Like air and water, geomagnetic field is one of the indispensable environmental conditions for human survival. Like a natural barrier, the geomagnetic field surrounding the Earth protects the life on Earth from the threat of high temperature, high speed solar winds and the bombardment of cosmic high-energy particles. The geomagnetic field originates from the magnetic fluid process of the outer core of the Earth and the current system of the solar system, so the geomagnetic field carries rich information about the inner and outer spaces of the Earth.1–2
Development of geomagnetism depends on the long-term accumulation of data. Geomagnetism is closely related to national economy and national defense construction. The research outcomes of geomagnetism directly or indirectly serve many fields, including navigation, communication, space environment monitoring, the forecast of natural disasters and climate changes, the exploration of energy and mineral resources, and so forth. A complete high-precision geomagnetic data set not only serves geomagnetism itself, but also provides the basis for research on, for example, seismics, geological structure, deep earth exploration and mineral exploration.3–5
In the study of geomagnetics and geospatial environments, ground station observation plays an important role. Study of the spatial distribution and long-term variation of the main magnetic field of the Earth mainly relies on data from geomagnetic station and geomagnetic complex points. In the study of geomagnetic origin and long-term variations of the main magnetic field, satellite magnetic data still cannot replace ground data. Indeed, studies of the origin of magnetic fields and the internal processes of the Earth have been drawing basic data from over-100-year station observation data, as well as hundreds-of-year sea magnetic data.6–8 With a large number of high-temporal resolution and high-precision geomagnetic data, scientists have made significant progress in sub-storm current systems, ionospheric-magnetospheric coupling and spatial weather forecasting. The long-term historical observation data provide a solid foundation for these studies.8–10 After decades of efforts, China has basically completed a nationwide digital geomagnetic observation network composed by 160 observation stations, which achieved the quasi-real-time transmission and automatic collection of observation data, and a large number of observation data have been accumulated since then.11
Beijing Ming Tombs station (BMT) was established in October 1985 and completed in August 1990. The station belongs to the Institute of Geology and Geophysics, Chinese Academy of Sciences. It is the first observation station entering the International Real-time Magnetic Observatory Network (INTERMAGNET). The station is located at about 600 meters northeast of Deshengkou Village, Changping District, Beijing. The geological base of this station is quartzite and gray matter rock. The magnetic background has a small local anomaly. After the land leveling and reconstruction, the distribution of the geomagnetic field has improved, in which the gradient of the total intensity of the magnetic field in the recording room and its surrounding area is less than 1 nT/m.
The geomagnetic observation room was built above ground. To ensure a stable instrument base, the foundation of external walls and 13 indoor white marble instrument frusta was built using limestone from 4 meters deep. There are three permanent signs in the southwest of the observation room, of which the commonly used standard sign No. 2 is about 700 meters from the observation room and has an azimuth angle of 207º31'50". The geomagnetic recording room is a semi-underground building. Its indoor temperature difference is less than 0.5℃, annual temperature difference about 12℃, and relative humidity less than 70%.12