[en] Saturn’s north ultraviolet aurora has been successfully observed twice between March and May 2011 with the STIS long-slit spectrograph on board the Hubble Space Telescope. Spatially resolved spectra at ∼12 Å spectral resolution have been collected at different local times from dawn to dusk to determine the amount of hydrocarbon absorption. For this purpose, the HST telescope slewed across the auroral oval from mid-latitudes up to beyond the limb while collecting spectral data in the timetag mode. Spectral images of the north ultraviolet aurora were obtained within minutes and hours with the UVIS spectrograph on board Cassini. Several daytime sectors and one nightside location were observed and showed signatures of weak absorption by methane present in (or above) the layer of the auroral emission. No absorption from other hydrocarbons (e.g. C2H2) has been detected. For the absorbed spectra, the overlying slant CH4 column varies from 3x1015 to 2x1016 cm-2, but no clear dependence on local time is identified. A Monte Carlo electron transport model is used to calculate the vertical distribution of the H2 emission and to relate the observed spectra to the energy of the primary auroral electrons. Assuming electron precipitation with a Maxwellian energy distribution into a standard model atmosphere, we find that the mean energy ranges from less than 3 to ∼10 keV. These results are compared with previous determinations of the energy of Saturn’s aurora based on ultraviolet spectra and limb images. We conclude that the energies derived from spectral methods indicate a wide range of electron energies while the nightside limb images suggest that the auroral precipitation is consistently soft. We emphasize the need for more realistic model atmospheres with temperature and hydrocarbon distributions appropriate to high-latitude conditions.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Gérard, Jean-Claude ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Département d'astrophys., géophysique et océanographie (AGO)
Gustin, Jacques ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Physique des atmosphères et des environnements planétaires
Pryor, Wayne
Grodent, Denis ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Physique des atmosphères et des environnements planétaires
Bonfond, Bertrand ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Radioti, Aikaterini ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Gladstone, G.R.
Clarke, J.T.
Nichols, J.D.
Language :
English
Title :
Remote sensing of the energy of auroral electrons in Saturn’s atmosphere: Hubble and Cassini spectral observations
Badman S.V., et al. Cassini VIMS observations of latitudinal and hemispheric variations in Saturn's infrared auroral intensity. Icarus 2011, 216:367-375.
Badman S.V., et al. Cassini observations of ion and electron beams at Saturn and their relationship to infrared auroral arcs. J. Geophys. Res. 2012, 117:A01211. 10.1029/2011JA017222.
Bonfond B., Grodent D., Gérard J.-C., Radioti A., Dols V., Delamere P.A., Clarke J.T. The Io UV footprint: Location, inter-spot distances and tail vertical extent. J. Geophys. Res. 2009, 114:A07224. 10.1029/2009JA014312.
Broadfoot A.L., et al. Extreme ultraviolet observations from Voyager 1 encounter with Saturn. Science 1981, 212:206-211.
Bunce E.J., et al. Origin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope. J. Geophys. Res. 2008, 113:A09209.
Clarke J.T., Moos H.W., Atreya S.K., Lane A.L. IUE detection of bursts of H LY α emission from Saturn. Nature 1981, 290:226-227. 10.1038/290226a0.
Clarke J.T., et al. Response of Jupiter's and Saturn's auroral activity to the solar wind. J. Geophys. Res. 2009, 114:A05210. 10.1029/2008JA013694.
Cowley S.W.H., et al. Auroral current systems in Saturn's magnetosphere: Comparison of theoretical models with Cassini and HST observations. Ann. Geophys. 2008, 26:2613-2630.
Dalgarno A., Yan M., Liu W. Electron energy deposition in a gas mixture of atomic and molecular hydrogen and helium. Astrophys. J. Suppl. Ser. 1999, 125:237-256.
Dziczek D., Ajello J.M., James G.K., Hansen D.L. Cascade contribution to the H2 Lyman band system from electron impact. Phys. Rev. A 2000, 61:64702-1-64702-4.
Esposito L.W., et al. The Cassini ultraviolet imaging spectrograph investigation. Space Sci. Rev. 2004, 115:299-361.
Garvey R.H., Green A.E.S. Energy-apportionment techniques based upon detailed atomic cross sections. Phys. Rev. A 1976, 14:946-953.
Garvey R.H., Porter H.S., Green A.E.S. An analytic degradation spectrum for H2. J. Appl. Phys. 1977, 48:190-193.
Geballe T.R., Jagod M.-F., Oka T. Detection of H3+ infrared emission lines in Saturn. Astrophys. J. Lett. 1993, 408:L109-L112.
Gérard J.-C., Dols V., Grodent D., Waite J.H., Gladstone G.R., Prangé R. Simultaneous observations of the Saturnian aurora and polar haze with the HST/FOC. Geophys. Res. Lett. 1995, 22:2685-2688.
Gérard J.-C., Grodent D., Gustin J., Saglam A., Clarke J.T., Trauger J.T. Characteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph. J. Geophys. Res. 2004, 109:A09207. 10.1029/2004JA010513.
Gérard J.-C., Bunce E.J., Grodent D., Crowley S.W.H., Clarke J.T., Badman S.V. Signature of Saturn's auroral cusp: Simultaneous Hubble Space Telescope FUV observations and upstream solar wind monitoring. J. Geophys. Res. 2005, 110:A11201. 10.1029/2005JA011094.
