Dynamics of Two-dimensional Type III Electron Beams in Randomly Inhomogeneous Solar Wind Plasmas

Abstract

The dynamics of a type III electron beam generating Langmuir wave turbulence and subsequent electromagnetic emissions is studied owing to two-dimensional Particle-In-Cell simulations performed in both homogeneous and randomly inhomogeneous solar wind plasmas. Important differences in the beam dynamics are highlighted between both cases, due to Langmuir waves’ transformations on the density fluctuations. This paper studies the dynamics of a weak beam interacting with Langmuir wave turbulence scattered by initially applied plasma density fluctuations, in terms of particle acceleration, non-Gaussian suprathermal electron tails, broadening and relaxation of velocity distributions, beam density localization, and electron diffusion or trapping in a turbulent plasma. Density fluctuations are the cause of beam acceleration during its relaxation stage; after Langmuir wave saturation, it gains up to half the energy lost during deceleration while wave turbulence is damping, exhibiting asymptotically a suprathermal tail of electrons carrying around 30% of its initial kinetic energy. Some important features observed for one-dimensional beams exciting Langmuir wave turbulence in randomly inhomogeneous plasmas can be recovered.

Electrostatic Wave Decay in the Randomly Inhomogeneous Solar Wind

“…The weak beam relative density, n b /n 0 = 5 × 10 −4 , enables us to work in the kinetic instability regime. The beam is injected parallel to the x-axis with a 2D Maxwellian velocity distribution function (Krafft & Savoini 2023).…”

“…All these features are in agreement with Figure 4(e), which shows the time variations of energies of ion acoustic and Langmuir waves, for ΔN = 0.025 and ΔN = 0.05. After reaching a maximum near ω p t ; 1000, the energy W  slowly decreases, due to the formation of a tail of accelerated beam electrons resulting from Langmuir wave scattering on random density fluctuations (Krafft et al 2013;Krafft & Savoini 2023). This effect is more prominent for larger ΔN than for smaller ones.…”

Mechanisms of Fundamental Electromagnetic Wave Radiation in the Solar Wind

“…Random density fluctuations are applied initially and evolve self-consistently, with typical wavelengths around 10 times larger than those of the Langmuir waves. A type III electron beam is injected along the x-axis, with a 2D Maxwellian velocity distribution (Krafft & Savoini 2023); drift and thermal velocities are v b = 0.25c and…”

“…As resonance conditions between waves required for EMD should be a priori hardly fulfilled, the question rises of whether EMD can occur in such plasmas. Moreover, Langmuir wave energy lost during conversion cannot be used further by other processes, not to mention that Langmuir wave turbulence itself also decays owing to the appearance of accelerated beam electrons resulting from wave scattering on density fluctuations (Krafft et al 2013;Krafft & Volokitin 2016Krafft & Savoini 2023).…”

“…between the beam particles of velocity v(v x , v y ) and the Langmuir waves of wavevectors k(k x , k y ) involves a second degree of freedom, so that the probability to find particles that can resonate with a given wave is larger than in a 1D plasma, where the resonance condition involves only the parallel direction (Krafft & Savoini 2023).…”