AURORAL LATITUDE PC5 FIELD LINE RESONANCES - QUANTIZED FREQUENCIES, SPATIAL CHARACTERISTICS, AND DIURNAL-VARIATION

On the basis of observations with the Goose Bay Radar, several station s of the the Canadian Auroral Network for the Origin of Plasmas in the Earth's Neighborhood (OPEN) Unified Study (CANOPUS) magnetometer arra y, and a CANOPUS meridian scanning photometer, recent work has indicat ed 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 lo cal 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-latit ude boundary layer. However, the reported stability of these frequenci es over long periods of time remains a puzzle, as it seems to be incon sistent with the dynamic behavior of the magnetosphere. In order to es tablish if these frequencies are also observed in the dayside magnetos phere, where one of the proposed source mechanisms predicts they shoul d be seen, we have examined the temporal and spatial characteristics o f Pc 5 pulsations recorded recently with the CANOPUS magnetometer arra y. Pure state filtering techniques were used to determine the Pc 5 sig nal characteristics and their temporal and spatial variation. The resu lts show that ULF wave signals with quantized frequencies seem to be o bserved throughout the dayside and dusk magnetosphere. However, freque ncies both higher than and between previously reported values were als o seen. The difficulty in correctly determining fundamental frequencie s and their stability in the presence of polychromatic, highly amplitu de-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 ch aracteristics prevailed in the local afternoon and evening. The reason for this asymmetry is unknown. The azimuthal wave numbers were genera lly small and showed a linear dependence on the Pc 5 signal frequency, implying antisunward phase velocities independent of the wave frequen cy.