Planetary Sciences: Solar System Objects: Saturn; Magnetospheric Physics: Auroral phenomena (2407); Magnetospheric Physics: Planetary magnetospheres (5443; 5737; 6033); Magnetospheric Physics: Solar wind/magnetosphere interactions; Magnetospheric Physics: Magnetic reconnection (7526; 7835)
Abstract :
[en] Model simulations by Bunce et al. (2005a) have shown that direct precipitation of electrons in Saturn's dayside cusp regions is not capable of producing significant FUV aurora. Instead, they suggested the possibility that the FUV bright emissions sometimes observed near noon are associated with reconnection occurring at the dayside magnetopause, possibly pulsed, analogous to flux transfer events seen at the Earth. Pulsed reconnection at the low-latitude dayside magnetopause when the IMF is directed northward (antiparallel to Saturn's magnetic field lines) is expected to give rise to pulsed twin-vortical flows in the magnetosphere and hence to bipolar field-aligned currents centered in the vortical flows closing in ionospheric Pedersen current. In the case of southward IMF and high-latitude lobe reconnection the model predicts that the vortical flows are displaced poleward of the open-closed field line boundary with reversed field-aligned currents compared with the former case. During January 2004, a unique campaign took place during which magnetic field and plasma instruments on board the Cassini-Huygens spacecraft measured the in situ solar wind and embedded interplanetary magnetic field while the Hubble Space Telescope simultaneously observed the far ultraviolet aurora in Saturn's southern hemisphere. The IMF was highly structured during this interval. The electric potential at Cassini is estimated from solar wind magnetic field and velocity measurements for the case of low-latitude or lobe reconnection. We show that a dayside FUV signature of intense electron precipitation is found poleward of or along the main oval during a period of minor compression period when the dayside reconnection voltage is estimated to be ~30-100 kV. Overall, we find that the conceptual model of Bunce et al. (2005a) provides a good estimate of the UV brightness and power for the case of northward IMF but somewhat underestimates the power for the southward IMF case, except if the speed of the vortical flow is larger than its value in the nominal model.
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)
Bunce, Emma J
Grodent, Denis ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Cowley, Stanley W.H.
Clarke, John T
Badman, Sarah V
Language :
English
Title :
Signature of Saturn's auroral cusp: Simultaneous Hubble Space Telescope FUV observations and upstream solar wind monitoring
Publication date :
01 November 2005
Journal title :
Journal of Geophysical Research. Space Physics
ISSN :
2169-9380
eISSN :
2169-9402
Publisher :
American Geophysical Union (AGU), Washington DC, United States
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.
Bunce, E. J., S. W. H. Cowley, and S. E. Milan (2005a), Interplanetary magnetic field control of Saturn's polar cusp aurora, Ann. Geophys, 23, 1405.
Bunce, E. J., S. W. H. Cowley, C. M. Jackman, J. T. Clarke, F. J. Crary, and M. K. Dougherty (2005b), Cassini observations of the interplanetary medium upstream of Saturn and their relation to the Hubble Space Telescope aurora data, Adv. Space Res., in press.
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.
Clarke, J. T., et al. (2005), Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Saturn, Nature, 433, 717.
Cowley, S. W. H. (1981), Magnetospheric asymmetries associated with the Y-component of the IMF, Planet. Space Sci., 29, 79.
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, and R. Prangé (2004), Saturn's polar ionospheric flows and their relation to the main auroral oval, Ann. Geophys., 22, 1379.
Cowley, S. W. H., S. V. Badman, E. J. Bunce, J. T. Clarke, J.-C. Gérard, D. Grodent, C. M. Jackman, S. E. Milan, and T. K. Yeoman (2005), Reconnection in a rotation-dominated magnetosphere and its relation to Saturn's auroral dynamics, J. Geophys. Res., 110, A02201, doi:10.1029/2004JA010796.
Crary, F. J., et al. (2005), Solar wind dynamic pressure and electric field as the main factors controlling Saturn's aurorae, Nature, 433, 720.
Dungey, J. W. (1961), Interplanetary field and the auroral zones, Phys. Rev. Lett., 6, 47.
