Solar Orbiter Observations of Ion Species during the Encounter with the Tail of Comet Leonard

<p>Around 17 December 2021, the Solar Orbiter spacecraft was predicted to have had its closest approach to comet C/2021 A1 (Leonard) with a minimum streamline distance < 0.01 AU. This encounter provided an unprecedented opportunity to investigate in situ comet Leonard's interaction with the solar wind and the composition of pick-up ions produced by ionization and dissociation of outgassed neutrals from its coma. It was a long-period comet originating from the Oort Cloud with a nucleus about 1 km in diameter, with ground-based telescope observations after its perihelion pass (at ~0.62 AU on 3 January 2022) indicating that it had subsequently disintegrated. Prior to perihelion, outbursts had been reported as well as variations in brightness, which had resulted in speculation about an impending disintegration. However, the dimming in November 2021, before the Solar Orbiter encounter, was argued to be due to a transition from outgassing dominated by carbon dioxide to water. Comet Leonard was the brightest comet of the year and noted for its spectacular ion tail with complex structures, including knots and streamers. Preliminary analysis of in situ Solar Orbiter observations have revealed tell-tale signatures of a cometary encounter around the time of predicted closest approach, such as evidence for magnetic field line draping. However, the clearest evidence has come from Solar Wind Analyzer-Heavy Ion Sensor (SWA-HIS) observations of singly-charged oxygen ions, which are typically not of solar origin and are usually produced when the solar wind interacts with a comet or other Solar System body. In this presentation we use SWA-HIS and EDP-STEP data to investigate aspects of the solar wind interaction and composition of cometary pick-up ions from this active, long-period comet shortly before its disintegration.</p>