Production of high-temperature plasmas during the early phases of A C9.7 flare

Abstract

Explosive chromospheric evaporation is predicted from some current solar flare models. In this paper, we analyze a flare with high time cadence raster scans with the EUV Imaging Spectrometer (EIS) on board the Hinode spacecraft. This observation covers an area of 240″ × 240 ″ with the 1 ″ slit in about 160 s. The early phases of a C9.7 flare that occurred on 2007 June 6 were well observed. The purpose of our analysis is to study for the first time the spatially resolved spectra of high-temperature plasma, especially from Fe xxiii and Fe xxiv, allowing us to explore the explosive chromospheric evaporation scenario further. Sections of raster images obtained between 17:20:09 and 17:20:29 (UT) show a few bright patches of emission from Fe xxiii/Fe xxiv lines at the footpoints of the flaring loops; these footpoints were not clearly seen in the images taken earlier, between 17:17:30 and 17:17:49 (UT). Fe xxiii spectra at these footpoints show dominating blueshifted components of -(300 to 400) km s-1, while Fe xv/xiv lines are nearly stationary; Fe xii lines and/or lower temperature lines show slightly redshifted features, and Fe viii and Si vii to He ii lines show ∼+50 km s-1 redshifted components. The density of the 1.5-2 MK plasma at these footpoints is estimated to be 3 ×1010 cm-3 by the Fe xiii/xiv line pairs around the maximum of the flare. High-temperature loops connecting the footpoints appear in the Fe xxiii/xxiv images taken over 17:22:49-17:23:08 (UT) which is near the flare peak. Line profiles of these high-temperature lines at this flare peak time show only slowly moving components. The concurrent cooler Fe xvii line at 254.8 Å is relatively weak, indicating the predominance of high-temperature plasma (>107 K) in these loops. The characteristics observed during the early phases of this flare are consistent with the scenario of explosive chromospheric evaporation. © 2010. The American Astronomical Society. All rights reserved.