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( 1 D ) and O( 1 S ) 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( 1 D ) diffusion coefficient was determined to be between 0.8 and 1.4 × 10 11 cm 2 s −1 . The quenched lifetime of the O( 1 D ) 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.
Observation of spatial VLF field structures in an artificially disturbed ionosphere is reported. The disturbed area with horizontal sizes ∼50 km in a quiet middle‐latitude ionosphere was produced by the powerful RF Sura heating facility (56°1′N, 46°1′E). Measurements were carried out onboard the DEMETER satellite while passing the disturbed area at height ∼700 km. Spectra broadening (Δ f < ±1 kHz) and considerable (up to 30 dB) increase of signal intensity of VLF transmitters' signals were observed. The VLF field and electron density irregularities have similar spatial structure. The characteristics of the VLF field in disturbed by RF heating area are analyzed.
In this work we report on the results of the ionospheric heating experiments, which were carried out at the Sura (Russia) and EISCAT/Heating (Norway) facilities during several heating campaigns in 2009 and 2010. We present experimental evidences for the influence of the electron density perturbations, induced by HF-heating in the midlatitude and highlatitude ionosphere, on the GNSS radio signals. Variations in the total electron content (TEC), proportional to the reduced phases of navigational signals, were studied. Examples of the identification of the heating-induced variations in TEC, including determination of the amplitudes and temporal characteristics are presented.
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.
We present some results of microwave measurements of ozone number density variations in the mesosphere (in the ionospheric D-region at altitudes of ~ 60 km) under conditions when the ionosphere was pumped by high-frequency powerful radio waves radiated by the Sura heating facility, which is located near Nizhny Novgorod, Russia (coordinates: 56.15°N, 46.13°E). Determination of the ozone number density in the upper atmosphere was made using the method of the ground-based microwave radiometry, which is based on spectral measurements of thermal atmospheric emissions detected in the frequency range of the ozone radiation. The decrease of microwave emission intensity for the ozone line in HF-pumped atmosphere was first revealed.
