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Effect of Plasma Composition on the Interpretation of Faraday Rotation

Faraday rotation (FR) is widely used to infer the orientation and strength of magnetic fields in astrophysical plasmas. Although the absence of electron-positron pairs is a plausible assumption in many astrophysical environments, the magnetospheres of pulsars and black holes and their associated jets may involve a significant pair plasma fraction. This motivates being mindful of the effect of positrons on FR. Here we derive and interpret exact expressions of FR for a neutral plasma of arbitrary composition. We focus on electron-ion-positron plasmas in which charge neutrality is maintained by an arbitrary combination of ions and positrons. Because a pure electron-positron plasma has zero FR, the greater the fraction of positrons the higher the field strength required to account for the same FR. We first obtain general formulae and then specifically consider parameters relevant to active galctic nuclei (AGN) jets to illustrate the significant differences in field strengths that FR measurements from radio frequency measurements. Complementarily, using galaxy cluster core plasmas as examples, we discuss how plasma composition can be constrained if independent measurements of the field strength and number density are available and combined with FR.