Top side electron density data obtained during the hours of ~2100--2400 LT from the ionosphere sounder on the Isis 2 satellite are used to give indications of possible latitude gradients in the magnetospheric plasma density distributions. Studies are made of the amplitude and polarization characteristics of magnetohydrodynamic waves that occur in the frequency range of ~10--30 mHz at L~3.2 to L~4.4 during time intervals of Isis 2 passes. It is found that reversals in the sense of wave polarization occur in the vicinity of latitudes where L shell gradients are observed in the electron density distributions. The results of this study give strong support for the importance of magnetosphere plasma density gradients and the plasmapause in coupling ULF driving energy sources to locally resonant magnetospheric field lines.