Quantitative simulation of a magnetospheric substorm 2. Comparison with observations

Abstract

Several results of the computer simulation of the behavior of the inner magnetosphere during the substorm‐type event of September 19, 1976, are discussed in detail. The model predicts a modest ring current injection, in to L ≈ 6, with total strength that is comparable to the strength estimated from the observed decrease in Dst. For the geosynchronous orbit region on the duskside, the model predicts a characteristic energy dispersion often observed by McIlwain and collaborators: energetic ions arrive first after substorm onset, followed by less energetic ions. The computed electric fields compare satisfactorily with electric fields measured from S3‐2, although there are detailed differences. Three general features on which the model and observations are in good agreement are (1) the magnitude and direction of the high‐latitude electric field, (2) the degree to which the low‐latitude ionosphere is shielded from the high‐latitude convection electric field, and (3) the fact that the poleward electric field on the duskside is significantly larger, on the average, than the equatorward electric field on the dawnside. The observations indicated one instance of rapid flow equatorward of the auroral zone, involving an electric field of more than 100 mV/m. This rapid subauroral flow was accurately predicted by the model. The predicted east‐west magnetic perturbations due to region 2 Birkeland currents agree satisfactorily with S3‐2 observations with regard to direction, total magnitude, and general location, but there is an important general discrepancy: in most cases, the actual Birkeland currents were distributed over a wider range of latitude than the model would predict. Speculations are presented as to possible explanations of the discrepancy. The model Birkeland currents agree satisfactorily with the averaged observations of Iijima and Potemra, in terms of direction, strength, and overall pattern. The model suggests a theoretical interpretation of the observed overlap region near midnight, where a region of upward Birkeland current is bounded on its equatorward and poleward sides by regions of downward current. The model provides a useful picture of the overall magnetosphere‐ionosphere current system. It also suggests that the observed asymmetry in the change of the horizontal magnetic field at low‐latitude ground stations during the main phase of a magnetic storm should not be interpreted simply as asymmetric development of the inner‐magnetospheric ring current and the associated region 2 Birkeland currents. Region 1 Birkeland currents, which connect to the outer magnetosphere, play a major role in the asymmetry of low‐latitude ΔH, while overhead Hall currents seem to play a lesser role. The model indicates that the total Joule heating during the event is approximately three times the increase in ring current energy, a result that is in apparent contradiction to some previous estimates. A general, but highly approximate, analytic argument is presented in support of this result of the simulation. Some simple formulas are presented that give rough estimates of global Joule heating rates from observable parameters.