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On optimum solar wind - magnetosphere coupling functions for transpolar voltage and planetary geomagnetic activity
  • Michael Lockwood,
  • Kathryn A McWilliams
Michael Lockwood
University of Reading, University of Reading

Corresponding Author:m.lockwood@reading.ac.uk

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Kathryn A McWilliams
University of Saskatchewan, University of Saskatchewan
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We use 65,133 hourly averages of transpolar voltage (ΦPC) from observations made over 25 years by the SuperDARN radars, along with simultaneous interpolated am geomagnetic index values, and study their optimum interplanetary coupling functions. We find lags of 18min. and 31min. for ΦPC and am, respectively, and fit using a general coupling function with four free fit exponents. To converge to a fit, we need to average interplanetary parameters and then apply the exponent which is a widely-used approximation: we show how and why this is valid for all interplanetary parameters, except the factor quantifying the effect of the clock angle of the interplanetary magnetic field, sind(θ/2), which should be computed at high time resolution and then averaged. We demonstrate the effect of the exponent d on the distribution, and hence weighting, of samples and show it causes d to be best determined from the requirement that the coupling function is a linear predictor, which yields d=2.50±0.07 for ΦPC and d=3.00±0.22 for am. To check for overfitting, fits are made to half the available data and tested against the other half. Ensembles of 1000 fits are used to study the effect of the number of samples on the distribution of errors in individual fits and on systematic biases in the ensemble means. We find only a weak dependence of solar wind density for ΦPC but a significant one for am. The optimum coupling functions are shown to be significantly different for ΦPC and am.
Dec 2021Published in Journal of Geophysical Research: Space Physics volume 126 issue 12. 10.1029/2021JA029946