Oblique Instability of Quasi-Parallel Whistler Waves in the Presence of Cold and Warm Electron Populations

Whistler waves propagating nearly parallel to the ambient magnetic field experience a nonlinear instability that generates oblique electrostatic waves, including whistlers near the resonance cone that resemble oblique chorus in the Earth's magnetosphere. Focusing on the generation of oblique whistlers, earlier analysis of the instability is extended to the case where low-energy background plasma consists of both a "cold" population with energy ~ eV and a "warm" electron component with energy ~100 eV. This is motivated by observations in the Earth's magnetosphere where oblique chorus waves were shown to interact resonantly with the warm electrons. The main results are: i) the instability producing oblique whistlers is sensitive to the shape of the electron distribution at low energies. In the whistler range of frequencies, two distinct peaks in the growth rate are typically present: one at low wavenumbers associated with the warm population and one at high wavenumbers associated with the cold population; ii) the instability producing oblique whistler waves persists in cases where the temperature of the cold population is relatively high, including cases where cold population is absent and only the warm population is included; iii) particle-in-cell simulations show that the instability leads to heating of the background plasma and formation of characteristic resonant plateau and beam features in the electron distribution. The plateau/beam features have been previously detected in spacecraft observations of oblique chorus waves. However, they were attributed to external sources and were proposed to be the mechanism generating oblique chorus. In the present scenario, the causality link is reversed: the instability generating oblique whistler waves is shown to be a possible mechanism to generate the plateau/beam features.