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Ann. Geophys., 24, 2645-2665, 2006
www.ann-geophys.net/24/2645/2006/
© European Geosciences Union 2006
Statistical study of the location and size of the electron edge of the Low-Latitude Boundary Layer as observed by Cluster at mid-altitudes
Y. V. Bogdanova1, C. J. Owen1, A. N. Fazakerley1, B. Klecker2, and H. Rème3
1Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking RH5 6NT, UK
2MPI für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
3Centre d'Etude Spatiale des Rayonnements, 31028 Toulouse Cedex 4, France
Abstract. The nature of particle precipitations at dayside mid-altitudes can be
interpreted in terms of the evolution of reconnected field lines. Due to the
difference between electron and ion parallel velocities, two distinct
boundary layers should be observed at mid-altitudes between the boundary
between open and closed field lines and the injections in the cusp proper. At
lowest latitudes, the electron-dominated boundary layer, named the "electron
edge" of the Low-Latitude Boundary Layer (LLBL), contains soft-magnetosheath
electrons but only high-energy ions of plasma sheet origin. A second layer,
the LLBL proper, is a mixture of both ions and electrons with characteristic
magnetosheath energies. The Cluster spacecraft frequently observe these two
boundary layers. We present an illustrative example of a Cluster mid-altitude
cusp crossing with an extended electron edge of the LLBL. This electron edge
contains 10–200 eV, low-density, isotropic electrons, presumably originating
from the solar wind halo population. These are occasionally observed with
bursts of parallel and/or anti-parallel-directed electron beams with higher
fluxes, which are possibly accelerated near the magnetopause X-line. We then
use 3 years of data from mid-altitude cusp crossings (327 events) to carry
out a statistical study of the location and size of the electron edge of the
LLBL. We find that the equatorward boundary of the LLBL electron edge is
observed at 10:00–17:00 magnetic local time (MLT) and is located typically between
68° and 80° invariant latitude (ILAT). The location
of the electron edge shows a weak, but significant, dependence on some of the
external parameters (solar wind pressure, and IMF BZ-
component), in agreement with expectations from previous studies of the cusp
location. The latitudinal extent of the electron edge has been estimated
using new multi-spacecraft techniques. The Cluster tetrahedron crosses the
electron and ion boundaries of the LLBL/cusp with time delays of 1–40 min between spacecraft. We reconstruct the motion of the electron
boundary between observations by different spacecraft to improve the accuracy
of the estimation of the boundary layer size. In our study, the LLBL electron
edge is distinctly observed in 87% of mid-altitude LLBL/cusp crossings with
clear electron and ion equatorward boundaries equivalent to 35% of all
LLBL/cusp crossings by Cluster. The size of this region varied between
0°–2° ILAT with a median value of 0.2°
ILAT. Generally, the size of the LLBL electron edge depends on the
combination of many parameters. However, we find an anti-correlation between
the size of this region and the strength of the IMF, the absolute values of
the IMF BY- and BZ-components and the solar
wind dynamic pressure, as is expected from a simple reconnection model for
the origin of this region.
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