Joule Heating and Anomalous Resistivity in the Solar Corona

TL;DR

This paper models Joule heating in the solar corona based on observed coronal currents, finding that classical resistivity estimates are insufficient for significant heating, but instabilities could enhance resistivity and heating.

Contribution

It introduces a model linking observed coronal currents with Joule heating, highlighting the potential role of instabilities in increasing effective resistivity.

Findings

Calculated Joule heating is much lower than theoretical estimates.

Classical resistivity is too low for significant coronal heating.

Current-driven instabilities may enhance resistivity and heating.

Abstract

Recent radioastronomical observations of Faraday rotation in the solar corona can be interpreted as evidence for coronal currents, with values as large as $2.5 \times 10^9$ Amperes (Spangler 2007). These estimates of currents are used to develop a model for Joule heating in the corona. It is assumed that the currents are concentrated in thin current sheets, as suggested by theories of two dimensional magnetohydrodynamic turbulence. The Spitzer result for the resistivity is adopted as a lower limit to the true resistivity. The calculated volumetric heating rate is compared with an independent theoretical estimate by Cranmer et al (2007). This latter estimate accounts for the dynamic and thermodynamic properties of the corona at a heliocentric distance of several solar radii. Our calculated Joule heating rate is less than the Cranmer et al estimate by at least a factor of $3 \times 10^5$. The currents inferred from the observations of Spangler (2007) are not relevant to coronal heating unless the true resistivity is enormously increased relative to the Spitzer value. However, the same model for turbulent current sheets used to calculate the heating rate also gives an electron drift speed which can be comparable to the electron thermal speed, and larger than the ion acoustic speed. It is therefore possible that the coronal current sheets are unstable to current-driven instabilities which produce high levels of waves, enhance the resistivity and thus the heating rate.