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Ann. Geophys., 24, 637-649, 2006
www.ann-geophys.net/24/637/2006/
© European Geosciences Union 2006
Observations of concentrated generator regions in the nightside magnetosphere by Cluster/FAST conjunctions
M. Hamrin1, O. Marghitu2,3, K. Rönnmark1, B. Klecker3, M. André4, S. Buchert4, L. M. Kistler5, J. McFadden6, H. Rème7, and A. Vaivads4
1Department of Physics, Umeå University, Umeå, Sweden
2Institute for Space Sciences, Bucharest, Romania
3Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
4Swedish Institute of Space Physics, Uppsala, Sweden
5Space Science Center, University of New Hampshire, NH, Durham, USA
6Space Sciences Lab., University of California at Berkley, USA, USA
7CESR-CNRS, Toulouse, France
Abstract. Here and in the companion paper, Marghitu et al. (2006), we
investigate plausible auroral generator regions in the nightside
auroral magnetosphere. In this article we use magnetically
conjugate data from the Cluster and the FAST satellites during a
3.5-h long event from 19-20 September 2001. Cluster is in the
Southern Hemisphere close to apogee, where it probes the plasma sheet
and lobe at an altitude of about 18 RE. FAST is below the
acceleration region at approximately 0.6 RE. Searching for clear
signatures of negative power densities,
E·J<0, in the Cluster data we can identify
three concentrated generator regions (CGRs) during our event. From
the magnetically conjugate FAST data we see that the observed
generator regions in the Cluster data correlate with auroral
precipitation. The downward Poynting flux observed by Cluster, as
well as the scale size of the CGRs, are consistent with the electron
energy flux and the size of the inverted-V regions observed by FAST.
To our knowledge, these are the first in-situ observations of
the crossing of an auroral generator region. The main contribution
to E·J<0 comes from the GSE EyJy. The
electric field Ey is weakly negative during most of our
entire event and we conclude that the CGRs occur when the duskward
current Jy grows large and positive. We find that our
observations are consistent with a local southward expansion of the
plasma sheet and/or rather complicated, 3-D wavy
structures propagating over the Cluster satellites. We find that the
plasma is working against the magnetic field, and that kinetic
energy is being converted into electromagnetic energy. Some of the
energy is transported away as Poynting flux.
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