Whistler-Langmuir oscillitons and their relation to auroral hiss

Abstract

Abstract.A new type of oscilliton (soliton with superimposed spatial oscillations) is described which arises in plasmas if the electron cyclotron frequency e is larger than the electron plasma frequency ω e , which is a typical situation for auroral regions in planetary magnetospheres. Both highfrequency modes of concern, the Langmuir and the whistler wave, are completely decoupled if they propagate parallel to the magnetic field. However, for oblique propagation two mixed modes are created with longitudinal and transverse electric field components. The lower mode (in the literature commonly called the whistler mode, e.g. Gurnett et al., 1983) has whistler wave characteristics at small wave numbers and asymptotically transforms into the Langmuir mode. As a consequence of the coupling between these two modes, with different phase velocity dependence, a maximum in phase velocity appears at finite wave number. The occurrence of such a particular point where phase and group velocity coincide creates the condition for the existence of a new type of oscillating nonlinear stationary structure, which we call the whistler-Langmuir (WL) oscilliton. After determining, by means of stationary dispersion theory, the parameter regime in which WL oscillitons exist, their spatial profiles are calculated within the framework of cold (non-relativistic) fluid theory. Particle-in-cell (PIC) simulations are used to demonstrate the formation of WL oscillitons which seem to play an important role in understanding electron beam-excited plasma radiation that is observed as auroral hiss in planetary magnetospheres far away from the source region.

Gap formation around <em>Ω</em>_e/2 and generation of low-band whistler waves by Landau-resonant electrons in the magnetosphere: Predictions from dispersion theory

“…This model starts from the fluid equations of a cold main plasma and a drifting warm minor electron population described by a water‐bag distribution. Details of the dispersion analysis are described in Sauer and Sydora (2010, 2011). Aside from the absence of strong damping of the beam modes in the fluid model, there is a reasonable agreement between both approaches.…”

Parametric Decay of Beam‐Generated Langmuir Waves and Three‐Wave Interaction in Plateau Plasmas: Implications for Type III Radiation

“…An important point is the evolution of the wave number spectrum of the electric field during and after saturation of the beam instability. Previous particle simulations (Sauer & Sydora, 2011, 2012 have shown that a wave number shift of the primary unstable Langmuir wave from k~ω e /V b to smaller values takes place, obviously caused by plateau formation and related effects. The detailed mechanism behind this conversion remained unclear, but the crucial role of ion dynamics was obvious.…”

“…Our present investigations of electromagnetic wave generation near to the electron plasma frequency ω e go back to particle‐in‐cell (PIC) simulations and their interpretation in the previous work by Sauer and Sydora (, ). There, beam‐plasma interaction in a magnetized plasma has been studied under different plasma conditions, in particular, by varying the ratio G = Ω e / ω e between the electron cyclotron frequency Ω e and the electron plasma frequency ω e .…”

“…The first, at kc / ω e ~ 4.5, is associated with the beam instability, according to k ~ ω e / V b . The other one at kc / ω e ~ 1, as discussed in Sauer and Sydora (, ), is related to the point where the Langmuir mode and the left‐ and right‐handed electromagnetic modes cross each other. The third region, finally, marks the second harmonic at ω ~ 2 ω e and k ~ 2 ω e / c , obviously generated by wave activity at kc / ω e ~ 1 due to resonant wave‐wave interaction.…”

“…The crucial question that emerged from the above results was to find a mechanism that would explain the increased wave activity in the region of mode coupling, which, with respect to the wave numbers, is widely separated from the beam instability. Inspired by the above described electromagnetic PIC simulations of beam-plasma interaction (Sauer & Sydora, 2011, 2012 and partly related to the recent investigations by Sauer et al (2017), we present here an approach which proposes two main steps for the transformation mechanism of electrostatic Langmuir waves into electromagnetic radiation. The first is the decay of the beam-generated Langmuir wave into scattered Langmuir waves of significantly lower wave number and ion acoustic waves via a hitherto unknown decay mechanism in a plateau plasma and the establishment of a quasi-stationary resonant three-wave interaction.…”