Growth and decay of a post-sunset equatorial anomaly at low latitudes: Control of E×B, neutral winds and daytime electrojet strength

Analyzing the 2 months ionospheric electron content (IEC) data acquired during a high solar activity period from a network of a stations whose sub-ionospheric points at 420 km were located within ± 1° of 84° meridian and covering a latitude belt of 3–21°N (magnetic), it has been shown that following sunset a fresh equatorial anomaly develops in continuation to the decaying daytime equatorial anomaly (DEA) in the low latitude belt. The occurrence of this post-sunset equatorial anomaly (PEA) is although a regular phenomenon but large day-to-day variations are seen in its strength (crest to trough ratio) and the latitude extent, PEA develops fully within 2–3 h of sunset and then decays. During its development crest of anomaly clearly shows a pole-ward movement and during decay it shows substantial equator-ward movement. While the trough of PEA is always seen over the magnetic equator, the crest of fully developed anomaly may be seen to lie at latitudes anywhere between 12°N and 21°N magnetic or more on different days. Thus the development and decay of PEA produce large latitudinal disturbance of F-region plasma in the lower latitude belt in the post-sunset hours and the occurrence of various low latitude phenomenon observed in IEC, e.g. post-sunset IEC decreases at equatorial latitudes, post-sunset IEC enhancements at 12°N or higher latitudes, pre-midnight IEC enhancements at equatorial latitudes, etc., have been attributed to the systematic development and decay of PEA. Also, the occurrence and development of PEA has been found to play an important role in the onset of equatorial plasma bubble associated ionospheric irregularities and their latitudinal growth. The equatorial ionization anomaly (EIA) is primarily caused by the ExB drifts whereas meridional winds in association with E×B drifts also seems to play some role in determining the crest of PEA.