• scintillation spectrum;
  • angle-of-arrival fluctuations;
  • solar conjunction

[1] In this study, we develop a complete theoretical approach to derive the angle-of-arrival fluctuations (AAF) of radio signals passing through the turbulent solar plasma medium during solar superior conjunctions. Using the power spectra of phase fluctuations measured at various solar elongation angles (or impact heliocentric distances) from the Cassini spacecraft, we have defined the dependence of the AAF variance on the heliocentric distance as ∼r−3.5 within a range very close to the Sun. This quantity decreases with increasing distance, with a slope significantly less steep than that previously expected. The AAF expression is theoretically derived by assuming a frozen turbulence and by converting a phase temporal variation into a spatial variation. To perform this calculation, the solar plasma medium is treated as an anisotropic ionized medium by applying the Booker electron irregularity spectrum model and the phase expression in term of the electron refractive index. Using the phase spectral measurements from the Cassini spacecraft during a solar superior conjunction, coefficients of the expression are calibrated, and the final AAF results are quantitatively obtained.