Author name code: aschwanden ADS astronomy entries on 2022-09-14 author:"Aschwanden, Markus J." ------------------------------------------------------------------------ Title: Interface Region Imaging Spectrograph (IRIS) Observations of the Fractal Dimension in the Solar Atmosphere Authors: Aschwanden, Markus J.; Vilangot Nhalil, Nived Bibcode: 2022arXiv220712894A Altcode: While previous work explored the fractality and self-organized criticality (SOC) of flares and nanoflares in wavelengths emitted in the solar corona (such as in hard X-rays, soft X-rays, and EUV wavelenghts), we focus here on impulsive phenomena in the photosphere and transition region, as observed with the {\sl Interface Region Imaging Spectrograph (IRIS)} in the temperature range of $T_e \approx 10^4-10^6$ K. We find the following fractal dimensions (in increasing order): $D_A=1.21 \pm 0.07$ for photospheric granulation, $D_A=1.29 \pm 0.15$ for plages in the transition region, $D_A=1.54 \pm 0.16$ for sunspots in the transition region, $D_A=1.59 \pm 0.08$ for magnetograms in active regions, $D_A=1.56 \pm 0.08$ for EUV nanoflares, $D_A=1.76 \pm 0.14$ for large solar flares, and up to $D_A=1.89 \pm 0.05$ for the largest X-class flares. We interpret low values of the fractal dimension ($1.0 \lapprox D_A \lapprox 1.5$) in terms of sparse curvi-linear flow patterns, while high values of the fractal dimension ($1.5 \lapprox D_A \lapprox 2.0$) indicate near space-filling transport processes, such as chromospheric evaporation. Phenomena in the solar transition region appear to be consistent with SOC models, based on their size distributions of fractal areas $A$ and (radiative) energies $E$, which show power law slopes of $\alpha_A^{obs}=2.51 \pm 0.21$ (with $\alpha_A^{theo}=2.33$ predicted), and $\alpha_E^{obs}=2.03 \pm 0.18$ (with $\alpha_E^{theo}=1.80$ predicted). Title: Reconciling Power-law Slopes in Solar Flare and Nanoflare Size Distributions Authors: Aschwanden, Markus J. Bibcode: 2022ApJ...934L...3A Altcode: 2022arXiv220312484A We unify the power laws of size distributions of solar flare and nanoflare energies. We present three models that predict the power-law slopes α E of flare energies defined in terms of the 2D and 3D fractal dimensions (D A , D V ): (i) the spatiotemporal standard self-organized criticality model, defined by the power-law slope α E1 =1 + 2/(D V + 2) = (13/9) ≍ 1.44; (ii) the 2D thermal energy model, α E2 = 1 + 2/D A = (7/3) ≍ 2.33; and (iii) the 3D thermal energy model, α E3 = 1 + 2/D V = (9/5) ≍ 1.80. The theoretical predictions of energies are consistent with the observational values of these three groups, i.e., α E1 = 1.47 ± 0.07, α E2 = 2.38 ± 0.09, and α E3 = 1.80 ± 0.18. These results corroborate that the energy of nanoflares does not diverge at small energies, since (α E1 < 2) and (α E3 < 2), except for the 2D model (α E2 > 2). Thus, while this conclusion does not support nanoflare scenarios of coronal heating from a dimensionality point of view, magnetic reconnection processes with quasi-1D or quasi-2D current sheets cannot be ruled out. Title: The Fractality and Size Distributions of Astrophysical Self-Organized Criticality Systems Authors: Aschwanden, Markus J. Bibcode: 2022ApJ...934...33A Altcode: The statistics of nonlinear processes in avalanching systems, based on the self-organized criticality (SOC) concept of Bak et al. (1988), predicts power-law-like size (or occurrence frequency) distribution functions. Following up on previous work, we define a standard SOC model in terms of six assumptions: (i) area fractality, (ii) volume fractality, (iii) the flux-volume proportionality, (iv) classical diffusion, (v) the Euclidean maximum at the event peak time, and (vi) the spatiotemporal fluence or energy of an avalanche event. We gather data of the fractal dimension and power-law slopes from 162 publications and assemble them in 28 groups (for instance, solar flare energies, or stellar flare energies), from which we find that 75% of the groups are consistent with the standard SOC model. Alternative SOC models (Lévy flight, flat-world, nonfractal) are slightly less correlated with the data. Outliers are attributed to small number statistics, background definition problems, inadequate fitting ranges, and deviations from ideal power laws. Title: New insight into the nature and origin of switchbacks thanks to a comprehensive catalogue of events Authors: Dudok de Wit, Thierry; Aschwanden, Markus; Bale, Stuart; Froment, Clara; Krasnoselskikh, Vladimir; Larosa, Andrea; MacDowall, Robert; Raouafi, Nour Bibcode: 2021AGUFMSH44B..05D Altcode: One of the most intriguing observations made by Parker Solar Probe is the omnipresence of sudden deflections of the magnetic field, called switchbacks or jets. One of the pathways towards understanding the nature and the origin of these structures consists in studying their statistical properties. This can be pursued only if we have a means for detecting and extracting each individual switchback. Here we provide a robust technique that allows to automatically detect and identify switchbacks based on their sudden deflection from the Parker spiral. This allows us to build a comprehensive catalogue, with thousands of events per solar encounter. Most importantly, this catalogue does not only include full reversals, but also smaller deflections, whose properties have been overlooked so far while they are essential for building a complete picture. Using this catalogue we provide new constraints on the origin of these structures and their radial evolution. Title: Global energetics of solar flares. XIII. The Neupert effect and acceleration of coronal mass ejections Authors: Aschwanden, Markus J. Bibcode: 2021arXiv211207759A Altcode: Our major aim is a height-time model $r(t)$ of the propagation of {\sl Coronal Mass Ejections (CMEs)}, where the lower corona is self-consistently connected to the heliospheric path. We accomplish this task by using the Neupert effect to derive the peak time, duration, and rate of the CME acceleration phase, as obtained from the time derivative of the {\sl soft X-ray (SXR)} light curve. This novel approach offers the advantage to obtain the kinematics of the CME height-time profile $r(t)$, the CME velocity profile $v(t)=dr(t)/dt$, and the CME acceleration profile $a(t)=dv(t)/dt$ from {\sl Geostationary Orbiting Earth Satellite (GOES)} and white-light data, without the need of {\sl hard X-ray (HXR)} data. We apply this technique to a data set of 576 (GOES X and M-class) flare events observed with GOES and the {\sl Large Angle Solar Coronagraph (LASCO)}. Our analysis yields acceleration rates in the range of $a_A = 0.1-13$ km s$^{-2}$, acceleration durations of $\tau_A = 1.2-45$ min, and acceleration distances in the range of $d_A = 3-1063$ Mm, with a median of $d_A=39$ Mm, which corresponds to the hydrostatic scale height of a corona with a temperature of $T_e \approx 0.8$ MK. The results are consistent with standard flare/CME models that predict magnetic reconnection and synchronized (primary) acceleration of CMEs in the low corona (at a height of ~0.1 R_sun), while secondary (weaker) acceleration may occur further out at heliospheric distances. Title: Memory and Scaling Laws in the Dynamics of Solar Flares Authors: Johnson, Jay; Nurhan, Yosia; Aschwanden, Markus; Homan, Jonathan; Ccopa Rivera, Elmer Alberto; Wing, Simon Bibcode: 2021AGUFMSH25E2131J Altcode: The solar magnetic activity cycle provides energy input that is released in intense bursts of radiation known as solar flares. As such, the dynamics of the activity cycle is embedded in the sequence of times between the flare events. Recent analysis [Snelling et al., 2020, Ashwanden and Johnson, 2021] shows that solar flares exhibit memory on different timescales. Information theory analysis shows that the time ordering of flare events is not random, but rather there is dependence between successive flares. The increased mutual information results from the clustering of flares, which we demonstrate by comparing the cumulative distribution function of successive flares with the cumulative distribution function of surrogate sequences of flares obtained by random permutation of flares within rate-variable Bayesian blocks during which it is assumed that the flare rate is constant. Differences between the cumulative distribution functions is substantial on a timescale around 3 hours, suggesting that flare recurrence on that timescale is more likely than would be expected if the waiting time were drawn from a nonstationary Poisson process. At longer waiting times, the waiting time distribution of flares exhibits a power law form. The power laws also reveal memory in the nonlinear time dependence of the flaring rate. We discuss how time variability in the underlying driver of flares leads to power laws, and in particular discuss how sinusoidal or impulsive driving affects the waiting time distribution of flares. Title: The Poissonian Origin of Power Laws in Solar Flare Waiting Time Distributions Authors: Aschwanden, Markus J.; Johnson, Jay R.; Nurhan, Yosia I. Bibcode: 2021ApJ...921..166A Altcode: 2021arXiv210713065A In this study we aim for a deeper understanding of the power-law slope, α, of waiting time distributions. Statistically independent events with linear behavior can be characterized by binomial, Gaussian, exponential, or Poissonian size distribution functions. In contrast, physical processes with nonlinear behavior exhibit spatiotemporal coherence (or memory) and "fat tails" in their size distributions that fit power-law-like functions, as a consequence of the time variability of the mean event rate, as demonstrated by means of Bayesian block decomposition in the work of Wheatland et al. In this study we conduct numerical simulations of waiting time distributions N(τ) in a large parameter space for various (polynomial, sinusoidal, Gaussian) event rate functions λ(t), parameterized with an exponent p that expresses the degree of the polynomial function λ(t) ∝ tp. We derive an analytical exact solution of the waiting time distribution function in terms of the incomplete gamma function, which is similar to a Pareto type II function and has a power-law slope of α = 2 + 1/p, in the asymptotic limit of large waiting times. Numerically simulated random distributions reproduce this theoretical prediction accurately. Numerical simulations in the nonlinear regime (p ≥ 2) predict power-law slopes in the range of 2.0 ≤ α ≤ 2.5. The self-organized criticality model yields a prediction of α = 2. Observations of solar flares and coronal mass ejections (over at least a half solar cycle) are found in the range of αobs ≍ 2.1-2.4. Deviations from strict power-law functions are expected due to the variability of the flare event rate λ(t), and deviations from theoretically predicted slope values α occur due to the Poissonian weighting bias of power-law fits. Title: The Solar Memory from Hours to Decades Authors: Aschwanden, Markus J.; Johnson, Jay R. Bibcode: 2021ApJ...921...82A Altcode: 2021arXiv210713621A Waiting-time distributions allow us to distinguish at least three different types of dynamical systems, including (i) linear random processes (with no memory); (ii) nonlinear, avalanche-type, nonstationary Poisson processes (with memory during the exponential growth of the avalanche rise time); and (iii) chaotic systems in the state of a nonlinear limit cycle (with memory during the oscillatory phase). We describe the temporal evolution of the flare rate λ(t) ∝ tp with a polynomial function, which allows us to distinguish linear (p ≍ 1) from nonlinear (p ≳ 2) events. The power-law slopes α of the observed waiting times (with full solar cycle coverage) cover a range of α = 2.1-2.4, which agrees well with our prediction of α = 2.0 + 1/p = 2.3-2.6. The memory time can also be defined with the time evolution of the logistic equation, for which we find a relationship between the nonlinear growth time τG = τrise/(4p) and the nonlinearity index p. We find a nonlinear evolution for most events, in particular for the clustering of solar flares (p = 2.2 ± 0.1), partially occulted flare events (p = 1.8 ± 0.2), and the solar dynamo (p = 2.8 ± 0.5). The Sun exhibits memory on timescales of ≲2 hr to 3 days (for solar flare clustering), 6-23 days (for partially occulted flare events), and 1.5 month to 1 yr (for the rise time of the solar dynamo). Title: Role of the Solar Minimum in the Waiting Time Distribution Throughout the Heliosphere Authors: Nurhan, Yosia I.; Johnson, Jay R.; Homan, Jonathan R.; Wing, Simon; Aschwanden, Markus J. Bibcode: 2021GeoRL..4894348N Altcode: 2021arXiv210505939N Many processes throughout the heliosphere such as flares, coronal mass ejections (CMEs), storms and substorms have abrupt onsets. The waiting time between these onsets provides key insights as to the underlying dynamical processes. We explore the tail of these waiting time distributions (WTDs) in the context of random processes driven by the solar magnetic activity cycle, which we approximate by a sinusoidal driver. Analytically, we find that the distribution of large waiting times of such a process approaches a power law slope of −2.5, which is primarily controlled by the conditions when the driving is minimum. We find that the asymptotic behavior of WTDs of solar flares, CMEs, geomagnetic storms, and substorms exhibit power laws that are in reasonable agreement with a sinusoidally driven nonstationary Poisson process. However, the WTD of substorms during solar minimum may be more consistent with prolonged periods of weak driving followed by abrupt increase in the rate. Title: Correlation of the Sunspot Number and the Waiting-time Distribution of Solar Flares, Coronal Mass Ejections, and Solar Wind Switchback Events Observed with the Parker Solar Probe Authors: Aschwanden, Markus J.; Dudok de Wit, Thierry Bibcode: 2021ApJ...912...94A Altcode: 2021arXiv210202305A Waiting-time distributions of solar flares and coronal mass ejections (CMEs) exhibit power-law-like distribution functions with slopes in the range of ατ ≍ 1.4-3.2, as observed in annual data sets during four solar cycles (1974-2012). We find a close correlation between the waiting-time power-law slope ατ and the sunspot number (SN), i.e., ατ = 1.38 + 0.01 × SN. The waiting-time distribution can be fitted with a Pareto-type function of the form N(τ) = N0 ${({\tau }_{0}+\tau )}^{-{\alpha }_{\tau }}$ , where the offset τ0 depends on the instrumental sensitivity, the detection threshold of events, and pulse pileup effects. The time-dependent power-law slope ατ(t) of waiting-time distributions depends only on the global solar magnetic flux (quantified by the sunspot number) or flaring rate, which is not predicted by self-organized criticality or magnetohydrodynamic turbulence models. Power-law slopes of ατ ≍ 1.2-1.6 were also found in solar wind switchback events, as observed with the Parker Solar Probe during the solar minimum, while steeper slopes are predicted during the solar maximum. We find that the annual variability of switchback events in the heliospheric solar wind and solar flare and CME rates (originating in the photosphere and lower corona) are highly correlated. Title: VizieR Online Data Catalog: Global energetics of solar flares. IX. (Aschwanden, 2019) Authors: Aschwanden, M. J. Bibcode: 2021yCat..18850049A Altcode: We analyzed the same data set of 173 solar flares presented in PaperI, which includes all M- and X-class flares observed with the Solar Dynamics Observatory (SDO) during the first 3.5yr of the mission (2010 June 1 to 2014 January 31). We use the 45s line-of-sight magnetograms from Helioseismic and Magnetic Imager (HMI/SDO) and make use of all coronal Extreme Ultraviolet Variability (EUV) channels of Atmospheric Imaging Assembly (AIA/SDO) (in the six wavelengths 94, 131, 171, 193, 211, 335Å), in the temperature range of T~0.6-16MK. The spatial resolution is ~1.6" for AIA, and the pixel size of HMI is 0.5".

(1 data file). Title: Finite System-size Effects in Self-organized Criticality Systems Authors: Aschwanden, Markus J. Bibcode: 2021ApJ...909...69A Altcode: 2021arXiv210103124A We explore upper limits for the largest avalanches or catastrophes in nonlinear energy dissipation systems governed by self-organized criticality. We generalize the idealized "straight" power-law size distribution and Pareto distribution functions in order to accommodate incomplete sampling, limited instrumental sensitivity, finite system-size effects, and "Black Swan" and "Dragon King" extreme events. Our findings are as follows. (i) Solar flares show no finite system-size limits up to L ≲ 200 Mm, but solar flare durations reveal an upper flare duration limit of ≲6 hr. (ii) Stellar flares observed with Kepler exhibit inertial ranges of E ≍ 1034-1037 erg, finite system-size ranges of E ≍ 1037-1038 erg, and extreme events at E ≍ (1-5) × 1038 erg. (iii) The maximum flare energies of different spectral type stars (M, K, G, F, A, giants) reveal a positive correlation with the stellar radius, which indicates a finite system-size limit imposed by the stellar surface area. Fitting our finite system-size models to terrestrial data sets (earthquakes, wildfires, city sizes, blackouts, terrorism, words, surnames, web links) yields evidence (in half of the cases) for finite system-size limits and extreme events, which can be modeled with dual power-law size distributions. Title: Self-organized Criticality in Stellar Flares Authors: Aschwanden, Markus J.; Güdel, Manuel Bibcode: 2021ApJ...910...41A Altcode: 2021arXiv210606490A Power-law size distributions are the hallmarks of nonlinear energy dissipation processes governed by self-organized criticality (SOC). Here we analyze 75 data sets of stellar flare size distributions, mostly obtained from the Extreme-Ultraviolet Explorer and the Kepler mission. We aim to answer the following questions for size distributions of stellar flares. (i) What are the values and uncertainties of power-law slopes? (ii) Do power-law slopes vary with time? (iii) Do power-law slopes depend on the stellar spectral type? (iv) Are they compatible with solar flares? (v) Are they consistent with SOC models? We find that the observed size distributions of stellar flare fluences (or energies) exhibit power-law slopes of αE = 2.09 ± 0.24 for optical data sets observed with Kepler. The observed power-law slopes do not show much time variability and do not depend on the stellar spectral type (M, K, G, F, A, giants). In solar flares, we find that background subtraction lowers the uncorrected value of αE = 2.20 ± 0.22 to αE = 1.57 ± 0.19. Furthermore, most of the stellar flares are temporally not resolved in low-cadence (30 minutes) Kepler data, which causes an additional bias. Taking these two biases into account, the stellar flare data sets are consistent with the theoretical prediction $N(x)\propto {x}^{-{\alpha }_{x}}$ of SOC models, i.e., αE = 1.5. Thus, accurate power-law fits require automated detection of the inertial range and background subtraction, which can be modeled with the generalized Pareto distribution, finite-system size effects, and extreme event outliers. Title: VizieR Online Data Catalog: Global energetics of solar flares. VIII. (Aschwanden+, 2019) Authors: Aschwanden+; Aschwanden, M. J.; Kontar, E. P.; Jeffrey, N. L. S. Bibcode: 2020yCat..18810001A Altcode: We use the same Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data of 191 flare events as previously analyzed in Paper III, using the Object Spectral Executive (OSPEX) software.

(1 data file). Title: Global Energetics of Solar Flares. XII. Energy Scaling Laws Authors: Aschwanden, Markus J. Bibcode: 2020ApJ...903...23A Altcode: 2020arXiv200704419A In this study we test 18 versions of five fundamental energy scaling laws that operate in large solar flares. We express scaling laws in terms of the magnetic potential field energy Ep, the mean potential field strength Bp, the free energy Efree, the dissipated magnetic flare energy Ediss, the magnetic length scale L, the thermal length scale Lth, the mean helically twisted flux tube radius R, the sunspot radius r, the emission-measure-weighted flare temperature Te, the electron density ne, and the total emission measure EM, measured from a data set of 173 GOES M- and X-class flare events. The five categories of physical scaling laws include (i) a scaling law of the potential field energy, (ii) a scaling law for helical twisting, (iii) a scaling law for Petschek-type magnetic reconnection, (iv) the Rosner-Tucker-Vaiana scaling law, and (v) the Shibata-Yokoyama scaling law. We test the self-consistency of these theoretical scaling laws with observed parameters by requiring two criteria: a cross-correlation coefficient of CCC > 0.5 between the theoretically predicted scaling laws and observed values, and a linear regression fit with a slope of α ≍ 1 within one standard deviation σ. These two criteria enable us (i) to corroborate some existing (or modified) scaling laws, (ii) to reject other scaling laws that are not consistent with the observations, (iii) to probe the dimensionality of flare geometries, and (iv) to predict various energy parameters based on tested scaling laws. Title: Global Energetics of Solar Flares. XI. Flare Magnitude Predictions of the GOES Class Authors: Aschwanden, Markus J. Bibcode: 2020ApJ...897...16A Altcode: 2020arXiv200704413A In this study we determine scaling relationships of observed solar flares that can be used to predict upper limits of the Geostationary Orbiting Earth Satellite (GOES)-class magnitude of solar flares. The flare prediction scheme is based on the scaling of the slowly varying potential energy Ep(t), which is extrapolated in time over an interval of Δt ≤ 24 hr. The observed scaling of the dissipated energy Ediss scales with the potential field energy as ${E}_{\mathrm{diss}}\propto {E}_{p}^{1.32}$ . In addition, the observed scaling relationship of the flare volume, $V\propto {E}_{\mathrm{diss}}^{1.17}$ , the multi-thermal energy, Eth ∝ V0.76, the flare emission measure $(\mathrm{EM})\propto {E}_{\mathrm{th}}^{0.79}$ , the EM-weighted temperature Tw, and the GOES flux, ${F}_{8}{(t)\propto {E}_{p}(t)}^{0.92}$ , allows us then to predict an upper limit of the GOES-class flare magnitude in the extrapolated time window. We find a good correlation (cross-correlation coefficient (CCC) ≍ 0.7) between the observed and predicted GOES-class flare magnitudes (in 172 X- and M-class events). This is the first algorithm that employs observed scaling laws of physical flare parameters to predict GOES flux upper limits, an important capability that complements previous flare prediction methods based on machine-learning algorithms used in space-weather forecasting. Title: Global Energetics of Solar Flares. X. Petschek Reconnection Rate and Alfvén Mach Number of Magnetic Reconnection Outflows Authors: Aschwanden, Markus J. Bibcode: 2020ApJ...895..134A Altcode: 2020arXiv200704404A We investigate physical scaling laws for magnetic energy dissipation in solar flares, in the framework of the Sweet-Parker model and the Petschek model. We find that the total dissipated magnetic energy Ediss in a flare depends on the mean magnetic field component Bf associated with the free energy Ef, the length scale L of the magnetic area, the hydrostatic density scale height λ of the solar corona, the Alfvén Mach number MA = V1/VA (the ratio of the inflow speed V1 to the Alfvénic outflow speed VA), and the flare duration τf, I.e., ${E}_{\mathrm{diss}}=(1/4\pi ){B}_{f}^{2}\ L\ \lambda \ {V}_{{\rm{A}}}\ {M}_{{\rm{A}}}\ {\tau }_{f}$ , where the Alfvén speed depends on the nonpotential field strength Bnp and the mean electron density ne in the reconnection outflow. Using MDI/Solar Dynamics Observatory (SDO) and AIA/SDO observations and 3D magnetic field solutions obtained with the vertical-current approximation non-linear force-free field code we measure all physical parameters necessary to test scaling laws, which represents a new method to measure Alfvén Mach numbers MA, the reconnection rate, and the total free energy dissipated in solar flares. Title: Torsional Alfvénic Oscillations Discovered in the Magnetic Free Energy during Solar Flares Authors: Aschwanden, Markus J.; Wang, Tongjiang Bibcode: 2020ApJ...891...99A Altcode: 2020arXiv200110103A We report the discovery of torsional Alfvénic oscillations in solar flares, which modulate the time evolution of the magnetic free energy Ef(t), while the magnetic potential energy Ep(t) is uncorrelated, and the nonpotential energy varies as Enp(t) = Ep + Ef(t). The mean observed time period of the torsional oscillations is Pobs = 15.1 ± 3.9 minutes, the mean field line length is L = 135 ± 35 Mm, and the mean phase speed is Vphase = 315 ± 120 km s-1, which we interpret as torsional Alfvénic waves in flare loops with enhanced electron densities. Most of the torsional oscillations are found to be decay-less, but exhibit a positive or negative trend in the evolution of the free energy, indicating new emerging flux (if positive), magnetic cancellation, or flare energy dissipation (if negative). The time evolution of the free energy has been calculated in this study with the Vertical-current Approximation (Version 4) Non-linear Force-free Field code, which incorporates automatically detected coronal loops in the solution and bypasses the non-force-freeness of the photospheric boundary condition, in contrast to traditional NLFFF codes. Title: Nonstationary Fast-driven, Self-organized Criticality in Solar Flares Authors: Aschwanden, Markus J. Bibcode: 2019ApJ...887...57A Altcode: 2019arXiv190908673A The original concept of self-organized criticality, applied to solar flare statistics, assumed a slow-driven and stationary flaring rate, which implies timescale separation (between flare durations and interflare waiting times). The concept reproduces power-law distributions for flare peak fluxes and durations, but predicts an exponential waiting time distribution. In contrast to these classical assumptions, we observe (i) multiple energy dissipation episodes during most flares, (ii) violation of the principle of timescale separation, (iii) a fast-driven and nonstationary flaring rate, (iv) a power-law distribution for waiting times Δt, with a slope of α Δt ≈ 2.0, as predicted from the universal reciprocality between mean flaring rates and mean waiting times, and (v) pulses with rise times and decay times of the dissipated magnetic free energy on timescales of 12 ± 6 minutes, and up to 13 times in long-duration (≲4 hr) flares. These results are inconsistent with coronal long-term energy storage, but require photospheric-chromospheric current injections into the corona. Title: Self-Organized Criticality in Solar and Stellar Flares. Are There Dragon-King Events ? Authors: Aschwanden, M. J. Bibcode: 2019AGUFMSH34A..07A Altcode: We search for outliers in extreme events of statistical size distributions of astrophysical data sets, motivated by the Dragon-King hypothesis of Sornette (2009), which suggests that the most extreme events in a statistical distribution may belong to a different population, and thus may be generated by a different physical mechanism, in contrast to the strict power law behavior of self-organized criticality (SOC) models. Identifying such disparate outliers is important for space weather predictions. Possible physical mechanisms to produce such outliers could be generated by sympathetic flaring. However, we find that Dragon-King events are not common in solar and stellar flares, identified in 4 out of 25 solar and stellar flare data sets only. Consequently, small, large, and extreme flares are essentially scale-free and can be modeled with a single physical mechanism. In very large data sets (N > 10,000), we find significant deviations from ideal power laws in almost all data sets. Neverthess, the fitted power law slopes constrain physcial scaling laws in terms of flare areas and volumes, which have the highest nonlinearity in their scaling laws. Title: Global Energetics of Solar Flares and Coronal Mass Ejections Authors: Aschwanden, Markus J.; Caspi, Amir; Cohen, Christina M. S.; Holman, Gordon; Jing, Ju; Kretzschmar, Matthieu; Kontar, Eduard P.; McTiernan, James M.; Mewaldt, Richard A.; O'Flannagain, Aidan; Richardson, Ian G.; Ryan, Daniel; Warren, Harry P.; Xu, Yan Bibcode: 2019JPhCS1332a2002A Altcode: We investigate the global energetics and energy closure of various physical processes that are energetically important in solar flares and coronal mass ejections (CMEs), which includes: magnetic energies, thermal energies, nonthermal energies (particle acceleration), direct and indirect plasma heating processes, kinetic CME energies, gravitational CME energies, aerodynamic drag of CMEs, solar energetic particle events, EUV and soft X-ray radiation, white-light, and bolometric energies. Statistics on these forms of energies is obtained from 400 GOES M- and X-class events during the first 3.5 years of the Solar Dynamics Observatory (SDO) mission. A primary test addressed in this study is the closure of the various energies, such as the equivalence of the dissipated magnetic energies and the primary dissipated are energies (accelerated particles, direct heating, CME acceleration), which faciliate the energy of secondary processes (plasma heating, shock acceleration) and interactions with the solar wind (aerodynamic drag). Our study demonstrates energy closure in the statistical average, while individual events may have considerable uncertainties, requiring improved nonlinear force-free field models, and particle acceleration models with observationally constrained low-energy cutoffs. Title: Global Energetics of Solar Flares. IX. Refined Magnetic Modeling Authors: Aschwanden, Markus J. Bibcode: 2019ApJ...885...49A Altcode: 2019arXiv190908672A A more accurate analytical solution of the vertical-current approximation nonlinear force-free field (VCA3-NLFFF) model is presented that includes, besides the radial (B r ) and azimuthal (B φ ) magnetic field components, a poloidal component ({B}θ \ne 0) as well. This new analytical solution is of second-order accuracy in the divergence-freeness condition and of third-order accuracy in the force-freeness condition. We reanalyze the sample of 173 GOES M- and X-class flares observed with the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO). The new code reproduces helically twisted loops with a low winding number below the kink instability consistently, avoiding unstable, highly twisted structures of the Gold-Hoyle flux rope type. The magnetic energies agree within {E}VCA3}/{E}W=0.99+/- 0.21 with the Wiegelmann (W-NLFFF) code. The time evolution of the magnetic field reveals multiple, intermittent energy buildup and releases in most flares, contradicting both the Rosner-Vaiana model (with gradual energy storage in the corona) and the principle of timescale separation (τ flare ≪ τ storage) postulated in self-organized criticality models. The mean dissipated flare energy is found to amount to 7% ± 3% of the potential energy, or 60% ± 26% of the free energy, a result that can be used for predicting flare magnitudes based on the potential field of active regions. Title: Self-organized Criticality in Solar and Stellar Flares: Are Extreme Events Scale-free? Authors: Aschwanden, Markus J. Bibcode: 2019ApJ...880..105A Altcode: 2019arXiv190605840A We search for outliers in extreme events of statistical size distributions of astrophysical data sets, motivated by the Dragon-King hypothesis of Sornette, which suggests that the most extreme events in a statistical distribution may belong to a different population, and thus may be generated by a different physical mechanism, in contrast to the strict power-law behavior of self-organized criticality models. Identifying such disparate outliers is important for space weather predictions. Possible physical mechanisms to produce such outliers could be generated by sympathetic flaring. However, we find that Dragon-King events are not common in solar and stellar flares, identified in 4 out of 25 solar and stellar flare data sets only. Consequently, small, large, and extreme flares are essentially scale-free and can be modeled with a single physical mechanism. In very large data sets (N ≳ 104) we find significant deviations from ideal power laws in almost all data sets. Nevertheless, the fitted power-law slopes constrain physical scaling laws in terms of flare areas and volumes, which have the highest nonlinearity in their scaling laws. Title: Global Energetics of Solar Flares. VIII. The Low-energy Cutoff Authors: Aschwanden, Markus J.; Kontar, Eduard P.; Jeffrey, Natasha L. S. Bibcode: 2019ApJ...881....1A Altcode: 2019arXiv190605835A One of the key problems in solar flare physics is the determination of the low-energy cut-off: the value that determines the energy of nonthermal electrons and hence flare energetics. We discuss different approaches to determine the low-energy cut-off in the spectrum of accelerated electrons: (i) the total electron number model, (ii) the time-of-flight model (based on the equivalence of the time-of-flight and the collisional deflection time), (iii) the warm target model of Kontar et al., and (iv) the model of the spectral cross-over between thermal and nonthermal components. We find that the first three models are consistent with a low-energy cutoff with a mean value of ≈10 keV, while the cross-over model provides an upper limit for the low-energy cutoff with a mean value of ≈21 keV. Combining the first three models we find that the ratio of the nonthermal energy to the dissipated magnetic energy in solar flares has a mean value of q E = 0.57 ± 0.08, which is consistent with an earlier study based on the simplified approximation of the warm target model alone (q E = 0.51 ± 0.17). This study corroborates the self-consistency between three different low-energy cutoff models in the calculation of nonthermal flare energies. Title: Global Energetics of Solar Flares. VII. Aerodynamic Drag in Coronal Mass Ejections Authors: Aschwanden, Markus J.; Gopalswamy, Nat Bibcode: 2019ApJ...877..149A Altcode: 2019arXiv190605804A The free energy that is dissipated in a magnetic reconnection process of a solar flare, generally accompanied by a coronal mass ejection (CME), has been considered as the ultimate energy source of the global energy budget of solar flares in previous statistical studies. Here we explore the effects of the aerodynamic drag force on CMEs, which supplies additional energy from the slow solar wind to a CME event, besides the magnetic energy supply. For this purpose, we fit the analytical aerodynamic drag model of Cargill and Vršnak et al. to the height-time profiles r(t) of LASCO/SOHO data in 14,316 CME events observed during the first 8 yr (2010-2017) of the Solar Dynamics Observatory era (ensuring EUV coverage with AIA). Our main findings are (1) a mean solar wind speed of w = 472 ± 414 km s-1, (2) a maximum drag-accelerated CME energy of E drag ≲ 2 × 1032 erg, (3) a maximum flare-accelerated CME energy of E flare ≲ 1.5 × 1033 erg, (4) the ratio of the summed kinetic energies of all flare-accelerated CMEs to the drag-accelerated CMEs amounts to a factor of 4, (5) the inclusion of the drag force slightly lowers the overall energy budget of CME kinetic energies in flares from ≈7% to ≈4%, and (6) the arrival times of CMEs at Earth can be predicted with an accuracy of ≈23%. Title: Helical Twisting Number and Braiding Linkage Number of Solar Coronal Loops Authors: Aschwanden, Markus J. Bibcode: 2019ApJ...874..131A Altcode: 2019arXiv190210612A Coronal loops in active regions are often characterized by quasi-circular and helically twisted (sigmoidal) geometries, which are consistent with dipolar potential field (PF) models in the former case, and with nonlinear force-free field models with vertical currents in the latter case. Alternatively, Parker-type nanoflare models of the solar corona hypothesize that a braiding mechanism operates between unresolved loop strands, which is a more complex topological model. In this study we use the vertical-current approximation of a nonpotential magnetic field solution (that fulfils the divergence-free and force-free conditions) to characterize the number of helical turns N twist in twisted coronal loops. We measure the helical twist in 15 active regions observed with Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager/SDO (Solar Dynamic Observatory) and find a mean nonpotentiality angle (between the potential and nonpotential field directions) of μ NP = 15° ± 3°. The resulting mean rotational twist angle is φ = 49° ± 11°, which corresponds to N twist = φ/360° = 0.14 ± 0.03 turns with respect to the untwisted PF, with an absolute upper limit of N twist ≲ 0.5, which is far below the kink instability limit of | {N}twist}| ≳ 1. The number of twist turns N twist corresponds to the Gauss linkage number N link in braiding topologies. We conclude that any braided topology (with | {N}link}| ≥slant 1) cannot explain the observed stability of loops in a force-free corona, nor the observed low twist number. Parker-type nanoflaring can thus occur in non-force-free environments only, such as in the chromosphere and transition region. Title: New Millennium Solar Physics Authors: Aschwanden, Markus J. Bibcode: 2019ASSL..458.....A Altcode: No abstract at ADS Title: The Minimum Energy Principle Applied to Parker's Coronal Braiding and Nanoflaring Scenario Authors: Aschwanden, Markus; van Ballegooijen, A. A. Bibcode: 2018csc..confE..52A Altcode: 2018arXiv180805269A Parker's coronal braiding and nanoflaring scenario predicts the development of tangential discontinuities and highly misaligned magnetic field lines, as a consequence of random buffeting of their footpoints due to the action of sub-photospheric convection. The increased stressing of magnetic field lines is thought to become unstable above some critical misalignment angle and to result into local magnetic reconnection events, which is generally referred to as Parker's ``nanoflaring scenario''. In this study we show that the minimum (magnetic) energy principle leads to a bifurcation of force-free field solutions for helical twist angles at |phi(t)| = pi, which prevents the build-up of arbitrary large free energies and misalignment angles. The minimum energy principle predicts that neighbored magnetic field lines are almost parallel (with misalignment angles of Delta mu 1.6-1.8 deg, and do not reach a critical misalignment angle prone to nanoflaring. Consequently, no nanoflares are expected in the divergence-free and force-free parts of the solar corona, while they are more likely to occur in the chromosphere and transition region. Title: Convection-driven Generation of Ubiquitous Coronal Waves Authors: Aschwanden, Markus J.; Gošic, Milan; Hurlburt, Neal E.; Scullion, Eamon Bibcode: 2018ApJ...866...73A Altcode: We develop a new method to measure the 3D kinematics of the subphotospheric motion of magnetic elements, which is used to study the coupling between the convection-driven vortex motion and the generation of ubiquitous coronal waves. We use the method of decomposing a line-of-sight magnetogram from MDI/SDO into unipolar magnetic charges, which yields the (projected) 2D motion [x(t), y(t)] and the (half) width evolution w(t) of an emerging magnetic element from an initial depth of d ≲ 1500 km below the photosphere. A simple model of rotational vortex motion with magnetic flux conservation during the emergence process of a magnetic element predicts the width evolution, i.e., w(t)/w 0 = [B(t)/B 0]-1/2, and an upper limit of the depth variation d(t) ≤ 1.3 w(t). While previous 2D tracing of magnetic elements provided information on advection and superdiffusion, our 3D tracing during the emergence process of a magnetic element is consistent with a ballistic trajectory in the upward direction. From the estimated Poynting flux and lifetimes of convective cells, we conclude that the Coronal Multi-channel Polarimeter-detected low-amplitude transverse magnetohydrodynamic waves are generated by the convection-driven vortex motion. Our observational measurements of magnetic elements appear to contradict the theoretical random-walk braiding scenario of Parker. Title: Toward a Quantitative Comparison of Magnetic Field Extrapolations and Observed Coronal Loops Authors: Warren, Harry P.; Crump, Nicholas A.; Ugarte-Urra, Ignacio; Sun, Xudong; Aschwanden, Markus J.; Wiegelmann, Thomas Bibcode: 2018ApJ...860...46W Altcode: 2018arXiv180500281W It is widely believed that loops observed in the solar atmosphere trace out magnetic field lines. However, the degree to which magnetic field extrapolations yield field lines that actually do follow loops has yet to be studied systematically. In this paper, we apply three different extrapolation techniques—a simple potential model, a nonlinear force-free (NLFF) model based on photospheric vector data, and an NLFF model based on forward fitting magnetic sources with vertical currents—to 15 active regions that span a wide range of magnetic conditions. We use a distance metric to assess how well each of these models is able to match field lines to the 12202 loops traced in coronal images. These distances are typically 1″-2″. We also compute the misalignment angle between each traced loop and the local magnetic field vector, and find values of 5°-12°. We find that the NLFF models generally outperform the potential extrapolation on these metrics, although the differences between the different extrapolations are relatively small. The methodology that we employ for this study suggests a number of ways that both the extrapolations and loop identification can be improved. Title: Order out of Randomness: Self-Organization Processes in Astrophysics Authors: Aschwanden, Markus J.; Scholkmann, Felix; Béthune, William; Schmutz, Werner; Abramenko, Valentina; Cheung, Mark C. M.; Müller, Daniel; Benz, Arnold; Chernov, Guennadi; Kritsuk, Alexei G.; Scargle, Jeffrey D.; Melatos, Andrew; Wagoner, Robert V.; Trimble, Virginia; Green, William H. Bibcode: 2018SSRv..214...55A Altcode: 2017arXiv170803394A Self-organization is a property of dissipative nonlinear processes that are governed by a global driving force and a local positive feedback mechanism, which creates regular geometric and/or temporal patterns, and decreases the entropy locally, in contrast to random processes. Here we investigate for the first time a comprehensive number of (17) self-organization processes that operate in planetary physics, solar physics, stellar physics, galactic physics, and cosmology. Self-organizing systems create spontaneous " order out of randomness", during the evolution from an initially disordered system to an ordered quasi-stationary system, mostly by quasi-periodic limit-cycle dynamics, but also by harmonic (mechanical or gyromagnetic) resonances. The global driving force can be due to gravity, electromagnetic forces, mechanical forces (e.g., rotation or differential rotation), thermal pressure, or acceleration of nonthermal particles, while the positive feedback mechanism is often an instability, such as the magneto-rotational (Balbus-Hawley) instability, the convective (Rayleigh-Bénard) instability, turbulence, vortex attraction, magnetic reconnection, plasma condensation, or a loss-cone instability. Physical models of astrophysical self-organization processes require hydrodynamic, magneto-hydrodynamic (MHD), plasma, or N-body simulations. Analytical formulations of self-organizing systems generally involve coupled differential equations with limit-cycle solutions of the Lotka-Volterra or Hopf-bifurcation type. Title: Alfvén wave dissipation in the solar chromosphere Authors: Grant, Samuel D. T.; Jess, David B.; Zaqarashvili, Teimuraz V.; Beck, Christian; Socas-Navarro, Hector; Aschwanden, Markus J.; Keys, Peter H.; Christian, Damian J.; Houston, Scott J.; Hewitt, Rebecca L. Bibcode: 2018NatPh..14..480G Altcode: 2018arXiv181007712G Magnetohydrodynamic Alfvén waves1 have been a focus of laboratory plasma physics2 and astrophysics3 for over half a century. Their unique nature makes them ideal energy transporters, and while the solar atmosphere provides preferential conditions for their existence4, direct detection has proved difficult as a result of their evolving and dynamic observational signatures. The viability of Alfvén waves as a heating mechanism relies upon the efficient dissipation and thermalization of the wave energy, with direct evidence remaining elusive until now. Here we provide the first observational evidence of Alfvén waves heating chromospheric plasma in a sunspot umbra through the formation of shock fronts. The magnetic field configuration of the shock environment, alongside the tangential velocity signatures, distinguish them from conventional umbral flashes5. Observed local temperature enhancements of 5% are consistent with the dissipation of mode-converted Alfvén waves driven by upwardly propagating magneto-acoustic oscillations, providing an unprecedented insight into the behaviour of Alfvén waves in the solar atmosphere and beyond. Title: Self-organizing systems in planetary physics: Harmonic resonances of planet and moon orbits Authors: Aschwanden, Markus J. Bibcode: 2018NewA...58..107A Altcode: 2017arXiv170108181A The geometric arrangement of planet and moon orbits into a regularly spaced pattern of distances is the result of a self-organizing system. The positive feedback mechanism that operates a self-organizing system is accomplished by harmonic orbit resonances, leading to long-term stable planet and moon orbits in solar or stellar systems. The distance pattern of planets was originally described by the empirical Titius-Bode law, and by a generalized version with a constant geometric progression factor (corresponding to logarithmic spacing). We find that the orbital periods Ti and planet distances Ri from the Sun are not consistent with logarithmic spacing, but rather follow the quantized scaling (Ri + 1 /Ri) =(Ti + 1 /Ti) 2 / 3 =(Hi + 1 /Hi) 2 / 3 , where the harmonic ratios are given by five dominant resonances, namely (Hi + 1 :Hi) =(3 : 2) ,(5 : 3) ,(2 : 1) ,(5 : 2) ,(3 : 1) . We find that the orbital period ratios tend to follow the quantized harmonic ratios in increasing order. We apply this harmonic orbit resonance model to the planets and moons in our solar system, and to the exo-planets of 55 Cnc and HD 10180 planetary systems. The model allows us a prediction of missing planets in each planetary system, based on the quasi-regular self-organizing pattern of harmonic orbit resonance zones. We predict 7 (and 4) missing exo-planets around the star 55 Cnc (and HD 10180). The accuracy of the predicted planet and moon distances amounts to a few percents. All analyzed systems are found to have ≈ 10 resonant zones that can be occupied with planets (or moons) in long-term stable orbits. Title: Statistical Properties of Photospheric Magnetic Elements Observed by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory Authors: Javaherian, M.; Safari, H.; Dadashi, N.; Aschwanden, M. J. Bibcode: 2017SoPh..292..164J Altcode: Magnetic elements of the solar surface are studied (using the 6173 Å Fe I line) in magnetograms recorded with the high-resolution Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI). To extract some statistical and physical properties of these elements (e.g. filling factors, magnetic flux, size, and lifetimes), we employed the region-based method called Yet Another Feature Tracking Algorithm (YAFTA). An area of 400×400 was selected to investigate the magnetic characteristics in 2011. The correlation coefficient between filling factors of negative and positive polarities is 0.51. A broken power-law fit was applied to the frequency distribution of size and flux. Exponents of the power-law distributions for sizes smaller and greater than 16 arcsec2 were found to be −2.24 and −4.04, respectively. The exponents of power-law distributions for fluxes lower and greater than 2.63 ×1019Mx were found to be −2.11 and −2.51, respectively. The relationship between the size [S ] and flux [F ] of elements can be expressed by a power-law behavior of the form of S ∝F0.69. The lifetime and its relationship with the flux and size of quiet-Sun (QS) elements during three days were studied. The code detected patches with lifetimes of about 15 hours, which we call long-duration events. We found that more than 95% of the magnetic elements have lifetimes shorter than 100 minutes. About 0.05% of the elements had lifetimes of more than six hours. The relationships between size [S ], lifetime [T ], and flux [F ] for patches in the QS yield power-law relationships S ∝T0.25 and F ∝T0.38, respectively. Executing a detrended-fluctuation analysis of the time series of new emerged magnetic elements, we found a Hurst exponent of 0.82, which implies a long-range temporal correlation in the system. Title: Exoplanet Predictions Based on Harmonic Orbit Resonances Authors: Aschwanden, Markus; Scholkmann, Felix Bibcode: 2017Galax...5...56A Altcode: 2017arXiv170507138A The current exoplanet database includes 5454 confirmed and candidate planets observed with the Kepler mission. We find 932 planet pairs from which we extract distance and orbital period ratios. While earlier studies used a logarithmic spacing, which lacks a physical model, we employ here the theory of harmonic orbit resonances, which contains quantized ratios instead, to explain the observed planet distance ratios and to predict undetected exoplanets. We find that the most prevailing harmonic ratios are (2:1), (3:2), and (5:3) in 73% of the cases, while alternative harmonic ratios of (5:4), (4:3), (5:2), and (3:1) occur in the other 27% of the cases. Our orbital predictions include 171 exoplanets, 2 Jupiter moons, 1 Saturn moon, 3 Uranus moons, and 4 Neptune moons. The accuracy of the predicted planet distances amounts to a few percent, which fits the data significantly better than the logarithmic spacing. This information may be useful for targeted exoplanet searches with Kepler data and to estimate the number of live-carrying planets in habitable zones. Title: Global Energetics of Solar Flares. VI. Refined Energetics of Coronal Mass Ejections Authors: Aschwanden, Markus J. Bibcode: 2017ApJ...847...27A Altcode: 2017arXiv170401993A In this study, we refine the coronal mass ejection (CME) model that was presented in an earlier study of the global energetics of solar flares and associated CMEs and apply it to all (860) GOES M- and X-class flare events observed during the first seven years (2010-2016) of the Solar Dynamics Observatory (SDO) mission. The model refinements include (1) the CME geometry in terms of a 3D volume undergoing self-similar adiabatic expansion, (2) the solar gravitational deceleration during the propagation of the CME, which discriminates between eruptive and confined CMEs, (3) a self-consistent relationship between the CME center-of-mass motion detected during EUV dimming and the leading-edge motion observed in white-light coronagraphs, (4) the equipartition of the CME’s kinetic and thermal energies, and (5) the Rosner-Tucker-Vaiana scaling law. The refined CME model is entirely based on EUV-dimming observations (using Atmospheric Imager Assembly (AIA)/SDO data) and complements the traditional white-light scattering model (using Large-Angle and Spectrometric Coronagraph Experiment (LASCO)/Solar and Heliospheric Observatory data), and both models are independently capable of determining fundamental CME parameters. Comparing the two methods, we find that (1) LASCO is less sensitive than AIA in detecting CMEs (in 24% of the cases), (2) CME masses below {m}{cme}≲ {10}14 g are underestimated by LASCO, (3) AIA and LASCO masses, speeds, and energies agree closely in the statistical mean after the elimination of outliers, and (4) the CME parameters speed v, emission measure-weighted flare peak temperature T e , and length scale L are consistent with the following scaling laws: v\propto {T}e1/2, v\propto {({m}{cme})}1/4, and {m}{cme}\propto {L}2. Title: Global Energetics in Solar Flares and Coronal Mass Ejections Authors: Aschwanden, Markus J. Bibcode: 2017SPD....4820002A Altcode: We present a statistical study of the energetics of coronal mass ejections (CME) and compare it with the magnetic, thermal, and nonthermal energy dissipated in flares. The physical parameters of CME speeds, mass, and kinetic energies are determined with two different independent methods, i.e., the traditional white-light scattering method using LASCO/SOHO data, and the EUV dimming method using AIA/SDO data. We analyze all 860 GOES M- and X-class flare events observed during the first 7 years (2010-2016) of the SDO mission. The new ingredients of our CME modeling includes: (1) CME geometry in terms of a self-similar adiabatic expansion, (2) DEM analysis of CME mass over entire coronal temperature range, (3) deceleration of CME due to gravity force which controls the kinetic and potentail CME energy as a function of time, (4) the critical speed that controls eruptive and confined CMEs, (5) the relationship between the center-of-mass motion during EUV dimming and the leading edge motion observed in white-light coronagraphs. Novel results are: (1) Physical parameters obtained from both the EUV dimming and white-light method can be reconciled; (2) the equi-partition of CME kinetic and thermal flare energy; (3) the Rosner-Tucker-Vaiana scaling law. We find that the two methods in EUV and white-light wavelengths are highly complementary and yield more complete models than each method alone. Title: Statistical Properties of Photospheric Magnetic Elements Observed by SDO/HMI Authors: Javaherian, Mohsen; Safari, Hossein; Dadashi, Neda; Aschwanden, Markus Josef Bibcode: 2017arXiv170709291J Altcode: Magnetic elements of the solar surface are studied in magnetograms recorded with the high-resolution Solar Dynamics Observatory / Helioseismic and Magnetic Imager . To extract some statistical and physical properties of these elements (e.g., filling factors, magnetic flux, size, lifetimes), the Yet Another Feature Tracking Algorithm (YAFTA), a region-based method, is employed. An area with 400$^{\prime\prime}\times$400$^{\prime\prime}$ was selected to investigate the magnetic characteristics during the year 2011. The correlation coefficient between filling factors of negative and positive polarities is 0.51. A broken power law fit was applied to the frequency distribution of size and flux. Exponents of the power-law distributions for sizes smaller and greater than 16 arcsec$^2$ were found to be -2.24 and -4.04, respectively. The exponents of power$-$law distributions for fluxes smaller and greater than 2.63$\times$10$^{19}$ Mx were found to be -2.11 and -2.51, respectively. The relationship between the size ($S$) and flux ($F$) of elements can be expressed by a power-law behavior in the form of $S\propto F~^{0.69}$. The lifetime and its relationship with the flux and size of quiet-Sun (QS) elements are studied during three days. The code detected patches with lifetimes of about 15 hours, which we call long-duration events. It is found that more than 95\% of the magnetic elements have lifetimes of less than 100 minutes. About 0.05\% of the elements were found with lifetimes of more than 6 hours. The relationships between the size (S), lifetime (T), and the flux (F) for patches in the QS, indicate the power$-$law relationships $S\propto T~^{0.25}$ and $F\propto T~^{0.38}$, respectively. Executing a detrended fluctuation analysis of the time series of new emerged magnetic elements, we find a Hurst exponent of 0.82, which implies long-range temporal correlation in the system. Title: The Width Distribution of Loops and Strands in the Solar Corona—Are We Hitting Rock Bottom? Authors: Aschwanden, Markus J.; Peter, Hardi Bibcode: 2017ApJ...840....4A Altcode: 2017arXiv170101177A In this study, we analyze Atmospheric Imaging Assembly (AIA) and Hi-C images in order to investigate absolute limits for the finest loop strands. We develop a model of the occurrence-size distribution function of coronal loop widths, characterized by the lower limit of widths w min, the peak (or most frequent) width w p , the peak occurrence number n p , and a power-law slope a. Our data analysis includes automated tracing of curvilinear features with the OCCULT-2 code, automated sampling of the cross-sectional widths of coronal loops, and fitting of the theoretical size distribution to the observed distribution. With Monte Carlo simulations and variable pixel sizes {{Δ }}x, we derive a first diagnostic criterion to discriminate whether the loop widths are unresolved ({w}p/{{Δ }}x≈ 2.5+/- 0.2) or fully resolved (if {w}p/{{Δ }}x≳ 2.7). For images with resolved loop widths, we can apply a second diagnostic criterion that predicts the lower limit of loop widths as a function of the spatial resolution. We find that the loop widths are marginally resolved in AIA images but are fully resolved in Hi-C images, where our model predicts a most frequent (peak) value at {w}p≈ 550 {km}, in agreement with recent results of Brooks et al. This result agrees with the statistics of photospheric granulation sizes and thus supports coronal heating mechanisms operating on the macroscopic scale of photospheric magneto-convection, rather than nanoflare braiding models on unresolved microscopic scales. Title: VizieR Online Data Catalog: Global energetics of solar flares. III. (Aschwanden+, 2016) Authors: Aschwanden, M. J.; Holman, G.; O'Flannagain, A.; Caspi, A.; McTiernan, J. M.; Kontar, E. P. Bibcode: 2017yCat..18320027A Altcode: This study entails the third part of a global flare energetics project, in which Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data of 191 M and X-class flare events from the first 3.5yrs of the Solar Dynamics Observatory mission are analyzed. We fit a thermal and a nonthermal component to RHESSI spectra, yielding the temperature of the differential emission measure (DEM) tail, the nonthermal power-law slope and flux, and the thermal/nonthermal cross-over energy eco. From these parameters, we calculate the total nonthermal energy Ent in electrons with two different methods: (1) using the observed cross-over energy eco as low-energy cutoff, and (2) using the low-energy cutoff ewt predicted by the warm thick-target bremsstrahlung model of Kontar et al. Based on a mean temperature of Te=8.6MK in active regions, we find low-energy cutoff energies of ewt=6.2+/-1.6keV for the warm-target model, which is significantly lower than the cross-over energies eco=21+/-6keV. Comparing with the statistics of magnetically dissipated energies Emag and thermal energies Eth from the two previous studies, we find the following mean (logarithmic) energy ratios with the warm-target model: Ent=0.41Emag, Eth=0.08Emag, and Eth=0.15Ent. The total dissipated magnetic energy exceeds the thermal energy in 95% and the nonthermal energy in 71% of the flare events, which confirms that magnetic reconnection processes are sufficient to explain flare energies. The nonthermal energy exceeds the thermal energy in 85% of the events, which largely confirms the warm thick-target model.

(1 data file). Title: Global Energetics of Solar Flares. V. Energy Closure in Flares and Coronal Mass Ejections Authors: Aschwanden, Markus J.; Caspi, Amir; Cohen, Christina M. S.; Holman, Gordon; Jing, Ju; Kretzschmar, Matthieu; Kontar, Eduard P.; McTiernan, James M.; Mewaldt, Richard A.; O'Flannagain, Aidan; Richardson, Ian G.; Ryan, Daniel; Warren, Harry P.; Xu, Yan Bibcode: 2017ApJ...836...17A Altcode: 2017arXiv170101176A In this study we synthesize the results of four previous studies on the global energetics of solar flares and associated coronal mass ejections (CMEs), which include magnetic, thermal, nonthermal, and CME energies in 399 solar M- and X-class flare events observed during the first 3.5 yr of the Solar Dynamics Observatory (SDO) mission. Our findings are as follows. (1) The sum of the mean nonthermal energy of flare-accelerated particles ({E}{nt}), the energy of direct heating ({E}{dir}), and the energy in CMEs ({E}{CME}), which are the primary energy dissipation processes in a flare, is found to have a ratio of ({E}{nt}+{E}{dir}+{E}{CME})/{E}{mag}=0.87+/- 0.18, compared with the dissipated magnetic free energy {E}{mag}, which confirms energy closure within the measurement uncertainties and corroborates the magnetic origin of flares and CMEs. (2) The energy partition of the dissipated magnetic free energy is: 0.51 ± 0.17 in nonthermal energy of ≥slant 6 {keV} electrons, 0.17 ± 0.17 in nonthermal ≥slant 1 {MeV} ions, 0.07 ± 0.14 in CMEs, and 0.07 ± 0.17 in direct heating. (3) The thermal energy is almost always less than the nonthermal energy, which is consistent with the thick-target model. (4) The bolometric luminosity in white-light flares is comparable to the thermal energy in soft X-rays (SXR). (5) Solar energetic particle events carry a fraction ≈ 0.03 of the CME energy, which is consistent with CME-driven shock acceleration. (6) The warm-target model predicts a lower limit of the low-energy cutoff at {e}c≈ 6 {keV}, based on the mean peak temperature of the differential emission measure of T e = 8.6 MK during flares. This work represents the first statistical study that establishes energy closure in solar flare/CME events. Title: VizieR Online Data Catalog: Global energetics of solar flares. IV. CME (Aschwanden, 2016) Authors: Aschwanden, M. J. Bibcode: 2017yCat..18310105A Altcode: We analyze observations from AIA/SDO for the 399 flare events of the primary data set that consists of all solar GOES M- and X-class flare events observed with Atmospheric Imaging Assembly (AIA)/SDO during the first 3.5 years of the SDO mission (2010 June-2014 January), which is identical to those analyzed in Papers I (Aschwanden+, 2014, J/ApJ/797/50), II (Aschwanden+ 2015SoPh..290.2733A) and III (Aschwanden+, 2016, J/ApJ/832/27) for other forms of energies.

