Electron Strahl and Halo Formation in the Solar Wind

Abstract

We propose a kinetic model describing the formation of the strahl and halo electron populations in the solar wind. We demonstrate that the suprathermal electrons propagating from the sun along the Parker-spiral magnetic field lines are progressively focused into a narrow strahl at heliospheric distances $r\lesssim 1$ AU, while at $r\gtrsim 1$ AU the width of the strahl saturates due to Coulomb collisions and becomes independent of the distance. Our theory of the strahl broadening does not contain free parameters and it agrees with the Wind observations at 1 AU to within $15-20\%$. This indicates that Coulomb scattering, rather than anomalous turbulent diffusion, plays a dominant role in strahl formation in these observations. We further propose that the nearly isotropic halo electron population may be composed of electrons that ran away from the sun as an electron strahl, but later ended up on magnetic field lines leading them back to the sun. Through the effects of magnetic defocusing and Coulomb pitch-angle scattering, a narrow source distribution at large heliocentric distances appears nearly isotropic at distances $\sim$1 AU. The halo electrons are, therefore, not produced locally; rather, they are the fast electrons trapped by magnetic field lines on global heliospheric scales. At the electron energies $K\lesssim 200\mathrm{eV}$, our theory is quite insensitive to the particular mechanism that produces a population of sunward-streaming suprathermal particles. At larger energies, however, our theory indicates that the scattering provided by Coulomb collisions alone is not sufficient to isotropize a narrow sunward-propagating electron beam.

arXiv comment9 pages, 3 figures

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Cited by in corpus (16)

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• Electron heat flux in the near-Sun environment
• 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
• Role of reconnection in inertial kinetic-Alfven turbulence
• 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
• Particle energization in colliding subcritical collisionless shocks investigated in the laboratory
• The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere
• Statistics of Solar Wind Electron Breakpoint Energies Using Machine Learning Techniques
• The Kinetic Expansion of Solar-Wind Electrons: Transport Theory and Predictions for the very Inner Heliosphere
• Parallel Velocity Mixing Yielding Enhanced Electron Heating During Magnetic Pumping
• The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons