  The potential role of magnetic fields and cosmic ray propagation for feedback
processes in the early Universe can be probed by studies of local starburst
counterparts with an equivalent star-formation rate. Archival data from the
WSRT was reduced and a new calibration technique introduced to reach the high
dynamic ranges needed for the complex source morphology of M82. This data was
combined with archival VLA data, yielding total power maps at 3cm, 6cm, 22cm
and 92cm. The data shows a confinement of the emission at wavelengths of 3/6cm
to the core region and a largely extended halo reaching up to 4kpc away from
the galaxy midplane at wavelengths of 22/92cm up to a sensitivity limit of
90muJy and 1.8mJy respectively. The results are used to calculate the magnetic
field strength in the core region to 98muG and to 24muG in the halo regions.
From the observation of free-free losses the filling factor of the ionised
medium could be estimated to 2%. We find that the radio emission from the core
region is dominated by very dense HII-regions and supernova remnants, while the
surrounding medium is filled with hot X-ray and neutral gas. Cosmic rays
radiating at frequencies higher than 1.4 GHz are suffering from high
synchrotron and inverse Compton losses in the core region and are not able to
reach the halo. Even the cosmic rays radiating at longer wavelengths are only
able to build up the observed kpc sized halo, when several starbursting periods
are assumed where the photon field density varies by an order of magnitude.
These findings together with the strong correlation between Halpha, PAH+, and
our radio continuum data suggests a magnetic field which is frozen into the
ionised medium and driven out of the galaxy kinematically.
