White--Light Flares Observed by YOHKOH

Astronomy and Astrophysics – Astronomy

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Scientific paper

The Wide Band Spectrometer (WBS) on the YOHKOH satellite has spectroscopic capabilities for measuring solar X-ray and gamma-ray emission in the energy range from 3 keV to 100 MeV. The WBS consists of three different types of spectrometers, soft X-ray spectrometer (SXS: two gas proportional counters covering the 3-40 keV band), hard X-ray spectrometer (HXS: NaI scintillation counter covering the 20-600 keV band), and gamma-ray spectrometer (GRS: two BGO scintillation counters covering the 0.3-100 MeV band). The WBS provides light curves in ten energy bands and the wide band energy spectrum which are essential for understanding of particle acceleration and plasma heating during solar flares. To date the WBS has recorded about 200 flares since last October. Of these, the four impulsive X-class flares of 1991 Oct. 24, 1991 Oct. 27, 1991 Nov. 15, and 1991 Dec. 3 produced high-energy photons up to about 10 MeV. In particular, the 1991 Oct. 27 flare (X6.1/3B) showed intense gamma-ray lines resulting from nuclear deexcitation and neutron capture (this gives evidence that ions were accelerated to 10-100 MeV/nuc), whereas the 1991 Dec. 3 flare (X2.0/2B) showed the intense gamma-ray spectrum which is dominated by electron bremsstrahlung. These spectral observations imply there are different types of particle acceleration processes in the flare sites. The time variation of hard X-ray spectral index is also discussed in connection with the electron acceleration.

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