Excitation of a Hall current generator by FAC closure, via an divergent Hall current, during the transient phase of M-I coupling

*Akimasa Yoshikawa[1]
Graduate School of Sciences, Kyushu University[1]

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 that is excited during the transient phase of MI coupling, plays a crucial role in the formation of the ionospheric rotational current system. Numerical simulation has been able to explain the details of 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.

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 that 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 time-scale for the rotational current system to accumulate or extract the poloidal magnetic energy of ionospheric surface wave. This depends on the inductance of the rotational current system, and the effective conductivity of the ionospheric rotational conduction-current. This characteristic time scale becomes the cause of an ionospheric inductive effect, such as a time delay or phase-lag between the source electromagnetic fiel of the FAC and the corresponding poloidal magnetic field on the ground. This latter causes an inductive shielding effect on the amplitude of the geomagnetic disturbance. Numerical simulation has been able to explain the details of 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.