Evolution of the Solar Wind Structure in the Acceleration Region during 1990-1993(STEP Interval).

The single-station measurements of interplanetary scintillation (IPS) at 2 and 8 GHz are used to study the radial distribution of solar wind velocity and density fluctuations near the sun. IPS observations in the ecliptic plane reveal the occurrence of the solar wind acceleration in the radial range between 10 and 30 Rs (solar radii), and also reveal the radial decrease of density fluctuations with a power index of approximately -2. Such structure is basically unchanged during the period from 1990 to 1993; i.e. the solar maximum to the declining phase. On the other hand, IPS observations at the high latitude region of the sun demonstrate a significant reduction of the turbulence level as well as a marked development of high speed streams in the low solar activity. This fact is in good agreement with the well-known solar cycle variation of the coronal hole. From the comparison between IPS data at 2 and 8 GHz, it is found that there is a slight but significant difference in the radial slope of the scintillation index variation. This might be attributed to the effect of either source size or spectral index of turbulence.