Coupling the Io plasma torus to Jupiter's ionosphere: Cassini observations of longitudinal variations

The ionization of ∼1 ton/second of neutral material from Io's atmosphere produces a dense ( ∼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 ( ∼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 ∼200^o, 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.