Analysis of the periodical characteristics of Sq index

Using the hourly data of the X component of geomagnetic field during 1960 to 1980 from Beijing and Guangzhou geomagnetic observatories of China, the Asq index is calculated based on the new geomagnetic activity index for describing the variation of Sq dynamo current intensity proposed by Xu (1992). The method of Fourier analysis is used to analyze the periodical characteristics of the Asq index. The result shows that a prominent 11-year cycle, as well as the annual and semiannual variations are clearly seen in the Fourier harmonics of the Asq index, which are closely related with the F107 index, suggesting the close relationship between the variations of the Asq index and the solar radiation. Because the correlation calculation with Delta(S-q) over bar (t) is derived from the quiet clays of each month, which contains the periodical information of the Sq field, the amplitudes of the 11-years cycle, annual and semiannual variations are smaller.