Magnetic Fields in the Milky Way and in Galaxies (revised version of
  March 2021)
  Most of the visible matter in the Universe is ionized, so that cosmic
magnetic fields are quite easy to generate and due to the lack of magnetic
monopoles hard to destroy. Magnetic fields have been measured in or around
practically all celestial objects, either by in-situ measurements of
spacecrafts or by the electromagnetic radiation of embedded cosmic rays, gas,
or dust. The Earth, the Sun, solar planets, stars, pulsars, the Milky Way,
nearby galaxies, more distant (radio) galaxies, quasars, and even intergalactic
space in clusters of galaxies have significant magnetic fields, and even larger
volumes of the Universe may be permeated by 'dark' magnetic fields. Information
on cosmic magnetic fields has increased enormously as the result of the rapid
development of observational methods, especially in radio astronomy. In the
Milky Way, a wealth of magnetic phenomena was discovered that are only partly
related to objects visible in other spectral ranges. The large-scale structure
of the Milky Way's magnetic field is still under debate. The available data for
external galaxies can well be explained by field amplification and ordering via
the dynamo mechanism. The measured field strengths and the similarity of field
patterns and flow patterns of the diffuse ionized gas give strong indication
that galactic magnetic fields are dynamically important. They may affect the
formation of spiral arms, outflows, and the general evolution of galaxies. In
spite of our increasing knowledge on magnetic fields, many important questions
on the origin and evolution of magnetic fields, like their first occurrence in
young galaxies, or the existence of large-scale intergalactic fields remained
unanswered. 'Cosmic magnetism' is a key science project for several existing
and planned radio telescopes, like LOFAR and the SKA.
