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Ann. Geophys., 24, 2533-2541, 2006
www.ann-geophys.net/24/2533/2006/
© European Geosciences Union 2006
Ionospheric long-term trends: can the geomagnetic control and the greenhouse hypotheses be reconciled?
A. V. Mikhailov
Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Troitsk, Moscow Region 142190, Russia
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.
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