The review is based in a report presented by the author at the RAS Physical Sciences Division's session in honor of Vitaly L Ginzburg's 90th birthday. It examines the current status of theoretical and experimental research on nonlinear phenomena arising when a powerful radio wave propagates in the ionosphere. The focus is on the modification of the ionosphere under the resonance excitation of natural plasma oscillations by radio waves. The upper-hybrid resonance gives rise to strong upper- and lower-hybrid plasma waves; excites strongly elongated ionospheric irregularities, and induces artificial ionospheric radio emission. Nonlinear processes are found to undergo complete transformation near double resonances, when the upper-hybrid frequency is close to a multiple of the electron gyromagnetic frequency. In the neighborhood of the Langmuir resonance, intense plasma waves and ion-sound waves are excited, electrons are effectively accelerated, and an artificial glow of the ionosphere appears.

https://iopscience.iop.org/article/10.1070/PU2007v050n11ABEH006212/pdf

Nonlinear effects in the ionosphere

A high frequency electromagnetic pump wave transmitted into the ionospheric plasma from the ground can stimulate electromagnetic radiation with frequencies around that of the ionospherically reflected pump wave. The numerous spectral features of these stimulated electromagnetic emissions (SEE) and their temporal evolution on a wide range of time scales are reviewed and related theoretical, numerical, and simulation results are discussed. On long (thermal) time scales the SEE constitutes a self-organization of the ionospheric plasma which depends on the interaction of nonlinear processes in a hierarchy of time scales in response to the electromagnetic pumping. Particularly, the appearance of the rich SEE spectrum is associated with the slow self-structuring of the plasma density into a spectrum of magnetic field-aligned density striations. The dependence of the SEE on electron gyroharmonic effects and the presence of density striations suggests that the existence of a magnetic field in the plasma is important for plasma turbulence to dissipate into non-thermal electromagnetic radiation during the long time quasi-stationary state of the turbulence evolution.

Stimulated electromagnetic emissions by high-frequency electromagnetic pumping of the ionospheric plasma

Infrared absorption and emission spectroscopy has a long and rich history in the development and application of molecular physics. Absorption and emission of infrared radiation is an important diagnostic means in determining the presence and concentration of molecular species in various environments, ranging from plasmas and flames to the Earth’s atmosphere and interstellar space. A quantitative analysis of these observations relies on a detailed knowledge of the Einstein A coefficients that characterize the spontaneous emission rates. These A coefficients can generally be inferred from absorption measurements, pro

/pdf/direct-measurement-of-the-radiative-lifetime-of-2k3v8bg2mw.pdf

Direct Measurement of the Radiative Lifetime of Vibrationally Excited OH Radicals

Temporal behaviour of artificial small-scale ionospheric irregularities: Review of experimental results

New and detailed experimental and theoretical results concerning the prominent downshifted maximum (DM) feature in spectra of stimulated electromagnetic emissions are reported. The experimental results were obtained at the Sura ionospheric modification facility in Russia by transmitting a powerful high-frequency ordinary mode pump wave into the ionospheric F region. We present detailed experimental results of the dependence of the DM on the pump frequency. Different frequency components of the DM have slightly different growth times after pump turn-on and are suppressed in different pump frequency ranges at harmonics of the electron cyclotron frequency. The pump frequency range near the cyclotron harmonics in which the DM does not develop, decreases significantly with increasing harmonic, from several kilohertz at the fourth harmonic to an extremely narrow range of 0.2 kHz or less at the seventh harmonic. We discuss conditions for plasma wave propagation to explain this dependence on the cyclotron harmonics.

Downshifted maximum features in stimulated electromagnetic emission spectra

Experimental results concerning the spectrum of stimulated electromagnetic emissions (SEE) in the sidebands of a powerful high-frequency electromagnetic ordinary mode pump wave are presented. The pump wave was vertically injected into the ionospheric F region from the Sura ionospheric modification facility in Russia. We report detailed measurements of the commonly observed continuum, downshifted maximum, and broad upshifted maximum emissions for pump frequencies ranging from the fourth to above the seventh electron cyclotron harmonic in the ionospheric plasma as well as observations of a new wideband emission occurring primarily in the upper sideband. The dependence of the SEE spectral structure on the pump frequency in relation to electron cyclotron harmonics is emphasized. All spectral features in the sidebands of the pump exhibit cyclotron harmonic effects.

Spectral structure of stimulated electromagnetic emissions between electron cyclotron harmonics

Steady state spectral features of stimulated electromagnetic emissions (SEEs) for their major emission components (DM, NC, BC, BUM, and BUS) are studied in a wide pump wave frequency range, from 4.3 to 9.5 MHz, i.e., from slightly above the 3rd to slightly above the 7th gyroharmonic frequency. Based on these systematic experimental data, new peculiarities in the behaviour of the SEE intensity and of the spectral properties, in relation to the gyroharmonic mode number, have been found. The experimental results, discussed in the paper, were collected during the years 1996–2000 at the Sura heating facility in Russia by modification of the ionospheric F region, using ordinary mode HF pump waves.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001GL013251

Spectral features of stimulated electromagnetic emission, measured in the 4.3–9.5 MHz pump wave frequency range

We present measurements of stimulated electromagnetic emission (SEE) excited in the ionosphere by a powerful high-frequency ordinary mode radio wave vertically injected from a ground-based transmitter, at frequencies f 0 near high harmonics of the ionospheric electron cyclotron frequency f c . A prominent lower-sideband SEE feature was found to be absent in an extremely narrow f 0 range of Δf 0 /f 0 ≃2×10 −5 . This absorption resonance width allows an estimate of f c with an accuracy of a few tens of Hz, corresponding to a magnetic field magnitude accuracy of about 1 nT

Narrow cyclotron harmonic absorption resonances of stimulated electromagnetic emission in the ionosphere.

The parameters of plasma disturbances at altitudes 660 and 840 km, measured by the instruments onboard the French DEMETER satellite and the US DMSP satellites passing through the magnetic flux tube footed at the region of intense modification of the F2 ionospheric layer by the high-power HF radio waves of the Sura heating facility, are presented. The formation of artificial enhanced-density plasma ducts in the outer ionosphere is observed experimentally. Conditions facilitating the formation of such ducts are pointed out.

G. P. Komrakov

Papers

Downshifted maximum features in stimulated electromagnetic emission spectra

Spectral structure of stimulated electromagnetic emissions between electron cyclotron harmonics

Spectral features of stimulated electromagnetic emission, measured in the 4.3–9.5 MHz pump wave frequency range

Narrow cyclotron harmonic absorption resonances of stimulated electromagnetic emission in the ionosphere.

Density ducts formed by heating the Earth’s ionosphere with high-power HF radio waves