Magneto-Thermal Coupling and Coronal Heating in Solar Active Regions Inferred from Microwave Observations

TL;DR

This study uses microwave observations to analyze coronal heating mechanisms in solar active regions, favoring steady or high-frequency heating models and revealing scaling laws for magnetic field and heating rate dependencies.

Contribution

It introduces a novel analysis combining microwave observations with synthetic modeling to constrain coronal heating mechanisms and parameters.

Findings

Wave transmission or reflection models best fit the observations.

Coronal magnetic field strength decreases with loop length following a -0.55 power law.

Volumetric heating rate weakly depends on magnetic field and decreases with loop length.

Abstract

The solar corona is much hotter than the photosphere and chromosphere, but the physical mechanism responsible for heating the coronal plasma remains unidentified yet. The thermal microwave emission, which is produced in strong magnetic field above sunspots, is a promising but barely exploited tool for studying the coronal magnetic field and plasma. We analyzed the microwave observations of eight solar active regions obtained with the Siberian Radioheliograph in years 2022-2024 in the frequency range of 6-12 GHz. We produced synthetic microwave images based on various coronal heating models, and determined the model parameters that provided the best agreement with the observations. The observations and simulations strongly favour either a steady-state (continuous) plasma heating process, or high-frequency heating by small energy release events with a short cadence. The average magnetic field strength in a coronal loop was found to decrease with the loop length, following a scaling law with the most probable index of about -0.55. In the majority of cases, the estimated volumetric heating rate was weakly dependent on the magnetic field strength, and decreased with the coronal loop length following a scaling law with the index of about -2.5. Among the known theoretical heating mechanisms, the model based on wave transmission or reflection in coronal loops acting as resonance cavities was found to provide the best agreement with the observations. The obtained results did not demonstrate a significant dependence on the emission frequency in the considered range.