POLARIZATION, PROPAGATION AND MHD WAVE MODES OF PI2 PULSATIONS - SABRE SAMNET RESULTS

The SABRE coherent radar has been employed in conjunction with the SAMNET magnetometer array to examine the wave characteristics of Pi2 pulsations at auroral/sub-auroral latitudes. The latitude ranges of the two systems have a small overlap, with the SABRE field of view being predominantly poleward of the magnetometer array. The Pi2 pulsations are here divided into two types. For waves in the SABRE field of view which have a compressional nature, as deduced by the ratio of the pulsation backscatter intensity to the pulsation line of sight Doppler velocity, similar wave period and propagation are observed on SABRE and SAMNET. These waves generally correlate with substorms where the SABRE/SAMNET system is distant from the substorm ionospheric currents, as deduced by the bays in the Z component magnetograms. In contrast two events have an Alfvenic mode structure in the SABRE field of view, with large ionospheric velocity modulations. These events have different periods and opposite azimuthal propagation on SABRE than at lower latitudes. The Alfvenic waves generally correlate with substorm breakups relatively close to the SABRE and SAMNET systems, as deduced from the Z component bays. The background ionospheric flow directions are also considered, and it is found that the flow inferred by SABRE often does not change at the onset of the substorm expansion phase, even when the substorm current wedge occurs in the SABRE local time sector. This indicates that the radar field of view is generally equatorward of the substorm enhanced ionospheric current system. It is concluded that Pi2 pulsations consist of an auroral zone Alfven wave and a lower latitude compressional mode, possibly due to a cavity mode wave source. Furthermore, sub-auroral latitudes are a transition region between these two characteristic wave types and the SABRE radar provides a technique to distinguish between the two sources.