The quantitative theory is developed for the frequency spectrum broadening and sideband structure of VLF signals in the upper ionosphere. The theory is based on a common theory of wave-wave interaction in plasma and includes the transformation effects of a quasi-monochromatic whistler wave into quasi-electrostatic whistler mode waves (l waves) near the resonance cone with their interaction with field-aligned plasma density (electric field) inhomogeneities. Such inhomogeneities can be ion-acoustic or ion cyclotron electrostatic waves. We consider both cases of regular (deterministic phases) and irregular (random phases) inhomogeneities. The spectral characteristics and interaction length of l waves and some nonstationary effects are considered, which permit us to obtain the quantitative interpretation of main experimental data. The conditions of high coherence of interacting waves are investigated, which are compared with the results of bicoherence analysis of sideband structure of VLF signals. The estimations of bicoherence value are given for such type of wave-wave interaction. ¿ American Geophysical Union 1993