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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