Princeton Center for Heliophysics Seminar
"An unexpected encounter in the inner Heliosphere: Solar Orbiter crossing through the ion tail of comet ATLAS. Structure of magnetic field draping and signatures from cometary pick-up ion waves close to the Sun."
Soon after the launch of the ESA Solar Orbiter in February 2020 it was suggested that the spacecraft would have flown through the ion and dust tails of comet C/2019 Y4 (ATLAS) just after perihelion, while its nucleus was at 0.25 Astronomical Units from the Sun. The encounter is expected to have taken place when Orbiter was also close to its first perihelion at 0.5 AU, then offering us the first opportunity to study the tail of a comet inside 1AU. Moreover, comet ATLAS fragmented just before perihelion, making such an unusual near-Sun encounter even more interesting and a unique event.
In this talk, I will briefly review some of the main plasma processes that characterise the interaction of an active comet with the surrounding solar wind, namely the draping of the interplanetary magnetic field around the nucleus, the pick-up of cometary ions and the generation of plasma waves by associated instabilities.
Then, I will present joint observations from all in-situ instruments onboard Solar Orbiter about this event and will discuss identified signatures of magnetic field draping and ion-scale waves. Despite fragmentation and the fact that the encounter took place at a significant distance downstream of the nucleus, the overall structure observed at large scale is consistent with that of a comet magnetotail - 2 magnetic lobes of opposite polarity surrounding a central dense and almost unmagnetised plasma sheet - and qualitatively in agreement with past comet-tail crossings. Moreover, at smaller scale, we identified several intervals of ion waves likely associated to instabilities driven by cometary Oxygen ions picked-up by the solar wind and whose polarisation properties are in good agreement with predictions from numerical simulations.
These results suggest that the spacecraft has indeed crossed the ion-tail of comet ATLAS at 0.5 AU and that events of this type could become more common in the near future thanks to the presence of Solar Orbiter, Parker Solar Probe and BepiColombo in the inner Heliosphere.