AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 25. HAD II: Celebrating Other Centennials
Historical, Oral, Monday, May 31, 1999, 10:00-11:30am, Marquette

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[25.03] Fifty Years of the Galactic Magnetic Field

E. N. Parker (Enrico Fermi Institute, University of Chicago, Chicago)

Scientific awareness of the galactic magnetic field had its beginning in the years 1948 - 1951 with Fermi's work on cosmic rays, and the observation of the polarization of dust reddened starlight by Hiltner and Hall, and the interpretation of the polarization in terms of magnetically aligned dust grains by Spitzer and Tukey and by Davis and Greenstein. The field was more or less along the local spiral arm, with large fluctuations on scales of 100 pc. Faraday rotation measurements indicated a strength of the order of a few microgauss, and dynamical considerations on the structure of the gaseous disk of the galaxy suggested similar values. By 1966 it was clear that the magnetic field and the cosmic ray gas were tied to the interstellar gas to form an unstable active tripart dynamical system. The system is continually inflated by blast waves from supernovae and particularly by the cosmic ray gas from supernovae, which extend lobes of the galactic magnetic field outward at speeds of the order of 50 km/sec in both directions from the disk of the Galaxy to form the galactic halo. The dissipation of shock waves and the dissipation of magnetic field in the halo evidently provide the multi-million degree gas responsible for the x-ray emission from the halo, best seen from galaxies other than our own. The concentration of magnetic field in and along the spiral density waves forming the spiral arms strongly suggests that the galactic magnetic field represents a progressive dynamo wave. However, the dynamo theory of the galactic magnetic field has yet to establish the nature of the field dissipation that makes the dynamo possible.

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