Longitudinal modulation of the O/N2 column density retrieved from TIMED/GUVI measurement

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.