Abstract :
To study the long-term trend of coupling between two regions, we have chosen a species that travels from one region to another. Nitric oxide is one such species which is produced in the thermosphere and is transported downwards. Evidences point out that there is increase in this transport rate. As a result, a decrease in the lower thermosphere and a possible increase in the mesosphere in [NO] are expected. A decrease in [NO] has indeed been reported in the lower thermosphere. However, a study of electron density data of Thumba (9°N, 77°E) at 80km where NO is ionized by Ly-α, does not show any increase in [NO], rather it shows a decreasing trend. Thermospheric nitric oxide could also penetrate to the upper stratosphere. In this region O3 is photo-chemically controlled. One of the major losses of O3 in this region is through NO. A decrease in [NO] in this region will decrease the loss rate of O3 and hence increase [O3]. Ozone is being measured at many places in India by Dobson spectrophotometer. The nearest place to Thumba where O3 is being measured is Kodaikanal (10°N, 77°E). An examination of Umkehr O3 data of Kodaikanal shows that at layers-7 and 9 i.e. in the upper stratosphere, there is an increasing trend of O3. It appears, therefore, that there is a decreasing trend of NO in the lower thermosphere, mesosphere and upper stratosphere. Though there is an increasing trend in the coupling strength of this region, the decreasing trend in NO is explainable by the decreasing trend in its production rate.
