Pc 3 pulsations were recorded at three stations along the L≂1. 78 shell and spanning 35¿ in longitude during July 1983. The pulsations recorded at Gough Island (10¿W) had higher frequencies than those recorded at two stations in South Africa (20¿E and 25¿E). Local time effects or small differences in the magnetic field configuration do not account for the differences in resonant frequency. We conclude that the higher pulsation frequency at Gough Island is due to a lower plasma mass density on that field line relative to the others and that oxygen plays a significant role. Factors which may contribute to the lower plasma mass density are the lower rate of production of O+ by solar ultraviolet radiation, F region heating by energetic particle precipitation, and the greater loss of O+ caused by meridional winds. ¿American Geophysical Union 1987