Six years of magnetic quick‐run records made at the low‐latitude M'Bour Observatory have been analyzed. It was found that the frequency of occurrence of pulsations Pc 3 and 4 presents a marked diurnal variation characterized by two principal maxima, at sunrise and around local noon. The curves of occurrence are compared with the diurnal variation of the critical frequency of the ionospheric F2 layer at Dakar. The morning maximum of pulsations corresponds to low values of foF2 which contrasts with high values corresponding to the noon maximum. In conclusion the frequency of occurrence of the geomagnetic pulsations Pc does not appear to be directly related to the critical frequency of the F2 layer. Copyright 1969 by the American Geophysical Union.
