Current Status and Main Scientific Outcomes of the CSES Mission*

ZEREN Zhima HUANG Jianping LIU Dapeng YANG Yanyan YAN Rui ZHAO Shufan ZHANG Zhenxia LIN Jian CUI Jing CHU Wei WANG Qiao LU Hengxin XU Song GUO Feng YANG Dehe ZHOU Na LIU Qinqin HUANG He WANG Jie TAN Qiao LI Wenjing Lü Fangxian ZHU Keying SHEN Xuhui

(National Institute of Natural Hazards, Ministry of Emergency Management, Beijing 100085)

Abstract This report briefly introduces the current status of the CSES (China Seismo-Electromagnetic Satellite)mission which includes the first satellite CSES 01 in-orbit (launched in February 2018),and the second satellite CSES 02 (will be launched in 2023) under development.The CSES 01 has been steadily operating in orbit for over four years,providing abundant global geophysical field data,including the background geomagnetic field,the electromagnetic field and wave,the plasma (in-situ and profile data),and the energetic particles in the ionosphere.The CSES 01 platform and the scientific instruments generally perform well.The data validation and calibration are vital for CSES 01,for it aims to monitor earthquakes by extracting the very weak seismic precursors from a relatively disturbing space electromagnetic environment.For this purpose,we are paying specific efforts to validate data quality comprehensively.From the CSES 01 observations,we have obtained many scientific results on the ionosphere electromagnetic environment,the seismo-ionospheric disturbance phenomena,the space weather process,and the Lithosphere-Atmosphere-Ionosphere coupling mechanism.

Key words CSES mission,Satellite platform,Scientific payloads,Data validation,Electromagnetic environment,Seismic-ionospheric disturbance,Space weather process

1 Introduction

It is widely accepted that the global geophysical field observations from satellites have significant application in natural disaster prevention and scientific research.Strong earthquakes are one of the most destructive natural hazards,claiming countless deaths and economic losses in human history.Promoting earthquake disaster prevention and reduction capability is a common issue faced by all countries worldwide.Driven by the objective of earthquake disaster prevention and mitigation,China proposed a stereoscopic earthquake monitoring system from ground to space early in 2003 by launching a series of Low Earth Orbit (LEO) electromagnetism and gravity satellites in the following decades[1].This CSES mission is called the Zhangheng mission,named after the ancient Chinese scientist Zhangheng who invented the world’s first seismo-scope in the second century[2].The electromagnetism satellite mission is called Zhangheng-01 (or ZH-1),and the gravity satellite mission is called Zhangheng-02 (or ZH-2).The electromagnetism satellite mission,also known as the China Seismo-Electromagnetic Satellite (CSES 01 or ZH-1),one satellite has already been launched in orbit on 2 February 2018,and the second satellite being approved in September 2018 and under development at present,the third one is under planning as a constellation around 2030.The gravity satellite mission is still under scientific demonstration discussion.

The scientific objectives of the ZH-1 mission are to obtain global observation on the background geomagnetic field,the electromagnetic field and waves,thein-situand profile ionospheric plasma parameters,and the energetic particles;to monitor and study the ionospheric perturbations which could be associated with the natural hazards (such as seismic activities,volcano eruptions,the intense geomagnetic storms) or human activities(e.g.,the electric power transmission system,or the artificial VLF radio wave transmitters).Besides that,the ZH-1 mission also supports the research on geophysics,space science,and radio science by providing a datasharing service for international cooperation and the scientific community.

On 2 February 2018,the first satellite of the ZH-1 mission,which is also abbreviated as the ZH-1(01) or CSES 01,was launched successfully in orbit.CSES 01 is the first space-based platform in China for earthquake observation and geophysical field measurement,which was approved in 2013 after ten years of scientific and engineering demonstration.CSES 02 is under construction and will be launched in 2023.The following sections will introduce the current status of CSES 01 and CSES 02 and some selected main scientific outcomes.

2 Current Status of CSES Mission

2.1 CSES 01

On 2 February 2018,the CSES 01 was launched into a sun-synchronous circular orbit at an altitude of 507 km in the topside ionosphere,and it has been steadily operating in space for over four years up to now.Among its scientific objectives,short-term earthquake prediction is listed as the top scientific goal.To serve the need for emergence response to disastrous earthquakes,such as the 2008 Mw 7.9 Wenchuan earthquake,CSES 01 is designed to provide real-time data over China’s territory by direct downlinking data to the ground segment.

