Electron strahl and halo formation in the solar wind

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 less than or similar to 1 au, while at r greater than or similar to 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 Wind observations of the strahl width at 1 au to within 15-20 per cent, for widths that are resolvable by the instrument. 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 halo electron population at energies K less than or similar to 200 eV 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. The halo electrons are therefore not produced locally; rather, they are the fast electrons trapped by magnetic field lines on global heliospheric scales. Through the effects of magnetic defocusing and Coulomb pitch-angle scattering, a narrow source distribution at large heliocentric distances appears nearly isotropic at distances similar to 1 au. At larger energies K greater than or similar to 200 eV, however, our theory indicates that the scattering provided by Coulomb collisions alone is not sufficient to isotropize a narrow sunward-propagating electron beam.