Stimulated Electromagnetic Emission Research Articles

Radio Pumping of Ionospheric Plasma with Orbital Angular Momentum

Experimental results are presented of pumping ionospheric plasma with a radio wave carrying orbital angular momentum (OAM), using the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. Optical emissions from the pumped plasma turbulence exhibit the characteristic ring-shaped morphology when the pump beam carries OAM. Features of stimulated electromagnetic emissions (SEE) that are attributed to cascading Langmuir turbulence are well developed for a regular beam but are significantly weaker for a ring-shaped OAM beam in which case upper hybrid turbulence dominates the SEE.

First observations of stimulated electromagnetic emission in the ionosphere modified by the spear heating facility on Spitsbergen

We present the first results of observations of the stimulated electromagnetic emission (SEE) in the ionosphere modified by the Space Plasma Exploration by Active Radar (SPEAR) heating facility. Observation of the SEE is the key method of ground-based diagnostics of the ionospheric plasma disturbances due to high-power HF radiation. The presented results were obtained during the heating campaign performed at the SPEAR facility in February–March 2007. Prominent SEE special features were observed in periods in which the critical frequency of the F 2 layer was higher than the pump-wave frequency (4.45 MHz). As an example, such special features as the downshifted maximum and the broad continuum in the region of negative detunings from the pump-wave frequency are presented. Observations clearly demonstrate that the ionosphere was efficiently excited by the SPEAR heating facility despite the comparatively low pump-wave power.

The polar-ionosphere phenomena induced by high-power radio waves from the spear heating facility

We present the results of experimental studies of specific features in the behavior of small-scale artificial field-aligned irregularities (AFAIs) and the DM component in the spectra of stimulated electromagnetic emission (SEE). Analysis of experimental data shows that AFAIs in the polar ionosphere are generated under different background geophysical conditions (season, local time, the presence of sporadic layers in the E region, etc.). It is shown that AFAIs can be excited not only in the F region, but also in “thick” sporadic E s layers of the polar ionosphere. The AFAIs were observed in some cycles of heating when the HF heater frequency exceeded the critical frequency by 0.3–0.5 MHz. Propagation paths of diagnostic HF radio waves scattered by AFAIs were modelled for geophysical conditions prevailing during the SPEAR heating experiments. Two components, namely, a narrow-banded one with a Doppler-spectrum width of up to 2 Hz and a broadband one observed in a band of up to 20 Hz, were found in the sporadic E s layer during the AFAI excitation. Analysis of the SEE spectra shows that the behavior of the DM component in time is irregular, which is possibly due to strong variations in the critical frequency of the F 2 layer from 3.5 to 4.6 MHz. An interesting feature observed in the SPEAR heating experiments is that the generation of the DM component was similar to the excitation of AFAIs when the heater frequency was up to 0.5 MHz higher than the critical frequency.

Ionospheric plasma density irregularities measured by stimulated electromagnetic emission

It is well known that ionospheric plasma turbulence can be conveniently generated by controlled injection of powerful high‐frequency radio beams from the ground. Irradiation of the ionosphere with such radio waves leads to the formation of plasma density structures, striations, and the generation of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission (SEE). In this paper we present experimental results of the dependence of SEE on decreasing excitation levels of the striations. In the experiments the frequency of the injected radio beam was varied near the fifth harmonic of the local ionospheric electron gyro frequency. We use the SEE measurements to obtain transverse length scales of the striations involved in the generation of the SEE. Our results show that different spectral features of the SEE display different temporal dynamics, suggesting that they are related to striations with different transverse length scales (1 ≲ l⊥ ≲ 25 m).

