The triggering and saturation mechanisms of the electrostatic waves having frequencies near 1 1/2 times the electron gyrofrequency in the magnetosphere are studied. Also studied is the corresponding evolution of the electron distribution function caused by the wave-particle interaction. The linear instabilities supporting these waves are found to require a cold species of electrons having density between 0.1 cm-3 and 1 cm-3 and a warm species whose distribution function has a relatively weak positive slope in the velocity space perpendicular to the background magnetic field. The corresponding unstable wavelengths are short and thus result in much smaller diffusion coefficients than those derived previously. Such instabilities when triggered by an enhanced injection of ionospheric cold electrons into the warm plasma clouds can support wave amplitudes that grow to nonlinear saturation levels much greater than 10 mV/m and as large as a few hundred millivolts per meter. The majority of the observed wave activity (1-10 mV/m) can be explained as marginal instabilities maintained by a convection-induced slow increase of the cold electrons in the plasma clouds. The evolution of the waves and plasma is described in a simple physical model for different plasma parameters.