Hybrid code simulations of the solar wind interaction with Pluto

Pluto's low gravity implies that the atmosphere is only weakly bound and that significant hydrodynamic outflow can exist. Though surface spectroscopy of Pluto has revealed methane frost, the dominant escaping neutral gas is thought to be N-2. These escaping neutrals are photoionized, and the heavy ions (N-2(+)) move away from Pluto in the direction perpendicular to the solar wind flow (i.e., nearly unmagnetized relative to the length scales of the plasma interaction region). The turning distance of the solar wind protons at the magnetic pileup boundary is large compared to the interaction region. As a result, large ion gyroradius effects determine Pluto's highly asymmetric interaction with the solar wind. We use a three-dimensional hybrid code (fluid electrons, kinetic ions) to investigate the geometry of the interaction region for a variety of possible atmospheric escape rates in anticipation of the New Horizons encounter with Pluto. We find considerable structuring in the wake region due to bi-ion waves and Kelvin-Helmholtz waves. The shock structures vary from a simple Mach cone for low escape rates (similar to 2 x 10(26) s(-1)) to a full detached bow shock for large escape rates (similar to 2 x 10(28) s(-1)).