Cassini evidence for rapid interchange transport at Saturn
André, N.; Persoon, A. M.; Coates, A. J.; Dougherty, M. K.; Hospodarsky, G. B.; Mitchell, D. G.; Paranicas, C.; Smith, H. T.; Young, D. T.; Menietti, J. D.; Mauk, B. H.; Hill, T. W.; Rymer, A. M.
United States, Netherlands, United Kingdom
Abstract
During its tour Cassini has observed numerous plasma injection events in Saturn's inner magnetosphere. Here, we present a case study of one "young" plasma bubble observed when Cassini was in the equatorial plane. The bubble was observed in the equatorial plane at ∼7 Saturn radii from Saturn and had a maximum azimuthal extent of ∼0.25 Rs (Rs=Saturn radius ∼60330 km). We show that the electron density inside the event is lower by a factor ∼3 and the electron temperature higher by over an order of magnitude compared to its surroundings. The injection contains slightly increased magnetic field magnitude of 49 nT compared with a background field of 46 nT. Modelling of pitch angle distributions inside the plasma bubble and measurements of plasma drift provide a novel way to estimate that the bubble originated between 9< L<11 and had an average radial propagation speed of ∼260+60/-70 km s -1. An independent estimate of the speed of the injection following theoretical work of Pontius et al. [1986. Steady State Plasma transport in a Corotation-Dominated Magnetosphere. Geophys. Res. Lett. 13(11), 1097-1100] based on the mass per unit flux gives a maximum radial propagation speeds of 140 km s -1. These results are similar to those found by Thorne et al. [1997. Galileo evidence for rapid interchange transport in the Io torus. Geophys. Res. Lett. 24, 2131] for one event observed in Jupiter's magnetosphere near Io. We therefore suggest this is evidence of the same process operating at both planets.