Control/Tracking Number: 06-A-262-SPD37 Activity: Standard Submission Current Date/Time: 4/18/2006 7:16:53 PM
Measuring the Temperature of Hot Flare Plasma Using RHESSI Fe and Fe/Ni Line Observations
Author Block: Amir Caspi1, S. Krucker2, R. P. Lin1 1Dept. of Physics and Space Sciences Laboratory, University of California, Berkeley, 2Space Sciences Laboratory, University of California, Berkeley.
Solar flares of GOES class M or X can produce "super-hot" (temperatures of ~30 MK or higher) thermal plasmas. Continuum emission from such flare plasmas can dominate the X-ray spectrum up to ~20-50 keV, making it difficult to separate from the non-thermal bremsstrahlung from accelerated, tens of keV electrons. Observations suggest that flare-accelerated electrons contribute significantly to heating of the thermal plasma (manifesting as the empirical "Neupert effect"), although recent studies also suggest that other heating mechanisms may be equally significant. Accurately characterizing the thermal plasma provides information about the heating and cooling mechanisms involved, and also improves our understanding of the non-thermal emission. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes solar photons with energies >3 keV, with a spectral resolution of ~1 keV FWHM, and is especially sensitive to flare plasmas above ~10 MK. RHESSI observes both the thermal continuum and the Fe and Fe/Ni line complexes at ~6.7 and ~8 keV. The fluxes and equivalent widths of the line complexes are strongly temperature-dependent and are useful as probes of the thermal plasma. We present an analytical method using the ratio of the line fluxes (Fe to Fe/Ni) as a temperature diagnostic to compare with and constrain the continuum temperature measurements. We discuss results of this analysis on two X-class flares (23/Jul/2002 and 02/Nov/2003) and compare our method to the equivalent width method of Phillips et al. (2005). We also present an application of this analysis by examining the "pre-impulsive" phase of two flares (23/Jul/2002 and 24/Aug/2002) which both appear to show strong non-thermal emission from a coronal source.
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