Coronal radio-sounding experiments were carried out using two-way coherent dual-frequency carrier signals of the ESA spacecraft ROSETTA in 2010 and MARS EXPRESS in 2010/2011. Differential frequency measurements recorded at both NASA and ESA tracking stations (sample rate: 1Hz) are analyzed in this paper. Spectral analysis of the S-band, X-band, and differential frequency records has shown that the r.m.s. frequency fluctuation of each signal can be described by a radial power-law function of the form σi=Ai(R/R⊙)−βi, where i=s, x, sx. The ratio of the coefficients As and Ax differs from the expected theoretical value As/Ax=fs/fx. This occurs because the X-band fluctuations underlie two-way propagation conditions while the S-band fluctuations are essentially the product of a one-way propagation experiment. The intensity of the frequency fluctuations decreases sharply at high heliolatitudes. The asymmetry of the frequency fluctuation intensity between ingress and egress is exploited to determine the solar wind speed at small heliocentric distances.

In the implementation of the space projects Rosetta and Mars Express, a large-scale series of experiments has been carried out on radio sounding circumsolar plasma by decimeter (S-band) and centimeter (X-band) signals of the Rosetta comet probe (from October 3 to October 31, 2010) and the Mars Express satellite of Mars (from December 25, 2010 to March 27, 2011). It was found that in the phase of ingress the spacecraft behind the Sun, the intensity of the frequency fluctuations increases in accordance with a power function whose argument is the solar offset distance of radio ray path, and when the spacecraft is removed from the Sun (the egress phase), frequency fluctuations are reduced. Periodic strong increases in the fluctuation level, exceeding by a factor of 3–12 the background values of this value determined by the regular radial dependences, are imposed on the regular dependences. It was found that increasing the fluctuations of radio waves alternates with the periodicity m × T or n × T, where m = 1/2, n = 1, аnd T is the synodic period of the Sun’s rotation (T ≈ 27 days). It was shown that the corotating structures associated with the interaction regions of different speed fluxes are formed in the area of solar wind acceleration and at distances of 6–20 solar radii already have a quasi-stationary character.

The coronal Faraday rotation (FR) experiments using the linearly polarized signals of the Helios-1 and Helios-2 interplanetary probes remain a unique investigation of the magnetic field of the solar corona and its aperiodic and quasi-periodic variations. The unexpectedly long lifetime of these spacecraft (1974 – 1986) enabled studies from very deep solar-activity minimum (1975 – 1976) into the strong activity maximum (1979). Important experimental data were also obtained for the rising (1977 – 1978) and declining (1980 – 1984) branches of the solar-activity cycle. Previous publications have presented results of the initial experimental data only for coronal-sounding experiments performed during individual solar-conjunction opportunities. This report is a more detailed analysis of the Helios FR measurements for the entire period 1975 – 1984. Radial profiles of the FR fluctuation (FRF) intensity recorded during the deepest solar-activity minimum in 1975 – 1976 are shown to differ distinctly from those during the strong solar-activity maximum in 1979. In particular, the decrease of the FRF intensity with solar-offset distance is substantially steeper in 1979 than in 1975/1976. In all cases, however, the FR data reveal quasi-periodic wave-like fluctuations in addition to the random background with a power-law spectrum. The dominant period of these fluctuations, recorded during 35 % of the total measurement time, is found to be close to five minutes. Large-scale FR variations at considerably longer periods (1.1 – 2.7 hours) were observed during 20 % of the measurement time. Knowing the intrinsic motion of the radio ray path from spacecraft to Earth and making a reasonable assumption about the solar-wind velocity, FRF observations at widely spaced ground stations have been used to estimate the velocity of coronal Alfvén waves. The velocity values range between 290 and 550 km s−1 at heliocentric distances between 3.5 and 4.5 R⊙ and are marginally lower (150 – 450 km s−1) at distances between 5.5 and 6.5 R⊙. Occasional FR variations with a period near 160 minutes and harmonics with periods 60, 30, and 20 minutes were also observed.

Fluctuations in the Faraday rotation of the plane of polarization of S-band (2.3 GHz) radio signals transmitted through the solar corona by the HELIOS-1 AND HELIOS-2 space probes are analyzed. Simultaneous measurements of the Faraday-rotation fluctuations at the Goldstone and Canberra stations have yielded estimates of the velocity of perturbations of the magnetic field in the circumsolar plasma at heliocentric distances of three to six solar radii. The velocity of these perturbations is a combination of the Alfvén and solar-wind speeds. Temporal spectra of the Faraday-rotation fluctuations are obtained based on a large volume of observational data obtained in various years in four cycles of radio-sounding experiments. Filtration of the input data using spectral, correlation, and wavelet analyses shows that trains of quasi-periodic oscillations of the magnetic field with various amplitudes and periods from 2 to 160 min are regularly present in the Faraday-rotation fluctuations. This quasi-periodic character of these perturbations supports their connection with Alfvén waves propagating in the circumsolar plasma.

