VLF wave data from the ISIS 2 satellite has revealed the existence of a new phenomenon in which coherent VLF signals from the Siple Station, Antarctica, VLF transmitter are observed to trigger a new type of VLF emission as these signals propagate upward to the satellite at 1400 km altitude through the ionosphere and low-altitude magnetosphere. The emissions have the form of band-limited impulses of approximately 20--30 ms duration. The bandwidth of the emissions is as much as 1 kHz and their amplitude is as much as 20 dB above that of the triggering signal. The emissions are thought to be the result of a rapidly evolving quasi electrostatic plasma instability triggered by the transmitter signals. The effect occurs generally when the transmitter signals lie just below a band of impulsive VLF hiss which is commonly observed to occur in the subauroral region poleward of the plasmapause. The impulsive VLF hiss band has a lower cutoff frequenccy in the range 3--4 kHz and appears to be the subauroral extension of the well-known auroral VLF impulsive hiss band. This phenomenon is possibly the transient analog of the recently reported spectral broadening effect [Bell et al., 1983b> but the connection is not clear. In the present paper we show examples of this new phenomenon and delineate the conditions under which it occurs. The role of precipitating energetic particles in producing the phenomenon is discussed.