 2003BAAA...46....5M Altcode: We have tested coronal heating models following two different approaches. In the first case, we compared the dependence of the coronal heating rate predicted by theoretical models with the observed one, deriving the scalings of parameters, such as: the density, temperature and intensity of the coronal magnetic field, with the length of magnetic field lines. To do so, we combined density and temperature measurements for 47 coronal loops with magnetic field models for 14 active regions. In the second case, we analyzed the long term evolution of an active region observed during seven rotations on the solar disk and we determined the dependence of the observed heating rate with the magnetic field density (bar{B}), after finding the scalings of plasma parameters with bar{B}. In both cases, we found that models based on the dissipation of stressed, current-carrying magnetic fields (called direct current models) are in better agreement with observations than models that attribute coronal heating to the dissipation of MHD waves injected at the base of the corona (called alternate current models). Taking into account that the photospheric field is concentrated in thin magnetic flux tubes, observations are in best agreement with models invoking a stochastic buildup of energy, current layers and MHD turbulence, within direct current models. Title: Coronal arcs and magnetic structure of the solar corona Authors: López Fuentes, M. C.; Klimchuk, J. A. Bibcode: 2003BAAA...46....2L Altcode: Since the strength of the coronal magnetic field decreases with distance from the photosphere, it is expected that coronal magnetic structures expand with height. Nevertheless, loops observed in EUV and Soft X-rays have constant cross section (Klimchuk 2000). In this study we compare a set of TRACE loops with flux tubes obtained from the force-free extrapolation of the coronal magnetic field. As boundary condition we use MDI magnetograms cotemporal with the TRACE observations. The observed width of the coronal loops in the studied set is constant. On the other hand, the corresponding modeled flux tubes expand with distance from the photosphere. To compare with observed quantities, we compute the width of the flux tubes as seen from the line of sight. In contrast with the nearly constant thickness of the observed loops, the width of the modeled flux tubes is highly variable. These results imply that the magnetic structure of the corona and the mechanism of coronal heating are much more complex than currently understood. 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: The mystery of Coronal Loops Authors: Klimchuk, J. A. Bibcode: 2003BAAA...46....2K Altcode: Coronal loops, beautiful plasma structures that outline arched magnetic fields, are often referred to as the fundamental building blocks of the million degree solar corona. Despite their importance and years of intensive study, coronal loops are in many ways enigmatic. In particular, recent observations have forced us to abandon our old belief that they are static structures maintained by steady heating. In this talk we review the status of coronal loop physics and describe an emerging paradigm of nanoflare heating. 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: Scaling Laws for Solar and Stellar Coronae Authors: Klimchuk, James A. Bibcode: 2002ASPC..277..321K Altcode: 2002sccx.conf..321K No abstract at ADS 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: 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: 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: Coronal loops Authors: Klimchuk, James Bibcode: 2002ocnd.confE..17K Altcode: No abstract at ADS Title: Observations and Modeling of Solar Coronal Loops Authors: Klimchuk, J. Bibcode: 2002cosp...34E1208K Altcode: 2002cosp.meetE1208K Coronal loops are often described as the fundamental building blocks of solar and stellar coronae. Clearly, therefore, a comprehensive understanding of coronae requires an explanation of the nature and origin of these loops, including the mechanism of their heating. Certain general aspects of coronal loops are reasonably well understood. For example, we know that the plasma is structured by the magnetic field and that strongly heated flux tubes tend to be hotter and denser than weakly heated flux tubes. Some observations suggest that loops are in quasi-static equilibrium, and scaling laws have been used to describe the relationships among physical variables and to test competing theories of coronal heating. Other observations raise serious doubts about whether the quasi-static description is valid. At this point, we cannot say with any certainty whether loops are isothermal or multithermal (i.e., monolithic structures or collections of unresolved strands) or whether they are heated steadily or in a highly episodic fashion (e.g., by nanoflares). This talk will address what we can learn about these important questions from a combination of observations and theoretical modeling. Title: Modeling the coronal loop of an X-ray bright point Authors: McMullen, R.; Longcope, D.; McKenzie, D.; Kankelborg, C.; Klimchuk, J. Bibcode: 2002ocnd.confE..28M Altcode: No abstract at ADS Title: Observation and Theory of Coronal Loop Structure Authors: Klimchuk, J. A. Bibcode: 2002mwoc.conf...65K Altcode: Following up on an initial study of 10 soft X-ray loops observed by Yohkoh (Klimchuk et al. 1992), we have carefully examined 43 additional Yohkoh loops and 24 EUV loops observed by TRACE, and we confirm our original finding that most coronal loops 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 times 513 times 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. 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: On the Correlation between Coronal and Lower Transition Region Structures at Arcsecond Scales Authors: Vourlidas, A.; Klimchuk, J. A.; Korendyke, C. M.; Tarbell, T. D.; Handy, B. N. Bibcode: 2001ApJ...563..374V Altcode: We compare the morphology of active region structures observed in the 171 Å (T~9×105 K) and Lyα (T~2×104 K) lines. The coronal data were obtained by the Transition Region and Coronal Explorer (TRACE) in support of the Very High Angular Resolution Ultraviolet Telescope (VAULT) sounding rocket launch, which acquired subarcsecond resolution images of an active region in the Lyα line, on 1999 May 7. Using a pair of calibrated, nearly simultaneous images, we find that: (i) a very good correlation exists between the Lyα and 171 Å intensities in the TRACE moss regions, (ii) we can identify several identical structures in some (but not all) moss areas, and (iii) the correlations are greatly reduced at the footpoints of the 171 Å large-scale loops. We derive a lower limit for the Lyα emission measure, under the assumption of effectively optically thin emission, and compare it to the 171 Å emission measure. As in previous studies, we find an excess of Lyα material compared to the amount expected for a thermal conduction-dominated corona-chromosphere transition region, even for structures that appear to be identical in the two wavelengths. This result implies that some other mechanism besides classical heat conduction from the corona must contribute to the observed Lyα intensities. The observations do not support the idea of a physically distinct cool loop component within active regions. Title: Spectroscopic Diagnostics of Nanoflare-heated Loops Authors: Klimchuk, J. A.; Cargill, P. J. Bibcode: 2001ApJ...553..440K Altcode: To evaluate the usefulness of spectroscopic techniques for diagnosing realistic solar plasmas and to better understand the physical origin of coronal heating, we have simulated observations of model coronal loops that are heated randomly and impulsively by nanoflares. We find that the emission measures, densities, and filling factors that are inferred from spectral line intensities (EMs, ns, and φs, respectively) are generally an excellent representation of the properties of the nanoflare-heated plasma. To better than 25% in most cases, EMs indicates the amount of material present in the ΔlogT=0.3 temperature interval centered on the peak of the line contribution function, ns indicates the average density of this material, and φs indicates the fraction of the total volume that the material occupies. Measurements with lithium-like lines are much less accurate, however. We provide diagnostic values and line intensities for many different spectral lines that can be compared directly with observations from the Coronal Diagnostic Spectrometer and Solar Ultraviolet Measurements of Emitted Radiation instruments on SOHO and from the future Extreme Ultraviolet Imaging Spectrometer instrument on Solar-B. Such comparisons will provide the first ever rigorous test of the nanoflare concept, which has enormous implications for understanding the mechanism of coronal heating. Title: Ultra-High Resolution Observations of the Upper Chromosphere: First Results From the NRL VAULT Sounding Rocket Payload Authors: Vourlidas, A.; Korendyke, C. M.; Dere, K. P.; Klimchuk, J. A. Bibcode: 2001AGUSM..SP61A03V Altcode: The Very high resolution Advanced ULtraviolet Telescope (VAULT) is a new spectroscopic imaging instrument. The instrument was launched on May 7, 1999 as a sounding rocket payload. The goal of the first VAULT flight was to obtain sub-arcsecond images of the Sun in the light of Lya (1216 Å). VAULT directly imaged an active region plage, fliaments and the fine structures in the supergranule boundaries and network with the unprecented spatial resolution of 0.33 arcseconds. We present the VAULT images and the first results from the comparison of the Lya data to observations from other instruments and in particular with a sequence of TRACE 171 Å images taken during the VAULT flight. 