ULF signatures of the polar cusp

Geomagnetic pulsations associated with the polar cusp have been detected at Cape Parry, Northwest Territories, Canada, using flux gate and induction magnetometers. Data from this station show enhanced power almost every day near local noon. It is this enhancement which is taken as the signature of the polar cusp. Maximum signal levels are observed prior to local noon, although the signals are present for several hours on each side of local noon. The spectrum of pulsations near the cusp is broadband, reaching from a few millihertz to above 1 Hz. On average the power spectrum decreases with frequency as f−2.6. Within this general decrease there are often found regions of enhanced power, principally near 5 mHz and near 40 mHz. We have identified the 5‐mHz band with fluctuations in ionospheric currents associated with the cusp boundary and its motions. The 40‐mHz band is presumed to be due to direct penetration of the hydromagnetic radiation in the cusp to the ground. From the measured spectrum we infer an rms fluctuation amplitude of the cusp currents of approximately 3×10³ A, and from this we estimate the total power dissipated by these fluctuations of approximately 4×107 W.