Magnetospheric Physics: Planetary magnetospheres (5443; 5737; 6030); Magnetospheric Physics: Auroral phenomena (2407); Magnetospheric Physics: Energetic particles; precipitating; Magnetospheric Physics: Magnetopause; cusp; and boundary layers; Planetology: Solar System Objects: Saturn
Abstract :
[en] We analyze a set of 15 FUV images obtained between October 1997 and January 2001 with the Hubble Space Telescope Imaging Spectrograph (STIS), providing a good view of Saturn's south auroral oval. It is found that the morphology and brightness distribution of the aurora are dynamical with variations occurring on time scales of hours or less. The dayside main oval lies between 70° and 80° and is generally brighter and thinner in the morning than in the afternoon sector. The afternoon sector is characterized by more diffuse emission at higher latitudes. Weak emission is also observed poleward of the main oval up to the pole. A spot of enhanced auroral precipitation, tentatively identified as the optical signature of the dayside cusp, is sometimes observed poleward of the main oval in the noon sector, especially during periods when the morning arc is not fully developed. A spiral structure of the main oval with arcs at two latitudes in the same sector is occasionally observed. The brightness of the main oval ranges from below the STIS threshold of 1 kR of H[SUB]2[/SUB] emission up to ~75 kR. The total electron precipitated power varies between 20 and 140 GW, that is, comparable to the Earth's active aurora but about two orders of magnitude less than on Jupiter. An increasing trend of the precipitated power between the 1997 and the 2000-2001 observations may be related to the rising solar activity. Six spectra of the aurora in the noon sector covering the 1200-1700 Å range are dominated by emissions of the Lyman-alpha line and H[SUB]2[/SUB] Werner and Lyman bands. Their comparison with a synthetic model of electron excited H[SUB]2[/SUB] emissions indicates the presence of a weak absorption below 1400 Å by a column of methane ranging between 7 × 10[SUP]15[/SUP] and 2 × 10[SUP]16[/SUP] cm[SUP]-2[/SUP]. The corresponding energy of the primary auroral electrons is estimated 12 +/- 3 keV, using a low-latitude model atmosphere based on Voyager occultation measurements. The main oval brightness and the characteristic electron energy are generally consistent with recent models of Saturn's aurora, which colocate the main oval with the narrow upward field-aligned current system associated with departure from plasma corotation near the open-closed field line boundary. The latitude of the bright morning arc is somewhat lower than model predictions based on the plasma flow velocity measured by Voyager in the middle magnetosphere.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Gérard, Jean-Claude ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Grodent, Denis ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Gustin, Jacques ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Saglam, Adem ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Clarke, John T
Trauger, John T
Language :
English
Title :
Characteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph
Publication date :
01 September 2004
Journal title :
Journal of Geophysical Research. Space Physics
ISSN :
2169-9380
eISSN :
2169-9402
Publisher :
American Geophysical Union (AGU), Washington, United States - District of Columbia
Barbosa, D. D. (1990), Auroral precipitation flux of ions and electrons in Saturn's outer magnetosphere, Planet. Space Sci., 38, 1295.
Ben Jaffel, L., V. Leers, and B. R. Sandel (1995), Dark auroral oval on Saturn discovered in Hubble Space Telescope ultraviolet images, Science, 269, 951.
Broadfoot, A. L., et al. (1981), Extreme ultraviolet observations from Voyager 1 encounter with Saturn, Science, 212, 206.
Bunce, E. J., S. W. H. Cowley, and T. K. Yeoman (2004), Jovian cusp processes: Implications for the polar aurora, J. Geophys. Res., 109, A09S13, doi:10.1029/2003JA010280.
Clarke, J. T., H. W. Moos, S. K. Atreya, and A. L. Lane (1981), IUE detection of bursts of H Ly α emission from Saturn, Nature, 290, 226.
Connemey, J. E. P., L. Davis Jr., and D. L. Chenette (1984), Magnetic field models, in Saturn, edited by T. Gehrels and M. S. Matthews, p. 378, Univ. of Ariz. Press, Tucson, Ariz.
Cowley, S. W. H., and E. J. Bunce (2001), Origin of the main auroral oval in Jupiter's coupled magnetosphere-ionosphere system, Planet. Space Sci., 49, 1067.
Cowley, S. W. H., and E. J. Bunce (2003), Corotation-driven magnetosphere-ionosphere coupling currents in Saturn's magnetosphere and their relation to the auroras, Ann. Geophys., 21, 1691.
Cowley, S. W. H., E. J. Bunce, T. S. Stallard, and S. Miller (2003), Jupiter's polar ionospheric flows: Theoretical interpretation, Geophys. Res. Lett., 30(5), 1220, doi:10.1029/2002GL016030.
Cowley, S. W. H., E. J. Bunce, and J. M. O'Rourke (2004a), A simple quantitative model of plasma flows and currents in Saturn's polar ionosphere, J. Geophys. Res., 109, A05212, doi:10.1029/2003JA010375.
Cowley, S. W. H., E. J. Bunce, and R. Prangé (2004b), Saturn's polar ionospheric flows and their relation to the main auroral oval, Ann. Geophys., 21, 1.
Davis, L., Jr., and E. J. Smith (1990), A model of Saturn's magnetic field based on all available data, J. Geophys. Res., 95, 15,257.
Dols, V., J.-C. Gérard, J. T. Clarke, J. Gustin, and D. Grodent (2000), Diagnostics of the Jovian aurora deduced from ultraviolet spectroscopy: Model and HST/GHRS observation, Icarus, 147, 251.
Dungey, J. W. (1961), Interplanetary field and the auroral zones, Phys. Rev. Lett., 6, 47.
