Conjugate cusp‐region ULF pulsation responses to the solar wind event of May 23, 1989

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We have studied a hydromagnetic wave event in the early noon sector of the cusp region, using HF radar data collected in the southern hemisphere 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 interplanetary shock. This shock triggered the first pulsation, a strong ssc, at 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 pulsation was clearly differentiated from the ssc by an abrupt 180° change of phase. A further such phase discontinuity at 1434 UT (near noon MLT) marked the start of a third distinct ULF pulsation with a clearly 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 observations with ground‐based magnetometer measurements; the pulsation current systems in the northern and southern hemispheres were found to be highly correlated and conjugate. The 3.3‐mHz pulsation was observed over a range of ∼10° in invariant latitude, and its amplitude showed a strong, broad maximum close to the latitude of the cusp as inferred from an independent radar‐satellite analysis of cusp signatures. The zonal E × B motion associated with the pulsation showed a linear latitudinal decrease in phase (25° per degree of latitude) while the meridional motion had nearly constant phase. The longitudinal phase variation corresponded to an m value of ∼10 with a source at later MLT. We discuss the possibility that the dominant 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.