The CSES Mission and Its Preliminary Results

SHEN Xuhui ,ZEREN Zhima ,YUAN Shigeng ,DAI Jianping ,HUANG Jianping ,ZHU Xinghong ,YANG Yanyan,YAN Rui,ZHAO Shufan,LIU Dapeng,ZHANG Zhenxia,WANG Qiao,CHU Wei,LU Hengxin,XU Song,GUO Feng,TAN Qiao,LI Wenjing,ZHOU Na

1 Institute of Crustal Dynamics,China Earthquake Administration,Beijing 100085

2 DFH Satellite Co.Ltd.,Beijing 100081

3 Beijing Special Engineering Design and Research Institute,Beijing 100028

Abstract:As the first satellite of the China national geophysical field observation series of satellite missions,the China Seismo-Electromagnetic Satellite (CSES) was designed upon an optimized CAST2000 platform for a sun synchronous orbit.Onboard CSES,there are total eight types of scientific payloads including the Search-coil Magnetometer,Electric Field Detector,High Precision Magnetometer,GNSS Occupation Receiver,Plasma Analyzer,Langmuir Probe,Energetic Particle Detector Package,and a Three-band Transmitter to individually acquire the global electromagnetic field,electromagnetic waves,ionospheric plasma parameters as well as energetic particles.Up to now,CSES has been operating normally in orbit for 2 years.By using the various sensor data acquired by CSES,we have achieved scientific research in the areas of the global geomagnetic field modeling,space weather,earthquake event analysis,the Lithosphere-Atmosphere-Ionosphere coupling mechanism and so on.

Key words:CSES mission,CSES IGFR 2020.0,LAIC,space weather,geomagnetic storm

1 INTRODUCTION

On February 2,2018,the China Seismo-Electromagnetic Satellite (CSES),also called ZHANGHENG 1 (ZH -1),was launched successfully into orbit on top of a LM-2D launch vehicle from the Jiuquan Satellite Launch Center[1].CSES is the first space-based platform in China for both earthquake observation and geophysical field measurement,which was first proposed earlier in 2003 and was approved in 2013 after ten years of scientific and engineering demonstration.Back at the beginning of the century,there was a hot scientific dispute about whether earthquakes are predictable or not,accompanied with a series of devastating earthquakes including the 2001 China KunlunshanMs8.1,2004 IndonesiaMs9.2,2008 China WenchuanMs8.0 and 2011 JapanMs9.0.On the assumption that earthquake prediction is a natural science based on observations,at the beginning of 2003,the Chinese government made the decision to develop a space-based observation system so as to explore new methods and theory for earthquake prediction,such as to better understand the physical processes of earthquake.

2 GENERAL CONCEPT OF CSES MISSION

2.1 Scientific Objectives

The CSES mission is the first satellite under the Chinese space-based geophysical field observation system and has considerable application prospects in the area of earthquake science,geophysics,space sciences and other areas.The scientific objectives of the mission are as follows:

1) To obtain global data of the electromagnetic field,plasma and energetic particles in the ionosphere,especially the real-time data when the satellite passes over Chinese territory.

2) To monitor and study the ionospheric perturbations which could be possibly associated with seismic activities,especially with those destructive ones.

3) To monitor and study the near-Earth space environment,and its disturbances caused by human activities.

4) To analyze the features of seismo-ionospheric perturbations,thus to explore the possibility for short-term earthquake forecasting in terms of satellite observation and to search for new approaches for short-term and imminent prediction.

5) To support the research on geophysics,space science as well as radio science etc.

6) To provide data sharing service for international cooperation and the scientific community.

2.2 The CSES Satellite System

The platform of CSES was designed based upon an optimized CAST2000 platform which offers a standard multi-mission platform at a very attractive cost.The configuration of CSES is shown in Figure 1.Technically,the platform architecture is generic,and adaptations are limited to relatively minor changes with several electrical interfaces and software modules.

The platform includes eight units:a data transmission subsystem (DTs),structure and mechanical subsystem (SMs),thermal control subsystem(TCs),attitude and orbital control subsystem (AOCs),power supply subsystem (PSs),telemetry and tele-command subsystem (TTCs),on board data handling subsystem (OBDHs) plus scientific payloads.

Satellite structure uses a dual-layer cabin design:payload layer cabin and platform layer cabin.The satellite is flying in the direction of satellite X axis,satellite Z axis is pointing towards nadir,while the satellite Y axis is then decided by right hand rule.The solar panel locates on the +Y side of satellite with 12°offset angle and can rotate around the satellite Y axis.

