RHESSI Line and Continuum Observations of Super-hot Flare Plasma

TL;DR

This study uses RHESSI observations to analyze the thermal plasma in a solar flare, identifying two distinct temperature components and their spatial and temporal evolution, revealing insights into flare heating and plasma dynamics.

Contribution

It provides the first detailed characterization of super-hot flare plasma with high-resolution imaging and spectroscopy, distinguishing spatially and thermally separate components.

Findings

Identification of a super-hot plasma component exceeding 30 MK

Spatial separation between super-hot and hot plasma regions

Correlation between Fe line ratios and super-hot continuum temperature

Abstract

We use RHESSI high-resolution imaging and spectroscopy observations from ~5 to 100 keV to characterize the hot thermal plasma during the 2002 July 23 X4.8 flare. These measurements of the steeply falling thermal X-ray continuum are well fit throughout the flare by two distinct isothermal components: a super-hot (T > 30 MK) component that peaks at ~44 MK and a lower-altitude hot (T < 25 MK) component whose temperature and emission measure closely track those derived from GOES measurements. The two components appear to be spatially distinct, and their evolution suggests that the super-hot plasma originates in the corona, while the GOES plasma results from chromospheric evaporation. Throughout the flare, the measured fluxes and ratio of the Fe and Fe-Ni excitation line complexes at ~6.7 and ~8 keV show a close dependence on the super-hot continuum temperature. During the pre-impulsive phase, when the coronal thermal and non-thermal continua overlap both spectrally and spatially, we use this relationship to obtain limits on the thermal and non-thermal emission.