Magnetic fields beneath active region coronal loops

TL;DR

This study investigates whether multipolar magnetic fields contribute to heating in active region coronal loops, concluding that unipolar fields are primarily responsible for heating, not the minor multipolar fields.

Contribution

The paper provides observational evidence that multipolar magnetic fields do not significantly contribute to coronal heating in active regions, challenging previous hypotheses.

Findings

Minor polarity fields are too weak and distant to influence coronal footpoints.

Reconnection between multipolar fields does not produce observable dynamical effects.

Unipolar fields are the main source of heating in active region coronal loops.

Abstract

We examine the hypothesis that multipolar magnetic fields advected by photospheric granules can contribute heating to the active chromosphere and corona. On 28 September 2020 the GRIS and HiFI+ instruments at the GREGOR telescope obtained data of NOAA 12773. We analyze Stokes profiles of spectral lines of Si I and He I, to study magnetic fields from photosphere to the upper chromosphere. Magnetogram and EUV data from the HMI and AIA instruments on the SDO spacecraft are co-aligned and studied in relation to the GRIS data. At coronal loop footpoints, minor polarity fields comprise just 0.2% and 0.02% of the flux measured over the 40" x 60" area observed in the photosphere and upper chromosphere, centered 320" from disk center. Significantly, the minority fields are situated >~ 12" from bright footpoints. We use physical arguments to show that any unresolved minority flux cannot reach coronal footpoints adjacent to the upper chromosphere. Even if it did, the most optimistic estimate of the energy released through chromospheric reconnection is barely sufficient to account for the coronal energy losses. Further, dynamical changes accompanying reconnection between uni- and multi- polar fields are seen neither in the He I data nor in narrow-band movies of the H alpha line core. We conclude that the hypothesis must be rejected. Bright chromospheric, transition region and coronal loop plasmas must be heated by mechanisms involving unipolar fields.