Evidence of a Weak Galactic Center Magnetic Field from Diffuse Low-Frequency Nonthermal Radio Emission

New low-frequency 74 and 330 MHz observations of the Galactic center ( GC) region reveal the presence of a large-scale (6° x 2°) diffuse source of nonthermal synchrotron emission. A minimum-energy analysis of this emission yields a total energy of ∼(φ(4/7) f(3/7)) x 10(52) ergs and a magnetic field strength of ∼ μ G (where φ is the proton to electron energy ratio and f is the filling factor of the synchrotron emitting gas). The equipartition particle energy density is 1.2(φ/f)(2/7) eV cm(-3), a value consistent with cosmic-ray data. However, the derived magnetic field is several orders of magnitude below the 1 mG field commonly invoked for the GC. With this field the source can be maintained with the supernova rate inferred from the GC star formation. Furthermore, a strong magnetic field implies an abnormally low GC cosmic-ray energy density. We conclude that the mean magnetic field in the GC region must be weak, of order 10 mG ( at least on size scales ≥ 125(n))