Variations in the spectral index of the galactic radio continuum emission in the northern hemisphere

A study is made of the radio continuum emission of the Galaxy in the North Celestial Hemisphere. The variation of spectral index over a range of frequencies between 38 and 1420 MHz is derived after consideration of the accuracy of the temperature scales and zero levels of the surveys. Comparison with the results of earlier low-resolution surveys made with scaled arrays or horns lends support to the procedure adopted for the fixing of base levels. Most of the features with large angular scales appear to be associated with the radio loops I and III even at considerable angular distances from their ridge lines. This casts some doubt on earlier deductions of the properties of a galactic halo from low-resolution surveys. The spectral index variation of the radio loops is discussed in the context of diffusive shock acceleration of cosmic ray electrons. It is suggested that the steepening of the spectrum is best accounted for if there is an upper energy limit to the shock acceleration in the loops in the region of 5–10 GeV.