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: Observational Signatures of Nanoflare-Heated Loops Authors: Klimchuk, J. A.; Cargill, P. J. Bibcode: 2001AGUSM..SP52B01K Altcode: After many years of intense study, the heating of the corona remains a largely unsolved problem. A number of ideas have been proposed involving the episodic release of small bursts of energy, termed ``nanoflares" by Parker based on his rough estimate of the energies involved. Adopting this concept, we investigate a model whereby individual coronal loops like those observed by Yohkoh, SOHO, and TRACE are comprised of many thousands of unresolved strands that are heated randomly and independently by nanoflares. The strands cool by thermal conduction and radiation until they are heated again by the next event. We simulate the observation of these model loops by broadband and spectroscopic instruments such as SXT, CDS, SUMER, and EIS. By varying the nanoflare parameters (primarily the mean energy of the events) and comparing with actual observations, we are able to test the model and constrain the parameters. We find that many real loops are compatible with the model, especially those hotter than 2 MK, but that many others are not. These latter loops are also incompatible with steady heating, and we suggest that they contain coronal material that is injected into the loop by a mechanism not directly related to coronal heating (e.g., not chromospheric evaporation). Title: Properties of EUV and X-ray emission in solar active regions Authors: Matthews, S. A.; Klimchuk, J. A.; Harra, L. K. Bibcode: 2001A&A...365..186M Altcode: Using observations from the Coronal Diagnostic Spectrometer (CDS) on SoHO and the Soft X-ray Telescope (SXT) on Yohkoh we investigate how the spatial properties of active region emission observed in the EUV and X-ray range varies with temperature. We examine the contrast per unit area of the EUV emission from a number of active regions, and employ correlation techniques and Fourier methods with which we obtain the two dimensional power spectrum of the intensity distribution for a number of images in emission lines formed at different temperatures. Integrating this over polar angle we find isotropic power-law behaviour at all temperatures in a number of topologically different active regions, with a tendency for flatter spectra at lower temperatures. The existence of power-law spectra indicates that there is no preferred length scale within the regions, at least not a resolvable one, while flatter spectra at lower temperatures indicate that the structures are relatively smaller in this temperature range, possibly providing support for the idea of a multi-component transition region (TR). Implications for various heating models are discussed. Title: Theory of Coronal Mass Ejections Authors: Klimchuk, J. A. Bibcode: 2001GMS...125..143K Altcode: Coronal mass ejections (CMEs) are extremely important phenomena, both for understanding the evolution of the global corona and for understanding and predicting space weather. Despite this importance, the physical explanation of CMEs remains largely confused. A variety of theoretical models have been proposed, and we here attempt to organize them according to a classification scheme that identifies and differentiates their essential physical attributes. We propose five distinct classes of models, which we present with the aid of simple analogues involving springs, ropes, and weights. We also indicate how some of the models appear to be inconsistent with certain observations. 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: 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: 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: Cross-Sectional Properties of Coronal Loops Authors: Klimchuk, J. A. Bibcode: 2000SoPh..193...53K Altcode: Careful examination of 43 soft X-ray loops observed by Yohkoh has revealed a number of interesting properties of the loop cross section. First, the loops tend to be only slightly (≈ 30%) wider at their midpoints than at their footpoints, implying less-than-expected expansion of the magnetic field. Second, the variation of width along each loop tends to be modest, implying that the cross section has an approximately circular shape. And third, cross-axis intensity profiles tend to be singly-peaked and simple, implying that the cross section is approximately uniformly filled on resolvable scales. We conclude that the energy which heats the plasma is either dissipated axially symmetrically on a scale equal to a loop diameter (≈ 11 000 km) or else is dissipated with any spatial structure, but on a scale much smaller than a loop diameter, and then transported laterally in an axisymmetric fashion (perhaps via conduction along chaotic field lines). In their present form, none of the theoretical ideas concerning the magnetic structure and heating of loops are obviously capable of explaining all of the observed properties. Title: Width Variations along Coronal Loops Observed by TRACE Authors: Watko, J. A.; Klimchuk, J. A. Bibcode: 2000SoPh..193...77W Altcode: We have measured width variations along coronal loops observed by TRACE in the 171, 195, and 284 Å bandpasses. The loops are not significantly thicker in the middle compared to near the footpoints, and there is no correlation between the footpoint-to-midpoint expansion and the loop length. This applies to both post-flare and non-flare loops. The observations conflict with our present understanding of active region magnetic fields, and they have important implications for the structure and heating of the corona. Title: Three-dimensional Stereoscopic Analysis of Solar Active Region Loops. II. SOHO/EIT Observations at Temperatures of 1.5-2.5 MK Authors: Aschwanden, Markus J.; Alexander, David; Hurlburt, Neal; Newmark, Jeffrey S.; Neupert, Werner M.; Klimchuk, J. A.; Gary, G. Allen Bibcode: 2000ApJ...531.1129A Altcode: In this paper we study the three-dimensional structure of hot (Te~1.5-2.5 MK) loops in solar active region NOAA 7986, observed on 1996 August 30 with the Extreme-ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO). This complements a first study (Paper I) on cooler (Te~1.0-1.5 MK) loops of the same active region, using the same method of Dynamic Stereoscopy to reconstruct the three-dimensional geometry. We reconstruct the three-dimensional coordinates x(s), y(s), z(s), the density ne(s), and temperature profile Te(s) of 35 individual loop segments (as a function of the loop coordinate s) using EIT 195 and 284 Å images. The major findings are as follows. (1) All loops are found to be in hydrostatic equilibrium, in the entire temperature regime of Te=1.0-2.5 MK. (2) The analyzed loops have a height of 2-3 scale heights, and thus only segments extending over about one vertical scale height have sufficient emission measure contrast for detection. (3) The temperature gradient over the lowest scale height is of order dT/ds~1-10 K km-1. (4) The radiative loss rate is found to exceed the conductive loss rate by about two orders or magnitude in the coronal loop segments, implying that the loops cannot be in quasi-static equilibrium, since standard steady-state loop models show that radiative and conductive losses are comparable. (5) A steady state could only be maintained if the heating rate EH matches exactly the radiative loss rate in hydrostatic equilibrium, requiring a heat deposition length λH of the half density scale height λ. (6) We find a correlation of p~L-1 between loop base pressure and loop length, which is not consistent with the scaling law predicted from steady-state models of large-scale loops. All observational findings indicate consistently that the energy balance of the observed EUV loops cannot be described by steady-state models. Title: Magnetic Field and Plasma Scaling Laws: Their Implications for Coronal Heating Models Authors: Mandrini, C. H.; Démoulin, P.; Klimchuk, J. A. Bibcode: 2000ApJ...530..999M Altcode: In order to test different models of coronal heating, we have investigated how the magnetic field strength of coronal flux tubes depends on the end-to-end length of the tube. Using photospheric magnetograms from both observed and idealized active regions, we computed potential, linear force-free, and magnetostatic extrapolation models. For each model, we then determined the average coronal field strength, <B>, in approximately 1000 individual flux tubes with regularly spaced footpoints. Scatter plots of <B> versus length, L, are characterized by a flat section for small L and a steeply declining section for large L. They are well described by a function of the form log=C1+C2logL+C3/2log(L2+S2), where C2~0, -3<=C3<=-1, and 40<=S<=240 Mm is related to the characteristic size of the active region. There is a tendency for the magnitude of C3 to decrease as the magnetic complexity of the region increases. The average magnetic energy in a flux tube, <B2>, exhibits a similar behavior, with only C3 being significantly different. For flux tubes of intermediate length, 50<=L<=300 Mm, corresponding to the soft X-ray loops in a study by Klimchuk & Porter (1995), we find a universal scaling law of the form ~Lδ, where δ=-0.88+/-0.3. By combining this with the Klimchuk & Porter result that the heating rate scales as L-2, we can test different models of coronal heating. We find that models involving the gradual stressing of the magnetic field, by slow footpoint motions, are in generally better agreement with the observational constraints than are wave heating models. We conclude, however, that the theoretical models must be more fully developed and the observational uncertainties must be reduced before any definitive statements about specific heating mechanisms can be made. Title: The spatial distribution of EUV emission in active regions Authors: Matthews, S. A.; Klimchuk, J. A.; Harra, L. K. Bibcode: 2000ssls.work...53M Altcode: The full version of this paper will be published elsewhere. We give here only an extended abstract. Title: Test on the parameter dependence of coronal heating models Authors: Démoulin, P.; Mandrini, C. H.; Klimchuk, J. A. Bibcode: 2000ssls.work...85D Altcode: The motivation of this work has been to provide observational constraints on coronal heating models by testing their predictions for the heating rate as a function of several physical parameters. In Mandrini et al. (1999), we have investigated how the magnetic field strength, , of coronal flux tubes depends on the end-to-end length, L, of the tube. For flux tubes of intermediate length, 50 ≤ L ≤ 300 Mm, corresponding to the soft X-ray loops in a study by Klimchuk & Porter (1995), we find a universal scaling law of the form ∝Lδ, where δ= -0.88±0.3. By combining this with the Klimchuk & Porter result that the heating rate scales as L-2, we can test different models of coronal heating. We find that models involving the gradual stressing of the magnetic field, by slow footpoint motions, are in generally better agreement with the observational constraints than are wave heating models. Title: Properties of Transition Region and Coronal Loops Authors: Matthews, S. A.; Klimchuk, J. A.; Harra-Murnion, L. K. Bibcode: 1999ESASP.446..489M Altcode: 1999soho....8..489M The magnetic field plays a vital role in governing the size, shape and dynamics of loops in the solar atmosphere and as such it seems reasonable to ask whether differences in the spatial distribution of these structures are indicative of differences in the form of the heating. Using observations from the Coronal Diagnostic Spectrometer (CDS) on SoHO and the Soft X-ray Telescope (SXT) on Yohkoh we investigate how the spatial distribution of EUV and X-ray emission in active regions varies with temperature. We examine various correlations between emission at different temperatures, the variation of contrast with temperature and employ Fourier methods to obtain the two dimensional power spectrum of the intensity distribution for a number of lines at different temperatures. Integrating this over polar angle we find isotropic power-law behaviour at all temperatures in a number of topologically