  The coupling state between ions and neutrals in the interstellar medium plays
a key role in the dynamics of magnetohydrodynamic (MHD) turbulence, but is
challenging to study numerically. In this work, we investigate the damping of
MHD turbulence in a partially ionized medium using 3D two-fluid (ions+neutrals)
simulations generated with the AthenaK code. Specifically, we examine the
velocity, density, and magnetic field statistics of the two-fluid MHD
turbulence in different regimes of neutral-ion coupling. Our results
demonstrate that when ions and neutrals are strongly coupled, the velocity
statistics resemble those of single-fluid MHD turbulence. Both the velocity
structures and kinetic energy spectra of ions and neutrals are similar, while
their density structures can be significantly different. With an excess of
small-scale sharp density fluctuations in ions, the density spectrum in ions is
shallower than that of neutrals. When ions and neutrals are weakly coupled, the
turbulence in ions is more severely damped due to the ion-neutral collisional
friction than that in neutrals, resulting in a steep kinetic energy spectrum
and density spectrum in ions compared to the Kolmogorov spectrum. We also find
that the magnetic energy spectrum basically follows the shape of the kinetic
energy spectrum of ions, irrespective of the coupling regime. In addition, we
find large density fluctuations in ions and neutrals and thus spatially
inhomogeneous ionization fractions. As a result, the neutral-ion decoupling and
damping of MHD turbulence take place over a range of length scales.
