Plasma line data gathered at the Arecibo Observatory are used to examine relative variations in topside F region differential photoelectron fluxes in the 5- to 20-eV range. A spectral feature not found in present theoretically calculated spectra is noted near 15 eV. A new approach to the interpretation of the measured spectra is taken, which allows a qualitative estimate of the relative importance of different energy loss mechanisms. The altitude variation of the observed photoelectron flux energy spectra at the higher altitudes (h?350 km) and the lower energies (E≲10 eV) agrees quantitatively with the expected variation of the spectrum calculated on the assumption that the dominant energy loss mechanism is to the ambient thermal electron gas, as given by Schunk and Hays (1971).