利用地球的非流体静态平衡结构改进地球极移模型

Polar motion of the Earth describes the movement of the Celestial Intermediate Pole with respect to Earth fixed coordinate system. Well-determined theoretical model of Earth’s polar motion can be of help in understating the mutual interaction among the layers within the Earth. Setting up an accurate empirical model for the Earth’s polar motion is of significant importance for related civil industries that are related to positioning. As for the polar motion of prograde diurnal band, the noise level of the empirical model can be constrained to 1μas based on relative methods. Such magnitude is much smaller than discrepancies between empirical model and semi-theoretical model given by IERS within this same frequency band. On the other hand, amplitude given by empirical model is much larger than that predicted by semi-theoretical model at certain frequency, implying some unknown resonance mechanism is needed near this frequency. Current theoretical model for the polar motion in prograde diurnal band applies an Earth-rotation model that does not include the effects induced by non-hydrostatic structure, such as triaxiality of the core, lateral inhomogeneity of the mantle and core and topography at the core-mantle boundary. This study intends to close the gap between semi-theoretical model and empirical model of prograde diurnal polar motion by taking the non-hydrostatic structure of the layers within the Earth into account.The major goal of this project are closing the gap between empirical model and semi-theoretical model of the prograde polar motion and constraining the parameters of lateral inhomogeneity structure within the Earth by using empirical model of prograde diurnal polar motion.

地球的极移描述了天球中间极相对于地固坐标系的运动。建立准确的地球极移的理论模型，有助于我们了解地球内部各圈层的相互作用。建立精确的地球极移的经验模型，对于与定位相关的国家各产业，也有着重要的应用意义。目前对于周日频段的极移，相关方法建立的经验模型噪声水平已达到1μas，远小于该频段经验模型与IERS给出的半理论模型之间的差距。此外，在某些特定频率，经验模型给出的振幅远大于半理论模型给出的振幅，表示在这些频率附近存在尚待解释的共振机制。IERS提供的半理论模型没有考虑非流体静态平衡导致的地核三轴性，地幔和地核的横向不均匀效应，以及核幔边界的起伏。本研究拟从地球内部圈层的非流体静态平衡结构入手，来解释周日频段极移的半理论模型和经验模型之间差距。本项目拟完成如下任务：缩小顺向周日极移经验模型和半理论模型之间的差距，并利用顺向周日极移的经验模型约束地球内部的横向不均匀结构参数。

地球的顺向周日极移模型被广泛应用于空间大地测量及相关学科中。目前地球的顺向周日极移的经验模型和理论模型存在差异，且差异超过了经验模型的噪声水平。现有的理论模型中没有包含一些非流体静力平衡结构，比如地核的三轴性。本研究以前人考虑地核的赤道扁率的顺向周日极移模型为基础，加入了三轴地核指向这个新参数，构建了新的考虑地核三轴性的顺向周日极移模型。然后用现有的经验模型和理论模型的差，反演地核的三轴性参数。结果表明，考虑地核三轴性可以有效减少现有理论模型和经验模型的差异。此外，利用不同的经验模型和理论模型的差异反演得到的地核赤道扁率有较好的一致性，三轴地核的指向参数一致性较差。后者可能和观测手段与观测数据的跨度有关，也可能由三轴地球和三轴地核的相对旋转有关。另外，我们发现前人在求解考虑椭球面边界的弹性运动方程时出现了纰漏。针对于此，我们对相关方程做了重新推导和求解。项目的研究成果对于我们认识地球的顺向周日极移及地核的三轴特性提供了新的思路，为我们了解地球内部的横向不均匀效应奠定了基础。