(1 data file). Title: Global Energetics of Solar Flares. IV. Coronal Mass Ejection Energetics Authors: Aschwanden, Markus J. Bibcode: 2016ApJ...831..105A Altcode: 2016arXiv160504952A This study entails the fourth part of a global flare energetics project, in which the mass m cme, kinetic energy E kin, and the gravitational potential energy E grav of coronal mass ejections (CMEs) is measured in 399 M and X-class flare events observed during the first 3.5 years of the Solar Dynamics Observatory (SDO) mission, using a new method based on the EUV dimming effect. EUV dimming is modeled in terms of a radial adiabatic expansion process, which is fitted to the observed evolution of the total emission measure of the CME source region. The model derives the evolution of the mean electron density, the emission measure, the bulk plasma expansion velocity, the mass, and the energy in the CME source region. The EUV dimming method is truly complementary to the Thomson scattering method in white light, which probes the CME evolution in the heliosphere at r ≳ 2 R , while the EUV dimming method tracks the CME launch in the corona. We compare the CME parameters obtained in white light with the LASCO/C2 coronagraph with those obtained from EUV dimming with the Atmospheric Imaging Assembly onboard the SDO for all identical events in both data sets. We investigate correlations between CME parameters, the relative timing with flare parameters, frequency occurrence distributions, and the energy partition between magnetic, thermal, nonthermal, and CME energies. CME energies are found to be systematically lower than the dissipated magnetic energies, which is consistent with a magnetic origin of CMEs. Title: Global Energetics of Solar Flares. III. Nonthermal Energies Authors: Aschwanden, Markus J.; Holman, Gordon; O'Flannagain, Aidan; Caspi, Amir; McTiernan, James M.; Kontar, Eduard P. Bibcode: 2016ApJ...832...27A Altcode: 2016arXiv160706488A This study entails the third part of a global flare energetics project, in which Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data of 191 M and X-class flare events from the first 3.5 years of the Solar Dynamics Observatory mission are analyzed. We fit a thermal and a nonthermal component to RHESSI spectra, yielding the temperature of the differential emission measure (DEM) tail, the nonthermal power-law slope and flux, and the thermal/nonthermal cross-over energy e co. From these parameters, we calculate the total nonthermal energy E nt in electrons with two different methods: (1) using the observed cross-over energy e co as low-energy cutoff, and (2) using the low-energy cutoff e wt predicted by the warm thick-target bremsstrahlung model of Kontar et al. Based on a mean temperature of T e = 8.6 MK in active regions, we find low-energy cutoff energies of {e}{wt}=6.2+/- 1.6 {keV} for the warm-target model, which is significantly lower than the cross-over energies {e}{co}=21+/- 6 {keV}. Comparing with the statistics of magnetically dissipated energies E mag and thermal energies E th from the two previous studies, we find the following mean (logarithmic) energy ratios with the warm-target model: {E}{nt}=0.41 {E}{mag}, {E}{th}=0.08 {E}{mag}, and {E}{th}=0.15 {E}{nt}. The total dissipated magnetic energy exceeds the thermal energy in 95% and the nonthermal energy in 71% of the flare events, which confirms that magnetic reconnection processes are sufficient to explain flare energies. The nonthermal energy exceeds the thermal energy in 85% of the events, which largely confirms the warm thick-target model. Title: The Global Energetics of Solar Flares and CMEs Authors: Aschwanden, Markus Bibcode: 2016usc..confE..58A Altcode: We present statistical results of a global flare energetics project, in which the mass, the kinetic energy, and the gravitational potential energy of coronal mass ejections (CMEs) is measured in 399 M and X-class flare events observed during the first 3.5 yrs of the Solar Dynamics Observatory (SDO) mission, using a new method based on the EUV dimming effect. The EUV dimming is modeled in terms of a radial adiabatic expansion process, which is fitted to the observed evolution of the total emission measure of the CME source region. The model derives the evolution of the mean electron density, the emission measure, the bulk plasma expansion velocity, the mass, and the energy in the CME source region. The EUV dimming method is truly complementary to the Thomson scattering method in white light, which probes the CME evolution in the heliosphere at r >2 R_sun, while the EUV dimming method tracks the CME launch in the corona. We compare the CME parameters obtained in white light with the LASCO/C2 coronagraph with those obtained from EUV dimming with AIA) onboard SDO for all identical events in both data sets. We investigate correlations between CME parameters, the relative timing with flare parameters, frequency occurrence distributions, and the energy partition between magnetic, thermal, non-thermal, and CME energies. CME energies are found to be systematically lower than the dissipated magnetic energies, which is consistent with a magnetic origin of CMEs. Title: Tracing the Chromospheric and Coronal Magnetic Field with AIA, IRIS, IBIS, and ROSA Data Authors: Aschwanden, Markus J.; Reardon, Kevin; Jess, Dave B. Bibcode: 2016ApJ...826...61A Altcode: 2016arXiv160202119A The aim of this study is to explore the suitability of chromospheric images for magnetic modeling of active regions. We use high-resolution images (≈ 0\buildrel{\prime\prime}\over{.} 2{--}0\buildrel{\prime\prime}\over{.} 3), from the Interferometric Bidimensional Spectrometer in the Ca II 8542 Å line, the Rapid Oscillations in the Solar Atmosphere instrument in the Hα 6563 Å line, the Interface Region Imaging Spectrograph in the 2796 Å line, and compare non-potential magnetic field models obtained from those chromospheric images with those obtained from images of the Atmospheric Imaging Assembly in coronal (171 Å, etc.) and in chromospheric (304 Å) wavelengths. Curvi-linear structures are automatically traced in those images with the OCCULT-2 code, to which we forward-fitted magnetic field lines computed with the Vertical-current Approximation Nonlinear Force Free Field code. We find that the chromospheric images: (1) reveal crisp curvi-linear structures (fibrils, loop segments, spicules) that are extremely well-suited for constraining magnetic modeling; (2) that these curvi-linear structures are field-aligned with the best-fit solution by a median misalignment angle of {μ }2≈ 4^\circ -7° (3) the free energy computed from coronal data may underestimate that obtained from cromospheric data by a factor of ≈ 2-4, (4) the height range of chromospheric features is confined to h≲ 4000 km, while coronal features are detected up to h = 35,000 km; and (5) the plasma-β parameter is β ≈ {10}-5{--}{10}-1 for all traced features. We conclude that chromospheric images reveal important magnetic structures that are complementary to coronal images and need to be included in comprehensive magnetic field models, something that is currently not accomodated in standard NLFFF codes. Title: The Vertical-current Approximation Nonlinear Force-free Field Code—Description, Performance Tests, and Measurements of Magnetic Energies Dissipated in Solar Flares Authors: Aschwanden, Markus J. Bibcode: 2016ApJS..224...25A Altcode: 2016arXiv160200635A In this work we provide an updated description of the Vertical-Current Approximation Nonlinear Force-Free Field (VCA-NLFFF) code, which is designed to measure the evolution of the potential, non-potential, free energies, and the dissipated magnetic energies during solar flares. This code provides a complementary and alternative method to existing traditional NLFFF codes. The chief advantages of the VCA-NLFFF code over traditional NLFFF codes are the circumvention of the unrealistic assumption of a force-free photosphere in the magnetic field extrapolation method, the capability to minimize the misalignment angles between observed coronal loops (or chromospheric fibril structures) and theoretical model field lines, as well as computational speed. In performance tests of the VCA-NLFFF code, by comparing with the NLFFF code of Wiegelmann, we find agreement in the potential, non-potential, and free energy within a factor of ≲ 1.3, but the Wiegelmann code yields in the average a factor of 2 lower flare energies. The VCA-NLFFF code is found to detect decreases in flare energies in most X, M, and C-class flares. The successful detection of energy decreases during a variety of flares with the VCA-NLFFF code indicates that current-driven twisting and untwisting of the magnetic field is an adequate model to quantify the storage of magnetic energies in active regions and their dissipation during flares. The VCA-NLFFF code is also publicly available in the Solar SoftWare. Title: 25 Years of Self-organized Criticality: Numerical Detection Methods Authors: McAteer, R. T. James; Aschwanden, Markus J.; Dimitropoulou, Michaila; Georgoulis, Manolis K.; Pruessner, Gunnar; Morales, Laura; Ireland, Jack; Abramenko, Valentyna Bibcode: 2016SSRv..198..217M Altcode: 2015SSRv..tmp...31M; 2015arXiv150608142M The detection and characterization of self-organized criticality (SOC), in both real and simulated data, has undergone many significant revisions over the past 25 years. The explosive advances in the many numerical methods available for detecting, discriminating, and ultimately testing, SOC have played a critical role in developing our understanding of how systems experience and exhibit SOC. In this article, methods of detecting SOC are reviewed; from correlations to complexity to critical quantities. A description of the basic autocorrelation method leads into a detailed analysis of application-oriented methods developed in the last 25 years. In the second half of this manuscript space-based, time-based and spatial-temporal methods are reviewed and the prevalence of power laws in nature is described, with an emphasis on event detection and characterization. The search for numerical methods to clearly and unambiguously detect SOC in data often leads us outside the comfort zone of our own disciplines—the answers to these questions are often obtained by studying the advances made in other fields of study. In addition, numerical detection methods often provide the optimum link between simulations and experiments in scientific research. We seek to explore this boundary where the rubber meets the road, to review this expanding field of research of numerical detection of SOC systems over the past 25 years, and to iterate forwards so as to provide some foresight and guidance into developing breakthroughs in this subject over the next quarter of a century. Title: 25 Years of Self-Organized Criticality: Solar and Astrophysics Authors: Aschwanden, Markus J.; Crosby, Norma B.; Dimitropoulou, Michaila; Georgoulis, Manolis K.; Hergarten, Stefan; McAteer, James; Milovanov, Alexander V.; Mineshige, Shin; Morales, Laura; Nishizuka, Naoto; Pruessner, Gunnar; Sanchez, Raul; Sharma, A. Surja; Strugarek, Antoine; Uritsky, Vadim Bibcode: 2016SSRv..198...47A Altcode: 2014arXiv1403.6528A; 2014SSRv..tmp...29A Shortly after the seminal paper "Self-Organized Criticality: An explanation of 1/ f noise" by Bak et al. (1987), the idea has been applied to solar physics, in "Avalanches and the Distribution of Solar Flares" by Lu and Hamilton (1991). In the following years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into the numerical SOC toy models, such as the discretization of magneto-hydrodynamics (MHD) processes. The novel applications stimulated also vigorous debates about the discrimination between SOC models, SOC-like, and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC studies from the last 25 years and highlight new trends, open questions, and future challenges, as discussed during two recent ISSI workshops on this theme. Title: 25 Years of Self-organized Criticality: Space and Laboratory Plasmas Authors: Sharma, A. Surjalal; Aschwanden, Markus J.; Crosby, Norma B.; Klimas, Alexander J.; Milovanov, Alexander V.; Morales, Laura; Sanchez, Raul; Uritsky, Vadim Bibcode: 2016SSRv..198..167S Altcode: Studies of complexity in extended dissipative dynamical systems, in nature and in laboratory, require multiple approaches and the framework of self-organized criticality (SOC) has been used extensively in the studies of such nonequilibrium systems. Plasmas are inherently nonlinear and many ubiquitous features such as multiscale behavior, intermittency and turbulence have been analyzed using SOC concepts. The role of SOC in advancing our understanding of space and laboratory plasmas as nonequilibrium systems is reviewed in this article. The main emphasis is on how SOC and related approaches have provided new insights and models of nonequilibrium plasma phenomena. Among the natural plasmas the magnetosphere, driven by the solar wind, is a prominent example and extensive data from ground-based and space-borne instruments have been used to study phenomena of direct relevance to space weather, viz. geomagnetic storms and substorms. During geomagnetically active periods the magnetosphere is far from equilibrium, due to its internal dynamics and being driven by the turbulent solar wind, and substorms are prominent features of the complex driven system. Studies using solar wind and magnetospheric data have shown both global and multiscale features of substorms. While the global behavior exhibits system-wide changes, the multiscale behavior shows scaling features. Along with the studies based on observational data, analogue models of the magnetosphere have advanced the understanding of space plasmas as well as the role of SOC in natural systems. In laboratory systems, SOC has been used in modeling the plasma behavior in fusion experiments, mainly in tokamaks and stellarators. Tokamaks are the dominant plasma confinement system and modeling based on SOC have provided a complementary approach to the understanding of plasma behavior under fusion conditions. These studies have provided insights into key features of toroidally confined plasmas, e.g., the existence of critical temperature gradients above which the transport rates increase drastically. The SOC models address the transport properties from a more general approach, compared to those based on turbulence arising from specific plasma instabilities, and provide a better framework for modeling features such as superdiffusion. The studies of space and laboratory plasmas as nonequilibrium systems have been motivated by features such as scaling and critical behavior, and have provided new insights by highlighting the properties that are common with other systems. Title: Preface Authors: Aschwanden, Markus J. Bibcode: 2016SSRv..198....1A Altcode: 2015SSRv..tmp..112A No abstract at ADS Title: A Self-Critique of Self-Organized Criticality in Astrophysics Authors: Aschwanden, Markus J. Bibcode: 2016IAUFM..29B.735A Altcode: The concept of ``self-organized criticality'' (SOC) was originally proposed as an explanation of 1/f-noise by Bak, Tang, and Wiesenfeld (1987), but turned out to have a far broader significance for scale-free nonlinear energy dissipation processes occurring in the entire universe. Over the last 30 years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into numerical SOC toy models. The novel applications stimulated also vigorous debates about the discrimination between SOC-related and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC models applied to astrophysical observations, attempt to describe what physics can be captured by SOC models, and offer a critique of weaknesses and strengths in existing SOC models. Title: Thresholded Power law Size Distributions of Instabilities in Astrophysics Authors: Aschwanden, Markus J. Bibcode: 2015ApJ...814...19A Altcode: 2015arXiv151001987A Power-law-like size distributions are ubiquitous in astrophysical instabilities. There are at least four natural effects that cause deviations from ideal power law size distributions, which we model here in a generalized way: (1) a physical threshold of an instability; (2) incomplete sampling of the smallest events below a threshold x0; (3) contamination by an event-unrelated background xb; and (4) truncation effects at the largest events due to a finite system size. These effects can be modeled in the simplest terms with a “thresholded power law” distribution function (also called generalized Pareto [type II] or Lomax distribution), N(x){dx}\propto {(x+{x}0)}-a{dx}, where x0 > 0 is positive for a threshold effect, while x0 < 0 is negative for background contamination. We analytically derive the functional shape of this thresholded power law distribution function from an exponential growth evolution model, which produces avalanches only when a disturbance exceeds a critical threshold x0. We apply the thresholded power law distribution function to terrestrial, solar (HXRBS, BATSE, RHESSI), and stellar flare (Kepler) data sets. We find that the thresholded power law model provides an adequate fit to most of the observed data. Major advantages of this model are the automated choice of the power law fitting range, diagnostics of background contamination, physical instability thresholds, instrumental detection thresholds, and finite system size limits. When testing self-organized criticality models that predict ideal power laws, we suggest including these natural truncation effects. Title: Benchmark Test of Differential Emission Measure Codes and Multi-thermal Energies in Solar Active Regions Authors: Aschwanden, Markus J.; Boerner, Paul; Caspi, Amir; McTiernan, James M.; Ryan, Daniel; Warren, Harry Bibcode: 2015SoPh..290.2733A Altcode: 2015arXiv150907546A; 2015SoPh..tmp..146A We compare the ability of 11 differential emission measure (DEM) forward-fitting and inversion methods to constrain the properties of active regions and solar flares by simulating synthetic data using the instrumental response functions of the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) and EUV Variability Experiment (SDO/EVE), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and the Geostationary Operational Environmental Satellite/X-ray Sensor (GOES/XRS). The codes include the single-Gaussian DEM, a bi-Gaussian DEM, a fixed-Gaussian DEM, a linear spline DEM, the spatial-synthesis DEM, the Monte-Carlo Markov Chain DEM, the regularized DEM inversion, the Hinode/X-Ray Telescope (XRT) method, a polynomial spline DEM, an EVE+GOES, and an EVE+RHESSI method. Averaging the results from all 11 DEM methods, we find the following accuracies in the inversion of physical parameters: the EM-weighted temperature Twfit/Twsim=0.9 ±0.1 , the peak emission measure EMpfit/EMpsim=0.6 ±0.2 , the total emission measure EMtfit/EMtsim=0.8 ±0.3 , and the multi-thermal energies Ethfit/EMthapprox=1.2 ±0.4 . We find that the AIA spatial-synthesis, the EVE+GOES, and the EVE+RHESSI method yield the most accurate results. Title: Blind Stereoscopy of the Coronal Magnetic Field Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Malanushenko, Anna Bibcode: 2015SoPh..290.2765A Altcode: 2015SoPh..tmp..147A; 2015arXiv150604713A We test the feasibility of 3D coronal-loop tracing in stereoscopic EUV image pairs, with the ultimate goal of enabling efficient 3D reconstruction of the coronal magnetic field that drives flares and coronal mass ejections (CMEs). We developed an automated code designed to perform triangulation of coronal loops in pairs (or triplets) of EUV images recorded from different perspectives. The automated (or blind) stereoscopy code includes three major tasks: i) automated pattern recognition of coronal loops in EUV images, ii) automated pairing of corresponding loop patterns from two different aspect angles, and iii) stereoscopic triangulation of 3D loop coordinates. We perform tests with simulated stereoscopic EUV images and quantify the accuracy of all three procedures. In addition we test the performance of the blind-stereoscopy code as a function of the spacecraft-separation angle and as a function of the spatial resolution. We also test the sensitivity to magnetic non-potentiality. The automated code developed here can be used for analysis of existing Solar TErrestrial RElationship Observatory (STEREO) data, but primarily serves for a design study of a future mission with dedicated diagnostics of non-potential magnetic fields. For a pixel size of 0.6 (corresponding to the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) spatial resolution of 1.4), we find an optimum spacecraft-separation angle of αs≈5. Title: A Self-Critique of Self-Organized Criticality in Astrophysics Authors: Aschwanden, Markus J. Bibcode: 2015IAUGA..2255145A Altcode: The concept of ``self-organized criticality'' (SOC) was originally proposed as an explanation of 1/f-noise by Bak, Tang, and Wiesenfeld (1987), but turned out to have a far broader significance for scale-free nonlinear energy dissipation processes occurring in the entire universe. Over the last 30 years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into numerical SOC toy models. The novel applications stimulated also vigorous debates about the discrimination between SOC-related and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC models applied to astrophysical observations, attempt to describe what physics can be captured by SOC models, and offer a critique of weaknesses and strengths in existing SOC models. Title: VizieR Online Data Catalog: Global energetics of solar flares. II. (Aschwanden+, 2015) Authors: Aschwanden, M. J.; Boerner, P.; Ryan, D.; Caspi, A.; McTiernan, J. M.; Warren, H. P. Bibcode: 2015yCat..18020053A Altcode: The dataset we are analyzing for this project on the global energetics of flares includes all M- and X-class flares observed with the Solar Dynamics Observatory (SDO) during the first 3.5yr of the mission (2010 June 1 to 2014 January 31), which amounts to 399 flare events, as described in Paper I (Aschwanden et al. 2014, J/ApJ/797/50). We attempt to calculate the thermal energies in all 399 cataloged events, but we encountered eight events with incomplete or corrupted Atmospheric Imaging Assembly (AIA) data, so that we are left with 391 events suitable for thermal data analysis.

AIA provides EUV images corresponding to an effective spatial resolution of ~1.6".

(1 data file). Title: VizieR Online Data Catalog: Global energetics of solar flares. I. (Aschwanden+, 2014) Authors: Aschwanden, M. J.; Xu, Y.; Jing, J. Bibcode: 2015yCat..17970050A Altcode: The data set we are analyzing for this project on the global energetics of flares includes all M- and X-class flares observed with the Solar Dynamics Observatory (SDO) during the first 3.5yr of the mission (2010 June 1 to 2014 January 31), which amounts to 399 flare events. Magnetic energies are determined for events that have a heliographic longitude of <~45° (177 events), of which 5 events contained incomplete or corrupted Atmospheric Imaging Assembly (AIA) data, so that we are left with 172 events suitable for magnetic data analysis.

The analyzed SDO data set includes EUV images observed with the AIA, as well as magnetograms from the Helioseismic and Magnetic Imager (HMI). The SDO started observations on 2010 March 29 and has produced essentially continuous data of the full Sun since then.

(1 data file). Title: Magnetic Energy Dissipation during the 2014 March 29 Solar Flare Authors: Aschwanden, Markus J. Bibcode: 2015ApJ...804L..20A Altcode: 2015arXiv150403301A We calculated the time evolution of the free magnetic energy during the 2014 March 29 flare (SOL2014 March 29T17:48), the first X-class flare detected by the Interface Region Imaging Spectrograph (IRIS). The free energy was calculated from the difference between the nonpotential field, constrained by the geometry of observed loop structures, and the potential field. We use Atmospheric Imager Assembly (AIA)/SDO and IRIS images to delineate the geometry of coronal loops in EUV wavelengths, as well as to trace magnetic field directions in UV wavelengths in the chromosphere and transition region. We find an identical evolution of the free energy for both the coronal and chromospheric tracers, as well as agreement between AIA and IRIS results, with a peak free energy of {{E}free}({{t}peak})≈ (45+/- 2)× {{10}30} erg, which decreases by an amount of {Δ }{{E}free}≈ (29+/- 3)× {{10}30} erg during the flare decay phase. The consistency of free energies measured from different EUV and UV wavelengths for the first time here, demonstrates that vertical electric currents (manifested in form of helically twisted loops) can be detected and measured from both chromospheric and coronal tracers. Title: Multi-thermal Energies of Solar Flares Authors: Ryan, Daniel; Aschwanden, Markus; Boerner, Paul; Caspi, Amir; McTiernan, James; Warren, Harry Bibcode: 2015TESS....130215R Altcode: Measuring energy partition in solar eruptions is key to understanding how different processes affect their evolution. In order to improve our knowledge on this topic, we are participating in a multi-study project to measure the energy partition of 400 M- and X-class flares and associated coronal mass ejections (CMEs). In this study we focus on the flare thermal energies of 391 of these events. We improve upon previous studies in the following ways: 1) We determine thermal energy using spatially resolved multi-thermal differential emission measures (DEMs) determined from AIA (Atmospheric Imaging Assembly) rather than relying on the isothermal assumption; 2) We determine flare volumes by thresholding these DEM maps rather than relying on single passband observations which may not show the full flare volume; 3) We analyze a greater number of events than previous similar studies to increase the statistical reliability of our results. We find that the thermal energies of these flares lie in the range 10^26.8—10^32 erg. These results are compared to those of Aschwanden et al. (2014) who examined a subset of these events. They determined the dissipated non-potential magnetic energy which is thought to be the total energy available to drive solar eruptions. For the 171 events common to both studies, we find that the ratio of flare thermal energy to dissipated magnetic energy ranges from 2%—40%. This is an order of magnitude higher than previously found by Emslie et al. (2012). This may be because Emslie et al. (2012) had to assume the amount of non-potential magnetic energy, or that they relied on the isothermal assumption to determine flare thermal energies. The improved results found here will help us better understand the role played by flare thermal processes in dissipating the overall energy of solar eruptions. Title: Magnetic and Hydrodynamic Energy Scaling Laws in Solar Flares Authors: Aschwanden, Markus; Boerner, Paul; Xu, Yan; Ju, Jing; Ryan, Dan; Caspi, Amir; McTiernan, James; Warren, Harry Bibcode: 2015TESS....140603A Altcode: We determine the dissipated non-potential magnetic energy and measure the multi-thermal energy in a sample of about 400 M and X-class flares observed with AIA and HMI during the first 4 years of the SDO mission. The free energy is determined with two nonlinear force-free field (NLFFF) models, one is based on the 3D vectorphotospheric magnetic field and the other uses forward-fitting of a vertical-current model to automatically traced coronal loops.The multi-thermal energy is measured with a spatial-synthesis differential emission measure (DEM) code, which yields a more comprehensive multi-thermal energy (being larger by an averagefactor of 14) than iso-thermal estimates. We show how the correlations and powerlaw-like size distributions of energies and other geometrical and physical parameters reveal magnetic and hydrodynamic scaling lawsthat are in agreement with recent statistical models of nonlinear dissipative systems governed by self-organized criticality. Title: Global Energetics of Solar Flares: II. Thermal Energies Authors: Aschwanden, Markus J.; Boerner, Paul; Ryan, Daniel; Caspi, Amir; McTiernan, James M.; Warren, Harry P. Bibcode: 2015ApJ...802...53A Altcode: 2015arXiv150205941A We present the second part of a project on the global energetics of solar flares and coronal mass ejections that includes about 400 M- and X-class flares observed with the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) during the first 3.5 yr of its mission. In this Paper II we compute the differential emission measure (DEM) distribution functions and associated multithermal energies, using a spatially-synthesized Gaussian DEM forward-fitting method. The multithermal DEM function yields a significantly higher (by an average factor of ≈14), but more comprehensive (multi-)thermal energy than an isothermal energy estimate from the same AIA data. We find a statistical energy ratio of {{E}th}/{{E}diss} ≈ 2-40% between the multithermal energy Eth and the magnetically dissipated energy Ediss, which is an order of magnitude higher than the estimates of Emslie et al. 2012. For the analyzed set of M- and X-class flares we find the following physical parameter ranges: L={{10}8.2}{{-10}9.7} cm for the length scale of the flare areas, {{T}p}={{10}5.7}{{-10}7.4} K for the DEM peak temperature, {{T}w}={{10}6.8}{{-10}7.6} K for the emission measure-weighted temperature, {{n}p}={{10}10.3}-{{10}11.8} cm-3 for the average electron density, E{{M}p}={{10}47.3}-{{10}50.3} cm-3 for the DEM peak emission measure, and {{E}th}={{10}26.8}-{{10}32.0} erg for the multithermal energies. The deduced multithermal energies are consistent with the RTV scaling law {{E}th,RTV}=7.3× {{10}-10} Tp3Lp2, which predicts extremal values of {{E}th,max }≈ 1.5× {{10}33} erg for the largest flare and {{E}th,min }≈ 1× {{10}24} erg for the smallest coronal nanoflare. The size distributions of the spatial parameters exhibit powerlaw tails that are consistent with the predictions of the fractal-diffusive self-organized criticality model combined with the RTV scaling law. Title: Magnetic Energies in Solar Active Regions and Flares Calculated from Automated Coronal Loop Tracing Authors: Aschwanden, M. J. Bibcode: 2014AGUFMSH13A4073A Altcode: Magnetic energies contained in solar active regions or dissipated in flares can now be calculated from coronal images (such as from AIA/SDO) and line-of-sight magnetograms (such as from HM I/SDO). The magnetogram provides a potential field solution, while automated tracing of coronal loops in different EUV wavelengths provide the misalignment angles between potential field lines and non-potential field lines. We present an automated code that uses data from AIA and HMI to calculate the free energy and dissipated energy in solar flares, based on a nonlinear force-free field approximation in terms of vertical currents that produce helical twists of coronal loops. We study the time evolution of free energy and energy dissipation during some 200 solar flares and compare it with the global energetics of flare and CME energies. The occurrence frequency distributions of dissipated magnetic energies follow closely the predicted powerlaw distribution functions of self-organized criticality models. The presented results provide for the first time statistics on magnetic energies dissipated in solar flares. Title: Global Energetics of Solar Flares. I. Magnetic Energies Authors: Aschwanden, Markus J.; Xu, Yan; Jing, Ju Bibcode: 2014ApJ...797...50A Altcode: 2014arXiv1410.8013A We present the first part of a project on the global energetics of solar flares and coronal mass ejections that includes about 400 M- and X-class flares observed with Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). We calculate the potential (Ep ), the nonpotential (E np) or free energies (E free = E np - Ep ), and the flare-dissipated magnetic energies (E diss). We calculate these magnetic parameters using two different NLFFF codes: the COR-NLFFF code uses the line-of-sight magnetic field component Bz from HMI to define the potential field, and the two-dimensional (2D) coordinates of automatically detected coronal loops in six coronal wavelengths from AIA to measure the helical twist of coronal loops caused by vertical currents, while the PHOT-NLFFF code extrapolates the photospheric three-dimensional (3D) vector fields. We find agreement between the two codes in the measurement of free energies and dissipated energies within a factor of <~ 3. The size distributions of magnetic parameters exhibit powerlaw slopes that are approximately consistent with the fractal-diffusive self-organized criticality model. The magnetic parameters exhibit scaling laws for the nonpotential energy, Enp \propto E_p1.02, for the free energy, Efree \propto E_p1.7 and Efree \propto B\varphi 1.0 L1.5, for the dissipated energy, Ediss \propto E_p1.6 and Ediss \propto Efree0.9, and the energy dissipation volume, V \propto Ediss1.2. The potential energies vary in the range of Ep = 1 × 1031-4 × 1033 erg, while the free energy has a ratio of E free/Ep ≈ 1%-25%. The Poynting flux amounts to F flare ≈ 5 × 108-1010 erg cm-2 s-1 during flares, which averages to F AR ≈ 6 × 106 erg cm-2 s-1 during the entire observation period and is comparable with the coronal heating rate requirement in active regions. Title: Homologous flare-CME events and their metric type II radio burst association Authors: Yashiro, S.; Gopalswamy, N.; Mäkelä, P.; Akiyama, S.; Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Chandra, R.; Manoharan, P. K.; Mahalakshmi, K.; Dwivedi, V. C.; Jain, R.; Awasthi, A. K.; Nitta, N. V.; Aschwanden, M. J.; Choudhary, D. P. Bibcode: 2014AdSpR..54.1941Y Altcode: Active region NOAA 11158 produced many flares during its disk passage. At least two of these flares can be considered as homologous: the C6.6 flare at 06:51 UT and C9.4 flare at 12:41 UT on February 14, 2011. Both flares occurred at the same location (eastern edge of the active region) and have a similar decay of the GOES soft X-ray light curve. The associated coronal mass ejections (CMEs) were slow (334 and 337 km/s) and of similar apparent widths (43° and 44°), but they had different radio signatures. The second event was associated with a metric type II burst while the first one was not. The COR1 coronagraphs on board the STEREO spacecraft clearly show that the second CME propagated into the preceding CME that occurred 50 min before. These observations suggest that CME-CME interaction might be a key process in exciting the type II radio emission by slow CMEs. Title: Determining the 3D Structure of the Corona Using Vertical Height Constraints on Observed Active Region Loops Authors: Gary, G. Allen; Hu, Qiang; Lee, Jong Kwan; Aschwanden, Markus J. Bibcode: 2014SoPh..289.3703G Altcode: 2014SoPh..tmp...87G The corona associated with an active region is structured by high-temperature, magnetically dominated closed and open loops. The projected 2D geometry of these loops is captured in EUV filtergrams. In this study using SDO/AIA 171 Å filtergrams, we expand our previous method to derive the 3D structure of these loops, independent of heliostereoscopy. We employ an automated loop recognition scheme (Occult-2) and fit the extracted loops with 2D cubic Bézier splines. Utilizing SDO/HMI magnetograms, we extrapolate the magnetic field to obtain simple field models within a rectangular cuboid. Using these models, we minimize the misalignment angle with respect to Bézier control points to extend the splines to 3D (Gary, Hu, and Lee 2014). The derived Bézier control points give the 3D structure of the fitted loops. We demonstrate the process by deriving the position of 3D coronal loops in three active regions (AR 11117, AR 11158, and AR 11283). The numerical minimization process converges and produces 3D curves which are consistent with the height of the loop structures when the active region is seen on the limb. From this we conclude that the method can be important in both determining estimates of the 3D magnetic field structure and determining the best magnetic model among competing advanced magnetohydrodynamics or force-free magnetic-field computer simulations. Title: The Compatibility of Flare Temperatures Observed with AIA, GOES, and RHESSI Authors: Ryan, Daniel F.; O'Flannagain, Aidan M.; Aschwanden, Markus J.; Gallagher, Peter T. Bibcode: 2014SoPh..289.2547R Altcode: 2014SoPh..tmp...31R; 2014arXiv1401.4098R We test the compatibility and biases of multi-thermal flare DEM (differential emission measure) peak temperatures determined with AIA with those determined by GOES and RHESSI using the isothermal assumption. In a set of 149 M- and X-class flares observed during the first two years of the SDO mission, AIA finds DEM peak temperatures at the time of the peak GOES 1 - 8 Å flux to have an average of Tp=12.0±2.9 MK and Gaussian DEM widths of log10T)=0.50±0.13. From GOES observations of the same 149 events, a mean temperature of Tp=15.6±2.4 MK is inferred, which is systematically higher by a factor of TGOES/TAIA=1.4±0.4. We demonstrate that this discrepancy results from the isothermal assumption in the inversion of the GOES filter ratio. From isothermal fits to photon spectra at energies of ϵ≈6 - 12 keV of 61 of these events, RHESSI finds the temperature to be higher still by a factor of TRHESSI/TAIA=1.9±1.0. We find that this is partly a consequence of the isothermal assumption. However, RHESSI is not sensitive to the low-temperature range of the DEM peak, and thus RHESSI samples only the high-temperature tail of the DEM function. This can also contribute to the discrepancy between AIA and RHESSI temperatures. The higher flare temperatures found by GOES and RHESSI imply correspondingly lower emission measures. We conclude that self-consistent flare DEM temperatures and emission measures require simultaneous fitting of EUV (AIA) and soft X-ray (GOES and RHESSI) fluxes. Title: The Compatibility of Flare Temperatures Observed with AIA, GOES, and RHESSI Authors: Ryan, Daniel; Aschwanden, Markus J.; O'Flannagain, Aidan M; Gallagher, Peter T Bibcode: 2014AAS...22412337R Altcode: In this talk we compare multi-thermal flare DEM peak temperatures determined with SDO/AIA with those determined by GOES/XRS and RHESSI using the isothermal assumption. In a set of 149 M- and X-class flares, AIA finds an average DEM peak temperature at the time of the GOES long channel peak of 12.0±2.9 MK and Gaussian DEM widths of log10(σT ) = 0.50±0.13. From GOES observations of the same 149 events, a mean temperature of 15.6±2.4 MK is inferred, which is higher by a factor of TGOES/TAIA = 1.4±0.4. We demonstrate that this discrepancy results from the isothermal assumption in the inversion of the GOES filter ratio. From isothermal fits to photon spectra at energies of 6-12 keV of 61 of these events, RHESSI finds the temperature to be higher (TRHESSI/TAIA = 1.9±1.0). We find that this is partly a consequence of the isothermal assumption. However, RHESSI is not sensitive to the low-temperature range of the DEM peak, and thus only samples the DEM’s high-temperature tail. This is expected to be the cause of further discrepancies. We conclude that self-consistent flare DEM temperatures require simultaneous fitting of EUV and SXR fluxes. Title: Magnetic Energy Dissipation in 200 Solar Flares Measured with SDO Authors: Aschwanden, Markus J. Bibcode: 2014AAS...22412306A Altcode: We present the first statistical study of magnetic energetics in solar flares. The amount of dissipated magnetic energy during solar flares provides the fundamental limit on the flare energy budget that is partitioned into the kinetic and potential energy of CMEs, acceleration of nonthermal particles, and radiation in soft X-rays, EUV, UV, and bolometric luminosity. The determination of the dissipated magnetic energy requires the calculation of nonlinear force-free field (NLFFF) solutions during flares,which can quantify the nonpotential E_N(t), the potential E_P(t), and the free magnetic energy E_{free}(t)=E_N(t)-E_P(t), which itself represents an upper limit on the magnetic energy dE_diss that can be dissipated during a flare. Here we developed a NLFFF forward-fitting code that fits a nonpotential field in terms of vertical currents with helically twisted field lines to automatically traced coronal loops from 7 AIA wavelength filters and apply it to 200 M- and X-class flares that havebeen observed during the first 4 years of the Solar Dynamics Observatory (SDO) mission. We cacluate the free energy with a cadence of 6 minutes during all 200 flares, and find significantmagnetic energy decreases dE_diss in almost all flares, in the order of E_diss ~ 10(31)-10(32) erg, which amounts to a fraction of dE_diss/E_P ~ 0.01-0.3 of the potential magnetic energy E_P. We find that the dissipated energy dE_diss cannot simply be determined by an energy difference before and after the flare, because the hydrodynamic evolution causes brightenings and dimmings of helically twisted loops (sigmoids) in the flare core region, which acts as a time-dependent illumination effect of nonpotential loop structures. Title: The Association of Solar Flares with Coronal Mass Ejections During the Extended Solar Minimum Authors: Nitta, N. V.; Aschwanden, M. J.; Freeland, S. L.; Lemen, J. R.; Wülser, J. -P.; Zarro, D. M. Bibcode: 2014SoPh..289.1257N Altcode: 2013arXiv1308.1465N We study the association of solar flares with coronal mass ejections (CMEs) during the deep, extended solar minimum of 2007 - 2009, using extreme-ultraviolet (EUV) and white-light (coronagraph) images from the Solar Terrestrial Relations Observatory (STEREO). Although all of the fast (v>900 km s−1), wide (θ>100) CMEs are associated with a flare that is at least identified in GOES soft X-ray light curves, a majority of flares with relatively high X-ray intensity for the deep solar minimum (e.g. ≳1×10−6 W m−2 or C1) are not associated with CMEs. Intense flares tend to occur in active regions with a strong and complex photospheric magnetic field, but the active regions that produce CME-associated flares tend to be small, including those that have no sunspots and therefore no NOAA active-region numbers. Other factors on scales similar to and larger than active regions seem to exist that contribute to the association of flares with CMEs. We find the possible low coronal signatures of CMEs, namely eruptions, dimmings, EUV waves, and Type III bursts, in 91 %, 74 %, 57 %, and 74 %, respectively, of the 35 flares that we associate with CMEs. None of these observables can fully replace direct observations of CMEs by coronagraphs. Title: The Magnetic Field of Active Region 11158 during the 2011 February 12-17 Flares: Differences between Photospheric Extrapolation and Coronal Forward-Fitting Methods Authors: Aschwanden, Markus J.; Sun, Xudong; Liu, Yang Bibcode: 2014ApJ...785...34A Altcode: 2014arXiv1402.5340A We developed a coronal nonlinear force-free field (COR-NLFFF) forward-fitting code that fits an approximate nonlinear force-free field (NLFFF) solution to the observed geometry of automatically traced coronal loops. In contrast to photospheric NLFFF codes, which calculate a magnetic field solution from the constraints of the transverse photospheric field, this new code uses coronal constraints instead, and this way provides important information on systematic errors of each magnetic field calculation method, as well as on the non-force-freeness in the lower chromosphere. In this study we applied the COR-NLFFF code to NOAA Active Region 11158, during the time interval of 2011 February 12-17, which includes an X2.2 GOES-class flare plus 35 M- and C-class flares. We calculated the free magnetic energy with a 6 minute cadence over 5 days. We find good agreement between the two types of codes for the total nonpotential EN and potential energy EP but find up to a factor of 4 discrepancy in the free energy E free = EN - EP and up to a factor of 10 discrepancy in the decrease of the free energy ΔE free during flares. The coronal NLFFF code exhibits a larger time variability and yields a decrease of free energy during the flare that is sufficient to satisfy the flare energy budget, while the photospheric NLFFF code shows much less time variability and an order of magnitude less free-energy decrease during flares. The discrepancy may partly be due to the preprocessing of photospheric vector data but more likely is due to the non-force-freeness in the lower chromosphere. We conclude that the coronal field cannot be correctly calculated on the basis of photospheric data alone and requires additional information on coronal loop geometries. Title: STEREO/ Extreme Ultraviolet Imager (EUVI) Event Catalog 2006 - 2012 Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre; Nitta, Nariaki V.; Lemen, James R.; Freeland, Sam; Thompson, William T. Bibcode: 2014SoPh..289..919A Altcode: 2013arXiv1306.3180A We generated an event catalog with an automated detection algorithm based on the entire EUVI image database observed with the two Solar Terrestrial Relations Observatory (STEREO)-A and -B spacecraft over the first six years of the mission (2006 - 2012). The event catalog includes the heliographic positions of some 20 000 EUV events, transformed from spacecraft coordinates to Earth-based coordinates, and information on associated GOES flare events (down to the level of GOES A5-class flares). The 304 Å wavelength turns out to be the most efficient channel for flare detection (79 % of all EUVI event detections), while the 171 Å (4 %), 195 Å (10 %), and the 284 Å channel (7 %) retrieve substantially fewer flare events, partially due to the suppressing effect of EUV dimming, and partially due to the lower cadence in the later years of the mission. Due to the Sun-circling orbits of STEREO-A and -B, a large number of flares have been detected on the farside of the Sun, invisible from Earth, or seen as partially occulted events. The statistical size distributions of EUV peak fluxes (with a power-law slope of αP=2.5±0.2) and event durations (with a power-law slope of αT=2.4±0.3) are found to be consistent with the fractal-diffusive self-organized criticality model. The EUVI event catalog is available on-line at secchi.lmsal.com/EUVI/euvi_autodetection/euvi_events.txt and may serve as a comprehensive tool to identify stereoscopically observed flare events for 3D reconstruction and to study occulted flare events. Title: A Macroscopic Description of a Generalized Self-organized Criticality System: Astrophysical Applications Authors: Aschwanden, Markus J. Bibcode: 2014ApJ...782...54A Altcode: 2013arXiv1310.4191A We suggest a generalized definition of self-organized criticality (SOC) systems: SOC is a critical state of a nonlinear energy dissipation system that is slowly and continuously driven toward a critical value of a system-wide instability threshold, producing scale-free, fractal-diffusive, and intermittent avalanches with power law-like size distributions. We develop here a macroscopic description of SOC systems that provides an equivalent description of the complex microscopic fine structure, in terms of fractal-diffusive transport (FD-SOC). Quantitative values for the size distributions of SOC parameters (length scales L, time scales T, waiting times Δt, fluxes F, and fluences or energies E) are derived from first principles, using the scale-free probability conjecture, N(L)dLvpropL -d , for Euclidean space dimension d. We apply this model to astrophysical SOC systems, such as lunar craters, the asteroid belt, Saturn ring particles, magnetospheric substorms, radiation belt electrons, solar flares, stellar flares, pulsar glitches, soft gamma-ray repeaters, black-hole objects, blazars, and cosmic rays. The FD-SOC model predicts correctly the size distributions of 8 out of these 12 astrophysical phenomena, and indicates non-standard scaling laws and measurement biases for the others. Title: Multiwavelength diagnostics of the precursor and main phases of an M1.8 flare on 2011 April 22 Authors: Awasthi, A. K.; Jain, R.; Gadhiya, P. D.; Aschwanden, M. J.; Uddin, W.; Srivastava, A. K.; Chandra, R.; Gopalswamy, N.; Nitta, N. V.; Yashiro, S.; Manoharan, P. K.; Choudhary, D. P.; Joshi, N. C.; Dwivedi, V. C.; Mahalakshmi, K. Bibcode: 2014MNRAS.437.2249A Altcode: 2013arXiv1310.6029A; 2013MNRAS.tmp.2720A We study the temporal, spatial and spectral evolution of the M1.8 flare, which occurred in the active region 11195 (S17E31) on 2011 April 22, and explore the underlying physical processes during the precursor phase and their relation to the main phase. The study of the source morphology using the composite images in 131 Å wavelength observed by the Solar Dynamics Observatory/Atmospheric Imaging Assembly and 6-14 keV [from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)] revealed a multiloop system that destabilized systematically during the precursor and main phases. In contrast, hard X-ray emission (20-50 keV) was absent during the precursor phase, appearing only from the onset of the impulsive phase in the form of foot-points of emitting loops. This study also revealed the heated loop-top prior to the loop emission, although no accompanying foot-point sources were observed during the precursor phase. We estimate the flare plasma parameters, namely temperature (T), emission measure (EM), power-law index (γ) and photon turn-over energy (ɛto), and found them to be varying in the ranges 12.4-23.4 MK, 0.0003-0.6 × 1049 cm-3, 5-9 and 14-18 keV, respectively, by forward fitting RHESSI spectral observations. The energy released in the precursor phase was thermal and constituted ≈1 per cent of the total energy released during the flare. The study of morphological evolution of the filament in conjunction with synthesized T and EM maps was carried out, which reveals (a) partial filament eruption prior to the onset of the precursor emission and (b) heated dense plasma over the polarity inversion line and in the vicinity of the slowly rising filament during the precursor phase. Based on the implications from multiwavelength observations, we propose a scheme to unify the energy release during the precursor and main phase emissions in which the precursor phase emission was originated via conduction front that resulted due to the partial filament eruption. Next, the heated leftover S-shaped filament underwent slow-rise and heating due to magnetic reconnection and finally erupted to produce emission during the impulsive and gradual phases. Title: Soft X-ray Fluxes of Major Flares Far Behind the Limb as Estimated Using STEREO EUV Images Authors: Nitta, N. V.; Aschwanden, M. J.; Boerner, P. F.; Freeland, S. L.; Lemen, J. R.; Wuelser, J. -P. Bibcode: 2013SoPh..288..241N Altcode: 2013arXiv1304.4163N With increasing solar activity since 2010, many flares from the backside of the Sun have been observed by the Extreme Ultraviolet Imager (EUVI) on either of the twin STEREO spacecraft. Our objective is to estimate their X-ray peak fluxes from EUVI data by finding a relation of the EUVI with GOES X-ray fluxes. Because of the presence of the Fe XXIV line at 192 Å, the response of the EUVI 195 Å channel has a secondary broad peak around 15 MK, and its fluxes closely trace X-ray fluxes during the rise phase of flares. If the flare plasma is isothermal, the EUVI flux should be directly proportional to the GOES flux. In reality, the multithermal nature of the flare and other factors complicate the estimation of the X-ray fluxes from EUVI observations. We discuss the uncertainties, by comparing GOES fluxes with the high cadence EUV data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We conclude that the EUVI 195 Å data can provide estimates of the X-ray peak fluxes of intense flares (e.g., above M4 in the GOES scale) to small uncertainties. Lastly we show examples of intense flares from regions far behind the limb, some of which show eruptive signatures in AIA images. Title: Multi-wavelength Observations of the Spatio-temporal Evolution of Solar Flares with AIA/SDO. II. Hydrodynamic Scaling Laws and Thermal Energies Authors: Aschwanden, Markus J.; Shimizu, Toshifumi Bibcode: 2013ApJ...776..132A Altcode: 2013arXiv1308.5198A In this study we measure physical parameters of the same set of 155 M- and X-class solar flares observed with AIA/SDO as analyzed in Paper I, by performing a differential emission measure analysis to determine the flare peak emission measure EM p , peak temperature Tp , electron density np , and thermal energy E th, in addition to the spatial scales L, areas A, and volumes V measured in Paper I. The parameter ranges for M- and X-class flares are log (EM p ) = 47.0-50.5, Tp = 5.0-17.8 MK, np = 4 × 109-9 × 1011 cm-3, and thermal energies of E th = 1.6 × 1028-1.1 × 1032 erg. We find that these parameters obey the Rosner-Tucker-Vaiana (RTV) scaling law T_p^2 \propto n_p L and HvpropT 7/2 L -2 during the peak time tp of the flare density np , when energy balance between the heating rate H and the conductive and radiative loss rates is achieved for a short instant and thus enables the applicability of the RTV scaling law. The application of the RTV scaling law predicts power-law distributions for all physical parameters, which we demonstrate with numerical Monte Carlo simulations as well as with analytical calculations. A consequence of the RTV law is also that we can retrieve the size distribution of heating rates, for which we find N(H)vpropH -1.8, which is consistent with the magnetic flux distribution N(Φ)vpropΦ-1.85 observed by Parnell et al. and the heating flux scaling law FH vpropHLvpropB/L of Schrijver et al.. The fractal-diffusive self-organized criticality model in conjunction with the RTV scaling law reproduces the observed power-law distributions and their slopes for all geometrical and physical parameters and can be used to predict the size distributions for other flare data sets, instruments, and detection algorithms. Title: A Nonlinear Force-Free Magnetic Field Approximation Suitable for Fast Forward-Fitting to Coronal Loops. III. The Free Energy Authors: Aschwanden, Markus J. Bibcode: 2013SoPh..287..369A Altcode: 2012arXiv1211.1708A; 2012SoPh..tmp..329A An analytical approximation of a nonlinear force-free magnetic field (NLFFF) solution was developed in Paper I, while a numerical code that performs fast forward-fitting of this NLFFF approximation to a line-of-sight magnetogram and coronal 3D loops has been described and tested in Paper II. Here we calculate the free magnetic energy Efree=EN−EP, i.e., the difference of the magnetic energies between the non-potential field and the potential field. A second method to estimate the free energy is obtained from the mean misalignment angle change Δμ=μP−μN between the potential and non-potential field, which scales as Efree/EP≈tan2(Δμ). For the four active regions observed with STEREO in 2007 we find free energies in the range of qfree=(Efree/EP)≈1 % - 10 %, with an uncertainty of less than ± 2 % between the two methods, while the free energies obtained from 11 other NLFFF codes exhibit a larger scatter of about ± 10 %. We also find a correlation between the free magnetic energy and the GOES flux of the largest flare that occurred during the observing period, which can be quantified by an exponential relationship, FGOES∝exp(qfree/0.015), implying an exponentiation of the dissipated currents. Title: A Nonlinear Force-Free Magnetic Field Approximation Suitable for Fast Forward-Fitting to Coronal Loops. I. Theory Authors: Aschwanden, Markus J. Bibcode: 2013SoPh..287..323A Altcode: 2012arXiv1207.2780A; 2012SoPh..tmp..181A We derive an analytical approximation of nonlinear force-free magnetic field solutions (NLFFF) that can efficiently be used for fast forward-fitting to solar magnetic data, constrained either by observed line-of-sight magnetograms and stereoscopically triangulated coronal loops, or by 3D vector-magnetograph data. The derived NLFFF solutions provide the magnetic field components Bx(x), By(x), Bz(x), the force-free parameter α(x), the electric current density j(x), and are accurate to second-order (of the nonlinear force-free α-parameter). The explicit expressions of a force-free field can easily be applied to modeling or forward-fitting of many coronal phenomena. Title: A Nonlinear Force-Free Magnetic Field Approximation Suitable for Fast Forward-Fitting to Coronal Loops. II. Numeric Code and Tests Authors: Aschwanden, Markus J.; Malanushenko, Anna Bibcode: 2013SoPh..287..345A Altcode: 2012arXiv1207.2783A; 2012SoPh..tmp..182A Based on a second-order approximation of nonlinear force-free magnetic field solutions in terms of uniformly twisted field lines derived in Paper I, we develop here a numeric code that is capable to forward-fit such analytical solutions to arbitrary magnetogram (or vector magnetograph) data combined with (stereoscopically triangulated) coronal loop 3D coordinates. We test the code here by forward-fitting to six potential field and six nonpotential field cases simulated with our analytical model, as well as by forward-fitting to an exactly force-free solution of the Low and Lou (Astrophys. J.352, 343, 1990) model. The forward-fitting tests demonstrate: i) a satisfactory convergence behavior (with typical misalignment angles of μ≈1 - 10), ii) relatively fast computation times (from seconds to a few minutes), and iii) the high fidelity of retrieved force-free α-parameters (αfitmodel≈0.9 - 1.0 for simulations and αfitmodel≈0.7±0.3 for the Low and Lou model). The salient feature of this numeric code is the relatively fast computation of a quasi-force-free magnetic field, which closely matches the geometry of coronal loops in active regions, and complements the existing nonlinear force-free field (NLFFF) codes based on photospheric magnetograms without coronal constraints. Title: Multi-wavelength Observations of the Spatio-temporal Evolution of Solar Flares with AIA/SDO. I. Universal Scaling Laws of Space and Time Parameters Authors: Aschwanden, Markus J.; Zhang, Jie; Liu, Kai Bibcode: 2013ApJ...775...23A Altcode: 2013arXiv1308.4936A We extend a previous statistical solar flare study of 155 GOES M- and X-class flares observed with AIA/SDO to all seven coronal wavelengths (94, 131, 171, 193, 211, 304, and 335 Å) to test the wavelength dependence of scaling laws and statistical distributions. Except for the 171 and 193 Å wavelengths, which are affected by EUV dimming caused by coronal mass ejections (CMEs), we find near-identical size distributions of geometric (lengths L, flare areas A, volumes V, and fractal dimension D 2), temporal (flare durations T), and spatio-temporal parameters (diffusion coefficient κ, spreading exponent β, and maximum expansion velocities v max) in different wavelengths, which are consistent with the universal predictions of the fractal-diffusive avalanche model of a slowly driven, self-organized criticality (FD-SOC) system, i.e., N(L)vpropL -3, N(A)vpropA -2, N(V)vpropV -5/3, N(T)vpropT -2, and D 2 = 3/2, for a Euclidean dimension d = 3. Empirically, we find also a new strong correlation κvpropL 0.94 ± 0.01 and the three-parameter scaling law Lvpropκ T 0.1, which is more consistent with the logistic-growth model than with classical diffusion. The findings suggest long-range correlation lengths in the FD-SOC system that operate in the vicinity of a critical state, which could be used for predictions of individual extreme events. We find also that eruptive flares (with accompanying CMEs) have larger volumes V, longer flare durations T, higher EUV and soft X-ray fluxes, and somewhat larger diffusion coefficients κ than confined flares (without CMEs). Title: Nonlinear Force-Free Magnetic Fields of Active Regions based on automated loop tracing in AIA/SDO images with DEM discrimination of chromospheric and coronal features Authors: Aschwanden, Markus J.; Boerner, P.; AIA/SDO Team Bibcode: 2013SPD....4420104A Altcode: We developed a forward-fitting code that computes a nonlinear force-free magnetic field (NLFFF) solution constrained by line-of-sight magnetograms from HMI/SDO and by coronal loop structures detected in EUV images from AIA/SDO. The 2D coordinates of coronal loop structures are detected with an improved version of the Oriented Coronal CUrved Loop Tracing (OCCULT-2) code, an automated pattern recognition algorithm that has demonstrated a quality and fidelity in loop tracing that matches visual perception. One fundamental limitation in the completeness of detecting coronal loops comes from the background confusion of coronal loop EUV emission with low-temperature (T=10^4-10^6 K) emission from the chromosphere and transition region, as well as T ~ 1.0 MK emission from reticulated ``moss structure'' that stems from the footpoints of hotter (T 2-8 MK) coronal loops. We employ a pixel-wise differential emission measure (DEM) analysis using the 7 coronal AIA filters in order to produce uncontaminated emission measure maps in coronal temperature ranges, which allows an improved performance of automated loop tracing. A nonlinear force-free magnetic field solution is then computed by forward-fitting of an analytical NLFFF solution of twisted coronal field lines to the automatically traced coronal loop coordinates. We demonstrate the performance of this magnetic field modeling for a number of solar active regions observed with SDO. The developed method is able to calculate the most realistic magnetic field models of solar active regions that match all available observable constraints. Title: A multiwavelength study of eruptive events on January 23, 2012 associated with a major solar energetic particle event Authors: Joshi, N. C.; Uddin, W.; Srivastava, A. K.; Chandra, R.; Gopalswamy, N.; Manoharan, P. K.; Aschwanden, M. J.; Choudhary, D. P.; Jain, R.; Nitta, N. V.; Xie, H.; Yashiro, S.; Akiyama, S.; Mäkelä, P.; Kayshap, P.; Awasthi, A. K.; Dwivedi, V. C.; Mahalakshmi, K. Bibcode: 2013AdSpR..52....1J Altcode: 2013arXiv1303.1251J We use multiwavelength data from space and ground based instruments to study the solar flares and coronal mass ejections (CMEs) on January 23, 2012 that were responsible for one of the largest solar energetic particle (SEP) events of solar cycle 24. The eruptions consisting of two fast CMEs (≈1400 km s-1 and ≈2000 km s-1) and M-class flares that occurred in active region 11402 located at ≈N28 W36. The two CMEs occurred in quick successions, so they interacted very close to the Sun. The second CME caught up with the first one at a distance of ≈11-12 Rsun. The CME interaction may be responsible for the elevated SEP flux and significant changes in the intensity profile of the SEP event. The compound CME resulted in a double-dip moderate geomagnetic storm (Dst∼-73nT). The two dips are due to the southward component of the interplanetary magnetic field in the shock sheath and the ICME intervals. One possible reason for the lack of a stronger geomagnetic storm may be that the ICME delivered a glancing blow to Earth. Title: Optimization of Curvilinear Tracing Applied to Solar Physics and Biophysics Authors: Aschwanden, Markus; De Pontieu, Bart; Katrukha, Eugene Bibcode: 2013Entrp..15.3007A Altcode: 2013arXiv1307.5046A We developed an automated pattern recognition code that is particularly well suited to extract one-dimensional curvi-linear features from two-dimensional digital images. A former version of this {\sl Oriented Coronal CUrved Loop Tracing (OCCULT)} code was applied to spacecraft images of magnetic loops in the solar corona, recorded with the NASA spacecraft {\sl Transition Region And Coronal Explorer (TRACE)} in extreme ultra-violet wavelengths. Here we apply an advanced version of this code ({\sl OCCULT-2}) also to similar images from the {\sl Solar Dynamics Observatory (SDO)}, to chromospheric H-$\alpha$ images obtained with the {\sl Swedish Solar Telescope (SST)}, and to microscopy images of microtubule filaments in live cells in biophysics. We provide a full analytical description of the code, optimize the control parameters, and compare the automated tracing with visual/manual methods. The traced structures differ by up to 16 orders of magnitude in size, which demonstrates the universality of the tracing algorithm. Title: Self-Organized Criticality Systems Authors: Aschwanden, M. J. Bibcode: 2013socs.book.....A Altcode: Contents: (1) Introduction - Norma B. Crosby ---

(2) Theoretical Models of SOC Systems - Markus J. Aschwanden ---

(3) SOC and Fractal Geometry - R. T. James McAteer ---

(4) Percolation Models of Self-Organized Critical Phenomena - Alexander V. Milovanov ---

(5) Criticality and Self-Organization in Branching Processes: Application to Natural Hazards - Álvaro Corral, Francesc Font-Clos ---

(6) Power Laws of Recurrence Networks - Yong Zou, Jobst Heitzig, Jürgen Kurths ---

(7) SOC computer simolations - Gunnar Pruessner ---

(8) SOC Laboratory Experiments - Gunnar Pruessner ---

(9) Self-Organizing Complex Earthquakes: Scaling in Data, Models, and Forecasting - Michael K. Sachs et al. ---

