Solar wind plasma and magnetic field measurements by Ulysses during its first polar orbit have allowed extensive investigations on the radial evolution of Alfvenic fluctuations in the polar solar wind from 1 to 5 AU. A noteworthy result is that the radial regime changes with heliocentric distance. Up to about 2.5 AU the dominant outward propagating fluctuations decrease more rapidly than the inward ones. Farther out, however, a steepening of the gradient for inward fluctuations is observed. A state is quickly reached where both kinds of fluctuation decline at approximately the same rate in the presence of a surviving excess of outward fluctuation energy. Recent simulations of the nonlinear development of the parametric instability for non-monochromatic Alfven waves have given results that closely resemble the observed polar turbulence behavior. Thus, it would seem that when the generation from velocity gradients becomes negligible, as in the case of polar wind, the parametric instability can play a major role in driving turbulence evolution.
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