Modelling solar low-lying cool loops with optically thick radiative losses

TL;DR

This study extends hydrodynamic simulations of small, cool magnetic loops in the solar atmosphere to include optically thick radiative losses, revealing their stability and contribution to the EUV emission, but with limitations in matching observed DEMs.

Contribution

It demonstrates that stable, quasi-static cool loops can exist with optically thick radiative losses and density-dependent heating, expanding previous models that used optically thin losses.

Findings

Stable cool loops are possible with optically thick losses.

DEM of static loops does not match observations in certain temperature ranges.

Dynamic loop models better reproduce observed DEMs.

Abstract

We investigate the increase of the DEM (differential emission measure) towards the chromosphere due to small and cool magnetic loops (height $\lesssim8$~Mm, $T\lesssim10^5$~K). In a previous paper we analysed the conditions of existence and stability of these loops through hydrodynamic simulations, focusing on their dependence on the details of the optically thin radiative loss function used. In this paper, we extend those hydrodynamic simulations to verify if this class of loops exists and it is stable when using an optically thick radiative loss function. We study two cases: constant background heating and a heating depending on the density. The contribution to the transition region EUV output of these loops is also calculated and presented. We find that stable, quasi-static cool loops can be obtained by using an optically thick radiative loss function and a background heating depending on the density. The DEMs of these loops, however, fail to reproduce the observed DEM for temperatures between $4.6<\log T<4.8$. We also show the transient phase of a dynamic loop obtained by considering constant heating rate and find that its average DEM, interpreted as a set of evolving dynamic loops, reproduces quite well the observed DEM.