Large‐scale solar wind flow around Saturn’s nonaxisymmetric magnetosphere

Abstract

The interaction between the solar wind and a magnetosphere is central to the dynamics of a planetary system. Here we address fundamental questions on the large‐scale magnetosheath flow around Saturn using a 3‐D magnetohydrodynamic (MHD) simulation. We find Saturn’s polar‐flattened magnetosphere to channel ~20% more flow over the poles than around the flanks at the terminator. Further, we decompose the MHD forces responsible for accelerating the magnetosheath plasma to find the plasma pressure gradient as the dominant driver. This is by virtue of a high‐β magnetosheath and, in turn, the high‐MA bow shock. Together with long‐term magnetosheath data by the Cassini spacecraft, we present evidence of how nonaxisymmetry substantially alters the conditions further downstream at the magnetopause, crucial for understanding solar wind‐magnetosphere interactions such as reconnection and shear flow‐driven instabilities. We anticipate our results to provide a more accurate insight into the global conditions upstream of Saturn and the outer planets.Key PointsPreferential flow over the poles with at least 20% more mass flux channeled over the poles than around the equator at SaturnMagnetosheath flow predominantly driven by pressure gradient forces owing to the high‐β regimeUnbalanced forces exerted on the magnetosheath plasma leading to twisting. Parker spiral IMF exhibits enhanced Bz in the magnetosheathPlain Language SummaryRotating gas giants are bulged along the equator and flattened along the poles owing to their embedded plasma disks. This paper addresses some fundamental questions on how the solar wind interacts with an oblate magnetosphere as the obstacle. Results show and quantify preferential flow over the poles compared to around the equator. The magnetospheres of gas giants can be thought of as somewhere between a sphere, where flow travels symmetrically around in all directions, and a wing, where flow travels entirely over and beneath. This leads to unbalanced forces exerted on the upstream plasma, which substantially changes the magnetic field conditions upstream of the planets. Such conditions are what control means of energy exchange between the solar wind and the planets, namely, reconnection and shear‐driven instabilities.