Venus Dayside and Nightside Ionosphere Variations During Corotating High-speed Streams: A Statistical Study using Pioneer Venus Orbiter Observations

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We present a statistical study of Venus ionospheric variability using about two years of Pioneer Venus Orbiter measurements of the major ions O 2 + , O + , and H + during quiet solar wind conditions, solar wind high-speed streams (HSSs), and preceding corotating interaction regions (CIRs). Under quiet solar wind conditions, the dayside ionosphere is characterized by a peak O 2 + density (∼41 × 10 4 cm −3 ) at an altitude of ∼164 km (the V 1 layer), an O + density peak (∼71 × 10 3 cm −3 ) at ∼203 km (the V 2 layer), and an H + density peak (∼8 × 10 2 cm −3 ) at ∼423 km, on average. The density of O 2 + decreases sharply below the O + density at an altitude of ∼192 km and below H + at ∼307 km. The ionosphere is characterized by prominent day–night asymmetries. The asymmetry in the peak ion densities (layer heights) is found to decrease (increase) systematically with altitude. CIRs and HSSs enhance the day–night asymmetries in the peak O 2 + and O + densities compared to their quiet time values. Considering average ionospheric parameters during the CIRs (and preceding high-density heliospheric plasma sheets), significant decreases are recorded in the dayside peak O 2 + density (∼18%), the nightside peak O + density (∼41%), the altitudes of the dayside O + (∼3%) and H + (∼19%) ion density peaks, and the dayside O 2 + /H + transition altitude (∼7%) compared to their values for the quiet ionosphere. During the HSS impacts, dayside (nightside) ionospheric O 2 + and O + peak densities exhibit decreases by ∼12% (∼64%) and ∼13% (∼44%), respectively, compared to their quiet ionospheric values. The results are discussed based on current understanding of the Venus ionospheric dynamics.