SpaceWeb@SkyNet.oulu.fi - last update: 25 July 1997, 1200 UT (RR) . Compressional Pc 3 are daytime, relatively low amplitude (a few nT) fluctuations of field amplitude with typical periods of 20-30 s ( ULF range). ... The correlations with the IMF cone angle suggest that the source population of waves is associated with the quasi-parallel region of the bow shock (the magnetopause is exposed to quasi-parallel region of the shock only for sufficiently radial IMF). ... Space Physics Text Book of Oulu ...