ULF high‐ and low‐m field line resonances observed with the Super Dual Auroral Radar Network

Abstract

Numerous field line resonance events have been observed with three HF radars (Saskatoon, Kapuskasing, and Goose Bay) of the Super Dual Auroral Radar Network (SuperDARN). The field line resonances cause oscillations in the F region plasma flows which are detected in the measured line of sight Doppler velocities. After analysis, it was found that the resonances were of two types: those with low azimuthal wave number, low‐m, and those with high azimuthal wave number, high‐m. The high‐m events showed many similarities with high‐m pulsations of previous reports including local time of most occurrences (noon‐dusk), pulsation frequencies, westward propagation, increase in phase with latitude, and north‐south polarization. The low‐m events exhibited typical field line resonance characteristics and were found near dusk and dawn with anti‐Sunward propagation. The most notable result was the fact that the high‐ and low‐m events shared many common features. They both were found to occur at the same discrete and stable frequencies. The most common frequencies were 1.3, 1.9, and 2.5‐2.6 mHz, which have previously been associated with magnetospheric waveguide modes. They also occurred at other less common frequencies, such as 1.5–1.6 mHz. Both types of events were localized in latitude with an inverse relation between frequency and latitude. Both were characterized by a wave packet structure with a duration of approximately 1 hour. The numerous features shared by the high‐ and low‐m resonances strongly suggest that they are caused by the same source mechanism. A dispersive waveguide model as a source for the field line resonances is discussed.