Multistation Observations of Pc1-2 ULF Pulsations in the Vicinity of the Polar Cusp

Geomagnetic pulsations with frequencies in the range 0.1-2.5 Hz have been recorded with a six station Antarctic magnetometer array ranging in latitude from -62.3 to -80.6° invariant. The observed spectral characteristics were compared with the cusp and boundary layer locations for selected days in November and December, 1986, and July, 1990, in order to determine wave source regions. Specific pulsation features have been identified with different magnetospheric regions. The cusp is characterised by intense unstructured 0.15-0.45 Hz noise which diminishes rapidly away from this region. These signals are likely to result from a mixture of ion-cyclotron, Kelvin-Helmholtz and drift-wave instabilities. Narrowband unstructured Pc2 emissions typically around 0.2 Hz and short discrete Pc1 packets or bursts with frequencies below 0.4 Hz, occur in the local morning at boundary layer latitudes, accompanied by generally enhanced power levels between about 0.1 and 0.5 Hz. These emissions are probably due to ion-cyclotron waves generated near or below the He+ gyrofrequency on boundary layer field lines. Propagation of such signals into the polar cap via the ionospheric waveguide is uncommon, perhaps due to the existence of severe ionospheric irregularity features near the dayside auroral oval. Structured and quasi-structured emissions such as hydromagnetic chorus are recorded at the foot of plasmatrough field lines and are consistent with ion-cyclotron wave generation and propagation on closed field lines in the outer magnetosphere.