Multi‐Decadal Trends in the Low Latitude foF2 Driven by Secular Magnetic Variations

Abstract Long‐term ionospheric trends have been widely studied, but their origin and magnitude remain subjects of debate. This study quantifies the linear trend in the F2‐region critical frequency ( foF2 ) and its local time dependence using observations from eight low‐latitude ionosonde stations. The trend is derived by fitting and removing background variations using a local empirical model of foF2 that uses known drivers. The resulting trends are largely negative, reaching a maximum magnitude of about −0.09 MHz/yr at the Cachoeira Paulista (22.70°S, 45.00°W) station around midnight. Only two of the eight stations examined exhibit positive trends, with the largest positive value observed over Fortaleza (3.90°S, 38.40°W), reaching +0.04 MHz/yr at 14 local time. We further estimate the “geometrical trend” of foF2 at these ionosonde stations by assuming that the URSI foF2 climatology remains constant in quasi‐dipole coordinates under secular variation of the Earth's magnetic field. Here, “geometrical trend” refers to the apparent foF2 trend component arising solely from displacement between magnetic and geographic coordinates. Our results indicate that the geometrical trend largely accounts for the observed daytime trend at five out of eight ionosondes. The largest discrepancy between the observed and geometrical trends occurs at nighttime in the eastern Brazilian sector, where the strongly negative observed trends are qualitatively consistent with estimated changes in vertical plasma transport induced by horizontal thermospheric winds. Discrepancies are also found at all local times for stations located near the geomagnetic equator, where the main magnetic field strength is decreasing and potentially affecting the E × B drift.