To meet the scientific goals,CSES is designed to carry eight scientific payloads,including the High Precision Magnetometer (HPM) for the total magnetic field observations[3–5];the three-axis Search-Coil Magnetometer (SCM),and the Electric Field Detector (EFD) for the electromagnetic field detection at a broad frequency range from DC to HF[6,7];a Langmuir Probe (LAP) and Plasma Analyzer Package (PAP) forin-situplasma parameters measurements[8,9];the High Energetic Particle Package (HEPP) and Italian Energetic Particle Detector(IEPD) for high energy particles[10,11];the GNSS Occultation Receiver (GOR) and Tri-Band Beacon (TBB) to measure electron density profiles[12,13].

Up to now,the scientific instruments generally perform well,except for the PAP,EFD,and TBB,which have certain defects in different aspects[14].PAP was contaminated after four months in orbit,leading to lower absolute values than expected;further evaluation suggests that the relative values of ion densities can be used in scientific applications.EFD is heavily interfered with by the satellite-internal communication over the equatorial region and has a high noise level in the HF band.The middle frequency band (400 MHz) of TBB (which needs the ground-based receivers to realize the coherent beacon system) malfunctioned after launch,so the data quality of TBB is still under evaluation.We suggest extra caution when using PAP,EFD,and TBB for scientific research.

The CSES 01 flies 15.2 orbits around the global Earth per day at the local time around 02:00 am (nightside) and 02:00 pm (dayside),respectively.It has a 5-day revisiting period for the same area.The scientific data are packed into HDF (Hierarchical Data Format) with the descending (orbit path from north polar to south polar) and ascending (from south to north) half orbits data files,respectively.There are five levels of data in total based on the working principles of each payload,which are described as follows.

Level 0:The data are reconstructed after a series of preprocessing,including frame synchronization,derandomization,decoding,and deformatting of the raw telemetry data.

Level 1:The data obtained after the general error elimination,physical units convert,format conversion,etc.based on the Level 0 data.

Level 2:The calibrated physical values correspond to Level 1 data with orbit information after coordination system transformation and necessary data inversion.

Level 2 A:Only EFD in the ULF band and GOR deliver the Level 2 A data.For EFD,Level 2 A is generated after eliminating theVs×Beffect in the ULF band,whereVsis the velocity of satellite,Bis the geomagnetic field;For GOR data,it is the TEC values obtained after conversion with precision orbit determination information on Level 2.

Level 3:Time sequential data along satellite orbits generated after resampling,necessary spectral analysis based on Level 2 data.

Level 4:The global interpolation maps of physical values from Level 2 data.

CSES 01 has been in orbit over 22868 circles until 16 March 2022,producing over 300 TB of scientific data.The Level 2 data products that can be directly used for scientific applications are accessible via the website to the international scientific community*https://www.leos.ac.cn.

Within the past two years,we have achieved much progress in data processing,data validation,and scientific application.

In the field of data processing,one progress is that a wave vector analysis tool for the electromagnetic waves of the CSES was developed by Huet al.[15],which can provide the waveform spectrum transform,Singular Value Decomposition (SVD),and Poynting flux methods.According to the specific electromagnetic wave events,we validated the algorithm code by comparison with DEMETER’s observations.We applied the tool to analyze the propagation feature of the waves,confirming that the CSES 01 has a good performance on the electromagnetic field observations.Qinget al.[16]evaluated the CSES Precise Orbit Determination (POD) based on GPS and BDS observations.Results show that the CSES orbit consistency can reach up to 3 cm in 3 D RMS,which can satisfy the centimeter-level requirements of the scientific application.See details in Refs.[15,16].

