  We examine with a particle-in-cell (PIC) simulation the collision of two
equally dense clouds of cold pair plasma. The clouds interpenetrate until
instabilities set in, which heat up the plasma and trigger the formation of a
pair of shocks. The fastest-growing waves at the collision speed c/5 and low
temperature are the electrostatic two-stream mode and the quasi-electrostatic
oblique mode. Both waves grow and saturate via the formation of phase space
vortices. The strong electric fields of these nonlinear plasma structures
provide an efficient means of heating up and compressing the inflowing upstream
leptons. The interaction of the hot leptons, which leak back into the upstream
region, with the inflowing cool upstream leptons continuously drives
electrostatic waves that mediate the shock. These waves heat up the inflowing
upstream leptons primarily along the shock normal, which results in an
anisotropic velocity distribution in the post-shock region. This distribution
gives rise to the Weibel instability. Our simulation shows that even if the
shock is mediated by quasi-electrostatic waves, strong magnetowaves will still
develop in its downstream region.
