Abstract
Abstract. Lower-hybrid (LH) oscillitons reveal one aspect of geocomplexities. They have been observed by rockets and satellites in various regions in geospace. They are extraordinary solitary waves the envelop of which has a relatively longer period, while the amplitude is modulated violently by embedded oscillations of much shorter periods. We employ a two-fluid (electron-ion) slab model in a Cartesian geometry to expose the excitation of LH oscillitons. Relying on a set of self-similar equations, we first produce, as a reference, the well-known three shapes (sinusoidal, sawtooth, and spiky or bipolar) of parallel-propagating ion-acoustic (IA) solitary structures in the absence of electron inertia, along with their Fast Fourier Transform (FFT) power spectra. The study is then expanded to illustrate distorted structures of the IA modes by taking into account all the three components of variables. In this case, the ion-cyclotron (IC) mode comes into play. Furthermore, the electron inertia is incorporated in the equations. It is found that the inertia modulates the coupled IA/IC envelops to produce LH oscillitons. The newly excited structures are characterized by a normal low-frequency IC solitary envelop embedded by high-frequency, small-amplitude LH oscillations which are superimposed upon by higher-frequency but smaller-amplitude IA ingredients. The oscillitons are shown to be sensitive to several input parameters (e.g., the Mach number, the electron-ion mass/temperature ratios, and the electron thermal speed). Interestingly, whenever a LH oscilliton is triggered, there occurs a density cavity the depth of which can reach up to 20% of the background density, along with density humps on both sides of the cavity. Unexpectedly, a mode at much lower frequencies is also found beyond the IC band. Future studies are finally highlighted. The appendices give a general dispersion relation and specific ones of linear modes relevant to all the nonlinear modes encountered in the text.
Highlights
Nonlinear waves have increasingly drawn much attention in the study of geocomplexities in last decades
In order to provide the most basic picture for the emergence and propagation of nonlinear solitary waves, and to gain important insights into the effects of electron inertia on the features of solitary structures, while still being able to illustrate the process clearly, we focus on a system composed of two components: isothermal electrons and adiabatic ions
Inspired by Sauer et al (2003)’s study in a dusty plasma case where an addition of a second ion population in a single-ion plasma leads to significant modifications of solitary waves, we specialize Kourakis and Shukla (2005) model to explain the formation of abundantly observed LH-oscillitons in space plasmas
Summary
Nonlinear waves have increasingly drawn much attention in the study of geocomplexities in last decades. In order to provide the most basic picture for the emergence and propagation of nonlinear solitary waves, and to gain important insights into the effects of electron inertia on the features of solitary structures, while still being able to illustrate the process clearly, we focus on a system composed of two components: isothermal electrons and adiabatic ions They are described by two-fluid equations under collision-free conditions in the Cartesian frame (x,y,z), including conservation equations of mass, momentum, and energy, plus four Maxwell’s equations. Based on these studies we focus on the evolving patterns of oscillitons to show the modulation of the electron inertia on lowfrequency solitary waves by introducing high-frequency oscillations into amplitudes
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