First Simultaneous In Situ Measurements of a Coronal Mass Ejection by Parker Solar Probe and STEREO-A

We present the first Parker Solar Probe mission (PSP)-observed coronal mass ejection (CME) that hits a second spacecraft before the end of the PSP encounter, providing an excellent opportunity to study short-term CME evolution. The CME was launched from the Sun on 2019 October 10 and was measured in situ at PSP on 2019 October 13 and at STEREO-A on 2019 October 14. The small, but not insignificant, radial (∼0.15 au) and longitudinal (∼8°) separation between PSP and STEREO-A at this time allows for both observations of short-term radial evolution as well as investigation of the global CME structure in longitude. Although initially a slow CME, magnetic field and plasma observations indicate that the CME drove a shock at STEREO-A and also exhibited an increasing speed profile through the CME (i.e., evidence for compression). We find that the presence of the shock and other compression signatures at 1 au are due to the CME having been overtaken and accelerated by a high speed solar wind stream (HSS). We estimate the minimum interaction time between the CME and the HSS to be ∼2.5 days, indicating the interaction started well before the CME arrival at PSP and STEREO-A. Despite alterations of the CME by the HSS, we find that the CME magnetic field structure is similar between the vantage points, with overall the same flux rope classification and the same field distortions present. These observations are consistent with the fact that coherence in the magnetic structure is needed for steady and continued acceleration of the CME.