Cassini observations of ion and electron beams at Saturn and their relationship to infrared auroral arcs

We present Cassini Visual and Infrared Mapping Spectrometer observations of infrared auroral emissions from the noon sector of Saturn's ionosphere revealing multiple intense auroral arcs separated by dark regions poleward of the main oval. The arcs are interpreted as the ionospheric signatures of bursts of reconnection occurring at the dayside magnetopause. The auroral arcs were associated with upward field-aligned currents, the magnetic signatures of which were detected by Cassini at high planetary latitudes. Magnetic field and particle observations in the adjacent downward current regions showed upward bursts of 100–360 keV light ions in addition to energetic (hundreds of keV) electrons, which may have been scattered from upward accelerated beams carrying the downward currents. Broadband, upward propagating whistler waves were detected simultaneously with the ion beams. The acceleration of the light ions from low altitudes is attributed to wave-particle interactions in the downward current regions. Energetic (600 keV) oxygen ions were also detected, suggesting the presence of ambient oxygen at altitudes within the acceleration region. These simultaneous in situ and remote observations reveal the highly energetic magnetospheric dynamics driving some of Saturn's unusual auroral features. This is the first in situ identification of transient reconnection events at regions magnetically conjugate to Saturn's magnetopause.

[1]  Christopher T. Russell,et al.  Magnetopause structure and the role of reconnection at the outer planets , 1997 .

[2]  M. Dougherty,et al.  Cassini observations of the Interplanetary Medium Upstream of Saturn and their relation to the Hubble Space Telescope aurora data , 2006 .

[3]  Emma J. Bunce,et al.  Variable morphology of Saturn's southern ultraviolet aurora , 2005 .

[4]  R. Walker,et al.  Magnetospheric convection at Saturn as a function of IMF BZ , 2007 .

[5]  Emma J. Bunce,et al.  Interplanetary magnetic field control of Saturn's polar cusp aurora , 2005 .

[6]  Philippe Zarka,et al.  Response of Jupiter's and Saturn's auroral activity to the solar wind , 2009 .

[7]  Emma J. Bunce,et al.  Reconnection in a rotation-dominated magnetosphere and its relation to Saturn's auroral dynamics , 2005 .

[8]  U. Inan,et al.  Wave normal and Poynting vector calculations using the Cassini radio and plasma wave instrument , 2001 .

[9]  Chihiro Tao,et al.  UV and IR auroral emission model for the outer planets: Jupiter and Saturn comparison , 2011 .

[10]  G. Marklund.  Electric Fields and Plasma Processes in the Auroral Downward Current Region, Below, Within, and Above the Acceleration Region , 2009 .

[11]  Vladimir Kalegaev,et al.  IMF dependence of the open-closed field line boundary in Saturn's ionosphere, and its relation to the UV auroral oval observed by the Hubble Space Telescope , 2007 .

[12]  S. Krimigis,et al.  Ion conics and electron beams associated with auroral processes on Saturn , 2009 .

[13]  R. H. Brown,et al.  The Cassini Visual And Infrared Mapping Spectrometer (Vims) Investigation , 2004 .

[14]  Emma J. Bunce,et al.  Cassini observations of the variation of Saturn's ring current parameters with system size , 2007 .

[15]  R. Elphic,et al.  FAST observations in the downward auroral current region: Energetic upgoing electron beams, parallel potential drops, and ion heating , 1998 .

[16]  K. Khurana,et al.  Anti-planetward auroral electron beams at Saturn , 2006, Nature.

[17]  Emma J. Bunce,et al.  Statistical characteristics of field‐aligned currents in Saturn's nightside magnetosphere , 2011 .

[18]  S. Miller,et al.  Ion winds in Saturn's southern auroral/polar region , 2004 .

[19]  J.-C. Gérard,et al.  Variation of Saturn's UV aurora with SKR phase , 2010 .

[20]  Christopher T. Russell,et al.  Modeling the size and shape of Saturn's magnetopause with variable dynamic pressure , 2006 .

[21]  Robert H. Brown,et al.  Location of Saturn's northern infrared aurora determined from Cassini VIMS images , 2010 .

[22]  Michelle F. Thomsen,et al.  Evidence for reconnection at Saturn's magnetopause , 2008 .

