# Kinetic Theory and Fast Wind Observations of the Electron Strahl

## TL;DR

This paper presents a kinetic model for the solar wind electron strahl, successfully predicting its angular width and energy dependence at 1 AU by matching observations, and highlights the importance of collisionality in shaping the strahl.

## Contribution

The study introduces a new kinetic model for the electron strahl in the solar wind that accurately predicts its properties at 1 AU by fitting to Wind satellite data.

## Key findings

- Model predicts strahl angular width and energy scaling at 1 AU.
- Strahl distribution is governed by the local temperature Knudsen number.
- Model overestimates strahl amplitude at larger heliocentric distances.

## Abstract

We develop a model for the strahl population in the solar wind -- a narrow, low-density and high-energy electron beam centered on the magnetic field direction. Our model is based on the solution of the electron drift-kinetic equation at heliospheric distances where the plasma density, temperature, and the magnetic field strength decline as power-laws of the distance along a magnetic flux tube. Our solution for the strahl depends on a number of parameters that, in the absence of the analytic solution for the full electron velocity distribution function (eVDF), cannot be derived from the theory. We however demonstrate that these parameters can be efficiently found from matching our solution with observations of the eVDF made by the Wind satellite's SWE strahl detector. The model is successful at predicting the angular width (FWHM) of the strahl for the Wind data at 1 AU, in particular by predicting how this width scales with particle energy and background density. We find the strahl distribution is largely determined by the local temperature Knudsen number $\gamma \sim |T dT/dx|/n$, which parametrizes solar wind collisionality. We compute averaged strahl distributions for typical Knudsen numbers observed in the solar wind, and fit our model to these data. The model can be matched quite closely to the eVDFs at 1 AU, however, it then overestimates the strahl amplitude at larger heliocentric distances. This indicates that our model may be improved through the inclusion of additional physics, possibly through the introduction of "anomalous diffusion" of the strahl electrons.