The longitudinal variation of mid-latitude hiss as measured on six balloons at latitudes of 35¿S--55¿S, shows a significant minimum at 70¿E--80¿E, about 1000 km east of the geomagnetic conjugate of the Soviet transmitter, UMS (17.1 kHz). It is suggested that the well documented pitch angle diffusion induced by the UMS signal removes the ability of the trapped electrons to maintain the amplification of the hiss to at least the reflection and re-ducting losses (~20 dB) at the ends of the echoing, ducted path. The eastward shift of the minimum from the conjugate of UMS is primarily caused by the time taken by the eastward drifting electrons to recover this ability, though the effect is enhanced by the east-west asymmetry in the propagation of hiss under the ionosphere. ¿ American Geophysical Union