Coronal energy release by MHD avalanches III. Identification of a reconnection outflow from a nanoflare

Abstract

Outflows perpendicular to the guide field are believed to be a possible signature of magnetic reconnection in the solar corona and specifically a way to detect the occurrence of ubiquitous small-angle magnetic reconnection. The aim of this work is to identify possible diagnostic techniques of such outflows in hot coronal loops with SDO/AIA and the forthcoming MUltislit Solar Explorer (MUSE), in a realistically dynamic coronal loop environment in which an MHD avalanche is occurring. We consider 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 synthesized its emission as detectable with AIA and MUSE. The outflow has a maximum temperature around 8 MK, a total energy of 1024 erg, a velocity of a few hundred km/s, and a duration of less than 1 min. We show the emission in the AIA 94 A channel (Fe XVIII line) and in the MUSE 108 A Fe XIX spectral line. his outflow shares many features with nanojets recently detected at lower temperatures. Its low emission measure makes, however, its detection difficult with AIA, but Doppler shifts can be measured with MUSE. Conditions become different in a later steady state phase when the flux tubes are filled with denser and relatively cooler plasma.