Nonlinear dynamics of the Farley-Buneman modes is investigated. It is shown that in the nonlinear regime, there exists a secondary nonlinear instability leading to excitation of low-frequency, long-wavelength modes that corresponds to the inverse energy cascade toward the longer wavelengths. The growth rate of the secondary instability is proportional to the squared amplitude of primary electron density fluctuations. ¿ 2001 American Geophysical Union