Electrostatic Wave Decay in the Randomly Inhomogeneous Solar Wind

Abstract

Despite a few space observations where Langmuir and ion acoustic waves are expected to participate in the mechanism of electrostatic decay, this is to date believed to be the main and fastest nonlinear wave process in the solar wind. However, in such a plasma where random density fluctuations are ubiquitous, the question of whether nonlinear wave processes play a significant role in Langmuir wave turbulence generated by electron beams associated with type III solar radio bursts remains still open. This paper provides several answers by studying, owing to two-dimensional challenging particle-in-cell simulations, the dynamics and the properties of the ion acoustic waves excited by such Langmuir wave turbulence and the role they play in the electrostatic decay. The impact on this process of plasma background density fluctuations and electron-to-ion temperature ratio is studied. Moreover, it is shown that, for a typical solar wind plasma with an average level of density fluctuations of a few percent of the ambient density and a temperature ratio of the order of 1, nonlinear induced scattering off ions occurs, with small intensity low-frequency quasi-modes and only in localized plasma regions where density is depleted or weakly perturbed by low-frequency turbulence.

Mechanisms of Fundamental Electromagnetic Wave Radiation in the Solar Wind

“…for ω p t  7000, highlighting that the EMD  +    f occurs in this time interval, as revealed by Figures 1-2. Nevertheless, the proportionality between energies' and energy products' time variations is not always very good (compare with Figures 3(c)-(d) of Krafft & Savoini (2024), where it is much better, the ESD being the prominent nonlinear wave process), and the relations µ…”

“…Note that the low-frequency excitation appearing at k x ∼ 0 in Figure 1, also visible in 1D simulations with modified Zakharov equations (Krafft et al 2015) or studies performed in the framework of weak turbulence theory (Ziebell et al 2015), corresponds to ion-acoustic waves of large wavelengths that do not play a role in the processes studied here. This structure appears likely due to linear beatings between close (in the frequency and wavelength domains) beam-driven Langmuir waves generated during the beam deceleration (see also Krafft & Savoini 2024).…”

“…Note that panels (a)-(f) are the continuation from ω p t ; 5500 to ω p t ; 13,500 of the left panels of Figure 2 in Krafft & Savoini (2024). All variables are normalized.…”

“…The dominant nonlinear wave interaction process in the solar wind is believed to be the ESD of Langmuir waves,  ¢ + ¢    (e.g., Tsytovich 1970;Cairns & Robinson 1995;Graham & Cairns 2013;Krafft & Savoini 2024), where ¢  and ¢  are backscattered Langmuir and ion-acoustic waves, respectively. It has a significantly larger growth rate than electromagnetic decay (EMD)  +   .…”

“… are backscattered and forward-propagating Langmuir waves, and ¢  and   are forward-and backward-propagating ion-acoustic modes, respectively (Krafft & Savoini 2024). The three-wave resonance conditions are written as w…”