Solar Wind Interactions with Comet C/2021 A1 Using STEREO HI and a Data-assimilative Solar Wind Model

Cometary tails display dynamic behavior attributed to interactions with solar wind structures. Consequently, comet-tail observations can serve as in situ solar wind monitors. During 2021 December, Comet Leonard (C/2021 A1) was observed by the STEREO-A heliospheric imager. The comet tail exhibited various signatures of interactions with the solar wind including bending, kink formation, and finally complete disconnection. In this study, we compare the timing of these events with solar wind structures predicted by the Heliospheric Upwind eXtrapolation model with a time-dependency (or HUXt) solar wind model using new solar wind data assimilation (DA) techniques. This serves both to provide the most accurate solar wind context to interpret the cometary processes, but also as a test of the DA and an example of how comet observations can be used in model validation studies. Coronal mass ejections, stream interaction regions (SIRs), and heliospheric current sheet (HCS) crossings were all considered as potential causes of the tail disconnection. The results suggest the tail disconnection at Comet Leonard was the result of it crossing the HCS and into an SIR. The timing of these features agree better with the DA model results than the non-DA model, showing the value of this approach. Case studies such as this expand our understanding of comet–solar wind interactions, and in demonstrating the utility of DA for solar wind modeling. We note that this could lead to comets acting as additional in situ measures for solar wind conditions for regions where no in situ spacecraft are available, potentially improving solar wind DA in the future.