A new technique is used to measure F region plamsa drifts at night. The technique involves observing the motion of density cavities generated by high-power radio waves in the ionosphere. The airglow signature of the cavities is recorded and analyzed with a ground-based, low-light-level imaging system. The heater-induced cavity optical tracer method optical tracer method is capable of providing velocity information with an error less than 1 m/s and a time resolution better than 60 s. ¿ American Geophysical Union 1989