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 （D_z） 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 D_z with multiple regression method. The result shows that D_z 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 D_z in solar maximum is obviously greater than that in solar minimum. The semiannual variation of solar activity modulates the amplitude of semiannual variation of D_z obviously. The phase of semiannual variation of D_z is in March- April （or September-October）, so the maximum of D_z 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.