Observations of Magnetic Fields - J.P. Vallée

The observational study of galactic magnetic fields dates back to 1949; an excellent review of the early 30 years has been made by Verschuur (1979). I review here the developments since then and the current state of our observational knowledge on the magnetic fields inside and outside the Milky Way galaxy, for objects with sizes greater than 1 parsec (= 3.2 light-years; = 3.1 × 1016 m). Included are the medium-scale magnetic fields in the isolated globules, dusty elongated clouds and narrow filaments, large interstellar superbubbles, and in large-scale magnetic fields in the spiral arms in our Galaxy and in objects outside our Galaxy out to cosmological distances. The large-scale magnetic fields can act as guides to the low density gas in its motion in the rarefied areas of the interstellar medium, and as tracers of the past dynamical histories of galaxies in motion, linking galactic dynamic with galactic dynamos. Medium-scale magnetic fields can play a support role, supporting clouds against outside pressures or against collapse due to self-gravity. Small-scale magnetic fields play a significant role on smaller-scale phenomena: propagation of cosmic-rays, shock waves, cosmic dust orientation, star formation (although there is little detailed discussion here of magnetic fields on star formation and objects with sizes < 1 parsec).