Ionospheric equivalent slab thickness ingestion into the NeQuick model

The ionospheric equivalent slab thickness (r), defined as the ratio of the vertical total electron content (vTEC) to the ionospheric F2-layer electron density maximum (NmF2), is a parameter providing useful information on the shape of the vertical electron density profile. However, the use of this information is of difficult practical application in empirical ionosphere models, such as the NeQuick, because by design they do not explicitly include r as a modelling parameter. In this work, we investigated the opportunity of using measured r values to improve the empirical modelling of the ionosphere vertical electron density profile by NeQuick. Measured r values were obtained through NmF2 observations and vTEC measurements obtained between 2001 and 2019 by an ionosonde and a ground-based GNSS receiver, respectively, co-located at Rome ionospheric station (41.8 degrees N, 12.5 degrees E; Italy). NeQuick r was obtained as the ratio between modelled NmF2 and vTEC values, the latter calculated by integration of the vertical profile. As a first step, r values modelled by NeQuick were compared with corresponding values measured at Rome station to highlight diurnal, seasonal, and solar activity differences. Then, measured r values were ingested in NeQuick through a three-parameter assimilation procedure which first assimilates F2-layer peak characteristics to constrain the F2-layer anchor point, and then assimilates vTEC to optimize the F2-layer shape through the NeQuick F2-layer thickness parameter, namely B2bot. The assimilation procedure provides information on how the NeQuick B2bot has to be modified to match measured r values, and then on how the shape of the F2-layer profile has to be changed accordingly. Our results highlight that, in many cases, the NeQuick B2bot has to be increased to match observations, which has implications on the modelling of the NeQuick bottomside and topside effective scale heights.