GALACTIC MAGNETIC-FIELD STRENGTHS, FROM 3 DIFFERENT METHODS - A CAUTIONARY NOTE

Recent cosmic-ray data have led to the suggestion that the magnetic fi eld B in late-type galaxies may be quite large, i.e. B > 3B(eq), where B-eq is the magnetic field derived from the equipartition method (e.g ., Chi and Wolfendale 1993; Zweibel 1993). Previous radio data had fed to the claim that the magnetic field B in late-type galaxies was clos e to B-eq, i.e. 0.3B(eq) < B < 3B(eq) (Duric 1990) and 0.75B(eq) < B < 1.25B(eq) (Hummel 1986). It is important now to evaluate whether the real magnetic field B is far or close to B-eq. Here, a statistical stu dy is carried out of the three methods often employed to give magnetic field strengths, namely (i) the Faraday rotation method, (ii) the Equ ipartition method, and (iii) the Cosmic-ray particle method. I use sta tistics with small observational samples to find that two independent methods (Faraday rotation B-fa and Equipartition B-eq) are converging on similar values of the magnetic field strength. For the available ob servational data, I find on average that 1.0B(eq) < B-fa < 1.2B(eq). T his convergent result, by two independent methods, strongly suggests t hat the real magnetic field B approximate to B-fa approximate to B-eq. The third method (cosmic-ray particle B-cr) appears at times to predi ct magnetic field strengths different than those of the other two meth ods. Thus a caution is in order; possible reasons for that divergence are discussed.