Ionospheric thermal radiation at radio frequencies

Radio noise on medium frequencies consists largely of man-made noise and atmospherics, but it should also contain a component due to thermal radiation from the ionosphere. This thermal radiation from the ionosphere has been identified and measured. It is very weak, corresponding to an effective temperature of about 300° K, or a field strength of about 10 ^-9 volts per metre in a frequency band of one kc/sec, under the conditions qf observation. It is necessary to minimize man-made noise and atmospherics by observing during the midday hours at places remote from electrical machinery. These observations introduce a new method for studying the electron temperature in the absorbing (and emitting) part of the ionosphere, which, for the frequencies used, is at a height of about 70 or 80 km. The measured temperatures, 240 to 290° K, agree with other observations of temperatures at these heights.