Spectra of Stimulated Electromagnetic Emission of the Ionosphere Sweeping of the Pump Wave Frequency Near Gyroharmonics. II. Discussion of the Results

Alternative mechanisms of generation of the stimulated electromagnetic emission (SEE) excited in the ionosphere by high-power radio waves are analyzed on the basis of measurements of the SEE spectra obtained during the pump-wave frequency sweeping near the forth (n = 4) and fifth (n = 5) harmonics of the electron gyrofrequency nfce [1] and their comparison with the existing physical models. A method for determination of the magnetic field strength and plasma density near the double-resonance region in the ionosphere is developed. It is shown that the generation of the broad upshifted maximum (BUM) feature in the SEE spectrum should occur several kilometers below the double-resonance altitude. A role of high-frequency plasma modes and small-scale magnetic field-aligned irregularities, excited under ionosphere pumping by a high-power radio wave, in the formation of SEE spectra is demonstrated. It is shown that the difference in the emission intensities for f0 ≲ nfce and f0 > nfce is related to different regions (altitudes) at which the plasma waves exist in these cases.

Spectra of Stimulated Electromagnetic Emission of the Ionosphere Upon Sweeping of the Pump Wave Frequency Near Gyroharmonics. I. Experimental Results

We present the results of experimental studies of the spectra of the stimulated electromagnetic emission excited in the ionosphere by powerful radio waves during the pump wave frequency sweeping near the forth (n = 4) and fifth (n = 5) harmonics of the electron cyclotron frequency nf ce. The frequency sweep was carried out for long (continuous) pumping in vertical and inclined directions (at 14° and 18° south of the zenith), as well as for the pulse diagnostic wave both with and without additional pumping far from the gyroharmonics. The dependences of the spectral features of the stimulated electromagnetic emission on the ratio between the pump-wave frequency f 0 (or on the diagnostic-wave frequency fDW) and nf ce were analyzed. It is found that near the multiple gyroresonance, different spectral features of the stimulated emission are quenched at the same frequency for different pump-wave frequencies. For a sufficiently large inclination of the pump wave beam from the vertical direction, the intensity of the stimulated electromagnetic emission is notably decreased for f 0 ≲ nf ce as compared with f 0 > nf ce.

Spatiotemporal evolution of radio wave pump‐induced ionospheric phenomena near the fourth electron gyroharmonic

On 12 November 2001, the European Incoherent Scatter (EISCAT) high‐frequency (HF) radio wave transmitter facility, operating in O‐mode at 5.423 MHz with 550 MW effective radiated power, produced artificial optical rings which appeared immediately at transmitter turn‐on and collapsed into blobs after ∼60 s while descending in altitude. A similar descent in altitude was observed in the EISCAT ultra high frequency (UHF) ion line enhancements. Likewise, the stimulated electromagnetic emission (SEE) spectra changed as the pump frequency approached the fourth electron gyroharmonic due to pump‐induced variations in electron concentration. Optical recordings were made from Skibotn at 630.0 and 557.7 nm and from Ramfjord in white light. The altitude of the initial optical ring and steady state blob has been estimated by triangulation. The evolution in altitude of the optical emissions, ion line enhancements, and SEE spectra all show a similar morphology but are generally not at exactly the same height. Typically, the optical height is close to and a few kilometers below that of the radar backscatter but sometimes above it, both of which are above the SEE generation altitude. There is evidence that upper hybrid (UH) waves, which propagate perpendicular to the magnetic field line, and Langmuir (L) waves, which propagate parallel to the magnetic field line, act simultaneously to accelerate electrons even in the steady state.

Recovery of the ponderomotive parametric instability after long pumping of the ionosphere

We show that during high frequency (HF) pumping of the overhead ionosphere with a pump wave frequency slightly above the local fifth electron gyroharmonic, when one switches the pump wave from continuous pumping to low duty cycle pulse pumping, the stimulated electromagnetic emission (SEE) features related to the ponderomotive parametric instability (PPI) near the pump wave reflection point, such as the ponderomotive narrow continuum and the narrow continuum maximum, recover and become dominating in the SEE spectra after a few seconds after the switch. Simultaneously, the ionospherically reflected pump wave signal exhibits a recovery of ponderomotive self-action and the damped quasi-periodical oscillations (spikes) of the reflected pump wave, also related to the PPI development. The intensity of the down-shifted maximum (DM) feature in the SEE spectra increases for about 1–2 s after the switch to the pulse mode, but later, together with PPI related phenomena recovery, the DM intensity decreases noticeably. The phenomena described are attributed to the relaxation of small-scale irregularities (striations). The latter provide strong scattering of the pump wave into upper hybrid waves, strong shielding of the pump wave reflection point, and generation of the upper-hybrid related DM feature.

