  Both global dynamics and turbulence in magnetized weakly collisional cosmic
plasmas are described by general magnetofluid equations that contain pressure
anisotropies and heat fluxes that must be calculated from microscopic plasma
kinetic theory. It is shown that even without a detailed calculation of the
pressure anisotropy or the heat fluxes, one finds the macroscale dynamics to be
generically unstable to microscale Alfvenically polarized fluctuations. Two
instabilities are considered in detail: the parallel firehose instability
(including the finite-Larmor-radius effects that determine the fastest growing
mode) and the gyrothermal instability (GTI). The latter is a new result - it is
shown that a parallel ion heat flux destabilizes Alfvenically polarized
fluctuations even in the absence of the negative pressure anisotropy required
for the firehose. The main conclusion is that both pressure anisotropies and
heat fluxes trigger plasma microinstabilities and, therefore, their values will
likely be set by the nonlinear evolution of these instabilities. Ideas for
understanding this nonlinear evolution are discussed. It is argued that cosmic
plasmas will generically be "three-scale systems," comprising global dynamics,
mesoscale turbulence and microscale plasma fluctuations. The astrophysical
example of cool cores of galaxy clusters is considered and it is noted that
observations point to turbulence in clusters being in a marginal state with
respect to plasma microinstabilities and so it is the plasma microphysics that
is likely to set the heating and conduction properties of the intracluster
medium. In particular, a lower bound on the scale of temperature fluctuations
implied by the GTI is derived.