The data validation and calibration are vital for CSES 01,for it aims to monitor earthquakes by extracting the very weak seismic precursors from a relatively disturbing space electromagnetic environment.For this purpose,we are paying specific efforts to validate data quality comprehensively.For the electromagnetic field detection payloads,we cross-calibrated the consistency of HPM,EFD,and SCM in their overlapped detection frequency range and firstly evaluated the timing system and the sampling time differences between EFD and SCM[17].A sampling time synchronization method was put forward for EFD and SCM waveform data.The consistency between FGM and SCM in the Ultra-Low-Frequency (ULF) range is validated by using the Magnetic Torque (MT) signal source as a reference.Yanget al.[18]validated the HPM data through comparison with the Swarm satellite constellation,and the result demonstrates a good data quality of the HPM.Based on the data validation work,the potential magnetic field disturbances are flagged in the second version of HPM data.To the ionospheric plasma parameters,Yanet al.[19]firstly discovered a Sudden Drop (SD) in the plasma potential (Vp) and floating potential (Vf) data and a Spike(SP) in the daysideVpandVfdata.According to the analysis ofI-Vcurves both inside and outside the SD and SP structures,we exclude the possibility of probe contamination and confirm that the scientific data of CSES LAP are reliable.These two regular features depend on the solar illumination conditions and the corresponding adjustment of the satellite-current system equilibrium.For data validation,see details in Ref.[17–19].

For the scientific application,the most important achievement is the CSES Global Geomagnetic Field Model (CGGM 2020.0),which was built at the end of 2019.The model is derived by solving a series of mathematical Spherical Harmonic (SH) Gauss coefficients.CGGM 2020.0 can provide a prediction of Earth’s static main field up to SH degree and order 15 and a linear secular variation up to degree and order 8 with an expansion in 2nd order B-splines.This model is validated by the International Association of Geomagnetism and Aeronomy (IAGA) and finally has been selected as one of 15 international candidate models for calculation of the 13th generation International Geomagnetic Reference Field (IGRF-13)*https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html.We have published the CGGM 2020.0 coefficient (in IGRF type) on the CSES scientific data sharing website**https://leos.ac.cn/#/article/info/236,and a CGGM 2020.0 Calculator is also provided to calculate magnetic field predictions in the given time and position.The corresponding papers were published in 2021;see details in Ref.[20,21].

Based on the CSES 01 observations,we have also progressed scientific research on the electromagnetic field environment,seismo-ionospheric disturbance phenomena,space weather,and Lithosphere-Atmosphere-Ionosphere coupling.Up to now,the incomplete statistics show that around 146 scientific papers have been published***https://leos.ac.cn/#/scientificResearch/pathPQBTbfcszmCmzTPdhipdnATjwMhJzcpb.Some of the new scientific results of CSES 01 achieved since 2020 are introduced in the following Section 3.

2.2 CSES 02

The CSES 02 project was proved in April 2018 and was officially initiated in September 2018.As a successor of CSES 01,the CSES 02 is the first operational satellite of the ZH-1 mission,which will directly serve for routine natural hazards monitoring.

CSES 02 also will provide the same physical field parameters as the CSES 01;its scientific payloads are mostly inherited from the ones from the CSES 01 but with specific optimizations based on CSES 01’s in-orbit performance.The payloads of CSES 02 include the High Precision Magnetometer (HPM02),Search Coil Magnetometer (SCM02),Electric Field Detector (EFD02),Langmuir Probe (LAP02),Plasma Analyzer Package(PAP02),Energetic Electron Spectrometer (EES,and HEPD02),GNSS Occultation Receiver (GOR02),Tri-Band Beacon Transmitter (TBB02),and Ionospheric Photometer (IPM).The IPM is newly added to improve the ionosphere and atmosphere tomography capability,and the EES is a substitution for the CSES 01’s HEPP.

The platform of CSES 02 is remodeled upon the CAST2000,which offers a standard multi-mission platform at a very attractive cost.Technically,the platform architecture is generic,and adaptations are limited to relatively minor changes in several electrical interfaces and software modules.The platform includes eight units:Data Transmission subsystem (DTs),Structure and Mechanism Subsystem (SMs),Thermal Control subsystem (TCs),Attitude and Orbital Control subsystem(AOCs),Power Supply subsystem (PSs),Telemetry and Telecommand subsystem (TTCs),Onboard Data Handling subsystem (OBDHs) and scientific payloads.Some performance improvements of CSES-02 are also made based on CSES 01;the working area enlarges from 65 degrees north-south latitude to the whole globe.CSES 02 flies together with CSES 01 in the same orbit space,with an orbital altitude of about 507 km and a high orbital inclination of 97.4°,the ascending node and descending node-Local Time (LT) is 02:00 and 14:00,respectively as shown inFig.1.

