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[1] The nightside oxygen exosphere of Venus is investigated for high and moderate solar activity by means of a Monte-Carlo model. Hot O atoms are assumed to be produced by dissociative recombination of O2+ and NO+ molecular ions and by charge transfer processes between ionospheric O+ ions and neutral O and H atoms. The model considers rotational and vibrational excitation of the initial energy distribution of hot O atoms, includes elastic, inelastic, and quenching collisions between the suprathermal atoms and the ambient neutral atmosphere species, and uses differential cross sections for the determination of the scattering angle in the collisions. The results indicate that dissociative recombination of O2+ is, like at Venus' dayside, the most efficient source of hot O atoms at the planet's nightside. For high solar activity, the nightside exospheric density of hot O atoms is about one order of magnitude lower compared to the dayside, although between 2–10 times higher than in previous studies.

[1] We present a Monte Carlo study of the atomic hot oxygen corona of Venus. In this model we consider elastic, inelastic, and quenching collisions between the traced hot particle and the ambient neutral atmosphere as well as differential cross sections to determine the scattering angle in the collisions. We also include rotational and vibrational excitation energies for the calculation of the initial energy of the produced hot oxygen atoms. Our results indicate that the differential cross sections and the fraction between elastic, inelastic, and quenching collisions are the most sensitive parameters which effect the corona density. We found that the hot O densities inferred from PVO observations can only be reproduced during high solar activity based on a forward scattering model but without inelastic and quenching collisions. The corona densities for low solar activity (VEX solar conditions) are about a factor of 2–3 smaller than for high solar activity.

[1] After more than two years in orbit still no Venus Express observations were published concerning the hot oxygen corona of Venus which could verify the corresponding controversial observations of Venera 11 and PVO, three decades ago. Based on recent energy and mass dependent collision cross sections, the energy distributions of hot atomic oxygen created via dissociative recombination of O2+ are calculated in the daytime thermosphere by means of a 3D Monte Carlo approach. The exosphere density is obtained from the corresponding energy density and angular distribution at 240 km altitude by using a test particle model which traces the ballistic trajectories of hot O atoms in the exosphere. Our study indicates that upon taking into account proper input parameters, the hot oxygen corona appears to be substantially less dense than suggested by previous simulations.

Figure 9. Simulation results in the terminator plane, i.e., looking from behind on Venus. The solar wind is orientated out of the plane (+x axis), the convective electric field is antiparallel to the (+z) axis and points away from the planet; thus, the background magnetic field completes the right-handed system (+y axis). Figure 10 displays (left) the solar wind density (nsw), (middle) the heavy ion density (nhi), and (right) the background magnetic field. An asymmetric bow shock, exhibiting a shocklet structure, is formed in front of the obstacle owing to the supersonic solar wind (left plot). The interplanetary magnetic field is draping around Venus generating an induced magnetotail on the nightside of the planet (right plot). See text for details. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, E00B98, doi:10.1029/2009JE003377, 2009