Trends in the solar quiet geomagnetic field variation linked to the Earth's magnetic field secular variation and increasing concentrations of greenhouse gases

摘要

Long-term trends in solar quiet geomagnetic field variation (Sq) are studied in connection to the Earth's magnetic field secular variations and increasing greenhouse gas concentrations. Sy is mainly caused by ionospheric current systems that flow in the E region and depends, among other variables, on the ionospheric conductivities. These conductivities in tum depend on the Earth's main magnetic field (B) and the electron concentration in the E region, for which f_oE is a measure of its peak value. Since B shows secular variation, induced long-term changes in Sy might be expected. Another possible mechanism that would be able to induce Sq trends is the increasing concentrations of greenhouse gases that produce a cooling effect in the upper atmosphere and, according to model predictions and experimental results, an increasing trend in f_oE. To detect if both mechanisms mentioned arc able to induce trends in Sy, the Sq variation of the horizontal intensity (H) of three magnetic observatories (Apia, Fredericksburg, and Hermanus), for which B is decreasing, is analyzed for the period 1960-2001. We find significant increasing trends (6.6%, 5.4%, and 9.9%, respectively) which may be partially accounted for by B secular variations in the respective sites. The Sq trend expected from the theoretically predicted f_oE increase is low (～0.5%), although positive, as is the observed trend.