Physics – Plasma Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p53c..04b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P53C-04
Physics
Plasma Physics
[2740] Magnetospheric Physics / Magnetospheric Configuration And Dynamics, [2756] Magnetospheric Physics / Planetary Magnetospheres, [7800] Space Plasma Physics
Scientific paper
Saturn's magnetic dipole axis is almost perfectly aligned with its rotation axis and thus one expects a totally axisymmetric magnetosphere. It was therefore suprising to find periodic phenomena in almost all of the magnetospheric measurements, but a lack of a longitudinal asymmetry to account for those phenomena. The Saturn Kilometric Radiation (SKR) was the first remote measurement to show clear periodicities, which was used to define a rotational period of the planet, in similar way that had been done for Jupiter. However, the SKR-derived period later changed by several minutes, which directly tells us that SKR is only loosely related to the true rotation period of the planet and is a magnetospheric and/or ionospheric phenomenon. The Cassini measurements have revealed periodic modulations close to the SKR-derived period in the magnetic field, energetic particles, energetic neutral atoms (ENA), and more. Any hypothesis attempting to explain the origin of the periodic behavior must be able to explain how the planetary rotation is communicated out to the largely subcorotating magnetosphere, and explain the mechanisms that relate the periodic modulations in the different measurements. In this paper we first review the magnetospheric observations of periodic modulations, the statistical analyses that have been performed and their interpretations. In light of those observational facts, we then discuss the hypotheses that have been put forward to explain the occurence of the magnetospheric periodicities, the mechanisms that relate them, and the underlying cause that relates it to planetary rotation. Hypotheses to be discussed include, but are not limited to (1) Cold plasma is released periodically down the tail like a dripping faucet, which causes energetic particle injections, which in turn are responsible for the periodic modulations in magnetic field and SKR; (2) The longitudinally asymmetric energetic particle pressure distribution drift around the planet with a period close to the SKR period and trigger plasmoid releases when it passes the stretched field around midnight; (3) The ionosphere displays a longitudinal asymmetric conductance, which affects magnetospheric dynamics; and (4) The magnetic field displays a longitudinal anomaly at low altitudes, which affects magnetospheric dynamics.
C:son Brandt Pontus
Carbary James F.
Hansen Kenneth Calvin
Khurana Krishan K.
Mitchell Donald G.
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