Other
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.p44a..04m&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P44A-04
Other
5734 Magnetic Fields And Magnetism, 5744 Orbital And Rotational Dynamics (1221), 2736 Magnetosphere/Ionosphere Interactions (2431), 2740 Magnetospheric Configuration And Dynamics, 2756 Planetary Magnetospheres (5443, 5737, 6033)
Scientific paper
The period of Saturn kilometric radiation (SKR) modulation established by Voyagers 1 and 2 in 1980 and 1981 (10 hours, 39 minutes, 22.4 +/- 7s) has been adopted by the International Astronomical Union as the official rotation period of Saturn. Other quantities seen to exhibit modulation at about the same period include the magnetic field, energetic electron spectral slope, and energetic neutral atom (ENA) emission. However first the Ulysses spacecraft, and later Cassini, have measured a significantly different the SKR period than the Voyagers (approximately 10 hours, 45minutes). This change is problematic, because if the field is truly locked to Saturns rotation, this would imply a huge change in angular momentum over a relatively short period. Furthermore, no consensus model has been accepted to explain how the effects of the rotation are communicated from the planetary body out to distances as large as over 20 Rs (Saturn radii). In this paper, we explore the possibility that the observed SKR period is not Saturns intrinsic rotation period, but rather stems from friction between the ionosphere and Saturns zonal wind flows. We suggest that the SKR location reflects a high conductivity anomaly in Saturns ionosphere, whereby rigid rotation is imposed on that part of the magnetosphere that connects via the magnetic field and field-aligned currents with this high conductivity anomaly (this is similar to the hypothesis of the camshaft model for the magnetic perturbation suggested by Espinosa et al., 2003). In that work, Espinosa et al. suggest that the high conductivity region exists because of a high order magnetic anomaly, that affects ionospheric conductivity locally. We extend that model to include a feed-back loop with the magnetosphere. In this scenario, a magnetospheric disturbance initially triggered by interaction with the field-aligned currents results in additional energy deposition in the ionosphere. This further increases the ionospheric conductivity, but more importantly ties the high conductivity region to the middle magnetospheric disturbance. The local zonal thermospheric winds, if they are in frictional equilibrium with the conducting ionosphere, will move the high conductivity region (and the rest of the ionosphere) at whatever velocity they are traveling. With the feedback between the magnetospheric heating and the ionospheric conductivity established, the field-aligned current remains rooted in the wind-convected ionosphere, and so the active (SKR) meridian will slowly drift away from the core-rooted magnetic anomaly. The process will be self-sustaining for a certain length of time, until it fizzles out (either because the magnetospheric instability is no longer sufficiently close to triggering, or because the source particle populations are depleted, or whatever). Some time later, when the magnetosphere has stored sufficient energy to prime the instability, it will go off again, again starting at the location of the magnetic anomaly. Espinosa, S. A., D. J. Southwood, and M. K. Dougherty, How can Saturn impose its rotation period in a noncorotating magnetosphere? J. Geophys. Res., 108(A2), 1086, doi:10.1029/2001JA005084, 2003
C:son Brandt Pontus
Carbary James F.
Dougherty K. M. K. M.
Gurnett Donald A.
Jones Gail
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