O. I. Yakovlev

Radio occultation data analysis by the radioholographic method

The radioholographic method is briefly described and tested by using data of 4 radio occultation events observed by the GPS/MET experiment on 9 February 1997. The central point of the radioholographic method (Pavelyev, 1998) is the generation of a radiohologram along the LEO satellite trajectory which allows the calculation of angular spectra of the received GPS radio wave field at the LEO satellite. These spectra are promising in view of detection, analysis and reduction of multipath/diffraction effects, study of atmospheric irregularities and estimation of bending angle error. Initial analysis of angular spectra calculated by the multiple signal classification (MUSIC) method gives evidence that considerable multibeam propagation occurs at ray perigee heights below 20 km and at heights around 80–120 km for the 4 GPS/MET occultation events. Temperature profiles obtained by our analysis (radioholographic method, Abel inversion) are compared with those of the traditional retrieval by the UCAR GPS/MET team (bending angle from slope of phase front, Abel inversion). In 3 of 4 cases we found good agreement (standard deviation σT∼1.5°K between both retrievals at heights 0–30 km).

Attenuation and scintillation of radio waves in the Earth's atmosphere from radio occultation experiments on satellite‐to‐satellite links

A theoretical analysis of refractive loss of radio waves by the Earths atmosphere in radio occultation measurements along the satellite-to-satellite line for various altitude profiles of the refractive index is given. Experimental results for refractive loss on the orbital spacecraft - geostationary satellite link are presented. Theoretical calculations are compared with experimental data, and a conclusion is drawn that the signal amplitude during radio occultation is strongly dependent on the layered structure of the refractive index profile. Amplitude scintillations of centimeter (λ1 = 2 cm ) and decimeter (λ2 = 32 cm ) radio waves used in radio occultation experiments are described. Dependences of the rms value of the amplitude scintillations versus the minimum altitude of the ray line for the two above wavelength bands are presented. The frequency spectra of the log-amplitude scintillations are analyzed together with the dependence of the rms amplitude on the wavelength. Experimental data are compared to the theory of scintillations in a turbulent atmosphere, and the altitude model of the structure constant of refractivity fluctuations is determined.

Investigation of the Venus atmosphere and surface by the method of radiosounding using Venera-9 and 10 satellites

Abstract Using the Venera-9 and 10 satellites radio occultation measurements of the atmosphere and bistatic radar measurements of the surface of the planet Venus were realized from October 1975 to March 1976. The altitude dependence of the molecular number density, pressure and temperature on the night and day sides were derived. An analysis is made of the stratified structure and turbulence in the atmosphere of Venus. The results of pressure measurements on the surface by the method of bistatic radar are presented. The diagrams and the tables of the parameters of the atmosphere are given.

Radio wave propagation in the turbulent solar plasma using three satellites

Abstract The radio wave propagation in the solar-wind plasma was investigated before and after Mars-2, Mars-7 and Venera-10s superior conjunction. It was found that the moving turbulent solar-wind plasma produced the amplitude and frequency fluctuations and the spectral broadening of monochromatic radio waves. When the Sun-Earth-spacecraft angle decreases from 6° to (0.6) the bandwidth of spectral broadening increases from (0.3) Hz to 300 Hz. The region of 2.5–4° is specific since the bandwidth of spectral broadening is independent of Sun-Earth-spacecraft angle. The spectrum of frequency fluctuations could be represented by a power law with a spectral index of 0.7 ± 0.2. The temporal frequency spectra of the amplitude have two distinct regions. In the low-frequency region the spectral density is approximately constant. The high-frequency portion of the amplitude spectrum follows the power law with a spectral index of 2.8 ± 0.4. The turbulence characteristics of a solar-wind plasma are determined from the experimental data obtained. The three-dimensional wave-number spectrum of irregularities is found to be close to the Kolmogorov-Oboukhov spectrum. The solar-wind density fluctuations decline with heliocentric distance very steeply, but in the distance interval of 7 × 10 6 – 12 × 10 6 km, there is a region of enhanced turbulence which could be caused by shock waves.

Sporadic structures and small-scale irregularity in the nighttime polar ionosphere in the period of high solar activity according to the data of radio occultation measurements on satellite-to-satellite paths

Results of the analysis of 327 sessions of radio occultation on satellite-to-satellite paths are presented. The data are taken in the nighttime polar ionosphere in the regions with latitudes of 67°–88°, and in the period of high solar activity from October 26, 2003 to November 9, 2003. Typical ionospheric changes in the amplitude and phase of decimeter radio waves on paths GPS satellites-CHAMP satellite are presented. It is demonstrated that these data make it possible to determine characteristics of the sporadic Es structures in the lower ionosphere at heights of 75–120 km. Histograms of distribution of the lower and upper boundaries, thickness, and intensity of the Es structures are presented. Dispersion and spectra of amplitude fluctuations of decimeter radio waves caused by small-scale irregularity of the ionospheric plasma are analyzed. The relation of the polar Es structures and intensity of small-scale plasma irregularity to various manifestations of solar activity is discussed. The efficiency of monitoring the ionospheric disturbances caused by shock waves of the solar wind by the radio occultation method on satellite-to-satellite paths is demonstrated.

