Solar Orbiter’s Passage through Comet Leonard’s Tail while in the He⁺ Focusing Cone

Abstract

The Solar Orbiter spacecraft crossed Comet Leonard’s ion tail on 2021 December 17 near 1 au. In and near the ion tail, significant amounts of singly charged oxygen (O⁺) ions were detected by the Heavy Ion Sensor on board the spacecraft. These ions are likely the result of outgassed water molecules from the comet that became ionized and disassociated into protons and O⁺ ions and that were then picked up by the interplanetary magnetic field and advected outward with the solar wind. At this time, the spacecraft was also located amidst the denser parts of the interstellar helium (He) neutrals that are focused here by their gravitational interaction with the Sun. Pickup He⁺ ions in greater numbers are also found in this region and form when neutrals interact with solar photons. Newly ionized ions can generate waves that propagate mainly along the background magnetic field before the waves scatter the ions toward isotropy. Spectral magnetic field analyses are employed to find mainly elliptically polarized waves associated with O⁺ and He⁺ from ring-beam instabilities. Waves associated with He⁺ are identified, but O⁺ waves are not seen. Visibility is concluded to be affected by the relative amplitude of the waves to turbulence, and the visibility increases when the sampling direction is more aligned with the background magnetic field.