Enhanced Langmuir Fluctuation by Strahl Electrons and Its Influence on the Formation of the Halo Component

It is well known that the solar wind electrons consist of the core, halo, and sometimes field-aligned strahl component. It is speculated that as the solar wind expands out the strahl component actually turns into the halo [Maksimovic et al., 2005; Stverak et al., 2005]. The detailed physics of strahl to halo transition is poorly understood, but it may involve pitch-angle scattering by whistler waves, finite collisions, and/or other waves. In the present paper we focus on the role of enhanced Langmuir fluctuation generated by the Strahl component and its subsequent role in generating the halo population. We model the strahl electrons by a drifting Gaussian beam, which leads to the excitation of enhanced Langmuir fluctuation. The enhanced electrostatic fluctuations then undergoes nonlinear scattering, whereby the wave momenta shift to longer wavelength regime. This results in acceleration/heating of the strahl electrons to suprathermal energy range. The resulting quasi-asymptotic electron velocity distribution resembles the core-halo system with a power-law energetic tail distribution. We demonstrate this process by means of a weak turbulence simulation.