Long-term trends of ionospheric electron density related to global warming

Abstract

Long-term trends of ionospheric electron density have been studied using vertical sounding measurements at 10 ionosonde stations from European, Asian, and American longitude sectors. The analysis focuses on studying the relationship of ionospheric F2 layer noontime peak electron density (NmF2) data covering a long time period of up to 71 years with the 30 cm solar radio flux index F30. The long-term behavior of 11-year sliding averages of noontime NmF2 data shows a substantial decrease from a stable reference level that is specific for each ionosonde station. The reference level is defined by a linear model of the noontime F2 layer 11-year sliding peak electron density NmF2* as a function of the associated 11-year sliding F30 solar activity index F30*. Whereas NmF2* is proportional to F30* within a small variability range of ± 1.5% over nearly two solar cycles until 1982, NmF2* decouples from this linear relationship with F30* afterwards. The deviation (reduction) may reach up to 20.6% in 2022 or up to about 5% per decade in the Northern hemisphere and up to 18.2% in 2022 or about 4% per decade in the Southern hemisphere. It is expected that such strong changes should have serious consequences for the accuracy of empirical ionosphere models utilizing a database that was established before the 1980s. For the first time, it has been demonstrated that there is a significant correlation between the observed long-term decrease in ionospheric electron density and the temperature anomaly (TA) measured at the Earth’s surface. This finding highlights a close connection between atmospheric changes at lower altitudes, as indicated by surface temperature records, and variations in the ionospheric electron density observed over extended periods. Similar to the temperature anomaly (TA), the concept of “electron density anomaly” (EDA) has been introduced to characterize deviations in ionospheric electron density from expected values. Analysis indicates that the EDA exhibits a more pronounced effect in the Northern Hemisphere compared to the Southern Hemisphere. A similar pattern occurs with the TA, which supports the idea that a shared physical mechanism may explain both the EDA and TA phenomena. This long-term reduction of the electron density reflects ongoing modifications in the structure and behavior of the Earth’s magnetosphereionosphere-thermosphere (MIT) system. The findings suggest that these changes are closely linked to the increasing concentrations of greenhouse gases accumulating in the thermosphere. As greenhouse gas levels rise, their effects extend beyond the lower atmosphere, impacting the upper atmospheric regions and contributing to observable trends in ionospheric electron density.