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

THE ROLE OF UNSTABLE SPORADIC-E LAYERS IN THEGENERATION OF MIDLATITUDE SPREAD-FC. Haldoupis 1 , M. C. Kelley 2 , G. C. Hussey 3 , and S. Shalimov 41: Physics Department, University of Crete, Iraklion, Crete, Greece2: School of Electrical and Computer Engineering, Cornell University, Ithaca, NY. USA3: Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Canada4: The Institute of Physics of the Earth, Moscow, Russia.1. IntroductionMid-latitude spread-F is identified in the ionograms by the multiplicity and height spreading ofthe F region trace, caused by strong undulations and irregular plasma structuring in the Fregion electron density. Many of these disturbances are attributed to the passage of large-scaleatmospheric gravity waves which can impose a wavelike altitude modulation in the F layer[Bowman, 1990], but also to electrodynamic forces and large-scale plasma instabilities [Kelleyand Fukao, 1991]. The latter applies in particular to mesoscale spread-F with scales from a fewtens to many tens of kilometres, which occurs overwhelmingly in the summer nighttime.Incoherent scatter radar (ISR) measurements from Arecibo, coherent-backscatter of aspectsensitiveF region irregularities, and airglow imaging observations, established a key propertyof the phenomenon, namely that it relates of plasma regions characterized by rapid turbulentupwelling and abrupt plasma uplifts. An uplift in the F region can be caused by upward andnorthward ExB plasma drifts driven by an eastward electric field and/or a meridionalsouthward wind. The rapid upwelling of meter-scale irregularities observed by Fukao et al.[1991], which can be accounted for only in terms of ExB plasma motions, and the largeelectric fields inside spread-F measured by Behnke [1979] and inferred by Swartz et al. [2002],imply that these uplifts are caused by eastward electric fields rather than southward winds.In this paper, we postulate that the eastward electric fields required for the F region uplifts, andtherefore the generation of spread-F, are mapped up along the field lines from within stronglyunstable sporadic E layers (E s ). This interpretation emerged from our present understanding ofthe unstable patchy E s and from growing evidence for a link between mesoscale spread-F andunstable sporadic E. Here we present a new mechanism for spread-F generation in which thekey role is played by polarization electric fields set up within westward-drifting E s plasmapatches and mapping up to F region. More details can be found in a paper by Haldoupis et al[2003] and a companion one by Kelley et al. [2003], both being in press in J.G.R.2. Unstable Sporadic E Layers and Connection to Spread-FThe term “unstable E s ” refers to the situation when midlatitude sporadic E is destabilised by thegradient-drift and/or the Farley plasma instabilities, which leads to the generation of aspectsensitiveplasma irregularities that can scatter radio waves. The E s -related echoes are found topossess mostly negative-mean Doppler shifts corresponding to northward and upward (away)velocities with magnitudes usually less than 100 m/s but also at times well above this limit,sometimes even exceeding 300 m/s [e.g., see Haldoupis et al., 1997; Swartz et al., 2002]. Sincethe echoes are due to ExB drifting irregularities, the observed velocities are attributable toenhanced eastward electric fields of several mV/m, well above the ambient dynamo fields atmidlatitude.Many radar studies show E s echoes to associate with unstable plasma regions or structures,presumably patches of sporadic E, which drift across the radar field of view with the neutralwind. The zonal scale lengths of these patches were found to extend from a few km to many86