Nonlinear beam generated plasma waves as a source for enhanced plasma and ion acoustic lines

Abstract

Observations by, for instance, the EISCAT Svalbard Radar (ESR) demonstrate that the symmetry of the naturally occurring ion line in the polar ionosphere can be broken by an enhanced, nonthermal, level of fluctuations (naturally enhanced ion-acoustic lines, NEIALs). It was in many cases found that the entire ion spectrum can be distorted, also with the appearance of a third line, corresponding to a propagation velocity significantly slower than the ion acoustic sound speed. It has been argued that selective decay of beam excited primary Langmuir waves can explain some phenomena similar to those observed. We consider a related model, suggesting that a primary nonlinear process can be an oscillating two-stream instability, generating a forced low frequency mode that does not obey any ion sound dispersion relation. At later times, the decay of Langmuir waves can give rise also to enhanced asymmetric ion lines. The analysis is based on numerical results, where the initial Langmuir waves are excited by a cold dilute electron beam. By this numerical approach, we can detect fine details of the physical processes, in particular, demonstrate a strong space-time intermittency of the electron waves in agreement with observations. Our code solves the full Vlasov equation for electrons and ions, with the dynamics coupled through the electrostatic field derived from Poisson's equation. The analysis distinguishes the dynamics of the background and beam electrons. This distinction simplifies the analysis for the formulation of the weakly nonlinear analytical model for the oscillating two-stream instability. The results have general applications beyond their relevance for the ionospheric observations.

Coherent amplitude modulation of electron-beam-driven Langmuir waves

“…The irregular Langmuir envelope modulation has been clearly reproduced in a series of particle-in-cell (PIC) simulations of electron-beam-plasma interaction (Omura et al, 1996;Muschietti et al, 1996;Matsukiyo et al, 2004;Usui et al, 2005) and in electrostatic Vlasov simulations (Umeda, 2006;Silin et al, 2007;Daldorff et al, 2011;Sauer and Sydora, 2012), but the simulations do not reveal the physical nature of the phenomenon. Coherent modulation and the associated double peak have as yet not been explained, neither theoretically nor in terms of numerical simulations.…”

A comparison of weak-turbulence and particle-in-cell simulations of weak electron-beam plasma interaction

“…Plasma wave generation may also modify streaming non-thermal electron populations in flaring coronal loops 14,15 . In the ionosphere, plasma a) Electronic mail: h.ratcliffe@warwick.ac.uk wave interactions may lead to naturally enhanced ionacoustic lines seen in radar observations 16,17 .…”

“…Vlasov simulations of weak beam-plasma interactions are less common. For example, Daldorff et al 17 considered ionospheric electron beams modifying ion-acoustic lines, but were forced to use unphysical pa-rameters such as a 1% beam density for computational reasons, altering the processes of Langmuir wave generation and scattering. Henri et al 23 considered finite Langmuir wave-packets and concluded that for sufficiently fast wave growth ion-sound wave damping could be overcome even in plasma of equal ion and electron temperature, and resonant wave-wave interactions would proceed as usual.…”

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.…”