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 Beq on x-axis versus that from the Faraday method Bfa on y-axis (open squares) and from the cosmic-ray method Bcr on y-axis (open circles). A dashed line shows the equality Bfa = Beq. The value of Bcr = 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.