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Many parts of the interstellar medium, such as HII regions, the Diffuse Ionized Gas, and star formation regions, consist of partially ionized plasmas. Ion-neutral collisions are therefore potentially an important process. There have been many suggestions in the literature that collisional processes may be extremely important in star formation regions, by determining a minimum irregularity or clump size. This conclusion is drawn from the linear theory of magnetohydrodynamic waves, which predicts that waves become evanescent when the wave frequency is roughly equal to the ion neutral collision frequency. This condition is fulfilled in the diffuse interstellar medium for waves with wavelengths of order $10^{16}$ cm. We have recently presented results on the interstellar density fluctuation spatial power spectrum which shows no evidence of a break or other spectral feature at the collisional scale. This result may indicate that a collisionally-determined minimum clump size is not physically relevant for the interstellar medium. A possible explanation for this result is that interstellar plasma turbulence does not behave like an ensemble of magnetohydrodynamic waves.
