9/12/2023 0 Comments Gu hara speed up gif![]() Peterson and Winckler ( 1959) discovered the first hard X-ray emission during a flare in 1958. In the late 1950s it became possible to observe the Sun in hard X-rays (≳ 10 keV) by balloons and rockets. These discoveries could only mean that the flare phenomena is not confined to thermal plasma, but includes high-energy particles and involves the corona. Forbush noticed ground-level cosmic ray enhancements associated with major solar flares. During a radio burst, the total solar luminosity in radio waves may increase by several orders of magnitude. Meter wave radio emissions, detected serendipitously in 1942 during military radar operations, revealed the presence of non-thermal electrons in the corona (Hey, 1983). Variations of source size, ejections of plasma blobs into interplanetary space, and blast waves (Moreton, 1964) were noted. 1.2 Brief history of flare observationsĪ few years after Carrington and Hodgson, the Sun was studied extensively in the H α line originating in the chromosphere, and the reports of flares became much more frequent, but also bewilderingly complex. Most manifestations seem to be secondary responses to the original energy release process, converting magnetic energy into particle energy, heat, waves, and motion. Nevertheless, it is better to define the flare phenomenon observationally as a brightening of any emission across the electromagnetic spectrum occurring at a time scale of minutes. It may be used as a guide for novices to distinguish flares from other plasma physical phenomena in the solar atmosphere also associated with brightenings, such as the expulsion of magnetic flux or dissipation of shock waves. ![]() However, one has to bear in mind that such a definition represents a specific, although widely accepted, interpretation of observations. The general use of the term “flare” today often alludes to a “sudden release of magnetic energy by reconnection”. ![]() The problem is even more acute in other languages, such as German and French, denoting flares with the equivalent to “eruption”, which may be confused with Coronal Mass Ejections (CMEs) that often happen simultaneously at the time of large flares. Thus the word “flare” is used in solar physics today in a rather ill-defined way, describing a syndrome of apparently related processes at various wavelengths. Local flaring of the Sun has been reported at all wavelengths accessible from the ground and from space. Hodgson observed the first flare in the continuum of white light, the localized, minute-long brightenings on the Sun have remained an enigma. While flare scenarios have slowly converged over the past decades, 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. Large flares influence interplanetary space and substantially affect the Earth’s lower ionosphere. Flare-like processes may be responsible for large-scale restructuring of the magnetic field in the corona as well as for its heating. 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. There is increasing evidence supporting reconnection of magnetic field lines as the basic cause. The interplay of observations with theory is important to deduce the geometry and to disentangle the various processes involved. 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. 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 be independent of coronal mass ejections (CMEs), 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. Space missions such as RHESSI, Yohkoh, TRACE, and SOHO have enlarged widely the observational base. This review focuses on recent observations in EUV, soft and hard X-rays, white light, and radio waves. Solar flares are observed at all wavelengths from decameter radio waves to gamma-rays at 100 MeV.
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