different active regions, with a tendency for flatter spectra at lower temperatures. The existence of power-law spectra indicates that there is no preferred length scale within the regions. Flatter spectra at lower temperatures are consistent with emission predominantly from smaller scale structures such as low-lying loops or footpoints. Title: Temperature and density in a polar plume - measurements from CDS/SOHO Authors: Young, P. R.; Klimchuk, J. A.; Mason, H. E. Bibcode: 1999A&A...350..286Y Altcode: A detailed analysis of a particularly intense polar plume observed on the 25th of October, 1996, by the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO) is presented. Above the limb, emission measure distributions derived for both the plume and a section of coronal hole background are found to be sharply peaked at approximately 1.0-1.1 million degrees in both regions. The temperature rises with height in the background, but no evidence is found for a rising temperature in the plume. The density of the background is approximately 10(8) electrons/cm(3) and falls with height. In the plume the density is between 3.8 and 9.5x 10(8) electrons/cm(3) , and exhibits no decrease with height up to 70 000 km. The plume base is visible on the solar surface and shows a strong brightening lying directly below the main body of the plume. This brightening has a temperature of 2 000 000 K, and a density of 2.5-5.6x 10(9) electrons/cm(3) . Images from lines formed at different temperatures suggest that the morphology of the base is consistent with an emerged bipole in a region of unipolar magnetic flux. A measurement of the Mg/Ne relative abundance is made at two transition region brightenings at the base of the plume. An enhancement of only 1.5 is found over the photospheric value. Considerations of the geometry of both the high temperature brightening at the base of the plume and the off-limb section give filling factors of 0.5 and 1.0, respectively. Title: Magnetic Field Scaling Laws and Their Implications for Coronal Heating Authors: Klimchuk, J. A.; Demoulin, P.; Mandrini, C. H. Bibcode: 1999AAS...194.2304K Altcode: 1999BAAS...31..861K Ever since it was realized, some 60 years ago, that the solar corona is two orders of magnitude hotter than the underlying photosphere, scientists have puzzled over the reason for these extreme conditions. A number of plausible ideas have been proposed, including the dissipation of MHD waves (AC models) and the dissipation of stressed, current-carrying magnetic fields (DC models), but it has proved difficult to establish which, if any, is correct. One approach to answering this fundamental question is to determine scaling laws relating the heating rate to observable physical parameters. Klimchuk & Porter (1995, Nature, 377, 131) showed that the heating rate varies inversely with the square of the length of coronal loops observed by Yohkoh. To compare this with the predictions of coronal heating theories, it is necessary to know also how the magnetic field strength in the loops varies with their length. By computing magnetic field extrapolation models based on both observed and synthetic distributions of active region surface fields, we have found that B ~ ( L(2) + S(2) )(c/2) , where B is the coronal field strength averaged along a loop, L is the loop length, S is the characteristic size of the active region, and -3 <= c <= -1, depending on the complexity of the region. More importantly, for the range of loop lengths studied by Klimchuk & Porter, 50 < L < 300 Mm, there is a universal scaling law of the form B ~ L(delta ,) where delta = -0.98 +/- 0.3. The details of these results will be presented, and their implications for theories of coronal heating will be discussed. It will be shown that DC models are in better agreement with the observations than are AC models. This work was supported in part by NASA grant W-19,200. Title: The Solar Flotilla Authors: Kahler, S. W.; Klimchuk, J. A.; Szabo, A.; Galvin, A. B. Bibcode: 1999AAS...194.6507K Altcode: 1999BAAS...31R.928K The Solar Flotilla is one of two candidate missions for the NASA Roadmap which would use a multispacecraft mission to explore the inner heliosphere. The Solar Flotilla will make particles and fields measurements at 10 or more points around the Sun within a distance of 0.2 to 0.4 AU. Identically instrumented microsatellites will be placed in orbit after a journey to Mercury where they will be injected into three principal orbit planes by 3-body interactions with the planet. Solar Flotilla measurements will address the question of how the solar output of plasma and magnetic flux varies with space and time, similar to the far better understood variations of radiant energy outputs, which can be measured remotely. Fundamental questions of magnetic flux and helicity expulsion can only be studied with such measurements. Additionally, mass flows and their variations can be understood on a global scale with measurements from the Solar Flotilla. The radial and azimuthal variations of turbulence, waves, and particle temperatures and abundances all relate to the heating of the solar corona and the acceleration of the solar wind. The spatial extents and shapes of interplanetary shocks and energetic particle distributions can be studied and used for improved space weather predictions. Together with imaging instruments at 1 AU we can explore the interplanetary extensions of short time-scale coronal features such as blobs and coronal mass ejections and long time-scale features such as plumes and streamers. The number of the spacecraft, their near-solar environment and great distance from Earth pose significant technological problems for operations and communications to be solved before deployment of the mission. We discuss further benefits and requirements of the proposed mission. 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: Three-dimensional Stereoscopic Analysis of Solar Active Region Loops. I. SOHO/EIT Observations at Temperatures of (1.0-1.5) × 106 K Authors: Aschwanden, Markus J.; Newmark, Jeffrey S.; Delaboudinière, Jean-Pierre; Neupert, Werner M.; Klimchuk, J. A.; Gary, G. Allen; Portier-Fozzani, Fabrice; Zucker, Arik Bibcode: 1999ApJ...515..842A Altcode: The three-dimensional structure of solar active region NOAA 7986 observed on 1996 August 30 with the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) is analyzed. We develop a new method of dynamic stereoscopy to reconstruct the three-dimensional geometry of dynamically changing loops, which allows us to determine the orientation of the mean loop plane with respect to the line of sight, a prerequisite to correct properly for projection effects in three-dimensional loop models. With this method and the filter-ratio technique applied to EIT 171 and 195 Å images we determine the three-dimensional coordinates [x(s), y(s), z(s)], the loop width w(s), the electron density ne(s), and the electron temperature Te(s) as a function of the loop length s for 30 loop segments. Fitting the loop densities with an exponential density model ne(h) we find that the mean of inferred scale height temperatures, Tλe=1.22+/-0.23 MK, matches closely that of EIT filter-ratio temperatures, TEITe=1.21+/-0.06 MK. We conclude that these cool and rather large-scale loops (with heights of h~30-225 Mm) are in hydrostatic equilibrium. Most of the loops show no significant thickness variation w(s), but we measure for most of them a positive temperature gradient (dT/ds>0) across the first scale height above the footpoint. Based on these temperature gradients we find that the conductive loss rate is about 2 orders of magnitude smaller than the radiative loss rate, which is in strong contrast to hot active region loops seen in soft X-rays. We infer a mean radiative loss time of τrad~40 minutes at the loop base. Because thermal conduction is negligible in these cool EUV loops, they are not in steady state, and radiative loss has entirely to be balanced by the heating function. A statistical heating model with recurrent heating events distributed along the entire loop can explain the observed temperature gradients if the mean recurrence time is <~10 minutes. We computed also a potential field model (from SOHO/MDI magnetograms) and found a reasonable match with the traced EIT loops. With the magnetic field model we determined also the height dependence of the magnetic field B(h), the plasma parameter β(h), and the Alfvén velocity vA(h). No correlation was found between the heating rate requirement EH0 and the magnetic field Bfoot at the loop footpoints. 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: 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: Report on new mission concept study: Stereo X-Ray Corona Imager mission Authors: Liewer, Paulett C.; Davis, John M.; de Jong, E. M.; Gary, G. A.; Klimchuk, James A.; Reinert, Richard P. Bibcode: 1998SPIE.3442...53L Altcode: Studies of the 3D structure and dynamics of the solar corona have been severely limited by the constraint of single viewpoint observations. The Stereo X-Ray Coronal Imager (SXCI) mission will send a single instrument, an X-ray telescope, into deep space expressly to record stereoscopic images of the solar corona. The SXCI spacecraft will be inserted into an approximately 1 ZAU heliocentric orbit leading Earth by approximately 25 degrees at the end of nine months. The SXCI x-ray telescope forms one element of a stereo pair, the second element being an identical x-ray telescope in Earth orbit placed there as part of the NOAA GOES program. X-ray emission is a powerful diagnostic of the corona and its magnetic fields, and 3D information on the coronal magnetic structure would be obtained by combining the data from the two x-ray telescopes. This information can be used to address the major solar physics questions of (1) what causes explosive coronal events such as coronal mass ejections, eruptive flares and prominence eruptions and (2) what causes the transient heating of coronal loops. Stereoscopic views of the optically thin corona will resolve some ambiguities inherent in single line-of-sight observations. Triangulation gives 3D solar coordinates of features which can be seen in the simultaneous images form both telescopes. As part of this study, tools were developed for determining the 3D geometry of coronal features using triangulation. Advanced technologies for visualization and analysis of stereo images were tested. Results of mission and spacecraft studies are also reported. 