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

Abstract

Abstract. 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 modi®cation for one of the non-diagonal terms (S hu ) in the conductivity tensor near the equatorial region is very important; the term in¯uences the pro®le of the ionospheric electric ®eld around the equator drastically. The proposed model can reproduce well the results representing the observed electric and magnetic ®eld signatures of geomagnetic sudden commencement. The new model is applied to two factors concerning polar-originating ionospheric current systems. First, the latitudinal pro®le of the DP2 amplitude in the daytime is examined, changing the canceling rate for the dawnto-dusk electric ®eld by the region 2 ®eld-aligned current. It is shown that the equatorial enhancement would not appear when the ratio of the total amount of the region 2 ®eld-aligned current to that of region 1 exceeds 0.5. Second, the north-south asymmetry of the magnetic ®elds 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 ®elds 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.

Climatology of F region zonal plasma drifts over Jicamarca

“…The prompt penetration drifts were obtained by first dividing the equatorward (horizontal) model electric field given in Figure 6 of Fejer et al [1990] by the sine of the dip angle and then calculated considering that in the Peruvian equatorial region a downward electric field of 1mV/m corresponds to an F region eastward drift velocity of 40 m/s. Other convection models [e.g., Peymirat et al, 2000;Tsunomura, 1999] and more recent RCM simulations also give very similar results. Figure 9 shows westward daytime prompt penetration drifts with largest values near noon and eastward drifts at night with very small magnitudes, except near dawn.…”

Daytime equatorial geomagnetic H field response to the growth phase and expansion phase onset of isolated substorms: Case studies and their implications

“…If the IMF direction-related change in the convection electric field is sudden and prominent, then the convection electric field can penetrate all the way to the dip equator through the polar ionosphere and manifest until such a time when the ring current shielding reasserts itself. Theoretical and numerical simulations studies consistently show that a sudden (idealized) increase of magnetospheric convection (represented by the polar cap potential drop and region 1 field-aligned currents (FACs)) leads to a short-lived disturbance in the equatorial zonal electric field with a polarity in phase with the quiet time pattern, namely, eastward on the dayside and westward on the nightside [e.g., Btanc, 1983b; Spiro et at., 1988; Denisenko and Zamay, 1992; Tsunomura, 1999]. Kikuchi et at.…”

“…It follows conceptually that a sudden reduction in a large-scale electric field will be accompanied by a similar reduction in the polar cap potential drop and region 1 FAC and a westward ionospheric electric field disturbance (decrease in the dayside equatorial electrojet current) close to the dip equator, before shielding by ring current gains control. This over-shielding effect is just the opposite of what happens with a sudden increase of the polar cap potential drop and has been extensively studied [e.g., Blanc, 1983b;Spiro et al, 1988;Denisenko and Zamay, 1992;Tsunomura, 1999;Peymirat et al, 2000]. We therefore interpret the negative baylike perturbation in H field with marked dip equator enhancement noticed precisely with the onset of expansion phase of the substorms on August 19 and February 12 as the signatures of penetration electric fields associated with the expansion phase reduction in convection electric field.…”

“…Sudden and prominent southward turnings of Bz are also observed to lead to electric field disturbances of opposite polarity, i.e., in phase with the quiet time diurnal pattern, but their occurrence is noticed to be not as frequent as with northward wings in Bz [Fejer, 1986]. Semianalytical and numerical simulation studies helped to understand the short-lived electric field disturbances at low and equatorial latitudes in terms of the penetration of magnetospheric or high-latitude electric fields due to under-shielding and over-shielding effects [e.g., Kamide and Matsushita, 1981;Blanc, 1983b;Spiro et al, 1988;Denisenko and Zamay, 1992;Tsunomura, 1999;Peymirat et al, 2000].…”

Longitudinal structure of Pc3 pulsations on the ground near the magnetic equator

“…[43] There are some numerical simulations of the polaroriginating two-dimensional ionospheric current system [Tsunomura and Araki, 1984;Alperovich et al, 1996;Tsunomura, 1999]. In order to interpret PRI of SC in terms of the ionospheric current, Tsunomura and Araki [1984] assumed field-aligned currents (FACs) at high latitude as the source of the ionospheric current and solved the equation of the current continuity in the thin-shell ionosphere with a realistic conductivity.…”