Formation of the Lyman continuum during solar flares

McLaughlin, S. A., Milligan, R. O. , Kerr, G. S., Monson, A. J., Simões, P. J. A. and Mathioudakis, M. (2023) Formation of the Lyman continuum during solar flares. Astrophysical Journal, 944(2), 186. (doi: 10.3847/1538-4357/acaf66)

Abstract

The Lyman continuum (LyC; <911.12 Å) 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 nonthermal energy deposition in the chromosphere during flares, we analyzed 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, b1, and the color temperature, Tc, in response to a range of nonthermal 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 nonthermal electron flux of the beam. Generally, b1 decreased from 102–103 to closer to unity during solar flares, indicating a stronger coupling to local conditions, while Tc increased from 8–9 to 10–16 kK. Tc was found to be approximately equal to the electron temperature of the plasma when b1 was at a minimum. Both optically thick and optically thin components of LyC were found to be 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 upward. We put these results in the context of current and future missions.

• Item Type: Articles
• Additional Information: S.A.M. would like to thank the Science and Technology Facilities Council (UK) for the award of a PGR studentship. A.J.M. acknowledges funding from the Science Technology Funding Council (STFC) Grant Code ST/T506369/1. R.O.M. and G.S. K. would like to acknowledge support from NASA Heliophysics Supporting Research grant NNH19ZDA001N and R.O.M. thanks the Science and Technologies Facilities Council (UK) for the award of an Ernest Rutherford Fellowship (ST/ N004981/2). P.J.A.S. acknowledges support from the Fundo de Pesquisa Mackenzie (MackPesquisa), CNPq (contract 307612/2019-8), and FAPESP, the Sao Paulo Research Foundation (contract 2013/24155-3).
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Kerr, Graham and Simoes, Dr Paulo and Milligan, Dr Ryan
• Authors: McLaughlin, S. A., Milligan, R. O., Kerr, G. S., Monson, A. J., Simões, P. J. A., and Mathioudakis, M.
• College/School: College of Science and Engineering > School of Physics and Astronomy
• Journal Name: Astrophysical Journal
• Publisher: IOP Publishing
• ISSN: 0004-637X
• ISSN (Online): 1538-4357
• Published Online: 23 February 2023
• Copyright Holders: Copyright © 2023 The Authors
• First Published: First published in Astrophysical Journal 944(2): 186
• Publisher Policy: Reproduced under a Creative Commons License