Heliospheric tomography using interplanetary scintillation observations: 2. Latitude and heliocentric distance dependence of solar wind structure at 0.1–1 AU

Abstract

Interplanetary scintillation is a useful means to measure the solar wind in regions inaccessible to in situ observation. However, interplanetary scintillation measurements involve a line‐of‐sight integration, which relates contributions from all locations along the line of sight to the actual observation. We have developed a computer assisted tomography (CAT) program to reduce the adverse effects of the line‐of‐sight integration. The program uses solar rotation and solar wind motion to provide three‐dimensional perspective views of each point in space accessible to the interplanetary scintillation observations and optimizes a three‐dimensional solar wind speed distribution to fit the observations. We analyzed IPS speeds observed at the Solar‐Terrestrial Environment Laboratory and confirmed that (1) the solar wind during the solar minimum phase has a dominant polar high‐speed solar wind region with speeds of about 800 km s −1 and within 30° of the solar equator speeds decrease to 400 km s −1 as observed by Ulysses, and (2) high‐speed winds get their final speed of 750–900 km s −1 within 0.1 AU, and consequently, that acceleration of the solar wind is small above 0.1 AU.