On the temperature of the solar wind

Abstract

Solar wind provides an example of a weakly collisional plasma expanding from a thermal source in the presence of spatially diverging magnetic field lines. Observations show that in the inner heliosphere, the electron temperature declines with the distance approximately as $T_e(r)\sim r^{-0.3}\dots r^{-0.7}$, which is significantly slower than the adiabatic expansion law $\sim r^{-4/3}$. Motivated by such observations, we propose a kinetic theory that addresses the non-adiabatic evolution of a nearly collisionless plasma expanding from a central thermal source. We concentrate on the dynamics of energetic electrons propagating along a radially diverging magnetic flux tube. Due to the conservation of their magnetic moments, the electrons form a beam collimated along the magnetic field lines. Due to weak energy exchange with the background plasma, the beam population slowly loses its energy and heats the background plasma. We propose that no matter how weak the collisions are, at large enough distances from the source a universal regime of expansion is established where the electron temperature declines as $T_e(r)\propto r^{-2/5}$. This is close to the observed scaling of the solar wind temperature in the inner heliosphere. Our first-principle kinetic derivation may thus provide an explanation for the slower-than-adiabatic temperature decline in the solar wind. More broadly, it may be useful for describing magnetized winds from G-type stars.

9 pages, 3 figures

References in corpus (6)

• Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
• Recent progress in astrophysical plasma turbulence from solar wind observations
• Scattering of Strahl Electrons in the Solar Wind between 0.3 and 1 au: Helios Observations
• Ensemble Simulations of Proton Heating in the Solar Wind via Turbulence and Ion Cyclotron Resonance
• Nature of stochastic ion heating in the solar wind: testing the dependence on plasma beta and turbulence amplitude
• Quasilinear Approach of the Whistler Heat-Flux Instability in the Solar Wind

Cited by in corpus (16)

• Electron heat flux in the near-Sun environment
• The Radial Evolution of the Solar Wind as Organized by Electron Distribution Parameters
• Proton and electron temperatures in the solar wind and their correlations with the solar wind speed
• Stability of superthermal strahl electrons in the solar wind
• Preferential Proton over Electron heating from coherent structures during the first perihelion of Parker Solar Probe
• A drift kinetic model for the expander region of a magnetic mirror
• Arguments for the physical nature of the triggered ion-acoustic waves observed on the Parker Solar Probe
• The Kinetic Expansion of Solar-Wind Electrons: Transport Theory and Predictions for the very Inner Heliosphere
• Ion kinetic effects and instabilities in the plasma flow in the magnetic mirror
• Concept for a Space-based Near-Solar Neutrino Detector
• A Kinetic Model of Solar Wind Acceleration Driven by Ambipolar Electric Potential and Velocity-Space Diffusion
• Excitation of ion-acoustic waves by non-linear finite-amplitude standing Alfvén waves
• Quasi-parallel anti-sunward propagating whistler waves associated to the electron-deficit in the near-Sun solar wind: Particle-in-Cell simulation
• The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons
• The role of the thermal properties of electrons on the dispersion properties of Alfvén waves in space plasmas
• Ambipolar electric field and potential in the solar wind estimated from electron velocity distribution functions