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Azimuthal magnetic fields in saturn's magnetosphere: Effects associated with plasma sub-corotation and the magnetopause-tail current system

Bunce, E. J. and Cowley, S. W. H. and Wild, J. A. (2003) Azimuthal magnetic fields in saturn's magnetosphere: Effects associated with plasma sub-corotation and the magnetopause-tail current system. Annales Geophysicae, 21 (8). pp. 1709-1722. ISSN 0992-7689

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    Abstract

    We calculate the azimuthal magnetic fields expected to be present in Saturn’s magnetosphere associated with two physical effects, and compare them with the fields observed during the flybys of the two Voyager spacecraft. The first effect is associated with the magnetosphere-ionosphere coupling currents which result from the sub-corotation of the magnetospheric plasma. This is calculated from empirical models of the plasma flow and magnetic field based on Voyager data, with the effective Pedersen conductivity of Saturn’s ionosphere being treated as an essentially free parameter. This mechanism results in a ‘lagging’ field configuration at all local times. The second effect is due to the day-night asymmetric confinement of the magnetosphere by the solar wind (i.e. the magnetopause and tail current system), which we have estimated empirically by scaling a model of the Earth’s magnetosphere to Saturn. This effect produces ‘leading’ fields in the dusk magnetosphere, and ‘lagging’ fields at dawn. Our results show that the azimuthal fields observed in the inner regions can be reasonably well accounted for by plasma sub-corotation, given a value of the effective ionospheric Pedersen conductivity of ∼1–2mho. This statement applies to field lines mapping to the equator within ∼8 RS (1 RS is taken to be 60 330 km) of the planet on the dayside inbound passes, where the plasma distribution is dominated by a thin equatorial heavy-ion plasma sheet, and to field lines mapping to the equator within ∼15 RS on the dawn side outbound passes. The contributions of the magnetopause-tail currents are estimated to be much smaller than the observed fields in these regions. If, however, we assume that the azimuthal fields observed in these regions are not due to sub-corotation but to some other process, then the above effective conductivities define an upper limit, such that values above ∼2 mho can definitely be ruled out. Outside of this inner region the spacecraft observed both ‘lagging’ and ‘leading’ fields in the post-noon dayside magnetosphere during the inbound passes, with ‘leading’ fields being observed both adjacent to the magnetopause and in the ring current region, and ‘lagging’ fields being observed between. The observed ‘lagging’ fields are consistent in magnitude with the sub-corotation effect with an effective ionospheric conductivity of ∼1–2 mho, while the ‘leading’ fields are considerably larger than those estimated for the magnetopause-tail currents, and appear to be indicative of the presence of another dynamical process. No ‘leading’ fields were observed outside the inner region on the dawn side outbound passes, with the azimuthal fields first falling below those expected for sub-corotation, before increasing, to exceed these values at radial distances beyond ∼15–20 RS , where the effect of the magnetopause-tail currents becomes significant. As a by-product, our investigation also indicates that modification and scaling of terrestrial magnetic field models may represent a useful approach to modelling the three-dimensional magnetic field at Saturn.

    Item Type: Article
    Journal or Publication Title: Annales Geophysicae
    Uncontrolled Keywords: Planetary DCS-publications-id ; art-672 ; DCS-publications-credits ; iono ; DCS-publications-personnel-id ; 104
    Subjects: T Technology > T Technology (General)
    Departments: Faculty of Science and Technology > School of Computing & Communications
    Faculty of Science and Technology > Physics
    ID Code: 6616
    Deposited By: ep_importer_dcs
    Deposited On: 21 Apr 2008 11:53
    Refereed?: Yes
    Published?: Published
    Last Modified: 26 Jul 2012 18:15
    Identification Number:
    URI: http://eprints.lancs.ac.uk/id/eprint/6616

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