Abstract
Abstract. Long-term variability has previously been observed in the relative magnitude of annual and semi-annual variations in the critical frequency (related to the peak electron concentration) of the ionospheric F2 layer (foF2). In this paper we investigate the global patterns in such variability by calculating the time varying power ratio of semi-annual to annual components seen in ionospheric foF2 data sequences from 77 ionospheric monitoring stations around the world. The temporal variation in power ratios observed at each station was then correlated with the same parameter calculated from similar epochs for the Slough/Chilton data set (for which there exists the longest continuous sequence of ionospheric data). This technique reveals strong regional variation in the data, which bears a striking similarity to the regional variation observed in long-term changes to the height of the ionospheric F2 layer. We argue that since both the height and peak density of the ionospheric F2 region are influenced by changes to thermospheric circulation and composition, the observed long-term and regional variability can be explained by such changes. In the absence of long-term measurements of thermospheric composition, detailed modelling work is required to investigate these processes.
Highlights
The annual variation in the peak electron concentration of the ionospheric F region has long been known to vary with geomagnetic location
This global variation shares regional similarities with global trends seen in long-term changes in hmF2
Since both these parameters are influenced by thermospheric composition and/or circulation, we argue that changes to thermospheric composition may be responsible for global variability in both parameters, though the cause of such global compositional changes is still the subject of much research
Summary
The annual variation in the peak electron concentration of the ionospheric F region has long been known to vary with geomagnetic location. The ionosphere above other stations, such as Stanley in the Falkland Islands, is dominated by a semi-annual variation, with peak F region electron concentrations occurring at the equinoxes. For a station at a similar geographic latitude but on a “far from pole” longitude, such as Stanley, a semi-annual variation results as compositional changes between equinox and winter months are relatively small compared with the associated change in solar zenith angle. This leads to ionospheric densities that peak at the equinoxes
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