Two New Results in Solar Wind Turbulence

We have created a data base 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 magenetic clouds. Every attempt was made to identify samples from 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. We have also investigated the nature of turbulent magnetic dissipation range. Focussing on the spectral properties at spacecraft frequencies ≥ 0.5 Hz, we show that while the inertial range at lower frequencies displays a tightly constrained range of spectral indexes, the dissipation range exhibits a broad range of power law indexes. We show that the explanation for this variation lies with the dependence of the dissipation range spectrum on the rate of energy cascade through the inertial range such that steeper spectral forms result from greater cascade rates.