Lyndsay Fletcher

Coronal Loop Oscillations Observed with the Transition Region and Coronal Explorer

We report here, for the —rst time, on spatial oscillations of coronal loops, which were detected in extreme-ultraviolet wavelengths (171 with the T ransition Region and Coronal Explorer, in the tem- Ae ) perature range of MK. The observed loop oscillations occurred during a —are that began at T e B 1.0¨1.5 1998 July 14, 12:55 UT and are most prominent during the —rst 20 minutes. The oscillating loops connect the penumbra of the leading sunspot to the —are site in the trailing portion. We identi—ed —ve 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 —xed nodes at the footpoints). We investigate diUerent MHD wave modes and —nd that the fast kink mode with a period q \ 205(L /1010 cm~3)1@2 cm)(n e /109 (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 —are site with a radial speed of B700 km s~1. Because the observed loop oscillation periods are compa- rable to photospheric 5 minute oscillations, a resonant coupling between the two systems is possible. We further —nd evidence for global extreme-UV dimming in the entire active region possibly associated with a coronal mass ejection. Subject headings: Sun: coronaSun: —aresSun: oscillationsSun: UV radiation

An Observational Overview of Solar Flares

We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era. Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections. We also discuss flare soft X-ray spectroscopy and the energetics of the process. The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory. The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations.

Impulsive Phase Flare Energy Transport by Large-Scale Alfvén Waves and the Electron Acceleration Problem

The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the preflare coronal magnetic field. Hard X-ray observations imply that a substantial fraction of flare energy released during the impulsive phase is converted to the kinetic energy of mildly relativistic electrons (10-100 keV). The liberation of the magnetic free energy can occur as the coronal magnetic field reconfigures and relaxes following reconnection. We investigate a scenario in which products of the reconfiguration?large-scale Alfv?n wave pulses?transport the energy and the magnetic field changes rapidly through the corona to the lower atmosphere. This offers two possibilities for electron acceleration. First, in a coronal plasma with -->? < me/mp, the waves propagate as inertial Alfv?n waves. In the presence of strong spatial gradients, these generate field-aligned electric fields that can accelerate electrons to energies on the order of 10 keV and above, including by repeated interactions between electrons and wave fronts. Second, when they reflect and mode-convert in the chromosphere, a cascade to high wavenumbers may develop. This will also accelerate electrons by turbulence, in a medium with a locally high electron number density. This concept, which bridges MHD-based and particle-based views of a flare, provides an interpretation of the recently observed rapid variations of the line-of-sight component of the photospheric magnetic field across the flare impulsive phase, and offers solutions to some perplexing flare problems, such as the flare number problem of finding and resupplying sufficient electrons to explain the impulsive-phase hard X-ray emission.

A Trace white light and Rhessi hard X-ray study of flare energetics

In this paper we investigate the formation of the white-light (WL) continuum during solar flares and its relationship to energy deposition by electron beams inferred from hard X-ray emission. We analyze nine flares spanning GOES classifications from C4.8 to M9.1, seven of which show clear cospatial RHESSI hard X-ray and TRACE WL footpoints. We characterize the TRACE WL/UV continuum energy under two simplifying assumptions: (1) a blackbody function, or (2) a Paschen-Balmer continuum model. These set limits on the energy in the continuum, which we compare with that provided by flare electrons under the usual collisional thick-target assumptions. We find that the power required by the white-light luminosity enhancement is comparable to the electron beam power required to produce the HXR emission only if the low-energy cutoff to the spectrum is less than 25 keV. The bulk of the energy required to power the white-light flare (WLF) therefore resides at these low energies. Since such low-energy electrons cannot penetrate deep into a collisional thick target, this implies that the continuum enhancement is due to processes occurring at moderate depths in the chromosphere.

