11:00 〜 11:15
[PEM14-02] Improvement of the computer assisted tomography of interplanetary scintillation observations
キーワード：太陽風、惑星間空間シンチレーション、乱流、トモグラフィー
The computer assisted tomography (CAT) of interplanetary scintillation (IPS) observations allows reconstruction of global distribution of the solar wind speed. Earlier studies demonstrated that the solar wind data derived from the CAT analysis are comparable to those from in situ measurements. However, comparison between the CAT analysis and in situ measurements at the Earth orbit for the period after 2009 showed that the solar wind speeds derived from IPS observations are systematically higher than those from in situ observations. In this study, we examined two effects as a cause for this discrepancy; one is the effect of the number of IPS data used for the CAT analysis, and the other is the effect of the empirical relation between solar wind speed V and density fluctuations ΔNe; V ∝ ΔNeα, which is assumed in the CAT analysis. As the result, we found that the latter can explain the observed discrepancy if α varies with time, whereas the former is unlikely. Instead, the effect of the number of IPS data can account for variations of rms deviations and correlations between IPS and in situ observations. The index of α=-0.5 is optimal to analyze IPS observations between 1985 and 2008, and a positive and larger value of alpha (i.e. α=+1.0) yields better agreement between IPS and in situ observations. By using these optimal values for α, the CAT analysis of IPS observations is improved, and this leads to better understanding of global heliosphere. The result obtained here also suggests that physical properties of solar wind micro-turbulence may evolve over long timescale. If the momentum flux is invariant among different solar wind flows, the result obtained here implies that fractional density fluctuations of the solar wind for the recent period depend on the speed more strongly than those for the past period.