RHESSIMicroflare Statistics. II. X‐Ray Imaging, Spectroscopy, and Energy Distributions

Abstract

We present the first statistical analysis of the thermal and nonthermal X-ray emission of all 25,705 microflares (RHESSI) observed between 2002 March and 2007 March. These events were found by searching the 6-12 keV energy range (see Paper I) and are small active region flares, from low (GOES) C class to below A class. Each microflare is automatically analyzed at the peak time of the 6-12 keV emission: the thermal source size is found by forward-fitting the complex visibilities for 4-8 keV, and the spectral parameters (temperature, emission measure, power-law index) are found by forward-fitting a thermal plus nonthermal model. The resulting wealth of information we determine about the events allows a range of the thermal and nonthermal properties to be investigated. In particular, we find that there is no correlation between the thermal loop size and the flare magnitude, indicating that microflares are not necessarily spatially small. We present the first thermal energy distribution of RHESSI flares and compare it to previous thermal energy distributions of transient events. We also present the first nonthermal power distribution of RHESSI flares and find that a few microflares have unexpectedly large nonthermal powers up to 10²⁸ erg s⁻¹. The total microflare nonthermal energy, however, is still small compared to that of large flares as it occurs for shorter durations. These large energies and difficulties in analyzing the steep nonthermal spectra suggest that a sharp broken power law and thick-target bremsstrahlung model may not be appropriate for microflares.