Diffusion des Echos au Voisinage des Fr quences Critiques de F 2

Abstract

From the study of ionospheric data recorded at Poitiers (46? N.) it appears that two sorts of scattered echoes take place in the neighbourhood of F2 critical frequencies. The first one seems to be produced by the multiplication of magneto-ionic components. This scattering ends with ground sunrise; it exists during both quiet and perturbed ionospheric conditions; it is far more frequent and stronger when the nights are longer. Its existence seems to be connected with nocturnal thermal and convective disorder. The second sort of scattering is mainly recorded during the second part of the night, after F2 penetration, at a virtual height of about 800 to 1,200 km. The sharp lower boundary of these reflections, which disappears a few moments after sunrise, leads to the postulation of an ionospheric G region, whose existence has been often discussed. This sort of scattering is connected with magnetically disturbed conditions and, chiefly, with meteoritic periods.