The Width of Magnetic Ejecta Measured near 1 au: Lessons from STEREO-A Measurements in 2021-2022

Coronal mass ejections (CMEs) are large-scale eruptions with a typical radial size at 1 au of 0.21 au but their angular width in interplanetary space is still mostly unknown, especially for the magnetic ejecta (ME) part of the CME. We take advantage of STEREO-A angular separation of 20 degrees-60 degrees from the Sun-Earth line from 2020 October to 2022 August, and perform a two-part study to constrain the angular width of MEs in the ecliptic plane: (a) we study all CMEs that are observed remotely to propagate between the Sun-STEREO-A and the Sun-Earth lines and determine how many impact one or both spacecraft in situ, and (b) we investigate all in situ measurements at STEREO-A or at L1 of CMEs during the same time period to quantify how many are measured by the two spacecraft. A key finding is that out of 21 CMEs propagating within 30 degrees of either spacecraft only four impacted both spacecraft and none provided clean magnetic cloud-like signatures at both spacecraft. Combining the two approaches, we conclude that the typical angular width of an ME at 1 au is similar to 20 degrees-30 degrees, or 2-3 times less than often assumed and consistent with a 2:1 elliptical cross section of an ellipsoidal ME. We discuss the consequences of this finding for future multi-spacecraft mission designs and for the coherence of CMEs.