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.