Ionograms recorded from four ionosonde stations along the Western Pacific (WestPac) chain (about 122 degreesE geographic, 192 degreesE geomagnetic) are employed to study the occurrence of an additional layer at F-region altitudes during the 1-15 March 1998 WestPac campaign. It was found that the appearance of the additional layer at the local noontime hours is a typical phenomenon at Parepare (4 degreesS geographic, 14.8 degreesS geomagnetic). The additional layer was not clearly observed at Cebu (0.4 degreesS geomagnetic) and Manila (3.7 degreesN geomagnetic), and was not observed at Chung-Li (14.2 degreesN geomagnetic) during the campaign. Furthermore, the additional layer was not seen from any of the station on I I March 1998, a magnetically disturbed day. These results indicate that the fountain effect (produced by ExB motion) plays an important role in the formation of the additional layer. However, they also suggest the dynamics of the layer fori-nation are in some way influenced by the location of the station relative to the geographic equator. (C) 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.