Stimulated plasma instability and nonlinear phenomena in the ionosphere

Abstract

Several hundred topside ionograms, corresponding to both high‐ and low‐power sounder operation, were used to investigate stimulated wave‐particle interactions in the ionosphere. The investigation combined the benefits of high frequency resolution Alouette 2 analog sounder data with modern digital graphics techniques. One of the main conclusions of the study is that the sounder pulse can cause significant plasma heating when the plasma parameter ƒN/ƒH is confined to specific ranges, where ƒN is the plasma frequency and ƒH is the electron cyclotron frequency. This heating is indicated by a variety of diffuse plasma signal returns that appear to be generated in a plasma which is driven into a state of turbulence by the sounder pulse. The signals are observed primarily in the range of ƒz ≲ nƒH ≲ ƒT where ƒT is the upper hybrid frequency, ƒz is the cutoff of the electromagnetic z wave (the lower‐frequency branch of the extraordinary mode), and n is an integer ≥ 1. The case n = 1 corresponds to a newly discovered diffuse resonance observed at a frequency designated by ƒDNT which is confined between ƒN and ƒT. It is observed over nearly the complete limits of the above frequency range, i.e.,ƒz ≲ ƒH ≲ ƒT. The characteristics of the ƒDNT resonance reveal a dramatic change as the ambient plasma parameter ƒN/ƒH increases from ƒN/ƒH < 1 to ƒN/ƒH > 1. The phenomena associated with n > 1 (some reported here for the first time) are restricted to narrower frequency limits of nƒH within the ƒz to ƒT range. Such restrictions of the stimulated phenomena to certain ƒN/ƒH values suggest that the source of the heating is the absorption of energy from slowly propagating sounder‐stimulated plasma waves rather than directly from the RF fields associated with the brief sounder pulse. The sequence of diffuse resonances observed at frequencies below ƒT and between the nƒH harmonics has previously been attributed to a sounder‐initiated instability combined with a nonlinear sustaining mechanism. Several theories have been proposed. The present observations provide strong support for the Harris instability generation process and the nonlinear Landau damping maintaining process for the long‐duration diffuse resonances, and indicate that the nonlinear three‐wave interaction process may be important for the short‐duration diffuse resonances. The so‐called Q resonances observed at frequencies above ƒT and between the nƒH harmonics have previously been attributed to nearly pure Bernstein mode waves stimulated by the sounder pulse. The present observations, however, indicate that these resonances have some characteristics which imply that generation processes in a sounder‐stimulated plasma turbulence may be involved. These Q resonances, which are often observed to ‘float’ away from the zero time delay baseline, are here found to occasionally have nonfloating subsidiary components. The short‐time‐duration resonances observed at the harmonics and sum and differrence frequencies of the long‐duration plasma resonances at ƒN, ƒT, ƒH, and 2ƒH were found to occur only when ƒT < 2ƒH. Such a dependence on plasma conditions suggests that they are the result of nonlinear processes taking place in the ambient plasma rather than in the instrumentation. One class of stimulated diffuse signal returns is only observed when ƒN/ƒH is relatively large ( ≳ 4), suggesting that plasma heating may be more efficient under these conditions. Such conditions are shown to be ideal for exciting high‐order nƒH resonances in that they can then be observed even when the antenna deviates considerably from a near‐parallel orientation with respect to the local magnetic field direction.