Spacecraft radio scattering observations of the power spectrum of electron density fluctuations in the solar wind

Abstract

Solar wind electron density power spectra in the solar equatorial region are inferred from observations of phase scintillations and spectral broadening made with the Viking, Helios, and Pioneer spacecraft. The heliocentric distance range covered is 2–215 RS, and for some observations close to the sun the spectra extend to fluctuation frequencies as high as 100 Hz. For heliocentric distances ≳20 RS the equivalent spacecraft‐measured one‐dimensional density spectrum Vne is well modeled by a single power law (f−α) in the frequency range 10−4‐5 × 10−2 Hz. The mean spectral index α is 1.65, very close to the Kolmogorov value of 5/3. Under the assumption of constant solar wind speed, Vne varies as R−3.45, where R is heliocentric distance. Within 20 RS, Vne can still be modeled by a single power law over the frequency range 10−3‐10¹ Hz, but the spectral index becomes smaller, α ∼ 1.1. The flattening of the density spectrum within 20 RS is presumably associated with energy deposition in the near‐sun region and acceleration of the solar wind.

The relation between gas density and velocity power spectra in galaxy clusters: High-resolution hydrodynamic simulations and the role of conduction

“…The observed power spectrum of the solar wind density, which is also consistent with the famous −5/3 slope, has further proven that turbulent processes are a key component shaping the dynamics of astrophysical plasmas (e.g. Woo & Armstrong 1979;Marsch & Tu 1990). In a similar way, the wealth of information contained in the power spectrum extracted from the hot plasma filling galaxy clusters can help us to significantly advance our knowledge of the intracluster medium (ICM) astrophysics.…”

Radio occultation experiment of the Venus atmosphere and ionosphere with the Venus orbiter Akatsuki

“…The density fluctuation of the interplanetary plasma can also be an error source for small solar offset distances of the ray path. Based on the results of solar occultation measurements (Woo and Armstrong, 1979;Imamura et al, 2005), the error is estimated to be ∼3000 cm −3 for the solar offset distance of 0.1 AU, and decreases to ∼300 cm −3 for 0.3 AU, given the typical time scale of traversing the Venus ionosphere of 10 s. It should be noted that these uncertainties vary with time.…”

“…The temporal variations of the phase and the intensity of the received signal provide information on the spatial spectrum (power law) of density inhomogeneities, the anisotropy of inhomogeneities, the inner turbulence scale, and the bulk velocity of plasma (Pätzold et al, 1996;Imamura et al, 2005;Efimov et al, 2010). Based on the noise level estimation (Section 5.4) and previous occultation results (Woo and Armstrong, 1979), we expect the phase power spectrum will be obtained up to ∼10 Hz for the solar offset distance of 10 solar radii; such a spectrum would enable us to study, for example, the excess power at small scales (100-600 km) observed by the Nozomi radio occultation (Imamura et al, 2005). The minimum solar offset distance of ∼1.5 solar radii will occur on June 25, 2011 in the renewed cruising plan.…”

Tracking heliospheric disturbances by interplanetary scintillation

“…Note that more realistic values of would not produce any significant changes in the analysis, since model calculations of the g-value are insensitive to . The solar wind turbulence is known to be well described by the Kolmogorov spectrum (q=11/3) for all distances except for the vicinity of the Sun (Woo and Armstrong, 1979). Therefore, we assumed q=11/3 in the analysis.…”