Long-term Trends in the Low Latitude foF2 Driven by Secular Variation of Earth's Magnetic Field

Long-term ionospheric trends have been widely studied, yet their origin and magnitude remain debated. This study quantifies trends driven by the secular variation (SV) of Earth’s main magnetic field and examines their dependence on local time. Using the long-term evolution of quasi-dipole coordinates and IRI-2020 foF2 climatology, we predict SV-induced changes of up to 10% in foF2 and 20% in NmF2 over the Equatorial Ionization Anomaly region between 2000 and 2024. Observations from two Brazilian ionosonde stations show strong agreement with these predictions in both trend direction and local time variation. The predicted SV-driven trends explain most of the observed daytime trends in the Brazilian sector, while nighttime discrepancies point to additional negative influences, likely related to long-term thermospheric cooling. This work provides a framework to separate natural SV-driven variations from potential anthropogenic effects.