MAGE Simulation for the Sep 24 05-06 UT Interval
We plot in Figure 3 the MAGE simulation results every 10 minutes over
the 1-hour period. Six time intervals are highlighted by the cyan color.
The IMF conditions are given in the top panel of Figure 3. The first
interval is for the southward IMF condition (S-IMF) and followed by two
northward IMF (N-IMF) intervals. Then a transition from northward to
southward IMF (0-IMF) occurred and was followed by two southward IMF
(S-IMF) intervals. The above changes in IMF Bz orientation provide an
opportunity to examine how the IMF Bz component impacts the
magnetosphere-ionosphere system from high to low latitudes. The IEF
dawn-dusk component is also a good indicator for the ionospheric
dawn-dusk potential, which determines the high latitude convection and
penetrating electric field. IEF switched from positive to negative, then
positive again in response to the changing sign of the IMF Bz.
The potential maps for the six IMF Bz conditions are shown in Figure 4a.
All the variables from MAGE used in figures of this paper are selected
from the pressure level 5.625 roughly at 405 km (the 50th vertical
grid). The noon is on top of the plot and midnight at the bottom. The
first image of an S-IMF case has a typical two-cell ion convection
pattern with a CPCP of 84.7 kV at 0504 UT. In the second case (N-IMF at
0515 UT), the two-cell convection pattern remains but with a smaller
CPCP of 51.2 kV. Even though the IMF turns northward, the anti-sunward
convection within the polar cap remains probably due to the delay in the
response of the nightside convection to changes in solar wind IMF
conditions [e.g., Lu et al., 2002]. Only at 0525UT the second N-IMF
case, a four-cell convection pattern appears with CPCP further reduced
to 49.3 kV. The four-cell pattern is consistent with the N-IMF
condition. The N-IMF case is followed by near zero IMF-Bz (0-IMF) at
0534UT when the four-cell pattern remains and the CPCP drops further to
37.4 kV. As soon as the IMF turns southward (S-IMF) at 0545 UT the
two-cell convection pattern returns and CPCP increases to 105 kV. As the
IMF remains southward (S-IMF) at 0555 UT, the two-cell pattern persists
and the CPCP raises to 125 kV.
To illustrate how the penetrating electric field acts onto the low
latitudes, we plot the same potential map for the entire northern
hemisphere from the pole to the equator in Figure 4b. In the first case,
the low latitude high potential is connected to the high latitude
convection cell on the dawnside, which produces a potential drop from a
morning peak and a nightside valley at the equator (7.5 kV). The yellow
arrows indicate the electric fields on the dayside (eastward) and
nightside (westward). In the second case at 0515 UT, the equatorial
potential peak is on the dayside and the potential valley is on the
nightside towards dawn with a potential drop ~ 6 kV. It
is noteworthy that there is no large dawn-dusk potential drop on either
the dayside or nightside. The electric fields direct from noon to dawn
and dusk and is very weak. The high latitude dawnside convection cell
potential does not penetrate to the low latitudes. The same pattern
persists at 0525 and 0534 UT as well. As IMF Bz turns southward (S-IMF),
an equatorial potential peak occurs on the dawnside and a strong
dawn-dusk potential drop is applied to the equatorial region
(~9 kV) on both the dayside and nightside. To correlate
the interplanetary electric field to the equatorial potential drop we
list the values of these parameters in Table 1. We also plot these
values in Figure 5. Because of the apparent correlation between the CPCP
and the equatorial potential drop, we performed a linear fit between the
CPCP and the equatorial potential drop shown on the rightside of Figure
5. The equatorial potential drop is about 14% of the CPCP.