Generation of large sheet‐like ionospheric plasma irregularities at Arecibo

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.