Spectral Index of the Diffuse Radio Background Measured from 100 to 200 Mhz

The mean absolute brightness temperature of the diffuse radio background was measured as a function of frequency in a continuous band between 100 and 200 MHz over an effective solid angle of ∼ π str at high Galactic latitude. A spectral brightness temperature index of β = 2.5 ± 0.1 (αS = 0.5) was derived from the observations, where the error limits are 3σ and include estimates of the instrumental systematics. Zenith drift scans with central declination δ = −26.5◦ and spanning right ascensions 0 < α < 10 h yielded little variation in the mean spectral index. The mean absolute brightness temperature at ν = 150 MHz was found to reach a minimum of T = 237 ± 10 K at α = 2.5 h. Combining these measurements with those of Haslam et al. (1982) yields a spectral index of β = 2.52± 0.04 between 150 < ν < 408 MHz. Subject headings: Galaxy: general — techniques: spectroscopic — radio continuum: general