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 (IRIS). 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 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 thermal heating mostly. 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 and also on the propagation of the condensation.

26 pages, 13 figures, accepted by the Astrophysical Journal

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• Spectroscopic observations of a flare-related coronal jet
• Diagnosing the Optically Thick/Thin Features Using the Intensity Ratio of Si IV Resonance Lines in Solar Flares
• Simultaneous Observations of Chromospheric Evaporation and Condensation during a C-class Flare
• The Transition Region of Solar Flare Loops
• Imaging and Spectroscopic Observations of the Dynamic Processes in Limb Solar Flares
• A new view of the solar interface region from the Interface Region Imaging Spectrograph (IRIS)
• Spectroscopic Case Studies of Four Long-duration Transition-region Explosive Events
• Revisiting the Spectral Features of Ellerman Bombs and UV Bursts. I. Radiative Hydrodynamic Simulations