Dependence of foF2 long-term changes on solar activity and longitude Original Research Article

The analysis of long-term changes of the F2-layer critical frequency at mid-latitudes was carried out. The additional dependence of quiet monthly median values, foF2m, on sliding average over 11 years of Wolf number values was found. This allowed to isolate a linear on time trend of relative average annual values of foF2 which was not connected significantly to solar action. It is found that this trend is the sum of the trend P on account of the greenhouse effect, which almost does not depend on longitude, and the trend because of secular variations of the geomagnetic field M whose dependence on longitude is essential.