Do we need to consider electrons’ kinetic effects to properly model a planetary magnetosphere? The case of Mercury

While a global full particle-in-cell (PIC) model of Earth’s magnetosphere cannot yet achieve enough resolution to be quantitatively accurate, the full model of smaller planets is becoming possible and can be used to learn what extra effects the full electron kinetic physics brings. From smaller planets we can learn much relevant also for the Earth. 

In this presentation, we investigate the global magnetosphere of Mercury 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 (1836 m_ion/m_e) 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 effected 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 included. The region with the most energetic plasma is 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.

Reference: Lapenta, G., Schriver, D. Walker, R.J.,  Berchem, J., Echterling, N.F.,  El Alaoui, M., Travnicek, P., (2022) Do we need to consider electron kinetic effects to properly model a planetary magnetosphere: the case of Mercury, arXiv:2201.01653, submitted.