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 ...

Using substorm currents derived from the Rice computer simulation of the substorm event of September 19, 1976, we have computed theoretical magnetograms as a function of universal time for various stations. A theoretical Dst has also been computed. Our computed magnetograms were obtained by integrating the Biot‐Savart law over a maze of approximately 2700 wires and bands that carry the ring currents, the Birkeland currents, and the horizontal ionospheric currents. Ground currents and dynamo currents were neglected. Computed contributions to the magnetic field perturbation from eleven different kinds of currents are displayed (e.g., ring currents, northern hemisphere Birkeland currents). On the basis of comparison of theoretical results with corresponding observations, we make the following remarks. First, overall agreement of theory and data is generally satisfactory, especially for stations at high and mid‐magnetic latitudes. Second, model results suggest that the ground magnetic field perturbations arise from very complicated combinations of different kinds of currents and that the magnetic field disturbances due to different but related currents often cancel each other, despite the fact that complicated inhomogeneous conductivities in our model prevent rigorous application of Fukushima's theorem. Third, both the theoretical and observed Dst decrease during the expansion phase of the substorm, but data indicate that Dst relaxes back toward its initial value within about an hour after the peak of the substorm. This effect exists qualitatively in the computer simulation if we sharply reduce the assumed polar cap potential drop and conductivity at the end of the substorm. Fourth, the dawn‐dusk asymmetry in the horizontal component of magnetic field disturbance at low latitudes in a substorm is essentially due to a net downward Birkeland current at noon, net upward current at midnight, and generally antisunward flowing electrojets; it is not due to a physical partial ring current injected into the duskside of the inner magnetosphere.