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Experiments involving the radio sounding of solar plasma with signals from the Venus Express and Mars Express spacecraft, which were carried out from October 6 to December 6, 2006, provided data on frequency fluctuations of the radio signals for the western and eastern coronal limbs. The processing of this data provided information on the intensity of the frequency fluctuations, a parameter of plasma turbulence. High peaks of the intensity of frequency fluctuation were noted against the background of radial dependence; they can be explained by the passage of the compressed part of a corotating disturbance through a radio ray path. A delay in these events indicates a high-level connection of the fluctuations in the sounding radio-signal frequency within the same coronal region.

An experiment on radio sounding of the circumsolar plasma by signals from the Mars Express spacecraft in 2013 was carried out in the period from March 4 through May 31. The investigated characteristics were the frequency of signals in the centimeter and decimeter ranges and the differential frequency. A number of events were recorded in which the intensity of the frequency fluctuations of the signals probing the plasma was several times higher than the background values. As shown by the analysis of observations of solar activity, such as an increase in the flux of X-rays and SOHO LASCO coronagraph data, this is explained by the passage of disturbed plasma streams generated in the solar corona through the spacecraft radio communication path with the Earth. The comparison of the radio transmission data with the results of measurements of the parameters of the near-Earth plasma using the Wind spacecraft in adjacent periods of time. As a result of the analysis of the data on the proton concentration, it became clear that sharp increases in both the average values and fluctuations of this characteristic were also observed near the Earth. The time lag between the events observed in the circumsolar and near-Earth plasma shows that the cause of the disturbances is the increased activity of the same coronal region rotating with the Sun.

Coronal mass ejections from active regions on the Sun can be observed as series of individual events separated in time in the inner solar wind and near-Earth plasma. To find such events we have analyzed experimental data of dual-frequency solar wind radio sounding by the Rosetta and Mars Express spacecraft. The cycles of experiments performed in 2010 and 2011 measured the frequency fluctuations of X- and S-band signals. The temporal variations in the level of frequency fluctuations measured in the inner solar wind have been compared with the time variations in average plasma parameters recorded near Earth’s orbit. Since radio-sounding cycles were sufficiently long, we have managed to record events where significant amplifications of frequency fluctuations in the inner solar wind and increases in the plasma concentration near Earth’s orbit are associated with flare processes in the same active region on the Sun. Here, the amplifications of frequency fluctuations on the east limb occur earlier than near Earth’s orbit, and those on the west limb occur later. The time shift for the west limb turns out to be less than for the east limb. The sign of the time shift and the ratios between its numerical values depend on active region displacement relative to the central meridian due to the rotation of the Sun.

Results of experiments on polarized radio sounding of the outer solar corona using the Helios spacecraft from 1975 to 1984 are presented. The characteristic parameters of the temporal spectra of fluctuations in the Faraday rotation of the plane of polarization for heliocentric distances from 3.5 to 5.5 solar radii are obtained. The absolute level of these fluctuations and, consequently, the level of fluctuations of the magnetic field, is almost independent of the solar activity. It is well known that the global structure of the solar wind varies with the solar cycle such that there is slow solar wind at low latitudes and fast solar wind at high latitudes during solar minima. In contrast, a slow solar wind dominates at all latitudes during solar maxima. One explanation for the invariance of the fluctuations observed by sounding the circumsolar plasma is that the mean magnetohydrodynamic turbulence of the low-latitude, slow solar wind depends weakly on the phase of the solar cycle.

This is an open access article under the CC BY-NC-ND license., This work uses a subset of “quiet” MaRS ionospheric dayside observations (MaRSquiet, 2004–2017) and a 1-D photochemical model (IonA-2) to investigate the potential formation processes of the excess electron densities merged with the base of the main ionosphere (Mm). 42% of the investigated MaRS observations contain identified Mm, which occur in a large variety of shapes ranging from smoothly decreasing electron densities to peak structures below the base of M1. The Mm appear over the full range of accessible solar zenith angles (50° - 90°) and are found between approximately 70 and 110 km altitude. Their base is found on average deeper in the atmosphere than the base of the averaged undisturbed MaRS electron density profiles. This indicates a dependence of the Mm formation on energy sources that penetrate deep into the atmosphere. This is supported by a strong positive correlation with increasing solar activity when solar flares, coronal mass ejections, and enhanced short solar X-ray and Ly-α intensities are more common. No relationship is found between the Mm occurrence rate and the magnitude/inclination of the weak crustal crustal magnetic field in MaRSquiet. Investigations with the IonA-2 photochemical model for undisturbed and flare conditions show that the ionization of the local neutral atmosphere by solar X-ray radiation, The Mars Express Radio Science Experiment (MaRS) is funded by the German Space Agency (DLR) under the Grant 50QM1802. Support for Mars Express Radio Science at Stanford University is provided by NASA through a JPL Contract. Support for the Multimission Radio Science Support Team is provided by NASA/JPL. Portions of this research were performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with NASA. We thank everyone involved in the Mars Express project at ESTEC, ESOC, ESAC, JPL, and the ESTRACK and DSN ground stations for their continuous support. K. P. and M. P. acknowledge funding for this project by the Deutsche Forschungsgemeinschaft (DFG) under Grant PA 525/11-1, PA525/11-2 and PA 525/14-1. F.G.G. is funded by the Spanish Ministerio de Ciencia, Innovacion y Universidades, the Agencia Estatal de Investigacion and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award to the Instituto de Astrofisica de Andalucia (SEV-2017-0709). Solar Irradiance Platform historical irradiances are provided courtesy of W. Kent Tobiska and Space Environment Technologies. These historical irradiances have been developed with partial funding from the NASA UARS, TIMED, and SOHO missions. We acknowledge support for work conducted on the development of the Mars Climate Database from the European Space Agency (under ESTEC Contract 11369/95/NL/JG(SC)) and from CNES. CNRS (LMD group), the IAA and the UK Particle Physics and Astronomy Research Council (AOPP, Oxford Group) also provided support during the development of the Martian General Circulation Models.

