Evolving phase propagation in an intermediate-m ULF wave driven by substorm-injected particles

An ultra-low frequency (ULF) wave was simultaneously observed in the ionosphere by the Super Dual Auroral Radar Network (SuperDARN) radar at Hankasalmi, Finland and on the ground by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers with close proximity to the radar. The onset time of the wave event was around 03:00 magnetic local time. Fourier wave analysis of the event suggests a wave pe23 riod of about 1340 s with an equatorward latitudinal and eastward longitudinal wave phase propagation, and an effective azimuthal wave number of 17±1, in the intermediate range of those observed in ULF waves. This wave has been interpreted as resulting from drift26 ing electrons of energies of 13±5 keV in a drift resonance condition linked to energetic particle populations during a magnetospheric substorm. The latitudinal phase charac28 teristics of this wave experienced temporal evolution, believed to be caused by additional injected particle populations associated with the same substorm driving the wave, which resulted in an observed loss of HF backscatter. This observation of a unique type of tem31 poral evolution in the phase propagation characteristics of ULF waves enhances current understanding about the structure, dynamics and source of these types of ULF waves.