2-DIMENSIONAL SIMULATIONS OF SUPERCRITICAL QUASI-PARALLEL SHOCKS - UPSTREAM WAVES, DOWNSTREAM WAVES, AND SHOCK RE-FORMATION

Two-dimensional hybrid (particle ions, massless fluid electrons) simul ations of quasi-parallel collisionless shocks are carried out in order to investigate the upstream wave properties, the shock re-formation p rocess, and the downstream turbulence. The two-dimensional simulations confirm the results of earlier one-dimensional simulations. When back streaming diffuse ions are retained re-formation of a shock with an up stream magnetic field - shock normal angle Of THETA(Bno) = 30-degrees occurs as a result of upstream low-frequency waves which steepen, beco me pulsationlike structures and take over as the re-formed shock. The upstream waves are initially aligned with the shock normal; later in t he run the waves become more and more aligned with the upstream magnet ic field. However, when approaching the shock, the wave vectors are re fracted in the region of increasing diffuse ion density into the shock normal direction so that shock re-formation is again coherent along t he shock surface. In addition, re-formation on a smaller scale and out of phase along the shock front is due to more or less specularly refl ected ions. Re-formation Of a THETA(Bno) = 10-degrees shock is due to locally at the shock ramp emerging waves. These are attributed to the so-called interface instability in the region of partial overlap betwe en the incident cold solar wind and part of the hot downstream distrib ution. These waves emerge in phase along the shock surface and thus re -formation is in this more parallel case also coherent along the shock . At medium Alfven Mach number (M(A) approximately 5) shocks, upstream waves which are aligned with the upstream magnetic field are convecte d into the shock and produce ripples on the shock surface. At higher M ach number (M(A) approximately 9) the shock surface becomes less coher ent and the local value of the shock normal - magnetic field angle var ies greatly. The re-formation length scale is larger than in the lower Mach number case. The turbulence downstream reflects the two mechanis ms of shock reformation: in the THETA(Bno) = 30-degrees case the upstr eam pulsations are mode converted when convected through the shock lay er. In the THETA(Bno) = 10-degrees case the downstream turbulence resu lts from the local instability at the shock front.