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<p>The turnover phenomenon observed for the radial variation of interplanetary scintillation (IPS) represents transition between weak and strong scattering of radio waves, and its location from the Sun (the turnover distance) serves as a reliable measure of solar-wind density fluctuations. In this study, we determined the turnover distances for 36 sources from IPS observations during 2008 – 2022 (Cycle 23/24 minimum through Cycle 25 rising phase) using the 327&#xa0;MHz radio telescope at Toyokawa Observatory of the Institute for Space-Earth Environmental Research, Nagoya University. Such turnover-distance measurements over a long period using multiple sources provide unique information on the global distribution of solar-wind density fluctuations evolving with time, leading to improved understanding of the heliospheric response to the solar cycle. At Cycle 24 maximum, large values of the turnover distance were observed for all latitudes, whereas smaller values were observed at Cycle 23/24 and 24/25 minima particularly over the poles. These results suggest that the solar-wind density fluctuations at around 0.2 AU from the Sun increase (decrease) at solar maximum (minimum), and that their distribution in latitude were nearly uniform at solar maximum and non-uniform at minimum. The decrease in the turnover distance over the poles at solar minimum is ascribed to the effect of the polar coronal hole. Although the results obtained here were generally consistent with IPS observations made in Cycles 21 and 22, the turnover distances of the equator observed at Cycles 23/24 and 24/25 minima were smaller than those at the Cycle 20/21 and 21/22 minima, suggesting a marked reduction in density fluctuations of the equator at Cycles 23/24 and 24/25 minima, which is regarded as a manifestation of the weak solar cycle. A significant negative correlation was found between the turnover distance and the sharpness of the turnover. This means that the density gradient becomes flat (steep) when the solar-wind density fluctuations decrease (increase), providing with important implications on formation of large-scale density structure in the heliopshere.</p>

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Global Distribution of Solar-Wind Density Fluctuations Derived from Interplanetary Scintillation Observations During 2008 – 2022

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  • 发表日期：

    2026年6月22日

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  • Munetoshi Tokumaru

摘要

The turnover phenomenon observed for the radial variation of interplanetary scintillation (IPS) represents transition between weak and strong scattering of radio waves, and its location from the Sun (the turnover distance) serves as a reliable measure of solar-wind density fluctuations. In this study, we determined the turnover distances for 36 sources from IPS observations during 2008 – 2022 (Cycle 23/24 minimum through Cycle 25 rising phase) using the 327 MHz radio telescope at Toyokawa Observatory of the Institute for Space-Earth Environmental Research, Nagoya University. Such turnover-distance measurements over a long period using multiple sources provide unique information on the global distribution of solar-wind density fluctuations evolving with time, leading to improved understanding of the heliospheric response to the solar cycle. At Cycle 24 maximum, large values of the turnover distance were observed for all latitudes, whereas smaller values were observed at Cycle 23/24 and 24/25 minima particularly over the poles. These results suggest that the solar-wind density fluctuations at around 0.2 AU from the Sun increase (decrease) at solar maximum (minimum), and that their distribution in latitude were nearly uniform at solar maximum and non-uniform at minimum. The decrease in the turnover distance over the poles at solar minimum is ascribed to the effect of the polar coronal hole. Although the results obtained here were generally consistent with IPS observations made in Cycles 21 and 22, the turnover distances of the equator observed at Cycles 23/24 and 24/25 minima were smaller than those at the Cycle 20/21 and 21/22 minima, suggesting a marked reduction in density fluctuations of the equator at Cycles 23/24 and 24/25 minima, which is regarded as a manifestation of the weak solar cycle. A significant negative correlation was found between the turnover distance and the sharpness of the turnover. This means that the density gradient becomes flat (steep) when the solar-wind density fluctuations decrease (increase), providing with important implications on formation of large-scale density structure in the heliopshere.