The worldwide behaviour of the twelve-month period (seasonal and non-seasonal) variation of the maximum electron densityN in theF2 layer appears noticeably different at noon and at midnight. In each case, it is controlled by the geomagnetic field and by the solar activity but, whilst at noon it shows large asymmetric features in the two hemispheres, at midnight, on the other hand, it is fairly symmetrical. It is not possible to account for the experimental results by assuming some external source of radiation producing non-seasonal effects. One is led to assume some terrestrial cause of asymmetry: for example, some effect of general circulation in the upper atmosphere. With respect to the correlation of electron density with solar activity, there appears again some noticeable asymmetry in the two hemispheres; in particular, the northern hemisphere appears more influenced by the solar hydrogen filaments, especially for increasing latitude. The explanation of these asymmetries, in terms of some direct influence of the sun, is also puzzling, so that, in this case, one is led to think of some more or less unknown effect of solar corpuscular (or electromagnetic) radiation on the upper atmosphere, for example on its movements or its conductivity. At the present stage of our knowledge, we cannot exclude the possibility of a similar cause for both the asymmetries of the twelve-month period variation and of the correlations with solar activity. Further observational evidence is required.