Ionosphere of Venus: First Observations of Day-Night Variations of the Ion Composition

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Abstract
The Bennett radio-frequency ion mass spectrometer on the Pioneer Venus orbiter is returning the first direct composition evidence of the processes responsible for the formation and maintenance of the nightside ionosphere. Early results from predusk through the nightside in the solar zenith angle range 63° (dusk) to 120° (dawn) reveal that, as on the dayside, the lower nightside ionosphere consists of F1and F2 layers dominated by O2+ and O+, respectively. Also like the dayside, the nightside composition includes distributions of NO+, C+, N+, H+, He+, CO2+, and 28+ (a combination of CO+ and N2+). The surprising abundance of the nightside ionosphere appears to be maintained by the transport of O+ from the dayside, leading also to the formation of O2+ through charge exchange with CO2. Above the exobase, the upper nightside ionosphere exhibits dramatic variability in apparent response to variations in the solar wind and interplanetary magnetic field, with the ionopause extending to several thousand kilometers on one orbit, followed by the complete rertnoval of thermal ions to altitudes below 200 kilometers on the succeeding orbit, 24 hours later. In the upper ionosphere, considerable structure is evident in many of the nightside ion profiles. Also evident are horizontal ion drifts with velocities up to the order of 1 kilometer per second. Whereas the duskside ionopause is dominated by O+ H+ dominates the topside on the dawnside of the antisolar point, indicating two separate regions for ion depletion in the magnetic tail regions.