  When a pulsar is moving through a partially ionized medium, a fraction of
neutral Hydrogen atoms penetrate inside the pulsar wind and can be
photo-ionized by the nebula UV radiation. The resulting protons remains
attached to the magnetic field of the light leptonic pulsar wind enhancing its
inertia and changing the flow dynamics of the wind. We present here the first
numerical simulations of such effect in the tails of bow shock nebulae. We
produce a set of different models representative of pulsars moving in the
interstellar medium with different velocities, from highly subsonic to
supersonic, by means of 2D hydrodynamic relativistic simulations. We compare
the different tail morphologies with results from theoretical models of mass
loading in bow shocks. As predicted by analytical models we observe a fast
sideways expansion of the tail with the formation of secondary shocks in the
ISM. This effect could be at the origin of the head-and-shoulder morphology
observed in many BSPWNe.
