Generation of Weak Double Layers and Low-Frequency Electrostatic Waves in the Solar Wind

Abstract

We propose that the mechanism for the generation of weak double layers (WDLs) and low-frequency coherent electrostatic waves, observed by Wind in the solar wind at 1 AU, might be slow and fast ion-acoustic solitons and double layers. The solar wind plasma is modelled as a fluid of hot protons and hot $\alpha$ particles streaming with respect to protons, and suprathermal electrons having a $\kappa$-distribution. The fast ion-acoustic mode is similar to the ion-acoustic mode of a proton–electron plasma and can support only positive-potential solitons. The slow ion-acoustic mode is a new mode that occurs due to the presence of $\alpha$ particles. This mode can support both positive and negative solitons and double layers. The slow ion-acoustic mode can exist even when the relative streaming, $U_{0}$, between $\alpha$ particles and protons is zero, provided that the $\alpha$ temperature, $T_{i}$, is not exactly equal to four times the proton temperature, $T_{p}$. An increase of the $\kappa$-index leads to an increase in the critical Mach number, maximum Mach number, and the maximum amplitude of both slow and fast ion-acoustic solitons. The slow ion-acoustic double layer can explain the amplitudes and widths, but not the shapes, of the observed WDLs in the solar wind at 1 AU by Wind spacecraft. The Fourier transform of the slow ion-acoustic solitons/double layers would produce broadband low-frequency electrostatic waves having main peaks between 0.35 kHz to 1.6 kHz, with an electric field in the range of $E = (0.01\,\mbox{--}\,0.7)~\mbox{mV}\,\mbox{m}^{-1}$, in excellent agreement with the observed low-frequency electrostatic wave activity in the solar wind at 1 AU.