Kilometer-scale layered structures inside spread-F

We report the first incoherent scatter radar observations-made at Arecibo O bservatory-of remarkable bottom-side (< 280 km) F-region sheet-like plasma structures with. horizontal/vertical-scales of less than or similar to1 km. These structures are an apparent manifestation of spread-F, appear to give rise to VHF coherent (FAI) scattering, and are clearly electrodynamic in o rigin involving a form of the Perkins spread-F instability process in the p resence of relatively large E-fields and horizontal plasma gradients. The n et instability process appears to be associated with downwelling features c haracteristic of the nighttime F-region. These structures-that may be self- similar on scales of similar to1-100 km-indicate considerable electrodynami c activity in the mid-latitude F-region on scales that have profound space- weather implications especially regarding deep scintillations on earth-spac e communications links.