Frequency functions of Venus nightside ion densities

The variability of the total and major ion densities in the nightside Venus ionosphere are studied by deriving an experimental frequency function for these quantities. The frequency function N(i) derived for the total ion density is found to be insensitive to division of the parent set into two approximately equal subsets representing dawn and dusk hemispheres, an upper and lower altitude interval, or two solar zenith angle intervals. The N(i) function is reasonably well peaked near the median value, and 78 percent 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 N(i).