Trends in the ionospheric E-region

A review of the methods used to extract f(o) E long-term trends from ionosonde observations is presented. The f(o)E trends revealed with the proposed approach may be positive/negative depending on the phase (decreasing/increasing) of the geomagnetic activity long-term variations - so called geomagnetic control, which also takes place in the F2-layer. This "natural" dependence is observed until the end of 1960s (on some stations later), then the geomagnetic control breaks down and positive trends or irregular f(o)E variations appear on many stations. The trends before the "break down" effect can be attributed to solar and geomagnetic activity long-term variations, which can be removed, the residual f(o)E trends being small and insignificant. Positive f(o)E trend observed over the last 30-year period can be explained by a 2.5 times decrease of [NO] at the E-region heights. The predicted "greenhouse" effects are too small even under a doubling CO2 scenario, therefore the observed f(o)E trends cannot be used either to confirm or to reject the "greenhouse" hypothesis. (C) 2006 Published by Elsevier Ltd.