Analysis Of The Planetary Waves In The Mesosphere And Lower Thermosphere

Many scale waves exist in mesosphere and lower thermosphere (MLT) region—about 80-120 km, and these waves can be identified in the periodic variations of atmospheric wind, temperature, density, surface pressure, ionized components and airglow. Planetary waves are part of such waves and are one of the most important dynamic processes in the upper atmosphere. If deeply studying the origin, evolution and dissipation of the planetary waves, and also their temporal/spatial characteristics, we can further know how the atmospheric layers dynamically interact on each other and availably explain many phenomena in the upper atmosphere and ionosphere.In the present study, the wind data observed by Wuhan meteor radar (30.5oN, 114.3oE), Adelaide MF radar (35oS, 138oE) and Yamagawa MF radar (31.2oN, 130.6oE) were used to investigate the spatial characteristics and seasonal variability of the MLT 16-day waves and 6.5-day waves which belong to quasi-periodic waves and propagate westly. We also discussed the potential sources of these waves. Furthermore, we used the Wuhan wind data and foEs data to survey the possible mechanism of the summer 2-day waves in the MLT region affect the summer abnormity of midlatitude Es layer. Main results of our study are outlined as follows.1. The 16-day waves in the mesosphere and lower thermosphere over Wuhan and AdelaideOn the whole, the zonal components of the 16-day waves are larger than the meridional ones at both sites except for several occasional intervals. The strong 16-day waves generally appear in the spring/autumn and winter months, but summer waves occasionally occur. Through analyzing the wave phase variations with height, we found that the 16-day waves in summer hemisphere may come from the winter southern hemisphere. The 16-day wave penetrates to the MLT region along lines of weak eastward background wind. However, the characteristics of the 16-day waves at both sites also show some differences. 16-day waves are very intensive at greater heights in Wuhan MLT region but at lower heights over Adelaide. 2. The 6.5-day waves in the mesosphere and lower thermosphere over Wuhan, Adelaide and YamagawaThe 6.5-day planetary waves in the MLT region over three stations have prominent seasonal variability: these waves exhibit strong energies near equinoxes and weak activities near solstices in both zonal and meridional winds, however, the winter waves occasionally occur. In our near four year observations, we do not find the quasi-biennial variation of mesospheric 6.5-day waves, which was observed in equatorial MLT region. Similar as mentioned in the results about 16-day waves, the strong 6.5-day waves appear in the higher altitude range over Wuhan and Yamagawa (midlatitude of the northern hemisphere) but in the lower altitude range over Adelaide (midlatitude of the northern hemisphere).Strong waves appear not only in the westward background winds but also in the eastward mean flows. This result does not consist with the equatorial observation, i.e., the zonal 6.5-day wave behave intensively when the background wind is westward in the equatorial MLT region. Moreover, it is worthy of note that partial occurrences of the zonal 6.5-day waves are near when and where the background winds alternate between easterlies and westerlies. It implies the potential relation between the atmospheric instability due to the reverse of background winds and the 6.5-day waves. During the global scale 6.5-day wave events of April-May in 2003, waves propagated westward and had little phase variation with height. The spring 6.5-day waves over Wuhan and Yamagawa had the main periods near 6-7 days, and main periods of the autumn 6.5-day waves over Adelaide fell in the whole width of 5-7 days. The periods of 6.5-day waves near spring equinox are more concentrative than those near autumn equinox. Generally, the episodes of 6.5-day wave activity last 3-4 weeks.3. The possible causation of the forming process of Es led by the 2-day planetary waves from lower atmosphereIn the summer of 2002-2004, there were many planetary waves besides the 27-day solar period in Wuhan foEs data. Among these waves, the strongest was 2-day oscillations; furthermore, the 2-day oscillations in 2003 had the maximum amplitudes. However, we found the summer 2-day waves in Wuhan wind were strongest in 2002 not in 2003.Through studying the nonlinear interactions between the summer 2-day oscillations and the tide waves in foEs and wind data, we found that the 2-day periodic modulation of tide waves was largest in 2003 and smallest in 2004. We also found that the 2-day waves in wind agreed well with the 2-day oscillations in foEs on the occurrence duration. This phenomena support the theory that Es is affected indirectly by the planetary waves through the action of the diurnal and semidiurnal tides which are strongly modulated by the same planetary waves. But, this mechanism may be only one of reasons why planetary waves exist in foEs.