Acceleration phenomena of high-speed wind observed at 0.1–0.3 AU with interplanetary scintillation

Velocity interplanetary scintillation (IPS) observations have been obtained at a frequency of 327 MHz by the Solar-Terrestrial Environment Laboratory since the early 1980s, covering the distance range from 0.1 to 1 AU. The IPS technique measures high-latitude streams which have low speeds (⩽400 km/s) within 0.3 AU and high speeds (⩾700 km/s) beyond 0.3 AU. The foot points of these streams were located in the HeI coronal holes or at their boundaries. Since the IPS measurement can be biased by line-of-sight integration, we investigated whether the acceleration observation could be a result of the biasing effect. We found that if the velocity dependence of turbulence level is expressed by δNe=V−α, α should be equal to 0.5 at distances beyond 0.3 AU. If α increases to 2.2 within 0.3 AU, the IPS speed in high latitude stream near the sun can be biased lower by about 200 km/s. However, a much larger α near the sun is required to explain a speed difference of 300 km/s.