Microflares and the Statistics of X-ray Flares

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.Comment: To be published in Space Science Reviews (2011

Flare X-ray Observations of AB Doradus: Evidence of Stellar Coronal Opacity

X-ray spectra of the late-type star AB Dor, obtained with the XMM-Newton satellite are analysed. AB Dor was particularly active during the observations. An emission measure reconstruction technique is employed to analyse flare and quiescent spectra, with emphasis on the Fe XVII 15-17 A wavelength region. The Fe XVII 16.78 A/15.01 A line ratio increases significantly in the hotter flare plasma. This change in the ratio is opposite to the theoretical predictions and is attributed to the scattering of 15.01 A line photons from the line-of-sight. The escape probability technique indicates an optical depth of about 0.4 for the 15.01 A line. During the flare, the electron density is 4.4*10^10 cm^-3 and the fractional Fe abundance is 0.5 +/- 0.1 of the solar photospheric value. Using these parameters a path length of about 8,000 km is derived. There is no evidence for opacity in the quiescent X-ray spectrum of the star.Comment: 13 pages, 3 figures, to be published in ApJ Letter