Radio frequency observations of noise in the upper ionosphere were made with a 12 m electric dipole and a narrow-band receiver which either swept from 0.25 to 4.0 MHz every 16 s or operated at a fixed frequency of 2 MHz. Several distinct types of noise were observed. One is similar to that previously studied by a number of space vehicles close to the electron gyro- and plasma frequencies. Evidence is presented that simple theories based on the generation of electromagnetic waves by Cerenkov radiation in a cold plasma with subsequent propagation are inadequate to account for this noise. Two other kinds of noise observed may be locally generated; one at least, which has an impulsive character, by an interaction between the satellite and the plasma. As the satellite crosses the auroral regions, an interesting series of apparently related phenomena is observed when comparison is made with data from other experiments on the satellite. This is most clearly defined in the ‘daytime cusp’ region. An enhancement of the radio noise is accompanied by intense bursts of electrons in the moderate energy range at and below 1 keV, typical of this region, and by a sharp dip in the ambient electron density. The latter is presumably the daytime extension of the ionospheric ‘trough’.
Gray whales strand more often on days with increased levels of atmospheric radio-frequency noise
