A possible cause for the generation of unusually strong similar to 20-and 30-hour oscillations in the neutral wind of the lower thermosphere

The harmonic relationship between the diurnal and semidiurnal tides gives r ise to an elementary mathematical relationship that has intriguing conseque nces for secondary waves produced by non-linear interactions between the di urnal tide and planetary waves. A speculative theory is developed which pre dicts that, under certain conditions, these secondary waves can be amplifie d by non-linear interaction with the semidiurnal tide. A peculiar feature o f dynamics in the MLT region above Bulgaria is the presence of strong oscil lations with periods near 20 and 30 h, especially in the zonal wind compone nt. Observational evidence from a meteor radar at Yambol, Bulgaria suggests that the 20- and 30-h signals are produced as the result of non-linear int eractions of the type proposed by the novel theory. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.