  Magnetic fields play an important role in shaping the structure and evolution
of the interstellar medium (ISM) of galaxies, but the details of this
relationship remain unclear. With SKA1, the 3D structure of galactic magnetic
fields and its connection to star formation will be revealed. A highly
sensitive probe of the internal structure of the magnetoionized ISM is the
partial depolarization of synchrotron radiation from inside the volume.
Different configurations of magnetic field and ionized gas within the
resolution element of the telescope lead to frequency-dependent changes in the
observed degree of polarization. The results of spectro-polarimetric
observations are tied to physical structure in the ISM through comparison with
detailed modeling, supplemented with the use of new analysis techniques that
are being actively developed and studied within the community such as Rotation
Measure Synthesis. The SKA will enable this field to come into its own and
begin the study of the detailed structure of the magnetized ISM in a sample of
nearby galaxies, thanks to its extraordinary wideband capabilities coupled with
the combination of excellent surface brightness sensitivity and angular
resolution.
