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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 at the VLA and even as early as 1962 at the Pulkovo Radio Telescope. Pulsars were exploited to measure coronal RM at the MPIfR Effelsberg 100-m Telescope. Extending these single raypath observations, an ambitious project has been proposed to use the future LOFAR facility for constructing a coronal RM 'image'. Different time scales of FR variations are related to different physical phenomena. Among the observed effects are: (a) slow variations associated with the changing geometry and rotation of the corona; (b) random oscillations probably arising from a rich spectrum of coronal Alfvén waves; (c) rapid changes in RM caused by transient events such as coronal mass ejections (CMEs). [ABSTRACT FROM AUTHOR]

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 radio ray paths, implying that the outward solar wind transport in the CME may be described as an ordered flow. The density turbulence power spectrum becomes steeper behind the CME leading front but becomes unusually flat near the trailing edge. It is shown that the effects observed in the Galileo radio-sounding data are associated with the same CME observed first (6 January) by the SOHO/LASCO coronagraphs near the Sun and later (10-11 January) in the solar wind on the WIND spacecraft near the Earth (e.g. abrupt changes in ion density, peaks in the velocity, interplanetary magnetic field enhancements). [ABSTRACT FROM AUTHOR]

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 p1 = 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 (p1 = 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 < 10R⊙ 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 < 30R⊙ 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.© 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

A statistical analysis of the Faraday-rotation fluctuations (FRFs) of linearly polarized radio signals from the Helios 1 and Helios 2 spacecraft shows that the FRF time power spectra can be of three types. Spectra of the first type are well fitted by a single power law in the range of fluctuation frequencies 1-10 mHz. Spectra of the second type are a superposition of a power law and two quasi-harmonic components with fluctuation frequencies of about v[sub 1] = 4 mHz (fundamental frequency) and v[sub 2] = 8 mHz (second harmonic). Spectra of the third type exhibit only one of the two quasi-harmonic components against the background of a power law. The spectral density of the quasi-harmonic components can be represented by a resonance curve with a fairly broad [Δ[sub v] ≈ (0.5-1.3)v[sub 1, 2]] distribution relative to the v = v[sub 1, 2] peak. The intensity of the quasi-harmonic FRF has a radial dependence that roughly matches the radial dependence for the background FRF, while their period at the fundamental frequency is approximately equal to the period of the well-known 5-min oscillations observed in the lower solar atmosphere. The fluctuations with 5-min periods in FRF records can be explained by the presence in the outer corona of isolated trains of Alfvén waves generated at the base of the chromosphere-corona transition layer and by acoustic waves coming from deeper layers. [ABSTRACT FROM AUTHOR]