Cédric Cox

Monte Carlo model of electron transport for the calculation of Mars dayglow emissions

[1] A model of the photoelectron collision-induced component of the Mars dayglow using recent cross sections and solar flux is described. The calculation of the photoelectron source of excitation is based on a stochastic solution of the Boltzmann equation using the direct simulation Monte Carlo method. The neutral atmosphere is taken from outputs of a global circulation model, and recent inelastic collision cross sections are adopted. The calculated vertical profiles of the CO Cameron bands and CO2 doublet emissions integrated along the line of sight compare well with the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) limb profiles observed with the SPICAM spectrograph on board Mars Express made at Ls = 166 during the summer season at northern midlatitudes. The comparison shows agreement to within the uncertainties of the excitation cross sections. Seasonal changes in the brightness and the altitude of the emission peaks are predicted with intensity variations in the range 15–20%.

Distribution of the ultraviolet nitric oxide Martian night airglow: Observations from Mars Express and comparisons with a one‐dimensional model

Limb observations with the SPICAM ultraviolet spectrometer on board the Mars Express orbiter revealed ultraviolet nightglow emission in the δ (190-240 nm) and γ(225 -270 nm) bands of nitric oxide. This emission arises from radiative recombination between O( 3 P) and N( 4 S) atoms that are produced on the day side and form excited NO molecules on the night side. In this study, we analyze the night limb observations obtained during the MEX mission. In particular, we describe the variability of the emission brightness and its peak altitude. We examine possible correlations with latitude, local time, magnetic field strength or solar activity. We show that the altitude of maximum emission varies between 55 and 92 km while the brightness is in the range 0.2 to 10.5 kR. The total vertical emission rate ranges from 8 to 237 R with an average value of 36 ± 52 R. The observed topside scale height of the emission profile varies between 3.8 and 11.0 km, with a mean value of 6 ± 1.7 km. We use a chemical-diffusive atmospheric model where the eddy coefficient, whose value in the Mars thermosphere is uncertain, is a free parameter to match the observed peak altitude of the emission. The model solves the continuity equation for O( 3 P), N( 4 S), and NO using a finite volume method on a one-dimensional grid. We find that the downward flux of N atoms at 100 km varies by two orders of magnitude, ranging from 10 7 to 10 9 atoms cm -2 s -1 .

Concurrent observations of the ultraviolet nitric oxide and infrared O2 nightglow emissions with Venus Express

Two prominent features of the Venus nightside airglow are the nitric oxide δ and γ bands produced by radiative association of O and N atoms in the lower thermosphere and the O2 infrared emission generated by three-body recombination of oxygen atoms in the upper mesosphere. The O2 airglow has been observed from the ground, during the Cassini flyby, and with VIRTIS on board Venus Express. It now appears that the global structure of the two emissions shows some similarities, but the statistical location of the region of strongest emission is not coincident. The Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) ultraviolet spectrograph has collected a large number of spectra of the Venus nitric oxide nightside airglow. Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) images have been obtained at the limb and in the nadir-viewing mode and have provided new information on the horizontal and vertical distribution of the emission. We present the first concurrent observations of the two emissions observed with Venus Express. We show that nadir observations generally indicate a low degree of correlation between the two emissions observed quasi-simultaneously at a common location. A statistical study of limb profiles indicates that the altitude and the brightness of the two airglow layers generally do not covary. We suggest that this lack of correlation is explained by the presence of strong horizontal winds in the mesosphere-thermosphere transition region. They carry the downflowing atoms over large distances in such a way that regions of enhanced NO emission generally do not coincide with zones of bright O2 airglow.

Mars ultraviolet dayglow variability: SPICAM observations and comparison with airglow model

mean brightness of 21.6 ± 7.2 kR with a peak located at 119.1 ± 7.0 km. We show that the brightnessintensityoftheairglowsismainlycontrolledbythesolarzenithangleandbysolar activity. Moreover, during Martian summer of year 2005, an increase of the airglow peak altitudehasbeenobservedbetweenLs=120°and180°.Wedemonstratethatthisvariationis due to a change in the thermospheric local CO2 density, in agreement with observations performed by stellar occultation. Using a Monte Carlo one‐dimensional model, we also show that the main features of the emission profiles can be reproduced for the considered set of data. However, we find it necessary to scale the calculated intensities by a fixed factor.