Radio Astronomy Techniques of Observing Magnetic Fields: The Galaxy

Abstract

Optical techniques were first used to detect magnetic fields in cosmic objects. Hale used the Zeeman effect to detect solar magnetic fields in 1908. Babcock extended the Zeeman technique to show the existence of magnetic fields in Ap stars in 1946. Optical polarization observations were made as early as 1920 by W.F. Meyer who observed the Hubble variable nebula NGC 2261. Polarization observations of the Andromeda nebula were made by Öhman in 1942. However at first the interpretation of these optical polarization observations was in terms of scattered light only. The theoretical work of Davis and Greenstein suggested that optical polarization could also be due to dust grains aligned in magnetic fields. Observations of Hiltner and Hall supported this interpretation. Extensive surveys of starlight polarization were made by many observers giving information about nearby magnetic fields in the Galaxy. Optical polarization observations of galaxies gave some information on the Magellanic Clouds and other nearby objects but due to lack of sensitivity progress was slow.