Parametric decay of current‐driven Langmuir waves in plateau plasmas: Relevance to solar wind and foreshock events

Abstract

Langmuir amplitude modulation in association with type III radio bursts is a well‐known phenomenon since the beginning of space observations. It is commonly attributed to the superposition of beam‐excited Langmuir waves and their backscattered counterparts as a result of parametric decay. The dilemma, however, is the discrepancy between fast beam relaxation and long‐lasting Langmuir wave activity. Instead of starting with an unstable electron beam, our focus in this paper is on the nonlinear response of Langmuir oscillations that are driven after beam stabilization by the still persisting current of the (stable) two‐electron plasma. The velocity distribution function of the second population forms a plateau (index h) with a point at which ∂fh∂v∼0 associated with weak damping over a more or less extended wave number range k. As shown by particle‐in‐cell simulations, this so‐called plateau plasma drives primarily Langmuir oscillations at the plasma frequency (ωe) with k = 0 over long times without remarkable change of the distribution function. These Langmuir oscillations act as a pump wave for parametric decay by which an electron‐acoustic wave slightly below ωe and a counterstreaming ion‐acoustic wave are generated. Both high‐frequency waves have nearly the same amplitude, which is given by the product of plateau density and velocity. Beating of these two wave types leads to pronounced Langmuir amplitude modulation, in reasonable agreement with solar wind and terrestrial foreshock observations made by the Wind spacecraft.

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

“…Earth and Planetary Physics doi: 10.26464/epp2020020 145 kc/ω e ~ 7, accompanied by mode splitting similar to the case of electron plasma waves (Sauer et al, 2017(Sauer et al, , 2019. The frequency of the most unstable low-band whistler wave is around ω/Ω e = 0.3, which is very close to the resonance frequency given by ω/Ω e = cosθ.…”

“…An optimum propagation angle θ ~ 75° was found. The maximum growth rate γ / Ω e ~ 0.02 occurs where the beam mode ω ~ kV b intersects the whistler mode at kc / ω e ~ 7, accompanied by mode splitting similar to the case of electron plasma waves (Sauer et al, 2017, 2019). The frequency of the most unstable low‐band whistler wave is around ω / Ω e = 0.3, which is very close to the resonance frequency given by ω / Ω e = cos θ .…”

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

“…Whereas one of the peaks was interpreted by beam excitation, for the other one different mechanisms have been considered, such as modulational instability, classical parametric decay, and particle trapping. In own studies (Baumgärtel, ; Sauer & Sydora, , ; Sauer et al, ), Langmuir oscillations driven by the beam/plateau current have been introduced as a new concept. In particular, the interpretation of Langmuir amplitude modulations in the recent paper by Sauer et al () by parametric decay of current‐driven Langmuir oscillations is very similar to the present approach.…”

“…In own studies (Baumgärtel, 2013;Sauer & Sydora, 2015Sauer et al, 2017), Langmuir oscillations driven by the beam/plateau current have been introduced as a new concept. In particular, the interpretation of Langmuir amplitude modulations in the recent paper by Sauer et al (2017) by parametric decay of current-driven Langmuir oscillations is very similar to the present approach. There are only differences regarding the origin of the Langmuir oscillations.…”

“…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.…”