Fig.1 Orbit design of CSES 01 and CSES 02,and their footprints on the ground

More in-depth international cooperation has been carried out in the CSES 02 project,especially between China and Italy.The Sino-Italy cooperation team is called the CSES-Limadou team which was built during the CSES 01 project on the energetic particle payload and corresponding scientific applications.Italy’s CSESLimadou team is led by the ASI (Italy Space Agency),and is composed of scientists and engineers from the INFN,INAF-IAPS,University of Trento,University of Rome Tor Vergata.The successful cooperation led to a very good foundation for the in-depth cooperation on CSES 02,which got approved by the two countries’leaders.In March 2019,China’s President Xi Jinping visited Rome and witnessed the signing of the cooperation agreement on CSES 02 between CNSA (China National Space Agency) and ASI.In the CSES 02 project,the Italy side is responsible for developing the Electric Field Detector (EFD02) and the High Energy Particle Detector (HEPD02).The outbreak of the COVID-19 pandemic caused a delay in the design and procurement of the Italy side.So the HEPD02 and EFD02 have been particularly affected by the worsening global shortage of semiconductors and raw materials.However,the Italy team is trying their best to minimize the impacts of this situation.The flight model of HEPD02 and EFD02 will be tested at acceptance levels,calibrated,and will be delivered to China by the end of 2022.

3 Selected Main Scientific Outcomes

3.1 Electromagnetic Environment Revealed by the CSES Mission

The ionosphere is a highly dynamic region because it is a coupling area between the Lithosphere,Atmosphere,the inner magnetosphere,and the solar wind.Especially in the high-latitude ionosphere,there are energetic particles precipitated from the radiation belts and a variety of intense electromagnetic emissions are excited.The most common and typical ELF/VLF whistler-mode waves in the high-latitude ionosphere include ionospheric hiss waves[22,23],chorus waves (which occasionally appear)[24,25],and quasiperiodic waves[26,27].Zhimaet al.[2]found the Quasiperiodic waves (QP) accompanied by simultaneous energetic electron precipitations in the highlatitude ionosphere from CSES 01 data,as shown inFig.2.The new features of QP waves observed by CSES 01 are the well-pronounced rising-tone structures and very short repetition periods that previous studies do not often report.The majority of QP waves appear at geomagnetic latitudes from 50° to 65°,andLshell from 3.5 to 4,mainly inside the plasmapause.The QP waves obliquely propagate towards decreasingLshell directions with right-handed polarization,with wave normal angles varying from 30° to 50°.

Fig.2 ELF/VLF QP waves recorded by CSES satellite in the high-latitude upper ionosphere on 26 February 2018.(a)(b) Power Spectral Density values (PSD) of the magnetic field and the electric field.(c)(d) Density of H+ and He+.(e) Drifting velocity of ions.(f) Density of electron.Data are displayed as a function of Universal Time (UT),geomagnetic longitude (mlon),geomagnetic latitude (mlat) and L shell,respectively

The global distribution ofNe/Tederived from CSES was revealed by Yanet al.[28].Results show that the large-scale ionospheric structures,such as the Equatorial Ionization Anomaly (EIA),the longitudinal Wavenumber (WN3/4),the Weddell Sea Anomaly (WSA),the northern Midlatitude Summer Nighttime Anomaly(MSNA),and the midlatitude ionospheric trough,are well represented by the CSES measurements.For the global distribution ofTeat dayside,a clear ETA structure is found in the equatorial region,showing seasonal variations.A notable feature ofTemeasured by CSES is the abnormal increases/decreases in the dayside/nightsideTeover the WSA region in winter,which is consistent with previous research.Fig.3shows the global distributions ofNeat 02:00 LT of CSES (up panels) and Swarm (bottom panels) under the quiet geomagnetic conditions.We observed that the global distributionsNewith WSA and MSNA from CSES and Swarm are quite consistent during conjunction periods of the two satellite although the different absolute values ofNe.

Fig.3 Global distributions of Ne at 02:00 LT of CSES (up panels) and Swarm (bottom panels) under the quiet geomagnetic conditions.From left to right:Ne measurements during 12–19 July 2018,18–27 November 2018,and 7–16 April 2019,corresponding to roughly at the summer solstice,winter solstice,and spring equinox,respectively.For CSES observations,Ne value range from 0 to 2.5×1010 or 3×1010 m–3,while for Swarm one,Ne is mainly from 0 to 2.5×1011 m–3

More details about the electromagnetic field,the ionospheric plasma distribution can be found in Ref.[2,28].

