Proceedings with Extended Abstracts (single PDF file) - Radio ...

tens of km (mostly between 20 and 100 km) and to drift westward prior to 0200 h local timewith the neutral wind and speeds ranging from about 30 to 150 m/s. The inferred largewestward winds during unstable E s can help set up strong eastward polarization fields inside aplasma patch which then can drive the gradient-drift instability, and even the Farley ( modifiedtwo-stream) instability. The existence of such fields is now understood in terms of apolarization process proposed by Haldoupis et al. [1997], which is the same as that at themagnetic equator but with the geometry turned on its side. This requires a sporadic E plasmapatch with sharp, horizontal conductivity gradients at its edges that play the same role thatvertical gradients play at the magnetic equator. In the presence of an ambient meridional field,a much stronger zonal field will build up inside the patch to maintain divergence-free currentflow, just as a vertical polarization field builds up at the equator (which actually drives theequatorial electrojet) so that the vertical Hall and Pedersen currents nearly cancel.Many studies show that unstable E s associates with “spread E s ” which refers to patchy andspatially structured sporadic E rather than to continuous blanketing-type layers. This implies anelectrodynamic link between unstable E s and spread-F, given the close relationship betweenspread E s and spread-F reported in several ionosonde studies {e.g., see Bowman, 1990).Following suggestions that spread-F occurs in conjunction with unstable Es, we consideredtesting this theory by revisiting an older data set comprised of common-volume observations of50-MHz Doppler backscatter and ionosonde recordings carried out over the southern AegeanSea during summer 1996. The experiment included the Sporadic E SCATter experiment(SESCAT), a 50-MHz continuous-wave Doppler radar located on the northern coastline ofCrete that was capable of observing coherent echoes from a fixed E region area about 160 kmto the north, and a Canadian Advanced Digital Ionosonde (CADI) that was placed beneath theSESCAT field of view on the island of Milos. For the purposes of this work, we have tried acomparison between CADI spread-F and SESCAT observations. Since midlatitude backscatteris strictly a nighttime phenomenon, we considered only the 10-hour period from 1900 to 0500LT when vertical incidence ionograms were recorded every 2 minutes. The idea was to inspectthe data using quick-look plots and search for a relationship between the two phenomena.Surprisingly, we found a one-to-one relation in the occurrence of strong to moderate SESCATechoes and CADI spread-F. During nights with a few short lived, low-intensity echoes or noechoes at all, spread-F was absent in the measured ionograms. The inspection of the two datasets indeed suggests a connection between the two phenomena. A quantitative analysisbetween the occurrence of the two phenomena, that is, the unstable E s and spread-F, led to agood degree of correlation signified by a linear correlation coefficient near 0.8.3. A New Mechanism for Generating Mesoscale Spread-FBased on the SESCAT/CADI comparisons and published results elsewhere, we conclude thatthere must be a connection between mesoscale spread-F and unstable sporadic E. We postulatethat this relation is sustained through electrical coupling of the two ionospheric regions, whichallows mapping of electric fields up (down) the Earth's magnetic-field lines. Next, weintroduce a new idea which can serve as a mechanism for generating mesoscale spread-F atmidlatitude, as illustrated in Figure 1.The schematic in Figure 1 shows highly conducting sporadic-E patches of metallic-ion plasmawith abrupt boundaries drifting with the neutral wind in the nighttime E region. In therectangular coordinate system, x is pointing horizontally to the east, y is perpendicular to themagnetic field pointing southward and downward, and z is along B. For simplicity, we ignoreany bulk meridional motions and assume that the patches are drifting westward, as suggestedby experiment. In accordance with the Haldoupis et al. [1997] polarisation mechanism, thetotal meridional field E y = E y 0 + U x xB, can drive stronger eastward polarization fields, E x p ≅87