Empirical high‐latitude electric field models

Abstract

Electric field measurements from the Dynamics Explorer 2 satellite have been analyzed to extend the empirical models previously developed from dawn‐dusk OGO 6 measurements (J. P. Heppner, 1977). The analysis embraces large quantities of data from polar crossings entering and exiting the high latitudes in all magnetic local time zones. Paralleling the previous analysis, the modeling is based on the distinctly different polar cap and dayside convective patterns that occur as a function of the sign of the Y component of the interplanetary magnetic field. The objective, which is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns, is met by deriving one pattern (model BC) for the northern hemisphere with a +Y interplanetary magnetic field (IMF) and southern hemisphere with a −Y IMF and two patterns (models A and DE) for the northern hemisphere with a − Y IMF and southern hemisphere with a +Y IMF. The most significant large‐scale revisions of the OGO 6 models are (1) on the dayside where the latitudinal overlap of morning and evening convection cells reverses with the sign of the IMF Y component, (2) on the nightside where a westward flow region poleward from the Harang discontinuity appears under model BC conditions, and (3) magnetic local time shifts in the positions of the convection cell foci. The modeling above was followed by a detailed examination of cases where the IMF Z component was clearly positive (northward). Neglecting the seasonally dependent cases where irregularities obscure pattern recognition, the observations range from reasonable agreement with the new BC and DE models, to cases where different characteristics appeared primarily at dayside high latitudes. The analysis of these differences for strong +Bz conditions, using passes through different MLT zones, led to finding that the deviations from both the BC and DE models could be modeled by extending the evening cell foci toward, and beyond, noon with a deformation that roughly resembles a rotational twist of the extended evening cell. The morning cell accommodation to the model BC deformation produces a narrow tongue of eastward convection near 75° invariant latitude on the dayside. The model DE deformation compresses the morning cell toward the night hours such that the sunward convection in the morning cell is diverted to polar latitudes in the 0400 to 0700 MLT sector. In both cases, the deformations of the two‐cell patterns lead to sunward convection in dayside polar regions while maintaining the integrity of the nightside convection pattern. Thus the nightside dilemmas that plague three‐ and four‐cell models designed to explain sunward convection in polar regions under +Bz conditions do not appear.