电离层电子密度冬季异常的变化规律与形成机制研究

The winter anomaly of ionospheric F2 layer is the phenomenon that the maximum daytime electron density occurs in winter instead of summer. This variation of the electron density is opposite to the seasonal effects of the ionospheric ionization due to solar radiation. This anomaly favors the location at the middle latitude in the geographic coordinate, but at the subauroral latitudes (near magnetic pole location). This is due to the contribution from auroral heating on the O/N2 ratio in addition to the seasonal variations of this ratio. In winter, the O/N2 at the near-pole-location is much stronger than that at the far-from-the-pole location (at middle latitude in the geographic coordinate, but at latitudes lower than the subauroral region), thus is assumed to contribute greatly to the observed winter anomaly there; while at the far-from-pole location the equinoctial peaks of the electron density or semi-annual variation of the electron density dominates. The above explanation of the winter anomaly has a long history and rarely considers the effects of the plasma dynamics. However, recently, a new O/N2 pattern is revealed through observations from TIMED/GUVI, which show stronger O/N2 ratio in the far-from-pole region while weaker O/N2 ratio in the near-pole region. This longitudinal pattern of O/N2 is unexpected, and thus the formation of the winter anomaly of the electron density needs further investigation. Our present work will construct the global pattern of the winter anomaly from various data base, including the COSMIC, the ionosonde as well as the incoherent scatter radar observation. The winter anomaly patterns between the northern and southern hemispheres will be compared and their mechanisms will be examined, by a comparison with the global pattern from horizontal wind and the observational O/N2 ratio. Further, the NCAR-TIEGCM model, which is a time-dependent, self-consistent, three-dimensional and physics-based model of the global thermosphere/ionosphere system, will be applied to examine the physical mechanisms of the winter anomaly, which involves the relative contribution from various factors, such as the solar flux, the O/N2 ratio, the horizontal wind and the plasma drift, and also the interaction among these factors.

电离层冬季异常，即白天电子密度在全年冬季最大而非出现在预期的夏季的现象，已得到广泛研究，但以往研究主要考虑热层大气成分而忽视动力学过程等因素的影响。且近年来观测表明，热层大气成分在近极和远极地区的相对大小及其对电子密度季节变化的可能影响与以往人们期待的不一致，因此冬季异常的形成机制是亟待解决的科学问题。申请者拟运用全球多种电离层观测进行南北半球对比分析，将冬季异常现象与全球中性成分和热层风对比，研究其全球变化特性和形成机制。同时借助三维自洽的热层-电离层电动力学耦合模型开展系统的理论模拟研究，以辨明太阳辐射、中性大气成分、中性风和电场漂移等因素对全球冬季异常现象的相对贡献和相互影响。

以往人们认为，由极区加热驱动的极区次级环流是引起中性大气成分O/N2比值在冬季显著增加，出现强冬季电子密度异常并随太阳活动变化的主要原因，但O/N2变化的具体机制尚不清楚，且动力学过程的影响较少考虑。本项目计划借助观测和模拟，辨明太阳辐射、中性大气成分、中性风和电场漂移等因素对电离层电子密度的季节变化（主要是冬季异常）的贡献和相互影响的机制。基于以上研究目标，本项目在执行期间（2019-2022年），在中性大气成分的变化机制，太阳辐射对电离层/热层的影响，以及电离层变化受动力学等多种过程的相对影响方面，开展了多项研究工作，主要包括：（1）基于TIEGCM理论模型对O/N2随季节和太阳辐射变化响应过程中的各物理和化学过程进行了定量的研究，揭示了O/N2变化主要受化学过程和增强的热层环流的影响。（2）我们的研究也揭示了原子O对太阳辐射加热的响应时间快于N2分子，并发现了可引起O/N2季节变化的极区极光能量的世界时/经度变化。上述研究很好地揭示了O/N2随季节和太阳活动变化的机理，且O/N2比值可能受到影响极区的太阳风和行星际磁场的影响，对理解电子密度的冬季异常及其随太阳活动的变化具有重要参考意义。（3）本项目也阐明了O/N2比值、电场漂移以及中性风等过程对多种电离层现象/变化的相对影响，如赤道电子密度峰值高度的异常结构的形成，电离层F1层出现率的经度变化，以及低层大气潮汐对电离层总电子含量增强的贡献等。这些结果均为探讨电离层电子密度的季节变化提供了多种可能性，为电离层建模和电离层天气预报提供了重要的基础和前提。基于这些研究成果，在地学高水平杂志发表SCI论文15篇，培养了博士毕业生3名（汪令敏、李振兴、李娜）和在读博士、硕士研究生3名（黎中黎，朱晟霆，王一涵）。协助培养博士后黄福庆和程雪涛。