Numerical analysis of global ionospheric current system including the effect of equatorial enhancement

A modeling method is proposed to derive a two-dimensional ionospheric layer conductivity, which is appropriate to obtain a realistic solution of the p olar-originating ionospheric current system including equatorial enhancemen t. The model can be obtained by modifying the conventional, thin shell cond uctivity model. It is shown that the modification for one of the non-diagon al terms (Sigma(theta phi)) in the conductivity tensor near the equatorial region is very important; the term influences the profile of the ionospheri c electric field around the equator drastically. The proposed model can rep roduce well the results representing the observed electric and magnetic fie ld signatures of geomagnetic sudden commencement. The new model is applied to two factors concerning polar-originating ionospheric current systems. Fi rst, the latitudinal profile of the DP2 amplitude in the daytime is examine d, changing the canceling rate for the dawn-to-dusk electric field by the r egion 2 field-aligned current. It is shown that the equatorial enhancement would not appear when the ratio of the total amount of the region 2 field-a ligned current to that of region exceeds 0.5. Second, the north-south asymm etry of the magnetic fields in the summer solstice condition of the ionosph eric conductivity is examined by calculating the global ionospheric current system covering both hemispheres simultaneously. It is shown that the posi tive relationship between the magnitudes of high latitude magnetic fields a nd the conductivity is clearly seen if a voltage generator is given as the source, while the relationship is vague or even reversed for a current gene rator. The new model, based on the International Reference Ionosphere (IRI) model, can be applied to further investigations in the quantitative analys is of the magnetosphere-ionosphere coupling problems.