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

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 Bo. The simulation demonstrates that this instability leads to substantial strahl velocity scattering perpendicular to Bo. 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.