Kinetic-scale physics of multi-species solar wind

Interaction with a comet

¹ Swedish Institute of Space Physics, Kiruna, Sweden
² Department of Physics, Umeå University, Umeå, Sweden

^★ Corresponding author: anja.moslinger@umu.se

Received: 16 October 2024
Accepted: 25 July 2025

Abstract

Context. The solar wind affects the plasma environment around all Solar System bodies. A strong solar wind dynamic pressure pushes plasma boundaries closer to these objects. For small objects, kinetic effects on scales smaller than an ion gyroradius play an important role and species with various mass-per-charge may act differently. In this case, the solar wind composition can be important.

Aims. Protons are the dominant ion species in the solar wind, however, the density of alpha particles can sometimes increase significantly. We analyse the effect of different solar wind alpha-to-proton ratios on the plasma boundaries of a weakly outgassing comet (Q ≈ 1 − 2 × 10²⁶ s⁻¹). In addition, we investigate the energy transfer between the solar wind ions, the cometary ions, and the electromagnetic fields.

Methods. Using the hybrid model Amitis, we simulated two different alpha-to-proton ratios and analysed the resulting plasma structures. We calculated the power density (E ∙ J) of all three ion species (solar wind protons, alphas, and cometary ions) to identify load and generator regions. The integrated 1D power density shows the evolution of the power density from the upstream solar wind to downstream of the nucleus.

Results. A higher alpha-to-proton ratio leads to a larger comet magnetosphere, but weaker magnetic field pile-up. The protons transfer energy to the fields and the cometary ions in the entire upstream region and the pile-up layer. Upstream of the nucleus, alphas are inefficient in transferring energy and can act as a load, especially for low alpha-to-proton ratios. The transfer of energy from alphas to cometary ions happens further downstream due to their larger inertia.

Conclusions. For a multi-species solar wind, the mass loading and energy transfer upstream of the pile-up layer will be most efficient for the species with the lowest inertia. Protons represent such a species, and different gyroradii of the ions result in distinct flow patterns for each individual species.