[en] The magnetospheric cusp is a region connecting the interplanetary environment to the ionosphere and enabling solar wind particles to reach the ionosphere. We report the detection of several isolated high-latitude auroral emissions with the Ultraviolet Imaging Spectrograph of the Cassini spacecraft. We suggest that these auroral spots, located in the dawn-to-noon sector and poleward of the main emission, are the ionospheric signatures of the magnetospheric cusp, in agreement with some previous observations with the Hubble Space Telescope. The high-latitude cusp auroral signature has been associated with high-latitude lobe reconnection in the presence of a southward interplanetary magnetic field. The occurrence rate of the polar cusp aurora suggests that lobe reconnection is frequent at Saturn. Several auroral imaging sequences reveal a quasiperiodic brightening of the polar cusp aurora with a period in the range of 60 to 70 min. Similar pulsations in the energetic electron fluxes and in the azimuthal component of the magnetic field are simultaneously observed by Cassini instruments, suggesting the presence of field-aligned currents. Pulsed dayside magnetopause reconnection is a likely common triggering process for the cusp auroral brightenings at Saturn and the quasiperiodic pulsations in the high-latitude energetic electron fluxes.
Research center :
STAR - Space sciences, Technologies and Astrophysics Research - ULiège
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Palmaerts, Benjamin ; Université de Liège - ULiège > Département d'Astrophysique, Géophysique et Océanographie (AGO) > Laboratoire de Physique Atmosphérique et Planétaire (LPAP)
Radioti, Aikaterini ; Université de Liège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Roussos, E.; Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
Grodent, Denis ; Université de Liège > Département d'astrophys., géophysique et océanographie (AGO) > Labo de physique atmosphérique et planétaire (LPAP)
Gérard, Jean-Claude ; Université de Liège > Département d'astrophys., géophysique et océanographie (AGO) > Département d'astrophys., géophysique et océanographie (AGO)
Krupp, N.; Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
Mitchell, D. G.; Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, USA
Arridge, C. S., et al. (2016), Cassini observations of Saturn's southern polar cusp, J. Geophys. Res. Space Physics, 121, 3006–3030, doi:10.1002/2015JA021957.
Badman, S. V., et al. (2012), Cassini observations of ion and electron beams at Saturn and their relationship to infrared auroral arcs, J. Geophys. Res., 117, A01211, doi:10.1029/2011JA01722.
Badman, S. V., A. Masters, H. Hasegawa, M. Fujimoto, A. Radioti, D. Grodent, N. Sergis, M. K. Dougherty, and A. J. Coates (2013), Bursty magnetic reconnection at Saturn's magnetopause, Geophys. Res. Lett., 40, 1027–1031, doi:10.1002/grl.50199.
Belenkaya, E. S., S. W. H. Cowley, C. J. Meredith, J. D. Nichols, V. V. Kalegaev, I. I. Alexeev, O. G. Barinov, W. O. Barinova, and M. S. Blokhina (2014), Magnetospheric magnetic field modelling for the 2011 and 2012 HST Saturn aurora campaigns—Implications for auroral source regions, Ann. Geophys., 32, 689–704, doi:10.5194/angeo-32-689-2014.
Bunce, E. J., S. W. H. Cowley, and S. E. Milan (2005), Interplanetary magnetic field control of Saturn's polar cusp aurora, Ann. Geophys., 23, 1405–1431, doi:10.5194/angeo-23-1405-2005.
Bunce, E. J., S. W. H. Cowley, I. I. Alexeev, C. S. Arridge, M. K. Dougherty, J. D. Nichols, and C. T. Russell (2007), Cassini observations of the variation of Saturn's ring current parameters with system size, J. Geophys. Res., 112, A10202, doi:10.1029/2007JA012275.
Bunce, E. J., et al. (2008), Origin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope, J. Geophys. Res., 113, A09209, doi:10.1029/2008JA013257.
Cargill, P. J., et al. (2005), Cluster at the magnetospheric cusps, Space Sci. Rev., 118, 321–366, doi:10.1007/s11214-005-3835-0.