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: 3D-Stereoscopic Analysis of Solar Active Region Loops Observed with SOHO/EIT Authors: Aschwanden, M. J.; Newmark, J. S.; Delaboudiniere, J. -P.; Neupert, W. M.; Klimchuk, J. A.; Gary, G. Allen; Portier-Fozzani, F.; Zucker, A. Bibcode: 1998cee..workE..19A Altcode: The three-dimensional (3D) structure of solar active region NOAA 7986 observed on 1996 August 30 with the Extrem-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SoHO) is analyzed. We develop a new method of Dynamic Stereoscopy to reconstruct the 3D geometry of dynamically changing loops, which allows us to determine the orientation of the loop plane with respect to the line-of-sight, a prerequisite to correct properly for projection effects in 3D loop models. With this method and the filter-ratio technique applied to EIT 171 angle and 195 angle images we determine the 3D coordinates [x(s),y(s),z(s)], the loop width w(s), the electron density n_e(s), and the electron temperature T_e(s) as function of the loop length s for 30 loop segments. Fitting the loop densities with an exponential density model n_e(h) we find that the mean of inferred scale height temperatures, T_elambda = 1.22 plus or minus 0.23 MK, matches closely that of EIT filter-ratio temperatures, T_eEIT = 1.21 plus or minus 0.06 MK. We conclude that these cool and rather large-scale loops (with heights of h ~30-225 Mm), which dominate the EIT 171 angle images, are in hydrostatic equilibrium. Most of the loops show no significant thickness variation w(s), but we can measure for most of them a positive temperature gradient (dT/ds > 0) across the first scale height above the footpoint. Based on these temperature gradients we find that the conductive loss rate is about two orders of magnitude smaller than the radiative loss rate, which is in strong contrast to hot active region loops seen in SXR. We infer a mean radiative loss time of τrad ~40 minutes. For steady state models, the heating rate has to balance the radiative loss, i.e. the heating rate has to scale with the squared density (E_H propto n_e^2). From potential-field extrapolations we determine also the magnetic field strength B(s), the plasma beta-parameter beta(s), and the Alfven velocity v_A(s) along the loops, and discuss the findings in the context of coronal heating models. Title: Theory of spicules, jets, plumes and other solar eruptions Authors: Klimchuk, J. A. Bibcode: 1998ESASP.421..233K Altcode: 1998sjcp.conf..233K No abstract at ADS Title: A Self-consistent Model for the Resonant Heating of Coronal Loops: The Effects of Coupling with the Chromosphere Authors: Ofman, L.; Klimchuk, J. A.; Davila, J. M. Bibcode: 1998ApJ...493..474O Altcode: We present the first model of resonant heating of coronal loops that incorporates the dependence of the loop density on the heating rate. By adopting the quasi-static equilibrium scaling law ρ ~ Q5/7, where ρ is the density and Q is the volumetric heating rate, we are able to approximate the well-known phenomena of chromospheric evaporation and chromospheric condensation, which regulate the coronal density. We combine this scaling law with a quasi-nonlinear MHD model for the resonant absorption of Alfvén waves in order to study the spatial and temporal dependence of the heating. We find that the heating is concentrated in multiple resonance layers, rather than in the single layer of previous models, and that these layers drift throughout the loop to heat the entire volume. These newfound properties are in much better agreement with coronal observations. Title: A Self-Consistent Model for the Resonant Heating of Coronal Loops: the Effects of Coupling with the Chromosphere Authors: Klimchuk, J. A.; Ofman, L.; Davila, J. M. Bibcode: 1997SPD....28.0504K Altcode: 1997BAAS...29..909K The physical nature of coronal heating remains one of the great problems of solar physics. One of the several theories that are being pursued is the resonant absorption of MHD waves. While promising in several respects, this theory has suffered from a glaring deficiency: the computed heating is incompatible with both the assumed density and the observed structure of coronal loops. We present the first model of resonant heating of coronal loops that incorporates the dependence of the loop density on the heating rate. By adopting the quasi-static equilibrium scaling law rho ~ Q(5/7) , where rho is the density and Q is the volumetric heating rate, we are able to approximate the well-known phenomena of chromospheric evaporation and chromospheric condensation, which regulate the coronal density. We combine this scaling law with a linearized MHD model for the resonant absorption of Alfven waves to study the spatial and temporal dependence of the heating. We find that the heating is concentrated in multiple resonance layers, rather than the single layer of previous models, and that these layers drift throughout the loop to heat the entire volume. These new properties are in much better agreement with coronal observations, including recent observations from the CDS and EIT instruments on SOHO, as well as earlier observations from the SXT instrument on Yohkoh. Title: A Nanoflare Explanation for the Heating of Coronal Loops Observed by Yohkoh Authors: Cargill, P. J.; Klimchuk, J. A. Bibcode: 1997ApJ...478..799C Altcode: The nanoflare model of Cargill (1994a) is used to model active region loops observed by the Yohkoh Soft X-ray Telescope (SXT). Using observed information concerning the dimensions and energy-loss rate of each loop, a range of loop models with different temperatures, emission measures, and filling factors is generated. For hot loops (T > 4 × 106 K), it is shown that filling factors less than 0.1 can fit the data, although the uncertainties can be quite large. For cool loops (T ~ 2 × 106 K), the model cannot reproduce the observed temperature and emission measure for any value of the filling factor. Earlier work of Porter & Klimchuk suggested that some of these loops cannot be explained by a steady state heating model either. It is proposed that there may exist two distinct classes of loops and that coronal material is injected into the cooler loops by a mechanism that is not directly related to heating (e.g., not chromospheric evaporation). Title: The Occurrence Rate of Soft X-Ray Flares as a Function of Solar Activity Authors: Feldman, U.; Doschek, G. A.; Klimchuk, J. A. Bibcode: 1997ApJ...474..511F Altcode: In this paper we investigate the occurrence rate of soft X-ray solar flares observed by the Geostationary Operational Environmental Satellites (GOES). The analysis includes all flares classified as equal to or brighter than A1 and covers the time period from 1993 November to 1995 July. We find a power-law relationship between the number of flares per hour and peak X-ray brightness in the 1-8 Å range. The average power-law index for dN/dF, where N is the number of events per hour and F, the GOES flux, is about -1.88 +/- 0.21. A similar result was found from previous work based on uncollimated GOES-type observations concerning flares brighter than about C2. This index is independent of the background flux level (which is related to the solar activity level) to within our statistical uncertainties. We obtain the FWHM distribution of flare lifetimes from our sample and find that the distribution is independent of X-ray brightness class. We extrapolate the soft X-ray flare occurrence rate obtained for the Sun to other very active solar-like stars. 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: Magnetic Reconnection Following Coronal Mass Ejections Authors: Klimchuk, J. A. Bibcode: 1996AAS...188.3306K Altcode: 1996BAAS...28..868K It is well known that large-scale soft X-ray arcades form and grow in the aftermath of coronal mass ejections. This phenomenon is usually interpreted as evidence for magnetic reconnection occurring at progressively greater heights along the vertical current sheet that is created when the coronal field is stretched open by the eruption. Closed magnetic loops are formed and heated by the reconnection, and they subsequently fill with hot, dense plasma evaporated from the chromosphere. The loops then cool and fade from view, but as they do, new hot loops are created above. In this way the arcade slowly grows, shell by shell. This picture is very appealing and seems to agree qualitatively with many aspects of the observations, but is it correct? We have studied the disk event of 1993 Jan 26 and found a startling inconsistency: the rate at which the arcade plasma is observed to be heated is two orders of magnitude smaller than the rate at which energy is expected to be extracted from the magnetic field. Is the standard interpretation fundamentally wrong? We suggest that it is essentially correct, but that only a small fraction ( ~ 1%) of the open magnetic field actually reconnects. We discuss a possible reason for this involving 3D effects, as well as the interesting consequences that it has for the detailed magnetic structure of fully formed coronal arcades. Title: Broadband Imaging Spectroscopy with the Solar Radio Telescope Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Hudson, H. S.; Klimchuk, J. A.; Petrosian, V.; White, S. M. Bibcode: 1996ASPC...93..430B Altcode: 1996ress.conf..430B No abstract at ADS Title: The Heating of Soft X-ray Coronal Loops Authors: Klimchuk, J. A.; Porter, L. J. Bibcode: 1996mpsa.conf...39K Altcode: 1996IAUCo.153...39K No abstract at ADS Title: Post-Eruption Arcades and 3-D Magnetic Reconnection (Invited) Authors: Klimchuk, James A. Bibcode: 1996ASPC..111..319K Altcode: 1997ASPC..111..319K Soft X-ray emitting arcades are often observed to form and grow in the aftermath of coronal eruptions. They are generally interpreted as evidence for magnetic reconnection occurring at progressively greater heights along a vertical current sheet that is created by the eruption. This "standard model" explains many of the morphological aspects of the observations very well. The author has examined the energetics of an event observed by Yohkoh and finds that the rate of magnetic energy conversion expected from the model exceeds the observed rate of arcade plasma heating by perhaps as much as a factor of 100. He takes this as evidence that reconnection is being inhibited, and suggests that the cause may be three-dimensional effects involving the interlinking of field lines. Title: Soft X-Ray Loops and Coronal Heating Authors: Porter, Lisa J.; Klimchuk, James A. Bibcode: 1995ApJ...454..499P Altcode: We have measured the temperatures, pressures, and lengths of 47 nonflaring coronal loops observed by the Soft X-Ray Telescope on the Yohkoh satellite. The median temperature is 5.7 x 106 K, and the median pressure is 1.6 x 1016 cm-3 K. We have carefully examined the possible random and systematic errors in the measurements and have found, through simulated observations, that the errors produced by photon statistical noise can sometimes greatly exceed the values given by commonly used error expressions, derived here and elsewhere. Furthermore, the measurements are not normally distributed and therefore are not amenable to standard statistical analysis. We have used nonparametric methods to look for statistical relationships and find that temperature and length are uncorrelated and that pressure varies inversely with length to approximately the first power.

