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\begin{document}

\title{Penetrating Electric Field Simulated by the MAGE and Observed by ICON}



\author[1]{Qian Wu}%
\author[2]{Wenbin Wang}%
\author[3]{Dong Lin}%
\author[4]{Chaosong Huang}%
\author[5]{Yongliang Zhang}%
\affil[1]{National Center for Atmospheric Research (UCAR)}%
\affil[2]{HAO/NCAR}%
\affil[3]{National Center for Atmospheric Research}%
\affil[4]{Air Force Research Laboratory}%
\affil[5]{The Johns Hopkins University Applied Physics Laboratory}%


\vspace{-1em}



  
  \date{June 16, 2024}


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\selectlanguage{english}
\begin{abstract}
Using the newly developed, Multiscale Atmosphere-Geospace Environment
(MAGE) model, we simulated the penetrating electric field in the
equatorial region under different interplanetary magnetic field (IMF) Bz
conditions during September 2020. Two intervals were selected for
detailed analysis with the vertical ion drift data from the NASA ICON.
The MAGE simulations show that in southward IMF (S-IMF) cases, the
dawn-dusk electric potential drop at the equator is about 14\% of the
cross polar cap potential difference. The dawn-dusk potential drop at
the equator varies instantaneously with the changes in the IMF Bz or
interplanetary electric field, which in turn alters the vertical ion
drift. The daytime changes of the equatorial vertical ion drift in
response to the penetrating electric field related to the IMF Bz are
only half of that during the nighttime, due mostly to the E-region
dynamo. MAGE simulation shows pre-reversal enhancement (PRE) during
southward IMF cases, but the PRE was absent in the ICON IVM
observations. Further observations and modeling are needed to resolve
this discrepancy.%
\end{abstract}%



\sloppy


\textbf{Introduction}

Penetrating electric field is a high latitude dawn-dusk field applied to
the mid and low latitudes at a much short time scale than
thermosphere-ionosphere interaction process such as ion neutral
collision. Penetration electric fields have been intensively studied in
the past {[}e.g., Nishida, 1968; Kikuchi et al., 2008; W. Wang et al.,
2008; Huang, 2019; Huang et al., 2005; Maruyama et al., 2005; Kelley et
al., 2003; Fejer et al., 2007; Lu et al, 2012{]}.

While the basic underlying physics is known, many aspects of the
penetrating electric field (PEF) remain to be addressed. For example, it
is difficult to simulate the penetrating electric field, which changes
rapidly, using a first principle thermosphere and ionosphere model
driven by empirical high latitude convection models such as \emph{Heelis
et al.} {[}1982{]} and \emph{Weimer} {[}2005{]}. Lu et al. {[}2012{]}
used the AMIE (Assimilative Mapping of Ionospheric Electrodynamics)
{[}Richmond and Kamide, 1988{]} driven TIMEGCM
(Thermosphere-Ionosphere-Mesosphere-Electrodynamic General Circulation
Model) model to simulate penetrating electric field effects. While AMIE
uses real data as inputs, the derived convection pattern at high
latitudes still relies heavily on empirical data information. The case
examined by Lu et al. {[}2012{]} was a very strong storm event (November
9-10, 2004), which produced 120 m/s vertical ion drift at the equator.
High latitude model inputs with a high time resolution will enable the
examination of the penetrating electric field in great details and
improve the understanding of observed results, especially under weak to
moderate geomagnetic activity conditions. One of the outstanding
questions is how the penetration electric field is related to changes in
the IMF Bz and the cross polar cap potential (CPCP). Penetrating
electric field effect becomes complicated when the IMF Bz changes back
and forth between northward and southward.

Towards the goal of better understanding the penetrating electric field,
we use a coupled magnetosphere ionosphere model (MAGE, Multiscale
Atmosphere-Geospace Environment) to simulate the penetrating electric
field effect on the equatorial ionosphere {[}e.g., Lin et al., 2021;
Pham et al., 2022{]}, specifically on the vertical ion drift. The MAGE
model provides much higher and more dynamic high latitude inputs of
convection pattern and auroral precipitation by coupling with a
magnetospheric model. The objective is to show how the IMF Bz component
controls the equatorial vertical ion drift, through the CPCP and
penetrating electric field. In addition to the latest modeling tools, we
also will use the NASA ICON mission ion drift measurement to observe the
effect of the penetrating electric field from an equatorial orbiting
satellite platform. MAGE simulations are used to facilitate
interpretation of the observational results.

