Solar cycle evolution of ICME sheath regions at 1 AU

Aims. We investigate the evolution of interplanetary coronal mass ejection (ICME) sheath regions at 1 AU across solar cycles 23, 24, and the rising phase of 25, focusing on their variability and turbulence in relation to upstream solar wind conditions and the global heliospheric state.Methods. Using a dataset of over 900 ICME sheath events, we applied statistical metrics such as the interquartile range (IQR) and the turbulence index (TI) to quantify variability and turbulence. The analysis compares full and rising phases of solar cycles and examines both local ICME parameters (e.g., sheath total pressure, non-radial flows) and global interplanetary indicators such as open solar flux (OSF).Results. From solar cycle 23 to solar cycle 24, the sheath total pressure and magnetic field strength decreased by over 40% and 25%, respectively, accompanied by reduced turbulence and variability. In contrast, the rising phase of solar cycle 25 shows increased magnetic complexity, particularly in non-radial field components, despite stable bulk parameters. Non-radial flow patterns also shift from tangentially dominated in solar cycle 23 and solar cycle 24 to normal-dominated in solar cycle 25, suggesting changes in ICME orientation and sheath formation mechanisms. No significant correlation is found between OSF and sheath properties, indicating that local solar wind and ICME-specific factors are the primary drivers of sheath evolution.Conclusions. The study reinforces the importance of upstream solar wind dynamics in relation to variations in plasma and magnetic field measured components of ICME sheaths. The derived trends in turbulence, magnetic orientation, and flow geometry suggest that sheath regions are sensitive indicators of solar cycle phase and should be considered as distinct, structured components in ICME modeling.FullText for HTML: https://doi.org/10.1051/0004-6361/202557295

**研究目标**：我们旨在探究第23、24太阳周及第25太阳周上升阶段中，1天文单位（astronomical unit, AU）处的行星际日冕物质抛射（interplanetary coronal mass ejection, ICME）鞘区的演化特性，重点关注其与上游太阳风条件及全球日球层状态相关的可变性与湍流特征。
**研究方法**：本研究使用涵盖900余次ICME鞘层事件的数据集，运用四分位距（interquartile range, IQR）、湍流指数（turbulence index, TI）等统计指标对可变性与湍流特性进行量化分析。本研究对比了各太阳周的完整阶段与上升阶段，并同时考察了局域ICME参数（如鞘区总压、非径向流）与全局行星际指标（如开放太阳通量（open solar flux, OSF））。
**研究结果**：从第23太阳周到第24太阳周，鞘区总压与磁场强度分别下降超40%与25%，伴随湍流与可变性的减弱。与之相反，第25太阳周的上升阶段则呈现出更强的磁场复杂性，尤其是非径向磁场分量，尽管整体等离子体参数保持稳定。非径向流型也从第23、24太阳周的切向主导转变为第25太阳周的法向主导，这暗示ICME取向与鞘区形成机制发生了变化。未发现开放太阳通量与鞘区特性间存在显著相关性，表明局域太阳风与ICME自身相关因素是鞘区演化的主要驱动因素。
**研究结论**：本研究进一步证实了上游太阳风动力学对于ICME鞘区观测到的等离子体与磁场分量变化的重要性。所得到的湍流、磁场取向与流场几何的演化趋势表明，鞘区是太阳周阶段的敏感指示器，且在ICME建模中应作为独立的结构化组件加以考量。
全文HTML链接：https://doi.org/10.1051/0004-6361/202557295