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.