Solar wind interaction with comet 67P around perihelion

Abstract

Near perihelion, when comet 67P was most active, the Rosetta spacecraft resided inside the comet induced magnetosphere. The solar wind magnetic field was still present, but the solar wind ions were mostly gone, Rosetta was in the solar wind ion cavity. The solar wind was not completely gone though, there were sporadic occurrences of solar wind ions. Observations from this period could thus shed light on the solar wind - comet interaction for a medium activity comet. Solar wind ions with a broad energy and angular distribution would indicate a fully developed cometosheath pushed closer to the nucleus. We present the first results from a study of all detected sporadic events.

Introduction

The Rosetta spacecraft followed comet 67P from August 2014 to end of September 2016. This covered heliocentric distances from more than 3 AU down to a comet perihelion at 1.2 AU. The comet activity and thus also the comet gas and plasma cloud expanded significantly as the comet approached perihelion. From May 2015 to the end of 2016 the Rosetta spacecraft was mostly well the solar wind ion cavity, a region devoid of solar wind ions (Behar et al. 2017). RPC-ICA identification of ion species was based on a manual inspection of daily mass channel data. Very sporadic occurrences of the solar wind that did not stand out over the noise level in this daily summed data is missing from the level 4 mass separated data set in the Planetary Science Archive.

Method

The solar wind ion  occurrences we report here were too short-lived to be seen in daily averages. They are not present in the standard mass separated data set delivered to PSA. An automatic algorithm was used to detect potential cases. A total of 285 ion velocity distributions with potential solar wind ions around perihelion were found and manually inspected. Of these 174 passed the manual inspection. We used a selection criteria of at least 20 counts detected within the solar wind ion mass channels (H⁺, He²⁺ and He⁺) in one energy level and azimuthal sector. A sample inspection figure is shown in Fig. 1 with data from May 28 2014. The lower left panel shows the “energy- mass matrix” with mass channel (anode) on the x-axis and energy (eV) on the y axis. Protons are seen at mass channels 26 and 27. The ions at lower mass channels are water ions. This panel is the most important to verify that we indeed see H+ and He2+ and not water ions of cometary origin.

Figure 1: An example of the data figures used to inspect potential cases of sporadic occurrence of solar wind ions in the time period when Rosetta was mostly inside the solar wind ion cavity, from May 2014 to and of 2016. The upper left panel shows solar wind ion counts summed over all energies as function of sector corresponding to azimuths of 0°-360° (x-axis), and elevation from approximately -45° to +45° from the instrument symmetry plane (y axis). The upper left panel shows solar wind counts as function of sector and energy channel. The lower left channel shows the ion counts as function of mass channel (x-axis) and energy in eV (y-axis). The lower right panel is a line plot of solar wind ion counts as function of energy. The sample data is from 28 May 2015 at 05:58 UT.

Results

We do see broad energy distributions of protons at times. This indicates that the solar wind pushed the cometosheath closer to the nucleus for these cases. Usually the angular width of the signal is not more than three sectors of 22.5° width.  A more careful analysis will be made to see if this fits expectations of a fully developed cometosheath. 

The next step in the analysis is to look at the data in a better field of view plot as we did in Moeslinger et al (2023). This is shown in Fig. 2. The upper panel shows flow directions and fluxes of cometary ions with energy above 40 eV, the lower panel shows solar wind ions. One can see that solar wind ions are seen in the lower half of the field of view, cometary ions in the upper half. Both species are coming from somewhere in between the sun (yellow dot) and the comet nucleus (grey star).

Figure 2: Plot of cometary ions (upper panel) and solar wind ions (lower panel) in the RPC-ICA field of view. The colour indicates the median energy and the intensity the logarithm of the particle flux. See Moeslinger et al. (2023) for more details on this type of plot.

 At other cases the angular and energy extent of the observed ions were small, energies were often well below typical solar wind energies. What appeared to be almost undisturbed solar wind was seen on some occasions. On several occasions only H⁺ or only He²⁺ was seen in a narrow energy and angular range, reminiscent of reported observations of solar wind precipitation observed at Mars (Stenberg et al. 2011).

We put the observations in the context of the magnetic field data and other plasma data. We end by noting what implications our results have for a Comet Interceptor flyby of a moderately active comet like 67P at perihelion.

References

Behar, E.,  H. Nilsson, M. Alho,  C. Goetz,  B. Tsurutani, The birth and growth of a solar wind cavity around a comet - Rosetta observations, Monthly Not. Roy. Astr. Soc., 469, S396 – S403, 2017

Moeslinger, A.,  Wieser, G. S.,  Nilsson, H.,  Gunell, H.,  Williamson, H. N.,  LLera, K., et al. (2023).  Solar wind protons forming partial ring distributions at comet 67P. Journal of Geophysical Research: Space

Stenberg, G., H. Nilsson, Y. Futaana, S. Barabash, A. Fedorov, and D. A. Brain, Observational evidence of alpha-particle capture at Mars, Geophys. Res. Lett., doi:10.1029/2011GL047155,, 2011