Shocks are solutions to the hydrodynamical equations. However, these are 
valid only on scales much larger than the mean free path of particles. 
In dilute plasmas such as the interstellar medium we speak of collisionless 
shocks to indicate that electromagnetic fields are responsible for the 
dissipation of bulk-flow energy.
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a) Discuss whether or not you expect a different shock thickness for shocks that
propagate into an ionized medium, as opposed to neutral gas.
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b) For an ionized upstream medium, do you expect a different mean free path of electrons and ions? If so, what might be the consequences?