A Coronal Mass Ejection and Magnetic Ejecta Observed In Situ by STEREO-A and Wind at 55$^\circ$ Angular Separation

TL;DR

This study analyzes a wide coronal mass ejection observed in situ by two spacecraft separated by 55°, revealing insights into its shape, propagation, and interaction with high-speed streams, challenging some common expectations about CME-HSS interactions.

Contribution

It provides one of the widest multi-spacecraft CME detections, offering new insights into CME shape, extent, and interactions with high-speed streams.

Findings

CME observed at both spacecraft with consistent magnetic ejecta signatures

Differences in ME duration and electron pitch-angle data challenge common models

CME associated with a wide filament eruption and propagating between spacecraft

Abstract

We present an analysis of {\it in situ} and remote-sensing measurements of a coronal mass ejection (CME) that erupted on 2021 February 20 and impacted both the Solar TErrestrial RElations Observatory (STEREO)-A and the {\it Wind} spacecraft, which were separated longitudinally by 55$^\circ$. Measurements on 2021 February 24 at both spacecraft are consistent with the passage of a magnetic ejecta (ME), making this one of the widest reported multi-spacecraft ME detections. The CME is associated with a low-inclined and wide filament eruption from the Sun's southern hemisphere, which propagates between STEREO-A and {\it Wind} around E34. At STEREO-A, the measurements indicate the passage of a moderately fast ($\sim 425$~km\,s$^{-1}$) shock-driving ME, occurring 2--3 days after the end of a high speed stream (HSS). At {\it Wind}, the measurements show a faster ($\sim 490$~km\,s$^{-1}$) and much shorter ME, not preceded by a shock nor a sheath, and occurring inside the back portion of the HSS. The ME orientation measured at both spacecraft is consistent with a passage close to the legs of a curved flux rope. The short duration of the ME observed at {\it Wind} and the difference in the suprathermal electron pitch-angle data between the two spacecraft are the only results that do not satisfy common expectations. We discuss the consequence of these measurements on our understanding of the CME shape and extent and the lack of clear signatures of the interaction between the CME and the HSS.