Aspect scattering of short-wave radio signals on artificial ionospheric nonuniformities

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.

Section: Introductionmentioning

confidence: 90%

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

Blagoveshchenskaya

Борисова

Kornienko

et al. 2008

“…Hereafter we will use the term "irregularities" instead of "small-scale artificial ionospheric irregularities." Intense small-scale stratification of the ionospheric plasma into electron number density irregularites strongly stretched along the magnetic field was discovered even in the first experiments [1][2][3]. The spatial scale of irregularities ranges from several units to several tens of meters across the magnetic field and several kilometers along the magnetic field.…”

Section: Introductionmentioning

confidence: 97%

“…The spatial scale of irregularities ranges from several units to several tens of meters across the magnetic field and several kilometers along the magnetic field. Extensive experimental studies of irregularities were performed in the last three decades using different HF pumping facilities such as "Sura" and the facility in Zimyonki (Russia), EISCAT/HEATING and SPEAR (Norway), HAARP (USA), and the observatory in Arecibo (Puerto-Rico) located at mid-and high latitudes (see, e.g., [3][4][5][6][7][8][9][10][11][12][13]). Since artificial irregularities lead to an intense scattering of radio waves, they are explored by methods based on the inverse and aspect scattering of diagnostic HF and VHF signals by irregularities.…”

Section: Introductionmentioning

confidence: 99%

Some distinctive features in the behavior of small-scale artificial ionospheric irregularities at mid-and high latitudes

Blagoveshchenskaya

Борисова

Kornienko

et al. 2007

Self Cite

We present the results of experimental studies of some features in the behavior of small-scale artificial irregularities (SSAIs) at mid-and high latitudes based on the "Sura" and EISCAT/ HEATING HF facilities. Observations were performed by the method of aspect scattering using a network of diagnostic paths having a common reception point located near St. Petersburg. We found that an extremely long duration of the second (slow) stage of SSAI relaxation of up to 5 min occurs in the evening hours when the ionosphere above the "Sura" facility is illuminated by the Sun, but the solar terminator travels through the magnetically conjugated ionosphere. The conjecture is made that the processes initiated by the terminator are mostly responsible for secondary ionospheric turbulence maintaining the irregularities above "Sura." A drastic increase in the Doppler spectra width of the scattered signals is revealed when the magnetically conjugate point of the ionosphere is located on the shade side of the terminator, but the ionosphere above the "Sura" facility is still lighted. It is assumed that the "run away" of photoelectrons from the day to the night side could reduce the threshold of excitation of artificial irregularities, leading to an increase in their intensity. The presence of fairly intense scattered signals was detected from the "Sura" and EISCAT/HEATING experimental results both under conditions of pulsed HF heating after continuous heater-on periods and cycled HF heating by short pulses. In the case of pulsed heating by short pulses with duration τ p < 100 ms and average radiated power P a below the threshold power P thr of the SSAI generation cutoff the irregularities can be maintained due only to striction parametric instabilities. The excitation of irregularites under the cycled HF pumping with the pulse duration τ p = 384 ms for P a comparable with P thr was detected. The aspect-angle dependence, or the so-called magnetic zenith effect, was found in the SSAI intensity. The residual turbulence aftereffects played a significant role in the SSAI development.

“…One of the main effects of such an impact is intense small-scale stratification of the ionospheric plasma into magnetic field-aligned electron-density irregularities [1][2][3]. These irregularities cause intense aspect scattering of the meter-and decameter-wavelength radio emission.…”

Section: Introductionmentioning

confidence: 99%

Consistent quasi-periodic variations of the geomagnetic pulsation level and doppler frequency shift of decametric radio waves aspect-scattered by artificial field-aligned ionospheric irregularities

Черногор

Vertogradov

Uryadov

et al. 2011

Self Cite

We present the results of experimental studies of consistent quasi-periodic variations of the geomagnetic-pulsation level and effective Doppler frequency shift of decametric radio waves aspectscattered by artificial field-aligned ionospheric irregularities. For processing of signals, we used a system spectral analysis and a correlation analysis. It is shown that quasi-periodic variations of the effective Doppler frequency shift are caused by geomagnetic-field pulsations with an amplitude of 0.5-1.0 nT and a period of 50-150 s. Estimated amplitudes of the quasi-periodic and aperiodic drift velocities of the ionospheric irregularities appeared close to 5-10 and 15-30 m/s, respectively.