3.2 Seismo-ionospheric Disturbances and LAIC Mechanism

Since the CSES launched on 2 February 2018,there are 38 strong shallow EQs (M 7+,depth shallower than 100 km) occurred worldwide*https://www.ceic.ac.cn/until December 2021,the epicenter distribution is shown inFig.4.The previous studies[29–31]demonstrate that the seismo-ionospheric perturbation phenomena predominately appear during the shallow strong EQs,so we mainly focus on the strong EQs with a depth shallower than 30 km.The possible seismic ionospheric disturbances are listed inTable 1.Besides the single case studies,some statistical studies were also recently carried out using CSES data.For example,Zhuet al.[32]statistically examined the about 2.5 years ofNedata from CSES during the M 4.8+EQs worldwide.The results show that the significant variations of ionospheric parameters related to earthquakes mainly occurred 1 to 7 days and 13 to 15 days before the earthquakes,respectively,and within 200 km from the epicenters.

Table 1 Possible seismo-ionospheric disturbances recorded by the CSES during the shallow strong EQs

Fig.4 Global earthquake activity occurred after the launch of CSES from February 2018 to 1 December 2021.The overlapped black lines are the orbit trajectories of the CSES

The Lithosphere-Atmosphere-Ionosphere is a coupling system;the electromagnetic emissions,the chemistry gases emitted by the stressed rocks in the EQ preparation zones lead to the variation geophysical and geochemical field,thus directly impacting the groundbased instruments.The seismic signals also can couple into the atmosphere,breaking the chemical reaction balance there,leading to the formation of local electric currents,air ionization,atmospheric gas condensation,the plasma parameter irregularity[33].Therefore,besides CSES data,the CSES scientific application center also collects multi-source data.At present,the multi-source observations from infrared/hyperspectral remoting sensing satellites,the ground-based electromagnetic field instruments,the ionosondes,and the ground-based GNSS receivers can be collected at the first time to comparatively explore the seismic signals with the CSES data.

For example,Liuet al.[34]applied a multi-parametric approach to climatological data before the Ms 8.0 2008 Wenchuan and Ms 7.0 2013 Lushan Earthquakes(EQs) to detect anomalous changes associated with the preparing phase of those large seismic events.The results show a chain of processes occurred within two months before the EQs:AOD anomalous response is the earliest,followed by SKT,TCWV,and SLHF in the EQs.A close spatial relation between the seismogenicLongmenshan Fault (LMSF) zone and the extent of the detected anomalies indicates that some changes occurred within the faults before the EQs,as shown inFig.5.

Fig.5 MERRA-2 AOD at 18:00 UTC 2013 in the region of EQ epicenter (indicated by a star in the maps).Grey lines indicate main seismic faults in the research area;on the 49–53th,78–80th days,respectively,with the value on 23th (20 February 2013) subtracted.Latitude (North) and longitude (East) are in degrees

We also pay increasing effort to study the Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) mechanisms.There are mainly three coupling mechanisms:electromagnetic wave,electric field,and geochemistry channel.Zhaoet al.[35]constructed a LAIC model for ELF EM wave radiated from a current source in the Lithosphere.The simulated EM field at the altitude of the CSES is compared with the sensitivity of EM sensors onboard the CSES.The results illustrate that an earthquake with a magnitude over 6.0 may be detected by the EM sensors of the CSES,as shown inFig.6.It is noted whether the anomaly can be detected depends on the focal depth,seismogenic environment,and ionospheric parameters.

Fig.6 (a)(b) is a comparison between a simulated EM field at CSES altitude from an M6 earthquake with different source depths and sensitivity of CSES EM sensors (10 km,15 km;lithospheric conductivity is 10–4 S·m–1);(c)(d) is comparison under different lithospheric conductivity and sensitivity of CSES EM sensors (source depth is 15 km;lithospheric conductivity is 10–4 S·m–1 and 10–5 S·m–1)

The CSES magnetic data are helpful in the study of the lithospheric magnetic field signal caused by magnetized rocks in the crust and uppermost mantle.Wanget al.[36]derived a lithospheric magnetic anomaly map over China and surrounding regions which is consistent with the lithospheric part of the CHAOS-7 model (Fig.7).In particular,it reveals four major magnetic anomalies containing long-wavelength signals at the altitude of Low-Earth-Orbiting satellites.

