  We report on a detailed study of magnetic fluctuations in the JET pedestal,
employing basic theoretical considerations, gyrokinetic simulations, and
experimental fluctuation data, to establish the physical basis for their
origin, role, and distinctive characteristics. We demonstrate quantitative
agreement between gyrokinetic simulations of microtearing modes (MTMs) and two
magnetic frequency bands with corresponding toroidal mode numbers n=4 and 8.
Such disparate fluctuation scales, with substantial gaps between toroidal mode
numbers, are commonly observed in pedestal fluctuations. Here we provide a
clear explanation, namely the alignment of the relevant rational surfaces (and
not others) with the peak in the omega star profile, which is localized in the
steep gradient region of the pedestal. We demonstrate that a global treatment
is required to capture this effect. Nonlinear simulations suggest that the MTM
fluctuations produce experimentally-relevant transport levels and saturate by
relaxing the background electron temperature gradient, slightly downshifting
the fluctuation frequencies from the linear predictions. Scans in
collisionality are compared with simple MTM dispersion relations. At the
experimental points considered, MTM growth rates can either increase or
decrease with collision frequency depending on the parameters thus defying any
simple characterization of collisionality dependence.
