Validating a Non-conventional Method for Expansion of Coronal Mass Ejections (CMEs) and Investigating the Evolution of a CME Substructures Using Solar Orbiter and Wind Observations

Abstract: We present a validation of our recently proposed non-conventional method, Constant Acceleration Accounted Perspective (CAAP), for estimating the instantaneous expansion speed of coronal mass ejection (CMEs), even when only single-point in situ observations are available. This validation is enabled by the radial alignment of SolO and Wind spacecraft (0.13 AU radial and 2.3 deg angular separation), providing simultaneous observations of the center (at Wind) and trailing edge (at SolO) of a CME associated magnetic cloud (MC) during 3-5 November 2021, allowing a direct measurement of its instantaneous expansion speed. These measurements are compared with CAAP-derived instantaneous expansion speed estimates at both spacecraft. The favorable spacecraft configuration also enables tracking the temporal evolution of CME substructures, including the shock, sheath, and MC. A discrepancy is noted between the low-inclination MC axis estimated from minimum variance analysis (MVA) and the highly inclined ENW-type MC axis suggested by visual inspection of in situ measurements. We also observe an apparent increase in the magnetic flux within the MC from SolO to Wind, indicating a noticeable deviation from magnetic flux conservation. During the CME's propagation from SolO to Wind, the shock becomes unexpectedly stronger at Wind, while the sheath thickness remains nearly the same, likely due to MC acceleration from back compression by a high-speed stream and ambient solar wind variability. Our results demonstrate the applicability of the CAAP method and the importance of accounting for temporal evolution in CME substructures for space weather studies.