Poleward propagation of pulsations near the cusp

Abstract

During 1985 magnetometer stations were operated at Cape Parry and Sachs Harbor, N.W.T., Canada in order to investigate the spectrum of geomagnetic pulsations at high latitudes and near the polar cusp. Each day, as the stations move under the dayside cusp, the level of pulsation power increases by several orders of magnitude. The spectrum is broadband, reaching from below 5 mHz to above 1 Hz with enhancements near 5 and 40 mHz. The 5 mHz power is produced, in part, by discrete pulses which may be interpreted as due to evolving current patterns in the overhead ionosphere. The pulses have amplitudes of 50–100 nT, temporal widths of a few minutes and show time delays between the two stations which indicates poleward motion at 1–5 km s −1. Consistently, for every pulse, the time delay depends upon the component of the magnetometer used. The largest time delays occur for the Z-component and these are about twice the delays measured using the H- and D-components. This dependence may imply the current filament is evolving in shape as it moves poleward. While the number of pulses present in the data varies from day to day, presumably controlled by solar wind conditions, when they are present the pulses occur at 10–20 min intervals throughout the times when the stations are near the dayside cusp. The amplitudes, occurrence frequency and poleward motion are consistent with the identification of these pulses as the ground signatures of currents associated with flux transfer events.