Long‐Term Trends in Sporadic E Critical Frequency Using Multi‐Decadal Ionosonde Data

Sporadic E (Es) layers are thin ionization structures in the ionospheric E region that can strongly affect high‐frequency (HF) and even very‐high‐frequency (VHF) radio propagation. However, their long‐term variability remains poorly understood, particularly in relation to local time and seasonal dependence. Here, we investigate multidecadal trends in the sporadic E critical frequency (foEs) using more than 30 years of manually scaled ionosonde observations from 14 stations spanning low‐, mid‐, and high latitudes in both hemispheres. The results reveal pronounced regional and latitudinal differences. Mid‐latitude stations in the Southern Hemisphere exhibit predominantly positive foEs trends across most local times and seasons, whereas Northern Hemisphere mid‐latitude stations show positive trends mainly during summer and weak or negative trends during other seasons. High‐latitude stations generally display negative trends. Removing the influence of the solar cycle produces only minor changes in the trend estimates, indicating that solar variability is not the primary driver of the observed long‐term changes. The spatial and seasonal patterns of the trends are consistent with recent model predictions linking increasing carbon dioxide concentrations to enhanced vertical ion convergence through changes in thermospheric wind shear, particularly at mid‐latitudes. These findings highlight the importance of resolving diurnal and seasonal variability when assessing long‐term ionospheric change.