TY - JOUR

T1 - The Kinetic Expansion of Solar-wind Electrons

T2 - Transport Theory and Predictions for the Very Inner Heliosphere

AU - Jeong, Seong-Yeop

AU - Verscharen, Daniel

AU - Vocks, Christian

AU - Abraham, Joel B.

AU - Owen, Christopher J.

AU - Wicks, Robert T.

AU - Fazakerley, Andrew N.

AU - Stansby, David

AU - Berčič, Laura

AU - Nicolaou, Georgios

AU - Agudelo Rueda, Jeffersson A.

AU - Bakrania, Mayur

N1 - Funding information: D.V. is supported by STFC Ernest Rutherford Fellowship ST/P003826/1. C. V. is supported by the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation)—VO 2123/1-1. J. B. A. is supported by STFC grant ST/T506485/1. D.V., C.J.O., A.N.F., D.S., and L.B. are supported by STFC Consolidated Grant ST/S000240/1. R. T. W. is funded by STFC Consolidated Grant ST/V006320/1. J.A.A.R. is supported by the ESA Networking/Partnering Initiative (NPI) contract 4000127929/19/NL/MH/mg and the Colombian program Pasaporte a la Ciencia, Foco Sociedad—Reto 3, ICETEX grant 3933061. M. B. is supported by a UCL Impact Studentship, joint funded by the ESA NPI contract 4000125082/18/NL/MH/ic. We appreciate helpful discussions with Christopher Chen. This work was discussed at the "Joint Electron Project" at MSSL.

PY - 2022/3/11

Y1 - 2022/3/11

N2 - 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 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 fast-magnetosonic/whistler instability in the near-Sun environment.

AB - 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 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 fast-magnetosonic/whistler instability in the near-Sun environment.

KW - 360

KW - The Sun and the Heliosphere

UR - http://www.scopus.com/inward/record.url?scp=85127084702&partnerID=8YFLogxK

U2 - 10.3847/1538-4357/ac4805

DO - 10.3847/1538-4357/ac4805

M3 - Article

VL - 927

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 2

M1 - 162

ER -