Annu. Rev. Astron. Astrophys. 1996. 34: 155-206 Copyright © 1996 by Annual Reviews. All rights reserved

2.3. Small-Scale Field Structures

Any unresolved field structures will lead to beam depolarization and thus to polarizations significantly below the theoretical limit of P₀ 75%. This effect is independent of wavelength and can be used to estimate the spatial scale and strength of field irregularities using observations at short wavelengths, where Faraday effects are weak.

At longer wavelengths, varying field orientations along the line of sight give rise to dispersion in rotation measures (Faraday dispersion), which also leads to depolarization (Burn 1966). Faraday dispersion is expected to arise from small H II regions (of 1 pc in size) in the thin galactic disk (Ehle & Beck 1993) as well as from larger scale fluctuations ( 10-100 pc) in the diffuse ionized medium of the thick disk (e.g. Krause 1993, Neininger et al 1993). This effect makes the Faraday rotation angle no longer proportional to ² because the effective Faraday depth decreases with increasing . It was recently discovered that at wavelengths greater than or equal to 10 cm, galaxies are generally not transparent topolarized radio waves (Sukumar & Allen 1991, Beck 1991, Horellou et al 1992). Even at 6 cm complete Faraday depolarization may occur in spiral arms or in the plane of edge-on galaxies.

To obtain full rotation measures, only observations in the Faraday-thin regime ( 6 cm) should be used Vallée 1980, Beck 1993). Rotation measures between longer wavelengths are lower and are weighted to regions near to the observer. Variations in Faraday depth may also lead to a spatial variation of the observed RM, which complicates the interpretation of observations. On the other hand, Faraday depolarization allows the study of layers at different depths sampled at different wavelengths (EM Berkhuijsen et al, in preparation).