The investigation of the solar wind transsonic region at meter wavelengths

Abstract

Radio astronomy experiments designed to probe the interplanetary plasma close to the Sun have been carried out at the wavelength λ = 2.92 m using the occultation method. The experiments are based on a modification of the occultation method by which the sources, in this case quasars, are used to measure the radial dependence of the apparent source size, the scattering angle θ(R). The radial dependence of this source size θ(R) reveals that an anomalous enhancement in the scattering appears at radial distances from about 16 to 30 R⊙, which is associated with the solar wind transsonic region. It is shown by analysis of the theoretical equations that the radial profiles of both the source size θ(R) and the scintillation index m(R) are consistent in the near solar interplanetary medium (R ≲ 40 R⊙). Combining these two independent applications of the occultation method extends the range of the investigation and provides a powerful new diagnostic of the solar wind transsonic region.