RHESSI Microflares with Quiet Microwave Emission

摘要

We statistically study RHESSI microflares, or "weak events," from the first month of 2003. In total, 94 events are selected from the RHESSI flare catalog in the 3-6 and 12-25 keV energy ranges. The sample differs from those of previous studies in that the events are characterized by having quiet microwave emission, based on observations from the Nobeyama Radio Polarimeters. These microflares cluster around GOES class C1.1 and are associated with active regions. Consistent with previous findings, the thermal plasma observed by RHESSI is found to be hot, 10 MK T 15 MK, with low emission measure (1046 cm?3 EM 1047 cm?3) and density (1.0 × 109 cm?3 ne 12 × 109 cm?3). The spectral fitting requires a steep nonthermal power-law component, with median index of 8.1. Further evidence for the presence of nonthermal electrons is that the estimated nonthermal energy is comparable to the thermal energy, having a median value of ~4.0 × 1028 ergs for the events studied here. Using the correlation between the time derivative of the light curve at 3-6 keV and the 12-25 keV hard X-ray flux, we investigate the Neupert effect in these microflares. Similar to findings for large flares, roughly half of these microflares (46 of 94) display the Neupert effect. RHESSI imaging shows that each event has a single source at 3-12 keV. Such characteristics indicate that some in situ heating mechanism is at work in the local corona surrounding microflares, with the low microwave emission resulting from a lack of nonthermal electron propagation to the chromosphere.