Tests of a boundary layer model of field‐aligned currents using S3–3 data

Abstract

Data obtained from the low altitude, polar orbiting S3–3 satellite are utilized to test the model of field‐aligned currents and electric fields within the auroral zone developed by Siscoe and Maynard [1991]. This model, for which the cross polar cap potential drop is the input parameter, combines the boundary layer‐driven field‐aligned current model of Siscoe et al. [1991] for Region 1 with a model for Region 2 currents based on Vasyliunas [1972]. Many general features of the observations are reproduced by the model. The main discrepancies may be explained by two simplifications which were made in the model: (1) the time independence of the boundary layer, in particular, of the scale length of magnetosphere‐ionosphere coupling, l and (2) the uniform ionospheric conductivity. When the expected time dependence of l is taken into account, the disagreement between the model gradients, latitudinal extents, and maxima in the fields and those observed in the S3–3 data in the poleward (Region 1) auroral zone is readily accounted for in the context of a refined boundary layer model. The fact that the observed Region 2 currents are usually larger and narrower than the predicted currents may be due to the underestimate of the Region 1 current explained above and the fact that the ionospheric conductivity was constant. These results suggest that the two most important modifications needed to improve the model are the inclusion of local‐time dependence in the magnetosphere‐ionosphere coupling parameter (as was done by Phan et al. [1989]) and of latitude and local time dependence in the ionospheric conductivity. There is also some evidence that the low latitude boundary layer (LBL) parameters have cross polar cap potential dependencies which must be included in a model of this type. The S3–3 field data used the convection reversal as a marker for the polar cap boundary, whereas the model used the poleward edge of the low‐latitude boundary layer. Comparison of the energetic particle signatures with the electric field data indicated that this was a small effect on the determination of the cross polar cap potential drop. For the events shown herein, the potential drop across the low‐latitude boundary layer was usually less than a few kV due to the existence of sunward convection in the equatorward portion of the layer.