Penetration of auroral electric fields to the equator during a substorm

We have studied the negative magnetic bay associated with the substorm that occurred on April 20, 1993, and have found that it is markedly enhanced at the daytime dip equator, coherent with that at afternoon subauroral latitu des. The amplitude of the negative bay decreases monotonously with the lati tude, but it is amplified at the dip equator by a factor of 2.5 compared to the low-latitude negative bay. This latitudinal profile implies that in ad dition to the three-dimensional current system in the magnetosphere, DP ion ospheric currents originating in the polar ionosphere contribute greatly to negative bays. Penetration of the convection electric field and the effect of a shielding electric field due to Region 2 (R2) field-aligned currents (FACs) are examined on the basis of European Incoherent Scatter (EISCAT) an d International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetomete r observations made in the afternoon sector. The northward electric field a t EISCAT (66 degrees corrected geomagnetic latitude (CGMLAT)) is well corre lated with the magnetic field X component at Nurmijarvi (56 degrees CGMLAT) during the presubstorm period, but the coherency breaks down during the su bstorm cycle. By assuming that the R2 FACs intensify the northward electric field at EISCAT but reduce it at Nurmijarvi, we demonstrate that the R2 FA Cs grow concurrently, although delay by some 17 min, with the convection el ectric field. Our analytical results indicate that the convection electric field decreases abruptly during the substorm and that the shielding electri c field overcomes the convection electric field at around the peak of the n egative bay, owing to its delayed reaction. The equatorial negative bay is thus due to an overshielding effect caused by the electric field associated with the R2 FACs.