A study of medium-scale Travelling Ionospheric Disturbances observed at mid-latitudes.

Travelling Ionospheric Disturbances (T.I.D.s) are perturbations of the ionized regions of the upper atmosphere caused by the passage of acoustic-gravity waves (AGW). These oscillations fall into two distinct classes, labelled large and medium-scale, respectively. Large-scale T.I.D.s propagate equatorwards from the auroral zone and are well correlated with auroral and magnetic activity. The generation mechanism for medium-scale T.I.D.s, however is less well understood. This thesis presents the analysis of observations of medium-scale T.I.D.s taken over a three-year period by means of the HF Doppler technique. Previous T.I.D. observations are reviewed and compared with the present findings. Agreement is found for all T.I.D. characteristics observed, with the exception of the propagation direction, for which no seasonal dependence is evident. As previously reported, no correlation is found between the occurrence of these waves and auroral indices (Ap, Kp, AE). The neutral thermospheric wind can act as an azimuthal filter for AGW through the processes of ducting, critical coupling and reflection. The diurnal rotation of the wind vector thus introduces a similar rotation in the observed T.I.D. propagation direction. This phenomenon is illustrated by observational data and is reproduced theoretically by tracing hypothetical wave paths through a modelled three-dimensional wind profile. A semi-diurnal variation in the propagation direction has been observed which may indicate that lower atmospheric tides may significantly influence upward-propagating gravity-waves. A reverse ray-tracing analysis has been performed to locate the source of the observed waves; for the majority of waves, the generation region lies to the WNW within a range of about 1000 km. Auroral sources do not appear to be important for waves observed at mid-latitudes. A possible source is the meteorological jet stream and a moderate correlation is found between the jet speed at 300 mb and wave occurrence over Leicester.