Based on data of Digisonde Portable Sounder-4 (DPS-4) in 1995 -1997. we have analyzed the seasonal variations of F region at Zhongshan Station (69. 4°S, 76. 4°E). During the summer of Zhongshan Station, F region ionization is mainly controlled by the solar ultraviolet radiation. Similar to the phenomena in mid-latitude area, the value f0F2 is changed with local time. During equinox scasons, soft electron precipitation from the cusp/cleft region seems significant, f0F2 is changed with rnagnetic local time, and shows the magnetic noon phenomenon. In winter, the effect of the solar radiation on the F region is less than that of summer. Instead, F region is affected by particle precipitation from cusp/cleft region as well as polar plasma convection, therefore, the diurnal variation of f0F2 is more complex and shows two peaks. F region occurs all day in summer. and seldom appears at midnight in equinox. In winter, F region shows two minimums, one is at midnight and the other is at afternoon cusp. Further analysis of the F region spread indicates that in winter the aurora oval passes over the Zhongshan Station is at 1 100 UT-1 500 UT.
