Large-scale ionospheric plasma irregularities, generated by O-mode heater waves at Arecibo, are shown for the first time to have sheet-like structures. The irregularities are aligned with the magnetic meridional plane and have scale sizes ranging from a few hundred meters to a few kilometers. This interpretation is based on detailed considerations of sequential measurements of radar backscatter power, the controlling magnetic field geometry, and the deduced E¿B plasma drift. The alignment of O-mode-generated irregularities with the magnetic meridional plane, and their disappearance during X-mode heating intervals are consistent with predictions of the thermal filamentation instability model. ¿ 1998 American Geophysical Union