Evidence for nonlinear coupling of planetary waves and tides in the lower thermosphere over Bulgaria Dora Pancheva* Geophysical Institute, Bulgarian Academy of Sciences, So®a, Bulgaria Received 23 November 1998; accepted 7 June 1999 Abstract Analyses of hourly values of zonal and meridional wind near 95 km observed by meteor radar at Yambol (42.58N, 26.68E) during January 1991±June 1992 indicate the presence of planetary waves with prevailing periods of 1.5±2.5, 4±6, 9±10 and 16±18 days. About 20% of the whole power of atmospheric motions is connected with these waves, so they play an important role in the dynamics of the mesosphere-lower thermosphere (MLT) region. By dynamic spectral analysis applied to the hourly neutral wind and to the calculated hourly values of tidal amplitudes it has been demonstrated that there is considerable modulation of tidal amplitudes by planetary waves in the neutral wind, as this process is better expressed in the semidiurnal tides. The nonlinear interaction between tides and planetary waves is studied by bispectral analysis. The results of these analyses indicate again that the nonlinear interactions between semidiurnal tides and planetary waves with periods 2±20 days are stronger than those of the diurnal tides and planetary waves. A peculiar feature of dynamics in the MLT region above Bulgaria is the presence of strong oscillations with periods of 20 and 30 h, which indicate signi®cant nonlinear coupling between them. 7 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction The latitudinal and vertical average behaviour of at- mospheric tides as a function of season in the meso- sphere and lower thermosphere (MLT) region is quite well understood (Manson et al., 1989; Vincent et al., 1989). However, the strength of the solar tides in the MLT region can be rather variable with time scales of a few days to three weeks (synoptic periods) (Avery et al., 1989; Fraser et al., 1989; Williams and Virdi, 1989; Huuskonen et al., 1991). Variability in the amplitude and phase of the solar tides can be caused by a num- ber of mechanisms: (1) changes in the tidal forcing; (2) propagation through changing background winds; (3) additional energy near the tidal frequencies due to local or synoptic scale disturbances; (4) nonlinear inter- actions between the tides, or between the tides and pla- netary waves (Cevolani and Kingsley, 1992; Pancheva and Mukhtarov, 1994; Mitchell et al., 1996). Some ob- servations indicate the presence of subsidiary com- ponents close to tidal periods and this eect can be explained by nonlinear interactions between tides and planetary waves (Teitelbaum and Vial, 1991). The pro- cess of nonlinear coupling can generate two secondary waves whose frequencies are the sum and dierence of the frequencies of the primary waves (tide and plane- tary wave). Then these two secondary waves beat with the primary tide, modulating its amplitude with the planetary wave period. Modulation of the semidiurnal tidal amplitudes with a period near 2 days is demon- Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 115±132 1364-6826/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S1364-6826(99)00032-2 * Present address: University of Wales, Department of Phy- sics, Aberystwyth, Ceredigion SY23 3BZ, UK. Tel.: +44- 1970-621902; fax: +44-1970-622826. E-mail address: ddp@aber.ac.uk (D. Pancheva).