Ionosphere of Venus: First Observations of the Dayside Ion Composition Near Dawn and Dusk

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Abstract
The first in situ measurements of the composition of the ionosphere of Venus are provided by independent Bennett radio-frequency ion mass spectrometers on the Pioneer Venus bits and orbiter spacecraft, exploring the dawn and duskside regions, respectively. An extensive composition of ion species, rich in oxygen, nitrogen, and carbon chemistry is idenitified. The dominant topside ion is O+, with C+, N+, H+, and He+ as prominent secondary ions. In the lower ionosphere, the ionzization peak or F1 layer near 150 kilometers reaches a concentration of about 5 x l03 ions per cubic centimeter, and is composed of the dominant molecular ion, O2+, with NO+, CO+, and CO2+, constituting less than 10 percent of the total. Below the O+ peak near 200 kilometers, the ions exhibit scale heights consistent with a neutral gas temperature of about 180 K near the terminator. In the upper ionosphere, scale heights of all species reflect the effects of plasma transport, which lifts the composition upward to the often abrupt ionopause, or thermal ion boundary, which is observed to vary in height between 250 to 1800 kilometers, in response to solar wind dynamics.