| Summary: | Travelling Ionospheric Disturbances (T.I.D.s) are perturbations of the plasma in the ionosphere caused by the passage of acoustic gravity waves. These oscillations fall into two distinct classes, labelled large scale (LS) and medium scale (MS), respectively. These TIDs have frequently been observed at high-latitude and their source of generation may be the magnetic activity and/or the weather system within the region. This thesis presents the analysis of TIDs observations taken over a eight year (1981- 1989) period by means of an advanced Incoherent Scatter radars (EISCAT radar system) at F-region high-latitude, employing CP-2 common programme. An eight year synoptic study of TID effects in electron density measured at various heights is presented. A similar investigation of the field aligned ion velocity has also been undertaken. These two data sets have been computed and the wave disturbance in the parameter quantified. Large scale disturbances occured more frequently than those of medium scale and both are observed most frequently during daytime and at the first equinox. There are two dominant azimuths for wave propagation southwest and northeast. A very clear example of wave activity was observed by EISCAT facility on April 25th, 1989 between 0800-1300 UT in the measurements of Ne, Te, Ti, and Vi, extending to hundreds of kilometres height in the ionosphere. The wave trace amplitude shows amplification with height. The relative phase difference between the TID amplitude in Ne/Ne, Te and Ti with Vi has been established. Comparative cases have been presented between the theoretical and the observed values to test Hooke's formula and Hines dispersion relation. It has been noticed that there was no correlation between the three-hourly Kp indices, the hourly AE indices or the international daily sunspot number Ri indices with the average values of the TIDs spectral power. Good correlation was found between the low cloud type occurrence and the MSTIDs main parameters. A method of estimating the horizontal distance of the wave source from the radar position is presented.
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