On the longitudinal structure of the transient day-to-day variation of thesemidiurnal tide in the mid-latitude lower thermosphere - I. Winter season

The longitudinal structure of the day-to-day variations of semidiurnal tide amplitudes is analysed based on coordinated mesosphere/lower thermosphere wind measurements at several stations during three winter campaigns. Possib le excitation sources of these variations are discussed. Special attention is given to a nonlinear interaction between the semidiurnal tide and the da y-to-day mean wind variations. Data processing includes the S-transform ana lysis which takes into account transient behaviour of secondary waves. It i s shown that strong tidal modulations appear during a stratospheric warming and may be caused by aperiodic mean wind variations during this event.