Measurement results are presented for the velocity of density irregularities in solar wind plasma, which modulate the frequency of coherent radio signals from the Galileo spacecraft in the S-band. The measurements have been performed by the spaced receiving technique at ground monitoring stations. The data cover a range of heliocentric distances from 7 to 72 solar radii at low heliolatitudes near solar minimum. It is shown that acceleration of slow solar wind continues up to distances of about 30 solar radii. In the overlapping ranges of distances from the Sun, the results are qualitatively consistent with LASCO SOHO data for the same periods, with the previous results obtained by analyzing the amplitude fluctuations of signals from the Venera 15 and 16 spacecraft, and with the more recent measurements acquired using the Mars Express spacecraft.

In 2013 and 2015, investigations of the internal solar wind were carried out using the method of two-frequency radio sounding by signals from the Mars Express European spacecraft. The values of the S- and X-bands’ frequency and the differential frequency were registered with a sampling rate of 1s at the American and European networks of ground-based tracking stations. The spatial distribution of the frequency fluctuation’s level has been studied. It has been shown that the intensity of frequency fluctuation considerably decreases at high heliolatitudes. In some radio sounding sessions, quasiperiodic oscillations of sub-mHz band have been observed in the temporal spectra of frequency fluctuations; they are supposed to be associated with the density inhomogeneities, the sizes of which are close to the turbulence outer scale.

The Mars Express spacecraft is operating in Mars orbit since early 2004. The Mars Express Radio Science Experiment (MaRS) employs the spacecraft and ground station radio systems (i) to conduct radio occultations of the atmosphere and ionosphere to obtain vertical profiles of temperature, pressure, neutral number densities and electron density, (ii) to conduct bistatic radar experiments to obtain information on the dielectric and scattering properties of the surface, (iii) to investigate the structure and variation of the crust and lithosphere in selected target areas, (iv) to determine the mass, bulk and internal structure of the moon Phobos, and (v) to track the MEX radio signals during superior solar conjunction to study the morphology of coronal mass ejections (CMEs). Here we report observations, results and discoveries made in the Mars environment between 2004 and 2014 over almost an entire solar cycle. © 2016 Elsevier Ltd.

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.

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 Alfven 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 Alfven waves propagating in the circumsolar plasma.

The electron density distributions of the lower ionospheres of Mars and Venus are mainly dependent on the solar X-ray and EUV flux and the solar zenith angle. The influence of an increasing solar flux is clearly seen in the increase of the observed peak electron density and total electron content (TEC) of the main ionospheric layers. The model “Ionization in Atmospheres” (IonA) was developed to compare ionospheric radio sounding observations, which were performed with the radio science experiments MaRS on Mars Express and VeRa on Venus Express, with simulated electron density profiles of the Mars and Venus ionospheres. This was done for actual observation conditions (solar flux, solar zenith angle, planetary coordinates) from the bases of the ionospheres to ∼160 km altitude. IonA uses models of the neutral atmospheres at ionospheric altitudes (Mars Climate Database (MCD) v4.3 for Mars; VenusGRAM/VIRA for Venus) and solar flux information in the 0.5–95 nm wavelength range (X-ray to EUV) from the SOLAR2000 data base. The comparison between the observed electron density profiles and the IonA profiles for Mars, simulated for a selected MCD scenario (background atmosphere), shows that the general behavior of the Mars ionosphere is reproduced by all scenarios. The MCD “low solar flux/clear atmosphere” and “low solar flux/MY24” scenarios agree best (on average) with the MaRS set of observations, although the actual Mars atmosphere seemed to be still slightly colder at ionospheric altitudes. For Venus, the VenusGRAM model, based on VIRA, is too limited to be used for the IonA simulation of electron density profiles. The behavior of the V2 peak electron density and TEC as a function of solar zenith angle are in general reproduced, but the peak densities and the TEC are either over- or underestimated for low or high solar EUV fluxes, respectively. The simulated V2 peak altitudes are systematically underestimated by 5 km on average for solar zenith angles less than 45° and the peak altitudes rise for zenith angles larger than 60°. The latter is the opposite of the observed behavior. The explanation is that VIRA and VenusGRAM are valid only for high solar activity, although there is also very poor agreement with VeRa observations from the recent solar cycle, in which the solar activity increases to high values. The disagreement between the observation and simulation of the Venus electron density profiles proves, that the true encountered Venus atmosphere at ionospheric altitudes was denser but locally cooler than predicted by VIRA.

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 Alfven 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.

