Based on the thermal structure, atmosphere is classified in to different regions. The atmospheric region between ~ 10 km to 90 km is generally referred to as middle atmosphere and the region above is upper atmosphere. Absorption of solar UV radiation with the dissociation of O2 and O3 dominates in the middle atmospheric region and lower thermosphere region respectively. In upper thermosphere solar EUV is the principle source of ionization and heating. The middle atmosphere is the region where most of the atmospheric waves and oscillations originate. These cover a wide range of periods from few minutes to few years (from atmospheric gravity waves to Quasi Biennial Oscillations – QBO). A leakage of energy from the middle atmosphere has profound effects in the upper atmospheric and ionospheric region. Thus short and long scale variability present in the middle atmosphere can be linked with the variability in the upper atmospheric and ionospheric regions.

Barotropically and baroclinically unstable regions can develop due to horizontal and vertical shear in the mean zonal wind, presumably driven and sustained by the momentum deposition, due to the dissipating gravity waves etc. This suggests that these instabilities may interact strongly with planetary waves, that propagating upwards from the troposphere. There is a possibility that a significant part of the planetary wave disturbances of the troposphere, may propagate in to the upper atmosphere. Thus any upward propagating momentum (from planetary / gravity), passing through the atmosphere has a potential to amplify and grow. This potential mechanism assumes a vertical propagation of a quasi 16 – day oscillations in to the ionosphere and can suggest the energy dissipation of the oscillation in the ionosphere.

Quasi Biennial Oscillation is the key parameter which controls both lower and upper atmospheric dynamics and chemistry. The stratospheric QBO in winds has been considered as one of the factors affecting the day to day variability of the Equatorial Ionization Anomaly - EIA and the modulation of semidiurnal tides. Atmospheric density at F2 region heights is much lower than that at lower thermospheric heights, so the oscillations are expected to be caused by the respective oscillations in the middle atmosphere. It means that a fraction of energy reaches the F region heights. Most of the energy deposited in the thermosphere is ultimately carried from mesosphere through molecular and eddy conduction process. In such a situation planetary waves play a crucial role in controlling the dynamics of the two regions.

For studying the variations and periodicities of different parameters in the middle atmosphere, we have a good data base extending from ground level to about 120 km altitude, which includes the measurement of winds and temperature using high altitude balloon flights (Radiosonde), RH 200 rocket flights and Radar observations from TERLS (Trivandrum: 8 0N, 77 0E), viz. SKiYMET Meteor Wind Radar (35.25 MHz), Partial Reflection Radar (2.5 MHz). Complementary to these, different satellite data (COSMIC/FORMOSAT, SABER/TIMED etc) and data from the co-located facilities like magnetometer / ionosonde are also used for the above mentioned scientific studies.

As mentioned above the lower middle and upper atmosphere regions compose a strongly coupled system in which phenomena occurring at one height can have profound effects elsewhere The mesosphere lower thermosphere (MLT) region is a critical region in the vertical coupling since here the physical processes filter and shape the flux of waves and tides ascending through the mesosphere into the overlaying thermosphere- ionosphere system. The dynamics of the MLT region which is dominated by atmospheric tides, planetary and gravity waves of large amplitudes establish a wind system of the lower thermosphere, whose interaction with the ionospheric plasma produces the dynamo electric fields and currents that control the quiet time electrodynamic processes of the upper atmospheric region.

The assimilated data and mesospheric winds provided by the above network of experiments have been used to study coupling processes due to planetary waves in the middle and upper atmosphere. It is observed that there is a signature of strong vertical coupling between the two regions, especially during winter months. However, not all observed disturbances in mesospheric winds can be explained by the simple propagation of planetary waves from below. In addition to the vertical coupling there is also weaker horizontal "inter-hemispheric" coupling during equinoxes. The variability of the large-scale oscillations driven by the combined action of the wind system and magnetic field is investigated by analysing data from (Trivandrum: 8 0N, 77 0E), station and the response of the ionosphere to planetary waves from below is studied.