From January 27 to 30, 1986, while Voyager 2 was outbound from Uranus, the planetary radio astronomy experiment recorded strong (~3--6 dB) modulations of the broadband smooth Uranian radio emission (Warwick et al., 1987 a) at frequencies below 500 kHz. These fluctuations exhibited simultaneously two different time scales (~100 s and ~10 s) and were exclusively observed outside the Uranian magnetopause during six planetary rotations. The modulations were only present during a part of the total duration of each broadband smooth episode. This phenomenon is interpreted as being due to propagation effects through the magnetopause. The long-period modulation is due to the influence of ~8000-km wavelength surface waves on Uranus' downstream magnetopause. Such waves have only been studied in detail for the inbound passage (Lepping et al., 1987). The short-period oscillation might be the signature of a second magnetopause ripple period, similar to that measured by Aubry et al. (1971) for the Earth's magnetopause. There is strong evidence that the surface wave fluctuations causing the scintillations are induced by the solar wind interaction with the magnetosphere. We discuss why these modulations are confined to certain periods of time and to a certain frequency range. ¿ American Geophysical Union 1992