L. N. Samoznaev

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.

The spectrum of magnetic field irregularities in the solar corona and in interplanetary space

Abstract 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

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.

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)υ1,2] distribution relative to the v=v1, 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.

Dual-frequency radio sounding with spacecraft signals: Quasi-periodic perturbations in the circumsolar plasma

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.

Frequency fluctuation of spacecraft radio signals in the circumsolar plasma

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.

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.

Faraday rotation fluctutation spectra observed during solar occultation of the Helios spacecraft

Faraday rotation (FR) measurements using linearly polarized radio signals from the two Helios spacecraft were carried out during the period from 1975 to 1984. This paper presents the results of a spectral analysis of the Helios S-band FR fluctuations observed at heliocentric distances from Fig. 3 to 16 R⊙ during the superior conjunctions 1979–1984. The mean intensity of the FR fluctuations usually does not exceed the noise level for solar offsets greater than ≃15 R⊙. The rms FR fluctuation amplitude increases rapidly as the radio ray path approaches the Sun. Good representations of the radial variation were obtained with a two-term empirical formula. The equivalent two-dimensional FR fluctuation spectra are well modeled by a single power-law over the frequency range from 1 to 50 mHz. There is a tendency for an increase of the spectral index from 1.2 at solar offset distances R>10 R⊙ to 1.6 at R<6 R⊙, corresponding to a range for the three-dimensional spectral index p=2.2–2.6. FR fluctuations thus display ...

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 12.8–36.9Rs (where Rs 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 25Rs, which are elongated in the radial direction with anisotropy coefficients from 2.3 to 3.0. The inhomogeneities at heliocentric distances exceeding 30Rs become close to isotropic.

Quasi-periodic Faraday-rotation fluctuations of the polarization plane during radio sounding of the circumsolar plasma

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.