Spatial-temporal variation characteristics of geomagnetic field in China and model accuracy evaluation

Since 1968, the International Association of Geomagnetism and Aeronomy （IAGA） has continuously published the International Geomagnetic Reference Field （IGRF） models. In 2019, the 13th generation IGRF model （IGRF-13） was released; at the same year, the U.S. National Centers for Environmental Information （NCEI） and the British Geological Survey （BGS） jointly issued the World Magnetic Model 2020 （WMM2020）. This study aims to use the IGRF-13 model, along with data from Chinese geomagnetic observatories, to conduct an in-depth analysis of the spatio-temporal variation characteristics of the seven elements of the geomagnetic field: magnetic declination D, magnetic inclination I, horizontal intensity H, northward component X, eastward component Y, vertical component Z, and total intensity F from 2000 to 2020.

Furthermore, this study evaluates the applicability and accuracy of the IGRF-13 and WMM2020 models in China using data from 28 Chinese geomagnetic observatories. By studying the morphological changes and trends of these elements over the aforementioned period, we aim to gain a deeper understanding of the evolution process of the geomagnetic field and provide a detailed description of geomagnetic activities in China.

We selected geomagnetic observatories within China that have continuous observation records. Subsequently, the IGRF-13 model was utilized to calculate the geomagnetic field model values spanning the period from 2000 to 2020. By analyzing these model values, we studied the change characteristics of the seven elements of the geomagnetic field （D, I, H, X, Y, Z, F） in different time intervals, each interval being five years long, throughout the period from 2000 to 2020.

Twelve geomagnetic observatories were chosen from different regions across eastern, southern, western, northern, and central China, all with long-term and continuous observation records and located at similar latitudes and longitudes. For the data from 2000 to 2020, we performed a first-difference analysis on the time series of H, Z, F, so as to explore the trends and short-term fluctuations of these three components over time, thereby achieving a better understanding of the dynamic changes in the geomagnetic field.

Meanwhile by using the IGRF-13 model, we computed the model values of the total intensity of the geomagnetic field in China at five-year intervals spanning from 1960 to 2020. The analysis of these data enabled us to identify the long-term trends in the changes of the geomagnetic field and detect any potential periodic patterns. Comprehending these historical changes and predicting future trends is crucial for understanding the evolution of the geomagnetic field.

To evaluate the applicability and accuracy of the IGRF-13 and WMM2020 within the China, we adopted the difference and root mean square error （RMSE） to compare the differences between the model predicted-values and the actual measurements obtained from Chinese geomagnetic observatories. By this way, we could distinctly identify the advantages and disadvantages of the two models for China.

After a detailed analysis of the geomagnetic field in China from 2000−2020, this study has drawn the following main conclusions:

1） The seven elements of the geomagnetic field displayed alterations in varying degrees over different time periods. The moving directions of the isolines of the seven elements are also defferent. During the three periods from 2000 to 2005, from 2005 to 2010, and from 2010 to 2015, the isolines for all seven geomagnetic elements demonstrated a consistent movement direction. Conversely, during the period from 2015 to 2020, the moving direction reversed. This indicates that the geomagnetic field is characterized not only by gradual long-term trends but also by relatively abrupt short-term fluctuations, which can be affected by a multitude of internal and external factors.

During the period from 2000 and 2020, the geomagnetic field in China generally exhibited a trend of gradual strengthening. The region with the maximum change amplitude was located in the northwestern area, where the maximum increase about 1000 nT. In contrast, the northeastern region experienced the smallest increase. The isolines generally ran from northwest to southeast, indicating that the changes in the geomagnetic field during this period had distinct spatial distribution characteristics.

2） By analyzing the overall changes in the geomagnetic field at five-year interval from 1960 to 2020, we found that the variations of the geomagnetic field exhibit a “trough-peak-trough” pattern, with a cycle of approximately 30 years. Based on this cyclical characteristic, it is predicted that the geomagnetic field in China will show a downward trend in the next decade.

3） The accuracy of the IGRF-13 and WMM2020 models was evaluated by comparing their values with actual observations. The resulting average absolute errors for the D, I, H, X, Y, Z, and F components were as follows: 0, −0.1′, 2 nT, 1 nT, 1 nT, −1 nT, and −1 nT. The RMS errors are D＝0.2′, I＝0.0′, H＝0 nT, X＝1 nT, Y＝1 nT, and Z＝1 nT, and F＝1 nT. These RMS errors are consistent and fall within the accuracy range of the global model estimation, indicating that both models can accurately reflect the long-term variation characteristics of the geomagnetic field in the China.