We investigate the detailed processes at work in the drift of magnetic fields in molecular clouds. To the frictional force, whereby the magnetic force is transmitted to neutral molecules, ions contribute more than half only at cloud densities nH 104 cm-3, and charged grains contribute more than about 90% at nH 106 cm-3. Thus, grains play a decisive role in the process of magnetic flux loss. Approximating the flux loss time tB by a power law tB ∝ B-γ, where B is the mean field strength in the cloud, we find γ ≈ 2, characteristic of ambipolar diffusion, only at nH 107 cm-3, at which ions and the smallest grains are pretty well frozen to the magnetic fields. At nH 107 cm-3, γ decreases steeply with nH, and finally at nH ≈ ndec ≈ a few × 1011 cm-3, at which the magnetic fields effectively decouple from the gas, γ 1 is attained, reminiscent of Ohmic dissipation, although flux loss occurs about 10 times faster than by pure Ohmic dissipation. Because even ions are not very well frozen at nH 107 cm-3, ions and grains drift slower than the magnetic fields. This insufficient freezing makes tB more and more insensitive to B as nH increases. Ohmic dissipation is dominant only at nH 1 × 1012 cm-3. While ions and electrons drift in the direction of the magnetic force at all densities, grains of opposite charges drift in opposite directions at high densities, at which grains are major contributors to the frictional force. Although magnetic flux loss occurs significantly faster than by Ohmic dissipation even at very high densities, such as nH ≈ ndec, the process going on at high densities is quite different from ambipolar diffusion, in which particles of opposite charges are supposed to drift as one unit.
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