Living Reviews in Solar Physics
Solar flares are observed at all wavelengths from decameter radio waves to gamma-rays beyond 1 GeV. This review focuses on recent observations in EUV, soft and hard X-rays, white light, and radio waves. Space missions such as RHESSI, Yohkoh, TRACE, SOHO, and more recently Hinode and SDO have enlarged widely the observational base. They have revealed a number of surprises: Coronal sources appear before the hard X-ray emission in chromospheric footpoints, major flare acceleration sites appear to
... on sites appear to be independent of coronal mass ejections, electrons, and ions may be accelerated at different sites, there are at least 3 different magnetic topologies, and basic characteristics vary from small to large flares. Recent progress also includes improved insights into the flare energy partition, on the location(s) of energy release, tests of energy release scenarios and particle acceleration. The interplay of observations with theory is important to deduce the geometry and to disentangle the various processes involved. There is increasing evidence supporting magnetic recon-This article is a revised version of https://doi.org/10.12942/lrsp-2008-1. Change summary Major revision, updated and expanded. Change details Significant new results from RHESSI and SDO are included. In particular, Sect. 3.4 on "Emissions from above the coronal source" was added (including two new figures) and Sect. 3.2 on "Coronal hard X-ray sources" was expanded. New observations of flares in white light are reported (replaced Fig. 25 with a movie), as well as recent multi-wavelength observations in radio waves and gamma-rays combined with hard X-rays. Four references were removed and 49 new references added. Electronic supplementary material The online version of this article (nection as the basic cause. While this process has become generally accepted as the trigger, it is still controversial how it converts a considerable fraction of the energy into non-thermal particles. Flare-like processes may be responsible for large-scale restructuring of the magnetic field in the corona as well as for its heating. Large flares influence interplanetary space and substantially affect the Earth's ionosphere. Flare scenarios have slowly converged over the past decades, but every new observation still reveals major unexpected results, demonstrating that solar flares, after 150 years since their discovery, remain a complex problem of astrophysics including major unsolved questions.