Statistical properties of ionospheric stimulated electromagnetic emissions

Abstract. We have analysed the statistical properties of the stimulated electromagnetic emissions (SEE) spectral features in the steady state, reached after a long period of continuous HF pumping of the ionosphere in experiments performed at the Sura ionospheric radio research facility in Russia. Using a digital filter bank method, we have been able to analyse complex valued signals within narrow frequency bands. Each of the SEE spectral features are thereby separated into a number of narrow spectral components. Statistical tests were performed for all these spectral components and the distributions of the spectral amplitudes and phases were evaluated. Also, a test for sinusoidal components was performed. These tests showed that all observed SEE features were indistinguishable from coloured Gaussian noise. The test results exclude that the SEE features can be the result of a single isolated coherent process, but does not rule out that there could be many statistically independent parametric wave-wave processes taking place simultaneously at various parts of the HF-pumped ionosphere, as long as the superposition from all these is incoherent. Furthermore, from the test results, we cannot exclude the possibility that the waveforms of some, or all, of the SEE features may be chaotic.

Competition between Langmuir and Upper-Hybrid Turbulence in a High-Frequency-Pumped Ionosphere

We show how the secondary escaping radiation, also known as stimulated electromagnetic emission (SEE), from the ionosphere irradiated by a high-intensity radio beam, can be used to study both reflection altitude ponderomotive parametric instabilities and upper-hybrid altitude thermal parametric instabilities. This has allowed us to observe the transfer of energy from smaller to higher sideband frequency offsets and to identify a new transient SEE feature.

Properties of the thermal narrow-continuum component in the spectrum of stimulated ionospheric emission

We present the results of experimental studies of the properties of the thermal narrow-continuum component NCth of stimulated electromagnetic emission (SEE) of the ionosphere observed in a narrow region between the pump frequency and the downshifted maximum (DM). Spectral and dynamic characteristics of the NCth component are considered and the dependence of its properties on the frequency and power of the pumping wave as well as on the zenith angle of a high-power radio-wave beam in the geomagnetic-meridian plane is analyzed. It is proved that the NCth generation is determined by the thermal (resonant) parametric instability near the level of the upper-hybrid resonance for a high-power radio wave. New experimental data on the properties of the main DM thermal component in the spectrum of stimulated electromagnetic emission, which we used as the reference component in most measurements, are presented.

Features of Excitation of Stimulated Electromagnetic Emission of the Ionosphere Modified by an Oblique High-Power Radio Wave

We present the results of measuring the characteristics of the stimulated electromagnetic emission (SEE) of the ionosphere with variation in the zenith angle of a pump beam of high-power O-mode radio waves in the geomagnetic-meridian plane. The experiments were performed at the midlatitude heating facility “Sura.” It is established that the maximum intensity of the DM and BC components of SEE is observed for southward inclination angles θ ≈ 8°--12° of the antenna beam, for which the most intense generation of artificial small-scale ionospheric irregularities also takes place. Based on the results of measurements near the fourth and fifth harmonics of the electron gyrofrequency, it is found that the first component of the BUM (BUM-1) is generated only when the pump wave reaches the plasma-resonance region. This allows one to assume that, unlike the second component of the BUM (BUM-2), whose generation is determined by development of instability in the upper-hybrid resonance region, the BUM-1 generation mechanism should be related to processes of interaction between a high-power radio wave and the plasma in the plasma-resonance region.

