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The coupling state between ions and neutrals in the interstellar medium playsa key role in the dynamics of magnetohydrodynamic (MHD) turbulence, but ischallenging to study numerically. In this work, we investigate the damping ofMHD turbulence in a partially ionized medium using 3D two-fluid (ions+neutrals)simulations generated with the AthenaK code. Specifically, we examine thevelocity, density, and magnetic field statistics of the two-fluid MHDturbulence in different regimes of neutral-ion coupling. Our resultsdemonstrate that when ions and neutrals are strongly coupled, the velocitystatistics resemble those of single-fluid MHD turbulence. Both the velocitystructures and kinetic energy spectra of ions and neutrals are similar, whiletheir density structures can be significantly different. With an excess ofsmall-scale sharp density fluctuations in ions, the density spectrum in ions isshallower than that of neutrals. When ions and neutrals are weakly coupled, theturbulence in ions is more severely damped due to the ion-neutral collisionalfriction than that in neutrals, resulting in a steep kinetic energy spectrumand density spectrum in ions compared to the Kolmogorov spectrum. We also findthat the magnetic energy spectrum basically follows the shape of the kineticenergy spectrum of ions, irrespective of the coupling regime. In addition, wefind large density fluctuations in ions and neutrals and thus spatiallyinhomogeneous ionization fractions. As a result, the neutral-ion decoupling anddamping of MHD turbulence take place over a range of length scales.

Damping of MHD Turbulence in A Partially Ionized Medium