Auroral current systems in Saturn's magnetosphere: comparison of theoretical models with Cassini and HST observations

The first simultaneous observations of fields and plasmas in Saturn's high-latitude magnetosphere and UV images of the conjugate auroral oval were obtained by the Cassini spacecraft and the Hubble Space Telescope (HST) in January 2007. These data have shown that the southern auroral oval near noon maps to the dayside cusp boundary between open and closed field lines, associated with a major layer of upward-directed field-aligned current (Bunce et al., 2008). The results thus support earlier theoretical discussion and quantitative modelling of magnetosphere-ionosphere coupling at Saturn (Cowley et al., 2004), that suggests the oval is produced by electron acceleration in the field-aligned current layer required by rotational flow shear between strongly sub-corotating flow on open field lines and near-corotating flow on closed field lines. Here we quantitatively compare these modelling results (the "CBO" model) with the Cassini-HST data set. The comparison shows good qualitative agreement between model and data, the principal difference being that the model currents are too small by factors of about five, as determined from the magnetic perturbations observed by Cassini. This is suggested to be principally indicative of a more highly conducting summer southern ionosphere than was assumed in the CBO model. A revised model is therefore proposed in which the height-integrated ionospheric Pedersen conductivity is increased by a factor of four from 1 to 4 mho, together with more minor adjustments to the co-latitude of the boundary, the flow shear across it, the width of the current layer, and the properties of the source electrons. It is shown that the revised model agrees well with the combined Cassini-HST data, requiring downward acceleration of outer magnetosphere electrons through a ~10 kV potential in the current layer at the open-closed field line boundary to produce an auroral oval of ~1° width with UV emission intensities of a few tens of kR.

[1]  K. Szego,et al.  Cassini Plasma Spectrometer Investigation , 1996, Optics & Photonics.

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

[3]  Barry H. Mauk,et al.  A model for the azimuthal plasma velocity in Saturn's magnetosphere , 2004 .

[4]  Philippe Zarka,et al.  Detailed study of FUV Jovian auroral features with the post-COSTAR HST faint object camera , 1998 .

[5]  David J. Southwood,et al.  A new perspective concerning the influence of the solar wind on the Jovian magnetosphere , 2001 .

[6]  T. Hill,et al.  The Jovian auroral oval , 2001 .

[7]  V. Vasyliūnas,et al.  Plasma distribution and flow , 1983 .

[8]  Donald A. Gurnett,et al.  A Saturnian longitude system based on a variable kilometric radiation period , 2006 .

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

[10]  Stanley W. H. Cowley,et al.  A model of the plasma flow and current in Saturn's polar ionosphere under conditions of strong Dungey cycle driving , 2006 .

[11]  Jonathan D. Nichols,et al.  Oscillation of Saturn's southern auroral oval , 2008 .

[12]  Emma J. Bunce,et al.  Corotation-driven magnetosphere-ionosphere coupling currents in Saturn's magnetosphere and their relation to the auroras , 2003 .

[13]  Travis W. Hill,et al.  Inertial limit on corotation , 1979 .

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

[15]  Michel Blanc,et al.  Saturn's Auroral Response to the Solar Wind: Centrifugal Instability Model , 2006 .

[16]  Emma J. Bunce,et al.  Jupiter's polar ionospheric flows: Measured intensity and velocity variations poleward of the main auroral oval , 2003 .

[17]  Emma J. Bunce,et al.  Saturn's Polar Ionospheric Flows and Their Relation to the Main Auroral Oval , 2022 .

[18]  Emma J. Bunce,et al.  Cassini observations of planetary‐period magnetic field oscillations in Saturn's magnetosphere: Doppler shifts and phase motion , 2006 .

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

[20]  J. Richardson,et al.  Thermal ions at Saturn: Plasma parameters and implications , 1986 .

[21]  John D. Richardson,et al.  A plasma density model for Saturn based on Voyager observations , 1990 .

[22]  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 .

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

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

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

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

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

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

[29]  M. Dougherty,et al.  The Variable Rotation Period of the Inner Region of Saturn's Plasma Disk , 2007, Science.

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

[31]  David J. Southwood,et al.  Saturnian magnetospheric dynamics: Elucidation of a camshaft model , 2007 .

[32]  J. Waite,et al.  Magnetospheric energization by interaction between planetary spin and the solar wind , 1984 .

[33]  I. Sandahl,et al.  Some Characteristics of the Parallel Electric Field Acceleration of Electrons Over Discrete Auroral Arcs as Observed from Two Rocket Flights , 1978 .

[34]  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 .

[35]  Edward J. Smith,et al.  A model of Saturn's magnetic field based on all available data , 1990 .

[36]  J. H. Waite,et al.  Hubble Space Telescope imaging of Jupiter's UV aurora during the Galileo orbiter mission , 1998 .

[37]  Emma J. Bunce,et al.  Origin of the main auroral oval in Jupiter's coupled magnetosphere–ionosphere system , 2001 .

[38]  Emma J. Bunce,et al.  A simple axisymmetric model of magnetosphere-ionosphere coupling currents in Jupiter's polar ionosphere , 2005 .

[39]  K.-H. Glassmeier,et al.  The Cassini Magnetic Field Investigation , 2004 .

[40]  Stephen Knight,et al.  Parallel electric fields , 1973 .

[41]  N. André,et al.  Derivation of density and temperature from the Cassini–Huygens CAPS electron spectrometer , 2008 .