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.