Pc 3 and Pc 4 activity during a long period of low interplanetary magnetic field cone angle as detected across the Institute of Geological Sciences Array

Strong Pc 3 and Pc 4 waves were recorded across the Institute of Geological Sciences ground magnetometer array for a period of about 12 hours on August 20, 1978, when the cone angle of the interplanetary magnetic field was less than 45°. The peak frequencies in the Pc 3 band varied little from station to station and were approximately the same as the frequency of the upstream waves simultaneously observed from ISEE 1 and 2. However, only certain pairs of stations saw signal waveforms having high coherence. In contrast, the waves in the Pc 4 band were seen on the surface of the Earth only at low latitudes and were not enhanced in the upstream waves. The peak frequency of Pc 4 waves on the ground was a strong function of latitude. With the exception of the ending one hour early of Pc 4 activity at a single station, both Pc 3 and Pc 4 signals ceased abruptly when the interplanetary magnetic field (IMF) cone angle increased to large angles. Although these observations exhibit the global control of occurrence of Pc 3 and Pc 4 waves by the IMF, the cross correlation analysis of the temporal variations of Pc 3 and Pc 4 power implies that the Pc 4 waves were from the nightside or the flank of the magnetosphere, while the Pc 3 waves were from the subsolar region.