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The temporal variations of the low latitude nighttimespread F (SF) observed by DPS-4 digisonde at low latitude Hainanstation (geog. 19.5° N, 109.1° E, dip lat. 9.5° N) during thedeclining solar cycle 23 from March 2002 to February 2008 arestudied. The spread F measured by the digisonde were classified intofour types, i.e., frequency SF (FSF), range SF (RSF), mixed SF(MSF), and strong range SF (SSF). The statistical results show thatMSF and SSF are the outstanding irregularities in Hainan, MSF mainlyoccurs during summer and low solar activity years, whereas SSFmainly occurs during equinoxes and high solar activity years. TheSSF has a diurnal peak before midnight and usually appears during20:00–02:00 LT, whereas MSF peaks nearly or after midnight and occursduring 22:00–06:00 LT. The time of maximum occurrence of SSF is laterin summer than in equinoxes and this time delay can be caused by thelater reversal time of the E×B drift insummer. The SunSpot Number (SSN) dependence of each type SF is different duringdifferent season. The FSF is independent of SSN during each season;RSF with SSN is positive relation during equinoxes and summer and isno relationship during the winter; MSF is significant dependence onSSN during the summer and winter, and does not relate to SSN duringthe equinoxes; SSF is clearly increasing with SSN during equinoxesand summer, while it is independent of SSN during the winter. Theoccurrence numbers of each type SF and total SF have the same trend,i.e., increasing as Kp increases from 0 to 1, and then decreasing asincreasing Kp. The correlation with Kp is negative for RSF, MSF, SSFand total SF, but is vague for the FSF

The statistical properties of spread F observed at Hainan station during the declining period of the 23rd solar cycle