Factors controlling the occurrence of Pc 3 magnetic pulsations at synchronous orbit

The dependence of the occurrence probability of Pc 3 pulsations (frequency of 0.022–0.100 Hz) at synchronous orbit on factors such as local time and solar wind parameters is studied. Power spectra calculated from the magnetic field observed at the ATS 6 satellite for a 1‐year period from June 1974 to May 1975 were examined to identify Pc 3 pulsation occurrence. Pc 3 pulsations are usually observed in the dayside magnetosphere. The average diurnal occurrence pattern is a maximum at 0900 LT, with the pattern shifting to earlier local times in winter and to later local times in summer. Solar wind velocity V SW and the direction of the interplanetary magnetic field with respect to the sun‐earth line, θ XB , significantly affect the occurrence probability. Large V SW and small θ XB are favorable for the occurrence of Pc 3 pulsations. The average occurrence probability over 0600–1600 LT changes from 10% for V SW ≲400 km/s and θ XB ≳60° to 80% for V SW ≳600 km/s and θ XB ≲20°. The result suggests that the solar wind controls Pc 3 occurrence. However, the study of the occurrence probability alone is not sufficient to distinguish between the most likely sources, that is, the magnetosheath turbulence and the Kelvin‐Helmholtz instability at the magnetopause.