An observationally constrained model of strong magnetic reconnection in the solar chromosphere

Atmospheric stratification and estimates of heating rates

Institute for Solar Physics, Dept. of Astronomy, Stockholm University, AlbaNova University Centre, 10691 Stockholm, Sweden
e-mail: carlos.diaz@astro.su.se

Received: 10 December 2020
Accepted: 8 February 2021

Abstract

Context. The evolution of the photospheric magnetic field plays a key role in the energy transport into the chromosphere and the corona. In active regions, newly emerging magnetic flux interacts with the pre-existent magnetic field, which can lead to reconnection events that convert magnetic energy into thermal energy.

Aims. We aim to study the heating caused by a strong reconnection event that was triggered by magnetic flux cancelation.

Methods. We use imaging and spectropolarimetric data in the Fe I 6301& 6302 Å, Ca II 8542 Å, and Ca II K spectral lines obtained with the CRISP and CHROMIS instruments at the Swedish 1-m Solar Telescope. These data were inverted with the STiC code by performing multi-atom, multi-line, non-local thermodynamic equilibrium inversions. These inversions yielded a three-dimensional model of the reconnection event and surrounding atmosphere, including temperature, velocity, microturbulence, magnetic field, and radiative loss rate.

Results. The model atmosphere shows the emergence of magnetic loops with a size of several arcseconds into a pre-existing predominantly unipolar field. Where the reconnection region is expected to be, we see an increase in the chromospheric temperature of roughly 2000 K as well as bidirectional flows of the order of 10 km s⁻¹ emanating from there. We see bright blobs of roughly 0.2 arcsec in diameter in the Ca II K, moving at a plane-of-the-sky velocity of the order of 100 km s⁻¹ and a blueshift of 100 km s⁻¹, which we interpret as ejected plasmoids from the same region. This scenario is consistent with theoretical reconnection models, and therefore provides evidence of a reconnection event taking place. The chromospheric radiative losses at the reconnection site are as high as 160 kW m⁻², providing a quantitative constraint on theoretical models that aim to simulate reconnection caused by flux emergence in the chromosphere.