Auroral latitude Pc 5 field line resonances: Quantized frequencies, spatial characteristics, and diurnal variation

Abstract

On the basis of observations with the Goose Bay Radar, several stations of the the Canadian Auroral Network for the Origin of Plasmas in the Earth's Neighborhood (OPEN) Unified Study (CANOPUS) magnetometer array, and a CANOPUS meridian scanning photometer, recent work has indicated the existence of Pc 5 ULF waves with discrete and remarkably stable frequencies (1.3, 1.9, 2.7 and 3.3 mHz with <10% variation) in the local midnight and early morning (predawn) sectors. These ULF waves were interpreted as the consequence of field line resonances driven by MHD waveguide/cavity modes, possibly excited by impulsive stimulation of the magnetopause or by Kelvin‐Helmholtz instabilities in the low‐latitude boundary layer. However, the reported stability of these frequencies over long periods of time remains a puzzle, as it seems to be inconsistent with the dynamic behavior of the magnetosphere. In order to establish if these frequencies are also observed in the dayside magnetosphere, where one of the proposed source mechanisms predicts they should be seen, we have examined the temporal and spatial characteristics of Pc 5 pulsations recorded recently with the CANOPUS magnetometer array. Pure state filtering techniques were used to determine the Pc 5 signal characteristics and their temporal and spatial variation. The results show that ULF wave signals with quantized frequencies seem to be observed throughout the dayside and dusk magnetosphere. However, frequencies both higher than and between previously reported values were also seen. The difficulty in correctly determining fundamental frequencies and their stability in the presence of polychromatic, highly amplitude‐modulated signals is noted. The signal characteristics observed in the postnoon sector were found to be significantly different compared with those seen at prenoon. While local morning and noon Pc 5 revealed typical field line resonance characteristics, toroidal mode signal characteristics prevailed in the local afternoon and evening. The reason for this asymmetry is unknown. The azimuthal wave numbers were generally small and showed a linear dependence on the Pc 5 signal frequency, implying antisunward phase velocities independent of the wave frequency.