The Zeeman effect can be used to measure interstellar magnetic fields. It is the only existing method for measuring magnetic field strengths in molecular clouds (Troland).
The 21 cm line of H I is an example of a radio frequency spectral line that is suitable for Zeeman effect measurements. An external magnetic field splits the upper level of the 21-cm line into three levels. The splitting between the highest and the lowest levels is much smaller than the typical line width in most cases; the observed splitting is then proportional only to the parallel component of the magnetic field. To detect the splitting, one observes the difference between the two circular polarizations. This difference is the frequency derivative of the line profile with an amplitude proportional to (Bpar/del nu), where del nu is the line width (Verschuur & Kellerman).
Zeeman splitting was first detected in the 21-cm line by Verschuur. Other radio frequency spectral lines originating in molecular clouds come from molecules. However, the Zeeman effect in molecular lines is almost always undetectably small. The 1665 and 1667 MHz lines of OH are very sensitive probes of the Zeeman effect in molecular clouds.
Zeeman effect observations are useful in studying the interstellar magnetic field because they provide an estimate of the field strength. Further, they sample relatively localized regions along the line-of-sight where the spectral line originates. The disadvantage of Zeeman effect measurements lies in the fact the only the radial component of the field is usually measured. Also, field reversals within the telescope beam can lead to cancellation of the observed effect. The latter problem can usually be solved by using aperture synthesis techniques, which provide higher spatial resolution of the Zeeman effect, mitigating the problem of field cancellation across the beam of a single dish.
In spite of its limitations, the Zeeman effect in radio frequency spectral lines provides a strict lower limit on the magnetic field. Since it is the only available technique to probe field strengths, its limitations must be accepted as the fundamental current limitations upon our ability to observationally measure magnetic fields.
1. Troland, T. H. Observational Aspects of Magnetic Fields [Return to document]
2. Verschuur & Kellerman, Galactic and Extragalactic Radio Astronomy[Return to document]
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