Altitude extension of NCAR-TIEGCM (TIEGCM‑X) and evaluation

The upper boundary height of the traditional community general circulation model of the ionosphere‑thermosphere system is too low to be applied to the topside ionosphere/thermosphere study. In this study, the National Center for Atmospheric Research Thermosphere‑Ionosphere‑Electrodynamics General Circulation Model (NCAR‑TIEGCM) was successfully extended upward by four scale heights from 400–600 km to 700–1200 km depending on solar activity, named TIEGCM‑X. The topside ionosphere and thermosphere simulated by TIEGCM‑X agree well with the observations derived from a topside sounder and satellite drag data. In addition, the neutral density, temperature, and electron density simulated by TIEGCM‑X are morphologically consistent with the NCAR‑TIEGCM simulations before extension. The latitude‑altitude distribution of the equatorial ionization anomaly derived from TIEGCM‑X is more reasonable. During geomagnetic storm events, the thermospheric responses of TIEGCM‑X are similar to TIEGCM. However, the ionospheric storm effects in TIEGCM-X are stronger than those in TIEGCM and are even opposites at some middle and low latitudes due to the presence of more closed magnetic field lines. DMSP observations prove that the ionospheric storm effect of TIEGCM-X is more reasonable. The well‑validated TIEGCM‑X has significant potential applications in ionospheric/thermospheric studies, such as the responses to storms, low‑latitude dynamics, and data assimilation.