  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(delta n,
delta B_parallel) between proton density fluctuations delta n and the
field-aligned (compressible) component of the magnetic field delta B_parallel
is negative and close to -1. The typical dependence of C(delta n,delta
B_parallel) on the ion plasma beta_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 Alfven wave mode rather than the (fast)
whistler mode.
