Observations and simulations of the Na I D1 line profiles in an M-class solar flare

TL;DR

This study combines high-resolution observations and radiative hydrodynamic simulations to analyze the Na I D1 line profiles during an M-class solar flare, revealing how electron beam heating affects line emission and asymmetries.

Contribution

It provides the first detailed comparison of observed and simulated Na I D1 line profiles in a solar flare, elucidating the effects of electron beam heating on line formation.

Findings

Na I D1 line intensity increases during the flare.

Line profiles show a central reversal with blue wing asymmetry.

Simulations match observed profiles, showing heating causes emission reversal.

Abstract

We study the temporal evolution of the Na I D1 line profiles in the M3.9 flare SOL2014-06-11T21:03 UT, using high spectral resolution observations obtained with the IBIS instrument on the Dunn Solar Telescope combined with radiative hydrodynamic simulations. Our results show a significant increase in line core and wing intensities during the flare. The analysis of the line profiles from the flare ribbons reveal that the Na I D1 line has a central reversal with excess emission in the blue wing (blue asymmetry). We combine RADYN and RH simulations to synthesise Na I D1 line profiles of the flaring atmosphere and find good agreement with the observations. Heating with a beam of electrons modifies the radiation field in the flaring atmosphere and excites electrons from the ground state $\mathrm{3s~^2S}$ to the first excited state $\mathrm{3p~^2P}$, which in turn modifies relative population of the two states. The change in temperature and the population density of the energy states make the sodium line profile revert from absorption into emission. Analysis of the simulated spectra also reveals that the Na I D1 flare profile asymmetries are produced by the velocity gradients generated %and opacity effects in the lower solar atmosphere.