We selected two geomagnetically active intervals during September 2020.
The first interval is from September 24, 5-6 UT when the IMF Bz turned
northward first and then turned southward within one hour, which is
highlighted (blue shaded) in the IMF Bz and By subplot of Figure 1. The
figure also includes solar wind speed and density, and the
interplanetary electric field (IEF) dawn-dusk component. The IMF Bz
variations led to changes in the IEF, which affects the dawn-dusk
electric potential and penetrating electric field. The second interval
is from September 26, 9-10 UT with the interval highlighted in Figure 2.
In this case, IMF Bz turned southward within the highlighted interval.
Moreover, we have ICON observations of the equatorial ExB meridional ion
drift, which is upward at the magnetic dip equator, for comparison. The
paper is organized by (1) a brief description of the model setup, (2)
discussion of the simulation results for the two intervals, (3)
comparison between the model results and ICON ion drift observations,
and (4) additional discussion on the ICON ion drift data.

\textbf{Model Simulation Setup}

The MAGE model {[}Pham et al., 2022; Lin et al., 2021{]} consists of the
Grid Agnostic MHD with Extended Research Applications (GAMERA) model of
the magnetosphere {[}Zhang et al. 2019{]}, the Rice Convection Model
(RCM) of the ring current {[}Toffoletto et al. 2003{]}, the
Thermospheric Ionosphere Electrodynamics General Circulation Model
(TIEGCM) of the ionosphere-thermosphere {[}Richmond et al., 1992{]}, and
the RE-developed Magnetosphere-Ionosphere Coupler/Solver (REMIX)
{[}Merkin and Lyon, 2010{]} that links different components of MAGE. As
aforementioned, the standalone TIEGCM is traditionally driven by
empirical high latitude ion convection models {[}e.g., Heelis et al.
1982; Weimer 2005{]} which simply use the three-hourly Kp index or
selected solar wind parameters as inputs. These empirical models do not
capture fast temporal variations of high latitude convection to address
questions of the dynamic changes of global ionospheric electric fields,
such as the penetrating electric field which can induce nearly
instantaneous responses in the ionospheric thermospheric system in the
equatorial region. In this study the magnetospheric model of GAMERA
provides the dynamic features of electric potential at high latitudes
for the TIEGCM at a cadence of every 10 s. Moreover, the MAGE model can
resolve mesoscale convection structures such as subauroral polarization
streams (SAPS) that are absent in the empirical models {[}Lin et al.,
2021{]}. In addition, the MAGE also incorporates the RCM to simulate the
ring current effect that can have implication for the penetrating
electric field. It is also noted that the ring current may provide the
shielding effect to the penetrating electric field. The model setup is
shown in Figure 1 of Lin et al., {[}2021{]}. In this study, the MAGE
model was driven by solar wind and IMF data obtained from the CDAWeb
OMNI database with one-minute resolution. The simulated interval starts
from 19 September 2020. Specifically, we focus on the TIEGCM outputs for
the ionosphere-thermosphere, which have a spatial resolution of 1.25
degree latitudinally and longitudinally and 0.25 scale height
vertically. The vertical direction consists of 57 grids spanning from
the altitude of \textasciitilde{}97 km to \textasciitilde{}600 km. The
time step is 5 seconds. We save the results every 5 minutes.

\textbf{ICON Observation}

We also used the ion drift data from the NASA ICON mission {[}Immel et
al., 2018{]} for a comparison with the MAGE simulation results. ICON is
an equatorial mission for studying equatorial ionospheric connection to
the lower atmosphere. It carries ion velocity meter (IVM), which measure
ion drift, density, and composition {[}Heelis et al., 2017{]}. In this
study, we will only examine the IVM ExB meridional ion drift, which at
the magnetic dip equator is vertically upward pointing. IVM ExB ion
drifts are decomposed into magnetically zonal (horizonal, eastward is
positive) and meridional (within the magnetic meridian plain, upward is
positive). Hence, at the low latitudes, the meridional ion drift is a
good approximation of the vertical ion drift. IVM samples the ion drift
at 1 second cadence. The IVM data quality depends on the ion density. In
our selected cases, we only used the data from noon to evening magnetic
local time when the ion density is relatively high.

\textbf{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 \textasciitilde{} 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
(\textasciitilde{}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.

\textbf{Table 1. IEF, CPCP, and Equatorial Potential}\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
Time (UT) & 0504 & 0515 & 0525 & 0534 & 0545 & 0555\tabularnewline
\midrule
\endhead
IEF (mV/m) & 1.3 & -3.3 & -3.1 & -0.4 & 2.2 & 3.3\tabularnewline
CPCP (kV) & 84.7 & 51.2 & 49.3 & 37.4 & 105 & 125\tabularnewline
Equatorial Potential Drop (kV) & 7.5 & 1.5 & 1.5 & 0.0 & 9.0 &
12\tabularnewline
\bottomrule
\end{longtable}

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.

