Quantitative experimental verification of the magnetic conjugacy of geosynchronous orbit and the auroral zone

A fundamental consensus of the first International Conference on Substorms (ICS-1) was that important auroral process are related to processes in space that occur near the inner edge of the plasma sheet. Numerical magnetic field models and popular opinion suggest that the aurora in the ionosphere map magnetically to the equatorial ionosphere at distances of perhaps 6 to 10 R_E. This study tests those propositions quantitatively by comparing the predictions of five magnetic field models with measured magnetic conjunctions between low-and high-altitude satellites. The mapping is tested observationally by comparing electron energy spectra obtained by the Magnetospheric Plasma Analyzer (MPA) at geosynchronous orbit and by the DMSP spacecraft. Typically, the spectra match well for only a few seconds so the accuracy is better than one degree. We then compare the measured magnetic footpoints of geosynchronous orbit with the footpoints predicted by five magnetospheric field models: Tsyganenko-89, Tsyganenko-87, Tsyganenko-82, Oslen-Pfitzer, and Hilmer-Voigt. Based on a set of over 100 measured magnetic conjunctions we confirm that geosynchronous orbit generally has its magnetic footpoint in the auroral zone but that there is significant variation. Statistically the uncertainty in the mapping given by magnetic field models is approximately ±3°. Only about 25-30% of the time did the field model predict the conjunction to within ±1° and as much as 20% of the time the field model could be off by more than ±5°. Although there are significant differences between the mappings predicted by various magnetic field models but that there is no clear "winner" in predicting the observed mapping. We also suggest that this technique provides an excellent opportunity for testing future magnetic field models and for determining the appropriate parameterizations for those models.