The Wagging Plasma Tail of Comet C/2020 S3 (Erasmus)

Abstract

Long-period comet C/2020 S3 (Erasmus) reached perihelion at 0.398 au on UT 2020 December 12.67, making it a bright, near-Sun object. Images taken between mid-November and December 2020 using the HI-1 camera and COR2 coronagraph onboard STEREO-A, as well as the LASCO/C3 coronagraph onboard SoHO, show significant variations in the plasma tail position angles. To analyze these variations, a simple technique was developed to calculate the aberration angles. These angles are defined as the angle between the sun-comet line and the tail axis, measured in the orbital plane. The aberration angles were found to range from $1.2^\circ$ to $46.8^\circ$, with an average (median) value of approximately $20.3^\circ$ ($16.3^\circ$). By considering the aberration angles, the solar wind radial velocities during the observations were inferred to range from 73.9 km/s to 573.5 km/s, with mean (median) values of approximately 205.5 km/s (182.3 km/s). Throughout the observations, two periods were identified where the tails showed forward tilting, which cannot be explained by aberration alone. In one case, this anomalous position angle was sustained for at least 11 days and is possibly due to co-rotating interaction regions. In the other case, the tail exhibited dramatic excursions from 180$^\circ$ to 150$^\circ$ back to 210$^\circ$ over a limited period of around 34 hours. This behavior is tentatively explained as a consequence of the interaction with a halo Coronal Mass Ejection that was launched from NOAA 12786 and arrived at comet C/2020 S3 during the time when the tail displayed its wagging behavior.