  Field-line localized ballooning modes have been observed at the edge of high
confinement mode plasmas in ASDEX Upgrade with rotating 3D perturbations
induced by an externally applied n = 2 error field and during a moderate level
of edge localized mode-mitigation. The observed ballooning modes are localized
to the field-lines which experience one of the two zero-crossings of the radial
flux surface displacement during one rotation period. The localization of the
ballooning modes agrees very well with the localization of the largest growth
rates from infinite-n ideal ballooning stability calculations using a realistic
3D ideal magnetohydrodynamic equilibrium. This analysis predicts a lower
stability with respect to the axisymmetric case. The primary mechanism for the
local lower stability is the 3D distortion of the local magnetic shear.
