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Session 26 - Solar Flares II.
Oral session, Monday, June 10
Wisconsin Center,

[26.09] Nonthermal Microwave Emission from Soft X-ray Transient Brightenings.

D. E. Gary (Caltech), M. Hartl (Harvard), T. Shimizu (High Altitude Observatory)

Soft X-ray transient brightenings (TBs) are small enhancements, generally associated with active region loops, that last for 5-10 min. Shimizu et al. (1994) has shown that the rate of energy release in TBs has the same form as that for flares, and if they are interpreted as tiny flares they would extend the flare energy release rate to lower energy by two orders of magnitude. However, in initial investigation of radio counterparts of TBs near 15 GHz (Gopalswamy et al 1995; White et al. 1995) showed no conclusive evidence for nonthermal electrons, which brings into question whether TBs are flare-like energy releases. The presence of nonthermal electrons is most easily seen at somewhat lower radio frequencies, which led us to search for such emission in radio data from the OVRO Solar Array in the range 1-18 GHz.

Using soft X-ray observations from Yohkoh, we identified 34 transient brightenings in solar active region AR7172 observed from 20-29 May 1992. A comparison with radio data from OVRO yielded the following new results: 1) enhanced radio emission can be positively associated with TBs in 12 events (35%), with another 17 (50%) showing a possible association; 2) a number of the positive associations show radio peaks that occur near the onset of the soft X-ray enhancement, with a Neupert-Effect-like relationship (mean delay of the soft X-ray peak relative to the microwave peak is 2.5 min); 3) the radio power spectra of those events intense enough to give good spectra indicate the presence of a nonthermal electron population; 4) the correlation between peak radio flux and peak soft X-ray flux is weak. For the 7 events with the clearest Neupert-Effect relationship, there is a much higher correlation between integrated radio flux and peak soft X-ray flux. Together these results suggest that TBs are the analog of solar flares extended below the subflare energy range. We may now state that the low-energy extension of flare events is not sufficient to heat the corona, but the study of TBs can shed light on flare processes that may be masked in larger flares.

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