DISPERSED ION STRUCTURES AT THE POLEWARD EDGE OF THE AURORAL OVAL - LOW-ALTITUDE OBSERVATIONS AND NUMERICAL MODELING

被引:54
作者
BOSQUED, JM
ASHOURABDALLA, M
ELALAOUI, M
PEROOMIAN, V
ZELENYI, LM
ESCOUBET, CP
机构
[1] CNRS, RECH PLASMA GRP E, F-31055 TOULOUSE, FRANCE
[2] UNIV CALIF LOS ANGELES, INST GEOPHYS & PLANETARY PHYS, LOS ANGELES, CA 90024 USA
关键词
D O I
10.1029/93JA01143
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
We have compared the AUREOL 3 (A3) observations of auroral ion precipitation, particularly ion beams, with the results from the global kinetic model of magnetotail plasma of Ashour-Abdalla et al. (1993). We have identified 101 energetic 2-20 keV H + velocity dispersed precipitating ion structures (VDIS) with fluxes above 10(-3) ergs. cm-2. s-1 in the A3 record between the end of 1981 and mid-1984. These beams display a systematic increase in energy with increasing latitude and were observed in a narrow region within less than 1 deg in latitude of the polar cap boundary. The VDIS are the most distinctive feature in the auroral zone of the plasma sheet boundary layer. We report first on a statistical analysis of the possible relationships between magnetic activity or substorm phase and the VDIS properties. The VDIS are found on 15-18% of the A3 orbits. In general their frequency of occurrence is not correlated with activity. However in the 2200-0200 MLT sector, the probability of observing more energetic VDIS increases for larger values of the AE index. Our particle simulations of the precipitating ions have been extended by using a series of modified versions of the Tsyganenko (1989) magnetic field model and by varying the cross-magnetosphere electric field. In the simulations, plasma from a mantle source is subject to strong nonlinear acceleration, forming beams which flow along the PSBL. Only 3 to 4% of these beams precipitate into the ionosphere to form the VDIS while the majority return to the equatorial plane after mirroring and form the thermalized central plasma sheet. The final energy and the dispersion of the beams in the model depend on the amplitude of the cross-tail electric field. Two unusual observations of low-energy (< 5 keV) 0 + VDIS, shifted by 4-degrees-5-degrees in invariant latitude equatorward of H + VDIS are analyzed in detail. The sparsity of such 0 + events and the absence of the changes in the flux and frequency of occurrence indicate a solar wind origin for the plasma. Finally, large-scale kinetic modeling, even with its simplifications and assumptions (e.g., static magnetic field, solar wind source), reproduces low-altitude auroral ion features fairly well; it may therefore be presented as an appropriate framework into which data on energization and transport of the hot plasma, obtained in the equatorial plane, could be inserted in the near future.
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页码:19181 / 19204
页数:24
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