Comparing a spherical harmonic model of the global electric field distribution with Astrid‐2 observations

Electric field measurements provided by the double probe instrument on the Astrid‐2 satellite are compared with the empirical Weimer electric field model for all magnetic local times, except between 11 and 13 MLT, and poleward of 55° corrected geomagnetic latitude (CGLat). We focus the model evaluation on its ability to predict the latitudinal locations of the convection reversal boundaries for two‐cell convection patterns and to estimate the magnitude of the electric field above 55° CGLat. A total number of 780 polar cap passes are employed from the Northern Hemisphere between January and July 1999. The measured average electric field magnitude in the dawn‐dusk meridian plane above 55° CGLat is generally 25% larger than the predicted field independent of the interplanetary magnetic field (IMF) direction. The model shows a better correspondence with the observed electric field for southward IMF than for northward IMF, with most cases centered around B z = −1.5 nT and r = 0.88. However, the agreement for northward IMF is promising, and a few examples are shown to corroborate this fact. The observed and predicted convection reversal boundary locations along the satellite track for southward IMF are on the average found 2–3° CGLat apart in the dawn‐dusk meridian plane but may be as far apart as 9° CGLat. An initial investigation of the relative timing of a 20‐min averaging window for the IMF along the 20–25 min polar cap crossing suggests that a time‐dependent transfer function may be found that applies a higher weight to the input solar wind data early in the pass and a lower weight later in the pass for an IMF window that corresponds to the first half of the crossing and the opposite weight versus time dependence for an IMF window corresponding to the last half of the crossing.