NEW INSIGHTS INTO THE LARGE l992 JULY 15-l7 FLARE ON AU MICROSCOPIl: THE FIRST DETECTION OF POSTERUPTIVE ENERGY RELEASE ON A RED DWARF STAR

摘要

Extren1e Ultraviolet Explorer (EUVE) observations of the long-duration l992 July l5--17 flaring event on the red dwarf star AU Mic are discussed. The flare was detected in the EUVE Deep Survey (DS) Lexan/B band (65--l90 A) and was made up of a large impulsive event corresponding to an increase in the extreme ultraviolet (EUV) emission by a factor of l5 relative to quiescence, followed by a slow expo- nential decay interspersed with another smaller impulsive event and further decay. As observed in thc EUV, the flare lasted for more than half a day. A similar exponential decay of the flux in the Fe XVIIl λ93.9 line was detected in the EUVE short wavelength (SW) spectrometer following the first powerful impulsive phase. The flare decay in the 65--l90 A band and in the Fe XVIII line during this prolonged event lasted a factor of l0 or more longer than would have been the case had the plasma cooling been due only to radiative losses at typical flare plasma densities. An analysis of thc simultaneous DS Lexan/B data and the Fe XVIIl line flux has allowed us to determine the approximate temporal behavior of the flaring plasma temperature (d log T/'dt≈0.5 day~-1) and of the emission measure of the EUV source. The total energy emitted in the 1--2000 A range during the first l2 hours or so of thc flaring event was 3 x l0~35 crgs. Solar analogs of processes providing similar long-duration soft X-ray radiation arc discussed f (1) a coronal mass ejection (CME), (2) a system of giant postflare arches, which can form in powerful solar flares after a CME; (3) typical soft X-ray flaring 1o