Long-term Behaviour of the Ionospheric F2 Layer Peak Height on a Global Scale.

Thomas Ulich and Esa Turunen

Geophysical Observatory, FIN-99600 Sodankylä, Finland.

Extensive model calculations show that increasing amounts greenhouse gas concentrations in the air cause a cooling of the entire middle and upper atmosphere. Based on a scenario of doubling the CO2 and CH4 concentrations, most authors predict a cooling of the order of 10 K in the mesosphere. Observational evidence of cooling is scarce, but tentatively existing. Latest temperature measurements by lidar and from satellites show cooling trends. However, these cover only short time intervals, in the order of one or two solar cycles. Evidence in the form of increasing occurence of noctilucent clouds is suggested. Such observations are difficult to quantify reliably in a long-term data set. Lowering of the reflection heights of low frequency radio waves has been presented as evidence of cooling. At thermospheric altitudes Rishbeth (1990) predicted a cooling by 50 K to cause lowering of the altitude of maximum electron density in the F2 region by 20 km. Observational evidence for such lowering was presented by Bremer (1992), who investigated almost 40 years of data from ionospheric vertical soundings at one single location in mid-latitudes. We examine a large set of ionospheric data from different stations all around the world. While clear cooling is evident at many sites, some ionosonde records do not show such a trend. The interpretation of these observations is discussed.

Presented in session: 2.18