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

Abstract

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°–60° 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° 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 ∼20°–30°, 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.

Exploring the Impact of the Aging Effect on Inferred Properties of Solar Coronal Mass Ejections

“…However, such measurements are subject to many constraints. For instance, it has been shown recently that the profiles of the magnetic field components of the same CME probed by two different spacecraft separated by even just a few degrees can show strong discrepancies (Winslow et al 2022;Lugaz et al 2023;Regnault et al 2023b). This is consistent with the finding presented by Lugaz et al (2018) where the coherence scale of the magnetic field components amounted to about 15°.…”

Azimuthal Size Scales of Solar Wind Periodic Density Structures

“…These studies do not distinguish between different solar wind structures-that is, corotating interaction regions, coronal mass ejections, and PDSs, among others. A recent study using STEREO-A and Wind in situ measurements showed that magnetic ejecta are also elongated in the direction perpendicular to the Sun-Earth line (Lugaz et al 2024). In interpreting the trend in our occurrence rate distribution, the separation along the Z GSE component is unlikely to play a role, due to the relatively small values.…”