Abstract We present an analysis of 23 years of thermal plasma measurements in the topside ionosphere from the Defense Meteorological Satellite Program (DMSP) spacecraft. The H + /O + ratio and density vary dramatically with the solar cycle; cross‐correlation coefficients between E 10.7 and the daily averaged densities are greater than 0.85. The ionospheric parameters also vary dramatically with season, particularly at latitudes away from the equator where the solar zenith angle varies greatly with season. There are also 27 day solar rotation periodicities in the density, associated with periodicities in the directly measured solar EUV flux. Empirical orthogonal function analysis captures over 95% of the variation in the density in the first two principal components. The first principal component (PC1) is clearly associated with the solar EUV while the second principal component (PC2) is clearly associated with the solar zenith angle variation. The magnitude of the variation of the response of the topside ionosphere to solar EUV variability is shown to be closely related to the ionospheric composition. This is interpreted as the result of the effect of composition on the scale height in the topside ionosphere and the “pivot effect” in which the variation in density near the F 2 peak is amplified by a factor of e at an altitude a scale height above the F 2 peak. When the topside ionosphere is H + dominated during solar minimum, DMSP may be much less than a scale height above the F 2 peak while during solar maximum, when it is O + dominated, DMSP may be several scale heights above the F 2 peak.