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

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 R S , and for some observations close to the sun the spectra extend to fluctuation frequencies as high as 100 Hz. For heliocentric distances ≳20 R S the equivalent spacecraft‐measured one‐dimensional density spectrum V ne 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, V ne varies as R −3.45 , where R is heliocentric distance. Within 20 R S , V ne 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 R S is presumably associated with energy deposition in the near‐sun region and acceleration of the solar wind.