Thermal Conduction and Electron Heating in Solar Coronal Mass Ejections

Abstract

Massive eruptions of plasma and magnetic fields from the solar corona, called Coronal Mass Ejections (CMEs), are significant drivers of space weather phenomena and can cause extreme geomagnetic storms if Earth-directed. Therefore, reliable estimates of CME arrival times and a thorough understanding of CME dynamics are crucial for space weather forecasting. If CMEs were to expand adiabatically, their temperature at 1 AU would be about a few degrees of kelvin. However, the observed proton temperatures are as high as $\approx 10^5$ K. This discrepancy suggests that either there is sufficient thermal conduction from the Sun to the CME interior or substantial plasma heating happening inside the CMEs. We examine the first possibility in this work by considering electron thermal conduction. We have computed the CME propagation velocities and electron thermal conduction front velocities for a collection of 38 Earth-directed CMEs using semi-empirical models, remote sensing images from SOHO/LASCO and STEREO/SECCHI coronagraphs, as well as in situ data from WIND spacecraft. The conduction velocities are estimated for purely Spitzer thermal conductivity and two different types of turbulence-modified anomalous thermal conductivities. Comparison between the CME propagation and conduction velocities shows that thermal conduction is much faster than CME propagation for Spitzer conductivity, while it is less fast for Kolmogorov turbulence-modified conductivity. The two speeds become comparable for conductivity modified by Kraichnan turbulence. These results are consistent across all 38 events. This seems to imply that thermal conduction is sufficient to explain the high electron temperature in the CME interior, and thus CME expansion can be modelled as nearly isothermal. However, thermal conduction cannot be an explanation for protons, since it is very inefficient. We have further calculated the heating rate of electron-proton equilibration and found that it is quite small. Our results, therefore, justify the need for invoking additional heating mechanisms (such as turbulent heating) for protons.