Measurements of Coronal and Chromospheric Magnetic Fields using Polarization Observations by the Nobeyama Radioheliograph

TL;DR

This study demonstrates how polarization observations from the Nobeyama Radioheliograph can be used to measure magnetic fields in the solar corona and chromosphere, providing insights into active region magnetic structures.

Contribution

It introduces a method to derive coronal and chromospheric magnetic fields from polarization and spectral data using radio observations, validated by comparison with ultraviolet and magnetic field data.

Findings

Coronal magnetic field around 70 G was measured at active region edges.

Circular polarization degree ranged from 0.5% to 1.7%.

Derived magnetic fields are 20-50% of photospheric fields.

Abstract

Coronal and chromospheric magnetic fields are derived from polarization and spectral observations of the thermal free-free emission using the Nobeyama Radioheliograph (NoRH). In magnetized plasma, the ordinary and extraordinary modes of free-free emission have different optical depths. This creates a circularly polarized component in an atmosphere with a temperature gradient. We observed an active region on April 13, 2012 to derive its coronal and chromospheric magnetic fields. The observed degree of circular polarization was between 0.5 % and 1.7 %. The radio circular polarization images were compared with ultraviolet images observed by the Atmospheric Imaging Assembly and the photospheric magnetic field observed by the Helioseismic and Magnetic Imager, both on board the Solar Dynamic Observatory. At the edge of the active region, the radio circular polarization was emitted mainly from coronal loops, and the coronal magnetic field was derived to be about 70 G. At the center of the active region, the chromospheric and coronal components cannot be separated. The derived magnetic field is about 20 % to 50 % of the corresponding photospheric magnetic field, which is an emission-measure-weighted average of the coronal and chromospheric magnetic fields.