Statistical Study of Chromospheric Evaporation in Impulsive Phase of Solar Flares

• Published in: The Astrophysical journal Vol. 871; no. 1; pp. 2 - 14
• Main Authors: Sadykov, Viacheslav M, Kosovichev, Alexander G, Sharykin, Ivan N, Kerr, Graham S
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 20.01.2019; IOP Publishing
• Subjects: Astrophysics; Atmospheric models; Doppler effect; Energy; Energy flux; Evaporation; Fluxes; High energy electrons; Iron; Line spectra; methods: statistical; Radiation; Radiative transfer; Solar atmosphere; Solar flares; Spectroscopy; Sun: chromosphere; Sun: flares; Sun: UV radiation; techniques: spectroscopic; X ray imagery; X ray telescopes
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aaf6b0

We present a statistical study of chromospheric evaporation in solar flares using simultaneous observations by the RHESSI X-ray telescope and the Interface Region Imaging Spectrograph UV spectrograph. The results are compared with radiation hydrodynamic flare models from the F-CHROMA RADYN database. For each event, we study spatially resolved Doppler shifts of spectral lines formed in the transition region (C ii 1334.5 ) and hot coronal plasma (Fe xxi 1354.1 ) to investigate the dynamics of the solar atmosphere during the flare impulsive phase. We estimate the energy fluxes deposited by high-energy electrons using X-ray imaging spectroscopy and assuming the standard thick-target model. Using the RADYN flare models, the RH 1.5D radiative transfer code, and the Chianti atomic line database, we calculate C ii and Fe xxi line profiles and compare with the observations. While the RADYN models predict a correlation between the Doppler shifts and deposited energy flux for both lines, this was only observed in the C ii data. Several quantitative discrepancies are found between the observations and models: the Fe xxi Doppler shifts are substantially stronger in the models than in the data, and the C ii mean blueshifts are absent in the observations but predicted by the models. The transition energies between "gentle" and "explosive" evaporation regimes estimated from the observations ( erg cm−2 s−1) and derived from the models ( erg cm−2 s−1) are comparable with each other. The results illustrate relationships among the processes of chromospheric evaporation, the response of the colder layers, and the flare energy flux deposited by high-energy electrons, although demonstrating discrepancy between analyzed observations and RADYN models.