Housekeeping data is exchanged onboard CSES through the CAN bus,the OBDH Central Computer is used as a host and all other equipment are guests.Onboard telemetry adopts the TM package to communicate.The satellite AOCs uses Earth oriented 3-axis stabilization.The 3 star trackers,2 groups of gyros and 1 digital sun sensor are used to measure the attitude.

A reaction wheel and magnetic torque are used to maintain the zero-momentum control.A propulsion system is used for attitude complementary control and orbital maintenance.A S-band telecommunication system assisted by GPS positioning is used for TMTC subsystem.The power supply subsystem is composed of a 80 A·h Li-ion battery and GaInP2/GaAs/Ge solar cell panel.

The main orbital parameters of CSES are listed in Table 1.The distance between neighboring tracks is around 2650 km each day,and is reduced to 530 km with a revisit period of 5 days.

Figure 1 The configuration of CSES satellite platform

The payloads,status,and relating physical parameters of the CSES satellite are listed in Table 2.

2.3 The CSES Ground Segment

The ground segment of CSES consists of a science and application center,satellite ground networks,field verification bases and a comparison system for satellite-ground measurement.The science and application center is responsible for mission operation and control,data management and service,as well as earthquake science application,operated by the China Earthquake Administration (Figure 2).

Table 1 Main parameters of CSES platform

Table 2 The scientific payloads,relating physical values as well as status

3 CURRENT STATUS OF CSES OPERATION IN ORBIT

The commissioning phase of the satellite was completed in November 2018,9 months after launch.As of now,CSES has been operating in orbit for 2 years and collected scientific data from more than 10000 orbits,except for some minor issue with the Plasma Analyzer Package,all payloads are working well.All devices including the platform are working in good condition.Except that PAP is contaminated in orbit,EFD has little higher noise in HF band than expected and TBB gets malfunction of 400 MHz band.

After the commission phase,every 3 -4 months the satellite performs an orbital maneuver for ground track control so as to have a perfect recursive visit with respect to all geophysical locations.The latest ground track keeping maneuver was performed on January 20,2020.The remaining fuel is about 35 kg(42 kg before launch),and based upon the current usage,the fuel is sufficient to cover the entire life span of the satellite.

The satellite implements 6 -8 times data transmission downlink and 4 times TMTC duplex links to the ground segment each day.Both the data transmission link and TMTC link work steadily and efficiently.

Normally all payloads switch off over ±65° of latitude,however,the satellite can extend the working zone to cover the polar region according to the specific experiments.

In order to provide reliability information for all payloads,the performance of the satellite platform is evaluated every 6 months in terms of positioning accuracy,attitude information accuracy and potential charging control,meanwhile the thermal control system and timing system have been monitored continuously.So far,the major requirement specifications for the CSES mission have been met.The satellite is working stably and continuously providing scientific data with high duty cycle.

Figure 2 Data flow between satellite and ground units

The ground receiving system consists of 4 stations,which are Miyun Station,Kashi Station,Sanya Station and SouthWest Station.The orbit prediction error is ＜ 30 s.All interfaces between satellite and ground station such as coding,data rate,and modulation satisfy the requirements,and the transmission efficiency from all ground stations to data station is fully satisfactory.

The data application system,which is in charge of the mission operational center,runs the control management subsystem and data processing subsystem and are generally running stably,in which a parallel processing architecture is embedded allowing automatic data process operation,and a manual interactive process operation.So far,the success rate for level-0 to level-2 data processing has reached more than 90%.The data distribution website is opened and ready for users to access.

4 THE PRELIMINARY RESULTS OF THE CSES MISSION

As a result of the data acquired by CSES,we have achieved a series of scientific researches including global geomagnetic field modeling and ionospheric environment,space weather and earthquake events analyzing,the Lithosphere-Atmosphere-Ionosphere coupling mechanism research and so on.

4.1 The Global Geomagnetic Field Reference Model

The International Geomagnetic Reference Field (IGRF)model is used to describe the main field and the secular variation of Earth’s geomagnetic field.The building of IGRF model is an international effort organized by the International Association of Geomagnetism and Aeronomy (IAGA) since 1900.This is the first time that China has response to the open call from IAGA and started to build a candidate model.Until the final submission of IGRF-13 candidate model on October1,2019,CSES had accumulated one and half years’ dataset.Using this dataset,with the method described in paper[2],the main field for 2020 totally based on CSES’s data was built (we call it CSES-IGRF 2020.0),becoming the only set without using Swarm data in all of the 12 candidate models of IGRF-13.Figure 3 shows the declination and inclination calculated from our candidate model.

To validate our candidate model,we also make some comparison with the prediction of our parent model with that of the CHAOS6-x9 model[3].The result illustrates that the differences between the CSES parent model and CHAOS6-x9 are of a zonal nature,with amplitudes of about 20 nT for Br values.These differences most likely reflect some of the systematic boom deformation along the CSES orbits.We note,however,that such differences remain within a reasonable acceptable level for typical IGRF candidate models.

