Midlatitude transition bays and their relation to the spatial movement of overhead current systems

An analysis of the occurrence frequency of midlatitude geomagnetic bays gives strong support to the polar electrojet configuration of Akasofu, Chapman, and Meng. Modifications of that current system, combined with an emphasis on the importance of differential movement of the overhead current system with respect to an observer, result in consistent explanations of the form of most geomagnetic bay phenomena. A study is made of phenomena that we term ‘transition bays,’ and the dependence of the form of geomagnetic bays on latitude is investigated. It is shown that D bays show a drop in occurrence frequency around local midnight relative to the number of events before and after. This dip is shown to identify the ‘demarcation line’ for the average current system for the bay. Subauroral and polar-cap stations may lie in a ‘cancellation zone’ so that the H component of the disturbance will have its contribution from the electrojet almost canceled by the contribution from the overhead return current. It is shown that there are two separate causes of geomagnetic bays, and their manifestations are discussed in detail.