Miniature Magnetized Shocks from Plasma Collision with Minimagnetospheres

Minimagnetospheres have been found to exist above the lunar surface, resulting from the solar wind plasma interaction with localized magnetic patches on the Moon's crust. The size of these objects are on the order of the plasma kinetic scales, lying beyond the validity of magnetohydrodynamics, and therefore constitute unique "laboratories" to investigate the role of kinetic effects in magnetosphere formation/dynamics. In this work we investigate the conditions under which collisionless magnetized shocks are formed due to plasma interaction with such small-scale (order of the plasma kinetic scales) magnetic obstacles. We have performed multidimensional particle-in-cell (PIC) simulations, that capture both electron and ion kinetics from first principles, in order to accurately describe the important microphysical processes associated with these scenarios. We observe the clear formation of a magnetized shock when the typical size of the magnetic obstacle is greater than ~ 2 ion-Larmor-radii. This condition may be fulfilled in lunar minimagnetospheres, whose dimensions are on the order of the ion inertial length, only for low Mach number shocks (<2). The effective size of the magnetic obstacle, however, is strongly dependent on the relative orientation of its own field to that of the plasma; antiparallel field configurations increase the effective size of the magnetic obstacle, allowing the clear formation of a shock, whereas in parallel field configurations the effective size of the magnetic obstacle is decreased, inhibiting shock formation in some cases. PIC simulations further capture electron-scale surface instabilities that modulate the magnetopause boundary and other streaming instabilities resulting from the interaction between the upstream and reflected plasma.