Galopeau, P., P. Zarka, and D. Lequéau (1995), Source location of Saturn's kilometric radiation: The Kelvin-Helmholtz instability hypothesis, J. Geophys. Res., 100, 26,397.
Geballe, T. R., M.-F. Jagod, and T. Oka (1993), Detection of H 3+ emission lines in Saturn, Astrophys. J., 408, L109.
Gérard, J.-C., and V. Singh (1982), A model of energetic electrons and EUV emission in the Jovian and Saturnian atmospheres and implications, J. Geophys. Res., 87, 4525.
Gérard, J.-C., V. Dols, D. Grodent, J. H. Waite, G. R. Gladstone, and R. Prangé (1995), Simultaneous observations of the Saturnian aurora and polar haze with the HST/FOC, Geophys. Res. Lett., 22, 2685.
Gérard, J.-C., D. Grodent, V. Dols, and J. H. Waite Jr. (1998), The longitudinal variation of the color ratio of the Jovian ultraviolet aurora: A geometric effect?, Geophys. Res. Lett., 25, 1601.
Gérard, J.-C., J. Gustin, D. Grodent, P. Delamere, and J. T. Clarke (2002), The excitation of the FUV Io tail on Jupiter: Characterization of the electron precipitation, J. Geophys. Res., 107(A11), 1394, doi:10.1029/2002JA009410.
Gérard, J.-C., J. Gustin, D. Grodent, J. T. Clarke, and A. Grard (2003), Spectral observations of transient features in the FUV Jovian polar aurora, J. Geophys. Res., 108(A8), 1319, doi:10.1029/2003JA009901.
Grodent, D., J. H. Waite, and J.-C. Gérard (2001), A self-consistent model of the Jovian auroral thermal structure, J. Geophys. Res., 106, 12,933.
Grodent, D., J. T. Clarke, J. H. Waite Jr., S. W. H. Cowley, J.-C. Gérard, and J. Kim (2003a), Jupiter's polar auroral emission, J. Geophys. Res., 108(A10), 1366, doi:10.1029/2003JA010017.
Grodent, D., J. T. Clarke, J. Kim, J. H. Waite Jr., and S. W. H. Cowley (2003b), Jupiter's main auroral oval observed with HST-STIS, J. Geophys. Res., 108(A11), 1389, doi:10.1029/2003JA009921.
Gustin, J., D. Grodent, J.-C. Gérard, and J. T. Clarke (2002), Spatially resolved far ultraviolet spectroscopy of the Jovian aurora, Icarus, 156, 91.
Gustin, J., J.-C. Gérard, D. Grodent, S. W. H. Cowley, J. T. Clarke, and A. Grard (2004), Energy-flux relationship in the Jovian aurora deduced from HST-STIS spectral observations, J. Geophys. Res., 109, doi:10.1029/2003JA010365, in press.
Hill, T. W. (2001), The Jovian auroral oval, J. Geophys. Res., 106, 8101.
Kaiser, M. L., M. D. Desch, W. S. Kurth, A. Lecacheux, F. Genova, B. M. Pederson, and D. R. Evans (1984), Saturn as a radio source, in Saturn, edited by T. Gehrels and M. S. Matthews, p. 378, Univ. of Ariz. Press, Tucson, Ariz.
McGrath, M. A., and J. T. Clarke (1992), H I Lyman alpha emission from Saturn (1980-1990), J. Geophys. Res., 103, 20,237.
Moses, J. I., B. Bézard, E. Lellouch, H. Feuchtgruber, G. R. Gladstone, and M. Allen (2000), Photochemistry of Saturn's atmosphere I. Hydrocarbon chemistry and comparisons with ISO observations, Icarus, 143, 244.
Pallier, L., and R. Prangé (2001), More on the structure of the high-latitude Jovian aurorae, Planet. Space Sci., 49, 1159.
Phan, T. D., et al. (2003), Extended magnetic reconnections at the Earth's magnetopause from detection of bi-directional jets. Nature, 404, 848.
Sandel, B. R., and A. L. Broadfoot (1981), Morphology of Saturn's aurora, Nature, 292, 679.
Sandel, B. R., et al. (1982), Extreme ultraviolet observations from the Voyager 2 encounter with Saturn, Science, 292, 679.
Shemansky, D. E., and J. M. Ajello (1983), The Saturn spectrum in the EUV: Electron excited hydrogen, J. Geophys. Res., 88, 459.
Smith, G. R., D. E. Shemansky, J. B. Holberg, A. L. Broadfoot, B. R. Sandel, and J. C. McConnell (1983), Saturn's upper atmosphere from the Voyager 2 EUV solar and stellar occultations, J. Geophys. Res., 88, 8667.
Stallard, T., S. Miller, G. E. Ballester, D. Rego, R. D. Joseph, and L. M. Trafton (1999), The H3+ latitudinal profile of Satum, Astrophys. J., 521, L149.
Stallard, T., S. Miller, S. W. H. Cowley, and E. J. Bunce (2003), Jupiter's polar ionospheric flow: Measured intensity and veleocity variations poleward of the main auroral oval, Geophys. Res. Lett., 30(5), 1221, doi:10.1029/2002GL016031.
Trauger, J. T., et al. (1998), Saturn's hydrogen aurora: Wide field and planetary camera 2 imaging from the Hubble Space Telescope, J. Geophys. Res., 103, 20,237.
Vasyliunas, V. M. (1983), Plasma distribution and flow, in Physics of the Jovian Magnetosphere, edited by A. J. Dessler, p. 395, Cambridge Univ. Press, New York.
Waite, J. H., Jr., T. E. Cravens, J. U. Kozyra, A. F. Nagy, S. K. Atreya, and R. H. Chen (1983), Electron precipitation and related acronomy of the Jovian thermosphere and ionosphere, J. Geophys. Res., 88, 6143.