Natural velocity of magnetic islands.

The phase velocity of magnetic islands is calculated in the semicollisional regime with cold ions. Two solution branches arise, corresponding to islands propagating with the ions and with the electrons. For the ion branch the phase velocity and the polarization current are small. For the electron branch, the phase velocity depends on the ratio of W, the half-width of the island, and rho(s), the Larmor radius calculated with the electron temperature. For W >>rho(s) the phase velocity is larger than the electron drift velocity and the polarization current is destabilizing. For W << rho(s), the situation is reversed provided that the density and temperature gradients point in the same direction.