Optimization of Solar-Wind Speed Models Using Interplanetary Scintillation Observations

Improvement of the model providing the boundary condition of the solar-wind speed near the Sun is essential for gaining a better forecast of space weather. We optimized the parameters of the distance from the coronal hole boundary (DCHB) model and the Wang–Sheeley (WS) model, which enabled the determination of solar-wind speed from observations of the Sun’s magnetic field. In this study, we used solar-wind speed data derived from interplanetary scintillation (IPS) observations at the Institute for Space-Earth Environmental Research (ISEE) for six Carrington rotations in Solar Cycle 23 as reference data. A comparison of IPS observations and optimized DCHB models demonstrated strong-to-moderate positive correlations and small deviations, except for solar maximum data. The degraded correlation at the solar maximum is ascribed to the effect of the rapid structural evolution of the solar wind and coronal magnetic field. The performance of the optimized DCHB model was better than that of the optimized WS model. To solve a limitation of the DCHB model in reproducing slow-wind speeds, we propose a modified version of the DCHB model and optimize it for IPS observations. The optimized solutions for the modified DCHB model demonstrate performance comparable to that of the original model. The results obtained in this study suggest that the DCHB acts better as a controlling parameter for the solar-wind speed than the expansion factor and that both the optimized DCHB model and its modified version are useful for improving the estimation of the solar-wind speed at the source surface from magnetograph observations.