Gérard J.-C., et al. Altitude of Saturn's aurora and its implications for the characteristic energy of precipitated electrons. Geophys. Res. Lett. 2009, 36:L02202. 10.1029/2008GL036554.
Grodent D., Gérard J.-C., Cowley S.W.H., Bunce E.J., Clarke J.T. The global morphology of Saturn's southern ultraviolet aurora. J. Geophys. Res. 2005, 110:A01207. 10.1029/2004JA010717.
Grodent D., Gustin J., Gérard J.-C., Radioti A., Bonfond B., Pryor W.R. Small scale structures in Saturn's ultraviolet aurora. J. Geophys. Res. 2011, 116:A09225. 10.1029/2011JA016818.
Gustin J., Feldman P.D., Gérard J.-C., Grodent D., Vidal-Madjar A., Ben Jaffel L., Desert J.-M., Moos H.W., Sahnow D.J., Weaver H.A., Wolven B.C., Ajello J.M., Waite J.H., Roueff E., Abgrall H. Jovian auroral spectroscopy with FUSE: Analysis of self-absorption and implications for electron precipitation. Icarus 2004, 171:336-355.
Gustin J., Gérard J.-C., Pryor W., Feldman P.D., Grodent D., Holsclaw G. Characteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra. Icarus 2009, 200:176-187.
Gustin J., Bonfond B., Grodent D., Gérard J.-C. Conversion from HST ACS and STIS auroral counts into brightness, precipitated power, and radiated power for H2 giant planets. J. Geophys. Res. 2012, 117:A07316. 10.1029/2012JA017607.
Jackman C.H., Garvey R.H., Green A.E.S. Electron impact on atmospheric gases: I. Updated cross sections. J. Geophys. Res. 1977, 82:5081-5090.
Kameta K., Kouchi N., Ukai M., Hatano Y. Photoabsorption, photoionization, and neutral-dissociation crosssections of simple hydrocarbons in the vacuum ultraviolet range. J. Elec. Spect. Rel. Phen. 2002, 123:225-238.
Kimble R.A., et al. The on-orbit performance of the Space Telescope Imaging Spectrograph. Astrophys. J. 1998, 492:L83. 10.1086/311102.
Kurth W.S., et al. Auroral processes. Saturn from Cassini-Huygens 2009, Springer. M.K. Dougherty, L.W. Esposito, S.M. Krimigis (Eds.).
Lee A.Y.T., Yung Y.L., Cheng B.M., Bahou M., Chung C.-Y., Lee Y.P. Enhancement of deuterated ethane on Jupiter. Astrophys. J. 2001, 551:L93-L96.
McGrath M.A., Clarke J.T. H I Lyman alpha emission from Saturn (1980-1990). J. Geophys. Res. 1992, 103(20):237.
Melin H., Miller S., Stallard T., Trafton L.M., Geballe T.R. Variability in the H3+ emission of Saturn: Consequences for ionisation rates and temperature. Icarus 2007, 186:234-241.
Moses J.I., Bézard B., Lellouch E., Feuchtgruber H., Gladstone G.R., Allen M. Photochemistry of Saturn's atmosphere. I. Hydrocarbon chemistry and comparisons with ISO observations. Icarus 2000, 143:244-298.
Müller-Wodarg I.C.F., Mendillo M., Yelle R.V., Aylward A.D. A global circulation model of Saturn's thermosphere. Icarus 2006, 180:147-160.
Nichols, et al. Variation of Saturn's UV aurora with SKR phase. J. Geophys. Res. 2010, 37:L15102. 10.1029/2010GL044057.
Radioti A., et al. Bifurcations of the main auroral ring at Saturn: Ionospheric signatures of consecutive reconnection events at the magnetopause. J. Geophys. Res. 2011, 116:A11209. 10.1029/2011JA016661.
Sandel B.R., Shemansky D.E., Broadfoot A.L., Holberg J.B., Smith G.R. Extreme ultraviolet observations from the Voyager 2 encounter with Saturn. Science 1982, 215:548-553.
Shemansky D.E., Ajello J.M. The Saturn spectrum in the EUV: Electron excited hydrogen. J. Geophys. Res. 1983, 88:459-464.
Shematovich V.I., Bisikalo D.V., Gérard J.C. A kinetic model of the formation of the hot oxygen geocorona: 1. Quiet geomagnetic conditions. J. Geophys. Res. 1994, 99:23217-23228.
Shyn T.W., Sharp W.E. Angular distributions of electrons elastically scattered from H2. Phys. Rev. A 1981, 24:1734-1740.
Smith C.G.A., Aylward A.D., Millward G.H., Miller S., Moore S.L.E. An unexpected cooling effect in Saturn's upper atmosphere. Nature 2007, 445:399-401.
Stallard T., Miller S., Melin H., Lystrup M., Dougherty M.K., Achilleos N. Saturn's auroral/polar H3+ infrared emission I. General morphology and ion velocity structure. Icarus 2007, 189:1-13.
Stallard T.S., Melin H., Miller S., Badman S.V., Brown R.H., Baines K.H. Peak emission altitude of Saturn's H3+ aurora. Geophys. Res. Lett. 2012, 39:L15103. 10.1029/2012GL052806.
Talboys, et al. Statistical characteristics of field-aligned currents in Saturn's nightside magnetosphere. J. Geophys. Res. 2011, 116:A04213.
Trauger J.T., et al. Saturn's hydrogen aurora: Wide field and planetary camera 2 imaging from the Hubble Space Telescope. J. Geophys. Res. 1998, 103(20):237.