Plasma Motion inside Flaring Regions Revealed by Doppler Shift Information from SDO/EVE Observations
Plasma motions within flaring regions provide key information for us to understand the flare processes. Here, we study two X-class flares near the solar disk center, one on 2014 January 7 and the other on 2012 March 7, by using 10 extreme ultraviolet (EUV) emission lines from the Solar Dynamics Observatory/EUV Variability Experiment (EVE). The EVE plasma dynamic spectrum chart, a 2D map of Doppler shift against temperature and time, is constructed based on a spectroscopic analysis of the EUV lines. Three kinds of plasma motion are identified in the plasma dynamic spectrum charts: chromospheric evaporation (100-200 km s-1) above 1 MK, cooling inside post-flare loops (approximately 150 km s-1) between 0.3 and 1 MK, and condensation at footpoints (<30 km s-1) below 0.3 MK. We find that the chromospheric evaporation and condensation at footpoints started in the impulsive phase almost simultaneously, while the cooling occurred later in the gradual phase, with a time delay of more than 10 minutes, probably implying the timescale of evaporation movement and heat loss. Atmospheric Imaging Assembly observations and differential emission measure (DEM) analyses suggest that the cooled plasma moves downward within the cold transition region (TR) loops, from top to feet, which are below the hot coronal loops. Besides, the reversal temperature between blue/redshifts is close to 1 MK, implying that the boundary of upflowing/downflowing plasma is located at the lower corona or the upper TR.