Dynamic instability in the lower thermosphere inferred from irregular sporadic E layers

Simultaneous observations of an irregular sporadic E layer from the Arecibo incoherent scatter radar and a coherent scatter radar located on St. Croix are presented. The layers exhibit periodic structuring which is attributed to shear instability in the neutral flow. Estimates of the time-varying vector neutral wind profiles in which the layer was embedded are analyzed and shown to be shear unstable in the Richardson number sense. In addition to the calculation of the Richardson number values, we present an eigenvalue analysis of the model of Miles (1961) and Howard (1961) for the observed wind profiles. The calculated eigenmodes have dominant Kelvin-Helmholtz modes for the estimated flow that are propagating to the southwest with phase speeds near 50 m/s and horizontal wavelengths between 10-15 km. The growth times for the waves would have been as little as about 1 min. These features are in reasonable agreement with the observed of E-s-layer structure. The Miles-Howard model has been analyzed extensively in the past using both analytic and numerical techniques, but calculations of eigenmodes for the equations in a case with background winds that have turning and speed shear have not been carried out previously, as far as we know. The difficulties associated with the calculation are related to identifying the fastest growing modes among the large number of modes that satisfy the equations. The technique and the relationship of the solutions to the observed sporadic E layer wave structure are described.