The coronal Faraday rotation (FR) experiments using the linearly polarized signals of the HELIOS-1 and HELIOS-2 interplanetary probes remain as a unique investigation of the magnetic field of the solar corona and its aperiodic and quasi-periodic variations. The unexpectedly long lifetime of these spacecraft (1974-1986) enabled studies from very deep solar activity minimum (1975-1976) into the strong activity maximum (1979). Important experimental data were also obtained for the rising (1977-1978) and declining (1980-1984) branches of the solar activity cycle. Previous publications have presented results of the initial experimental data only for coronal sounding experiments performed during individual solar conjunction opportunities. This report is a more detailed analysis of the HELIOS FR measurements for the entire period 1975-1984. Radial profiles of the FR fluctuation intensity recorded during the deepest solar activity minimum in 1975-1976 are shown to differ distinctly from those during the strong solar activity maximum in 1979. In particular, the decrease of the FR fluctuation intensity with solar offset distance is substantially higher in 1979 in comparison with 1975/1976. In all cases, however, the FR data reveal the presence of quasi-periodic wave-like fluctuations in addition to aperiodic variations. The dominant period of these fluctuations, recorded during 35 percent of the total measurement time, is found to be close to five minutes. Large-scale FR variations at considerably longer periods (1.1-2.7 hours) were observed during 20 percent of the measurement time. Observations of FR fluctuations at widely-spaced ground stations have been used to estimate the velocity of coronal Alfven waves. The velocity values range between 290 and 550 km/s at heliocentric distances between 3.5 and 4.5 solar radii and are marginally smaller (150-450 km/s) at distances between 5.5 and 6.5 solar radii.

Coronal radio-sounding experiments were carried out using two-way coherent dual-frequency carrier signals of the ESA spacecraft Rosetta (ROS) in 2006. Frequency measurements recorded at both NASA and ESA tracking stations (sample rate: 1 Hz) are analyzed in this work. Spectral analysis of the S-band, X-band, and differential frequency records has shown that the mean frequency fluctuation of each signal can be described by a radial power-law function of the form σ i =A i (R/R⊙)−mi , where i=x,s,sx. The ratio of the coefficients A s and A x is not the expected theoretical value A s/A x=f s/f x. This occurs because the X-band fluctuations underlie a two-way propagation mode while the S-band fluctuations are essentially the product of a one-way propagation experiment. Results are compared with similar, but not identical, two-way radio propagation experiments performed during the 1991 solar conjunction of the Ulysses spacecraft.

The statistical characteristics of Faraday-rotation fluctuations (FRFs) of a radio-wave polarization plane are analyzed when the circumsolar plasma is sounded by the signals of a Helios 1 space probe. The time spectra and auto- and crosscorrelation functions of FRFs, which are measured simultaneously at two ground stations separated by large distances, have been determined. The wavelet spectra of FRFs are presented, and the opportunities of the wavelet transform and other techniques used to reveal quasi-periodic FRFs with different periods are investigated. It is demonstrated that the quasi-periodic magnetic field oscillations, which are excited by the trains of Alfvén waves propagating through the solar supercorona (their period varies between 2 and 160 min), are observed at the heliocentric distances of 3–12 solar radii.

We have performed spectral processing of the data of experiments on radio sounding of circumsolar plasma by coherent S- and X-band signals from the spacecraft Ulysses, Mars Express, Rosetta, and Venus Express carried out from 1991 to 2009. The experiments were realized in the mode of coherent response, when a signal stabilized by the hydrogen standard is transmitted from the ground station to a spacecraft, received by the onboard systems, and retransmitted to the Earth with conserved coherence. Thus, the signal sounding the coronal plasma passes twice through the medium: on the propagation path ground station — spacecraft and on the same path in the opposite direction. The spectra of frequency fluctuations in both the bands are obtained and, using them, the radial dependences of fluctuation intensities are found, which can be approximated by a power law. It is shown that the ratio of intensities of frequency fluctuations in the S- and X-bands is comparable with the theoretical value and characterizes the degree of correlation of irregularities of the electron density along the propagation path ground station — spacecraft and back. Analysis of the correlation of frequency fluctuations on the two paths allows one to get a lower estimate of the outer scale of the circumsolar plasma turbulence. For heliocentric distances R = 10 solar radii (R S ) the outer scale is larger than 0.25R S .

