Broadening of solar wind strahl pitch-angles by the electron/electron instability: Particle-in-cell simulations

[1] Solar wind electrons in the suprathermal energy range from about 70 eV to greater than 1 keV usually are observed as two distinct, relatively tenuous components: an approximately isotropic halo and a strongly anisotropic "strahl'' directed along the magnetic field. This manuscript describes a two-dimensional electromagnetic particle-in-cell simulation in a magnetized, homogeneous, collisionless plasma with such a two-component representation of suprathermal electrons. In this computation the electrostatic electron/electron instability is driven by a strahl with a large average speed along the background magnetic field B-o. The simulation demonstrates that this instability leads to substantial strahl velocity scattering perpendicular to B-o. The most important result here is that, for strahl speeds large compared to the halo thermal speed, after instability saturation the width of the electron pitch-angle distribution exhibits a maximum as a function of electron energy.