Penetration of high-latitude electric fields into low latitudes

Abstract

Simulation studies of ionospheric electric fields with special emphasis placed on the electrical coupling between high and low latitudes are presented by means of the algorithm developed by Kamide and Matsushita ( J. geophys. Res. 84 p. 4083, 1979) to derive the horizontal electric fields in the global ionosphere generated by field-aligned currents in auroral latitudes. The ionospheric electric potential is obtained from a numerical solution of the steady-state current continuity equation assuming anisotropic conductivities as well as upward and downward currents along auroral field lines. These assumptions are based on our current knowledge of auroral phenomena and geomagnetic variations as well as rocket and satellite measurements of field-aligned currents and radar observations of the ionospheric conductivity distribution. We demonstrate the importance of the basic assumptions leading to the main features observed during both quiet and disturbed times. In the simplest model when the conductivity is taken as constant, the values of the electric potential decay very slowly with an increase of the colatitude in middle and low latitudes. On the other hand, when the conductivity gradient is realistically included in terms of both day-night asymmetry of the quiet-time conductivity and slight auroral enhancements in the night side auroral belt for weakly disturbed times, the decay of the electric field toward low latitudes occurs rapidly. Furthermore, the inclusion of the field-aligned currents in the equatorward half of the auroral belt results in extensive reduction of the electric field in low latitudes, and produces a shielding effect against the penetration of the high-latitude electric field into low latitudes. It is also found that there is a notable asymmetry in the decay rate of the electric field toward low latitudes between the morning and evening sectors. In both quiet and disturbed cases, the electric field in the evening sector decays more rapidly with latitudes than that in the morning sector, a result from the difference in the sign of the longitudinal gradient of the ionospheric conductivity. How the electric field of the high latitude origin can (or cannot) penetrate deep into low latitudes is discussed in light of the magnetospheric convection near the so-called Alfvén layer.