IMF control of geomagnetic activity

Abstract

Geomagnetic activity is produced by changes in the solar wind velocity and southward turnings of the interplanetary magnetic field. Two decades of study of this coupling have established a number of empirical relations. The technique of linear prediction filtering has been particularly useful in defining impulse response functions relating solar wind and geomagnetic parameters. For example, the response function relating the rectified solar wind electric field to the AL index is approximately a Rayleigh function with time constant of one hour. The Fourier transform of this function gives the transfer function of the magnetosphere, which is a low pass filter. Filtering analysis shows that less than half the variance of the AL index is predictable by the solar wind implying that the magnetosphere is not a simple, passive element as assumed. When response functions are calculated for different levels of activity we find that they are bi-modal for moderate levels of activity and uni-modal for strong activity. A simple bi-modal model of the magnetospheric response can account for 90% of the variance in AL, provided the parameters are varied from substorm to substorm. This result suggests that both growth phase and expansion phase currents are very closely proportional to the rectified solar wind electric field, but the parameters in this relation depend on internal conditions about which the solar wind has no information. We perform a simple simulation of the magnetosphere incorporating the idea that magnetic activity is the result of superposing two types of currents, those directly driven by the solar wind and those driven by somewhat random unloading of energy stored in the magnetotail by the driven processes. The model response functions have the same behavior as real data, i.e. bi-modal response for moderate activity and uni-modal response for strong activity. The near-earth neutral line model provides a possible explanation for why two types of processes are superimposed in the magnetic variations.