Mercier criterion for the lower helical modes

A system of ideal MHD equations describing plasma perturbations near the resonance surface with margin q=m/n is considered. It is shown that under all m and n numbers the system indicated is reduced to one and the same equation identical to the Mercier mode equation derived within m>>1. A conclusion is made that in all helical type perturbations there is an oscillation branch, called a quasirearrangement mode, which is characterized by an oscillating structure near the resonance surface. This mode becomes unstable under the Mercier criterion violation