Chromosphere of the quiet sun -- I. Shock and current-sheet dynamics and heating

TL;DR

This study uses advanced simulations to quantify the roles of shocks and current sheets in heating the quiet Sun's chromosphere, revealing shocks dominate lower layers while current sheets are key in upper regions.

Contribution

It provides a detailed analysis of shock and current sheet contributions to chromospheric heating using physics-based criteria in radiation-magnetohydrodynamics simulations.

Findings

Shocks contribute up to 59% of mechanical heating in the lower chromosphere.

Current sheets become dominant in the upper chromosphere as plasma conditions change.

Overall, shocks and current sheets account for 66% of the chromospheric heating.

Abstract

The solar chromosphere is a crucial interface between the solar interior and its interplanetary environment, regulating how energy is locally deposited into heat and transported into the upper atmospheric layers. Despite significant progress, the dominant processes responsible for chromospheric heating remain debated, particularly under quiet-Sun (QS) conditions. We aim to disentangle and quantify the respective roles of shocks and current sheets (CS) in QS chromospheric modeling. We use a simulation performed with the radiation-magnetohydrodynamics code Bifrost. In order to identify shocks and CS events across space and time, we develop and apply physics-based criteria, allowing us to describe their dynamics and evaluate their contributions to both dissipative (viscous and ohmic) and mechanical (including compressive work) heating. Shocks are found to dominate the energy deposition in the lower chromosphere (up to $59\%$ of the mechanical heating), while CS become the primary contributor in the upper chromosphere, as both plasma $\beta$ and Mach number $Ma$ drop. Overall, $66\%$ of the mechanical chromospheric heating is powered by the combined action of shocks and CS. These results support a multi-process view of the chromospheric heating in the QS, dominated by shocks, CS, and non-steep gradient dynamics. In addition to viscous and ohmic dissipation, compressive heating can play a major role locally in the model, particularly in chromospheric shock structures, where it offsets non-reversibly cooling from expansion and radiation, and therefore constitutes a key heating contribution to consider in the energy budget. This study further highlights the need for next-generation observations to resolve the intermittent and small-scale nature of chromospheric dynamics, in order to bring new constraints on the coupling between the different layers of the solar atmosphere.