The Weimer potential has a negative bias averaging -13 kV, while the AMPERE-derived potential has a positive bias averaging 19 kV. This is reflected in the large evening cell of Weimer, whereas the AMPERE potential has a more pronounced morning cell.

4. Discussion

The high-latitude electric potential has been determined from AMPERE field-aligned currents and conductances modeled by SAMI3. This is a development of the MIX approach first demonstrated by Merkin and Lyon, 2010. An outcome of the use of SAMI3 in solving for the potential is that the model can be used to predict TEC in the polar caps, based on those potential solutions. This allows for independent validation of the technique against GPS-derived images of TEC from the MIDAS algorithm (Mitchell and Spencer, 2003; Spencer and Mitchell, 2007). Applied to the case of 23 May 2014, a period of moderate geomagnetic disturbance (KP reached 5+ between 21-24 UT), the AMPERE-derived potential results in high-latitude TEC predictions that are much closer to the observations than SAMI3 run with the Weimer (2005) model. Important biases remain in both versions of the model, especially at lower latitudes (45 – 60° N) where TEC is overestimated in the evening sector and underestimated in the morning sector. This skews the location of the tongue of ionization to later local times in both versions of the model. Comparison of the derived potentials against DMSP velocity data indicate good agreement in the evening cell, but some discrepancies in the morning cell. The bulk of F-region plasma is typically found post-noon, so the effects of this discrepancy should be limited in terms of formation of tongues of ionization and patches. The problem might be caused by low conductances in the morning cell, which would be expected to reduce the magnitude of field-aligned currents observed by AMPERE, or by low ion densities at DMSP altitudes affecting the performance of the drift meter.
The new AMPERE-derived potential and its relatively good performance in predicting high-latitude TEC serves as an indication of the large degree of uncertainty in high-latitude potential models. The Weimer potential is substantially larger than the AMPERE-derived potential (77 kV vs 60 kV) and is skewed towards the evening cell at -13 kV versus a +19 kV skew towards the morning cell in AMPERE. The two potentials also show a variety of smaller scale differences in terms of latitudinal extent and the shape of the dayside cusp. The comparison between the AMPERE-derived potential and the observed TEC data in Figure 5 shows a close match between the shape of the potential around the dayside cusp and the path of the tongue of ionization. This indicates that AMPERE’s underlying dataset (the Iridium constellation of 66 operational satellites reporting magnetic perturbations every 10 minutes) has sufficient spatio-temporal resolution to capture the major features of polar cap plasma convection at scales of 100s of km and larger.
The conductance model represents a major source of uncertainty in the estimation of electric potentials from FAC observations. During testing, we assessed several conductance options, including constant conductances, solar EUV parameterization and the empirical model of Robinson et al. (2020), and found that none of them produced an improvement over the internal SAMI3/Hardy conductance. Likewise the turbulent Pedersen conductance term of Dimant and Oppenheim (2011), whose effect is to reduce the strength of the electric potential, was not found to improve agreement with independent data in this case. In fact the AMPERE-derived potential was weaker than the Weimer potential even without consideration of the turbulent conductance term. It may be that competing biases caused this outcome. Future efforts towards accurate, global characterization of the ionospheric conductance will be useful in the application of this technique.
The high-latitude potential is of major importance to high-latitude ionospheric dynamics, but is not the only source of uncertainty in modeling the plasma distribution there. Other important factors include the reservoir of photo-ionized plasma on the subauroral dayside, and the plasma lifetimes in the polar cap (e.g. Chartier et al., 2019). Various SAMI3 driver options were tested with the aim of matching observed subauroral TEC levels (results not shown in this paper). These included the Flare Irradiance Spectral Model by Chamberlin et al. (2007) and the neutral atmosphere of the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model by Roble and Ridley (1994). General conclusions about those models should not be drawn from this analysis but, in this case study, none of those options was able to match the observed levels of TEC as well as the configuration shown here. Future operational systems would benefit from ionospheric density assimilation schemes to better specify the sub-auroral plasma that feeds into the polar caps.

Conclusions

A case study of 23 May 2014 (a day with moderate geomagnetic activity) demonstrates the potential for integrating high-latitude electric potential estimates based on AMPERE observations into SAMI3. The new technique is useful in predicting high-latitude TEC. The AMPERE-derived potential is in good agreement with DMSP ion drifts overall, and closely matches the tongue of ionization observed in MIDAS GPS-derived TEC images of the northern polar cap. In this case, SAMI3’s predictions of high-latitude TEC are much closer to the data when using AMPERE-derived potentials than when using the Weimer potential. The Weimer cross-polar cap potential is substantially larger than the AMPERE potential at 77 kV versus 60 kV. At least in this case, this investigation demonstrates that the AMPERE data has sufficient spatio-temporal resolution to predict TEC variations at scales of 100s of km and above.

Acknowledgements

The authors acknowledge support of National Science Foundation (NSF) CEDAR grant #1922930 and NASA Heliophysics grant 80NSSC21K1557. VGM acknowledges support from the NASA DRIVE Science Center for Geospace Storms (CGS) under grant 80NSSC20K0601 and an LWS grant 80NSSC19K0080. AMPERE development, data acquisition, and science processing at JHU/APL were supported by NSF awards ATM #0739864 and ATM #1420184. AMPERE data used in this paper are publicly available through the AMPERE web site (http://ampere.jhuapl.edu). MIDAS GPS-derived TEC data were used courtesy of the University of Bath, Claverton, Bath, UK. Input GPS data were obtained from the International GNSS Survey mirrors at http://garner.ucsd.edu/, ftp://geodesy.noaa.gov and ftp://data-out.unavco.org/. Solar and geomagnetic indices were obtained from https://omniweb.gsfc.nasa.gov/ and http://wdc.kugi.kyoto-u.ac.jp/. DMSP ion drift data were retrieved from http://cedar.openmadrigal.org/showExperiment?experiment_list=100118297 (UT Dallas files were used). DMSP processing code is available here: https://github.com/alexchartier/mix. Model output has been posted to https://zenodo.org/record/5218739#.YR10lNNKhb8

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