Experimental radio sounding data of the circumsolar plasma, recorded with the help of the Ulysses, Mars Express, Venus Express, and Rosetta spacecraft, have been systematically analyzed. These data were processed via spectral and correlation methods with the aim of detecting quasi-periodic frequency fluctuations. Integral wavelet spectra have been constructed for the purpose of identifying periodic radio frequency fluctuations at different periods. Comparative analysis of the methods for detecting the quasi-periodic disturbances of the radio frequency in the circumsolar plasma has been carried out. It is demonstrated that frequency fluctuations with periods of 1–80 min are observed regularly in radio sounding experiments of the solar wind plasma.

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 .

Coronal radio sounding experiments were carried out using the dual-frequency signals of the spacecraft Ulysses, Galileo, Mars Express, Venus Express, and Rosetta. The change in differential frequency recorded at the NASA and ESA ground stations, a quantity sensitive only to the plasma along the radio ray path from spacecraft to receiver, has been analyzed in this work. This large volume of observational data provides evidence for the occasional presence of a quasi-periodic component (QPC) in the derived frequency fluctuation spectra. First seen in data from the Mars Express conjunction in 2004, further evidence for the QPC has now been found in data recorded at other solar conjunction opportunities from 1991 to 2009, thereby better defining the statistical characteristics of the QPC. The level of QPC spectral density is a factor of three higher than the expected power-law background level. The characteristic frequency of the spectral density maximum is roughly 4 mHz, corresponding to a QPC fluctuation period of about 4 min. The bandwidth of the spectral line is comparable to the maximum frequency. The QPC are observed at heliocentric distances between 3 and 40 solar radii, both in equatorial regions and at high heliolatitudes. The QPC is detected with an occurrence frequency of about 20% and is occasionally accompanied by its second harmonic. The most likely progenitors of the QPC are quasi-periodic electron density fluctuations associated with magnetosonic waves, which are generated locally from nonlinear interactions of 5-min band Alfven waves propagating from the coronal base.

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 Alfven waves.

Frequency fluctuation data of monochromatic radio waves propagating through the circumsolar plasma in 1975–2002 are systematized and analyzed. The radial dependences of the intensity of the frequency fluctuations are obtained for the decimeter radio waves in the circumsolar plasma from the results of radio sounding using the Ulysses and Galileo spacecrafts. It is demonstrated that the radial profile of the rms frequency fluctuations can be approximated by a power-law function whose exponent is determined by the intensity of the plasma inhomogeneities, the velocity of solar wind, the spectral index of the spatial spectrum of the plasma fluctuations, and the outer scale of turbulence. It is also shown that three different frequency fluctuation regimes are found in the solar wind acceleration region and in the inner and outer regions of the stable solar wind.

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.

The two-position radio sounding of the solar wind by the Galileo and Cassini spacecraft has been first performed. These spacecraft followed the Sun from east to west from May 12 to 24, 2000 and sounded the regions spaced in radial directions by several millions of kilometers. Stable correlation has been revealed between fluctuation effects detected in spatially spaced radio-sounding paths of disturbed plasma structures of the coronal mass ejection (CME) type. The radio effects have been found to correlate also with the data on the solar wind density near the Earth orbit. It has been shown that the two-position radio-sounding method together with the data on solar radiation in the X-ray and optic ranges and with the results of local plasma measurements provides information on the structure and velocity of the propagation of CMEs from the photosphere to the Earth orbit. In the most powerful event recorded on May 13, 2000, the CME velocity at the heliocentric distances of about 15R⊙ (R⊙ is the solar radius) reached 1200 km/s. At (15–25) R⊙, the velocity was about 1300 km/s. At distances larger than 25R⊙, disturbance was decelerated from 1300 to 450 km/s near the Earth orbit.

Spectral and correlation analyses of the frequency fluctuations of radio waves measured in the course of the radio sounding of the circumsolar plasma by the signals of the Galileo spacecraft in 1995–2002 are performed. The radial dependences of the fluctuation intensities are obtained, and disturbed plasma structures (similar to coronal mass ejections) that pass through the radio ray path are detected. The radio-sounding data are compared with the results of plasma measurements on the Wind satellite. A relationship between events in near-Earth space and the circumsolar plasma is established. The correlation of radio wave frequency fluctuations along different ray paths in the circumsolar plasma is studied. The exponents of the spatial spectrum of turbulence and the solar wind velocity are determined at different heliocentric distances.

The statistical characteristics of amplitude fluctuations of radio signals propagating through cir- cumsolar plasma at various distances from the Sun are investigated during various phases of solar activity. Pri- mary data obtained in large-scale series of radio-probing sessions are systemized. During these sessions, cir- cumsolar plasma was probed with signals from spacecraft launched by Russia and other countries. It is found that the dependences of the relative level of amplitude fluctuations (scintillation index) on the solar-offset dis- tance of a radio beam can be approximated with power functions with exponents depending on the heliocentric distance. When the distance between a probed region and the photosphere is large, this exponent is close to the value 1.5. As the Sun is approached, the exponent grows to a value of 1.8-2.05 reached at a heliocentric distance of about four solar radii. The scintillation index is calculated theoretically with allowance for the radial depen- dence of turbulence characteristics, such as the spectral index and outer scale.

