In Situ Heating of the 2007 May 19 CME Ejecta Detected by STEREO/PLASTIC and ACE

TL;DR

This study models the heating and evolution of a 2007 CME using a spheromak magnetic field configuration and compares it with in situ measurements from STEREO and ACE, revealing limitations of the model in explaining rapid heating and high charge states.

Contribution

It introduces a spheromak-based model for CME magnetic topology and assesses its ability to reproduce observed ion charge states and heating patterns.

Findings

Spheromak model can explain some heating between 1.1 R_sun and Earth.

Model struggles to account for rapid heating at lower heights.

High Fe charge states outside the flux rope require additional energization mechanisms.

Abstract

In situ measurements of ion charge states can provide unique insight into the heating and evolution of coronal mass ejections when tested against realistic non-equilibrium ionization modeling. In this work we investigate the representation of the CME magnetic field as an expanding spheromak configuration, where the plasma heating is prescribed by the choice of anomalous resistivity and the spheromak dynamics. We chose as a test case, the 19 May 2007 CME observed by STEREO and ACE. The spheromak is an appealing physical model, because the location and degree of heating is fixed by the choice of anomalous resistivity and the spheromak expansion rate which we constrain with observations. This model can provide the heating required between 1.1$R_{\sun}$ and earth orbit to produce charge states observed in the CME flux rope. However this source of heating in the spheromak alone has difficulty accounting for the rapid heating to Fe$^{8 - 11+}$ at lower heights, as observed in STEREO EUVI due to the rapid radiative cooling that occurs at the high densities involved. Episodes of heating and cooling clearly unrelated to spheromak expansion are observed prior to the eruption, and presumably still play a role during the eruption itself. Spheromak heating is also not capable of reproducing the high Fe charge states (Fe$^{16+}$ and higher) seen in situ exterior to the flux rope in this CME. Thus while the spheromak configuration may be a valid model for the magnetic topology, other means of energization are still required to provide much of the rapid heating observed.