Positive ion mass spectrometer observations in the E region

Positive ion mass spectrometer observations of the auroral zone ionosphere above Fort Churchill and the midlatitude ionosphere above White Sands are reviewed. The auroral zone ionosphere, either day or night, resembles the midlatitude daytime ionosphere. Decay of the ionosphere at sunset and into the night is seen from measurements made above White Sands. The midlatitude nighttime ionosphere in the E region is sustained at an ion density of about 10 3 ions/cm 3 by resonantly scattered ionizing solar radiation. Sporadic‐ E events are seen as concentration of metallic atomic ions, primarily of Mg + and Fe + .