On post‐midnight field‐aligned irregularities observed with a 30.8‐MHz radar at a low latitude: Comparison with<i>F</i>‐layer altitude near the geomagnetic equator

<jats:p>We investigated the relationship between post‐midnight<jats:italic>F</jats:italic>‐region field aligned irregularities (FAIs) and<jats:italic>F</jats:italic>‐layer altitude by analyzing data of a 30.8‐MHz radar installed 5at Kototabang, Indonesia (0.2°S, 100.3°E; geomagnetic latitude 10.4°S) and an ionosonde installed at Chumphon, Thailand (10.7°N, 99.4°E; geomagnetic latitude 3.3°N). Chumphon is located near the geomagnetic equator on approximately the same meridian as Kototabang. Case studies show that the altitude of the<jats:italic>F</jats:italic>‐layer rose at Chumphon a half hour before the post‐midnight FAIs appeared at Kototabang. The Doppler velocity of the<jats:italic>E</jats:italic>‐region FAIs observed simultaneously by the 30.8‐MHz radar was downward, indicating that the<jats:italic>F</jats:italic>‐layer uplift was not caused by the electric field. We also investigated seasonal variations of the post‐midnight FAI occurrence and the<jats:italic>F</jats:italic>‐layer altitude. Both the post‐midnight FAIs and the uplift of the<jats:italic>F</jats:italic>‐layer were frequently seen around midnight between May and August. The seasonal variation of the midnight<jats:italic>F</jats:italic>‐layer uplift around the geomagnetic equator coincided with that of the post‐midnight FAI occurrence at Kototabang. These results suggest that the uplift of the<jats:italic>F</jats:italic>‐layer would play an important role in the generation of post‐midnight FAIs. We evaluated the linear growth rate of the Rayleigh‐Taylor instability based on the altitude of the<jats:italic>F</jats:italic>‐layer observed at Chumphon. The result shows that the uplift of the<jats:italic>F</jats:italic>‐layer can enhance the growth rate because gravity‐driven eastward electric current increases. Therefore, we interpret that the observed FAIs were accompanied by plasma bubble, the growth rate of which was reinforced by the uplifted<jats:italic>F</jats:italic>‐layer.</jats:p>