The effect of the ionosphere on the detectability, polarization, structure and decay of low frequency magnetospheric hydromagnetic waves has been a topic of interest for thirty years. There are two classes of signal behavior in the atmosphere and ionosphere, one where the ionosphere has little effect and one where the ionospheric conductivity controls the reflection, the amplitude detected on the ground and rate of signal decay. It has been assumed that the latter type of behavior is pertinent in most practical applications. Recent interest in global modes of the magnetospheric cavity requires a reassessment of the assumption. We investigate one simple case, the hydromagneic surface wave, and discuss what happens in more complicated modes. We conclude that signals in the vicinity of field line resonance will be strongly coupled to the local ionosphere but far away will not. ¿ American Geophysical Union 1988