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

We present a validation of our recently proposed non-conventional method, Constant Acceleration Accounted Perspective (CAAP), for estimating the instantaneous expansion speed of a coronal mass ejection (CME)-associated magnetic cloud (MC) using single-point in situ observations. Unique validation is enabled by the rare radial alignment of SolO and Wind, allowing simultaneous observations of the MC center (at Wind) and trailing edge (at SolO) during 3–5 November 2021 to measure its instantaneous expansion speed. This measurement is compared with estimates from both our CAAP method and the conventional approach. The radial alignment of SolO and Wind (0.13 AU radial and 2.3^∘ angular separation) enables tracking the temporal evolution of CME substructures such as shock, sheath, and MC. We note a discrepancy between the estimated low-inclined MC axis from minimum variance analysis (MVA) and the highly inclined ENW-type MC indications from the in situ observations. The magnetic flux embedded in the MC is noted to increase from SolO to Wind as reflected in the significant increase in the MC's radial size and disproportionate decrease of its axial magnetic field. During the evolution of CME between both locations, the shock is unexpectedly stronger at Wind than at SolO, and the sheath size is almost equal at both spacecraft. This is explained by the combined role of accelerated MC, due to its back compression from the high-speed stream, and ambient medium variability between both locations. Our results support the usefulness of the recently proposed non-conventional (CAAP) method and understanding temporal variability in CME substructures.