OVRO, BBSO, BATSE, and YOHKOH observations of a twin solar flare

Haimin Wang, Dale Gary, H. Zirin, N. Nitta, R. A. Schwartz, T. Kosugi

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We present the results of studies on two solar flares that occurred on 1993 February 11: an M1.1 flare at 18:07 UT and an M2.7 flare at 18:31 UT. Our study was based on comprehensive observations by the following observatories: Owens Valley Radio Observatory, which obtains 1-18 GHz microwave images; Big Bear Solar Observatory, which obtains magnetograms, Hα and He D3 filtergrams; BATSE on board Compton Gamma Ray Observatory, which obtains high-resolution hard X-ray spectra; and the Japanese satellite Yohkoh, which obtains high-resolution soft and hard X-ray images. We find the following: (1) While the optical and hard X-ray emissions are confined to a small loop near the leading spot of the active region for both flares, a large-scale soft X-ray loop connects from the leading to the following spot 160″ away. In low-frequency microwaves (<4 GHz), sources appear at each end of the big loop, and the source near the following spot (away from the Hα flare site) dominates at frequencies <2.8 GHz. For both flares, as frequency increases, the source near the leading spot becomes dominant, and the source near the following spot vanishes gradually. (2) As frequency increases, the centroid of the leading microwave source moves progressively downward until it reaches the footpoint at high frequencies. (3) For the M2.7 event, in the compact loop near the leading spot, two footpoints are seen in both soft and hard X-rays. The dominant hard X-ray source has a softer spectrum than the weaker one, suggesting that the weaker one may become dominant at the higher energies (>100 keV) responsible for the microwave emission. The high-frequency microwave emission is better associated with this latter footpoint. (4) The large soft X-ray loop in the M2.7 flare is the postflare loop of the M1.1 flare. This flare is associated with a different compact loop which is 40″ away from the main flare. (5) For the M2.7 flare, the microwave brightness temperature spectra in the sources at the two ends of the big loop require very different source parameters. The primary source near the leading spot can be explained by nonthermal gyrosynchrotron emission from electrons with a power-law energy index (δ) of 5.3. The same group of electrons can explain the observed BATSE hard X-ray spectra. The low-frequency radio source near the following spot is due to either a thermal component, or a nonthermal component with a steep energy index (δ = 9.4). Based on the available information, we cannot distinguish these two possibilities.

Original languageEnglish (US)
Pages (from-to)403-411
Number of pages9
JournalAstrophysical Journal
Issue number2 PART I
StatePublished - 1996

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Sun: Flares
  • Sun: Radio radiation
  • Sun: UV radiation
  • Sun: X-rays, gamma rays


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