We have assembled a data base of over 800 intervals of solar wind observations recorded by the ACE spacecraft at 1 AU and examined the computed spectral characteristics of the magnetic fluctuations in the inertial range. We find a strong correlation between the variance anisotropy of the magnetic fluctuations, the proton plasma beta, and the amplitude of the total magnetic fluctuation. It is not possible to say whether the proton plasma beta and magnetic fluctuation amplitude are related by in situ dynamics, but it seems unlikely. It does seem clear that the variance anisotropy for solar wind magnetic field fluctuations is linked to either the proton beta or the amplitude of the fluctuations or both. Several theories for interplanetary magnetic fluctuations predict this. There are important implications for the nature of solar wind turbulence and the rate of spectral cascade. This analysis appears to support published explanations for the observed compressive component.
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