Collisionless Subcritical Shocks

Subcritical shocks have Mach numbers below critical. This chapter gives a brief overview as a preparatory for the discussion of collisionless supercritical shocks in the next chapters. For subcritical shocks (anomalous) resistive or viscous dissipation in combination with dispersion is high enough to stabilise the shock transition. Observations of subcritical shocks in space are sparse, mostly restricted to interplanetary space. We discuss the theory of subcritical shocks, define the dissipation length and discuss the cold plasma subcritical shock model. Finally, the reasons for collisionless dissipation in subcritical shocks are illuminated.