Abstract
Electrostatic streaming instabilities have been proposed as the generation mechanism for the electrostatic solitary waves observed in various space plasma environments. Past studies on the subject have been mostly based on the kinetic theory and particle simulations. In this paper, we extend our recent study based on one-dimensional fluid theory and particle simulations to two-dimensional regimes for both bi-streaming and bump-on-tail streaming instabilities in electron-ion plasmas. Both linear fluid theory and kinetic simulations show that for bi-streaming instability, the oblique unstable modes tend to be suppressed by the increasing background magnetic field, while for bump-on-tail instability, the growth rates of unstable oblique modes are increased with increasing background magnetic field. For both instabilities, the fluid theory gives rise to the linear growth rates and the wavelengths of unstable modes in good agreement with those obtained from the kinetic simulations. For unmagnetized and weakly magnetized systems, the formed electrostatic structures tend to diminish after the long evolution, while for relatively stronger magnetic field cases, the solitary waves may merge and evolve to steady one-dimensional structures. Comparisons between one and two-dimensional results are made and the effects of the ion-to-electron mass ratio are also examined based on the fluid theory and kinetic simulations. The study concludes that the fluid theory plays crucial seeding roles in the kinetic evolution of electrostatic streaming instabilities.
Highlights
Electrostatic streaming instabilities have been proposed as the generation mechanism for the electrostatic solitary waves observed in various space plasma environments
We extend our recent study based on one-dimensional fluid theory and particle simulations to two-dimensional regimes for both bi-streaming and bump-on-tail streaming instabilities in electron-ion plasmas
Both linear fluid theory and kinetic simulations show that for bi-streaming instability, the oblique unstable modes tend to be suppressed by the increasing background magnetic field, while for bump-on-tail instability, the growth rates of unstable oblique modes are increased with increasing background magnetic field
Summary
Observational evidences for the existence of electrostatic solitary waves (ESWs) in space plasma environments have been overwhelming.[1–9] As shown in many studies based on particle-in-cell simulations, ESWs may be generated by the electrostatic streaming instability,[10–13] including both bistreaming and bump-on-tail instabilities associated with the ESWs observed in the auroral region and magnetotail plasma sheet boundary layer, respectively.[3–7] In multi-dimensional systems and via streaming instabilities, solitary structures, may form in a steady manner only for certain magnitudes of background magnetic field and for unmagnetized or weakly magnetized plasmas the formed solitary structures may inevitably be destroyed in the nonlinear evolution process.[14,15] The simulation results show that the formed solitary structures may tend to evolve into one- or twodimensional structures in the nonlinear evolution process of two- and three-dimensional particle simulations.[16]. Based on the linear kinetic theory and two-dimensional particle simulations, Miyake et al.[15] have examined the effects of the background magnetic field on the bump-on-tail instability. Environments, in this study we extend the model calculations to two-dimensionality and oblique propagation to examine the role of fluid theory in the evolution and formation of electrostatic solitary waves in multi-dimensional streaming instabilities. The plasma system under consideration consists of electrons and ions embedded in the background static magnetic field, which has important applications to various space plasma environments as already shown in many studies based on the kinetic simulations. For the applications to the observed ESWs occurring in the magnetosphere, two types of electron streaming instability with bi-streaming and bump-on-tail velocity distributions are examined in this study.[3,10,14–18,20–22] Comparisons between one and two-dimensional results from linear theory and nonlinear simulations are made. The study may help to clarify the role of fluid theory in the complex evolution of kinetic streaming instabilities
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