MULTIPOINT OBSERVATIONS OF A DAYSIDE TRANSIENT EVENT

Many different processes may take place at the magnetopause. To distin guish the various mechanisms requires simultaneous multipoint observat ions in the dayside magnetosphere. Such coincident observations were m ade by a number of spacecraft located in the solar wind, at the magnet opause, at geosynchronous orbit, and at polar orbit on September 23, 1 986. Beginning at 1110 UT, the B(z) component of the IMF underwent thr ee periodic cycles, each with a duration of about 7 min, and there wer e 4 solar wind pressure pulses with a period of 10 min, as recorded by the IMP 8 satellite. The ISEE satellite pair crossed the magnetopause near local noon at 1125 UT and recorded two bipolar events in the nor mal component of the magnetic field at 1115 and 1120 UT. In addition, ISEE 1 observed a bipolar event at 1118 UT that ISEE 2 did not. Period ic oscillations and compressions of the magnetic field were then subse quently seen at geosynchronous orbit in the dawn sector. Greenland mag netometer stations also located in the morning sector recorded transie nt responses to the solar wind pressure pulses. The East Coast station s (near local noon) apparently observed a different vortex at 1122 UT. Viking auroral images show an activation of a localized feature polew ard of the dayside auroral distribution at 1122 UT. An auroral enhance ment, wen at 1128 UT in the afternoon sector, moved eastward at a spee d of 5 km s-1 and decayed in intensity by 1141 UT. Clear Pc 5 pulsatio ns were seen equatorward of this activation. Differences in both in si tu measurements and ionospheric responses to the two bipolar events le ad to the conclusion that the 1118 UT event can be interpreted as bein g associated with reconnection at the same time that the magnetopause was in motion.