Large-scale longitudinal variation in ionospheric height and equatorial spread F occurrences observed by ionosondes



[1] Variations in ionospheric height associated with the prereversal enhancement (PRE) at two equatorial ionosonde stations separated by 6.34° in longitude were studied for the first time. The stations used were in Chumphon (10.7°N, 99.4°E, 3.3 Mag. Lat.), Thailand and Bac Lieu (9.3°N, 105.7°E, 1.6 Mag. Lat.), Vietnam. Variations in the virtual height of the bottomside of the F region (hF) at 2.5 MHz observed at these two stations were analyzed for a period in March and April 2006. When the equatorial spread F (ESF) was not observed, hF variations at the two stations were very similar, with a local time shift corresponding to the longitude separation. However, when ESF was observed, they were often significantly different. Our results show that hF enhancement, which could be interpreted as being due to the eastward electric field enhancement, is quite localized in longitude when ESF occurs.