The only remote diagnostic of conditions within the outer planets' magnetospheres is the highly variable flux of low-frequency radio waves. As at the other radio planets, Neptune radio emission also manifests, on a time scale of days, major intensity fluctuations that are indicative of a solar wind energy-coupling process of some kind. We find that the merging of interplanetary magnetic field lines with Neptune's magnetosphere is the best predictor of emitted radio energy. By contrast, viscouslike energy coupling processes, such as might be caused by solar wind density of bulk speed fluctuations, are apparently ineffective in driving the radio emission. ¿American Geophysical Union 1991