Amplitude fluctuations of decimeter and centimeter radio waves emmitted by the Venera-15 and Venera-16 space probes during propagation through the solar plasma

Results are presented of investigations into the fluctuations of centimeter and decimeter radio waves in the solar plasma. The experimental dependence of the scintillation index on distance of closest point of approach in the range 2.3-100 solar radii is given. Dependence of the scintillation index on wavelength and solar activity is discussed. The dependence of the variance of the fluctuations of the electron concentration on distance to the sun is obtained. It is shown that there is a continuously present region of heightened plasma inhomogeneity in the region of space around the sun.

Role of radio wave reflection during radio occultation by the solar corona

The authors present a theoretical analysis of reflection of radio waves by the sun when the plasma around the sun occults a spacecraft. The frequency difference is calculated between direct and reflected waves, smearing of the energy spectra, and absorption and refractive attenuation of the radio waves. It is shown that in the decimeter range, it may be possible to detect the reflected ray by the methods of frequency and time selection. The authors show that it is possible to perform bistatic radio location of the sun.

Effect of the solar-wind shock wave on the polar ionosphere according to the radio occultation data on satellite-to-satellite paths

The polar lower ionosphere is distinguished by its variability due to various effects of solar activity. They result in the appearance of sporadic E S structures, in excitation of the broad spectrum of plasma inhomogeneities, and in variations of the electron concentration. The goal of the present study is to reveal regularities in the variations of the night-side polar lower ionosphere on the basis of radio wave measurements on links from navigation GPS satellites to CHAMP satellite. These ionosphere variations are stipulated by the effect of the flare solar activity. To this end, it was necessary to determine the altitude profiles of the electron concentration N e ( h ) and to find the characteristics of both sporadic E S structures and plasma inhomogeneities. We used the data of 327 radio occultation events of the night side polar ionosphere in regions of latitudes higher than 65° N for the time period from October 25 to November 9, 2003, when particularly strong solar activity was observed. In the radio occultation events of the ionosphere, we have registered the amplitude E and the increments of the phase path ψ for two coherent signals with the frequencies of f 1 = 1.5 GHz and f 2 = 1.2 GHz [1‐3]. In order to determine the excitation degree for small-scale inhomogeneity of the ionosphere plasma, we analyzed random fluctuations of the radio-wave amplitude δ E . These fluctuations were subjected to the standard procedure of statistical processing in order to obtain the root-mean-square amplitude deviation σ . The fluctuations δ E are caused mostly by plasma inhomogeneities located in the ionosphere F region at altitudes h ≈ 200–300 km. The analysis of these fluctuations has shown that the quantity σ is subjected to variations with the minimum values being σ min ≈ 1 .5% and with the maximum fluctuation level attaining 20%. In Fig. 1, dots indicate values of root-mean-square deviations σ for the fluctuations of the amplitude E , which were registered within the period of October 26 to November 9, 2003 (measurement dates are indicated at the horizontal axis). In the beginning of this period, the fluctuation intensity attained σ ≈ 2%, whereas later, on October 27, an increase in the fluctuations was observed. The maximum of the field-intensity fluctuations took place on October 29, 30, and 31 when σ attained 10‐14%. Starting from November 1, the fluctuation intensity began to decrease, and on November 4, elevated values of σ were recorded again. In order to determine the amplitude fluctuations σ A more objectively, we have excluded the slight effect of continually present small fluctuations with σ min = 1.5%. We have used here the relationship = σ 2 – and have realized smoothing the dependence as a function of time by the sliding mean technique with time interval 6-hour.

Detection of wave phenomena in the solar corona using radio occultation methods

Abstract Solar wind irregularities modulate the radio waves propagating in the solar corona and supercorona. The magnitude of the fluctuations is determined by the intensity of electron density irregularities and their characteristic period by the irregularities velocity. Detailed analysis shows however that the velocity registered in radio occultation experiments is actually a combination of two velocities, namely the velocity of plasma flows and that of the magnetohydrodynamic waves. This feature may manifest itself in several maxima in the correlation functions of the frequency fluctuations in signals registered at several ground stations. The interaction of plasma flows with the magnetosonic waves propagating from and toward the Sun results correspondingly in higher and lower apparent velocity of the irregularities. It was proved in radio occultation data obtained using spacecraft Venera-15,-16, Viking and Voyager.

Radio Wave Propagation Phenomena from GPS Occultation Data Analysis.

The RO remote sensing can be performed with any two cooperating satellites located on opposite sides with respect to the Earth’s limb and moving to radio shadow. Several RO missions are working now aboard the Low Earth Orbit satellites. These missions provide global monitoring of the atmosphere and ionosphere of the Earth at different altitudes with high spatial resolution and accuracy. Their data are very important for meteorology, weather prediction. The RO data can be used to detect the climate changes, connections between the ionospheric, atmospheric processes, and solar activity, and to estimate conditions for radio navigation and radio location.