Tidelike Periodicities of Ionospheric Sporadic E Observed at Low Latitude Hainan Station

The time series of the ionospheric Sporadic E (Es) critical frequency (f₀E_s) and its virtual height (h'E_s) were performed on a large database of DPS-4 ionograms recorded at Hainan station (19.5°N, 109.1°E) in 2004 to investigate the features of tidelike periodicity variations in low latitude. On yearly spectra, there was pronounced 8-hour periodicity in low latitude Es in addition to the well known 24-and 12-hour tidal variations. The yearly average phases of 24-, 12-, and 8-hour periodicities for f₀E_s (h'E_s) were at 13:00LT (13:00LT), 11:00LT (09:00LT), and 09:30LT (09:30LT). On every two-month spectra, the results showed that (1) the 24-hour periodicity in Es was the most pronounced in each season; (2) The 12-hour periodicity in Es was outstanding in each season except in January-February months; (3) The 8-hour periodicity was remarkable in equinoxes and July-August months for f₀E_s, and it was notable in almost each season for h'E_s; (4) The 6-hour periodicity mainly occurred in November and December for f₀E_s, whereas for h'E_s it appeared more in summer and winter, and weakly in equinox months. It is clearly distinct from previous results by the fact that the 6-and 8-hour periodicities in Es layer at Hainan station can occur in winter or other seasons. The 6-and 8-hour periodicities in Es layer most probably are caused by the quarter-diurnal and terdiurnal tide components, which needs to be investigated further by combining the observations and modeling of local tide.