ABSTRACT
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
and in
, where
and
are the perpendicular and parallel temperatures of the protons, B is the magnetic field strength, and
is the proton density. If the amplitude of the compressive fluctuations is large enough,
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
away from the marginal stability boundary until the fluctuating value of
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
is close to unity.
