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 posmoon 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 asynmaetry 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.

Recent magnetometer and HF radar observations have indicated the existence of multiple, discrete field line resonances with remarkably stable frequencies of 1.3, 1.9, 2.6, and 3.4 mHz, apparently driven by MHD waveguide/cavity modes in the magnetosphere. Given the dynamic nature of the magnetosphere, the apparent stability of these frequencies is difficult to understand, and it is therefore important to examine this question on the grounds of a larger database. In this paper we present a statistical survey of the occurrence of Pc 5 field line resonance frequencies based on 1 year of data from several stations of the CANOPUS magnetometer array. Pure state filtering techniques were used to investigate various statistical aspects of auroral latitude Pc 5 characteristics, with emphasis on the occurrence of preferential, discrete spectral peaks. A number of case studies exploiting the full spatial extent of the CANOPUS array were carried out to complement the statistics. Although the results provide further support for the existence of discrete (spatially monochromatic) field line resonances, we find that the apparently stable frequencies do not seem to be particularly distinguished from other Pc 5 frequencies, except perhaps the 1.9‐mHz peak. Discrete field line resonance characteristics were also observed for other sets of frequencies, and we therefore conclude that the apparently stable waveguide/cavity mode frequencies do not necessarily represent a unique set. The implications of these results with respect to various possible Pc 5 wave generation mechanisms are discussed.

Data obtained from the Australia Wide Array of Geomagnetic Stations (AWAGS) have provided a unique opportunity to investigate the spatial variation of long‐period (Pc 5) ULF wave signal characteristics as a result of plasma waves in low latitudes. Pure state filtering techniques were applied in order to detect highly polarized Pc5 events and derive the wave signal parameters, i.e., the wave frequency, amplitude, polarization, and phase. The results indicate that the Pc 5 signal frequency was virtually independent of latitude and longitude, while the amplitude decreased considerably toward lower latitudes. The spatial variation of the signal polarization was found to be small. However, a statistical analysis of the signal polarization revealed a systematic diurnal variation. Counterclockwise sense of rotation (viewed in the direction of the ambient magnetic field) with the major axis of the polarization ellipse in the northeast quadrant was observed in the local morning, while clockwise sense of rotation with the polarization major axis aligned mainly in the north‐south direction was seen in the local afternoon. Interstation phase analysis yielded small phase variations across the entire station array, indicating azimuthal wave numbers of 1

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In order to identify the generation mechanisms of low-latitude Pc3-4 geomagnetic pulsations, data were obtained from a meridiona! chain of inductionmagnetometers spanning L values from 1.4 to 2.7 (-30 ø to -52 ø geomagnetic latitude). The spatial structure of Pc3-4 signal parameters was examined by means of spectral, polarization and interstation phase analysis. This paper describes three typical individual events whose spectral, polarization and phase characteristics indicate the existence of field line resonances at low latitudes within the plasmasphere. The spatial phase structure shows a local minimum and indicates phase motion toward the resonance region. Resonance region widths of AL --0.2 to AL = 0.8, corresponding to north-south ionospheric scale lengths of 250 and 1500 km or more, respectively, are seen. The coupling of field line resonances to global compressional modes is considered to be a likely generation mechanism of these pulsations. ]R•FRODUCTIONIt is generally accepted that geomagnetic pulsations are a manifestation of hydromagnetic (hm) wave activity generated by a variety of physical processes resulting principally from the interaction of the solar wind plasma with the Earth's magnetic field.Several mechanisms that could account for the transfer of energy from the solar wind through the magnetosphere to the surface of the Earth have been proposed. One of these is theKelvin-Helmholtz instability, which can couple ULF energy from surface waves on the magnetopauseto field line resonances within the magnetosphere [Southwood, 1974; Chen and Hasegawa 1974a]. The transmission of upstream ion cyclotron wave energy into the magnetosphere through the subsolar or cusp regions, at times when the angle between the Sun-Earth line and the direction of the interplanetary magnetic field (IMF) is small, has also been suggested as a source of Pc34 pulsations [Yumoto and $aito, 1983; Yumoto et al., 1984, 1985; Engebretson et al., 1986, 1987]. More recently, theories of global cavity modes have been developed. These describe standing compressional waves in the magnetosphere which may couple to transverse resonances [Kivelson and Southwood, 1985, 1986; Allan et al., !985, 1986a,b; Krauss-Varban and Patel, 1988; Zhu and Kivelson, 1988 Lysak, 1989, 1991]. The global compressional mode has a discrete spectrum determined by the radial structure of the magnetosphere. The transverse mode, on the other hand, has a continuous spectrum (i.e., latitude dependent frequency) due to inhomogeneities of the magnetic field and plasma distribution. Resonances occur at positions where the eigenfrequencies of both modes match. Near a resonance, the signals are expected to display predominantly toroidal mode characteristics, that is, mainly linear polarization in the azimuthal (E-W) direction. The resonant L shell is characterized by a polarization reversal associated with a relative