(10) Wildfires and the Forest-Fire Model - Stefan Hergarten ---

(11) SOC in Landslides - Stefan Hergarten ---

(12) SOC and Solar Flares - Paul Charbonneau ---

(13) SOC Systems in Astrophysics - Markus J. Aschwanden --- Title: Height of shock formation in the solar corona inferred from observations of type II radio bursts and coronal mass ejections Authors: Gopalswamy, N.; Xie, H.; Mäkelä, P.; Yashiro, S.; Akiyama, S.; Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Chandra, R.; Manoharan, P. K.; Mahalakshmi, K.; Dwivedi, V. C.; Jain, R.; Awasthi, A. K.; Nitta, N. V.; Aschwanden, M. J.; Choudhary, D. P. Bibcode: 2013AdSpR..51.1981G Altcode: 2013arXiv1301.0893G Employing coronagraphic and EUV observations close to the solar surface made by the Solar Terrestrial Relations Observatory (STEREO) mission, we determined the heliocentric distance of coronal mass ejections (CMEs) at the starting time of associated metric type II bursts. We used the wave diameter and leading edge methods and measured the CME heights for a set of 32 metric type II bursts from solar cycle 24. We minimized the projection effects by making the measurements from a view that is roughly orthogonal to the direction of the ejection. We also chose image frames close to the onset times of the type II bursts, so no extrapolation was necessary. We found that the CMEs were located in the heliocentric distance range from 1.20 to 1.93 solar radii (Rs), with mean and median values of 1.43 and 1.38 Rs, respectively. We conclusively find that the shock formation can occur at heights substantially below 1.5 Rs. In a few cases, the CME height at type II onset was close to 2 Rs. In these cases, the starting frequency of the type II bursts was very low, in the range 25-40 MHz, which confirms that the shock can also form at larger heights. The starting frequencies of metric type II bursts have a weak correlation with the measured CME/shock heights and are consistent with the rapid decline of density with height in the inner corona. Title: Kelvin-Helmholtz Instability of the CME Reconnection Outflow Layer in the Low Corona Authors: Foullon, Claire; Verwichte, Erwin; Nykyri, Katariina; Aschwanden, Markus J.; Hannah, Iain G. Bibcode: 2013ApJ...767..170F Altcode: New capabilities for studying the Sun allow us to image for the first time the magnetic Kelvin-Helmholtz (KH) instability developing at the surface of a fast coronal mass ejecta (CME) less than 150 Mm above the solar surface. We conduct a detailed observational investigation of this phenomenon, observed off the east solar limb on 2010 November 3, in the EUV with SDO/AIA. In conjunction with STEREO-B/EUVI, we derive the CME source surface position. We ascertain the timing and early evolution of the CME outflow leading to the instability onset. We perform image and spectral analysis, exploring the CME plasma structuring and its parabolic flow pattern. As we evaluate and validate the consistency of the observations with theoretical considerations and predictions, we take the view that the ejecta layer corresponds to a reconnection outflow layer surrounding the erupting flux rope, accounting for the timing, high temperature (~11.6 MK), and high flow shear (~680 km s-1) on the unstable CME northern flank and for the observed asymmetry between the CME flanks. From the irregular evolution of the CME flow pattern, we infer a shear gradient consistent with expected spatial flow variations across the KH-unstable flank. The KH phenomenon observed is tied to the first stage of a linked flare-CME event. Title: Automated Temperature and Emission Measure Analysis of Coronal Loops and Active Regions Observed with the Atmospheric Imaging Assembly on the Solar Dynamics Observatory (SDO/AIA) Authors: Aschwanden, Markus J.; Boerner, Paul; Schrijver, Carolus J.; Malanushenko, Anna Bibcode: 2013SoPh..283....5A Altcode: 2011SoPh..tmp..384A We developed numerical codes designed for automated analysis of SDO/AIA image datasets in the six coronal filters, including: i) coalignment test between different wavelengths with measurements of the altitude of the EUV-absorbing chromosphere, ii) self-calibration by empirical correction of instrumental response functions, iii) automated generation of differential emission measure [DEM] distributions with peak-temperature maps [Tp(x,y)] and emission measure maps [EMp(x,y)] of the full Sun or active region areas, iv) composite DEM distributions [dEM(T)/dT] of active regions or subareas, v) automated detection of coronal loops, and vi) automated background subtraction and thermal analysis of coronal loops, which yields statistics of loop temperatures [Te], temperature widths [σT], emission measures [EM], electron densities [ne], and loop widths [w]. The combination of these numerical codes allows for automated and objective processing of numerous coronal loops. As an example, we present the results of an application to the active region NOAA 11158, observed on 15 February 2011, shortly before it produced the largest (X2.2) flare during the current solar cycle. We detect 570 loop segments at temperatures in the entire range of log(Te)=5.7 - 7.0 K and corroborate previous TRACE and AIA results on their near-isothermality and the validity of the Rosner-Tucker-Vaiana (RTV) law at soft X-ray temperatures (T≳2 MK) and its failure at lower EUV temperatures. Title: Hydrodynamic Scaling Laws and Solar Flare Statistics from AIA Authors: Aschwanden, Markus J. Bibcode: 2013enss.confE..75A Altcode: We present a statistical solar flare study of 155 GOES M- and X-class flares observed with AIA/SDO in all 7 coronal wavelengths (94, 131, 171, 193, 211, 304, 335 A) and investigate the wavelength-dependence of scaling laws and statistical distributions. Except for the 171 and 193 A wavelengths, which are affected by EUV dimming caused by coronal mass ejections (CMEs), we find near-identical size distributions of geometric (flare size L, area A, volume V, fractal dimension D2), temporal (flare duration D), and spatio-temporal parameters (diffusion coefficient, spreading exponent, and maximum expansion velocity) in different wavelengths, which are consistent with the universal predictions of the fractal-diffusive avalanche model of a slowly-driven self-organized criticality (FD-SOC) system, i.e., N(L) L^(-3), N(A) A^(-2), N(V) V^(-5/3), N(D) D^(-2), D2=3/2, for a Euclidean dimension d=3. We perfom also a differential emission measure (DEM) analysis in all flares to determine the flare peak emission measure EM_p, peak temperature T_p, electron density n_p, and thermal energy E_th. We find that these parameters obey the Rosner-Tucker-Vaiana (RTV) scaling law T_p^2 n_p L and H T^(7/2) L^(-2) during the flare peak time t_p of maximum density n_p, when energy balance between the heating rate H and the conductive and radiative loss rates is achieved for a short instant, and thus enables the applicability of the RTV scaling law. The application of the RTV scaling law predicts powerlaw distributions for all physical parameters, which we demonstrate with numerical Monte-Carlo simulations as well as with analytical calculations. A consequence of the RTV law is also that we can retrieve the size distribution of heating rates, for which we find N(H) H^(-1.8), which is consistent with the magnetic flux distribution N(Phi) Phi^(-1.85) observed by Parnell et al. (2009) and the heating flux scaling law F_H H L B/L of Schrijver et al. (2004). The fractal-diffusive self-organized criticality model in conjunction with the RTV scaling law reproduces the observed powerlaw distributions and their slopes for all geometrical and physical parameters and can be used to predict the size distributions for other flare datasets, instruments, and detection algorithms. Title: Recent Advances in Observations of Coronal EUV Waves Authors: Liu, Wei; Ofman, Leon; Aschwanden, Markus J.; Nitta, Nariaki; Zhao, Junwei; Title, Alan M. Bibcode: 2013enss.confE..67L Altcode: MHD waves can be used as seismological tools to decipher otherwise elusive physical parameters of the solar corona, such as the magnetic field strength and plasma density. Recent high cadence, high resolution, full-disk imaging observations from SDO/AIA have opened a new chapter in understanding these waves. Various types of EUV waves associated with flares/CMEs have been discovered or observed in unprecedented detail. In this talk, we will review such new observations, focusing on the following topics and their interrelationships: (1) quasi-periodic fast waves traveling along coronal funnels within CME bubbles at speeds up to 2000 km/s, associated with flare pulsations at similar frequencies; (2) quasi-periodic wave trains within broad, diffuse pulses of global EUV waves (so-called "EIT waves") running ahead of CME fronts; (3) interactions of global EUV waves with local coronal structures on their paths, such as flux-rope coronal cavities and their embedded filaments (kink oscillations) and coronal holes or active regions (deflections). We will discuss the implications of these observations on coronal seismology, on their roles in transporting energy through different parts of the solar atmosphere, and on understanding their associated eruptive flares/CMEs. Title: Nonlinear Force-free Magnetic Field Fitting to Coronal Loops with and without Stereoscopy Authors: Aschwanden, Markus J. Bibcode: 2013ApJ...763..115A Altcode: 2012arXiv1212.2996A We developed a new nonlinear force-free magnetic field (NLFFF) forward-fitting algorithm based on an analytical approximation of force-free and divergence-free NLFFF solutions, which requires as input a line-of-sight magnetogram and traced two-dimensional (2D) loop coordinates of coronal loops only, in contrast to stereoscopically triangulated three-dimensional loop coordinates used in previous studies. Test results of simulated magnetic configurations and from four active regions observed with STEREO demonstrate that NLFFF solutions can be fitted with equal accuracy with or without stereoscopy, which relinquishes the necessity of STEREO data for magnetic modeling of active regions (on the solar disk). The 2D loop tracing method achieves a 2D misalignment of μ2 = 2.°7 ± 1.°3 between the model field lines and observed loops, and an accuracy of ≈1.0% for the magnetic energy or free magnetic energy ratio. The three times higher spatial resolution of TRACE or SDO/AIA (compared with STEREO) also yields a proportionally smaller misalignment angle between model fit and observations. Visual/manual loop tracings are found to produce more accurate magnetic model fits than automated tracing algorithms. The computation time of the new forward-fitting code amounts to a few minutes per active region. Title: Multi-wavelength diagnostics of thermal and non-thermal characteristics in 22 April 2011 confined flare Authors: Awasthi, Arun K.; Jain, Rajmal; Aschwanden, Markus J.; Uddin, Wahab; Srivastava, Abhishek K.; Chandra, Ramesh; Gopalswamy, Nat; Nitta, Nariaki; Yashiro, Seiji; Manoharan, P. K.; Prasad Choudhary, Debi; Joshi, N. C.; Dwivedi, Vidya Charan; Mahalakshmi, K. Bibcode: 2013ASInC...9...71A Altcode: We study the spatial, spectral and temporal characteristics of thermal and non-thermal emission in an M1.8 flare, which occurred in NOAA AR 11195 (S17E31) on 22 April 2011. This study quantifies spatial and temporal correlation of thermal and non-thermal emissions in precursor, impulsive as well as gradual phase of energy release employing multi-wavelength observation from SDO, HESSI and SOXS missions. Based on spectral fitting analysis performed on the X-ray emission observed by RHESSI as well as SOXS missions in low energy and high energy respectively, we define that <20 keV emission corresponds to thermal and >20 keV emission to be non-thermal counterpart of the emission. Therefore, we construct X-ray images employing RHESSI observation in energy bands 6-20 and 20-100 keV over the time integration of 30s. We report co-spatial X-ray emission in various phases of emission. We also report absence of non-thermal counterpart in the X-ray emission in precursor phase however visible at the commencement of main phase. To characterize thermal and non-thermal signatures, we overlay the X-ray image contours on the Hα and EUV observations from GONG and SDO/AIA respectively. We report thermal emission in the precursor phase to be co-spatial to UV counterpart. In contrast, we report absence of emission in the EUV wavebands i.e. 1600 and 1700 Å which, in principle, correspond to temperature minimum zone and photosphere during the precursor phase. This confirms the absence of non-thermal emission as appeared in X-ray emission during the precursor phase. Further, during the impulsive as well as in gradual phase, thermal and non-thermal emissions have been found to be originated from a compact source, co-spatial in nature. Analysis of Line of sight (LOS) magnetic field observations from SDO/HMI does not reveal noticeable changes in the positive and negative fluxes as well as magnetic-field gradient during this event. In contrast, Hα emission observed by GONG has revealed the filament eruption as the trigger of flare. This suggests filament eruption to be driver of this event, consistent with the CSHKP model of solar flare. Title: Coronal Seismology with ATST Authors: Aschwanden, M. J. Bibcode: 2012ASPC..463..133A Altcode: We give a brief summary on the current status of coronal seismology and anticipate research opportunities for ATST in this discipline. Given the optical/infrared spectral range and the high-resolution magnetic field capabilities of ATST (≍ 0.05″-0.1″), the potential of exploring coronal seismology includes: (1) Optical detection of coronal waves and oscillations, (2) high-resolution magnetic field modeling with accurate determination of Alfvénic speeds, and (3) correlative studies that investigate the coupling between photospheric waves (detected in optical wavelengths) and coronal waves, which will provide insights into the generation mechanism of coronal waves, the origin and efficiency of coronal heating by waves, and diagnostics on flare and CME processes by global waves. Title: Quantifying Coronal Dimming as Observed in EUV and X-ray Images in Eruptive Events Authors: Nitta, N. V.; Aschwanden, M. J.; Boerner, P.; Hill, S. M.; Lemen, J. R.; Liu, W.; Schrijver, C.; Wuelser, J. Bibcode: 2012AGUFMSH41A2097N Altcode: Data from SOHO have shown that coronal dimming is closely related with coronal mass ejections (CMEs). In particular, dimming areas in EIT 195 A images often match the lateral extension of the associated CMEs. In this presentation, we summarize how CMEs compare with dimming as identified at different wavelengths and by other instruments, such as Yohkoh SXT, TRACE, GOES (12-15) SXI, STEREO EUVI and SDO AIA. Emphasis is placed on recent data, since the combination of AIA and STEREO data can lead us to better characterize CMEs and to more accurately estimate how much mass is ejected. We discuss technical issues that arise when quantifying dimming as a proxy for a CME. The issues include instrument calibration, effects of heating and cooling and integration along the line of sight. We also touch on the relation of dimming with globally propagating coronal fronts, which are routinely isolated in running difference images, and its implications on the magnitudes of the associated CMEs. Title: Measurements of the Mass of Coronal Mass Ejections from the EUV Dimming Observed with STEREO Authors: Aschwanden, M. J. Bibcode: 2012AGUFMSH44A..02A Altcode: The masses of Coronal Mass Ejections (CMEs) have traditionally been determined from white-light coronagraphs (based on Thomson scattering of electrons), as well as from EUV dimming observed with one spacecraft. Here we report on an improved method of measuring CME masses based on EUV dimming observed with the dual STEREO/EUVI spacecraft in multiple temperature filters that includes 3D volume and density modeling in the dimming region and background corona. As a test we investigate 8 CME events with previous mass determinations from STEREO/COR2, of which 6 cases are reliably detected with EUVI using our automated multi-wavelength detection code. We find CME masses in the range of m_CME = (2-7) * 10^(15) g. The agreement between the two EUVI/A and B spacecraft is m_A/m_B =1.3+/-0.6$ and the consistency with white-light measurements by COR2 is m_EUVI/m_COR2 = 1.1+/-0.3. The consistency between EUVI and COR2 implies no significant mass backflows (or inflows) at r < 4 R_sun and adequate temperature coverage for the bulk of the CME mass in the range of T = 0.5-3.0 MK. The temporal evolution of the EUV dimming allows us also to model the evolution of the CME density n_e(t), volume V(t), height-time h(t), and propagation speed v(t) in terms of an adiabatically expanding self-similar geometry. We determine e-folding EUV dimming times of t_D=1.3+/-1.4 hrs. We test the adiabatic expansion model in terms of the predicted detection delay dt = 0.7 hr between EUVI and COR2 for the fastest CME event (2008-Mar-25) and find good agreement with the observed delay dt = 0.8 hr. Title: Solar Stereoscopy with STEREO/EUVI A and B Spacecraft from Small (6) to Large (170) Spacecraft Separation Angles Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre; Nitta, Nariaki; Lemen, James Bibcode: 2012SoPh..281..101A Altcode: 2012SoPh..tmp..197A; 2012arXiv1207.2787A We performed for the first time stereoscopic triangulation of coronal loops in active regions over the entire range of spacecraft separation angles (αsep≈6,43,89,127,and 170). The accuracy of stereoscopic correlation depends mostly on the viewing angle with respect to the solar surface for each spacecraft, which affects the stereoscopic correspondence identification of loops in image pairs. From a simple theoretical model we predict an optimum range of αsep≈22 - 125, which is also experimentally confirmed. The best accuracy is generally obtained when an active region passes the central meridian (viewed from Earth), which yields a symmetric view for both STEREO spacecraft and causes minimum horizontal foreshortening. For the extended angular range of αsep≈6 - 127 we find a mean 3D misalignment angle of μPF≈21 - 39 of stereoscopically triangulated loops with magnetic potential-field models, and μFFF≈15 - 21 for a force-free field model, which is partly caused by stereoscopic uncertainties μSE≈9. We predict optimum conditions for solar stereoscopy during the time intervals of 2012 - 2014, 2016 - 2017, and 2021 - 2023. Title: GeV Particle Acceleration in Solar Flares and Ground Level Enhancement (GLE) Events Authors: Aschwanden, Markus J. Bibcode: 2012SSRv..171....3A Altcode: 2012SSRv..tmp....5A; 2010arXiv1005.0029A; 2012SSRv..tmp....7A Ground Level Enhancement (GLE) events represent the most energetic class of solar energetic particle (SEP) events, requiring acceleration processes to boost ≳1 GeV ions in order to produce showers of secondary particles in the Earth's atmosphere with sufficient intensity to be detected by ground-level neutron monitors, above the background of cosmic rays. Although the association of GLE events with both solar flares and coronal mass ejections (CMEs) is undisputed, the question arises about the location of the responsible acceleration site: coronal flare reconnection sites, coronal CME shocks, or interplanetary shocks? To investigate the first possibility we explore the timing of GLE events with respect to hard X-ray production in solar flares, considering the height and magnetic topology of flares, the role of extended acceleration, and particle trapping. We find that 50% (6 out of 12) of recent (non-occulted) GLE events are accelerated during the impulsive flare phase, while the remaining half are accelerated significantly later. It appears that the prompt GLE component, which is observed in virtually all GLE events according to a recent study by Vashenyuk et al. (Astrophys. Space Sci. Trans. 7(4):459-463, 2011), is consistent with a flare origin in the lower corona, while the delayed gradual GLE component can be produced by both, either by extended acceleration and/or trapping in flare sites, or by particles accelerated in coronal and interplanetary shocks. Title: The Spatio-temporal Evolution of Solar Flares Observed with AIA/SDO: Fractal Diffusion, Sub-diffusion, or Logistic Growth? Authors: Aschwanden, Markus J. Bibcode: 2012ApJ...757...94A Altcode: 2012arXiv1208.1527A We explore the spatio-temporal evolution of solar flares by fitting a radial expansion model r(t) that consists of an exponentially growing acceleration phase, followed by a deceleration phase that is parameterized by the generalized diffusion function r(t)vpropκ(t - t 1)β/2, which includes the logistic growth limit (β = 0), sub-diffusion (β = 0-1), classical diffusion (β = 1), super-diffusion (β = 1-2), and the linear expansion limit (β = 2). We analyze all M- and X-class flares observed with Geostationary Operational Environmental Satellite and Atmospheric Imaging Assembly/Solar Dynamics Observatory (SDO) during the first two years of the SDO mission, amounting to 155 events. We find that most flares operate in the sub-diffusive regime (β = 0.53 ± 0.27), which we interpret in terms of anisotropic chain reactions of intermittent magnetic reconnection episodes in a low plasma-β corona. We find a mean propagation speed of v = 15 ± 12 km s-1, with maximum speeds of v max = 80 ± 85 km s-1 per flare, which is substantially slower than the sonic speeds expected for thermal diffusion of flare plasmas. The diffusive characteristics established here (for the first time for solar flares) is consistent with the fractal-diffusive self-organized criticality model, which predicted diffusive transport merely based on cellular automaton simulations. Title: First Three-dimensional Reconstructions of Coronal Loops with the STEREO A+B Spacecraft. IV. Magnetic Modeling with Twisted Force-free Fields Authors: Aschwanden, Markus J.; Wuelser, Jean-Pierre; Nitta, Nariaki V.; Lemen, James R.; DeRosa, Marc L.; Malanushenko, Anna Bibcode: 2012ApJ...756..124A Altcode: 2012arXiv1207.2790A The three-dimensional coordinates of stereoscopically triangulated loops provide strong constraints for magnetic field models of active regions in the solar corona. Here, we use STEREO/A and B data from some 500 stereoscopically triangulated loops observed in four active regions (2007 April 30, May 9, May 19, and December 11), together with SOHO/MDI line-of-sight magnetograms. We measure the average misalignment angle between the stereoscopic loops and theoretical magnetic field models, finding a mismatch of μ = 19°-46° for a potential field model, which is reduced to μ = 14°-19° for a non-potential field model parameterized by twist parameters. The residual error is commensurable with stereoscopic measurement errors (μSE ≈ 8°-12°). We developed a potential field code that deconvolves a line-of-sight magnetogram into three magnetic field components (Bx , By , Bz ), as well as a non-potential field forward-fitting code that determines the full length of twisted loops (L ≈ 50-300 Mm), the number of twist turns (median N twist = 0.06), the nonlinear force-free α-parameter (median α ≈ 4 × 10-11 cm-1), and the current density (median jz ≈ 1500 Mx cm-2 s-1). All twisted loops are found to be far below the critical value for kink instability, and Joule dissipation of their currents is found to be far below the coronal heating requirement. The algorithm developed here, based on an analytical solution of nonlinear force-free fields that is accurate to second order (in the force-free parameter α), represents the first code that enables fast forward fitting to photospheric magnetograms and stereoscopically triangulated loops in the solar corona. Title: Automated Solar Flare Statistics in Soft X-Rays over 37 Years of GOES Observations: The Invariance of Self-organized Criticality during Three Solar Cycles Authors: Aschwanden, Markus J.; Freeland, Samuel L. Bibcode: 2012ApJ...754..112A Altcode: 2012arXiv1205.6712A We analyzed the soft X-ray light curves from the Geostationary Operational Environmental Satellites over the last 37 years (1975-2011) and measured with an automated flare detection algorithm over 300,000 solar flare events (amounting to ≈5 times higher sensitivity than the NOAA flare catalog). We find a power-law slope of α F = 1.98 ± 0.11 for the (background-subtracted) soft X-ray peak fluxes that is invariant through three solar cycles and agrees with the theoretical prediction α F = 2.0 of the fractal-diffusive self-organized criticality (FD-SOC) model. For the soft X-ray flare rise times, we find a power-law slope of α T = 2.02 ± 0.04 during solar cycle minima years, which is also consistent with the prediction α T = 2.0 of the FD-SOC model. During solar cycle maxima years, the power-law slope is steeper in the range of α T ≈ 2.0-5.0, which can be modeled by a solar-cycle-dependent flare pile-up bias effect. These results corroborate the FD-SOC model, which predicts a power-law slope of α E = 1.5 for flare energies and thus rules out significant nanoflare heating. While the FD-SOC model predicts the probability distribution functions of spatio-temporal scaling laws of nonlinear energy dissipation processes, additional physical models are needed to derive the scaling laws between the geometric SOC parameters and the observed emissivity in different wavelength regimes, as we derive here for soft X-ray emission. The FD-SOC model also yields statistical probabilities for solar flare forecasting. Title: On the Oscillatory and Non-oscillatory Loop Systems and Dynamical Processes during the X2.1 Solar Flare on 06 September 2011 Authors: Srivastava, Abhishek K.; Jain, Rajmal; Prasad Choudhary, Debi; Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat; Awasthi, Arun Kumar; Chandra, Ramesh; Kayshap, Pradeep; Joshi, N. C.; Manoharan, P. K.; Norris, Max; Makela, Pertti; Mahalakshmi, K.; Elamathi, E. Bibcode: 2012cosp...39.1882S Altcode: 2012cosp.meet.1882S No abstract at ADS Title: Magnetic Flux Imbalance in Active Regions NOAA 11283 and NOAA 11302 Authors: Prasad Choudhary, Debi; Jain, Rajmal; Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat; Awasthi, Arun Kumar; Chandra, Ramesh; Srivastava, Abhishek K.; Manoharan, P. K.; Norris, Max; Mahalakshmi, K.; Elamathi, E.; Uddin, Wahab; Yashiro, Seiji Bibcode: 2012cosp...39..334P Altcode: 2012cosp.meet..334P We investigate the magnetic flux imbalance of two active regions NOAA 11302 and NOAA 11283 during their disk passage. The active region NOAA 11302 appeared in the east limb on September 23, 2011 as beta-gamma complexity and produced 73 c-class, 27 M-class and 2 X-class flares many of which were associated with CMEs during the disk passage. The active region NOAA 11283 appeared on the east limb on September 1, 2011 as beta-gamma complexity and produced 16 c-class, 9 m-class and 2 x-class flares and CMEs. Both these active regions were of similar size but the evolution of magnetic complexity during their disk passage was very different. None of them made second disk passage. These two active regions represent two different class of activity. Among several reasons, the magnetic flux imbalance of the active regions result due to the presence of electric current with in the active regions. The high cadence full disk magnetograms obtained using the GONG and SDO-HMI instruments serve as the primary data source of this investigation. We relate the change in the magnetic flux imbalance with the flare occurrence in these two contrasting active regions. Title: Height of Shock Formation in the Solar Corona Inferred from Observations of Type II Radio Bursts and Coronal Mass Ejections Authors: Gopalswamy, Nat; Jain, Rajmal; Prasad Choudhary, Debi; Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Awasthi, Arun Kumar; Srivastava, Abhishek K.; Joshi, N. C.; Manoharan, P. K.; Makela, Pertti; Mahalakshmi, K.; Elamathi, E.; Uddin, Wahab; Yashiro, Seiji; Akiyam, Sachiko Bibcode: 2012cosp...39..653G Altcode: 2012cosp.meet..653G No abstract at ADS Title: Quasi-periodic Fast-mode Wave Trains within a Global EUV Wave and Sequential Transverse Oscillations Detected by SDO/AIA Authors: Liu, Wei; Ofman, Leon; Nitta, Nariaki V.; Aschwanden, Markus J.; Schrijver, Carolus J.; Title, Alan M.; Tarbell, Theodore D. Bibcode: 2012ApJ...753...52L Altcode: 2012arXiv1204.5470L We present the first unambiguous detection of quasi-periodic wave trains within the broad pulse of a global EUV wave (so-called EIT wave) occurring on the limb. These wave trains, running ahead of the lateral coronal mass ejection (CME) front of 2-4 times slower, coherently travel to distances >~ R /2 along the solar surface, with initial velocities up to 1400 km s-1 decelerating to ~650 km s-1. The rapid expansion of the CME initiated at an elevated height of 110 Mm produces a strong downward and lateral compression, which may play an important role in driving the primary EUV wave and shaping its front forwardly inclined toward the solar surface. The wave trains have a dominant 2 minute periodicity that matches the X-ray flare pulsations, suggesting a causal connection. The arrival of the leading EUV wave front at increasing distances produces an uninterrupted chain sequence of deflections and/or transverse (likely fast kink mode) oscillations of local structures, including a flux-rope coronal cavity and its embedded filament with delayed onsets consistent with the wave travel time at an elevated (by ~50%) velocity within it. This suggests that the EUV wave penetrates through a topological separatrix surface into the cavity, unexpected from CME-caused magnetic reconfiguration. These observations, when taken together, provide compelling evidence of the fast-mode MHD wave nature of the primary (outer) fast component of a global EUV wave, running ahead of the secondary (inner) slow component of CME-caused restructuring. Title: Coronal Mass Ejections and Type II Radio Bursts from Active Region 11158 Authors: Yashiro, Seiji; Jain, Rajmal; Prasad Choudhary, Debi; Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat; Awasthi, Arun Kumar; Chandra, Ramesh; Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi, N. C.; Manoharan, P. K.; Makela, Pertti; Mahalakshmi, K.; Elam, E. Bibcode: 2012cosp...39.2205Y Altcode: 2012cosp.meet.2205Y The NOAA active region (AR) 11158 emerged at around S20E50 on 2011 February 10 as two bipoles and quickly developed into a large complex region. During 2011 February 13-17, AR 11158 produced 48 flares (>C1 level) including the first X-class flare of 15 February, 2011 in solar cycle 24. The 48 flares can be divided into four groups based on their location within the AR. We examined their associations of coronal mass ejections (CMEs) and metric type II radio bursts in order to find preferred locations of both the phenomena. We found that, out of 48 flares, 15 had associated CMEs, occurring frequently at the eastern edge of the AR. We also found that six flares were associated with type II radio bursts and all of them were associated with CMEs also. No type II was associated with the CME-less flare. This suggests that the CME association is a necessary condition for a flare to be associated with a metric type II burst. Title: Multi-wavelength diagnostics of thermal and non-thermal sources in the 22 April 2011 flare event Authors: Awasthi, Arun Kumar; Jain, Rajmal; Prasad Choudhary, Debi; Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat; Chandra, Ramesh; Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi, N. C.; Manoharan, P. K.; Norris, Max; Mahalakshmi, K.; Elamathi, E.; Uddin, Wahab Bibcode: 2012cosp...39...75A Altcode: 2012cosp.meet...75A No abstract at ADS Title: A Study of the 12 June 2010 C6.1/SF flare associated with a CME, surge and energetic particles Authors: Uddin, Wahab; Jain, Rajmal; Manoharan, P. K.; Prasad Choudhary, Debi; Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat; Awasthi, Arun Kumar; Chandra, Ramesh; Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi, N. C.; Norris, Max; Makela, Pertti; Mahalaksh, K. Bibcode: 2012cosp...39.2026U Altcode: 2012cosp.meet.2026U In this paper, we present the multiwavelength analysis of the C6.1/SF flare on 12 June 2010 from NOAA AR 11081. The flare was observed by various ground based (ARIES H-alpha; HIRAS Radio) and space borne observatories (SDO, STEREO, SOHO, GOES). The flare was accompanied by a spray/surge and a slow coronal mass ejection (CME) that propagated with a speed of ~382 km/s. The eruption was associated with a weak solar energetic particle (SEP) event. The solar source of the eruption was a rapid emerging flux region. The eruption was also associated with the three major types of radio bursts (type II, III and IV). The interesting observation is the shock production (type II burst and SEP event) by a relatively slow CME. We interpret the results in the light of existing theories. Title: Solar Energetic Particle Events and Associated CMEs during the Rising Phases of Solar Cycle 23 and 24 - A Comparative study Authors: Chandra, Ramesh; Jain, Rajmal; Prasad Choudhary, Debi; Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat; Awasthi, Arun Kumar; Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi, N. C.; Manoharan, P. K.; Makela, Pertti; Mahalakshmi, K.; Elamathi, E.; Uddi, Wahab Bibcode: 2012cosp...39..303C Altcode: 2012cosp.meet..303C No abstract at ADS Title: Self-Organized Criticality Systems in Astrophysics (Chapter 13) Authors: Aschwanden, Markus J. Bibcode: 2012arXiv1207.4413A Altcode: Chapter 13: SOC Systems in Astrophysics --- Content list: 13.1 Theory -- 13.1.1 The Sacle-Free Probability Theorem - 13.1.2 The Fractal-Diffusive Spatio-Temporal Relationship - 13.1.3 Size Distributions of Astrophysical Observables - 13.1.4 Scaling Laws for Thermal Emission of Astrophysical Plasmas - 13.1.5 Scaling Laws for Astrophysical Acceleration Mechanisms - 13.2 Observations -- 13.2.1 Lunar Craters - 13.2.2 Asteroid Belt - 13.2.3 Saturn Ring - 13.2.4 Magnetospheric Substorms and Auroras - 13.2.5 Solar Flares - 13.2.6 Stellar Flares - 13.2.7 Pulsars - 13.2.8 Soft Gamma-Ray Repeaters - 13.2.9 Black-Hole Objects - 13.2.10 Blazars - 13.2.11 Cosmic Rays - 13.3 Conclusions Title: Flares in the Crab Nebula Driven by Untwisting Magnetic Fields Authors: Sturrock, Peter; Aschwanden, Markus J. Bibcode: 2012ApJ...751L..32S Altcode: 2012arXiv1205.0039S The recent discovery of PeV electrons from the Crab Nebula, produced on rapid timescales of one day or less with a sharply peaked gamma-ray spectrum without hard X-rays, challenges traditional models of diffusive shock acceleration followed by synchrotron radiation. Here, we outline an acceleration model involving a DC electric field parallel to the magnetic field in a twisted toroidal field around the pulsar. Sudden developments of resistivity in localized regions of the twisted field are thought to drive the particle acceleration, up to PeV energies, resulting in flares. This model can reproduce the observed timescales of T ≈ 1 day, the peak photon energies of U Φ, rr ≈ 1 MeV, maximum electron energies of U e, rr ≈ 1 PeV, and luminosities of L ≈ 1036 erg s-1. Title: SDO/AIA Detection of Quasi-periodic Wave Trains Within Global EUV ("EIT") Waves and Their Coronal Seismology Implications Authors: Liu, Wei; Ofman, L.; Aschwanden, M. J.; Nitta, N.; Schrijver, C. J.; Title, A. M.; Tarbell, T. D. Bibcode: 2012AAS...22051501L Altcode: The nature of global EUV waves (so-called "EIT waves") has long been under debate because of instrumental limitations and projection effects when viewed on the solar disk. We present here high cadence SDO/AIA observations of global EUV waves occurring on the limb. We report newly discovered quasi-periodic wave trains located in the low corona within a broad, diffuse pulse of the global EUV wave ahead of the lateral CME front/flank. These waves coherently travel to large distances on the order of 1 solar radii with initial velocities up to 1400 km/s. They have dominant 1-3 minute periodicities that often match the X-ray pulsations of the accompanying flare, suggestive of a causal connection. In addition, recently discovered quasi-periodic fast propagating (QFP) waves of 1000-2000 km/s (Liu, Title, Zhao et al. 2011 ApJL) are found in the funnel of coronal loops rooted at the flare kernel. These waves are spatially confined within the CME bubble and rapidly disappear while approaching the CME front, suggestive of strong damping and/or dispersion. These observations provide new evidence of the fast-mode wave nature of the primary, fast component of a global EUV wave, running ahead of a secondary, slow component of CME-caused restructuring of the coronal magnetic field. We suggest that the two types of quasi-periodic waves are both integral parts of global coronal dynamics manifested as a CME/flare eruption, and they have important implications for global and local coronal seismology. Title: Flares in the Crab Nebula Driven by Untwisting Magnetic Fields Authors: Sturrock, Peter A.; Aschwanden, M. J. Bibcode: 2012AAS...22040708S Altcode: The recent discovery of PeV electrons from the Crab nebula, produced on rapid time scales of one day or less with a sharply peaked gamma-ray spectrum without hard X-rays, challenges traditional diffusive shock acceleration models followed by synchrotron radiation. Here we outline an acceleration model involving a DC electric field, parallel to the magnetic field, in a twisted toroidal field in the nebula.. Sudden developments of resistivity in localized regions of the twisted field are thought to drive the flares and associated particle acceleration up to PeV energies. This model can reproduce the observed time scales of about 1 day, the peak photon energies of about 1 MeV, maximum electron energies of about 1 PeV, and a luminosity of about 1036 erg s-1. Title: Constraining Differential Emission Measure and Energy Estimates for Microflares and Active Regions by Combining SDO/AIA and RHESSI Authors: Inglis, Andrew; Christe, S.; Aschwanden, M. Bibcode: 2012AAS...22050905I Altcode: Direct diagnostics of the fundamental parameters of solar coronal phenomena remains an active and challenging goal. Constrained, spatially resolved values of many parameters, such as the magnetic field strength, temperature, emission measure, and energy, are often difficult to achieve. However, the advent of the Atmospheric Imaging Assembly (AIA) on board SDO provides us with greater opportunity to pursue these measurements.

Here, we present differential emission measure (DEM) analysis of a selection of recent microflares and hot active regions, utilising a combination of high temperature RHESSI data, and forward-fitting temperature mapping, a procedure developed by Achwanden et al. (2011) for SDO/AIA. This procedure models the plasma temperature as a Gaussian distribution, and uses the instrument response functions to find the distribution which best reproduces the observed fluxes in each AIA wavelength. The accuracy of the method is examined, including an investigation of uncertainties and the temperature range over which the DEM is reasonably constrained. We also study the ability of the method to accurately represent the flux in all of the AIA wavelengths simultaneously. This technique is combined with RHESSI spectral data to produce complementary time-dependent DEM measurements for four microflares from June - August 2011, and one set of post-flare loops from 2010 October 16. In general, the high temperature emission measure fitted by RHESSI has a steeper spectral index than that observed with AIA. However, we illustrate how the high temperature RHESSI data can be used to further inform the AIA fitting procedure, improving the results. We also expand on the AIA forward fitting to produce spatial energy maps of microflares and active regions, also allowing estimates of the total energy of these regions to be made. Title: Force-Free Magneto-Stereoscopy of Coronal Loops Authors: Aschwanden, Markus J.; Malanushenko, A.; Wuelser, J.; Nitta, N.; Lemen, J. R.; DeRosa, M. Bibcode: 2012AAS...22041103A Altcode: We derive an analytical approximation of nonlinear force-free magnetic field solutions (NLFFF) that can efficiently be used for fast forward-fitting to solar magnetic data, constrained either by observed line-of-sight magnetograms and stereoscopically triangulated coronal loops, or by 3D vector-magnetograph data. We test the code by forward-fitting to simulated data, to force-free solutions derived by Low and Lou (1990), and to active regions observed with STEREO/EUVI and SOHO/MDI. The forward-fitting tests demonstrate: (i) a satisfactory convergence behavior (with typical misalignment angles of 1-10 deg), (ii) a high fidelity of retrieved force-free alpha-parameters, and (iii) relatively fast computation times (from seconds to minutes). The novel feature of this NLFFF code is the derivation of a quasi-forcefree field based on coronal constraints, which bypasses the non-forcefree photosphere of standard magnetograms. Applications range from magnetic modeling of loops to the determnination of electric currents, twist, helicity, and free (non-potential) energy in active regions. Title: Theoretical Models of Self-Organized Criticality (SOC) Systems Authors: Aschwanden, Markus J. Bibcode: 2012arXiv1204.5119A Altcode: In this chapter 2 of the e-book "Self-Organized Criticality Systems" we summarize the classical cellular automaton models, which consist of a statistical aspect that is universal to all SOC systems, and a physical aspect that depends on the physical definition of the observable. Then we derive some general analytical formulations of SOC processes, such as the exponential-growth SOC model and the fractal-diffusive SOC model, which also have universal validity for SOC processes, while specific applications to observations require additional physical scaling laws (e.g., for astrophysical or geophysical observations). Finally we discuss alternative SOC processes, SOC-related, or non-SOC processes, such as: self-organization (without criticality), forced SOC model, Brownian motion or classical diffusion, hyper-diffusion and Levy flight, nonextensive Tsallis entropy, turbulence, percolation, phase transitions, network systems, and chaotic systems. We synthesize a metrics that specifies which observational SOC properties are shared by these processes. Title: Force-free Magnetic Fields and Electric Currents inferred from Coronal Loops and Stereoscopy Authors: Aschwanden, Markus J.; Boerner, P.; Schrijver, C. J.; Malanushenko, A. Bibcode: 2012decs.confE.105A Altcode: Force-free magnetic fields are considered to be a natural state of the low plasma-beta corona. There exist about a dozen of numerical nonlinear force-free field (NLFFF) computation codes that are able to caclulate a divergence-free and force-free solution of the magnetic field, by extrapolation from a lower boundary condition that is specified with 3D vector magnetograph data. However, significant differences in the solutions have been found among the different NLFFF codes, as well as in comparison with stereoscopically triangulated 3D coordinates of coronal loops, exhibiting field misalignment angles of 20-40 degrees. Each calculation of a NLFFF solution is computing-intensive and no code is fast enough to enable forward-fitting to observations. Here we derive an analytical approximation of NLFFF solutions that is accurate to second order and can efficiently be used for forward-fitting to coronal loops. We demonstrate the accurcay of the NLFFF forward-fitting code by reproducing the Low and Lou (1990) analytical model withg an accuracy of <5 degres. Further, we show examples of fitted NLFFF solutions to STEREO observations of coronal loops. Future NLFFF fits are expected based on line-of-sight magnetograms and automated loop tracings only, without requiring vector field and STEREO data. Title: A statistical fractal-diffusive avalanche model of a slowly-driven self-organized criticality system Authors: Aschwanden, M. J. Bibcode: 2012A&A...539A...2A Altcode: 2011arXiv1112.4859A
Aims: We develop a statistical analytical model that predicts the occurrence frequency distributions and parameter correlations of avalanches in nonlinear dissipative systems in the state of a slowly-driven self-organized criticality (SOC) system.
Methods: This model, called the fractal-diffusive SOC model, is based on the following four assumptions: (i) the avalanche size L grows as a diffusive random walk with time T, following L ∝ T1/2; (ii) the energy dissipation rate f(t) occupies a fractal volume with dimension DS; (iii) the mean fractal dimension of avalanches in Euclidean space S = 1,2,3 is DS ≈ (1 + S)/2; and (iv) the occurrence frequency distributions N(x) ∝ x - αx based on spatially uniform probabilities in a SOC system are given by N(L) ∝ L - S, with S being the Eudlidean dimension. We perform cellular automaton simulations in three dimensions (S = 1,2,3) to test the theoretical model.
Results: The analytical model predicts the following statistical correlations: F ∝ LDS ∝ TDS/2 for the flux, P ∝ LS ∝ TS/2 for the peak energy dissipation rate, and E ∝ FT ∝ T1 + DS/2 for the total dissipated energy; the model predicts powerlaw distributions for all parameters, with the slopes αT = (1 + S)/2, αF = 1 + (S - 1)/DS, αP = 2 - 1/S, and αE = 1 + (S - 1)/(DS + 2). The cellular automaton simulations reproduce the predicted fractal dimensions, occurrence frequency distributions, and correlations within a satisfactory agreement within ≈ 10% in all three dimensions.
Conclusions: One profound prediction of this universal SOC model is that the energy distribution has a powerlaw slope in the range of αE = 1.40 - 1.67, and the peak energy distribution has a slope of αP = 1.67 (for any fractal dimension DS = 1,...,3 in Euclidean space S = 3), and thus predicts that the bulk energy is always contained in the largest events, which rules out significant nanoflare heating in the case of solar flares.

Movie included with Fig. 1 is available in electronic form at http://www.aanda.org Title: SDO/AIA Observations of Various Coronal EUV Waves Associated with Flares/CMEs and Their Coronal Seismology Implications Authors: Liu, Wei; Ofman, Leon; Aschwanden, Markus J.; Nitta, Nariaki; Zhao, Junwei; Title, Alan M. Bibcode: 2012decs.confE..87L Altcode: MHD waves can be used as diagnostic tools of coronal seismology to decipher otherwise elusive critical physical parameters of the solar corona, such as the magnetic field strength and plasma density. They are analogous to acoustic waves used in helioseismology, but with complexities arising from the magnetic field and nonlinearity. Recent high cadence, high resolution, full-disk imaging observations from SDO/AIA have opened a new chapter in understanding these waves. Various types of EUV waves associated with flares/CMEs have been discovered or observed in unprecedented detail. In this presentation, we will review such new AIA observations, focusing on the following topics and their interrelationships: (1) quasi-periodic fast waves traveling along coronal funnels within CME bubbles at speeds up to 2000 km/s, associated with flare pulsations at similar frequencies; (2) quasi-periodic wave trains within broad, diffuse pulses of global EUV waves (so-called EIT waves) running ahead of CME fronts; (3) interactions of global EUV waves with local coronal structures on their paths, such as flux-rope coronal cavities and their embedded filaments (kink oscillations) and coronal holes/active regions (deflections). We will discuss the implications of these observations on coronal seismology, on their roles in transporting energy through different parts of the solar atmosphere, and on understanding their associated eruptive flares/CMEs. Title: SDO/AIA Observations of Quasi-periodic Fast (~1000 km/s) Propagating (QFP) Waves as Evidence of Fast-mode Magnetosonic Waves in the Low Corona: Statistics and Implications Authors: Liu, W.; Ofman, L.; Title, A. M.; Zhao, J.; Aschwanden, M. J. Bibcode: 2011AGUFMSH33A2043L Altcode: Recent EUV imaging observations from SDO/AIA led to the discovery of quasi-periodic fast (~2000 km/s) propagating (QFP) waves in active regions (Liu et al. 2011). They were interpreted as fast-mode magnetosonic waves and reproduced in 3D MHD simulations (Ofman et al. 2011). Since then, we have extended our study to a sample of more than a dozen such waves observed during the SDO mission (2010/04-now). We will present the statistical properties of these waves including: (1) Their projected speeds measured in the plane of the sky are about 400-2200 km/s, which, as the lower limits of their true speeds in 3D space, fall in the expected range of coronal Alfven or fast-mode speeds. (2) They usually originate near flare kernels, often in the wake of a coronal mass ejection, and propagate in narrow funnels of coronal loops that serve as waveguides. (3) These waves are launched repeatedly with quasi-periodicities in the 30-200 seconds range, often lasting for more than one hour; some frequencies coincide with those of the quasi-periodic pulsations (QPPs) in the accompanying flare, suggestive a common excitation mechanism. We obtained the k-omega diagrams and dispersion relations of these waves using Fourier analysis. We estimate their energy fluxes and discuss their contribution to coronal heating as well as their diagnostic potential for coronal seismology. Title: A Statistical Fractal-Diffusive Avalanche Model of a Slowly-Driven Self-Organized Criticality System Authors: Aschwanden, M. J. Bibcode: 2011AGUFMSH51C2018A Altcode: We develop a statistical analytical model that predicts the occurrence frequency distributions and parameter correlations of avalanches in nonlinear dissipative systems in the state of slowly-driven self-organized criticality (SOC). This model, called the fractal-diffusive SOC model, is based on the following four assumptions: (i) The avalanche size L grows as a diffusive random walk with time T, following L ∝ T1/2; (ii) The instantaneous energy dissipation rate P occupies a fractal volume with dimension DS, which predicts the relationships P ∝ LDS ∝ TDS/2 and the total dissipated energy E ∝ P T ∝ T1+DS/2; (iii) The mean fractal dimension of avalanches in Euclidean space S=1,2,3 is DS ≈ (1+S)/2; and (iv) The occurrence frequency distributions N(x) ∝ xx based on spatially uniform probabilities in a SOC system are given by N(L) ∝ L-S, which predicts powerlaw distributions for all parameters, with the slopes α T=(1+S)/2, α P=1+(S-1)/D_S, and α E=1+(S-1)/(D_S+2). We test the predicted fractal dimensions, occurrence frequency distributions, and correlations with numerical simulations of cellular automaton models in three dimensions S=1,2,3 and find satisfactory agreement within ≈ 10%. One profound prediction of this universal SOC model is that the energy distribution has a powerlaw slope in the range of α E=1.40-1.67 (for any fractal dimension) and thus predicts that the bulk energy is always contained in the largest events, which rules out significant nanoflare heating in the case of solar flares.<br /><br /><img class="jpg" border=0 width=600px src="/meetings/fm11/program/images/SH51C-2018_A.jpg">

Title: The State of Self-organized Criticality of the Sun during the Last Three Solar Cycles. II. Theoretical Model Authors: Aschwanden, Markus J. Bibcode: 2011SoPh..274..119A Altcode: 2010arXiv1010.0986A The observed power-law distributions of solar-flare parameters can be interpreted in terms of a nonlinear dissipative system in a state of self-organized criticality (SOC). We present a universal analytical model of an SOC process that is governed by three conditions: i) a multiplicative or exponential growth phase, ii) a randomly interrupted termination of the growth phase, and iii) a linear decay phase. This basic concept approximately reproduces the observed frequency distributions. We generalize it to a randomized exponential growth model, which also includes a (log-normal) distribution of threshold energies before the instability starts, as well as randomized decay times, which can reproduce both the observed occurrence-frequency distributions and the scatter of correlated parameters more realistically. With this analytical model we can efficiently perform Monte-Carlo simulations of frequency distributions and parameter correlations of SOC processes, which are simpler and faster than the iterative simulations of cellular automaton models. Solar-cycle modulations of the power-law slopes of flare-frequency distributions can be used to diagnose the thresholds and growth rates of magnetic instabilities responsible for solar flares. Title: The State of Self-organized Criticality of the Sun During the Last Three Solar Cycles. I. Observations Authors: Aschwanden, Markus J. Bibcode: 2011SoPh..274...99A Altcode: 2010arXiv1006.4861A We analyze the occurrence-frequency distributions of peak fluxes [P], total fluxes [E], and durations [T] of solar flares over the last three solar cycles (during 1980 - 2010) from SMM/HXRBS, CGRO/BATSE, and RHESSI hard X-ray data. From the synthesized data we find powerlaw slopes with mean values of αP=1.73±0.07 for the peak flux, αE=1.62±0.12 for the total flux, and αT=1.99±0.35 for flare durations. We find a tendency of an anti-correlation of the powerlaw slope of peak fluxes with the flare rate or sunspot number as a function of the solar cycle. The occurrence powerlaw slope is always steeper by Δα≈0.1 during a solar-cycle minimum compared with the previous solar-cycle maximum, but the relative amplitude varies for each cycle or instrument. Since each solar cycle has been observed with a different instrument, part of the variation could be attributed to instrumental characteristics and different event selection criteria used in generating the event catalogs. The relatively flatter powerlaw slopes during solar maxima could indicate more energetic flares with harder electron-energy spectra, probably due to a higher magnetic complexity of the solar corona. This would imply a non-stationarity (or solar-cycle dependence) of the coronal state of self-organized criticality. Title: Transient phenomena and thermal evolution during the Boss' Day flare of 2010 October 16 with AIA, EVE and RHESSI Authors: Inglis, A.; Christe, S.; Aschwanden, M. J.; Dennis, B. R. Bibcode: 2011AGUFMSH41A1902I Altcode: On 2010 October 16th, Boss' Day, SDO/AIA observed an M1.6 flare, its first flare using automatic exposure control. This flare also exhibited a number of interesting features, including a faint wavefront, visible in the 171 A and 193 A channels of AIA with an estimated speed of ~700-900 km/s, a CME with a much lower velocity of ~300 km/s, a large plasma ribbon ignited by the flare, late-phase post-flare loops situated away from the main flare kernel, and lastly a vertically polarised kink mode excited far from the flare kernel which was subject to a recent coronal seismological study by Aschwanden & Schrijver (2011). We present an investigation of a number of these features. We find that the wavefront observed in AIA is most likely a signature of the CME observed at later times with LASCO, thus indicating that the CME experiences significant deceleration. A deceleration scenario is also supported by the fact that the observed wavefront velocity, which is only visible some time after the flare, is too slow to be consistent with the timing of the flare eruption. The differential emission measure and energetics of the late-phase post-flare loop are also investigated using coordinated AIA, EVE, and RHESSI observations. We find that the temperature in the late-phase loops is initially modelled well by a single dominant temperature profile in each pixel. However, this model breaks down as the loop cools suggesting a broadening of the differential emission measure as a function of time. This case study illustrates the limitations of single-temperature modelling of the solar corona, but also reveals the diagnostic potential of multi-wavelength AIA studies. Title: Modeling Super-fast Magnetosonic Waves Observed by SDO in Active Region Funnels Authors: Ofman, L.; Liu, W.; Title, A.; Aschwanden, M. Bibcode: 2011ApJ...740L..33O Altcode: Recently, quasi-periodic, rapidly propagating waves have been observed in extreme ultraviolet by the Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) instrument in about 10 flare/coronal mass ejection (CME) events thus far. A typical example is the 2010 August 1 C3.2 flare/CME event that exhibited arc-shaped wave trains propagating in an active region (AR) magnetic funnel with ~5% intensity variations at speeds in the range of 1000-2000 km s-1. The fast temporal cadence and high sensitivity of AIA enabled the detection of these waves. We identify them as fast magnetosonic waves driven quasi-periodically at the base of the flaring region and develop a three-dimensional MHD model of the event. For the initial state we utilize the dipole magnetic field to model the AR and include gravitationally stratified density at coronal temperature. At the coronal base of the AR, we excite the fast magnetosonic wave by periodic velocity pulsations in the photospheric plane confined to a funnel of magnetic field lines. The excited fast magnetosonic waves have similar amplitude, wavelength, and propagation speeds as the observed wave trains. Based on the simulation results, we discuss the possible excitation mechanism of the waves, their dynamical properties, and the use of the observations for coronal MHD seismology. Title: Solar Stereoscopy and Tomography Authors: Aschwanden, Markus J. Bibcode: 2011LRSP....8....5A Altcode: We review stereoscopic and tomographic methods used in the solar corona, including ground-based and space-based measurements, using solar rotation or multiple spacecraft vantage points, in particular from the STEREO mission during 2007-2010. Stereoscopic and tomographic observations in the solar corona include large-scale structures, streamers, active regions, coronal loops, loop oscillations, acoustic waves in loops, erupting filaments and prominences, bright points, jets, plumes, flares, CME source regions, and CME-triggered global coronal waves. Applications in the solar interior (helioseismic tomography) and reconstruction and tracking of CMEs from the outer corona and into the heliosphere (interplanetary CMEs) are not included. Title: Coronal Seismology in the SDO Era: AIA Observations of Various Coronal Waves Associated with CMEs/Flares Authors: Liu, Wei; Ofman, Leon; Aschwanden, Markus J.; Nitta, Nariaki; Zhao, Junwei; Title, Alan M. Bibcode: 2011sdmi.confE..49L Altcode: MHD waves, as critical diagnostic tools of coronal seismology, can be used to decipher otherwise elusive physical parameters of the solar corona, such as the magnetic field strength and plasma density. They are analogous to acoustic waves used in helioseismology. Recent high cadence, high resolution, full-disk imaging observations from SDO/AIA have opened a new chapter in understanding these waves. Various types of waves associated with flares and/or CMEs have been discovered. In this presentation, we will review such new AIA observations, focusing on the following topics: (1) fine structures in CME-related global EUV waves (so-called EIT waves), including a diffuse pulse superimposed with multiple sharp fronts or "ripples" (Liu et al. 2010, ApJL); (2) quasi-periodic fast waves traveling in coronal funnels at speeds up to 2000 km/s and associated with flares pulsating at similar frequencies (Liu et al. 2011, ApJL); (3) interaction of global EUV waves with local coronal structures on their paths, such as flux-rope coronal cavities (triggered kink oscillations, Liu et al. in preparation) and coronal holes/active regions (deflection). We will discuss the implications of these observations on coronal seismology and on understanding their associated flares and CMEs. We also anticipate to exchange ideas with helioseismologists at this workshop, in a hope to bring together coronal seismology and helioseismology techniques to advance our understanding of solar oscillations from the interior to the upper atmosphere. Title: Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares Authors: Holman, G. D.; Aschwanden, M. J.; Aurass, H.; Battaglia, M.; Grigis, P. C.; Kontar, E. P.; Liu, W.; Saint-Hilaire, P.; Zharkova, V. V. Bibcode: 2011SSRv..159..107H Altcode: 2011SSRv..tmp..162H; 2011SSRv..tmp..242H; 2011SSRv..tmp..260H; 2011SSRv..tmp...86H; 2011arXiv1109.6496H High-energy X-rays and γ-rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at energies above several tens of keV has been the collisional thick-target model. After the launch of the Reuven Ramaty High Energy Solar Spectroscopic Imager ( RHESSI) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed. Title: Coronal Loop Oscillations Observed with Atmospheric Imaging Assembly—Kink Mode with Cross-sectional and Density Oscillations Authors: Aschwanden, Markus J.; Schrijver, Carolus J. Bibcode: 2011ApJ...736..102A Altcode: 2011arXiv1105.2191A A detailed analysis of a coronal loop oscillation event is presented, using data from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO) for the first time. The loop oscillation event occurred on 2010 October 16, 19:05-19:35 UT and was triggered by an M2.9 GOES-class flare, located inside a highly inclined cone of a narrow-angle coronal mass ejection. This oscillation event had a number of unusual features: (1) excitation of kink-mode oscillations in vertical polarization (in the loop plane), (2) coupled cross-sectional and density oscillations with identical periods, (3) no detectable kink amplitude damping over the observed duration of four kink-mode periods (P=6.3 minutes), (4) multi-loop oscillations with slightly (≈10%) different periods, and (5) a relatively cool loop temperature of T ≈ 0.5 MK. We employ a novel method of deriving the electron density ratio external and internal to the oscillating loop from the ratio of Alfvénic speeds deduced from the flare trigger delay and the kink-mode period, i.e., ne /ni = (vA /v Ae)2 = 0.08 ± 0.01. The coupling of the kink mode and cross-sectional oscillations can be explained as a consequence of the loop length variation in the vertical polarization mode. We determine the exact footpoint locations and loop length with stereoscopic triangulation using STEREO/EUVI/A data. We model the magnetic field in the oscillating loop using Helioseismic and Magnetic Imager/SDO magnetogram data and a potential-field model and find agreement with the seismological value of the magnetic field, B kink = 4.0 ± 0.7 G, within a factor of two. Title: Direct Imaging of Quasi-periodic Fast Propagating Waves of ~2000 km s-1 in the Low Solar Corona by the Solar Dynamics Observatory Atmospheric Imaging Assembly Authors: Liu, Wei; Title, Alan M.; Zhao, Junwei; Ofman, Leon; Schrijver, Carolus J.; Aschwanden, Markus J.; De Pontieu, Bart; Tarbell, Theodore D. Bibcode: 2011ApJ...736L..13L Altcode: 2011arXiv1106.3150L Quasi-periodic propagating fast mode magnetosonic waves in the solar corona were difficult to observe in the past due to relatively low instrument cadences. We report here evidence of such waves directly imaged in EUV by the new Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory. In the 2010 August 1 C3.2 flare/coronal mass ejection event, we find arc-shaped wave trains of 1%-5% intensity variations (lifetime ~200 s) that emanate near the flare kernel and propagate outward up to ~400 Mm along a funnel of coronal loops. Sinusoidal fits to a typical wave train indicate a phase velocity of 2200 ± 130 km s-1. Similar waves propagating in opposite directions are observed in closed loops between two flare ribbons. In the k-ω diagram of the Fourier wave power, we find a bright ridge that represents the dispersion relation and can be well fitted with a straight line passing through the origin. This k-ω ridge shows a broad frequency distribution with power peaks at 5.5, 14.5, and 25.1 mHz. The strongest signal at 5.5 mHz (period 181 s) temporally coincides with quasi-periodic pulsations of the flare, suggesting a common origin. The instantaneous wave energy flux of (0.1-2.6) × 107 erg cm-2 s-1 estimated at the coronal base is comparable to the steady-state heating requirement of active region loops. Title: A New Method for Modeling the Coronal Magnetic Field with STEREO and Submerged Dipoles Authors: Sandman, A. W.; Aschwanden, M. J. Bibcode: 2011SoPh..270..503S Altcode: 2011SoPh..tmp..109S Recent magnetic modeling efforts have shown substantial misalignment between theoretical models and observed coronal loop morphology as observed by STEREO/EUVI, regardless of the type of model used. Both potential field and non-linear force-free field (NLFFF) models yielded overall misalignment angles of 20 - 40 degrees, depending on the complexity of the active region (Sandman et al., Solar Phys.259, 1, 2009; DeRosa et al., Astrophys. J.696, 1780, 2009) We demonstrate that with new, alternative forward-fitting techniques, we can achieve a significant reduction in the misalignment angles compared with potential field source surface (PFSS) models and NLFFF models. Fitting a series of submerged dipoles to the field directions of stereoscopically triangulated loops in four active regions (30 April, 9 May, 19 May, and 11 December 2007), we find that 3 - 5 dipoles per active region yield misalignment angles of ∼ 11° - 18°, a factor of two smaller than those given by previously established extrapolation methods. We investigate the spatial and temporal variation of misalignment angles with subsets of loops for each active region, as well as loops observed prior to and following a flare and filament eruption, and find that the spatial variation of median misalignment angles within an active region (up to 75%) exceeds the temporal variation associated with the flare (up to 40%). We also examine estimates of the stereoscopic error of our analysis. The corrected values yield a residual misalignment of 7° - 13°, which is attributed to the non-potentiality due to currents in the active regions. Title: 3-D reconstruction of active regions with STEREO Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre Bibcode: 2011JASTP..73.1082A Altcode: We review data analysis and physical modeling related to the 3-D reconstruction of active regions in the solar corona, using stereoscopic image pairs from the STEREO/EUVI instrument. This includes the 3-D geometry of coronal loops (with measurements of the loop inclination plane, coplanarity, circularity, and hydrostaticity), the 3-D electron density and temperature distribution (which enables diagnostics of hydrostatic, hydrodynamic, and heating processes), the 3-D magnetic field (independent of any theoretical model based on photospheric extrapolations), as well as the 3-D reconstruction of CME phenomena, such as EUV dimming, CME acceleration, CME bubble expansion, and associated Lorentz forces that excite MHD kink-mode oscillations in the surroundings of a CME launch site. The mass of CMEs, usually measured from white-light coronagraphs, can be determined independently from the EUV dimming in the CME source region. The detailed 3-D density and temperature structure of an active region can be modeled using the method of instant stereoscopic tomography with orders of magnitude higher spatial resolution than with standard solar-rotation tomography. Title: SDO/AIA Observations of a Global EUV Disturbance Traveling into a Coronal Cavity and Its Subsequent Oscillations: New Evidence of Fast Mode MHD Waves Authors: Liu, Wei; Aschwanden, M. J.; Ofman, L.; Nitta, N. V.; Tarbell, T. D. Bibcode: 2011SPD....42.0906L Altcode: 2011BAAS..43S.0906L We report new SDO/AIA observations of a global EUV disturbance that propagates at 600 km/s and sweeps through a coronal cavity, instigating its bodily transverse oscillations. The high temporal resolution and large FOV of AIA allow us to clearly see, for the first time, the timing coincidence between the onsets of the oscillations and the arrival of the disturbance at increasing distances covering 300 Mm in the neighborhood of the cavity. There is a time delay of the oscillations from the near side to the far side of the cavity, which is consistent with the travel time of the global perturbation. In addition, we find a fine structure consisting of evenly spaced pulses of periods 100-120 s within the global disturbance. In contrast, the CME loop expansion falls behind the global disturbance at a smaller velocity of 200 km/s. These observations suggests that this global disturbance is a real fast mode MHD wave that continues propagating into the cavity, rather than an apparent wave caused by CME expulsion that is not expected to penetrate through a topological separatrix, including the flux rope cavity boundary here. The cavity and its hosted prominence have oscillation amplitudes of 20 km/s and periods of 20-30 minutes. Such unusually long periods, compared with a few minutes commonly observed in coronal loops, likely reflect kink mode oscillations of the long cavity flux rope of a large length (a fraction of the solar radius). Title: Energy-Dependent Timing of Thermal Emission in Solar Flares Authors: Jain, Rajmal; Awasthi, Arun Kumar; Rajpurohit, Arvind Singh; Aschwanden, Markus J. Bibcode: 2011SoPh..270..137J Altcode: 2011SoPh..tmp...60J; 2011SoPh..tmp...83J; 2011arXiv1103.5546J We report solar flare plasma to be multi-thermal in nature based on the theoretical model and study of the energy-dependent timing of thermal emission in ten M-class flares. We employ high-resolution X-ray spectra observed by the Si detector of the "Solar X-ray Spectrometer" (SOXS). The SOXS onboard the Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May 2003. Firstly we model the spectral evolution of the X-ray line and continuum emission flux F(ε) from the flare by integrating a series of isothermal plasma flux. We find that the multi-temperature integrated flux F(ε) is a power-law function of ε with a spectral index (γ)≈−4.65. Next, based on spectral-temporal evolution of the flares we find that the emission in the energy range E=4 - 15 keV is dominated by temperatures of T=12 - 50 MK, while the multi-thermal power-law DEM index (δ) varies in the range of −4.4 and −5.7. The temporal evolution of the X-ray flux F(ε,t) assuming a multi-temperature plasma governed by thermal conduction cooling reveals that the temperature-dependent cooling time varies between 296 and 4640 s and the electron density (ne) varies in the range of ne=(1.77 - 29.3)×1010 cm−3. Employing temporal evolution technique in the current study as an alternative method for separating thermal from nonthermal components in the energy spectra, we measure the break-energy point, ranging between 14 and 21±1.0 keV. Title: Multi-thermal observations of the 2010 October 16 flare:heating of a ribbon via loops, or a blast wave? Authors: Christe, Steven; Inglis, A.; Aschwanden, M.; Dennis, B. Bibcode: 2011SPD....42.2237C Altcode: 2011BAAS..43S.2237C On 2010 October 16th SDO/AIA observed its first flare using automatic exposure control. Coincidentally, this flare also exhibited a large number of interesting features. Firstly, a large ribbon significantly to the solar west of the flare kernel was ignited and was visible in all AIA wavelengths, posing the question as to how this energy was deposited and how it relates to the main flare site. A faint blast wave also emanates from the flare kernel, visible in AIA and observed traveling to the solar west at an estimated speed of 1000 km/s. This blast wave is associated with a weak white-light CME observed with STEREO B and a Type II radio burst observed from Green Bank Observatory (GBSRBS). One possibility is that this blast wave is responsible for the heating of the ribbon. However, closer scrutiny reveals that the flare site and the ribbon are in fact connected magnetically via coronal loops which are heated during the main energy release. These loops are distinct from the expected hot, post-flare loops present within the main flare kernel. RHESSI spectra indicate that these loops are heated to approximately 10 MK in the immediate flare aftermath. Using the multi-temperature capabilities of AIA in combination with RHESSI, and by employing the cross-correlation mapping technique, we are able to measure the loop temperatures as a function of time over several post-flare hours and hence measure the loop cooling rate. We find that the time delay between the appearance of loops in the hottest channel, 131 A, and the cool 171 A channel, is 70 minutes. Yet the causality of this event remains unclear. Is the ribbon heated via these interconnected loops or via a blast wave? Title: Solar Corona Loop Studies with the Atmospheric Imaging Assembly. I. Cross-sectional Temperature Structure Authors: Aschwanden, Markus J.; Boerner, Paul Bibcode: 2011ApJ...732...81A Altcode: 2011arXiv1103.0228A We present a first systematic study on the cross-sectional temperature structure of coronal loops using the six coronal temperature filters of the Atmospheric Imaging Assembly (AIA) instrument on the Solar Dynamics Observatory (SDO). We analyze a sample of 100 loop snapshots measured at 10 different locations and 10 different times in active region NOAA 11089 on 2010 July 24, 21:00-22:00 UT. The cross-sectional flux profiles are measured and a cospatial background is subtracted in six filters in a temperature range of T ≈ 0.5-16 MK, and four different parameterizations of differential emission measure (DEM) distributions are fitted. We find that the reconstructed DEMs consist predominantly of narrowband peak temperature components with a thermal width of σlog (T) <= 0.11 ± 0.02, close to the temperature resolution limit of the instrument, consistent with earlier triple-filter analysis from the Transition Region and Coronal Explorer by Aschwanden & Nightingale and from EIS/Hinode by Warren et al. or Tripathi et al. We find that 66% of the loops could be fitted with a narrowband single-Gaussian DEM model, and 19% with a DEM consisting of two narrowband Gaussians (which mostly result from pairs of intersecting loops along the same line of sight). The mostly isothermal loop DEMs allow us also to derive an improved empirical response function of the AIA 94 Å filter, which needs to be boosted by a factor of q 94 = 6.7 ± 1.7 for temperatures at log (T) <~ 6.3. The main result of near-isothermal loop cross-sections is not consistent with the predictions of standard nanoflare scenarios, but can be explained by flare-like heating mechanisms that drive chromospheric evaporation and upflows of heated plasma coherently over loop cross-sections of w ≈ 2-4 Mm. Title: Direct Imaging by SDO/AIA of Quasi-periodic Propagating Fast Mode Magnetosonic Waves of 2000 km/s in the Solar Corona Authors: Liu, Wei; Title, A. M.; Zhao, J.; Ofman, L.; Schrijver, C. J.; Aschwanden, M. J.; De Pontieu, B.; Tarbell, T. D. Bibcode: 2011SPD....42.2114L Altcode: 2011BAAS..43S.2114L Quasi-periodic, propagating fast mode magnetosonic waves in the corona were difficult to observe in the past due to relatively low instrument cadences. We report here unprecedented evidence of such waves directly imaged in EUV by the new SDO/AIA instrument. In the 2010 August 1 C3.2 flare/CME event, we find arc-shaped wave trains of 1-5% intensity variations emanating near the flare kernel and propagating outward along a funnel of coronal loops. Sinusoidal fits to a typical wave train indicate a phase velocity of 2350 +/- 210 km/s. Similar waves propagating in opposite directions are observed in closed loops between two flare ribbons. In the k-omega diagram of the Fourier wave power, we find a bright ridge that represents the dispersion relation and can be well fitted with a straight line passing through the origin, giving an equal phase and group velocity of 1630 +/- 760 km/s averaged over the event. This k-omega ridge shows a broad frequency distribution with prominent power at four non-harmonic frequencies, 5.5, 14.5, 25.1, and 37.9 mHz, among which the 14.5 mHz (period: 69 s) signal is the strongest. The signal at 5.5 mHz (period: 181 s, same as chromospheric 3-minute oscillations) temporally coincides with flare pulsations, suggesting a common origin of possibly quasi-periodic magnetic reconnection. The instantaneous wave energy flux of (0.1-2.6)e7 ergs/cm2/s estimated at the coronal base is comparable to the steady-state heating requirement of active region loops. Title: Temperature Analysis of Coronal Loop Cross-Sections: Monolithic vs. Nanoflare Heating Authors: Aschwanden, Markus J.; Boerner, P. Bibcode: 2011SPD....42.0504A Altcode: 2011BAAS..43S.0504A We present a first systematic study on the cross-sectional temperature structure of coronal loops using the six coronal temperature filters of the Atmospheric Imaging Assembly (AIA) instrument on the Solar Dynamics Observatory (SDO). We analyze a sample of 100 loop snapshots measured at 10 different locations and 10 different times in active region NOAA 11089 on 2010 July 24, 21:00-22:00 UT.

