Evidence for Chromospheric Evaporation in the Late Gradual Flare Phase from [ITAL]SOHO[/ITAL]/CDS Observations

Abstract

Using extreme-ultraviolet (EUV) spectroheliograms from the first intentional postflare observations with the Coronal Diagnostic Spectrometer (CDS) on board SOHO, we determine relative line-of-sight velocities and their temporal evolution during the gradual flare phase of an M6.8 two-ribbon flare that occurred on 1998 April 29. Dopplergrams in lines of O V, Fe XVI, and Fe XIX, with formation temperatures Tmax of, respectively, 0.25, 2.0, and 8.0 MK show strong velocity gradients coincident with the Hα ribbons, visible in Big Bear Solar Observatory (BBSO) images. These gradients are perpendicular to and moving with the Hα ribbons. Bright downflowing plasma seems to be prevalent in the regions, between the ribbons and the magnetic neutral line, that coincide with the ends of postflare loops seen with the Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. The plasma on the outer side of the ribbons is less bright in the EUV but shows strong relative blueshifts. This pattern of upflows and downflows demonstrates, for the first time in transition region and coronal lines, the existence of chromospheric evaporation during the late gradual phase of a flare and provides evidence for ongoing reconnection.