A combination of simulated VLF noise, 200 Hz wide, superimposed on a variable amplitude constant frequency test signal (≈3 kHz) is transmitted from Siple Station, Antarctica, and received at Lake Mistissini, Quebec. As the test signal is slowly ramped up in power a ''threshold'' level is reached at which growth and triggering of emissions begin (coherent wave instability). Results show that sufficiently strong simulated noise surpresses the coherent wave instability, which corresponds to increasing the threshold level. This experiment is repeated at progressively lower levels of the simulated noise, until the threshold level for growth and triggering on the test signal no longer changes. At this point the simulated noise power is estimated to equal typical background noise levels due to magnetospheric hiss in the interaction region (near the equatorial plane in a duct at L≈4). These results suggest that unducted magnetospheric hiss is responsible for the threshold effect. ¿ American Geophysical Union 1992