Comparison of Arecibo E‐region data and sporadic‐E theory: a measurement of the diffusion coefficient

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Abstract
Incoherent scatter measurements using multiple‐pulse techniques were performed at Arecibo to study the E region with a height resolution of 2.4 km. These simultaneous measurements of ion temperature, ion‐neutral collision frequency, and electron density, as well as horizontal and vertical components of ion drift velocity, have allowed comparison between theories of sporadic‐E and experiment. Sporadic‐E layers were consistently observed to descend along the curve where the vertical ion drift velocity (Vz) was zero and its gradient ∂ Vz/∂z was negative, as predicted by redistribution theory. Assuming no diffusion, the calculated effective recombination coefficient at the layer peak is more than an order of magnitude too small for NO+ or O2+, and varies with height in a manner which is unacceptable for a recombination coefficient. This indicates that the layer is composed of long‐lived ions (probably metallic), and that the peak layer density is controlled by diffusion rather than recombination. By ignoring recombination, the diffusion coefficient was estimated from the layer thickness and the vertical gradient in the vertical ion velocity. The results were compared with independent values calculated from the measurements of ion temperature and ion‐neutral collision frequency. The two estimates were in good agreement over the range 110 to 120 km, showing that electron densities in sporadic‐E layers are primarily determined by molecular diffusion.