On the morphology of Stimulated Electromagnetic Emission spectra in a wide pump wave frequency range

Results of systematic study of Stimulated Electromagnetic Emission (SEE) steady state spectrum dependence on the pump wave frequency f 0 are presented. Investigations of the HF-pumped ionosphere were performed at the SURA facility for the frequency range 4.3 < f 0 < 9.5 MHz with stepping of 1–50 kHz including the vicinities of the electron gyroharmonics nf ce from n = 4 to 7. The results are based on the data collected during 1996–2004. The main tendencies in a behavior of the most prominent SEE spectral features (their peak and total intensities, and positions in the spectrum) versus f 0 are established and studied in detail. A comparative analysis of peculiarities of different spectral features allows to improve a determination of the electron gyrofrequency in the ionosphere from the SEE spectra, as well as provides a base for deeper understanding nonlinear processes in the pump-driven upper hybrid turbulence responsible for the SEE generation.

Experimental Studies of the Effects Observed During the Nonlinear Interaction of Two High-Power Radio Waves in a Magnetoplasma

We present the results of experiments on modification of the ionospheric F region by two high-power (Peff ≈ 20 MW) O-mode electromagnetic waves. The experiments were performed at the “Sura” heating facility of the Radiophysical Research Institute (Nizhny Novgorod, Russia) in May 23–27, 1993 at the pump frequencies near the 4th, 5th, or 6th harmonics of the electron gyrofrequency. Ionospheric perturbations were diagnosed by measuring the stationary spectral characteristics of the stimulated electromagnetic emission (SEE) of the ionospheric plasma. We determine the features of variation in the spectral characteristics of particular SEE components during the simultaneous heating of the ionospheric plasma by two radio waves in comparison with the case of a monochromatic pump wave. We observed the effect of enhanced generation of the broad up-shifted maximum (BUM) by the higher-frequency pump wave. This is accompanied by strong suppression of the BUM induced by the lower-frequency pump wave. It is shown that the effects observed during the two-frequency heating of the ionosphere have well pronounced gyroharmonic properties, i.e., depend on both the electron-gyroharmonic number and the frequency detuning of the pump waves from a harmonic of the electron gyrofrequency. We also pointed out that a change in the properties of artificial small-scale ionospheric irregularities (striations) excited by high-power radio waves is not the cause of a change in the properties of the down-shifted maximum and the BUM during the two-frequency modification of the ionospheric plasma. Ways for the further development of these studies are discussed.

Simultaneous observations of the 557.7 nm airglow and stimulated electromagnetic emission during HF pumping of the ionosphere with diagnostic schedule: first results

The first results on simultaneous observation for artificial airglow emission at 557.7 nm (green line) and stimulated electromagnetic emission (SEE) during HF pumping of the ionosphere with specially designed HF pulsing sequence are reported. Ionospheric radio pumping was performed at the “Sura” radio facility situated near Niznhny Novgord, Russia. The measurements of airglow were handled at the Astronomical Observatory of Kazan State University (220 km to the east of “Sura” facility) by the astronomical telescope AZT-14 connected with the electric photometer. During several “Sura” runs enhancements of the airglow intensity in time with pump wave pulses were observed after the pump wave switch from continuous transmission to low duty cycle pulse transmission. The airglow and SEE intensities and SEE decay rates were correlated.

Comments on Stimulated Electromagnetic Emissions in the Ionospheric Plasma

The present paper reconsiders the pump wave enhanced fluctuation spectrum that was derived in the seventies.

