Estimating the Rate of Cessation of Magnetospheric Activity in AMPERE Field-Aligned Currents

The decay of magnetospheric activity when driving is turned off (to a minimum) was measured in total field-aligned current estimates from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) project. Events of distinct northward turnings of the interplanetary magnetic field were identified, with prolonged periods of stable southward driving conditions followed by northward interplanetary magnetic field conditions. All but 4 of 43 identified events exhibit a well-defined exponential decay in the total hemispheric field-aligned current following the northward turning. A superposed epoch analysis yields a generic decay constant of 0.9, corresponding to an e-folding time of 1.1 hr. A statistical analysis of the ensemble of events also reveals a seasonal variation in the decay parameter with faster decay observed in the winter than in the summer hemisphere. This result can be understood in terms of stronger/weaker line tying of the ionospheric foot points of magnetospheric field lines for higher/lower conductivity.