Inference of magnetospheric and ionospheric electrical properties from the decay of geomagnetic activity

Abstract

The three-dimensional current system characteristic of directly driven magnetospheric substorm activity that couples the magnetosphere to the ionosphere can be represented by an LCR circuit where the source of the emf, the capacitance, and the resistance are in parallel. An earlier study (G. Rostoker and F. Pascal. Can. J. Phys. 68, 74 (1990).) has shown that the impulse response of the magnetosphere can be represented by the response of such an LCR circuit to a sudden increase in the crosstail potential drop ε, where the circuit parameters, (ε, L, C, R) are all permitted to vary with time as the current in the circuit asymptotically approaches its new level. In this paper we examine the response of the directly driven system to sudden northward turnings of the interplanetary magnetic field. We find that the directly driven westward electrojet in the dawn sector decays in an approximately exponential fashion following the northward turning, and that the decay time is nearly linearly dependent on the value of the auroral electrojet (AE) at the time of the northward turning. We use the relationship between the characteristic time constant of the decay and the magnitude of AE at the time of the northward turning to develop quantitative expressions describing the magnitudes of the electrical bulk parameters, L, C, and R as a function of the level of magnetospheric activity.