Coupling between field-aligned current impulses and Pi1 noise bursts

The estimated densities of local field-aligned currents related to nighttim e substorm onset or magnetic impulse events at cusp latitudes may exceed th e threshold for the excitation of high-frequency turbulence in the topside ionosphere. The evolution of an ion-acoustic instability is shown to lead t o quasi-oscillations around the saturation level. Consequent variations in anomalous conductivity will result in pulsed electron precipitation and noi se generation in the Pi1 band. This proposed mechanism gives a natural inte rpretation to the experimentally observed coupling between localized magnet ic disturbances, electron precipitation/acceleration, and bursts of high-fr equency ULF noise. The alternative interpretations based on mechanisms of i on cyclotron instability stimulation and ionospheric Alfven resonator excit ation can be applied only under specific plasma conditions.