Desroche, M., F. Bagenal, P. A. Delamere, and N. Erkaev (2013), Conditions at the magnetopause of Saturn and implications for the solar wind interaction, J. Geophys. Res. Space Physics, 118, 3087–3095, doi:10.1002/jgra.50294.
Dougherty, M. K., et al. (2004), The Cassini magnetic field investigation, Space Sci. Rev., 114, 331–383, doi:10.1007/s11214-004-1432-2.
Dougherty, M. K., et al. (2005), Cassini magnetometer observations during Saturn orbit insertion, Science, 307(5713), 1266–1270, doi:10.1126/science.1106098.
Esposito, L. W., et al. (2004), The Cassini Ultraviolet Imaging Spectrograph investigation, Space Sci. Rev., 115, 299–361, doi:10.1007/s11214-004-1455-8.
Fasel, G. J., L. C. Lee, and R. W. Smith (1993), A mechanism for the multiple brightenings of dayside poleward-moving auroral forms, Geophys. Res. Lett., 20(20), 2247–2250, doi:10.1029/93GL02487.
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, A71091, doi:10.1029/2001JA900161.
Fuselier, S. A., H. U. Frey, K. J. Trattner, S. B. Mende, and J. L. Burch (2002), Cusp aurora dependence on interplanetary magnetic field Bz, J. Geophys. Res., 107, A71111, doi:10.1029/2001JA900165.
Fuselier, S. A., S. M. Petrinec, K. J. Trattner, M. Fujimoto, and H. Hasegawa (2007), Simultaneous observations of fluctuating cusp aurora and low-latitude magnetopause reconnection, J. Geophys. Res., 112, A11207, doi:10.1029/2007JA012252.
Fuselier, S. A., R. Frahm, W. S. Lewis, A. Masters, J. Mukherjee, S. M. Petrinec, and I. J. Sillanpaa (2014), The location of magnetic reconnection at Saturn's magnetopause: A comparison with Earth, J. Geophys. Res. Space Physics, 119, 2563–2578, doi:10.1002/2013JA019684.
Gérard, J.-C., D. Grodent, J. Gustin, and A. Saglam (2004), Characteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph, J. Geophys. Res., 109, A09207, doi:10.1029/2004JA010513.
Gérard, J.-C., E. J. Bunce, D. Grodent, S. W. H. Cowley, J. T. Clarke, and S. V. Badman (2005), Signature of Saturn's aurora cusp: Simultaneous Hubble Space Telescope FUV observations and upstream solar wind monitoring, J. Geophys. Res., 110, A11201, doi:10.1029/2005JA011094.
Gérard, J.-C., B. Bonfond, J. Gustin, D. Grodent, J. T. Clarke, D. Bisikalo, and V. Shematovich (2009), Altitude of Saturn's aurora and its implications for the characteristic energy of precipitated electrons, Geophys. Res. Lett., 36, L02202, doi:10.1029/2008GL036554.
Grodent, D. (2015), A brief review of ultraviolet auroral emissions on giant planets, Space Sci. Rev., 187(1), 23–50, doi:10.1007/s11214-014-0052-8.
Grodent, D., J. Gustin, J.-C. Gérard, A. Radioti, B. Bonfond, and W. R. Pryor (2011), Small-scale structures in Saturn's ultraviolet aurora, J. Geophys. Res., 116, A09225, doi:10.1029/2011JA016818.
Jasinski, J. M., et al. (2014), Cusp observation at Saturn's high-latitude magnetosphere by the Cassini spacecraft, Geophys. Res. Lett., 41, 1382–1388, doi:10.1002/2014GL059319.
Jasinski, J. M., J. A. Slavin, C. S. Arridge, G. Poh, X. Jia, N. Sergis, A. J. Coates, G. H. Jones, and J. H. Waite Jr. (2016), Flux transfer event observation at Saturn's dayside magnetopause by the Cassini spacecraft, Geophys. Res. Lett., 43, 6713–6723, doi:10.1002/2016GL069260.
