Solar variability and cooling of the upper atmosphere as seen by long-term measurements of cosmic radio noise.

Thomas Ulich, Esa Turunen and Hilkka Ranta

Geophysical Observatory, FIN-99600 Sodankylä, Finland

Model estimates of the effects of increasing greenhouse gas emissions predict cooling of the upper atmosphere. Experimental evidence for the cooling is, however, scarce and mostly based on short time scales. Since the absorption of cosmic radio noise depends on electron-neutral collision frequency, which in turn is a function of temperature, the cosmic radio noise measurements reflect long-term changes of temperature. We analyse the long-term recordings of the Finnish riometer chain for signs of cooling of the upper atmosphere and of solar variability. The Finnish riometer chain comprises 8 riometers, some of which started operation already in 1964; the chain covers the large latitudinal range between 60° 31' N and 69° 45' N.