Insights Into the Shape and Sheath Structure of the 23 April 2023 Coronal Mass Ejection From Simultaneous Multi‐Spacecraft Measurements

Abstract Information about the shape and structure of coronal mass ejections (CMEs) when they reach Earth is usually limited as one must rely on single‐point in situ measurements or extrapolate from coronal and inner heliospheric white‐light imaging. The 23–24 April 2023 CME, which was measured simultaneously by Wind and STEREO‐A when separated by about 10 in longitude allows us to test existing paradigms. This includes for example, the idea that the CME sheath region gets wider as one moves away from the CME center line (the CME nose), or that the magnetic ejecta (ME) is more force‐free closer to the nose. We find that, contrary to those expectations, STEREO‐A, which was farther away from the CME central direction than Wind based on remote observations, measures a shorter sheath region and a ME closer to the force‐free expectations than what was measured by Wind . The data also allow us to compare local measurements, such as the shock normal with those obtained from timing information that may reflect the global structure of the CME. We then compare the measurements in the sheath region over the different near‐Earth spacecraft. We determine that, for this event, there is no simple monotonic relationship between the coherence of the measurements, as quantified by correlation and root mean square difference, and the distance away from the Sun‐Earth line. Lastly, we briefly describe the existence of a sub‐Alfvénic solar wind region during the passage of the ME at both STEREO‐A and Wind . Overall, this case study showcases the additional information gained from having multiple simultaneous spacecraft measurements and how our current paradigm about the CME structure may need to be revised.