Electrostatic solitary waves (ESWs) have been observed in the Earth's magnetosphere, solar wind, lunar wake, and also in other planetary magnetospheres. The observed characteristics of the ESWs have been interpreted in terms of models based either on Bernstein-Green-Kruskal (BGK) modes/phase space holes or ion- and electron-acoustic solitons. However, the space community has favored the models based on BGK modes/phase space holes. In this review, current understanding of the fluid models for ion-and electron-acoustic solitons and double layers in multi-component plasmas is presented. The relationship between the theoretical models and space observations of ESWs is emphasized. Two specific applications of ion- and electron-acoustic solitons to the occurrence of weak double layers and coherent electrostatic waves in the solar wind and the lunar wake are discussed by comparing the observations and theoretical predictions. It is concluded that models based on ion- and electron-acoustic solitons/double layers provide a plausible interpretation for the ESWs observed in space plasmas.
A linear analysis of electrostatic waves propagating parallel to the ambient field in a four component homogeneous, collisionless, magnetised plasma comprising fluid protons, fluid He++, electron beam, and suprathermal electrons following kappa distribution is presented. In the absence of electron beam streaming, numerical analysis of the dispersion relation shows six modes: two electron acoustic modes (modes 1 and 6), two fast ion acoustic modes (modes 2 and 5), and two slow ion acoustic modes (modes 3 and 4). The modes 1, 2 and 3 and modes 4, 5, and 6 have positive and negative phase speeds, respectively. With an increase in electron beam speed, the mode 6 gets affected the most and the phase speed turns positive from negative. The mode 6 thus starts to merge with modes 2 and 3 and generates the electron beam driven fast and slow ion acoustic waves unstable with a finite growth. The electron beam driven slow ion-acoustic waves occur at lower wavenumbers, whereas fast ion-acoustic waves occur at a large value of wavenumbers. The effect of various other parameters has also been studied. We have applied this analysis to the electrostatic waves observed in lunar wake during the first flyby of the ARTEMIS mission. The analysis shows that the low (high) frequency waves observed in the lunar wake could be the electron beam driven slow (fast) ion-acoustic modes.
The coupling of electrostatic ion cyclotron and ion acoustic waves is examined in three component magnetized plasma consisting of electrons, protons, and alpha particles. In the theoretical model relevant to solar wind plasma, electrons are assumed to be superthermal with kappa distribution and protons as well as alpha particles follow the fluid dynamical equations. A general linear dispersion relation is derived for such a plasma system which is analyzed both analytically and numerically. For parallel propagation, electrostatic ion cyclotron (proton and helium cyclotron) and ion acoustic (slow and fast) modes are decoupled. For oblique propagation, coupling between the cyclotron and acoustic modes occurs. Furthermore, when the angle of propagation is increased, the separation between acoustic and cyclotron modes increases which is an indication of weaker coupling at large angle of propagation. For perpendicular propagation, only cyclotron modes are observed. The effect of various parameters such as number density and temperature of alpha particles and superthermality on dispersion characteristics is examined in details. The coupling between various modes occurs for small values of wavenumber.
Kinetic dispersion of the ion acoustic waves has been explored for an unmagnetized five component plasma system comprising of Venusian protons, Venusian oxygen ions, Venusian electrons, solar wind protons, and kappa electrons. The solar wind protons and electrons are assumed to be streaming along the ambient magnetic field. The plasma parameters for this study have been obtained from Lundin et al. [Icarus 215(2), 751–758 (2011)] for the dawn dusk meridian of Venus Express with the data from the ASPERA-4 ion mass analyzer. Our analysis revealed that two modes, viz., ion acoustic mode and beam driven mode, are excited for the considered plasma parameters. The ion acoustic mode exists due to the Venusian ions, and its growth rate is influenced by the solar wind beam electrons. The beam driven mode's existence and its growth rate depend on the solar wind beam protons. We conjecture that the ion acoustic mode and the beam driven mode could be useful in explaining the electrostatic noise in the Venusian ionosphere in the range of several hundreds Hz to 1 kHz and several tens kHz, respectively.
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