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.