Abstract
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 ~ 1–2 mho. 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.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)
Highlights
Analysis of magnetometer data obtained during the three flybys of Saturn’s magnetosphere undertaken to date, by Pioneer-11 in 1979, and by Voyagers-1 and -2 in 1980 and 1981, respectively, have revealed a number of surprising features of Saturn’s magnetic field
The modelled fields are again too small to make a significant contribution out to radial distances of ∼10 RS, but become comparable to the observed azimuthal field at distances beyond. These results suggest that the azimuthal magnetic fields which are due to the magnetopause-tail current system are not a significant effect during the dayside inbound passes, though they would need to be included in any careful treatment, as indicated by the Voyager-2 results
In this paper we have for the first time quantitatively examined two mechanisms that will produce azimuthal magnetic fields in Saturn’s magnetosphere, and have compared the calculated fields with those observed during the Saturn flybys of the two Voyager spacecraft
Summary
Analysis of magnetometer data obtained during the three flybys of Saturn’s magnetosphere undertaken to date, by Pioneer-11 in 1979, and by Voyagers-1 and -2 in 1980 and 1981, respectively, have revealed a number of surprising features of Saturn’s magnetic field. The main surprise has been that, within the limitations of the data coverage, the internal field of the planet has been found to be closely symmetric about the spin axis, consisting of the sum of axisymmetric dipole, quadrupole, and octupole terms (Connerney et al, 1982, 1984; Davis and Smith, 1990). Small-amplitude azimuthal fields were observed inside the magnetosphere, ∼5–10 nT in magnitude, which have been attributed to a number of effects. One such feature, de- To Sun Magnetopause. Bunce et al.: Azimuthal magnetic fields in Saturn’s magnetosphere w
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.