HR: 08:30h
AN: SH31A-01    [Abstracts]
TI: Solar Hard X-ray Emissions in the Decay Phase of Gradual X-class Flares
AU: * Krucker, S
EM: krucker@ssl.berkeley.edu
AF: Space Science Lab UC Berkeley, 7 Gauss Way, Berkeley, CA 94720-7450 United States
AU: Lin, R P
EM: rlin@ssl.berkeley.edu
AF: Space Science Lab UC Berkeley, 7 Gauss Way, Berkeley, CA 94720-7450 United States
AB: Solar hard X-ray emissions are most often observed during the impulsive phase of solar flares and are produced by non-thermal electrons that lose their energy in the chromosphere by collisions. During large gradual flares, however, HXR emissions can last well into the decay phase showing progressively hardening X-ray spectra (e.g. Cliver et al. 1986). Here we present X-ray observations of such events obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). RHESSI provides for the first time high resolution (1 keV) spectral observations with simultaneous imaging down to 2". In about half of all X-class flares seen by RHESSI, enhanced HXR emission lasting well into decay phase (up to two hours after the flare onset) is observed. These emissions are generally ~10 times fainter and show much less time variations than HXR bursts occurring during the impulsive phase. The X-ray spectra are found to be hard and can generally be fitted with broken power law functions. First results from one of the most prominent events (2005 January 19, 8UT) show clear spectral hardening. Over 15 minutes, the spectral power law index hardens below the break (from 2.6 to 2.1) as well as above it (from 3.3 to 2.6). Furthermore, the break energy increases from 40 to 60 keV. Imaging reveals that the emission come from footpoints of post flare loops suggesting that it is produced in a thick target. No HXR emission is seen to originate from the post flare loops. We are presently investigating the importance of trapping of energetic electrons in these events.
DE: 7519 Flares
DE: 7554 X rays, gamma rays, and neutrinos
DE: 7807 Charged particle motion and acceleration
SC: SPA-Solar and Heliospheric Physics [SH]
MN: 2005 Joint Assembly