Changes in the Earth's magnetic field over the past century: Effects on the ionosphere‐thermosphere system and solar quiet (Sq) magnetic variation

We investigated the contribution of changes in the Earth's magnetic field to long‐term trends in the ionosphere, thermosphere, and solar quiet (Sq) magnetic variation using the Coupled Magnetosphere‐Ionosphere‐Thermosphere (CMIT) model. Simulations with the magnetic fields of 1908, 1958, and 2008 were done. The strongest differences occurred between ~40°S–40°N and ~100°W–50°E, which we refer to as the Atlantic region. The height and critical frequency of the F 2 layer peak, h m F 2 and f o F 2 , changed due to changes in the vertical E × B drift and the vertical components of diffusion and transport by neutral winds along the magnetic field. Changes in electron density resulted in changes in electron temperature of the opposite sign, which in turn produced small corresponding changes in ion temperature. Changes in neutral temperature were not statistically significant. Strong changes in the daily amplitude of the Sq variation occurred at low magnetic latitudes due to the northward movement of the magnetic equator and the westward drift of the magnetic field. The simulated changes in h m F 2 , f o F 2 , and Sq amplitude translate into typical trends of ±1 km/decade (night) to ±3 km/decade (day), −0.1 to +0.05 MHz/decade, and ±5 to ±10 nT/century, respectively. These are mostly comparable in magnitude to observed trends in the Atlantic region. The simulated Atlantic region trends in h m F 2 and f o F 2 are ~2.5 times larger than the estimated effect of enhanced greenhouse gases on h m F 2 and f o F 2 . The secular variation of the Earth's magnetic field may therefore be the dominant cause of trends in the Atlantic region ionosphere.