The observed lifetimes of the loops are much longer than their computed cooling times, suggesting that the loops are in a state of quasi-static equilibrium. This has allowed us to use simple scaling law theory to infer that the volumetric heating rate in the loops (averaged along the loop axis) varies inversely with length to approximately the second power. This is an important constraint for distinguishing among competing theories of coronal heating, and we discuss the results in the context of three specific models. Title: Solar Rotation Stereoscopy in Microwaves Authors: Aschwanden, Markus J.; Lim, Jeremy; Gary, Dale E.; Klimchuk, James A. Bibcode: 1995ApJ...454..512A Altcode: We present here the first stereoscopic altitude measurements of active region sources observed at microwave frequencies (10-14 GHz The active region NOAA 7128 was observed with the Owens Valley Radio Observatory (OVRO) on 1992 April 13, 14, 15, and 16 as it passed through the central meridian. From white-light data of the underlying sunspot we determined the rotation rate of the active region, which was found to have a relative motion of dL/dt = +0°.240 day-1 with respect to the standard photospheric differential rotation rate. Based on this rotation rate we determine for the microwave sources stereoscopic altitudes of 3.3-11.0 Mm above the photosphere. The altitude spectrum h(v) of the right circular polarization (RCP) main source shows a discontinuity at 12 GHz and can be satisfactorily fitted with a dipole model with a transition from the second to the third harmonic level at 12 GHz. The dominance of the third harmonic for frequencies above 12 GHz occurs because the second harmonic level drops below the transition region, at a height of 2.6±0.6 Mm according to the microwave data. The altitude spectrum h(v) serves also to invert the temperature profile T(h) from the optically thick parts of the radio brightness temperature spectrum TB(ν[h]). The microwave emission in both circular polarizations can be modeled with gyroresonance emission, with x-mode for RCP and o-mode in LCP, with the same harmonics at each frequency, but different emission angles in both modes. The contributions from free-free emission are negligible in both polarizations, based on the peak emission measure of EM ≍ 6 × 1028 cm-5 observed in soft X-rays by Yohkoh/SXT.

This study demonstrates that the height dependence of the coronal magnetic field B(h) and the plasma temperature T(h) in an active region can be inverted from the stereoscopic altitude spectra h(v) and the observed brightness temperature spectra TB(ν). Title: Scaling of heating rates in solar coronal loops Authors: Klimchuk, James A.; Porter, Lisa J. Bibcode: 1995Natur.377..131K Altcode: THE gas of the solar corona is at a temperature of several million degrees, orders of magnitude hotter than the underlying photosphere. The nature of the physical process that heats the solar corona (and the coronae of solar-type stars more generally) has been a long-standing puzzle. A number of plausible heating mechanisms have been proposed, but observations have so far been unable to discriminate between them1. Here we show that coronal heating exhibits scaling properties that should provide a powerful diagnostic of the underlying mechanism. The coronal magnetic field organizes the coronal plasma into loop-like features, which form the basic structural elements of the corona2. We demonstrate that the pressures and lengths of the coronal loops are statistically related, suggesting that the heating rate scales inversely with approximately the square of the loop length. Existing coronal heating theories make different predictions about what this scaling should be, and a model3-4 of energy dissipation by stressed coronal magnetic fields appears at present to be the most consistent with our observational result. Title: A Comparison of Active Region Temperatures and Emission Measures Observed in Soft X-Rays and Microwaves and Implications for Coronal Heating Authors: Klimchuk, J. A.; Gary, D. E. Bibcode: 1995ApJ...448..925K Altcode: We have determined active region temperatures and emission measures using both broad-band soft X-ray images from the Yohkoh satellite and spatially and spectrally resolved microwave data from the Owens Valley Radio Observatory (OVRO). This work differs from previous work in that the microwave temperatures and emission measures are directly measured from the microwave spectrum, and are not model-dependent. The soft X-ray temperatures and emission measures are ≍2.5 times greater than the corresponding microwave values, on average. Detailed error analysis indicates that the temperature differences are real, but that the emission measure differences may not be.

We have simulated Yohkoh and OVRO observations of idealized plane-parallel and nested-loop coronal models. The plane-parallel model reproduces the observed temperature differences if the coronal temperature decreases exponentially with height from a maximum value of 4 × 106 K at the base to an asymptotic value of ≍106 K. The nested-loop model, which assumes quasi-static loop equilibrium, also reproduces the observed temperature differences and indicates that the volumetric coronal heating rate varies inversely with loop length to a power greater than 2. Both models predict microwave emission measures that are larger than observed. We suggest that a more complex model is required to explain the observed emission measures and that more than one coronal heating mechanism may be operative in solar active regions.

We present derivations of the temperature and emission measure uncertainties that result from random and systematic errors in the Yohkoh observations. The expression for the random error emission measure uncertainty is different from that used previously and is especially important for observations of nonflaring plasmas. 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: A Solar Radio Telescope for the Future: Science Summary from the SRT Workshop Authors: Gary, D. E.; Bastian, T. S.; Hudson, H. S.; Hurford, G. J.; Klimchuk, J. A.; Petrosian, V.; White, S. M. Bibcode: 1995SPD....26..801G Altcode: 1995BAAS...27..971G No abstract at ADS Title: The Cross Sectional Properties of Coronal Loops Authors: Klimchuk, J. A. Bibcode: 1995SPD....26..705K Altcode: 1995BAAS...27Q.966K No abstract at ADS Title: A Solar Radio Telescope for the Future: Strawman Concept from the SRT Workshop Authors: Hurford, G. J.; Bastian, T. S.; Gary, D. E.; Hudson, H. S.; Klimchuk, J. A.; Petrosian, V.; White, S. M. Bibcode: 1995SPD....26..802H Altcode: 1995BAAS...27..971H No abstract at ADS Title: The Possible Role of MHD Waves in Heating the Solar Corona Authors: Porter, Lisa J.; Klimchuk, James A.; Sturrock, Peter A. Bibcode: 1994ApJ...435..482P Altcode: The possible role of waves in the heating of the solar corona has been investigated. A general dispersion relation has been derived for waves propagating in a homogeneous plasma subject to dissipation by viscosity and thermal conduction. The dissipation mechanisms have been incorporated self-consistently into the equations, and no assumptions about the strength of the damping have been made. Solutions of the sixth-order dispersion relation provide information on how the damping of both slow and fast mode waves depends upon the plasma density, temperature, field strength, and angle of propagation relative to the background magnetic field. We provide a detailed comparison to the standard approach, which is to solve for the wave quantities in the absence of dissipation and then to use these quantities in expressions for the heating due to viscosity and thermal conduction. Title: The Possible Role of High-Frequency Waves in Heating Solar Coronal Loops Authors: Porter, Lisa J.; Klimchuk, James A.; Sturrock, Peter A. Bibcode: 1994ApJ...435..502P Altcode: We investigate the role of high-frequency waves in the heating of solar active region coronal loops. We assume a uniform background magnetic field, and we introduce a density stratification in a direction perpendicular to this field. We focus on ion compressive viscosity as the damping mechanism of the waves. We incorporate viscosity self-consistently into the equations, and we derive a dispersion relation by adopting a slab model, where the density inside the slab is greater than that outside. Such a configuration supports two types of modes: surface waves and trapped body waves. In order to determine under what conditions these waves may contribute to the heating of active regions, we solve our dispersion relation for a range of densities, temperatures, magnetic field strengths, density ratios, wavevector magnitudes, wavevector ratios, and slab widths. We find that surface waves exhibit very small damping, but body waves can potentially damp at rates needed to balance radiative losses. However, the required frequencies of these body waves are very high. For example, the wave frequency must be at least 5.0/s for a slab density of 109,5/cc, a slab temperature of 106,5 K, a field strength of 100 G, and a density ratio of 5. For a slab density of 1010/cc, this frequency increases to 8.8/s. Although these frequencies are very high, there in no observational evidence to rule out their existence, and they may be generated both below the corona and at magnetic reconnection sites in the corona. However, we do find that, for slab densities of 1010/cc or less, the dissipation of high-frequency waves will be insufficient to balance the radiative losses if the magnetic field strength exceeds roughly 200 G. Because the magnetic field is known to exceed 200 G in many active region loops, particularly low-lying loops and loops emanating from sunspots, it is unlikely that high-frequency waves can provide sufficient heating in these regions. 