Articles | Volume 29, issue 9
Ann. Geophys., 29, 1655–1662, 2011
Ann. Geophys., 29, 1655–1662, 2011

  28 Sep 2011

28 Sep 2011

The distribution of the ring current: Cluster observations

Q.-H. Zhang1, M. W. Dunlop2, M. Lockwood2, R. Holme3, Y. Kamide1, W. Baumjohann4, R.-Y. Liu1, H.-G. Yang1, E. E. Woodfield5, H.-Q. Hu1, B.-C. Zhang1, and S.-L. Liu1 Q.-H. Zhang et al.
  • 1SOA Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai, China
  • 2SSTD, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, UK
  • 3Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, UK
  • 4Space Research Institute, Austrian Academy of Sciences, Graz, Austria
  • 5Department of Communications Systems, University of Lancaster, Lancaster, UK

Abstract. Extending previous studies, a full-circle investigation of the ring current has been made using Cluster 4-spacecraft observations near perigee, at times when the Cluster array had relatively small separations and nearly regular tetrahedral configurations, and when the Dst index was greater than −30 nT (non-storm conditions). These observations result in direct estimations of the near equatorial current density at all magnetic local times (MLT) for the first time and with sufficient accuracy, for the following observations. The results confirm that the ring current flows westward and show that the in situ average measured current density (sampled in the radial range accessed by Cluster ~4–4.5 RE) is asymmetric in MLT, ranging from 9 to 27 nA m−2. The direction of current is shown to be very well ordered for the whole range of MLT. Both of these results are in line with previous studies on partial ring extent. The magnitude of the current density, however, reveals a distinct asymmetry: growing from 10 to 27 nA m−2 as azimuth reduces from about 12:00 MLT to 03:00 and falling from 20 to 10 nA m−2 less steadily as azimuth reduces from 24:00 to 12:00 MLT. This result has not been reported before and we suggest it could reflect a number of effects. Firstly, we argue it is consistent with the operation of region-2 field aligned-currents (FACs), which are expected to flow upward into the ring current around 09:00 MLT and downward out of the ring current around 14:00 MLT. Secondly, we note that it is also consistent with a possible asymmetry in the radial distribution profile of current density (resulting in higher peak at ~4–4.5 RE). We note that part of the enhanced current could reflect an increase in the mean AE activity (during the periods in which Cluster samples those MLT).