HR: 1340h
AN: SH13A-0285    [Abstracts]
TI: Pre-Impulsive Hard X-Ray Emission from Coronal Sources in X-Class Flares
AU: * Caspi, A
EM: cepheid@ssl.berkeley.edu
AF: Department of Physics, University of California, Berkeley, CA 94720-7300 United States
AU: * Caspi, A
EM: cepheid@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450 United States
AU: Krucker, S
EM: krucker@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450 United States
AU: Lin, R P
EM: rlin@ssl.berkeley.edu
AF: Department of Physics, University of California, Berkeley, CA 94720-7300 United States
AU: Lin, R P
EM: rlin@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450 United States
AB: The GOES-class X4.8 event on 23 July 2002 exhibits significant hard X-ray emission prior to the impulsive phase of the flare. Images from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) show that most of the emission appears to originate from a coronal source. Analysis of the Fe (~6.7 keV) and Fe/Ni (~8 keV) line complexes seen in RHESSI spectra suggests that there is little or no thermal continuum emission above ~15 keV, thus implying that non-thermal electron bremsstrahlung dominates the spectrum even to low energies. The X3.1 event on 24 Aug 2002 displays similar characteristics, and its position at the west limb occults the footpoints and shows that the bulk of the HXR emission unambiguously originates in the corona. Preliminary analysis shows that the pre-impulsive spectra are well-fit by a broken power-law characteristic of non-thermal emission. The spectral indices above and below the break energy differ by ~2, suggesting that we may be observing the transition between thin- and thick-target bremsstrahlung. The break energy increases with time, which suggests an average electron pitch angle of ~45°, assuming an increasing column density derived from the thermal emission measure. We analyze and compare the RHESSI X-ray spectra for the pre-impulsive phases of both 24 Aug 2002 and 23 July 2002, using the Fe and Fe/Ni line complexes to constrain the thermal parameters. We derive the time-varying characteristics of the electron populations and discuss the implications for acceleration and heating of electrons in the corona during this period.
UR: http://sprg.ssl.berkeley.edu/~cepheid/agu2005/
DE: 7509 Corona
DE: 7519 Flares
DE: 7554 X-rays, gamma rays, and neutrinos
DE: 7594 Instruments and techniques
DE: 7845 Particle acceleration
SC: SPA-Solar and Heliospheric Physics [SH]
MN: Fall Meeting 2005