The role of plasma heating and expansion in the energetics of solar coronal mass ejections

Abstract

Coronal Mass Ejections (CMEs) are bodily expulsions of hot plasma and magnetic fields from the solar corona. Such ejections, which are often directed towards the Earth cause geomagnetic storms, which substantially affect technologies we use on a routine basis. This has prompted extensive investigations of the manner in which CMEs are initiated in the solar corona and the manner in which they propagate through the heliosphere. However, there has not been as much attention devoted to the energy expended in expanding and heating the CME as it propagates. These are crucial issues, the answers to which can substantially impact our understanding of CME dynamics.

Were the CMEs to expand adiabatically from near the sun to the earth, then their temperature would be no more than a few degrees of Kelvin. But the observed temperatures of CME plasma intercepted near the Earth are around a hundred thousand kelvin. Furthermore, our understanding of laboratory tokamak plasmas suggests that expanding magnetic flux ropes (such as CMEs) should contract in cross-section, whereas CMEs are observed to expand. These suggest that our understanding of CME driving and thermodynamics is far from complete.

We examine a well-studied sample of CMEs that have been tracked from their origin at the Sun to their interception at the Earth. We seek to reconcile their propagation and expansion profiles in the context of a flux rope model that accounts for Lorentz forces as well as gas pressure. We obtain observationally motivated constraints on the evolution of temperature, polytropic index and plasma beta of these CMEs. These results can also allow us to investigate how turbulent fluctuations inside the CME are possibly dissipated. Taken together, our results are expected to provide inputs to an operational model of CME propagation from the Sun to the Earth.