A statistical analysis of transverse wave activity in the Pc 5 frequency band using satellite magnetic field data from a 6 month interval shows a symmetric distribution along the geostationary orbit in the magnetosphere, whereas observations on the ground display an asymmetric distribution with regard to the noon sector with maximum wave activity on the morningside. As a possible cause of these different local time variations of the wave activity, the ionospheric screening effect is discussed, which depends on the radial scale lengths or resonance width of the waves as well as on their azimuthal wave number. It is suggested in this study that a local time variation of the resonance width of the waves is caused by a variation of the radial gradient of the plasma density along the geostationary orbit, leading to a slowly varying efficiency of the ionospheric screening effect with local time. Local time variations of the resonance width thus will be of equal importance to explain the observed asymmetry as, for example, local time variations of wave sources causing local time variations of the azimuthal wave number. ¿ 2000 American Geophysical Union