Intermittency is a de¿ning characteristic of turbulence, and can be described as "the nonuni-form distriubtion of eddy formations in a stream" [55]. Spatial intermittency occurs when turbulent eddies are generated over a wide range of scales but do not ¿ll the ¿uid volume, only irregu-lar sub-volumes [56]; this is associated with sharp gradients and coherent structures. Temporal intermittency occurs when ¿uctuations appear as "bursts" in time with varying durations and is characteristic of the turbulent cascade. Matthaeus and co-authors review the properties of spatial and temporal intermittency in both ¿uids and plasmas [57]. Intermittency is related to nonlinear phase-space structures called "holes" and "clumps" which were originally found in electrostatic turbulence simulations [58, 59]. Temporal intermittency has been associated with the inverse cas-cade of two-dimensional turbulence, as described in Section 1.3.
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