# In what cases should plasmas be considered compressible?

I was listening to a lecture where it was mentioned incompressibility was assumed during derivation of an electrostatic instability, but incompressibility was not assumed for an electromagnetic instability. Although I was not given further explanation for the physical reasoning for the intuition behind this step for these particular derivations. Thus, more generally, when should incompressibility be approximately valid (e.g. fluid flow is much smaller than sound speed)?

• @EuklidAlexandria - That is not true. There are pressure-balance modes in plasmas that require compressibility to exist. – honeste_vivere Aug 30 '18 at 14:34