Abstract
Abstract. A relationship between foE trends and geomagnetic activity long-term variations has been revealed for the first time. By analogy with earlier obtained results on the foF2 trends it is possible to speak about the geomagnetic control of the foE long-term trends as well. Periods of increasing geomagnetic activity correspond to negative foE trends, while these trends are positive for the decreasing phase of geomagnetic activity. This "natural" relationship breaks down around 1970 (on some stations later) when pronounced positive foE trends have appeared on most of the stations considered. The dependence of foE trends on geomagnetic activity can be related with nitric oxide variations at the E-layer heights. The positive foE trends that appeared after the "break down" effect may also be explained by the [NO] decrease which is not related to geomagnetic activity variations. But negative trends or irregular foE 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 foE behavior after the end of the 1960s.Key words. Ionosphere (ionosphere-atmosphere interactions; ionospheric disturbances)
Highlights
Ionospheric parameter long-term trends have been widely discussed during the last decade
This interest is due to possible anthropogenic impact on the Earth’s atmosphere, and the ionospheric trends may serve as an indicator of such changes in the upper atmosphere
The main results of our analysis may be summarized as follows: 1. Using a newly proposed approach to the foE trend analysis, it was shown for the first time the relationship between foE trends and geomagnetic activity long-term variations
Summary
Ionospheric parameter long-term trends have been widely discussed during the last decade. The main efforts have been directed towards the F2-region parameter long-term trends analysis, since these observations are the most abundant and consistent, while trends in the Eregion were considered only by some researchers (Givishvili and Leshchenko, 1993, 1995, 1996, 1998; Bremer, 1998, 2001; Sharma et al, 1999) The result of these analyses is that positive foE trends are the most probable and are presumably due to a decrease in the neutral NO concentration at the E-region heights (Danilov and Smirnova, 1997; Bremer, 1998; Danilov, 2001). Since the late 1950s, this ratio demonstrates a pronounced decrease which can be related only to a [NO] decrease at the E-region heights
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