Keywords:sporadic E, spread F
In this work, we examine the relation of Es-layer parameters to spread-F generation in the nighttime ionospheres at low- and mid-latitudes during 1996. The data of Es-layer parameters and spread-F are obtained from the Chungli (24.9°N, 121.2°E) and Kokubunji (35.7°N, 139.5°E) ionograms. The Es-layer parameters include foEs (critical frequency of Es-layer), fbEs (blanketing frequency of Es-layer), and Δf (=foEs-fbEs). After the pre-midnight and post-midnight data are classified by seasons, the Es-layer and spread-F events are categorized into each frequency interval of Es-layer parameters. At Chungli, an increasing trend is found in post-midnight of all three seasons, for the occurrence probabilities of spread-F versus foEs. For the occurrence probabilities of spread-F versus Δf, the increasing trend also exists in pre-midnight of the J-months and in post-midnight of all seasons. The increasing trends can be found in the results at Kokubunji, for the occurrence probabilities of spread-F versus foEs and Δf. The increasing trends indicate that polarization electric fields generated in Es-layer can assist to produce spread-F, through the electrodynamical coupling of Es-layer and F-region On the other hand, the results of fbEs show that fbEs is irrelevant to the spread-F generation.