The maximum electron concentration of the nighttime E layer, the valley above it, and the appearance of the intermediate layer are described in detail from analyses of the low-frequency ionograms (50–2000 khz) obtained at Boulder for three nights of quiet, moderate, and severe geomagnetic activity. Electron concentration profiles are obtained from the ionograms by a method giving some information about the width and depth of valley minimums in the nighttime E region. These parameters are obtained by analysis of parts of the ordinary and extraordinary radio echo traces, and then cross-checked by a ray-tracing process from which the complete ordinary and extraordinary ray curves (including the extraordinary ray below the gyrofrequency) are calculated for comparison. The profiles consistently show a deep, wide valley on magnetically quiet nights, and are filled in with increasing ionization as magnetic disturbance becomes more severe. An intermediate layer develops during strong disturbances. The latitude variation of nocturnal ionization in the intermediate region is shown. Possible causes of the nighttime E layer are discussed in connection with the results obtained.