Investigating the local and global expansion of ICMEs using multi-spacecraft observations

Multi-spacecraft observations of interplanetary coronal mass ejections (ICMEs) across varying longitudinal and radial separations provide valuable insights into their general properties, expansion, and interactions with the solar wind environment during propagation. By tracking the properties of individual events, we often find significant variability compared to average trends. The global expansion is well determined by measurements of the magnetic field strength with increasing heliocentric distance, however, determining the local expansion requires measurements of the solar wind plasma speed, mostly only available at 1 au prior to the launch of Parker Solar Probe and Solar Orbiter.  

Previous studies have found weak correlations between global and local expansion measures. In this study, we use the HELIO4CAST lineup catalogue (https://helioforecast.space/lineups) which includes ICMEs observed by Parker Solar Probe, Solar Orbiter, BepiColombo, STEREO A, and Wind. We investigate the local expansion of ICMEs measured at spacecraft in the inner heliosphere and 1 au for individual events, comparing these to the global expansion rate. We present examples of events that follow previously determined relationships and those that deviate, including events where there are discrepancies between local expansion at different spacecraft, demonstrating the limitations of such measurements for constraining space weather forecasts.