Explaining the Evolution of Ion Velocity Distributions at a Low Activity Comet

• Published in: Journal of geophysical research. Space physics 2024-09, Vol.129 (9), p.n/a
• Main Authors: Moeslinger, A., Gunell, H., Nilsson, H., Fatemi, S., Stenberg Wieser, G.
• Format: Article
• Language: English
• Subjects: Alpha particles; Alpha rays; Cometary ions; Cometary magnetospheres; Distribution functions; Electric fields; Evolution; Flow distribution; Flow pattern; Ion velocity; Ions; Magnetic fields; Protons; Solar wind; Tracing; Velocity distribution
• ISSN: 2169-9380; 2169-9402; 2169-9402
• DOI: 10.1029/2024JA032757

At a low activity comet the plasma is distributed in an asymmetric way. The hybrid simulation code Amitis is used to look at the spatial evolution of ion velocity distribution functions (VDFs), from the upstream solar wind (SW) to within the comet magnetosphere where the SW is heavily mass‐loaded by the cometary plasma. We find that the spatial structures of the ions and fields form a highly asymmetric induced magnetosphere. The VDFs of SW and cometary ions vary drastically for different locations in the comet magnetosphere. The shape of the VDFs differ for different species. The SW protons show high anisotropies that occasionally resemble partial rings, in particular at small cometocentric distances. A second, decoupled, proton population is also found. Solar wind alpha particles show similar anisotropies, although less pronounced and at different spatial scales. The VDFs of cometary ions are mostly determined by the structure of the electric field. We perform supplementary dynamic particle backtracing to understand the flow patterns of SW ions that lead to these anisotropic distributions. This tracing is needed to understand the origin of cometary ions in a given part of the comet magnetosphere. The particle tracing also aids in interpreting observed VDFs and relating them to spatial features in the electric and magnetic fields of the comet environment. Key Points Hybrid simulations with the Amitis code for a low activity comet show the formation of an asymmetric induced magnetosphere The velocity distributions of solar wind protons form partial rings in the simulation as previously reported by observations Backtracing the cometary ions in the tail shows that the shape of their velocity distributions is driven by electric field structures