ELF-VLF (0.3--30 kHz) wave data measured at ground-based observatories for 250 sudden commencements were analyzed for amplitude and spectral modifications and correlated with magnetic field and precipitating particle observations. Changes in ELF-VLF wave activity at high-latitude stations were observed in 50--60% of the events studied and for approximately 80% of the events when the observing station was on the dayside. Characteristic, well-defined modifications of both coherent and incoherent ELF-VLF wave emissions were observed, including wave growth of the order of 20 dB, increases in the upper frequency limit of the waves, and enhanced triggering of discrete emissions. Wave growth generally occurred first at lower frequencies and with increasing delay at upper frequencies. The growth rate for the incoherent wave emissions (0.3--2.7 dB/s) was found to be at least 2 orders of magnitudes less than known growth rates for coherent waves. Measurable particle precipitation inferred form cosmic noise signal absorption was observed to begin simultaneous to within 5 s of the wave growth onset in a significant number of cases, suggesting that at least a part of the observed precipitaiton results from wave-induced scattering. ¿ American Geophysical Union 1990