ON THE AMPLIFICATION OF ELF EMISSIONS BY CHARGED PARTICLES IN THE EXOSPHERE, WITH SPECIAL REFERENCE TO THE FREQUENCY BAND AROUND THE PROTON CYCLOTRON FREQUENCY,
KIRUNA GEOPHYSICAL OBSERVATORY (SWEDEN)
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Using a collisionless warm-plasma model having different temperatures along and perpendicular to the geomagnetic field, and a macroscopic drift along the field lines, possible amplification and absorption mechanisms for transverse waves propagating along the field lines were investigated with particular regard to frequencies near the proton cyclotron frequency. A search was made, notably, for frequency-selective effects that might account for the observations, in high latitudes, of narrow band emissions centered at approximately 700 cps, which is close to the proton cyclotron frequency in the ionosphere. No such effects were found. The only ELF emissions which readily may be amplified in the exosphere are those of right-handed polarisation. Streaming protons may cause the amplification. ELF emissions of left-handed polarization may be amplified by high-energy electrons. From the theory applied in this study, it follows that instabilities caused by anisotropic temperatures possibly are factors in the generation of L-mode emissions at frequencies below the proton cyclotron frequency, in which case the protons will constitute the important factor.