Lower-hybrid (LH) oscillitons evolved from ion-acoustic (IA)/ion-cyclotron (IC) solitary waves: effect of electron inertia

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 highfrequency, 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 electronion 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 areCorrespondence to: J. Z. G. Ma (johnzg.ma@asc-csa.gc.ca) 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.

Whistler-Langmuir oscillitons and their relation to auroral hiss

“…A significant feature of this mode is the appearance of a phase velocity maximum at finite wave numbers (kc/ω e < 1), which results from the different (phase velocity-wave number) dependence of both merging modes, and the associated gap above it. As known from previous studies of nonlinear stationary structures on other wave branches (multi-ion waves, whistler waves in plasmas with G < 1 and lowerhybrid waves; see Sauer et al, 2001;Dubinin et al, 2003;Ma and Hirose, 2010), such mode characteristics are favorable for the existence of oscillitons. For obvious reasons, we use the term whistler-Langmuir oscilliton to describe the mixed mode nonlinear stationary wave structure considered in this work.…”

“…Ma and Hirose (2010) have shown that lower-hybrid (LH) oscillitons may evolve from ion-acoustic/ion-cyclotron solitary waves if in the underlying set of self-similar equations the effect of electron inertia is taken into account. Furthermore, the relevance of the LH solitary structures in explaining small-scale density depletions observed by rocket and satellite experiments has been discussed.…”

Nonlinear electrostatic ion-acoustic &quot;oscilliton&quot; waves driven by charge non-neutrality effects

“…On the contrary, if the condition is not met, just as in regions at higher altitudes in geospace, Ma and Hirose (2010) illustrated, in the same geometry and in the presence of the electron inertia, that the multi-frequency "oscilliton" structures are driven, which features an LF solitary envelope in the IC mode, while the HF oscillations come from the lower-hybrid (LH) mode. In addition, the IA constituent, which has a higher frequency but much smaller amplitude, superimposes upon the LH mode.…”

“…Nevertheless, similar to the charge non-neutrality condition, inertia does play an important role for the excitation of oscillitons, as argued already by McKenzie, Verheest, and their co-workers (e.g., McKenzie, 2002;Verheest, 2005, and references therein). In a related paper, Ma and Hirose (2010) illustrated the effect of the electron mass on the formation of nonlinear LH oscillitons, following their work on linear LH waves . A noticeable result is that the electron mass drives IA/IC solitary waves to evolve into nonlinear LH oscillitons.…”

“…We thus exclude the magnetic filed variation (i.e., under an electrostatic condition which is in agreement with the low β condition), and neglect the electron inertia due to m e m i , in the present article, so as to minimize external distractions from the focus on the effect of the charge nonneutrality condition. It should be mentioned here that, in the case including variation and inertia, Ma and Hirose (2010) showed that electromagnetic "lower-hybrid oscillitons" are triggered.…”