V. E. Andreev

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 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.

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 ...

Outer scale of coronal turbulence near the sun

Radio frequency fluctuations have been recorded using the Galileo carrier signal during the spacecrafts 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.

East-West scattering level asymmetry of the solar corona

Abstract 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.

Turbulence Regimes of the Solar Wind in the Region of its Acceleration and Initial Stage of Supersonic Motion

Coronal radio sounding experiments were carried out during the solar conjunctions of the spacecraft Ulysses and Galileo, providing information on the solar wind plasma over a wide range of heliocentric distances and heliolatitudes on both East and West limbs of the Sun. An important component of these investigations is to identify the turbulence regimes of the solar wind in its acceleration and initial supersonic regions. This work concentrates on the variation of the spectral index of the temporal frequency fluctuation spectrum αf. The analysis leads to the following preliminary conclusions: (1) At low heliolatitudes the turbulence becomes ‘developed’, with αf reaching the Kolmogorov value of 2/3, at distances beyond 2O R⊙; (2) At high heliolatitudes (poleward of 65°) the solar wind turbulence remains undeveloped out to distances of at least 30 R⊙; (3) At distances close to the Sun (less than 7 R⊙) the spectrum sometimes becomes a double power‐law with small spectral index αf ≃ 0.03–0.11 at low fluctuati...

Properties of coronal Alfvén waves from polarized radio occulatation experiments

Abstract 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.

Fine Structure of the Solar Wind Turbulence Inferred from Simultaneous Radio Occultation Observations at Widely-Spaced Ground Stations

Coronal radio sounding experiments with the Ulysses spacecraft at superior conjunction provided numerous opportunities for simultaneous observations of the downlink signals at two widely‐spaced ground stations. In some instances the duration of these observations extended for up to four hours, thereby allowing studies of solar wind turbulence dynamics at spatial scales comparable with the corona‐projected distance between ground stations (a few thousand km). The frequency and phase fluctuations produced by electron density inhomogeneities are normally quite well correlated on these scales. The spectral index of the temporal frequency fluctuation spectra varied over a wide range during the observations. The cross‐correlation coefficient reached maximal values (≈ 0.5) when the spectral index was high (≈ 1), but no correlation could be detected when the spectral index became small (< 0.4). Similar behavior in many of the data sets implies that this is a common, if not permanent, feature of the solar wind. Po...

Properties of Alfvén waves in the outer solar corona from two-station faraday rotation observations

Abstract 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.