Hrd X-ray and Microwave Observations of Microflares |
|
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
|
 
 
 
 
|
|