Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p11b1262k&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P11B-1262
Physics
2736 Magnetosphere/Ionosphere Interactions (2431), 2740 Magnetospheric Configuration And Dynamics, 5737 Magnetospheres (2756), 6939 Magnetospheric Physics (2700)
Scientific paper
Explanations of the periodic modulation of Io-associated decametric radiation from Jupiter invoke the rotating tilted dipole moment of Jupiter as a clock. Saturn, with an axially symmetric dipole moment, lacks such a clock, yet the corresponding kilometric radiation (SKR) is also periodically modulated (with a slowly drifting period). It remains unclear what controls the varying intensity of the emissions although it is widely accepted that the SKR power is generated in regions of intense field-aligned currents principally in the morning sector. Here we propose a model based on examination of the magnetospheric magnetic field that can account for the observed properties of SKR. Large scale perturbations of the azimuthal component of the magnetic field, corresponding to upward field- aligned currents, have been detected at invariant latitudes above 70 degrees on the high inclination orbits of the Cassini spacecraft in late 2006. The currents peak in intensity in the outer morning-side magnetosphere in a region where flux tubes extended in the antisolar direction drape over lower latitude flux tubes that remain roughly in meridian planes. The current strength is modulated by the rotating cam magnetic field, the periodic field structure observed deeper inside the magnetosphere, with intensification occurring when the peak upward cam currents rotate into the morning sector. Noting a strong morning-afternoon asymmetry of the magnetic structure of the outer magnetosphere, we argue that the spatial localization of the most intense SKR emissions can be understood if the power intensifies where plasma rotates into the morning sector, a region where the distance to the magnetopause diminishes steadily and the rotating cam structure encounters a strong field shear. The field shear to which we appeal is imposed by interaction with the solar wind, whose link to SKR properties has been noted in the past. Although there is evidence that SKR sources are found in both northern and southern regions at magnetically conjugate locations, it is not clear whether the northern hemisphere source is as strong as that in the south. Depending on whether the dominant emissions occur in one or two hemispheres, we require slightly different but closely related scenarios for ways in which the rotating cam structure produces peak SKR power.
Galland Kivelson Margaret
Southwood David J.
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