Invert The Lithospheric Magnetic Field In China Mainland By Combining CHAMP And Swarm Satellite Data

The earth's lithospheric magnetic field originates from the crust and upper mantle of magnetic rocks and reflects a 3D spatial distribution of rock magnetism.Given its different rock magnetic environments,magnetic carriers,temperature pressure conditions,and tectonic evolutions,the lithospheric magnetic field carries a rich and complex geological structures and tectonic movement information.The continuous high-precision orbital observation of CHAMP for 10 years provides a unique database for the study of the earth's main magnetic field and its long-term variation.During the period of the CHAMP mission,a series of high-precision lithosphere magnetic field models have been issued around the world,and the corresponding spatial resolution has also been improved.Although early domestic and foreign scholars have theoretically analyzed the separation of various field sources and the parameterization of the model using the CHAMP satellite data,the single satellite has difficulty achieving quasi-synchronous observation of the earth's main and changing magnetic field.This bottleneck undoubtedly constrains the effective separation of the earth's various field sources.The successful launch of the Swarm constellation not only provides the most accurate measurement of the geomagnetic field to date but also realizes the global observation of the geomagnetic field from 1h to several days.This mission brings new vigor and vitality in constructing the earth's magnetic field models.Data processing of high space-time coverage sampling,differential gradient measurement,and optimization of Swarm constellation has provided sufficient data and technical support for the complete separation of various field sources and observing the earth's real-time changing magnetic field.Based on the reviewing,digesting and summerizing of the research results at home and abroad,this article deeply discussed the refinement of satellite magnetic data and the method for separating magnetic anomaly signal of lithosphere.A complete set of software systems of satellite magnetic data preprocessing process and model inversion is designed,and the abnormal model of lithospheric magnetic field in mainland China is established.The distribution of magnetic anomaly is also explained from the perspective of tectonic structure,lithospheric thermal states and seismicity.The main research results of this study are as follows:?1?Different space-time reference systems' definations and the pratical formula for mutual conversion have been given based on the space-time reference systems that involved in the satellite magnetic data and geomagnetic field model.The conversion process among the station,J2000 inertial,geomagnetic,geocentric polar?spherical?,and spherical crown coordinate systems are also discussed.The calculation method of diurnal longitude and magnetic world time is analyzed.Consistency between the observed values and the model reference framework results of internal and external source field is obtained.?2?The accuracy of commonly used earth's main magnetic field models in mainland China is analyzed.The observation data of 31 stations in magnetically quiet days from January to September in 2015,and EMM2015?16-720?model are used to evaluate the accuracy and applicability of IGRF12/EMM2015/POMME10/MCO_SHA₂D models in China.It is providing reliable reference for the signal stripping of main magnetic field.?3?The basic theory and analytical expression of geomagnetic modeling are investigated.On the basis of the potential theory of geomagnetic field,the Gaussian spherical harmonic expression of the earth's main magnetic field is deduced in detail,besides the calculation model of ionosphere-magnetosphere and its induced magnetic field is also provided.Both mathematical expressions of spherical cap harmonic analysis and modified spherical crown harmonic analysis of local geomagnetic anomaly are comprehensively analyzed under Neumann and Dirichlet boundary condition.The calculation method of non-integer-order Schmidt semi-normalized associated Legendre function is also discussed.?4?The rejection method of observation and gross error data of satellite geomagnetic disturbance is systematically analyzed.The magnetic static data are extracted during the magnetically quiet days on the basis of the geomagnetic index condition?Dst/Kp?,local condition Sq and geomagnetic latitude.The quality control of vector observation data is achieved through model test method,scalar calibration method,false observation method and,data quality identification?Quality/Flags?.The idea of data stratification is proposed,and the method of data grid equalization on different levels is provided.The processing flow and corresponding procedure of data function modules are designed and presented.?5?The separation method of non-lithospheric magnetic field signal during the magnetically quiet days is explored.The main magnetic field is corrected based on MCO_SHA₂D model.For the correction of the external field and its induced magnetic field,the idea of implementing an effective separation of large-scale external field signals during the magnetically quiet days based on the high-pass filter of the Hamming window function is proposed.The feasibility of this method is tested based on the latest MIO_SHA₂D and MMA_SHA₂C ionospheric-magnetospheric magnetic field models.The method of determining the filter window length is also given,thereby providing reference for the parameter design of the filter.?6?Based on the partition modified spherical crown harmonic analysis,WHU-MLI01 model of lithospheric magnetic field in mainland China is built,and the spatial resolution of WHU-MLI01 is better than 150 km at satellite altitude.Considering the small ball crown can provide improved spatial resolution in ensuring the accuracy of the model.Therefor in the process of actual modeling,mainland China is divided into three sub-regions to independent modeling.An iterative inversion method of seamless splicing between neighboring ball crowns and the practical method of weakening the model boundary effect are also introduced.The accuracy and stability of WHU-MLI01 model in horizontal and vertical directions are tested by associating with the satellite observation,the MF7 and MLI_SHA₂D high-precision lithosphere magnetic field model.The results show a small error of the model fitting,and the mean square errors of XYZ geomagnetic components are better than 0.3nT,0.3nT,and 0.25nT comparing with satellite observations and MF7 model,the mean square errors of XYZ geomagnetic components are better than 0.5nT comparing with MLI_SHA₂D model.The boundary effect in the horizontal direction of the WHU-MLI01 model is quite obvious,but it is can be fully eliminated by moderately amplified the half-angle of the cap,and the model can provide stable spatial continuation in the vertical direction.?7?The relationship between the distribution characteristics and the related physical properties of lithosphere magnetic anomalies in mainland China is comprehensively analyzed.An obvious correlation exists between the anomalous strength and shape distribution of lithosphere and the major tectonic units,geothermal heat,and seismicity in the continental region.The distribution of magnetic anomalies in the lithosphere is consistent with that of the regional tectonics,and can clearly reflect the direction and characteristics of the major tectonic units in the continent.The intensity of magnetic anomalies can truely reflect the thermal equilibrium and tectonic motion in the depth of crust.It is believed that the positive anomalies of lithosphere mainly correspond to the ancient,stable land,and continental nuclei.Besides the terrestrial heat flows in these areas are relatively low,and the seismic activities are less than normal.The negative anomalies are mainly distributed in the active orogenic belts?suture zone?or rift basins,and the values of heat flow and seismicity are relatively high in these areas.The weak magnetic properties of lithosphere may be related to the widespread distribution of weak magnetic material of rock mass itself.For example,the negative magnetic anomalies of the Yangtze craton?except the Sichuan Basin?may be closely related to the sedimentation within the Yangtze plate and the thermal demagnetization process of the lithosphere.Such as the North China craton that has been destroyed and remodeled during the long geological evolution.The strong negative anomaly and high heat flow also indicate that the material flow in the deep crust may be in the process of thermal reform.For example,in the Plateau region,the Indian plate continues to downthrust northward.The friction generates heat between the plates accelerates the material convection at the upper mantle and deep crust.This condition is accompanied by the upward continuous migration of thermal substances,which leads to Curie isothermal surface upward.In addition,strong negative anomaly and high heat flow also mean that the deep crustal tectonic movement may be active.