COMMENTS ON ALFVEN WAVES AND PLASMA DRIFT IN A SLIGHTLY IONIZED-GAS

It is well known that there exist two critical wavelengths for Alfven waves in a slightly ionized medium. One of the two is lambda(1) = pi V -A tau(ni), where V-A is the Alfven speed in the medium and tau(ni) is the friction timescale for neutral particles due to ions. The other i s lambda(2) = 4 pi V(A)()tau(in), where V-A(*) is the Alfven speed in the ions and tau(in) is the friction timescale for the ions, respecti vely. In typical molecular clouds, lambda(2) is much smaller than lamb da(1). We let lambda() denote the intermediate range: lambda(2) < lam bda() < lambda(1), where the so-called ion-neutral damping of the Alf ven waves occurs. In this paper, we show that magnetic forces affect t he neutral fluid even at lambda(). This is possible when lambda(2) is much larger than the mean free path Lambda of the neutral particles f or neutral-neutral collision. When tau(in), is much longer than the co llision timescale among the neutral particles, tau(nn), the communicat ion timescale of the magnetic fields and the neutral fluid is tau(in). If tau(in) much greater than tau(nn), the neutral fluid can adjust it self to the magnetic forces during tau(in). For typical molecular clou ds, it is estimated that Lambda much less than lambda(2) and tau(nn) m uch less than tau(in) (much less than tau(ni)). It may suggest that ma gnetically subcritical cloud cores contract quasi-statically with magn etic flux loss, even if the size is lambda(). On the other hand, if a disturbance scale of the magnetic held is smaller than lambda(2), the magnetic stress can escape as Alfven waves that couple almost exclusi vely with the ions. In addition, we point out that it is difficult for the Jeans length to become smaller than lambda(2) in isolated molecul ar clouds or cores. Observational implications are also presented.