Long-term radio scintillation variations in the circumsolar plasma

Long-term scintillation measurements of the solar wind formation zone at so lar elongations ranging from 1 degrees-8 degrees (Sun impact parameters: 4- 30 R.) were recorded using the water maser source IRC-20431 at the waveleng th lambda=1.35 cm during its annual solar occultations in December 1981-199 8. Dramatic changes in the spatial dependence of the scintillation index we re recorded over the course of the 11-year solar cycle. Markedly diminished scattering, attributed to a pronounced heliolatitude effect, was observed at the closest solar approach distances in the years around solar activity minimum. From parallel investigations of the solar magnetic field structure it was determined that the field strength at the source of the solar wind streamlines is the governing factor for the solar wind acceleration process . Particularly apparent in the scintillation data during solar activity min imum is the increasing role of the polar coronal holes with their associate d open magnetic field structure. The dependence of the solar scattering int ensity on heliolatitude fades in the years of high solar activity as the le vel of scintillations increases at polar latitudes.