Comet 2I/Borisov was only the second interstellar object known to have entered the solar system. The highly eccentric, hyperbolic orbit of 2I/Borisov and its high inclination to the ecliptic plane demonstrated that the comet had an extrasolar system origin (e.g., Manzini et al. 2020). Polarimetric observations of this comet suggested a remarkably smooth, pristine coma with a high concentration of carbon monoxide, that had likely never interacted with the solar wind of either the Sun or any other star. Comet 2I/Borisov could have formed around a red dwarf – a smaller, fainter type of star than the Sun – though other kinds of stars are possible (e.g., Bagnulo et al. 2021). Figure 1 displays an image of 2I/Borisov taken with the W. M. Keck Observatory Low-Resolution Imaging Spectrometer on 24 November 2019, when the comet was 2 AU from the Sun, along with an image of the Earth to show the size scale.

Figure 1. Image of Comet 2I/Borisov taken with the Keck Observatory on 24 November 2019, when the comet was located ~2 AU from the Sun (Credit: P. van Dokkum, G. Laughlin, C. Hsieh, S. Danieli, Yale University).

Probing Plasma Tail of 2I/Borisov

Generally comets have shown two types of tails, respectively, the dust tail formed by the solar radiation pressure, which triggers sublimation of the outer surface of coma and flow of dust particles and the plasma tail caused by the complex interaction between the solar wind and the coma. The plasma tails have opening angles of a few degrees, pointing in the anti-solar direction, but are longer than the dust tails. The electron density irregularities in the plasma tail scatter the plane radio wave front from a compact background radio source passing through it causing scintillation to be observed in the intensity of the radio source (e.g., Salter and Manoharan 2019). At the Arecibo and Green Bank observatories, we took advantage of the passage of the plasma tail of 2I/Borisov in front of a number of compact extragalactic radio sources to investigate the properties of its plasma tail using radio scintillation observations at the P band (302–352 MHz), 820 MHz, and the L band (1120–1730 MHz).

Observations and Analysis

The path of Comet 2I/Borisov from mid-October to mid-December 2019 is shown in Figure 2. The positions of the radio sources selected for observation as the plasma tail of the comet passed over them are also marked along the path of the comet. During this period, the Sun–comet and Earth–comet distances lay in the ranges 2.5–2.0 and 2.9–2.4 AU,  respectively. On each day, the occulted sources were tracked for ~2 – 2.5 hours and the total power time series were sampled at 2-ms rate and by performing Fourier transformation of 1-minute data block the temporal power spectrum (P(f)) was computed  (e.g., Manoharan et al. 2001). For the occultation source B1019+083, the plot of the scintillation index (i.e., ${m\,\,=\,\,\sqrt{\int P(f)\, {\rm d}f}/{\rm <I>}}$), as a function of time, computed from the power spectra of 1-minute data stretches from Arecibo observations at L-band on 31 October 2019, is shown in Figure 3(a). The scintillation index reached a maximum at 13 UT as the line of sight to the source approached the central part of the ∼ plasma tail. After this time, the apparent scintillation decreased, as the telescope pointing reached the tracking limit and the source moved out of the telescope beam.

Figure 2. Path of Comet 2I/Borisov from mid-October to mid-December 2019 (Manoharan et al. 2022).

The temporal power spectra of B1019+083, for selective time intervals of the above scintillation plot, also showed a systematic evolution in the spectral shape as the line of sight to the source approached the central part of the plasma tail. Figure 3(b) displays average spectra corresponding to (i) the start of the L-band observations at 12:26 UT (blue spectrum), (ii) close to the peak of the scintillations at 12:57 UT (red spectrum), and (iii) 13:17 UT, when the telescope pointing lay outside of the plasma tail and at an off-source region (green spectrum). When the line of sight to the radio source lay close to the centerline of the tail (~12:57 UT), the power spectrum became broad, extending up to 10 Hz, and its amplitude increased.

Figure 3. (a) Scintillation indices from Arecibo observations of B1019+083 at the L band on 31 October 2019. (b) Power spectra of the intensity fluctuations observed on B1019+083 on 31 October at different times (red, blue and green spectra) and 05 November 2019 (black spectrum) (Manoharan et al. 2022).

Conclusions

The presence and absence of scintillation at different perpendicular distances from the central axis of the plasma tail of 2I/Borisov suggests a narrow tail of less than 6 arcmin at a distance of 10 arcmin ( $~10^6$ km) from the comet nucleus. Data recorded during the occultation of B1019+083 on 31 October 2019 with the Arecibo Telescope covered the width of the plasma tail from its outer region to the central axis. The excess level of L-band scintillation indicates a plasma density enhancement of 15–20 times that of the background solar wind. The evolving shape of the observed scintillation power spectra across the tail from its edge to the central axis suggests a density spectrum flatter than Kolmogorov and that the plasma density irregularity scales present in the tail range between 10 and 700 km. The discovery of a high-frequency spectral excess corresponding to irregularity scales much smaller than the Fresnel scale suggests the presence of smallscale density structures in the plasma tail, likely caused by interaction between the solar wind and the plasma environment formed by the comet.

Based on the recent paper: P. K. Manoharan, Phil Perillat, C. J. Salter, Tapasi Ghosh, Shikha Raizada, Ryan S. Lynch, Amber Bonsall-Pisano, B. C. Joshi, Anish Roshi, Christiano Brum and Arun Venkataraman, Probing the Plasma Tail of Interstellar Comet 2I/Borisov, 2022, The Planetary Science Journal, 3:266 (12pp). DOI:10.3847/PSJ/aca09f

References
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*Full list of authors: P. K. Manoharan, Phil Perillat, C. J. Salter, Tapasi Ghosh, Shikha Raizada, Ryan S. Lynch, Amber Bonsall-Pisano, B. C. Joshi, Anish Roshi, Christiano Brum and Arun Venkataraman