Compressive fluctuations are a minor yet significant component of
astrophysical plasma turbulence. In the solar wind, long-wavelength compressive
slow-mode fluctuations lead to changes in $\beta_{\parallel \mathrm p}\equiv
8\pi n_{\mathrm p}k_{\mathrm B}T_{\parallel \mathrm p}/B^2$ and in $R_{\mathrm
p}\equiv T_{\perp \mathrm p}/T_{\parallel \mathrm p}$, where $T_{\perp \mathrm
p}$ and $T_{\parallel \mathrm p}$ are the perpendicular and parallel
temperatures of the protons, $B$ is the magnetic field strength, and
$n_{\mathrm p}$ is the proton density. If the amplitude of the compressive
fluctuations is large enough, $R_{\mathrm p}$ crosses one or more instability
thresholds for anisotropy-driven microinstabilities. The enhanced field
fluctuations from these microinstabilities scatter the protons so as to reduce
the anisotropy of the pressure tensor. We propose that this scattering drives
the average value of $R_{\mathrm p}$ away from the marginal stability boundary
until the fluctuating value of $R_{\mathrm p}$ stops crossing the boundary. We
model this "fluctuating-anisotropy effect" using linear Vlasov--Maxwell theory
to describe the large-scale compressive fluctuations. We argue that this effect
can explain why, in the nearly collisionless solar wind, the average value of
$R_{\mathrm p}$ is close to unity.
