ON THE ROLE OF ELECTRIC-FIELD DIRECTION IN THE FORMATION OF SPORADIC E-LAYERS IN THE SOUTHERN POLAR-CAP IONOSPHERE

Measurements of the occurrence of sporadic E (Es)-layers and F-region electric fields were obtained with a modern, HF digital ionosonde loca ted at Casey, Antarctica (66.3 degrees S, 110.5 degrees E, 81 degrees S CGM latitude) during the late austral summer of 1995/96. The occurre nce of Es-layers was inferred from the presence of appropriate traces in normal swept-frequency ionograms, and the electric fields were infe rred from F-region ''drift-mode'' velocities assuming that the plasma convection velocities given by E x B/B-2 were measured, on average, by the interferometer. The theory of formation of high-latitude Es-layer s predicts that electric fields directed toward the south west (SW) sh ould be particularly effective at producing thin layers in the souther n hemisphere. Our measurements made at a true polar cap station are co nsistent with this expectation, and are contrasted with observations m ade by incoherent scatter radars in the northern hemisphere, which als o show the importance of SW electric fields, whereas the same theory p redicts that NW electric fields should be important at northern latitu des. We reconcile the interhemispheric differences with simple calcula tions of ion convergence driven by the electric fields specified by th e IZMI-RAN electrodynamic model (IZMEM) in both hemispheres. The impor tance of the interplanetary magnetic field in the control of high-lati tude Es formation is emphasised as an important adjunct to space weath er modelling and forecasting. (C) 1998 Elsevier Science Ltd. All right s reserved.