Observed amplitude constraints on Alfvenic fluctuations in the solar wind

Theoretical work on Alfven waves in magnetized plasmas predicts upper limits to fluctuation amplitudes resulting from various instabilities. Here we use plasma and magnetic field data from the NASA Wind spacecraft to show that the amplitudes of perpendicular velocity and magnetic field fluctuations are constrained. The observed high beta threshold is apparently consistent with a self-excited pressure anisotropy instability and the Alfven ratio is also suppressed. At low beta, the constraint is consistent with a simple growth rate for parametric instability of monochromatic Alfven waves. These results can guide expectations for measurements in the inner heliosphere by Solar Probe Plus.