Suprathermal electron spectra in the Venus ionosphere

In this study, a multistream kinetic model is used to derive the suprathermal electron intensity in the Venus ionosphere, to be compared with the observations made by the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) Electron Spectrometer (ELS) instrument onboard Venus Express (VEx) on 18 May 2006. Input parameters of the model are chosen to be consistent with the detailed ephemeris information. We consider two cases for the magnetic field line configuration. The first assumes vertical magnetic field lines, while in the second the configuration is consistent with the VEx Magnetometer (MAG) measurements for that particular orbit. The model calculations suggest that the energy spectrum of suprathermal electrons not only depends on the local photoelectron production but is also strongly influenced by transport as well as energy degradation. We confirm the postulation of Coates et al. (2008) that the spectral features seen in the ASPERA-4 ELS data are signatures of O+(S-4(0), P-2(0), and D-2(0)) production by solar HeII 30.4 nm photons at altitudes where vertical transport is important. We also identify additional photoelectron peaks, in particular the peak associated with the ionization of atmospheric He by the solar HeII 30.4 nm photons. The combination of the identified photoelectron peaks provides a validation of the proposed spacecraft potential value of similar to-5 eV derived from the O+-related features by Coates et al. (2008). The model results indicate that the condition of local equilibrium is satisfied for suprathermal electrons below similar to 155 km on Venus. However, transport becomes effective at higher altitudes, significantly altering the secondary electron production rate and leading to strongly anisotropic suprathermal electron flow. For the case with the MAG-derived, slanted magnetic field configuration, we make a data-model comparison in terms of both the absolute magnitude of the suprathermal electron intensity and the appearance of spectral peaks. The comparison suggests that the actual magnetic field lines are probably inclined more vertically than a simple extrapolation from the MAG measurements.