Large-scale coordinated observations of Pc5 pulsation events

HF (high-frequency) radars belonging to SuperDARN (Super Dual Auroral Radar Network) receive backscatter over substantial fields of view which, when combined, allow for simultaneous returns over extensive regions of the polar caps and midlatitudes. This makes them ideal instruments for the observation of pulsations in the Pc5 (15 mHz) frequency band. Relatively few pulsation events observed by multiple radars have been reported in the literature. Here we describe observations of three such events which extend over more than 120 degrees of magnetic longitude in the Northern Hemisphere and one of which is also detected in the Southern Hemisphere. All three events show characteristics of field line resonances. In one case the pulsation has also been observed by magnetometers under or near the radar fields of view. The extensive longitudinal coverage allows accurate determination of azimuthal wave numbers. These are at the upper end of the lower values associated with external sources such as those in the solar wind. Such sources imply antisunward flow. However, the azimuthal wave number is negative, implying westward propagation at magnetic local times on both sides of noon, as would be expected from drift-bounce resonance with positive particles. Quiet conditions and a very low ring current during the events argue against this. The identification of the source of pulsations from a number of different mechanisms remains a problem of interest.