Excitation of a Hall‐current generator by field‐aligned current closure, via an ionospheric, divergent Hall‐current, during the transient phase of magnetosphere–ionosphere coupling

Abstract

To clarify the process by which an ionospheric current system is formed by field‐aligned current (FAC) closure in the ionosphere, an inclusive formulation of magnetosphere–ionosphere (MI) coupling is undertaken. The “Hall‐current generator”, which is excited during the transient phase of MI coupling, plays a crucial role in the formation of the ionospheric rotational‐current system. It extracts energy from the FAC system through the divergent Hall‐current and pumps it into the rotational Hall‐current. The energy of the rotational current accumulates as an evanescent poloidal magnetic field, associated with the ionospheric surface wave. This accumulated energy is also fed back to the FAC system through the change in energy flow of the Hall‐current generator. It is found that there is a typical timescale for the rotational‐current system to accumulate or extract the poloidal magnetic energy of ionospheric surface waves. This depends on the inductance of the rotational‐current system and the effective conductivity of the ionospheric rotational current. This characteristic timescale becomes the cause of an ionospheric inductive effect, such as a time delay or phase lag between the source electromagnetic field of the FAC and the corresponding poloidal magnetic field on the ground. The latter causes an inductive shielding effect on the amplitude of the geomagnetic disturbance. Numerical simulation has been able to explain the details of the physical process that occurs when the incident FAC is developing and decaying, and how the energy and current are redistributed into the other elements during the transient MI‐coupling process.