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.