angeo-24-3533-2006.pdf (5.01 MB)
A statistical analysis of the location and width of Saturn's southern auroras
journal contributionposted on 2020-05-14, 13:30 authored by SV Badman, SWH Cowley, J-C Gerard, D Grodent
A selection of twenty-two Hubble Space Telescope images of Saturn's ultraviolet auroras obtained during 1997–2004 has been analysed to determine the median location and width of the auroral oval, and their variability. Limitations of coverage restrict the analysis to the southern hemisphere, and to local times from the post-midnight sector to just past dusk, via dawn and noon. It is found that the overall median location of the poleward and equatorward boundaries of the oval with respect to the southern pole are at ~14° and ~16° co-latitude, respectively, with a median latitudinal width of ~2°. These median values vary only modestly with local time around the oval, though the poleward boundary moves closer to the pole near noon (~12.5°) such that the oval is wider in that sector (median width ~3.5°) than it is at both dawn and dusk (~1.5°). It is also shown that the position of the auroral boundaries at Saturn are extremely variable, the poleward boundary being located between 2° and 20° co-latitude, and the equatorward boundary between 6° and 23°, this variability contrasting sharply with the essentially fixed location of the main oval at Jupiter. Comparison with Voyager plasma angular velocity data mapped magnetically from the equatorial magnetosphere into the southern ionosphere indicates that the dayside aurora lie poleward of the main upward-directed field-aligned current region associated with corotation enforcement, which maps to ~20°–24° co-latitude, while agreeing reasonably with the position of the open-closed field line boundary based on estimates of the open flux in Saturn's tail, located between ~11° and ~15°. In this case, the variability in location can be understood in terms of changes in the open flux present in the system, the changes implied by the Saturn data then matching those observed at Earth as fractions of the total planetary flux. We infer that the broad (few degrees) diffuse auroral emissions and sub-corotating auroral patches observed in the dayside sector at Saturn result from precipitation from hot plasma sub-corotating in the outer magnetosphere in a layer a few Saturn radii wide adjacent to the magnetopause, probably having been injected either by Dungey-cycle or Vasyliunas-cycle dynamics on the nightside.
This work is based on observations made by the NASA/ESA Hubble Space Telescope, obtained at the SpaceTelescope Science Institute, which is operated by the AURA Inc forNASA. We thank J. T. Clarke for use of the January 2004 HST images. The Voyager data shown in Fig. 7 were supplied by the Planetary Data Center at UCLA. Research at Leicester was supported byPPARC grant PPA/G/O/2003/00013. S. V. Badman was supported by a PPARC studentship and S. W. H. Cowley by a Royal Society Leverhulme Trust Senior Research Fellowship. J.-C. Gérard and D. Grodent were supported by the Belgian Fund for Scientific Research (FNRS) and the PRODEX programme of the European Space Agency.
CitationBadman, S. V., Cowley, S. W. H., Gérard, J.-C., and Grodent, D.: A statistical analysis of the location and width of Saturn's southern auroras, Ann. Geophys., 24, 3533–3545, https://doi.org/10.5194/angeo-24-3533-2006, 2006.
Author affiliation/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy
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Science & TechnologyPhysical SciencesAstronomy & AstrophysicsGeosciences, MultidisciplinaryMeteorology & Atmospheric SciencesGeologyASTRONOMY & ASTROPHYSICSGEOSCIENCES, MULTIDISCIPLINARYMETEOROLOGY & ATMOSPHERIC SCIENCESmagnetospheric physicsauroral phenomenamagnetosphere-ionosphere interactionsplanetary magneto-spheresEXTREME-ULTRAVIOLET OBSERVATIONSINTERPLANETARY MAGNETIC-FIELDHUBBLE-SPACE-TELESCOPESOLAR-WINDALPHA EMISSIONMAGNETOSPHEREOVALIONOSPHERECURRENTSCOMPRESSIONS