Distinguishing between Waves and Turbulence: Variance Anisotropy in MHD

Spacecraft observations and theoretical models indicate that solar windfluctuations have elements interpretable as turbulence and elementsinterpretable as waves. Distinguishing between these can be problematic, particularly when using measurements obtained from a single spacecraft. In nonlinear fluid systems, features that are suggestive of linearized wave modes can also be characteristic of nonlinear activity such as turbulent cascades of energy. For example, in simulations of strong MHD turbulence, nonlinear fluctuations are mostly polarized in the same sense as linear Alfven waves are, but often with little indication that wave behaviour is prominent in the dynamics. Here we report on results from simulations of hydrodynamic and MHDsystems which do not support linearized waves. We focus on the degreeof variance anisotropy which develops, finding that it can be quite similar to that associated with systems that do support linearized waves. Thus variance anisotropy is not necessarily diagnostic of wave activity and should be interpreted with considerable care. The diagnostic sensitivity/acuity of other measures, including the Alfven ratio, will also be reported on. The relevance of these issues to solar wind observations will be discussed.