HR: 1340h
AN: SM33A-1253    [Abstracts]
TI: Coupling the Io plasma torus to Jupiter's ionosphere: Cassini observations of longitudinal variations
AU: Steffl, A J
EM: steffl@colorado.edu
AF: University of Colorado/LASP, 392 UCB, Boulder, CO 80309-0392 United States
AU: Delamere, P A
EM: delamere@lasp.colorado.edu
AF: University of Colorado/LASP, 392 UCB, Boulder, CO 80309-0392 United States
AU: * Bagenal, F
EM: bagenal@colorado.edu
AF: University of Colorado/LASP, 392 UCB, Boulder, CO 80309-0392 United States
AB: The ionization of $\sim$1 ton/second of neutral material from Io's atmosphere produces a dense ($\sim$2000 cm$^{-3}$) plasma torus. Plasma coupling works to enforce corotation between the torus and Jupiter's magnetic field; however, significant corotational breakdown is found in the middle magnetosphere between 20-30 R$_J$ in the centrifugal equator plane and, interestingly, the breakdown point maps to Jupiter's main auroral oval. In addition, the torus is known to deviate from corotation by up to 3% near Io's orbit (6 R$_J$). Recent observations of the torus by the Cassini Ultraviolet Imaging Spectrograph (UVIS) have revealed significant azimuthal variations in ion composition along with roughly 1.4% deviation from corotation ($\sim$1 km/s at the orbit of Io). We find that the amplitude of the azimuthal compositional asymmetry varies with time. For the minor ion species of S II and S IV, the amplitude varies between 5-20%, while the major ion species of S III and O II remain relatively constant with amplitudes ranging between 2-5%. This amplitude of the azimuthal compositional asymmetry appears to be modulated by the magnetic longitude of the plasma, such that when the peak in the S II mixing ratio (and correspondingly, the minimum in S IV) is aligned with a System III longitude of $\sim$200$^\circ$, the amplitude is greatest. We are able to model this behavior with a longitudinal modulation of corotational lag and/or a hot electron source. We will discuss the implication of the Cassini observations with regard to magnetosphere-ionosphere coupling at Jupiter.
DE: 5719 Interactions with particles and fields
DE: 5737 Magnetospheres (2756)
DE: 5757 Remote sensing
DE: 5780 Tori and exospheres
SC: SPA-Magnetospheric Physics [SM]
MN: 2004 AGU Fall Meeting