CONJUGATE CUSP-REGION ULF PULSATION RESPONSES TO THE SOLAR-WIND EVENTOF MAY 23, 1989

We have studied a hydromagnetic wave event in the early noon sector of the cusp region, using HF radar data collected in the southern hemisp here and ground-based magnetic data from both hemispheres, There were three distinct pulsations, which appear to have been driven by changes in the solar wind, beginning with the passage of a powerful interplan etary shock, This shock triggered the first pulsation, a strong ssc, a t 1346 UT, It was followed at 1402 UT by a second pulsation, which had a stable dominant frequency of 3.3 mHz and lasted about half an hour. Although this frequency was close to that of the ssc, the new pulsati on was clearly differentiated from the ssc by an abrupt 180 degrees ch ange of phase, A further such phase discontinuity at 1434 UT (near noo n MLT) marked the start of a third distinct ULF pulsation with a clear ly lower frequency of 2.8 mHz, which continued for another half hour, until 1505 UT, and was correlated with the magnitude of the IMF. These latter two pulsations yielded a successful calibration of the radar o bservations with ground-based magnetometer measurements; the pulsation current systems in the northern and southern hemispheres were found t o be highly correlated and conjugate. The 3.3-mHz pulsation was observ ed over a range of similar to 10 degrees in invariant latitude, and it s amplitude showed a strong, broad maximum close to the latitude of th e cusp as inferred from an independent radar-satellite analysis of cus p signatures, The zonal E x B motion associated with the pulsation sho wed a linear latitudinal decrease in phase (25 degrees per degree of l atitude) while the meridional motion had nearly constant phase. The lo ngitudinal phase variation corresponded to an m value of similar to 10 with a source at later MLT. We discuss the possibility that the domin ant wave activity near 3 mHz in the latter two pulsations was due to a hydromagnetic surface wave at the magnetopause stimulated by the ssc and modified by the forcing action of the solar wind.