Phone e. Email confirmation optional. Marketing permission What does this mean in detail? Your rights This declaration of consent may be withdrawn at any time by sending an email with the subject "Unsubscribe" to news rohde-schwarz. Get Information. Your request has been sent successfully. We will contact you shortly. In addition, the waves will be focused or defocused at scales of the Fresnel zone. Different AIT diagnostics are based on these effects. They include coherent and incoherent radars, vertical ionospheric sounding, and ionospheric radioscopy scintillation technique.
The HF waves radiated by a powerful heater were scattered into the ionospheric waveguide. This waveguide is formed in a valley region between the electron density peaks of the E- and F-layers Davies, The waveguide is located at high altitude where the electron collision frequency drops and thus the wave attenuation becomes very low.
That allows the radio waves to propagate to long distances. The key problem of providing the waveguide propagation regime is the feeding of the ionospheric waveguide, which is located in the valley between the E and F ionospheric regions.
In the indicatrices of the resonance signal scattering by AIT a fraction of energy propagates within the sliding angle along the waveguide axes thus providing its feeding. It is known that effective conditions for the waveguide excitation, as well as signal landing from the waveguide, are created by the regular horizontal gradients which appear during sunset and sunrise in the ionosphere.
Such observations are useful for the comparable analysis of behavior of signals emitted by the HF heating facilities and for the identification of mechanisms related to the waveguide feeding. An even more interesting opportunity for channeling of the HF signals occurs due to the aspect scattering of radio waves by the field aligned plasma irregularities FAI , when the scattering vector is parallel to the Earth surface. Since the Arecibo HF beam is vertical, the aspect scattered wave will be oriented almost horizontally toward the South.
Such geometry provides unique opportunity to channel the radio wave energy into the ionospheric waveguide and excites the whispering gallery modes Budden and Martin, ; Erukhinov et al.
The whispering gallery modes require only the ionospheric F region curvature, and it does not depend on the E region existence. Those conditions can be fulfilled during the nighttime.
If the wave emission can be produced parallel to the Earth surface at the ionospheric altitudes, it can provide the energy to enter and exit from the wave channel. We will show later on in this paper that by choosing the proper conditions of the aspect scattering of the HF signals by FAIs, one can execute such opportunity. This paper is aimed to study the excitation of the ionospheric waveguide due to scattering of the HF heating wave by the artificial ionospheric turbulence AIT.
All three experiments used similar methodology. In those experiments the heating signals are caused by the resonance scattering of the emission off the decameter scale irregularities being of the order of wavelength of the incident wave.
The control of the scattering characteristics is provided by the suitable choice of the heating regime and the Sun illumination of the ionosphere. This way, the ionospheric waveguide can be fed at the beginning of the transmitting line. The energy extraction from the ionospheric wave guide is provided either by refraction off the natural horizontal gradients of the electron concentration for example during sunset and sunrise or by scattering off the natural irregularities near the receiver's location.
The most convenient conditions for this process occur when the E-layer that screens the energy extraction from the waveguide is absent, i. Nevertheless, most of the radio link should be Sun illuminated while the E-layer which serves as a lower boundary of the wave guide exists. Consider that the receiver is located in the Antarctic, which is a high latitude region and as such acquires high levels of natural turbulence, it is likely to find here natural irregularities of the decameter scale even during quiet ionospheric conditions.
Note that all discussed results were obtained under quiet geophysical conditions quiet ionosphere and unperturbed magnetic field. The results of the heating campaigns will be discussed in the chronological order in which they were performed. The first successful experiment Zalizovski et al.
The HF operating frequencies varied from 4. Both O- and X-modes were used for the heating. The HF facility radiated power varied from to kW. In addition, the HF radiation of the RWM station of time and frequency service located close to Moscow Russia was continuously recorded as a test radiation in Antarctica.
The layout of experiment is shown in Figure 2. Figure 2. Layout of the experiment. The experiments were conducted during the fall when sunrise terminator line crossed simultaneously through the interaction region over Tromso and the most remote receiving site in Antarctica.
We should emphasize the peculiarity of the signals received across all three detection sites. It is well-accepted that the narrowband stable signal was formed by the side lobes radiation of the antenna. It propagated along the radio paths by the ordinary hop and multi hops mechanisms.
Weak variations of the Doppler shift and amplitude of the narrowband signal were not correlated at different paths. The broadband signal component behaved differently. Variations of the Doppler shift and spectral density were well-correlated at all three radio paths.
Figure 3. The facility operated in a 5 min on, 5 min off regime. We used two frequencies in order to excite the upper ionosphere plasma oscillations having the combination frequency. The propagation conditions along the radio paths were such that the multiple hop mechanism had not operated.
All three panels show well-correlated quasiperiodic variations of the Doppler frequency spectra. The correlation coefficient between the pairs of signals detected by the different receivers was higher than 0. Analysis of the geophysical background during the experiment shows that the magnetic field above the HF heating facility experienced similar quasiperiodic variations.
