Variation of the main magnetic field in Chinese mainland during 20th century

In order to investigate the variation of main magnetic field in Chinese mainland during 20th century, the newest global modes, such as IGRF11, CALS3K. 4 (3K. 4) and GUFM1, were firstly adopted to calculate and analyze the mean values of annual rate of change (MAC) during 1900—1990. Combining the global mode, CM4, and regional model, Taylor polynomial (TY), we analyzed the distribution of annual rate of change (ARC) and MAC with interval of 10 years, based on survey data during 1960—2000, and then made a comparison with that of three global models. Results showed that the MACs are very close based on the models IGRF11, 3K. 4 and GUFM1 throughout the 20th century. These three models demonstrated the similar distribution of ARC in China during 1960—2000, which is resulted from the same modeling theory and closed truncation levels. The distributions of ARC based on global and TY models are basically consistent, that is, the X component of ARC decreases with years, and Y, Z, F firstly decrease and then increase. However, there were some tiny differences between global models and TY model, which is caused by the higher density of valid survey data and the usage of regional model. Results also showed that the MAC from TY model is consistent with that from CM4, indicating that the CM4 model is able to ideally reflect the variation of main magnetic field in Chinese mainland.