Preface to the Special Issue on the Macao Science Satellite-1 Mission

The Macao Science Satellite-1(MSS-1)mission(https://mss.must.edu.mo/)is marked by a new high-precision constellation of satellites that will survey the Earth’s geomagnetic and space environment.MSS-1 consists of two satellites that are to be launched in the near future.Since these two low Earth orbit(LEO)satellites will operate in circular orbits,with an inclination of about 41°,they are expected to provide essential measurements covering the Earth’s lower-latitude regions—including,especially,the South Atlantic Anomaly(SAA).This special issue presents 18 articles to provide the international scientific community with details regarding the mission’s goals,relevant scientific research,on-board payloads,and international collaborations.Contributors are members of the scientific and engineering groups involved in the mission.In this preface,we categorize the articles and give some brief comments or editor’s recommendations.

High-precision modeling of tide-induced 3-D magnetic field and analysis of geomagnetic satellite orbit requirements

Ground-based magnetic observatories and geomagnetic satellites can observe the induced magnetic field generated by the motion of seawater containing sodium and chlorine ions.Calculating the three-dimensional(3-D)spatial distribution of tide-induced magnetic fields(TIMF)is crucial for inverting the electrical conductivity structure of the oceanic lithosphere.It also serves as an essential basis for designing optimal geomagnetic observatories and satellite orbits.However,existing methods for simulating TIMF suffer from limitations in inaccurately modeling realistic coastlines,heterogeneous land and sea surface properties,and complex deep Earth structures,thereby the interpretational level of TIMF data is reduced.To overcome this issue,we developed a tetrahedral-based finite element method for simulating TIMF,which can efficiently approximate realistic coastlines,heterogeneous land and sea surface properties,and complex deep Earth structures.Firstly,we derived the boundary value problem for the seawater motion-induced electromagnetic field,which was solved using the vector finite element method based on tetrahedral elements.Secondly,using the latest ocean depth and seafloor sediment layer models,we constructed a 3-D conductivity model of the Earth,which includes realistic coastlines,heterogeneous land and sea conductivity distributions.We then computed the TIMF using the M_(2)tidal source as an example and validated our method by comparing it with results obtained from spherical harmonic finite element and integral equation methods.Finally,utilizing the computed high-precision M_(2),N_(2),and O1 TIMF signals,we marked global observatories capable of observing strong M_(2),N_(2),and O1 TIMF signals and predicted alternative stations suitable for tide signal observations.Additionally,we calculated TIMF at heights of 450 and 200 km for the Macao Science Satellite 1 and its subsequent satellites.The results indicate that the amplitude of the tidal-induced magnetic field at 200 km is approximately twice that at 450 km.The maximum amplitudes of M_(2),N_(2),and O1 TIMF at 200 km are eight,two,and three times the measurement accuracy of the magnetic sensing payload(0.5 nT),respectively.The 200 km orbit has great potential for detecting high-resolution electrical structures of the seafloor lithosphere and asthenosphere in regions such as New Zealand,southern Iceland,the southern Indian Ocean,the Ross Sea region of Antarctica,and the Sea of Okhotsk.It also holds the potential for studying large-scale oceanic dynamic processes and properties.

3D finite-element modeling of Earth induced electromagnetic field and its potential applications for geomagnetic satellites

The accumulated large amount of satellite magnetic data strengthens our capability of resolving the electrical conductivity of Earth’s mantle.To invert these satellite magnetic data,accurate and efficient forward modeling solvers are needed.In this study,a new finite-element based forward modeling solver is developed to accurately and efficiently compute the induced electromagnetic field for a realistic 3D Earth.Firstly,the nodal-based finite element method with linear shape function on tetrahedral grid is used to assemble the final system of linear equations for the magnetic vector potential and electric scalar potential.The FGMRES solver with algebraic multigrid(AMG)preconditioner is used to quickly solve the final system of linear equations.The weighted moving least-square method is employed to accurately recover the electromagnetic field from the numerical solutions of magnetic vector and electric scalar potentials.Furthermore,a local mesh refinement technique is employed to improve the accuracy of the estimated electromagnetic field.At the end,two synthetic models are used to verify the accuracy and efficiency of our newly developed forward modeling solver.A realistic 3D Earth model is used to simulate the induced magnetic field at 450 and 200 km altitudes which are the planned flying altitudes of Macao’s geomagnetic satellites.The simulation indicates that(1)the amplitude of the mantle-induced magnetic field can reach 10–30 nT at 450 km altitude,which is 10–30%of the primary magnetic field.The induced magnetic field at 200 km altitude has larger amplitudes.These mantleinduced magnetic fields can be measured by Macao geomagnetic satellites;(2)the amplitude of the ocean-induced magnetic field can reach 5–30 nT at satellite altitudes,which needs to be carefully considered in the interpretation of satellite magnetic data.We are confident that our newly developed forward modeling solver will become a key tool for interpreting satellite magnetic data.

The state-of-the-art of the China Seismo-Electromagnetic Satellite mission

The China Seismo-Electromagnetic Satellite(CSES) mission was proposed in 2003 and approved in 2013 after ten years' scientific and engineering demonstrations. To meet the requirement of scientific objectives, the satellite is designed to be in a sunsynchronous orbit with an altitude of 507 km and descending node time of 14:00 LT. The CSES satellite carries 8 instruments,including search-coil magnetometer(SCM), electric field detector(EFD), high precision magnetometer(HPM), GNSS occultation receiver(GOR), plasma analyzer package(PAP), langmuir probe(LAP), high energetic particle package(HEPP) and detector(HEPD), and tri-band beacon(TBB), among which HEPD is provided by Italian Space Agency. The CSES satellite was launched successfully on February 2, 2018, and is planned to operate for 5 years. The CSES mission is the first satellite in China to measure geophysical fields, which will have a lot of application prospects in the study of seismology, geophysics, space sciences, and so on.