GENERATION OF LARGE SHEET-LIKE IONOSPHERIC PLASMA IRREGULARITIES AT ARECIBO

Large-scale ionospheric plasma irregularities, generated by O-mode hea ter waves at Arecibo, are shown for the first time to have ''sheet-lik e'' structures. The irregularities are aligned with the magnetic merid ional plane and have scale sizes ranging from a few hundred meters to a few kilometers. This interpretation is based on detailed considerati ons of sequential measurements of radar backscatter power, the control ling magnetic field geometry, and the deduced E x B plasma drift. The alignment of O-mode-generated irregularities with the magnetic meridio nal plane, and their disappearance during X-mode heating intervals are consistent with predictions of the thermal filamentation instability model.