Cometary volatiles ejected from a comet’s nucleus are ionized when in the inner Solar System, transported away from the Sun by the solar wind (SW), and are partially visible as an ion tail. An in-situ encounter with this cometary plasma can provide unique information on comets’ composition, and the SW’s interactions with it. Incidental in-situ ion tail encounters by spacecraft are surprisingly commonplace, but are often hard to identify based solely on in-situ plasma measurements, and solely comparing the positions of spacecraft and comets does not provide a complete picture of the geometry of such tail crossings.
Here, we present the results of a method that provides relatively accurate information on serendipitous spacecraft-comet encounters, using in-situ data to find and refine our understanding of tail and coma crossings. SW velocity measurements made by a spacecraft anti-sunward of a comet can be used to extrapolate the flow of the SW back towards the Sun, in which the minimum distance between these extrapolated SW packets and known comets is a measure of the likelihood of an ion tail crossing. This method provides a better picture of the geometry of an encounter than SW speed estimates, and can uncover previously unknown crossings.
During the last few decades, comet 73P/Schwassmann-Wachmann 3 has undergone multiple fragmentation events. In May-June 2006, such fragments passed sunward of Sun-Earth L1, where the ACE and Wind spacecraft were stationed. A flux of cometary pickup ions was recognised in ACE/SWICS and Wind/STICS data, and was identified as a cometary ion tail crossing by Gilbert et al. (ApJ 815, 12, 10pp, 2015). Using proton velocities from ACE/SWICS data, it is shown that the encounter was with the flank of the extended coma of 73P’s many fragments. This is supported by the lack of an observed magnetic field draping signature or other Heliospheric Magnetic Field (HMF) modification characteristic of an active comet’s ion tail.
In August 2011, Comet C/2010 X1 (Elenin) passed directly sunward of spacecraft STEREO-B, during which there was a flux in water-group cometary ions detected by STEREO-B/PLASTIC, identified as an ion tail encounter by Galvin et al. (AGU Fall Meeting 2013, abstract P31A-1789, 2013). Using proton velocities from PLASTIC data, a previously unknown ion tail crossing of comet 73P fragment AM is identified 3 weeks after the comet Elenin tail crossing. Recently fragmented comets such as 73P provide opportunities to study their chemically pristine interior composition, and the transport of cometary ions by the SW allows for measurements to be made millions of kilometres from the fragments.