The triggering and saturation mechanisms of the electrostatic waves having frequencies near 1½ 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.

High-frequency electrostatic microinstabilities in magnetospheric plasmas are considered in detail. Rather special plasma parameters are found to be required to match the theoretical wave spectrum with satellite observations in the magnetosphere. In particular it is necessary to have a cold and a warm species of electrons such that (1) the warm component has an anomalous velocity distribution function that is nonmonotonic in υ⊥ and is the source of free energy driving the instabilities, (2) the density ratio of the cold component to the hot component is greater than about 10−2, and (3) the temperature ratio of the two components for cases of high particle density is no less than 0.1. These requirements and the corresponding instability criteria are satisfied only in the trapping region (4 ≤ L ≤ 10); this is also the region in which the waves are most frequently observed. The range of unstable wavelengths and an estimate of the diffusion coefficient are also obtained. The waves are found to induce strong diffusion in velocity space for low-energy electrons (≲1 kev) during periods of moderate wave amplitude (≲10 mv/m). Electrons with energies of up to 100 kev can be strongly diffused when the wave amplitude is large (∼100 mv/m). Geophysical implications are discussed; predicted results compare favorably with available observations pertaining to electrostatic waves, the particle distribution function, particle acceleration, and pitch angle diffusion in the magnetosphere.

It is demonstrated that it is the width of the positive derivative region of the υ⊥ distribution function which determines whether the flutelike microinstabilities of a magnetized plasma occur at high densities.