Statistics of Solar Wind Electron Breakpoint Energies Using Machine Learning Techniques

Abstract

Solar wind electron velocity distributions at 1 au consist of a thermal "core" population and two suprathermal populations: "halo" and "strahl". The core and halo are quasi-isotropic, whereas the strahl typically travels radially outwards along the parallel and/or anti-parallel direction with respect to the interplanetary magnetic field. With Cluster-PEACE data, we analyse energy and pitch angle distributions and use machine learning techniques to provide robust classifications of these solar wind populations. Initially, we use unsupervised algorithms to classify halo and strahl differential energy flux distributions to allow us to calculate relative number densities, which are of the same order as previous results. Subsequently, we apply unsupervised algorithms to phase space density distributions over ten years to study the variation of halo and strahl breakpoint energies with solar wind parameters. In our statistical study, we find both halo and strahl suprathermal breakpoint energies display a significant increase with core temperature, with the halo exhibiting a more positive correlation than the strahl. We conclude low energy strahl electrons are scattering into the core at perpendicular pitch angles. This increases the number of Coulomb collisions and extends the perpendicular core population to higher energies, resulting in a larger difference between halo and strahl breakpoint energies at higher core temperatures. Statistically, the locations of both suprathermal breakpoint energies decrease with increasing solar wind speed. In the case of halo breakpoint energy, we observe two distinct profiles above and below 500 km/s. We relate this to the difference in origin of fast and slow solar wind.

arXiv commentPublished in Astronomy & Astrophysics, 11 pages, 10 figures

References in corpus (8)

• Solar Orbiter: Exploring the Sun-heliosphere connection
• Origins of the slow and the ubiquitous fast solar wind
• Scattering of Strahl Electrons in the Solar Wind between 0.3 and 1 au: Helios Observations
• Self-induced Scattering of Strahl Electrons in the Solar Wind
• Whistler wave generation by halo electrons in the solar wind
• The Origin of Non-Maxwellian Solar Wind Electron Velocity Distribution Function: Connection to Nanoflares in the Solar Corona
• Electron Strahl and Halo Formation in the Solar Wind
• Suprathermal electron strahl widths in the presence of narrow-band whistler waves in the solar wind

Cited by in corpus (4)

• Parker Solar Probe Observations of Suprathermal Electron Flux Enhancements Originating from Coronal Hole Boundaries
• Radial evolution of thermal and suprathermal electron populations in the slow solar wind from 0.13 to 0.5 au : Parker Solar Probe Observations
• The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons
• Ambipolar electric field and potential in the solar wind estimated from electron velocity distribution functions