The cross-sectional flux profiles are measured and a cospatial background is subtracted in 6 filters in a temperature range of T ≈ 0.5-16 MK, and 4 different parameterizations of differential emission measure (DEM) distributions are fitted. We find that the reconstructed DEMs consist predominantly of narrowband peak temperature components with a thermal width of &#963log(T) ≤ 0.11±0.02, close to the temperature resolution limit of the instrument, consistent with earlier triple-filter analysis from TRACE by Aschwanden and Nightingale (2005) and from EIS/Hinode by Warren et al. (2008) or Tripathi et al. (2009). We find that 66% of the loops could be fitted with a narrowband single-Gaussian DEM model, and 19% with a DEM consisting of two narrowband Gaussians (which mostly result from pairs of intersecting loops along the same line-of-sight). The mostly isothermal loop DEMs allow us also to derive an improved empirical response function of the AIA 94 [[Unable to Display Character: &#506]] filter, which needs to be boosted by a factor of q94 = 6.7± 1.7 for temperatures at log(T) </≈ 6.3. The main result of near-isothermal loop cross-sections is not consistent with the predictions of standard nanoflare scenarios, but can be explained by flare-like heating mechanisms that drive chromospheric evaporation and upflows of heated plasma coherently over loop cross-sections of w ≈ 2-4 Mm. Title: Modeling Fast Magnetosonic Waves Observed by SDO in Active region Funnels Authors: Ofman, Leon; Liu, W.; Title, A.; Aschwanden, M. Bibcode: 2011SPD....42.2104O Altcode: 2011BAAS..43S.2104O Recently, quasi-periodic, propagating waves have been observed in EUV by the SDO/AIA instrument in about 10 flare/CME events thus far. A typical example is the waves associated with the 2010 August 1 C3.2 flare/CME that exhibited arc-shaped wave trains propagating in an active region magnetic funnel with 5% intensity variations at speeds in the range of 1000-2000 km/s. The fast temporal cadence and high sensitivity of AIA enabled the detection of these waves. We identify them as fast magnetosonic waves driven quasi-periodically at the base of the flaring region, and develop a three-dimensional MHD model of the event. For the initial state we utilize the dipole magnetic field to model the active region, and include gravitationally stratified density at coronal temperature. At the coronal base of the active region we excite the fast magnetosonic wave by periodic velocity pulsations in the photospheric plane confined to the funnel of magnetic field line. The excited fast magnetosonic waves have similar amplitude, wavelength and propagation speeds as the observed wave trains. Based on the simulation results, we discuss the possible excitation mechanism of the waves, their dynamical properties, and the use of the event for coronal MHD seismology. Title: Simulating Coronal Emission in Six AIA Channels Using Quasi-Static Atmosphere Models and Non-Linear Magnetic Field Models Authors: Malanushenko, Anna; Schrijver, C.; DeRosa, M.; Aschwanden, M.; Wheatland, M. S.; van Ballegooijen, A. A. Bibcode: 2011SPD....42.2116M Altcode: 2011BAAS..43S.2116M We present the results of simulations of the EUV coronal emission in AIA channels. We use a non-linear force-free model of magnetic field constructed in such a way that its field lines resemble the observed coronal loops in EUV. We then solve one-dimensional quasi-steady atmosphere model along the magnetic field lines (Schrijver & Ballegooijen, 2005). Using coronal abundances from CHIANTI and AIA response functions we then simulate the emission that would be observed in AIA EUV channels. The resulting intensities are compared against the real observations in a manner similar to that in Aschwanden et. al., 2011. The study is similar to those by Lindquist et. al., 2008, with a few important differences. We use a model of the coronal magnetic field that resembles the topology observed in EUV, we study EUV emission of cool loops (rather than SXR) and we make use of high resolution and cadence AIA and HMI data. Title: Determining the Structure of Solar Coronal Loops Using Their Evolution Authors: Mulu-Moore, Fana M.; Winebarger, Amy R.; Warren, Harry P.; Aschwanden, Markus J. Bibcode: 2011ApJ...733...59M Altcode: Despite significant progress in understanding the dynamics of the corona, there remain several unanswered questions about the basic physical properties of coronal loops. Recent observations from different instruments have yielded contradictory results about some characteristics of coronal loops, specifically as to whether the observed loops are spatially resolved. In this paper, we examine the evolution of coronal loops through two extreme-ultraviolet filters and determine if they evolve as a single cooling strand. We measure the temporal evolution of eight active region loops previously studied and found to be isothermal and resolved by Aschwanden & Nightingale. All eight loops appear in "hotter" TRACE filter images (Fe XII 195 Å) before appearing in the "cooler" (Fe IX/Fe X 171 Å) TRACE filter images. We use the measured delay between the two filters to calculate a cooling time and then determine if that cooling time is consistent with the observed lifetime of the loop. We do this twice: once when the loop appears (rise phase) and once when it disappears (decay phase). We find that only one loop appears consistent with a single cooling strand and hence could be considered to be resolved by TRACE. For the remaining seven loops, their observed lifetimes are longer than expected for a single cooling strand. We suggest that these loops could be formed of multiple cooling strands, each at a different temperature. These findings indicate that the majority of loops observed by TRACE are unresolved. Title: Self-Organized Criticality in Astrophysics Authors: Aschwanden, Markus J. Bibcode: 2011soca.book.....A Altcode: Chapter 1: Self-organized criticality phenomena

Chapter 2: Numerical SOC models

Chapter 3: Anaytical SOC models

Chapter 4: Statistics of random processes

Chapter 5: Waiting time distributions

Chapter 6: Event detection methods

Chapter 7: Occurrence frequency distributions

Chapter 8: Fractal geometry

Chapter 9: Physical SOC models

Chapter 10: SOC-like models Title: First Results on Coronal Loop Analysis with AIA/SDO Authors: Aschwanden, M. J. Bibcode: 2010AGUFMSH11A1599A Altcode: A decisive question with regard to coronal heating is whether the cross-sectional structure of coronal loops is isothermal or multi-thermal. If the cross-section of an observed coronal loop has an isothermal structure, this indicates macroscopically resolved heating mechanisms, such as chromospheric heating by a flare-like process that channels upflow of heated plasma through the diverging canopy geometry in the transition region and fills coronal fluxtubes with diameters corresponding to the separation of photospheric magneto-convection cells, observed as granulation with spatial scales of about 1000 km. Such diameters of coronal fluxtubes can be resolved with AIA/SDO. On the other side, if the observed loop cross-sections have a multi-thermal structure, this could indicate a substructure with unresolved loop strands with different temperatures, as they would be produced by theoretical nanoflare heating scenarios. Using the 6 coronal filters of AIA/SDO we model the differential emission measure distribution of the smallest resolved coronal loops in active region NOAA 11089, observed on 2010 July 24. We present the first results of DEM modeling that reveals statistics of iso-thermal and multi-thermal loops and their dependence on other physical parameters (temperature, density, loop widths). We discuss the consequences for coronal heating theories. Title: Uncovering Mechanisms of Coronal Magnetism via Advanced 3D Modeling of Flares and Active Regions Authors: Fleishman, Gregory; Gary, Dale; Nita, Gelu; Alexander, David; Aschwanden, Markus; Bastian, Tim; Hudson, Hugh; Hurford, Gordon; Kontar, Eduard; Longcope, Dana; Mikic, Zoran; DeRosa, Marc; Ryan, James; White, Stephen Bibcode: 2010arXiv1011.2800F Altcode: The coming decade will see the routine use of solar data of unprecedented spatial and spectral resolution, time cadence, and completeness. To capitalize on the new (or soon to be available) facilities such as SDO, ATST and FASR, and the challenges they present in the visualization and synthesis of multi-wavelength datasets, we propose that realistic, sophisticated, 3D active region and flare modeling is timely and critical, and will be a forefront of coronal studies over the coming decade. To make such modeling a reality, a broad, concerted effort is needed to capture the wealth of information resulting from the data, develop a synergistic modeling effort, and generate the necessary visualization, interpretation and model-data comparison tools to accurately extract the key physics. Title: 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: Bootstrapping the Coronal Magnetic Field with STEREO: Unipolar Potential Field Modeling Authors: Aschwanden, Markus J.; Sandman, Anne W. Bibcode: 2010AJ....140..723A Altcode: 2010arXiv1004.4920A We investigate the recently quantified misalignment of αmis ≈ 20°-40° between the three-dimensional geometry of stereoscopically triangulated coronal loops observed with STEREO/EUVI (in four active regions (ARs)) and theoretical (potential or nonlinear force-free) magnetic field models extrapolated from photospheric magnetograms. We develop an efficient method of bootstrapping the coronal magnetic field by forward fitting a parameterized potential field model to the STEREO-observed loops. The potential field model consists of a number of unipolar magnetic charges that are parameterized by decomposing a photospheric magnetogram from the Michelson Doppler Imager. The forward-fitting method yields a best-fit magnetic field model with a reduced misalignment of αPF ≈ 13°-20°. We also evaluate stereoscopic measurement errors and find a contribution of αSE ≈ 7°-12°, which constrains the residual misalignment to αNP ≈ 11°-17°, which is likely due to the nonpotentiality of the ARs. The residual misalignment angle, αNP, of the potential field due to nonpotentiality is found to correlate with the soft X-ray flux of the AR, which implies a relationship between electric currents and plasma heating. Title: A Universal Scaling Law for the Fractal Energy Dissipation Domain in Self-Organized Criticality Systems Authors: Aschwanden, Markus J. Bibcode: 2010arXiv1008.0873A Altcode: Nonlinear dissipative systems in the state of self-organized criticality release energy sporadically in avalanches of all sizes, such as in earthquakes, auroral substorms, solar and stellar flares, soft gamma-ray repeaters, and pulsar glitches. The statistical occurrence frequency distributions of event energies $E$ generally exhibit a powerlaw-like function $N(E)\propto E^{-\alpha_E}$ with a powerlaw slope of $\alpha_E \approx 1.5$. The powerlaw slope $\alpha_E$ of energies can be related to the fractal dimension $D$ of the spatial energy dissipation domain by $D=3/\alpha_E$, which predicts a powerlaw slope $\alpha_E=1.5$ for area-rupturing or area-spreading processes with $D=2$. For solar and stellar flares, 2-D area-spreading dissipation domains are naturally provided in current sheets or separatrix surfaces in a magnetic reconnection region. Thus, this universal scaling law provides a useful new diagnostic on the topology of the spatial energy dissipation domain in geophysical and astrophysical observations. Title: Reconciliation of Waiting Time Statistics of Solar Flares Observed in Hard X-rays Authors: Aschwanden, Markus J.; McTiernan, James M. Bibcode: 2010ApJ...717..683A Altcode: 2010arXiv1002.4869A We study the waiting time distributions of solar flares observed in hard X-rays with ISEE-3/ICE, HXRBS/SMM, WATCH/GRANAT, BATSE/CGRO, and RHESSI. Although discordant results and interpretations have been published earlier, based on relatively small ranges (<2 decades) of waiting times, we find that all observed distributions, spanning over 6 decades of waiting times (Δt ≈ 10-3-103 hr), can be reconciled with a single distribution function, N(Δt) vprop λ0(1 + λ0Δt)-2, which has a power-law slope of p ≈ 2.0 at large waiting times (Δt ≈ 1-1000 hr) and flattens out at short waiting times Δt <~ Δt 0 = 1/λ0. We find a consistent breakpoint at Δt 0 = 1/λ0 = 0.80 ± 0.14 hr from the WATCH, HXRBS, BATSE, and RHESSI data. The distribution of waiting times is invariant for sampling with different flux thresholds, while the mean waiting time scales reciprocically with the number of detected events, Δt 0 vprop 1/n det. This waiting time distribution can be modeled with a nonstationary Poisson process with a flare rate λ = 1/Δt that varies as f(λ) vprop λ-1exp - (λ/λ0). This flare rate distribution requires a highly intermittent flare productivity in short clusters with high rates, separated by relatively long quiescent intervals with very low flare rates. 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: Bootstrapping the Coronal Magnetic Field with STEREO/EUVI Authors: Aschwanden, Markus; Sandman, Anne Bibcode: 2010EGUGA..1214426A Altcode: The 3D coronal magnetic field obtained from stereoscopically triangulated loops has been compared with standard photospheric magnetogram extrapolations. We found a large misalignment of 20-40 deg, depending on the complexity of an AR (Sandman et al. 2009; DeRosa et al. 2009). These studies prove that the magnetic field in the photosphere is not force-free and fundamentally cannot reproduce the coronal magnetic field. Bootstrapping with coronal loop 3D geometries are required to improve modeling of the coronal field. Such coronal field bootstrapping methods are currently developed using stereoscopically triangulated loops from STEREO/EUVI and preliminary results show already a significantly reduced misalignment of 10-20 deg. Title: Coronal Dimming And Waves Observed In Flare-Associated CMEs Authors: Nitta, Nariaki; Aschwanden, M.; Freeland, S.; Lemen, J.; Wuelser, J.; Zarro, D. Bibcode: 2010AAS...21640614N Altcode: 2010BAAS...41..882N The relationship between solar flares and coronal mass ejections (CMEs) is still an active area of research. It is studied from various aspects. Our goal is to understand the importance of magnetic reconnection in launching CMEs and that of magnetic field environment of the flaring regions to determine how eruptive flares are. We have studied the association of solar flares during 2007-2009 with CMEs, using primarily extreme-ultraviolet (EUV) and inner coronagraphic images from the Solar Terrestrial Relations Observatory (STEREO). While energetic CMEs tend to accompany a flare, flares with relatively high soft X-ray intensity for the extended solar minimum conditions are often found without an associated CME, even though the underlying photospheric magnetic field is strong and complex. In contrast, some of the regions hosting flare-associated CMEs have weak photospheric field, sometimes not even classified as active regions because of no sunspots. Out of several signatures in low coronal images previously raised as proxies for CMEs, large-scale dimming that persists for at least an hour is found to be a sufficient condition. Waves in EUV images, on the other hand, may not necessarily signal an appreciable CME that is still clearly observed beyond, for example, 5 Rsun, unless the concurrent dimming is substantial. This suggests that waves detected in EUV images may have more than one origins. We present the result of the survey and discuss a number of well-observed cases focusing on the properties of the flares and CMEs with respect to the extents of dimming and wave. Title: Bootstrapping the Coronal Magnetic Field with STEREO Authors: Aschwanden, Markus J. Bibcode: 2010AAS...21630601A Altcode: The 3D coronal magnetic field obtained from stereoscopically triangulated loops has been compared with standard photospheric magnetogram extrapolations. We found a large misalignment of 20-40 deg, depending on the complexity of an AR (Sandman et al. 2009; DeRosa et al. 2009). These studies prove that the magnetic field in the photosphere is not force-free and fundamentally cannot reproduce the coronal magnetic field. Bootstrapping with coronal loop 3D geometries are required to improve modeling of the coronal field. Such coronal field bootstrapping methods are currently developed using stereoscopically triangulated loops from STEREO/EUVI and preliminary results show already a significantly reduced misalignment of 10-20 deg. Title: A Code for Automated Tracing of Coronal Loops Approaching Visual Perception Authors: Aschwanden, Markus J. Bibcode: 2010SoPh..262..399A Altcode: 2010SoPh..tmp...45A; 2010SoPh..tmp...57A We develop a new numerical code with automated feature extraction, customized for tracing of coronal loops, a method we call Oriented Coronal CUrved Loop Tracing (OCCULT), which for the first time breaks even with the results of visual tracing. The method used is based on oriented-directivity tracing of curvi-linear features, but in contrast to other general feature-extraction algorithms, it is customized for solar EUV and SXR images by taking advantage of the specific property that coronal loops have large curvature radii compared with their widths. We evaluate the performance of this new code by comparing the cumulative distribution of loop lengths, the median and maximum loop lengths, the completeness of detection, and the congruency of the detected features with other numerical codes and visual tracings. We find that the new code closely approaches the results of visual perception and outperforms the other existing numerical codes. This algorithm is useful for the 3D reconstruction of the geometry, motion, and oscillations of coronal loops, with single or stereoscopic spacecraft, as well as for modeling of the loop hydrodynamics and the coronal magnetic field. Title: Image Processing Techniques and Feature Recognition in Solar Physics Authors: Aschwanden, Markus J. Bibcode: 2010SoPh..262..235A Altcode: This review presents a comprehensive and systematic overview of image-processing techniques that are used in automated feature-detection algorithms applied to solar data: i) image pre-processing procedures, ii) automated detection of spatial features, iii) automated detection and tracking of temporal features (events), and iv) post-processing tasks, such as visualization of solar imagery, cataloguing, statistics, theoretical modeling, prediction, and forecasting. For each aspect the most recent developments and science results are highlighted. We conclude with an outlook on future trends. Title: Self-Organized Criticality in Solar Physics and Astrophysics Authors: Aschwanden, Markus J. Bibcode: 2010arXiv1003.0122A Altcode: The concept of "self-organized criticality" (SOC) has been introduced by Bak, Tang, and Wiesenfeld (1987) to describe the statistics of avalanches on the surface of a sandpile with a critical slope, which produces a scale-free powerlaw size distribution of avalanches. In the meantime, SOC behavior has been identified in many nonlinear dissipative systems that are driven to a critical state. On a most general level, SOC is the statistics of coherent nonlinear processes, in contrast to the Poisson statistics of incoherent random processes. The SOC concept has been applied to laboratory experiments (of rice or sand piles), to human activities (population growth, language, economy, traffic jams, wars), to biophysics, geophysics (earthquakes, landslides, forest fires), magnetospheric physics, solar physics (flares), stellar physics (flares, cataclysmic variables, accretion disks, black holes, pulsar glitches, gamma ray bursts), and to galactic physics and cosmology. Title: CME-related Phenomena and Solar Flares Authors: Nitta, Nariaki; Aschwanden, Markus; Freeland, Samuel; Lemen, James; Wuelser, Jean-Pierre; Zarro, Dominic Bibcode: 2010cosp...38.1792N Altcode: 2010cosp.meet.1792N The relationship between solar flares and coronal mass ejections (CMEs) is still an active area of research. It is studied from various aspects. Our goal is to understand the importance of magnetic reconnection in launching CMEs and that of magnetic field environment of the flaring regions to determine how eruptive flares are. We have studied the association of solar flares dur-ing 2007-2009 with CMEs, using primarily extreme-ultraviolet (EUV) and inner coronagraphic images from the Solar Terrestrial Relations Observatory (STEREO). While energetic CMEs tend to accompany a flare, flares with relatively high soft X-ray intensity for the extended solar minimum conditions are often found without an associated CME, even though the underlying photospheric magnetic field is strong and complex. In contrast, some of the regions hosting flare-associated CMEs have weak photospheric field, sometimes not even classified as active regions because of no sunspots. Out of several signatures in low coronal images previously raised as proxies for CMEs, large-scale dimming that persists for at least an hour is found to be a sufficient condition. Waves in EUV images, on the other hand, may not necessarily signal an appreciable CME that is still clearly observed beyond, for example, 5 Rsun, unless the concur-rent dimming is substantial. This suggests that waves detected in EUV images may have more than one origins. We present the result of the survey and discuss a number of well-observed cases focusing on the properties of the flares and CMEs with respect to the extents of dimming and wave. Title: Energy-dependent timing of thermal emission in solar flares Authors: Jain, Rajmal; Rajpurohit, Arvind; Awasthi, Arun; Aschwanden, Markus Bibcode: 2010cosp...38.2965J Altcode: 2010cosp.meet.2965J A study of thermal emission in solar flares using high-resolution X-ray spectra observed by the Si detector onboard "Solar X-ray Spectrometer" (SOXS) has been conducted. The SOXS onboard GSAT-2 Indian spacecraft was launched by GSLV-D2 rocket on 08 May 2003. With this we investigate the energy dependent timing of thermal emission in solar flares. Firstly we model the spectral-temporal evolution of the X-ray flux F(e,t) assuming multi-temperature plasma governed by thermal conduction cooling. This model is found in agreement with the temperature and emission measure derived from the fitting of the spectra observed by the Si detector. We investigate 10 M-class flares and found that the emission in the energy range e=6 -20 keV is dominated by temperatures T=15 -50 MK while the power-law index (gamma) of the thermal spectrum varies over 2.7 -4.3. The temperature-dependent cooling time varies between 22 and 310 s. The electron density (ne) obtained for the flares under investigation ranges between 0.03 and 5X1011 cm-3 suggests that conduction cooling of thermal X-ray plasma dominates over radiative cooling in the initial phase of the flare. The current study also provides an alternative method for separating thermal from non-thermal spectra, which enables us to measure the break-energy point to be varying between 17 and 220.7 keV. Title: Joint STEREO-Hinode Observations of Coronal Dimming and Waves Associated with a CME/Flare Event Authors: Nitta, Nariaki; Aschwanden, Markus; Frank, Zoe; Slater, Gregory; Tarbell, Theodore; Zarro, Dominic Bibcode: 2010cosp...38.2928N Altcode: 2010cosp.meet.2928N During the solar minimum between cycles 23 and 24, a number of minor flares from unimpressive active regions were associated with large-scale dimming and waves as observed by the EUV Imager (EUVI) on STEREO. We present a detailed study on one of the CME/flare events that was observed also by the instruments on Hinode. We analyze SOT Ca II data to explore the origin of the disturbances and EIS slit spectra to determine the timings of the upflow and associated turbulence that are presumably correlated with coronal dimming. The sequence of phenomena as captured by different instruments on STEREO, Hinode and SOHO helps us identify the key physical processes that gave rise to the event. Title: Spatial Scaling Law in Solar Flares Authors: Aschwanden, Markus Bibcode: 2010cosp...38.2995A Altcode: 2010cosp.meet.2995A How do the observed macroscopic scales observed in solar flares relate to the theoretically implied microscopic scales of energy release and particle acceleration processes. We approach this question by investigating the fast milli-second time scales observed during the impulsive flare phase in hard X-ray and radio wavelengths and their relation to the spatial scales observed in hard and soft X-ray images. Previous studies with Yohkoh and CGRO data established a scaling law between the temporal and spatial scales. We investigate also the fractal spatial patterns of flares observed at arc-second spatial resolution with TRACE and deduce the fractal dimension in 2D and filling factors in 3D. We discuss the implications of these observational results for the spatial scaling of the underlying magnetic reconnection processes. Title: The 3D Geometry, Motion, and Hydrodynamic Aspects of Oscillating Coronal Loops Authors: Aschwanden, Markus J. Bibcode: 2009SSRv..149...31A Altcode: We transition from two-dimensional (2D) imaging observations of kink-mode loop oscillations in the solar corona to three-dimensional (3D) reconstructions by exploring two new methods: (1) De-projection of 2D loop tracings using the strategy of curvature radius maximization in 3D space, based on the assumption of force-free magnetic fields; and (2) stereoscopic triangulation of epipolar loop coordinates using coaligned images from the STEREO EUVI/A and B spacecraft. Both methods reveal new features of oscillating loops: non-circularity, non-planarity, and helical geometries. We extend the 3D reconstruction techniques into the time domain and find indications of circularly polarized (helical) kink-mode oscillations, in contrast to linearly polarized modes assumed previously. We discuss also hydrodynamic effects of coronal loops in non-equilibrium state that are essential for the detection and modeling of kink-mode oscillations. Title: 4D Modeling of CME Expansion and EUV Dimming Observed with STEREO/EUVI Authors: Aschwanden, M. J. Bibcode: 2009AGUFMSH41A1644A Altcode: This is the first attempt to model the kinematics of a CME launch and the resulting EUV dimming quantitatively with a self-consistent model. Our 4D-model assumes self-similar expansion of a spherical CME geometry that consists of a CME front with density compression and a cavity with density rarefaction, satisfying mass conservation of the total CME and swept-up corona. The model contains 14 free parameters and is fitted to the 2008 March 25 CME event observed with STEREO/A and B. Our model is able to reproduce the observed CME expansion and related EUV dimming during the initial phase from 18:30 UT to 19:00 UT. The CME kinematics can be characterized by a constant acceleration (i.e., a constant magnetic driving force). While the observations of EUVI/A are consistent with a spherical bubble geometry, we detect significant asymmetries and density inhomogeneities with EUVI/B. This new forward-modeling method demonstrates how the observed EUV dimming can be used to model physical parameters of the CME source region, the CME geometry, and CME kinematics. Title: Hydrodynamic Modeling of Coronal Loops with Hinode and STEREO Authors: Aschwanden, M. J. Bibcode: 2009ASPC..415..234A Altcode: The hydrodynamic evolution of impulsively-heated coronal loops and their subsequent cooling can now be modeled with multi-wavelength imaging instruments in soft X-ray (SXR) and extreme ultraviolet (EUV) wavelengths. Using analytical approximations to the hydrodynamic evolution of the density n(s,t) and temperature T(s,t) of an impulsively-heated loop (as a function of the loop length coordinate s and time t) we show an example how lightcurves observed with HINODE/XRT, EIS, GOES, and STEREO/EUVI can be modeled with a forward-fitting method in order to infer the maximum heating rate, the heating duration, and the cooling time of a heated loop during a small B1-class flare on 2007 February 1, previously analyzed by Warren et al. (2007). Title: The Hydrodynamic Evolution of Impulsively Heated Coronal Loops: Explicit Analytical Approximations Authors: Aschwanden, Markus J.; Tsiklauri, David Bibcode: 2009ApJS..185..171A Altcode: We derive simple analytical approximations (in explicit form) for the hydrodynamic evolution of the electron temperature T(s, t) and electron density n(s, t), for one-dimensional coronal loops that are subject to impulsive heating with subsequent cooling. Our analytical approximations are derived from first principles, using (1) the hydrodynamic energy balance equation, (2) the loop scaling laws of Rosner-Tucker-Vaiana and Serio, (3) the Neupert effect, and (4) the Jakimiec relationship. We compare our analytical approximations with 56 numerical cases of time-dependent hydrodynamic simulations from a parametric study of Tsiklauri et al., covering a large parameter space of heating rates, heating timescales, heating scale heights, loop lengths, for both footpoint and apex heating, mostly applicable to flare conditions. The average deviations from the average temperature and density values are typically ≈20% for our analytical expressions. The analytical approximations in explicit form provide an efficient tool to mimic time-dependent hydrodynamic simulations, to model observed soft X-rays and extreme-ultraviolet light curves of heated and cooling loops in the solar corona and in flares by forward fitting, to model microflares, to infer the coronal heating function from light curves of multi-wavelength observations, and to provide physical models of differential emission measure distributions for solar and stellar flares, coronae, and irradiance. Title: First Measurements of the Mass of Coronal Mass Ejections from the EUV Dimming Observed with STEREO EUVI A+B Spacecraft Authors: Aschwanden, Markus J.; Nitta, Nariaki V.; Wuelser, Jean-Pierre; Lemen, James R.; Sandman, Anne; Vourlidas, Angelos; Colaninno, Robin C. Bibcode: 2009ApJ...706..376A Altcode: The masses of coronal mass ejections (CMEs) have traditionally been determined from white-light coronagraphs (based on Thomson scattering of electrons), as well as from extreme ultraviolet (EUV) dimming observed with one spacecraft. Here we develop an improved method of measuring CME masses based on EUV dimming observed with the dual STEREO/EUVI spacecraft in multiple temperature filters that includes three-dimensional volume and density modeling in the dimming region and background corona. As a test, we investigate eight CME events with previous mass determinations from STEREO/COR2, of which six cases are reliably detected with the Extreme Ultraviolet Imager (EUVI) using our automated multi-wavelength detection code. We find CME masses in the range of m CME = (2-7) × 1015 g. The agreement between the two EUVI/A and B spacecraft is mA /mB = 1.3 ± 0.6 and the consistency with white-light measurements by COR2 is m EUVI/m COR2 = 1.1 ± 0.3. The consistency between EUVI and COR2 implies no significant mass backflows (or inflows) at r < 4 R sun and adequate temperature coverage for the bulk of the CME mass in the range of T ≈ 0.5-3.0 MK. The temporal evolution of the EUV dimming allows us to also model the evolution of the CME density ne (t), volume V(t), height-time h(t), and propagation speed v(t) in terms of an adiabatically expanding self-similar geometry. We determine e-folding EUV dimming times of tD = 1.3 ± 1.4 hr. We test the adiabatic expansion model in terms of the predicted detection delay (Δt ≈ 0.7 hr) between EUVI and COR2 for the fastest CME event (2008 March 25) and find good agreement with the observed delay (Δt ≈ 0.8 hr). Title: Comparison of STEREO/EUVI Loops with Potential Magnetic Field Models Authors: Sandman, A. W.; Aschwanden, M. J.; DeRosa, M. L.; Wülser, J. P.; Alexander, D. Bibcode: 2009SoPh..259....1S Altcode: The Solar Terrestrial Relations Observatory (STEREO) provides the first opportunity to triangulate the three-dimensional coordinates of active region loops simultaneously from two different vantage points in space. Three-dimensional coordinates of the coronal magnetic field have been calculated with theoretical magnetic field models for decades, but it is only with the recent availability of STEREO data that a rigorous, quantitative comparison between observed loop geometries and theoretical magnetic field models can be performed. Such a comparison provides a valuable opportunity to assess the validity of theoretical magnetic field models. Here we measure the misalignment angles between model magnetic fields and observed coronal loops in three active regions, as observed with the Extreme Ultraviolet Imager (EUVI) on STEREO on 30 April, 9 May, and 19 May 2007. We perform stereoscopic triangulation of some 100 - 200 EUVI loops in each active region and compute extrapolated magnetic field lines using magnetogram information from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). We examine two different magnetic extrapolation methods: (1) a potential field and (2) a radially stretched potential field that conserves the magnetic divergence. We find considerable disagreement between each theoretical model and the observed loop geometries, with an average misalignment angle on the order of 20° - 40°. We conclude that there is a need for either more suitable (coronal rather than photospheric) magnetic field measurements or more realistic field extrapolation models. Title: 4-D modeling of CME expansion and EUV dimming observed with STEREO/EUVI Authors: Aschwanden, M. J. Bibcode: 2009AnGeo..27.3275A Altcode: 2009arXiv0908.1913A This is the first attempt to model the kinematics of a CME launch and the resulting EUV dimming quantitatively with a self-consistent model. Our 4-D-model assumes self-similar expansion of a spherical CME geometry that consists of a CME front with density compression and a cavity with density rarefaction, satisfying mass conservation of the total CME and swept-up corona. The model contains 14 free parameters and is fitted to the 25 March 2008 CME event observed with STEREO/A and B. Our model is able to reproduce the observed CME expansion and related EUV dimming during the initial phase from 18:30 UT to 19:00 UT. The CME kinematics can be characterized by a constant acceleration (i.e., a constant magnetic driving force). While the observations of EUVI/A are consistent with a spherical bubble geometry, we detect significant asymmetries and density inhomogeneities with EUVI/B. This new forward-modeling method demonstrates how the observed EUV dimming can be used to model physical parameters of the CME source region, the CME geometry, and CME kinematics. Title: Can We Use STEREO/EUVI to Improve Boundary Conditions for Magnetic Modeling? Authors: Sandman, Anne; Aschwanden, Markus J.; Alexander, David Bibcode: 2009shin.confE..19S Altcode: The STEREO mission provides a unique opportunity to observe active region loops simultaneously from two different vantage points in space. Using the stereoscopic data, we can triangulate 3D coordinates for active region loops and use them to perform quantitative comparisons between observed loop geometries and theoretical magnetic field models. These comparisons provide some insight into the validity of magnetic field models, but recent work (DeRosa et al. 2009, Sandman et al. 2009) has shown that potential and non-potential models yield equally poor agreement with observed loop structures. In order to improve the results of magnetic modeling we must resolve the discrepancy between the typically force-free domain (the corona) and non-force-free boundary condition (the photosphere or chromosphere). We seek to address this discrepancy by 'correcting' the boundary condition using observational constraints. The magnetic field in the low corona cannot currently be mapped directly, but using STEREO 3D loop coordinates we can constrain the orientation of the magnetic field in the low corona. In a previous study we measured the misalignment angles between model magnetic fields and observed coronal loops in three active regions, as observed with STEREO/EUVI on April 30, May 9, and May 19, 2007. We now attempt to use these misalignment angles to modify the magnetogram input to the magnetic field model such that the modified model has minimal misalignment with the observed coronal loops near the base of the corona. We present the results of a test case exploring the potential and limitations of this technique. Title: The possible role of vortex shedding in the excitation of kink-mode oscillations in the solar corona Authors: Nakariakov, V. M.; Aschwanden, M. J.; van Doorsselaere, T. Bibcode: 2009A&A...502..661N Altcode: We propose a model for the excitation of horizontally polarised transverse (kink) magnetoacoustic oscillations of solar coronal loops by upflows associated with coronal mass ejections. If the magnetic field in the plasma that is dragged in the vertical direction by the flow is parallel to the loop, the phenomenon of vortex shedding causes the appearance of a quasi-periodic horizontal force that is applied to alternating sides of the loop. The period of the force is determined by the flow speed and the loop's minor radius. The oscillations are excited the most effectively when the force is in resonance with the natural frequency of the kink oscillations. This model can explain the selectivity of the excitation of the oscillations and the initial growth of the oscillation amplitude. Title: Seismology of a Large Solar Coronal Loop from EUVI/STEREO Observations of its Transverse Oscillation Authors: Verwichte, E.; Aschwanden, M. J.; Van Doorsselaere, T.; Foullon, C.; Nakariakov, V. M. Bibcode: 2009ApJ...698..397V Altcode: The first analysis of a transverse loop oscillation observed by both Solar TErrestrial RElations Observatories (STEREO) spacecraft is presented, for an event on the 2007 June 27 as seen by the Extreme Ultraviolet Imager (EUVI). The three-dimensional loop geometry is determined using a three-dimensional reconstruction with a semicircular loop model, which allows for an accurate measurement of the loop length. The plane of wave polarization is found from comparison with a simulated loop model and shows that the oscillation is a fundamental horizontally polarized fast magnetoacoustic kink mode. The oscillation is characterized using an automated method and the results from both spacecraft are found to match closely. The oscillation period is 630 ± 30 s and the damping time is 1000 ± 300 s. Also, clear intensity variations associated with the transverse loop oscillations are reported for the first time. They are shown to be caused by the effect of line-of-sight integration. The Alfvén speed and coronal magnetic field derived using coronal seismology are discussed. This study shows that EUVI/STEREO observations achieve an adequate accuracy for studying long-period, large-amplitude transverse loop oscillations. Title: Nonlinear Force-Free Magnetic Field Modeling of AR 10953: A Critical Assessment Authors: De Rosa, Marc L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.; Lites, B. W.; Aschwanden, M. J.; Amari, T.; Canou, A.; McTiernan, J. M.; Régnier, S.; Thalmann, J. K.; Valori, G.; Wheatland, M. S.; Wiegelmann, T.; Cheung, M. C. M.; Conlon, P. A.; Fuhrmann, M.; Inhester, B.; Tadesse, T. Bibcode: 2009SPD....40.3102D Altcode: Nonlinear force-free field (NLFFF) modeling seeks to provide accurate representations of the structure of the magnetic field above solar active regions, from which estimates of physical quantities of interest (e.g., free energy and helicity) can be made. However, the suite of NLFFF algorithms have failed to arrive at consistent solutions when applied to (thus far, two) cases using the highest-available-resolution vector magnetogram data from Hinode/SOT-SP (in the region of the modeling area of interest) and line-of-sight magnetograms from SOHO/MDI (where vector data were not available). One issue is that NLFFF models require consistent, force-free vector magnetic boundary data, and vector magnetogram data sampling the photosphere do not satisfy this requirement. Consequently, several problems have arisen that are believed to affect such modeling efforts. We use AR 10953 to illustrate these problems, namely: (1) some of the far-reaching, current-carrying connections are exterior to the observational field of view, (2) the solution algorithms do not (yet) incorporate the measurement uncertainties in the vector magnetogram data, and/or (3) a better way is needed to account for the Lorentz forces within the layer between the photosphere and coronal base. In light of these issues, we conclude that it remains difficult to derive useful and significant estimates of physical quantities from NLFFF models. Title: A Critical Assessment of Nonlinear Force-Free Field Modeling of the Solar Corona for Active Region 10953 Authors: De Rosa, Marc L.; Schrijver, Carolus J.; Barnes, Graham; Leka, K. D.; Lites, Bruce W.; Aschwanden, Markus J.; Amari, Tahar; Canou, Aurélien; McTiernan, James M.; Régnier, Stéphane; Thalmann, Julia K.; Valori, Gherardo; Wheatland, Michael S.; Wiegelmann, Thomas; Cheung, Mark C. M.; Conlon, Paul A.; Fuhrmann, Marcel; Inhester, Bernd; Tadesse, Tilaye Bibcode: 2009ApJ...696.1780D Altcode: 2009arXiv0902.1007D Nonlinear force-free field (NLFFF) models are thought to be viable tools for investigating the structure, dynamics, and evolution of the coronae of solar active regions. In a series of NLFFF modeling studies, we have found that NLFFF models are successful in application to analytic test cases, and relatively successful when applied to numerically constructed Sun-like test cases, but they are less successful in application to real solar data. Different NLFFF models have been found to have markedly different field line configurations and to provide widely varying estimates of the magnetic free energy in the coronal volume, when applied to solar data. NLFFF models require consistent, force-free vector magnetic boundary data. However, vector magnetogram observations sampling the photosphere, which is dynamic and contains significant Lorentz and buoyancy forces, do not satisfy this requirement, thus creating several major problems for force-free coronal modeling efforts. In this paper, we discuss NLFFF modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT, STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process illustrate three such issues we judge to be critical to the success of NLFFF modeling: (1) vector magnetic field data covering larger areas are needed so that more electric currents associated with the full active regions of interest are measured, (2) the modeling algorithms need a way to accommodate the various uncertainties in the boundary data, and (3) a more realistic physical model is needed to approximate the photosphere-to-corona interface in order to better transform the forced photospheric magnetograms into adequate approximations of nearly force-free fields at the base of the corona. We make recommendations for future modeling efforts to overcome these as yet unsolved problems. Title: Tracking The 3-d Structure Of Erupting Filaments With The Stereo/secchi Euvi Authors: Wuelser, Jean-Pierre; Aschwanden, M.; Lemen, J.; Nitta, N.; Sandman, A. Bibcode: 2009SPD....40.2605W Altcode: In May 2007, the STEREO/SECCHI EUVI observed several erupting filaments that were associated with a CME. The stereoscopic observations in both He II and Fe IX allow a detailed study of the 3-D geometry and dynamics of the filaments as they erupt. Deconvolution of the EUVI imagery with the instrument point spread function enhances image contrast and detail, and improves the accuracy of the 3-D analysis. The quantitative results of this analysis may potentially lead to a better understanding of the early CME process. Title: Can We Use STEREO/EUVI to Improve Boundary Conditions for Magnetic Modeling? Authors: Sandman, Anne; Aschwanden, M. J.; Alexander, D. Bibcode: 2009SPD....40.1220S Altcode: The STEREO mission provides a unique opportunity to observe active region loops simultaneously from two different vantage points in space. Using the stereoscopic data, we can triangulate 3D coordinates for active region loops and use them to perform quantitative comparisons between observed loop geometries and theoretical magnetic field models. These comparisons provide some insight into the validity of magnetic field models, but recent work (DeRosa et al. 2009, Sandman et al. 2009) has shown that potential and non-potential models yield equally poor agreement with observed loop structures. In order to improve the results of magnetic modeling we must resolve the discrepancy between the typically force-free domain (the corona) and non-force-free boundary condition (the photosphere or chromosphere). We seek to address this discrepancy by "correcting” the boundary condition using observational constraints. The magnetic field in the low corona cannot currently be mapped directly, but using STEREO 3D loop coordinates we can constrain the orientation of the magnetic field in the low corona. In a previous study we measured the misalignment angles between model magnetic fields and observed coronal loops in three active regions, as observed with STEREO/EUVI on April 30, May 9, and May 19, 2007. We now use these misalignment angles to modify the magnetogram input to the magnetic field model. The resulting model field has minimal misalignment with the observed coronal loops near the base of the corona, allowing for a more realistic field extrapolation. Title: Solar Flare and CME Observations with STEREO/EUVI Authors: Aschwanden, M. J.; Wuelser, J. P.; Nitta, N. V.; Lemen, J. R. Bibcode: 2009SoPh..256....3A Altcode: STEREO/EUVI observed 185 flare events (detected above the GOES class C1 level or at > 25 keV with RHESSI) during the first two years of the mission (December 2006 - November 2008), while coronal mass ejections (CMEs) were reported in about a third of these events. We compile a comprehensive catalog of these EUVI-observed events, containing the peak fluxes in soft X rays, hard X rays, and EUV, as well as a classification and statistics of prominent EUV features: 79% show impulsive EUV emission (coincident with hard X rays), 73% show delayed EUV emission from postflare loops and arcades, 24% represent occulted flares, 17% exhibit EUV dimming, 5% show loop oscillations or propagating waves, and at least 3% show erupting filaments. We analyze an example of each EUV feature by stereoscopic modeling of its 3D geometry. We find that EUV emission can be dominated by impulsive emission from a heated, highly sheared, noneruptive filament, in addition to the more common impulsive EUV emission from flare ribbons or the delayed postflare EUV emission that results from cooling of the soft-X-ray-emitting flare loops. Occulted flares allow us to determine CME-related coronal dimming uncontaminated from flare-related EUV emission. From modeling the time evolution of EUV dimming we can accurately quantify the initial expansion of CMEs and determine their masses. Further, we find evidence that coronal loop oscillations are excited by the rapid initial expansion of CMEs. These examples demonstrate that stereoscopic EUV data provide powerful new methods to model the 3D aspects in the hydrodynamics of flares and kinematics of CMEs. Title: Classifying Coronal Loops as Isothermal or Multi-thermal Using the Loops' Evolution Authors: Mulu, Fana; Winebarger, A. R.; Warren, H. P.; Aschwanden, M. J. Bibcode: 2009SPD....40.1218M 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 poster, we introduce a new technique to determine if an observed loop is isothermal or multi-thermal. We will determine the evolution of loops in multiple filter images from the Transition Region and Coronal Explorer (TRACE). Our new technique will calculate the delay of the loop between different filter images, calculate a cooling time, and determine if that cooling time is consistent with the observed lifetime. We will present preliminary results that show if the temperature structure across coronal loops is isothermal or multi-thermal. Title: First Measurements of the Mass of Coronal Mass Ejections from the EUV Dimming Observed with Stereo EUVI A and B Spacecraft Authors: Aschwanden, Markus J.; Nitta, N. V.; Wuelser, J.; Lemen, J. R.; Sandman, A.; Vourlidas, A.; Colaninno, R. C. Bibcode: 2009SPD....40.2116A Altcode: The masses of Coronal Mass Ejections (CMEs) have traditionally been determined from white-light coronagraphs, based on the Thomson scattering of electrons. Here we develop a new method of measuring CME masses from the EUV dimming seen with EUV imaging telescopes in multiple temperature filters. As a test we compare the CME masses measured by STEREO/EUVI A and B with those previously determined by STEREO/COR2, for a set of 8 CME events of which we detected 7 with EUVI and determined the masses in 6 cases. We find CME masses in the range of m = (2-7) x 10(15) g. The agreement between the two EUVI/A and B spacecraft is mA/mB =1.3 +/- 0.6 and the consistency with white-light measurements by COR2 is mEUVI/mCOR2 = 1.1 +/- 0.3. The consistency between EUVI and COR2 implies no significant mass backflows (or inflows) at r < 4 R and adequate temperature coverage for the bulk of the CME mass in the range of T = 0.5-3.0 MK. The temporal evolution of the EUV dimming allows us also to model the evolution of the CME density, volume, height-time, and propagation speed in terms of an adiabatically expanding self-similar geometry. We test this model with the predicted detection delay between EUVI and COR2 for the 2008-Mar-25 event. Title: The CME-Flare Relation Revisited With STEREO Observations Authors: Nitta, Nariaki; Aschwanden, M.; Freeland, S.; Lemen, J.; Wuelser, J.; Zarro, D. Bibcode: 2009SPD....40.2105N Altcode: We study the association of solar flares since March 2007 with coronal mass ejections (CMEs), using images taken by the EUV Imager (EUVI), COR1 and COR2 coronagraphs on board STEREO. This is done by searching EUVI data for low coronal signatures attributable to CMEs, such as dimming, EUV waves and eruptions, following them to COR1 and COR2 fields of view. Base and running difference images (after correcting for differential rotation) as well as raw images in all the four filters of EUVI on STEREO A and STEREO B are viewed as movies to find the CME-related signatures. The COR1 data are particularly helpful for connecting the EUVI signatures with CMEs observed by COR2. Only 2 (out of 11) M-class flares and 7 (out of 64) C-class flares are convincingly associated with CMEs traceable beyond 5 Rs. There are also a handful of less intense (B-class and A-class) flares associated with CMEs. We discuss the "calibration" of the low coronal signatures with actual CMEs, quantitatively re-defining them to be used as reliable proxies for CMEs. Radio observations are also found to be of use to distinguish flares associated and not associated with CMEs. Lastly we consider the CME association of flares in terms of the the following items about the flaring active regions: their basic properties, their relations with more global field, and local (spatial or temporal) changes therein. This study may help us understand the effect of (reconnection-driven) flare processes on the initiation and subsequent dynamics of CMEs. Title: First Three-Dimensional Reconstructions of Coronal Loops with the STEREO A+B Spacecraft. III. Instant Stereoscopic Tomography of Active Regions Authors: Aschwanden, Markus J.; Wuelser, Jean-Pierre; Nitta, Nariaki V.; Lemen, James R.; Sandman, Anne Bibcode: 2009ApJ...695...12A Altcode: Here we develop a novel three-dimensional (3D) reconstruction method of the coronal plasma of an active region by combining stereoscopic triangulation of loops with density and temperature modeling of coronal loops with a filling factor equivalent to tomographic volume rendering. Because this method requires only a stereoscopic image pair in multiple temperature filters, which are sampled within ≈1 minute with the recent STEREO/EUVI instrument, this method is about four orders of magnitude faster than conventional solar rotation-based tomography. We reconstruct the 3D density and temperature distribution of active region NOAA 10955 by stereoscopic triangulation of 70 loops, which are used as a skeleton for a 3D field interpolation of some 7000 loop components, leading to a 3D model that reproduces the observed fluxes in each stereoscopic image pair with an accuracy of a few percents (of the average flux) in each pixel. With the stereoscopic tomography we infer also a differential emission measure distribution over the entire temperature range of T ≈ 104-107, with predictions for the transition region and hotter corona in soft X-rays. The tomographic 3D model provides also large statistics of physical parameters. We find that the extreme-ultraviolet loops with apex temperatures of Tm lsim 3.0 MK tend to be super-hydrostatic, while hotter loops with Tm ≈ 4-7 MK are near-hydrostatic. The new 3D reconstruction model is fully independent of any magnetic field data and is promising for future tests of theoretical magnetic field models and coronal heating models. Title: New Aspects on Particle Acceleration in Solar Flares from RHESSI Observations Authors: Aschwanden, M. J. Bibcode: 2009AsJPh..17..423A Altcode: No abstract at ADS Title: Coronal Mass Ejections Associated With Impulsive Solar Flares - Observations With SECCHI EUVI On STEREO Authors: Nitta, N. V.; Lemen, J. R.; Wuelser, J.; Aschwanden, M. J.; Freeland, S. L.; Zarro, D. M. Bibcode: 2008AGUFMSH13B1538N Altcode: Long-duration flares, sometimes referred to as Long Decay Events (LDEs), are known to be unmistakable signatures of coronal mass ejections (CMEs), and often of fast and large ones. Short-duration or impulsive flares, on the other hand, do not as frequently accompany CMEs, even though X-ray plasmoid ejections seen in some of these flares may suggest that all flares are eruptive irrespective of durations. Some of these ejections in X-ray or EUV images could be failed ejections, however, meaning that they do not move into interplanetary medium. A complementary, and perhaps more reliable signature of a CME in the low corona may be large-scale dimming typically observed at 1-2 MK. We report on high cadence observations of SECCHI EUVI on STEREO that show this phenomenon in weak impulsive flares more frequently than expected. We systematically study flare periods with good data coverage. In order to avoid false dimming, we use both base and running difference images after carefully co-aligning the image pairs. Some of the dimming events were observed in more than one channel and at two widely separated view angles, letting us better understand the nature of dimming especially in terms of the associated CME. We discuss how the properties of dimming are reflected in CME parameters, how to distinguish the impulsive flares with large- scale effects from those that are confined, and whether similar events could account for orphan ICMEs without a clearly associated CME near the Sun. Title: Nonlinear Force-Free Magnetic Field Modeling of the Solar Corona: A Critical Assessment Authors: De Rosa, M. L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.; Lites, B. W.; Aschwanden, M. J.; McTiernan, J. M.; Régnier, S.; Thalmann, J.; Valori, G.; Wheatland, M. S.; Wiegelmann, T.; Cheung, M.; Conlon, P. A.; Fuhrmann, M.; Inhester, B.; Tadesse, T. Bibcode: 2008AGUFMSH41A1604D Altcode: Nonlinear force-free field (NLFFF) modeling promises to provide accurate representations of the structure of the magnetic field above solar active regions, from which estimates of physical quantities of interest (e.g., free energy and helicity) can be made. However, the suite of NLFFF algorithms have so far failed to arrive at consistent solutions when applied to cases using the highest-available-resolution vector magnetogram data from Hinode/SOT-SP (in the region of the modeling area of interest) and line-of-sight magnetograms from SOHO/MDI (where vector data were not been available). It is our view that the lack of robust results indicates an endemic problem with the NLFFF modeling process, and that this process will likely continue to fail until (1) more of the far-reaching, current-carrying connections are within the observational field of view, (2) the solution algorithms incorporate the measurement uncertainties in the vector magnetogram data, and/or (3) a better way is found to account for the Lorentz forces within the layer between the photosphere and coronal base. In light of these issues, we conclude that it remains difficult to derive useful and significant estimates of physical quantities from NLFFF models. Title: Using STEREO/EUVI to Study Active Region Magnetic Fields Authors: Sandman, A.; Aschwanden, M.; Wuelser, J.; De Rosa, M.; Alexander, D. Bibcode: 2008AGUFMSH13B1523S Altcode: We examine the effect of linear transformations on the misalignment between model magnetic fields and coronal loops in active regions, as observed with STEREO/EUVI on three separate occasions between April 30 and May 19, 2007. We perform stereoscopic triangulation of some 100 EUVI loops in each active region, and identify the tangent vectors along every loop. Using magnetogram information from SOHO/MDI we compute a 3D potential field and interpolate the magnetic field vector at every position along the EUVI loops. The angle between the loop tangent vector and the magnetic field vector provides a measure of the misalignment angle between the observed field configuration and the model. We then transform the field in a way that preserves the divergence-free condition while injecting electric currents into the system. With this modified field we repeat our calculation of the misalignment angles between the magnetic field vectors and the EUV loop tangent vectors, quantifying the improvement of the transformed magnetic field model. Results of this type of magnetic modeling are presented for three active regions. Title: Tracking the 3-D Structure of an Erupting Filament With the STEREO/SECCHI EUVI Authors: Wuelser, J.; Aschwanden, M. J.; Lemen, J. R.; Nitta, N.; Sandman, A. Bibcode: 2008AGUFMSH12A..01W Altcode: On 2007 May 19, the STEREO/SECCHI EUVI observed an erupting filament that was associated with a CME. The stereoscopic observations in both He II and Fe IX allow a detailed study of the 3-D geometry and dynamics of the filament as it erupts. Deconvolution of the EUVI imagery with the instrument point spread function enhances image contrast and detail, and improves the accuracy of the 3-D analysis. The quantitative results of such an analysis may lead to a better understanding of the early CME process. Title: Instant Stereoscopic Tomography of Active Regions with STEREO/EUVI Authors: Aschwanden, M. J.; Wuelser, J.; Nitta, N.; Lemen, J.; Sandman, A. Bibcode: 2008AGUFMSH13B1555A Altcode: We develop a novel 3D reconstruction method of the coronal plasma of an active region by combining stereoscopic triangulation of loops with density and temperature modeling of coronal loops with a filling factor equivalent to tomographic volume rendering. Because this method requires only a stereoscopic image pair in multiple temperature filters, which are sampled within ~1 minute with the recent STEREO/EUVI instrument, this method is about 4 orders of magnitude faster than conventional solar rotation-based tomography. We reconstruct the 3D density and temperature distribution of active region NOAA 10955 by stereoscopic triangulation of 70 loops, which are used as a skeleton for a 3D field interpolation of some 7000 loop components, leading to a 3D model that reproduces the observed fluxes in each stereosocpic image pair with an accuracy of a few percent (of the average flux) in each pixel. With the stereoscopic tomography we infer also a differential emission measure (DEM) distribution over the entire temperature range of T~0.01-10 MK, with predictions for the transition region and hotter corona in soft X-rays. The tomographic 3D model provides also large statistics of physical parameters. We find that the EUV loops with apex temperatures of T = 1- 3 MK tend to be super-hydrostatic, while hotter loops with T = 4-7 MK are near-hydrostatic. The new 3D reconstruction model is fully independent of any magnetic field data and is promising for future tests of theoretical magnetic field models and coronal heating models. Title: The Effect of Radiative Cooling on Coronal Loop Oscillations Authors: Aschwanden, Markus J.; Terradas, Jaume Bibcode: 2008ApJ...686L.127A Altcode: Coronal loops that exhibit kink-mode oscillations have generally been assumed to have a constant density and temperature during the observed time interval. Analyzing their intensities in an EUV wave band, however, clearly shows that their brightness varies in a way that is consistent with a temperature cooling through the EUV passband, which limits their detection time, observed damping time, and number of observable periods. We study kink-mode oscillations of eight loops observed during the so-called harmonica event on 2001 April 15, 21:58-22:27 UT in the 171 Å band. We find loop densities of ne = (1.4 +/- 0.6) × 109 cm-3, loop widths of w = 2.0 +/- 2.6 Mm, and e-folding cooling times of τcool = 17 +/- 7 minutes, when they cool through the peak temperature T = 0.95 MK of the 171 Å band. We conclude that oscillations of a single loop cannot be detected longer than 10-20 minutes in one single filter and appropriate light curve modeling is necessary to disentangle the subsequent oscillation phases of multiple near-cospatial loops. Title: First 3D Reconstructions of Coronal Loops with the STEREO A+B Spacecraft. II. Electron Density and Temperature Measurements Authors: Aschwanden, Markus J.; Nitta, Nariaki V.; Wuelser, Jean-Pierre; Lemen, James R. Bibcode: 2008ApJ...680.1477A Altcode: Using the stereoscopically derived three-dimensional (3D) geometry of 30 loops observed with STEREO EUVI (described in Paper I) we determine here the electron density profiles ne(s) and electron temperature profiles Te(s) from a triple-filter analysis of the stereoscopic images taken in the wavelengths of λ = 171, 195, and 284 Å. The statistical results of our analysis of seven complete loops are: observed loop widths wobs = 2.6 +/- 0.1 Mm, corresponding to effective loop widths of w = 1.1 +/- 0.3 Mm if corrected for the instrumental point-spread function; loop flux ratios floop/ftotal = 0.11 +/- 0.04; mean loop (DEM peak) temperatures Tp = 1.1 +/- 0.2 MK; DEM temperature Gaussian widths σDEM = 0.35 +/- 0.04 MK; temperature variations along loops σT/Tp = 0.24 +/- 0.05; (resolution-corrected) loop base densities ne = (2.2 +/- 0.5) × 109 cm-3 loop lengths of L = 130 +/- 67 Mm; and all quantities are found to agree between STEREO A and B within a few percent. The temperature profiles T(s) along loops are found to be nearly constant, within the uncertainties of the background subtraction. The density profiles ne(s) are consistent with the gravitational stratification of hydrostatic loops, ne(h) = nbaseexp (- h/λT) , defined by the temperature scale heights λT and stereoscopically measured from the height profiles h(s) . The stereoscopic 3D reconstruction allows us for the first time to accurately measure the loop length L and to test loop scaling laws. We find that the observations are not consistent with equilibrium solutions, but rather display the typical overpressures of loops that have been previously heated to higher temperatures and cool down in a nonequilibrium state, similar to earlier EIT and TRACE measurements. Title: First Three-Dimensional Reconstructions of Coronal Loops with the STEREO A and B Spacecraft. I. Geometry Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre; Nitta, Nariaki V.; Lemen, James R. Bibcode: 2008ApJ...679..827A Altcode: We present one of the first triangulations and 3D reconstructions of coronal loops, using the EUVI telescopes of the two STEREO A and B spacecraft. The first triangulation of coronal loops was performed in an active region, observed with STEREO A and B on 2007 May 9 with a spacecraft separation angle of αsep = 7.3°, at a wavelength of 171 Å. We identify 30 loop structures (7 complete loops and 23 partial segments) and compute their 3D coordinates (x,y,z) (the full 3D coordinates are available as an electronic file). We quantify the height range, the stereoscopic height measurement errors, the loop plane inclination angles, and the coplanarity and circularity of the analyzed loops. The knowledge of the exact 3D geometry of a loop with respect to the observer's line of sight has important consequences for determining the correct vertical density scale height (used in hydrostatic models), the aspect angle of loop cross sections (used in inferring electron densities from optically thin emission measures), the absolute flow speeds (used in siphon flow models), the correct loop length (used in loop scaling laws), and the 3D vectors of the coronal magnetic field (used in testing theoretical magnetic field extrapolation models). The hydrodynamic and magnetic modeling of the analyzed loops will be described in subsequent papers. Title: Exploring large-scale coronal magnetic field over extended longitudes by STEREO/EUVI and its effect on solar wind prediction Authors: Nitta, N. V.; De Rosa, M. L.; Zarro, D. M.; Wuelser, J.; Aschwanden, M. J.; Lemen, J. R. Bibcode: 2008AGUSMSH23A..06N Altcode: The potential field source surface (PFSS) model forms the basis of a wide range of heliospheric science and applications, including prediction of the solar wind speed near Earth. Experience shows that the model sometimes works quite well, but not always. Possible reasons for failure include deviation of the interplanetary magnetic field from the nominal Parker spiral, violation of the assumptions used in the model such as the discontinuity at the source surface, and the lack of simultaneous full-surface magnetograms. Here we study the impact of the lack of simultaneous full-surface magnetograms, using observations of the corona over an extended longitude range made possible by the EUVI on board the STEREO mission. In spite of the lack of magnetographs on STEREO, EUVI data with a growing separation angle between spacecraft A and B at least allow us to locate major active regions and coronal holes in the area not seen from Earth. The PFSS extrapolations and their input synoptic maps are compared with EUVI data to measure how well the model fits the observations. These comparisons are discussed in terms of the solar wind speed predicted by the model and observed at L1 by ACE. Title: Tracing the 3-D coronal structure during CMEs with STEREO/SECCHI EUVI observations Authors: Wuelser, J.; Aschwanden, M.; De Rosa, M.; Lee, C.; Lemen, J.; Nitta, N.; Sandman, A. Bibcode: 2008AGUSMSH31A..05W Altcode: STEREO/SECCHI EUVI observations of solar coronal loops, filaments, and dimming regions provide unique information on the 3-D topology of the coronal magnetic field above active regions and its evolution during coronal mass ejections (CMEs). Active Region #10956 produced several CMEs during its passage across the solar disk in May 2007, some of them showing filament eruptions and dimming. The SECCHI/EUVI instrument on STEREO obtained high cadence observations in multiple lines simultaneously from both STEREO spacecraft. 3-D reconstructions of coronal features over the course of a CME show significant changes of the field topology. Comparisons with the potential field topology from magnetic field extrapolations show the degree of non-potentiality of the real field and changes in the vicinity of the CME onset. We present initial results of this study. Title: Solar Flares and CMEs observed with STEREO/EUVI Authors: Aschwanden, M. J.; Wuelser, J.; Nitta, N. V.; Lemen, J. R. Bibcode: 2008AGUSMSP44A..01A Altcode: STEREO/EUVI observed a respectable number of small flares and CMEs during the first year of its mission. A comprehensive survey between Dec 2006 and Jan 2008 yields: 10 GOES >M1-class flares, 20 GOES >C5-class flares, 80 >C1-class flares, 180 flares with >25 keV hard X-ray emission observed by RHESSI, at least 35 flare events with associated CME reports by LASCO or STEREO COR-1, and 25 flare events are occulted by either STEREO A or B. Some flares associated with CMEs clearly show an EUV dimming in the active region below the launched CME, for which we determine for the first time the 3D geometry with the two STEREO A and B spacecraft, allowing us to model the volume, density, and ejected mass from the corona. Some flares show impulsive signatures in EUV, simultaneously occurring with hard X-rays pulses, indicative of the primary heating of the chromospheric plasma. In larger flares, the bulk of the EUV emission is always substantially delayed to the soft X-ray emission, as a result of the plasma cooling from soft X-ray to EUV temperatures, from which we can estimate the total thermal flare energy using hydrodynamic models. Title: Fundamental physical processes in coronae: Waves, turbulence, reconnection, and particle acceleration Authors: Aschwanden, Markus J. Bibcode: 2008IAUS..247..257A Altcode: 2007arXiv0711.0007A; 2007IAUS..247..257A Our understanding of fundamental processes in the solar corona has been greatly progressed based on the space observations of SMM, Yohkoh, Compton GRO, SOHO, TRACE, RHESSI, and STEREO. We observe now acoustic waves, MHD oscillations, turbulence-related line broadening, magnetic configurations related to reconnection processes, and radiation from high-energy particles on a routine basis. We review a number of key observations in EUV, soft X-rays, and hard X-rays that innovated our physical understanding of the solar corona, in terms of hydrodynamics, MHD, plasma heating, and particle acceleration processes. Title: Comparison of Five Numerical Codes for Automated Tracing of Coronal Loops Authors: Aschwanden, Markus J.; Lee, Jong Kwan; Gary, G. Allen; Smith, Michael; Inhester, Bernd Bibcode: 2008SoPh..248..359A Altcode: The three-dimensional (3D) modeling of coronal loops and filaments requires algorithms that automatically trace curvilinear features in solar EUV or soft X-ray images. We compare five existing algorithms that have been developed and customized to trace curvilinear features in solar images: i) the oriented-connectivity method (OCM), which is an extension of the Strous pixel-labeling algorithm (developed by Lee, Newman, and Gary); ii) the dynamic aperture-based loop-segmentation method (developed by Lee, Newman, and Gary); iii) unbiased detection of curvilinear structures (developed by Steger, Raghupathy, and Smith); iv) the oriented-direction method (developed by Aschwanden); and v) ridge detection by automated scaling (developed by Inhester). We test the five existing numerical codes with a TRACE image that shows a bipolar active region and contains over 100 discernable loops. We evaluate the performance of the five codes by comparing the cumulative distribution of loop lengths, the median and maximum loop length, the completeness or detection efficiency, the accuracy, and flux sensitivity. These algorithms are useful for the reconstruction of the 3D geometry of coronal loops from stereoscopic observations with the STEREO spacecraft, or for quantitative comparisons of observed EUV loop geometries with (nonlinear force-free) magnetic field extrapolation models. Title: Theoretical modeling for the stereo mission Authors: Aschwanden, Markus J.; Burlaga, L. F.; Kaiser, M. L.; Ng, C. K.; Reames, D. V.; Reiner, M. J.; Gombosi, T. I.; Lugaz, N.; Manchester, W.; Roussev, I. I.; Zurbuchen, T. H.; Farrugia, C. J.; Galvin, A. B.; Lee, M. A.; Linker, J. A.; Mikić, Z.; Riley, P.; Alexander, D.; Sandman, A. W.; Cook, J. W.; Howard, R. A.; Odstrčil, D.; Pizzo, V. J.; Kóta, J.; Liewer, P. C.; Luhmann, J. G.; Inhester, B.; Schwenn, R. W.; Solanki, S. K.; Vasyliunas, V. M.; Wiegelmann, T.; Blush, L.; Bochsler, P.; Cairns, I. H.; Robinson, P. A.; Bothmer, V.; Kecskemety, K.; Llebaria, A.; Maksimovic, M.; Scholer, M.; Wimmer-Schweingruber, R. F. Bibcode: 2008SSRv..136..565A Altcode: 2006SSRv..tmp...75A We summarize the theory and modeling efforts for the STEREO mission, which will be used to interpret the data of both the remote-sensing (SECCHI, SWAVES) and in-situ instruments (IMPACT, PLASTIC). The modeling includes the coronal plasma, in both open and closed magnetic structures, and the solar wind and its expansion outwards from the Sun, which defines the heliosphere. Particular emphasis is given to modeling of dynamic phenomena associated with the initiation and propagation of coronal mass ejections (CMEs). The modeling of the CME initiation includes magnetic shearing, kink instability, filament eruption, and magnetic reconnection in the flaring lower corona. The modeling of CME propagation entails interplanetary shocks, interplanetary particle beams, solar energetic particles (SEPs), geoeffective connections, and space weather. This review describes mostly existing models of groups that have committed their work to the STEREO mission, but is by no means exhaustive or comprehensive regarding alternative theoretical approaches. Title: Solar flare physics enlivened by TRACE and RHESSI Authors: Aschwanden, Markus J. Bibcode: 2008JApA...29..115A Altcode: No abstract at ADS Title: Keynote address: Outstanding problems in solar physics Authors: Aschwanden, Markus J. Bibcode: 2008JApA...29....3A Altcode: No abstract at ADS Title: Solar Flare Geometries. I. The Area Fractal Dimension Authors: Aschwanden, Markus J.; Aschwanden, Pascal D. Bibcode: 2008ApJ...674..530A Altcode: In this study we investigate for the first time the fractal dimension of solar flares and find that the flare area observed in EUV wavelengths exhibits a fractal scaling. We measure the area fractal dimension D2, also called the Hausdorff dimension, with a box-counting method, which describes the fractal area as A(L) ~ LD2. We apply the fractal analysis to a statistical sample of 20 GOES X- and M-class flares, including the Bastille Day 2000 July 14 flare, one of the largest flares ever recorded. We find that the fractal area (normalized by the time-integrated flare area Af) varies from near zero at the beginning of the flare to a maximum of A(t)/Af = 0.65 +/- 0.12 after the peak time of the flare, which corresponds to an area fractal dimension in the range of 1.0lesssim D2(t) lesssim 1.89 +/- 0.05. We find that the total EUV flux Ftot(t) is linearly correlated with the fractal area A(t) . From the area fractal dimension D2, the volume fractal dimension D3 can be inferred (subject of Paper II), which is crucial to inferring a realistic volume filling factor, which affects the derived electron densities, thermal energies, and cooling times of solar and stellar flares. Title: Solar Flare Geometries. II. The Volume Fractal Dimension Authors: Aschwanden, Markus J.; Aschwanden, Pascal D. Bibcode: 2008ApJ...674..544A Altcode: Based on the area fractal dimension D2 of solar flares measured in Paper I, we carry out modeling of the three-dimensional (3D) flare volume here and derive an analytical relation between the volume fractal scaling V(L) ~ LD3 and the area fractal scaling A(L) ~ LD2. The 3D volume model captures a flare arcade with a variable number of flare loops; its fractal structure is not isotropic, but consists of aligned one-dimensional substructures. The geometry of the arcade model has three free parameters and makes some simplifying assumptions, such as semicircular loops, east-west orientation, location near the equator, and no magnetic shear. The analytical model predicts the scaling of the area filling factor qA(nloop) and volumetric filling factor qV(nloop) as a function of the number of loops nloop, and allows one to predict the volume filling factor qV(qA) and volume fractal dimension D3(D2) from the observationally measured parameters qA and D2. We also corroborate the analytical model with numerical simulations. We apply this fractal model to the 20 flares analyzed in Paper I and find maximum volume filling factors with a median range of qV ≈ 0.03-0.08 (assuming solid filling for loop widths of lesssim1 Mm). The fractal nature of the flare volume has important consequences for correcting electron densities determined from flare volume emission measures and density-dependent physical quantities, such as the thermal energy or radiative cooling time. The fractal scaling has also far-reaching consequences for frequency distributions and scaling laws of solar and stellar flares. Title: Solar Active Regions: A Transition from Morphological Observations to Physical Modeling (Opening Keynote Address) Authors: Aschwanden, M. J. Bibcode: 2008ASPC..383....1A Altcode: Although solar active regions can easily be identified from their sunspot groups, they keep some of the most challenging secrets of solar physics, regarding their subphotospheric origin, photospheric emergence, magnetic field structure, magnetic helicity, wave generation, propagation and dissipation, plasma heating and cooling, plasma flows, fractal geometry and intermittency, and magnetic instabilities leading to flares, CMEs, and coronal dimming. In this opening talk we review how morphological observations of active regions have gradually evolved into a more physics-based modeling approach over the last decades. Title: An Observational Test That Disproves Coronal Nanoflare Heating Models Authors: Aschwanden, Markus J. Bibcode: 2008ApJ...672L.135A Altcode: All theoretical models of nanoflare heating in the solar corona predict loops with an unresolved multistrand structure, which is in discrepancy with the quasi-isothermal cross sections of the finest coronal loop structures observed with TRACE, which have spatially resolved widths of w ≈ 1000-2000 km. We suggest modifying the theoretical models by relocating the hypothetical nanoflare events from their coronal location down to the chromosphere/transition region. Title: Scaling Laws of Solar and Stellar Flares Authors: Aschwanden, Markus J.; Stern, Robert A.; Güdel, Manuel Bibcode: 2008ApJ...672..659A Altcode: 2007arXiv0710.2563A In this study we compile for the first time comprehensive data sets of solar and stellar flare parameters, including flare peak temperatures Tp, flare peak volume emission measures EMp, and flare durations τf from both solar and stellar data, as well as flare length scales L from solar data. Key results are that both the solar and stellar data are consistent with a common scaling law of EMp propto T4.7p, but the stellar flares exhibit ≈250 times higher emission measures (at the same flare peak temperature). For solar flares we observe also systematic trends for the flare length scale L(Tp) propto T0.9p and the flare duration τF(Tp) propto T0.9p as a function of the flare peak temperature. Using the theoretical RTV scaling law and the fractal volume scaling observed for solar flares, i.e., V(L) propto L2.4, we predict a scaling law of EMp propto T4.3p, which is consistent with observations, and a scaling law for electron densities in flare loops, np propto T2p/L propto T1.1p. The RTV-predicted electron densities were also found to be consistent with densities inferred from total emission measures, np = (EMp/qVV)1/2, using volume filling factors of qV = 0.03-0.08 constrained by fractal dimensions measured in solar flares. Solar and stellar flares are expected to have similar electron densities for equal flare peak temperatures Tp, but the higher emission measures of detected stellar flares most likely represent a selection bias of larger flare volumes and higher volume filling factors, due to low detector sensitivity at higher temperatures. Our results affect also the determination of radiative and conductive cooling times, thermal energies, and frequency distributions of solar and stellar flare energies. Title: 3D Geometry of Coronal Loops Measured with STEREO / EUVI Authors: Aschwanden, M. J.; Wuelser, J.; Nitta, N.; Lemen, J. Bibcode: 2007AGUFMSH41B..04A Altcode: Using images from the STEREO/EUVI A and B spacecraft we developed an accurate method that performs stereoscopic triangulation and reconstruction of the 3D geometry of curvi-linear structures in the solar corona, such as loops, filaments, prominence threads, or wave features. We test the coalignment of stereoscopic images and establish an accuracy of better than <0.1 pixels in east-west direction, <0.3 pixels in north-south direction, and <0.05 degrees in roll angle. We reconstruct the 3D geometry of some 100 coronal loops loops in active regions in May 2007, when the spacecraft had a separation angle of ~10 degrees. We find that complete loops or incomplete segments of loops can only be reconstructed up to altitudes of about one hydrostatic scale height, which is h<50 Mm at a coronal temperature of T=1 MK. The determination of the 3D geometry of coronal loops is an important and necessary step to model their hydrodynamic structure. We show also that this method can be used to determine quantitatively the eigen-motion, oscillation, twisting, expansion, acceleration, or other dynamics of coronal loops, erupting filaments, and MHD wave fronts, in particular in association with flares and CMEs. Title: 3D Topology of Prominences Measured with STEREO/EUVI Authors: Slater, G. L.; Aschwanden, M. J. Bibcode: 2007AGUFMSH32A0771S Altcode: Using images from the STEREO/EUVI A and B spacecraft and stereoscopic triangulation methods developed by Aschwanden et al for the reconstruction of the 3D geometry of curvi-linear structures in the solar corona, we determine three dimensional model reconstructions of several long-lived prominences as well as their topology - in particular the twist and number of helical turns. The derived geometries are analyzed together with photospheric magnetograms and potential field extrapolations (PFFS) and compared to various models for the magnetic confinement and support of prominence plasmas. Title: Stereoscopic Observations of Low Coronal Ejections With and Without CMEs Authors: Nitta, N. V.; Wülser, J.; Aschwanden, M. J.; Lemen, J. R. Bibcode: 2007AGUFMSH32A0775N Altcode: Yohkoh soft X-ray images of solar flares have frequently shown characteristic ejections during the impulsive phase. They are thought to be plasmoids that hold important information on magnetic reconnection. These ejections are intimately associated with coronal mass ejections (e.g., Nitta & Akiyama 1999; Kim et al. 2005). They probably represent internal structures of CMEs, i.e., high-temperature counterparts of filament eruptions. However, their relation with ejections seen at low temperatures has not been studied systematically, although TRACE has revealed many beautiful examples. In this work we study ejections observed by the SECCHI EUVI on STEREO. Some of them are associated with CMEs, and others are not. Using pairs of EUVI images from spacecraft A and B, the trajectories of ejecta in individual channels (with representative temperatures 0.1-2 MK) are reconstructed in 3D. We discuss these ejections at different temperatures in the context of CMEs. Specifically, we ask what kinetic properties are correlated with CMEs and how they are related with CME manifestations in the low corona such as dimming and waves. Title: 3-D reconstruction of CME related transient coronal phenomena observed with the STEREO/SECCHI Extreme Ultraviolet Imager Authors: Wuelser, J.; Aschwanden, M. J.; Lemen, J. R.; Nitta, N. Bibcode: 2007AGUFMSH32A0772W Altcode: One of the primary objectives of the SECCHI investigation on STEREO is to study the initiation of CMEs in the low corona, and to better understand CME related changes of the three-dimensional coronal structure. The SECCHI Extreme Ultraviolet Imagers (EUVI) have been observing the solar corona from two significantly different vantage points since about March 2007. They have since captured several CMEs, including a few during the SECCHI campaign in May 2007, at an observatory separation angle of about 7-8 degrees. Observations at relatively small separation angles allow for easier identification of features in two views, which is critical for visually aided tie- point tools, as well as for more automated 3-D reconstruction methods. EUVI movies taken during the early onset of a CME show a range of transient phenomena, including coronal ejecta that can be tracked into the coronagraph fields of view, erupting filaments that trail the coronal ejecta, displacement of active region loops, coronal dimming, and "EIT" waves. We present preliminary results of our first 3-D reconstruction attempts on a selection of such CME related phenomena, with emphasis on coronal ejecta and active region loop displacements. Title: Early Evolution of CMEs as Observed by SECCHI EUVI on STEREO Authors: Lemen, J. R.; Nitta, N. V.; Wülser, J.; Aschwanden, M. J. Bibcode: 2007AGUFMSH32A0768L Altcode: We report on two eruptions associated with small (C1 and B8) flares that occurred in AR 956 on 2007 May 19 and 20, as observed stereoscopically by EUVI. The separation of the two spacecraft was approximately 9 degrees. Pairs of images in 171~Å\ and 304~Å\ from two view angles are used to constrain the trajectories of the ejecta or filaments that appear to be responsible for the associated CMEs; they were not homologous. We study how the 3D motions of the ejecta in the low corona correspond to the CMEs at higher altitudes. We also discuss the possible relation between the early CME propagation and the coronal magnetic field topology inferred from EUV loops in EUVI and TRACE 171~Å\ images. Title: 3D Magnetic Modeling of Active Regions Using STEREO/EUVI Authors: Sandman, A.; Aschwanden, M. J.; Alexander, D.; Wuelser, J. Bibcode: 2007AGUFMSH32A0770S Altcode: With the recent availability of stereoscopic data from the Extreme Ultraviolet Imager (EUVI) on the Solar Terrestrial Relations Observatory (STEREO) we have an unprecedented opportunity to investigate the accuracy of 3D magnetic field models. These data will be put to best use by modeling techniques that make no assumptions about the nature of the field (that it is potential, force-free, etc.). The Gary-Alexander radial stretching method use a series of transformations to map a simple potential field to a more complicated target field, and compare the transformed field lines with observed coronal structures in the EUV. Unlike many other simulation techniques, this approach requires only that the field remain divergence-free and continuous at the photosphere. Here we apply this transformation method to STEREO/EUVI data. We obtain a 3D potential field extrapolation using an MDI magnetogram, and utilize stereoscopy to derive the 3D field line coordinates from pairs of EUV images at 171Å. By comparing the 3D coordinates of the transformed model field lines with those of the real field lines as seen by EUVI, we can place constraints on the distribution of magnetic field and current in an active region. Title: Astrophysics in 2006 Authors: Trimble, Virginia; Aschwanden, Markus J.; Hansen, Carl J. Bibcode: 2007SSRv..132....1T Altcode: 2007arXiv0705.1730T The fastest pulsar and the slowest nova; the oldest galaxies and the youngest stars; the weirdest life forms and the commonest dwarfs; the highest energy particles and the lowest energy photons. These were some of the extremes of Astrophysics 2006. We attempt also to bring you updates on things of which there is currently only one (habitable planets, the Sun, and the Universe) and others of which there are always many, like meteors and molecules, black holes and binaries. Title: RHESSI Timing Studies: Multithermal Delays Authors: Aschwanden, Markus J. Bibcode: 2007ApJ...661.1242A Altcode: We investigate the energy-dependent timing of thermal emission in solar flares using high-resolution spectra and demodulated time profiles from the RHESSI instrument. We model for the first time the spectral-temporal hard X-ray flux f(ɛ,t) in terms of a multitemperature plasma governed by thermal conduction cooling. In this quantitative model we characterize the multitemperature differential emission measure distribution (DEM) and nonthermal spectra with power-law functions. We fit this model to the spectra and energy-dependent time delays of a representative data set of 89 solar flares observed with RHESSI during 2002-2005. Eliminating weak flares, we find 65 events suitable for fitting and obtain in 44 events (68%) a satisfactory fit that is consistent with the theoretical model. The best-fit results yield a thermal-nonthermal crossover energy of ɛth=18.0+/-3.4 keV, nonthermal spectral indices of γnth=3.5+/-1.1 (at ~30-50 keV), thermal multispectral indices of γth=6.9+/-0.9 (at ~10-20 keV), and thermal conduction cooling times of τc0=101.6+/-0.6 s at ɛth=1 keV (or T0=11.6 MK), which scale with temperature as τc(T)~T with β=2.7+/-1.2, consistent with the theoretically expected scaling of τc(T)~T-5/2 for thermal conduction cooling. The (empirical) Neupert effect is consistent with this theoretical model in the asymptotic limit of long cooling times. This study provides clear evidence that all analyzed flares are consistent with the model of a multitemperature plasma distribution and with thermal conduction as dominant cooling mechanism (at flare temperatures of T>~10 MK). Our modeling of energy-dependent time delays provides an alternative method for separating multithermal from nonthermal spectral components based on information in the time domain, in contrast to previous spectral fitting methods. Title: The Third Solar Dimension (Invited Parker Lecture) Authors: Aschwanden, Markus J. Bibcode: 2007AAS...21010401A Altcode: 2007BAAS...39..230A Over the last two decades we acquired stunning images of the Sun in EUV, soft X-ray, and hard X-ray wavelengths, which show us magnetic loops and arcades in the solar corona, quiescent and eruptive filaments, flares and coronal mass ejections (CMEs). Physical modeling of these phenomena requires a reconstruction of the 3-dimensional (3D) geometry, which was mostly accomplished with 3D extrapolations of the photospheric field, using theoretical models of magnetic potential fields and force-free fields. The

