Kinetic-Scale Physics of a Multi-Species Solar Wind

TL;DR

This study investigates how varying alpha-to-proton ratios in the solar wind influence plasma boundaries and energy transfer processes around a comet, revealing that species composition significantly affects magnetosphere size and energy dynamics.

Contribution

It provides the first detailed analysis of multi-species solar wind effects on cometary plasma boundaries using hybrid simulations, highlighting the role of ion composition in kinetic-scale physics.

Findings

Higher alpha-to-proton ratios enlarge the magnetosphere

Protons predominantly transfer energy to electromagnetic fields

Alpha particles act as load or generator depending on conditions

Abstract

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. Protons are the dominant ion species in the solar wind; however, sometimes the density of alpha particles increases significantly. We analyse the effect of different solar wind alpha-to-proton ratios on the plasma boundaries of the induced cometary magnetosphere. In addition, we investigate the energy transfer between the solar wind ions, the cometary ions, and the electromagnetic fields. Using the hybrid model Amitis, we simulate two different alpha-to-proton ratios and analyse the resulting plasma structures. We calculate the power density (E.J) of all three ion species (solar wind protons and 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. 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. 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, typically protons, since different ion gyroradii give different flow patterns for the individual species.