Pressure-Driven Current Systems in the Solar System

Abstract:

The ring current systems of Earth, Saturn and Jupiter are compared. At Earth, the ring current is primarily associated with energetic particle pressure gradients and its morphology and dynamics are governed primarily by the direction of the interplanetary magnetic field (IMF). For southward IMF the ring current is asymmetric (storm "main phase") with a pressure peak that is fixed around the midnight sector, where the bulk of the pressure lies in the approximate range of 10-300 keV protons. Depending on the duration of the southward IMF, O+ may dominate the pressure. The main phase ring current dictates the structure of the inner magnetosphere. For northward IMF, the ring current becomes symmetric around Earth. The asymmetric main-phase pressure distributions drive a 3D current system that couples the magnetosphere and ionosphere, often referred to as the Region-2 current system. At Saturn, the plasma pressure of the energetic protons and O+ in the approximate same (10-300 keV) energy range dominates roughly outside 9 RS as a result of periodic large-scale injections in the midnight sector. Because of the strong corotational electric field at Saturn, the injected energetic particle distributions form a PRC system drifting around the planet, also distorting the magnetic field in a periodic fashion as seen by the relatively slow-moving Cassini spacecraft. At Jupiter, the current systems around the planet form a magnetodisc, whose formation and stability is dependent on the centrifugal forces of the radially outward diffusing low-energy plasma. However, the structure of the magnetodisc is likely also formed as a result of pressure anisotropies in the energetic particles. The force balance of the three systems is contrasted against each other and their roles in the magnetic structure and ionospheric coupling of the inner magnetosphere are discussed.