Do we need to consider electron kinetic effects to properly model a planetary magnetosphere: the case of Mercury

TL;DR

This study compares hybrid and full particle-in-cell simulations of Mercury's magnetosphere to evaluate the importance of electron kinetic effects, revealing significant impacts on particle energization and localized physical processes.

Contribution

It demonstrates that including electron kinetic physics alters specific physical processes and energization mechanisms, providing a more detailed understanding of Mercury's magnetospheric dynamics.

Findings

Enhanced shock reformation in the foreshock region.

Sharper Kelvin-Helmholtz ripples on the magnetopause.

Thinner current sheets in the magnetotail with kinetic electrons.

Abstract

The magnetosphere of Mercury is studied using an implicit full particle in cell simulation (PIC). We use a hybrid simulation where ions are full particles and electrons are considered as a fluid to start a full PIC simulation where electrons are also particles and follow their distribution function. This approach allows us to estimate the changes introduced by the electron kinetic physics. We find that the overall macroscopic state of the magnetosphere of Mercury is little affected but several physical processes are significantly modified in the full PIC simulation: the foreshock region is more active with more intense shock reformation, the Kelvin-Helmholtz rippling effects on the nightside magnetopause are sharper, and the magnetotail current sheet becomes thinner than those predicted by the hybrid simulation. The greatest effect of the electron physics, comes from the processes of particle energization. Both species, not just the electrons, are found to gain more energy when kinetic electron processes are taken into account. The region with the most energetic plasma is found on the dusk side of the tail where magnetic flux ropes are formed due to reconnection. We find that the ion and electron energization is associated with the regions of reconnection and the development of kinetic instabilities caused by counter-streaming electron populations. The resulting electron distributions are highly non Maxwellian, a process that neither MHD nor hybrid models can describe.