Langmuir turbulence excited by an electron beam in a plasma with long-wave density fluctuations

International audienceThe interaction of the electron beam propagating in a nonuniform solar wind plasma with Langmuir waves is considered. Plasma waves are described one dimensional system of Zakharov equations, the beam dynamics is modeled using particles moving in the total electric field of the Langmuir waves. The role of external plasma density fluctuations, which are known to be present in the solar wind, is taking into account. It is shown that at low density fluctuations delta n /n₀ delta n /n₀ >3k(2) L_d(2) , plasma inhomogeneity has a decisive influence on the relaxation process. First, the growth of waves on the linear stage becomes localized, thereby clearly identifiable wave packets or clumps dominate the wave spectrum. This effect is due to the kinematics of wave propagation in conjunction with the resonant wave-particle interaction. The wave's packets start to grow more likely in regions of high density gradients. The second important feature in the relaxation of the beam in an inhomogeneous plasma is to build distribution of accelerated electrons with velocities exceeds the initial drift velocity of the beam. In this case the velocity distribution of electrons in the beam expands in the both direction to lower and higher velocities, the density of the accelerated electrons can reach more than 10 ÷ 20% of the beam density and the flux of energy in the group accelerated particles reaches 40% or more of the initial beam energy flux. In addition, the wave profiles obtained in the simulation results are in good agreement with the observed in situ on the spacecraft STEREO and others who show frequent presence of spatially localized Langmuir waves in the solar wind and correlated with the presence of superthermal electrons during type III radio bursts or in electron foreshock regions. Finally the processes of nonlinear parametric decay of Langmuir wave packets waves in an inhomogeneous plasma with the participation of ion-acoustic is considered on the base of the simulations