The character and features of dispersion distortions of an ultrashort radio pulse in the form of a sinusoidal train propagating along the spacecraft-Earth interplanetary path are theoretically analyzed. It is established that the instantaneous field oscillation frequency in the distorted pulse is completely determined by the value of the total electron content of the path. It is shown that, theoretically, one of the main characteristics of the solar wind can be measured from the frequency of the detected signal.

A large volume of coronal radio sounding has been accumulated over the years 1991–2009 by the ULYSSES, MARS-EXPRESS, and ROSETTA spacecraft. A description of the dual-frequency radio sounding investigations of the circumsolar plasma and the methods for processing of the signal frequency fluctuations are presented. It is demonstrated that a quasi-periodic sporadic component with a period of 3–8 min is present in the radio frequency fluctuation temporal spectra. The characteristics of the quasi-periodic oscillations are studied for various heliolatitudes and distances from the Sun. Wavelet analysis is applied to two radio tracking sessions in order to assess its effectivity in the detection and analysis of waves at specific periods. It is argued that the quasi-periodic component in the frequency perturbations is most probably caused by fast magnetoacoustic waves generated locally via nonlinear interactions with propagating coronal Alfvén waves.

The magnetic field created by this antenna was measuredat the distance 1200 km from the transmitter, in the Bare-ntsburg Observatory, by a three-component inductionmagnetometer. The radiation experiments were conductedwithin a few days, mostly at night, when the IAR formationprobability is high.Themainresultoftheseexperimentsisthatinanumberofcases, a nonmonotonic dependence of the received signal onthe frequency was detected for the horizontal magnetic fieldcomponent(Fig.5b,c).WebelievethatthisisduetotheIARinfluenceonthepropagatingsignal,becausetheschemeoftheexperimentallowedinvestigatingthefieldintheregionwithasignificant contribution of the wave propagating in the outerwaveguide (above the F-layer) to the total field.Wealsoestimatedtheeffectivenessofvariousionosphericmodels for interpretation of the results of the conductedmeasurements. We showed that the results of the interna-tional reference ionosphere (IRI) model calculations agreepoorly with the experimental data. The upper atmospheremodel (UAM) calculations better correspond to the experi-ment,buttheyarealsoimperfect.Theresultsofexperimentalionospheric studies (such as satellite tomography) should beused to refine the ionospheric characteristics obtained bymodel calculations.

A new procedure for analyzing the data obtained during radio probing of the Venusian ionosphere is proposed. The experimental results of the two-frequency probing with the use of the “Venera 15” and “Venera 16” orbiters that provide evidence that the daytime Venusian ionosphere has a layered structure are presented. It is shown that there exists a lower part of the daytime ionosphere at altitudes of 80 to 120 km.

неоднородности становятся близкими к изотропным. м разрешением ( с для частоты и с для интенсивности), что позволило исследовать анизотропию неоднородностей и пространственный спектр турбулентности околосолнечной плазмы. Анализ данных радиозондирования показал, что индекс мерцаний и интенсивность флуктуаций частоты уменьшаются с увеличением прицельного расстояния приблизительно по степенному закону. На основе измерений амплитудных флуктуаций и оценок скорости солнечного ветра, полученных по разнесенным наблюдениям, показано, что для гелиоцентрических расстояний менее , мелкомасштабные, размеры порядка 50 км, неоднородности вытянуты вдоль радиального направления с коэффициентом анизотропии от 2.3 до 3.0. Для гелиоцентрических расстояний более неоднородности становятся близкими к изотропным.

Data on the spatial distributions of turbulence characteristics in the inner solar wind are reported. Spectral indices for the outer and inner turbulence scales have been obtained in radio occultation experiments using signals from several spacecraft at different phases of the solar cycle. The characteristics of turbulence in the slow, low-latitude solar wind remain, on average, constant during the solar cycle. The outer turbulence scale in the fast, high-latitude solar wind appreciably exceeds that of the slow, low-latitude wind at the solar minimum. The new data confirm that the transition from the acceleration region to the steady-flow region is accompanied by a change in the turbulence regime. This change in the turbulence regime takes place at greater distances from the Sun for the fast than for the slow solar wind.