The First Quantitative Study of Tail Regrowth of CME-Driven Disconnection in Comet C/2023 P1 Nishimura Observed by SoloHI.

(Solar Orbiter Nugget #83 by S. B. Shaik^1,2, G. Stenborg³, P. Hess², A. Vourlidas³, K. Battams², and R. Colaninno²)

Introduction

The ion (plasma) tail of a comet acts as a sensitive tracer of heliospheric conditions, as it responds to variations in solar wind velocity, density, and interplanetary magnetic field (IMF)[1, 2]. During the comet’s passage close to the Sun, the interplanetary magnetic field carried by the solar wind and draped around the comet’s ionized coma can reconnect with solar magnetic structures, leading to the detachment of the tail defined as a tail disconnection event [TDE; 3, 4]. The principal heliospheric drivers for TDEs are the heliospheric current sheet [HCS; 5, 6] crossings, stream interaction regions [SIRs; 7], and coronal mass ejections [CMEs; 8, 9, 10].

After a TDE, the tail rapidly reforms as the cometary material continues to ionize. Although numerous TDEs have been documented since the 1970s, quantitative determinations of the timescale for tail reformation, and of how this timescale depends on local plasma conditions, have rarely been obtained, primarily because of limitations in time cadence, field of view (FOV), and spatial resolution. As a result, tail reformation has been described as instantaneous on observing timescales [4]. Here, we analyze the tail dynamics of comet C/2023 P1 [Nishimura; 11] using unique observations from the Solar Orbiter Heliospheric Imager [SoloHI; 12] onboard the Solar Orbiter spacecraft [13, 14]. SoloHI’s spatial and temporal resolution, along with a 40^o wide FOV, enable the detailed tracking of the rapid tail evolution in the inner heliosphere. Four TDEs were identified during the observing period (Table 1), each coinciding with CME passages, pointing to CME-driven magnetic reconnection as the dominant trigger. We analyze TDE 4 to investigate the tail regrowth rate and the disconnection dynamics leading up to the CME crossing.
 

Table 1. Disconnection events during the comet observing period.

Results

Disconnection Events and CME dynamics

Figure 1a illustrates the comet orbit and the observing geometry of SoloHI. The overall evolution of the comet and its interactions with CMEs can be viewed in the accompanying movie. We perform a 3D reconstruction of the CME associated with TDE 4 using observations from LASCO/C2, C3, SECCHI-A/COR2, and SoloHI, from which we derive a deprojected CME nose speed of ∼560±15 km/s. From the best fit parameters, the comet’s trajectory indicates that the comet encounters and traverses the CME flank (green shaded region in Figure 1b) at a heliocentric distance of ∼64 R_⊙. From the reconstruction and observing geometry, we estimate the CME speed at the comet’s position as ∼380±20 km/s.

Figure 1. Trajectory of comet C/2023 P1 relative to Solar Orbiter. (a) Side view showing the comet’s position with respect to the ecliptic plane on 2023 September 11, where solid (dashed) blue segments denote motion above (below) the ecliptic, and the inset marks the SoloHI field of view. (b) Top-down view of the ecliptic plane from solar north pole, showing the comet’s path and the associated CME (green shading), illustrating the CME–comet interaction geometry.

Tail Regrowth and Detached Tail Dynamics

In Figure 2, it can be seen that the tail initially develops a kink, marked by the orange arrow in panels (a) and (b), followed by the detachment of the tail segment, marking the onset of TDE. The disconnection occurs far from the nucleus, indicating interaction of the CME flank along the ion tail. After the CME passage, a new, straighter tail progressively regrows to its pre-event extent. To quantify tail reformation, we measured the deprojected tail length in successive SoloHI images. The tail grows approximately linearly with time, at a rate of ∼86±7 km/s, and recovers its pre-event extent over ∼24 h. This rate depends on the ion production rate of the comet, and on the re-draping of the magnetic field as the post-CME environment relaxes [15, 16].

Figure 2. Time-sequence of selected SoloHI image frames of the C/2023 P1 comet–CME interaction in a zoomed subfield. Orange circles mark the connected ion tail, and magenta circles indicate the disconnected segment. Each panel is annotated with the corresponding stage of the disconnection.

We also track the motion of the detached tail segment to assess its dynamics relative to the CME. From the comet–spacecraft geometry, we derive a deprojected antisunward drift speed of ∼295±20 km/s, slightly lower than the local CME flank speed of ∼380±20 km/s. This drift speed indicates that the detached material is likely advected by the CME flank as it sweeps across the comet.

A movie illustrating the overall evolution of the comet, its interactions with CMEs, and associated TDEs, can be found below, and can be downloaded here.

The movie first presents the wide-field evolution in SoloHI detector tiles 3 (left) and 2 (right), followed by a zoomed-in view of the selected subfield.

Conclusions

We investigate the comet C/2023 P1 interactions with CMEs during its passage through the inner heliosphere through SoloHI imaging observations and identify four TDEs within seven days. Contrary to the prevailing expectations that HCS crossings, SIRs, and CMEs drive TDEs, we find that all four events observed here are associated with CME crossings. Following the TDE, the ion tail regrows to its pre-event length (1.9×10⁶ km) over ∼24 h, corresponding to a linear growth rate of ∼86±7 km /s. Our study provides the first quantitative measurement of cometary tail regrowth, enabled by SoloHI’s sensitivity and favorable viewing geometry at heliocentric distances ≤0.32 au. A pronounced pre-event kink likely reflects interaction with a CME-driven disturbance ahead of the flank, while the actual disconnection occurs only when the CME’s flank magnetic structure reaches the comet. This study provides quantitative constraints on cometary tail reformation, showing that ion tails respond sensitively to the CME magnetic structure and reform as a gradual, quantifiable process responding to the evolving ambient environment rather than appearing instantaneously on observational timescales.

This work has been published in Shaik et al., ApJ, 999, 60 (2026) DOI: 10.3847/1538-4357/ae3bdb.

Affiliations

(1) George Mason University, Fairfax, VA 22030, USA
(2) U.S. Naval Research Laboratory, Washington, DC 20375, USA
(3) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA

Acknowledgements

Solar Orbiter is a mission of international cooperation between the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA), operated by ESA. The Solar Orbiter Heliospheric Imager (SoloHI) instrument was designed, built, and is now operated by the US Naval Research Laboratory with support from the NASA Heliophysics Division, Solar Orbiter Collaboration Office under DPR NNG09EK11I. The APL team is supported by the NASA grant 80NSSC22K1028. The NRL effort was also supported by the Office of Naval Research.

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