Vlasov simulation of amplitude-modulated Langmuir waves

Abstract

Mechanisms for the generation of Langmuir wave packets are studied by performing a one-dimensional electrostatic Vlasov simulation. The present simulation of a weak-electron-beam instability without ion dynamics suggests two new processes for the amplitude modulation of Langmuir waves. The beam instability excites Langmuir modes over a wide wave number range, but the saturation of the most unstable Langmuir mode “filters” the growth of sideband modes. Specific upper and lower sideband modes linearly grow to a high saturation level. Then the primary Langmuir mode is amplified and strongly modulated through interaction with the sideband modes.

Vlasov simulation of Langmuir wave packets

“…On the other hand, The GEOTAIL spacecraft observation in the magnetotail has shown that the amplitude of Langmuir wave packets is several hundred µV/m, which is too small for the Langmuir decay instability. The computer simulations have also demonstrated that amplitude-modulated Langmuir waves with a very small amplitude can be generated during a very-weak-electron-beam instability without ion dynamics (Akimoto et al, 1996;Usui et al, 2005;Umeda, 2006;Silin et al, 2007), which is consistent with the GEO-TAIL observation. These observations and simulations suggest that Langmuir wave packets with a small amplitude and a large amplitude are generated by different mechanisms with small amplitude packets generated by electron dynamics while large amplitude packets likely involve ion dynamics as well.…”

“…The amplitude modulation and packet formation of Langmuir waves were commonly observed in self-consistent kinetic simulations of electron beam instabilities (Muschietti et al, 1995(Muschietti et al, , 1996Akimoto et al, 1996;Matsukiyo et al, 2004;Usui et al, 2005;Umeda, 2006;Silin et al, 2007).…”

“…In the nonlinear trapping theory, Langmuir waves are modulated in space due to nonlinear trapping of electrons by the electrostatic potential of the Langmuir waves. However, a recent Vlasov simulation of a very-weak-electron-beam instability has demonstrated that Langmuir waves are not directly modulated by the nonlinear trapping but are modulated by nonlinear interaction between the most unstable primary Langmuir mode and its sideband modes (Umeda, 2006). The amplitude-modulated Langmuir waves have a broadband wavenumber spectrum, which indicates a turbulent feature rather than a coherent feature.…”

“…Electron phase-space-density holes (e.g. Berk and Roberts, 1967) or electrostatic solitary waves (Matsumoto et al, 1994) are coherent nonlinear electrostatic structures, while harmonic Langmuir waves and Langmuir wave packets have an incoherent quasi-power-law wavenumber spectrum which indicate a turbulent feature Gaelzer et al, 2003;Umeda et al, 2003;Umeda, 2006;Silin et al, 2007). The present study is aimed at the generation of amplitude-modulated Langmuir waves and Langmuir wave packets which is called a Langmuir turbulence in space plasmas.…”

Ion dynamics in electron beam–plasma interaction: particle-in-cell simulations

“…2005; Baumgärtel, 2013) and electrostatic Vlasov simulations (Umeda, 2006;Silin et al, 2007;Umeda, 2007;Umeda and Ito, 2008;Daldorff et al, 2011;Sauer and Sydora, 2012). Studies which include ion dynamics (in some cases with a reduced proton-to-electron mass ratio) show clearly that the heavy particles indeed contribute to the evolution of the instability.…”