Magnetic flux circulation in the rotationally-driven giant magnetospheres

Abstract:

The giant planet magnetodiscs are shaped by the radial transport of plasma originating in the inner magnetosphere. Along with plasma transport, magnetic flux transport is a key aspect of the stretched magnetic field configuration of the magnetodisc. While net mass transport is outward (ultimately lost to the solar wind), magnetic flux conservation requires a balanced two-way transport process involving magnetic reconnection. A key property of flux transport is the azimuthal bend forward or bend back of the magnetic field. The bend back configuration is an expected property of the magnetodisc with net mass outflow, but the bend forward configuration can be achieved with the rapid inward motion of mostly empty flux tubes following thin current sheet reconnection. We present a comprehensive analysis of current sheet crossings in Saturn's magnetosphere using Cassini MAG data from 2004 to 2012 in an attempt to quantify the circulation of magnetic flux, emphasizing local time dependence. We find that the bend forward cases are limited mostly to the post-noon sector, indicating that much of the reconnection returning flux to the inner magnetosphere occurs in the subsolar and dusk sector. We also find a complex and patchy network of reconnection sites, supporting the idea that plasma can be lost on small-scales through a ``drizzle''-like process rather than a single extended X-line as originally envisioned for the Vasyliunas cycle. Auroral implications for the observed flux circulation and comparisons with Jupiter will also be presented.