Probably they were due to the excitation of the resonance magnetic field micro-pulsations Pc 3. The key question which explains synchronization of the spectra at three different radio paths is how to identify the region which scatters the signals. It is obvious that such region is located in the perturbed area of the ionosphere above the HF heater.
Accordingly, the observed effect was called self-scattering SS of the powerful radio wave by the artificial ionospheric turbulence AIT Zalizovski et al. In the example shown in Figure 3 quasi-periodic variations of the spectral characteristics of the SS signals were due to propagation of the MHD wave through the scattering region thus causing the AIT modulations.
The detected effect of self-scattering was also observed during experiments using Sura heater Kagan et al. At the Sura SS was studied using the heating frequency close to the fourth electron gyroharmonic. In fact, one of the transmitters generated the probe signal having a higher frequency than the ionosphere heating frequency.
The probe signal was radiated continuously regardless of the heater operation. In some cases, digisondes DPS-4 Reinisch et al. A continuously operated probe transmitter allowed us to estimate the relaxation time of the signal scattering irregularities after the HF heater was switched off Galushko et al. When developing the layout of the experiment, we considered the existing potentialities to control the spatial AIT spectrum.
As described by Najmi et al. It was gradually increased from 5. The heating at each frequency was made by a long pulse of s duration. The pulse consisted of 10 sub-pulses of 10 s each. The ERP was stepped up from 0. This waveguide is formed between the electron density peaks of the E- and F-regions.
The waveguide is located at high altitude where the electron collision frequency drops and thus the wave attenuation becomes low. This allows the radio waves to propagate to super long distances. During the experiment, starting at about 03 UT, i. The details of the receiver and of the data acquisition system are presented in Najmi et al. The SEE signals are driven by the non-linear interaction of the injected HF wave with the ionospheric plasma that results in broadband emissions at frequencies different from the injected HF frequencies Thide et al.
The traces are averaged over 10 s of the heating time. Variations of the effective radiated power ERP are revealed in the figure by the color traces.
Figure 4. Variations of the effective radiated power ERP are shown in the figure by the color traces. They are effectively reflected by the artificial striations in the decimeter range. Thus, the radar detected strong scattering due to the ionospheric heating.
Figure 5 shows the time series of the received power at UAS on June 6th, Here the heating frequencies are given in MHz. The intensity of the received HF signals vs.
The blue trace shows the measured data while the red trace is the 10 s moving average. The intensity of detected signal strongly depends on the heating frequency f h.
As the higher frequencies tend to be reflected by higher regions, these are able to reach much greater distances as a result of the geometry. Whilst it is possible to reach considerable distances using the F region as already described, on its own this does not explain the fact that radio signals are regularly heard from opposite sides of the globe using HF propagation with the ionosphere.
This occurs because the signals are able to undergo several "reflections". Once the signals are returned to earth from the ionosphere, they can be reflected back upwards by the earth's surface, and again they are able to undergo another "reflection" by the ionosphere.
Naturally the signal is reduced in strength at each "reflection", and it is also found that different areas of the Earth reflect radio signals differently. As might be anticipated the surface of the sea is a very good reflector, whereas desert areas are very poor.
This means that signals that are "reflected" back to the ionosphere by the Pacific or Atlantic oceans will be stronger than those that use the Sahara desert or the red centre of Australia. In reality, the state of the ionosphere is not as clean and clinical as we might like, and there are many ways in which signals can be reflected multiple times achieve very long distances, sometimes being reflected on to another reflection by the ionosphere.
Sometimes they may be ducted between the layers or regions. It is not just the Earth's surface and the reflections that introduce losses into the signal path. In fact the major cause of loss is the D region, even for frequencies high up into the HF portion of the spectrum.
One of the reasons for this is that the signal has to pass through the D region twice for every reflection by the ionosphere. This means that to get the best signal strengths it is necessary signal paths enable the minimum number of hops to be used.
This is generally achieved using frequencies close to the maximum frequencies that can support communications using ionospheric propagation, and thereby using the highest regions in the ionosphere. In addition to this the level of attenuation introduced by the D region is also reduced. This means that a radio signal on 20 MHz for example will be stronger than one on 10 MHz if propagation can be supported at both frequencies. This can be a key factor when trying to establish two way radio communications.
The ionisation in the ionosphere is chiefly caused by radiation from the Sun. As a result the state of the Sun and the radiation received from it governs the state of the ionosphere and HF propagation. The Sun: The Sun is a fascinating star - discovering all about it is fascinating it is own right.
Despite this, our Sun is the main source of radiation that creates the ionosphere. Their presence leads to higher levels of radiation being emitted and therefore this affects HF propagation.
Sunspots have been recognised in the surface of the Sun for very many years, and their affect of radio propagation was noted once the way in which signals travelled over long distances started to be understood.
It was found that there was a correlation between sunspots and the conditions for HF radio propagation and radio communications.
Solar disturbances: From time to time, massive disturbances occur on the surface of the Sun. Solar flares, and coronal mass ejections, CMEs also give rise to increased levels of radiation which in turn affects HF propagation.
0コメント