Observations and modelling of Helium lines in solar flares

TL;DR

This study uses radiative hydrodynamic simulations to analyze how helium line intensities respond to electron beam heating in solar flares, revealing their sensitivity to energy flux and implications for flare diagnostics.

Contribution

The paper demonstrates the sensitivity of helium line intensities to electron beam heating parameters and compares simulation results with observations, highlighting the impact of heating rate on helium line ratios.

Findings

He II 304 Å line is more sensitive to chromospheric heating than He I 584 Å.

He line ratio increases with larger heating rates in simulations and observations.

Spectral index of electrons influences the He ratio in simulations but not in observations.

Abstract

We explore the response of the He II 304 {\AA} and He I 584 {\AA} line intensities to electron beam heating in solar flares using radiative hydrodynamic simulations. Comparing different electron beams parameters, we found that the intensities of both He lines are very sensitive to the energy flux deposited in the chromosphere, or more specifically to the heating rate, with He II 304 {\AA} being more sensitive to the heating than He I 584 {\AA}. Therefore, the He line ratio increases for larger heating rates in the chromosphere. A similar trend is found in observations, using SDO/EVE He irradiance ratios and estimates of the electron beam energy rate obtained from hard X-ray data. From the simulations, we also found that spectral index of the electrons can affect the He ratio but a similar effect was not found in the observations.