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.
Abstract. We investigate the electrodynamic coupling between auroral omega bands and the inner magnetosphere. The goal of this study is to determine the features to which omega bands map in the magnetosphere. To establish the auroralmagnetosphere connection, we appeal to the case study analysis of the data rich
Bistatic Auroral Radar System radar events showing a repetitive equatorward (or radarward) traveling band signature in the range‐time‐intensity plots have been analyzed. The associated pulsations have many features in common with previously reported high‐m storm time Pc 5 pulsations. The periods, azimuthal wave numbers (m), Dst dependence, and polarization ellipses are similar. One difference is also seen. The pulsation phase velocity of our events often has a significant equatorward component in the ionosphere. However, of the three cases for which the phase velocity direction in the magnetospheric equatorial plane was determined, one had phase velocities close to azimuthal at both locations, one had an azimuthal phase velocity in the magnetosphere but an equatorward component of the phase velocity in the ionosphere, and the final event had equatorward or inward components of phase velocity at both locations. These events are very similar to the equatorward traveling bands events observed by the Sweden and Britain Auroral Radar Experiment system (Tian et al., 1991; Yeoman et al., 1992) except that our m values are distinctly larger. If coupling to energetic protons is involved in exciting these pulsations, this difference implies that the energy of the protons involved decreases with increasing L of the interaction location. Finally, our data do not lend themselves to the plasmapause field line resonance interpretation of Tian et al. (1991).
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