Ph.D Thesis, Sodankylä Geophysical Observatory Publications, 87, 2000

Solar Activity Variations
and Long-Term Trends in the Ionosphere

Thomas Ulich

Dept of Physical Sciences, University of Oulu, Oulu, Finland


Sunspot numbers have traditionally been used as a measure for solar activity. Recently, a solar activity parameter, viz. the length of the sunspot cycle, was found to correlate well with the long-term evolution of northern hemispheric temperature. The question implied by this result is what is the relative importance of external (e.g. solar), internal (i.e. Earth-bound nature), and anthropogenic effects on climate change.

In the first chapter, a novel technique for estimating sunspot cycle length is introduced, which is based upon determining the time at which half of the sunspots of a given cycle have occurred (Median Time). Traditional methods are shown to have uncertainties of several months in sunspot cycle length due to uncertainties in the determination of the times of sunspot minima and maxima. The median method, however, is very stable with respect to the choice of sunspot minima. The method is able to reduce the uncertainty in cycle length by a factor of 30-50.

The later chapters discuss the possible ionospheric effects of changed concentrations of greenhouse gases in the atmosphere. The mesosphere and lower thermosphere are expected to cool by up to 50 K if the concentrations of mesospheric carbon dioxide and methane were doubled. Consequently, the height of the maximal electron concentration is predicted to lower down as a consequence of a cooled and thus shrinking upper atmosphere. This so-called F2-layer peak height can be obtained from standard ionosonde parameters by means of different empirical relations.

The F2-layer above Sodankylä, Finland, has lowered at a rate of about 4 km per decade since 1958. The F2-layer heights differ to some extent from one another depending on the empirical relations employed for their estimation. Moreover, they are governed by a number of other influences, which require possible long-term trends to be extracted by means of multi-parameter models. The compositon of these models is somewhat arbitrary and each model yields slightly different trends. While at Sodankylä the trends are rather consistent, at other ionosonde stations certain combinations of a specific empirical height formula and a trend model give misleading results. In a further analysis of altogether 66 ionosondes world-wide, trends and their errors are studied in detail based upon four empirical formulae for F2-layer peak height as well as twelve different trend models.

When applying any of the discussed methods to the data of a given ionosonde, great care must be taken in order to verify its applicability. It is shown that the ionosphere clearly changes on time scales much longer than a sunspot cycle. However, a large variety of trends is found which cannot be explained simply by increased amounts of atmospheric greenhouse gases.

ISBN 951-42-6023-6, ISSN 1456-3673.

Tiedote lehdistölle (Press release, in Finnish)

The thesis is available as a PDF file from the author via e-mail. This Ph.D. Thesis is (c)2000 by Sodankylä Geophysical Observatory, all rights reserved. If you want to use this material for publications, education, presentations, or whatever, you have to contact the author first. The same applies for any reproduction by whatever means.