We show, via an analysis of ground-based magnetic field data and fixed-frequency VLF signals propagating in the whistler mode in the magnetospheric hydromagnetic waves (having periods of ~60--120 s) observed on the ground have their wave ellipse orientations rotated by &pgr;/2 from their orientations in the magnetosphere. This analysis provides a detailed nonstatistical confirmation of recent theoretical predictions of the rotation effect of the ionosphere/atmosphere/ground on hydromagnetic waves.