Coronal energy release by MHD avalanches

III. Identification of a reconnection outflow from a nanoflare

¹ Dipartimento di Fisica & Chimica, Università di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy
² INAF-Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy
³ Harvard–Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02193, USA
⁴ Lockheed Martin Solar & Astrophysics Laboratory, 3251 Hanover St, Palo Alto, CA 94304, USA
⁵ Rosseland Centre for Solar Physics, University of Oslo, P.O. Box 1029 Blindern N-0315 Oslo, Norway
⁶ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern N-0315 Oslo, Norway
⁷ SETI Institute, 339 Bernardo Ave, Suite 200, Mountain View, CA 94043, USA

^⋆ Corresponding author; gabriele.cozzo@unipa.it

Received: 30 September 2024
Accepted: 2 February 2025

Abstract

Context. Outflows perpendicular to the guide field are believed to be a possible signature of magnetic reconnection in the solar corona. Specifically, outflows can help detect the occurrence of ubiquitous small-angle magnetic reconnection.

Aims. The aim of this work is to identify possible diagnostic techniques of such outflows in hot coronal loops with the Atmospheric Image Assembly (AIA) on board the Solar Dynamics Observatory and the forthcoming MUltislit Solar Explorer (MUSE), in a realistically dynamic coronal loop environment where a magnetohydrodynamic (MHD) avalanche is occurring.

Methods. We considered a 3D MHD model of two magnetic flux tubes, including a stratified, radiative, and thermal-conducting atmosphere, twisted by footpoint rotation. The faster rotating flux tube becomes kink-unstable and soon involves the other one in the avalanche. The turbulent decay of this magnetic structure on a global scale leads to the formation, fragmentation, and dissipation of current sheets, driving impulsive heating akin to a nanoflare storm. We captured a clear outflow from a reconnection episode soon after the initial avalanche and synthesised its emission as detectable with AIA and MUSE.

Results. The outflow has a maximum temperature around 8 MK, total energy of 10²⁴ erg, velocity of a few hundred km/s, and duration of less than 1 min. We show the emission in the AIA 94 Å channel (Fe XVIII line) and in the MUSE 108 Å Fe XIX spectral line.

Conclusions. This outflow shares many features with nanojets recently detected at lower temperatures. However, its low emission measure makes its detection difficult with AIA, while Doppler shifts can be measured with MUSE. Conditions become different in the later steady-state phase, when the flux tubes are filled with denser and relatively cooler plasma.