Defense Meteorological Satellite Program (DMSP) spacecraft at 840 km observe a 27-day variation in plasma density and temperature at all subauroral latitudes. At the peak of the solar cycle, evening-sector variations are ~40--50% in plasma density and ~5--10% in electron temperature. The percent of variation decreases with decreasing solar activity to or below the threshold of detectability for the DMSP sensors. We compare in situ densities with simultaneous observations of total electron content but find that similar variations are not present in a consistent manner. Thus we conclude that the variations exist mostly as topside phenomena. However, comparisons with variations in the radio flux at 10.7 cm (F10.7), a standard proxy for solar EUV, indicate that the topside variations are driven by the solar EUV flux. When compared with the variations of several alternative proxies for the solar EUV flux, we find that only one of them correlates better than F10.7. Because the topside ionosphere couples with the plasmasphere, we suggest that similar 27-day variations should appear in plasmaspheric parameters.