Magnetic activity variations of the nighttime E region at Arecibo

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 10 4 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.