Seven coronal radio-sounding campaigns were carried out during the active lifetime of the Galileo spacecraft in the years 1994–2002. The observational data analyzed in the present work are S -band frequency fluctuation measurements recorded during the solar conjunctions at different phases of solar activity cycle #23, specifically: periods near solar maximum (three conjunctions), near solar minimum (three conjunctions) and during the ascending phase (one conjunction). These data are all applicable to low heliographic latitudes, i.e. to the slow solar wind. The rms frequency fluctuation and power-law index of the frequency fluctuation temporal spectra are determined as a function of heliocentric distance. The turbulence power spectrum tends to be flatter inside ca. 20 solar radii during all phases of the solar cycle. This coincides with a transition in the flow from the inner acceleration region to the outer region of constant velocity. The radial falloff rate and absolute level of the rms frequency fluctuation are essentially invariant over the solar cycle.

Frequency fluctuations of the Galileo S-band radio signal were recorded nearly continuously during the spacecraft’s solar conjunction from December 1996 to February 1997. A strong propagating disturbance, most probably associated with a coronal mass ejection (CME), was detected on 7 February when the radio ray path proximate point was on the west solar limb at about 54 solar radii from the Sun. The CME passage through the line of sight is characterized by a significant increase in the fluctuation intensity of the recorded frequency and by an increase in the plasma speed from about 234 km s −1 up to about 755 km s −1 . These velocity estimates are obtained from a correlation analysis of frequency fluctuations recorded simultaneously at two widely-separated ground stations. The density turbulence power spectrum is found to steepen behind the CME front. The Galileo radio-sounding data are compared with SOHO/LASCO observations of the CME in the corona and with WIND spacecraft data near the Earth’s orbit.

The Hera Saturn entry probe mission is proposed as an M--class mission led by ESA with a contribution from NASA. It consists of one atmospheric probe to be sent into the atmosphere of Saturn, and a Carrier-Relay spacecraft. In this concept, the Hera probe is composed of ESA and NASA elements, and the Carrier-Relay Spacecraft is delivered by ESA. The probe is powered by batteries, and the Carrier-Relay Spacecraft is powered by solar panels and batteries. We anticipate two major subsystems to be supplied by the United States, either by direct procurement by ESA or by contribution from NASA: the solar electric power system (including solar arrays and the power management and distribution system), and the probe entry system (including the thermal protection shield and aeroshell). Hera is designed to perform in situ measurements of the chemical and isotopic compositions as well as the dynamics of Saturn's atmosphere using a single probe, with the goal of improving our understanding of the origin, formation, and evolution of Saturn, the giant planets and their satellite systems, with extrapolation to extrasolar planets. Hera's aim is to probe well into the cloud-forming region of the troposphere, below the region accessible to remote sensing, to the locations where certain cosmogenically abundant species are expected to be well mixed. By leading to an improved understanding of the processes by which giant planets formed, including the composition and properties of the local solar nebula at the time and location of giant planet formation, Hera will extend the legacy of the Galileo and Cassini missions by further addressing the creation, formation, and chemical, dynamical, and thermal evolution of the giant planets, the entire solar system including Earth and the other terrestrial planets, and formation of other planetary systems., Comment: Accepted for publication in Planetary and Space Science

Faraday rotation (FR) observations of radio sources near solar conjunction yield information on the coronal magnetic field at heliospheric distances not reached by in situ exploration. Measurements of FR yield the rotation measure (RM), a wavelength independent quantity defined as the integral along the raypath of the product of the electron density times the raypath-parallel component of the magnetic field. Independent observations or models of the coronal electron density are required in order to extract information about the magnetic field. The radio sounding sources can be either artificial (spacecraft) or natural, but they must be at least partially linearly polarized. The most extensive campaign of coronal radio sounding polarization measurements using a spacecraft was the Helios Faraday Rotation Experiment, which was conducted over the duration of the Helios 1 (1974–1984) and Helios 2 (1976–1980) missions. Other coronal FR experiments have been carried out using natural continuum sources recently a...

Properties of plasma turbulence in the solar wind can be determined from radio frequency fluctuation measurements recorded during solar conjunctions. Noteworthy recent results were obtained from radio occultation experiments performed with the spacecraft Galileo in the interval 1994–2002 and Ulysses in 1991–1997. The power spectral index was calculated for the range of heliocentric distances 5 RS

Frequency and amplitude fluctuations of the Galileo S-band radio signal were recorded nearly continuously during the spacecraft's solar conjunction from December 1996 to February 1997. A strong propagating disturbance associated with a CME was detected on 8 January when the radio ray path proximate point was on the east solar limb at about 32 solar radii from the Sun. Characteristics of the CME passage through the Galileo/Earth line of sight include: (a) a noticeable change of the average frequency of the radio signal, (b) a significant increase in the fluctuation level of the radio frequency and amplitude, and (c) a dual-velocity solar wind configuration with distinctly different values of the velocity (200 km/s and 360 km/s). These velocity estimates are obtained from a correlation analysis of frequency fluctuations recorded simultaneously at two widely-separated ground stations. The frequency fluctuation cross-correlation functions are relatively narrow during the passage of the disturbance through the...

