Correlation between magnetic and electric field perturbations in the field‐aligned current regions deduced from DE 2 observations

Abstract

Satellite observations have shown high correlation between magnetic and electric field perturbations in the high‐latitude field‐aligned current regions. The high correlation has been interpreted by two models. In the first, the Static model, the observed perturbations are regarded as being static spatial variations, and the ratio of the orthogonal magnetic and electric field components ΔBz/Ex represents the height‐integrated ionospheric Pedersen conductivity ΣP. In the second, Alfvén wave model, the observed perturbations are interpreted as being Doppler‐shifted Alfvén waves, and the inverse of the ratio gives the Alfvén wave velocity VA. In this paper we investigate changes of this ratio with spatial scale length, using the DE 2 observations. The ratio ΔBz/Ex is found to change little with scale length for variations of scale lengths longer than 64 km, or 8.0 s in time. While for variations of smaller scale lengths, which are obtained using numerical filters with cutoff periods shorter than 4.0 s, the same ratio shows a significant dependence on scale length. The calculated ratios are nearly equal to ΣP based on an ionospheric model for long‐wavelength structures and to 1/VA for short‐wavelength variations. The transition from the former to the latter usually begins around 4.0‐8.0 s on the time scale. On the dayside the correlation between ΔBz and Ex is generally high, and the transition is clearly seen. Thus the static model is applicable to valuations of scale lengths greater than 8.0 s (or 64 km); while the Alfvén wave effect becomes increasingly dominant for scale lengths less than 4.0 s (or 32 km). For scale lengths below about 5 km (∼0.6 s) the short‐circuiting effect at ionospheric altitudes higher than the altitudes at which the horizontal Pedersen closure current usually flows becomes appreciable. However, this effect alone cannot explain the observed decrease in the ratio ΔBz/μ0Ex. The relation between the ratio ΔBz/μ0Ex and the solar zenith angle is consistent with the relationship between the Pedersen conductivity and the solar zenith angle in the published conductivity models.