Polar Cap Ionospheric Oscillations in the ULF Frequency Range Observed With SuperDARN HF Radar

Abstract

Pc3-4 waves are recorded as geomagnetic pulsations with periods of 6-100s. They are generated at the bowshock and propagate to mid and auroral latitudes as Alfvén waves along closed magnetic field lines. At these latitudes Pc3-4 waves have been studied on the ground using magnetometers and in the ionosphere using HF radar. These waves have also been observed using magnetometers at polar latitudes even though there is no known propagation mechanism to the “open” field lines of the polar cap regions.

In this work we used PolarDARN stations at Rankin Inlet and Inuvik to attempt the first study of Pc3-4 waves in the polar cap regions using radar. In ground scatter data, Doppler velocity oscillations with frequencies in the Pc3-4 range were found to be a common daytime occurrence. The oscillations are spatially coherent and in phase along the beam’s line of sight, matching lower latitude observations. However, upon further study it became apparent that the characteristics of the oscillations are different from those known for Pc3-4 waves.

The observed oscillations have a diurnal trend that shows peaks in activity at 7:00 and 14:00MLT, where Pc3-4 oscillations have a diurnal peak at 10:30-11:00 MLT. In addition, poor coherence was observed between oscillations in radar and ground magnetic field variations at the nearby Taloyoak magnetometer. Further confounding the problem, we found that although the oscillations were coherent along the line-of-sight of the radar, poor coherence is observed when comparing oscillations in different beams separated by similar spatial scales. This finding counters both the spatial coherence observed along the beam’s line of sight and the spatial coherence of Pc3-4 waves at auroral latitudes. We conclude that it is unlikely that the observed oscillations are the result of Pc3-4 ULF waves. We instead propose that the observed Doppler velocity oscillations are caused by a change in the ionization along the ray’s path due to auroral particle precipitation.