Radio noise levels within and above the ionosphere

The principal results obtained from the background noise experiment in the Alouette satellites are summarized. The various natural emissions, most of which can reach intensity levels as high as 80 dB above receiver threshold, are identified in the recordings with reference to the characteristic frequencies of the ionosphere local to the spacecraft and are described in turn. Galactic noise is the predominant signal above fT, with some occasional solar noise contributions; emissions are described and values are presented for the galactic spectrum between 0.6 and 12 MHz. Below fT, four distinct ionospheric emissions are recognized and tentatively associated with plasma processes. They are 1) a noise band below fHwhich is thought to result from cyclotron emission from energetic electrons, 2) a band that occurs mainly between fHand fzS and which probably results from a spacecraft interaction with the local plasma, 3) a noise band between fzS and fTwhich is believed to result from transverse electrostatic oscillations, and 4) a band at frequencies above fNwhich probably results from longitudinal electrostatic oscillations in the ionosphere. These ionospheric emissions show an association with ionization irregularities; at times, also, overtones and intermodulation products of such emissions can be clearly recognized at frequencies above fT.