Studies on atmospheric tides and planetary waves in the mesosphere-lower thermosphere (MLT) region using SuperDARN HF radars and meteor radar.

Abstract

In this work, observational results of atmospheric dynamics caused by upward propagating atmospheric waves (tides, planetary waves and their interactions) in mesosphere-lower thermosphere (MLT) region are presented. This study is imperative as it contributes toward an understanding of various physical and dynamical processes that take place in this region. The seasonal and inter-annual variations of tides are investigated using MLT winds recorded simultaneously by SuperDARN HF radars situated at Halley (75°S, 26°W), SANAE (72°S, 3°W) and Syowa (69°S, 36°E) from 1998 to 2007. The seasonal variation of tides was found to be characterized by maximum amplitudes in summer and minimum amplitudes in winter. The semidiurnal tides showed additional enhancement of amplitude in autumn. The seasonal behavior of the diurnal tide (semidiurnal tide) was found to be similar to that of tropospheric specific humidity (stratospheric ozone mixing ratio) which suggests a forcing mechanism as a possible source of tidal variation. Long-term variation of semidiurnal tide was found to be correlated to F10.7 solar flux, which suggests solar activity as a possible driver of the semidiurnal tide variation. The variability of tides prior and post 2002 sudden stratospheric warming (SSW) event was studied using MLT winds derived from SuperDARN HF radars at Halley, SANAE and Syowa. Forcing mechanism using the ozone mixing ratio was found to be a possible source of semidiurnal tide (SDT) variability before the SSW event (160-250). Nonlinear interaction between planetary waves and tides on the other hand, was found to be a possible source responsible for the SDT variation just before, during and after the SSW event (250-300). Nonlinear interaction between planetary waves and tides in the MLT region was studied using wind velocity data collected from meteor radar located at Rothera (68°S, 68°W) Antarctica during the year 2005. Wavelet analysis conducted on the wind data showed that the MLT region is dominated by SDT’s and planetary waves with period ~ 5, 10, 16 and 23 days. Further analysis showed that SDT’s are modulated at the periods of ~5, ~16 and ~23 days. However, non-linear interaction between the SDT and 16-day planetary wave was found to be mostly responsible for the variability of the SDT than the interaction between the SDT and 5- as well as 23-day planetary. Study on the coupling between neutral atmosphere and ionosphere was conducted using SuperDARN HF radar and magnetic field data, both data sets were recorded from SANAE. The results showed that the quasi-16-day periodicity observed in the ionosphere most probably originated from the neutral atmosphere. This was established on the basis of the travel time of oscillation from the neutral atmosphere to the ionosphere. Modulation of semidiurnal tide at quasi-16-day periodicity was found to be the mechanism responsible for the neutral atmosphere/ionosphere coupling through ionospheric electrodynamo effect. Magnetosphere/ionosphere coupling was also observed at quasi-20- and -23-day periodicity using Dst index as the magnetospheric parameter. Solar-ionosphere coupling on the other hand was not observed.