EXTREME-ULTRAVIOLET SPECTROSCOPIC OBSERVATION OF DIRECT CORONAL HEATING DURING A C-CLASS SOLAR FLARE

With the Coronal Diagnostic Spectrometer operating in rapid cadence (9.8 s) stare mode during a C6.6 flare on the solar disk, we observed a sudden brightening of Fe XIX line emission (formed at temperature T approximate to 8 MK) above the pre-flare noise without a corresponding brightening of emission from ions formed at lower temperatures, including He I (0.01 MK), O V (0.25 MK), and Si XII (2 MK). The sudden brightening persisted as a plateau of Fe XIX intensity that endured more than 11 minutes. The Fe XIX emission at the rise and during the life of the plateau showed no evidence of significant bulk velocity flows, and hence cannot be attributed to chromospheric evaporation. However, the line width showed a significant broadening at the rise of the plateau, corresponding to nonthermal velocities of at least 89 km s(-1) due to reconnection outflows or turbulence. During the plateau He I, O V, and Si XII brightened at successively later times starting about 3.5 minutes after Fe XIX, which suggests that these brightenings were produced by thermal conduction from the plasma that produced the Fe XIX line emission; however, we cannot rule out the possibility that they were produced by a weak beam of nonthermal particles. We interpret an observed shortening of the O V wavelength for about 1.5 minutes toward the middle of the plateau to indicate new upward motions driven by the flare, as occurs during gentle chromospheric evaporation; relative to a quiescent interval shortly before the flare, the O V upward velocity was around -10 km s(-1).