Enhanced Ionospheric Ray-Tracing: Advanced Electron Collision and Horizontal Gradient Modeling in the IONORT-ISP-WC System

This manuscript analyzes IONORT-ISP-WC, an advanced ionospheric ray-tracing tool improving HF radio wave propagation predictions. It significantly upgrades IONORT-ISP by integrating a double-exponential collision frequency model for the D-layer (primary HF absorption), extending the ISP 3-D electron density grid to 65 km, and increasing spatial resolution from 2° x 2° to 1° x 1°. A central focus is the detailed examination and local_ionort Fortran implementation of horizontal gradients in ionospheric electron density profiles. This reveals a robust framework for incorporating these gradients. Crucially, electx_grid now actively calculates horizontal gradients (previously commented), though full Taylor optimization is a high-priority future development to leverage the high-resolution grid for unparalleled accuracy. IONORT-ISP-WC underwent rigorous validation against observed and synthetic oblique ionograms (from IONORT-IRI-WC, based on the climatological IRI model). Results demonstrate its superior Maximum Usable Frequency (MUF) prediction accuracy. This underscores assimilative models' value in capturing dynamic ionospheric conditions, especially with meticulous horizontal gradient accounting. MUF prediction discrepancies are primarily attributed to real-time assimilation data limitations (availability, geographical distribution). This report positions IONORT-ISP-WC as a robust, reliable, cutting-edge operational tool for diverse space weather applications. It outlines crucial future developments: comprehensive validation of advanced horizontal gradient modeling and strategic enhancement of global data assimilation networks for higher accuracy and resilience.