The Spectrum of the Radio Background Between 13 and 404 MHz

Abstract

Measurements of the brightness temperature of the radio background over a range of declinations at frequencies of 13.15, 17.5, and 81.5 MHz are described. Geometrically scaled aerials having beamwidths of 17° × 12° and 53° × 12° have been used, and the results are compared to deduce the spectrum of the background between these frequencies. The observed variation of the spectral index across the sky is fitted by a model in which the galactic contribution has temperature spectral indices 2.38 ± 0.03 and 2.46 ± 0.04 for two different regions of sky, superimposed upon an isotropic background due to radio galaxies, amounting to 30° ± 7°K at 178 MHz. The galactic contributions are considered to be associated with radiation from the local spiral arm and from the inter-arm region, respectively.