Measurements of frequency fluctuations in radio signals generated by the GALILEO spacecraft from January 6 to 11, 1997 are presented and analyzed. The passage of a coronal mass ejection observed by the SOHO/LASCO coronagraph on January 6, 1997 through the radio-communication path between the spacecraft and a ground station was recorded. Radio sounding was carried out at a carrier frequency of 2295 MHz at a heliocentric distance of about 32 solar radii, with the signal being received at three ground stations. As the mass ejection intersected the propagation path, the mean frequency of the signal increased and several-hour-long frequency fluctuations were enhanced. A spectral analysis of the frequency fluctuations shows that the regime and level of plasma turbulence are substantially different in different sections of the quiet solar wind and the disturbed plasmoid. A correlation between the intensity and temporal spectrum of the frequency fluctuations is found. The plasma density in the leading part of the coronal mass ejection exceeds the mean background value by more than an order of magnitude. Our correlation analysis of the frequency fluctuations recorded simultaneously at two widely separated measuring points shows that two flow components with different velocities—the quiet solar wind and a perturbed stream—crossed the communication path. The radio-sounding data are compared with observations of the coronal mass ejection by the SOHO/LASCO coronagraph and plasma measurements near the Earth’s orbit using the WIND satellite. A pronounced correlation is found between the variations in the mean frequency of the sounding signal and the plasma density in near-Earth space.

The Venus Express Radio Science Experiment (VeRa) uses radio signals at wavelengths of 3.6 and 13 cm (“X”- and “S”-band, respectively) to investigate the Venus surface, neutral atmosphere, ionosphere, and gravity field, as well as the interplanetary medium. An ultrastable oscillator (USO) provides a high quality onboard reference frequency source; instrumentation on Earth is used to record amplitude, phase, propagation time, and polarization of the received signals. Simultaneous, coherent measurements at the two wavelengths allow separation of dispersive media effects from classical Doppler shift. VeRa science objectives include the following: (1) Determination of neutral atmospheric structure from the cloud deck (approximately 40 km altitude) to 100 km altitude from vertical profiles of neutral mass density, temperature, and pressure as a function of local time and season. Within the atmospheric structure, search for, and if detected, study of the vertical structure of localized buoyancy waves, and the presence and properties of planetary waves. (2) Study of the H 2 SO 4 vapor absorbing layer in the atmosphere by variations in signal intensity and application of this information to tracing atmospheric motions. Scintillation effects caused by radio wave diffraction within the atmosphere can also provide information on small-scale atmospheric turbulence. (3) Investigation of ionospheric structure from approximately 80 km to the ionopause ( (4) Observation of forward-scattered surface echoes obliquely reflected from selected high-elevation targets with anomalous radar properties (such as Maxwell Montes). More generally, such bistatic radar measurements provide information on the roughness and density of the surface material on scales of centimeters to meters. (5) Detection of gravity anomalies, thereby providing insight into the properties of the Venus crust and lithosphere. (6) Measurement of the Doppler shift, propagation time, and frequency fluctuations along the interplanetary ray path, especially during periods of superior conjunction, thus enabling investigation of dynamical processes in the solar corona.

The results of several sets of measurements of the frequency of radio signals during coronal-sounding experiments carried out from 1991 to 2000 using the ULYSSES and GALILEO spacecraft are presented and analyzed. The S-band signals (carrier frequency f = 2295 MHz) were received at the three 70-m widely spaced ground stations of the NASA Deep Space Network. As a rule, the frequency-fluctuation spectra at frequencies above 1 mHz are power-laws. At small heliocentric distances, R < 10R ⊙ (R ⊙ is the solar radius), the spectral index is close to zero; this corresponds to a spectral index for the one-dimensional turbulence spectrum p 1 = 1. The index of the frequency-fluctuation spectra in the region of the supersonic solar wind at distances R > 30 R ⊙ is between 0.5 and 0.7 (p 1 = 1.5–1.7). The results demonstrate a substantial difference between the turbulence regimes in these regions: in the region of the established solar wind, the power-law spectra are determined by nonlinear cascade processes that pump energy from the outer turbulence scale to the small-scale part of the spectrum, whereas such cascade processes are absent in the solar wind acceleration region. Near the solar minimum, the change in the turbulence regime of the fast, high-latitude solar wind occurs at greater distances than for the slow, low-latitude solar wind. Spectra with a sharp cutoff at high frequencies have been detected for the first time. Such spectra are observed only at R < 10 R ⊙ and at sufficiently low levels of the electron density fluctuations. The measured cutoff frequencies are between 10 and 30 mHz; the cutoff frequency tends to increase with heliocentric distance. The variance of the plasma-density fluctuations has been estimated for the slow, low-latitude solar wind. These estimates suggest that the relative fluctuation level at distances 7 R ⊙ < R < 30 R ⊙ does not depend on heliocentric distance. The cross correlation of the frequency fluctuations recorded at widely spaced ground stations increases with the index of the frequency-fluctuation spectrum. At distances R ≈ 10 R ⊙, the rate of temporal changes in irregularities on the scale of several thousand kilometers is less than or comparable to the solar wind velocity.

Measurements of the motion of plasma density inhomogeneities in the inner solar wind are presented. The speeds were estimated using a cross-correlation analysis of radio frequency fluctuations of the Galileo spacecraft measured simultaneously at widely spaced ground stations. The radial projections of the correlation baselines on the pattern plane were of the order of several thousand kilometers. For cross-correlation functions calculated with comparatively short averaging times, we find that a pronounced two-velocity configuration is occasionally observed over the range of heliocentric distances 20 R ⊙ R ⊙ . The typical mean speed for such observations is about 300–400 km/s and the difference between the two predominant speeds is about 150–200 km/s. These results may indicate that the density fluctuations are associated with slow magnetosonic waves propagating in opposite directions at the local speed of sound in the reference frame moving with the mean solar wind speed. Quite reasonable estimates of the solar wind speed and speed of sound are obtained from this model. Another possible explanation of the two-velocity structures is that two independent solar wind streams are present simultaneously along different segments of the radio ray path.

