Topside Ionization and Effect on Altitudinal Evolution of Nocturnal Ionospheric Irregularities

Topside ionospheric electron density ( N e ) and its control on the variabilities in nocturnal ionospheric irregularities is examined globally using electron density measurements from COSMIC‐2 and Swarm satellite, along with the Swarm satellite‐based Ionospheric Plasma Irregularity (IPIR) index for low and high solar activity (LSA and HSA) periods of 2019–2020 and 2023–2024 respectively. Seasonal and solar cycle variations of topside N e and IPIR indices are examined, highlighting their direct relationship. The study also reveals the connection between the F region peak altitude at the equator and the peak electron density (NmF 2 ) at the crest region. The longitudinal pattern of topside N e is controlled by the Equatorial Ionization Anomaly (EIA), and this pattern is similarly reflected in topside irregularities. Three or four peaked structures are observed in the postsunset longitudinal pattern of topside irregularities and electron density during the March–April (MA) and June–July (JJ) months, while 2 peaked structures manifest in December–January (DJ) months for HSA and LSA. These patterns are attributed to the combined effects of topside N e associated modulation of F region dynamo, magnetic meridian alignment with sunset terminator, and lower atmospheric tides. The relationship between the altitudinal extent of irregularities with topside N e and N e gradients at the geomagnetic equator in Trivandrum shows a direct linear dependence on a day‐to‐day basis. The results suggest that the background topside N e has a significant effect on the strength and altitudinal extent of ionospheric irregularities on a day‐to‐day level.