  Ion cyclotron emission (ICE) is detected during edge localised modes (ELMs)
in the KSTAR tokamak at harmonics of the proton cyclotron frequency in the
outer plasma edge. The emission typically chirps downward (occasionally upward)
during ELM crashes, and is driven by confined 3MeV fusion-born protons that
have large drift excursions from the plasma core. We exploit fully kinetic
simulations at multiple plasma densities to match the time-evolving features of
the chirping ICE. This yields a unique, very high time resolution diagnostic of
the collapsing edge pedestal density.