The Cassini Faraday rotation experiment improves the current understanding of the coronal magnetic field by making the first measurements of the magnetic field within two solar radii of the south pole and by allowing the separation of the changing electron density from the magnetic field during transient crossings of the line of sight. Simultaneous ranging data to Cassini also contributes to the growing body of empirical electron density models by providing electron density data at solar maximum at varying latitudes including over the poles. Faraday rotation data in the solar corona were collected in 2002 and 2003 using the Cassini spacecraft in cruise to Saturn. Although Cassini primarily transmits in right-hand polarization, enough left-hand is produced to enable polarization measurements. We show that during solar conjunction Faraday rotation is measurable with Cassini. In the X- and Ka-bands both datasets currently show an asymmetric diurnal pattern in polarization not associated with the ionosphere. The Ka-band is much more sensitive to pointing accuracy and experienced periods of power drop outs which caused the left-circularly polarized signal to drop into the noise on occasion during the 2003 conjunction. Interpretation of the dataset is deferred to future papers.

Measurements of solar wind velocity have been derived from simultaneous coronal sounding observations of radio amplitude scintillations at both S-band and X-band during the solar conjunction of the Ulysses spacecraft in August 1991. The signal amplitude was recorded with an averaging time of 1 s. A cross-correlation analysis between S- and X-band amplitude fluctuations shows that the fluctuation signature at S-band appears to be shifted to earlier times with respect to the X-band recording. The time difference is proportional to the coronal separation of the ray paths and inversely proportional to the apparent velocity of plasma inhomogeneities across the ray paths. Preliminary estimates of solar wind speed obtained using model calculations of the differential refraction are found to lie near the expected transition from subsonic to supersonic velocities at solar offset distances of the order of 6–8 R⊙. As a byproduct of the investigation, we find that the transition from weak to saturated scintillation occurs at about 16 R⊙ for S-band and 7 R⊙ for X-band.

The characteristics of plasma turbulence in the inner solar wind, as deduced from radio frequency fluctuation measurements recorded during solar conjunction, are reviewed. Special emphasis is placed on results from the radio occultation experiments performed with the Galileo and Ulysses spacecraft in the interval 1991–2000. Estimates of the power spectral index are obtained over the range of heliocentric distances 5 R ⊙ R R ⊙ and the radial evolution of this parameter is discussed. Low-heliolatitude data from Galileo are compared with high-heliolatitude data from Ulysses during the period of solar activity minimum. Unusual power spectra with a sharp break at high fluctuation frequencies are detected for the first time in the solar wind acceleration region. The observations are interpreted within the framework of a theoretical model based on local generation of density fluctuations via nonlinear interactions of Alfven waves propagating away from the Sun.

Coronal sounding experiments were carried out using the S-band signals of the Galileo spacecraft during the period of high solar activity in 1999–2000. The parameter studied in this work is the frequency of the downlink carrier signal, which was recorded at the widely spaced ground stations of the NASA Deep Space Network. The measurements apply to both the initial thermal acceleration region of the solar wind at heliocentric distances R in the range from 5 to 10R⊙ (R⊙ = solar radius) and the extended acceleration region (10R⊙

Results of two-way mode coronal radio sounding experiments using the Helios-1,-2, Pioneer-10,-11, and Viking-1 spacecraft are presented. These observational data apply to the low-latitude solar wind during solar activity minimum. Enhancements of the Doppler scintillation variance were detected for spacecraft located closer to the effective scattering layer with respect to the variance measured for a more distant spacecraft at the same time and solar elongation. These enhancements can be explained by the assumption that the radio signal passes twice through the same solar wind density irregularities for close spacecraft but through uncorrelated irregularities when the heliocentric distance becomes too large. Estimates of the outer scale of solar wind turbulence at different heliocentric distances are found from a comparison of Doppler scintillations from near and distant spacecraft.

A new method is developed for solar wind speed measurements based on simultaneous observations of radio wave temporal intensity and frequency fluctuations. An estimate of the solar wind convection velocity is obtained from a combination of the frequency (Doppler) fluctuation spectral power at low fluctuating frequency and the power in the flat part of the intensity scintillation spectra below the Fresnel frequency. This new technique was applied to measurements taken during the Ulysses spacecraft coronal sounding experiment in 1991. Comparison of the speed estimates with earlier ones derived from simultaneous Doppler scintillation measurements at widely-spaced ground stations shows a fairly good agreement.

Radio sounding experiments on of the solar plasma were carried out by the GALILEO spacecraft using S-band (2295 MHz) signals in 1995–1996 a period of minimum solar activity. Equatorial regions at heliocentric distances of 7–80 solar radii were studied. The frequency of the received signal was detected by three ground stations. By carrying out continuous observations of unprecedented duration and processing the data using spectral and correlation methods, we have obtained reliable information on large-scale inhomogeneities of the solar-wind density for the first time. The outer turbulence scale increases with heliocentric distance, the dependence being close to linear. We estimate the outer turbulence scale and analyze its dependence on distance from the Sun and local plasma parameters for a model in which the outer scale is formed due to competition between the linear amplification of Alfven waves in the irregular, moving solar-wind plasma and the nonlinear transfer of turbulent energy to higher frequencies. A comparison of predictions for various specific cases of this model with the observational data suggests that the main nonlinear processes responsible for the formation of the inertial range of the spectrum on the investigated scales are three-wave decay processes involving Alfven and magnetoacoustic waves.

