Coronal rain formation in a two-fluid approximation

¹ Centre for mathematical Plasma Astrophysics, KU Leuven, 3001 Leuven, Belgium
² Department of Physics and Technology, University of Bergen, Norway

^⋆ Corresponding author: beatrice.braileanu@uib.no

Received: 23 March 2025
Accepted: 6 May 2025

Abstract

Context. Coronal rain results from thermal instability in the solar corona, where runaway in situ cooling causes plasma to condense and drain along the magnetic lines. Coronal rain is observed in 3D spine-fan magnetic configurations. The reconnection of the magnetic field lines around the null point creates jets, seen as denser structures traveling along the field lines. As these dense regions evolve, thermal instability can set in and ultimately form coronal rain.

Aims. In this paper we study the importance of partial ionization effects in the formation of coronal rain during the late evolution of 3D spine-fan magnetic reconnection in the solar corona.

Methods. We used a two-fluid model consisting of neutral and charged particles coupled by collisions, whereby ionization recombination processes were taken into account. The 3D magnetic field configuration consists of two spine-fan structures and associated null points. We used localized resistivity around the nulls to induce reconnection by driving the bottom boundary below one of the two null points. To trigger the thermal instability, we investigate here how magnetic reconnection generates flows that lead to the accumulation of higher-density structures along magnetic field lines.

Results. The dynamics associated with the spine-fan magnetic reconnection produces current sheets around the null point and flows along the field lines. Blobs similar to coronal rain start to appear after 400 seconds in the simulation domain, and follow the field lines from the direction of the perturbed null point. The temperature drop is accompanied by the recombination of charged particles. This leads to a drastic increase in the neutral density contrast within the rain blobs, while charges demonstrate the typical two orders of magnitude increase in density that is found in single fluid settings.

Conclusions. Recombination effects become important in coronal rain evolution when the temperature drops considerably in the condensed structures. The neutrals are slowed down by recombination, producing a decoupling in velocity at the size of the blob, but inside the condensing structure the neutrals can move faster across the field lines, creating small-scale structures. The incomplete elastic collisional coupling produces decoupling in velocity, and, associated with this, more efficient frictional heating for the neutrals, followed by a decoupling in temperature. This study presents a novel two-fluid approach to coronal rain, showing that incorporating two-fluid effects is essential to accurately capture its dynamics.