Planetary Waves And Their Roles In Atmospheric Coupling

Planetary waves and their roles in mesosphere/thermosphere/ionosphere coupling are key research topics in the recent years. Their momentum and energy deposition, when propagating from lower atmosphere to upper atmosphere, changes the background temperature and wind, and thus influences the global circulation, e.g. sudden stratosphere warming. The dissipation of the planetary waves in the mesopause region can change the atmospheric compositions and thus the thermosphere photochemistry. The planetary wave can also influence the F region ionosphere through E region wind dynamo. In addition, the planetary waves can also modulate the tides, which then transmit the planetary wave signals up into the thermosphere/ionosphere due to their long vertical wavelengths and thus low dissipations. Therefore, planetary waves are important chains that can cause couplings between different atmosphere layers, and thus the study of planetary waves can expand our knowledge not only on atmospheric dynamics but also on space weather forecast, which ensures the human explorations in near earth space. In this paper, we will study the influence of sudden stratosphere warming (SSW) on quasi2-day wave (Q2DW) and the possible solar cycle modulations, which are important reasons for its inter-annual variability. Then the morphology of the planetary wave signals in the ionosphere is generalized and the individual roles of the meridional and zonal components in E region wind dynamo are also investigated.Although the seasonal variations of the Q2DW are evident, its inter-annual variability is complicated. The TIMED (Thermosphere Ionosphere and Mesosphere Electric Dynamics (TIMED) satellite has been operating since February2002, which has provided neutral atmospheric temperature and horizontal wind for nearly13year. The temperature profile measured by SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument can cover the altitude of～20-110km, and the horizontal winds from TIDI instrument extend from85to105km. The global coverage of satellite observations facilitates the study of planetary waves from in different aspects, e.g. period and zonal wave numbers. We first studied the inter-annual variability of the Q2DW during2002-2011with both SABER temperature and TIDI wind observations, including the wave number3(W3) and wave number4(W4) components. We found that the W3Q2DW activities are the strongest in January2006during the past10years, with maximum temperature, meridional wind and zonal wind perturbations of～15K,～65m/s and～35m/s, respectively. Nevertheless, the period of the Q2DW during January2006was only～43hours, which was the shortest period。The abnormally strong W3Q2DW during January2006is most likely related to the strong SSW event in the winter stratosphere at the same time. In addition, we found the W3Q2DW in January reached minima during2008-2009, when the W4Q2DW in July achieved maximum amplitudes. This is consistent with the solar minimum and suggests different responses of W3and W4Q2DWs to solar cycle variations.The TIME-GCM was then utilized to study the influence of Sudden Stratospheric Warming (SSW) on Q2DW. Our results showed that the mean flow instabilities in the northern (winter) hemisphere mesopause region can provide additional source for the amplification of W3, but are weakened during SSW period. In our simulations, the mean flow instabilities induced by the deceleration or reversal of the stratosphere westerly do not provide evident amplifications for W3. The summer easterly in the southern hemisphere is enhanced during SSW period, which results in larger wave guide for W3and thus facilitate its propagation. The energy of W3is partially transferred to a child wave through the nonlinear interaction between W3and wave number1stationary planetary wave (SPW1), which is the wave number2(W2) Q2DW. The EP flux of W2showed that it can propagate in both winter and summer hemispheres, and result in more symmetric global temperature and wind structures compared with W3. In the summer hemisphere, the W2can be amplified by the mean flow instability at polar region of upper stratosphere and lower mesosphere. In the winter hemisphere, the nonlinear interactions between W3and SPW1at middle and low latitudes between50and100km provide evident additional source for W2. The W2is stronger during strong SSW event, which is due to the stronger parent wave of SPW1and the more favorable background condition for its propagation.A medium frequency (MF) radar, which can measure the horizontal wind between80and100km, was operated at Hawaii during1990-2006. This provides valuable datasets for the study of solar cycle modulations on Q2DW. The satellite observations have shown that the W3and W4dominate the Q2DW activities in January and July, respectively, thus the MF radar observations during January and July are also analyzed separately. The analysis results showed that both the meridional and zonal wind perturbations of the Q2DW in January are in-phase with solar cycle variations with correlation coefficient of0.45and0.6, respectively. But the Q2DW reached maximum amplitudes in1993and2003, which lagged the solar maxima by1-2years. The correlations between the Q2DW in July and the solar cycle are more complicated than in January. The meridional wind component was nearly in-phase with solar cycle with a correlation coefficient of0.4, while the zonal wind component was nearly out of phase with solar cycle with a correlation coefficient of-0.7.We also studied the roles of E region wind dynamo in mesosphere/thermosphere/ionosphere coupling through planetary waves. Both the ground based GPS total electron content (TEC) and the COSMIC satellite electron density profiles showed that the ionosphere can exhibit similar planetary wave signals to those in the mesosphere, which provided strong evidence for the neutral-ion couplings through planetary waves. We also found that both the latitudinal and vertical structures of the ionospheric responses agree well with the equatorial ionosphere anomaly, with maximum perturbations on both sides of the equator and a minimum at the equator. In addition, the responses reached maxima between1400and1800LT, which also agree well with the ionospheric fountain effect. The planetary waves most possibly modulated the fountain effect by superposing their wind perturbations on the background, which then transmitted the planetary wave signals up into the F region ionosphere through E×B vertical plasma drift. The roles of meridional and zonal wind in E region wind dynamo were then studied separately with control TIME-GCM simulations. It was shown that both the W2and W3can induce vertical plasma drift directly but with different characteristics, due to their different latitudinal wind structures and phases. Specifically, the vertical drifts induced by the meridional and zonal wind perturbations of W2are nearly in-phase with each other, which result stronger total plasma drift and a maximum at the equator; the vertical plasma drifts from the meridional and zonal wind components of W3have larger phase discrepancies and are nearly out of phase at the equator, which result in weaker vertical plasma drift perturbations and a minimum at the equator. Our TIME-GCM simulations clearly showed that the amplitude, phase, latitudinal and vertical structures of planetary waves are of significant factors in the neutral-ion couplings.In this paper, we studied the influence of SSW on Q2DW and the solar cycle modulations on Q2DW with both observations and numerical simulations, which expanded our knowledge on the inter-annual variability of Q2DW. The generalization of the planetary wave morphology in the ionosphere and the investigation of the individual role of meridional and zonal wind components are significant supplement to the mechanism study of neural-ion coupling.