How Do Changes of Geomagnetic Field and Greenhouse Gas Concentration Affect the Upper Ionosphere in the Long Term?

Observing the topside ionosphere using ground-based instruments is very challenging. However, slab thickness, defined as the ratio of total electron content (TEC) to NmF2, provides a useful proxy for characterizing the electron density distribution above the F2 peak. It therefore offers a valuable means of investigating the long-term variability of the upper region. On the other hand, model simulation can easily investigate the long-term variation of the electron density profile above the peak directly without the observational limitations inherent in ground-based measurements. This study aims to bridge observational and modeling studies of long-term topside ionospheric variability by investigating the long-term evolution of slab thickness simulated by numerical model and the physical mechanisms responsible for its variation. In this study, we use the Whole Atmosphere Community Climate Model with thermosphere-ionosphere extension (WACCM-X) to investigate long-term changes in this region from 1965 to 2008. Four model simulations are designed to separate the influences of increasing atmospheric greenhouse gas concentrations, varying solar activity, and secular changes in Earth's geomagnetic field. The model results show that increasing greenhouse gas concentrations lead to a global decrease in slab thickness, particularly in regions where the diffusive equilibrium assumption is valid. Whereas secular changes in the geomagnetic field produce more complex regional patterns, especially near the geomagnetic equator and in the auroral regions. Furthermore, slab thickness is closely linked to plasma temperature and electron density structure, indicating that comparing modeled and observed slab thickness can help improve our understanding of long-term variability in the topside ionosphere.