  Synchrotron radiation images from runaway electrons (REs) in an ASDEX Upgrade
discharge disrupted by argon injection are analyzed using the synchrotron
diagnostic tool SOFT and coupled fluid-kinetic simulations. We show that the
evolution of the runaway distribution is well described by an initial hot-tail
seed population, which is accelerated to energies between 25-50 MeV during the
current quench, together with an avalanche runaway tail which has an
exponentially decreasing energy spectrum. We find that, although the avalanche
component carries the vast majority of the current, it is the high-energy seed
remnant that dominates synchrotron emission. With insights from the
fluid-kinetic simulations, an analytic model for the evolution of the runaway
seed component is developed and used to reconstruct the radial density profile
of the RE beam. The analysis shows that the observed change of the synchrotron
pattern from circular to crescent shape is caused by a rapid redistribution of
the radial profile of the runaway density.
