The distribution of electrons in the nighttime ionosphere

The mean nighttime electron density all over the world at a series of heights below the peak of the F2 layer in the quiet ionosphere has been deduced from an analysis of ionosonde data. The results are considerably more accurate than any that have hitherto been available, since a method of analysis has been used in which the low-lying ionization, corresponding to a plasma frequency below the lowest frequency for which echoes are observed, is taken into account. A number of independent theoretical papers published in the recent past predicted that the ionization in the nighttime ionosphere would assume what is now known as an ‘α-Chapman’ distribution. The results of the survey presented here show that the experimental observations of the electron density-height profiles are in remarkably close agreement with these predictions. The observed form of the variation of the electron density at fixed heights with latitude at night shows little or no seasonal effects and differs considerably from the daytime form. These latitude distributions remain quantitatively unexplained.