RHESSI and SOHO CDS Observations of Explosive Chromospheric Evaporation

Abstract

Simultaneous observations of explosive chromospheric evaporation are presented using data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory. For the first time, cospatial imaging and spectroscopy have been used to observe explosive evaporation within a hard X-ray emitting region. RHESSI X-ray images and spectra were used to determine the flux of nonthermal electrons accelerated during the impulsive phase of an M2.2 flare. When we assumed a thick-target model, the injected electron spectrum was found to have a spectral index of ~7.3, a low-energy cutoff of ~20 keV, and a resulting flux of >=4×10¹⁰ ergs cm^-2 s^-1. The dynamic response of the atmosphere was determined using CDS spectra; we found a mean upflow velocity of 230+/-38 km s^-1 in Fe XIX (592.23 Å) and associated downflows of 36+/-16 and 43+/-22 km s^-1 at chromospheric and transition region temperatures, respectively, relative to an averaged quiet-Sun spectra. The errors represent a 1 σ dispersion. The properties of the accelerated electron spectrum and the corresponding evaporative velocities were found to be consistent with the predictions of theory.