Excitation and decay of solar wind-driven flows in the magnetosphere-ionosphere system

The paper discusses basic concepts of the form of high-latitude ionospheric flows and their excitation and decay in the light of recent high time-resolution measurements made by ground-based radars. To investigate the excitation and decay of the two reconnection-driven components of a flow, a concept of a flow-free equilibrium configuration for a magnetosphere which contains a given (arbitrary) amount of open flux is introduced. Reconnection events act either to create or destroy open flux, thus causing departures of the system from the equilibrium configuration. Flow is then excited, which moves the system back toward equilibrium with the changed amount of open flux. The overall time scale associated with the excitation and decay of the flow is about 15 min. The response of the system to both impulsive (flux transfer event) and continuous reconnection is discussed in these terms.