Effects of sporadic E-layer characteristics on spread-F generation in the nighttime ionosphere near a northern equatorial anomaly crest during solar minimum

This study is to know how the characteristics of sporadic E-layer (E s -layer) affect the generation of spread-F in the nighttime ionosphere near the crest of equatorial ionization anomaly during solar minimum. The data of E s -layer parameters and spread-F are obtained from the Chungli ionograms of 1996. The E s -layer parameters include foE s (critical frequency of E s -layer), fbE s (blanketing frequency of E s -layer), and Δ f (≡foE s −fbE s ). Results show that the nighttime variations of foE s and fbE s medians (Δ f medians) are different from (similar to) that of the occurrence probabilities of spread-F. Because the total number of E s -layer events is greater than that of spread-F events, the comparison between the medians of E s -layer parameters and the occurrence probabilities of spread-F might have a shortfall. Further, we categorize the E s -layer and spread-F events into each frequency interval of E s -layer parameters. For the occurrence probabilities of spread-F versus foE s , an increasing trend is found in post-midnight of all three seasons. The increasing trend also exists in pre-midnight of the J-months and in post-midnight of all seasons, for the occurrence probabilities of spread-F versus Δ f . These demonstrate that the spread-F occurrence increases with increasing foE s and/or Δ f . Moreover, the increasing trends indicate that polarization electric fields generated in E s -layer assist to produce spread-F, through the electrodynamical coupling of E s -layer and F-region. Regarding the occurrence probabilities of spread-F versus fbE s , the significant trend only appears in post-midnight of the E-months. This implies that fbE s might not be a major factor for the spread-F formation.