He I 10830 {\AA} Dimming During Solar Flares, I. The Crucial Role of Non-Thermal Collisional Ionisations

TL;DR

This study uses radiation hydrodynamics simulations to demonstrate that non-thermal collisional ionisation is essential for He I 10830 Å line dimming during solar flares, highlighting its diagnostic potential for flare energy transport.

Contribution

The paper provides the first detailed simulation comparison showing non-thermal collisional ionisation is necessary for He I 10830 Å dimming, distinguishing it from photoionisation effects.

Findings

Non-thermal collisional ionisation causes He I 10830 Å dimming.

Dimming is more prominent with higher low-energy cutoff values.

Dimming persists longer in weaker and more gradual flares.

Abstract

While solar flares are predominantly characterised by an intense broadband enhancement to the solar radiative output, certain spectral lines and continua will, in theory, exhibit flare-induced dimmings. Observations of transitions of orthohelium He I $\lambda\lambda 10830${\AA} and the He I D3 lines have shown evidence of such dimming, usually followed by enhanced emission. It has been suggested that non-thermal collisional ionisation of helium by an electron beam, followed by recombinations to orthohelium, is responsible for overpopulating the those levels, leading to stronger absorption. However it has not been possible observationally to preclude the possibility of overpopulating orthohelium via enhanced photoionisation of He I by EUV irradiance from the flaring corona followed by recombinations. Here we present radiation hydrodynamics simulations of non-thermal electron beam-driven flares where (1) both non-thermal collisional ionisation of Helium and coronal irradiance are included, and (2) only coronal irradiance is included. A grid of simulations covering a range of total energies deposited by the electron beam, and a range of non-thermal electron beam low-energy cutoff values, were simulated. In order to obtain flare-induced dimming of the He I 10830{\AA} line it was necessary for non-thermal collisional ionisations to be present. The effect was more prominent in flares with larger low-energy cutoff values and longer lived in weaker flares and flares with a more gradual energy deposition timescale. These results demonstrate the usefulness of orthohelium line emission as a diagnostic of flare energy transport.