Low-frequency electrostatic fluctuations in the polar cap E region were detected by an instrumented rocket payload. The basic characteristics of the fluctuations, such as speed and direction of propagation, were obtained by a correlation analysis. The bispectrum, in particular, provided indications of nonlinear couplings. Harmonic wave interactions were observed for a wide altitude range. Evidence for cascading of wave energy toward small wavelengths was obtained. The accuracy of the statistical analysis was tested by a Monte Carlo simulation. ¿American Geophysical Union 1993