  Magneto-opitcal studies of a c-oriented epitaxial MgB2 film show that below
10 K the global penetration of vortices is dominated by complex dendritic
structures abruptly entering the film. We suggest that the observed behavior is
due to a thermo-magnetic instability which is supported by vortex dynamics
simulations. The instability is also responsible for large fluctuations in the
magnetization curves in MgB2 at low temperatures.
