Magnetohydrodynamic (MHD) activity during edge localized modes (ELMs) has been monitored in the ASDEX tokamak. Besides a fast inward shift of the plasma column, a helical MHD perturbation has been found to accompany ELMs. For the discharges investigated, the helical instability has a poloidal mode number m of 3 or 4 and a toroidal mode number n of 1, and it propagates in the direction of the electron diamagnetic drift. It seems to be identical with the Toi mode, which has been observed to disappear during L-H transitions.
2D ECE measurements of type-I edge localized modes at ASDEX Upgrade