Abstract
Abstract. The ionospheric F2-layer parameter long-term trends are considered from the geomagnetic control concept and the greenhouse hypothesis points of view. It is stressed that long-term geomagnetic activity variations are crucial for ionosphere long-term trends, as they determine the basic natural pattern of foF2 and hmF2 long-term variations. The geomagnetic activity effects should be removed from the analyzed data to obtain real trends in ionospheric parameters, but this is not usually done. Only a thermosphere cooling, which is accepted as an explanation for the neutral density decrease, cannot be reconciled with negative foF2 trends revealed for the same period. A more pronounced decrease of the O/N2 ratio is required which is not provided by empirical thermospheric models. Thermospheric cooling practically cannot be seen in foF2 trends, due to a weak NmF2 dependence on neutral temperature; therefore, foF2 trends are mainly controlled by geomagnetic activity long-term variations. Long-term hmF2 variations are also controlled by geomagnetic activity variations, as both parameters, NmF2 and hmF2 are related by the F2-layer formation mechanism. But hmF2 is very sensitive to neutral temperature changes, so strongly damped hmF2 long-term variations observed at Slough after 1972 may be considered as a direct manifestation of the thermosphere cooling. Earlier revealed negative hmF2 trends in western Europe, where magnetic declination D<0 and positive trends at the eastern stations (D>0), can be related to westward thermospheric wind whose role has been enhanced due to a competition between the thermosphere cooling (CO2 increase) and its heating under increasing geomagnetic activity after the end of the 1960s.
Highlights
Long-term trends of ionospheric parameters are widely discussed during the last 15 years. These trends both in the F2 and E regions are very small and have no practical importance, they may serve as an indicator of long-term changes in the Earth’s upper atmosphere, and their investigation may be interesting from this point of view
Despite obvious contradictions with the ionospheric trend observations, the greenhouse hypothesis remains very popular. This is due to a general interest in the anthropogenic impact on the ecological system and on the Earth’s upper atmosphere
Not denying the very fact of the thermospheric density decrease, it should be kept in mind that the mechanism of this decrease may not be totally related to the thermosphere cooling
Summary
Long-term trends of ionospheric parameters are widely discussed during the last 15 years. These trends both in the F2 and E regions are very small and have no practical importance, they may serve as an indicator of long-term changes in the Earth’s upper atmosphere, and their investigation may be interesting from this point of view. Despite obvious contradictions with the ionospheric trend observations, the greenhouse hypothesis remains very popular. This is due to a general interest in the anthropogenic impact on the ecological system and on the Earth’s upper atmosphere. Not denying the very fact of the thermospheric density decrease, it should be kept in mind that the mechanism of this decrease may not be totally related to the thermosphere cooling
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