Fig.7 Lithospheric magnetic anomaly map over China and surrounding regions at average 507 km altitude(a) derived from CSES data and (b) given by the CHAOS-7 model.Abbreviations:TMA,Tarim magnetic high anomaly;SCMA,Sichuan magnetic high anomaly;SGMA,Songliao-Greater Khingan magnetic high anomaly;HMLA,Himalayan-Tibetan magnetic low anomaly

Based on previous studies and the CSES observations,we suggest that monitoring the short-term precursors from a stereoscopic system between the lithosphereionosphere is possible[14].However,it is admitted that it still has a long way to identify the seismo-ionospheric precursors from the electromagnetism satellites correctly.Due to the complexity of the EQ preparation mechanism and the limitation of observation technology and data analysis methods,there are many challenges,and we need a multidisciplinary perspective to explore this topic.

See more details about the seismic-ionospheric perturbations phenomenon and the LAIC mechanism revealed by CSES observations in Zhimaet al.[14],Zhuet al.[32],Liuet al.[34],Zhaoet al.[37,35],and Wanget al.[36].

3.3 Space Weather Process

The high-quality measurements of the space environment by instruments onboard CSES provide us an opportunity to investigate the electromagnetic signal,ionospheric disturbance,and high energy particle acceleration and loss mechanisms during space weather events.In the four years of operation,certain numbers of geomagnetic storms occurred,and CSES’s multi-type payloads show a very good response capability to the process of the space weather event.

Yanget al.[38]investigated the multi-payload response to an intense storm event that started on 25 August 2018,with a minimumDstof–176 nT.Fig.8presents dayside plasma and electric field observations from LAP,PAP,EFD,and GOR.The electron and oxygen density (temperature) are greatly enhanced (reduced)during the main and early recovery phases,indicating that this is a positive storm event.Further analysis reveals a simultaneous variation of electric field and plasma parameters,implying that electric field penetration quite probably is the cause of this positive storm.NmF2is enhanced by a factor of about 2 during storm time,and the position ofhmF2moves upward by ＞100 km for the two selected events,which can again support the statement that electric field penetration should play an important role in this positive storm event.

Liuet al.[39]detailedly reported the oxygen ion densityvariation during this storm;the relative variation ofobserved by the PAP appears consistent with theDstindex,showing a high negative correlation.The values ofDstindex on 24 and 25 August are higher than–30 nT,while the values offor orbit No.3087_0 and 3102_0 orbits are low during this period.On August 26,as theDstindex rapidly drops to the lowest value,thefor orbit No.3117_0 orbit also increases to the maximum value.TheDstindex gradually increases from 27 to 29 August,and thefor the orbits No.3132_0,3147_0,and 3163_0 orbits decrease accordingly.From August 30 to 31,theDstindex increased above–30 nT,and thefor orbits No.3178_0 and 3193_0 orbits continued to decrease.The above results indicate that the relative variation ofwell reflects the evolution processing of this positive geomagnetic storm event.

Zhanget al.[40]found that,at extremely lowLshells(L≈2),a weak flux enhancement (increased by 2–3 times) of hundreds of keV electrons and the corresponding formation of butterfly PADs appeared during the geomagnetic storm in August 2018.According to a numerical simulation of wave and particle interaction model,magnetosonic waves are thought to play an important role in electron acceleration and formation of butterfly PADs during this storm.During this storm,the chorus waves were proven to be able to play a significantly important role in diffusing and accelerating the 1–3 MeV electrons even in extremely lowLshells (L≈3) during storms[41].The CSES’s energetic particle data present that the loss mechanism of protons was energy dependence which is consistent with some previous studies.For protons at low energy 2–20 MeV,the fluxes were decreased during the storm’s main phase and did not come back quickly during the recovery phase,which is likely to be caused by the Coulomb collision to neutral atmosphere density variation[42].At higher energy 30–100 MeV,it was confirmed that the magnetic field line curvature scattering plays a significant role in the proton loss phenomenon during this storm shown in (seeFig.9).At the highest energies ＞ 100 MeV,the fluxes of protons kept a stable level and did not exhibit a significant loss during this storm.

