We present the observational and modeling study focused on the major factors determining the spatiotemporal structure of the high-latitude ionospheric plasma density enhancement—the tongue of ionization (TOI) structure—during the 2015 St. Patrick's Day geomagnetic storm. We use the Global Self-consistent Model of the Thermosphere, Ionosphere, Protonosphere (GSM TIP) to reproduce the plasma density distribution, and the results are compared with the observational data as deduced from the ground-based global positioning system total electron content and in situ plasma probe measurements at different altitudes. Both the simulation and observation results show that a large-scale TOI-like structure of enhanced plasma density extends from the dayside midlatitude region toward the central polar cap along the antisunward cross-polar convection flow. We reveal an important role of the clockwise convection cell rotation for the modification of TOI structure. According to model results during the storm main phase, the neutral thermospheric composition, particularly the “tongue” in n(N₂), modifies the spatial structure of TOI in such a way that (1) the near-pole region of enhanced plasma density is shifted to the duskside and, (2) at F region heights, the TOI is split into the dusk and dawn branches. The signature of TOI in the topside ionosphere considerably differs from that in the F region because of a lesser influence of the neutral composition changes at higher altitudes. Model results revealed that at plasmaspheric heights, the TOI structure appears in both the dawn and dusk convection cells.