Thermal and kinetic coronal rain diagnostics with Mg II h & k lines^⋆

¹ Departament de Física, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain
e-mail: matheus.akriginsky@uib.es
² Institute of Applied Computing & Community Code (IAC3), UIB, Spain

Received: 31 October 2023
Accepted: 20 January 2024

Abstract

Aims. The aim of this work is to assess the temperature and velocity diagnostics of coronal rain clumps, as observed in the lines formed in the upper chromosphere.

Methods. We performed the temperature diagnostics using inversions of data from nine spectroscopic observations obtained with the IRIS spectrograph in the Mg II h & k lines. The sensitivity to the temperature of the emission peaks of these lines was exploited to determine the temperature of the coronal rain plasma using inversions of the spectroscopic profiles. Additional relationships between different spectral features of these lines, derived from the use of 3D radiative transfer line synthesis applied to simulations, were employed in order to derive the line-of-sight (LoS) velocities in different parts of the coronal rain plasma.

Results. For the first time, spectroscopic inversions of coronal rain were successfully performed. Temperatures derived from the inversions yield coronal rain clump temperatures at the formation height of the emission peaks of the Mg II h & k lines in the range between 5000 and 7000 K. This narrow range of values remains consistent among all the different observations used in this work. We obtained LoS velocities of up to 40 km s⁻¹, which are consistent with the motion of the plasma being mostly constrained to the plane of the sky, as the coronal rain was mostly detected shortly after its formation and the observations took place in the disc. Furthermore, velocity diagnostics led to the detection of larger velocities at higher layers of the coronal rain plasma in some cases. This increased velocity seems to indicate that at some point (at least) during the fall of coronal rain clumps towards the chromosphere, the material in the upper part of the coronal rain plasma is falling with greater velocity than the material below it. The conditions of the temperature and density of the coronal rain plasma where the Mg II h line forms appear to be slightly different that those of the Mg II k line, with the former found at slightly colder and denser parts of the plasma.