The localized electron cyclotron resonance heating power that can suppress sawteeth reconnection often drives $m=2\mathrm{}$ tearing modes in a tokamak operating at constant current. The dynamics of mode onset and coupled mode evolution is described in detail and compared with a nonlinear theoretical model that identifies the effects of mode coupling, finite inertia of the rotating islands, and wall braking.

• Received 30 July 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.6038