[en] 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 similar to 14 degrees and similar to 16 degrees co-latitude, respectively, with a median latitudinal width of similar to 2 degrees. These median values vary only modestly with local time around the oval, though the poleward boundary moves closer to the pole near noon (similar to 12.5 degrees) such that the oval is wider in that sector (median width similar to 3.5 degrees) than it is at both dawn and dusk (similar to 1.5 degrees). It is also shown that the position of the auroral boundaries at Saturn are extremely variable, the poleward boundary being located between 2 degrees and 20 degrees co-latitude, and the equatorward boundary between 6 degrees and 23 degrees, 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 similar to 20 degrees-24 degrees 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 similar to 11 degrees and similar to 15 degrees. 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.
Grodent, Denis ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Language :
English
Title :
A statistical analysis of the location and width of Saturn's southern auroras
Publication date :
2006
Journal title :
Annales Geophysicae
ISSN :
0992-7689
eISSN :
1432-0576
Publisher :
European Geosciences Union, Katlenburg-Lindau, Germany
Badman, S. V., Bunce, E. J., Clarke, J. T., Cowley, S. W. H., Gérard, J.-C., Grodent, D., and Milan, S. E.: Open flux estimates in Saturn's magnetosphere during the January 2004 Cassini-HST campaign, and implications for reconnection rates, J. Geophys. Res., 110, A11216, doi:10.1029/2005JA011240, 2005.
Broadfoot, A. L., Sandel, B. R., Shemansky, D. E., Holberg, J. B., Smith, G. R., Strobel, D. F., McConnell, J. C., Kumar, S., Hunten, D. M., Atreya, S. K., Donahue, T. M., Moos, H. W., Bertaux, J. L., Blamont, J. E., Pomphrey, R. B., and Linik, S.: Extreme ultraviolet observations from Voyager 1 encounter with Saturn, Science, 212, 206–211, 1981.
Bunce, E. J., Cowley, S. W. H., and Milan, S. E.: Interplanetary magnetic field control of Saturn's polar cusp aurora, Ann. Geophys., 23, 1405–1431, 2005.
Clarke, J. T., Moos, H. W., Atreya, S. K., and Lane, A. L.: IUE detection of bursts of H Ly α emission from Saturn, Nature, 290, 226–230, 1981.
Connerney, J. E. P., Acuntilde;a, M. H., and Ness, N. F.: Currents in Saturn's magnetosphere, J. Geophys. Res., 88, 8779–8789, 1983.
Cowley, S. W. H. and Bunce, E. J.: Origin of the main auroral oval in Jupiter's coupled magnetosphere-ionosphere system, Planet. Space Sci., 49, 1067–1088, 2001.
Cowley, S. W. H. and Bunce, E. J.: Modulation of jovian middle magnetosphere currents and auroral precipitation by solar wind-induced compressions and expansions of the magnetosphere: Initial conditions and steady state, Planet. Space Sci., 51, 31–56, 2003a.
Cowley, S. W. H. and Bunce, E. J.: Modulation of Jupiter's main auroral oval emissions by solar wind-induced expansions and compressions of the magnetosphere, Planet. Space Sci., 51, 57–79, 2003b.
Cowley, S. W. H. and Bunce, E. J.: Corotation-driven magnetosphere- ionosphere coupling currents in Saturn's magnetosphere and their relation to the auroras, Ann. Geophys., 21, 1691–1707, 2003c.
Cowley, S. W. H., Bunce, E. J., and Prangé, R.: Saturn's polar ionospheric flows and their relation to the main auroral oval, Ann. Geophys., 22, 1379–1394, 2004a.
Cowley, S. W. H., Bunce, E. J., and O'Rourke, J. M.: A simple quantitative model of plasma flows and currents in Saturn's polar ionosphere, J Geophys Res., 109, A05212, doi:10.1029/2003JA010375, 2004b.
