The hybrid NeQuick-Digisonde-TaD (HyNT) model

The hybrid NeQuick-Digisonde-TaD (HyNT) model is based on the TaD extrapolation concept applied over the bottomside 3D electron density grids, which are calculated with the NeQuick model and interpolated with Digisonde data. The NeQuick is used as the background model in three dimensions, i.e. latitude, longitude, altitude. The 3D NeQuick grids, are updated with the Digisonde-extracted electron density at the Digisonde locations. This ingestion is achieved with interpolation. The interpolated 2D grid of foF2 and hmF2 and the corresponding GNSS-TEC maps are used as input to the TaD model, to obtain the extrapolated profile at each grid point. The NeQuick model is used for electron density reconstruction. The latest version of the NeQuick model, typically referred to as NeQuick 2 (Nava et al., 2008), is an empirical model of the electron density profile (Ne) from ground to ~20,000 km (covering the ionosphere and plasmasphere). It is driven by solar activity indices, most often effective sunspot number (R12) or global TEC. NeQuick is designed for quiet-time ionospheric conditions, for average behaviour across seasons and solar cycles. The model produces smooth, climatological profiles using a 3-segment semi-Epstein function. The TSM-assisted Digisonde (TaD) Profiler (TaD model) is a topside profiler for the electron density profile based on empirical equations derived from topside sounding data of the Alouette/ISIS database and ingests the Digisonde observations at the height of the maximum electron density and the TEC parameters to adjust the profiler with the actual conditions of the ionosphere. The model has three components: (a) the Topside Sounders Model (TSM) subroutine (Kutiev and Marinov 2007; Kutiev et al. 2006) that provides the empirical functions for the O+/H+ transition height (hT), the topside electron density scale height (HT) and their ratio RT = HT/hT, derived solely from the Alouette/ISIS data; (b) the Topside Sounders Model Profiler (TSMP) subroutine (Kutiev et al. 2009a, 2009b) that offers analytical formulas for obtaining the shape of the vertical plasma distribution in the topside ionosphere and plasmasphere based on TSM parameters and on the F layer maximum density (NmF2), its height (hmF2) and its scale height (Hm) at its lower boundary, derived from Digisondes. This profiler models separately the O+ and H+ density distributions in transition region between the topside F region and plasmasphere, extracted from the analysis of the electron density profiles from ISIS-1; (c) the final TaD subroutine that performs the necessary transformations to the Digisonde autoscaled scale height so that the integrated TSMP electron density from the F layer peak to GNSS orbits can be finally adjusted to the measured GNSS TEC at the Digisonde location (Belehaki et al. 2012; Kutiev et al. 2012). The 1-dimensional (1D) Electron Density Distribution (EDD) version of the TaD Model takes as input (a) the ionospheric characteristics at the hmF2 altitude, from a Digisonde sounder that can be retrieved from the SAO file and (b) the TEC parameter, which is the vertical electron content at the station location (i.e., the height integral of the electron density from 90 to 22000 km); the vertical TEC can be extracted from a TEC map such as the maps released by DLR or CODE, but it can also be calculated based on a single station solution code, with RINEX data received by a GNSS receiver co-located with the Digisonde. The topside profile is extrapolated with the TaD model as described above. This version does not provide the electron density profile (Ne) below hmF2 but calculates the bottomside TEC (bTEC) by approximating Ne distribution by the a-Chapman formula. The output of the code is the electron density profile over the ionosonde station.