We report the first incoherent scatter radar observations-made at Arecibo Observatory-of remarkable bottom-side (<280 km) F-region sheet-like plasma structures with horizontal/vertical-scales of ≲1 km. These structures are an apparent manifestation of spread-F, appear to give rise to VHF coherent (FAI) scattering, and are clearly electrodynamic in origin involving a form of the Perkins spread-F instability process in the presence of relatively large E-fields and horizontal plasma gradients. The net instability process appears to be associated with downwelling features characteristic of the nighttime F-region. These structures-that may be self-similar on scales of ~1--100 km-indicate considerable electrodynamic activity in the mid-latitude F-region on scales that have profound space-weather implications especially regarding deep scintillations on earth-space communications links. ¿ 2001 American Geophysical Union