Observations of the galactic radio emission between 1.5 and 10 MHz from the Alouette Satellite

Abstract

Since September 29, 1962, a sweep-frequency receiver covering the range 0.5 to 12 MHz has been operating in a 1000 km height, 80.5° inclination orbit about the earth for about 5 hours a day on the average. Only a very small portion of the data has yet been analyzed, and from this the spectrum of the galactic radio emission between 1.5 and 5 MHz has been determined fairly reliably, but it has not been possible from these data to extend this up to 10 MHz with any accuracy. Although there is no provision for absolute gain calibration of the receiver in flight, the data are so consistent as to give confidence to the pre-flight calibration. The greatest uncertainty hinges on the effects of the ionization on the antenna impedances. To evaluate these, galactic noise measurements have been made under a variety of local electron densities and different magnetic field strengths : these values have then been extrapolated to zero electron density and the free space values obtained by this extrapolation procedure are taken as a measure of the galactic emission. At 2.3 MHz the brightest region of the galaxy is centred on the south galactic pole and has a temperature of about 1.8 × 107 °K. The lowest temperature, about 5.0 × 106 °K, at 2.3 MHz was found for the region centred on R. A. 9 hr., dec + 75°. At this same frequency the brightness temperature versus frequency curve has a elope of — 1.7; at 1.5 MHz the slope is — 1.3 and at 5.0 MHz the slope is — 2.2.