After five years of "alone work", Zhang Heng-1 has expanded its global geophysical field and space environment perception and modeling capabilities

In order to promote the solution of the scientific problem of earthquake prediction, the China Earthquake Administration, the State Administration of Science, Technology and Industry for National Defense, the Ministry of Science and Technology, the Chinese Academy of Sciences and others jointly organized the Xiangshan Conference on February 18-19, 2003, and officially launched the demonstration work of my country's geophysical field satellite program. On February 2, 2018, as the first satellite of my country's geophysical field satellite program and the first space-based platform of the seismic stereoscopic observation system, the "Zhang Heng-1" electromagnetic monitoring test satellite was successfully launched into orbit at the Jiuquan Satellite Launch Center. As of February 2, 2023, the "Zhang Heng-1" satellite has successfully completed its five-year design life and will continue to serve in orbit beyond its service life.

The main scientific objectives of the Zhangheng-1 satellite are: to conduct electromagnetic information analysis and research on earthquakes of magnitude 7 or above worldwide and magnitude 6 or above in China on the basis of real-time monitoring of changes in the space electromagnetic environment, to preliminarily explore the information characteristics and mechanisms of ionospheric response changes before and after earthquakes, to study the interaction and effects of the Earth system, especially the ionosphere, with other related spheres, and to provide space electromagnetic environment monitoring data application services to national security, aerospace, navigation, communications and other related fields.

In the past five years, what has the Zhang Heng-1 satellite done?

During the period when Zhang Heng-1 was in orbit, China successfully acquired two basic data sets covering the entire globe, namely the geomagnetic field and the low-frequency electromagnetic spectrum, for the first time , and fully recorded the information of more than 500 strong earthquakes above magnitude 6 and nearly 60 strong earthquakes above magnitude 7 worldwide . On this basis, two scientific models, namely the global reference geomagnetic field and the coupling mechanism of lithosphere, atmosphere and ionosphere, were systematically constructed for the first time, reaching the international leading level; the case dissection and statistical analysis of significant earthquakes around the world were comprehensively carried out, and a preliminary understanding of the spatiotemporal statistical laws of earthquake electromagnetic and ionospheric precursors was obtained. At the same time, the analysis and research of space weather disasters and volcanic disaster effects were also carried out, which provided positive support for further promoting the scientific exploration of earthquake prediction and the monitoring and early warning of multi-hazard disaster chains.

1. Global geomagnetic field and low-frequency electromagnetic spectrum data collection fills the domestic gap

Since its launch, the satellite has recorded the global geomagnetic field and the low-frequency electromagnetic spectrum from ULF to HF bands. Based on the low-frequency electromagnetic spectrum data observed by the satellite, an autonomous, controllable, precise, and time-varying low-frequency electromagnetic radiation background field can be established (Figure 1), and low-frequency electromagnetic radiation disturbances can be dynamically sensed, effectively improving the ionospheric environment monitoring and early warning capabilities. The observed electromagnetic fluctuations are in the DC-3.5MHz frequency band, covering signals in multiple frequency bands such as ULF, ELF, VLF, and HF, including whistler waves, hisses, quasi-periodic discrete fluctuations and other rich fluctuations. Some of these electromagnetic fluctuations also lead to the acceleration and sedimentation of high-energy particles through the wave-particle interaction mechanism. The collection of these geomagnetic field and low-frequency electromagnetic spectrum data fills the domestic gap and plays an important role in the research of geospace physics and natural disaster prevention and control in my country.

Figure 1 Global ULF band electromagnetic spectrum background given by Zhang Heng satellite

2. Breakthrough progress has been made in the application of satellite electromagnetic data

The global earthquake cases during the satellite's orbit were systematically studied, and the preliminary conclusion is that there is a high probability that there are electromagnetic ionospheric precursor anomalies for earthquakes above magnitude 6. On February 17, 2018, a strong earthquake of magnitude 7.1 occurred in Mexico. On February 15 and 16, the Zhangheng-1 satellite continuously recorded the low-frequency electromagnetic radiation phenomenon before the earthquake, which was also the first earthquake phenomenon of magnitude 7 or above recorded by the Zhangheng-1 satellite. On February 6, 2023, Turkey experienced a double earthquake of magnitude 7.8. The Zhangheng-1 satellite data analysis also found that electromagnetic waves and plasma precursor anomalies appeared intermittently about 20 days before the earthquake. The results of earthquake case dissection and comprehensive statistics preliminarily show that during the gestation of earthquakes above magnitude 6, there is a high probability that obvious electromagnetic radiation and ionospheric disturbance anomalies will appear a few days before the earthquake and within hundreds of kilometers from the epicenter. Among them, the deviation in the aftershocks of the location where the anomaly occurs is related to the geomagnetic longitude and latitude where it is located. The probability of low-frequency electromagnetic radiation and ionospheric disturbance precursors appearing before earthquakes above magnitude 6 is more than 60%.

