How does the magnetosphere go to sleep?

Energy and circulation in the Earth's magnetosphere and ionosphere are largely determined by conditions in the solar wind and interplanetary magnetic field. When the driving from the solar wind is turned off (to a minimum), we expect the activity to die down but exactly how this happens is not known.

A recent study utilized global MHD simulations to addressed the questions of what constitutes the quietest state for the magnetosphere and how it is approached following a northward turning in the IMF that minimizes the driving. An exponential decay with a decay time of about 1 h was observed in several integrated parameters related to different aspects of magnetospheric activity, including the total field-aligned current into and out of the ionosphere.

The same time rate of change for the cessation of activity has also been measured in total field aligned current estimates from the AMPERE project, adding observational support to this finding. The observational study also revealed both a seasonal and a day/night variation in the decay parameter, with faster decay observed in the winter than in the summer hemisphere and on the nightside than on the dayside.

Decay time averages varied between 0.8 and 2.0 h for these scenarios. The results can be understood in terms of stronger/weaker line tying of the ionospheric foot points of magnetospheric field lines for higher/lower conductivity. Additional global modeling results with varying conductance scenarios for the ionosphere confirm this interpretation and provide a quantitative understanding of the effect.