Research on daily variation of vertical component of geomagnetic field in China

Based on the observation data from geomagnetic stations in China, we first statistically analyzed the minimum time （local time） of the vertical component at 15 geomagnetic stations based on its morphological characteristics of the daily variation curves. We then focused on the distribution characteristics of these minimum times within the Chinese region and their relationship with the longitude of the stations. Secondly, we selected 27 geomagnetic stations to analyze the annual average daily variation amplitudes of the vertical component during periods of high and low solar activity, aiming to achieve the relationship between the daily variation amplitudes of the vertical component and sunspot activity. This analysis was conducted for different seasons, including the months around the equinoxes, summer solstice, and winter solstice, as well as for years with high and low solar activity. The results show that: ① The minimum time of the vertical component of the geomagnetic field in China is mainly concentrated around local noon, showing a significant negative correlation with the longitude of the stations, and this time distribution follows a normal distribution pattern. ② The daily variation amplitude of the vertical component of the geomagnetic field is significantly influenced by sunspot activity, by a correlation coefficient of 0.929 with the number of sunspots, indicating a strong positive correlation. The daily variation amplitude of the vertical component exhibits a cyclic change of about 11 years. ③ The daily variation amplitude of the vertical component shows seasonal differences, being more active in the months around the summer solstice compared to the months around the winter solstice, indicating distinct seasonal impacts. These findings provide insights into the daily variation characteristics and patterns of the geomagnetic field and offer references for understanding Earth's internal activities.