Global Distribution of the Solar Wind Speed Reconstructed from Improved Tomographic Analysis of Interplanetary Scintillation Observations between 1985 and 2019

Computer-assisted tomography (CAT) for interplanetary scintillation (IPS) observations enables the determination of the global distribution of solar wind speed. We compared solar wind speeds derived from the CAT analysis of IPS observations between 1985 and 2019 with in situ observations conducted by the near-Earth and Ulysses spacecraft. From this comparison, we found that solar wind speeds from the IPS observations for 2009–2019 were systematically higher than the in situ observations, whereas those for the period until 2008 were in good agreement with the in situ observations. Further, we found that the discrepancy between IPS and the in situ observations is improved by changing the power index of the empirical relation between the solar wind speed and density fluctuations. The CAT analysis using an optimal value for the power index determined from the comparison between IPS and in situ observations revealed long-term variations in the solar wind speed distribution over three cycles, leading to a better understanding of the time-varying global heliosphere. We found that polar solar winds become highly anisotropic at the Cycle 24/25 minimum, which is a peculiar aspect of this minimum. The IPS observations showed general agreement with the Parker Solar Probe observations around the perihelion of Orbit 1; this supports the reliability of the CAT analysis. The results of this study suggest that the physical properties of solar wind microturbulence may vary with a long-term decline in the solar activity, which provides important implication on the solar wind acceleration.