Radio frequency fluctuations have been recorded using the Galileo carrier signal during the spacecraft's solar conjunctions. On these occasions the solar proximate point along the radio ray path to Earth ranged from 7 to 80 R⊙. Regions of low-speed solar wind (equatorial streamer belt) were investigated in 1995/96. Temporal power spectra and variances of the frequency scintillations, and in some cases estimates of the speeds of large-scale density irregularities, have been computed. Owing to the exceptionally high stability of the radio signals, the range of the Doppler scintillation power spectra could be extended down to unprecedentedly low frequencies (∼0.01 mHz). Some of the Galileo spectra display a low-frequency turnover at frequencies ∼0.1 mHz that might be evidence of the density turbulence outer scale in the inner slow solar wind.

Coronal radio sounding experiments with spacecraft near solar conjunction show a steady tendency toward enhanced radio frequency fluctuations during the ingress phase with respect to the egress phase. This behavior can be explained by the higher mean relative velocity of the solar wind density inhomogeneities traversing the radio ray path. Indeed, the relative velocity is the sum of the velocity of the inhomogeneities (solar wind +/− possible wave velocity) plus the ray path velocity during ingress and the difference of these components during egress. There are isolated exceptions to this rule (e.g., the Galileo spacecraft conjunctions in 1994/95 and 1995/96). These anomalous cases, which were observed in the years preceeding solar activity minimum, can be explained by the strongly asymmetric structure of the rotating corona over the course of the conjunction.

A Doppler Wind Experiment (DWE) will be performed during the Titan atmospheric descent of the ESA Huygens Probe. The direction and strength of Titan's zonal winds will be determined with an accuracy better than 1 m s−1 from the start of mission at an altitude of ∼160 km down to the surface. The Probe's wind-induced horizontal motion will be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit and propagation effects. It is also planned to record the frequency of the Probe signal using large ground-based antennas, thereby providing an additional component of the horizontal drift. In addition to the winds, DWE will obtain valuable information on the rotation, parachute swing and atmospheric buffeting of the Huygens Probe, as well as its position and attitude after Titan touchdown. The DWE measurement strategy relies on experimenter-supplied Ultra-Stable Oscillators to generate the transmitted signal from the Probe and to extract the frequency of the received signal on the Orbiter. Results of the first in-flight checkout, as well as the DWE Doppler calibrations conducted with simulated Huygens signals uplinked from ground (Probe Relay Tests), are described. Ongoing efforts to measure and model Titan's winds using various Earth-based techniques are briefly reviewed.

The large-scale stream structure of the solar wind flow is studied in the main acceleration zone from 10 to 40 solar radii from the Sun. Three independent sets of experimental data were used: radio astronomical observations of radio wave scattering using the large radio telescopes of the Lebedev Physical Institute; dual-frequency Doppler solar wind speed measurements from the Ulysses Solar Corona Experiment during the spacecraft's two solar conjunctions in summer 1991 and winter 1995; solar magnetic field strength and configuration computed from Wilcox Solar Observatory data. Both the experimental data on the position of the transonic region of the solar wind flow and the solar wind speed estimates were used as parameters reflecting the intensity of the solar wind acceleration process. Correlation studies of these data with the magnetic field strength in the solar corona revealed several types of solar wind flow differing in their velocities and the location of their primary acceleration region.

Faraday rotation fluctuations (FRF) of S-band polarized radio signals observed during the 1981 solar conjunction of the Helios spacecraft are used to investigate Alfven waves in the solar corona. The occultation geometry enabled coronal radio sounding of the heliographic equatorial region at solar offset distances from 3–12 R ⊙ . Spectral analysis was performed with the FRF data to determine the turbulence regime of the magnetic field irregularities. Cross-correlation functions of signals received simultaneously at two widelyseparated ground stations were used to obtain the propagation speed of Alfven waves traversing the ray paths.

Results are presented of a spectral analysis of Faraday rotation fluctuations observed during the solar occultations of the Helios spacecraft. The temporal spectra of Faraday rotation fluctuations in the frequency range between 10 −3 Hz and 10 −2 Hz usually assume the form of a power law. The power-law exponent is ⋍1.6 at heliocentric distances 3 R ⊙ r ⊙ (R ⊙ = solar radius) and ⋍1.1 at distances 6 R ⊙ r ⊙ . The Faraday rotation data are compared with in situ measurements of magnetic field fluctuations in the range of solar distances 0.3 r

A spectral analysis of coronal Faraday rotation (FR) data obtained with the linearly polarized signals of the two Heliosspacecraft reveals that about one-third of the temporal FR spectra contain a distinct spectral line superposed onto the background power-law spectrum. The most prevalent frequency of this quasi-harmonic component (QHC) is about 4 mHz, corresponding to a 4–5 min periodic oscillation of the coronal magnetic field. Physical reasons for the existence of QHC Alfven fluctuations in the inner solar wind are discussed. FR fluctuations (FRF) are considered to arise from both a turbulent background as well as an isolated Alfven wave train of finite extent and duration. An estimate can be made for the conditions under which the isolated wave train is observed above the ever present background. It is shown that the wave train must have a sufficiently long duration and transverse wavelength. It is suggested that the QHC at periods near 4–5 min in the FRF spectra are most probably produced by outward-propagating Alfven waves excited initially in the anisotropic structures of the chromospheric network.

