Kinetic Theory and Fast Wind Observations of the Electron Strahl

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 $\gamma \sim |TdT/dx|/n$, 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.

References in corpus (3)

• Whistler mode waves and the electron heat flux in the solar wind: Cluster observations
• Experimental determination of whistler wave dispersion relation in the solar wind
• Suprathermal electron strahl widths in the presence of narrow-band whistler waves in the solar wind

Cited by in corpus (22)

• The multi-scale nature of the solar wind
• The statistical properties of solar wind temperature parameters near 1 AU
• Electron energy partition across interplanetary shocks: I. Methodology and Data Product
• Coronal Electron Temperature inferred from the Strahl Electrons in the Inner Heliosphere: Parker Solar Probe and Helios observations
• Electron energy partition across interplanetary shocks: II. Statistics
• Electron Strahl and Halo Formation in the Solar Wind
• On the temperature of the solar wind
• Precision Electron Measurements in the Solar Wind at 1 au from NASA's Wind Spacecraft
• Kinetic theory of the electron strahl in the solar wind
• Stability Analysis of Core-Strahl Electron Distributions in the Solar Wind
• On quasi-parallel whistler waves in the solar wind
• Electron energy partition across interplanetary shocks: III. Analysis
• Whistler instability driven by the sunward electron deficit in the solar wind
• A Quasi-Linear Diffusion Model for Resonant Wave-Particle Instability in Homogeneous Plasma
• Stability of superthermal strahl electrons in the solar wind
• The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere
• Electron Heat Flux in the Solar Wind: Generalized Approaches to Fluid Transport with a Variety of Skewed Velocity Distributions
• The Kinetic Expansion of Solar-Wind Electrons: Transport Theory and Predictions for the very Inner Heliosphere
• Electron Groups in Solar Wind and Gas Discharge Plasmas
• Ambipolar electric field and potential in the solar wind estimated from electron velocity distribution functions
• The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons
• Radial evolution of thermal and suprathermal electron populations in the slow solar wind from 0.13 to 0.5 au : Parker Solar Probe Observations