Observations of incoherent scattering of electromagnetic waves at UHF from Langmuir waves by a new scheme involving linear frequency modulation (chirping) of a UHF transmitter and the demodulation (dechirping) of the received signals have been applied during HF heating experiments. These observations show that the high power HF wave used for ionospheric modification creates small-scale plasma depletions instantly on a time scale of 5 ms. For a plasma frequency of 5.1 MHz, plasma frequency gradient of the order of 50 kHz/km, and power density input of the HF heater wave of 8.0¿10-5 W/m2 the depletion ranged from 3 to 5%. This appears to provide direct evidence that the HF-induced modifications involve Langmuir waves trapped in density cavities. ¿American Geophysical Union 1987