Wavenumber‐4 Patterns of the Sporadic E Over the Middle‐ and Low‐Latitudes

The COSMIC radio occultation observations are used to investigate the wavenumber‐4 (WN4) structure of the Sporadic E (Es) layer during 2007–2018. The WN4 pattern is observed in the Es occurrence rate over the middle‐ and low‐latitudes. This pattern is stronger at the low‐latitudes than at the middle‐latitudes. It shows the peak intensity in latitude bands between 5° and 15° dip latitudes in both hemispheres. The WN4 structure occurs predominantly during daytime. It appears above 110 km in the morning, then descends slowly down to about 105 km, and finally disappears after sunset. The WN4 structure moves eastward with a speed of approximately 90°/day. It is prominent in boreal autumn and summer, and disappears in boreal winter. The dominant role of the DE3 tide for the formation of the WN4 structure is supported by similar features between the Es occurrence rate and the DE3 tide of the zonal wind shear, such as the phase, eastward speed, seasonal variation, symmetrical distribution, and strong activity at the low‐latitudes. Key Points The wavenumber‐4 (WN4) structure is observed in the sporadic E layer over the middle‐ and low‐latitudes The diurnal, seasonal, and altitudinal variations of the WN4 pattern are analyzed The diurnal eastward non‐migrating tide with zonal wavenumber‐3 plays the dominant role in the formation of the WN4 structure