  Magneto-optical imaging is used to visualize vortex avalanches in MgB2 films
at 4K. Avalanches ranging from 50 to 50000 vortices were detected. The size
distribution function has a clear peak whose position moves towards larger
sizes as the applied field increases. This field dependence as well as
variation of flux density profile during an avalanche are well described by a
proposed model assuming a thermal origin of the avalanches. The model is based
on the adiabatic approach and takes into account nonlocal electrodynamics in
thin superconductors. The threshold field for thermal avalanches is predicted
to be much smaller than that for thick superconductors, in agreement with the
experiment.
