Nature and the nonlinear evolution of electrostatic waves associated with the AMPTE solar wind releases

Abstract

To understand the generation and the nonlinear evolution of the electrostatic waves observed during the AMPTE (Active Magnetosphere Particle Tracer Explorers) solar wind releases, a detailed investigation is conducted. Previous linear studies have indicated that two distinct sets of instabilities may be responsible for the generation of these waves. One set consists of ion acoustic type instabilities which are insensitive to the presence of a background magnetic field, while the other group corresponds to the modified two‐stream instabilities and requires the solar wind flow to be across the ambient magnetic field. In order to establish which set of instabilities are more viable for the generation of the observed electrostatic waves a detailed linear Vlasov theory has been conducted by numerically solving the full electromagnetic dispersion relation. In addition, both the plasma wave and the magnetic field measurements by the IRM (Ion Release Module) spacecraft were used to correlate the frequency and the power of the observed waves with the magnitude and the direction of the solar wind magnetic field. The results of these analyses indicate that the ion acoustic type instabilities have growth rates that are an order of magnitude or more larger than those of the modified two‐stream instabilities. Similarly, the observations show no correlation between the magnitude and the direction of the ambient magnetic field on the one hand and the wave frequency and amplitude on the other hand, thus indicating that the ion acoustic type instabilities are the likely generation mechanism. In order to investigate the nonlinear evolution of these instabilities and discern their role in the coupling of the released ions to the solar wind, one‐dimensional full particle as well as fluid electron electrostatic simulations have been performed. The results show that both the solar wind protons and the released ions can be heated and accelerated in the directions oblique to the solar wind flow velocity.