4.1.4. Comparisons
Vallée (1995a) found that two independent methods (Faraday rotation B_{fa} and Equipartition B_{eq}) are converging on similar values of the magnetic field strength. Observations show that B_{eq} agrees with B_{fa} to within the errors involved, and B_{fa} and B_{eq} are derived independently of each other since different Stokes parameters are involved (Stokes Q and U for B_{fa} ; Stokes I for B_{eq}). For the available observational data, he found on average that 1.0 B_{eq} < B_{fa} < 1.2 B_{eq} . This convergent result, by two independent methods, strongly suggests that the real magnetic field B B_{fa} B_{eq}.
Figure 11 shows a plot of the magnetic field values, with B_{eq} on the x-axis, and B_{fa} on the y-axis (open squares) and also B_{cr} on the y-axis (open circles). A dashed line shows the equality B_{eq} = B_{fa} . The cosmic-ray particle B_{cr} appears in some cases to predict magnetic field strengths different than those of the other two methods, since the basic diffusion assumption ("well mixed" cosmic rays, gas, and magnetic fields) may not apply; cosmic-ray electrons from a galactic nucleus can find it difficult to diffuse quickly over a whole galactic disk. The value of B_{cr} = 120µG in Fig. 8 belongs to a part of the galaxy M82, as derived by Chi and Wolfendale (1993).
Figure 11. Plot showing the magnetic field values from the equipartition method B_{eq} on x-axis versus that from the Faraday method B_{fa} on y-axis (open squares) and from the cosmic-ray method B_{cr} on y-axis (open circles). A dashed line shows the equality B_{fa} = B_{eq}. The value of B_{cr} = 120 µG for M82 was derived by Chi & Wolfendale (1993). Caution is needed in interpreting the larger Bcr values (open circles), as some inherent assumptions in that method may not be valid. Each symbol (open square or open circle) stands for a different galaxy. See Vallée (1995a) for more details. |