Variations in the Arctic upper atmosphere since the early 20th century: Relation to Arctic surface warming and cooling

We have studied long-term ionospheric variations using data from the Tromsø ionosonde since 1932. To investigate Arctic ionospheric variations in relation to the Arctic warming and cooling before the 1970s, we have developed a Tromsø ionosonde database that makes it possible to study the daily variations over about 90 years. During the 1940s to 1960s, when the Arctic surface was cooling, a gradually increasing trend in the critical frequency of the F2 layer, foF2, was observed in the Tromsø ionosonde data with a rate of 0.022 ± 0.011 MHz/year. This can be explained by the Earth's atmosphere global energy balance. As for the trend of foF2 since 2001, there is a clear decrease with a rate of -0.046 ± 0.007 MHz/year. These trends are qualitatively consistent with the changes in the upper atmosphere that can be inferred from the cooling and warming of the Arctic surface. Studying these changes is important for understanding the coupling of the whole atmosphere. Further studies will require verification of the plausible scenario using global models.