On the basis of the phenomenological equations, effects of a high frequency external electric field on a weakly ionized plasma are investigated. It is shown that if the frequency of the applied field is close to the electron plasma frequency, the plasma oscillation couples strongly with the ion acoustic oscillation. If in particular the intensity of the external field exceeds some threshold value, the plasma becomes unstable, namely a plasma wave and a certain low frequency wave are excited simultaneously. Two types of instability are obtained, depending on the density of the neutral atoms. At high atomic densities, the excited low frequency mode becomes nonoscillatory, while at low densities it oscillates with the frequency of the ion acoustic wave. The latter case has a threshold value which is substantially lowered compared with the former. Typical values of threshold are evaluated and are shown to range from 0.01 to 100V/cm.
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