Apparent temperature anisotropies due to wave activity in the solar wind

TL;DR

This paper models the effects of large-amplitude plasma waves on particle distributions in the solar wind, explaining observed temperature anisotropies and wave-particle interactions through a kinetic framework validated by spacecraft data.

Contribution

It introduces a new model of velocity distribution functions in a plasma wave, linking wave activity to observed anisotropies and nonresonant interactions in the solar wind.

Findings

High wave activity explains key features of measured particle distributions.

Time-averaging leads to apparent temperature anisotropies.

Model aligns with Helios spacecraft measurements.

Abstract

The fast solar wind is a collisionless plasma permeated by plasma waves on many different scales. A plasma wave represents the natural interplay between the periodic changes of the electromagnetic field and the associated coherent motions of the plasma particles. In this paper, a model velocity distribution function is derived for a plasma in a single, coherent, large-amplitude wave. This model allows one to study the kinetic effects of wave motions on particle distributions. They are by in-situ spacecraft measured by counting, over a certain sampling time, the particles coming from various directions and having different energies. We compare our results with the measurements by the Helios spacecraft, and thus find that by assuming high wave activity we are able to explain key observed features of the measured distributions within the framework of our model. We also address the recent discussions on nonresonant wave--particle interactions and apparent heating. The applied time-averaging procedure leads to an apparent ion temperature anisotropy which is connected but not identical to the intrinsic temperature of the underlying distribution function.