Comparison of bottomside ionospheric profile parameters (B0 and B1) extracted from FORMOSAT-7/COSMIC-2 GNSS Radio occultations with Digisondes and IRI-2016 model

Abstract

The bottomside thickness (B0) and shape (B1) are two important parameters considered in the bottomside electron density profile (EDP) specification of the International Reference Ionosphere (IRI) model that is the most widely accepted global empirical ionospheric model. In the present study, we investigated B0 and B1 as derived from least-square fitting of FORMOSAT-7/COSMIC-2 (F7/C2) radio occultation (RO) profiles by employing certain quality constraints in accordance to coincident Digisonde soundings. We performed a comparative analysis of the local time diurnal, seasonal and latitudinal behaviour of B0 and B1 parameters from a huge database of F7/C2 profiles accumulated during the last two years (2020 and 2021) with estimations from the IRI-2016 submodels: Bil-2000, Gul-1987, and ABT-2009, referring to the bottomside profile characteristics during the low solar activity period of 25th solar cycle. The results reveal daytime underestimation and nighttime overestimation features of IRI submodels. The typical pre-sunrise enhancement in B0 is clearly discernible in the F7/C2 observations which could not be reproduced in any of the IRI submodels. Moreover, the equatorial bottomside thickness anomaly (EBTA) presenting hemispheric asymmetry is not evidenced in the observed as well as modelled parameters. The sharp changeover in B0 magnitude from summer to winter solstice hemisphere and spring to fall equinox hemisphere in Gul-1987 indicates its inability to accurately represent the B0 variation at the equatorial and low latitude sector. Despite the fact that ABT-2009 performs relatively better than Gul-1987 sub-model based on F7/C2 and Digisonde datasets in different seasons, we demonstrate that there is room for possible improvement in terms of B0 and B1 representation.