3D distribution of the coronal plasma could also be reconstructed by means of solar rotation stereoscopy and tomography. The most recent solar space mission is STEREO, launched in 2006, which provides us true stereoscopic images of unprecedented clarity and should reveal us the full 3D magnetic topology of flares and CMEs. The ULYSSES spacecraft, as well as the future missions Solar Orbiter and Sentinels, will fly out of the ecliptic plane and will provide us a 3D perspective of the heliosphere. Title: First 3d Triangulation Of Coronal Loops With Stereo/euvi Authors: Aschwanden, Markus J.; Wuelser, J.; Lemen, J.; Nitta, N. Bibcode: 2007AAS...210.2810A Altcode: 2007BAAS...39..137A The orbits of the two STEREO spacecraft A(head) and B(ehind) move to progressively larger stereoscopic separation angles a, with a=1.2 deg on March 1, a=3.0 deg on April 1, a=6.0 deg on May 1, and a=10.5 deg on June 1. This range of small-angle separation enables the first 3D triangulation of coronal features. Active region loops at 1 MK have a scale height of 50 Mm, for which the parallax effect amounts up to 7 EUVI pixels at a 10 deg separation angle. We present the first triangulations of active region loops, with the goal to reconstruct the 3D geometry along entire loop lengths. Such 3D reconstructions yield the inclination angles of loop planes, which allow us to test the relation between projected and vertical hydrostatic density scale heights. Another important application is how the 3D geometry of the stereoscopically reconstructed loops relates to theoretical (potential, linear and nonlinear force-free) magnetic field extrapolations. We attempt also to reconstruct the 3D geometry of filaments and to track their motion in 3D before eruption and onset of flares and CMEs. - This work is supported by the NASA STEREO under NRL contract N00173-02-C-2035. Title: First Assessments Of EUVI Performance On STEREO SECCHI Authors: Lemen, James; Wuelser, J. P.; Nitta, N.; Aschwanden, M. Bibcode: 2007AAS...210.2801L Altcode: 2007BAAS...39..135L The SECCHI investigation on the STEREO mission contains two Extreme Ultraviolet Imagers (EUVI), one on the ahead spacecraft and one on the behind spacecraft. EUVI views the solar disk using multilayer-coated normal-incidence optics that image onto 1.6 arcsec-per-pixel back-thinned CCD detectors. Four wavelength bandpasses are observed in series, 17.1 nm (Fe IX), 19.5 nm (Fe XII), 28.4 nm (Fe XV), and 30.4 nm (He II), covering the chromospheric and coronal plasma temperatures.Science operations began in January 2007 and both EUVIs are working very well. The fine pointing system effectively removes low frequency spacecraft pointing jitter, so the image resolution quality is very good, approaching 3.5 arcsec. We present early on-orbit assessments of the performance of both EUVIs and updated predictions for the temperature dependent instrument response functions, which are compared to early observations. The lunar transit observed by the EUVI on the behind spacecraft is used to assess the point spread function.

This work is supported by the NASA STEREO mission under NRL contract N00173-02-C-2035. Title: Low Coronal Manifestations Of Coronal Mass Ejections As Observed By STEREO EUVI Authors: Nitta, Nariaki; Wuelser, J.; Lemen, J.; Aschwanden, M.; Attrill, G. Bibcode: 2007AAS...210.2805N Altcode: 2007BAAS...39..136N Data from SOHO/EIT have tremendously advanced our knowledge about the initiation of coronal mass ejections (CMEs) as observed by SOHO/LASCO. The unequivocal EUV manifestations of CMEs include dimming and waves, typically observed at EIT's 195 A channel. In this presentation, we report mainly on these two observables in two similar events as observed on 2007 January 24 and 25 by the Extreme Ultraviolet Imagers (EUVI), which are part of SECCHI on STEREO. The source region of the two events was most likely AR 0940, which was located 20 degrees and 10 degrees behind the east limb at the times of the events. Images in 171 A and 195 A were taken at a basic cadence of 10 minutes, slightly better than that of the EIT movie sequence. But what is remarkable is the availability of nearly simultaneous (dt = 11 sec) full-disk images in the two wavelengths. We give detailed comparisons of the wave propagations and dimming regions as observed at 171 A and 195 A, and discuss their relations with the white-light CMEs and their associated flares. This work has been supported by NASA STEREO mission contract N00173-02-C-2035 through NRL. Title: Stereo Impact Investigation Early Mission Observations of The Solar Wind Structure, Its Sources and Its Interplanetary Consequences Authors: Luhmann, Janet; Schroeder, P.; Lin, R. P.; Larson, D. E.; Lee, C. O.; Sauvaud, J.; Acuna, M. H.; von Rosenvinge, T.; Mewaldt, R. A.; Davis, A. J.; Mueller-Mellin, R.; Mason, G. M.; Russell, C. T.; Jian, L.; Galvin, A. B.; Howard, R. A.; Aschwanden, M.; Arge, C. N.; MacNeice, P.; Chulaki, A.; Petrie, G. Bibcode: 2007AAS...21011907L Altcode: 2007BAAS...39..244L STEREO carries two main in-situ investigations to measure the solar wind plasma and interplanetary energetic particles and magnetic fields at 1 AU at the increasingly separated STEREO sites. These measurements have provided the opportunity to study the connections between the quiet corona and the solar wind structure during the first months of the mission's operation. In addition to the possibility of observing the same features at multiple sites, in combination with the SECCHI imaging investigation and SOHO and ACE the data allow us to explore specific questions regarding solar wind source regions and stream structure geometry. For example, not all high speed-low speed stream interaction regions have similar character; some exhibit more substructure and some have stronger locally accelerated particle signatures than others. We investigate this and other features taking into account the coronal source differences. Title: STEREO's in-situ perspective on the solar minimum solar wind structure Authors: Luhmann, J. G.; Larson, D.; Schroeder, P.; Lee, C. O.; Sauvaud, J.; Acuna, M. H.; Galvin, A. B.; Russell, C. T.; Jian, L.; Arge, C. N.; Odstrcil, D.; Riley, P.; Howard, R. A.; Aschwanden, M.; MacNeice, P.; Chulaki, A. Bibcode: 2007AGUSMSH34A..04L Altcode: STEREO multipoint measurements of the solar wind structure with the IMPACT and PLASTIC investigations, near Earth but off the Sun-Earth line, allow its sources and structure to be examined at solar minimum when such studies are particularly straightforward. With the aid of 3D models of the heliosphere available at the CCMC, we map the in-situ observations to their solar sources using a combination of the open field regions inferred from the SECCHI EUVI imagers and SOHO EIT, and the magnetogram-based models of the corona and solar wind. Our ultimate goal is the continuous tracking of solar wind source regions as the STEREO mission progresses, as well as the use of the mappings to deduce the distinctive properties of solar wind from different types of sources Title: The Coronal Heating Paradox Authors: Aschwanden, Markus J.; Winebarger, Amy; Tsiklauri, David; Peter, Hardi Bibcode: 2007ApJ...659.1673A Altcode: The ``coronal heating problem'' has been with us over 60 years, and hundreds of theoretical models have been proposed without an obvious solution in sight. In this paper we point out that observations show no evidence for local heating in the solar corona, but rather for heating below the corona in the transition region and upper chromosphere, with subsequent chromospheric evaporation as known in flares. New observational evidence for this scenario comes from (1) the temperature evolution of coronal loops, (2) the overdensity of hot coronal loops, (3) upflows in coronal loops, (4) the Doppler shift in coronal loops, (5) upward propagating waves, (6) the energy balance in coronal loops, (7) the magnetic complexity in the transition region, (8) the altitude of nanoflares and microflares, (9) the cross section of elementary loops, as well as (10) 3D MHD simulations of coronal heating. The phrase ``coronal heating problem'' is therefore a paradoxical misnomer for what should rather be addressed as the ``chromospheric heating problem'' and ``coronal loop filling process.'' This paradigm shift substantially reduces the number of relevant theoretical models for coronal heating in active regions and the quiet Sun, but our arguments do not apply to coronal holes and the extended heliospheric corona. Title: A Statistical Model of the Inhomogeneous Corona Constrained by Triple-Filter Measurements of Elementary Loop Strands with TRACE Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Boerner, Paul Bibcode: 2007ApJ...656..577A Altcode: Here we present the first quantitative model of the inhomogeneous solar corona, which we call the composite and elementary loops in a thermally inhomogeneous corona (CELTIC) model. We develop a self-consistent statistical model that quantifies the distributions of physical parameters, i.e., the distributions of loop widths, N(w,Te), electron densities, N(ne,Te), electron temperatures, N(Te), and statistical correlations between them. The parameterized distributions are constrained by the observed triple-filter fluxes of the EUV corona measured at some 18,000 loop positions with TRACE in the temperature range of Te~0.7-2.7 MK, as well as by the individual loop parameters (w,ne,Te) measured at these positions in ~240 detected loops, mostly sampled in active regions. The CELTIC model is inverted from the TRACE data and reproduces both the fluxes of the composite (active region and quiet Sun) background corona and the distributions of loop parameters in a self-consistent way. The best-fit values constrain a statistical correlation between the density and temperature, i.e., e>~e>α, with α=0.9+/-0.6, and between the loop width and temperature, i.e., ~e>β, with β=1.3+/-0.7, which can be related to the thermal pressure in a regime with a high plasma-β parameter. A possible explanation is a heating process located in the lower transition region or the upper chromosphere (e.g., as reproduced in the recent MHD simulations of Gudiksen and Nordlund), which produces sufficiently high electron densities, high plasma-β parameters, and thermally homogeneous loop cross sections as observed in elementary loop strands. Title: The Localization of Particle Acceleration Sites in Solar Flares and CMEs Authors: Aschwanden, Markus J. Bibcode: 2007sdeh.book..361A Altcode: No abstract at ADS Title: The Sun Authors: Aschwanden, Markus J. Bibcode: 2007ess..book...71A Altcode: The Sun is the central body and energy source of our solar system. The Sun is our nearest star, but otherwise it represents a fairly typical star in our galaxy, classified as G2-V spectral type, with a radius of r o ≈700,000 km, a mass of m o≈ 21033 g, a luminosity of L o≈ 3.8 10 26 W, and an age of t o≈ 4.6 10 9 years (Table 1). The distance from the Sun to our Earth is called an astronomical unit (AU) and amounts to R150 106 km. The Sun lies in a spiral arm of our galaxy, the Milky Way, at a distance of 8.5 kiloparsecs from the galactic center. Our galaxy contains R10 12 individual stars, many of which are likely to be populated with similar solar systems, according to the rapidly increasing detection of extrasolar planets over the last years; the binary star systems are very unlikely to harbor planets because of their unstable, gravitationally disturbed orbits. The Sun is for us humans of particular significance, first because it provides us with the source of all life, and second because it furnishes us with the closest laboratory for astrophysical plasma physics, magneto-hydrodynamics (MHD), atomic physics, and particle physics. The Sun still represents the only star from which we can obtain spatial images, in many wavelengths. Title: From solar nanoflares to stellar giant flares: Scaling laws and non-implications for coronal heating Authors: Aschwanden, Markus J. Bibcode: 2007AdSpR..39.1867A Altcode: In this study we explore physical scaling laws applied to solar nanoflares, microflares, and large flares, as well as to stellar giant flares. Solar flare phenomena exhibit a fractal volume scaling, V( L) ≲ L1.9, with L being the flare loop length scale, which explains the observed correlation EM∝Tp4 between the total emission measure EMp and flare peak temperature Tp in both solar and stellar flares. However, the detected stellar flares have higher emission measures EMp than solar flares at the same flare peak temperature Tp, which can be explained by a higher electron density that is caused by shorter heating scale height ratios sH/ L ≈ 0.04-0.1. Using these scaling laws we calculate the total radiated flare energies EX and thermal flare energies ET and find that the total counts C are a good proxy for both parameters. Comparing the energies of solar and stellar flares we find that even the smallest observed stellar flares exceed the largest solar flares, and thus their observed frequency distributions are hypothetically affected by an upper cutoff caused by the maximum active region size limit. The powerlaw slopes fitted near the upper cutoff can then not reliably be extrapolated to the microflare regime to evaluate their contribution to coronal heating. Title: Particle Acceleration in Solar Flares and Escape into Interplanetary Space Authors: Aschwanden, Markus J. Bibcode: 2006GMS...165..189A Altcode: We review the physics of particle acceleration and kinematics in solar flares under the particular aspect of their escape and propagation into interplanetary space. The topics include the magnetic topology in acceleration regions, the altitude of flare acceleration regions, evidence for bi-directional acceleration, the asymmetry of upward versus downward acceleration, and particle access to interplanetary space. Title: Astrophysics in 2005 Authors: Trimble, Virginia; Aschwanden, Markus J.; Hansen, Carl J. Bibcode: 2006PASP..118..947T Altcode: 2006astro.ph..6663T We bring you, as usual, the Sun and Moon and stars, plus some galaxies and a new section on astrobiology. Some highlights are short (the newly identified class of gamma-ray bursts, and the Deep Impact on Comet 9P/Tempel 1), some long (the age of the universe, which will be found to have the Earth at its center), and a few metonymic, for instance the term ``down-sizing'' to describe the evolution of star formation rates with redshift. Title: The Localization of Particle Acceleration Sites in Solar Flares and CMES Authors: Aschwanden, Markus J. Bibcode: 2006SSRv..124..361A Altcode: 2006SSRv..tmp..109A We review the particular aspect of determining particle acceleration sites in solar flares and coronal mass ejections (CMEs). Depending on the magnetic field configuration at the particle acceleration site, distinctly different radiation signatures are produced: (1) If charged particles are accelerated along compact closed magnetic field lines, they precipitate to the solar chromosphere and produce hard X-rays, gamma rays, soft X-rays, and EUV emission; (2) if they are injected into large-scale closed magnetic field structures, they remain temporarily confined (or trapped) and produce gyrosynchrotron emission in radio and bremsstrahlung in soft X-rays; (3) if they are accelerated along open field lines they produce beam-driven plasma emission with a metric starting frequency; and (4) if they are accelerated in a propagating CME shock, they can escape into interplanetary space and produce beam-driven plasma emission with a decametric starting frequency. The latter two groups of accelerated particles can be geo-effective if suitably connected to the solar west side. Particle acceleration sites can often be localized by modeling the magnetic topology from images in different wavelengths and by measuring the particle velocity dispersion from time-of-flight delays. Title: Temperature and Electron Density Along Elementary Coronal Loop Strands as Observed by TRACE Authors: Nightingale, Richard W.; Aschwanden, M. J. Bibcode: 2006SPD....37.0104N Altcode: 2006BAAS...38..215N Using the measured data of our initial triple-filter study originally observed by TRACE (with its < 1" resolution) in all three extreme-ultraviolet (EUV) filters at 171, 195, and 284 A (Aschwanden and Nightingale, 2005), where we disentangled the multi-thread structure of 78 coronal loops into elementary threads with isothermal cross-sections, we analyze now the 1-dimensional functions of temperature T(s), electron density n(s), and loop width w(s) along these finest observed loop threads. These functions of physical parameters determined with triple-filter data and subarcsecond resolution are believed to be the least background-contaminated measurements of coronal loop structures available today and thus provide suitable input for hydrodynamic loop models in the temperature range of T=0.7-2.7 MK. This work was supported in part by NASA under the TRACE contract NAS5-38099. Title: Coronal magnetohydrodynamic waves and oscillations: observations and quests Authors: Aschwanden, Markus J. Bibcode: 2006RSPTA.364..417A Altcode: No abstract at ADS Title: Stellar magnetic energy release at small-scales - microflares and their relevance for coronal heating Authors: Aschwanden, M. Bibcode: 2006cosp...36.3417A Altcode: 2006cosp.meet.3417A Recent data analysis of EUV images from TRACE and SoHO EIT have provided abundant statistics on physical parameters and their occurrence distributions of small-scale phenomena in the solar corona and transition region The observations indicate that many small-scale phenomena occur in the transition region and lowest region of the solar corona often revealing a signature of cooling plasma in the 1-2 MK temperature range that is confined in small-scale loops like miniature versions of larger flares These phenomena have therefore been aptly been named microflares and nanoflares also supported by the signatures of nonthermal electrons that accompany them - On the theoretical side there is the notion that nanoflares occur throughout the corona in small magnetic reconnection events driven by the braiding of coronal loops as consequence of the random footpoint motion as envisioned by Gene Parker In this talk we will discuss the discrepancy between theoretical expectations and observational constraints as well as discuss future ways to reconcile theory with observations Title: Physics of the Solar Corona. An Introduction with Problems and Solutions (2nd edition) Authors: Aschwanden, Markus J. Bibcode: 2005psci.book.....A Altcode: This paperback is the second edition of the original textbook published in Aug 2004, with an addition of some 170 problems and solutions, written for graduate students, post-Docs and researchers. It provides a systematic introduction into all phenomena of the solar corona, including the Quiet Sun, flares, and CMEs, covering the latest results from Yohoh, SoHO, TRACE, and RHESSI. The contents are:

1 - Introduction

2 - Thermal Radiation

3 - Hydrostatics

4 - Hydrodynamics

5 - Magnetic Fields

6 - Magnetohydrodynamics (MHD)

7 - MHD Oscillations

8 - Propagating MHD Waves

9 - Coronal Heating

10 - Magnetic Reconnection

11 - Particle Acceleration

12 - Particle Kinematics

13 - Hard X-rays

14 - Gamma-Rays

15 - Radio Emission

16 - Flare Plasma Dynamics

17 - Coronal Mass Ejections Title: Discriminating Composite from Elementary Coronal Loops Authors: Aschwanden, M. J. Bibcode: 2005AGUFMSH13B..01A Altcode: We establish three simple criteria to discriminate between elementary and composite loop structures, based on observations from high-resolution (TRACE) and low-resolution (EIT) imagers. Both datasets contain triple-filter data with a temperature diagnostic in the range of T=0.7-2.8 MK, which we use for forward-fitting of a differential emission measure (DEM) distribution to constrain the temperature range of each structure. From the TRACE dataset we find for the finest structures a mean width of w=1.5±0.4 Mm, a flux-to-background contrast of c=0.18±0.13, and a temperature range of dT=0.07±0.10 MK, which we identify as elementary strands, defined in terms of their thermal homogeneity. The loop structures observed with EIT have loop widths w, flux contrasts c, and temperature ranges dT that are all about an order of magnitude larger, and thus clearly constitute composite structures, consisting of many loop strands. These two contrasting observations resolve previous controversies about the basic thermal structure of coronal loops and yield a simple discrimination rule: Elementary loop strands (1) are near-isothermal (dT < 0.15 MK), (2) have a small width (w < 2 Mm), and (3) have a faint contrast (c < 0.3), while virtually all wider and higher-contrast loop features are most likely to be multi-thermal composites and have a broad DEM. Title: Examples of Elementary Coronal Loop Strands as Observed by TRACE Authors: Nightingale, R. W.; Aschwanden, M. J. Bibcode: 2005AGUFMSH41B1127N Altcode: A large study has recently been completed (in press at Astrophys. Jour.) of coronal loop structures observed by TRACE (with its < 1" resolution) in all three extreme-ultraviolet (EUV) filters at 171, 195, and 284 A. One of the results is that all of the fitted, uncontaminated loops with widths < 2000 km are consistent with a single temperature cross-section. This, in turn, leads to the conclusion that TRACE has resolved "elementary" or "monolithic" loop strands, in terms of thermal homogeneity over the loop cross-section. This further implies, due to the inhibition of cross-field transport in the corona (because the plasma-beta parameter is low), that the loop heating occurs below the corona in the transition region or chromosphere, where the plasma-beta values are higher allowing for possible cross-field transport. Several cases of measurements and results of acceptable isothermal fits of "elementary" loop strands (not shown in the original paper) will be presented. Also to be displayed are comparisons between extracted original images and high-pass filtered images utilized in the analysis and conclusions above. This work was supported in part by NASA under the TRACE contract NAS5-38099. Title: Three Criteria to Discriminate between Elementary and Composite Coronal Loops Authors: Aschwanden, Markus J. Bibcode: 2005ApJ...634L.193A Altcode: In this Letter, we establish three simple criteria to discriminate between elementary and composite loop structures, based on observations from high-resolution (TRACE) and low-resolution (EIT) imagers. Both data sets contain triple-filter data with a temperature diagnostic in the range of T~0.7-2.8 MK, which we use for forward-fitting of a differential emission measure (DEM) distribution to constrain the temperature range of each structure. From the TRACE data set, we find for the finest structures a mean width of w=1.4+/-0.3 Mm, a flux-to-background contrast of c~0.18+/-0.13, and a temperature range of dT=0.07+/-0.10 MK, which we identify as elementary strands, defined in terms of their thermal homogeneity. The loop structures observed with EIT have loop widths w, flux contrasts c, and temperature ranges dT that are all about an order of magnitude larger and thus clearly constitute composite structures, consisting of many loop strands. These two contrasting observations resolve previous controversies about the basic thermal structure of coronal loops and yield a simple discrimination rule: elementary loop strands (1) are near-isothermal (dT<~0.2 MK), (2) have a small width (w<~2 Mm), and (3) have a faint contrast (c<~0.3), while virtually all wider and higher contrast loop features are most likely multithermal composites and have a broad DEM. Title: Elementary Loop Structures in the Solar Corona Analyzed from TRACE Triple-Filter Images Authors: Aschwanden, Markus J.; Nightingale, Richard W. Bibcode: 2005ApJ...633..499A Altcode: This study represents the first quantitative analysis of the multithread structure of coronal loops. We analyzed a set of 234 fine loop threads observed with TRACE triple-filter images at wavelengths of 171, 195, and 284 Å. The cross-sectional flux profiles are simultaneously forward fitted in all three filters with the superposition of a cospatial Gaussian and linear background functions. We fit a general multitemperature differential emission measure (DEM) distribution to each cross section, as well as the special case of a single-temperature or isothermal DEM. We perform these forward fits at ~18,000 loop positions and find that this geometric model could be fitted in ~3500 cases (within a χred<=1.5), while all other cases require more complex geometric models of the loop cross section, secondary loops, and background. The major result of this study is that the vast majority (84%) of the acceptable DEM fits are isothermal. Temperatures are measured over the whole sensitivity range of 0.7-2.8 MK, but with a higher probability near the peak sensitivities of the three filters. We conclude that we indeed resolve ``elementary'' or ``monolithic'' loop strands with TRACE, in terms of isothermal homogeneity. Virtually all earlier studies detected ensembles of multiple strands, while our detected loop strands exhibit much smaller widths (w~1.4+/-0.3 Mm) and also smaller signal-to-background ratios (14%+/-10%). We suggest that the temperature homogeneity of coronal loops up to widths of w<~2000 km is related to their magnetic mapping to photospheric granulation (convection) cells. Title: Foreword Authors: Gallagher, Peter; Berghmans, David; Aschwanden, Markus Bibcode: 2005SoPh..228....1G Altcode: No abstract at ADS Title: The Elementary Structure of Coronal Loops Authors: Aschwanden, M. J. Bibcode: 2005AGUSMSP13B..02A Altcode: There is the question whether the plasma of coronal loops consists of unresolved strands or not, given the best spatial resolution of <1" we have today with EUV telescopes such as TRACE. Since the hypothetical loop strands would be thermally insulated from each other, due to the inhibited cross-field diffusion in the coronal plasma that has a plasma-beta parameter significantly smaller than unity, we expect that loop strands have arbitrarily different heating rates and temperatures. Consequently, both resolved as well as unresolved loop strands are expected not to be co-spatial in different temperature filters. We present the results of a triple-filter analysis of a larger number of coronal loop structures observed with TRACE in the 171, 195, and 284 A filters, where we measure the cospatiality of loop substructures with sub-arcsecond accuracy in near-simultaneous measurements with triple filters. In virtually all loop structures we identify multiple resolved substructures that are not cospatial in triple temperature filters, but are cospatial in dual temperature filters and consistent with a single temperatures over partial loop length segments. From these results we conclude that the cross-section of heated loop strands is resolved on ~1" spatial scale, but no thermal equilibrium is achieved over the entire length of coronal loops, and thus loops are always asymmetric in their temperature and density structure. Title: 2D Feature Recognition And 3d Reconstruction In Solar Euv Images Authors: Aschwanden, Markus J. Bibcode: 2005SoPh..228..339A Altcode: EUV images show the solar corona in a typical temperature range of T >rsim 1 MK, which encompasses the most common coronal structures: loops, filaments, and other magnetic structures in active regions, the quiet Sun, and coronal holes. Quantitative analysis increasingly demands automated 2D feature recognition and 3D reconstruction, in order to localize, track, and monitor the evolution of such coronal structures. We discuss numerical tools that "fingerprint" curvi-linear 1D features (e.g., loops and filaments). We discuss existing finger-printing algorithms, such as the brightness-gradient method, the oriented-connectivity method, stereoscopic methods, time-differencing, and space-time feature recognition. We discuss improved 2D feature recognition and 3D reconstruction techniques that make use of additional a priori constraints, using guidance from magnetic field extrapolations, curvature radii constraints, and acceleration and velocity constraints in time-dependent image sequences. Applications of these algorithms aid the analysis of SOHO/EIT, TRACE, and STEREO/SECCHI data, such as disentangling, 3D reconstruction, and hydrodynamic modeling of coronal loops, postflare loops, filaments, prominences, and 3D reconstruction of the coronal magnetic field in general. Title: Astrophysics in 2004 Authors: Trimble, Virginia; Aschwanden, Markus Bibcode: 2005PASP..117..311T Altcode: In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars (§ 4), the Moon and Planets (§ 3), a truly binary pulsar (§ 5), a kinematic apology (§ 6), the whole universe (§§ 7 and 8), reconsideration of old settled (§ 9) and unsettled (§ 10) issues, and some things that happen only on Earth, some indeed only in these reviews (§§ 10 and 11). Title: Coronal heating and the appearance of solar and stellar coronae Authors: Schrijver, C. J.; Sandman, A. W.; Aschwanden, M. J.; De Rosa, M. L. Bibcode: 2005ESASP.560...65S Altcode: 2005csss...13...65S No abstract at ADS Title: The Nature and Excitation Mechanisms of Acoustic Oscillations in Solar and Stellar Coronal Loops Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.; Aschwanden, M. J. Bibcode: 2004ESASP.575..114T Altcode: 2004soho...15..114T; 2004astro.ph..9556T In the recent work of Nakariakov et al. (2004), it has been shown that the time dependences of density and velocity in a flaring loop contain pronounced quasi-harmonic oscillations associated with the 2nd harmonic of a standing slow magnetoacoustic wave. That model used a symmetric heating function (heat deposition was strictly at the apex). This left outstanding questions: A) is the generation of the 2nd harmonic a consequence of the fact that the heating function was symmetric? B) Would the generation of these oscillations occur if we break symmetry? C) What is the spectrum of these oscillations? Is it consistent with a 2nd spatial harmonic? The present work (and partly Tsiklauri et al. (2004b)) attempts to answer these important outstanding questions. Namely, we investigate the physical nature of these oscillations in greater detail: we study their spectrum (using periodogram technique) and how heat positioning affects the mode excitation. We found that excitation of such oscillations is practically independent of location of the heat deposition in the loop. Because of the change of the background temperature and density, the phase shift between the density and velocity perturbations is not exactly a quarter of the period, it varies along the loop and is time dependent, especially in the case of one footpoint (asymmetric) heating. We also were able to model successfully SUMER oscillations observed in hot coronal loops. Title: The Role of Observed MHD Oscillations and Waves for Coronal Heating Authors: Aschwanden, M. J. Bibcode: 2004ESASP.575...97A Altcode: 2004soho...15...97A No abstract at ADS Title: STEREO/SECCHI Simulations of CMEs and Flares using TRACE Images Authors: Aschwanden, M. J.; Lemen, J.; Nitta, N.; Metcalf, T.; Wuelser, J.; Alexander, D. Bibcode: 2004AGUFMSH22A..02A Altcode: We simulate 3D models of EUV images of flare and CME events, using TRACE EUV movies. TRACE movies show 2D images in projection along a particular line-of-sight. We simulate 3D models of erupting filaments, flare loops, and postflare loops using: (1) a ``finger printing'' technique to trace linear structures in 2D images; (2) geometric 3D models based on force-free fields and curvature radius maximization of flare loop and flux rope structures; (3) conservation of velocity and acceleration parameters; (4) multi-temperature plasma filling according to hydrodyamic scaling laws; and (5) 2D projections from secondary line-of-sights that correspond to viewpoints of the secondary STEREO spacecraft. From such simulations we envision to illustrat 3D time-dependent models, what would be observed at the two STEREO spacecraft positions as well as from a near-Earth spacecraft such as SoHO. These simulations are used to test STEREO analysis software and to investigate what physical parameters and geometric 3D reconstructions can be retrieved from STEREO/SECCHI data. Title: The Coronal Heating Mechanism as Identified by Full-Sun Visualizations Authors: Schrijver, Carolus J.; Sandman, Anne W.; Aschwanden, Markus J.; De Rosa, Marc L. Bibcode: 2004ApJ...615..512S Altcode: We constrain the properties of the mechanism(s) responsible for the bulk of the heating of the corona of the Sun by simulating, for the first time, the appearance of the entire solar corona. Starting from full-sphere magnetic field maps for 2000 December 1 and 8, when the Sun was moderately active, we populate nearly 50,000 coronal field lines with quasi-static loop atmospheres. These atmospheres are based on heating flux densities FH that depend in different ways on the loop half-length L, the field strength B at the chromospheric base, the loop expansion with height, and the heating scale height. The best match to X-ray and EUV observations of the corona over active regions and their environs is found for FH~4×1014B1.0+/-0.3/L1.0+/-0.5 (in ergs cm-2 s-1 for B in Mx cm-2 and L in cm), while allowing for substantial loop expansion with increasing height, and for a heating scale height that is at least a sizeable fraction of the loop length. This scaling for coronal heating points to DC reconnection at tangential discontinuities as the most likely coronal heating mechanism, provided that the reconnection progresses proportional to the Alfvén velocity. The best-fit coronal filling factor equals unity, suggesting that most of the corona is heated most of the time. We find evidence that loops with half-lengths exceeding ~100,000 km are heated significantly more than suggested by the above scaling, possibly commensurate with the power deposited in the open field of coronal holes. Title: Tomographic 3D-Modeling of the Solar Corona with FASR Authors: Aschwanden, Markus J.; Alexander, David; de Rosa, Marc L. Bibcode: 2004ASSL..314..243A Altcode: 2003astro.ph..9501A The Frequency-Agile Solar Radiotelescope (FASR) literally opens up a new dimension, in addition to the 3D Euclidian geometry—the frequency dimension. The 3D geometry is degenerated to 2D in all images from astronomical telescopes, but the additional frequency dimension allows us to retrieve the missing third dimension by means of physical modeling. We call this type of 3D reconstruction Frequency Tomography. In this study we simulate a realistic 3D model of an active region, composed of 500 coronal loops with the 3D geometry [x(s), y(s), z(s)] constrained by magnetic field extrapolations and the physical parameters of the density ne(s) and temperature Te(s) given by hydrostatic solutions. We simulate a series of 20 radio images in a frequency range of ν=0.1-10 GHz, anticipating the capabilities of FASR, and investigate what physical information can be retrieved from such a dataset. We discuss also forward-modeling of the chromospheric and Quiet Sun density and temperature structure, another primary goal of future FASR science. Title: Physics of the Solar Corona. An Introduction Authors: Aschwanden, Markus J. Bibcode: 2004psci.book.....A Altcode: This textbook is written for graduate students, post-Docs, and researchers. It provides a systematic introduction into all phenomena of the solar corona, including the Quiet Sun, flares, and CMEs, covering the latest results from Yohoh, SoHO, TRACE, and RHESSI. The contents are:

1 - Introduction

2 - Thermal Radiation

3 - Hydrostatics

4 - Hydrodynamics

5 - Magnetic Fields

6 - Magnetohydrodynamics (MHD)

7 - MHD Oscillations

8 - Propagating MHD Waves

9 - Coronal Heating

10 - Magnetic Reconnection

11 - Particle Acceleration

12 - Particle Kinematics

13 - Hard X-rays

14 - Gamma-Rays

15 - Radio Emission

16 - Flare Plasma Dynamics

17 - Coronal Mass Ejections Title: Flare-generated acoustic oscillations in solar and stellar coronal loops Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.; Aschwanden, M. J. Bibcode: 2004A&A...422..351T Altcode: 2004astro.ph..2261T Long period longitudinal oscillations of a flaring coronal loop are studied numerically. In the recent work of Nakariakov et al. (2004) it has been shown that the time dependence of density and velocity in a flaring loop contain pronounced quasi-harmonic oscillations associated with the 2nd harmonic of a standing slow magnetoacoustic wave. In this work we investigate the physical nature of these oscillations in greater detail, namely, their spectrum (using the periodogram technique) and how heat positioning affects mode excitation. We found that excitation of such oscillations is practically independent of the location of the heat deposition in the loop. Because of the change of the background temperature and density, the phase shift between the density and velocity perturbations is not exactly a quarter of the period; it varies along the loop and is time dependent, especially in the case of one footpoint (asymmetric) heating. Title: Pulsed Particle Injection in a Reconnection-Driven Dynamic Trap Model in Solar Flares Authors: Aschwanden, Markus J. Bibcode: 2004ApJ...608..554A Altcode: The time structure of hard X-ray emission during solar flares shows subsecond pulses, which have an energy-dependent timing that is consistent with electron time-of-flight delays. The inferred time-of-flight distances imply an injection height about 50% above the soft X-ray-bright flare loops, where electrons are injected in a synchronized way. No physical injection mechanism is known that can account for the energy synchronization and duration of subsecond pulses. Here we propose a model in terms of dynamic loss cone angle evolution of newly reconnected magnetic field lines that relax from the cusp at the reconnection point into a force-free configuration, which can explain the subsecond pulse structure of injected particles as well as the observed correlation between the pulse duration and flare loop size. This quantitative model predicts that the pulse duration of hard X-ray or radio pulses scales with tw~2LB/vA (s), which is the Alfvénic transit time through the magnetic outflow region of a Petschek-type X-point with magnetic length scale LB, and thus provides a direct diagnostic of the magnetic reconnection geometry. It also demonstrates that the observed pulse durations are primarily controlled by the injection time rather than by the acceleration timescale. Title: Radiative hydrodynamic modeling of the Bastille-Day flare (14 July, 2000). I. Numerical simulations Authors: Tsiklauri, D.; Aschwanden, M. J.; Nakariakov, V. M.; Arber, T. D. Bibcode: 2004A&A...419.1149T Altcode: 2004astro.ph..2260T A 1D loop radiative hydrodynamic model that incorporates the effects of gravitational stratification, heat conduction, radiative losses, external heat input, presence of helium, and Braginskii viscosity is used to simulate elementary flare loops. The physical parameters for the input are taken from observations of the Bastille-Day flare of 2000 July 14. The present analysis shows that: a) the obtained maximum values of the electron density can be considerably higher (4.2 × 1011 cm-3 or more) in the case of footpoint heating than in the case of apex heating (2.5 × 1011 cm-3); b) the average cooling time after the flare peak takes less time in the case of footpoint heating than in the case of apex heating; c) the peak apex temperatures are significantly lower (by about 10 MK) for the case of footpoint heating than for apex heating (for the same average loop temperature of about 30 MK). This characteristic would allow to discriminate between different heating positioning; d) in both cases (of apex and footpoint heating), the maximum obtained apex temperature Tmax is practically independent of the heating duration σt, but scales directly with the heating rate EH0; e) the maximum obtained densities at the loop apex, nemax, increase with the heating rate EH0 and heating duration σt for both footpoint and apex heating. In Paper II we will use the outputs of these hydrodynamic simulations, which cover a wide range of the parameter space of heating rates and durations, as an input for forward-fitting of the multi-loop arcade of the Bastille-day flare. Title: Solar Coronal Heating Inferred from Full-disk Models of Coronal Emission Authors: Schrijver, C. J.; Sandman, A. W.; De Rosa, M. L.; Aschwanden, M. J. Bibcode: 2004AAS...204.9501S Altcode: 2004BAAS...36Q.826S The appearance of the corona as viewed by different instruments, as well as its global spectral irradiance, sensitively depends on how coronal heating scales with the properties of the coronal magnetic field. We explore a variety of scaling dependences by simulating the appearance of the full-disk solar corona as viewed by SOHO/EIT and by YOHKOH/SXT, based on observed photospheric magnetic fields combined with a potential-field source-surface model. This leads us to conclude that the best match to X-ray and EUV observations of the corona over active regions and their environments is found for a heating flux density going into the corona that scales linearly with the field strength at the coronal base and roughly inversely with loop length. This scaling points to DC reconnection at tangential discontinuities as the most likely coronal heating mechanism, provided that the reconnection progresses at a rate proportional to the Alfven velocity. We also find that the best-fit coronal filling factor equals unity, suggesting that most of the corona is heated most of the time. We find evidence that loops with half lengths exceeding approximately 100,000 km are heated significantly more than suggested by the above scaling, possibly commensurate with the power deposited in the open field of coronal holes. Title: TRACE Triple-Filter Analysis - Spatial Widths of Monolithic Coronal Loops Authors: Aschwanden, M. J. Bibcode: 2004AAS...204.9506A Altcode: 2004BAAS...36..827A We focus on the question of elementary plasma structures in the solar corona. The crucial issue under investigation is whether we observe with current high-resolution instrumentation (<1", such as with TRACE) monolithic structures in the corona, i.e., elementary loops or loop threads that are homogeneous in temperature and density in any given cross-section. The search and identification of monolithic structures has fundamental consequences in theory (hydrodynamic modeling, coronal heating function, cross-field diffusion processes) and for the planning and design of future high-resolution EUV and soft X-ray telescopes. The identification of monolithic structures is a necessary prerequisite to apply hydrodynamic models, to determine the time-dependent heating function and cooling processes, and to measure the iron abundance and first-ionization potential (FIP) effect. We analyze a number of loop structures from TRACE triple-filter data (171, 195, 284 A) and present quantitative results about the temperature width of the differential emission measure distribution of the finest resolved coronal loops in the temperature range of T=0.7-2.5 MK. We measure the width range of monolithic loop structures, for which we find upper limits in the range of 1"-2". A consequence is that loop structures observed with instruments of lower resolution (e.g., with Yohkoh, EIT, CDS) represent composite temperature structures and have to be modeled as such. -We acknowledge support from NASA for TRACE, LWS, and SEC projects. Title: Astrophysics in 2003 Authors: Trimble, Virginia; Aschwanden, Markus J. Bibcode: 2004PASP..116..187T Altcode: Five coherent sections appear this year, addressing solar physics, cosmology (with WMAP highlights), gamma-ray bursters (and their association with Type Ia supernovae), extra-solar-system planets, and the formation and evolution of galaxies (from reionization to assemblage of Local Group galaxies). There are also eight incoherent sections that deal with other topics in stellar, galactic, and planetary astronomy and the people who study them. Title: Acoustic Oscillations in Solar and Stellar Flaring Loops Authors: Tsiklauri, D.; Nakariakov, V. M.; Kelly, A.; Aschwanden, M. J.; Arber, T. D. Bibcode: 2004ESASP.547..473T Altcode: 2004soho...13..473T No abstract at ADS Title: Solar Magnetic Loops Observed with TRACE and EIT Authors: Aschwanden, M. J.; Title, A. M. Bibcode: 2004IAUS..219..503A Altcode: 2003IAUS..219E.219A We review major discoveries and new physical results that have been obtained from the TRACE mission over the last 4 years such as: (1) the temperature and density inhomogeneity of the coronal plasma (2) hydrostatic and non-hydrostatic loops (3) plasma flows in loops (4) transverse oscillations in the MHD kink-mode (5) the spatial heating function of loops (6) intermittent heating and cooling time scales (7) iron abundance enhancements (8) magnetic nullpoints and separator regions (9) highly fragmented postflare loop arcades and (10) nanoflare loop phenomena. We transform the physical properties as measured by TRACE for the Sun to stellar coronae and show how information on heating and cooling processes can be obtained from stellar differential emission measure (DEM) distributions. Title: On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy Authors: Aschwanden, Markus J.; Metcalf, Thomas R.; Krucker, Säm; Sato, Jun; Conway, Andrew J.; Hurford, G. J.; Schmahl, Edward J. Bibcode: 2004SoPh..219..149A Altcode: 2003astro.ph..9499A We compare the photometric accuracy of spectra and images in flares observed with the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft. We test the accuracy of the photometry by comparing the photon fluxes obtained in different energy ranges from the spectral-fitting software SPEX with those fluxes contained in the images reconstructed with the Clean, MEM, MEM-Vis, Pixon, and Forward-fit algorithms. We quantify also the background fluxes, the fidelity of source geometries, and spatial spectra reconstructed with the five image reconstruction algorithms. We investigate the effects of grid selection, pixel size, field of view, and time intervals on the quality of image reconstruction. The detailed parameters and statistics are provided in an accompanying CD-ROM and web page. We find that Forward-fit, Pixon, and Clean have a robust convergence behavior and a photometric accuracy in the order of a few percent, while MEM does not converge optimally for large degrees of freedom (for large field of view and/or small pixel sizes), and MEM-Vis suffers in the case of time-variable sources. This comparative study documents the current status of the RHESSI spectral and imaging software, one year after launch. Title: 2D MHD Modelling of Heated Coronal Loops Compared to TRACE Observations Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H. C.; Tsinganos, K.; Aschwanden, M. J. Bibcode: 2004hell.conf...31P Altcode: No abstract at ADS Title: Acoustic oscillations in solar and stellar flaring loops Authors: Nakariakov, V. M.; Tsiklauri, D.; Kelly, A.; Arber, T. D.; Aschwanden, M. J. Bibcode: 2004A&A...414L..25N Altcode: 2004astro.ph..2223N Evolution of a coronal loop in response to an impulsive energy release is numerically modelled. It is shown that the loop density evolution curves exhibit quasi-periodic perturbations with the periods given approximately by the ratio of the loop length to the average sound speed, associated with the second standing harmonics of an acoustic wave. The density perturbations have a maximum near the loop apex. The corresponding field-aligned flows have a node near the apex. We suggest that the quasi-periodic pulsations with periods in the range 10-300 s, frequently observed in flaring coronal loops in the radio, visible light and X-ray bands, may be produced by the second standing harmonic of the acoustic mode. Title: Magnetohydrodynamic Sausage-Mode Oscillations in Coronal Loops Authors: Aschwanden, Markus J.; Nakariakov, Valery M.; Melnikov, Victor F. Bibcode: 2004ApJ...600..458A Altcode: 2003astro.ph..9493A A recent study by Nakariakov and coworkers pointed out that the dispersion relation of MHD sausage-mode oscillations has been incorrectly applied to coronal loops, neglecting the highly dispersive nature of the phase speed and the long-wavelength cutoff of the wavenumber. In the light of these new insights, we revisit previous observations that have been interpreted in terms of MHD sausage-mode oscillations in coronal loops and come to the following conclusions: (1) fast sausage MHD-mode oscillations require such a high electron density imposed by the wavenumber cutoff that they can only occur in flare loops, and (2) in the previously reported radio observations (ν~100 MHz to 1 GHz) with periods of P~0.5-5 s, the fast sausage MHD-mode oscillation is likely to be confined to a small segment (corresponding to a high harmonic node) near the apex of the loop, rather than involving a global oscillation over the entire loop length. The recent microwave and soft X-ray observations of fast periods (P~6-17 s) by Asai and coworkers and Melnikov and coworkers, however, are consistent with fast sausage MHD oscillations at the fundamental harmonic. Title: 3D Numerical Simulations of Impulsively Generated MHD Waves in Solar Coronal Loops Authors: Selwa, M.; Murawski, K.; Kowal, G.; Nakariakov, V.; Aschwanden, M.; Oliver, R.; Ballester, J. L. Bibcode: 2004ESASP.547..495S Altcode: 2004soho...13..495S Impulsively generated magnetohydrodynamic waves in a typical EUV solar coronal loop are studied numerically with a use of the three-dimensional FLASH code. Our results reveal several 3D effects such as distinctive time signatures which are collected at a detection point inside the loop. A slow magnetosonic wave generates a significant variation in a mass density profile with a time-scale of the order of s. A fast kink wave affects a mass density too but its magnitude is much lower than in the case of a slow wave. Time-scales which are associated with the fast kink wave are generally lower than in the case of a slow wave; they are in the range of a dozen or so seconds. Temporal signatures of a fast sausage wave reveal s oscillations in the quasi-periodic phase. Impulses which are launched outside the loop excite few seconds oscillations in the mass density. Time-signatures depend on a position of the detection point; they are usually more complex further out from the exciter. Title: Communications, Navigation, and Timing Constraints for the Solar Imaging Radio Array (SIRA) Authors: Lemaster, E. A.; Byler, E. A.; Aschwanden, M. J. Bibcode: 2003AGUFMSH42C0553L Altcode: The Solar Imaging Radio Array (SIRA) is a proposed NASA mission to measure solar radio emissions in the 30kHz to 30MHz region of the electromagnetic spectrum. The baseline design consists of 16 separated spacecraft in an irregular pattern several kilometers across. Each spacecraft is equipped with a pair of crossed dipole antennas that together form a 16-element radio interferometer for Fourier-type image reconstruction (120 baselines in the UV-plane). The required close coordination between this formation of spacecraft places many unique constraints on the SIRA communications, navigation, control, and timing architectures. Current specifications call for knowledge of the relative locations of the spacecraft to approximately meter-level accuracy in order to maintain primary instrument resolution. Knowledge of the relative timing differences between the clocks on the spacecraft must likewise be maintained to tens of nanoseconds or better. This in turn sets a minimum bound on the regularity of communications updates between spacecraft. Although the actual positions of the spacecraft are not tightly constrained, enough control authority and system autonomy must be present to keep the spacecraft from colliding due to orbital perturbations. Each of these constraints has an important effect on the design of the architecture for the entire array. This paper examines the engineering requirements and design tradeoffs for the communications, navigation, and timing architectures for SIRA. Topics include the choice of navigation sensor, communications methodology and modulation schemes, and clock type to meet the overall system performance goals while overcoming issues such as communications dynamic range, bandwidth limitations, power constraints, available antenna beam patterns, and processing limitations. In addition, this paper discusses how the projected use of smaller spacecraft buses with their corresponding payload and cost limits has important consequences for the overall system design. Title: Observational Tests of Damping by Resonant Absorption in Coronal Loop Oscillations Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Andries, Jesse; Goossens, Marcel; Van Doorsselaere, Tom Bibcode: 2003ApJ...598.1375A Altcode: 2003astro.ph..9470A One of the proposed damping mechanisms of coronal (transverse) loop oscillations in the kink mode is resonant absorption as a result of the Alfvén speed variation at the outer boundary of coronal loops. Analytical expressions for the period and damping time exist for loop models with thin nonuniform boundaries. They predict a linear dependency of the ratio of the damping time to the period on the thickness of the nonuniform boundary layer. Ruderman and Roberts used a sinusoidal variation of the density in the nonuniform boundary layer and obtained the corresponding analytical expression for the damping time. Here we measure the thickness of the nonuniform layer in oscillating loops for 11 events, by forward-fitting of the cross-sectional density profile ne(r) and line-of-sight integration to the cross-sectional fluxes F(r) observed with TRACE 171 Å. This way we model the internal (ni) and external electron density (ne) of the coronal plasma in oscillating loops. This allows us to test the theoretically predicted damping rates for thin boundaries as a function of the density ratio χ=ne/ni. Since the observations show that the loops have nonuniform density profiles, we also use numerical results for damping rates to determine the value of χ for the loops. We find that the density ratio predicted by the damping time, χLEDA=0.53+/-0.12, is a factor of ~1.2-3.5 higher than the density ratio estimated from the background fluxes, χ=0.30+/-0.16. The lower densities modeled from the background fluxes are likely to be a consequence of the neglected hotter plasma that is not detected with the TRACE 171 Å filter. Taking these corrections into account, resonant absorption predicts damping times of kink-mode oscillations that are commensurable with the observed ones and provides a new diagnostic of the density contrast of oscillating loops. Title: Turbulence, Waves and Instabilities in the Solar Plasma Authors: Erdélyi, R.; Petrovay, K.; Roberts, B.; Aschwanden, M. Bibcode: 2003twis.book.....E Altcode: Significant advances have been made recently in both the theoretical understanding and observation of small-scale turbulence in different layers of the Sun, and in the instabilities that give rise to them. The general development of solar physics, however, has led to such a degree of specialization as to hinder interaction between workers in the field. This book therefore presents studies of different layers and regions of the Sun, but from the same aspect, concentrating on the study of small-scale motions. The main emphasis is on the common theoretical roots of these phenomena, but the book also contains an extensive treatment of the observational aspects.

