Pc5 ULF pulsations associated with waveguide modes observed with the IMAGE magnetometer array

Several recent publications have reported high-latitude radar observations of field line resonances (FLRs) displaying discrete and very stable frequencies in the Pc5 range. It is believed that these quantized frequency FLRs may be associated with cavity/waveguide harmonics of the outer magnetosphere but the prevalence and reported stability of the frequencies remains controversial. We examine the statistics of 137 Pc5 pulsation events identified from 3 months magnetometer data from the International Monitor for Auroral Geomagnetic Effects (IMAGE) and show that these discrete frequencies are also prominent in ground-based data but find no evidence to support their reported day-to-day stability. However, our observations are consistent with FLRs in the outer magnetosphere being driven by waveguide mode harmonics. Further, we examine amplitude, phase, polarization, and azimuthal wave number characteristics and show that these FLRs have features which are strongly local time dependent. Our analysis indicates that the local time dependence of the waveguide modes' magnetopause boundary condition may be critical in determining where Pc5 pulsation resonances can develop.