CME propagation: Where does solar wind drag take over?

TL;DR

This study analyzes how solar wind drag affects CME trajectories, finding it explains fast CME dynamics but not slower ones, which often propagate at constant speeds beyond 15-50 solar radii, challenging previous assumptions.

Contribution

It provides new insights into the height-dependent influence of solar wind drag on CME propagation, especially for slow CMEs.

Findings

Solar wind drag explains fast CME dynamics.

Slow CMEs often propagate at constant speeds beyond 15-50 R_sun.

Drag models below 15-50 R_sun cannot fully account for slow CME trajectories.

Abstract

We investigate the Sun-Earth dynamics of a set of eight well observed solar coronal mass ejections (CMEs) using data from the STEREO spacecraft. We seek to quantify the extent to which momentum coupling between these CMEs and the ambient solar wind (i.e., the aerodynamic drag) influences their dynamics. To this end, we use results from a 3D flux rope model fit to the CME data. We find that solar wind aerodynamic drag adequately accounts for the dynamics of the fastest CME in our sample. For the relatively slower CMEs, we find that drag-based models initiated below heliocentric distances ranging from 15 to 50 $R_{\odot}$ cannot account for the observed CME trajectories. This is at variance with the general perception that the dynamics of slow CMEs are influenced primarily by solar wind drag from a few $R_{\odot}$ onwards. Several slow CMEs propagate at roughly constant speeds above 15--50 $R_{\odot}$. Drag-based models initiated above these heights therefore require negligible aerodynamic drag to explain their observed trajectories.