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4.1.4. Comparisons

Vallée (1995a) found that two independent methods (Faraday rotation Bfa and Equipartition Beq) are converging on similar values of the magnetic field strength. Observations show that Beq agrees with Bfa to within the errors involved, and Bfa and Beq are derived independently of each other since different Stokes parameters are involved (Stokes Q and U for Bfa ; Stokes I for Beq). For the available observational data, he found on average that 1.0 Beq < Bfa < 1.2 Beq . This convergent result, by two independent methods, strongly suggests that the real magnetic field B approx Bfa approx Beq.

Figure 11 shows a plot of the magnetic field values, with Beq on the x-axis, and Bfa on the y-axis (open squares) and also Bcr on the y-axis (open circles). A dashed line shows the equality Beq = Bfa . The cosmic-ray particle Bcr 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 Bcr = 120µG in Fig. 8 belongs to a part of the galaxy M82, as derived by Chi and Wolfendale (1993).

Figure 11

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.

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