Signature of the quasi-27-day oscillation in the MLT and its relation with solar irradiance and convection

摘要

Intermittent occurrence of the quasi-27-day oscillation is observed in the mesosphere and lower thermosphere (MLT) zonal wind in the long term database over three southern hemispheric Brazilian locations, i.e. Sao Joao do Cariri (7.4 degrees S, 36.5 degrees W), Cachoeira Paulista (22.7 degrees S, 45 degrees W) and Santa Maria (29.7 degrees S, 53.7 degrees W). The oscillation shows a peak amplitude of similar to 15 m/s in the lower MLT. To determine the plausible sources of the quasi-27-day oscillation, the variation of the solar Ly-alpha flux and outgoing longwave radiation (proxy for convection) have been looked into. The oscillation shows considerable consistency with the solar UV flux implying potential solar influence on excitation. The oscillation in the MLT also exhibits good correlation with the outgoing longwave radiation at Cachoeira Paulista indicating plausible influence of lower atmospheric convective activity. Non-concurrent occurrence of the oscillation among the observational stations indicates potential role of local geophysical conditions. The zonal background wind in the MLT might cause dissipation of the upward propagating waves (modulated by 27-day oscillation) and hence imprint the lower atmospheric 27-day signature in the MLT.