On the depletion of ionization in the outer magnetosphere during magnetic disturbances

Diffusion of plasma across a homogeneous magnetic field (B) orthogonally bounded by two conducting planes is examined. Random curl-free electric fields (E), such that E·B = 0, consisting of fluctuations with periods greater than the time it takes to establish the magnetic field lines as equipotentials, are considered to cause the diffusion. The diffusion coefficient is 2 E2¯τB−2, where τ is the correlation time of the random electric field and E2¯ its variance. Within the geomagnetic field there is a region of noisy geomagnetic fluctuations of periods in excess of 100–200 seconds. In this noise region diffusion changes the equatorial radial density profile from the quiet n ∝ R−3 into n ∝ R−4 (approximately). It is suggested that the diffusion explains the first-order decrease in density in the outer magnetosphere observed with whistler techniques during magnetic disturbances. In the process of diffusion caused by disturbance there is a net energization of ambient plasma sufficient to make a major contribution to the main phase of a magnetic storm.