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Hrd X-ray and Microwave Observations of Microflares View all abstracts by submitter

Jiong Qiu   Submitted: 2004-06-04 16:05

In this paper, we study solar microflares using the coordinated hard X-ray and microwave observations obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during its open-shutter operation mode and the Owens Valley Solar Array (OVSA). The events in our study are selected in the energy range of 12-25 keV and are relatively large microflares with an average GOES soft X-ray incremental flux at the B2.0 level. A total of 760 microflares are identified from the RHESSI burst catalog. Out of the 200 microflares that fall into the OVSA observing window, about 40% are detected in microwaves. Using these hundreds of events as samples, we study the event distribution with respect to the flux, the solar activity, and active regions, in comparison with flares of larger scales. Non-thermal properties of microflares are investigated through spectral analysis of X-rays and microwaves. (1) We find that the event frequency distribution with respect to the RHESSI peak count rates at 12-25 keV can be accurately described with a power-law function down to 8 cts s-1, the power-law index being 1.75pm0.03, consistent with previous studies. (2) Similar to large flares, the occurrence rate of microflares is correlated with solar activity. The studied samples of microflares are mostly produced by active regions as suggested by the large percentage of events detected by OVSA that observes target active regions. However, all active regions do not have equal productivity, and certain active regions are a lot more productive than other regions. (3) While some large and complex active regions are predominantly productive in both very weak and strong events, we also find an active region that produces many microflares and C-class events but does not produce powerful events. %It may be that frequent occurrence of small-scale %events in some active regions helps relax the configuration and %prevent build-up to large bursts. (4) Analysis of energy-dependent time profiles suggests that there is a pronounced temporal correlation between the time derivative of soft X-rays and 14-20 keV hard X-rays, i.e., the Neupert effect, in about half of the studied events. (5) Albeit small, many microflares exhibit hard X-ray emission at over 10 keV and microwave emission at around 10 GHz. Spectral analysis in these two wavelengths corroborates the non-thermal nature of these emissions. (6) In a limited number of samples, the RHESSI spectral fitting yields a photon spectral index of 4.5-7, and microwave spectral analysis on the same events shows that the power-law index of the electron spectrum is in the range of 2-5. The discrepancy in the electron spectrum index derived from hard X-rays and microwaves is substantially greater than previously reported in big flares, hinting at the existence of high-energy, microwave emitting electrons that have a much hardened spectrum compared with electrons emitting hard X-rays.

Authors: Jiong Qiu, Chang Liu, Dale E. Gary, Gelu, M. Nita, Haimin Wang
Projects: RHESSI

Publication Status: ApJ, 2004, 612
Last Modified: 2004-06-04 16:05
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