4.2 Space Weather Events Records

With the rapid development of technology,the risk of damage from space weather is increasing.Therefore,monitoring and prediction of space weather related parameters has become essential in modern society.Since the launch of CSES,several geomagnetic storms and many space electromagnetic wave events have occurred,with the one occurring on August 25,2018 being the most intense storm,providing us with a very good opportunity to check the capabilities and performance of the satellite’s payloads and also to discuss possible mechanisms by using the varied instruments on the same platform.To monitor the ionosphere’s status during a storm,it is important to know background features under quiet conditions.The unique orbital design of CSES makes it possible to revisit orbital observations at the same place every five days at the same local time.For this particular event,observations from August 5 -6,2018 (the quietest revisiting day since August) are introduced as a reference for comparison with the disturbance storm time.The study shows that all investigated parameters simultaneously respond to the different phases of the geomagnetic storm,verifying the measuring capabilities of the assembled payloads onboard CSES.The main results are summarized as follows (see reference [4] for more detailed information).

As presented in Figure 4,the variation of the magnetic field detected from HPM well reflects the response to storm activity.The residual field (reflecting currents from the ionosphere and magnetosphere) is enhanced during the storm’s initial phase and then reduced sharply with the development of the storm’s main phase.During the recovery phase,the residual field slowly returns to a lower value.Fortunately,Swarm Alpha was very close in terms of LT to CSES during this storm event,which can help us to cross-check the measured values.The results indicate that the magnetic fields detected from the two satellites fit well with each other,so the magnetic field data from CSES exhibits very good quality.

Figure 3 The declination (upper figure) and inclination (lower figure) calculated from CSES-IGRF 2020.0 candidate model

In fact,the upper ionosphere is a highly dynamic region with a variety of intense electromagnetic emissions,especially the whistler-mode emissions at extremely/very low frequency(ELF/VLF) range.The most common and typical ELF/VLF whistler-mode waves in the high-latitude ionosphere mainly include hiss,chorus and quasi-periodic (QP) waves[5-7].

Figure 4 The residual magnetic field evolution for CSES and Swarm during August 25 -26

Figure 5 shows the QP wave events represented by the sum of power spectral density values (PSD) of three components of magnetic field over a frequency range from 300 Hz to 2 kHz.It can be seen that there exists fixed time-separations (i.e.,periods) between adjacent wave elements during each event at a broad frequency range from the proton cyclotron frequency predominantly up to 1.5 kHz,occasionally up to about 2 kHz.The proton cyclotron frequency is computed by the total magnetic field measured by HPM.For a more detailed description on wave structures,we extracted the separation time between adjacent wave elements for each event,and the results show that the periods between adjacent wave-packages mainly vary from about 8 s to about 23 s,and the higher latitude,the shorter periods were observed.

CSES’s observations are consistent with the previous understanding that the QP wave is almost a dayside phenomenon[8],however,compared to the similar technically designed satellite DEMETER in the upper ionosphere,CSES seems to record more well pronounced rising-tone structures.

Figure 5 The first quasiperiodic electromagnetic wave event recorded by CSES satellite in the high-latitude upper ionosphere on February 26,2018

4.3 The Lithosphere-Atmosphere-Ionosphere Coupling Mechanism Revealed by VLF Transmitters

The Earth,atmosphere,ionosphere and magnetosphere are the physical space coupled together.Mantle convection drives plate movement,which leads to seismic activities,and will have a response in the near Earth space[9-12].There are mainly three kinds of Seismo-Lithosphere-Atmosphere-Ionosphere coupling mechanisms at present.The electric field mechanism and the electromagnetic wave mechanism are used as pre-earthquake mechanism and behave well,while the acoustic gravity wave mechanism could explain the co-seismic phenomena well[13].

By using the data from the CSES satellite,we investigated the VLF signals in the ionosphere propagating from groundbased transmitters.We employed a full-wave method[14-16]to seek a solution to the Maxwell equations for waves in a horizontally-stratified medium.Considering the observation distance from the source to satellite is just 507 km,which is much smaller than the radius of the Earth,the Earth’s curvature is neglected.The features of VLF signals from four ground-based VLF stations (NWC,GBZ,NAA and NPM) recorded by CSES are presented.There is a good correlation between the CSES observation and full-wave simulation results.These conclusions demonstrate that the night-side VLF band electromagnetic observation of the ZH-1 satellite is stable and reliable[17].

