Kilometer‐scale layered structures inside spread‐F

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.