E. N. Sergeev

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

Abstract Features of artificial small-scale ionospheric irregularities (ASI) induced at F-region heights by powerful HF waves are discussed. The investigations presented here were performed during the past two decades at the Zimenki and Sura heating facilities, located at middle latitudes near Nizhniy Novgorod, Russia, as well as at the Gissar facility located at a lower latitude near Dushanbe, Tadzhikistan. The measurements were made by a variety of diagnostic methods employing artificial field-aligned scattering of HF and VHF radio waves, sounding of the disturbed region by means of low-power probing waves and testing of the artificial turbulence by means of stimulated electromagnetic emission (SEE). The dependence of ASI on such parameters as transverse scale length of the artificial irregularities, their location in the disturbed region, the power of the heating wave, the duration of HF radiation, geophysical conditions, aftereffects of the preceding modification, and the schedule of heater operation is considered for both the development stage after pump turn-on and the decay stage after pump turn-off. The temporal evolution of ASI spectral characteristics during the heater period is discussed. An empirical model for the ASI, based on the data available, was elaborated and verified by computer simulation of SEE generation.

Optical remote sensing of the thermosphere with HF pumped artificial airglow

Optical emissions excited by high-power radio waves in the ionosphere can be used to measure a wide variety of parameters in the thermosphere. Powerful high-frequency (HF) radio waves produce energetic electrons in the region where the waves reflect in the F region. These hot or suprathermal electrons collide with atomic oxygen atoms to produce localized regions of metastable O(1D) and O(1S) atoms. These metastables subsequently radiate 630.0 and 557.7 nm, respectively, to produce clouds of HF pumped artificial airglow (HPAA). The shapes of the HPAA clouds are determined by the structure of large-scale (≈10 km) plasma irregularities that occur naturally or that develop during ionospheric heating. When the HF wave is operated continuously, the motion of the airglow clouds follows the E × B drift of the plasma. When the HF wave is turned off, the airglow clouds decay by collisional quenching and radiation, expand by neutral diffusion, and drift in response to neutral winds. Images of HPAA clouds, obtained using both continuous and stepped radio wave transmissions, are processed to yield the electric fields, neutral wind vectors, and diffusion coefficients in the upper atmosphere. This technique is illustrated using data that were obtained in March 1993 and 1995 at the ionospheric modification facility near Nizhny Novgorod, Russia. Analysis of HPAA clouds yields zonal plasma drifts of 70 m s−1 eastward at night. On the basis of artificial airglow from energetic electrons generated at 260 km the zonal neutral wind speed was estimated to be 96 m s−1 and the O(1D) diffusion coefficient was determined to be between 0.8 and 1.4 × 1011 cm2 s−1. The quenched lifetime of the O(1D) was determined to be 29.4 s. The diffusion and quenching rates are directly related to the atomic and molecular concentrations in the thermosphere. Improvements in the remote-sensing technique may be obtained if the intensity of the artificial airglow emissions is increased. High-power radio transmissions employing pulse sequences and tuning near electron cyclotron harmonics were attempted to increase the optical emissions. Both of these, however, produced reduced intensity, and consequently, continuous transmission at frequencies away from electron gyro harmonics is the preferred heating regime.

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

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.

Broad continuum feature of stimulated electromagnetic emission

Abstract Computer simulation results of the broad continuum (BC) feature in stimulated electromagnetic emission from the ionosphere are compared with experimental data. The simulations were performed in the framework of a theoretical model of the BC generation based on the double transformation of electromagnetic waves to upper hybrid (UH) waves and back due to the scattering on artificial magnetic field-aligned small-scale irregularities (FAI) of plasma density. An induced scattering of the UH waves on thermal ions produces a wide spectrum of the SEE. An empirical model of the FAI spectrum, development and decay was used in the simulations. It is shown that the main observed properties of the BC feature can be interpreted within the framework of the model considered. The relations of the FAI intensity and spectral shape, the pump power, and other parameters to BC peculiarities as well as possibilities for artificial ionospheric turbulence diagnostics by means of BC measurements are discussed.

Influence of small-scale irregularities on the characteristics of the overshoot effect in the temporal evolution of stimulated electromagnetic emission. Part 1: Development stage

AbstractWe present the results of experimental studies of the evolution of stimulated electromagnetic emission of the ionosphere (SEE) under the F-layer modification by powerful HF radio waves in a broad range of the pump wave frequencies. We compare the parameters of the overshoot effect in SEE evolution to the observations of anomalous attenuation and field-aligned scattering of radio waves. We show the overshoot effect to result from the anomalous attenuation under scattering at artificial small-scale ionospheric irregularities. We found the characteristic time scales of the overshoot effect development to decrease and its value to increase as the pump wave frequency decreases from 6 to 4 MHz; this is attributed to the observed increase of the irregularities amplitude at scales

PECULIARITIES IN THE EVOLUTION OF THE BUM OF STIMULATED RADIO EMISSION OF THE IONOSPHERE

l \bot \sim 3 - 10m

Temporal evolution of HF-excited plasma waves, measured at different pump frequencies by stimulated electromagnetic emission (SEE)

m.

Ionospheric plasma density irregularities measured by stimulated electromagnetic emission

Results of measurements of the characteristics of stimulated electromagnetic emission induced in ionospheric plasma by pulsed high-power radio waves (diagnostic emission) presented. The dependences of the properties of diagnostic emission on the parameters of the diagnostic wave, ionospheric conditions, and the level of development of artificial ionospheric turbulence are determined, and criteria are given for selection of optimal diagnostic conditions for ionospheric-plasma sounding. Results of experiments on the sounding of artificial ionospheric turbulence by means of diagnostic emission are provided. It is established that with transition from daytime to evening conditions, the characteristic times of emission decay are sharply increased. This is attributed to an increase in the natural perturbation of the F-region.