[23]  Emma J. Bunce,et al.  Origin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope , 2008 .

[24]  Stanley W. H. Cowley,et al.  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 , 2000 .

[25]  M. Shay,et al.  Diamagnetic suppression of component magnetic reconnection at the magnetopause , 2003, physics/0305021.

[26]  V. Bailey.  The Interplanetary Magnetic Field , 1963, Nature.

[27]  D. Grodent,et al.  Solar wind dynamic pressure and electric field as the main factors controlling Saturn's aurorae , 2005, Nature.

[28]  B. Mauk,et al.  Equatorial electron beams and auroral structuring at Jupiter , 2007 .

[29]  Kevin H. Baines,et al.  Simultaneous Cassini VIMS and UVIS observations of Saturn's southern aurora: Comparing emissions from H, H2 and H3+ at a high spatial resolution , 2011 .

[30]  Emma J. Bunce,et al.  Signature of Saturn's auroral cusp:simultaneous Hubble Space Telescope FUV observations and upstream solar wind monitoring , 2005 .

[31]  N Achilleos,et al.  Cassini Magnetometer Observations During Saturn Orbit Insertion , 2005, Science.

[32]  Richard D. Starr,et al.  Mercury's Magnetosphere After MESSENGER's First Flyby , 2008, Science.

[33]  Denis Grodent,et al.  Bifurcations of the main auroral ring at Saturn: ionospheric signatures of consecutive reconnection events at the magnetopause , 2011 .

[34]  Emma J. Bunce,et al.  Magnetic field structure of Saturn's dayside magnetosphere and its mapping to the ionosphere: Results from ring current modeling , 2008 .

[35]  M. Dougherty,et al.  Auroral electron distributions within and close to the Saturn kilometric radiation source region , 2011 .

[36]  Denis Grodent,et al.  Characteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph , 2004 .

[37]  Denis Grodent,et al.  A statistical analysis of the location and width of Saturn's southern auroras , 2006 .

[38]  Emma J. Bunce,et al.  Complex structure within Saturn’s infrared aurora , 2008, Nature.

[39]  D. Grodent,et al.  Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter , 2005, Nature.

[40]  Rudolf A. Treumann,et al.  Auroral plasma physics , 2002 .

[41]  R. Treumann,et al.  In Situ Measurements in the Auroral Plasma , 2003 .

[42]  Christopher T. Russell,et al.  A survey of dayside flux transfer events observed by ISEE 1 and 2 magnetometers , 1984 .

[43]  Emma J. Bunce,et al.  Open flux estimates in Saturn's magnetosphere during the January 2004 Cassini‐HST campaign, and implications for reconnection rates , 2005 .

[44]  Edmond C. Roelof,et al.  Recurrent energization of plasma in the midnight-to-dawn quadrant of Saturn's magnetosphere, and its relationship to auroral UV and radio emissions , 2009 .

[45]  C. Russell,et al.  Flux transfer events at the Jovian magnetopause , 1985 .

[46]  M. Kivelson,et al.  Cassini observations of a Kelvin-Helmholtz vortex in Saturn's outer magnetosphere , 2010 .

[47]  S. M. Krimigis,et al.  Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan , 2004 .

[48]  Denis Grodent,et al.  Small-scale structures in Saturn's ultraviolet aurora , 2011 .

[49]  J. Slavin,et al.  In situ observations of the effect of a solar wind compression on Saturn's magnetotail , 2010 .

[50]  Emma J. Bunce,et al.  A simple quantitative model of plasma flows and currents in Saturn's polar ionosphere , 2004 .

[51]  K. Baines,et al.  Cassini VIMS observations of latitudinal and hemispheric variations in Saturn’s infrared auroral intensity , 2011 .

[52]  Emma J. Bunce,et al.  In situ observations of a solar wind compression‐induced hot plasma injection in Saturn's tail , 2005 .

[53]  Edward J. Smith,et al.  Interplanetary magnetic field at ∼9 AU during the declining phase of the solar cycle and its implications for Saturn's magnetospheric dynamics , 2004 .

[54]  M. Grande,et al.  Cassini Plasma Spectrometer Investigation , 2004 .

[55]  J. H. Waite,et al.  Detection of H3+ on Jupiter , 1989, Nature.