Formation and evolution of pair-Alfvén shocks with PIC simulations

We consider the recently discovered pair-Alfvén shock wave occurring in collisionless electron–positron plasmas. We perform a series of particle-in-cell studies in one and two dimensions in order to determine the stability conditions for such a shock and the mechanisms which sustain its growth. Building on our previous simulations, which established that these shocks are initially mediated by the Weibel instability before becoming Alfvénic, we demonstrate that the shock is sustained by self-generated Alfvén waves overtaking the shock in the upstream plasma. As a result, growth is only possible when the guiding magnetic field strength, hence the Alfvén speed, is sufficiently small, ωc ≲ 0.4 (in normalized units). Furthermore, the production of the waves in the upstream is dependent on a resonance between the Alfvén wave mode and the thermal noise in the plasma, which is inhibited at high magnetization. This explains the conditional absence of this type of shock identified previously.