Whistler mode waves and the electron heat flux in the solar wind: Cluster observations

TL;DR

This study analyzes Cluster spacecraft data to identify and characterize long-lived whistler mode waves in the solar wind, revealing their role in regulating electron heat flux especially at higher electron beta values.

Contribution

First comprehensive analysis of long-lived whistler waves in the solar wind, linking their occurrence to specific plasma conditions and heat flux regulation mechanisms.

Findings

Whistler waves are observed in about 10% of data, lasting from seconds to hours.

Presence of whistler waves correlates with low background turbulence and high electron heat flux.

Whistler waves influence heat flux regulation when electron beta exceeds 3.

Abstract

The nature of the magnetic field fluctuations in the solar wind between the ion and electron scales is still under debate. Using the Cluster/STAFF instrument, we make a survey of the power spectral density and of the polarization of these fluctuations at frequencies $f\in[1,400]$ Hz, during five years (2001-2005), when Cluster was in the free solar wind. In $\sim 10\%$ of the selected data, we observe narrow-band, right-handed, circularly polarized fluctuations, with wave vectors quasi-parallel to the mean magnetic field, superimposed on the spectrum of the permanent background turbulence. We interpret these coherent fluctuations as whistler mode waves. The life time of these waves varies between a few seconds and several hours. Here we present, for the first time, an analysis of long-lived whistler waves, i.e. lasting more than five minutes. We find several necessary (but not sufficient) conditions for the observation of whistler waves, mainly a low level of the background turbulence, a slow wind, a relatively large electron heat flux and a low electron collision frequency. When the electron parallel beta factor $\beta_{e\parallel}$ is larger than 3, the whistler waves are seen along the heat flux threshold of the whistler heat flux instability. The presence of such whistler waves confirms that the whistler heat flux instability contributes to the regulation of the solar wind heat flux, at least for $\beta_{e\parallel} \ge$ 3, in the slow wind, at 1 AU.