\textbf{MAGE Simulation and ICON IVM Observation for September 26, 09-10
UT Interval}

We selected this interval because of the availability of the ICON ion
drift data. Figure 8 shows the IMF and geomagnetic parameters in the
1-hour interval from 09 to 10 UT on September 26, 2021. We selected four
intervals for analysis, which are highlighted. The first two are N-IMF
cases followed by two S-IMF cases. Figure 9 shows the equatorial
vertical ion drift during the four cases. In addition, we also plot the
ICON ExB meridional drift in both magenta and cyan vectors from 09-10
UT. Note that ICON is a low-inclination satellite and takes
\textasciitilde{}50 minutes to fly over the dayside. In contrast, the
simulated ion drifts are taken at a specific UT. Hence, the simulation
match in the time only with a small portion of the plotted ICON data. To
highlight that portion of observational data, the magenta color
represents the drifts from ICON at the same time as the model map plot.
Just north of the ICON satellite track (at the bottom of the ICON drift
vector) is the simulated ExB meridional ion drift from the MAGE plotted
as a black line. The MAGE simulated ion drift is projected in the exact
same direction as the definition of the IVM ExB meridional ion drift for
an easy comparison. The vertical ion drift for the N-IMF and S-IMF cases
are like those in Figure 6 during the daytime. During the S-IMF, there
are strong upward ion drifts. Yet the ICON ion drifts show strong
downward trend. Those ICON IVM ion drifts appear to have a large offset.

To address the issue, we calculated 6-min MLT median values from the day
(orange line) and plot them with the raw 1 second data between 09 -- 10
UT (green line) in Figure 10. As we can see that both raw data and 6-min
MLT median values all show a downward drift at 18 MLT. The vertical ion
drift reverses at 18 MLT in typical conditions {[}e.g. Aol et al.,
2020{]}, when we should expect zero vertical ion drift. After consulting
with the IVM team from U. of Texas at Dallas, we applied a shift based
on the 6-minute MLT median value at 18 MLT in the ExB meridional ion
drift to bring the drift to zero at 18 MLT (blue line). We use the
one-day 6-minute MLT median data for the shift to avoid any
fast-oscillating effects of penetrating electric field on the
correction.

We plot the corrected ICON ion drift in Figure 11. Comparing the
simulated ExB ion drift along the ICON track with the corrected ICON
IVM, we noticed significant differences. First, at earlier UT and MLT
hours, the ICON IVM has stronger upward drift than the simulations at
0924 and 0935 UT. Unfortunately, when the S-IMF returns at 0945 UT, the
ICON satellite was located close to 18 MLT when the ion drift is near
zero. Hence, we could not see the strong upward ion drift measured by
ICON (at earlier MLT hours), even though the model show enhancement. At
0954 UT, the IMF was still southward (S-IMF), the ICON satellite passed
18 MLT and entered the region where downward ion drift is expected. ICON
IVM did indeed show downward (magenta vectors). However, the MAGE
simulation shows a sizeable PRE in the ion drift, which ICON IVM did not
see. Even the IVM data beyond 0954 UT in the cyan color did not show any
signature of the PRE. There was no sign of PRE in the N-IMF cases in the
simulation. The first sign of PRE appeared at 0945 UT during the first
S-IMF case. It is possible that penetrating electric field during the
N-IMF suppressed the PRE. We do notice that ICON did not see PRE during
the S-IMF case whereas the model predicts one. We are still searching
for the cause for the discrepancy on both the observation and simulation
side.

\textbf{Discussion}

\emph{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.

\emph{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.

\emph{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.

\emph{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.

\emph{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.

\emph{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.

\textbf{Summary}

Using the MAGE model, we were able to show how the penetrating electric
field varies with IMF Bz and is applied to the low latitudes. We
summarize our findings as follows: 1. MAGE shows during the S-IMF cases,
the dawn-dusk potential at the equator is about 1 tenth of the CPCP. 2.
The dawn-dusk potential at the equator is applied instantaneously as the
IMF Bz turns. 3. The daytime response of the equatorial vertical ion
drift is about half of that during the nighttime, due probably to the
E-region conductance influence. 4. MAGE simulation shows PRE during the
S-IMF case, and PRE is absent in the ICON IVM observation. Further
observations and modeling are needed to resolve this discrepancy.