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: 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 Eruptions Observed by YOHKOH Authors: Klimchuk, J. A.; Acton, L. W.; Harvey, K. L.; Hudson, H. S.; Kluge, K. L.; Sime, D. G.; Strong, K. T.; Watanabe, Ta. Bibcode: 1994xspy.conf..181K Altcode: No abstract at ADS Title: Interplanetary Consequences of Transient Coronal Events Authors: Watanabe, Ta.; Kojima, M.; Kozuka, Y.; Tsuneta, S.; Lemen, J. R.; Hudson, H.; Joselyn, J. A.; Klimchuk, J. A. Bibcode: 1994xspy.conf..207W Altcode: No abstract at ADS Title: Photospheric Magnetic Field Measurement Errors and the Inferred Properties of Coronal Magnetic Fields Authors: Klimchuk, James A.; Canfield, Richard C. Bibcode: 1994ASPC...68..233K Altcode: 1994sare.conf..233K No abstract at ADS Title: Eruptive-Prominence Related Coronal Disturbances Observed with YOHKOH SXT Authors: Watanabe, T.; Kozuka, Y.; Ohyama, M.; Kojima, M.; Yamaguchi, K.; Watari, S.; Tsuneta, S.; Joselyn, J. A.; Harvey, K. L.; Acton, L. W.; Klimchuk, J. A. Bibcode: 1994step.conf...85W Altcode: No abstract at ADS Title: Coronal/Interplanetary Disturbances Associated with a Solar Filament Disappearance on September 28, 1991 Authors: Watanabe, T.; Kozuka, Y.; Ohyama, M.; Kojima, M.; Yamaguchi, K.; Watari, S.; Tsuneta, S.; Joselyn, J. A.; Harvey, K. L.; Acton, L. W.; Klimchuk, J. A. Bibcode: 1994step.conf...89W Altcode: No abstract at ADS Title: Collisional Damping of Magnetoacoustic Waves in the Solar Corona Authors: Porter, Lisa A.; Sturrock, Peter A.; Klimchuk, James A. Bibcode: 1993BAAS...25.1203P Altcode: No abstract at ADS 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: Comparison of Coronal Temperatures and Emission Measures Determined from X-Ray and Microwave Observations Authors: Klimchuk, J. A.; Gary, D. E. Bibcode: 1993BAAS...25.1179K Altcode: No abstract at ADS Title: Static and dynamic loop models and their observational signatures. Authors: Klimchuk, James A. Bibcode: 1992ESASP.348..167K Altcode: 1992cscl.work..167K The magnetically-closed regions of the outer solar atmosphere can be studied in terms of one-dimensional hydrodynamic loop models. These regions include the bright plasma loops that are readily visible in EUV and X-ray images as well as the fainter, more diffuse-appearing plasma that surrounds them. The author reviews the basic theoretical properties of static, steady-state, and time-dependent loop models, and relates these properties to observations of emission measure distributions and Doppler shifts. Both existing observations and future observations from the Solar and Heliospheric Observatory (SOHO) are considered. In particular, the dependence of the models on the form of the energy input is emphasized. Title: Thickness Variations along Coronal Loops Observed by the Soft X-Ray Telescope on YOHKOH Authors: Klimchuk, James A.; Lemen, James R.; Feldman, Uri; Tsuneta, Saku; Uchida, Yutaka Bibcode: 1992PASJ...44L.181K Altcode: It has been suggested that observed coronal loops have constant thicknesses. However, if plasma loops coincide with magnetic loops, then we might expect many loops to be significantly broader at their tops than at their footpoints (since, on average, magnetic fields must diverge with height in the solar corona). It is important to understand how the thicknesses of loops vary along their lengths, since such a variation is related to the distribution of electric currents in the corona and is therefore relevant to solar flares and coronal heating. We here present preliminary results of our investigation of thickness variations along coronal loops observed with the Soft X-ray Telescope (SXT) on board the Yohkoh satellite. Title: Coronal/Interplanetary Disturbances Associated with Disappearing Solar Filaments Authors: Watanabe, Takashi; Kozuka, Yukio; Ohyama, Masamitsu; Kojima, Masayoshi; Yamaguchi, Kisuke; Watari, Shin-Ichi; Tsuneta, Saku; Joselyn, Jo A.; Harvey, Karen L.; Acton, Loren W.; Klimchuk, James. A. Bibcode: 1992PASJ...44L.199W Altcode: We discuss two examples of coronal/interplanetary disturbances associated with the disappearance of a 35(deg) long quiescent filament occurring near the solar disk center on 1991 September 28 (McAllister et al. 1992, Publ. Astron. Soc. Japan, 44, L205) and with a 25(deg) long eruptive prominence at the eastern solar limb taking place on 1991 November 7. Bright soft X-ray arcades were observed for both cases with the Yohkoh SXT, about 2--3 hr after the onset of each Hα event. For the erupting prominence on November 7, the arcade did not appear before the prominence reached a height of about 0.3 solar radii above the limb. This suggests that magnetic reconnection occurred below the relevant Hα structures. A transient coronal hole was formed in the immediate vicinity of the disappearing filament on September 28. Formation of the new coronal hole is suggested to be a cause of the filament disappearance. An interplanetary disturbance was detected by radio scintillation (IPS) observations immediately after the filament disappeared. 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: Thickness Variations Along Coronal Loops Observed by Yohkoh Authors: Klimchuk, J. A.; Kluge, K.; Lemen, J. R.; Feldman, U.; Uchida, Y. Bibcode: 1992AAS...180.2304K Altcode: 1992BAAS...24Q.760K No abstract at ADS Title: Cylindrically Symmetric Force-free Magnetic Fields Authors: Porter, Lisa J.; Klimchuk, James A.; Sturrock, Peter A. Bibcode: 1992ApJ...385..738P Altcode: The magnetofrictional method was used to study the energy buildup in stressed coronal fields possessing cylindrical symmetry. Four different nonlinear, force-free magnetic-field configurations were examined. It was determined that, in all cases, a reasonable amount of twist in the field lines can produce enough free magnetic energy to power a typical flare. Furthermore, it was found that the rate of energy buildup is enhanced if the greatest twist and/or the magnetic flux is concentrated closer to the neutral line. It is thought that the open-field configuration (a configuration for which the field lines extend to infinity and the current is confined to a current sheet separating outgoing and incoming field lines) is the limiting state as one imposes infinite shear. The results of this work do not contradict this theory once numerical errors are taken into account. Title: The Practical Application of the Magnetic Virial Theorem Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E. Bibcode: 1992ApJ...385..327K Altcode: The magnetic energy of solar active regions is computed via the application of the magnetic virial theorem together with vector magnetograph data. In order to investigate how errors in the vector magnetograph measurements produce errors in the virial theorem energy, the effects of realistic errors on known magnetic fields are simulated numerically. These include errors due to random polarization noise, crosstalk between different polarization signals, systematic polarization bias, and seeing-induced crosstalk. Analytical expressions for the energy errors which apply under certain idealized conditions are derived. The results are useful for evaluating the ability of vector magnetographs to provide suitable data for the accurate determination of magnetic energies using the virial theorem. Title: Three-dimensional Force-free Magnetic Fields and Flare Energy Buildup Authors: Klimchuk, J. A.; Sturrock, P. A. Bibcode: 1992ApJ...385..344K Altcode: The 'magneto-frictional' method is used to compute fully 3D models of force-free magnetic fields. Beginning with a potential field produced by a point dipole buried below the solar surface, the magnetic footpoints at the photosphere are displaced to investigate the buildup of magnetic energy. Reasonable footpoint shearing displacements are found to be able to increase the total magnetic energy by at least one-third. The energy buildup is greater when the shearing displacements are concentrated closer to the magnetic neutral line. Roughly half of the energy buildup is free magnetic energy. The absolute quantity of free magnetic energy (10 exp 30-33 ergs, depending on the scaling of the models) is sufficient to explain solar flares. No evidence for 'loss of equilibrium' was found. 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: The Practical Application of the Magnetic Virial Therom: Analytical Results Authors: Rhoads, J. E.; Klimchuk, J. A.; Canfield, R. C. Bibcode: 1991BAAS...23.1055R Altcode: No abstract at ADS Title: The Practical Application of the Magnetic Virial Theorm: Simulated Magnetograph Observations Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E. Bibcode: 1991BAAS...23.1031K Altcode: No abstract at ADS Title: Simulated SXT Observations of Coronal Loops Authors: Dixon, W. W.; Klimchuk, J. A.; Sturrock, P. A.; Lemen, J. R. Bibcode: 1991LNP...387..297D Altcode: 1991fpsa.conf..297D We have simulated the appearance of two static coronal loops as they might be observed by the Soft X-ray Telescope onboard the Solar-A spacecraft. One loop corresponds to a non-flaring active region loop, and the other corresponds to a post-flare loop. We find that the loops have fundamentally different appearances: the quiescent loop is brightest at its apex, while the hotter post-flare loop is brightest at, or near, its base (depending on the particular X-ray filter assumed for the observations). Title: The Practical Application of the Magnetic Virial Theorem Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E. Bibcode: 1991LNP...387..219K Altcode: 1991fpsa.conf..219K We have performed simulated vector magnetograph observations to study the effects of random and systematic magnetic field measurement errors on the magnetic energies that may be estimated using the virial theorem. 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: Shear-induced Inflation of Coronal Magnetic Fields Authors: Klimchuk, James A. Bibcode: 1990ApJ...354..745K Altcode: Using numerical models of force-free magnetic fields, the shearing of footprints in arcade geometries leading to an inflation of the coronal magnetic field was examined. For each of the shear profiles considered, all of the field lines become elevated compared with the potential field. This includes cases where the shear is concentrated well away from the arcade axis, such that Bz, the component of field parallel to the axis, increases outward to produce an inward Bz squared/8 pi magnetic pressure gradient force. These results contrast with an earlier claim, shown to be incorrect, that field lines can sometimes become depressed as a result of shear. It is conjectured that an inflation of the entire field will always result from the shearing of simple arcade configurations. These results have implications for prominence formation, the interplanetary magnetic flux, and possibly also coronal holes. Title: Flare Energy Buildup and the Stressing of 3-D Coronal Magnetic Fields Authors: Klimchuk, J. A.; Sturrock, P. A. Bibcode: 1990BAAS...22..900K Altcode: No abstract at ADS Title: Cylindrically-Symmetric Force-Free Magnetic Fields Authors: Porter, L. J.; Klimchuk, J. A.; Sturrock, P. A. Bibcode: 1990BAAS...22..853P Altcode: No abstract at ADS Title: Force-free Magnetic Fields: Is There a ``Loss of Equilibrium''? Authors: Klimchuk, J. A.; Sturrock, P. A. Bibcode: 1989ApJ...345.1034K Altcode: This paper examines concept in solar physics that is known as loss of equilibrium in which a sequence of force-free magnetic fields, said to represent a possible quasi-static evolution of solar magnetic fields, reaches a critical configuration beyond which no acceptable solution of the prescribed form exists. This concept is used to explain eruptive phenomena ranging from solar flares to coronal mass ejections. Certain sequences of force-free configurations are discussed that exhibit a loss of equilibrium, and it is argued that the concept is devoid of physical significance since each sequence is defined a way that does not represent an acceptable thought experiment. For example, the sequence may be defined in terms of a global constraint on the boundary conditions, or the evolution of the sequence may require the creation of mgnetic flux that is not connected to the photosphere and is not present in the original configuration. The global constraints typically occur in using the so-called generating function method. An acceptance thought experiment is proposed to specify the field configuration in terms of photospheric boundary conditions comprising the normal component of the field and the field-line connectivity. Consider a magnetic-field sequence that, when described in terms of a generating function, exhibits a loss of equilibrium and show that, when one instead defines the sequence in terms of the corresponding boundary conditions, the sequence is well behaved. 