  Microtearing modes (MTMs) are unstable in the shallow gradient region just
inside the top of the pedestal in the spherical tokamak experiment MAST, and
may play an important role in the pedestal evolution. The linear properties of
these instabilities are compared with MTMs deeper inside the core, and further
detailed investigations expose the basic drive mechanism. MTMs near the MAST
pedestal top are not well described by existing theories. In particular the
growth rate of the dominant edge MTM does not peak at a finite collision
frequency, as frequently reported for MTMs further into the core. Our study
suggests that the edge MTM is driven by a collisionless trapped particle
mechanism that is sensitive to magnetic drifts. This drive is enhanced in the
outer region of MAST by a high magnetic shear and a high trapped particle
fraction. Observations of similar modes on conventional aspect ratio devices
suggests this drive mechanism may be somewhat ubiquitous towards the edge of
current day and future hot tokamaks.
