Cassini observations of the variation of Saturn's ring current parameters with system size

[1] We have examined residual magnetic field vectors observed in Saturn's magnetosphere during the first 2 years of the Cassini mission and have fit them to a simple axisymmetric model of the ring current in the middle magnetosphere. We then examine the variations of the ring current parameters with size of the magnetosphere. In addition, we obtain secondary parameters, including the value of the axial field at the center of the ring (equivalently Saturn's Dst) Bz0, the total current IT flowing in the modeled ring current region, and the ratio of the ring current magnetic moment relative to the magnetic moment of Saturn's dipole field, kRC. Results show that the derived parameters increase significantly with system size, due principally to the increasing radius of the outer edge of the ring. We consider the implications of the response of the magnetic moment of the ring current to changing magnetospheric size, by theoretical consideration of the magnetic moment of individual particles in the ring current. The strong positive correlation of the ring current magnetic moment with system size suggests a system in which the ring current is dominated by inertia currents, rather than by thermal effects as in the case of the Earth, with magnetosphere-ionosphere coupling maintaining the angular velocity of the plasma. The variations of Saturn's ring current parameters with system size found in this study are shown to be closely compatible with the size variations in response to the solar wind dynamic pressure recently determined from Cassini data.

[1]  Christopher T. Russell,et al.  Shape of the low-latitude magnetopause: Comparison of models , 2000 .

[2]  David B. Beard,et al.  Shape of the geomagnetic field solar wind boundary , 1964 .

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

[4]  J. Connerney,et al.  Modeling the Jovian current sheet and inner magnetosphere , 1981 .

[5]  Christopher T. Russell,et al.  Location and shape of the Jovian magnetopause and bow shock , 1998 .

[6]  Emma J. Bunce,et al.  A note on the ring current in Saturn's magnetosphere: Comparison of magnetic data obtained during the Pioneer-11 and Voyager-1 and -2 fly-bys , 2003 .

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

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

[9]  R. L. McNutt,et al.  Force balance in the magnetospheres of Jupiter and Saturn , 1983 .

[10]  James Bishop,et al.  Thermospheric atomic hydrogen densities and fluxes from dayside Lyman α measurements , 2001 .

[11]  Norbert Sckopke,et al.  A general relation between the energy of trapped particles and the disturbance field near the Earth , 1966 .

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

[13]  Elena S. Belenkaya,et al.  Modeling of the Jovian Magnetosphere , 2005 .

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

[15]  J. Connerney,et al.  Saturn's ring current and inner magnetosphere , 1981, Nature.

[16]  Christopher T. Russell,et al.  A new functional form to study the solar wind control of the magnetopause size and shape , 1997 .

[17]  John E. P. Connerney,et al.  Reply [to “Comments on ‘The Z3 model of Saturn's magnetic field and the Pioneer 11 vector helium magnetometer observations’ by J. E. P. Connerney, M. H. Acuña, and N. F. Ness”] , 1985 .

[18]  R. McNutt.  Force Balance in Outer Planet Magnetospheres , 1984 .

[19]  Y. Feldstein,et al.  Modeling of geomagnetic field during magnetic storms and comparison with observations , 2001 .

[20]  Mario H. Acuna,et al.  Currents in Saturn's magnetosphere , 1983 .

[21]  Emma J. Bunce,et al.  A global magnetic model of Saturn's magnetosphere and a comparison with Cassini SOI data , 2006 .

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

[23]  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 .

[24]  John D. Richardson,et al.  An extended plasma model for Saturn , 1995 .

[25]  Barry H. Mauk,et al.  Particle and field stress balance within a planetary magnetosphere , 1985 .

[26]  Emma J. Bunce,et al.  A note on the vector potential of Connerney et al.'s model of the equatorial current sheet in Jupiter's magnetosphere , 2001 .

[27]  J. R. Spreiter,et al.  Solar-wind flow past objects in the solar system , 1970 .

[28]  E. Smith,et al.  Saturn's Magnetic Field and Magnetosphere , 1980, Science.

[29]  J. Connerney,et al.  Magnetic Field Studies by Voyager 2: Preliminary Results at Saturn , 1982, Science.

[30]  Giacomo Giampieri,et al.  Modelling of the ring current in Saturn's magnetosphere , 2004 .

[31]  E. Parker,et al.  Hydromagnetic theory of geomagnetic storms , 1959 .

[32]  J. Connerney,et al.  Magnetic field studies by voyager 1: preliminary results at saturn. , 1981, Science.

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