Jinks, S. L., et al. (2014), Cassini multi-instrument assessment of Saturn's polar cap boundary, J. Geophys. Res. Space Physics, 119, 8161–8177, doi:10.1002/2014JA020367.
Kan, J. R., C. S. Deehr, L. H. Lyu, and P. T. Newell (1996), Ionospheric signatures of patchy-intermittent reconnection at dayside magnetopause, J. Geophys. Res., 101(A5), 10,939–10,946, doi:10.1029/96JA00462.
Khurana, K. K., C. S. Arridge, H. Schwarzl, and M. K. Dougherty (2006), A model of Saturn's magnetospheric field based on latest Cassini observations, Eos Trans. AGU, 87(36), Jt. Assem. Suppl., Abstract P44A-01.
Krimigis, S. M., et al. (2004), Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan, Space Sci. Rev., 114, 233–329, doi:10.1007/s11214-004-1410-8.
Kuo, H., C. T. Russell, and G. Le (1995), Statistical studies of flux transfer events, J. Geophys. Res., 100(A3), 3513–3519, doi:10.1029/94JA02498.
Lai, H. R., H. Y. Wei, C. T. Russell, C. S. Arridge, and M. K. Dougherty (2012), Reconnection at the magnetopause of Saturn: Perspective from FTE occurrence and magnetosphere size, J. Geophys. Res., 117, A05222, doi:10.1029/2011JA017263.
Lamy, L., R. Prangé, W. Pryor, J. Gustin, S. V. Badman, H. Melin, T. Stallard, D. G. Mitchell, and P. C. Brandt (2013), Multispectral simultaneous diagnosis of Saturn's aurorae throughout a planetary rotation, J. Geophys. Res. Space Physics, 118, 4817–4843, doi:10.1002/jgra.50404.
Masters, A., D. G. Mitchell, A. J. Coates, and M. K. Dougherty (2011), Saturn's low-latitude boundary layer: 1. Properties and variability, J. Geophys. Res., 116, A06210, doi:10.1029/2010JA016421.
Masters, A., J. P. Eastwood, M. Swisdak, M. F. Thomsen, C. T. Russell, N. Sergis, F. J. Crary, M. K. Dougherty, A. J. Coates, and S. M. Krimigis (2012), The importance of plasma β conditions for magnetic reconnection at Saturn's magnetopause, Geophys. Res. Lett., 39, L08103, doi:10.1029/2012GL051372.
Meredith, C. J., I. I. Alexeev, S. V. Badman, E. S. Belenkaya, S. W. H. Cowley, M. K. Dougherty, V. V. Kalegaev, G. R. Lewis, and J. D. Nichols (2014), Saturn's dayside ultraviolet auroras: Evidence for morphological dependence on the direction of the upstream interplanetary magnetic field, J. Geophys. Res. Space Physics, 119, 1994–2008, doi:10.1002/2013JA019598.
Milan, S. E., M. Lester, S. W. H. Cowley, and M. Brittnacher (2000a), Dayside convection and auroral morphology during an interval of northward interplanetary magnetic field, Ann. Geophys., 18, 436–444, doi:10.1007/s00585-000-0436-9.
Milan, S. E., M. Lester, S. W. H. Cowley, and M. Brittnacher (2000b), Convection and auroral response to a southward turning of the IMF: Polar UVI, CUTLASS, and IMAGE signatures of transient magnetic flux transfer at the magnetopause, J. Geophys. Res., 105(A7), 15,741–15,755, doi:10.1029/2000JA900022.
Mitchell, D. G., W. S. Kurth, G. B. Hospodarsky, N. Krupp, J. Saur, B. H. Mauk, J. F. Carbary, S. M. Krimigis, M. K. Dougherty, and D. C. Hamilton (2009), Ion conics and electron beams associated with auroral processes on Saturn, J. Geophys. Res., 114, A02212, doi:10.1029/2008JA013621.
Mitchell, D. G., J. F. Carbary, E. J. Bunce, A. Radioti, S. V. Badman, W. R. Pryor, G. B. Hospodarsky, and W. S. Kurth (2016), Recurrent pulsations in Saturn's high latitude magnetosphere, Icarus, 263, 94–100, doi:10.1016/j.icarus.2014.10.028.
