2nd European Space Weather Week, ESTEC, Noordwijk, The Netherlands, 14-18 November 2005

Signs of global change in the upper atmosphere

Th. Ulich

Sodankylä Geophysical Observatory, Sodankylä, Finland.

Abstract

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 a broad range of different instruments. These experiments include radiowave propagation studies, measurements of absorption of cosmic radio noise, and in situ measurements by rocket-borne instruments to name but a few. However, vertical ionospheric sounders (ionosondes) provide possibly the most important data sets for studies of the long-term behaviour of the upper atmosphere. Ionosondes are in operation at a large number of observatories world-wide and their data are readily available from data centres. They typically make one measurement per hour from which a multitude of parameters can be obtained. For instance, at Sodankylš 13 standard parameters are extracted from each measurement on a routine basis and soundings are performed half-hourly. Here we give an overview of long-term change observed in the upper atmosphere with emphasis on ionosonde observations. Some critical issues of comparability of findings by various authors are discussed.