Properties of ELF electromagnetic waves in and above the Earth's ionosphere deduced from plasma wave experiments on the OV1-17 and Ogo 6 satellites

Abstract

An analysis of ac electric field data obtained on board the OV1-17 satellite and ac magnetic field data obtained on board the Ogo 6 satellite has been made during the northern hemisphere spring and summer of 1969 with the purpose of studying extreme low frequency (ELF) electromagnetic waves above the earth's ionosphere. The results are in basic agreement with a number of previous ground-based and low-altitude satellite experiments in that the peak signal was observed at high latitudes outside the statistical location of the plasmapause on the day side of the earth, that ELF chorus was very often observed in conjunction with the steady ELF hiss emissions, that the winter hemisphere signal was considerably smaller than that observed in summer or in equinoctial months, and that the emission strength and region of occurrence are asymmetric about magnetic noon. Observations of such strong hiss signals outside the plasmasphere are somewhat surprising in light of Ogo 3 and Ogo 5 measurements which show steady ELF hiss to be closely confined to the plasmasphere at high altitudes during normal circumstances. The present study supports the hypothesis that hiss leaks out of the plasmasphere and refracts downward into the lower ionosphere; such a model predicts the observed summer-winter asymmetry and the poleward skewing of the ELF peak signal strength with decreasing altitude. The observation reported here that the high-latitude boundary for ELF signals in the ionosphere is very near the low-latitude boundary for long-wavelength ionospheric irregularities and, at least in the morning hours, very near the horizontal density gradient due to precipitation of magnetosheath plasma in the cusp suggests that these variations in the medium act to reflect the waves and to increase the high-latitude intensity further. The relationship between signal strength and magnetic activity shown by these data is in agreement with other in situ measurements but not with some ground-based data. It is argued, however, that the anticorrelation observed at high-latitude ground stations is due to an equatorward displacement of the peak wave intensity region with increasing magnetic activity; such an equatorward displacement is shown in our results. It is also shown that a significant component of wave electric field is parallel to the wave number k and hence that Landau resonant effects may occur at low altitudes between outer zone radiation belt particles and ELF hiss.