Experimental study of magnetospheric convection

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We have used ISEE-1 measurements of the plasmapause position in stationary situations to determine the convection electric field. The results are very coherent and lead to a relationship between the electric field strength and the K m index for two types of models. The resulting field varies from nearly zero in quiet situations to 0.6 mV/m for K m = 6, but does not depend much on the model in the region where the plasmapause is found. The interpretation of the plasmapause, defined as the frontier between a region of dense cold plasma close to the Earth and a region of dilute plasma, as being the limit between closed and open drift orbits for zero energy particles is widely admitted [1] [2] [3] [4], even though other interpretations have been given [5]. Several authors have used measurements of the plasma density in the equatorial region to determine the plasmapause and deduce an estimation of the convection electric field [6] [7]. Several two-dimensional semi-empirical models have been proposed to interpret the measurements: uniform convection electric field [1] [2] [7] [8], models taking into account the screening effect of the hot particles [9] [10], more sophisticated ones using expansion in a large serie of terms [11] [12]. All these models could account for the measurements, with a dependence of the electric field on the magnetic agitation index Kp. In this work, we have used the electron density measurements provided by the relaxation sounder flown on-board the ISEE-1 satellite [13] to determine an empirical relationship between the electric field and the magnetic agitation, for two types of models: a uniform convection electric field [8], a model taking into account the modification of this field by the hot particles [9] [10]. For both of these models the electric field derives from a potential of the form [14]: ø γ=A γR γsin ψ where A is a coefficient related to the intensity of the electric field, R the geocentric distance and ψ the geomagnetic longitude. Furthermore γ = 1 holds for to J.L. Pieplu, M. Renard and J.P. Thouvenin who took care of preprocessing of the data at CNES. This work was performed under CNES contract 214.