  We present results of two-dimensional fully kinetic Particle-In-Cell
simulation in order to shed light on the role of whistler waves in the
scattering of strahl electrons and in the heat flux regulation in the solar
wind. We model the electron velocity distribution function as initially
composed of core and strahl populations as typically encountered in the
near-Sun solar wind as observed by Parker Solar Probe. We demonstrate that, as
a consequence of the evolution of the electron velocity distribution function,
two branches of the whistler heat flux instability can be excited, which can
drive whistler waves propagating in the direction oblique or parallel to the
background magnetic field. First, oblique whistler waves induce pitch-angle
scattering of strahl electrons, towards higher perpendicular velocities. This
leads to the broadening of the strahl pitch angle distribution and hence to the
formation of a halo-like population at the expense of the strahl. Later on, the
electron velocity distribution function experiences the effect of parallel
whistler waves, which contributes to the redistribution of the particles
scattered in the perpendicular direction into a more symmetric halo, in
agreement with observations. Simulation results show a remarkable agreement
with the linear theory of the oblique whistler heat flux instability. The
process is accompanied by a significant decrease of the heat flux carried by
the strahl population.
