Venus upper atmosphere structure

Abstract

From analysis of the orbiter atmospheric drag (OAD) data obtained from the orbital decay of the Pioneer Venus orbiter from December 9, 1978, to August 7, 1979, atmospheric densities have been determined and tabulated near 16°N latitude between 140 and 190 km for all times of day. Maximum daytime densities on Venus are approximately 8 × 10−13 g cm−3 at an altitude of 150 km (dropping by a factor of 24 at night) and 7 × 10−14 g cm−3 at 170 km (dropping by a factor of 82 at night). Comparative atmospheric densities on earth at 150 km are a factor of 3.5 higher during the day with only a 1% diurnal variation. An atmospheric composition, temperature, and density model based on OAD vertical structure measurements is presented. The inferred exospheric temperatures and atomic oxygen concentrations are surprisingly insensitive to model assumptions. This model indicates that atomic oxygen is the major species in the Venus atmosphere above 145 km at night and above 160 km during the day with mixing ratios near 140 km in excess of 0.1. Atomic oxygen concentrations determined from drag measurements near 170 km range from 1 × 109 cm−3 during the day to 3 × 107 cm−3 at night. Exospheric temperatures inferred from OAD measurements are found to vary from 100°K at night to approximately 300°K during the day. The very low exospheric temperatures discovered at night, lower than temperatures near 100 km, are inconsistent with the concept of a planetary thermosphere, in which temperature increases with altitude. We have adopted the term ‘cryosphere’ to define this region of cooling where temperature decreases with altitude. The phase of the diurnal temperature variation is consistent with theoretical models of an atmosphere with mean motion to the west more rapid than planetary rotation superimposed on solar‐antisolar motion. The very low nighttime temperatures and our observation of an absence of the predicted nighttime atomic oxygen bulge suggest substantial downward eddy transport of heat as well as of atomic oxygen. Finally, the OAD data indicate that the neutral upper atmosphere of Venus is unexpectedly insensitive to both solar extreme ultraviolet variations and variations in the solar wind. Thus, the Venus upper atmosphere may not be controlled by the major heating mechanisms generally assumed for planetary thermospheres.

Structure, luminosity, and dynamics of the Venus thermosphere

“…A value of T o of 275 + 15 K was derived from the He densities measured by the Bus Neutral Mass Spectrometer at 08" 30 local time, 37.9 ~ S latitude (von Zahn et al, 1980). Keating et al (1980) constructed an empirical model from the tAD data obtained in the early PV orbits; dayside values of Too were about 280-300 K. Airglow emission profiles measured by the OUVS were consistent with an exospheric temperature of about 275 K . Hedin et al (1983) derived a global average temperature of 228 K from ONMS data taken between December 1978 and August 1980 for a narrow latitude range of a few degrees around 16 ~ N. Variations with solar activity during this limited time period were observed to be only 10 ~o of the change seen in the terrestrial thermosphere.…”

“…A magnetic sector neutral mass spectrometer (BNMS) aboard the bus measured neutral densities from about 650 km to below the homopause, near 128 km (e.g., von Zahn, 1977;von Zahn et al, 1980). The orbiter payload included a quadrupole neutral mass spectrometer (ONMS) to measure thermospheric densities of CO 2, O, CO, N2, N, andHe (e.g., Niemann et al, 1979, 1980a, b), and an atmospheric drag experiment (OAD) to measure the total mass density profile (e.g., Keating, 1977;Keating et al, 1980). The orbiter ultraviolet spectrometer (OUVS) was designed to measure intensities of airglow emission features of neutral atmospheric atoms, molecules, and ions.…”

Ion dynamics in the Venus ionosphere

“…As a result of numerous observations made by the Pioneer Venus Orbiter (PVO) (Keating et al, 1980;Niemann et al, 1980) knowledge of the structure of the upper atmosphere of Venus during periods of solar maximum is quite complete. These observations have been supplemented by observations of the Pioneer Venus entry bus (Von Zahn et aL, 1980) during morning conditions near solar maximum.…”

Venus Atmospheric Thermal Structure and Radiative Balance

“…As for SOIR, the hydrostatic equilibrium and constant mean molecular weight are assumed to calculate the number density profiles. For both instruments, the retrieval technique needs to assume the mixing ratio of CO 2 , which is taken from Keating et al (1980). The VEXADE and torque experiments measured the total mass density of the atmosphere in the North polar region, around the apogee of the Venus Express orbit, during the aerobraking phases (Persson 2015;Rosenblatt et al 2012).…”