2011 SuperDARN Workshop
Short-period Doppler shift variations in the polar cap: ULF waves or something else?
G. Scoular, P. Ponomarenko, and J.-P. St.-Maurice
University of Saskatchewan, Saskatoon, SK, Canada
abstract. Daytime Pc3-4 waves (10-50 mHz) are generated at the bow shock and propagate through the inner magnetosphere to the ground as Alfven waves in the ?closed? field line geometry. These waves have also been detected in the polar cap, but their propagation mode for the ?open? field lines remains largely unknown. To address this issue, we ran a pilot study of ionospheric ULF wave signatures in the northern polar cap. For this purpose we analysed Doppler shift variations in the HF ground scatter echoes across the 5-60 mHz frequency range as measured by the PolarDARN radars at Rankin Inlet and Inuvik. Previous ground magnetometer studies showed that the high-latitude Pc3-4 waves exhibit relatively long spatial coherence, distinct band-limited spectral shape, and occurrence/power maximum near MLT noon. In contrast, our observations revealed a dominance of Doppler shift variations that exhibit low spatial coherence, a featureless power-law spectrum and no connection to the ground magnetic field variations, while their power has two diurnal peaks before and after 12 MLT. This rather unexpected outcome of our studies has forced us to look for alternative explanations of the observed Doppler shift variations.