Theoretical studies indicate that electric potential saturation in a polar cap ionosphere is regulated by the limited reconnection rate at the magnetopause associated with field-aligned currents (FACs), predicting the possible saturation of FACs under large interplanetary electric field (IEF). However, recent statistical studies have shown that observed FACs increase linearly with IEF. To reconcile this disparity, numerical experiments are conducted to explore the response of FACs under large IEF. Results show that FACs may exhibit either linear or saturated responses, depending on the Alfvén Mach number. The magnetospheric closure paths of region 1 currents differ significantly between the linear and saturated conditions. In the linear case, essential parts of the region 1 current extend toward dayside and connect to the magnetopause currents, whereas these parts of currents almost disappear in the saturation state. Saturation usually occurs under sub-Alfvénic solar wind conditions, so the observed FACs generally grow linearly with IEF.

中文翻译：

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为什么观察到的磁场对齐电流不会随着行星际电场的增加而饱和？


理论研究表明，极帽电离层中的电势饱和受与场对齐电流 （FAC） 相关的磁层顶处有限重连速率的调节，从而预测了在大行星际电场 （IEF） 下 FAC 可能达到的饱和。然而，最近的统计研究表明，观察到的 FAC 随 IEF 线性增加。为了调和这种差异，进行了数值实验以探索 FACs 在大型 IEF 下的响应。结果表明，FAC 可能表现出线性或饱和响应，具体取决于 Alfvén 马赫数。区域 1 电流的磁层闭合路径在线性和饱和条件之间有很大差异。在线性情况下，区域 1 电流的重要部分向日边延伸并连接到磁层顶电流，而电流的这些部分在饱和状态下几乎消失。饱和通常发生在亚阿尔夫尼克太阳风条件下，因此观测到的 FAC 通常随 IEF 线性增长。