Link: http://www.springer.com/east/home?SGWID=5-102-22-3362=5696-0&changeHeader=true Title: Radio Coverage from Chromosphere to Earth: FASR-LOFAR-SIRA Synergy Authors: Gary, D. E.; Kassim, N.; Gopalswamy, N.; Aschwanden, M. J. Bibcode: 2003AGUFMSH42E..02G Altcode: Radio emission is uniquely sensitive to a number of key plasma parameters (magnetic field, temperature, density, high-energy electrons, and various plasma waves) over heights ranging without gaps from the chromosphere, throughout the corona and heliosphere, to the Earth. Two ground-based radio arrays, the Frequency Agile Solar Radiotelescope (FASR) and the Low Frequency Array (LOFAR), together with the space-based Solar Imaging Radio Array (SIRA) are planned that will for the first time provide direct imaging of disturbances over this vast height range through interferometric imaging over their equally impressive frequency range of 24 GHz to 30 kHz. We describe the science goals of these instruments, focusing especially on the science addressed jointly by all three instruments. Among the examples are (1) simultaneous imaging of CMEs, flaring loops, and shock-associated (type II) emission and (2) imaging the propagation of electrons on open field lines (type III), from their acceleration point through the corona and heliosphere to the point where they are measured in situ by near-Earth spacecraft. In addition to spatially relating the different phenomena, the spectral information is rich in quantitative diagnostics. We give some examples of the revolutionary results we can expect from the combined instruments. Title: Coronal heating and the appearance of the solar corona Authors: Schrijver, C. J.; Sandman, A.; De Rosa, M. L.; Aschwanden, M. J. Bibcode: 2003AGUFMSH32A1104S Altcode: The details of the dependence of coronal heating on the conditions within the corona determine the appearance of the corona as viewed by different instruments. For example, strong fields at the base of short loops cause relatively hot, X-ray bright loops, whereas the much weaker fields over the quiet Sun result in cooler, EUV bright loops. Any dependence of the volume heating rates on local conditions (such as height or field strength) has a signature in the thermal profiles along the loops, translating into an appearance that depends on the instrumental pass band. In this preliminary study, we explore how such dependences of coronal heating on coronal conditions affect the appearance of the solar corona, and investigate the consequences for the global EUV and X-ray spectral irradiance. These results will eventually be used to compute the solar spectral irradiance in the EUV and X-rays for quiescent conditions throughout the solar cycle. Title: Probing Solar Energetic Particles with SIRA Authors: Aschwanden, M. J.; Nitta, N.; Lemaster, E.; Byler, E.; Gary, D.; Kassim, N.; Gopalswamy, N. Bibcode: 2003AGUFMSH42C0555A Altcode: The space-based SIRA (Solar Imaging Radio Array) will provide a powerful capability to track high energy particles from solar flare and CME sites through interplanetary/heliospheric space all the way to Earth. Together with two other overlapping planned radio interferometers, i.e., FASR (Frequency-Agile Solar Radiotelescope) and LOFAR (Low-Frequency Array) the entire plasma frequency range from 30 GHz all the way down to the plasma frequency cutoff of 30 kHz at 1 AU will be covered. These instruments will track the magnetic trajectory of high energy particles, beam-driven radio emission, and localize the acceleration sites in the corona or interplanetary shocks. We simulate some CME and type III events, as they will be mapped with these instruments, using realistic scattering functions of radio waves on coronal and heliospheric density inhomogeneities. Title: 2D MHD modelling of compressible and heated coronal loops obtained via nonlinear separation of variables and compared to TRACE and SoHO observations Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H. C.; Tsinganos, K.; Aschwanden, M. J. Bibcode: 2003A&A...409.1065P Altcode: 2003astro.ph..3373P An analytical MHD model of coronal loops with compressible flows and including heating is compared to observational data. The model is constructed via a systematic nonlinear separation of the variables method used to calculate several classes of exact MHD equilibria in Cartesian geometry and uniform gravity. By choosing a particularly versatile solution class with a large parameter space we are able to calculate models whose loop length, shape, plasma density, temperature and velocity profiles are fitted to loops observed with TRACE, SoHO/CDS and SoHO/SUMER. Synthetic emission profiles are also calculated and fitted to the observed emission patterns. An analytical discussion is given of the two-dimenional balance of the Lorentz force and the gas pressure gradient, gravity and inertial forces acting along and across the loop. These models are the first to include a fully consistent description of the magnetic field, 2D geometry, plasma density and temperature, flow velocity and thermodynamics of loops. The consistently calculated heating profiles which are largely dominated by radiative losses and concentrated at the footpoints are influenced by the flow and are asymmetric, being biased towards the upflow footpoint. Title: Review of Coronal Oscillations - An Observer's View Authors: Aschwanden, Markus J. Bibcode: 2003astro.ph..9505A Altcode: Recent observations show a variety of oscillation modes in the corona. Early non-imaging observations in radio wavelengths showed a number of fast-period oscillations in the order of seconds, which have been interpreted as fast sausage mode oscillations. TRACE observations from 1998 have for the first time revealed the lateral displacements of fast kink mode oscillations, with periods of ~3-5 minutes, apparently triggered by nearby flares and destabilizing filaments. Recently, SUMER discovered with Doppler shift measurements loop oscillations with longer periods (10-30 minutes) and relatively short damping times in hot (7 MK) loops, which seem to correspond to longitudinal slow magnetoacoustic waves. In addition, propagating longitudinal waves have also been detected with EIT and TRACE in the lowest density scale height of loops near sunspots. All these new observations seem to confirm the theoretically predicted oscillation modes and can now be used as a powerful tool for ``coronal seismology'' diagnostic. Title: A New Method to Constrain the Iron Abundance from Cooling Delays in Coronal Loops Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Winebarger, Amy R.; Warren, Harry P. Bibcode: 2003ApJ...588L..49A Altcode: 2003astro.ph..9506A Recent observations with the Transition Region and Coronal Explorer reveal that the time delay between the appearance of a cooling loop in different EUV temperature filters is proportional to the loop length, Δt12~L. We model this cooling delay in terms of radiative loss and confirm this linear relationship theoretically. We derive an expression that can be used to constrain the coronal iron enhancement αFe=AcorFe/AphFe relative to the photospheric value as function of the cooling delay Δt12, flux F2, loop width w, and filling factor qw<=1. With this relation, we find upper limits on the iron abundance enhancement of αFe<=4.8+/-1.7 for 10 small-scale nanoflare loops, and αFe<=1.4+/-0.4 for five large-scale loops, in the temperature range of T~1.0-1.4 MK. This result supports the previous finding that low first ionization potential elements, including Fe, are enhanced in the corona. The same relation constitutes also a lower limit for the filling factor, which is qw>=0.2+/-0.1 and qw>=0.8+/-0.2 for the two groups of coronal loops. Title: A New Method to Constrain the Iron Abundance from Cooling Delays in Coronal Loops Authors: Aschwanden, M. J.; Schrijver, C. J.; Winebarger, A. R.; Warren, H. P. Bibcode: 2003SPD....34.1701A Altcode: 2003BAAS...35..837A Recent observations with TRACE reveal that the time delay between the appearance of a cooling loop in different EUV temperature filters is proportional to the loop length, dt12 ∼ L . We model this cooling delay in terms of radiative loss and confirm this linear relationship theoretically. We derive an expression that can be used to constrain the coronal iron enhancement AFe=AFecor/AFePh relative to the photospheric value as function of the cooling delay dt12, flux F2, loop width w, and filling factor qw < 1. With this relation we find upper limits on the iron abundance enhancement of AFe < 4.8 +/- 1.7 for 10 small-scale nanoflare loops, and AFe < 1.4 +/- 0.4 for 5 large-scale loops, in the temperature range of T ∼ 1.0-1.4 MK. This result supports the previous finding that low-FIP elements, including Fe, are enhanced in the corona. The same relation constitutes also a lower limit for the filling factor, which is qw > 0.2 +/- 0.1 and qw > 0.8 +/- 0.2 for the two groups of coronal loops. Title: Astrophysics in 2002 Authors: Trimble, Virginia; Aschwanden, Markus J. Bibcode: 2003PASP..115..514T Altcode: This has been the Year of the Baryon. Some low temperature ones were seen at high redshift, some high temperature ones were seen at low redshift, and some cooling ones were (probably) reheated. Astronomers saw the back of the Sun (which is also made of baryons), a possible solution to the problem of ejection of material by Type II supernovae (in which neutrinos push out baryons), the production of R Coronae Borealis stars (previously-owned baryons), and perhaps found the missing satellite galaxies (whose failing is that they have no baryons). A few questions were left unanswered for next year, and an attempt is made to discuss these as well. Title: Flare electron energy budgets - what is RHESSI telling us? Authors: Brown, J. C.; Kontar, E.; MacKinnon, A. L.; Aschwanden, M. J. Bibcode: 2002ESASP.506..253B Altcode: 2002ESPM...10..253B; 2002svco.conf..253B We address the idea that energetic particles may play a key role in the dissipation and transport of energy in flares. After three decades of predictions of spatial, spectral and temporal distributions of hard X- and γ-rays, the various models can now be quantitatively tested against RHESSI high resolution spectral imaging data. It is shown that RHESSI results for a number of HXR flares are in very good agreement with predictions of the basic thick target model (Brown 1971) regarding source height as a function of energy and of global HXR spectrum. A single power-law injection spectrum and purely collisional transport (no wave generation) fit well the decrease of source peak height with increasing energy for very plausible chromospheric density structures. When the target ionisation drop across the transition one is included, the global HXR spectrum agrees well with observed "knee" spectra without any feature added to a scale-less power-law electron injection spectrum. This result favours statistically distributed, as opposed to single large scale, E-field acceleration. Whether energetic electron beams actually dominate flare energy transport still depends on accurate inference of the low energy thermal/nonthermal spectral transition though RHESSI results to date support the idea. The ion energy budget is also briefly mentioned. Title: Chromospheric density and height measurements of the 2002-Feb-20 flare observed with RHESSI Authors: Aschwanden, Markus J.; Brown, John C.; Kontar, Eduard P. Bibcode: 2002ESASP.506..275A Altcode: 2002ESPM...10..275A; 2002svco.conf..275A We present the first chromospheric density and height measurements made with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft during the flare of 2002-Feb-22, 11:06 UT. Thanks to the high energy resolution of the germanium-cooled hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids z(ɛ) as a function of increasing hard X-ray energy ɛ, as expected in the thick-target bremsstrahlung model of Brown. The altitude of hard X-ray emission as a function of photon energy ɛ can be characterized by a powerlaw function in the ɛ = 15-50 keV energy range, viz. z(ɛ) ≍ 2.3 (ɛ/20 keV)-1.3 Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density model n(z), which follows the powerlaw function ne(z) = 1.25×1013 (z/1 Mm)-2.5 cm-3. This density is comparable with models based on optical/UV spectrometry in the chromospheric height range, while at a height of h≍1000-2500 km, it is more consistent with the "spicular extended-chromosphere model" inferred from radio sub-mm observations. In coronal heights of the flare loop, the RHESSI inferred desities are comparable with soft X-ray and radio observations. Title: Coronal Loops Heated by Magnetohydrodynamic Turbulence. I. A Model of Isobaric Quiet-Sun Loops with Constant Cross Sections Authors: Chae, Jongchul; Poland, Arthur I.; Aschwanden, Markus J. Bibcode: 2002ApJ...581..726C Altcode: Several recent papers have presented new observational results indicating that many coronal loops in active regions are nearly isothermal. It is expected that quiet-Sun loops may have similar thermal structures, since quiet-Sun differential emission measures look similar to those in active regions. In the quiet Sun, it is well known from observations that the nonthermal velocity inferred from the excess broadening of a line over thermal broadening reaches a peak of about 30 km s-1 around 3×105 K and then decreases with temperature, having a value of about 20 km s-1 at 1×106 K. In the present work, we make the assumption that the observed nonthermal velocities are a manifestation of magnetohydrodynamic (MHD) turbulence and present a model of static, isobaric coronal loops heated by turbulence. Instead of solving the MHD equations, we adopt simple energy spectra in MHD turbulence and infer the heating rate as a function of temperature from the observed nonthermal velocities. By solving the steady state energy equation of a loop in which temperature monotonically increases with height, we obtain the following results: (1) The heating rate is predominantly near the footpoints and decreases with the loop arc length. (2) There is a critical temperature above which the loop cannot be maintained in a steady state. (3) The loop is denser and is more isothermal than uniformly heated loops, being compatible with recent observations. (4) The theoretical differential emission measures are in good agreement with the empirical values at temperatures above 105 K. Below this temperature, we still have a large discrepancy. (5) It is possible to explain the observed strong correlation between intensity and nonthermal velocity of a spectral line in the quiet Sun. Our results support the idea that quasi-statically driven MHD turbulence of the direct current (DC) type in the stratified medium (transition region and corona) is a viable mechanism for coronal heating. Title: Overview of the US system for post-docs, contractors, and careers in solar physics (Invited review) Authors: Aschwanden, Markus J. Bibcode: 2002ESASP.506..991A Altcode: 2002svco.conf..991A; 2002ESPM...10..991A This article is intended to offer young physicists specific information on job opportunities and careers in solar physics in the United States (US), a traditional post-Doc country for many young European physicists. There live about 500 solar physicists in the United States, which are employed at universities and colleges (43%), in government laboratories (35%), in private companies (8%), or have no official affiliation (14%). We provide a brief overview of the affiliations of the US solar physicists, where the academic institutions are located, what the government laboratories consist of, and what private companies have the largest contracts in the solar physics business. We compile also some demographic and sociological statistics from the larger groups of US astronomers and physicists, that may be of interest for prospective post-Docs. Title: Damping of coronal loop oscillations by resonant absorption of quasi-mode kink oscillations Authors: Goossens, M.; Andries, J.; Aschwanden, M. J. Bibcode: 2002ESASP.506..629G Altcode: 2002svco.conf..629G; 2002ESPM...10..629G Damped quasi-mode kink oscillations in cylindrical flux tubes are capable of explaining the observed rapid damping of the coronal loop oscillations when the ratio of the inhomogneity length scale to the radius of the loop is allowed to vary from loop to loop. They do not need to invoke anomalously low Reynolds numbers. The theoretical expressions for the decay time by Hollweg & Yang (1988) and Ruderman & Roberts (2002) are used to estimate the ratio of the length scale of inhomogneity compared to the loop radius for a collection of loop oscillations. Title: Chromospheric Height and Density Measurements in a Solar Flare Observed with RHESSI II. Data Analysis Authors: Aschwanden, Markus J.; Brown, John C.; Kontar, Eduard P. Bibcode: 2002SoPh..210..383A Altcode: We present an analysis of hard X-ray imaging observations from one of the first solar flares observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February 2002. The data were obtained from the 22 February 2002, 11:06 UT flare, which occurred close to the northwest limb. Thanks to the high energy resolution of the germanium-cooled hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids z(ε) as a function of increasing hard X-ray energy ε, as expected in the thick-target bremsstrahlung model of Brown. The altitude of hard X-ray emission as a function of photon energy ε can be characterized by a power-law function in the ε=15-50 keV energy range, viz., z(ε)≈2.3(ε/20 keV)−1.3 Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density model n(z), as derived in Paper I, which follows the power-law function ne(z)=1.25×1013(z/1 Mm)−2.5 cm−3. This density is comparable with models based on optical/UV spectrometry in the chromospheric height range of h≲1000 km, suggesting that the collisional thick-target model is a reasonable first approximation to hard X-ray footpoint sources. At h≈1000-2500 km, the hard X-ray based density model, however, is more consistent with the `spicular extended-chromosphere model' inferred from radio sub-mm observations, than with standard models based on hydrostatic equilibrium. At coronal heights, h≈2.5-12.4 Mm, the average flare loop density inferred from RHESSI is comparable with values from hydrodynamic simulations of flare chromospheric evaporation, soft X-ray, and radio-based measurements, but below the upper limits set by filling-factor insensitive iron line pairs. Title: The Differential Emission Measure Distribution in the Multiloop Corona Authors: Aschwanden, Markus J. Bibcode: 2002ApJ...580L..79A Altcode: This is a rebuttal of a recent Letter on the inadequacy of temperature measurements in the solar corona through narrowband filter and line ratios by Martens et al. We simulate the differential emission measure (DEM) distribution of a multiloop corona and find that the temperature profile of individual loops can be retrieved with narrowband filter ratios. The apparently flat DEM distributions constructed from Coronal Diagnostics Spectrometer line fluxes by Schmelz et al. are an artifact of a smoothing function (in temperature), while the unsmoothed DEM distribution reveals multiple peaks of near-isothermal loops. Title: Chromospheric Height and Density Measurements in a Solar Flare Observed with RHESSI I. Theory Authors: Brown, John C.; Aschwanden, Markus J.; Kontar, Eduard P. Bibcode: 2002SoPh..210..373B Altcode: We obtain a theoretical description of the height (z) distribution of flare hard X-rays in the collisional thick-target model as a function of photon energy ε. This depends on the target atmosphere density structure n(z) and on the beam spectral index δ. We show that by representing the data in terms of the 1-D function z(ε) defining where the emission peaks as a function of ε it is possible to derive n(z) from data on z(ε). This is done first on the basis of a simple stopping depth argument then refined to allow for the dependence on spectral index δ. The latter is worked out in detail for the case of a parameterization n(z)=n0 (z/z0)−b which yields numerical results for z(ε) well fit by z(ε)∼ε−α, with α dependent on δ, which is also found to fit well to actual observations. This enables derivation of flare loop n(z) in terms of n0,b from RHESSI data in an entirely novel way, independent of other density diagnostic methods, and also of how n(z) varies with time in flares such as by evaporation, as detailed in companion Paper II. Title: Reconstruction of RHESSI Solar Flare Images with a Forward Fitting Method Authors: Aschwanden, Markus J.; Schmahl, Ed; Team, The Rhessi Bibcode: 2002SoPh..210..193A Altcode: We describe a forward-fitting method that has been developed to reconstruct hard X-ray images of solar flares from the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), a Fourier imager with rotation-modulated collimators that was launched on 5 February 2002. The forward-fitting method is based on geometric models that represent a spatial map by a superposition of multiple source structures, which are quantified by circular gaussians (4 parameters per source), elliptical gaussians (6 parameters), or curved ellipticals (7 parameters), designed to characterize real solar flare hard X-ray maps with a minimum number of geometric elements. We describe and demonstrate the use of the forward-fitting algorithm. We perform some 500 simulations of rotation-modulated time profiles of the 9 RHESSI detectors, based on single and multiple source structures, and perform their image reconstruction. We quantify the fidelity of the image reconstruction, as function of photon statistics, and the accuracy of retrieved source positions, widths, and fluxes. We outline applications for which the forward-fitting code is most suitable, such as measurements of the energy-dependent altitude of energy loss near the limb, or footpoint separation during flares. Title: The RHESSI Imaging Concept Authors: Hurford, G. J.; Schmahl, E. J.; Schwartz, R. A.; Conway, A. J.; Aschwanden, M. J.; Csillaghy, A.; Dennis, B. R.; Johns-Krull, C.; Krucker, S.; Lin, R. P.; McTiernan, J.; Metcalf, T. R.; Sato, J.; Smith, D. M. Bibcode: 2002SoPh..210...61H Altcode: The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observes solar hard X-rays and gamma-rays from 3 keV to 17 MeV with spatial resolution as high as 2.3 arc sec. Instead of focusing optics, imaging is based on nine rotating modulation collimators that time-modulate the incident flux as the spacecraft rotates. Starting from the arrival time of individual photons, ground-based software then uses the modulated signals to reconstruct images of the source. The purpose of this paper is to convey both an intuitive feel and the mathematical basis for this imaging process. Following a review of the relevant hardware, the imaging principles and the basic back-projection method are described, along with their relation to Fourier transforms. Several specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable to RHESSI imaging are briefly described. The characteristic strengths and weaknesses of this type of imaging are summarized. Title: Coronal loop oscillations. An interpretation in terms of resonant absorption of quasi-mode kink oscillations Authors: Goossens, M.; Andries, J.; Aschwanden, M. J. Bibcode: 2002A&A...394L..39G Altcode: Damped quasi-mode kink oscillations in cylindrical flux tubes are capable of explaining the observed rapid damping of the coronal loop oscillations when the ratio of the inhomogeneity length scale to the radius of the loop is allowed to vary from loop to loop, without the need to invoke anomalously low Reynolds numbers. The theoretical expressions for the decay time by Hollweg & Yang (\cite{hollweg1988}) and Ruderman & Roberts (\cite{ruderman2002}) are used to estimate the ratio of the length scale of inhomogeneity compared to the loop radius for a collection of loop oscillations. Title: The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) Authors: Lin, R. P.; Dennis, B. R.; Hurford, G. J.; Smith, D. M.; Zehnder, A.; Harvey, P. R.; Curtis, D. W.; Pankow, D.; Turin, P.; Bester, M.; Csillaghy, A.; Lewis, M.; Madden, N.; van Beek, H. F.; Appleby, M.; Raudorf, T.; McTiernan, J.; Ramaty, R.; Schmahl, E.; Schwartz, R.; Krucker, S.; Abiad, R.; Quinn, T.; Berg, P.; Hashii, M.; Sterling, R.; Jackson, R.; Pratt, R.; Campbell, R. D.; Malone, D.; Landis, D.; Barrington-Leigh, C. P.; Slassi-Sennou, S.; Cork, C.; Clark, D.; Amato, D.; Orwig, L.; Boyle, R.; Banks, I. S.; Shirey, K.; Tolbert, A. K.; Zarro, D.; Snow, F.; Thomsen, K.; Henneck, R.; Mchedlishvili, A.; Ming, P.; Fivian, M.; Jordan, John; Wanner, Richard; Crubb, Jerry; Preble, J.; Matranga, M.; Benz, A.; Hudson, H.; Canfield, R. C.; Holman, G. D.; Crannell, C.; Kosugi, T.; Emslie, A. G.; Vilmer, N.; Brown, J. C.; Johns-Krull, C.; Aschwanden, M.; Metcalf, T.; Conway, A. Bibcode: 2002SoPh..210....3L Altcode: RHESSI is the sixth in the NASA line of Small Explorer (SMEX) missions and the first managed in the Principal Investigator mode, where the PI is responsible for all aspects of the mission except the launch vehicle. RHESSI is designed to investigate particle acceleration and energy release in solar flares, through imaging and spectroscopy of hard X-ray/gamma-ray continua emitted by energetic electrons, and of gamma-ray lines produced by energetic ions. The single instrument consists of an imager, made up of nine bi-grid rotating modulation collimators (RMCs), in front of a spectrometer with nine cryogenically-cooled germanium detectors (GeDs), one behind each RMC. It provides the first high-resolution hard X-ray imaging spectroscopy, the first high-resolution gamma-ray line spectroscopy, and the first imaging above 100 keV including the first imaging of gamma-ray lines. The spatial resolution is as fine as ∼ 2.3 arc sec with a full-Sun (≳ 1°) field of view, and the spectral resolution is ∼ 1-10 keV FWHM over the energy range from soft X-rays (3 keV) to gamma-rays (17 MeV). An automated shutter system allows a wide dynamic range (>107) of flare intensities to be handled without instrument saturation. Data for every photon is stored in a solid-state memory and telemetered to the ground, thus allowing for versatile data analysis keyed to specific science objectives. The spin-stabilized (∼ 15 rpm) spacecraft is Sun-pointing to within ∼ 0.2° and operates autonomously. RHESSI was launched on 5 February 2002, into a nearly circular, 38° inclination, 600-km altitude orbit and began observations a week later. The mission is operated from Berkeley using a dedicated 11-m antenna for telemetry reception and command uplinks. All data and analysis software are made freely and immediately available to the scientific community. Title: Analytical Approximations to Hydrostatic Solutions and Scaling Laws of Coronal Loops Authors: Aschwanden, Markus J.; Schrijver, Carolus J. Bibcode: 2002ApJS..142..269A Altcode: We derive accurate analytical approximations to hydrostatic solutions of coronal loop atmospheres, applicable to uniform and nonuniform heating in a large parameter space. The hydrostatic solutions of the temperature T(s), density ne(s), and pressure profile p(s) as a function of the loop coordinate s are explicitly expressed in terms of three independent parameters: the loop half-length L, the heating scale length sH, and either the loop-top temperature Tmax or the base heating rate EH0. The analytical functions match the numerical solutions with a relative accuracy of <~10-2-10-3. The absolute accuracy of the scaling laws for loop base pressure p0(L, sH, Tmax) and base heating rate EH0(L, sH, Tmax), previously derived for uniform heating by Rosner et al., and for nonuniform heating by Serio et al., is improved to a level of a few percent. We generalize also our analytical approximations for tilted loop planes (equivalent to reduced surface gravity) and for loops with varying cross sections. There are many applications for such analytical approximations: (1) the improved scaling laws speed up the convergence of numeric hydrostatic codes as they start from better initial values, (2) the multitemperature structure of coronal loops can be modeled with multithread concepts, (3) line-of-sight integrated fluxes in the inhomogeneous corona can be modeled with proper correction of the hydrostatic weighting bias, (4) the coronal heating function can be determined by forward-fitting of soft X-ray and EUV fluxes, or (5) global differential emission measure distributions dEM/dT of solar and stellar coronae can be simulated for a variety of heating functions. Title: Observations and models of coronal loops: from Yohkoh to TRACE Authors: Aschwanden, Markus J. Bibcode: 2002ESASP.505..191A Altcode: 2002IAUCo.188..191A; 2002solm.conf..191A We review highlights of recent observations of coronal loops in EUV and soft X-rays from Yohkohand TRACE. We review (1) hydrostatic loops, (2) non-hydrostatic loops, (3) oscillating loops, (4) nanoflare loops, and (5) coronal heating, in the light of new observations, with a critical discussion of previous interpretations and theoretical models. Title: Particle Acceleration and Kinematics in Solar Flares Authors: Aschwanden, Markus J. Bibcode: 2002paks.book.....A Altcode: This book presents a synthesis of what we learned about particle acceleration and kinematics from recent solar flare observations with the Yohkoh, SoHO, TRACE, CGRO spacecraft and radio instruments over the last decade. It deals with the topology of magnetic reconnection regions, discusses the geometry, small-scale dynamics, and electromagnetic fields of acceleration region in solar flares, provides a systematic description of the relativistic kinematics of particle acceleration, propagation, time-of-flight measurements, particle trapping, precipitation, and the resulting emissions in gamma rays, hard X-rays, and radio wavelengths. It is the first monograph on these solar flare topics, written for the level of graduate students and researchers in the field of solar physics, astrophysics, and magnetospheric physics. Link: http://www.wkap.nl/prod/b/1-4020-0725-6 Title: Damping Time Scaling of Coronal Loop Oscillations Deduced from Transition Region and Coronal Explorer Observations Authors: Ofman, L.; Aschwanden, M. J. Bibcode: 2002ApJ...576L.153O Altcode: The damping mechanism of recently discovered coronal loop transverse oscillations provides clues to the mechanism of coronal heating. We determine the scaling of the damping time with the parameters of the loops observed in extreme ultraviolet by the Transition Region and Coronal Explorer. We find excellent agreement of the scaling power to the power predicted by phase mixing and poor agreement with the power predicted by the wave leakage or ideal decay of the cylindrical kink mode mechanisms. Phase mixing leads to rapid dissipation of the Alfvén waves due to the variation of the Alfvén speed across the wave front and formation of small scales. Our results suggest that the loop oscillations are dissipated by phase mixing with anomalously high viscosity. Title: Nanoflare Statistics from First Principles: Fractal Geometry and Temperature Synthesis Authors: Aschwanden, Markus J.; Parnell, Clare E. Bibcode: 2002ApJ...572.1048A Altcode: We derive universal scaling laws for the physical parameters of flarelike processes in a low-β plasma, quantified in terms of spatial length scales l, area A, volume V, electron density ne, electron temperature Te, total emission measure M, and thermal energy E. The relations are specified as functions of two independent input parameters, the power index a of the length distribution, N(l)~l-a, and the fractal Haussdorff dimension D between length scales l and flare areas, A(l)~lD. For values that are consistent with the data, i.e., a=2.5+/-0.2 and D=1.5+/-0.2, and assuming the RTV scaling law, we predict an energy distribution N(E)~E with a power-law coefficient of α=1.54+/-0.11. As an observational test, we perform statistics of nanoflares in a quiet-Sun region covering a comprehensive temperature range of Te~1-4 MK. We detected nanoflare events in extreme-ultraviolet (EUV) with the 171 and 195 Å filters from the Transition Region and Coronal Explorer (TRACE), as well as in soft X-rays with the AlMg filter from the Yohkoh soft X-ray telescope (SXT), in a cospatial field of view and cotemporal time interval. The obtained frequency distributions of thermal energies of nanoflares detected in each wave band separately were found to have power-law slopes of α~1.86+/-0.07 at 171 Å (Te~0.7-1.1 MK), α~1.81+/-0.10 at 195 Å (Te~1.0-1.5 MK), and α~1.57+/-0.15 in the AlMg filter (Te~1.8-4.0 MK), consistent with earlier studies in each wavelength. We synthesize the temperature-biased frequency distributions from each wavelength and find a corrected power-law slope of α~1.54+/-0.03, consistent with our theoretical prediction derived from first principles. This analysis, supported by numerical simulations, clearly demonstrates that previously determined distributions of nanoflares detected in EUV bands produced a too steep power-law distribution of energies with slopes of α~2.0-2.3 mainly because of this temperature bias. The temperature-synthesized distributions of thermal nanoflare energies are also found to be more consistent with distributions of nonthermal flare energies determined in hard X-rays (α~1.4-1.6) and with theoretical avalanche models (α~1.4-1.5). Title: Astrophysics in 2001 Authors: Trimble, Virginia; Aschwanden, Markus J. Bibcode: 2002PASP..114..475T Altcode: During the year, astronomers provided explanations for solar topics ranging from the multiple personality disorder of neutrinos to cannibalism of CMEs (coronal mass ejections) and extra-solar topics including quivering stars, out-of-phase gaseous media, black holes of all sizes (too large, too small, and too medium), and the existence of the universe. Some of these explanations are probably possibly true, though the authors are not betting large sums on any one. The data ought to remain true forever, though this requires a careful definition of ``data'' (think of the Martian canals). Title: The Electron Number Problem Revisited with RHESSI Flare Observations Authors: Aschwanden, M. J.; Alexander, D.; Metcalf, T.; Nitta, N. Bibcode: 2002AAS...200.7608A Altcode: 2002BAAS...34..776A The Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observed a number of solar flares since its launch on February 5, 2002. We analyze first images and spectra from this new mission, using a variety of image reconstruction methods, such as Clean, Pixon, Maximum Entropy (MEM), and Forward-Fitting. We obtain context images of the flare regions in EUV from SoHO/EIT and TRACE, and soft X-ray light curves from GOES. From RHESSI we reconstruct hard X-ray images with full uv-coverage in time intervals of the spin period ( 4 s). RHESSI provides a substantially higher spatial resolution ( 2") and spectral resolution than previous data from Yohkoh/HXT. A longstanding problem in solar flare physics is the so-called electron number problem, which challenges a reconciliation between (1) the rate of injected electrons inferred from the observed hard X-ray photon spectrum in the framework of the thick-target model and the (2) the replenishment rate of electrons in the coronal acceleration volume. Thanks to the high spectral resolution of RHESSI we can for the first time properly separate the thermal and the non-thermal electron spectral components, and this way determine the injection rate of nonthermal electrons more accurately. In addition, imaging in many energy bands allows us to trace propagation and energy loss between the coronal acceleration site and the chromospheric thick-target site with higher accuracy, taking into account the partial energy loss of trapped and precipitating electrons. The goal of this investigation is to quantify the geometry, density, and inflows in the coronal acceleration region, which provides crucial constraints for the underlying magnetic reconnection process. Title: Radio Frequency-Tomography of Solar Flares Authors: Aschwanden, M. J. Bibcode: 2002AAS...200.4905A Altcode: 2002BAAS...34..722A The Frequency-Agile Solar Radiotelescope (FASR) is designed to produce simultaneous images of solar phenomena at many frequencies. A data cube with a stack of multiple frequency images can be used for tomographic reconstruction of the 3D density and temperature distribution of flares, based on the free-free emission at cm and mm wavelengths. We simulate a set of multi-frequency images for the Bastille-Day flare of 2000-July-14, based on EUV observations from TRACE and soft X-ray observations from Yohkoh. The 3D model consists of some 200 postflare loops with observationally constrained densities and temperatures. The temporal evolution involves flare plasma heating, a phase of conductive cooling, followed by a phase of radiative cooling. The images simulated at different microwave frequencies reveal a sequence of optically-thick free-free emission layers, which can be "pealed off" like onion shells with increasing radio frequency. We envision a tomographic method that yields information on the density and temperature structure of flare systems and their evolution. Comparison with EUV and soft X-ray based 3D models will also allow to quantify wave scattering at radio frequencies and provide information on small-scale inhomogeneities and wave turbulence. Besides the thermal free-free emission, radio images contain also information on coherent emission processes, such as plasma emission from electron beams and loss-cone emission from gyroresonant trapped particles, conveying information on particle acceleration processes. Title: Transverse oscillations in coronal loops observed with TRACE I. An Overview of Events, Movies, and a Discussion of Common Properties and Required Conditions Authors: Schrijver, Carolus J.; Aschwanden, Markus J.; Title, Alan M. Bibcode: 2002SoPh..206...69S Altcode: We study transverse loop oscillations triggered by 17flares and filament destabilizations; only 2 such cases have been reported in the literature until now. Oscillation periods are estimated to range over a factor of ∼15, with most values between 2 and 7 min. The oscillations are excited by filament destabilizations or flares (in 6% of the 255 flares inspected, ranging from about C3 to X2). There is no clear dependence of oscillation amplitude on flare magnitude. Oscillations occur in loops that close within an active region, or in loops that connect an active region to a neighboring region or to a patch of strong flux in the quiet Sun. Some magnetic configurations are particularly prone to exhibit oscillations: two active regions showed two, and one region even three, distinct intervals with loop oscillations. The loop oscillations are not a resonance that builds up: oscillations in loops that are excited along their entire length are likely to be near the fundamental resonance mode because of that excitation profile, but asymmetrically excited oscillations clearly show propagating waves that are damped too quickly to build up a resonance, and some cases show multiple frequencies. We discuss evidence that all oscillating loops lie near magnetic separatrices that outline the large-scale topology of the field. All magnetic configurations are more complicated than a simple bipolar region, involving mixed-polarities in the interior or vicinity of the region; this may reflect that the exciting eruptions occur only in such environments, but this polarity mixing likely also introduces the large-scale separatrices that are involved. Often the oscillations occur in conjunction with gradual adjustments in loop positions in response to the triggering event. We discuss the observations in the context of two models: (a) transverse waves in coronal loops that act as wave guides and (b) strong sensitivity to changes in the field sources for field lines near separatrices. Properties that favor model b are (1) the involvement of loops at or near separatrices that outline the large-scale topology of the field, (2) the combined occurrence of oscillations and loop translations, (3) the small period spread and similar decay time scale in a set of oscillating loops in one well-observed event, and (4) the existence of loops oscillating in antiphase with footpoints close together in two cases. All other properties are compatible with either model, except the fact that almost all of the oscillations start away from the triggering event, suggestive of an outward-pushing exciting wave more in line with model a. The spread in periods from event to event suggests that the oscillations may reflect the properties of some driver mechanism that is related to the flare or mass ejection. Title: 3D coronal observations with the solar cycle Authors: Portier-Fozzani, Fabrice; Inhester, Bernd; Papadopoulo, Théodore; Bijaoui, Albert; Aschwanden, Markus Bibcode: 2002ESASP.477..159P Altcode: 2002scsw.conf..159P The number of observed aurorae and variable disturbances in the Earth magnetosphere is correlated with the 11 years solar cycle. These phenomena result from 3D solar magnetic field variations and could have many consequences such as loss of satellites control, uncertainties in plane positions, high level of irradiance received by astronauts with high particles, etc... Recent progress in 3D visualization and reconstruction (with stereoscopy and tomography techniques) made it possible to follow the evolution of coronal plasma structures frozen in by the magnetic field. Loops geometries - including their twists measurements -, arcade loops structures or filament shapes, are determined by 3D techniques with SOHO, using the solar rotation. A 3D analysis of the flare formation on April 4th, 1997 shows an emerging flux near a sigmoid loop with magnetic fields reconnection. Other observations emphasize the role of the helicity (observed for example as a twist in many structures) in filament eruptions and CME formations. Detwisting processes from EIT images to coronographic data show ejections of material into the interplanetary space. Title: Transverse Oscillations in Coronal Loops Observed with TRACE II. Measurements of Geometric and Physical Parameters Authors: Aschwanden, Markus J.; De Pontieu, Bart; Schrijver, Carolus J.; Title, Alan M. Bibcode: 2002SoPh..206...99A Altcode: We measure geometric and physical parameters oftransverse oscillations in 26 coronal loops, out of the 17 events described in Paper I by Schrijver, Aschwanden, and Title (2002). These events, lasting from 7 to 90 min, have been recorded with the Transition Region and Coronal Explorer (TRACE) in the 171 and 195 Å wavelength bands with a characteristic angular resolution of 1", with time cadences of 15-75 seconds. We estimate the unprojected loop (half) length L and orientation of the loop plane, based on a best-fit of a circular geometry. Then we measure the amplitude A(t) of transverse oscillations at the loop position with the largest amplitude. We decompose the time series of the transverse loop motion into an oscillating component Aosc(t) and a slowly-varying trend Atrend(t). We find oscillation periods in the range of P=2-33 min, transverse amplitudes of A=100-8800 km, loop half lengths of L=37 000-291 000 km, and decay times of td=3.2-21 min. We estimate a lower limit of the loop densities to be in the range of nloop=0.13-1.7×109 cm−3. The oscillations show (1) strong deviations from periodic pulses, (2) spatially asymmetric oscillation amplitudes along the loops, and (3) nonlinear transverse motions of the centroid of the oscillation amplitude. From these properties we conclude that most of the oscillating loops do not fit the simple model of kink eigen-mode oscillations, but rather manifest flare-induced impulsively generated MHD waves, which propagate forth and back in the loops and decay quickly by wave leakage or damping. In contrast to earlier work we find that the observed damping times are compatible with estimates of wave leakage through the footpoints, for chromospheric density scale heights of ≈400-2400 km. We conclude that transverse oscillations are most likely excited in loops that (1) are located near magnetic nullpoints or separator lines, and (2) are hit by a sufficiently fast exciter. These two conditions may explain the relative rarity of detected loop oscillations. We show that coronal seismology based on measurements of oscillating loop properties is challenging due to the uncertainties in estimating various loop parameters. We find that a more accurate determination of loop densities and magnetic fields, as well as advanced numerical modeling of oscillating loops, are necessary conditions for true coronal seismology. Title: Effects of Temperature Bias on Nanoflare Statistics Authors: Aschwanden, Markus J.; Charbonneau, Paul Bibcode: 2002ApJ...566L..59A Altcode: Statistics of solar flares, microflares, and nanoflares have been gathered over an energy range of some 8 orders of magnitude, over E~1024-1032 ergs. Frequency distributions of flare energies are always determined in a limited temperature range, e.g., at T~1-2 MK if the 171 and 195 Å filters are used from an extreme ultraviolet telescope (the Solar and Heliospheric Observatory/EUV Imaging Telescope or the Transitional Region and Coronal Explorer). Because the electron temperature Te and the thermal energy E=3nekBTeV are statistically correlated in flare processes, statistics in a limited temperature range introduce a bias in the frequency distribution of flare energies, N(E)~E-aE. We demonstrate in this Letter that the power-law slope of nanoflare energies, e.g., aE~1.9, as determined in a temperature range of T~1.1-1.6 MK (195 Å), corresponds to a corrected value of a'E~1.4 in an unbiased, complete sample. This corrected value is in much better agreement with predictions from avalanche models of solar flares. However, it also implies that all previously published power-law slopes of EUV nanoflares, covering a range of aE~1.8-2.3, correspond to unbiased values of aE<2, which then poses a serious challenge to Parker's hypothesis of coronal heating by nanoflares. Title: Constraining the Properties of Nonradiative Heating of the Coronae of Cool Stars and the Sun Authors: Schrijver, Carolus J.; Aschwanden, Markus J. Bibcode: 2002ApJ...566.1147S Altcode: The dominant mechanism that heats the coronae of the Sun and of other cool stars remains to be identified, despite numerous solar and stellar studies. We address the problem from a statistical point of view, by approximating the emission expected from the ensemble of loops in stellar coronae. We develop a prototype of a global atmospheric, empirical model that employs (1) simulations of the surface magnetic field of the Sun and active stars throughout sunspot cycles, (2) potential field computations of the corresponding coronal field, and (3) an approximation of atmospheres for 2000 coronal loops for randomly selected field lines in each flux configuration, representative of all environments from very quiet to the interior of active regions. The latter requires specification of the flux density PH that passes through the base of the loops to heat the corona. We parameterize PH as a function of the base field strength Bbase (in G), loop half-length l (in Mm), and footpoint velocity v (in km s-1). We find a best fit for a heating flux density of PH~2×107(Bbase/100)1.0+/- 0.5(l/24)-0.7+/-0.3(v/0.4)0.0+/-0.5 ergs cm-2 s-1 (the allowed ranges of the exponents are shown). This parameterization matches the observed soft X-ray losses from the coronae of the Sun and more active stars with rotation periods down to 5 days, throughout their activity cycles, as well as the characteristic coronal temperatures, and the relationships between disk-averaged radiative and magnetic flux densities. We compare this parameterization to models previously published in the literature and find that dissipation of current layers and turbulence are the most likely candidate heating mechanisms, for which both low-frequency driving and high-frequency driving meet the criteria comparably well. We find, moreover, that the heating scale length of ~20 Mm inferred from solar observations matches the characteristic e-folding height of the field strength over solar active regions, which suggests that coronal heating depends on the local field strength. Our modeling suggests that there is no need for a strong selection mechanism to determine which loops are heated and which are not, but that the sensitive dependence of the heating on the base field strength causes the appearance of a corona that consists of bright loops embedded in less bright environments. We compare the differential emission measures for the simulated coronae to those of the Sun and more active cool stars, and we also discuss the apparently weak velocity dependence of the best-fit parameterization for PH. Title: Particle acceleration and kinematics in solar flares - A Synthesis of Recent Observations and Theoretical Concepts (Invited Review) Authors: Aschwanden, Markus J. Bibcode: 2002SSRv..101....1A Altcode: We review the physical processes of particle acceleration, injection, propagation, trapping, and energy loss in solar flare conditions. An understanding of these basic physical processes is inexorable to interpret the detailed timing and spectral evolution of the radiative signatures caused by nonthermal particles in hard X-rays, gamma-rays, and radio wavelengths. In contrast to other more theoretically oriented reviews on particle acceleration processes, we aim here to capitalize on the numerous observations from recent spacecraft missions, such as from the Compton Gamma Ray Observatory (CGRO), the Yohkoh Hard X-Ray Telescope (HXT) and Soft X-Ray Telescope (SXT), and the Transition Region and Coronal Explorer (TRACE). High-precision energy-dependent time delay measurements from CGRO and spatial imaging with Yohkoh and TRACE provide invaluable observational constraints on the topology of the acceleration region, the reconstruction of magnetic reconnection processes, the resulting electromagnetic fields, and the kinematics of energized (nonthermal) particles. Title: Reconciliation of the Coronal Heating Function between Yohkoh and TRACE Authors: Aschwanden, Markus J. Bibcode: 2002mwoc.conf...57A Altcode: Mixed results on the determination of the coronal heating function are quoted in literature. There seems to be a polarization between Yohkoh, SoHO/EIT, and TRACE results. Essentially, heating functions EH(s) derived from the temperature-broadband instrument SXT/Yohkoh yield best fits for uniform or looptop heating (Priest et al. 1998, 2000; Wheatland, Sturrock, & Acton 1997), while the same heating function derived from temperature-narrowband instruments like EIT and TRACE yield best fits for footpoint heating with scale heights of sH l ≅15 Mm. The problem seems to be rooted in oversimplified modeling of filter-ratio temperatures, using single-temperature models for every line-of-sight, although the broadband response of Yohkoh/SXT is sensitive to emission measure-weighted temperatures of the entire dEM(T)/dT distribution above T > 1.5 MK. We revisit previous analyses of Yohkoh data and demonstrate with forward-fitting methods of multi-temperature models or continuous dEM(T)/dT distributions that both the Yohkoh/SXT and TRACE data can be reconciled, both yielding coronal heating function with scale heights of sH lapprox 15 Mm, for active region loops as well as for quiet Sun region. This result has important consequences for the identification of the long-sought physical mechanisms responsible for coronal heating. Title: Transverse oscillations in coronal loops observed with TRACE Authors: Schrijver, C. J.; Aschwanden, M. J.; De Pontieu, B.; Title, A. M. Bibcode: 2001AGUFMSH11A0703S Altcode: TRACE discovered transverse oscillations in coronal loops associated with a flare three years ago, and until recently only two such events were known. We have now identified a total of 17 events that trigger some form of loop oscillations. Oscillation periods are estimated to range over a factor of ~ 15, with most values between 2 and 7 min. The oscillations are excited by filament destabilizations or flares (in 6%\ of the 255 flares inspected, ranging from about C3 to X2). Oscillations occur in loops that close within an active region, or in loops that connect an active region to a neighboring region or to a patch of strong flux in the quiet Sun. Some magnetic configurations are particularly prone to exhibit oscillations: two active regions showed two, and one region even three, distinct intervals with loop oscillations. The loop oscillations are not a resonance that builds up: oscillations in loops that are excited along their entire length are likely to be near the fundamental resonance mode because of that excitation profile, but asymmetrically excited oscillations clearly show propagating waves that are damped too quickly to build up a resonance, and some cases show multiple frequencies. We discuss evidence that all oscillating loops lie near magnetic separatrices that outline the large-scale topology of the field. Often the oscillations occur in conjunction with gradual adjustments in loop positions in response to the triggering event. We discuss the observations in the context of two models, and evaluate the contraints on coronal properties that can be deduced from them. >http://vestige.lmsal.com/TRACE/POD/TRACEoscillations.html</a> Title: Evolution of Magnetic Flux Rope in the Active Region NOAA 9077 on 14 July 2000 Authors: Yan, Yihua; Aschwanden, Markus J.; Wang, Shujuan; Deng, Yuanyong Bibcode: 2001SoPh..204...27Y Altcode: The finite energy force-free magnetic fields of the active region NOAA 9077 on 14 July 2000 above the photosphere were reconstructed. We study the evolution of the 3D magnetic field structures in AR 9077 and compare the reconstructed field lines with TRACE EUV 171 Å flare loops during the flare maximum, which confirms the process that flaring loops extended from lower sheared level to higher arcades. We also demonstrate the 3D magnetic field evolution before the 3B/X5.7 flare on 14 July and the magnetic structure after the flare on 15 July. This shows that the helical magnetic structures were significantly changed, suggesting that the flux rope was indeed erupted during the energetic flare at 10:24 UT on 14 July. Title: Flare Plasma Cooling from 30 MK down to 1 MK modeled from Yohkoh, GOES, and TRACE Observations during the Bastille-Day Event (14 July 2000) Authors: Aschwanden, M. J.; Alexander, D. Bibcode: 2001AGUFMSH32B..05A Altcode: We present an analysis of the evolution of the thermal flare plasma during the 2000-Jul-14, 10 UT, Bastille-Day flare event, using spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and TRACE. The spatial structure of this double-ribbon flare consists of a curved arcade with some 100 post-flare loops which brighten up in a sequential manner from highly-sheared low-lying to less-sheared higher-lying bipolar loops. We reconstruct an instrument-combined, average differential emission measure distribution dEM(T)/dT that ranges from T=1 MK to 40 MK and peaks at T0=10.9 MK. We find that the time profiles of the different instrument fluxes peak sequentially over 7 minutes with decreasing temperatures from T≈ 30 MK to 1 MK, indicating the systematic cooling of the flare plasma. From these temperature-dependent relative peak times tpeak(T) we reconstruct the average plasma cooling function T(t) for loops observed near the flare peak time, and find that their temperature decrease is initially controlled by conductive cooling during the first 188 s, T(t) ~ [1+(t/τcond)]-2/7, and then by radiative cooling during the next 592 s, T(t) ~ [1-(t/τrad)]3/5. From the radiative cooling phase we infer an average electron density of ne=4.2x 1011 cm-3, which implies a filling factor near 100% for the brightest observed 23 loops with diameters of ≈ 1.8 Mm that appear simultaneously over the flare peak time and are fully resolved with TRACE. We reproduce the time delays and fluxes of the observed time profiles near the flare peak self-consistently with a forward-fitting method of a fully analytical model. The total integrated thermal energy of this flare amounts to Ethermal=2.6 x 1031 erg. >http://www.lmsal.com/~aschwand/publications/publ.html</a> Title: Flare Plasma Cooling from 30 MK down to 1 MK modeled from Yohkoh, GOES, and TRACE observations during the Bastille Day Event (14 July 2000) Authors: Aschwanden, Markus J.; Alexander, David Bibcode: 2001SoPh..204...91A Altcode: We present an analysis of the evolution of the thermal flare plasma during the 14 July 2000, 10 UT, Bastille Day flare event, using spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and TRACE. The spatial structure of this double-ribbon flare consists of a curved arcade with some 100 post-flare loops which brighten up in a sequential manner from highly-sheared low-lying to less-sheared higher-lying bipolar loops. We reconstruct an instrument-combined, average differential emission measure distribution dEM(T)/dT that ranges from T=1 MK to 40 MK and peaks at T0=10.9 MK. We find that the time profiles of the different instrument fluxes peak sequentially over 7 minutes with decreasing temperatures from T≈30 MK to 1 MK, indicating the systematic cooling of the flare plasma. From these temperature-dependent relative peak times tpeak(T) we reconstruct the average plasma cooling function T(t) for loops observed near the flare peak time, and find that their temperature decrease is initially controlled by conductive cooling during the first 188 s, T(t)∼[1+(t/τcond)]−2/7, and then by radiative cooling during the next 592 s, T(t)∼[1−(t/τrad)]3/5. From the radiative cooling phase we infer an average electron density of ne=4.2×1011 cm−3, which implies a filling factor near 100% for the brightest observed 23 loops with diameters of ∼1.8 Mm that appear simultaneously over the flare peak time and are fully resolved with TRACE. We reproduce the time delays and fluxes of the observed time profiles near the flare peak self-consistently with a forward-fitting method of a fully analytical model. The total integrated thermal energy of this flare amounts to Ethermal=2.6×1031 erg. Title: Measurement of coronal magnetic twists during loop emergence of NOAA 8069 Authors: Portier-Fozzani, F.; Aschwanden, M.; Démoulin, P.; Neupert, W.; EIT Team; Delaboudinière, J. -P. Bibcode: 2001SoPh..203..289P Altcode: Emerging coronal loops were studied with extreme ultraviolet observations performed by SOHO/EIT on 5 and 6 August 1997 for NOAA 8069. Physical parameters (size and twist) were determined by a new stereoscopic method. The flux tubes were measured twisted when first observed by EIT. After emerging, they de-twisted as they expanded, which corresponds to a minimization of the energy. Different scenarios which take into account the conservation of the magnetic helicity are discussed in relation with structure and temperature variations. Title: Revisiting the Determination of the Coronal Heating Function from Yohkoh Data Authors: Aschwanden, Markus J. Bibcode: 2001ApJ...559L.171A Altcode: Results on the coronal heating function seem to strongly depend on the employed type of multitemperature modeling along the line of sight. Instruments with broadband temperature filters cause more temperature confusion than those with narrowband temperature filters. A possible bias of broadband filters is the hydrostatic weighting of multitemperature loop systems, which mimic a temperature increase with altitude and thus yield a preference for looptop heating. In this Letter we revisit a loop system previously analyzed by Priest et al., for which they found that the coronal heating is likely to be uniform in the temperature range between 1.6 and 2.2 MK. As an alternative scenario, we use standard hydrostatic solutions here (with vanishing conductive flux in the transition region). We show that hydrostatic solutions with a uniform heating function throughout the corona and transition region lead to unphysical solutions for the column depth and the altitude of the loop footpoints, while a footpoint-heating model yields acceptable physical solutions for a heating scale height of sH~13+/-1 Mm. The positive temperature gradient with height (which is also found in filter-ratio temperatures of other Yohkoh data) can be explained by the hydrostatic weighting bias resulting from hot loops (Tmax~2.6 MK) embedded in a cooler (Tmax~1.0 MK) background corona. Title: An Evaluation of Coronal Heating Models for Active Regions Based on Yohkoh, SOHO, and TRACE Observations Authors: Aschwanden, Markus J. Bibcode: 2001ApJ...560.1035A Altcode: Recent soft X-ray and EUV data from space observations with Yohkoh, the Solar and Heliospheric Observatory (SOHO), and the Transition Region and Coronal Explorer (TRACE) established three important observational constraints for coronal heating models: (1) coronal loops in active regions have an overdensity that can be supplied only by upflows of heated chromospheric plasma, (2) chromospheric upflows have been observed frequently in coronal loops, and (3) the coronal heating function has been localized in the lower corona within a height range of λH<~10 Mm above the photosphere. Although these three observational facts have been derived from active region loops, the part of the solar corona that is topologically connected to active regions makes up >~80% of the heating energy requirement (at a typical day around the maximum of the solar cycle) and thus constitutes the majority of the energy budget of the coronal heating problem at large. We discuss and compare a comprehensive set of theoretical models of coronal heating under the aspect of whether they can satisfy these observational constraints. We find that conventional direct current (DC) and alternating current (AC) coronal heating models that consider coronal loops as homogeneous flux tubes (in density and temperature) do not predict these observed effects, while refined models that include gravity and the transition region can reproduce them. In particular, magnetic reconnection models that spawn chromospheric evaporation satisfy the observational constraints the easiest. Our main conclusion is that the coronal heating problem can be solved only by tapping energization processes in the chromosphere and transition region. Title: Astrophysics in 2000 Authors: Trimble, Virginia; Aschwanden, Markus J. Bibcode: 2001PASP..113.1025T Altcode: It was a year in which some topics selected themselves as important through the sheer numbers of papers published. These include the connection(s) between galaxies with active central engines and galaxies with starbursts, the transition from asymptotic giant branch stars to white dwarfs, gamma-ray bursters, solar data from three major satellite missions, and the cosmological parameters, including dark matter and very large scale structure. Several sections are oriented around processes-accretion, collimation, mergers, and disruptions-shared by a number of kinds of stars and galaxies. And, of course, there are the usual frivolities of errors, omissions, exceptions, and inventories. Title: An Assessment of Coronal Heating Models based on Yohkoh, SoHO, and TRACE Observations Authors: Aschwanden, M. J. Bibcode: 2001AGUSM..SP61A08A Altcode: Recent soft X-ray and EUV data from space observations with Yohkoh, SoHO, and TRACE established three important observational constraints for coronal heating models: (1) Coronal loops have an overdensity that can only be supplied by upflows of heated chromospheric plasma, (2) chromospheric upflows have been observed in a large number of coronal loops, and (3) the coronal heating function has been localized in the lower corona within a height range of λH <~ 10-20 Mm above the photosphere. We discuss and compare a comprehensive set of theoretical models on coronal heating under the aspect whether they can satisfy these observational constraints. We find that conventional DC and AC coronal heating models that consider coronal loops as homogeneous flux tubes (in density and temperature), detached from the transition region, do not predict these observed effects, while refined models that include gravity and chromospheric models can reproduce the observed effects. Alternatively, most of the magnetic reconnection models previously used for flares, can reproduce the observed effects, because heating of the chromospheric plasma at the loop footpoints is involved with subsequent chromospheric evaporation like in flares. Therefore, coronal magnetic reconnection processes that work with lower heating efficiency than in flares, as well as chromospheric and photospheric reconnection processes should be considered as serious candidates for coronal heating models. Our main conclusion is that the coronal heating problem cannot be solved without including dynamic processes (and related heating effects) in the chromosphere and transition region zone. Title: Critical Issues of Nanoflare Statistics Authors: Aschwanden, M. J. Bibcode: 2001AGUSM..SP52B08A Altcode: The accuracy of solar microflare and nanoflare statistics became a critical issue because the extrapolation of their power-law distribution at the lower flux or energy threshold is often used as estimate of their total energy budget to meet the coronal heating requirement. We review and quantify a number of aspects that play a role in the determination of their frequency distribution: (1) event definition and discrimination, (2) sampling completeness, (3) observing cadence and exposure times, (4) pattern recognition algorithms, (5) density and energy model, (6) line-of-sight integration, (7) physical parameter limits, (8) wavelength dependence, (9) deviations from power-law functions, and (10) error estimates of power-law slopes. We discuss critical issues in the concept of event definitions, which includes flares, subflare bursts, radio bursts, coherent as well as intermittent temporal fine structure. Finally we review critical issues in physical concepts of nanoflares, which range from Parker's coronal tangential discontinuities down to miniature flare loops observed in EUV in the transition region. We conclude that power-law slopes from different data analyses cannot be properly compared without taking into account the involved measurement biases and theoretical model concepts. Title: Modeling of Coronal EUV Loops Observed with TRACE. I. Hydrostatic Solutions with Nonuniform Heating Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Alexander, David Bibcode: 2001ApJ...550.1036A Altcode: Recent observations of coronal loops in EUV wavelengths with the Transition Region and Coronal Explorer (TRACE) and the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) demonstrated three new results that cannot be explained by most of the existing loop models: (1) EUV loops are near-isothermal along their coronal segments, (2) they show an overpressure or overdensity compared with the requirements of steady state loops with uniform heating, and (3) the brightest EUV loops exhibit extended scale heights up to 4 times the hydrostatic scale height. These observations cannot be reconciled with the classical RTV (Rosner, Tucker, & Vaiana) model, they do not support models with uniform heating, and they even partially violate the requirements of hydrostatic equilibrium. In this study we are fitting for the first time steady state solutions of the hydrodynamic equations to observed intensity profiles, permitting a detailed consistency test of the observed temperature T(s) and density profiles ne(s) with steady state models, which was not possible in previous studies based on scaling laws. We calculate some 500 hydrostatic solutions, which cover a large parameter space of loop lengths (L~4-300 Mm), of nonuniform heating functions (with heating scale heights in the range of λH~1-300 Mm), approaching also the limit of uniform heating (λH>>L). The parameter space can be subdivided into three regimes, which contain (1) solutions of stably stratified loops, (2) solutions of unstably stratified loops (in the case of short heating scale heights, λH,Mm~LMm), and (3) a regime in which we find no numerical solutions (when λH,Mm<~LMm). Fitting the hydrostatic solutions to 41 EUV loops observed with TRACE (selected by the criterion of detectability over their entire length), we find that only 30% of the loops are consistent with hydrostatic steady state solutions. None of the observed EUV loops is consistent with a uniform heating function while in quasi-steady state. Those loops compatible with a steady state are found to be heated near the footpoints, with a heating scale height of λH=12+/-5 Mm, covering a fraction λH/L=0.2+/-0.1 of the loop length. These results support coronal heating mechanisms operating in or near the chromosphere and transition region. Title: Temperature Tomography of the Soft X-Ray Corona: Measurements of Electron Densities, Tempuratures, and Differential Emission Measure Distributions above the Limb Authors: Aschwanden, Markus J.; Acton, Loren W. Bibcode: 2001ApJ...550..475A Altcode: We analyze long-exposure and off-pointing Yohkoh/SXT data of the solar corona observed on 1992 August 26. We develop a new (temperature) tomography method that is based on a forward-fitting method of a four-parameter model to the observed soft X-ray fluxes F1(h) and F2(h) of two SXT wavelength filters as a function of height h. The model is defined in terms of a differential emission measure (DEM) distribution dEM(h, T)/dT, which includes also a temperature dependence of density scale heights λn(T)=qλλT and allows us to quantify deviations (qλ≠1) from hydrostatic equilibrium (qλ=1). This parametrization facilitates a proper line-of-sight integration and relates the widely used filter ratio temperature TFR to the peak of the DEM distribution. A direct consequence of the multi-scale height atmosphere is that the filter ratio temperature TFR(h) is predicted to increase with height, even if all magnetic field lines are isothermal. Our model fitting reveals that coronal holes and quiet-Sun regions are in perfect hydrostatic equilibrium but that coronal streamers have a scale height that exceeds the hydrostatic scale height by a factor of up to qλ<~2.3, which underscores the dynamic nature of coronal streamers. Our density measurements in coronal holes are slightly lower than most of the white-light polarized brightness inversions and seem to come closer to the requirements of solar wind models. Our DEM model provides also a physical framework for the semiempirical Baumbach-Allen formula and quantifies the temperature ranges and degree of hydrostaticity of the K, L, and F coronae. Title: The New Solar Corona Authors: Aschwanden, Markus J.; Poland, Arthur I.; Rabin, Douglas M. Bibcode: 2001ARA&A..39..175A Altcode: We focus on new observational capabilities (Yohkoh, SoHO, TRACE), observations, modeling approaches, and insights into physical processes of the solar corona. The most impressive new results and problems discussed in this article can be appreciated from the movies available on the Annual Reviews website and at http://www.lmsal.com/pub/araa/araa.html. "The Sun is new each day." Heraclites (ca 530-475 BC) "Everything flows." Heraclites (ca 530-475 BC) Title: Solar Flares: Nonthermal Electrons Authors: Aschwanden, M. Bibcode: 2000eaa..bookE2291A Altcode: Solar flares energize particles to different levels, usually subdivided into (1) thermal, (2) nonthermal or energetic and (3) high energetic (figure 1). The lowest energetic level is a thermal particle distribution (where particles have a Maxwellian velocity distribution), produced by Coulomb collisions between the particles of the heated flare plasma. Because the coronal plasma is heated to ... Title: Evidence for Nonuniform Heating of Coronal Loops Inferred from Multithread Modeling of TRACE Data Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Alexander, David Bibcode: 2000ApJ...541.1059A Altcode: The temperature Te(s) and density structure ne(s) of active region loops in EUV observed with TRACE is modeled with a multithread model, synthesized from the summed emission of many loop threads that have a distribution of maximum temperatures and that satisfy the steady state Rosner-Tucker-Vaiana (RTV) scaling law, modified by Serio et al. for gravitational stratification (called RTVSp in the following). In a recent Letter, Reale & Peres demonstrated that this method can explain the almost isothermal appearance of TRACE loops (observed by Lenz et al.) as derived from the filter-ratio method. From model-fitting of the 171 and 195 Å fluxes of 41 loops, which have loop half-lengths in the range of L=4-320 Mm, we find that (1) the EUV loops consist of near-isothermal loop threads with substantially smaller temperature gradients than are predicted by the RTVSp model; (2) the loop base pressure, p0~0.3+/-0.1 dynes cm-2, is independent of the loop length L, and it agrees with the RTVSp model for the shortest loops but exceeds the RTVSp model up to a factor of 35 for the largest loops; and (3) the pressure scale height is consistent with hydrostatic equilibrium for the shortest loops but exceeds the temperature scale height up to a factor of ~3 for the largest loops. The data indicate that cool EUV loops in the temperature range of Te~0.8-1.6 MK cannot be explained with the static steady state RTVSp model in terms of uniform heating but are fully consistent with Serio's model in the case of nonuniform heating (RTVSph), with heating scale heights in the range of sH=17+/-6 Mm. This heating function provides almost uniform heating for small loops (L<~20 Mm), but restricts heating to the footpoints of large loops (L~50-300 Mm). Title: Time Variability of the ``Quiet'' Sun Observed with TRACE. II. Physical Parameters, Temperature Evolution, and Energetics of Extreme-Ultraviolet Nanoflares Authors: Aschwanden, Markus J.; Tarbell, Ted D.; Nightingale, Richard W.; Schrijver, Carolus J.; Title, Alan; Kankelborg, Charles C.; Martens, Piet; Warren, Harry P. Bibcode: 2000ApJ...535.1047A Altcode: We present a detailed analysis of the geometric and physical parameters of 281 EUV nanoflares, simultaneously detected with the TRACE telescope in the 171 and 195 Å wavelengths. The detection and discrimination of these flarelike events is detailed in the first paper in this series. We determine the loop length l, loop width w, emission measure EM, the evolution of the electron density ne(t) and temperature Te(t), the flare decay time τdecay, and calculate the radiative loss time τloss, the conductive loss time τcond, and the thermal energy Eth. The findings are as follows: (1) EUV nanoflares in the energy range of 1024-1026 ergs represent miniature versions of larger flares observed in soft X-rays (SXR) and hard X-rays (HXR), scaled to lower temperatures (Te<~2 MK), lower densities (ne<~109 cm-3), and somewhat smaller spatial scales (l~2-20 Mm). (2) The cooling time τdecay is compatible with the radiative cooling time τrad, but the conductive cooling timescale τcond is about an order of magnitude shorter, suggesting repetitive heating cycles in time intervals of a few minutes. (3) The frequency distribution of thermal energies of EUV nanoflares, N(E)~10-46(E/1024)-1.8 (s-1 cm-2 ergs-1) matches that of SXR microflares in the energy range of 1026-1029, and exceeds that of nonthermal energies of larger flares observed in HXR by a factor of 3-10 (in the energy range of 1029-1032 ergs). Discrepancies of the power-law slope with other studies, which report higher values in the range of a=2.0-2.6 (Krucker & Benz; Parnell & Jupp), are attributed to methodical differences in the detection and discrimination of EUV microflares, as well as to different model assumptions in the calculation of the electron density. Besides the insufficient power of nanoflares to heat the corona, we find also other physical limits for nanoflares at energies <~1024 ergs, such as the area coverage limit, the heating temperature limit, the lower coronal density limit, and the chromospheric loop height limit. Based on these quantitative physical limitations, it appears that coronal heating requires other energy carriers that are not luminous in EUV, SXR, and HXR. Title: Time Variability of the ``Quiet'' Sun Observed with TRACE. I. Instrumental Effects, Event Detection, and Discrimination of Extreme-Ultraviolet Microflares Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Tarbell, Ted D.; Wolfson, C. J. Bibcode: 2000ApJ...535.1027A Altcode: The Transition Region and Coronal Explorer (TRACE) observed a ``quiet-Sun'' region on 1999 February 17 from 02:15 UT to 3:00 UT with full resolution (0.5" pixel size), high cadence (125 s), and deep exposures (65 and 46 s) in the 171 Å and 195 Å wavelengths. We start our investigation of the time variability of ``quiet-Sun'' images with a detailed analysis of instrumental and nonsolar effects, such as orbital temperature variations, filtering of particle radiation spikes, spacecraft pointing drift, and solar rotation tracking. We quantify the magnitude of various noise components (photon Poisson statistics, data digitization, data compression, and readout noise) and establish an upper limit for the data noise level, above which temporal variability can safely be attributed to solar origin. We develop a pattern recognition code that extracts spatiotemporal events with significant variability, yielding a total of 3131 events in 171 Å and 904 events in 195 Å. We classify all 904 events detected in 195 Å according to flarelike characteristics and establish a numerical flare criterion based on temporal, spatial, and dynamic cross-correlation coefficients between the two observed temperatures (0.9 and 1.4 MK). This numerical criterion matches the visual flare classification in 83% of the cases and can be used for automated flare search. Using this flare discrimination criterion we find that only 35% (and 25%) of the events detected in 171 (and 195) Å represent flarelike events. The discrimination of flare events leads to a frequency distribution of peak fluxes, N(ΔF)~ΔF-1.83+/-0.07 at 195 Å, which is significantly flatter than the distribution of all events. A sensitive discrimination criterion of flare events is therefore important for microflare statistics and for conclusions on their occurrence rate and efficiency for coronal heating. Title: Multi-Thread Modeling of Coronal Loops with TRACE Data Authors: Nightingale, R. W.; Aschwanden, M. J.; Alexander, D.; Reale, F.; Peres, G. Bibcode: 2000SPD....31.0211N Altcode: 2000BAAS...32..812N The temperature Te(s) and density structure ne(s) of active region loops in EUV observed with TRACE is modeled with a multi-thread model. The model loops are synthesized from the summed emission of many loop threads that have a distribution of maximum temperatures and that satisfy the steady-state Rosner-Tucker-Vaiana (RTV) scaling law, modified by Serio et al. for gravitational stratification (RTVSp). From model-fitting of the 171 and 195 Angstroms fluxes of 41 loops, which have loop half lengths in the range of L=4-320 Mm, we find: (1) The EUV loops can be explained by near-isothermal loop threads in the temperature range of Te ~ 0.8-1.6 MK with substantially smaller temperature gradients than predicted by the RTVSp model, (2) the loop base pressure, p0 ~ 0.3+/- 0.1 dyne cm-2, is independent of the loop length L, it agrees with the RTVSp model for the shortest loops, but exceeds the RTVSp model up to a factor of 35 for the largest loops, and (3) the pressure scale height is consistent with hydrostatic equilibrium for the shortest loops, but exceeds the temperature scale height up to a factor of ~ 3 for the largest loops. This work was supported by the TRACE project at LMSAL (contract NAS5-38099). Ref.: Aschwanden,M.J., Nightingale,R.W., Alexander,D., Reale,F., and Peres,G. 2000, ApJ, subm., ``Evidence for Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling of TRACE Data'', URL="ftp://sag.lmsal.com/pub/aschwand/2000_reale.ps.gz" Title: What TRACE Observations tell us about Heating of Coronal Loops Authors: Aschwanden, M. J.; Nightingale, R. W.; Alexander, D.; Reale, F.; Peres, G. Bibcode: 2000SPD....31.0210A Altcode: 2000BAAS...32..812A We analyzed the temperature Te(s) and density structure ne(s) of active region loops in EUV observed with TRACE (see SPD abstract by Nightingale et al.). The observational data indicate that cool EUV loops with maximum temperatures of Tmax ~ 0.8-1.6 MK cannot be explained with the static steady-state scaling law of Rosner, Tucker, & Vaiana (1978) or Serio et al. (1981), in terms of uniform heating. However, they are fully consistent with Serio's model (which includes gravitation and a heating scale height) in the case of nonuniform heating, with heating scale heights in the range of sH=17 +/- 6 Mm. This heating function provides almost uniform heating for small loops (L < 20 Mm), but restricts heating to the footpoints of large loops (L ~ 50-300 Mm). Another observational result of cool EUV loops is that the pressure scale height exceeds the hydrostatic scale height by a factor of ql ~ 1-3. This suggests that the pressure balance of these EUV loops may not be governed by hydrostatic equilibrium, but rather indicates a non-steady state, e.g. caused by dynamic mass flows and/or intermittent heating. Chromospheric upflows may explain the extended scale heights as well as the quasi-isothermal temperature structure of EUV loops. We review and discuss the differences between cool (T ~ 1-2 MK) EUV loops and hot (T ~ 2-8 MK) soft X-ray loops concerning loop scaling laws, radiative equilibrium, hydrostatic equilibrium, and heating function. - This work is supported by the TRACE project at LMSAL (contract NAS5-38099) Ref.: Aschwanden,M.J., Nightingale,R.W., Alexander,D., Reale,F., and Peres,G. 2000, ApJ, subm., ``Evidence for Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling of TRACE Data'' Title: Summary of Posters on Solar Physics Authors: Aschwanden, M. J. Bibcode: 2000IAUS..195..447A Altcode: No abstract at ADS Title: The Effect of Hydrostatic Weighting on the Vertical Temperature Structure of the Solar Corona Authors: Aschwanden, Markus J.; Nitta, Nariaki Bibcode: 2000ApJ...535L..59A Altcode: 2000astro.ph..4093A We investigate the effect of hydrostatic scale heights λ(T) in coronal loops on the determination of the vertical temperature structure T(h) of the solar corona. Every method that determines an average temperature at a particular line of sight from optically thin emission (e.g., in EUV or soft X-ray wavelengths) of a mutlitemperature plasma is subject to the emission measure-weighted contributions dEM(T)/dT from different temperatures. Because most of the coronal structures (along open or closed field lines) are close to hydrostatic equilibrium, the hydrostatic temperature scale height introduces a height-dependent weighting function that causes a systematic bias in the determination of the temperature structure T(h) as function of altitude h. The net effect is that the averaged temperature seems to increase with altitude, dT(h)/dh>0, even if every coronal loop (of a multitemperature ensemble) is isothermal in itself. We simulate this effect with differential emission measure distributions observed by SERTS for an instrument with a broadband temperature filter such as Yohkoh/Soft X-Ray Telescope and find that the apparent temperature increase due to hydrostatic weighting is of order ΔT~T0h/rsolar. We suggest that this effect largely explains the systematic temperature increase in the upper corona reported in recent studies (e.g., by Sturrock et al., Wheatland et al., or Priest et al.), rather than being an intrinsic signature of a coronal heating mechanism. Title: Electron Trapping and Precipitation in Asymmetric Solar Flare Loops Authors: Aschwanden, M. J.; Fletcher, L.; Sakao, T.; Kosugi, T.; Hudson, H. Bibcode: 2000IAUS..195..375A Altcode: Acceleration, propagation, and energy loss of particles energized in solar flares cannot be studied separately because their radiative signatures observed in the form of hard X-ray bremsstrahlung or radio gyrosynchrotron emission represent a convolution of all these processes. We analyze hard X-ray emission from solar flares using a kinematic model that includes free-streaming electrons (having an energy-dependent time-of-flight delay) as well as temporarily trapped electrons (which are pitch-angle scattered by Coulomb collisional scattering) to determine various physical parameters (trapping times, flux asymmetry, loss-cone angles, magnetic mirror ratios) in flare loops with asymmetric magnetic fields. Title: Electron Kinematics near the Loss-Cone Authors: Fletcher, L.; Aschwanden, M. J. Bibcode: 2000SPD....31.0247F Altcode: 2000BAAS...32..819F With the upcoming launch of the HESSI satellite, we expect that problems of non-thermal electron transport and radiation signatures will once more be the subject of some attention, since this is an integral part of the calculation of the spectral and spatial behavior of the radiative signatures which will be observed by HESSI. Problems of particle transport in coronal magnetic traps are often treated by making simple geometrical and timescale arguments for the fractions of accelerated particles which are trapped and precipitate from coronal loops. Such arguments are used to calculate the populations of, for example, directly precipitating and trap-precipitating particles (which can in principle be identified from hard X-ray time-series), or coronal versus footpoint emission ratios (which can be studied from spatially resolved HXR data). Using numerical simulation and analytic arguments we have studied the dynamics of particles within coronal traps, paying particular attention to the behavior in the vicinity of the loss-cone. We find that over a broad range of normally-assumed coronal parameters, such as mirror-ratio, loop length and loop density, (a) electrons cannot pass easily from the trap region to the loss-cone, so that (b) there is no collisionless trap-precipitating component and (c) a large fraction of accelerated particles will lose their entire energy budget within the coronal loop. We discuss what this means for our current understanding of the solar flare environment and our interpretation of radiative signatures. This work was supported by the Yohkoh/SXT project at LMSAL (NASA grant NAS8-40801) and by the U.K. Particle Physics and Astronomy Research Council. Title: Astrophysics in 1999 Authors: Trimble, Virginia; Aschwanden, Markus J. Bibcode: 2000PASP..112..434T Altcode: The year 1999 saw the arrival of a star with three planets, a universe with three parameters, and a solar corona that could be heated at least three ways. In addition, there were at least three papers on every question that has ever been asked in astrophysics, from ``Will the Universe expand forever?'' to ``Does mantle convection occur in one or two layers?'' The answers generally were, ``Yes,'' ``No,'' and ``None of the above,'' to each of the questions. The authors have done their best to organize the richness around centers defined by objects, methods, and madnesses. 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: Hard X-Ray Timing Experiments with HESSI Authors: Aschwanden, M. J. Bibcode: 2000ASPC..206..197A Altcode: 2000hesp.conf..197A No abstract at ADS Title: Quadrupolar Magnetic Reconnection in Solar Flares. I. Three-dimensional Geometry Inferred from Yohkoh Observations Authors: Aschwanden, Markus J.; Kosugi, Takeo; Hanaoka, Yoichiro; Nishio, Masanori; Melrose, Donald B. Bibcode: 1999ApJ...526.1026A Altcode: We analyze the three-dimensional geometry of solar flares that show so-called interacting flare loops in soft X-ray, hard X-ray, and radio emission, as previously identified by Hanaoka and Nishio. The two flare loops that appear brightest after the flare are assumed to represent the outcome of a quadrupolar magnetic reconnection process, during which the connectivity of magnetic polarities is exchanged between the four loop footpoints. We parameterize the three-dimensional geometry of the four involved magnetic field lines with circular segments, additionally constrained by the geometric condition that the two pre-reconnection field lines have to intersect each other at the onset of the reconnection process, leading to a 10 parameter model. We fit this 10 parameter model to Yohkoh Soft and Hard X-Ray Telescopes (SXT and HXT) data of 10 solar flares and determine in this way the loop sizes and relative orientation of interacting field lines before and after reconnection. We apply a flare model by Melrose to calculate the magnetic flux transfer and energy released when two current-carrying field lines reconnect to form a new current-carrying system in a quadrupolar geometry. The findings and conclusions are the following. (1) The pre-reconnection field lines always show a strong asymmetry in size, consistent with the scenario of newly emerging small-scale loops that reconnect with preexisting large-scale loops. (2) The relative angle between reconnecting field lines is nearly collinear in half of the cases, and nearly perpendicular in the other half, contrary to the antiparallel configuration that is considered to be most efficient for magnetic reconnection. (3) The angle between interacting field lines is reduced by ~10°-50° after quadrupolar reconnection. (4) The small-scale flare loop experiences a shrinkage by a factor of 1.31+/-0.44, which is consistent with the scaling law found from previous electron time-of-flight measurements, suggesting that electron acceleration occurs near the cusp of quadrupolar configurations. (5) The large-scale loop is found to dominate the total induction between current-carrying loops, providing a simple estimate of the maximum magnetic energy available for flare energy release because of current transfer, which scales as ΔEI~1029.63(r2/109 cm)(I2/1011A)2 (with r2 the curvature radius and I2 the current of the large-scale loop) and is found to correlate with observed flare energies deduced from soft X-ray and hard X-ray fluxes. Most of the energy is transferred to small-scale loops that have one-half of the large-scale current (I1=I2/2). (6) The quadrupolar reconnection geometry provides also a solution of Canfield's dilemma of the offset between the maximum of vertical currents and the HXR flare loop footpoints. (7) The quadrupolar geometry provides not only a framework for interacting double-loop flares, but it can also be considered as a generalized version of (cusp-shaped) single-loop flares. Title: Time Variability of EUV Brightenings in Coronal Loops Observed with TRACE Authors: Nightingale, Richard W.; Aschwanden, Markus J.; Hurlburt, Neal E. Bibcode: 1999SoPh..190..249N Altcode: We analyze coronal loops in active region 8272, observed with TRACE on 23 July 1998 during a 70-min interval with a cadence of 1.5 min, in the temperature range of T≈0.9-1.6 MK. We focus on a compact dipolar loop system with a linear size of ≈30 000 km. In this compact loop system we detect about 20 EUV brightenings at any instant of time and in each wavelength. We measure the centroid position of these EUV brightenings and determine their cospatiality in subsequent time frames. We find that EUV brightenings are not cospatial in subsequent time intervals (Δt=90 s), but are almost randomly distributed in space and time. Title: Do EUV Nanoflares Account for Coronal Heating? Authors: Aschwanden, Markus J. Bibcode: 1999SoPh..190..233A Altcode: Recent observations with EUV imaging instruments such as SOHO/EIT and TRACE have shown evidence for flare-like processes at the bottom end of the energy scale, in the range of Eth≈1024-1027 erg. Here we compare these EUV nanoflares with soft X-ray microflares and hard X-ray flares across the entire energy range. From the observations we establish empirical scaling laws for the flare loop length, L(T)∼T, the electron density, ne(T)∼T2, from which we derive scaling laws for the loop pressure, p(T)∼T3, and the thermal energy, Eth∼T6. Extrapolating these scaling laws into the picoflare regime we find that the pressure conditions in the chromosphere constrain a height level for flare loop footpoints, which scales with heq(T)∼T−0.5. Based on this chromospheric pressure limit we predict a lower cutoff of flare loop sizes at L∖min≲5 Mm and flare energies E∖min≲1024 erg. We show evidence for such a rollover in the flare energy size distribution from recent TRACE EUV data. Based on this energy cutoff imposed by the chromospheric boundary condition we find that the energy content of the heated plasma observed in EUV, SXR, and HXR flares is insufficient (by 2-3 orders of magnitude) to account for coronal heating. Title: Radio and Hard X-ray Observations of Flares and their Physical Interpretation Authors: Aschwanden, M. Bibcode: 1999spro.proc..307A Altcode: We review a selection of observations in radio, hard X-rays (HXR) and soft X-rays (SXR) that constrain geometrical and physical requirements for solar flare models. Guided by observations of interacting flare loops we discuss a flare model based on shear-driven quadrupolar reconnection, which explains single-loop and double-loop flares in a unified picture. We interpret various observational findings in the light of this unified flare model: - topology and geometry of interacting flare loops, - localization of particle acceleration region, - scale invariance of electron time-of-flight path and flare loop geometry, - density and magnetic field diagnostic in acceleration region, - bi-directionality of injected electron beams, - electron beam trajectories and correlated HXR pulses, - bifurcation of directly-precipitating and trap-precipitating electrons, - density and magnetic field diagnostic of trap region, - elementary time scales and dynamics in acceleration region. Title: Particle Acceleration and Kinematics in Solar Flares and the Solar Corona Authors: Aschwanden, M. J. Bibcode: 1999ESASP.448.1015A Altcode: 1999ESPM....9.1015A; 1999mfsp.conf.1015A No abstract at ADS Title: Coronal Loop Oscillations Observed with the Transition Region and Coronal Explorer Authors: Aschwanden, Markus J.; Fletcher, Lyndsay; Schrijver, Carolus J.; Alexander, David Bibcode: 1999ApJ...520..880A Altcode: We report here, for the first time, on spatial oscillations of coronal loops, which were detected in extreme-ultraviolet wavelengths (171 Å) with the Transition Region and Coronal Explorer, in the temperature range of Te~1.0-1.5 MK. The observed loop oscillations occurred during a flare that began at 1998 July 14, 12:55 UT and are most prominent during the first 20 minutes. The oscillating loops connect the penumbra of the leading sunspot to the flare site in the trailing portion. We identified five oscillating loops with an average length of L=130,000+/-30,000 km. The transverse amplitude of the oscillations is A=4100+/-1300 km, and the mean period is T=280+/-30 s. The oscillation mode appears to be a standing wave mode (with fixed nodes at the footpoints). We investigate different MHD wave modes and find that the fast kink mode with a period τ=205(L/1010 cm)(ne/109 cm-3)1/2(B/10 G)-1 s provides the best agreement with the observed period. We propose that the onset of loop oscillations in distant locations is triggered by a signal or disturbance that propagates from the central flare site with a radial speed of ~700 km s-1. Because the observed loop oscillation periods are comparable to photospheric 5 minute oscillations, a resonant coupling between the two systems is possible. We further find evidence for global extreme-UV dimming in the entire active region possibly associated with a coronal mass ejection. Title: Deconvolution of Directly Precipitating and Trap-precipitating Electrons in Solar Flare Hard X-Rays. III.Yohkoh Hard X-Ray Telescope Data Analysis Authors: Aschwanden, Markus J.; Fletcher, Lyndsay; Sakao, Taro; Kosugi, Takeo; Hudson, Hugh Bibcode: 1999ApJ...517..977A Altcode: We analyze the footpoint separation d and flux asymmetry A of magnetically conjugate double footpoint sources in hard X-ray images from the Yohkoh Hard X-Ray Telescope (HXT). The data set of 54 solar flares includes all events simultaneously observed with the Compton Gamma Ray Observatory (CGRO) in high time resolution mode. From the CGRO data we deconvolved the direct-precipitation and trap-precipitation components previously (in Paper II). Using the combined measurements from CGRO and HXT, we develop an asymmetric trap model that allows us to quantify the relative fractions of four different electron components, i.e., the ratios of direct-precipitating (qP1, qP2) and trap-precipitating electrons (qT1, qT2) at both magnetically conjugate footpoints. We find mean ratios of qP1=0.14+/-0.06, qP2=0.26+/-0.10, and qT=qT1+qT2=0.60+/-0.13. We assume an isotropic pitch-angle distribution at the acceleration site and double-sided trap precipitation (qT2/qT1=qP2/qP1) to determine the conjugate loss-cone angles (α1=42deg+/-11deg and α2=52deg+/-10deg) and magnetic mirror ratiosat both footpoints (R1=1.6,...,4.0 and R2=1.3,...,2.5). From the relative displacement of footpoint sources we also measure altitude differences of hard X-ray emission at different energies, which are found to decrease systematically with higher energies, with a statistical height difference of hLo-hM1=980+/-250 km and hM1-hM2=310+/-300 km between the three lower HXT energy channels (Lo, M1, M2). Title: Quadrupolar Magnetic Reconnection in Solar Flares Authors: Aschwanden, M. J.; Kosugi, T.; Hanaoka, Y.; Nishio, M.; Melrose, D. B. Bibcode: 1999AAS...194.3107A Altcode: 1999BAAS...31..869A We analyze the 3-dimensional (3D) geometry of solar flares that show so-called interacting flare loops in soft X-ray, hard X-ray, and radio emission, as previously identified by Hanaoka and Nishio. The two flare loops that appear brightest after the flare are assumed to represent the outcome of a quadrupolar magnetic reconnection process, during which the connectivity of magnetic polarities is exchanged between the four loop footpoints. We fit a 10-parameter 3D-model to Yohkoh SXT and HXT data of 10 solar flares and determine this way the pre-reconnection and post-flare geometry of interacting flare loops. We apply a flare model of Melrose to calculate the magnetic flux transfer and energy released when two current-carrying field lines reconnect to form a new current-carrying system in a quadrupolar geometry. Some findings are: (1) The pre-reconnection field lines always show a strong asymmetry in size, consistent with the scenario of new-emerging small-scale loops that reconnect with pre-existing large-scale loops. (2) The relative angle between reconnecting field lines is near-collinear in half of the cases, and near-perpendicular in the other half, contrary to the anti-parallel configuration suggested in the model of Heyvaerts et al. (3) The shear angle between interacting field lines reduces by 10-50 deg after quadrupolar reconnection. (4) The small-scale flare loop experiences a shrinkage by a factor of 1.31+0.44, which is consistent with the scaling law found from previous electron time-of-flight measurements, suggesting that electron acceleration occurs near the cusp of quadrupolar configurations. (5) The large-scale loop is found to dominate the total induction between current-carrying loops, providing a simple estimate of the maximum magnetic energy available for flare energy release due to current transfer, which scales as E=10(29.63) [r2/10(9) cm] [I2/10(11) A](2) , (with r2 the curvature radius and I2 the current of the large-scale loop) and is found to correlate with observed flare energies deduced from soft X-ray and hard X-ray fluxes. Most of the energy is transferred to small-scale loops that have half of the large-scale current I1=I2/2. (6) The quadrupolar reconnection geometry provides also a solution of ``Canfield's dilemma" of the offset between the maximum of vertical currents and the HXR flare loop footpoints. Title: YOHKOH Observations at the Y2K Solar Maximum Authors: Aschwanden, M. J. Bibcode: 1999AAS...194.8003A Altcode: 1999BAAS...31..965A Yohkoh will provide simultaneous co-aligned soft X-ray and hard X-ray observations of solar flares at the coming solar maximum. The Yohkoh Soft X-ray Telescope (SXT) covers the approximate temperature range of 2-20 MK with a pixel size of 2.46\arcsec, and thus complements ideally the EUV imagers sensitive in the 1-2 MK plasma, such as SoHO/EIT and TRACE. The Yohkoh Hard X-ray Telescope (HXT) offers hard X-ray imaging at 20-100 keV at a time resolution of down to 0.5 sec for major events. In this paper we review the major SXT and HXT results from Yohkoh solar flare observations, and anticipate some of the key questions that can be addressed through joint observations with other ground and space-based observatories. This encompasses the dynamics of flare triggers (e.g. emerging flux, photospheric shear, interaction of flare loops in quadrupolar geometries, large-scale magnetic reconfigurations, eruption of twisted sigmoid structures, coronal mass ejections), the physics of particle dynamics during flares (acceleration processes, particle propagation, trapping, and precipitation), and flare plasma heating processes (chromospheric evaporation, coronal energy loss by nonthermal particles). In particular we will emphasize on how Yohkoh data analysis is progressing from a qualitative to a more quantitative science, employing 3-dimensional modeling and numerical simulations. Title: Time Variability of Coronal Loops observed by TRACE Authors: Nightingale, R. W.; Aschwanden, M. J.; Hurlburt, N. E. Bibcode: 1999AAS...194.7802N Altcode: 1999BAAS...31..961N We attempt the 3-dimensional reconstruction of a set of coronal loops during the period of July 18 - 23, 1998, which has been observed by TRACE with a cadence of a few minutes in the temperature range of 1-1.5 MK. Using the method of ``Dynamic Stereoscopy'' and a filter-ratio technique we obtain density n_e(s,t) and temperature T_e(s,t) profiles along the loop length s with respect to time t. Based on these measurements we calculate the radiative E_R(s,t) and conductive E_C(s,t) losses, and attempt to constrain the heating function E_H(s,t) as a function of loop coordinate and time. We test whether the required heating function corresponds to a steady-state or is governed by episodic heating. In particular we investigate which time intervals are subject to continuous (or recurrent) heating and which are dominated by radiative cooling. From this study we shed some light on theoretical loop models (e.g., Rosner-Tucker-Vaiana steady-state model) and the resulting scaling laws. 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: Astrophysics in 1998 Authors: Trimble, Virginia; Aschwanden, Markus Bibcode: 1999PASP..111..385T Altcode: From Alpha (Orionis and the parameter in mixing-length theory) to Omega (Centauri and the density of the universe), the Greeks had a letter for it. In between, we look at the Sun and planets, some very distant galaxies and nearby stars, neutrinos, gamma rays, and some of the anomalies that arise in a very large universe being studied by roughly one astronomer per 10^7 Galactic stars. Title: Tetsuya Watanabe, Takeo Kosugi and Alphonse C. Sterling (eds.), Observational Plasma Astrophysics: Five Years of Yohkoh and Beyond, Astrophysics and Space Science Library 229 Authors: Aschwanden, Markus Bibcode: 1999SSRv...87..539A Altcode: No abstract at ADS Title: Nonthermal Flare Emissions Authors: Aschwanden, M. J. Bibcode: 1999mfs..conf..273A Altcode: Statistical Aspects of Hard X-Ray Flares Variability during the Solar Activity Cycle Periodicities during the Solar Activity Cycle Frequency Distributions and Correlations The Concept of Self-Organized Criticality Particle Acceleration and Injection Fast Time Structures in Hard X-Rays The Concept of a "Statistical Flare" Electron Beam Signatures in Hard X-Rays and Radio Pulsed Injection of Particles Second-Step Acceleration Particle Trapping and Precipitation Incoherent Radiation from Trapped Particles Temporal Aspects Spectral Analysis Imaging Data Coherent Emission from Trapped Particles Beam-Driven Emission in the Trap Loss-Cone-Driven Emission Pulsation Mechanisms in the Trap Precipitation Signatures Flare Diagnostics from Hard X-Ray/Radio Observations Title: The Electron Injection Function and Energy-dependent Delays in Thick-Target Hard X-Rays Authors: Brown, John C.; Conway, Andrew J.; Aschwanden, Markus J. Bibcode: 1998ApJ...509..911B Altcode: We analytically and numerically study the relationship between an energy-dependent electron injection spectrum, F0(E0, t), and the resulting bremsstrahlung photon spectrum, J(ɛ, t), with the goal of exploring whether injection functions could explain energy-dependent time delays observed in solar flare hard X-rays (HXRs) without any time-of-flight effects. We calculate the inversion of the bremsstrahlung photon spectrum (for the Kramers cross section) and find that the timing of the electron injection function depends on the time derivative of the second spectral derivative of the photon spectrum. To match the observed delays, a systematic softening of the electron injection spectrum is required over the duration (~1 s) of individual HXR pulses. This requirement is exactly the same as that which occurs in the time-of-flight model, except there the softening is due to spatial dispersion of injected electrons of different energy E0. We show that such a softening injection rate is not consistent with acceleration models where the electron acceleration times are comparable with the HXR pulse lengths, but it can be consistent with models where the acceleration times are very short since the injection spectrum variations are then governed by spectral variations in the acceleration rate. We conclude that acceleration mechanisms cannot be ruled out on the basis of HXR light curves alone as an alternative to time-of-flight effects. Observations of HXR images and of the relationship of HXRs to soft X-ray loops strongly suggest, however, that time-of-flight effects must be important and must be included in attempts to infer primary accelerator properties from HXR light curves. We also conclude that the agreement of the time-of-flight model with observed energy-dependent HXR delays, and the properties of any acceleration model contributing to this trend, puts strong constraints on the timescales involved in the accelerator. Title: High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission for the next (2000) solar maximum Authors: Lin, Robert P.; Hurford, Gordon J.; Madden, Norman W.; Dennis, Brian R.; Crannell, Carol J.; Holman, Gordon D.; Ramaty, Reuven; von Rosenvinge, Tycho T.; Zehnder, Alex; van Beek, H. Frank; Bornmann, Patricia L.; Canfield, Richard C.; Emslie, A. Gordon; Hudson, Hugh S.; Benz, Arnold; Brown, John C.; Enome, Shinzo; Kosugi, Takeo; Vilmer, Nicole; Smith, David M.; McTiernan, Jim; Hawkins, Isabel; Slassi-Sennou, Said; Csillaghy, Andre; Fisher, George; Johns-Krull, Chris; Schwartz, Richard; Orwig, Larry E.; Zarro, Dominic; Schmahl, Ed; Aschwanden, Markus; Harvey, Peter; Curtis, Dave; Pankow, Dave; Clark, Dave; Boyle, Robert F.; Henneck, Reinhold; Michedlishvili, Akilo; Thomsen, K.; Preble, Jeff; Snow, Frank Bibcode: 1998SPIE.3442....2L Altcode: The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum. Title: Wavelet Analysis of Solar Flare Hard X-Rays Authors: Aschwanden, Markus J.; Kliem, Bernhard; Schwarz, Udo; Kurths, Jürgen; Dennis, Brian R.; Schwartz, Richard A. Bibcode: 1998ApJ...505..941A Altcode: We apply a multiresolution analysis to hard X-ray (HXR) time profiles f(t) of solar flares. This method is based on a wavelet transform (with triangle-shaped wavelets), which yields a dynamic decomposition of the power at different timescales T, the scalogram P(T, t). For stationary processes, time-averaged power coefficients, the scalegram S(T), can be calculated. We develop an algorithm to transform these (multiresolution) scalegrams S(T) into a standard distribution function of physical timescales, N(T). We analyze 647 solar flares observed with the Compton Gamma Ray Observatory (CGRO), recorded at energies >=25 keV with a time resolution of 64 ms over 4 minutes in each flare. The main findings of our wavelet analysis are:

