The longitudinal variation of thermospheric O/N2 column density ratio is studied, based on the TIMED/GUVI measurements collected during 2002-2007. In October, a four-peaked longitudinal fluctuation in O/N2 is found. The wavenumber-4 fluctuation in equatorial O/N2 is generally stronger at lower solar activity, and maximizes in the morning of boreal summer and autumn. In boreal winter, the wavenumber-4 component gives way to the wavenumber-3. Such seasonal variation and solar activity dependence are consistent with those of the diurnal eastward-propagating mode with wavenumber-3, but the local time dependence is not. The daytime wavenumber-4 pattern in O/N2 does not shift eastward in local time as expected if it is directly connected to the DE3. No definite explanation is found for this structure, but a consideration of various mechanisms suggests that the vertical wind effect may be the most potential candidate.