Initial Observations of the Nightside Ionosphere of Venus from Pioneer Venus Orbiter Radio Occultations

Abstract

Pioneer Venus orbiter dual-frequency radio occultation measurements have produced many electron density profiles of the nightside ionosphere of Venus. Thirty-six of these profiles, measured at solar zenith angles (chi) from 90.60 degrees to 163.5 degrees , are discussed here. In the "deep" nightside ionosphere (chi > 110 degrees ), the structure and magnitude of the ionization peak are highly variable; the mean peak electron density is 16,700 +/- 7,200 (standard deviation) per cubic centimeter. In contrast, the altitude of the peak remains fairly constant with a mean of 142.2 +/- 4.1 kilometers, virtually identical to the altitude of the main peak of the dayside terminator ionosphere. The variations in the peak ionization are not directly related to contemporal variations in the solar wind speed. It is shown that electron density distributions similar to those observed in both magnitude and structure can be produced by the precipitation on the nightside of Venus of electron fluxes of about 108 per square centimeter per second with energies less than 100 electron volts. This mechanism could very likely be responsible for the maintenance of the persistent nightside ionosphere of Venus, although transport processes may also be important.

Meteoric Layers in Planetary Atmospheres

“…Below the main peak, one or more secondary ionospheric layers have been found. Examples of such layers have been found at Venus by Pioneer Venus (Kliore et al 1979) and Mars by Mariner IV (Fjeldbo et al 1966), Mars 4 and 5 (Savich et al 1976) and Mars Express (Pätzold et al 2005) among others, see Fig. 2.…”

“…The altitude of the main peak is located at 142.2 ± 4.1 km, very close to the main peak of the dayside terminator ionosphere. The peak density is characterized by a great variability, with a magnitude ranging from 23 × 10 3 to 40 × 10 3 cm −3 (Kliore et al 1979). A double-peak structure appears during two closely spaced orbits, 55 and 57, and on orbit 57 the structure appeared in both the entry and exit measurements.…”

Structure, luminosity, and dynamics of the Venus thermosphere

“…The model parameters were based on PV data, including the pre-dawn bulge in the H densities inferred by Brinton et al (1980), and the high ion temperatures measured by the ORPA . Alternative O] densities were taken from the PV OIMS measurements and the PV orbiter radio occultation (ORO) profiles (Kliore et al, 1979). The H 2 abundance was assumed to be less than 0.5 ppm, in accord with the mesospheric photochemical models (Yatteau, 1983;Yung and DeMore, 1982).…”

Impacts of Cosmic Dust on Planetary Atmospheres and Surfaces

“…This is close to the peak of meteoric ablation (Fig. 2), and so was tentatively attributed to a layer of metallic ions and electrons, although it was recognized that direct ionization by energetic electron or proton precipitation could also be responsible (Kliore et al 1979;Molina-Cuberos et al 2008). The main ionospheric peak in Titan was located at 1180 ± 150 km during the Voyager I fly-by (Bird et al 1997).…”

“…This evidence was obtained from radio occultation measurements, where the attenuation of radio waves transmitted from a spacecraft through a planet's atmosphere and received at Earth can be used to determine the electron density profile in the atmosphere. Pioneer Venus measurements show that the main ion layer in the Venus night-side ionosphere peaks around 142 km, with a second, intermittent peak around 120 km (Kliore et al 1979). This is close to the peak of meteoric ablation (Fig.…”