Long-term trends of <i>f</i>oE and geomagnetic activity variations

• Main Authors: A. V. Mikhailov, B. A. de la Morena
• Format: Article
• Language: English
• Published: Copernicus Publications 2003-03-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/21/751/2003/angeo-21-751-2003.pdf

A relationship between <i>f</i>oE trends and geomagnetic activity long-term variations has been revealed for the first time. By analogy with earlier obtained results on the <i>f</i>oF2 trends it is possible to speak about the geomagnetic control of the <i>f</i>oE long-term trends as well. Periods of increasing geomagnetic activity correspond to negative <i>f</i>oE trends, while these trends are positive for the decreasing phase of geomagnetic activity. This &quot;natural&quot; relationship breaks down around 1970 (on some stations later) when pronounced positive <i>f</i>oE trends have appeared on most of the stations considered. The dependence of <i>f</i>oE trends on geomagnetic activity can be related with nitric oxide variations at the E-layer heights. The positive <i>f</i>oE trends that appeared after the &quot;break down&quot; effect may also be explained by the [NO] decrease which is not related to geomagnetic activity variations. But negative trends or irregular <i>f</i>oE variations on some stations for the same time period require some different mechanism. Chemical pollution of the lower thermosphere due to the anthropogenic activity may be responsible for such abnormal <i>f</i>oE behavior after the end of the 1960s.<br><br><b>Key words. </b>Ionosphere (ionosphere-atmosphere interactions; ionospheric disturbances)