An updated compilation of observational data concerning the relationship between the interstellar magnetic field strength and the gas density is presented. Pulsar and Zeeman-effect data provide the only reliable information about the (B, n) relationship, and they now span nearly six orders of magnitude in gas density. Field strengths show no evidence of increase over the density range 0.1 to about 100/cu cm. At higher densities, a modest increase in field strength is observed in some regions, in line with theoretical expectations for self-gravitating clouds. In two regions of the interstellar medium, the magnetic field is unusually high; however, these are not locales where self-gravitation is important. Despite the consistency between observations and theory, questions still exist about how the magnetic field strength remains constant for densities up to about 100/cu cm. Further Zeeman effect studies and a better theoretical understanding of the formation of interstellar clouds and complexes will be necessary to answer these questions.
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Troland et al. (1986) studied this question.