Modelling the Venusian airglow

Abstract

Context. Modelling of the Venusian ionosphere fluorescence is required, to analyse data being collected by the SPICAV instrument onboard Venus Express. Aims. We present the modelling of the production of excited states of O, CO and N 2 , which enables the computation of nightglow emissions. In the dayside, we compute several emissions, taking advantage of the small influence of resonant scattering for forbidden transitions. Methods. We compute photoionisation and photodissociation mechanisms, and the photoelectron production. We compute electron impact excitation and ionisation, through a multi-stream stationary kinetic transport code. Finally, we compute the ion recombination using a stationary chemical model. Results. We predict altitude density profiles for O( 1 S) and O( 1 D) states, and emissions corresponding to their different transitions. They are found to agree with observations. In the nightside, we discuss the different O( 1 S) excitation mechanisms as a source of green line emission. We calculate production intensities of the O( 3 S) and O( 5 S) states. For CO, we compute the Cameron bands and the Fourth Positive bands emissions. For N 2 , we compute the LBH, first and Second Positive bands. All values are compared successfully to experiments when data are available. Conclusions. For the first time, a comprehensive model is proposed to compute dayglow and nightglow emissions of the Venusian upper atmosphere. It relies on previous works with noticeable improvements, both on the transport side and on the chemical side. In the near future, a radiative-transfer model will be used to compute optically-thick lines in the dayglow, and a fluid model will be added to compute ion densities.

Comprehensive calculation of the energy per ion pair or <I>W</I> values for five major planetary upper atmospheres

“…We use the TRANS-* family of codes adapted to Earth , Venus (Gronoff et al, , 2008, Mars (Simon et al, 2009), Titan (Lilensten et al, 2005;) and Jupiter (Ménager et al, 2010) as discussed below. The TRANS-* codes solve the 1-D kinetic transport Boltzmann equation for suprathermal electrons including updated elastic, ionisation, excitation and dissociative cross sections.…”

Transmission spectrum of Venus as a transiting exoplanet

“…We use their profile assuming an upper boundary temperature of 200 K at 100 km. The tropospheric temperature profile is extrapolated from the tropopause (220 K at 61 km) down to the surface assuming a constant temperature lapse rate of +8.5 K km −1 , yielding a surface temperature of 740 K. Above 100 km, the temperature profile corresponds to the neutral component of the ionosphere and follow those used by Gronoff et al (2008), initially calculated by Hedin et al (1983). The pressure profile p(z) is calculated across the whole altitude range (0-400 km) assuming the atmosphere behaves as a perfect gas at hydrostatic equilibrium, p(z) = p 0 exp − dz/H(z) , where p 0 = 93 bar is the surface pressure and…”

“…Above this altitude, X CO 2 follows the model of Gronoff et al (2008). In addition to CO 2 , we are considering molecular nitrogen (X N 2 = 3.5%), molecular and atomic oxygen (O 2 and O i), carbon monoxide (CO), water (H 2 O), and sulfur dioxide (SO 2 ).…”

“…The mixing ratio profiles for CO, SO 2 , and H 2 O are extrapolated from the values from Cotton et al (2011), de Bergh et al (2006, respectively. Mixing ratios of O 2 and O i are considered to be nonzero only in the upper atmosphere, where these species are produced; there, their mixing ratio profiles follow those calculated by Gronoff et al (2008). We finally include mesospheric ozone (O 3 ) following its recent detection by Montmessin et al (2011).…”

Sensitivity of upper atmospheric emissions calculations to solar/stellar UV flux

“…These diagnostics can be made through models such as the TRANS* family models, developed to study planetary ionospheres such as Earth (Lilensten & Blelly 2002), Mars (Witasse et al 2002(Witasse et al , 2003Simon et al 2009), Venus (Gronoff et al 2008), Titan (Lilensten et al 2005a(Lilensten et al , 2005bGronoff et al 2009aGronoff et al , 2009b, and Jupiter (Menager et al 2010). The TRANS* family models are 1D kinetic models which solve the Boltzmann equation for the suprathermal electrons along a vertical (or a magnetic field line) in the atmosphere.…”