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Soft-gamma-ray repeaters (SGRs) are galactic X-ray stars that emit numerous short-duration (about 0.1 s) bursts of hard X-rays during sporadic active periods. They are thought to be magnetars: strongly magnetized neutron stars with emissions powered by the dissipation of magnetic energy. Here we report the detection of a long (380 s) giant flare from SGR 1806-20, which was much more luminous than any previous transient event observed in our Galaxy. (In the first 0.2 s, the flare released as much energy as the Sun radiates in a quarter of a million years.) Its power can be explained by a catastrophic instability involving global crust failure and magnetic reconnection on a magnetar, with possible large-scale untwisting of magnetic field lines outside the star. From a great distance this event would appear to be a short-duration, hard-spectrum cosmic gamma-ray burst. At least a significant fraction of the mysterious short-duration gamma-ray bursts may therefore come from extragalactic magnetars.

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.

Abstract.Soft X-ray images of solar active regions frequently show S-or inverse-S (sigmoidal) morphology. We have studied the Yohkoh Soft X-Ray Telescope video movie for 1993 and 1997. We have classified active regions according to morphology (sigmoidal or non-sigmoidal) and nature of activity (eruptive or non-eruptive). As well, we have used NOAA sunspot areas for each region as a measure of size. We find that regions are significantly more likely to be eruptive if they are either sigmoidal or large.

Solar flare occurrence follows a power-law distribution against total flare energy. W: dN dW with an index e ~ 1.8 as determined by several studies. This implies (a) that microflares must have a different, steeper distribution if they are energetically significant, and (b) there must be a high-energy cutoff of the observed distribution. We identif~ the distinct 'soft' distribution needed for coronal heating, if such a distribution exists, with Parker's nanoflares.

The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the pre-flare 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 Alfvén wave pulses -transport the energy and magnetic-field changes rapidly through the corona to the lower atmosphere. This offers two possibilities for electron acceleration. Firstly, in a coronal plasma with β < m e /m p , the waves propagate as inertial Alfvén 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 wavefronts. Secondly, 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 1 Carried out while a Visiting Researcher, Space Sciences Laboratory, University of California, Berkeley, CA finding and resupplying sufficient electrons to explain the impulsive-phase hard X-ray emission.Subject headings: Sun:flares,corona; waves; acceleration of particles IntroductionStrong chromospheric hard X-ray emission and strong UV and white-light emission mark the impulsive phase of a solar flare. These signatures are usually interpreted in terms of the well-known "thick-target model" (Brown 1971;Hudson 1972) in which fast electrons lose energy in Coulomb collisions and ionizing collisions in the chromosphere, heating and producing bremsstrahlung en route. The inefficiency of the bremsstrahlung process in a cold thick target implies that a large fraction of flare energy resides in these electrons (Kane & Donnelly 1971;Lin & Hudson 1976;Holman et al. 2003), and calculations under the assumptions of the thick-target model yield numbers on the order of 10 34 − 10 37 electrons accelerated per second (e.g. Miller 1997;Holman et al. 2003). Various strands of evidence have led to the commonly-accepted idea that the particle acceleration takes place in the solar corona, following which the electrons propagate into the lower atmosphere where they heat, and generate the observed hard X-ray radiation. Extensive theoretical work over four decades (which we will not attempt to summarize here) has elucidated the basics and the specifics of numerous dif...

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We present X-ray imaging and spectral analysis of all microflares the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed between March 2002 and March 2007, a total of 25,705 events. These microflares are small flares, from low GOES C Class to below A Class (background subtracted) and are associated with active regions. They were found by searching the 6-12 keV energy range during periods when the full sensitivity of RHESSI's detectors was available (see paper I). Each microflare is automatically analyzed at the peak time of the 6-12 keV emission: the thermal source size is found by forward-fitting the complex visibilities for 4-8 keV, and the spectral parameters (temperature, emission measure, power-law index) are found by forward fitting a thermal plus nonthermal model. The combination of these parameters allows us to present the first statistical analysis of the thermal and non-thermal energy at the peak times of microflares. On average a RHESSI microflare has a fitted thermal loop width 8 Mm (11 ′′ ), length 23 Mm (32 ′′ ) and volume 1×10 27 cm 3 , temperature 13 MK, emission measure 3 × 10 46 cm −3 and density of 6 × 10 9 cm −3 . There is no correlation between the loop size and the flare magnitude, either flux in the loop or GOES class, indicating that microflares are not necessarily spatially small. There is also no clear correlation between the thermal parameters except between the RHESSI and GOES emission measures, the GOES values are generally twice the RHESSI emission measures. The microflare thermal energy at the time of peak emission in 6-12 keV ranges over 10 26 to 10 30 erg and has a median value of 10 28 erg. The frequency distribution of the thermal energy deviates from a power-law at low and high energies arising from a deficiency of events due to instrumental and selection effects. It is difficult to compare this energy distribution to previous thermal energy distributions of transient events, as the work sought nanoflares through imaging in EUV or soft X-rays and covered just a few hours. There are large uncertainties in the majority of the non-thermal parameters, due to the steep spectra down to low energies. We typically find a power-law index of 7 above a break energy of 9 keV, which corresponds to a low-energy cut-off in the electron distribution as low as 12 keV. The resulting non-thermal power estimates, covering 10 25 to 10 28 erg s −1 with median value of 10 26 erg s −1 , therefore have large uncertainties as well. The few microflares with unexpectedly large non-thermal powers 10 28 erg s −1 have the smallest uncertainties, of about 10%. The total non-thermal energy however is still small compared to that of large flares as it occurs for shorter durations.