SCINTILLATION NEAR THE F-LAYER TROUGH OVER NORTHERN EUROPE

Results are presented of amplitude and phase scintillation observations made during a two and a half year period at Lerwick in the Shetland Islands, a station in the subauroral zone in the vicinity of the F layer trough and the scintillation boundary. Transmissions from the multisatellite Navy Navigation Satellite System have been used to obtain both latitudinal and temporal coverage for the observations. More than 19,000 satellite passes have been monitored yielding in excess of 750,000 values of S4 and sigma-phi for amplitude and phase scintillation, respectively. The paper presents examples of scintillation occurrence as a function of latitude for both long term and diurnal time scales. A dominant feature is the enhanced occurrence of scintillation in response to increased solar activity. This is accompanied by an advance in the scintillation boundary to lower latitudes on a scale comparable to that associated with geomagnetic activity. The position of the premidnight minimum in boundary latitude has moved by some 10 deg in response to the increased solar activity between autumn 1987 and autumn 1989. The paper also discusses the importance of propagation and irregularity geometry to scintillation measurements and by comparison with model studies concludes that the observations are consistent with scintillations arising from irregularities of a field-aligned rodlike nature with low aspect ratio rather than sheets extended along L shells.