Figure 2 Composition of the Zhangheng-1 satellite

The Zhangheng-1 satellite has a good response capability to various natural disaster events. At present, space weather disasters have become an important factor affecting social economy. During its time in orbit, the Zhangheng-1 satellite fully recorded the spatiotemporal development of multiple space weather disaster events. For example, during the large magnetic storm that occurred in August 2018, various payloads such as electromagnetic fields, plasma, and energy particles observed the disturbance of geophysical field parameters during the initial phase, main phase, and recovery phase of the magnetic storm and its global distribution characteristics (Figure 3).

Figure 3 Zhangheng-1 satellite’s response to the strong magnetic storm on August 26, 2018

The solar X-ray monitor carried by Zhang Heng-1 also observed the enhancement of X-rays and the occurrence of solar proton events during solar flares and coronal mass ejections . These observations of solar activity are in good agreement with those of NOAA and GOES satellites. Zhang Heng-1 also observed ionospheric changes and strong disturbances in the communication and navigation environment before and after the Tonga volcanic eruption.

The first global reference geomagnetic field model in my country has been successfully constructed, reaching the international leading level. Based on the Zhang Heng-1 satellite data, the global reference geomagnetic field model CGGM 2020.0 (Figure 4) was established. The model was evaluated by the International Association of Geomagnetism and Aerophysics (IAGA) and met the modeling accuracy requirements of the International Global Geomagnetic Reference Field IGRF and was selected as the new generation IGRF model IGRF2020.0. This is the only Chinese model since IGRF began to be updated for more than a century, indicating that my country has fully mastered the key technologies of global geomagnetic field modeling and filled the gap in my country's ability to obtain global geomagnetic field strategic information resources.

Figure 4 Zhangheng-1 Global Reference Geomagnetic Field Model CGGM 2020.0

At present, various data are being integrated to construct high-resolution and high-precision global and regional geomagnetic field models, providing important strategic information resource guarantees for emergency combined navigation, global resource exploration, national security, and the construction of the "Belt and Road" and a community with a shared future for mankind.

3. Systematically develop a low-frequency electromagnetic wave cross-sphere propagation model to lay a theoretical foundation for the development of space geophysics science

Based on the massive observation constraints of the Zhangheng-1 satellite, a full-wave model of the propagation and penetration of low- frequency electromagnetic waves from the underground/surface to the atmosphere and ionosphere was established (Figure 5). It is concluded that some wave modes and energy of low-frequency electromagnetic waves can reach the ionosphere and be received by the satellite . This model system breaks through the cross-sphere coupling propagation mechanism of low-frequency electromagnetic waves, innovatively develops the Earth's lithosphere-atmosphere-ionosphere coupling model, and verifies the scientificity and reliability of the model with the help of ground-based low-frequency communications, scattering radars and related satellite observation signals, confirming that low-frequency electromagnetic waves generated by earthquakes can penetrate the lithosphere, atmosphere and ionosphere to reach the height of the Zhangheng-1 satellite, and thus be observed by the satellite.

Figure 5. Model of low-frequency electromagnetic wave propagation across the lithosphere and ionosphere and its attenuation in the ionospheric waveguide.

The establishment of this model corrects the limitations of previous understanding of the propagation of electromagnetic waves between the Earth's lithosphere, atmosphere and ionosphere, and provides theoretical support for my country to develop full-process monitoring and early warning of major global natural disasters and to carry out high-resolution detection of hidden near-surface objects using space-based low-frequency electromagnetic waves. It has laid an important foundation for major natural disaster monitoring, underground resource development and underground space management, and has scientifically broadened the research on Earth system science and the Earth's key spheres.

Compared with national strategic needs and international scientific frontiers, the development of Zhang Heng-1 satellite and geophysical field satellite program is still in its infancy. On the basis of systematic development of optical and microwave technologies, we will accelerate the development of geophysical field satellite program, build an independent high-resolution and high-precision geophysical field dynamic model, and create global natural disaster monitoring and early warning, extreme conditions communication and navigation environment management, and support capabilities for sustainable development of a habitable Earth.