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Ann. Geophys., 20, 329-340, 2002
www.ann-geophys.net/20/329/2002/
© European Geosciences Union 2002
Statistical study of transient plasma structures in magnetotail lobes and plasma sheet boundary layer: Interball-1 observations
E. E. Grigorenko1,2, A. Fedorov3,*, and L. M. Zelenyi2
1Skobeltzyn Institute of Nuclear Physics of Moscow State University, Vorobjevy Gory, 117234, Moscow, Russia
2Space Research Institute, 84/32, Profsojuznaja, 117810, Moscow, Russia
3CESR - CNRS, 9, av. de Colonel-Roche, 31400, Toulouse, France
*on leave from Space Research Institute, Moscow
Correspondence to: E. E. Grigorenkoe, 31; (grig@afed.iki.rssi.ru)
Abstract. The present paper is
devoted to the statistical study of plasma structures observed at the interface
between magnetospheric lobes and the plasma sheet at distances of 15–25 RE .
The majority of the registered structures are bursty earthward ion streams with
energies about 10–30 keV known as "beamlets". The paper describes
the results of beamlet statistical analysis for different interplanetary
magnetic fields (IMF), clock-angles and various magnetospheric conditions. It
is shown that the energy of beamlets increases monotonously with a distance
from the neutral sheet (NS) during slightly southward IMF and quiet
magnetospheric conditions. Under these conditions, beamlets are observed up to
5–6 RE over the NS. In contrast, the beamlet energy decreases with increasing
distance from the NS if the IMF is northward, and the region of beamlet
observation becomes much wider up to 12 RE above the NS. The same (inverse)
beamlet energy dependence on the distance from the NS is also registered during
active magnetospheric periods. Yet, the plasma structures of various duration
with quasi-isotropic ion velocity distributions resembling the plasma of the
plasma sheet (PS) were observed in the same region. Statistical study performed
for various IMF clock-angles demonstrates the clear difference in the spatial
distributions of such plasma structures for two extreme IMF directions. When
IMF is primarily northward, PS-like plasma fills the lobe region almost
entirely and could be associated with the PS expansions. For primarily
southward IMF time intervals, the probability of observing PS-like structures
of long duration sharply decreases with the increasing distance from the NS,
but PS-like structures of short durations (< 500 s) are distributed almost
uniformly up to 12 RE above the NS. This feature cannot be described in terms
of the PS flapping motions and requires another explanation. Relevance and
limitations of the existing models attempting to explain the generation
mechanisms of these transient structures are briefly discussed in the
concluding part of the paper.
Key words. Magnetospheric physics (magnetotail;
magnetotail boundary layers; plasma sheet)
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