A new type of Doppler velocity fluctuations in HF ground scatter from the polar cap

Abstract

We report a new type of variations in Doppler velocity of HF ground scatter echoes from the polar cap at f∼10 mHz. Similar fluctuations from lower latitudes are usually associated with large-scale dayside Pc3-4 ULF waves. However, the polar cap oscillations exhibit a puzzling anisotropy in spatial coherence along and across the radar's line of sight. Furthermore, in contrast to Pc3-4 waves, these fluctuations show no ground magnetic signatures and display a pronounced gap in power/occurrence around local noon. We hypothesize that localized, ≤100 km, auroral particle precipitations near the radar site can modulate Doppler shift of the radio waves entering the ionosphere. In the ground scatter returns, due to the geometrical spread of the rays propagating through the ionosphere to the ground, these variations would appear to have a much larger line-of-sight scale. Key Points A new type of Doppler fluctuations was observed in the polar cap using HF radars These fluctuations are distinct from those caused by ULF waves We suggest that they might be caused by localized particle precipitations ©2013. American Geophysical Union. All Rights Reserved.