The nightside equatorial vertical ion drift response to the IMF Bz variations is shown in Figure 6a. In the background, the ionospheric F-region peak electron density (NmF2) is plotted. In the first S-IMF case at 0504 UT, the magnetic equatorial ion drift at night is large and downward. Towards the left (dusk), the model successfully simulates the Pre-Reversal Enhancement (PRE). The nighttime downward ion drift is consistent with the dawn-dusk potential (westward electric field) shown in Figure 4b. As soon as the IMF turns northward (N-IMF) at 0515 UT, the ion drift reduces and is close to zero (slightly upward), even though the two-cell convection still exists at high latitudes (Figure 4a). At 0525 UT during another N-IMF case, the ion drift is still close to zero but with an upward speed larger than that in the prior N-IMF case. In the next 0-IMF case at 0534 UT, the ion drift stays close to zero and with a smaller upward speed than the case earlier. As the southward IMF returns (S-IMF) at 0545 UT, the ion drift turns downward again. The ion drift stays downward during the same S-IMF case at 0555 UT in accordance with the strong westward electric field shown in Figure 4b.

The daytime variations of equatorial vertical ion drift are plotted in Figure 6b. Opposite to that in the nighttime, the daytime ion drift is mostly upward. In the first case of S-IMF at 0504 UT, there are relatively large upward ion drifts corresponding to a strong westward electric field (Figure 4b). As the IMF turns northward, the upward ion drift reduces by about 50% in the afternoon. In the morning, the ion drift reverses to downward. That is consistent with the dayside electric field displayed in Figure 4b, where the N-IMF case has dayside electric field westward in the morning and eastward in the afternoon. The same kind of configuration maintains until 0534 UT for both N-IMF and 0-IMF conditions. As the IMF turns southward (S-IMF) at 0545 UT, strong upward ion drifts appear at all dayside local times. The same configuration exists at 0555 UT (S-IMF) as well reflecting the strong eastward electric field (Figure 4b).

To make sure that we are seeing penetrating electric field, we need to examine the thermospheric zonal winds at the magnetic equator (Figure 7). If the ion drift variations shown in Figures 6a and 6b were not due to the penetrating electric field, they might be caused by thermospheric wind variations. Figure 7 shows almost unchanged neutral winds during the 09 to 10 UT time interval. That rules out the possibility of neutral wind dynamo causing the vertical ion drift variations in Figures 6a and 6b.