Statistical Study of Chromospheric Evaporation in Impulsive Phase of Solar Flares

Abstract

We present a statistical study of chromospheric evaporation in solar flares using simultaneous observations by the RHESSI X-ray telescope and the IRIS 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\,Å) and hot coronal plasma (Fe\,XXI\,1354.1\,Å) 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; the C\,II mean blueshifts are absent in the observations but predicted by the models. The transition energy between `gentle' and `explosive' evaporation regimes estimated from the observations ($2-8\times 10^9$~erg~cm${}^{-2}$~s${}^{-1}$) and derived from the models ($2.2-10.1\times 10^9$~erg~cm${}^{-2}$~s${}^{-1}$) are comparable with each other. The results illustrate relationships among the processes of chromospheric evaporation, response of the colder layers, and the flare energy flux deposited by high-energy electrons, although demonstrating discrepancy between analyzed observations and RADYN models.

arXiv comment31 page, 7 figures, 3 tables

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• The F-CHROMA grid of 1D RADYN flare models
• Exploring mutual information between IRIS spectral lines. I. Correlations between spectral lines during solar flares and within the quiet Sun
• Bridging High-Density, Electron Beam Coronal Transport and Deep Chromospheric Heating in Stellar Flares
• Exploring mutual information between IRIS spectral lines. II. Calculating the most probable response in all spectral windows
• Response of SDO/HMI observables to heating of the solar atmosphere by precipitating high-energy electrons
• Simultaneous Observations of Chromospheric Evaporation and Condensation during a C-class Flare
• A new view of the solar interface region from the Interface Region Imaging Spectrograph (IRIS)
• Connecting Atmospheric Properties and Synthetic Emission of Shock Waves Using 3D RMHD Simulations of Quiet Sun
• Plasma dynamics in the flaring loop observed by RHESSI
• Observational Evidence Linking Loop Length and Thermal-Nonthermal Peak Timing in Solar Flares