Authors: F. Regnault (University of New Hampshire), N. Al-Haddad (University of New Hampshire), N. Lugaz (University of New Hampshire), C. J. Farrugia (University of New Hampshire), W. Yu (University of New Hampshire), B. Zhuang (University of New Hampshire) and E. E. Davies (Austrian Space Weather Office)

Simultaneous in situ measurements of coronal mass ejections (CMEs), including both plasma and magnetic field, by two spacecraft in radial alignment have been extremely rare. Here, we report on one such CME measured by Solar Orbiter (SolO) and Wind on 2021 November 3–5, while the spacecraft were radially separated by a heliocentric distance of 0.13 au and angularly by only 2.2°.

We focus on the magnetic cloud (MC) part of the CME. We find notable changes in the shape of the magnetic field strength and  in the speed profile inside the MC as the CME propagated from SolO to Wind. We observe a greater speed at the spacecraft that is further away from the Sun without any clear compression signatures at the MC rear that could have contributed to the acceleration we measure. Both spacecraft are really close to each other and the estimated fast magnetosonic wave speed inside the MC suggests that time evolution cannot be responsible for the observed discrepancies. We then infer that spatial variations over 2.2° of the MC structure are at the heart of the observed discrepancies.

Moreover, using MC properties at SolO, we estimate a propagation time of 8 hours to Wind whereas in reality the CME arrived at Wind 4 hours after it was observed at SolO. We also observe a 60 % relative error on the component of the magnetic field used to determine the geoeffectiveness of a CME. These results show that even small separations as low as 2.2° (or 0.03 au in arclength) between spacecraft can have a large impact on event properties observed, posing an issue for current CME model resolutions and affecting our forecasting capabilities.