geomagnetic control concept

Abstract. A previous approach to the ionospheric long-term trend analysis has been applied to the foF1 observations from Slough and Rome in order to investigate a possible relation-ship between the foF1 and the long-term variation of geo-magnetic activity. A 40-year period, starting in 1962, has been used for the analysis. According to the results obtained earlier for F2 and E-region trends, geomagnetic control of the long-term variation has also been revealed for the foF1. Thus, it is now possible to speak about the geomagnetic con-trol of the ionospheric trends in the whole ionosphere. This is not surprising as the Earth’s ionosphere is a single entity that is strongly controlled, either directly or indirectly, by the magnetic field. As with the F2-region, this geomagnetic control is provided via neutral composition and temperature changes. A very long-term (centennial) increase in geomag-netic activity in the 20th century is seen in the long-term foF1 variations as well. After its removal, the residual foF1 trends are very small and insignificant. In principal, this means that the observed foF1 long-term variations have a natural origin and can be attributed to solar and geomagnetic activity long-term variations. However, the situation in the thermosphere has been changing since 1997 and available foF2 observa-tions at the two stations reveal information about the “break down ” of the geomagnetic control in the F2-region. Possible reasons of these changes are discussed. Keywords. Atmospheric composition and structure (Ther-mosphere – composition and chemistry) – Ionosphere (Ionosphere-atmosphere interactions; Mid-latitude iono-sphere