Chromospheric Evaporation in Solar Flare Loop Strands Observed with the Extreme-ultraviolet Imaging Spectrometer on Board Hinode
The entire profile of the Fe XXIII line at 263.8 Å, formed at temperature ≈14 MK, was blueshifted by an upward velocity -122 ± 33 km s-1 when it was first detected by the Extreme-ultraviolet Imaging Spectrometer operating in rapid cadence (11.18 s) stare mode during a C1 solar flare. The entire profile became even more blueshifted over the next two exposures, when the upward velocity reached its maximum of -208 ± 14 km s-1 before decreasing to zero over the next 12 exposures. After that, a weak, secondary blueshifted component appeared for five exposures, reached a maximum upward velocity of -206 ± 33 km s-1, and disappeared after the maximum line intensity (stationary plus blueshifted) was achieved. Velocities were measured relative to the intense stationary profile observed near the flare's peak and early during its decline. The initial episode during which the entire profile was blueshifted lasted about 156 s, while the following episode during which a secondary blueshifted component was detected lasted about 56 s. The first episode likely corresponds to chromospheric evaporation in a single loop strand, while the second corresponds to evaporation in an additional strand, as described in multi-strand flare loop models proposed by Hori et al. and Warren & Doschek. Line emission from progressively cooler ions (Fe XVII, XVI, and XIV) brightened at successively later times, consistent with cooling of flare-heated plasma.