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: The Formation of Solar Prominences Authors: Antiochos, S. K.; Klimchuk, J. A. Bibcode: 1989BAAS...21.1185A Altcode: No abstract at ADS Title: Shear-induced inflation of coronal magnetic fields Authors: Klimchuk, James A. Bibcode: 1989STIN...9014178K Altcode: Using numerical models of force-free magnetic fields, the shearing of footprints in arcade geometries leading to an inflation of the coronal magnetic field was examined. For each of the shear profiles considered, all of the field lines become elevated compared with the potential field. This includes cases where the shear is concentrated well away from the arcade axis, such that Bz, the component of field parallel to the axis, increases outward to produce an inward Bzsquared/8 pi magnetic pressure gradient force. These results contrast with an earlier claim, shown to be incorrect, that field lines can sometimes become depressed as a result of shear. It is conjectured that an inflation of the entire field will always result from the shearing of simple arcade configurations. These results have implications for prominence formation, the interplanetary magnetic flux, and possibly also coronal holes. Title: Shear-Induced Inflation of Coronal Magnetic Fields Authors: Klimchuk, J. A. Bibcode: 1989BAAS...21..864K Altcode: No abstract at ADS Title: Magnetic properties of Civ Doppler shift patterns Authors: Klimchuk, James A. Bibcode: 1989SoPh..119...19K Altcode: The relationship between Doppler shift patterns observed in the transition region and magnetic field patterns observed in the photosphere is studied using coaligned pairs of CIV Dopplergrams and FeI magnetograms. Categories of magnetic features are defined - including neutral lines, unipolar regions, strong field regions, weak field regions, and magnetic boundaries - and from these, magnetic associations are determined for 159 V0 lines separating areas of relative blueshift and redshift observed in and around active regions. The cases are subdivided on the basis of whether blueshifts or redshifts are observed on the side of the V0 line nearest the solar limb. Title: Force-Free Magnetic Fields: Is there a "Loss of Equilibrium?" Authors: Klimchuk, J. A.; Sturrock, P. A. Bibcode: 1989BAAS...21R.855K Altcode: No abstract at ADS Title: Largescale Magnetic Field Phenomena Authors: Harrison, R. A.; Bentley, R. D.; Brosius, J.; Dwivedi, B. N.; Jardine, M.; Klimchuk, J. A.; Kundu, M. R.; Pearce, G.; Saba, J.; Sakurai, T.; Schmahl, E. J.; Schmelz, J.; Sime, D. G.; Steele, C. D. C.; Sun, M. T.; Tappin, S. J.; Waljeski, K.; Wang, A. H.; Wu, S. T. Bibcode: 1989tnti.conf....1H Altcode: No abstract at ADS Title: Coronal Magnetic Fields Produced by Photospheric Shear Authors: Klimchuk, J. A.; Sturrock, P. A.; Yang, W. -H. Bibcode: 1988ApJ...335..456K Altcode: The magnetofrictional method for computing force-free fields is used to examine the evolution of the magnetic field of a line dipole, when there is relative shearing motion between the two polarities. It is found that the energy of the sheared field can be arbitrarily large compared with the potential field. It is also found that it is possible to fit the magnetic energy, as a function of shear amplitude, by a simple functional form. The fit parameters depend only on the distribution of normal field in the photosphere and the form of the shearing displacement. They show that the energy is relatively more enhanced if the shear occurs: (1) where the normal field is strongest; and/or (2) in the inner region of the dipole, near the axis; and/or (3) over a large fraction of the dipole area. Title: Force-free magnetic fields: Is there a loss of equilibrium Authors: Klimchuk, James A.; Sturrock, Peter A. Bibcode: 1988STIN...8921717K Altcode: This paper examines concept in solar physics that is known as loss of equilibrium in which a sequence of force-free magnetic fields, said to represent a possible quasi-static evolution of solar magnetic fields, reaches a critical configuration beyond which no acceptable solution of the prescribed form exists. This concept is used to explain eruptive phenomena ranging from solar flares to coronal mass ejections. Certain sequences of force-free configurations are discussed that exhibit a loss of equilibrium, and it is argued that the concept is devoid of physical significance since each sequence is defined a way that does not represent an acceptable thought experiment. For example, the sequence may be defined in terms of a global constraint on the boundary conditions, or the evolution of the sequence may require the creation of magnetic flux that is not connected to the photosphere and is not present in the original configuration. The global constraints typically occur in using the so-called generating function method. An acceptable thought experiment is proposed to specify the field configuration in terms of photospheric boundary conditions comprising the normal component of the field and the field-line connectivity. Consider a magnetic-field sequence that, when described in terms of a generating function, exhibits a loss of equilibrium and show that, when one instead defines the sequence in terms of the corresponding boundary conditions, the sequence is well behaved. Title: Heating-related Flows in Cool Solar Loops Authors: Klimchuk, J. A.; Mariska, J. T. Bibcode: 1988ApJ...328..334K Altcode: The authors have investigated the effects of spatial and temporal variations in the heating of cool loop models in an attempt to explain the net redshifts that are observed on the Sun. In none of the situations considered are the induced flows able to satisfactorily reproduce the observations. In the case of asymmetric heating, the end-to-end flows can be as fast as 20 km s-1, but the downflowing leg is neither appreciably faster nor appreciably brighter than the upflowing leg; no net redshift is produced. In the case of symmetric heating, the downflows can also be large, but they are restricted to temperatures that are well below 105K. Neither situation would give rise to the ⪆7 km s-1 disk-averaged redshifts seen in emission lines of species like C IV. Title: Coronal magnetic fields produced by photospheric shear Authors: Klimchuk, James A.; Sturrock, Peter A.; Yang, Wei-Hong Bibcode: 1988cmfp.book.....K Altcode: The magneto-frictional method for computing force-free fields examines the evolution of the magnetic field of a line dipole, when there is relative shearing motion between the two polarities. The energy of the sheared field can be arbitrarily large compared with the potential field. It is possible to fit the magnetic energy, as a function of shear amplitude, by a simple functional form. The fit parameters depend only on the distribution of normal field in the photosphere and the form of the shearing displacement. The energy is relatively more enhanced if the shear occurs: (1) where the normal field is strongest; (2) in the inner region of the dipole, near the axis; or (3) over a large fraction of the dipole area. Title: Magnetic properties of C 4 Doppler shift patterns Authors: Klimchuk, James A. Bibcode: 1988STIN...8915062K Altcode: The relationship between Doppler shift patterns observed in the transition region and magnetic field patterns observed in the photosphere is studied using coaligned pairs C IV Dopplergrams and Fe I magnetograms. Categories of magnetic features are defined--including neutral lines, unipolar regions, strong field regions, weak field regions, and magnetic boundaries--and from these, magnetic associations are determined for 159 V0 lines separating areas of relative blueshift and redshift observed in and nearby to active regions. The cases are subdivided on the basis of whether blueshifts or redshifts are observed on the side of the V0 line nearest the limb. Two of the main results are that V0 lines associated with neutral lines tend to have limbward blueshifts, while V0 lines associated with unipolar regions tend to have limbward redshifts. These and other results provide supportive evidence for the active region model proposed recently by Klimchuk, in which relative redshifts occur where strong vertical fields penetrate the surface, and relative blueshifts occur where these same fields have spread out to become horizontal. It is likely that the relative blueshifts correspond to absolute Doppler shifts of very small amplitude, possibly even redshifts. Title: Coronal Magnetic Fields Produced by Photospheric Shear Authors: Klimchuk, J. A.; Sturrock, P. A.; Yang, W. -H. Bibcode: 1988BAAS...20..716K Altcode: No abstract at ADS Title: On the Large-Scale Dynamics and Magnetic Structure of Solar Active Regions Authors: Klimchuk, James A. Bibcode: 1987ApJ...323..368K Altcode: The author has studied sets of carefully coaligned C IV Dopplergrams, photospheric magnetograms, and Hα filtergrams to infer the flow properties of active regions and the relationship of these flows to the active region magnetic fields. The combined data show that active regions can be naturally divided into three basic parts: strong field regions, weak field corridors between strong fields of opposite polarity, and surrounding weak field areas. An idealized topological model in which the vertical fields of strong field regions diverge very rapidly with height to become essentially horizontal in the adjacent low-lying areas is proposed. The picture is similar to canopy structures. 