206
O. I. Korablev
3.3. Emissions
The CO dayglow at 4.7 μm on Venus has been observed using the long-slit
high-resolution spectrograph CSHELL at NASA IRTF with a resolving power of
4 × 10
4
. The CO (1–0) dayglow is optically thick, its intensity weakly depends
on the CO abundance and it proves poorly accessible for diagnostics of the Venus
atmosphere. Six observed lines of the CO dayglow at the hot (2–1) band show a
significant limb brightening typical of an optically thin airglow. Vertical intensi‑
ties of the CO (2–1) band corrected for viewing angle and the Venus reflection
are constant at 3.3 MR in the latitude range of ±50° at a solar zenith angle of 64°.
Rotational temperatures of the CO (2–1) dayglow should reflect ambient temper‑
ature near 111 km. The observed temperatures are slightly higher on the south
with a mean value of 203 K [Krasnopolsky, 2014].
The EUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution
by the UVIS instrument during the Cassini flyby of in 1999 are analized. Emis‑
sions from OI, OII, NI, Cl and CII and CO have been identified and their disc
average intensity has been determined. The O, N(2) and CO densities are copared
to the empirical VTS3 model [Gerard et al., 2011].
A model of the O
2
(a
1
Δ
g
) nightglow limb profiles perturbed by the action of
gravity waves propagating in the Venus’ upper atmosphere is considered. The
observational data are from VIRTIS/Venus Express. A high variability observed
in the shape of the nightglow limb profiles between 80 and 120 km, often char‑
acterized by the presence of a double peak, suggests the occurrence of the grav‑
ity waves. A well-known theory used to study terrestrial density fluctuations
induced by propagation of gravity waves is used. The retrieved vertical wave‑
lengths and amplitudes of the waves at ~100 km are 7–16 km and 3–14% respec‑
tively. Temperature fluctuations exceed 40% at higher altitudes (115–120 km)
thus inducing either wave breaking or dissipation. Intrinsic horizontal phase
velocities are expected to vary between 32 and 85 m s
‑1
[Altieri et al., 2014].
3.3. Clouds
A glory has been observed recently by Venus Express orbiter. Glory is an
optical phenomenon that poses stringent constraints on the cloud properties. It is
possible to constrain two properties of the particles at the cloud tops (about 70
km altitude) which are responsible for a large fraction of the solar energy ab‑
sorbed by Venus: A very accurate estimate of the cloud particles size of 1.2 μm
with a very narrow size distribution, and the refractive index higher than that of
sulfuric acid previously proposed for the clouds composition. Assuming that the
species contributing to the increase of the refractive index is the same as the
207
Planetary
Atmospheres
unknown UV absorber, it is possible to constrain the list of candidates: Either
small ferric chloride (FeCl
3
) cores inside sulfuric
acid particles or elemental
sulfur coating their surface. Both species have been suggested in the past as
candidates for the as yet unknown UV absorber [Markiewicz et al., 2014].
Since the discovery of ultraviolet markings on Venus, their observations have
been a powerful tool to study the morphology, motions and dynamical state at
the cloud top level. The cloud top morphology has been monitored by the Venus
Monitoring Camera (VMC) on Venus Express mission. The camera acquires
images in four narrow-band filters centered at 365, 513, 965 and 1010 nm with
spatial resolution from 50 km at apocentre to a few hundred of meters at pericen‑
tre. The VMC experiment provides a significant improvement in the Venus im‑
aging as compared to the capabilities of the earlier missions. The VMC observa‑
tions revealed multiple morphology structures and provides an unprecedented
dataset [Titov et al., 2012].
The cloud top structure on Venus was studied by joint analysis of the data
from VIRTIS and the atmospheric temperature sounding
by the Radio Science
experiment (VeRa) onboard Venus Express. The cloud top altitude and aerosol
scale height are derived by fitting VIRTIS spectra at 4–5 μm with temperature
profiles taken from the VeRa radio occultation. A gradual descent of the cloud
top from 67.2 ± 1.9 km in low latitudes to 62.8 ± 4.1 km at the pole and decrease
of the aerosol scale height from 3.8 ± 1.6 km to 1.7 ± 2.4 km is observed. These
changes correlate with the mesospheric temperature field. In the cold collar and
high latitudes the cloud top position remarkably coincides with the sharp minima
in temperature inversions suggesting importance of radiative cooling in their
maintenance. This behavior is consistent with the earlier observations. Spectral
trend of the cloud top altitude derived from a comparison with the earlier obser‑
vations in 1.6–27 μm wavelength range is qualitatively consistent with sulfuric
acid composition of the upper cloud and suggests that
particle size increases from
equator to the pole [Lee et al., 2012].
3.4. Structure and Dynamics
Six years of continuous monitoring of Venus by European Space Agency’s
Venus Express orbiter provides an opportunity to study dynamics of the atmos‑
phere our neighbor planet. Venus Monitoring Camera (VMC) on-board the or‑
biter has acquired the longest and the most complete
so far set of Venus UV
images. These images enable a study the cloud level circulation by tracking
motion of the cloud features. Total number of wind vectors derived is 45,600 for
the manual tracking and 391,600 for the digital method. This allowed to deter‑
mine
the mean circulation, its long‑term
and diurnal trends, orbit‑to‑orbit