High-frequency electrostatic waves in the magnetosphere

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.