We present evidence for the occasional observations of rapid (few minute) changes in the localization region of hydromagnetic waves in the earth's magnetosphere. These changes in the wave localization region, which can last for several hours, can be used to deduce the cold plasma density distributions in the magnetosphere in the L range of observations of the wave resonance location. We find that the plasma density distribution can vary as L-8 in the range L=3.2 to L=4.4 during the periods. The plasmapause is likely to be in the vicinity of the observing stations at these times. The cause of these variations in the wave resonance location remains unknown.