2.1.3. Radio Zeeman Effect
Another method to determine the magnetic field strengths uses the Zeeman effect. It involves the separation of two opposite circularly polarized components of the radiation (Stokes parameter V). This separation is proportional to the strength of the component of the magnetic field parallel to the line-of-sight. Zeeman splitting at centimeter and millimeter wavelengths, acting on circularly polarized radiation emitted by neutral atoms (HI) or molecules (OH, SiO, H2O, CN), allows the study of magnetic fields in large HI filaments in low density gas outside clouds, and in substellar hot spots at high thermal gas densities and small object sizes. The instruments most employed are located at Green Bank, Hat Creek, Very Large Array, and Effelsberg. A recent review can be found in Heiles (1987) and Crutcher (1994). Zeeman splitting provides a component of the magnetic field strength in some individual clouds, but there are not enough such data to shed light on the mid-scale and large-scale magnetic fields in the Milky Way galaxy (e.g., Heiles et al. 1993; Heiles 1996b).
At low magnetic field values near 10 µGauss, many Zeeman detection claims by Heiles & Troland (1982), Heiles (1988), Goodman & Heiles (1994), and others, have been followed by several counter-claims that deny many previous Zeeman detection claims in nearby HI clouds, based on instrumental problems having to do with the 'squint' effect of single-dish telescopes, or the polarized sidelobes of the telescopes used, creating spurious signals capable of mimicking magnetic fields (e.g., Verschuur 1995a, 1995b; Wielebinski 1995; Kazès and Crutcher 1986).
At magnetic field levels near 100 µGauss, claims of detections of the Zeeman effect near a few star-forming regions were made (e.g., Roberts et al. 1996). Some counter-claims have appeared (e.g., Crutcher et al. 1996b) could not detect the Zeeman effect above the detector noise in molecular cloud cores with gas densities of 106 cm-3, putting upper limits of around 200 µGauss, which is well below the expected virial values of about 600 µGauss. A 100 µGauss detection (Güsten and Fiebig, 1990) has been withdrawn (Troland & Heiles, 1986).
At magnetic field levels near 500 µGauss, detections by Zeeman effect of a magnetic field of about 500 µGauss in Sagittarius B2 near the Galactic center were made (e.g., Crutcher et al. 1996a). A counter-claim has appeared, i.e., no Zeeman detections leading to an upper limit of 500 µGauss toward objects within 3 parsecs of the Galactic Center (e.g., Marshall et al. 1995; see also Section 2.5 here). Values near 500 µGauss claimed in S106 (Roberts et al., 1995; Kazès et al., 1988) have been reduced to 140 µGauss (Verschuur, 1996).
At magnetic field values near 1000 µGauss and above, Zeeman detections of magnetic field are toward objects that are dense (> 106 cm-3) and small (< 0.1 parsec). Whereas some Zeeman information have been obtained so far in some atomic or molecular clouds, still no global Zeeman information has been obtained on the Milky Way's galactic magnetic fields. And no extragalactic Zeeman detection has been possible so far. Thus at the moment, the Zeeman method is limited by sensitivity and instrumental problems to small-scale objects with high-strength magnetic fields in our own Milky Way galaxy, and a significant improvement in detector sensitivity and in telescope structure is necessary to help advance further.