A diurnal modulation of the lunar tide in the upper atmosphere

Analysis of geomagnetic data from several stations in the equatorial region shows that the amplitude of the lunar semimonthly tide frequently does not have the expected symmetry about noon in its diurnal variation. There is a distinct tendency for larger amplitudes in the morning. At many stations this effect is largest in equinoctial months, though there are regional differences. Analysis of data from different years confirms its presence. The interaction between solar and lunar tides in the middle atmosphere may be responsible for the observed modulation.