The Nature of Plasma
The nature of any physical system depends upon the scale on which it is observed; a flower is hardly floral when viewed on the subnuclear scale. The authors identify the most important scales for plasma physics. Complexity in a plasma is rather different, being expressed temporally as much as spatially. It is characterized in the first place by the excitation of a enormous variety of dynamical modes. In particular, magnetized plasmas are anisotropic, responding in different ways to forces that are parallel or perpendicular to the direction. Characteristic plasma behavior is observed only on time scales longer than the plasma period and on length scales larger than the Debye length; the statistical character of that behavior is controlled by the plasma parameter. Indeed, collective plasma dynamics—the occurrence of macroscopic plasma and charge densities, of plasma waves, instabilities, vortices and so on—are most commonly associated with weakly coupled plasmas.