Self-Similarity of the Electron Strahl: Wind Data

The solar wind strahl is a narrow, field-aligned population of high-energy electrons that originate in the solar corona. The beam-like shape of the strahl in velocity space is believed to come from the competition of two physical processes: the mirror force tends to narrow this population, while Coulomb collisions wave-particle interactions tend to broaden it. Using data from the Wind satellites's SWE strahl detector, we investigate the detailed shape of the strahl and compare with predictions from a kinetic ``self-similar'' model. The asymptotic solution to the kinetic equation in the regime of the strahl predicts this population's functional form. Importantly, the detailed shape of the strahl distribution is determined entirely by a single local observable, the temperature Knudsen number γ ∼ |T dT/dr|/n. The model is successful at predicting the angular width (FWHM) of the strahl for the Wind data at 1 AU, for a wide range of Knudsen numbers. We argue that self-similarity is an effective framework for explaining the dynamics of electrons in the ambient solar wind; however, since this model is derived assuming conditions typical of the inner heliosphere, future measurements from the Solar Probe and Solar Orbiter missions will be necessary to definitively test the model's effectiveness.