Excitation of strong Langmuir turbulence in plasmas near critical density: Application to HF heating of the ionosphere

Abstract

Results are presented for an extensive study of strong Langmuir turbulence (SLT) in plasmas excited near the critical density by intense coherent radiation beams. The nominal parameters for HF heating experiments imply that the ionospheric plasma is in such a state. A large body of experimental data exists, particularly from incoherent scatter radar (ISR) measurements of HF‐enhanced fluctuation levels. Long‐time simulations of Zakharov's model of SLT and related theoretical arguments have led to new conclusions and insights: (1) linear parametric instabilities may play a role only during the first few milliseconds after heater turn‐on in a quiescent ionosphere, but there is also the possibility of “direct nucleation” of cavitons in preexisting density fluctuations; (2) both possibilities lead to Langmuir collapse; (3) the turbulence is sustained by nucleation of trapped electric fields in burnt‐out density cavities from previous collapses; (4) the nucleation‐collapse‐burnout scenario explains several features of the observed ISR plasma line power spectra in early‐time, low‐duty cycle experiments and predicts new features; (5) ISR spectra obtained at early times in low‐duty cycle heating experiments are consistent with the spectra of uncorrelated caviton events; (6) these spectra contain a “free mode” peak which is due to the radiation of free Langmuir waves by collapsing cavitons; this peak has recently been observed; (7) sharp spectral peaks observed in strong spectra in longer‐time, high‐duty cycle or CW heating can arise in the SLT model from spatio‐temporal caviton correlations, provided overdense domains exist and a Bragg resonance condition is satisfied; (8) correlation models can explain all the sharp features including the decay line, the cascade, the narrow oscillating two‐stream instability line, and the anti‐Stokes line; these models do not involve parametric instabilities; (9) the characteristic structure of the ISR spectrum is maintained over a much wider range of angles relative to the geomagnetic field than is the case for weak turbulence predictions; (10) the altitude dependence of the plasma line is consistent with observation without postulating large‐scale density modifications; and (11) the nonlinear absorption rate of heater energy is much larger than collisional absorption, the absorbed energy going mainly into suprathermal electrons.