  Plasma equilibria reconstructed from the Mega-Amp Spherical Tokamak (MAST)
have sufficient resolution to capture plasma evolution during the short period
between edge-localized modes (ELMs). Immediately after the ELM steep gradients
in pressure, P, and density, ne, form pedestals close to the separatrix, and
they then expand into the core. Local gyrokinetic analysis over the ELM cycle
reveals the dominant microinstabilities at perpendicular wavelengths of the
order of the ion Larmor radius. These are kinetic ballooning modes (KBMs) in
the pedestal and microtearing modes (MTMs) in the core close to the pedestal
top. The evolving growth rate spectra, supported by gyrokinetic analysis using
artificial local equilibrium scans, suggest a new physical picture for the
formation and arrest of this pedestal.
