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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 25, issue 4
Ann. Geophys., 25, 905-914, 2007
https://doi.org/10.5194/angeo-25-905-2007
© Author(s) 2007. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 25, 905-914, 2007
https://doi.org/10.5194/angeo-25-905-2007
© Author(s) 2007. This work is distributed under
the Creative Commons Attribution 3.0 License.

  08 May 2007

08 May 2007

Europa's Alfvén wing: shrinkage and displacement influenced by an induced magnetic field

M. Volwerk1,2, K. Khurana3, and M. Kivelson3,4 M. Volwerk et al.
  • 1Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria
  • 2Max-Planck-Institute for Extraterrestrial Physics, 85741 Garching, Germany
  • 3Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA 90095-1567, USA
  • 4Department of Earth and Space Sciences, UCLA, Los Angeles, CA, 90095-1567, USA

Abstract. The Galileo magnetometer data are used to investigate the structure of the Alfvén wing during three flybys of Europa. The presence of an induced magnetic field is shown to shrink the cross section of the Alfvén wing and offset it along the direction radial to Jupiter. Both the shrinkage and the offset depend on the strength of the induced field. The entry and exit points of the spacecraft into and out of the Alfvén wings are modeled to determine the angle between the wings and the background magnetic field. Tracing of the Alfvén characteristics in a model magnetic field consisting of Jupiter's background field and an induced field in Europa produces an offset and shrinking of the Alfvén wing consistent with the geometric modeling. Thus we believe that the Alfvén wing properties have been determined correctly. The Alfvén wing angle is directly proportional to the local Alfvén velocity, and is thus a probe for the local plasma density. We show that the inferred plasma density can be understood in terms of the electron density measured by the plasma wave experiment. When Europa is located in the Jovian plasma sheet the derived mass-per-charge exceeds the previous estimates, which is a result of increased pickup of sputtered ions near the moon. The estimated rate of O2+ pickup agrees well with the results from numerical models.

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