Results from the meridional ionosonde network located in Southeast Asia (SEALION) demonstrate the interesting nature of the F 3 layer, showing its generation mechanism. Ionograms obtained on 16 November 2004 and 31 March 2005 at Chiang Mai (CMU; geographic latitude 18.8¿N, geographic longitude 98.9¿E, and magnetic latitude 13.2¿N), Chumphon (CPN; 10.7¿N, 99.4¿E, and 3.2¿N) and Kototabang (KTB; 0.2¿S, 100.3¿E, and 10.1¿S) showed significant differences between CPN near the magnetic equator, and CMU and KTB in the magnetic low-latitude region. The simultaneous magnetic conjugate observations of the F 3 layer achieved using the SEALION ionosonde network data showed clear dependences of the F 3 layer on the magnetic latitude. It is suggested that these magnetic latitude dependences of the F 3 layer can be explained by considering the plasma diffusion effects along the magnetic field lines in the magnetic low-latitude region.