Various parametric decay channels of lower hybrid waves (LHW) in a topside ionospheric plasma are considered in order to find out whether VLF waves generated by precipitating keV electrons could be converted into waves having larger perpendicular wave number and thus interacting more effectively with ionospheric ions. In dipole approximation it is found that the most promising decay channel is a decay into a LHW and a nonresonant quasi-mode. However, use of the dipole approximation can be criticized, and a consideration of the wave vector of the pump suggests that also the decay into hot ion plasma waves or ion Bernstein waves and nonresonant quasi-modes is of importance. It is also noticed that the theory of parametric processes could be applied in interpretation of certain sounding rocket data where artificially produced LHW's are detected together with various other wave modes.