1. In strong flares, the shortest detected timescales are found in the range Tmin ~ 0.1-0.7 s. These minimum timescales are found to correlate with the flare loop size r (measured from Yohkoh images in 46 flares), according to the relation Tmin(r) ~ 0.5(r/109 cm) s. Moreover, these minimum timescales are subject to a cutoff, Tmin(ne) >~ TDefl(ne), which corresponds to the electron collisional deflection time at the loss-cone site of the flare loops (inferred from energy-dependent time delays in CGRO data).

2. In smoothly varying flares, the shortest detected timescales are found in the range Tmin ~ 0.5-5 s. Because these smoothly varying flares exhibit also large trap delays, the lack of detected fine structure is likely to be caused by the convolution with trapping times.

3. In weak flares, the shortest detected timescales cover a large range, Tmin ~ 0.5-50 s, mostly affected by Poisson noise.

4. The scalegrams S(T) show a power-law behavior with slopes of βmax ~ 1.5-3.2 (for strong flares) over the timescale range of [Tmin, Tpeak]. Dominant peaks in the timescale distribution N(T) are found in the range Tpeak ~ 0.5-102 s, often coinciding with the upper cutoff of N(T). These observational results indicate that the fastest significant HXR time structures detected with wavelets (in strong flares) are related to physical parameters of propagation and collision processes. If the minimum timescale Tmin is associated with an Alfvénic crossing time through elementary acceleration cells, we obtain sizes of racc ~ 75-750 km, which have a scale-invariant ratio racc/r ~ 0.03 to flare loops and are consistent with cell sizes inferred from the frequency bandwidth of decimetric millisecond spikes. Title: Deconvolution of Directly Precipitating and Trap-Precipitating Electrons in Solar Flare Hard X-Rays. I. Method and Tests Authors: Aschwanden, Markus J. Bibcode: 1998ApJ...502..455A Altcode: We develop and test a numerical code that provides a self-consistent deconvolution of energy-dependent hard X-ray (HXR) time profiles I(ɛ, t) into two HXR-producing electron components, i.e., directly precipitating and trap-precipitating electrons. These two HXR components can be physically distinguished because their energy-dependent time delays have an opposite sign. The deconvolution is based on the following model assumptions: (1) nonthermal electrons are injected from the acceleration site into coronal flare loops by an injection function f(E, α, t) that consists of synchronized pulses in energy E and pitch angle α, (2) electrons with initially small pitch angles (α <= α0) precipitate directly to the HXR emission site, (3) electrons with initially large pitch angles (α >= α0) are temporarily trapped and precipitate after the collisional deflection time, and (4) nonthermal electrons lose their energy by Coulomb collisions and emit thick-target HXR bremsstrahlung in a high-density (fully collisional) site (near or inside the chromosphere). The numerical deconvolution provides a self-consistent determination of three physical parameters: (1) the electron time-of-flight distance lTOF between the acceleration/injection site and the HXR emission site, (2) the electron density ne in the trap region, and (3) the fraction of HXR-emitting electrons that precipitate directly, qprec, which relates to the loss cone angle by qprec0) = (1 - cos α0) for isotropic pitch angle distributions. This yields the magnetic mirror ratio R = Bloss/Binj = 1/sin20) between the injection and loss cone site. With this method, we measure for the first time magnetic field ratios in coronal loops by means of HXR data. Based on this ratio, together with the knowledge of the photospheric field at the footpoint, a direct measurement of the magnetic field in the coronal acceleration region can be obtained. We simulate energy-dependent HXR data I(ɛ, t) with typical solar flare parameters (lTOF = 15,000 km, ne = 1011 cm-3, qprec = 0.5) and test the accuracy of the inversion code. We perform the inversion in 30 different simulations over the entire physically plausible parameter space and demonstrate that a satisfactory inversion of all three physical parameters lTOF, ne, and qprec is achieved in a density range of ne = 1010-1012 cm-3 for precipitation ratios of qprec = 0.1-0.9 and for signal-to-noise ratios of >~100 (requiring HXR count rates of >~104 counts s-1). Applications of this inversion method to solar flare observations in hard X-rays (CGRO/BATSE, Yohkoh/Hard X-Ray Telescope) and microwaves (Nobeyama) will be presented in subsequent papers. Title: Deconvolution of Directly Precipitating and Trap-Precipitating Electrons in Solar Flare Hard X-Rays. II. Compton Gamma Ray Observatory Data Analysis Authors: Aschwanden, Markus J.; Schwartz, Richard A.; Dennis, Brian R. Bibcode: 1998ApJ...502..468A Altcode: Based on the deconvolution method developed in the first paper of this series, we present here the data analysis of 20-200 keV hard X-ray (HXR) data from the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) recorded during 103 solar flares in 1991-1995. These are all of the flares simultaneously observed by CGRO with high time resolution (64 ms) and by Yohkoh in flare mode. The deconvolution method takes the measured HXR count rates as function of energy and time, I(ɛ, t), and computes the following self-consistently: the electron injection function n(E, t), the directly precipitating electron flux nprec(E, t), the trapped-precipitating flux ntrap(E, t), the fraction of directly precipitating electrons (qprec), the electron time-of-flight distance (lTOF), and the electron density at the loss cone site of the trap (ne). We find that the electron time-of-flight distances (lTOF = 20.0 +/- 7.3 Mm) inferred with the deconvolution method are fully consistent with those obtained earlier using a Fourier filter method. The trap electron densities (ne = 1010.96+/-0.57 cm-3) obtained from deconvolving the e-folding decay times of HXR pulses (according to the trap model of Melrose & Brown) are found to be statistically a factor of 1.5 lower than those inferred from cross-correlation delays. The fraction qprec of directly precipitating electrons, measured for the first time here, is found to have a mean (and standard deviation) of qprec = 0.42 +/- 0.16. Based on this precipitation fraction, we infer loss cone angles of α0 ~ 20°-70° and magnetic mirror ratios of R = Bloss/Binj ~ 1.2 - 3 (with a median value of Rmedian = 1.6) between the loss cone site and injection/acceleration site, assuming an isotropic pitch angle distribution at the injection site. The TOF distances and mirror ratios constrain magnetic scale heights in flare loops to λB = 10-150 Mm. The fact that this two-component model (with free-streaming and trapped electrons) satisfactorily fits the energy-dependent time delays in virtually all flares provides strong evidence that electron time-of-flight differences and collisional scattering times dominate the observed HXR timing, while the injection of electrons appears to be synchronized (independent of energy) and does not reveal the timing of the acceleration process. Title: New features in type IV solar radio emission: combined effects of plasma wave resonances and MHD waves Authors: Chernov, G. P.; Markeev, A. K.; Poquerusse, M.; Bougeret, J. L.; Klein, K. -L.; Mann, G.; Aurass, H.; Aschwanden, M. J. Bibcode: 1998A&A...334..314C Altcode: An intense and complex type IV solar radio burst over the time period 1992 0217 08-12 was recorded simultaneously by 3 spectrographs, ARTEMIS (100-500 MHz), OSRA (200-400 MHz) and IZMIRAN (180-270 MHz), and by the Nan\c cay radioheliograph. For ~ 2 hours, the event exhibited strong pulsations on various time scales and ``zebra patterns'' with new features: sudden frequency shifts of the whole pattern, splitting of individual zebra stripes into two stripes, structuration of the upper-frequency split stripes into emission dots in phase with ~ 0.2 s pulsations. Another new and spectacular feature was a ~ 10 MHz bandwidth emission at the high frequency cut-off of the whole event, oscillating between 350 and 450 MHz in phase with ~ 3 min pulsations, and itself structured by the ~ 0.2 s pulsations. Another property observed for the first time was that the circular polarization of zebra patterns changed sign during the event, possibly due to magnetic field reversal at some point of a long-lasting magnetic reconnection process in the upper corona. According to a classical picture, electrons are accelerated in the current sheet and trapped in the magnetic arch. Pulsations are due to MHD waves affecting the whole arch and electron beams as well. We confront two existing theories to these new observational features. The l+w=> t model (Chernov 1976, 1989), based on Langmuir wave - whistler coupling at normal and anomalous Doppler resonance, can account for all the observed fine structures of zebra patterns, and gives a plausible magnetic field of 11x 10(-4) T in the source. The Winglee & Dulk (1986) model, based on electron-cyclotron maser emission of upper-hybrid waves at double plasma resonance, seems the most adequate to account for the evolving emission line, with its source in the dense current sheet. Title: G. M. Simnett, C. E. Alissandrakis, and L. Vlahos (eds.), Solar and Heliospheric Plasma Physics, Proceedings of the 8th European Meeting on Solar Physics Authors: Aschwanden, Markus Bibcode: 1998SoPh..179..441A Altcode: No abstract at ADS Title: Impulsively generated MHD waves and their detectability in solar coronal loops Authors: Murawski, K.; Aschwanden, M. J.; Smith, J. M. Bibcode: 1998SoPh..179..313M Altcode: Impulsively generated magnetohydrodynamic waves in solar coronal loops, with arbitrary plasma β, are studied numerically by a flux-corrected transport algorithm. Numerical results show that the total reflection which occurs in the region of low Alfvén speed leads to trapped fast kink magnetosonic waves. These waves propagate along the slab and exhibit periodic, quasi-periodic, and decay phases. As a consequence of the difference in wave propagation speeds, the time signatures of the slow magnetosonic waves are delayed in time in comparison to the time signatures of the fast magnetosonic and Alfvén waves. An interaction between the waves can generate a longer lasting and complex quasi-periodic phase of the fast wave. We discuss also the observational detectability of such MHD waves in optical, radio, and soft X-ray wavelenghts. Title: Book Review: Solar and heliospheric plasma physics / Springer, 1997 Authors: Aschwanden, Markus Bibcode: 1998SoPh..179..443A Altcode: No abstract at ADS Title: Logistic Avalanche Processes, Elementary Time Structures, and Frequency Distributions in Solar Flares Authors: Aschwanden, Markus J.; Dennis, Brian R.; Benz, Arnold O. Bibcode: 1998ApJ...497..972A Altcode: We analyze the elementary time structures (on timescales of ~0.1-3.0 s) and their frequency distributions in solar flares using hard X-ray (HXR) data from the Compton Gamma Ray Observatory (CGRO) and radio data from the radio spectrometers of Eidgenoessische Technische Hochschule (ETH) Zurich. The four analyzed data sets are gathered from over 600 different solar flares and include about (1) 104 HXR pulses at >=25 and >=50 keV, (2) 4000 radio type III bursts, (3) 4000 pulses of decimetric quasi-periodic broadband pulsation events, and (4) 104 elements of decimetric millisecond spike events.

The time profiles of resolved elementary time structures have a near-Gaussian shape and can be modeled with the logistic equation, which provides a quantitative measurement of the exponential growth time τG and the nonlinear saturation energy level WS of the underlying instability. Assuming a random distribution (Poisson statistics) of saturation times tS (with an e-folding constant tSe), the resulting frequency distribution of saturation energies WS or peak energy dissipation rates FS = (dW/dt)t=tS has the form of a power-law function, i.e., N(FS)~FS, where the power-law index α is directly related to the number of e-folding amplifications by the relation α = (1 + τG/tSe). The same mathematical formalism is used to generate power-law distributions, as in Rosner & Vaiana, but the distribution of energies released in elementary flare instabilities is not related to the global energy storage process. We assume Poissonian noise for the unamplified energy levels in unstable flare cells, implying an exponential frequency distribution of avalanche energies WS or fluxes FS in the absence of coherent amplifications. Also, in the case of coherent amplification, the Poissonian noise introduces exponential rollovers of the power law at the low and high ends of the frequency distributions.

We fit both power-law and exponential functions to the observed frequency distributions of elementary pulse fluxes N(F) in each flare separately. For HXR pulses, one-half of the flares are better fitted with power-law frequency distributions, demanding coherent amplification of the underlying energy dissipation mechanism, e.g., current exponentiation occurring in the magnetic tearing instability. The majority of type III burst flares are best fitted with power-law distributions, consistent with the interpretation in terms of beam-driven coherent emission. The frequency distributions of decimetric pulsations and decimetric millisecond spikes are found to fit exponential functions, in contrast to the expected power laws for coherent emission mechanisms generally proposed for these radio burst types. A coherent emission mechanism can be reconciled with the observed exponential frequency distributions only if nonlinear saturation occurs at a fixed amplification factor for all elementary pulses or spikes, for example, if it is produced by an oscillatory compact source (in the case of decimetric pulsations) or by a fragmented source with similar spatial cell sizes (in the case of decimetric millisecond spikes). Title: Coordinated Observations with SOHO, YOHKOH and VLA Authors: Aschwanden, Markus J.; Bastian, Tim S.; Nitta, Nariaki; Newmark, Jeff; Thompson, Barbara J.; Harrison, Richard A. Bibcode: 1998ASPC..155..311A Altcode: 1998sasp.conf..311A No abstract at ADS Title: 3-Dimensional Models of Active Region Loops Authors: Aschwanden, M. J.; Neupert, W. M.; Newmark, J.; Thompson, B. J.; Brosius, J. W.; Holman, G. D.; Harrison, R. A.; Bastian, T. S.; Nitta, N.; Hudson, H. S.; Zucker, A. Bibcode: 1998ASPC..155..145A Altcode: 1998sasp.conf..145A No abstract at ADS Title: 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: What did YOHKOH and Compton Change in Our Perception of Particle Acceleration in Solar Flares? Authors: Aschwanden, M. J. Bibcode: 1998ASSL..229..285A Altcode: 1998opaf.conf..285A No abstract at ADS Title: Logistic Avalanche Processes, Elementary Time Structures, Frequency Distributions, and Wavelet Analysis of Solar Flares Authors: Aschwanden, M. J.; Dennis, B. R.; Schwartz, R. A.; Benz, A. O.; Kliem, B.; Schwarz, U.; Kurths, J. Bibcode: 1998cee..workE..10A Altcode: Do elementary time scales exist in solar flares that could provide us a clue on the spatial fragmentation of the primary energy release process? Or is there a continuous distribution, composed of a hierarchy of unresolved time structures? We present results of two recent studies on these fundamental questions. We applied a multi-resolution analysis (using triangle-shaped wavelet transforms) to 647 solar flares observed with the COMPTON Gamma Ray Observatory (CGRO) at hard X-ray energies of >= 25 keV with a time resolution of 64 ms. From the wavelet scalegrams we infer a distribution of time scales N(T) for each flare and find a cutoff for the shortest detected time scales Tmin that is independent of the Poisson noise for strong flares. These shortest time scales Tmin are found to correlate with the flare loop radius r (Fig.1), i.e. Tmin = 0.5 (r/10^9 cm)s (measured with Yohkoh) and the collisional deflection time, Tmin > ~tDefl(n_e), (determined from the electron density n_e of trapped electrons by measuring energy-dependent time delays detected with CGRO). From these observations we infer spatial sizes of racc = 75-750 km for elementary acceleration cells. In a second study we determined the frequency distributions of elementary time structures in over 600 flares, based on some 10^4 hard X-ray pulses at 25 and 50 keV, 4000 radio type III bursts, 4000 decimetric quasi-periodic broadband pulsation events, and 10^4 decimetric millisecond spike events. All elementary time structures have a quasi-Gaussian shape and can be modeled with the logistic equation, which describes the exponential growth phase and nonlinear saturation (caused by the limited amount of available free energy) of a general instability. We derive a theoretical description of frequency distributions in terms of this logistic avalanche model and find that the power-law slope of observed frequency distributions provides a powerful diagnostic on coherent versus incoherent instabilities. Title: Solar Active Region Movies seen by the SOHO Extreme-ultraviolet Telescope Authors: Newmark, J. S.; Thompson, B.; Gurman, J. B.; Delaboudiniere, J. P.; Aschwanden, Markus; Mason, Helen Bibcode: 1997AAS...191.7307N Altcode: 1997BAAS...29.1321N The Extreme-ultraviolet Imaging Telescope (EIT) on board the SOHO (SOlar and Heliospheric Observatory) satellite provides wide-field images of the corona and transition region on the solar disc and up to 1.4 solar radii above the limb. Its normal incidence multilayer-coated optics select spectral emission lines from FeIX (171 Ang), FeXII (195 Ang), FeXV (284 Ang), and HeII (304 Ang) with 2.6 arcsecond resolution which allow us to describe Solar activity over a wide temperature range. EIT is providing unique EUV observations of the structure and evolution of active regions. Here we show movies of active region 8059 from July 3-10, 1997. The high temporal variability of the AR loops is very evident. Initial temperature and density diagnostics are explored as well as a comparison with diagnostics from the SOHO-CDS instrument. Title: The Structure of the Solar Corona above Sunspots as Inferred from Radio, X-Ray, and Magnetic Field Observations Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J. Bibcode: 1997ApJ...489..403V Altcode: We present observations of a solar active region, NOAA/USAF no. 7123, during 1992 April 3-10. The database includes high-angular-resolution radio, soft X-ray, magnetograph, and Hα observations. The radio observations include VLA maps in the Stokes I and V parameters at 4.7 and 8.4 GHz. The soft X-ray observations were obtained by the Soft X-Ray Telescope on board the Yohkoh satellite, the magnetograms were obtained at Kitt Peak, Mt. Wilson, and Big Bear, and the Hα data were obtained at Big Bear.

The lead sunspot in the active region is studied here. In particular, the polarization properties and brightness temperature spectrum are used to constrain the thermal structure of the corona over the sunspot. It is found that the 4.7 GHz emission of the sunspot is polarized in the sense of the ordinary mode, in contradiction with simple gyroresonance models that predict that the spot should be polarized in the sense of the extraordinary mode. We model the spectral and temporal evolution of the polarization structure in two frequencies, 4.7 and 8.4 GHz, using gyroresonance models to fit one-dimensional brightness temperature profiles across the spot in each polarization and frequency. The constraints provided by the X-ray and magnetic field observations help us to derive a qualitatively self-consistent picture for the daily evolution of the spot. We attribute the excess of the o-mode emission to the magnetic field configuration and to the temperature inhomogeneities across the spot. Namely, we find that (1) the umbral and penumbral environments are distinct, with the X-rays and the o-mode radio emission coming from the hotter penumbral loops, while the observed x-mode emission originates from the cooler umbral loops; (2) there exist temperature inhomogeneities in both the radial and vertical direction over the spot; and (3) the umbral magnetic field remains more confined in the corona than that predicted by a dipole model. Instead, a field configuration based on the magnetohydrostatic equilibrium model of Low gives a better agreement with the observations. Title: Electron Trapping Times and Trap Densities in Solar Flare Loops Measured with COMPTON and YOHKOH Authors: Aschwanden, Markus J.; Bynum, Robert M.; Kosugi, Takeo; Hudson, Hugh S.; Schwartz, Richard A. Bibcode: 1997ApJ...487..936A Altcode: We measure energy-dependent time delays of ~20-200 keV hard X-ray (HXR) emission from 78 flares observed simultaneously with the Compton Gamma Ray Observatory and Yohkoh. Fast time structures (<~1 s) are filtered out, because their time delays have been identified in terms of electron time-of-flight (TOF) differences from directly precipitating electrons (Aschwanden et al.). For the smooth HXR flux, we find systematic time delays in the range of τS = t50 keV-t200 keV ~ -(1 ... 10) s, with a sign opposite to TOF delays, i.e., the high-energy HXRs lag the low-energy HXRs.

We interpret these time delays of the smooth HXR flux in terms of electron trapping, and we fitted a model of the collisional deflection time tDefl(E)~E3/2n-1e to the observed HXR delays in order to infer electron densities nTrape in the trap. Independently, we determine the electron density nSXRe in flare loops from soft X-ray (SXR) peak emission measures EM= n2edh, using loop width (w) measurements to estimate the column depth dh ~ w. Comparing the two independent density measurements in HXR and SXR, we find a mean ratio of qe=nTrape/nSXRe~1, with a relatively small scatter by a factor of ~2. Generally, it is likely that the SXR-bright flare loops have a higher density than the trapping regions (when qe < 1), but they also are subject to filling factors less than unity (when qe > 1). Our measurements provide comprehensive evidence that electron trapping in solar flares is governed in the weak-diffusion limit, i.e., that the trapping time corresponds to the collisional deflection time, while pitch-angle scattering by resonant waves seems not to be dominant in the 20-200 keV energy range. The measurements do not support a second-step acceleration scenario for energies <=200 keV. Title: Joint Time-Frequency Analysis of Solar Flare Hard X-Ray Observations Authors: Dennis, B. R.; Aschwanden, M. J. Bibcode: 1997SPD....28.0161D Altcode: 1997BAAS...29..890D Many flares have an impulsive phase in hard X-rays followed by a period of more gradually varying emission. This general trend in the timescale of the variations from <1 s early in a flare to as long as several minutes in the later phases can tell us about the energy release and particle acceleration processes if we know how to interpret it. We are investigating several methods of quantifying this change including the use of the short-time Fourier transform, the Gabor transform, and various types of wavelet analysis tools. Preliminary results will be presented from applying these techniques to HXRBS and BATSE hard X-ray time profiles of several flares. Title: Electron Trapping in Flares Explored with COMPTON and YOHKOH Authors: Aschwanden, M. Bibcode: 1997SPD....28.1802A Altcode: 1997BAAS...29..922A We measure energy-dependent time delays of ~20-200 keV hard X-ray (HXR) emission from 78 flares simultaneously observed with the Compton Gamma Ray Observatory (CGRO) and Yohkoh. Fast time structures (≲1 s) are filtered out, because their time delays have been identified in terms of electron time-of-flight (TOF) differences from directly precipitating electrons. For the smooth HXR flux we find systematic time delays in the range of τ_S=t_{50 keV}-t_{200 keV} ~ -(1...10) s, with a sign opposite to TOF delays, i.e. the high-energy HXRs lag the low-energy HXRs. We interpret these time delays of the smooth HXR flux in terms of electron trapping and fit a model of the collisional deflection time t(Defl) (E) ~ E(3/2) n_e(-1) to the observed HXR delays to infer electron densities n_e(Trap) in the trap. Independently we determine the electron density n_e(SXR) in flare loops from soft X-ray (SXR) peak emission measures EM=int n_e(2) dh, using loop width (w) measurements to estimate the column depth dh ~ w. Comparing the two independent density measurements in HXR and SXR we find a mean ratio of q_e=n_e(Trap}/n_e({SXR)) ~ 1, with a relatively small scatter by a factor of ~ 2. It is likely that the SXR-bright flare loops have generally a higher density than the trapping regions (when q_e<1), but are also subject to filling factors less than unity (when q_e>1). Our measurements provide comprehensive evidence that electron trapping in solar flares is governed in the weak diffusion limit, i.e. that the trapping time corresponds to the collisional deflection time, while pitch-angle scattering by resonant waves seems not to be dominant in the 20-200 keV energy range. The measurements do not support a second-step acceleration scenario for energies <= 200 keV. Title: Electron Densities in Solar Flare Loops, Chromospheric Evaporation Upflows, and Acceleration Sites Authors: Aschwanden, Markus J.; Benz, Arnold O. Bibcode: 1997ApJ...480..825A Altcode: We compare electron densities measured at three different locations in solar flares: (1) in soft X-ray (SXR) loops, determined from SXR emission measures and loop diameters from Yohkoh Soft X-Ray Telescope maps [nSXRe=(0.2-2.5)×1011 cm-3] (2) in chromospheric evaporation upflows, inferred from plasma frequency cutoffs of decimetric radio bursts detected with the 0.1-3 GHz spectrometer Phoenix of ETH Zürich [nupflowe=(0.3-11)×1010 cm-3] and (3) in acceleration sites, inferred from the plasma frequency at the separatrix between upward-accelerated (type III bursts) and downward-accelerated (reverse-drift bursts) electron beams [nacce=(0.6-10)×109 cm-3].

The comparison of these density measurements, obtained from 44 flare episodes (during 14 different flares), demonstrates the compatibility of flare plasma density diagnostics with SXR and radio methods. The density in the upflowing plasma is found to be somewhat lower than in the filled loops, having ratios in a range nupflowe/nSXRe=0.02-1.3, and a factor of 3.6 higher behind the upflow front. The acceleration sites are found to have a much lower density than the SXR-bright flare loops, i.e., nacce/nSXRe= 0.005-0.13, and thus must be physically displaced from the SXR-bright flare loops. The scaling law between electron time-of-flight distances l' and loop half-lengths s, i.e., l'/s = 1.4 +/- 0.3, recently established by Aschwanden et al. suggests that the centroid of the acceleration region is located above the SXR-bright flare loop, as envisioned in cusp geometries (e.g., in magnetic reconnection models). Title: Relative Timing of Microwave and HXR Bursts Authors: Bastian, T. S.; Aschwanden, M. J. Bibcode: 1997SPD....28.1803B Altcode: 1997BAAS...29..922B The close correlation between microwave and hard X-ray (HXR) emission during flares has often been cited as evidence that the same population of energetic electrons is responsible for both types of emission. The two emissions differ in detail, however. Imaging observations have demonstrated that the two are not necessarily cospatial and timing observations have demonstrated that the microwaves are often significantly delayed with respect to HXR emission, typically by several seconds, but occassionally by much longer times. Such delays are in seeming conflict with the thick target model for HXR emission in its simplest form, and with the idea that microwave and HXR emissions result from essentially the same population of electrons. One way to reconcile the delay between microwaves and HXRs in the thick target model is to suppose that that electron trapping is significant (e.g., Cornell et al., ApJ, 279, 875). For a magnetic trap containing a plasma of constant density, high energy electrons have a longer lifetime against collisions than low energy electrons (tau_ {def} ~ E(3/2) ). Hence the energetic electrons responsible for the microwaves remain in the trap longer and the microwave emission they emit peaks later than the HXR emission. Another possibility is that higher energy electrons are accelerated later than lower energy electrons (so-called ``second-step'' acceleration models; e.g., Bai and Dennis 1985, ApJ, 292, 699). To explore the question in detail we have assembled a sample of 16 flares observed simultaneously in microwaves by the Nobeyama radioheliograph and in HXRs by the BATSE instrument on board the CGRO in burst trigger mode. The former imaged the flares at 17 GHz with an angular resolution of ~ 10'' and a time resolution of either 50 msec or 1 sec. The latter obtained medium energy resolution spectra (16 channels) between 20-200 keV with a time resolution of 16 or 64 msec. We present preliminary results of our analysis. Title: Coronal and Interplanetary Particle Beams Authors: Aschwanden, Markus J.; Treumann, Rudolf A. Bibcode: 1997LNP...483..108A Altcode: 1997cprs.conf..108A In this report we attempt to synthesize the results of a series of working group discussions focused on the topics of particle beams and particle acceleration in the solar corona and in interplanetary space. We start our discussion of coronal beams with a standard flare scenario, established on recent X-ray (Yohkoh, CGRO) and radio observations, which constitutes a framework for the understanding of upward and downward accelerated electron beams and their secondary signatures, such as chromospheric evaporation. The second part is dedicated to interplanetary electron and ion beams, with emphasis on their relation to coronal beams, using recent spacecraft data from Ulysses and Wind. Interplanetary electron beams can often be traced back to coronal type III sources, while there is no such relation for interplanetary ion beams. In the third part, we briefly review acceleration mechanisms for coronal and interplanetary beams, separately for electrons and ions. Title: YOHKOH observation of the source regions of solar narrowband, millisecond spike events. Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J. Bibcode: 1997A&A...317..569K Altcode: The source regions of metric spike events are investigated on Yohkoh soft X-ray (SXR) maps. The spikes are identified by the spectrometer Phoenix between 300MHz and 360MHz and are associated with groups of type III bursts at lower frequencies reaching also the decametric range. The Very Large Array (VLA) provides simultaneously spatial information at 333MHz, 1446MHz and 4866MHz. Similar to the previous VLA observation of a metric spike event, the new data are consistent with a high altitude of the spike sources of about 5x10^10^cm above the photosphere. The additionally available SXR data for one of the presented events give the following new informations: (i) The spike sources occur near open field lines and near regions of a slightly enhanced SXR flux relative to the ambient plasma. (ii) Contrary to SXR observations of type III bursts without metric spike activity, no SXR jet is observed. (iii) At low altitude, a weak SXR enhancement occurs, peaking about 60 s after the spike event. The SXR source and the spike sources can be connected by potential field lines. The observations corroborate a model in which a metric spike event is attributed to an energy release region at high altitude, while upwards propagating electrons produce type III bursts and downward moving electrons are responsible for SXR emission of heated plasma. Title: The Scaling Law between Electron Time-of-Flight Distances and Loop Lengths in Solar Flares Authors: Aschwanden, Markus J.; Kosugi, Takeo; Hudson, Hugh S.; Wills, Meredith J.; Schwartz, Richard A. Bibcode: 1996ApJ...470.1198A Altcode: From the complete data set of solar flares simultaneously observed with the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) in the high-time resolution mode (64 ms) and the Hard X-ray Telescope (HXT) on board Yohkoh, we were able to determine the electron time-of-flight (TOF) distance l' and the flare loop geometry in 42 events. The electron TOF distances were determined from time delays (of ≍ 10-100 ms) of hard X-ray (HXR) pulses (measured in 16 channel spectra over ≍ 20-200 keV), produced by the velocity difference of the HXR-producing electrons. The flare loops were mostly identified from double footpoint sources in ≥30 keV HXT images, with radii in the range r = 3000-25,000 km. We find a scaling law between the electron TOF distance l' and the flare loop half-length 5 = r(π/2), having a mean ratio (and standard deviation) of l'/s = 1.4±0.3. In five flares, we observe coronal ≥ 30 keV HXR sources of the Masuda type in the cusp region above the flare loop and find that their heights are consistent with the electron TOF distance to the footpoints. These results provide strong evidence that particle acceleration in solar flares occurs in the cusp region above the flare loop and that the coronal HXR sources discovered by Masuda et al. are a signature of the acceleration site, probably controlled by a magnetic reconnection process. Title: Electron versus Proton Timing Delays in Solar Flares Authors: Aschwanden, Markus J. Bibcode: 1996ApJ...470L..69A Altcode: 1996astro.ph..7168A Both electrons and ions are accelerated in solar flares and carry nonthermal energy from the acceleration site to the chromospheric energy-loss site, but the relative amount of energy carried by electrons versus ions is subject of debate. In this Letter, we test whether the observed energy-dependent timing delays of 20--200 keV hard X-ray (HXR) emission can be explained in terms of propagating electrons versus protons. For a typical flare, we show that the timing delays of fast (<~1 s) HXR pulses is consistent with time-of-flight differences of directly precipitating electrons, while the timing delays of the smooth HXR flux is consistent with collisional deflection times of trapped electrons. We show that these HXR timing delays cannot be explained either by <=1 MeV protons (as proposed in a model by Simnett & Haines), because of their longer propagation and trapping times, or by ~40 MeV protons (which have the same velocity as ~20 keV electrons), because of their longer trapping times and the excessive fluxes required to generate the HXRs. Thus, the HXR timing results clearly rule out protons as the primary generators of >=20 keV HXR emission. Title: Electron Time-of-Flight Distances and Flare Loop Geometries Compared from CGRO and YOHKOH Observations Authors: Aschwanden, Markus J.; Wills, Meredith J.; Hudson, Hugh S.; Kosugi, Takeo; Schwartz, Richard A. Bibcode: 1996ApJ...468..398A Altcode: The distance between the coronal acceleration site and the chromospheric hard X-ray (HXR) emission site can be determined from velocity-dependent electron time-of-flight (TO F) differences in the framework of the thick-target model. We determine these electron TOF distances 1 with relative time delay measurements in the 30-300 keV energy range, using 16 channel data from BATSE/CGRO for the eight largest flares simultaneously observed with Yohkoh. We filter the HXR fine structure from the smoothly varying HXR flux with a Fourier filter in order to separate competing time delays. In the Yohkoh/HXT images we identify the corresponding flare loops that show ≥30 keV HXR footpoint emission and project the electron TOF distances into the loop plane, assuming a semicircular shape (with radius r). The flare loop radii vary in the range of r = 5600-17,000 km. In all eight flares we find that the projected electron TOF distance l' exceeds the loop half-length s = r(π/2), with a scale-invariant ratio of l'/s = 1.3±0.2. Projecting the electron TOF distances onto an open field line that extends to the cusp region above the flare loop, we find an average ratio of h/r = 1.7±0.4 for the height h of the acceleration site. This geometry is compatible with acceleration mechanisms operating in the cusp region, perhaps associated with magnetic reconnection processes above the flare loop. Alternatively, acceleration sites inside the flare loop cannot be ruled out (since l'/s < 2), but they do not provide a natural explanation for the observed length ratio l'/s. Large-scale electric DC field acceleration mechanisms are found to be less suitable to explain the observed HXR timing and pulse durations. Title: Electron Time-of-Flight Measurements during the Masuda Flare, 1992 January 13 Authors: Aschwanden, Markus J.; Hudson, Hugh; Kosugi, Takeo; Schwartz, Richard A. Bibcode: 1996ApJ...464..985A Altcode: The solar flare of 1992 January 13, 1729 UT, has become famous because Masuda's discovery of a hard X-ray loop-top source (Masuda 1994). Here we analyze energy-dependent time delays occurring in 30-120 keV hard X-ray (HXR) emission during this flare, observed by BATSE on board the Compton Gamma Ray Observatory with a time resolution of 64 ms. The purpose of this study is to reconstruct the kinematics of HXR-producing electrons from energy-dependent HXR delays, and to relate the inferred time-of-flight distance to the spatial geometry of the flare loop, as observed by SXT and HXT on board Yohkoh. The findings are the following:

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

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

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

This study provides the first quantitative localization of the electron acceleration site in a solar flare, and demonstrates that energy-dependent HXR delays offer a sensitive diagnostic for electron acceleration, propagation, and trapping in solar flares. Title: The Inversion of Electron Time-of-Flight Distances from Hard X-Ray Time Delay Measurements Authors: Aschwanden, Markus J.; Schwartz, Richard A. Bibcode: 1996ApJ...464..974A Altcode: The electron time-of-flight distance 1 between the acceleration site and the chromosphere can be measured during solar flares from energy-dependent hard X-ray (HXR) time delays τ(ɛ), based on the applicability of the thick-target model. The determination of the path length l represents an inversion problem because the time-dependent electron injection spectrum at the acceleration site, N(E, t, x = 0), is retarded by the propagation time tprop(E)=l/v(E) at the thick-target site, i.e., N(E, t, x = l) = N[E, t - tprop(E), x - 0], and has to be convolved with the bremsstrahlung cross section σ(ɛ, E) and the instrumental detector response function Ri(ɛ) to reproduce the observed HXR time profiles I(Ei, t) (in different detector channels i), from which the time delay differences τ(ɛi) - τ(ɛi) can be measured.