Cowley, S. W. H., Badman, S. V., Bunce, E. J., Clarke, J. T., Gérard, J.-C., Grodent, D., Jackman, C. M., Milan, S. E., and Yeoman, T. K.: Reconnection in a rotation-dominated magnetosphere and its relation to Saturn's auroral dynamics, J Geophys Res., 110, A02201, doi:10.1029/ 2004JA010796, 2005.
Davis Jr., L. and Smith, E. J.: A model of Saturn's magnetic field based on all available data, J. Geophys. Res., 95, 15 257–15 261, 1990.
Dungey, J. W.: Interplanetary field and the auroral zones, Phys Rev Lett., 6, 47–48, 1961.
Fairfield, D. H.: Average magnetic field configuration of the outer magnetosphere, J. Geophys. Res., 73, 7329–7338, 1968.
Gérard, J.-C., Dols, V., Grodent, D., Waite, J. H., Gladstone, G. R., and Prangé, R.: Simultaneous observations of the saturnian aurora and polar haze with the HST/FOC, Geophys. Res. Lett., 22, 2685–2688, 1995.
Gérard, J.-C., Grodent, D., Gustin, J., Saglam, A., Clarke, J. T., and Trauger, J. T.: Characteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph, J. Geophys. Res., 109, A09207, doi:10.1029/2004JA010513, 2004.
Gérard, J.-C., Bunce, E. J., Grodent, D., Cowley, S. W. H., Clarke, J. T., and Badman, S. V.: Signature of Saturn's auroral cusp: Simultaneous HST FUV observations and upstream solar wind monitoring, J. Geophys. Res., 110, A11201, doi:10.1029/2005JA011094, 2005.
Gérard, J.-C., Grodent, D., Cowley, S. W. H., Mitchell, D. G., Kurth, W. S., Clarke, J. T., Bunce, E. J., Nichols, J. D., Dougherty, M. K., Crary, F. J., and Coates, A. J.: Saturn's auroral morphology and activity during quiet magnetospheric conditions, J. Geophys. Res., in press, doi:10.1029/2006JA011965, 2006.
Gong, B. and Hill, T. W.: Variations of jovian and saturnian auroras induced by changes of solar wind dynamic pressure, Magnetospheres of the Outer Planets 2005 Programme and Abstracts, p 129, University of Leicester, Leicester, UK, 2005.
Grodent, D., Clarke, J. T., Kim, J., Waite Jr., J. H., and Cowley, S. W. H.: Jupiter's main auroral oval observed with HST-STIS, J. Geophys. Res., 108(A11), 1389, doi:10.1029/2003JA009921, 2003.
Grodent, D., Gérard, J.-C., Cowley, S. W. H., Bunce, E. J., and Clarke, J. T.: The global morphology of Saturn's southern ultraviolet aurora, J. Geophys. Res., 110, A07215, doi:10.1029/2004JA010983, 2005.
Hansen, C. J., Esposito, L., Stewart, I. A. F., Colwell, J., Hendrix, A., Pryor, W., Shemansky, D., and West, R.: Enceladus' water vapor plume, Science, 311, 422–425, 2006.
Hill, T. W.: Inertial limit on corotation, J. Geophys. Res., 84, 6554–6558, 1979.
Hill, T. W.: The jovian auroral oval, J Geophys. Res., 106, 8101–8108, 2001.
Jackman, C. M. and Cowley, S. W. H.: A model of the plasma flow and current in Saturn's polar ionosphere under conditions of strong Dungey-cycle driving, Ann. Geophys., 24, 1029–1055, 2006.
Judge, D. L., Wu, F. M., and Carlson, R. W.: Ultraviolet photometer observations of the Saturnian system, Science, 207, 431–434, 1980.
McGrath, M. A. and Clarke, J. T.: H I Lyman alpha emission from Saturn (1980–1990), J. Geophys. Res., 97, 13 691–13 703, 1992.
