Abstract: Geoelectric field is a key factor to make a connection between the space, the surface and interior of the Earth. It includes the induced currents due to the change of the space current system, and the conduction current caused by material migration in the deep earth. Based on the processing and analysis of the 10-years observation data from five fixed stations of Tianzhu Array in the northeastern margin of the Qinghai-Tibet Plateau, it is concluded that the telluric current in this area has an obvious annual variation with a period of 366 ±(<1) days, and its direction also has obvious seasonal variation characteristics. The telluric current direction changes once every 6 months, the time node of the direction change is April and September/October every year. We develop a semi-quantitative model, based on the following two results: ① the long-term trend of the telluric current is consistent with the annual trend of ionospheric Sq current; ② by calculating, it is found that the factors, the periodic change of regional climate, can lead to the structure change of surface electrical and further affect the seasonal trend of telluric current. These results expand the understanding of the annual ultra-low frequency telluric current and the knowledge and potential applications of the natural electromagnetic field.

Key words: Sq current, Tianzhu array, Telluric current, Climate and environment, Seasonal variation