SEMIANNUAL VARIATION OF GEOMAGNETIC Z COMPONENT AT LOW LATITUDE

Using the data collected in 4 geomagnetic stations located separately at Wuhan, Guangzhou, Quanzhou and Qiongzhong in the low latitude, the difference of geomagnetic Z component (Dz) between the diurnal average value and the average value of 1100-1300 LT at 5 quiet-day in every month have calculated. The amplitude and phase of semiannual variation in every year have obtained by analyzing the 12 Dz with multiple regression method. The result shows that Dz of 4 stations all have semiannual variation in every year. The amplitude of semiannual variation is related to the solar activity, and in general, the amplitude of semiannual variation of Dz in solar maximum is obviously greater than that in solar minimum. The semiannual variation of solar activity modulates the amplitude of semiannual variation of Dz obviously. The phase of semiannual variation of Dz is in March- April (or September-October), so the maximum of Dz appears in Equinox. The semiannual variation of geomagnetic Z component at low latitude indicates that the equato-rial electrojet have semiannual variation as well. The semiannual variation of the equatorial electrojet causes the f0F2 of ionosphere to appear semiannual variation by fountain effect. And this is a reasonable mechanism to produce semiannual variation of f0F2.