Heating of the magnetic chromosphere: observational constraints from Ca II 8542 spectra

TL;DR

This study uses high-resolution spectropolarimetric data to identify raised-core Ca II 8542 spectral profiles as indicators of localized heating in the Sun's magnetic chromosphere, providing new observational constraints for heating models.

Contribution

It reports the first observational evidence linking raised-core spectral profiles to elevated temperature canopies caused by magnetic field expansion and ohmic dissipation.

Findings

Raised-core profiles are abundant near magnetic bright points and plage.

These profiles indicate locations of increased chromospheric heating.

Comparison with simulations supports the association with magnetic field expansion and ohmic dissipation.

Abstract

The heating of the Sun's chromosphere remains poorly understood. While progress has been made on understanding what drives the quiet Sun internetwork chromosphere, chromospheric heating in strong magnetic field regions continues to present a difficult challenge, mostly because of a lack of observational constraints. We use high-resolution spectropolarimetric data from the Swedish 1-m Solar Telescope to identify the location and spatio-temporal properties of heating in the magnetic chromosphere. In particular, we report the existence of raised-core spectral line profiles in the Ca II 8542 line. These profiles are characterized by the absence of an absorption line core, showing a quasi-flat profile between +/- 0.5 {\AA}, and are abundant close to magnetic bright-points and plage. Comparison with 3D MHD simulations indicates that such profiles occur when the line-of-sight goes through an "elevated temperature canopy" associated with the expansion with height of the magnetic field of flux concentrations. This temperature canopy in the simulations is caused by ohmic dissipation where there are strong magnetic field gradients. The raised-core profiles are thus indicators of locations of increased chromospheric heating. We characterize the location and temporal and spatial properties of such profiles in our observations, thus providing much stricter constraints on theoretical models of chromospheric heating mechanisms than before.