On the sheet-like nature of 150 km (F1) radar echoes

[1] The so-called 150 km (or F1) echoes are shown to arise from field-aligned irregularities (FAI) that are embedded in a sheet-like region. Using a 49.92 MHz radar on Christmas Island, which has an antenna beam directed toward geographic east at a zenith angle of 29.5°, high-resolution Doppler-velocity variations within the radar scattering volume are used to infer that the sheet-like region could have both tilt and curvature. This finding is used to infer that plasma density (N) must be enhanced within the sheet-like region, and that FAI could be generated by an interchange instability, driven by a vertical electric field or a zonal neutral wind that acts on the horizontal component of the gradient in N. How tilted sheet-like structures could form in the F1 region is discussed.