We present the results of a study of Pc1 waves (0.2–5 Hz) recorded by the three spacecraft of NASA's Space Technology 5 (ST5) mission, which operated in a dawn‐dusk, 300 × 4500 km Sun‐synchronous orbit in a “string‐of‐pearls” configuration from 26 March through 23 June 2006. Regions with Pc1 wave activity are not only localized to rather narrow L shells but can appear and disappear on the time scales of ∼10 s to 10 min as examined by ST5. Only half of the 48 identified events were observed by all three spacecraft, and five events were observed by only one spacecraft. Only seven events were observed below L = 4, and only one was observed below L = 3.6, consistent with the relatively quiet geomagnetic conditions during this interval. The temporal occurrence distribution of Pc1 events was similar to that recorded at Halley, Antarctica (L = 4.56), during this same interval in that the number and intensity of events increased during magnetospheric compressions and during the recovery phase of magnetic storms, but they were reduced or absent during main phase and early recovery phase. This agreement suggests that if Pc1 events occur during main phase, their nearly universal absence in ground records cannot be ascribed to ionospheric screening effects or obscuration by irregular ULF noise generated in the ionosphere. These findings also suggest that although electromagnetic ion cyclotron waves might theoretically cause rapid depletion of radiation belt electrons during the main phase of storms, such waves cannot be assumed to occur during the main phase of all storms.
