IRIS Si iv Line Profiles at Flare Ribbons as Indications of Chromospheric Condensation

Abstract

We present temporal variations of the Si iv  line profiles at the flare ribbons in three solar flares observed by the Interface Region Imaging Spectrograph. In the M1.1 flare on 2014 September 6 and the X1.6 flare on 2014 September 10, the Si iv  line profiles evolve from wholly redshifted to red-wing enhanced with the flare development. However, in the B1.8 flare on 2016 December 2, the Si iv  line profiles are wholly redshifted throughout the flare evolution. We fit the wholly redshifted line profiles with a single-Gaussian function, but fit the red-asymmetric ones with a double-Gaussian function to deduce the corresponding Doppler velocities. In addition, we find that hard X-ray emission above 25 keV shows up in the two large flares, implying a nonthermal electron beam heating. In the microflare, there only appears weak hard X-ray emission up to 12 keV, indicative of a mostly thermal heating. We interpret the redshifts or red asymmetries of the Si iv  line at the ribbons in the three flares as spectral manifestations of chromospheric condensation. We propose that whether the line appears to be wholly redshifted or red-asymmetric depends on the heating mechanisms, as well as on the propagation of the condensation.