In this study, we solve this inversion problem by numerical forward integration of time-dependent electron injection spectra N(E, t) with Gaussian pulse shapes to obtain the convolved time-dependent HXR spectra I(ɛ, t), using specific detector response functions from the Burst and Transient Source Experiment/Compton Gamma Ray Observatory and the Hard X-Ray Burst Spectrometer/Solar Maximum Mission. We find that the timing of HXR pulses can be accurately represented with the (monoenergetic) photon energy ɛi that corresponds to the median of the channel count spectra Ci(ɛ) = I(ɛ)Ri(ɛ). We compute numerical conversion factors qE(ɛ, y, E0) that permit the conversion of the timing of photon energies ɛi(t) (for a power-law spectrum with slope γ and upper cutoff energy E0) into electron energies Ei(t) = qEɛi(t), from which kinematic parameters can be fitted to determine the electron time-of-flight path length l. We test the inversion procedure with numeric simulations and demonstrate that the inversion is accurate within σl/l ≤ 1% for noise-free data. This inversion procedure is applied to the Masuda flare (in this volume) to localize the electron acceleration region. Title: Hard X-ray timing Authors: Aschwanden, Markus J. Bibcode: 1996AIPC..374..300A Altcode: 1996hesp.conf..300A High-time resolution (64 ms) hard X-ray (HXR) data from BATSE/CGRO allow us to study the energy-dependent timing of acceleration, propagation, energy loss, and trapping of ≳20 keV HXR-emitting electrons during solar flares. In many flares two different HXR flux components can be distinguished: (1) the fine structure of a HXR time profile (containing sequences of subsecond pulses) exhibits delays of ≊10-100 ms for the low-energy electrons, while (2) the unmodulated HXR time profile (a smooth lower envelope to the fine structure) shows a delay of opposite sign and much larger magnitude, of typically 1-10 s. We model the timing of various acceleration mechanisms and find that the delay of the HXR pulses is dominated by time-of-flight differences rather than by acceleration time scales, while the timing of the unmodulated HXR flux is governed by trapping and collisional time scales. Title: Metric spikes and electron acceleration in the solar corona. Authors: Benz, A. O.; Csillaghy, A.; Aschwanden, M. J. Bibcode: 1996A&A...309..291B Altcode: Short and narrowband spikes near the starting frequency of metric type III bursts have been searched and analyzed in Zurich radio spectrometer data. We find that the probability for the occurrence of metric spikes is related to the starting frequency of type III bursts; the association rate increases to 34% for groups of type III bursts with starting frequencies below 500MHz. The frequency channel containing most of spikes has been cross-correlated in time with the frequency channels of type III bursts. The correlation is significant in all selected cases and is caused by the association of individual type III bursts with groups of spikes, proving beyond doubt that spikes and type III bursts are physically related. The cross-correlation also defines the average drift rate of the type III maximum and its relation to spikes. In the average, the peak time of metric spikes coincides with the correlated type III burst, extrapolated to the same frequency. This may be interpreted as the spike radiation being emitted at the same characteristic frequency as the type III emission. The generally higher polarization of spikes helps to distinguish them from type III bursts. Contrary to spikes at higher (decimetric) frequencies, 60% of metric spike events have the same sign of circular polarization as the associated type III bursts, 33% are opposite, while the polarization was not measurable for the rest of the type III bursts. Some of the metric spike events are associated with weak flare activity in Halpha_, but none has been found to be closely associated with a hard X-ray burst or a microwave event. Metric spikes are consistent with energy release at high altitudes producing low energy electron beams. If the spikes are caused by the same electron population as the type III bursts, its acceleration site must be below or in close proximity to the spike source. Title: The Scaling Law between Electron Time-of-Flight Distances and Loop Lengths in Solar Flares Authors: Aschwanden, M. J.; Kosugi, T.; Hudson, H. S.; Wills, M. J.; Schwartz, R. A. Bibcode: 1996AAS...188.2608A Altcode: 1996BAAS...28..858A ;}} ;}} From the complete dataset of solar flares simultaneously observed with the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory (CGRO) in the high-time resolution mode (64 ms) and the Hard X-Ray Telescope (HXT) onboard Yohkoh we were able to determine the electron time-of-flight (TOF) distance and the flare loop geometry in 42 events. The electron TOF distances l' were determined from hard X-ray (HXR) time delays (~ 10-100 ms) occurring in the 16-channel spectra (at ~ 20-200 keV), produced by the velocity difference of the HXR-producing electrons. The flare loops were mostly identified from double footpoint sources in >~ 30 keV HXT images, with radii ranging from r=3000 to r=25,000 km. We find a scaling law between the electron TOF distance l' and the flare loop half length s=r(pi /2), having a mean ratio (and standard deviation) of l'/s=1.41+/- 0.29. In 5 flares we observe coronal >~ 30 keV HXR sources of the Masuda-type in the cusp region above the flare loop, and find that their heights are consistent with the electron TOF distance to the footpoints. These results provide strong evidence that particle acceleration in solar flares occurs in the cusp region above the flare loop and that the coronal HXR sources discovered by Masuda are a signature of the acceleration site, probably controlled by a magnetic reconnection process. Title: On the Peculiar Radio Polarization of a Sunspot and the Distibution of the Coronal Plasma Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J. Bibcode: 1996AAS...188.3602V Altcode: 1996BAAS...28..873V A comprehensive set of microwave, soft x-ray and magnetic observations of solar active region NOAA/USAF 7123 was obtained during 4--9 April, 1992. The observations show an unusual 4.7 GHz source which is polarized in the sense of the ordinary mode over the umbra of the leading spot. A detailed analysis of the daily radio and soft X-ray maps of the spot indicates that (i) the umbral magnetic field is strongly confined, (ii) the umbral and penumbral loops have distinct atmospheres and (iii) temperature gradients may exist both radially and vertically over the spot. A possible scenario for the temporal evolution of the spot is also presented. Title: Soft X-Ray (Yohkoh) and Radio (VLA) Observations of Solar Narrowband, Millisecond Spike Events Authors: Krucker, Sam; Benz, Arnold O.; Aschwanden, Markus J. Bibcode: 1996ASPC..111..129K Altcode: 1997ASPC..111..129K The source regions of metric spike events are investigated on Yohkoh soft X-ray (SXR) maps. The spikes are identified by the spectrometer Phoenix between 300 MHz and 360 MHz and are associated with groups of type III bursts at lower frequencies reaching also the decametric range. The Very Large Array (VLA) provides simultaneously spatial information at 333 MHz and 1445 MHz. Similar to the previous VLA observation of a metric spike event, the new data are consistent with a high altitude of the spike sources of about 5×1010cm above the photosphere. The additionally available SXR data for one of the presented events give the following new information: (i) The spike sources occur near open field lines and near regions of a slightly enhanced SXR flux relative to the ambient plasma. (ii) Contrary to SXR observations of type III bursts without metric spike activity, no SXR jet is observed. (iii) At low altitude, a weak SXR enhancement occurs, peaking about 60 s after the spike event. The SXR source and the spike sources can be connected by potential field lines. The observations corroborate a model in which a metric spike event is attributed to an energy release region at high altitude, while upwards propagating electrons produce type III bursts and downward moving electrons are responsible for SXR emission of heated plasma. Title: Joint Radio and Soft X-Ray Imaging of an `Anemone' Active Region Authors: Vourlidas, A.; Bastian, T. S.; Nitta, N.; Aschwanden, M. J. Bibcode: 1996SoPh..163...99V Altcode: The Very Large Array and the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite jointly observed the rapid growth and decay of a so-called `anemone' active region on 3-6 April, 1992 (AR 7124). The VLA obtained maps of the AR 7124 at 1.5, 4.7, and 8.4 GHz. In general, discrete coronal loop systems are rarely resolved at 1.5 GHz wavelengths because of limited brightness contrast due to optical depth effects and wave scattering. Due to its unusual anemone-like morphology, however, several discrete loops or loop systems are resolved by both the VLA at 1.5 GHz and the SXT in AR 7124. Title: Do Electron Time-of-Flight Measurements Support Magnetic Reconnection Models? Authors: Aschwanden, Markus Bibcode: 1996ASPC..111..216A Altcode: 1997ASPC..111..216A Energy-dependent hard X-ray (HXR) timing measurements have became an important new tool to determine the electron time-of-flight (TOF) distance between the acceleration site and the HXR-emitting energy loss site during solar flares. This electron TOF distance l' was found to have a scale-invariant ratio to the flare loop half length s for a large range of flare loop sizes, i.e. l'/s = 1.4±0.3 (Aschwanden et al. 1996). The author discusses the consequences of this result, specifically whether it constrains the location of the electron acceleration to be inside or outside flare loops. In the latter case, it would support flare models with magnetic reconnection in the cusp region of the loop. Title: Electron Time-of-Flight Measurements Authors: Aschwanden, M. J. Bibcode: 1996mpsa.conf..209A Altcode: 1996IAUCo.153..209A No abstract at ADS Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft X-ray Maps Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S. Bibcode: 1996mpsa.conf..441K Altcode: 1996IAUCo.153..441K No abstract at ADS Title: Accuracy, Uncertainties, and Delay Distribution of Electron Time-of-Flight Measurements in Solar Flares Authors: Aschwanden, Markus J.; Schwartz, Richard A. Bibcode: 1995ApJ...455..699A Altcode: A systematic time lag of ≍20 ms between the 25-50 keV and 50-100 keV hard X-ray (HXR) emission has been recently discovered in solar flares. This was interpreted in terms of electron time-of-flight differences (Aschwanden, Schwartz, & Alt 1995c). Here we attempt to determine the accuracy and uncertainties of such energy-dependent time delay measurements using burst-trigger data from DISCSC/BATSE on the CGRO spacecraft, recorded with a time resolution of 64 ms. We evaluate the time delays by cross-correlating entire flare time profiles at different energies and evaluate the statistical uncertainty of a delay measurement with a Monte Carlo method, in which random noise is added to the raw data. We examine also uncertainties resulting from aliasing, incomplete sampling, and pulse pileup.

We measure the time delays τ = t25keV - t50keV in 622 flares, with a statistical uncertainty of u ≤ 32 ms in 29% of the events, or u < 64 ms in 65% of the events. The distribution f(τ) of the time delays from all flares can be characterized with three components: (1) a Gaussian peak at τ = 23.2 + 1.2 ms with a standard deviation of στ = 25.5 ms, (2) a power-law tail with a slope of - 2.0 for large positive delays (τ = 0.1-4 s), and (3) a power-law tail with a slope of - 1.3 for large negative delays ( |-τ| = 0.1-7.7 s). The percentages of flares in these three regimes are 15%, 69%, and 16%. Flares with short delays (|τ| ≤ 0.1 s) exhibit rapid fluctuations with subsecond pulses. Such rapid fluctuations are almost absent in flares with longer delays. We find also a systematic trend of softer spectra in flares with large positive delays.

We develop a simple physical model that combines electron time-of-flight differences in the thin-target and thick-target model. We are able to reproduce the observed time delay distribution in the range of |τ| ≲ 0.1 s, requiring a distribution of electron densities in the range of ne ≤ 3.0 × 1012 cm-3 and flare loop heights in the range of h ≤ 35,000 km. Large negative delays (τ ≲ -0.1 s) can be produced in low-density loops with efficient magnetic trapping. Large positive delays (τ ≳ 0.1 s) occur in flares with a strong thermal component due to the convolution of the injection profile with the heating and cooling function. This study demonstrates that energy-dependent HXR time delays can be used as a diagnostic and discriminator of flare models. Title: Active Region 7123: Its Peculiar Radio Polarization and the Distribution of the Umbral Coronal Plasma Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J. Bibcode: 1995AAS...18710105V Altcode: 1995BAAS...27.1426V A comprehensive set of microwave and soft x-ray observations of solar active region 7123 was obtained during 3--10 April, 1992. Here, we present a detailed analysis of the polarized radio emission from the sunspot-associated component of AR 7123. We use the VLA observations at 1.5, 4.7 and 8.4 GHz, supported by the available x-ray and magnetic data. We concentrate on understanding the 4.7 GHz o-mode polarization over the umbra of the leading spot of AR 7123 and its variation with aspect angle within the physical context provided by the x-ray observations (SXT/Yohkoh) and past work on umbral atmospheres. Title: Solar Electron Beams Detected in Hard X-Rays and Radio Waves Authors: Aschwanden, Markus J.; Benz, Arnold O.; Dennis, Brian R.; Schwartz, Richard A. Bibcode: 1995ApJ...455..347A Altcode: We present a statistical survey of electron beam signatures that are detected simultaneously at hard X-ray (HXR) and radio wavelengths during solar flares. For the identification of a simultaneous event we require a type III (normal-drifting or reverse-slope-drifting) radio burst that coincides (within ± 1 s) with a significant (≥ 3 σ HXR pulse of similar duration (≥ 1 s). Our survey covers all HXRBS/SMM and BATSE/CGRO flares that were simultaneously observed with the 0.1-1 GHz spectrometer Ikarus or the 0.1-3 GHz spectrometer Phoenix of ETH Zurich during 1980-1993. The major results and conclusions are as follows:

1. We identified 233 HXR pulses (out of 882) to be correlated with type III-like radio bursts: 77% with normal-drifting type III bursts, 34% with reverse-slope (RS)-drifting bursts, and 13% with oppositely drifting (III + RS) burst pairs. The majority of these cases provide evidence for acceleration of bidirectional electron beams.

2. The detailed correlation with type III-like radio bursts suggests that most of the subsecond fluctuations detectable in ≥ 25 keV HXR emission are related to discrete electron injections. This is also supported by the proportionality of the HXR pulse duration with the radio burst duration. The distribution of HXR pulse durations WX is found to have an exponential distribution, i.e., N(WX) ∝ exp (-WX/0.25 s) in the measured range of WX ≍ 0.5-1.5 s.

3. From oppositely drifting radio burst pairs we infer electron densities of ne = 109-1010 cm-3 at the acceleration site. From the absence of a frequency gap between the simultaneous start frequencies of upward and downward drifting radio bursts, we infer an upper limit of L ≤ 2000 km for the extent of the acceleration site and an acceleration time of Δt ≤ 3 ms for the (≥ 5 keV) radio-emitting electrons (in the case of parallel electric fields).

4. The relative timing between HXR pulses and radio bursts is best at the start frequency (of earliest radio detection), with a coincidence of ≲0.1 s in the statistical average, while the radio bursts are delayed at all other frequencies (in the statistical average). The timing is consistent with the scenario of electron injection at a mean coronal height of h ≍ 104 km. The radio-emitting electrons are found to have lower energies (≳ 5 keV) than the ≥ 25 keV HXR-emitting electrons.

5. The modulated HXR flux that correlates with electron beam signatures in radio amounts to 2%-6% of the total HXR count rate (for BATSE flares). The associated kinetic energy in electrons is estimated to be E = 4 × 1022-1027 ergs per beam, or Ne = 4 × 1028-1033 electrons per beam, considering the spread from the smallest to the largest flare detected by HXRBS.

6. The average drift rate of propagating electron beams is found here to be [dv/dt] = 0.10ν1.4 MHz km s-1 in the frequency range of ν = 200-3000 MHz, which is lower than expected from the Alvarez & Haddock relation for frequencies ≤ 550 MHz.

7. The frequency distributions of HXR fluxes (Fx) and radio type III burst fluxes (FR), which both can be characterized by a power law, are found to have a significantly different slope, i.e., N(Fx) ∝ Fx-1.87 versus N(FR) ∝ FR-1.28. The difference in the slope is attributed to the fundamental difference between incoherent and coherent emission processes.

In summary, these findings suggest a flare scenario in which bidirectional streams of electrons are accelerated during solar flares at heights of 10 km above the photosphere in rather compact regions (L ≲ 2000 km). The acceleration site is likely to be located near the top of flare loops (defined by HXR double footpoints) or in the cusp above, where electrons have also access to open field lines or larger arches. The observed bidirectionality of electron beams favors acceleration mechanisms with oppositely directed electric fields or stochastic acceleration in an X-type reconnection geometry. 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: First VLA observation of a solar narrowband, millisecond spike event. Authors: Krucker, S.; Aschwanden, M. J.; Bastian, T. S.; Benz, A. O. Bibcode: 1995A&A...302..551K Altcode: The first spatially resolved observation of solar, narrowband spikes in two dimensions is presented. The 'metric' spikes around 333MHz are classified by the broadband spectrometer Phoenix (ETH Zuerich), whereas the simultaneously observing Very Large Array (VLA) provides high angular resolution images of the solar disc. At lower frequencies, a group of associated type III bursts is detected. The spikes occur at high altitude (=~4.5x10^10^cm above the photosphere), and at least 3 separated locations of emission can be identified. The different spike sources are separated by up to 130" and show different degrees of polarization. Spikes and type III bursts have the same sense of circular polarization, and according to the extrapolated potential field lines, the polarization of the different spike sources is in x-mode. With a delay of 42s, a thermal source appears on the same extrapolated potential field lines as the spikes at the second frequency of the VLA (1446MHz). The location of the energy release relative to the spikes source is discussed. A scenario is proposed where the energy is released in or near the spike source, and in which the spikes, the type III bursts and the thermal source originate from the same energy release. Hot electrons expanding along the field lines generate a type III burst (upward direction) and heat the underlying dense plasma (thermal source). Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft X-Ray Maps Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S. Bibcode: 1995SoPh..160..151K Altcode: A solar type I noise storm was observed on 30 July, 1992 with the radio spectrometer Phoenix of ETH Zürich, the Very Large Array (VLA) and the soft X-ray (SXR) telescope on board theYohkoh satellite. The spectrogram was used to identify the type I noise storm. In the VLA images at 333 MHz a fully left circular polarized (100% LCP) continuum source and several highly polarized (70% to 100% LCP) burst sources have been located. The continuum and the bursts are spatially separated by about 100″ and apparently lie on different loops as outlined by the SXR. Continuum and bursts are separated in the perpendicular direction to the magnetic field configuration. Between the periods of strong burst activities, burst-like emissions are also superimposed on the continuum source. There is no obvious correlation between the flux density of the continuum and the bursts. The burst sources have no systematic motion, whereas the the continuum source shows a small drift of ≈ 0.2″ min−1 along the X-ray loop in the long-time evolution. The VLA maps at higher frequency (1446 MHz) show no source corresponding to the type I event. The soft X-ray emission measure and temperature were calculated. The type I continuum source is located (in projection) in a region with enhanced SXR emission, a loop having a mean density of «ne» = (1.5 ± 0.4) × 109 cm−3 and a temperature ofT = (2.1 ± 0.1) × 106 K. The centroid positions of the left and right circularly polarized components of the burst sources are separated by 15″-50″ and seem to be on different loops. These observations contradict the predictions of existing type I theories. Title: Electron Time-of-Flight Differences in Solar Flares Authors: Aschwanden, Markus J.; Schwartz, Richard A.; Alt, Daniel M. Bibcode: 1995ApJ...447..923A Altcode: In the thick-target model for hard X-ray (HXR) emission in solar flares, electron acceleration is assumed to occur in flaring loops at coronal heights, while HXR bremsstrahlung emission is produced in the chromosphere. Under this assumption, the velocity spectrum of the accelerated electrons causes time-of-flight differences that are expected to result in the lower energy HXRs to be delayed with respect to the higher energies. Here we report on the first observational evidence for such a delay. The electron time-of-flight differences between electrons that produce 25-50 keV and 50-100 keV HXR emission are found to have a distribution with a mean of τ = 16.7±1.9 ms and a standard deviation of στ = 16.8 ms. This result is based on the statistics of 5430 HXR pulses detected during 640 solar flares, recorded in the Discriminator Science Data (DISCSC) burst trigger mode with a time resolution of 64 ms by the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory (CGRO). From the time-of-flight differences we infer a mean altitude of the acceleration site of H = 7300±800 km (with a standard deviation of σH = 7300 km) above the level at which the electrons lose their energy. This derived mean loop height should be considered as a lower limit because it is based on the predominance of time-of-flight effects over opposite delay effects caused by pitch-angle scattering or trapping. For the electron density in the flare loops we find an upper limit of ne ≤ 4 × 1012 cm-3, based on the requirement that the electron travel time has to be shorter than the collision time. The relatively small time-of-flight differences correspond typically to only ≍3% of the HXR pulse duration, and, therefore, no rapid variation in the spectral slope of the observed HXR spectrum is expected. Title: Aspect Angle Dependence of the Polarized Radio Emission from AR 7123 Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.; Nitta, N. Bibcode: 1995SPD....26..701V Altcode: 1995BAAS...27..965V No abstract at ADS Title: Electron Time-of-Flight Measurements in Solar Flares Authors: Aschwanden, M. J.; Schwartz, R. A. Bibcode: 1995SPD....26.1208A Altcode: 1995BAAS...27..984A No abstract at ADS Title: Sequences of Correlated Hard X-Ray and Type III Bursts during Solar Flares Authors: Aschwanden, Markus J.; Montello, Maria L.; Dennis, Brian R.; Benz, Arnold O. Bibcode: 1995ApJ...440..394A Altcode: Acceleration and injection of electron beams in solar flares can be traced from radio type III (or type U) bursts and correlated hard X-ray pulses with similar timescales and nonthermal spectra. We perform a systematic survey of such correlated radio and hard X-ray (HXR) pulses with timescales of less than or approximately 2 s in flares simultaneously observed with the radio spectrometer Ikarus and the Hard X-Ray Burst Spectrometer (HXRBS) on solar maximum mission (SMM). We applied an epoch-folding technique to enhance correlated time patterns in burst sequences at the two wavelengths. We present the results from the strongest (10) flares with a HXRBS count rate greater than or = 3000 counts/s, which have a satisfactory signal-to-noise ratio for subsecond pulses. The major findings of this study are presented. These observations strongly suggest that particle acceleration in solar flares occurs in a pulsed mode where electron beams are simultaneously injected in upward and downward directions. Since the sequences of correlated HXR and radio bursts show identical durations and intervals at the two wavelengths, they are believed to reflect most directly the temporal dynamics of the underlying common accelerator. As a consequence, thick-target models should be reconsidered under the aspect of electron injection with pulse durations of 0.2-2.0 s and duty cycles of approximately = 50%. Title: Chromospheric Evaporation and Decimetric Radio Emission in Solar Flares Authors: Aschwanden, Markus J.; Benz, Arnold O. Bibcode: 1995ApJ...438..997A Altcode: We have discovered decimetric signatures of the chromospheric evaporation process. Evidence for the radio detection of chromospheric evaporation is based on the radio-inferred values of (1) the electron density, (2) the propagation speed, and (3) the timing, which are found to be in good agreement with statistical values inferred from the blueshifted Ca XIX soft X-ray line. The physical basis of our model is that free-free absorption of plasma emission is strongly modified by the steep density gradient and the large temperature increase in the upflowing flare plasma. The steplike density increase at the chromospheric evaporation front causes a local discontinuity in the plasma frequency, manifested as almost infinite drift rate in decimetric type III bursts. The large temperature increase of the upflowing plasma considerably reduces the local free-free opacity (due to the T-3/2 dependence) and thus enhances the brightness of radio bursts emitted at the local plasma frequency near the chromospheric evaporation front, while a high-frequency cutoff is expected in the high-density regions behind the front, which can be used to infer the velocity of the upflowing plasma. From model calculations we find strong evidence that decimetric bursts with a slowly drifting high-frequency cutoff are produced by fundamental plasma emission, contrary to the widespread belief that decimetric bursts are preferentially emitted at the harmonic plasma level. We analyze 21 flare episodes from 1991-1993 for which broadband (100-3000 MHz) radio dynamic spectra from Pheonix, hard X-ray data from (BATSE/CGRO) and soft X-ray data from Burst and Transient Source Experiment/Compton Gamma Ray Observatory (GOES) were available. Title: Imaging, Stereoscopy, and Tomography of the Solar Corona in Soft X-Rays and Radio Authors: Aschwanden, M. J. Bibcode: 1995LNP...444...13A Altcode: 1995cmer.conf...13A We review simultaneous imaging observations of the (quiet) solar corona in soft X-rays (or EUV) and radio, and outline recent developments that involve three-dimensional (3D) reconstruction techniques, such as stereoscopy and tomography. The 3D reconstruction of coronal structures involves not only accurate measurements of geometric parameters (position, altitude, rotation rate), but also the deconvolution of physical parameters (density, temperature, magnetic field) along the line-of-sight, which is most feasible with simultaneous observations in complementary wavelengths, e.g. in soft X-rays, EUV, and radio. Title: Joint Radio and Soft X-ray Imaging of an ``Anemone'' Active Region Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M.; Nitta, N. Bibcode: 1994AAS...185.8609V Altcode: 1994BAAS...26Q1465V The Very Large Array and the Soft X-ray Telescope aboard the Yohkoh satellite observed the rapid growth and decay of a so-called ``anemone'' active region on 3-6 April (AR 7124). In general, discrete coronal loop systems are rarely resolved at radio wavelengths due to optical depth effects and scattering. In the case of AR 7124, however, several discrete loops or loop systems are resolved by both the VLA and the SXT, probably due to its unusual ``anemone''-like morphology. Furthermore, the region exhibited a significant amount of variability, especially on April 3, marked by many subflares and intensity changes as has been reported in flare patrol observations. In the present study, physical parameters derived from multiband radio and soft x-ray techniques for discrete loops are compared. The temporal evolution of these parameters is also examined. Title: Electron beams in solar flares Authors: Aschwanden, Markus J.; Dennis, Brian R.; Benz, Arnold O. Bibcode: 1994umd..reptR....A Altcode: A list of publications resulting from this program includes 'The Timing of Electron Beam Signatures in Hard X-Ray and Radio: Solar Flare Observations by BATSE/Compton Gamma-Ray Observatory and PHOENIX'; 'Coherent-Phase or Random-Phase Acceleration of Electron Beams in Solar Flares'; 'Particle Acceleration in Flares'; 'Chromospheric Evaporation and Decimetric Radio Emission in Solar Flares'; 'Sequences of Correlated Hard X-Ray and Type 3 Bursts During Solar Flares'; and 'Solar Electron Beams Detected in Hard X-Rays and Radiowaves.' Abstracts and reprints of each are attached to this report. Title: Coherent-Phase or Random-Phase Acceleration of Electron Beams in Solar Flares Authors: Aschwanden, Markus J.; Benz, Arnold O.; Montello, Maria L. Bibcode: 1994ApJ...431..432A Altcode: Time structures of electron beam signatures at radio wavelengths are investigated to probe correlated versus random behavior in solar flares. In particular we address the issue whether acceleration and injection of electron beams is coherently modulated by a single source, or whether the injection is driven by a stochastic (possibly spatially fragmented) process. We analyze a total of approximately = 6000 type III bursts observed by Ikarus (Zurich) in the frequency range of 100-500 MHz, during 359 solar flares with simultaneous greater than or = 25 keV hard X-ray emission, in the years 1890-1983. In 155 flares we find a total of 260 continuous type III groups, with an average number of 13 +/- 9 bursts per group, a mean duration of D = 12 +/- 14 s, a mean period of P = 2.0 +/- 1.2 s, with the highest burst rate at a frequency of nu = 310 +/- 120 MHz. Pulse periods have been measured between 0.5 and 10 s, and can be described by an exponential distribution, i.e., N(P) varies as e -P/1.0s. The period shows a frequency dependence of P(nu)=46(exp-0.6)MHzs for different flares, but is invariant during a particular flare. We measure the mean period P and its standard deviation sigma p in each type III group, and quantify the degree of periodicity (or phase-coherence) by the dimensionless parameter sigma pP. The representative sample of 260 type III burst groups shows a mean periodicity of sigma p/P = 0.37 +/- 0.12, while Monte Carlo simulations of an equivalent set of truly random time series show a distinctly different value of sigma pP = 0.93 +/- 0.26. This result indicates that the injection of electron beams is coherently modulated by a particle acceleration source which is either compact or has a global organization on a timescale of seconds, in contrast to an incoherent acceleration source, which is stochastic either in time or space. We discuss the constraints on the size of the acceleration region resulting from electron beam propagation delays and from Alfvenic synchronization during a pulse period. We discuss two periodic processes in flares, which potentially control quasi-periodic particle acceleration: (1) MHD oscillations, and (2) current sheets with oscillatory dynamics. Title: Particle Acceleration in Flares Authors: Benz, A. O.; Kosugi, T.; Aschwanden, M. J.; Benka, S. G.; Chupp, E. L.; Enome, S.; Garcia, H.; Holman, G. D.; Kurt, V. G.; Sakao, T.; Stepanov, A. V.; Volwerk, M. Bibcode: 1994SoPh..153...33B Altcode: Particle acceleration is intrinsic to the primary energy release in the impulsive phase of solar flares, and we cannot understand flares without understanding acceleration. New observations in soft and hard X-rays, γ-rays and coherent radio emissions are presented, suggesting flare fragmentation in time and space. X-ray and radio measurements exhibit at least five different time scales in flares. In addition, some new observations of delayed acceleration signatures are also presented. The theory of acceleration by parallel electric fields is used to model the spectral shape and evolution of hard X-rays. The possibility of the appearance of double layers is further investigated. Title: VLA Stereoscopy of Solar Active Regions Authors: Aschwanden, M. J.; Bastian, T. S. Bibcode: 1994kofu.symp..357A Altcode: No abstract at ADS Title: Irradiance observations of the 1 8 Å solar soft X-ray flux from goes Authors: Aschwanden, Markus J. Bibcode: 1994SoPh..152...53A Altcode: 1994IAUCo.143...53A; 1994svs..coll...53A The solar 0.5-8 å soft X-ray flux was monitored by the NOAA Geostationary Operational Environmental Satellites (GOES) from 1974 to the present, providing a continuous record over two solar activity cycles. Attempts have been made to determine a soft X-ray (SXR) background flux by subtracting out solar flares (using the daily lowest flux level). The SXR background flux represents the quiescent SXR flux from heated plasma in active regions, and reflects similar (intermediate-term) variability and periodicities (e.g. 155-day period) as the SXR or hard X-ray (HXR) flare rate, although it is determined in non-flaring time intervals. The SXR background flux peaks late in Solar Cycle 21 (2-3 years after the sunspot maximum), similar to the flare rate measured in SXR, HXR, or gamma rays, possibly due the increasing complexity of coronal magnetic structures in the decay phase of the solar cycle. The SXR background flux appears to be dominated by postflare emission from the dominant active regions, while the contributions from the quiet Sun are appreciable in the Solar Minimum only (A1-level). Comparisons with full-disk integrated images from YOHKOH suggest that the presence of coronal holes can decrease the quietest SXR irradiance level by an additional order of magnitude, but only in the rare case of absence of active regions. Title: VLA Stereoscopy of Solar Active Regions. I. Method and Tests Authors: Aschwanden, Markus J.; Bastian, T. S. Bibcode: 1994ApJ...426..425A Altcode: We develop a new technique for extracting three-dimensional information from multiday solar Very Large Array (VLA) observations. While standard stereoscopic methods provide a three-dimensional view of an object by combining simultaneous observations from two different aspect angles, we relax the condition of simultaneity and exploit solar rotation to vary the aspect angle. The solar radio images are decomposed into Gaussian source components, which are then cross-correlated in maps from preceding and following days. This provides measurements of the three-dimensional position of correlated source centroids. In this first paper, we describe the stereoscopic method and perform tests with simulated and real radio maps (from the VLA at 20 cm), in order to study the accuracy of altitude measurements, and the limitations introduced by (i) source confusion, (ii) source motion, and (iii) the assumed differential rotation rate. The tests demonstrate that (i) the information content of a VLA map relevant for stereoscopic correlation can be conveniently represented in terms of a small number of Gaussian components; (ii) the fitting of the three-dimensional source position is stable within a numerical accuracy of less than or approximately equal to 0.02 map pixels, (iii) the relative accuracy of the altitude determination is uniform over the solar disk, and (iv) source confusion does not affect the accuracy of stereoscopic position measurements for sources with a signal-to-noise ratio of greater than or approximately equal to 36. Title: VLA Stereoscopy of Solar Active Regions. II. Altitude, Relative Motion, and Center-to-Limb Darkening of 20 Centimeter Emission Authors: Aschwanden, Markus J.; Bastian, T. S. Bibcode: 1994ApJ...426..434A Altcode: A newly developed method of stereoscopic correlation is applied to solar radio maps at a wavelength of 20 cm. On the basis of stereoscopic correlations between radio maps obtained on 6 different days we establish the presence of 66 radio source components associated with 22 active regions. We find the following statistical results for active region source structures at 20 cm: 1. The observed lifetime is consistent with an exponential distribution having an e-folding time scale of greater than 18 days. 2. The average altitude of 20 cm sources is 25 +/- 15 Mm; 90% of the sources are found in heights less than 40 Mm. 3. The average diameter of discrete source structures is 48 +/- 15 Mm, implying a vertical/horizontal aspect ratio of qA approx. = 0.5. 4. No significant source motion has been found with respect to the standard differential rotation rate of Omega = 13.45 deg - ((3 deg (sin2B)). 5. We find a statistical limb darkening, which can be described by the relation (TB(alpha))/TB(0)) = 0.4 + ((0.6(cos2 alpha)) for sources with TB greater than 0.5 MK. 6. The degree of source polarization is 15% +/- 10% and is independent of source location. 7. Bright sources (greater than or approximately equal to 0.5 MK), or equivalently, long-lived sources (greater than or approximately equal to 5 days) show a systematic variation of their altitude as function of the center-limb distance. Title: Periodic or random acceleration in solar flares? Authors: Aschwanden, Markus J.; Benz, Arnold O. Bibcode: 1994SSRv...68..193A Altcode: The issue whether acceleration and injection of electron beams is coherently modulated by a single quasi-periodic source, or whether the injection is driven by a stochastic process in time or (eventually fragmented) in space, is investigated by menas of a periodicity analysis of metric type III bursts. We analyze 260 continuous type III groups observed byIkarus (ETH Zurich) in the frequency range of 100 500 MHz during 359 solar flares with simultaneous ≥25 keV hard X-ray emission, in the years 1980 1983. Pulse periods have been measured between 0.5 and 10 s, and can be described by an exponential distribution, i.e.N(P) ∝e -P/1.0s. We measure the mean periodP and its standard deviation σp in each type III group, and quantify the degree of periodicity by the dimensionless parameter σp/P. The representative sample of 260 type III burst groups shows a mean periodicity of σp/P=0.37±0.12, while Monte-Carlo simulations of an equivalent set of truly random time series show a distinctly different value of σp/P=0.93±0.26. This result suggests that the injection of electron beams is periodically modulated by a particle acceleration source which is either compact or has a global organization on a time scale of seconds. Title: Pulsed Acceleration in Solar Flares Authors: Aschwanden, Markus J.; Benz, Arnold O.; Dennis, Brian R.; Kundu, Mukul R. Bibcode: 1994ApJS...90..631A Altcode: 1994IAUCo.142..631A We study the nonlinear dynamics of particle acceleration in solar flares by analyzing the time series of various quasi-periodic radio signatures during flares. In particular we present the radio and hard X-ray data of three flares which suppport the following tentative conclusions: (1) Particle acceleration and injection into magnetic structures occurs intrinsically in a pulsed mode (with a typical period of 1-2 s), produced by a single, spatially coherent, nonlinear system, rather than by a stochastic system with many spatially independent components ('statistical flare' produced by a fragmented primary energy release). (2) The nonlinear (quasi-periodic) mode of pulsed particle acceleration and injection into a coronal loop can be stabilized by phase locking with an MHD wave (oscillation) mode, if both periods are close to each other. (3) Pulsed injection of electron beams into a coronal loop may trigger nonlinear relaxational oscillations of wave-particle interactions. This is particularly likely when the limit cycles of both systems are similar. Title: The Timing of Electron Beam Signatures in Hard X-Ray and Radio: Solar Flare Observations by BATSE/Compton Gamma-Ray Observatory and PHOENIX Authors: Aschwanden, Markus J.; Benz, Arnold O.; Schwartz, Richard A. Bibcode: 1993ApJ...417..790A Altcode: We analyzed two solar flares of 1992 September 5 and 6, using the high time resolution (64 ms) hard X-ray data from BATSE/CGRO, and 100-3000 MHz radio (100 ms) dynamic spectra from PHOENIX. The broadband radio data reveal a separatrix frequency (at 620 and 750 MHz in the two flares) between normal- and reverse-drifting radio bursts, indicating a compact acceleration source where electron beams are injected in both the upward and downward direction. We find a mean injection rate of 1.2 bursts s-1 in one flare and more than 0.7 bursts s-1 in the other. From 12 broad-band, reverse-drifting radio bursts we find in five cases an unambiguous one-to-one correlation between the reverse-drifting radio bursts and hard X-ray (HXR) pulses of similar duration (400±220 ms). The high significance (15±6 σ) of the HXR pulses and the small scatter (±150 ms) in the relative timing strongly supports a close causal connection. The cross-correlation between HXR and radio pulses shows that the HXR pulses are coincident (within the instrumental time resolution) with the reverse-drifting bursts at the injection frequency (880±50 MHz), and lead the radio bursts by 270±150 ms at the highest observable frequency (1240±100 MHz). The average drift time of the downward propagating radio bursts is measured to 150 ms, corresponding to a drift rate of 2350 MHz s-1.

We examined various effects to model the observed timing of radio and HXR pulses (propagation delays, radio wave growth and damping, group velocity delays, radio wave scattering, radio wave ducting, light path differences, etc.). Assuming an exciter velocity of υR/c = 0.2±0.1 for the reverse-drifting radio bursts, we infer an altitude difference of H = 8000±3000 km between the injection site and the HXR source. The most likely explanation for the retarded radio emission seems to be a combination of the following two effects: (1) HXR-emitting (>25 keV) and radio-emitting electrons have different energies (the exciter velocity of the reverse-drifting radio bursts is associated with ≲5 keV electrons), and (2) a low (marginal) growth rate for plasma emission at the second harmonics. Delay effects caused by group velocity, collisional damping, wave scattering, and wave ducting are found to be minor (<30 ms each). Title: Quasi-periodic Particle Injection into Coronal Loops Authors: Aschwanden, Markus J.; Benz, Arnold O.; Dennis, Brian R.; Gaizauskas, Victor Bibcode: 1993ApJ...416..857A Altcode: We present observations of the flare of 1989 June 22, 1445 UT (in active region NOAA 5555), obtained with the Hard X-Ray Burst Spectrometer (HXRBS) on SMM at energies >52 keV, and with the broad-band radio spectrometer PHOENIX at ETH/Zurich in the frequency range of 100-2800 MHz. The radio emission is dominated by a ≲100% polarized decimetric continuum at 400-1400 MHz, peaking at 750 MHz. The decimetric radio flux is highly correlated with the 50-150 keV hard X-ray flux but is delayed by 3.5-5.4 s with respect to the hard X-rays. The HXR emission shows an excess of ≳10 fast (≳100 ms) spikes (according to Poisson statistics). The radio emission exhibits weak fine structure, consisting of ≍45 quasi-periodic pulses with a mean period of 1.6 s. The frequency-time drift pattern of this fine structure is found to be consistent with segments of inverted-U type bursts, suggesting quasi-periodic injection of electron beams into a loop system. The loop system has an average height of 68,000 km and expands with a velocity of 200 km s-1 due to flare heating. Chromospheric evaporation enhances the electron density near the footpoints. The type III- exciting electrons have a mean velocity of υ/c = 0.30±0.10 (22 keV) and propagate along inverted-U burst trajectories with a mean duration of 2.5 s. For those electrons which reach the mirror point near the opposite footpoint of the loop system, we calculate (from the density and loop length) a low energy cutoff of ≥ 8 keV due to collisional deflection, yielding a propagation velocity of v/c = 0.18 and a propagation delay of 5.1±1.0 s, which agrees well with the observed delay of 5.16 s between the cross-correlated HXR and radio flux. The ≥ 8 keV electrons provide free energy for a loss cone instability near the secondary footpoint, which is observed as decimetric continuum polarized in the same sense of circular polarization as the type III bursts. The constraints from the Hα flare position and the magnetic potential field extrapolation indicate that the loss cone emission is produced in the diverging field region above the umbra of the leading sunspot, which has a photospheric field strength of -1600 G.

This flare allows us to deconvolve quasi-periodic particle injection and subsequently triggered coherent radio emission from trapped particles in flare-associated loops. It demonstrates that quasi-periodic modes of particle acceleration, particle dynamics in mirror loops, and the resulting plasma instabilities can be efficiently diagnosed from correlated hard X-ray and radio signatures. Title: Frequency distributions of solar X-ray flare parameters Authors: Crosby, N.; Aschwanden, M.; Dennis, B. Bibcode: 1993AdSpR..13i.179C Altcode: 1993AdSpR..13..179C We have determined frequency distributions of flare parameters from the over 12,000 solar flares that were recorded with the Hard X-ray Burst Spectrometer (HXRBS) on SMM. These parameters include the total flare duration, peak counting rate, the peak hard X-ray flux, the total energy in electrons, and the peak hard X-ray flux in electrons (the latter two computed assuming a thick-target model). The energies were computed above a threshold energy between 25 and 50 keV. All the distributions can be represented by power-laws over a range of several orders of magnitude, above the HXRBS sensitivity threshold. Power-laws can be taken as evidence for self-organized criticality in solar flares. Correlations among these parameters are determined from linear regression fits as well as from the slopes from the frequency distributions. Variations of the frequency distributions were also investigated with respect to the solar cycle. Title: VLA Stereoscopy of Solar Active Region 7123 Authors: Aschwanden, M. J.; Bastian, T. S.; Nitta, N. Bibcode: 1993BAAS...25.1224A Altcode: No abstract at ADS Title: Frequency distributions and correlations of solar X-ray flare parameters Authors: Crosby, Norma B.; Aschwanden, Markus J.; Dennis, Brian R. Bibcode: 1993SoPh..143..275C Altcode: We have determined frequency distributions of flare parameters from over 12000 solar flares recorded with the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) satellite. These parameters include the flare duration, the peak counting rate, the peak hard X-ray flux, the total energy in electrons, and the peak energy flux in electrons (the latter two computed assuming a thick-target flare model). The energies were computed above a threshold energy between 25 and 50 keV. All of the distributions can be represented by power laws above the HXRBS sensitivity threshold. Correlations among these parameters are determined from linear regression fits as well as from the slopes of the frequency distributions. Variations of the frequency distributions were investigated with respect to the solar activity cycle. Title: The Coevolution of Decimetric Millisecond Spikes and Hard X-Ray Emission during Solar Flares Authors: Aschwanden, Markus J.; Guedel, Manuel Bibcode: 1992ApJ...401..736A Altcode: Results are presented of an analysis of a comprehensive data set of 27 solar flares with decimetric millisecond spikes between 1980 and 1989, simultaneously observed with the Zuerich radio spectrometers and the Hard X-ray Burst Spectrometer on the SMM spacecraft. Two contradictory relationships of the coevolution of hard X-ray and spiky radio emissions during flares are found: the temporal evolution of both emissions reveals a close functional dependence, but there is a substantial time delay between the two emissions. Five possible scenarios for the hard-X-ray-associated radio spike emission which may account for both their detailed coevolution and their substantial intervening time delay are discussed. All five scenarios are able to explain both the close coevolution of hard X-ray and radio emission as well as their mutual delay to some degree, but none of them can explain all observational aspects in a simple way. Title: 3D reconstruction methods of coronal structures by radio observations. Authors: Aschwanden, M. J.; White, S. M.; Bastian, T. S. Bibcode: 1992ESASP.348..217A Altcode: 1992cscl.work..217A The ability to carry out a three-dimensional reconstruction of structures in the solar corona would represent a major advance in our study of the physical properties in active regions and in flares. The authors describe several new methods which allow a geometric reconstruction of quasi-stationary coronal structures (e.g. active region loops) or dynamic structures (e.g. flaring loops): (1) steroscopy of multi-day imaging observations by the VLA. (2) Tomography of optically thin emission (in radio or soft X-rays). (3) Multi-frequency band imaging by the VLA. (4) Tracing of magnetic field lines by propagating electron beams. Title: Simultaneous Observations of Coronal Bright Points in X-Ray and Radio Wavelengths Authors: Nitta, Nariaki; Bastian, Timothy S.; Aschwanden, Markus J.; Harvey, Karen L.; Strong, Keith T. Bibcode: 1992PASJ...44L.167N Altcode: We present a first explicit comparison of coronal bright points in soft X-ray and radio wavelengths, using the Soft X-ray Telescope aboard the Yohkoh spacecraft and the Very Large Array. About half of the 33 compact sources indentified in a 20-cm full-disk map appear as X-ray bright points in the X-ray data. The other half apparently corresponds to unipolar regions with enhanced magnetic fields. Thus, the identification of radio bright points alone cannot reliably serve as a proxy for X-ray bright points. A preliminary analysis reveals that bright points commonly observed at 20 cm and in X-rays have temperatures of (1.4--2.9) times 10(6) K and emission measures of (0.4--2.5) times 10(45) cm(-3) . The observed brightness temperatures at 20 cm [(1--2.5) times 10(5) K] can be explained in terms of optically thin free-free emission from a plasma with these parameters. Title: Decimetric Solar Type U Bursts: VLA and PHOENIX Observations Authors: Aschwanden, Markus J.; Bastian, T. S.; Benz, A. O.; Brosius, J. W. Bibcode: 1992ApJ...391..380A Altcode: Observations of type U bursts, simultaneously detected by the VLA at 1.446 GHz and by the broadband spectrometer Phoenix in the 1.1-1.7 GHz frequency band on August 13, 1989 are reported. Extrapolations of the coronal magnetic field, assuming a potential configuration, indicate that the VLA 20 cm source demarcates an isodensity level. The source covers a wide angle of diverging magnetic field lines whose footpoints originate close to a magnetic intrusion of negative polarity into the main sunspot group of the active region with dominant positive polarity. The centroid of the 20-cm U-burst emission, which corresponds to the turnover frequency of the type U bursts and remains stationary during all U bursts, coincides with the apex of extrapolated potential field lines at a height of about 130,000 km. It is demonstrated that the combination of radio imaging and broadband dynamic spectra, combined with the magnetic field reconstruction from magnetograms, can constrain all physical parameters of a magnetic loop system. Title: VLA-PHOENIX-BATSE Studies of Impulsive Bursts Authors: Willson, R. F.; Lang, K. R.; Aschwanden, M. J. Bibcode: 1992AAS...180.3009W Altcode: 1992BAAS...24..776W No abstract at ADS Title: Self-Organized Criticality in Solar Flares Authors: Aschwanden, M. J.; Dennis, B. R. Bibcode: 1992AAS...180.3015A Altcode: 1992BAAS...24..777A No abstract at ADS Title: Solar Radio Pulsation Event Observed by the VLA and OVRO Authors: Aschwanden, M. J.; Bastian, T. S.; Gary, D. E. Bibcode: 1992AAS...180.4505A Altcode: 1992BAAS...24..802A We investigate interpretational aspects of the radio pulsation event which has been observed by the Very Large Array (VLA) and the Owens Valley Radio Observatory (OVRO) on December 21, 1990, 1930 UT, during the second MAX'91 observing campaign. The VLA was observing with a time resolution of 0.4 s at 0.33, 1.4 and 4.9 GHz, while OVRO used a time resolution of 0.2 s at 1.2, 1.4, 1.6, 1.8 and 2.0 GHz. The radio pulsation event was triggered by a C3.3 class flare in soft X-rays in active region 6412 (NOAA). The time profile of the quasi-periodic radio emission exhibits a period of 8.8 s and shows indications of secondary periodicities with faster periods. We investigate whether these periodicities can be explained in terms of (i) MHD eigen-modes in cylindrical fluxtubes (MHD surface modes and harmonic modes), or (ii) relaxational oscillations of a nonlinear dissipative system. We test these two options by means of Fourier spectra and by reconstruction of the ``strange attractor dimension'' of nonlinear systems. We attempt a three-dimensional reconstruction of the pulsating source by means of magnetic field extrapolation from the photospheric magnetogram and by using the constraints of the radio maps at multiple frequencies. Preliminary analysis indicates that plasma emission as well as gyrosynchrotron emission is coherently modulated in spatially diverging magnetic structures. We investigate whether the pulsating radio emission originates from plasma confined in a pulsating loop structure or whether the radio emission is driven by electron beams which are produced in a pulsating acceleration mechanism. The latter possibility would support a model where the primary energy release itself is governed by a nonlinear dissipative system, e.g. by oscillatory magnetic reconnection. Title: VLA, PHOENIX and BATSE observations of an X1 flare. Authors: Willson, Robert F.; Aschwanden, Markus J.; Benz, Arnold O. Bibcode: 1992NASCP3137..515W Altcode: 1992como.work..515W The authors present observations of an X1 flare (Jul 18, 1991) detected simultaneously with the VLA, the PHOENIX Digital Radio Spectrometer and the Burst and Transient Source Experiment (BATSE) aboard the Gamma Ray Observatory. The VLA was used to produce snapshot maps of the impulsive burst emission on timescales of 1.7 sec at both 20 and 91 cm. The results indicate electron acceleration in the higher corona several minutes before the onset of the hard X-ray burst detected by BATSE. Comparisons with high spectral and temporal observations by PHOENIX reveal a variety of radio bursts at 20 cm, such as type III bursts, intermediate drift bursts, and quasi-periodic pulsations during different stages of the X1 flare. The described X1 flare is unique in the sense that it appeared at the east limb, providing the most accurate information on the vertical structure of different flare tracers visible in radio wavelengths. Title: Characteristics of the impulsive phase of flares Authors: Benz, A. O.; Aschwanden, M. J. Bibcode: 1992LNP...399..106B Altcode: 1992esf..coll..106B; 1992LNP...399..106A; 1992IAUCo.133..106B The impulsive phase of flares is an observational concept, characterized by spiky emissions from -rays to radio waves. It is generally agreed that during this time a large fraction of the. original flare energy resides in energetic particles which are manifested in these emissions. Here we concentrate on recent decimeter and microwave observations that indicate a high level of fragmentation of this energy release when related to hard X-ray (HXR) flux. Recent attempts to characterize the flare and the distribution of the radio bursts in time and frequency by statistical methods are also reviewed. Title: The association of solar millisecond radio spikes with hard X-ray emission Authors: Guedel, M.; Benz, A. O.; Aschwanden, M. J. Bibcode: 1991A&A...251..285G Altcode: Conventional observational data regarding solar millisecond spikes are compared with data gathered simultaneously in the hard X-ray band by means of a statistical analysis. The analysis considers the association rate, correlation degree, and relative time delays between hard X-ray emissions (in the 25-438 keV range) and radio-spike events. About 95 percent of the radio-spike bursts occur during impulsive hard X-ray bursts, and approximately 43 percent of the compared events are characterized by hard X-ray time profiles that mimic the concentration of simultaneous radio spikes. The delay of the radio emission with respect to the hard X-ray bursts puts some constraints on the acceleration and propagation of particles. The time delays and the quantization into discrete radio events are theorized to be caused by the operation of the accelerator. Title: The attractor dimension of solar decimetric radio pulsations Authors: Kurths, J.; Benz, A. O.; Aschwanden, M. J. Bibcode: 1991A&A...248..270K Altcode: The temporal characteristics of decimetric pulsations and related radio emissions during solar flares are analyzed using statistical methods recently developed for nonlinear dynamic systems. The results of the analysis is consistent with earlier reports on low-dimensional attractors of such events and yield a quantitative description of their temporal characteristics and hidden order. The estimated dimensions of typical decimetric pulsations are generally in the range of 3.0 + or - 0.5. Quasi-periodic oscillations and sudden reductions may have dimensions as low as 2. Pulsations of decimetric type IV continua have typically a dimension of about 4. Title: Solar and Stellar Radio Spikes: Limits on the Saturation of the Electron-Cyclotron Maser Authors: Wentzel, Donat G.; Aschwanden, Markus J. Bibcode: 1991ApJ...372..688W Altcode: The solar millisecond radio 'spikes' have been explained in terms of X-mode radiation generated by a maser near the electron gyrofrequency, acting on fast coronal electrons with a loss cone. This maser is a phenomenon described by quasi-linear theory. It is sensitive to the small first-relativistic correction to the gyrofrequency. Thus, it might be disrupted rather easily by nonlinear effects. The maximum radiation density that can be reached before the radiation entrains (phase-locks) the electrons and saturates the maser is discussed. If the observed durations of solar radio spikes are a measure of the rate of scattering into the loss-cone, then the inferred energy density is at least two orders of magnitude less than the energy density at which entrainment sets in. Also, maser emission from auroral kilometric radiation does not reach wave energies critical for electron entrainment. Maser emissions from flare stars, however, show 3-4 orders of magnitude higher radio fluxes and brightness temperatures than for the solar case and are likely to be saturated by entrainment. Title: Coordinated VLA-PHOENIX-SMM Observations: Microwave Type U-Bursts Authors: Aschwanden, M. J.; Bastian, T. S.; Benz, A. O.; Dennis, B. R. Bibcode: 1991BAAS...23.1065A Altcode: No abstract at ADS Title: Temporally, Spectrally, and Spatially Resolved Observations of a Pulsating Solar Radio Burst Authors: Bastian, T. S.; Aschwanden, M. J.; Gary, D. E. Bibcode: 1991BAAS...23Q1072B Altcode: No abstract at ADS Title: Simultaneous Radio and Hard X-ray Observations by PHOENIX/Zurich and HXRBS/SMM during Max'91 (June 1989) Authors: Aschwanden, M. J.; Dennis, B. R.; Benz, A. O. Bibcode: 1991max..conf..234A Altcode: No abstract at ADS Title: Flare fragmentation and type III productivity in the 1980 June 27 flare Authors: Aschwanden, M. J.; Benz, A. O.; Schwartz, R. A.; Lin, R. P.; Pelling, R. M.; Stehling, W. Bibcode: 1990SoPh..130...39A Altcode: We present observations of the solar flare on 1980 June 27, 16:14-16:33 UT, which was observed by a balloon-borne 300 cm2 phoswich hard X-ray detector and by the IKARUS radio spectrometer. This flare shows intense hard X-ray (HXR) emission and an extreme productivity of (at least 754) type III bursts at 200-400 MHz. A linear correlation was found between the type III burst rate and the HXR fluence, with a coefficient of ≈ 7.6 × 1027 photons keV−1 per type III burst at 20 keV. The occurrence of ≈ 10 type III bursts per second, and also the even higher rate of millisecond spikes, suggests a high degree of fragmentation in the acceleration region. This high quantization of injected beams, assuming the thick-target model, shows up in a linear relationship between hard X-ray fluence and the type III rate, but not as fine structures in the HXR time profile. Title: Relaxation of the loss-cone by quasi-linear diffusion of the electron-cyclotron maser instability in the solar corona Authors: Aschwanden, Markus J. Bibcode: 1990A&AS...85.1141A Altcode: A self-consistent numeric two-dimensional code using kinetic-wave particle equations has been developed and applied to the maser dynamics of the solar corona. Time histories generated by the code reveal details of the evolution of the linear phase, the saturation and subsequent relaxation of the loss cone. Quantitative values are obtained for the saturation time, the amount of the converted free energy, the critical wave energy level for onset of maser diffusion, the velocity and pitch angle range of the resonant particles, and the relative importance of the different magnetoionic modes and harmonics. A wide range of initial conditions is covered by varying the loss-cone distribution and the ambient cold plasma parameters. Title: The saturation of the electron-cyclotron maser instability and the interpretation of solar millisecond spikes Authors: Aschwanden, M. J. Bibcode: 1990A&A...237..512A Altcode: A self-consistent numeric two-dimensional code of the kinetic wave-particle equations developed to investigate the maser dynamics in the solar context is applied to solar millisecond-spike observations in order to improve the diagnostic capabilities of the theory of the electron-cyclotron maser instablitity. Attention is given to the inhomogeneity of the magnetic field selecting magneto-ionic modes with relatively short saturation lengths and suppressing mechanisms such as collisional deflection, free-free absorption, and gyroresonance absorption. The time scales of maser saturation in respect to time scales of global particle changes in a magnetic loop are covered, relevant observations of solar millisecond spikes are described, and the interpretation in terms of physical parameters deduced from the quasi-linear maser simulations are presented. It is demonstrated that the quasi-linear simulations make it possible to constrain the physical parameters from the observed time scale and frequency. Title: The Evolution of Decimetric Millisecond Spikes and Hard X-Ray Emission During Solar Flares Authors: Aschwanden, M. J.; Güdel, M. Bibcode: 1990BAAS...22..823A Altcode: No abstract at ADS Title: Correlation of solar radio pulsations with hard X-ray emission Authors: Aschwanden, M. J.; Benz, A. O.; Kane, S. R. Bibcode: 1990A&A...229..206A Altcode: A systematic study of the correlation of quasi-periodic broad-band decimetric pulsations with hard X-ray (HXR) emission is carried out. It is found that, in 11 out of 56 simultaneously observed events, the decimetric quasi-periodic pulsations in the impulsive phase of flares are correlated. If events with concurring type III bursts are included, 19 cases of radio pulsations are correlated with HXR. Title: The Attractor Dimension of Solar Decimetric Radio Pulsations Authors: Kurths, J.; Benz, A. O.; Aschwanden, M. J. Bibcode: 1990PDHO....7..196K Altcode: 1990dysu.conf..196K; 1990ESPM....6..196K The authors have analyzed the temporal characteristics of decimetric pulsations and related radio emissions during solar flares. Title: The Time Scale of Radio Millisecond Spikes: Theory Versus Observations Authors: Aschwanden, M. J.; Gudel, M. Bibcode: 1989BAAS...21..847A Altcode: No abstract at ADS Title: On the Electron-Cyclotron Maser Instability. II. Pulsations in the Quasi-stationary State Authors: Aschwanden, M. J.; Benz, A. O. Bibcode: 1988ApJ...332..466A Altcode: In the previous paper we have studied the electron-cyclotron maser instability of a source in quasi-equilibrium. In a situation of weak diffusion such a state will remain for a limited time regardless of the particular loss-cone instability involved. Here our major interest is in oscillations set up by small disturbances around the steady state. Both the linear growth as well as the coupled diffusion rate are subject to a common oscillation period, since wave growth and diffusion are interlocked by a Lotka-Volterra type of coupled equations. Periods in the limit of small amplitude (limit cycle) of the self-organizing wave-particle system are investigated for the magnetoionic X- and O-mode and for harmonic numbers s = 1, 2 of the maser instability. Pulsation periods in the order of 1 s, as observed in the decimetric range of solar flare emission are found exist for (1) fundamental (s = 1) O-mode for 0.3 < ωpe ≲ 1.0, and (2) harmonic (s = 2) X-mode (for 1.0 ≲ ωpe 1.4). The period is sensitive to variations of the shape of the loss-cone and therefore not stable. The proposed model provides an interpretation of the observed quasi-periodic decimetric solar pulsations as well as some of similar stellar phenomena. Title: On the Electron-Cyclotron Maser Instability. I. Quasi-linear Diffusion in the Loss Cone Authors: Aschwanden, M. J.; Benz, A. O. Bibcode: 1988ApJ...332..447A Altcode: Emission and quasi-linear velocity diffusion by the electron-cyclotron maser instability are studied under conditions of continuous operation. For the first time, the maser-induced quasi-linear diffusion is computed with the same accuracy as the well-known linear growth rates, including integration along the actual resonance space in k-space. This permits the feedback of the emission on the particle distribution to be quantified. A novel result is the discovery of relatively long diffusion time scales for typical solar conditions. Oscillations set up by small disturbances around the steady state are considered. Periods in the limit of small amplitude of the self-organizing wave-particle system are investigated for the magnetoionic X- and O-mode and for harmonic numbers s = 1, 2 of the maser instability. Pulsation periods in the order of 1 s, as observed in the decimetric range of solar flare emission, are found to exist for fundamental O-mode and for harmonic X-mode. Title: Pulsations of the Radio Emission of the Solar Corona Authors: Aschwanden, M. J. Bibcode: 1988BAAS...20..682A Altcode: No abstract at ADS Title: Theory of Radio Pulsations in Coronal Loops Authors: Aschwanden, M. J. Bibcode: 1987SoPh..111..113A Altcode: `Pulsations' include a wide range of phenomena from strictly sinusoidal oscillations up to quasiperiodic fine structures, observed in the radio, microwave and X-ray frequency range. The various versions of pulsation models are reviewed and classified in three groups according to their driver mechanisms: (1) Magnetic flux tube oscillations (the emissivity of trapped particles is modulated by a standing or propagating MHD wave), (2) cyclic self-organizing systems of plasma instabilities (wave-particle, wave-wave interactions), and (3) modulation of acceleration (acceleration/injection of particles into the source). Observational references illustrate the applicability of the models. In conclusion, discrimination criteria of models are discussed, in order to give a key for interpretation of observations. Title: Pulsations of the radio emission of the solar corona: analysis of observations and theory of the pulsating electron-cyclotron maser Authors: Aschwanden, Markus Josef Bibcode: 1987PhDT.......103A Altcode: No abstract at ADS Title: The frequency-time drift of pulsations. Authors: Aschwanden, M. J.; Benz, A. O. Bibcode: 1986A&A...158..102A Altcode: A set of 1270 digitally recorded type III bursts and pulsations in the frequency range of 100 - 1000 MHz was analyzed by a structure recognizing program. Numerical fitting methods determined the drift rate with much higher accuracy than previous analysis of analog data. The analysis of the drift of solar radio bursts in frequency vs. time provides information not only about the dynamical parameters of the source but also reveals atmospherical properties. Type III events and pulsations are clearly distinguished in their drift distribution. Pulsations statistically exhibit 3 times higher drift rates than type III events, thus requiring considerably shorter local scale heights. The source direction of the type III bursts is preferentially upwards oriented, the pulsations show a downwards oriented anisotropy. Evidence is given that the pulsations have a different origin than the common type III burst, concerning the coronal environment as well as the emission mechanism. Outlines to construct models of decimetric pulsations are discussed. Title: The Polarization of Decimetric Pulsations Authors: Aschwanden, M. J. Bibcode: 1986SoPh..104...57A Altcode: A sample of 10 decimetric broadband pulsations were observed in 1980-1983 and analyzed in polarization. Half of the data set was 85-100% circularly polarized, the other half showed a mild polarization of 15-55%. The polarization is constant in time and frequency for the strongly polarized group. All the mildly polarized bursts originate from near the limb; the lower degree of circular polarization is likely to be caused by depolarization due to propagation effects. The degree of polarization is constant throughout the event, but varies in frequency for the mild polarized group. Following the leading spot hypothesis, the magneto-ionic mode of the emission was found to be extraordinary. The high circular polarization of the pulsations was interpreted to be determined by the emission mechanism itself, not by propagation effects or cut-offs (contrary to the metric type I noise storms). Implications for pulsation models are discussed. Title: Correlation of Solar Decimetric Radio Bursts with X-Ray Flares Authors: Aschwanden, M. J.; Wiehl, H. J.; Benz, A. O.; Kane, S. R. Bibcode: 1985SoPh...97..159A Altcode: Several hundred radio bursts in the decimetric wavelength range (300-1000 MHz) have been compared with simultaneous soft and hard X-ray emission. Long lasting (type IV) radio events have been excluded. The association of decimetric emission with hard X-rays has been found to be surprisingly high (48%). The association rate increases with bandwidth, duration, number of structural elements, and maximum frequency. Type III-like bursts are observed up to the upper limit of the observed band. This demonstrates that the corona is transparent up to densities of about 1010 cm−3, contrary to previous assumptions. This can only be explained in an inhomogeneous corona with the radio source being located in a dense structure. The short decimetric bursts generally occur during the impulsive phase, i.e. simultaneously with hard X-rays. The times of maximum flux are well correlated (within 2 s). The HXR emission lasts 4 times longer then the radio emission in the average. This work finds a close relationship between decimetric and HXR emission with sufficient statistics offering additional information on the flare process. Title: Different time constants of solar decimetric bursts in the range 100 1000 MHz Authors: Wiehl, H. J.; Benz, A. O.; Aschwanden, M. J. Bibcode: 1985SoPh...95..167W Altcode: Between 1980, January 1 and 1981, December 31 a total of 664 `decimetric pulsation' events, abbreviated DCIM, were observed with the Zürich spectrometers in the frequency range 100 to 1000 MHz. All of these events were recorded on film, allowing an effective resolution in time of 0.5 s, and 5 MHz in frequency. Some of these events were also recorded digitally with higher time and frequency resolution. Title: Decimetric Radio Emission During Solar Flares Authors: Benz, A. O.; Aschwanden, M. J.; Wiehl, H. J. Bibcode: 1985spit.conf..597B Altcode: No abstract at ADS