Onsager, T. G., and S. A. Fuselier (1994), Solar System Plasmas in Space and Time, chap. The location of magnetic reconnection for northward and southward interplanetary magnetic field, pp. 183–197, AGU, Washington, D. C.
Palmaerts, B., E. Roussos, N. Krupp, W. S. Kurth, D. G. Mitchell, and J. N. Yates (2016), Statistical analysis and multi-instrument overview of the quasi-periodic 1-hour pulsations in Saturn's outer magnetosphere, Icarus, 271, 1–18, doi:10.1016/j.icarus.2016.01.025.
Provan, G., T. K. Yeoman, and S. E. Milan (1998), CUTLASS Finland radar observations of the ionospheric signatures of flux transfer events and the resulting plasma flows, Ann. Geophys., 16, 1411–1422, doi:10.1007/s00585-998-1411-0.
Radioti, A., D. Grodent, J.-C. Gérard, S. E. Milan, B. Bonfond, J. Gustin, and W. Pryor (2011), Bifurcations of the main auroral ring at Saturn: Ionospheric signatures of consecutive reconnection events at the magnetopause, J. Geophys. Res., 116, A11209, doi:10.1029/2011JA016661.
Radioti, A., D. Grodent, J.-C. Gérard, B. Bonfond, J. Gustin, W. Pryor, J. M. Jasinski, and C. S. Arridge (2013), Auroral signatures of multiple magnetopause reconnection at Saturn, Geophys. Res. Lett., 40, 4498–4502, doi:10.1002/grl.50889.
Roussos, E., et al. (2016), Quasi-periodic injections of relativistic electrons in Saturn's outer magnetosphere, Icarus, 263, 101–116, doi:10.1016/j.icarus.2015.04.017.
Russell, C. T., and R. C. Elphic (1978), Initial ISEE magnetometer results: Magnetopause observations, Space Sci. Rev., 22, 681–715, doi:10.1007/BF00212619.
Sandholt, P. E., and C. J. Farrugia (2007), Role of poleward moving auroral forms in the dawn-dusk auroral precipitation asymmetries induced by IMF By, J. Geophys. Res., 112, A04203, doi:10.1029/2006JA011952.
Smith, M. F., and M. Lockwood (1996), Earth's magnetospheric cusps, Rev. Geophys., 34, 233–260, doi:10.1029/96RG00893.
Swisdak, M., B. N. Rogers, J. F. Drake, and M. A. Shay (2003), Diamagnetic suppression of component magnetic reconnection at the magnetopause, J. Geophys. Res., 108, A51218, doi:10.1029/2002JA009726.
Swisdak, M., M. Opher, J. F. Drake, and F. Alouani Bibi (2010), The vector direction of the interstellar magnetic field outside the heliosphere, Astrophys. J., 710, 1769–1775, doi:10.1088/0004-637X/710/2/1769.
Talboys, D. L., C. S. Arridge, E. J. Bunce, A. J. Coates, S. W. H. Cowley, and M. K. Dougherty (2009), Characterization of auroral current systems in Saturn's magnetosphere: High-latitude Cassini observations, J. Geophys. Res., 114, A06220, doi:10.1029/2008JA013846.
Talboys, D. L., E. J. Bunce, S. W. H. Cowley, C. S. Arridge, A. J. Coates, and M. K. Dougherty (2011), Statistical characteristics of field-aligned currents in Saturn's nightside magnetosphere, J. Geophys. Res., 116, A04213, doi:10.1029/2010JA016102.
Trenchi, L. (2008), Occurrence of reconnection jets at the dayside magnetopause: Double Star observations, J. Geophys. Res., 113, A07S10, doi:10.1029/2007JA012774.
Wild, J. A., et al. (2001), First simultaneous observations of flux transfer events at the high-latitude magnetopause by the Cluster spacecraft and pulsed radar signatures in the conjugate ionosphere by the CUTLASS and EISCAT radars, Ann. Geophys., 19, 1491–1508, doi:10.5194/angeo-19-1491-2001.
