4th IAGA/ICMA/CAWSES Workshop on Long-Term Changes and Trends in the Atmosphere, Sodankylš, Finland, 4-8 September 2006

Chasing trends in noisy ionospheric data: the Sodankylä Ionosonde

Th.Ulich1, M. A. Clilverd2, M. J. Jarvis2, H. Rishbeth3

1Sodankylä Geophysical Observatory, Sodankylä, Finland,
2British Antarctic Survey, Cambridge, England, U.K.,
3University of Southampton, Southampton, England, U.K.


About a solar cycle and a half ago it was suggested that while changes of the chemical composition of the lower atmosphere will lead to "global warming" in the troposphere, they will be accompanied by "global cooling" in the mesosphere and lower thermosphere. Since the 1930s, and most comprehensively since the IGY in 1957, the ionosphere has been monitored regularly by vertical ionospheric sounders (ionosondes). These radars make rather straightforward measurements of ionospheric properties and the results are readily available in electronic form.

Here we use high-quality measurements from the Sodankylš ionosonde to point out problems in determining the long-term behaviour of such data sets. We show that in some sense the observed trends do depend on the way they are obtained. Special emphasis is given to low-pass filtering. Finally, we propose a search and comparison scheme for trend-finding.