Centrifugally Driven Radial Convection of Plasma in Saturn's Inner Magnetosphere

[摘要] Theoretical studies have pointed out that in the inner part of a rapidly rotating magnetosphere such as that of Jupiter and Saturn, the dominant driving factor of radial plasma transport is the centrifugal interchange instability. After the in-situ observations of the Cassini spacecraft became available, the major observable signature of radial plasma convection is reported as a series of longitudinally localized injections and simultaneous drift dispersions of hot tenuous plasma from the outer magnetosphere. The Cassini Plasma Spectrometer (CAPS) and the Cassini Magnetospheric Imaging Instrument (MIMI) have observed signatures of these processes frequently, providing direct evidence for Saturn;;s magnetospheric centrifugally driven convective motions, in which the radial transport of plasma comprises hot, tenuous plasma moving inward and cooler, denser plasma moving outward. With methods similar to those of Hill et al. [2005], we study the statistics of the properties of such events by analyzing CAPS data from 26 Cassini orbits. A statistical picture of their major characteristics is developed, including the distributions of ages, longitudinal widths, radial distances, and longitudes and local times, which are all consistent with previous results. An unexpected longitude modulation of these events appears in the old (SLS) longitude system, while no such modulation seems to exist in 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 find that the inflow channels occupy only a small fraction (~ 7%) of the total available longitudinal space, indicating that the inflow sectors are much narrower than the outflow sectors. Furthermore, we assume that the plasma is largely confined to a thin equatorial sheet, and calculate its thickness by deriving the centrifugal scale height profile based on the CAPS observations. We also present the radial and longitudinal dependences of flux tube mass content, as well as the total ion mass between 5 and 10 Saturn 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