A Case Study of the 19 October 1989 Polar Cap Absorption Event Using the Imaging Riometer for Ionospheric Studies

Abstract : A polar cap absorption (PCA) event, beginning on 19 October 1989, is examined using the Imaging Riometer for Ionospheric Studies (IRIS) developed at the Univ. of Maryland. IRIS is a 49-beam (7 x 7) phased-array 38.2 MHz radiowave imaging system and operating at Amundsen-Scott South Pole Station, Antarctica. After applying a low-pass (>10 minute) filter, we perform a cross-correlation between the central beam and the other beams in the array. From the cross- correlation analysis, we attempt to determine whether there is any delay of effects during event onset and early main phase (plateau) within the array. For most of the period studied, no delay was detected; i.e. features of each beam's absorption time series were essentially simultaneous. However, significant delays (approx. 2 minutes) occurred at the equatorward edge during a local absorption peak occurring just prior to magnetic local noon. The peak is coincident with short-term energetic particle flux enhancements detected by GOES-7. Two possible mechanisms are proposed: a local time variation of cutoff rigidities possibly associated with the relative position of the dayside cusp, and a short-term change in energetic particle rigidity spectrum.