Linear mode coupling is often invoked to explain the conversion of electrostatic upper hybrid waves into terrestrial myriametric radiation. The efficiency of this conversion process can be studied by recently developed methods, which are based on the continuity equation for the wave distribution function. These estimates show that the efficiency of linear mode conversion is at least three orders of magnitude too low to account for the myriametric radiation observed in the magnetosphere. ¿ American Geophysical Union 1989