Abstract
Abstract. 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 (1–5 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° 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.
Highlights
Ultralow-frequency (ULF) pulsations arising from field line resonances and those in the Pc5 (1–5 mHz) are global magnetohydrodynamic (MHD) events in the magnetosphere, which may extend over several hours of local time and be observed at magnetically conjugate locations
Magnetometers, on the other hand, have poorer spatial resolution because the pulsation signal arises from the effects of the ionospheric currents associated with the pulsation and this is integrated over a transverse ionospheric region of dimensions comparable with the height of the ionosphere
A few pulsation events using multiple radars and coordinated magnetometers simultaneously observing a pulsation over an extensive range of longitude have been presented and analysed in the literature (e.g. Samson et al, 1991; Fenrich et al, 1995; Ziesolleck et al, 1998; James et al, 2013; Bland et al, 2014)
Summary
Ultralow-frequency (ULF) pulsations arising from field line resonances and those in the Pc5 (1–5 mHz) are global magnetohydrodynamic (MHD) events in the magnetosphere, which may extend over several hours of local time and be observed at magnetically conjugate locations. The magnetic conditions effectively rule out drift–bounce particle resonance Studies such as this one, with observations over a number of hours of local time, can determine azimuthal wave number m with reasonable confidence. SuperDARN has the ability to determine spectral information, azimuthal wave number, phase and group velocity, and polarization properties of the resonance These parameters are essential clues to the generation mechanism and their determination in these events presented an unusual scenario of toroidal resonances with sunward phase velocities during extremely magnetically quiet conditions.
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