Discussion
High Latitude and Low Latitude Reaction to the IMF Bz Variation
In the simulation of the September 24 05-06 UT case, the MAGE model simulated the penetrating electric field effect on the equatorial vertical ion drift. In the high latitudes, the MAGE magnetospheric input from GAMERA provides fast response to the varying IMF Bz conditions in convection pattern and CPCP. We noticed that the two-cell convection pattern does linger a bit as the IMF Bz turned northward at 0515 UT in Figure 4a, but its magnitude (CPCP) decreases compared with the southward IMF Bz case at the early UT. We note that in the daytime an extra convection cell already occurred in response to the northward turning of the IMF Bz. This is most likely related to the delay in the response of nightside convection to sudden solar wind and IMF condition changes [e.g., Lu et al., 2002]. While the high latitude nightside ion convection may have a longer memory, the equatorial electric field responds rapidly to high latitude convection changes. At 0515 UT, the strong downward ion drifts are gone on the nightside as well as the strong dawn-dusk potential near equator as shown Figure 6a. The dayside equatorial ion drifts also respond quickly at 0515 UT as shown in Figure 6b. Usually, one expects that the ion drifts at high latitudes react faster to changes in the solar wind and the magnetosphere. The simulation suggests that penetrating electric field acts on the equatorial region nearly instantaneously as shown in Figures 4b, 6a, and 6b. It appears that the ionospheric response to a S-IMF turning is a bit faster than a N-IMF turning at high latitudes depending on local time.
Penetrating Electric Field from the High Latitudes
The MAGE also shows that during S-IMF cases, the dawnside high latitude convection cell is connected to the low latitude high potential point. During N-IMF, the convection cells are closed off from the low latitudes (Figure 4b). In the case of S-IMF more high latitude magnetic fields lines are open and linked to outside of the magnetosphere and the solar wind dynamo can act on larger latitudinal range and apply high dawn-dusk potential to the low latitudes. During the N-IMF case, the polar cap is smaller and open field lines are far away from the low latitude region.
Dayside/Nightside Differences
Another point we should mention is the day/night difference in the equatorial ion drift reaction to the penetrating electric field. The nighttime vertical drift changes are twice as large as the daytime changes in the opposite direction. That is likely due to the daytime E-region conductance influence. Further study is needed.
Link Penetrating Electric Field to the IEF
Table 1 shows the links between the IEF, CPCP, and the equatorial dawn-dusk potential drop according to MAGE. When the IEF is positive, the equatorial dawn-dusk potential drop appears to be 14% of CPCP as shown in Figure 5. The IEF has larger relative increase than the CPCP and equatorial dawn-dusk potential in the MAGE model. That is understandable as the CPCP saturates near 100 kV when the IEF is above 3 mV/m [e.g., Shepherd et al., 2002]. MAGE model simulation appears to be consistent with that.
MAGE and ICON IVM Observation Comparison
While we wish to see that ICON were able to observe the penetrating electric field effect directly, we could not find ICON data during the early afternoon hours when the IVM usually provides high quality data. We do notice that the ICON upward ion drifts in the morning hours were stronger than the MAGE simulation results. That suggests that the real equatorial dawn-dusk potential may be larger than that simulated by MAGE.
PRE Discrepancy
One noticeable difference between the ICON IVM observation and MAGE simulation is the presence of the PRE in MAGE and absence in ICON IVM data. Right now, we do not have a definitive explanation for the discrepancy. The PRE in the MAGE simulations all occurred during the S-IMF cases. MAGE did not show any PRE during the N-IMF cases as the N-IMF condition induces downward vertical ion drift near the dusk. PRE is upward ion drift; hence N-IMF suppresses it.