The frequency with which the total ion density Ni and constituent densities O+, H+, and molecular ion density O2 ++M29 + (=[O2 +>+[NO +>+[CO +>+[N2 +>) fall between equal increments of the logarithm of the density is derived. The individual measurements are first normalized by dividing them by their median value at the altitude of measurement. This process removes the known altitude variation from the sets. The resulting frequency functions are assumed to result from temporal variations alone. The data used in deriving the functions are restricted to 120¿--180¿ solar zenith angle (SZA) and altitudes between 140 and 900 km. The frequency function derived for Ni is found to be insensitive to division of the parent set into two approximately equal subsets representing division into dawn and dusk hemispheres, an upper and lower altitude interval, or two SZA intervals. The Ni function is found to be reasonably well peaked near the median value, and 78% of the normalized values fall within a factor of 2.8 of the median value. The frequency of occurrence decreases more rapidly on the high side of the median than on the low side. The frequency functions for the constituent ions are similar to that for Ni, with the percentage of normalized densities falling within a factor of 2.8 of the median value ranging from 82% for O2 ++M29 + to 71% for H +. The similarities and dissimilarities between the frequency function for O + and those for H + and O2 ++M29 + are discussed in relation to the role of O + in producing the H + and O2 ++M29 + constituents. ¿ American Geophysical Union 1987