Centrifugally Driven Radial Convection of Plasmain Saturn's Inner Magnetosphere

[摘要] Theoretical studies have pointed out that in the inner part of a rapidly rotatingmagnetosphere such as that of Jupiter and Saturn, the dominant driving factor ofradial plasma transport is the centrifugal interchange instability. After the in-situobservations of the Cassini spacecraft became available, the major observablesignature of radial plasma convection is reported as a series of longitudinallylocalized injections and simultaneous drift dispersions of hot tenuous plasma from theouter magnetosphere. The Cassini Plasma Spectrometer (CAPS) and the CassiniMagnetospheric Imaging Instrument (MIMI) have observed signatures of theseprocesses frequently, providing direct evidence for Saturn's magnetosphericcentrifugally driven convective motions, in which the radial transport of plasmacomprises hot, tenuous plasma moving inward and cooler, denser plasma movingoutward.With methods similar to those of Hill et al. [2005], we study the statistics of theproperties of such events by analyzing CAPS data from 26 Cassini orbits. A statisticalpicture of their major characteristics is developed, including the distributions of ages,longitudinal widths, radial distances, and longitudes and local times, which are allconsistent with previous results. An unexpected longitude modulation of these eventsappears in the old (SLS) longitude system, while no such modulation seems to existin the new (SLS2/SLS3) longitude system. A Lomb periodogram analysis, however,reveals no significant periodic modulation of these events.We further extend the statistical sample of these injection/dispersion events and findthat the inflow channels occupy only a small fraction (~ 7%) of the total availablelongitudinal space, indicating that the inflow sectors are much narrower than theoutflow sectors. Furthermore, we assume that the plasma is largely confined to a thinequatorial sheet, and calculate its thickness by deriving the centrifugal scale heightprofile based on the CAPS observations. We also present the radial and longitudinaldependences of flux tube mass content, as well as the total ion mass between 5 and 10Saturn radii. Combining these results, we estimate a global plasma mass outflow rate~ 280 kg/so

[发布机构] Rice University

[效力级别] Astronomy