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We present the first PSP-observed CME that hits a second spacecraft beforethe end of the PSP encounter, providing an excellent opportunity to studyshort-term CME evolution. The CME was launched from the Sun on 10 October 2019and was measured in situ at PSP on 13 October 2019 and at STEREO-A on 14October 2019. The small, but not insignificant, radial (~0.15 au) andlongitudinal (~8 deg) separation between PSP and STEREO-A at this time allowsfor observations of both short-term radial evolution as well as investigationof the global CME structure in longitude. Although initially a slow CME,magnetic field and plasma observations indicate that the CME drove a shock atSTEREO-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 othercompression signatures at 1 au are due to the CME having been overtaken andaccelerated by a high speed solar wind stream (HSS). We estimate the minimuminteraction time between the CME and the HSS to be about 2.5 days, indicatingthe 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 fieldstructure is similar between the vantage points, with overall the same fluxrope classification and the same field distortions present. These observationsare consistent with the fact that coherence in the magnetic structure is neededfor steady and continued acceleration of the CME.

First simultaneous in situ measurements of a coronal mass ejection by Parker Solar Probe and STEREO-A