Abstract
Abstract. It is shown that after elimination of the dominant influence of solar activity, the height hmF2 of the maximum ionisation of the ionosphere (which is inversely linearly related to M(3000)F2 as described for example by the Shimazaki formula) shows a marked decrease during the recent solar minimum (2005 to 2009) of about 20 km. This is in accordance with results of studies of the decreased neutral density of the lower thermosphere and the density decrease at about 400 km during the unusual recent solar minimum. It is also shown that there is a trend reversal in the height of the ionosphere hmF2 in about 1963/1964 with a negative trend from 1942 to 1963 and a positive trend from 1964 to 2005. This is in contrast to the results reported in most other publications of ionospheric long-term trends. There is no significant trend in foF2 and, particularly, none of the effects described above for hmF2 could be found in foF2.
Highlights
1 Introduction A database of about 1.5 million monthly hourly median values of each of the two ionospheric parameters foF2 and M(3000)F2 measured worldwide between 1941 and 2009 was created recently (Damboldt and Suessmann, 2011), using data available in the World Data Centres (WDC). This large collection of measurements provides the basis for the study of long-term variations of the ionosphere, up to almost 7 solar cycles
For the derivation of trends in ionospheric properties it is advisable to remove the strong influence of the solar cycle if data is only available for a few solar cycles, which is the case here, as observations began only in 1941
In the present paper an alternative method is applied, namely the comparison with an ionospheric model, in this case with the CCIR ionospheric prediction model (ITU, 2010). This model considers solar activity as the main parameter and as an empirical model based on ionospheric observations made in the years around the International Geophysical Year (IGY) it inherently includes the effects of short-term variations of the Earths magnetic field
Summary
A database of about 1.5 million monthly hourly median values of each of the two ionospheric parameters foF2 and M(3000)F2 measured worldwide between 1941 and 2009 was created recently (Damboldt and Suessmann, 2011), using data available in the World Data Centres (WDC). This large collection of measurements provides the basis for the study of long-term variations of the ionosphere, up to almost 7 solar cycles
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