Formation Of The Lyman Continuum During Solar Flares

TL;DR

This study models the formation and evolution of the Lyman Continuum during solar flares, revealing how non-thermal electrons influence chromospheric conditions and LyC spectral features, with implications for solar observations.

Contribution

It provides the first detailed radiative hydrodynamic modeling of LyC formation during flares, linking spectral responses to non-thermal electron energy deposition.

Findings

LyC intensity increases by 4-5.5 orders of magnitude during flares.

The departure coefficient $b_1$ decreases, indicating stronger local coupling.

The color temperature $T_c$ rises to 10-16 kK, matching electron temperature at minimum $b_1$.

Abstract

The Lyman Continuum (LyC; $<911.12$\AA) forms at the top of the chromosphere in the quiet-Sun, making LyC a powerful tool for probing the chromospheric plasma during solar flares. To understand the effects of non-thermal energy deposition in the chromosphere during flares, we analysed LyC profiles from a grid of field-aligned radiative hydrodynamic models generated using the RADYN code as part of the F-CHROMA project. The spectral response of LyC, the temporal evolution of the departure coefficient of hydrogen, $b_1$, and the color temperature, $T_c$, in response to a range of non-thermal electron distribution functions, were investigated. The LyC intensity was seen to increase by 4-5.5 orders of magnitude during solar flares, responding most strongly to the non-thermal electron flux of the beam. Generally, $b_1$ decreased from $10^2$-$10^3$ to closer to unity during solar flares, indicating a stronger coupling to local conditions, while $T_c$ increased from $8$-$9$kK to $10$-$16$kK. $T_c$ was found to be approximately equal to the electron temperature of the plasma when $b_1$ was at a minimum. Both optically thick and optically thin components of LyC were found in agreement with the interpretation of recent observations. The optically thick layer forms deeper in the chromosphere during a flare compared to quiescent periods, whereas the optically thin layers form at higher altitudes due to chromospheric evaporation, in low-temperature, high-density regions propagating upwards. We put these results in the context of current and future missions.