Plasmapause formation at Saturn

Cassini observations during a rapid, high‐latitude, dawnside pass from Saturn's lobe to inner magnetosphere on 25 June 2009 provide strong evidence for the formation of a “plasmapause” at Saturn by Vasyliunas‐type nightside reconnection of previously mass‐loaded flux tubes. A population of hot, tenuous plasma that lies between the lobe and the dense inner magnetospheric plasma is consistent with a region formed by very recent injection from a reconnection region in the tail, including low density, high temperature, supercorotational flow, a significant O + content, and the near‐simultaneous observation of enhanced Saturn kilometric radiation emissions. The sharp boundary between that region and the cool dense inner magnetospheric plasma thus separates flux tubes that were involved in the reconnection from those that successfully traversed the nightside without mass loss. This event demonstrates that tail reconnection can strip off inner magnetospheric plasma in to at least dipole L  = 8.6. Clear evidence of flux tube interchange driven by the sharp boundary is found, both inward moving flux tubes of hotter plasma and, for the first time, the outward moving cool population. The outward moving cool regions have azimuthal sizes less than 1  R S , were probably created within the past 1.2 h, and have outflow speeds greater than about 5 km/s. At the outer edge of the reconnected region, there is also a possible signature of Dungey‐type lobe reconnection following the initial Vasyliunas‐type reconnection. Observations from this event are entirely consistent with previously described global MHD simulations of tail reconnection, plasmoid departure, and Saturnward injection of reconnected flux.