The Slow-Mode Nature of Compressible Wave Power in Solar Wind Turbulence

We use a large, statistical set of measurements from the Wind spacecraft at 1 AU, and supporting synthetic spacecraft data based on kinetic plasma theory, to show that the compressible component of inertial range solar wind turbulence is primarily in the kinetic slow mode. The zero-lag cross-correlation C (δ n , δ B ∥ ) between proton density fluctuations δ n and the field-aligned (compressible) component of the magnetic field δ B ∥ is negative and close to −1. The typical dependence of C (δ n , δ B ∥ ) on the ion plasma beta β i is consistent with a spectrum of compressible wave energy that is almost entirely in the kinetic slow mode. This has important implications for both the nature of the density fluctuation spectrum and for the cascade of kinetic turbulence to short wavelengths, favoring evolution to the kinetic Alfvén wave mode rather than the (fast) whistler mode.