  A joint analysis is done of the radio and X-ray observations of SN 1993J. It
is argued that neither synchrotron cooling behind the forward shock nor thermal
cooling behind the reverse shock is supported by observations. In order for
adiabatic models to be consistent, a reinterpretation of the radius of the
spatially resolved VLBI-source is needed during the first few hundred days.
Instead of reflecting the position of the forward shock, it is then associated
with the expansion of the Rayleigh-Taylor unstable region emanating from the
contact discontinuity. Although observations imply a constant ratio between the
energy densities in magnetic fields and relativistic electrons, they do not
appear to scale individually with the thermal energy density behind the forward
shock; rather, in adiabatic models, the evolution of the magnetic field
strength is best understood as scaling inversely with the supernova radius.
