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Ann. Geophys., 24, 619-635, 2006
www.ann-geophys.net/24/619/2006/
© European Geosciences Union 2006
Experimental investigation of auroral generator regions with conjugate Cluster and FAST data
O. Marghitu1,2, M. Hamrin3, B. Klecker2, A. Vaivads4, J. McFadden5, S. Buchert4, L. M. Kistler6, I. Dandouras7, M. André4, and H. Rème7
1Institute for Space Sciences, Bucharest, Romania
2Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
3Department of Physics, Umeå University, Umeå, Sweden
4Swedish Institute of Space Physics, Uppsala, Sweden
5Space Sciences Lab., University of California at Berkeley, USA
6Space Science Center, University of New Hampshire, Durham, USA
7CESR-CNRS, Toulouse, France
Abstract. Here and in the companion paper, Hamrin et al. (2006), we present experimental
evidence for the crossing of auroral generator regions, based on
conjugate Cluster and FAST data. To our knowledge, this is the first
investigation that concentrates on the evaluation of the power
density, E·J, in auroral generator regions,
by using in-situ measurements. The Cluster data we discuss
were collected within the Plasma Sheet Boundary Layer (PSBL), during
a quiet magnetospheric interval, as judged from the geophysical
indices, and several minutes before the onset of a small substorm,
as indicated by the FAST data. Even at quiet times, the PSBL is an
active location: electric fields are associated with plasma motion,
caused by the dynamics of the plasma-sheet/lobe interface, while
electrical currents are induced by pressure gradients. In the
example we show, these ingredients do indeed sustain the conversion
of mechanical energy into electromagnetic energy, as proved by the
negative power density, E·J<0. The plasma
characteristics in the vicinity of the generator regions indicate a
complicated 3-D wavy structure of the plasma sheet boundary.
Consistent with this structure, we suggest that at least part of the
generated electromagnetic energy is carried away by Alfvén waves,
to be dissipated in the ionosphere, near the polar cap boundary.
Such a scenario is supported by the FAST data, which show energetic
electron precipitation conjugated with the generator regions crossed
by Cluster. A careful examination of the conjunction timing
contributes to the validation of the generator signatures.
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