Statistical Study of Chromospheric Evaporation in Impulsive Phase of Solar Flares

TL;DR

This study compares observations of chromospheric evaporation during solar flares with advanced models, revealing both consistencies and discrepancies in plasma dynamics and energy flux predictions.

Contribution

It provides a comprehensive statistical analysis combining high-resolution observations with radiation hydrodynamic models to understand flare-driven chromospheric evaporation.

Findings

Models predict stronger Fe XXI Doppler shifts than observed.

Observed C II blueshifts are absent, contrary to model predictions.

Estimated energy fluxes for transition between evaporation regimes are consistent between data and models.

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\,{\AA}) and hot coronal plasma (Fe\,XXI\,1354.1\,{\AA}) 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.