On the Relationship Between E Region Scintillation and ENSO Observed by FORMOSAT‐3/COSMIC

Global Navigation Satellite System radio occultation signals often show extremely strong levels of scintillation when passing through the ionospheric E region. This is related to sporadic E ( E s )—dense layers of metallic ions that can form in the E region, influencing terrestrial and satellite radio propagation. In our report on the 2007–2014 variation of E region S 4 amplitude fluctuation indices measured by the FORMOSAT‐3/Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) satellite constellation, we find that the spatial and temporal variation of the maximum S 4 index in the E region is proportionate to the occurrence rate of extreme scintillation and by extension, sporadic E . We also find that the monthly median extreme S 4 amplitude fluctuation index in the E region midlatitudes shows a dependence on variation of the El Niño–Southern Oscillation (ENSO) in the troposphere that has not been previously reported. The ENSO‐related variation of the E region median extreme S 4 indices varies closely with the tropopause height, with both parameters lagging the Oceanic Niño Index by roughly 1 to 2 months, while also displaying a similar spectrum of periodicities. This similarity is especially strong in the southern midlatitudes. These results indicate that ENSO signatures can be transmitted to E s formation mechanisms, potentially through modulation of vertically propagating atmospheric tides that alter lower thermospheric wind shears. The end result is the modulation of the interannual variation of extreme E s values by ENSO.