Na+ Energization on Mercury's Dayside - Modeling and MESSENGER Observations

Data collected by the Fast Imaging Plasma Spectrometer aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft showed singly-charged sodium ions at energies of between 1 and 10 keV in Mercury's northern planetary cusp. These particles are likely generated by either photoionization or charge exchange, which would put their initial energy as ions at roughly 1 eV. No known acceleration mechanism local to the cusp can account for this energy gain. In this work, we therefore undertake fully-kinetic simulation of particle trajectories through the electric and magnetic fields output from a global MHD model of Mercury's magnetosphere, in search of a plausible explanation for the source of this energization. We find that, despite the low initial energy of these ions, even Na⁺ with initial energy of 1 eV are not permanently trapped within the closed field region of the planet's magnetosphere. Instead, these Na⁺ ions can be picked up by the Dungey-cycle return flow in the dayside magnetosphere, in some cases providing sufficient energy for their escape into the magnetosheath. They can then be rapidly picked up into the magnetosheath flow, where they are accelerated up to order 10 keV. Taken together, these results show that a 2-stage ion pickup process is very likely responsible for the energies of many of the Na⁺ ions observed in Mercury's northern magnetospheric cusp by MESSENGER.