Theoretical magnetograms based on quantitative simulation of a magnetospheric substorm

Abstract

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.

Antisunward net Birkeland current system deduced from the Oersted satellite observation

“… Chen et al [1982] predicted that the day‐night net Birkeland currents flow into the ionosphere around 1100 LT and flow out around 2200 LT. The local time distribution of the net current system can be derived from Figures 4 and 5 because the orbits of the Oersted satellite cover various local time sectors.…”

“…Even if the total intensity of the region 1 and the region 2 Birkeland currents are the same, the net Birkeland currents flow into the Earth's ionosphere around the noon and flow out around the midnight by the Hall conductivity effect. Chen et al [1982] predicted from their quantitative simulation of current system in the inner magnetosphere that the net Birkeland currents flow into the ionosphere around 1100 LT and flow out around 2200 LT, which is schematically illustrated in Figure 1a. The day‐night net Birkeland currents make the latitudinal profile of the magnetic field variation, Δ B ϕ , as illustrated in Figure 1b.…”

How the Earth's inner magnetosphere works: An evolving picture

“…This view seems, at first glance, to be in conflict with the main-phase ground-magnetometer observations. However, Crooker and Siscoe (1981), Harel et al (1981b) and Chen et al (1982) argued that it is not. They showed that the observed asymmetry can be explained quantitatively by the idea that the sum of region-1 and region-2 Birkeland currents is downward on the dayside and a net upward on the night side.…”

Unmodelled magnetic contributions in satellite-based models

“…the net field-aligned currents resulting from unbalanced Region 1 and Region 2 currents. These have been theoretically hypothesised by Crooker and Siscoe (1981) and by Chen et al (1982) to explain the dawn-dusk asymmetry of the disturbance observed in the north-south geomagnetic component and the features of the disturbance observed in the east-west component at low and mid-latitudes. Evidences of net field-aligned currents have been discovered later in satellite observations (e.g.…”