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
~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.