The Kinetic Expansion of Solarwind Electrons: Transport Theory and Predictions for the Very Inner Heliosphere
Abstract
We propose a transport theory for the kinetic evolution of solarwind electrons in the heliosphere. We derive a gyroaveraged 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 threedimensional kinetic equation, we develop a mathematical approach that combines the CrankNicolson scheme in velocity space and a finitedifference 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 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 the 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 β _{∥s } (i.e., the ratio of the 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 the Parker Solar Probe. Furthermore, we provide theoretical evidence that the electron strahl is not scattered by the oblique fastmagnetosonic/whistler instability in the nearSun environment.
 Publication:

The Astrophysical Journal
 Pub Date:
 March 2022
 DOI:
 10.3847/15384357/ac4805
 arXiv:
 arXiv:2201.00079
 Bibcode:
 2022ApJ...927..162J
 Keywords:

 Solar wind;
 Space plasmas;
 Heliosphere;
 Theoretical models;
 1534;
 1544;
 711;
 2107;
 Physics  Space Physics
 EPrint:
 Accepted in ApJ