Poinsar6 maps define topographpSof Vlasov distribution functions con&tent with ktbchastic dynads

In a recent paper [A. D. Bailey et al., Phys. Rev. Lett. 34,3124 (1993)], the authors presented direct planar laser induced. fluorescence measurements of the oscillatory ion fluid velocity field in the presence of a large amplitude drift-Alfvenj wave. Surprisingly, the measured speeds were an order of magnitude lower than predicted by standard fluid theory, yet the flow pattern was consistent with the fluid theory. A new model, based on the connection between stochasticity and bulk behavior, is presented which gives insights into the cause of this behavior. It is shown that when particle motion is stochastic, invariant sets of a ‘Poincare map’ define a flat-topped particle distribution function consistent with both the electromagnetic held driving the Vlasov equation and the fine-scale single particle dynamics. The approach is-described for the general case and explored for a slab model of the observed drift wave. 8 1995 American Institute of Physics.