Phase structure of low-latitude Pc3 - 4 pulsations

Physically realistic models of ULF wave generation and propagation must embody amplitude, spectrum and phase characteristics. Although observational programs often focus on the spectral properties of ULF signals, the characterization of their phase properties still remains unclear. Existing analytical procedures often make unrealistic assumptions about the spectral purity and stability of the waveform. This paper describes two techniques which may be used to evaluate the spectral and temporal evolution of phase properties in ULF signals. The procedures were tested on synthetic Pc3 pulsation-like signals before application to real multi-station low-latitude ground data. Phase analysis like this introduces a new dimension to the interpretation and modelling of ULF wave processes. For instance, the identification of harmonic and packet structures is greatly facilitated. We find that much of the apparent packet structure and phase skips in amplitude-time records is due to wave interference. However, real phase skips have been observed in multi-station ground data, and need not occur simultaneously or at the same location in both Ḣ and Ḋ components. Examination of multi-station phase-time properties allows group structure, spatial coherency, and group velocities to be inferred. In one example we infer group propagation in a south-easterly direction at 8-17 km s ^-1.