Plasma instability at (n+ ½)fcand its relationship to some satellite observations

A nonresonant instability having growth rates up to 0.28 ωc and frequency ω ∼ (n + ½)ωc can be driven by a distortion of the electron perpendicular velocity distribution, namely, a peak at υ⊥>0 whose thermal width is not too large. Naturally occurring emissions at (n + ½)ωc observed by Ogo 5 near the geomagnetic equator outside the plasmapause could be produced by a naturally occurring peak in the ambient υ⊥ distribution. Some of the “diffuse resonances” at ω ∼ (n + ½)ωc observed in Alouette 2 ionograms could be due to the very large RF sounder antenna field interacting with ambient electrons to produce a transient peak in υ⊥ distribution via a wave-particle coherent acceleration process, with a subsequent instability appearing as a signal return at or near (n + ½)ωc, for appropriate values of ωp/ωc.