Hydrogen Balmer line formation in solar flares affected by return currents

TL;DR

This study examines how electric return currents in solar flares influence hydrogen Balmer line profiles, revealing significant core emission increases and shape sensitivities due to nonthermal excitation effects.

Contribution

It introduces a model incorporating return current effects into Balmer line formation, highlighting their impact on line profiles in solar flare conditions.

Findings

Return currents significantly increase Balmer line core emission.

Line shapes are sensitive to beam flux and return-current electron density.

Nonthermal excitation by return currents alters traditional NLTE flare models.

Abstract

Aims. We investigate the effect of the electric return currents in solar flares on the profiles of hydrogen Balmer lines. We consider the monoenergetic approximation for the primary beam and runaway model of the neutralizing return current. Methods. Propagation of the 10 keV electron beam from a coronal reconnection site is considered for the semiempirical chromosphere model F1. We estimate the local number density of return current using two approximations for beam energy fluxes between $4\times 10^{11}$ and $1\times 10^{12} {\rm erg cm^{-2} s^{-1}}$. Inelastic collisions of beam and return-current electrons with hydrogen are included according to their energy distributions, and the hydrogen Balmer line intensities are computed using an NLTE radiative transfer approach. Results. In comparison to traditional NLTE models of solar flares that neglect the return-current effects, we found a significant increase emission in the Balmer line cores due to nonthermal excitation by return current. Contrary to the model without return current, the line shapes are sensitive to a beam flux. It is the result of variation in the return-current energy that is close to the hydrogen excitation thresholds and the density of return-current electrons.