Fig.9 Proton evolution during the large magnetic storm of August 2018 observed by HEPP-H onboard CSES satellite.The outer boundary of the inner radiation belt are denoted by red dotted lines before 26 August (quiet time) and black dotted lines after 26 August (storm time).The flux enhancement within the region of L ＞ 2.5 appearing from 26 August could come from the high-energy electron contamination

The temporal and spatial distributions of the ELF/VLF wave activities and energetic particle precipitations in the ionosphere during the intense storm were reported by Zhimaet al.[43]based on the CSES’s electromagnetic field observations.A good correlation of the ionospheric ELF/VLF wave activities with energetic particle precipitations during the various evolution phases of the geomagnetic storm is revealed by CSES (seeFig.10).The ELF/VLF whistler-mode waves recorded by CSES mainly include structure-less VLF waves,structured VLF quasiperiodic emissions,structure-less ELF hiss waves,etc.Wave vector analysis shows that the ELF/VLF whistler-mode waves mostly likely from the radiation belt obliquely propagate Earthward during storm time.The results suggest that particles in high latitude ionosphere most likely precipitate from the outer radiation belt due to interactions with ELF/VLF waves which are generated by strong temperature anisotropy after injection of energetic particle injections by the solar wind.

Fig.10 Variation of ionospheric wave intensity and energetic flux during the geomagnetic storm occurred in 2018

The CSES’s good performance on the observation of the space weather event allows us to provide the estimates of theDstindex on the CSES orbit-by-orbit basis and define it as the CSES-Dstindex.Fig.11presents a CSES (in red) and Swarm Alpha (in blue) basedDstindex from 1–31 August 2018,together with the groundbased determinedDst-index (in black).It is seen that the CSES and Swarm-basedDstindexes well capture the main variations in the ground-based determinedDstindex.It should be noted that the offset of 10 to 20 nT is expected since the ground-determinedDstindex is relative to an unknown offset,while the satellite determined value has the correct offset.As described in Yanget al.[18],using CSES CDSM and FGM data,we can also derive the storm time variation for ionosphere current systems,such as Sq,eastward Equatorial Electrojets(EEJs),Counter Equatorial Electrojet (CEJs),Field Aligned Currents (FACs).

More details about the CSES’s performance on the space weather event observations can be found in the works of Yanget al.[18,38],and Liuet al.[39],Zhanget al.[40–42],and Zhimaet al.[2,43].

4 Conclusions

Since the launch of CSES 01 in February 2018,the CSES 01 has been steadily operating in orbit for over four years,acquiring much global geophysical field data.Up to now,the CSES 01 platform and the scientific instruments generally perform well,except for certain payloads that have certain defects.CSES 01 has been in orbit over 22868 circles until 16 March 2022,producing over 300 TB of scientific data.The Level 2 data products are accessible via the website to the international scientific community*https://www.leos.ac.cn/.We are paying great efforts to produce high-quality data.

In the electromagnetic environment,the quasiperiodic waves accompanied by simultaneous energetic electron precipitations in the high-latitude ionosphere are recorded by CSES 01.The new features of QP waves observed by CSES are the well-pronounced rising-tone structures and very short repetition periods that previous studies do not often report.CSES well depicts the largescale ionospheric structures,such as the Equatorial Ionization Anomaly (EIA),the longitudinal wavenumber(WN3/4),etc.

There are 38 strong shallow EQs (M 7+,depth shallower than 100 km) worldwide from CSES 01’s launch to December 2021;pieces of evidence of the possible seismic ionospheric disturbances are accumulating.Besides CSES data,the CSES scientific application center also collects multi-source observations from infrared/hyperspectral remoting sensing satellites,the ground-based electromagnetic field instruments,the ionosondes,and the ground-based GNSS receivers to comparatively explore the seismic signals with the CSES data.For the seismic-ionospheric disturbance mechanism,we constructed a LAIC model for ELF electromagnetic wave propagation from the Lithosphere,and the simulation shows that CSES can sense the electromagnetic waves radiated by an earthquake with a magnitude over 6.0.

In the four years of operation,CSES’s multi-type payloads show a very good response capability to the process of the space weather event.The temporal and spatial distributions of the ELF/VLF wave activities and energetic particle precipitations in the ionosphere during the intense storm are well depicted by CSES 01.One important discovery is that CSES 01 captured a weak flux enhancement (increased by 2–3 times) of hundreds of keV electrons,and the corresponding formation of butterfly PADs appeared during the geomagnetic storm in August 2018.

AcknowledgmentsWe acknowledge the CSES scientific mission which was funded by China National Space Administration (CNSA) and China Earthquake Administration (CEA),the data in this study can be downloaded from the website https://www.leos.ac.cn/.