Statistical Study of Chromospheric Evaporation in Impulsive Phase of Solar Flares

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.