Theory of nearly perpendicular electrostatic plasma waves and comparison to Freja satellite observations

The linear and nonlinear two-fluid nature of oblique electrostatic plasma waves is explored and clarified. It is found that two distinct wave regimes exist corresponding to propagation angles greater than or less than epsilon = root m(e)/m(i) with respect to the perpendicular direction of the magnetic field. Propagation angles greater than epsilon correspond to either the electrostatic ion cyclotron wave or the shorter wavelength oblique ion acoustic wave, which are termed the fast ion cyclotron and fast ion acoustic waves respectively. Angles less than epsilon correspond to the inertial Alfven wave which is called the slow ion cyclotron wave and in the short wavelength limit is called the slow ion acoustic wave. The slow ion-acoustic wave is found to have a predominately ion-Boltzmann response. Initial value simulations having fast or slow ion cyclotron wave initial conditions of modest amplitudes result in nonlinear steepening and subsequent breakup into fast or slow ion acoustic waves respectively This mechanism of ion acoustic wave generation may partially explain the origin of ion acoustic activity detected by the Freja satellite within regions of ion and electron acceleration.