  A variety of studies of magnetised plasma turbulence invoke theories for the
advection of a passive scalar by turbulent fluctuations. Examples include
modelling the electron density fluctuations in the interstellar medium,
understanding the chemical composition of galaxy clusters and the intergalactic
medium, and testing the prevailing phenomenological theories of
magnetohydrodynamic turbulence. While passive scalar turbulence has been
extensively studied in the hydrodynamic case, its counterpart in MHD turbulence
is significantly less well understood. Herein we conduct a series of
high-resolution direct numerical simulations of incompressible, field-guided,
MHD turbulence in order to establish the fundamental properties of passive
scalar evolution. We study the scalar anisotropy, establish the scaling
relation analogous to Yaglom's law, and measure the intermittency of the
passive scalar statistics. We also assess to what extent the pseudo Alfven
fluctuations in strong MHD turbulence can be modelled as a passive scalar. The
results suggest that the dynamics of a passive scalar in MHD turbulence is
considerably more complicated than in the hydrodynamic case.