Long-term scintillation measurements of the solar wind formation zone at solar elongations ranging from 1°–8° (Sun impact parameters: 4–30 R⊙) were recorded using the water maser source IRC-20431 at the wavelength λ=1.35 cm during its annual solar occultations in December 1981–1998. Dramatic changes in the spatial dependence of the scintillation index were recorded over the course of the 11-year solar cycle. Markedly diminished scattering, attributed to a pronounced heliolatitude effect, was observed at the closest solar approach distances in the years around solar activity minimum. From parallel investigations of the solar magnetic field structure it was determined that the field strength at the source of the solar wind streamlines is the governing factor for the solar wind acceleration process. Particularly apparent in the scintillation data during solar activity minimum is the increasing role of the polar coronal holes with their associated open magnetic field structure. The dependence of the solar scattering intensity on heliolatitude fades in the years of high solar activity as the level of scintillations increases at polar latitudes.

A well-known method for studying the solar wind very close to the Sun (heliocentric distances: 4 to 40 solar radii) is by radio sounding between a spacecraft at superior conjunction and the Earth. The Ulysses Solar Corona Experiment was performed at the spacecraft's two solar conjunctions in summer 1991 and winter 1995, during which dual-frequency ranging and Doppler observations were conducted globally on a nearly continuous basis at the NASA Deep Space Network and other ground stations. The dual-frequency Doppler measurements were used to determine coronal plasma velocities by a cross-correlation analysis during those occasions when tracking data were recorded simultaneously at two well-separated ground stations. A ‘filtering’ technique was developed to suppress noise and enhance the 2-station correlations, a procedure particularly effective at small solar offsets. From the electron content measurements during the two solar conjunctions it was found that regions of higher electron density tend to occur when the two-station correlations yield slower outward flow velocities.

Radio sounding investigations were performed over a period of about 30 days during the 1991 solar occultation of the Ulysses spacecraft (August and September 1991). Dual-frequency Doppler and ranging measurements were obtained using the NASA Deep Space Network. For this experiment the radio ray path from Ulysses to Earth moved essentially parallel to the solar equator, sounding the circumsolar plasma on the east (ingress phase) and west (egress phase) solar limbs at solar offset distances from about 4 to 40 RS (solar radii). Dual-frequency, two-way Doppler data were used to determine the propagation speed of coronal plasma inhomogeneities by cross-correlation analysis between the uplink and downlink ray paths. The motion of Earth and spacecraft with respect to Sun results in a spatial separation (typically 20,000 km) between the uplink and downlink ray path during the round-trip travel time of the signal. The specific configuration of the Ulysses radio system (dual-frequency S/X band downlinks coherently locked to a S band uplink) provides the means for separating the uplink and downlink plasma contributions. The propagation speed of plasma inhomogeneities intersecting both ray paths can be determined by computing the cross-correlation of the Doppler data from both ray paths as a function of time lag. The time lag of maximum cross-correlation was used to calculate the propagation speed of plasma inhomogeneities. By evaluating the Ulysses data using this uplink and downlink cross-correlation method, significant cross-correlation peaks were obtained in the range of 7 to 34 RS. A high symmetry of the propagation speeds for both sides of the solar limb was found. An increase in propagation speed with increasing solar distance can be seen, starting from about 50 km/s at a distance of 7 RS up to 390 km/s at a distance of 29.7 RS (east) and from about 120 km/s at 9.8 RS up to 415 km/s at 33.8 RS (west). A significant enhancement (550–660 km/s) was observed at a distance of about 18 RS on both sides of the solar limb due to a solar event still being investigated.

A statistical study of Faraday rotation fluctuations (FRF) has been performed using polarization angle data of S-band (f = 2.3 GHz) radio spacecraft signals. The measurements were recorded during the recurring superior conjunctions of the Helios probes, during which the solar proximate point of the radio ray path reached heliocentric distances between 3 and 34 R⊙. The most commonly found temporal FRF spectra are power laws with an average spectral index ∼1.5 over the frequency range from 1 mHz < v < 10 mHz. The FRF variance decreases with heliocentric distance, the falloff exponent being ∼8 for R < 6 R⊙ and ∼3 for distances 8 < R < 6 R < 16 R⊙. The results are interpreted under the assumption that the FRF are produced by Alfven waves propagating in the coronal plasma. For the applicable range of heliocentric distances it is shown that Alfven waves are in a regime of free propagation and probably transfer much of their energy to the kinetic energy of the solar wind. The spatial power spectrum of magnetic field fluctuations is inferred to be strongly anisotropic, the irregularities extending along the background magnetic field with axial ratios of the order of 10.

Polarization angle measurements from coronal radio sounding experiments using the linearly polarized Helios signals are spectral analyzed to investigate Faraday rotation fluctuations at characteristic periods from 1 to 15 minutes. The level of fluctuation intensity of the spectra are determined for heliocentric distances between 3 and 16 solar radii. Assuming that the magnetic field fluctuations are isotropic and are generated by Alfven waves, a technique is developed for deriving the magnetic field amplitude of coronal Alfven waves from the Faraday rotation fluctuations.