Initial duct growth determined from cold‐start plasma‐line data recorded at Arecibo

Recent high‐resolution height observations of the HF‐induced plasma line have been made by Djuth et al. [1990]. The plasma line is first observed after 3 ms very close to the HF‐reflection height and not at the height expected from weak‐turbulence theory in a uniformly varying ionosphere. Natural field‐aligned ducts exist in the ionosphere. Those with a diameter of a few meters or more can trap Langmuir waves (L waves) generated by the parametric decay instability and propagating nearly parallel to the magnetic field B. The ponderomotive force of the trapped L waves near the highest HF interference maximum causes electrons to be expelled parallel to B and ions to be expelled perpendicular to B. The ion gyroradius is comparable to the smallest duct diameter that can trap L waves. Since it only takes a few milliseconds for the ions to move at their thermal velocity across the duct, the duct can grow in a few milliseconds to a level where L waves capable of backscattering the Arecibo radar can be trapped. Duct growth is eventually limited by Landau damping and/or escape of the ducted L waves. These control the lower‐height limit of the plasma line and terminate the overshoot.