13:45 〜 14:00
[PEM10-13] 中間領域の磁束管断面積拡大が及ぼす太陽風速度への影響
キーワード：太陽風、スードストリーマー、宇宙天気、数値シミュレーション
The solar wind is a key factor in the space weather and its forecasting. In particular, the temporal and spatial structure of the wind speed is closely linked to the occurrence of co-rotating interaction regions (CIRs) and the propagation of interplanetary coronal mass ejections (ICMEs). Most space weather forecasting models adopt empirical methods to specify the solar wind parameters at the inner boundary, aiming to reduce computational costs and enhance forecasting speed. One of the conventional empirical laws for the solar wind speed is the Wang-Sheeley relation (Wang & Sheeley 1990; Arge & Pizzo 2000), which describes an inverse correlation between the solar wind speed and the flux-tube expansion factor. However, it has been suggested that this model overestimates solar wind speeds in regions between coronal holes with the same magnetic polarity, known as pseudostreamers (Riley et al. 2015; Tokumaru et al. 2024). Pseudostreamers exhibit non-monotonic expansion of magnetic flux tubes, with the expansion factor reaching a peak (Panasenco et al. 2019), which may contribute to the overestimation. Observational studies also indicate that expansion height, rather than the expansion factor, is a key parameter (Dakeyo et al. 2024). These findings suggest that the shape of magnetic flux tubes in the intermediate region between the coronal base and the source surface plays a crucial role in determining solar wind speed.
To develop a novel empirical law for solar wind speed that incorporates the flux-tube shape in the intermediate region, we investigate the influence of magnetic flux tube shape variations in the intermediate region using physical simulations. To this end, we solved a one-dimensional wave-driven magnetohydrodynamic (MHD) simulation from the photosphere to interplanetary space, with calculations performed while varying the magnetic field and expansion factor. Since multiple parameters characterize the flux-tube shape, we investigate how the solar wind speed changes when a single parameter is varied, and the others are fixed. As a result, we find that variations in the magnetic flux tube shape in the intermediate region significantly alter solar wind speed by up to several hundred km/s. Moreover, we find that the integral value in the intermediate region is found to correlate more strongly with the solar wind speed than with the expansion factor at the source surface or the B/f value. These findings indicate that using an integrated value rather than a point value can improve prediction accuracy.
To develop a novel empirical law for solar wind speed that incorporates the flux-tube shape in the intermediate region, we investigate the influence of magnetic flux tube shape variations in the intermediate region using physical simulations. To this end, we solved a one-dimensional wave-driven magnetohydrodynamic (MHD) simulation from the photosphere to interplanetary space, with calculations performed while varying the magnetic field and expansion factor. Since multiple parameters characterize the flux-tube shape, we investigate how the solar wind speed changes when a single parameter is varied, and the others are fixed. As a result, we find that variations in the magnetic flux tube shape in the intermediate region significantly alter solar wind speed by up to several hundred km/s. Moreover, we find that the integral value in the intermediate region is found to correlate more strongly with the solar wind speed than with the expansion factor at the source surface or the B/f value. These findings indicate that using an integrated value rather than a point value can improve prediction accuracy.