The statistics of equatorial spread-F at the conjugate stations in Southeast Asia

Ionospheric disturbance is a major impediment to current communication systems with the signals propagating through the ionosphere. A signature of such disturbance when appearing in an ionogram, resulting from the ionosonde observation system, is known as spread-F. The spread-F statistics is therefore crucial to the design and operation of communication system. In this work, we analyze the equatorial spread-F (ESF) statistics at the conjugate stations: Chiangmai, Chumphon and Kototabang during the period of minimum solar activity from September 2008 to April 2009. The statistics of range type spread-F (RSF) occurrence after the local sunset are shown in terms of the monthly average percentage since it is related to the Equatorial Plasma Bubble (ESP). We found that the RSF occurrence is higher at Chumphon station, which is near the magnetic equator, than at Kototabang and Chiangmai stations. The maximum level occurs during March 2009 at Chumphon station and the higher rate mostly during the equinoctial months. In addition, we compare the observed RSF occurrence probability at Chumphon station with the IRI-2012 model prediction. The comparison results show that the IRI model overestimates our results in all seasons, most evidently in December Solstice, except the estimation in June solstice during 21:00-03:30 LT when IRI model underestimates the statistics of RSF occurrence. Nevertheless, the IRI model gives closer probability values in March equinox during 20:00-22:00 LT than other seasons.