A comparison between Pc 3–4 pulsations observed by GOES 7 and the CANOPUS magnetometer array

Abstract

We have analyzed 1 month of GOES 7 and Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer data and compared Pc 3–4 wave activity seen at geosynchronous orbit with simultaneous ground Pc 3–4 pulsations at auroral latitudes. We found that at geosynchronous orbit the majority of coherent Pc 3–4 events were azimuthally or radially polarized. A number of transverse events revealed clear field line resonance (FLR) signatures in the latitudinal power, polarization, and phase variations. However, the FLR characteristics often appeared to be masked by ionospheric screening and/or wave propagation effects. The azimuthal phase propagation was found to be antisunward for most events, although sunward phase propagation has also been observed. The ground Pc 3–4 occurrence, polarization, and longitudinal phase characteristics associated with transverse waves were found to be similar to those of auroral latitude Pc 5 pulsations. These similarities between transverse Pc 3–4 and auroral latitude Pc 5 characteristics may point to a common source mechanism for these pulsations. One event appeared consistent with the excitation of multiple waveguide modes. Most compressional Pc 3–4 activity appeared to be relatively broadband and virtually unpolarized. However, the center frequency of the dominant band was found to be well correlated with the interplanetary magnetic field magnitude. Power spectra from the ground stations usually contained significant signal power within the frequency band of the compressional perturbation seen by GOES 7. It is shown that at some frequencies, the broadband compressional waves may excite local field line resonances, whereas at other frequencies the compressional signal is transmitted to the ground without obvious resonance structures. The ground Pc 3–4 occurrence, spectral, polarization, and longitudinal phase characteristics associated with compressional waves were found to differ in several respects from the signal characteristics associated with transverse waves. These differences between transverse and compressional Pc 3–4 waves argue against upstream waves being the sole source of auroral latitude Pc 3–4 pulsations.