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

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.

💡 Research Summary

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The paper investigates the origin of cometary pickup ions detected by spacecraft during close passages of fragments of comet 73P/Schwassmann‑Wachmann 3 (hereafter 73P). The authors focus on two events: (1) the May–June 2006 encounter when several fragments of 73P passed sunward of the Sun‑Earth Lagrange point L1, and both ACE and Wind recorded unusually high fluxes of O⁺ and water‑group ions; and (2) the August 2011 passage when fragment 73P‑AM passed directly sunward of STEREO‑B, which observed a modest increase in water‑group ion counts.

To assess whether the spacecraft crossed the comet’s ion tail, the authors develop a new predictive tool called “Tailcatcher”. Tailcatcher uses the JPL Horizons ephemerides for all known fragments of 73P and the three‑dimensional proton velocity measured in situ by each spacecraft. By propagating the measured solar‑wind velocity vectors backward toward the Sun, the program reconstructs the three‑dimensional solar‑wind flow that would have carried cometary ions from the comet to the spacecraft. The minimum distance between this extrapolated solar‑wind packet and each comet fragment – the impact parameter – is taken as a quantitative measure of the likelihood of an ion‑tail crossing.

Applying Tailcatcher to the 2006 ACE/Wind data, the authors find that the impact parameters for the tracked large fragments never fall below the conservative threshold of 0.05 AU (≈7.5 × 10⁶ km), which is roughly twice the estimated diameter of the ion tail of comet Hyakutake (the benchmark case). Moreover, the timing of the minima does not coincide with the four distinct O⁺ flux peaks reported by Gilbert et al. (2015). This mismatch indicates that the large fragments themselves cannot account for the prolonged ion enhancements, which began about eight days before the fragments crossed the Sun‑L1 line and persisted for more than two weeks afterwards.

The authors therefore propose that the extended dust trail left by the multitude of smaller, often untracked, fragments of 73P acted as a diffuse source of cometary ions. Infrared images from Spitzer (Fig. 1) show a prominent dust ribbon spanning the region between the major fragments. The dust trail, enriched in volatile ices, would continuously outgas as it drifts away from the nucleus, creating a broad “ion sheet” that the solar wind can load with newly ionized particles. Because this sheet is much wider than a classic, narrow ion tail, spacecraft can encounter it even when the main fragments are poorly aligned.

The 2011 STEREO‑B event provides an independent test of this hypothesis. Using the PLASTIC proton velocity measurements, Tailcatcher calculates an impact parameter of <0.05 AU for fragment 73P‑AM at the time of a modest water‑group ion count increase. The ion enhancement aligns temporally with the impact‑parameter minimum, supporting the idea that even a small fragment’s surrounding dust trail can supply detectable pickup ions when the geometry is favorable. The authors note that PLASTIC’s limited mass resolution prevents a definitive identification of the ion species, but the coincidence of geometry and flux increase is compelling.

The discussion emphasizes several methodological limitations. Tailcatcher assumes that the solar‑wind velocity measured at the spacecraft is identical to that experienced by the comet and remains constant along the trajectory, an approximation that becomes less accurate for separations of several AU or for highly variable solar‑wind conditions. The method also does not model the intrinsic broadening of the ion tail or the detailed spatial distribution of the dust trail; instead, a conservative 0.05 AU threshold is adopted to accommodate these uncertainties. Despite these simplifications, the authors argue that Tailcatcher successfully reproduces known ion‑tail crossings (e.g., Hyakutake, ATLAS) and can uncover previously unrecognized events, as demonstrated for 73P.

In conclusion, the study provides strong evidence that the extended dust trail of comet 73P/Schwassmann‑Wachmann 3, rather than its large fragments alone, was the dominant source of the observed cometary pickup ions during the 2006 and 2011 encounters. This finding has broader implications for interpreting in‑situ plasma measurements near fragmented comets and for predicting future ion‑tail crossings by missions such as Solar Orbiter and Parker Solar Probe. Incorporating realistic dust‑trail outgassing and three‑dimensional solar‑wind propagation into comet–solar‑wind interaction models will be essential for a more accurate understanding of cometary plasma environments.