Observation of a coronal mass ejection in January 1997 using radio sounding of the near-solar plasma with the GALILEO spacecraft

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 outer scale of solar-wind turbulence from GALILEO coronal-sounding data

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.     

Properties of solar wind turbulence from radio occultation experiments with the NOZOMI spacecraft

Radio-sounding experiments using signals from the Japanese NOZOMI spacecraft to probe the circum solar plasma were performed from December 2000 through January 2001. They can be used to obtain information about the properties of the solar wind plasma in the region where it is accelerated at heliocentric distances of 12.8–36.9R _s (where R _s is the radius of the Sun). Measurements of the intensity and frequency of the received signals were carried out with high time resolution (～0.05 s for the frequency and ～0.0064 s for the intensity), making it possible to investigate the anisotropy of inhomogeneities and the spatial spectrum of the turbulence of the circum solar plasma. Analysis of these radio-sounding data has shown that the scintillation index and intensity of the frequency fluctuations decrease approximately according to a power law with increasing distance of the line of sight from the Sun. Measurements of the amplitude fluctuations and estimates of the solar wind velocity derived from spatially separated observations indicate the presence of small-scale inhomogeneities with sizes of the order of 50 km at heliocentric distances less than 25R _s , which are elongated in the radial direction with anisotropy coefficients from 2.3 to 3.0. The inhomogeneities at heliocentric distances exceeding 30R _s become close to isotropic.     

Quasi-harmonic faraday-rotation fluctuations of radio waves when sounding the outer solar corona

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 v₁=4 mHz (fundamental frequency) and v₂=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 [Δυ ≈ (0.5–1.3)υ_1,2] distribution relative to the v=v_{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 wellknown 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.     

Characteristics of Coronal Alfvén Waves Deduced from Helios Faraday Rotation Measurements

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 Alfvén waves propagating in the coronal plasma. For the applicable range of heliocentric distances it is shown that Alfvén 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.     

Spatial distribution of turbulence characteristics in the inner 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.     

Turbulence in the inner solar wind determined from frequency fluctuations of the downlink signals from the ULYSSES and GALILEO spacecraft

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 < 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.     

Five-minute magnetic field fluctuations in the solar wind acceleration region

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 Alfvén 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 Alfvén 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 Alfvén waves excited initially in the anisotropic structures of the chromospheric network.     

Two-Way Frequency Fluctuations Observed During Coronal Radio Sounding Experiments

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.     

Investigation of coronal mass ejections by the two-position radio sounding method

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.