The Normal E Region of the Ionosphere

The E layer is situated in the middle ionosphere; its lower boundary is at a level of about 100 km when the sun is vertical. Its maximum ionization density is of the order of 105 electrons per cc but this varies by about 50 to 60 per cent in the course of the sunspot cycle. The production of ionization in the E layer is due to solar photons, most probably of X-ray character. The disappearance of electrons in the E layer is by way of dissociative recombination between electrons and positive molecular ions. E-layer morphology is influenced to a slight extent by horizontal currents flowing in it across the horizontal geomagnetic field; in other words, the E layer is a motor. It is generally considered that such currents are produced by dynamo action in the E layer due mainly to a diurnal horizontal tidal motion of the conducting medium across the earth's vertical magnetic force. The rough parallelism of the intensity of these horizontal currents with E-layer conductivity during the sunspot cycle suggests the simple result that the diurnal horizontal motion itself does not vary substantially with solar activity.