Net radiative cooling rates and partial ionization in cool coronal condensations

¹ Astronomical Institute, The Czech Academy of Sciences, 251 65 Ondřejov, Czech Republic
² Center of Excellence ‘Solar and Stellar Activity’, University of Wroclaw, Wroclaw, Poland
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany

^⋆ Corresponding author: gunar@asu.cas.cz

Received: 27 January 2025
Accepted: 18 May 2025

Abstract

Aims. We provide tabulated net radiative cooling rates (NRCRs) in the plasma of cool coronal condensations, together with the electron densities and ionization degrees describing its partial ionization. These readily applicable rates result from combined effects of the radiative cooling and radiative heating in the dominant atomic transitions in hydrogen, Mg II , and Ca II .

Methods. These NRCRs represent realistic estimates based on 1D non-LTE (i.e. departures from the local thermodynamic equilibrium) radiative transfer modelling that uses 1D isothermal and isobaric prominence models. To construct easy-to-use NRCR tables, we employed the concept of voxels (volume pixels), which allowed us to incorporate the essential information about the location of the modelled plasma with respect to the source of illumination. We provide tabulated values of NRCRs, electron densities, and ionization degrees for a broad range of plasma parameters representing a wide variety of cool coronal condensations, such as prominences, cool coronal loops, spicules, jets, and coronal rain.

Results. The accuracy of the provided voxel-based NRCRs, when tested against non-LTE calculations, is high, often showing a difference of only a few per cent. However, in some cases, the differences increase by up to a factor of two, which is the consequence of the assumptions and simplifications adopted here. Despite such differences, the voxel-based NRCRs that incorporate both the optically thick and thin radiative processes are a significant improvement, compared to the optically thin radiative loss formulas, when dealing with the cool plasmas at temperatures below 30 000 K.

Conclusions. The provided NRCRs are tabulated for three different scenarios of the orientation of the modelled structure with respect to the source of illumination: (i) a vertical orientation where a surface receives illumination from one half of the solar disc, (ii) a horizontal orientation where the bottom surface receives illumination from the entire solar disc, and (iii) a horizontal orientation where the top surface does not receive any illumination from the solar disc. This allows the voxel-based NRCRs to be implemented even in complex multi-dimensional simulations of cool coronal condensations.