High-latitude geophysical studies with satellite Injun 3: 5. Very-low-frequency electromagnetic radiation

This is a preliminary report of very-low-frequency (VLF) electromagnetic radiation measurements made by the Injun 3 satellite in the altitude range 250 to about 2000 km over North America from December 1962 to March 1963. The radiation is received by a loop antenna on the magnetically oriented satellite. Amplitude is measured with narrow-band filters at six frequencies, the lowest being 0.7 kc/s and the highest being 8.8 kc/s. A wideband amplitude over the frequency band from 0.5 to 7.0 kc/s is also measured. In addition, the VLF signal modulates the telemetry transmitter so that a detailed spectral analysis can be made on the ground. It is found that the amplitude of naturally occurring VLF signals is commonly tens of decibels stronger at Injun 3 than on the ground, and this is to be understood as due to strong absorption of VLF energy in the ionosphere, as has been found previously in other experiments. The maximum observed signal strength over the band 0.5 to 7.0 kc/s is 5 × 10−2 gamma. The amplitude of VLF chorus, considered for local times between 0800 and 1300, shows a maximum at L = 5 over the three-day period studied. Simultaneous occurrences of VLF electromagnetic emission, auroral optical emissions, and particle precipitation into the atmosphere have been repeatedly observed. Two specific instances are discussed. In the more stable of the two, it is concluded that the auroral hiss was generated in the same magnetic shell in much the same region of the magnetosphere (near the outer boundary of trapping) as were the precipitated particles that caused the aurora. For this event it is estimated that, for the VLF radiation having frequencies less than 10 kc/s, the VLF energy flux at the satellite was about 8 × 10−7 erg cm−2 sec−1, the energy flux of precipitated electrons was about 10 ergs cm−2 sec−1, and that from the auroral light was about 0.6 erg cm−2 sec−1. These measurements suggest that, in this event, the electrons caused both the visible aurora and the VLF hiss emission.