In the same time,ground-based VLF EM signals can transmit across the atmosphere,expanding into the ionosphere[18],and interact with the energetic particles in the radiation belt[19].The electron precipitation belts induced by NWC VLF transmitter,which has attracted much attentions in the past decades,is also observed by the particle detectors onboard the ZH-1 satellite.

The onboard low-energy particle detector (HEPP-L)[20]can measure the electron fluxes with energy ranging from 0.1 keV to 3 MeV with wide pitch angles.So with the launch of the CSES satellite and the good performance of HEPP-L particle detector it provided a good opportunity for studying the electron precipitation belts of NWC.

Figure 7 shows the NWC electron precipitation between L-shells ofL=1.4 andL=1.8 labeled by the blue dots-line on the top panel.The flux level,energy spectra,pitch angle distribution is displayed on the below panels of Figure 7.From this figure,we can find the clear electron precipitation wisp structures distributing in the energy range of 0.1 -0.3 MeV.

4.4 The Ionospheric Disturbances Associated with Strong Earthquakes

Since the l980s,many countries have developed satellites capable of making electromagnetic observations.Electromagnetic precursors to seismic events have been reported since then[21,22],serving as a new observing technique for studying seismic electromagnetic effects.According to the Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) models,these effects can be explained by anomalous electric fields on the ground surface in either the positive or the negative direction[23,24].After the CSES satellite was launched on February 2,2018,the first earthquake withMs＞7.0 recorded by CSES was theMs7.1 earthquake occurred in Mexico on February 17.The disturbances of low-frequency electromagnetic waves and ionospheric plasma were found one day before the earthquake.

Figure 6 Comparison of electric field (top panel) and magnetic field (bottom panel) excited by VLF transmitters (NWC,NAA,NPM) between CSES satellite observation and simulations

Figure 7 NWC electron precipitation measured by HEPP-L particle detector onboard ZH-1

The CSES team examined the strong earthquake withMs＞6.0 in China and the earthquakes ofMs＞7.0 all over the world since the CSES launch,and explored the disturbance characteristics and mechanism of the ionospheric before and after the earthquake.

Based on these results,we conclude that during strong earthquake activities,CSES data are responsive to these earthquake events.However,all the measured parameters display similar background variations in absence of seismic activity since the ionosphere is affected by a number of other factors[25].Geomagnetic storm indexDstandKpcan be used to examine whether perturbations are from earthquakes or magnetic storms,but they cannot be used to rule out other possibilities.This is one of the challenges in studying earthquake precursors.

Figure 8 The enhancement of ULF/ELF at 155.5 Hz magnetic field before Mexico earthquake

Up to now,the mechanism of ionospheric perturbation related to earthquakes still remains poorly understood and needs a long-term further study.

Table 4 The possible ionospheric disturbances with global M≥7 earthquakes recorded by CSES

5 CONCLUSIONS

As the first satellite for the China national geophysical field observation satellite missions,CSES was designed to acquire the global electromagnetic fields and ionospheric background as well as their spatial-temporal change with natural and manmade events,especially for large natural hazards.Regarding to the objectives,the CSES satellite was designed based upon the CAST2000 platform with a sun synchronous orbit with its altitude of 507 km,descending local time of 14:00 and a 5 days revisit period.Onboard CSES,there are total 8 types payloads to measure the background magnetic field,the electromagnetic waves,ionospheric plasma parameters,and high energy particle,etc.

As of now,CSES has been operating in orbit for 2 years and collected scientific data for over 10,000 orbits,except for some minor issue with the Plasma Analyzer Package,entirety set of payloads are working in well.All devices including the platform are currently operating as normal.During its operation,CSES has acquired abundant data,such as multi-band waveform and spectral in electric and magnetic fields; in-situ plasma parameters(including densities and temperatures of electron and ion),electron density profiles and tomography and energetic particle flux and energy spectra.

By using all these kinds of data acquired by CSES,we have achieved a series of scientific researches including global geomagnetic field remodeling,the ionospheric background environment,space weather and earthquake events analyzing,the LAIC mechanism research and so on.All these data demonstrate that CSES mission will benefit not only earthquake science but also geophysics,space science,radio wave science and space weather monitoring.The growing number of earthquake case study (Ms＞6 in China orMs＞7 in the world) will be acquired.In addition,the obtained data are also very helpful for improving global geomagnetic field and ionosphere model.

Acknowledgments:This work made use of the data from CSES mission (http://www.leos.ac.cn/),a project funded by China National Space Administration (CNSA) and China Earthquake Administration (CEA).This scientific application of CSES data in this paper is supported by the National Key R&D Program of China (Grant No.2018YFC1503500),the APSCO Earthquake Research Project Phase II and ISSI-BJ (IT2019-33)project.