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

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 ( h ′ F ) 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, h ′ F 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 h ′ F enhancement, which could be interpreted as being due to the eastward electric field enhancement, is quite localized in longitude when ESF occurs.