  Galactic outflows driven by starbursts can modify the galactic magnetic
fields and drive them away from the galactic planes. Here, we quantify how
these fields may magnetize the intergalactic medium. We estimate the strength
and structure of the fields in the starburst galaxy M82 using thermal polarized
emission observations from SOFIA/HAWC+ and a potential field extrapolation
commonly used in solar physics. We modified the Davis-Chandrasekhar-Fermi
method to account for the large-scale flow and the turbulent field. Results
show that the observed magnetic fields arise from the combination of a
large-scale ordered potential field associated with the outflow and a
small-scale turbulent field associated with bow-shock-like features. Within the
central $900$ pc radius, the large-scale field accounts for $53\pm4$% of the
observed turbulent magnetic energy with a median field strength of $305\pm15$
$\mu$G, while small-scale turbulent magnetic fields account for the remaining
$40\pm5$% with a median field strength of $222\pm19$ $\mu$G. We estimate that
the turbulent kinetic and turbulent magnetic energies are in close
equipartition up to $\sim2$ kpc (measured), while the turbulent kinetic energy
dominates at $\sim7$ kpc (extrapolated). We conclude that the fields are frozen
into the ionized outflowing medium and driven away kinetically. The magnetic
field lines in the galactic wind of M82 are `open,' providing a direct channel
between the starburst core and the intergalactic medium. Our novel approach
offers the tools needed to quantify the effects of outflows on galactic
magnetic fields as well as their influence on the intergalactic medium and
evolution of energetic particles.

Astrophysics of Galaxies