Frey, H. U., S. B. Mende, T. J. Immel, S. A. Fuselier, E. S. Claflin, J.-C. Gérard, and B. Hubert (2002), Proton aurora in the cusp, J. Geophys. Res., 107(A7), 1091, doi:10.1029/2001JA900161.
Fuselier, S. A., B. J. Anderson, and T. G. Onsager (1997), Electron and ion signatures of field line topology at the low-shear magnetopause, J. Geophys. Res., 102, 4847.
Fuselier, S. A., H. U. Frey, K. J. Trattner, S. B. Mende, and J. L. Burch (2002), Cusp aurora dependence on IMF Bz, J. Geophys. Res., 107(A7), 1111, doi:10.1029/2001JA900165.
Gérard, J.-C., V. Dols, D. Grodent, J. H. Waite, G. R. Gladstone, and R. Prangé (1995), Simultaneous observations of the satumian aurora and polar haze with the HST/FOC, Geophys. Res. Lett., 22, 2685.
Gérard, J.-C., D. Grodent, J. Gustin, A. Saglam, J. T. Clarke, and J. T. Trauger (2004), Characteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph, J. Geophys. Res., 109, A09207, doi:10.1029/2004JA010513.
Grodent, D., J.-C. Gérard, S. W. H. Cowley, E. J. Bunce, and J. T. Clarke (2005), Variable morphology of Saturn's southern ultraviolet aurora, J. Geophys. Res., 110, A07215, doi:10.1029/2004JA010983.
Hill, T. W. (1979), Inertial limit on corotation, J. Geophys. Res., 84, 6554.
Jackman, C. M., N. Achilleos, E. J. Bunce, S. W. H. Cowley, M. K. Dougherty, G. H. Jones, and S. E. Milan (2004), Interplanetary magnetic field conditions at ∼9 A.U. during the declining phase of the solar cycle and its implications for Saturn's magnetospheric dynamics, J. Geophys. Res., 109, A11203, doi:10.1029/2004JA010614.
Kurth, W. S., et al. (2005), An Earth-like correspondence between Saturn's auroral features and radio emission, Nature, 433, 722.
McCrea, I. W., M. Lockwood, J. Moen, F. Pitout, P. Eglitis, A. D. Aylward, J.-C. Cerisier, A. Thorolfssen, and S. E. Milan (2000), ESR and EISCAT observations of the response of the cusp and cleft to IMF orientation changes, Ann. Geophys., 18, 1009.
McGrath, M. A., and J. T. Clarke (1992), H I Lyman alpha emission from Saturn (1980-1990), J. Geophys. Res., 103, 20,237.
Milan, S. E., M. Lester, S. W. H. Cowley, and M. Brittnacher (2000), Dayside convection and auroral morphology during an interval of northward interplanetary magnetic field, Ann. Geophys., 18, 436.
Newell, P. T., C.-I. Meng, D. G. Sibeck, and R. Lepping (1989), Some low-altitude cusp dependencies on the interplanetary magnetic field, J. Geophys. Res., 94, 8921.
Onsager, T. G., and S. A. Fuselier (1994), The location of magnetic reconnection for northward and southward interplanetary magnetic field, in Solar System Plasmas in Space and Time, Geophys. Monogr. Ser., vol. 84, edited by J. L. Burch and J. H. Waite Jr., p. 183, AGU, Washington, D. C.
Onsager, T. G., C. A. Kletzing, J. B. Austin, and H. MacKieman (1993), Model of magnetosheath plasma in the magnetosphere: Cusp and mantle particles at low altitudes, Geophys. Res. Lett., 20, 479.
Reiff, P. H., and J. L. Burch (1985), IMF By-dependent plasma flow and Birkeland currents in the dayside magnetosphere: 2. A global model for northward and southward IMF, J. Geophys. Res., 90, 1595.
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, 215, 548.
Sandholt, P. E., C. J. Farrugia, S. W. H. Cowley, M. Lester, and J.-C. Cerisier (2001), Excitation of transient lobe cell convection and auroral arc at the cusp poleward boundary during a transition of the interplanetary magnetic field from south to north, Ann. Geophys., 19, 487.
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.