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 f
T
, with some occasional solar noise contributions; emissions are described and values are presented for the galactic spectrum between 0.6 and 12 MHz. Below f
T
, four distinct ionospheric emissions are recognized and tentatively associated with plasma processes. They are 1) a noise band below f
H
which is thought to result from cyclotron emission from energetic electrons, 2) a band that occurs mainly between f
H
and f
z
S and which probably results from a spacecraft interaction with the local plasma, 3) a noise band between f
z
S and f
T
which is believed to result from transverse electrostatic oscillations, and 4) a band at frequencies above f
N
which 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 f
T
.