The Kinetic Expansion of Solar-Wind Electrons: Transport Theory and Predictions for the very Inner Heliosphere

Abstract

We propose a transport theory for the kinetic evolution of solar-wind electrons in the heliosphere. We derive a gyro-averaged kinetic transport equation that accounts for the spherical expansion of the solar wind and the geometry of the Parker-spiral magnetic field. To solve our three-dimensional kinetic equation, we develop a mathematical approach that combines the Crank--Nicolson scheme in velocity space and a finite-difference Euler scheme in configuration space. We initialize our model with isotropic electron distribution functions and calculate the kinetic expansion at heliocentric distances from 5 to 20 solar radii. In our kinetic model, the electrons evolve mainly through the combination of the ballistic particle streaming, the magnetic mirror force, and the electric field. By applying fits to our numerical results, we quantify the parameters of the electron strahl and core part of the electron velocity distributions. The strahl fit parameters show that the density of the electron strahl is around 7% of the total electron density at a distance of 20 solar radii, the strahl bulk velocity and strahl temperature parallel to the background magnetic field stay approximately constant beyond a distance of 15 solar radii, and ${\beta }_{\parallel s}$ (i.e., the ratio between strahl parallel thermal pressure to the magnetic pressure) is approximately constant with heliocentric distance at a value of about 0.02. We compare our results with data measured by Parker Solar Probe. Furthermore, we provide theoretical evidence that the electron strahl is not scattered by the oblique fast-magnetosonic/whistler instability in the near-Sun environment.

Accepted in ApJ

References in corpus (10)

• Evidence for Electron Landau Damping in Space Plasma Turbulence
• First in-situ Measurements of Electron Density and Temperature from Quasi-Thermal Noise Spectroscopy with Parker Solar Probe/FIELDS
• Misestimation of temperature when applying Maxwellian distributions to space plasmas described by kappa distributions
• Electron heat flux in the near-Sun environment
• Particle-In-Cell simulation of whistler heat flux instabilities in the solar wind: heat flux regulation and electron halo formation
• Alternative high plasma beta regimes of electron heat-flux instabilities in the solar wind
• Measurement of the Open Magnetic Flux in the Inner Heliosphere down to 0.13AU
• On the role of solar wind expansion as a source of whistler waves: scattering of suprathermal electrons and heat flux regulation in the inner heliosphere
• A Quasi-Linear Diffusion Model for Resonant Wave-Particle Instability in Homogeneous Plasma
• Stability of superthermal strahl electrons in the solar wind

Cited by in corpus (4)

• Thermal energy budget of electrons in the inner heliosphere: Parker Solar Probe Observations
• A Kinetic Model of Solar Wind Acceleration Driven by Ambipolar Electric Potential and Velocity-Space Diffusion
• 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