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: The Magnetic and Velocity Structure Adjacent to Solar Active Regions Authors: Athay, R. Grant; Klimchuk, J. A. Bibcode: 1987ApJ...318..437A Altcode: Results from a number of earlier papers relating velocity patterns observed in the C IV line at 154.8 nm to photospheric magnetic-field patterns are combined to develop a qualitative model of the magnetic-field geometry outside of the strong field areas of active regions. The motion is assumed to originate at the crests of magnetic arcades and flow downward along field lines, which are assumed to be elliptical in shape with the major axis in the photosphere. It is found that the ratio of the major axis to the minor axis of the ellipse must be less than two for fields under 100 G. Also, it is concluded that the magnetic neutral surfaces defined by the loci of horizontal field lines are often tilted at a large angle to the vertical at the altitude of the C IV emission. Title: Heating Related Flows in Cool Loops Authors: Klimchuk, J. A.; Mariska, J. T. Bibcode: 1987BAAS...19..932K Altcode: No abstract at ADS Title: C IV Doppler shifts observed in active region filaments. Authors: Klimchuk, J. A. Bibcode: 1986NASCP2442..183K Altcode: 1986copp.nasa..183K The Doppler shift properties of 21 active region filaments have been studied using C IV Dopplergram data. Most are associated with corridors of weak magnetic field that separate opposite polarity strong fields seen in photospheric magnetograms. A majority of the filaments are relatively blue shifted, although several lie very close to the dividing lines between relative blue and red shift. Only one filament in our sample is clearly red shifted. A new calibration procedure for Dopplergrams indicates that sizable zero point offsets are often required. The center-to-limb behavior of the resulting absolute Doppler shifts suggests that filament flows are usually quite small (<3 km/s). It is possible that they vanish. Title: Numerical simulations of a siphon mechanism for quiescent prominence formation. Authors: Poland, A. I.; Mariska, J. T.; Klimchuk, J. A. Bibcode: 1986NASCP2442...57P Altcode: 1986copp.nasa...57P Quiescent prominences represent a significant challenge to our understanding of the flow of mass and energy in the outer layers of the solar atmosphere. A small number of quiescent prominences contain as much mass as the entire corona (Athay, 1976). The problem then is how to get that much material into the relatively small volume of a prominence and maintain it at a temperature of 10,000 K in close proximity to material at one million K. The thermal insulation to conduction provided by the magnetic field explains the disparate temperatures. The mass source problem is less well understood. One method for supplying mass to the prominence is to siphon it from the chromosphere. The siphon mechanism begins with a magnetic loop that evolves into a configuration with a gravitational well, such as that described by Kippenhahn and Schluter (1957). This could be formed, for example, by a twist in the magnetic field. A gravitational well could also be formed by a condensation induced sag in the field. This could further enhance the condensation process. Once this well has formed, or as it is forming, the material in the well area of the loop must cool and condense to the point where radiative losses exceed any heat input. Additional material must also flow into the well from the underlying chromosphere to supply the mass required to form the prominence. One example from a series of numerical simulations that were performed to study the formation of quiescent prominences is presented. 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: Magnetic Shear. IV. Hale Regions 16740, 16815, and 16850 Authors: Athay, R. G.; Klimchuk, J. A.; Jones, H. P.; Zirin, H. Bibcode: 1986ApJ...303..884A Altcode: Dopplergrams made in C IV 1548 A are studied for evidence of velocity shear near H-alpha dark filaments and for large-scale flow convergent on active regions. The three regions studied support earlier conclusions that shear is a common property of active regions and that active regions may be the foci of converging plasma flow. Flow patterns near filaments show divergence or convergence as well as shear. Also the sense of the shear can be either cyclonic or anticyclonic. No preference is noted for convergence or divergence or for a particular sense of shear, and there appears to be no correlation between the sense of the shear and the sign of the velocity gradient normal to the filament. The close association of H-alpha dark filaments with shear lines leads to the suggestion that the filaments may arise from a cooling instability induced by the Bernoulli effect. 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 Large-Scale Dynamics and Structure of Solar Active Regions Observed in C IV Authors: Klimchuk, J. A. Bibcode: 1986BAAS...18R.702K Altcode: No abstract at ADS Title: Large-scale structure and dynamics of solar active regions observed in the far ultraviolet Authors: Klimchuk, James Andrew Bibcode: 1985PhDT.......145K Altcode: No abstract at ADS Title: Large-Scale Structure and Dynamics of Solar Active Regions Observed in the Far Ultraviolet. Authors: Klimchuk, J. A. Bibcode: 1985PhDT.........6K Altcode: In this thesis we examine high resolution Dopplergrams in CIV (1548) for the purpose of understanding the large -scale dynamics of solar active regions and the closely related problem of magnetic structure. The Dopplergrams are compared with photospheric magnetograms and H(,(alpha)) filtergrams in order to determine the relationship of the flows to the overall magnetic field topology. These observations sample three different heights in the atmosphere and can potentially be used to map the field from the photosphere up through the transition region. Vector properties of the flow and field are inferred from center-to-limb variations in the measured Doppler shift and longitudinal field component. We find that spatial correlations between features seen in Dopplergrams, magnetograms, and filtergrams are quite close, and that active regions can be naturally divided into three basic parts: strong field regions, weak field corridors between opposite polarities, and surrounding weak field areas. Strong field regions are relatively red shifted and contain magnetic fields that penetrate the photosphere in a nearly vertical fashion. Corridors and surrounding areas are relatively blue shifted, on the other hand, and contain fields that are mostly horizontal in the upper photosphere and chromosphere. The transition from vertical to horizontal field appears to be quite sharp and implies a magnetic topology that diverges very rapidly with height. Sizable uncertainties in the Dopplergram interpretation have prevented us from describing the transition region flows unambiguously. We can, however, identify two possible scenarios that are consistent with the data. In the most likely of the two, the flows vanish within corridors and surrounding areas, and they produce constant (across the disk) absolute red shifts of about 18 km/s within strong field regions. The physical causes of this last result are unclear, and it remains an important unsolved problem. Title: Observed Associations Between CIV Doppler Shifts and Photospheric Magnetic Fields in Active Regions Authors: Klimchuk, J. A. Bibcode: 1984BAAS...16..532K Altcode: No abstract at ADS Title: Outflow from the sun's polar corona Authors: Orrall, F. Q.; Rottman, G. J.; Klimchuk, J. A. Bibcode: 1983ApJ...266L..65O Altcode: New observations of systematic Doppler shifts of EUV resonance lines formed both in the low corona and transition region are reported. They were made with an improved, high-resolution, stable, rocket-borne spectrometer flown on 1981 November 23. The chord of the solar disk scanned by the spectrometer crossed the north polar cap hole and its low-latitude extension, and also a compact low-latitude hole near sun center. Within both holes, the lines were systematically shifted to shorter wavelengths relative to the rest of the solar disk. These observations strengthen the association of negative Doppler shifts with coronal holes and indicate that this 'blueshift signature', previously observed only within small low-latitude holes, is also characteristic of the low-density polar corona. The mean relative blueshift measured in lambda 625 Mg X (T = 10 to the 6.15 power K) within the polar hole was about 8 km/s. Title: Measurements of outflow from the base of solar coronal holes Authors: Rottman, G. J.; Orrall, F. Q.; Klimchuk, J. A. Bibcode: 1982ApJ...260..326R Altcode: New evidence is presented that EUV emission lines formed at the levels of the base of the corona and the transition region are systematically shifted to shorter wavelengths within coronal holes relative to the rest of the solar disk, and that moreover this shift increases with height in the atmosphere. Measurements were made with a rocket-borne EUV spectrometer having high spectroscopic resolution and stability flown on July 15, 1980. Repeated measurements were made along a chord of the solar disk that crossed a compact coronal hole near sun center identified on gamma 10830 He I spectroheliograms. The maximum measured shift corresponded to a velocity of 12 km/sec in gamma 625 Mg X and 7 km/sec in gamma 629 O V. If these velocities correspond to a true mass flux, they provide important data on the acceleration of coronal plasma in open magnetic field regions. These observed Doppler displacements are a strong and significant signature of coronal holes, now measured on three rocket flights. Title: Measurement of systematic outflow from the solar transition region underlying a coronal hole Authors: Rottman, G. J.; Klimchuk, J. A.; Orrall, F. Q. Bibcode: 1981ApJ...247L.135R Altcode: This letter presents measurements of small Doppler shifts in the line center position of 629A OV obtained with a new high-resolution EUV spectrometer flown aboard a sounding rocket. A major result is the detection of an apparent systematic outflow (relative to the quiet sun) of approximately 3 km/s average and 5 km/s maximum in the solar transition region underlying a well-defined low-latitude coronal hole. This is reminiscent of a similar apparent outflow observed by Cushman and Rense in the coronal line 303A Si XI. The hypothesis that this is evidence for acceleration of the high speed solar wind deep in the transition region and inner corona is explored briefly. Title: EUV Observations of High-Speed Downflows Over Sunspots Authors: Klimchuk, J. A.; Rottman, G. J. Bibcode: 1981BAAS...13..914K Altcode: No abstract at ADS Title: EUV Observations of Solar Mass Loss from the Lower Solar Atmosphere Authors: Rottman, G. J.; Orrall, F. Q.; Klimchuk, J. A. Bibcode: 1981BAAS...13..812R Altcode: No abstract at ADS Title: Mass Flux within Coronal Holes Authors: Orrall, F. Q.; Rottman, G. J.; Klimchuk, J. Bibcode: 1980BAAS...12..919O Altcode: No abstract at ADS Title: Velocity Fields Observed in Coronal Holes and the Underlying Transition Region Authors: Rottman, G. J.; Klimchuk, J. A.; Orrall, F. Q. Bibcode: 1980BAAS...12..919R Altcode: No abstract at ADS