Stimulated electromagnetic emissions during pump frequency sweep through fourth electron cyclotron harmonic

The frequency of a high‐power HF radio wave incident on the ionosphere was swept, using a computer‐controlled transmitter signal, in &lt;10 s within a 60‐kHz‐wide frequency band approximately centered on the fourth harmonic of the electron cyclotron frequency. Measurements of the spectral behavior of stimulated electromagnetic emissions (SEE) across this harmonic after preconditioning could thereby be made with unprecedented resolution, speed, and ionospheric stability. Comparison of local electron cyclotron frequency estimations based on the experimental data reveals discrepancies between certain downshifted maximum models and the empirical broad upshifted maximum (BUM) feature formula ΔfBUM = f0 − nfce. Weak emissions related to the BUM were discovered below the nominal BUM cutoff frequency. Finally, we observed that the intensity of certain SEE components differed depending on the whether the pump frequency sweep was ascending or descending.

Analyzing the processes of excitation and decay of plasma turbulence near the fifth electron gyroharmonic using stimulated electromagnetic emission of the ionosphere

We present the results concerning the dynamics of stimulated electromagnetic emission (SEE) in the vicinity of the 5th electron gyroharmonic in the ionospheric-modification experiment. Measurements of the dynamic characteristics of different SEE features were performed at the “Sura” heating facility (Radiophysical Research Institute (NIRFI), Nizhny Novgorod) with the use of specialized equipment of the Swedish Institute of Space Physics, Uppsala Division (IRFU) designed for signal spectral analysis with a high temporal-frequency resolution. Based on the measurements performed, we analyzed the following features of nonstationary HF plasma turbulence: the competition between excitation of the Langmuir and upper-hybrid plasma modes for different values of mismatch between the pump frequency and the gyroharmonic, the fine structure of nonstationary energy redistribution over the turbulence spectrum during its excitation and relaxation, and the appearance of anomalously strong collisionless dissipation of turbulence energy with increasing pump duration. Possible mechanisms of the observed phenomena are discussed.

Studying the features of transport processes in the upper ionosphere using HF-induced artificial ionospheric turbulence

We analyze measurements of the velocity at which the artificial plasma turbulence, induced in the regions of resonance interaction between a powerful wave and a plasma, spreads along the geomagnetic field. The experimental data were obtained in 1996–2000 during experiments at the “Sura” heating facility on HF modification of the ionospheric F region. The stimulated electromagnetic emission (SEE) of the ionosphere was used for diagnosing the artificial plasma turbulence. The spread velocity was usually higher than the ion thermal velocity. In many cases, this velocity was close to and sometimes even much higher than the electron thermal velocity. We consider the dependence of the evolution features of the observed phenomena on the scheme of measurements (frequency and power of pumping and diagnostic waves, their timing, the distance between the levels of pumping- and diagnostic-wave reflection, etc.). A possible effect of the ionospheric D and E regions on the features of the observed phenomena is discussed. Based on the measurements performed, we formulate the requirements for future experiments.

Full polarimetry measurements of stimulated electromagnetic emissions: First results

A theory and technique for the measurement of the full state of polarization (SOP) for wideband electromagnetic HF emissions from the ionosphere are introduced. The technique was employed in a recent experiment at the Sura ionospheric HF pumping facility near Nizhniy Novgorod, Russia, to measure the full SOP of stimulated electromagnetic emissions (SEE) in their steady state. Measurements were made for O‐mode polarized pump waves at two different frequencies: one below and one slightly above the fourth electron gyroharmonic frequency. The wideband spectral parameters measured in this experiment include the horizontal intensity, the degree of horizontal circular polarization, the degree of horizontal polarization and the tilt angle of the horizontal polarization ellipse. Evaluation of the data collected demonstrates that the technique can be used for HF polarimetry and that, in particular, such measurements provide useful new diagnostics for SEE. Among the most important results regarding the polarimetry of SEE were low degrees of circular polarization in the frequency range between the peak of the broad upshifted maximum and the pump; some SEE features had degree of circularity and tilt angle values that were consistent with slightly oblique waves indicative of a distributed SEE source region; and the pump was found to have changed from an initial left‐handed polarized state to a right‐handed polarized state during steady state.