A long period (~6 min) hydromagnetic wave was observed near L~4 in the southern hemisphere at both Halley Bay and Siple, Antarctica, to have a substantially larger amplitude than that observed in the northern hemisphere conjugate areas. At Halley Bay, as well as at Siple, the wave appeared quite monochromatic. Data from a station array conjugate to Siple in the northern hemisphere showed the wave to be localized near L~3.4. Whistler wave determinations of the equatorial electron densities show the wave to occur inside the plasmasphere. The wave period and localization region are no readily reconcilable with a cold, pure hydrogen plasma. The density of heavy ions that would be required within the plasmasphere to explain the observations appears excessive from present understanding of magnetosphere plasmas.