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

¹ INAF–Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
e-mail: csasso@oacn.inaf.it
² INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy

Received: 25 May 2015
Accepted: 21 August 2015

Abstract

Aims. We investigate the increase of the differential emission measure (DEM) towards the chromosphere due to small and cool magnetic loops (height ≲8 Mm, T ≲ 10⁵ K). In a previous paper, we analysed the conditions of existence and stability of these loops through hydrodynamic simulations, focussing on their dependence on the details of the optically thin radiative loss function.

Methods. In this paper, we extend those hydrodynamic simulations to verify if this class of loops exists and 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 extreme-UV output of these loops is also calculated and presented.

Results. We find that stable, quasi-static cool loops can be obtained 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 the observed DEM very well.