  The plasma response from an external n = 2 magnetic perturbation field in
ASDEX Upgrade has been measured using mainly electron cyclotron emission (ECE)
diagnostics and a rigid rotating field. To interpret ECE and ECE-imaging
(ECE-I) measurements accurately, forward modeling of the radiation transport
has been combined with ray tracing. The measured data is compared to synthetic
ECE data generated from a 3D ideal magnetohydrodynamics (MHD) equilibrium
calculated by VMEC. The measured amplitudes of the helical displacement around
the low field side midplane are in reasonable agreement with the one from the
synthetic VMEC diagnostics. Both exceed the prediction from the vacuum field
calculations and indicate the presence of a kink response at the edge, which
amplifies the perturbation. VMEC and MARS-F have been used to calculate the
properties of this kink mode. The poloidal mode structure of the magnetic
perturbation of this kink mode at the edge peaks at poloidal mode numbers
larger than the resonant components |m| > |nq|, whereas the poloidal mode
structure of its displacement is almost resonant |m| ~ |nq|. This is expected
from ideal MHD in the proximity of rational surfaces. The displacement measured
by ECE-I confirms this resonant response.
