Detailed diurnal variations of F2 layer ionization in true heights have been obtained over the Tamanrasset meridian (5°S to 30°N) for 20 almost consecutive days in June and July 1966. Our improved time and latitude resolution in charting the evolution of the maximum ionization parameters, ƒ0F2 and hMF2, through the entire intertropical range made it possible for us to follow the movements of the two areas of greatest density known as ionization ‘crests’; their diurnal displacement on either side of the magnetic equator was found to differ from theoretical simulations, even those which take neutral wind into account. The asymmetry of their pattern in latitude and local time varies from one day to another much more than expected. The ‘summer’ side northern crest changes most; the maximum latitude extension of this crest varies in a complex manner with ap magnetic activity indices. Fast evolution phases, during early crest formation, and the deviation of the ionization structure from its expected ‘arch’ magnetic symmetry suggest a new type of low-latitude transitory process.