Flare kernels may be smaller than you think: modelling the radiative response of chromospheric plasma adjacent to a solar flare

Author:

Osborne Christopher M J1ORCID,Fletcher Lyndsay12ORCID

Affiliation:

1. SUPA School of Physics and Astronomy, Kelvin Building, University of Glasgow , Glasgow G12 8QQ, UK

2. Rosseland Centre for Solar Physics, University of Oslo , PO Box 1029, Blindern, NO-0135 Oslo, Norway

Abstract

ABSTRACT Numerical models of solar flares typically focus on the behaviour of directly heated flare models, adopting magnetic-field-aligned, plane-parallel methodologies. With high spatial- and spectral-resolution ground-based optical observations of flares, it is essential also to understand the response of the plasma surrounding these strongly heated volumes. We investigate the effects of the extreme radiation field produced by a heated column of flare plasma on an adjacent slab of chromospheric plasma, using a two-dimensional radiative transfer model and considering the time-dependent solution to the atomic level populations and electron density throughout this model. The outgoing spectra of H α and Ca ii 854.2 nm synthesized from our slab show significant spatial-, time-, and wavelength-dependent variations (both enhancements and reductions) in the line cores, extending of the order of 1 Mm into the non-flaring slab due to the incident transverse radiation field from the flaring boundary. This may lead to significant overestimates of the sizes of directly heated flare kernels, if line-core observations are used. However, the radiation field alone is insufficient to drive any significant changes in continuum intensity, due to the typical photospheric depths at which they form, so continuum sources will not have an apparent increase in size. We show that the line formation regions near the flaring boundary can be driven upwards in altitude by over 1 Mm despite the primary thermodynamic parameters (other than electron density) being held horizontally uniform. This work shows that in simple models these effects are significant and should be considered further in future flare modelling and interpretation.

Funder

University of Glasgow

College of Science and Engineering

UK Research and Innovation

Science and Technology Facilities Council

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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