\textbf{Acknowledgement}

The work by Q.W. is supported by NASA grants 80NSSC20K0199,
80NAAC21K0014, NNX17AG69G and NSF grants AGS-2120511 and AGS-2054356.
W.W. is supported by NASA grants 80NSSC17K0013, 80NSSC20K0601,
80NSSC20K0356, 80NSSC19K0080, 80NSSC17K0679, and 80NSSC20K0199. D.L. is
supported by NASA grants 80NSSC17K0013, 80NSSC20K0601, 80NSSC21K0008,
80NSSC21K1677. C.H. is supported by NASA grants 80HQTR20T0015,
80HQTR20T0016, and NNH22OB17A. Y.Z. is supported by NASA grants
80NSSC20K0354 and 80NSSC21K1673. NCAR is supported by NSF. The ICON is a
NASA mission, and the ion drift data can be obtained from
https://spdf.gsfc.nasa.gov/pub/data/icon/l2/l2-7\_ivm-a/2020/ The MAGE
simulation data used in this study are available from
https://gdex.ucar.edu/dataset/287\_qwu.html.

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\textbf{Figures Captions}

\textbf{Figure 1.} Interplanetary parameters during September 24, 2020.
IMF Bz and By (first panel), solar wind speed (second panel), solar wind
density (third panel) and interplanetary electric field (IEF) dawn-dusk
component (forth panel). The time interval for detailed analysis is
highlighted with cyan color from 05 to 06 UT, when the IMF Bz turned
northward and southward.

\textbf{Figure 2.} Same as Figure 1 but for September 26, 2920.

\textbf{Figure 3.} Interplanetary parameters from September 24, 05-06
2020. The 6 samples represent S-IMF to N-IMF cases. For the S-IMF cases,
the IEF dawn-dusk component is positive. During the N-IMF cases, the IEF
is mostly negative.

\textbf{Figure 4a} . High latitude potential map during the September
24, 05-06, 2020. The noon is on the top and midnight at bottom. The
equatorial boundary is at 40N. Dawn is on the right and dusk on the
left. During the S-IMF cases, the two-cell convection pattern dominates.
In case of N-IMF, the potential map shows multi-cell convection
patterns. The Cross Polar Cap Potential (CPCP) is estimated based on the
maximum and minimum values. The CPCP is much larger during the S-IMF
than in the N-IMF cases.

\textbf{Figure 4b.} Same as Figure 5a but extended to the equator. By
extending to the equator, the penetrating electric field effect to the
equatorial region can be seen. The yellow arrows indicate the direction
of the electric field during the S-IMF and N-IMF cases on the day and
nightside of the equatorial region.

\textbf{Figure 5.} IEF, CPCP, and equatorial potential drop (dawn/dusk
during S-IMF). Least squares fit for CPCP vs equatorial potential drop.

\textbf{Figure 6a.} Nightside vertical ion drift at the magnetic equator
(black arrows downward drift shown as pointing southward in the figure)
of September 24, 05-06 UT. Background is the nmf2 from the MAGE. Dusk is
on the left. EIAs are clearly seen on the duskside. PRE is present in
the first case. The vertical ion drift varies with IMF Bz component and
equatorial electric field. During S-IMF cases, the vertical ion drifts
are mostly downward. The scale vector is for 20 m/s.

\textbf{Figure 6b.} Same as Figure 6a but for dayside vertical ion
drift. In most cases, the vertical ion drifts are upward.

\textbf{Figure 7.} The equatorial thermospheric zonal wind during
September 24 05-06 UT at \textasciitilde{} 400 km. The eastward zonal
winds are positive shown as pointing northward with black vectors in the
figure. The scale vector is for 20 m/s.

\textbf{Figure 8.} Interplanetary parameters for September 26, 09-10 UT
in the same format as Figure 4.

\textbf{Figure 9.} Comparison of the ICON ExB meridional ion drift (cyan
and magenta vector lines from the ICON satellite track) with simulated
ExB meridional ion drift (black line above the ICON satellite track) for
September 26, 09-10 UT. The magenta vectors coincide with the time of
the MAGE simulation for that particular subplot. As the ICON satellite
cannot sample all spatial locations in the subplot simultaneously, the
satellite only measures a subsection for the section matching the time
of simulation. The background is the nmf2. The equatorial vertical ion
drift is shown as black vectors as in Figure 6b.

\textbf{Figure 10.} ICON IVM ExB meridional ion drift offset correction.
The uncorrected IVM ion drift from 9-10 UT on Sep 26, 2020 (green) and
daily magnetic local time hourly median values (orange). The 6-minute
MLT median values have an offset of 37 m/s at 18 MLT, when the value on
average should be zero. We use the 37 m/s offset corrected the IVM
meridional ion drift (blue). The IVM data have issue with low ion
density and photoelectron contamination before 12 MLT. Only the data
after 12 MLT are used to compare with the model simulation.

\textbf{Figure 11.} Same as Figure 9, but for the corrected ICON IVM ExB
meridional ion drift comparison with the model simulation.\selectlanguage{english}
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