Modeling Sodium Energization at Mercury.

Mercury, the only other rocky planet in the Solar System with an intrinsic dipole field, exists in a unique and exciting parameter space due to the planet's relatively weak magnetic dipole and its proximity to the Sun. One of the more puzzling results revealed by MESSENGER, the first Mercury orbiter, is that Na+ (and other planetary ions) are regularly observed with energies of 1 to 10 keV in Mercury's northern magnetospheric cusp. Sodium is a dominant planetary particle at Mercury, and neutral sodium atoms are therefore a major component of the planet's exosphere. Na+ ions are mostly likely formed at energies on the order of 1 eV through photoionization of exospheric Na, and although traditional drifts and magnetospheric pickup are likely involved, there is no single mechanism yet hypothesized at Mercury that can readily account for the energization of these ions by three or more orders of magnitude. We look to computer modeling to bridge the gap between physical intuition about the creation of these ions and their energization up to order 1-10 keV. In this work, modeling of Na+ acceleration is undertaken through use of the Particle-in-Cell code the Adaptive Mesh Particle Simulator (AMPS) in combination with MHD simulations of Mercury from the model BATSRUS. Modeling reveals that a drift process can increase the perpendicular velocity (and therefore gyroradius) of sodium inside the closed-field region on the dayside such that it may transport across the magnetopause and enter into the magnetosheath, and it suggests that a pickup process in the sheath is the dominant method of acceleration of these heavy ions.