Letter to the editor Spatial and seasonal variations of the foF2 long-term trends

Using a method suggested by the authors earlier, the long-term trends of the F2-layer critical frequency, foF2 are derived for a set of ionospheric stations with a wide latitudinal and longitudinal coverage. All the trends are found to be negative. A pronounced dependence on geomagnetic latitude is found, the trend magnitude increasing with the latter. No globe scale longitudinal e€ect in trends is detected. For the majority of the stations there is also a pronounced seasonal e€ect, the trend magnitude being higher in summer than in winter.