Strong range SF observed in low latitude ionosphere over acsension IS in Atlantic ocean

In the ionogram obtained by sweep frequency radar named ionosonde, the spread echoes in the radio waves indicate the ionospheric plasma irregularities which can scatter, refract and reflect radio wave. The spread echoes are called ionospheric Spread F (SF). SF is considered to have different types: Range SF, Frequency SF, Mixed SF, and Branch SF. Recently some authors, using data from Asian and American continents, found an independent type of spread F named strong range SF (SSF) in low latitude ionosphere, and also found no BSF at low latitude. In this paper, with data from Digisonde at Acsension IS station (7.9°S, 345.6Έ Magnetic 2.3°S) in the low latitude region in Atlantic Ocean in 2004, the occurrence features of SF, especially SSF, were statistically studied. The results showed that there were RSF, SSF, FSF and MSF with occurrences of 5.2%, 23.6%, 26.5% and 44.7%, respectively. The SSF have high occurrence from January to March and from September to December, and have low occurrence from April to August. The SSF accounted for about 30% of all SF in summer and equinoxes, and about 5% of all SF in winter. The SSF mainly occurred at LT 21:00–23:00 in which the ionospheric scintillation just has high occurrence, which suggest that the SSF is associated with Equatorial plasma bubbles. The monthly occurrence variation and the appearance period of SSF were much different from other types of SF. Our results show that the SSF can take place in low latitude ionosphere not only over Asian and American continents, but also over Atlantic Ocean.