A study of Pc5 hydromagnetic waves with equatorward phase propagation

Abstract

Equatorward propagating bands of auroral radar backscatter of period 150–600 s are a commonly observed phenomenon on the SABRE VHF coherent radar, but until very recently were rarely observed on other coherent radar systems located at different latitudes. A recent statistical study (Mao Tian et al., 1991, Planet. Space Sci. 39, 1239) provided strong evidence that such propagating bands on SABRE were geomagnetic pulsations with resonant structures on the plasmapause. Here a detailed analysis of such pulsations observed by SABRE, together with the SAMNET and EISCAT cross magnetometer arrays, is presented. The pulsations have rapid westward azimuthal phase propagation independent of local time and a latitudinal phase change in both geographic and geomagnetic coordinate systems which, on average, exceeds the 180° phase change predicted by field line resonance theory. Furthermore, the events exhibit a strong attenuation between their ionospheric signature and their ground magnetometer signature. These results are consistent with an excitation mechanism involving wave-particle interactions, similar to that postulated for storm-time Pc5 pulsation events. The azimuthal phase propagation of the pulsations, if used to calculate a drift velocity mapped out to the equatorial plane, is consistent with that of protons of energy 35–70 keV undergoing gradient-curvature drift. It is suggested that these particles drift westwards and interact with the dusk plasmapause region, producing a resonant wave which is detected by the coherent radar as an equatorward propagating pulsation event.