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Using 13‐bit Barker‐coded pulses, high resolution (0.9 km) incoherent scatter measurements of nighttime E‐ and F‐region electron densities have been conducted at Arecibo. Electron density profile sequences are presented for four nights characterized by a wide range of magnetic activity. The data show a deep valley in electron density above 120 km during quiet periods which fills in with plasma to levels of 104 cm−3 during a magnetic storm (Kp ≃ 8). An intermediate layer frequently appears near 125 km, independently of magnetic activity. In view of the observations, the connection between nighttime valley content and magnetic activity is discussed.

Recent incoherent scatter electron density measurements made with 0.9‐km‐altitude resolution Barker‐coded pulses are combined with the Jacchia neutral atmosphere model to generate nighttime electrical conductivities for the lower ionosphere at Arecibo. Electron density profiles and derived conductivities are presented for nights characterized by low and high magnetic activity. A model for nighttime electron densities incorporating 17 nights of data is presented along with the computed electrical conductivities. The features of the model include: (1) a sharply defined lower boundary at 85 km, (2) a layer between 85 and 120 km having an average density of 2 × 10³ cm−3 and showing a weak symmetry about solar midnight due to the ionizing effect of scattered solar ultraviolet radiation, (3) a valley of low electron density above 120 km that is progressively filled in with ionization as magnetic activity increases, and (4) electron density enhancements in the upper E region occurring 4 hours after local midnight as a result of downward transport of plasma from the F region. Height‐integrated conductivities calculated from the model and the two nights of observations are also given.

A simplified model of plasma motion in the low‐latitude nighttime E region is used to investigate the role of uniform horizontal neutral winds in the formation of sporadic E layers. Numerical results show that thick layers may be formed in the mid‐E region by plasma compression at the altitude where ion‐neutral collisions destroy the ability of horizontal winds to move ionization vertically. It is hypothesized that this mechanism accounts for the appearance of early morning sporadic E layers that are observed to form near 125 km at low latitudes following descents of the F layer.