Multipoint Spacecraft Measurements of a Planar Magnetic Structure Within an ICME Structure in October 2021: From the Solar Surface to the Inner Heliosphere

Abstract We studied a coronal mass ejection (CME) structure that propagated through the inner heliosphere in October 2021, which was observed by BepiColombo, Solar Orbiter, PSP, STEREO‐A, and DSCOVR spacecraft located at different radial distances and heliospheric longitudes. Notably, four of the five spacecraft detected a dip‐like signature in the magnetic field embedded within the CME. We investigated the cause of the dip‐like signature on the basis of both in situ solar wind data and solar surface data. We examined the flare‐triggering structure of the M1.6 flare, which probably caused the CME; however, there was no conclusive evidence found linking the flare to the dip‐like signature observed in the ejected CME. We applied the minimum variance analysis (MVA) to the in situ magnetic field data to determine whether the dip‐like signature could represent a planar magnetic structure (PMS) embedded in the CME‐driven shock sheath. Our MVA results confirmed that the magnetic field associated with the dip‐like structure lies along a plane perpendicular to the minimum variance direction, consistent with the characteristics of the PMS previously reported in the literature. Additionally, we examined the shock parameters and found that the CME‐driven shock developed more rapidly than usual, probably because of the passage of a stream interaction region (SIR) during the initial phase of its propagation. This fact suggests that the interaction between the CME and the SIR played a significant role in the formation of the PMS, resulting in the observation of dip‐like structure downstream at most of the spacecraft.