  Supersonic turbulence in the interstellar medium (ISM) is believed to decay
rapidly within a flow crossing time irrespective of the degree of
magnetization. However, this general consensus of decaying magnetohydrodynamic
(MHD) turbulence relies on local isothermal simulations, which are unable to
take into account the roles of global structures of magnetic fields and the
ISM. Utilizing three-dimensional MHD simulations including interstellar cooling
and heating, we investigate decaying MHD turbulence within cold neutral medium
sheets embedded in warm neutral medium. Early evolution of turbulent kinetic
energy is consistent with previous results for decaying compressible MHD
turbulence characterized by rapid energy decay with a power-law form of
$E\propto t^{-1}$ and by short decay time compared to a flow crossing time. If
initial magnetic fields are strong and perpendicular to the sheet, however,
long-term evolution of the kinetic energy shows that a significant amount of
turbulent energy ($\sim 0.2E_0$) still remains even after ten flow crossing
times for models with periodic boundary conditions. The decay rate is also
greatly reduced as the field strength increases for such initial and boundary
conditions, but not if the boundary conditions are that for a completely
isolated sheet. We analyze velocity power spectra of the remaining turbulence
to show that in-plane, incompressible motions parallel to the sheet dominate at
later times.