Evidence for the Flare Trigger Site and Three-Dimensional Reconnection in Multiwavelength Observations of a Solar Flare

Based on a multiwavelength data set and a topological model for the magnetic field, we argue that a M1.9 flare which occurred on 1993 May shows evidence of three-dimensional coronal reconnection in a spine-fan configuration. Images from the Transition Region and Coronal Explorer allow the detailed examination of the structures involved in the flare and preflare in the 171 A (1 MK) EUV passband and the Lyα (10,000-20,000 K) passband. Yohkoh Hard X-ray Telescope maps the position of nonthermal electron precipitation and the Soft X-ray Telescope reveals preflare and flare heating on large and small scales. While the flare appears to be driven by changes in small-scale field close to the photosphere, near the interface between strong opposite magnetic polarities, the result is the disruption of large-scale field. We demonstrate how this observed activity on large and small scales, along with many other aspects of the flare, suggests a qualitative explanation in the three-dimensional reconfiguration of coronal magnetic field, following a small-scale flux cancellation at the photosphere.

Recent Advances in Understanding Particle Acceleration Processes in Solar Flares

We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered.

The Structure and Evolution of a Sigmoidal Active Region

Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections. Sigmoids, or S-shaped structures, may be indicators of twisted or helical magnetic structures, having an increased likelihood of eruption. We present here an analysis of a sigmoidal regions three-dimensional structure and how it evolves in relation to its eruptive dynamics. We use data taken during a recent study of a sigmoidal active region passing across the solar disk (an element of the third Whole Sun Month campaign). While S-shaped structures are generally observed in soft X-ray (SXR) emission, the observations that we present demonstrate their visibility at a range of wavelengths including those showing an associated sigmoidal filament. We examine the relationship between the S-shaped structures seen in SXR and those seen in cooler lines in order to probe the sigmoidal regions three-dimensional density and temperature structure. We also consider magnetic field observations and extrapolations in relation to these coronal structures. We present an interpretation of the disk passage of the sigmoidal region, in terms of a twisted magnetic flux rope that emerges into and equilibrates with overlying coronal magnetic field structures, which explains many of the key observed aspects of the regions structure and evolution. In particular, the evolving flux rope interpretation provides insight into why and how the region moves between active and quiescent phases, how the regions sigmoidicity is maintained during its evolution, and under what circumstances sigmoidal structures are apparent at a range of wavelengths.

Observations of conduction driven evaporation in the early rise phase of solar flares

In the classical flare picture, hard X-ray emission from the chromosphere is succeeded by soft-X-ray emission from hot plasma in the flare loop, the soft X-ray emission being a direct consequence of the impact of the non-thermal particle beam. However, observations of events exist in which a pronounced increase in soft X-ray emission is observed minutes before the onset of the hard X-ray emission. Such pre-flare emission clearly contradicts the classical flare picture. For the first time, the pre-flare phase of such solar flares is studied in detail. We want to explain the time evolution of the observed emission by means of alternative energy transport mechanisms such as heat conduction. RHESSI events displaying pronounced pre-flare emission were analyzed in imaging and spectroscopy. The pre-flare phase is characterized by purely thermal emission from a coronal source with increasing emission measure and density. After this earliest phase, a small non-thermal tail to higher energies appears in the spectra, becoming more and more pronounced. However, images still only display one X-ray source, implying that this non-thermal emission is coronal. The increase of emission measure and density indicates that material is added to the coronal region. The most plausible origin is evaporated material from the chromosphere. Energy provided by a heat flux is capable of driving chromospheric evaporation.

HIGH-RESOLUTION OBSERVATIONS OF PLASMA JETS IN THE SOLAR CORONA

We present recent observations of coronal jets, made by TRACE and Yohkoh/SXT on 28 May and 19 August 1998. The high spatial resolution of TRACE enables us to see in detail the process of material ejection; in the line of Fe ix (one million degrees) we see both bright emitting material and dark absorbing/scattering material being ejected, i.e., both hot and cold material, highly collimated and apparently ejected along the direction of the overlying field lines. Bright ejecta are seen simultaneously in Lyman α for one event and Yohkoh/SXT in the other. The jets on the two days are different in that the 19 August jet displays the morphology typical of a one-sided anemone jet while the 28 May jet exhibits a two-sided jet morphology. The 19 August jet shows evidence for rotation and an interesting bifurcation at large distances from the energy release site. We study the physical properties and energetics of these jetting events, and conclude that existing theoretical models capture the essential physics of the jet phenomena.

Deconvolution of Directly Precipitating and Trap-precipitating Electrons in Solar Flare Hard X-Rays. III.Yohkoh Hard X-Ray Telescope Data Analysis

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=42° ± 11° and α2=52° ± 10°) 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).