Comet 73P/Schwassmann-Wachmann 3's Dust Trail as a Source of Pickup Ions

TL;DR

This study investigates whether the extended debris trail from the fragmented comet 73P/Schwassmann-Wachmann 3 can be a significant source of cometary pickup ions beyond the visible major fragments. It introduces the Tailcatcher method, which uses 3D extrapolation of the solar wind from spacecraft measurements to estimate the likelihood of a tail immersion via an impact parameter threshold of $0.05\mathrm{AU}$. Analysis of the 2006 ACE/Wind event and the 2011 STEREO-B event reveals that observed $O^{+}$ and water-group ion flux enhancements align better with the extended debris trail than with the major fragments, supporting the trail as a substantive ion source. The results highlight the potential for dust trails to cause detectable ion sheets and suggest broad applicability of Tailcatcher to identify similar encounters in archival data and future missions, with implications for magnetic interactions and solar wind mass loading.

Abstract

73P/Schwassmann-Wachmann 3 is a short-period comet that has undergone multiple fragmentation events in the last few decades. During May-June 2006, while passing near Earth, multiple fragments of comet 73P passed sunward of Sun-Earth Lagrange Point 1, while cometary pickup ions were detected concurrently by instruments on both the ACE and Wind spacecraft, implying the crossing of one or more ion tail. Additionally, during August 2011, a fragment of 73P passed directly sunward of spacecraft STEREO-B. A detection of cometary ions is shown to originate at fragment 73P-AM. Solar wind velocity measurements are used to extrapolate the flow of the solar wind in 3 dimensions and, when compared with the positions of known comets and cometary fragments, estimate the separation between the cometary ion tail and the spacecraft. Using this technique, it is shown that the alignment of the major cometary fragments with the spacecraft was poor for the transport of cometary ions via the solar wind, but the encounter was near enough for immersion in the diffuse ion tail surrounding an extended dust trail within which the nucleus fragments reside. This implies that, at this distance, the extended trail of cometary debris was a significant source of cometary ions in the case of comet 73P.

Figures (6)

• Figure 1: Infrared image from NASA's Spitzer Space Telescope of fragmented comet 73P/Schwassman-Wachmann 3. A trail of material can be seen spanning the space between the large fragments. From spitzer.
• Figure 2: 73P fragment impact parameters, calculated using the Tailcatcher method, for Sun-Earth Lagrange Point 1, SEL1. Different colours represent the 71 known fragments of 73P at the time of detection. The $x$ axis is the spacecraft time, corresponding to the time at which a propagating SW plasma packet arrives at the spacecraft. The highlighted timeframe corresponds to the measurement of significant O$^{+}$ flux by both ACE and Wind gilbert2015.
• Figure 3: Histogram with 6-hour bins showing the binned times at which ACE crosses the orbital planes of the fragments of comet 73P.
• Figure 4: Plots A and B show incoming cometary O$^{+}$ flux measured by ACE/SWICS at energies 0.6 - 83 keV $e^{-1}$, and Wind/STICS at energies below 220 keV, respectively. Retrieved from gilbert2015.
• Figure 5: Top: Ion count rates recorded by STEREO-B/PLASTIC in the mass-per-charge range of water group ions (14-20). Ion fluxes are not available during this time period. Bottom: Impact parameters calculated for 3-dimensional solar wind velocities measured by STEREO-B, for fragments of comet 73P.