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Saturn's magnetosheath flows exhibit significant dawn/dusk asymmetry. The dawnside flows are reduced from expectation, suggesting significant momentum transport through the magnetopause boundary where the flow shear is maximized. It has been suggested that the solar wind interaction with the giant magnetospheres is, in fact, dominated by a viscous-like interaction governed by the Kelvin-Helmholtz instability. In three dimensions, the Kelvin-Helmholtz instability can generate small-scale and intermittent magnetic reconnection due, in part, to a twisted magnetic field topology. The net result is a field line threading of the magnetopause boundary and the generation of Maxwell shear stresses. Here we present three-dimensional hybrid simulations (kinetic ions and massless fluid electrons) of conditions similar to Saturn's dawnside magnetopause boundary to quantify the viscous-like, tangential drag. Using model-determined momentum fluxes, we estimate the effect on dawnside sheath flows and find very good agreement with observations.

Plain Language Summary Planets with magnetic fields generate a cavity in space known as a magnetosphere. At Earth, the solar wind is capable of flowing around the magnetospheric obstacle with minimal drag along the equatorial flanks. This does not appear to be the case at Saturn. A significant solar wind flow asymmetry was observed with the Cassini Spacecraft, indicating substantial drag on the dawnside boundary. Using high-resolution computer simulations that capture individual charged particle (ion) motion, we have demonstrated the basic physical processes responsible for this drag force. We find that strong flow shears generate vortices (i.e., the Kelvin-Helmholtz instability) that twist the magnetic field, directly connecting the solar wind and planetary magnetic fields in an intermittent and patchy manner. The drag force on the solar wind flow is estimated and found to be in good agreement with the simulations. This is the first three-dimensional computer simulation of Saturn's interaction with the solar wind that includes individual charged particle motion. Previous simulations have been limited to two dimensions and did not capture the complicated magnetic connectivity between Saturn and the solar wind.