  A two-dimensional electromagnetic particle-in-cell simulation with the
realistic ion-to-electron mass ratio of 1836 is carried out to investigate the
electrostatic collisionless shocks in relatively high-speed (~3000 km s^-1)
plasma flows and also the influence of both electrostatic and electromagnetic
instabilities, which can develop around the shocks, on the shock dynamics. It
is shown that the electrostatic ion-ion instability can develop in front of the
shocks, where the plasma is under counter-streaming condition, with highly
oblique wave vectors as was shown previously. The electrostatic potential
generated by the electrostatic ion-ion instability propagating obliquely to the
shock surface becomes comparable with the shock potential and finally the shock
structure is destroyed. It is also shown that in front of the shock the
beam-Weibel instability gradually grows as well, consequently suggesting that
the magnetic field generated by the beam-Weibel instability becomes important
in long-term evolution of the shock and the Weibel-mediated shock forms long
after the electrostatic shock vanished. It is also observed that the secondary
electrostatic shock forms in the reflected ions in front of the primary
electrostatic shock.
