The ionosphere is very active and complex due to photo‐ionization from the solar activity, while traditional empirical models can only give a rough description of its actual variations. Nowadays, global ionosphere maps (GIMs) derived from denser Global Navigation Satellite Systems (GNSS) world‐tracking data provide an excellent total electron content (TEC) data set for global ionospheric research and modeling. In this paper, long‐tern variations of 16‐year (2003–2018) TEC time series from GIMs are investigated by using the principal mode analysis (PCA) technique. We analyze the resulting modes in the time‐spectral domain and parameterize the main contributions in terms of solar and magnetospheric forcing, local solar time (LST), and annual variations. The results show that the TEC variability is strongly dependent on the geographical location of the Earth's magnetic field, and the Earth's diurnal rotation modulates its spatial patterns of variability. The latitudinal asymmetry in the global distribution of TEC variations is due to the effects caused by the irregular shape of the Earth's magnetic field along with its diurnal rotation. The analyses of residuals show that periodicities are correlated to the solar wind speed and magnetospheric forcing, especially those located near the southern dip pole at the night side. Furthermore, we found a TEC anomaly at about 15° from the South magnetic dip at the night side, more prominent around 52°S 155°E.

中文翻译：

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全球电离层图长期电离层TEC变化的新模式和新机制

由于太阳活动引起的电离，电离层非常活跃且复杂，而传统的经验模型只能粗略地描述其实际变化。如今，源自更密集的全球导航卫星系统（GNSS）世界跟踪数据的全球电离层图（GIM）为全球电离层研究和建模提供了出色的总电子含量（TEC）数据集。本文使用主模式分析（PCA）技术研究了GIM中16年（2003-2018）TEC时间序列的长期变化。我们分析了时光谱域中的结果模式，并根据太阳和磁层强迫，当地太阳时间（LST）和年度变化参数化了主要贡献。结果表明，TEC的可变性很大程度上取决于地球磁场的地理位置，并且地球的日旋转调节其可变性的空间模式。TEC变化的整体分布中的纬度不对称是由于地球磁场的不规则形状及其昼夜旋转引起的影响。残差分析表明，周期性与太阳风速和磁层强迫有关，特别是那些位于夜侧南倾极附近的周期性。此外，我们发现在夜侧南磁倾角约15°处有TEC异常，在52°S 155°E附近更为明显。TEC变化的整体分布中的纬度不对称是由于地球磁场的不规则形状及其昼夜旋转引起的影响。残差分析表明，周期性与太阳风速和磁层强迫有关，特别是那些位于夜侧南倾极附近的周期性。此外，我们发现在夜侧南磁倾角约15°处有TEC异常，在52°S 155°E附近更为明显。TEC变化的整体分布中的纬度不对称是由于地球磁场的不规则形状及其昼夜旋转引起的影响。残差分析表明，周期性与太阳风速和磁层强迫有关，特别是那些位于夜侧南倾极附近的周期性。此外，我们发现在夜侧南磁倾角约15°处有TEC异常，在52°S 155°E附近更为明显。