Inconsistencies Between Local and Global Measures of CME Radial Expansion as Revealed by Spacecraft Conjunctions

TL;DR

This study compares local and global measures of CME expansion using spacecraft data, revealing they are not directly related and highlighting the influence of magnetic pressure and solar wind on CME evolution.

Contribution

It provides the first direct comparison of local and global CME expansion measures and investigates how CME properties influence their expansion behavior.

Findings

Local and global expansion measures are not strongly correlated.

CME expansion depends on initial magnetic field strength inside the ME.

Expansion is driven by magnetic pressure near the Sun and solar wind pressure at 1 au.

Abstract

The radial expansion of coronal mass ejections (CMEs) is known to occur from remote observations; from the variation of their properties with radial distance; and from local in situ plasma measurements showing a decreasing speed profile throughout the magnetic ejecta (ME). However, little is known on how local measurements compare to global measurements of expansion. Here, we present results from the analysis of 42 CMEs measured in the inner heliosphere by two spacecraft in radial conjunction. The magnetic field decrease with distance provides a measure of their global expansion. Near 1 au, the decrease in their bulk speed provides a measure of their local expansion. We find that these two measures have little relation with each other. We also investigate the relation between characteristics of CME expansion and CME properties. We find that the expansion depends on the initial magnetic field strength inside the ME, but not significantly on the magnetic field inside the ME measured near 1 au. This is an indirect evidence that CME expansion in the innermost heliosphere is driven by the high magnetic pressure inside the ME, while by the time the MEs reach 1 au, they are expanding due to the decrease in the solar wind dynamic pressure with distance. We also determine the evolution of the ME tangential and normal magnetic field components with distance, revealing significant deviations as compared to the expectations from force-free field configurations as well as some evidence that the front half of MEs expand at a faster rate than the back half.