Equatorial F3-layer observations during the winter of 2004

Vertical ionospheric soundings were carried out in the equatorial zone above Pant Raja, district of Batu Pahat, Johor, Malaysia (lat. 1/spl deg/ 86' N, long. 103/spl deg/ 8' E) for the winter months of December 2004, January and February 2005. The vertical digital sounder is used to measure and to determine the critical frequency, maximum usable frequency and electron density at any given time. The presence of the F3-layer is quite prominent compared with mid- and high-latitude ionosphere. The greatest occurrence of the F3 layer during the winter of 2004 is in the period between 1100 LT to 1300 LT (13.3%) while the least is between 0700 LT to 0900 LT (1.2%). The mean critical frequency for the F3 layer is about 8 MHz (foF3) while the mean virtual height for the F3 layer is about 600 km (h'F3). The maximum virtual height of the F3 layer (h'F3) recorded is about 900 km on the 7th February 2005 at 1800 LT while the minimum is about 420 km on the 5th February 2005 at 1300 LT.