On the Structure of the Dayside Low-latitude Boundary Layer

T. M. Bauer, G. Paschmann, N. Sckopke, W. Baumjohann, and R. A. Treumann

Analyzing 33 AMPTE/IRM crossings of the dayside (08:00-16:00 LT) magnetopause, we study the structure of the low-latitude boundary layer. For 19 of the 33 crossings the transition from the magnetosheath to the magnetosphere is step-like and we can distinguish two sublayers: the outer (OBL) and inner boundary layer (IBL). The OBL is dominated by solar wind plasma whose density is somewhat lower and whose temperature is somewhat higher than in the magnetosheath. For 11 of the 33 crossings the change of the flow velocity from the magnetosheath to the OBL is in reasonable agreement with the prediction from tangential stress balance across a rotational discontinuity, which indicates that at least for these crossings the OBL is on open field lines. Further evidence for open field lines is provided by the occasional occurrence of ``D-shaped'' distributions of the solar wind population and heat flux due to escaping ring current particles. In the IBL the number densities of solar wind particles and magnetospheric particles are comparable. Like in the magnetosphere proper, the magnetospheric particles consist of a hot ( > 500 eV) ring current population and a cold ( < 50 eV) ionospheric population. The solar wind electrons in the IBL show a strong field-aligned temperature anisotropy. We find arguments for the IBL being on closed field lines. These are the change of the east-west component of the flow velocity from tailward in the OBL to stagnant in the IBL and the sharp increase of the density of ring current electrons.

The figure shows the phase space density of ions in the energy range 20 eV-4 keV measured on 84/09/21 in the magnetosphere proper, the IBL, the OBL, and the magnetosheath (from left to right). The distribution functions are shown in a two-dimensional cut through velocity space that contains the magnetic field direction, B (upward), and n×B (to the left), where n is the magnetopause normal. Moreover, the projections of the directions of the proton bulk flow, Vp, and the convection electric field, Ec = -Vp×B, are given. Black or white stars in the ion distributions give the projection of the de Hoffmann-Teller velocity, VHT, onto the cut. In the magnetosheath the solar wind population incident on the magnetopause is seen. The distribution of solar wind ions in the OBL exhibits the ``D shape'' with a velocity cutoff at VHT expected for ions transmitted along open field lines. In the IBL the density of solar wind ions is distinctly lower than in the OBL and their bulk flow is stagnant. There are no solar wind ions in the magnetosphere proper.

Bauer, T. M.: Particles and fields at the dayside low-latitude magnetopause. Dissertation, Ludwig-Maximilians-Universität München, 1997.

(Figure also available in postscript format ).

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