NIGHTTIME MID-LATITUDE F-REGION DYNAMICS AND SPREAD-F MORPHOLOGY (PUERTO RICO)

In this study simultaneous data from incoherent scatter radar and a Fabry-Perot interferometer were used to study mid-latitude nighttime spread-F at Arecibo, Puerto Rico. Four nights of simultaneous data were observed in which three showed occurrence of spread-F. The continuous scan method for taking incoherent scatter data was used and this enabled velocities to be measured with a sixteen-minute time resolution. The Fabry-Perot interferometer provided neutral wind velocities with a 45-minute resolution.;The determination of neutral wind velocity from the interferometer is found to have an uncertainty as large as 45 m/sec which is frequently larger than the neutral wind velocity. The north-south neutral velocity is found not the be in agreement with the more accurate incoherent scatter results. The latter is found to be well correlated with the motion of the F layer while the velocities obtained from the interferometer were not well correlated.;The comparison of the predictions of the flux-tube model with data showed that the time variation of total electron content and integrated pedersen conductivity were in reasonably good agreement with the model for Nov. 27, and for parts of Nov. 28 and Dec. 15-16. No good agreement was observed for Nov. 29. Cycle-averaged data were primarily used for the comparisons, and it was shown that good results could be obtained with this method. The Perkins and the Scannapieco instability models are compared with available data for equilibrium and growth time. Both models gave almost the same result for the quasi-equilibrium values of integrated Pedersen conductivity indicating that the east-west gradients in conductivity are small. Evaluation of growth time required use of the east-west neutral wind from the Fabry-Perot interferometer, which resulted in significant uncertainty in the growth time. In spite of this uncertainty, the Perkins model was able to give more reasonable values for the growth time and the onset of spread-F even though the predictions were not always consistent with the observations. The Scannapieco model, however, did not predict an unstable situation for the observations in his study.;Nighttime dynamics of the F region is discussed in detail, especially at the time of spread-F. One of the nights showed the presence of traveling ionospheric disturbances which was the probable cause of spread-F at that night.