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 polar-originating ionospheric current system including equatorial enhancement. The model can be obtained by modifying the conventional, thin shell conductivity model. It is shown that the modification for one of the non-diagonal terms (Σθφ) in the conductivity tensor near the equatorial region is very important; the term influences the profile of the ionospheric electric field around the equator drastically. The proposed model can reproduce well the results representing the observed electric and magnetic field signatures of geomagnetic sudden commencement. The new model is applied to two factors concerning polar-originating ionospheric current systems. First, the latitudinal profile of the DP2 amplitude in the daytime is examined, changing the canceling rate for the dawn-to-dusk electric field by the region 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-aligned current to that of region 1 exceeds 0.5. Second, the north-south asymmetry of the magnetic fields in the summer solstice condition of the ionospheric conductivity is examined by calculating the global ionospheric current system covering both hemispheres simultaneously. It is shown that the positive relationship between the magnitudes of high latitude magnetic fields and 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 generator. The new model, based on the International Reference Ionosphere (IRI) model, can be applied to further investigations in the quantitative analysis of the magnetosphere-ionosphere coupling problems.Key words. Ionosphere (electric fields and currents; equatorial ionosphere; ionosphere-magnetosphere interactions

DYNAMIC STRUCTURE OF THE DAYSIDE MAGNETOPAUSE DURING THE SI EVENT ON FEBRUARY 9, 1986

Dynamic structure of the dayside magnetopause is studied with the Si event observed on February 9, 1986. It is clarified that three successive magnetopause crossing occurred with the interval of about 150 second in association with the Si event on February 9, 1986.This special issue contains contributed papers presented at the symposium "Solar Terrestrial Events in February-March 1986" (January 19, 1989, Nagoya

ELECTRIC FIELD VARIATION OF THE IONOSPHERE AFTER THE SI ON FEBRUARY 9, 1986

The relationship is described between geomagnetic variations and HF Doppler frequencies for the oscillative variation following the Si event at 1748 UT on February 9, 1986. After comparing time differences between geomagnetic variations and HF Doppler frequencies, it is suggested that the electric fields variations deduced from the HF Doppler frequencies data for this event are due to compressional mode HM waves excited by the dynamic response of the magnetosphere to the solar wind pressure variations.This special issue contains contributed papers presented at the symposium "Solar Terrestrial Events in February-March 1986" (January 19, 1989, Nagoya