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

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.

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