Global variation in the long - term seasonal changes 1 observed in ionospheric F - region data

8 Long-term variability has previously been observed in the relative magnitude of annual and 9 semi-annual variations in the critical frequency (related to the peak electron concentration) of 10 the ionospheric F2 layer (foF2). In this paper we investigate the global patterns in such 11 variability by calculating the time varying power ratio of semi-annual to annual components 12 seen in ionospheric foF2 data sequences from 77 ionospheric monitoring stations around the 13 world. The temporal variation in power ratios observed at each station was then correlated 14 with the same parameter calculated from similar epochs for the Slough/Chilton dataset (for 15 which there exists the longest continuous sequence of ionospheric data). This technique 16 reveals strong regional variation in the data which bear a striking similarity to the regional 17 variation observed in long-term changes to the height of the ionospheric F2 layer. We argue 18 that since both the height and peak density of the ionospheric F2 region are influenced by 19 changes to thermospheric circulation and composition, that the observed long-term and 20 regional variability can be explained by such changes. In the absence of long-term 21 measurements of thermospheric composition, detailed modelling work is required to 22 investigate these processes. 23