We present X-ray imaging and spectral analysis of all microflares the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed between March 2002 and March 2007, a total of 25,705 events. These microflares are small flares, from low GOES C Class to below A Class (background subtracted) and are associated with active regions. They were found by searching the 6-12 keV energy range during periods when the full sensitivity of RHESSI's detectors was available (see paper I). 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 combination of these parameters allows us to present the first statistical analysis of the thermal and non-thermal energy at the peak times of microflares. On average a RHESSI microflare has a fitted thermal loop width 8 Mm (11 ′′ ), length 23 Mm (32 ′′ ) and volume 1×10 27 cm 3 , temperature 13 MK, emission measure 3 × 10 46 cm −3 and density of 6 × 10 9 cm −3 . There is no correlation between the loop size and the flare magnitude, either flux in the loop or GOES class, indicating that microflares are not necessarily spatially small. There is also no clear correlation between the thermal parameters except between the RHESSI and GOES emission measures, the GOES values are generally twice the RHESSI emission measures. The microflare thermal energy at the time of peak emission in 6-12 keV ranges over 10 26 to 10 30 erg and has a median value of 10 28 erg. The frequency distribution of the thermal energy deviates from a power-law at low and high energies arising from a deficiency of events due to instrumental and selection effects. It is difficult to compare this energy distribution to previous thermal energy distributions of transient events, as the work sought nanoflares through imaging in EUV or soft X-rays and covered just a few hours. There are large uncertainties in the majority of the non-thermal parameters, due to the steep spectra down to low energies. We typically find a power-law index of 7 above a break energy of 9 keV, which corresponds to a low-energy cut-off in the electron distribution as low as 12 keV. The resulting non-thermal power estimates, covering 10 25 to 10 28 erg s −1 with median value of 10 26 erg s −1 , therefore have large uncertainties as well. The few microflares with unexpectedly large non-thermal powers 10 28 erg s −1 have the smallest uncertainties, of about 10%. The total non-thermal energy however is still small compared to that of large flares as it occurs for shorter durations.

No abstract

No abstract

This review surveys the statistics of solar X-ray flares, emphasising the new views that RHESSI has given us of the weaker events (the microflares). The new data reveal that these microflares strongly resemble more energetic events in most respects; they occur solely within active regions and exhibit high-temperature/nonthermal emissions in approximately the same proportion as major events. We discuss the distributions of flare parameters (e.g., peak flux) and how these parameters correlate, for instance via the Neupert effect. We also highlight the systematic biases involved in intercomparing data representing many decades of event magnitude. The intermittency of the flare/microflare occurrence, both in space and in time, argues that these discrete events do not explain general coronal heating, either in active regions or in the quiet Sun.

This review discusses the complementary relationship between radio and hard X-ray observations of the Sun using primarily results from the era of the Reuven Ramaty High Energy Solar Spectroscopic Imager satellite. A primary focus of joint radio and hard X-ray studies of solar flares uses observations of nonthermal gyrosynchrotron emission at radio wavelengths and bremsstrahlung hard X-rays to study the properties of electrons accelerated in the main flare site, since it is well established that these two emissions show very similar temporal behavior. A quantitative prescription is given for comparing the electron energy distributions derived separately from the two wavelength ranges: this is an important application with the potential for measuring the magnetic field strength in the flaring region, and reveals significant differences between the electrons in different energy ranges. Examples of the use of simultaneous data from the two wavelength ranges to derive physical conditions are then discussed, including the case of microflares, and the comparison of images at radio and hard X-ray wavelengths is presented. There have been puzzling results obtained from observations of solar flares at millimeter and submillimeter wavelengths, and the comparison of these results with corresponding hard X-ray data is presented. Finally, the review discusses the association of hard X-ray releases with radio emission at decimeter and meter wavelengths, which is dominated by plasma emission (at lower frequencies) and electron cyclotron maser emission (at higher frequencies), both coherent emission mechanisms that require small numbers of energetic electrons. These comparisons show broad general associations but detailed correspondence remains more elusive.