Ion velocity distribution functions at a low activity comet

We study the transition period of induced comet magnetospheres between the simple deflection of the solar wind due to mass loading and a fully developed bow shock. We present experimental data from the Rosetta mission as well as hybrid simulations of a low-activity comet where no fully developed bow shock has formed. We focus on the shape and evolution of ion velocity distribution functions (VDFs) and their formation mechanisms.

Experimental results from the Ion Composition Analyzer (ICA) aboard the Rosetta mission show partial ring distributions of solar wind (SW) protons as well as cometary ions at comet 67P/Churyumov-Gerasimenko even at low-activity (Moeslinger et al., 2023a, 2023b). These measurements were taken at a heliocentric distance of 2.8 AU, approximately 30km from the comet nucleus. Such observations of partial ring distributions are atypical for this activity level. These partial ring distributions represent a significant deceleration of the bulk velocity of solar wind protons. The simultaneously observed alpha particles do not show significant deceleration or partial ring formation, presumably due to their larger gyroradii. The cometary pickup ions also show initial stages of partial ring formation, although at much lower velocities compared to the SW protons.

To further understand the formation of non-Maxwellian ion velocity distributions we simulate the comet magnetosphere with a hybrid model (Amitis, Fatemi et al., 2017). Partial ring distributions of SW protons also form in the model. They are seen in large parts of the comet magnetosphere, both close to the nucleus in the +E-hemisphere, as well as the magnetic field pile-up region in the -E-hemisphere. In general, the shape of the SW protons is non-Maxwellian throughout most of the magnetosphere less than a few hundred km upstream of the nucleus, and in the entire downstream region. The VDFs continuously evolve with position. An interesting feature is the appearance of secondary populations in the SW proton data in the -E-hemisphere, similar to reflected ions at shocks. SW alpha particles develop non-Maxwellian VDFs further downstream compared to the protons. The difference between alphas and protons can be traced back to their different mass/charge ratios. Since the size of the comet magnetosphere is similar to their gyroradii, SW protons and alpha particles interact with each other as well as the cometary plasma, and kinetic effects play a major role. Due to this interaction between the SW ions, the composition of the SW (alpha/proton ratio) affects the boundary formation and energy transfer between the ion species. The VDFs of cometary ions in the downstream region have complex shapes that are predominantly driven by the structure of the electric field in this region (Moeslinger et al., 2024).

In the simulations we successfully reproduce the partial-ring-shaped VDFs of SW protons observed by ICA at comet 67P at a low outgassing rate. The more Maxwellian-shaped distributions of the SW alpha particles also agree between observations and model. Observations of ion VDFs can give an indication of the spacecraft position within the comet magnetosphere. Studies and hybrid simulations like these, with a focus on ion velocity distribution functions, will be very useful for future comet missions like Comet Interceptor, where measurements at different points in the comet magnetosphere will be available.

References
Fatemi, S., Poppe, A. R., Delory, G. T., & Farrell, W. M. (2017). AMITIS: A 3D GPU-Based Hybrid-PIC Model for Space and Plasma Physics. Journal of Physics: Conference Series, 837(1). https://doi.org/10.1088/1742-6596/837/1/012017
Moeslinger, A., Gunell, H., Nilsson, H., Fatemi, S., & Stenberg Wieser, G. (2024). Explaining the Evolution of Ion Velocity Distributions at a low activity Comet. https://doi.org/10.22541/essoar.171415906.69609077/v1
Moeslinger, A., Stenberg Wieser, G., Nilsson, H., Gunell, H., Williamson, H. N., LLera, K., Odelstad, E., & Richter, I. (2023a). Solar Wind Protons Forming Partial Ring Distributions at Comet 67P. JGR: Space Physics, 128(2). https://doi.org/10.1029/2022JA031082
Moeslinger, A., Nilsson, H., Stenberg Wieser, G., Gunell, H., & Goetz, C. (2023b). Indirect Observations of Electric Fields at Comet 67P. JGR: Space Physics, 128(9), e2023JA031746. https://doi.org/10.1029/2023JA031746