IAGA 2005 Scientific Assembly, Toulouse, France, 18-29 July 2005

Long-term trends in foF2: a comparison of results of various methods

J. Lastovika1, A. V. Mikhailov2, Th. Ulich3, J. Bremer4, A. G. Elias5, N. O. de Adler5, V. Jara6, R. Abarca del Rio6, A. J. Foppiano6, E. Ovalle6 and A. D. Danilov7

1Institute of Atmospheric Physics, Prague, Czech Republic,
2IZMIRAN, Troitsk, Moscow Region, Russia,
3Sodankylä Geophysical Observatory , Sodankyla, Finland,
4Leibnitz Institute of Atmospheric Physics, Kuehlungsborn, Germany,
5Universidad Nacional de Tucuman, Tucuman, Argentina,
6Universidad de Concepcion, Concepcion, Chile,
7Institute of Applied Geophysics, Moscow, Russia.


Results of various authors on long-term trends in foF2, i.e. in the maximum electron density in the ionosphere, and their interpretation do not reveal a consistent pattern. Therefore a joint analysis of a carefully selected data set was performed by six teams, which used different approaches to trend determination. High-quality data of station Juliusruh (54.6°N, 13.4°E) for noon (average from 10-14 UT) with a small number of gaps were used for the period 1976-1996 (two solar cycles from minimum to minimum). Juliusruh is relatively sensitive to geomagnetic activity as almost subauroral station, which might play some role in interpretation of trend results. Various methods provide results, which differ to some extent, even when one co-author applies different methods. Another source of differences is application of various ways of removal (or at least large suppression) of the effect of solar (and geomagnetic) activity. Also interpretation is not unique - co-authors consider either the long-term change in geomagnetic activity, or the anthropogenic (greenhouse) effect to be predominantly responsible for trends. Nevertheless, there is some output from the joint analysis. All trends are either negative or statistically insignificant. Data corrections with sunspot number (R), F10.7 adjusted to the Sun-Earth distance, and observed F10.7 result in somewhat different trends; the observed F10.7 appears to be the best solar proxy. The Juliusruh dip angle increased over the period 1976-1996 from 68.6 to 68.8°, which resulted in a quite negligible effect on trends. The contradiction between the results of various authors has partly been explained or removed. Limited progress has been reached in interpretation of the observed trends in terms of geomagnetic or anthropogenic origin.