ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-26-3793-2008 Ionospheric F1 layer long-term trends and the geomagnetic control concept Mikhailov A. V. 1 Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Troitsk, Moscow Region 142190, Russia 28 11 2008 26 12 3793 3803 Copyright: © 2008 A. V. Mikhailov 2008 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/26/3793/2008/angeo-26-3793-2008.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/26/3793/2008/angeo-26-3793-2008.pdf

A previous approach to the ionospheric long-term trend analysis has been applied to the <I>fo</I>F1 observations from Slough and Rome in order to investigate a possible relationship between the <I>fo</I>F1 and the long-term variation of geomagnetic 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 <I>fo</I>F1. Thus, it is now possible to speak about the geomagnetic control 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 geomagnetic activity in the 20th century is seen in the long-term <I>fo</I>F1 variations as well. After its removal, the residual <I>fo</I>F1 trends are very small and insignificant. In principal, this means that the observed <I>fo</I>F1 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 <I>fo</I>F2 observations 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.

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