Corotation-driven magnetosphere-ionosphere coupling currents in Saturn's magnetosphere and their relation to the auroras
journal contributionposted on 24.01.2017, 11:44 by S. W. H. Cowley, E. J. Bunce
We calculate the latitude profile of the equatorward-directed ionospheric Pedersen currents that are driven in Saturn’s ionosphere by partial corotation of the magnetospheric plasma. The calculation incorporates the flattened figure of the planet, a model of Saturn’s magnetic field derived from spacecraft flyby data, and angular velocity models derived from Voyager plasma data. We also employ an effective height-integrated ionospheric Pedersen conductivity of 1 mho, suggested by a related analysis of Voyager magnetic field data. The Voyager plasma data suggest that on the largest spatial scales, the plasma angular velocity declines from near-rigid corotation with the planet in the inner magnetosphere, to values of about half of rigid corotation at the outer boundary of the region considered. The latter extends to ~ 15–20 Saturn radii (RS) in the equatorial plane, mapping along magnetic field lines to ~ 15° co-latitude in the ionosphere. We find in this case that the ionospheric Pedersen current peaks near the poleward (outer) boundary of this region, and falls toward zero over ~ 5°–10° equator-ward of the boundary as the plasma approaches rigid corotation. The peak current near the poleward boundary, integrated in azimuth, is ~ 6 MA. The field-aligned current required for continuity is directed out of the ionosphere into the magnetosphere essentially throughout the region, with the current density peaking at ~ 10 nA m-2 at ~ 20° co-latitude. We estimate that such current densities are well below the limit requiring field-aligned acceleration of magnetospheric electrons in Saturn’s environment ( ~ 70 nAm-2), so that no significant auroral features associated with this ring of upward current is anticipated. The observed ultraviolet auroras at Saturn are also found to occur significantly closer to the pole (at ~ 10°–15° co-latitude), and show considerable temporal and local time variability, contrary to expectations for corotation-related currents. We thus conclude that Saturn’s ‘main oval’ auroras are not associated with corotation-enforcing currents as they are at Jupiter, but instead are most probably associated with coupling to the solar wind as at Earth. At the same time, the Voyager flow observations also suggest the presence of radially localized ‘dips’ in the plasma angular velocity associated with the moons Dione and Rhea, which are ~ 1–2 RS in radial extent in the equatorial plane. The presence of such small-scale flow features, assumed to be azimuthally extended, results in localized several-MA enhancements in the ionospheric Pedersen current, and narrow bi-polar signatures in the field-aligned currents which peak at values an order of magnitude larger than those associated with the large-scale currents. Narrow auroral rings (or partial rings) ~ 0.25° co-latitude wide with intensities ~ 1 kiloRayleigh may be formed in the regions of upward field-aligned current under favourable circumstances, located at co-latitudes between ~ 17° and ~ 20° in the north, and ~ 19° and ~22° in the south.
We would like to thank Joe Mafi at the Planetary Data Center, UCLA, for supplying us with plasma and magnetic field data from the Voyager Saturn flybys. We would also like to thank John Clarke (Boston University) and Denis Grodent (Universite de Li ´ ege) for helpful comments on the relation between ` precipitated electron energy flux and auroral UV emission intensity. EJB was supported during the course of this study by PPARC Grant PPA/G/O/1999/00181 and SWHC by PPARC Senior Fellowship PPA/N/S/2000/00197.
CitationAnnales Geophysicae, 2003, 21 (8), pp. 1691-1707 (17)
Author affiliation/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy
VersionVoR (Version of Record)
Published inAnnales Geophysicae
PublisherEuropean Geosciences Union (EGU), Copernicus Publications, Springer Verlag (Germany)
Science & TechnologyPhysical SciencesAstronomy & AstrophysicsGeosciences, MultidisciplinaryMeteorology & Atmospheric SciencesGeologyASTRONOMY & ASTROPHYSICSGEOSCIENCES, MULTIDISCIPLINARYMETEOROLOGY & ATMOSPHERIC SCIENCESmagnetospheric physicscurrent systemsmagnetosphere-ionosphere interactionsplanetary magnetospheresHUBBLE-SPACE-TELESCOPEEXTREME ULTRAVIOLET OBSERVATIONSMAGNETIC-FIELDSOLAR-WINDKILOMETRIC RADIATIONEQUATORIAL CURRENTRADIO-EMISSIONALPHA EMISSIONCURRENT SHEETMODEL