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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 33, issue 9
Ann. Geophys., 33, 1195–1202, 2015
https://doi.org/10.5194/angeo-33-1195-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 33, 1195–1202, 2015
https://doi.org/10.5194/angeo-33-1195-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Regular paper 30 Sep 2015

Regular paper | 30 Sep 2015

The latitudinal structure of the nightside outer magnetosphere of Saturn as revealed by velocity moments of thermal ions

Z. Nemeth1, K. Szego1, L. Foldy1, M. G. Kivelson2,3, X. Jia3, K. M. Ramer3, S. W. H. Cowley4, G. Provan4, and M. Thomsen5 Z. Nemeth et al.
  • 1Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Budapest, Hungary
  • 2Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI, USA
  • 3Earth and Space Sciences, University of California, Los Angeles, CA, USA
  • 4Department of Physics and Astronomy, University of Leicester, Leicester, UK
  • 5Planetary Science Institute, Tucson, AZ, USA

Abstract. In this study we investigate the latitudinal behavior of the azimuthal plasma velocities in the outer magnetosphere of Saturn using the numerical ion moments derived from the measurements of the Cassini Plasma Spectrometer. One of the new results presented is that although these moments display some scatter, a significant positive correlation is found to exist between the azimuthal velocity and the plasma density, such that on average, the higher the density the higher the rotation speed. We also found that both the azimuthal velocity and the density anticorrelate with the magnitude of the radial component of the magnetic field and drop rapidly with increasing distance from the magnetic equator. The azimuthal velocities show periodic behavior with a period near the planetary rotation period, which can also be explained by the strong dependence on magnetic latitude, taking into account the flapping of the magnetodisk. It is thus found that the dense plasma near the magnetic equator rotates around the planet at high speed, while the dilute plasma at higher latitudes in the northern and southern hemispheres rotates significantly slower. The latitudinal gradient observed in the azimuthal speed is suggested to be a direct consequence of the sub-corotation of the plasma in the outer magnetosphere, with highest speeds occurring on field lines at lowest latitudes mapping to the rapidly rotating inner regions of the plasma sheet, and the speed falling as one approaches the lobe, where the field lines are connected to strongly sub-corotating plasma.

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The latitudinal structure of the nightside outer magnetosphere of Saturn was studied by using Cassini CAPS plasma measurements. It was found that the plasma density and the azimuthal velocity decrease simultaneously as the spacecraft moves away from the magnetic equator. The magnetosphere exhibits an “onion shell like” structure: the inner regions spin rapidly corotating with the planet, but moving outward and/or further away from the equatorial plane one can encounter slower and slower shells.
The latitudinal structure of the nightside outer magnetosphere of Saturn was studied by using...
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