Milan, S. E., Lester, M., Cowley, S. W. H., Oksavik, K., Brittnacher, M., Greenwald, R. A., Sofko, G., and Villain, J.-P.: Variations in polar cap area during two substorm cycles, Ann. Geophys., 21, 1121–1140, 2003.
Milan, S. E., Cowley, S. W. H., Lester, M., Wright, D. M., Slavin, J. A., Fillingim, M., Carlson, C. W., and Singer, H. J.: Response of the magnetotail to changes in the open flux content of the magnetosphere, J. Geophys. Res., 109, A04220, doi:10.1029/2003JA010350, 2004.
Milan, S. E., Provan, G., and Hubert, B.: Magnetic flux transport in the Dungey-cycle: A survey of dayside and nightside reconnection rates, J. Geophys. Res., in press, doi:10.1029/2006JA011642, 2006.
Ness, N. F., Acuntilde;a, M. H., Lepping, R. P., Connerney, J. E. P., Behannon, K. W., Burlaga, L. F., and Neubauer, F. M.: Magnetic field studies by Voyager 1: Preliminary results at Saturn, Science, 212, 211–217, 1981.
Nichols, J. D., Bunce, E. J., Clarke, J. T., Cowley, S. W. H., Gérard, J.-C., Grodent, D., and Pryor, W. R.: Response of Jupiter's UV auroras to interplanetary conditions as observed by the Hubble Space Telescope during the Cassini fly-by campaign, J. Geophys. Res., in press, doi:10.1029/2006JA012005, 2006.
Prangé, R., Pallier, L., Hansen, K. C., Howard, R., Vourlidas, A., Courtin, R., and Parkinson, C.: An interplanetary shock traced by planetary auroral storms from the Sun to Saturn, Nature, 432, 78–81, 2004.
Richardson, J. D.: Thermal ions at Saturn: plasma parameters and implications, J. Geophys. Res., 91, 1381–1389, 1986.
Richardson, J. D. and Sittler Jr., E. C.: A plasma density model for Saturn based on Voyager observations, J. Geophys. Res., 95, 12 019–12 031, 1990.
Sandel, B. R. and Broadfoot, A. L.: Morphology of Saturn's aurora, Nature, 292, 679–682, 1981.
Sandel, B. R., Shemansky, D. E., Broadfoot, A. L., Holberg, J. B., Smith, G. R., McConnell, J. C., Strobel, D. F., Atreya, S. K., Donahue, T. M., Moos, H. W., Hunten, D. M., Pomphrey, R. B., and Linik, S.: Extreme ultraviolet observations from the Voyager 2 encounter with Saturn, Science, 215, 548–553, 1982.
Southwood, D. J. and Kivelson, M. G.: A new perspective concerning the influence of the solar wind on Jupiter, J. Geophys. Res., 106, 6123–6130, 2001.
Trauger, J. T., Clarke, J. T., Ballester, G. E., Evans, R. W., Burrows, C. J., Crisp, D., Gallagher III, J. S., Griffiths, R. E., Hester, J. J., Hoessel, J. G., Holtzman, J. A., Krist, J. E., Mould, J. R., Sahai, R., Scowen, P. A., Stapelfeldt, K. R., and Watson, A. M.: Saturn's hydrogen aurora: Wide field and planetary camera 2 imaging from the Hubble Space Telescope, J Geophys Res., 103, 20 237–20 244, 1998.
Vasyliunas, V. M.: Plasma distribution and flow, in: Physics of the Jovian Magnetosphere, edited by: Dessler, A. J., p 395–453, Cambridge Univ. Press, Cambridge, UK, 1983.
Waite Jr., J. H., Combi, M. R., Ip, W.-H., Cravens, T. E., McNutt Jr, R. L., Kasprzak, W., Yelle, R., Luhmann, J., Niemann, H., Gell, D., Magee, B., Fletcher, G., Lunine, J., and Tseng, W.-L.: Cassini Ion and Neutral Mass Spectrometer: Enceladus plume composition and structure, Science, 311, 1419–1422, 2006.
