On quasi-parallel whistler waves in the solar wind

TL;DR

This paper analyzes the properties and stability of parallel and anti-parallel whistler waves in the solar wind, revealing differences in their frequencies, growth rates, and amplitudes, with implications for heat flux regulation.

Contribution

It provides a comprehensive linear stability analysis of anti-parallel whistler waves, showing they are less energetic and more difficult to detect than parallel waves in the solar wind.

Findings

Anti-parallel whistler waves have smaller frequencies and growth rates.

Anti-parallel waves saturate at amplitudes about ten times smaller.

Presence of anti-parallel waves is obscured by magnetic turbulence.

Abstract

The recent simulations showed that the whistler heat flux instability, which presumably produces the most of quasi-parallel coherent whistler waves in the solar wind, is not efficient in regulating the electron heat conduction. In addition, recent spacecraft measurements indicated that some fraction of coherent whistler waves in the solar wind may propagate anti-parallel to the electron heat flux, being produced due to a perpendicular temperature anisotropy of suprathermal electrons. We present analysis of properties of parallel and anti-parallel whistler waves unstable at electron heat fluxes and temperature anisotropies of suprathermal electrons typical of the pristine solar wind. Assuming the electron population consisting of counter-streaming dense thermal core and tenuous suprathermal halo populations, we perform a linear stability analysis to demonstrate that anti-parallel whistler waves are expected to have smaller frequencies, wave numbers and growth rates compared to parallel whistler waves. The stability analysis is performed over a wide range of parameters of core and halo electron populations. Using the quasi-linear scaling relation we show that anti-parallel whistler waves saturate at amplitudes of one order of magnitude smaller than parallel whistler waves, which is at about $10^{-3}\;B_0$ in the pristine solar wind. The analysis shows that the presence of anti-parallel whistler waves in the pristine solar wind is more likely to be obscured by turbulent magnetic field fluctuations, because of lower frequencies and smaller amplitudes compared to parallel whistler waves. The presented results will be also valuable for numerical simulations of the electron heat flux regulation in the solar wind.