Global assessment of topside effective scale heights in the low- and mid-latitude ionosphere using COSMIC-2 observations and IRI-2020 topside COR2 model

Abstract

The effective topside scale height (H_eff) governs the vertical distribution of electron density in the topside ionosphere, and its accurate representation is crucial for ionospheric modeling and space weather applications. This study examines the global distribution of H_eff spanning the low- and mid-latitude regions and its diurnal, seasonal, longitudinal variability, as well as statistical characteristics, using Constellation Observing System for Meteorology, Ionosphere, and Climate 2 (COSMIC-2) radio occultation observations during the solar cycle 25 (2019–2024). The results are compared with the corresponding predictions from the International Reference Ionosphere (IRI)—2020 topside sub-model IRI-COR2, and assimilated IRI-COR2 obtained by ingesting COSMIC-2-derived foF2 and hmF2 parameters into IRI-COR2. It is observed that H_eff values during high solar activity (HSA) consistently exceeded the corresponding values during low solar activity (LSA), across all datasets, seasons, and longitude sectors, confirming the strong solar cycle modulation of H_eff driven by enhanced solar EUV flux. An equatorial pre-sunrise enhancement of H_eff, centered near the magnetic equator between 3 and 6 local time, was identified across all datasets under both solar activity conditions, which could be associated with reduced recombination rates and plasma redistribution along magnetic field lines. The NDJ (November, December, January) consistently yielded the highest equatorial H_eff, whereas the FMA (February, March, April) showed the most suppressed values, consistent with inter-hemispheric thermospheric neutral wind asymmetry during the December solstice. IRI-COR2 systematically overestimates H_eff relative to COSMIC-2 with RMSEs of 31.81 km and 45.34 km during LSA and HSA, respectively, which is most pronounced near the magnetic equator. Assimilation of COSMIC-2 foF2 and hmF2 marginally reduced the RMSE to 31.35 km and 44.26 km, respectively, although systematic biases persisted, highlighting the need for full-profile assimilation to further improve topside ionospheric representation.