Interplanetary magnetic fluctuation anisotropy in the inertial range

[ 1] We have created a database of interplanetary magnetic field spectra from over 900 separate solar wind intervals at 1 AU using data from the Advanced Composition Explorer ( ACE) spacecraft. These intervals embrace a broad range of solar wind conditions including fast and slow wind conditions, rarefaction regions, shocked plasma, and magnetic clouds. Every attempt was made to develop a database that samples the broadest possible range of solar wind conditions without regard for occurrence frequency. We have examined the ratio of magnetic power in the component perpendicular to the mean field to that parallel to the mean field ( the so-called variance anisotropy) as measured in the high-frequency regime of the inertial range and find it to be strongly correlated to the proton beta. The variance anisotropy may be a proxy for the spectrum of density fluctuations in this region of the spectrum that is unresolved by ACE instruments and that is often unresolved by current flight hardware. The observed correlation with proton beta appears to be in keeping with predictions derived from magnetohydrodynamic turbulence concepts where the compressive component is driven by the incompressible turbulence in the low turbulent Mach number regime. This apparent agreement strongly suggests that the compressive component arises from in situ dynamics and has little if anything to do with solar origins.