A Comparison Between Venus Observations and the VTGCM to
Provide Interpretation of the Varying Temperatures, Winds, and CO
Density Distributions
1
1
2
3
A. S. Brecht , S. W. Bougher , M. Sornig , A. C. Vandaele
2
( University of Michigan, Ann Arbor, Michigan, USA I.Physikalisches Institut, University of Cologne,
3
Germany Belgian Institute for Space Aeronomy, Brussels, Belgium)
1
Summary
The Michigan thermospheric general circulation
model for Venus (VTGCM) produces results that
are comparable to recently obtained VEX data
and ground based observations. The VTGCM is
a three dimensional model that can calculate
temperatures, zonal winds, meridional winds,
vertical winds, and concentration of specific
species. The VTGCM also computes the O2 IR,
and NO UV nightglow intensity distributions.
This study will examine the modeled
temperatures, winds, and the CO density
distributions. Various maps and profiles are
created to specifically be compared to VEX and
ground based observations. Sensitivity tests will
be conducted to show possible sources of
variability. The study will help provide a better
understanding of the processes driving the
variations of the Venus middle and upper
atmosphere.
Introduction
Venus Express (VEX) has been observing
atmospheric properties to glean information
about the dynamics in the middle to upper
atmosphere of Venus; specifically the wind
system and the variability seen in the
observations. The Visible and InfraRed thermal
Imaging Spectrometer (VIRTIS) observes the O2
IR night airglow in the middle atmosphere, while
the
Spectroscopy
for
Investigation
of
Characteristics of the Atmosphere of Venus
(SPICAV) observes the NO UV night airglow.
These two nightglow intensities and distributions
provide information about the global circulation
(strength of winds and altitude variations). The
O2 IR nightglow layer is seen near 97 km around
the equator at midnight with a hemispheric
average integrated vertical intensity of 0.47 MR
[Soret et al. 2010]. The NO UV nightglow is
observed in a different altitude region; ~113 km.
It is usually slightly south of the equator at 0200
LT with a hemispheric average intensity of 1.2 kR
[Gérard et al. 2010]. Furthermore, when these
nightglow
emissions
are
observed
simultaneously they do not appear to be
correlated [Gérard et al., 2009]. One reason for
the lack of correlation is the presence of strong
horizontal winds that vary with altitude in the
lower thermosphere. SPICAV has also observed
the nightside temperature above 80 km. A highly
variable warm layer was discovered around 100
km and the peak temperature ranges between
190 K and 240 K [Bertaux et al. 2007]. The
Venus
Express
Radio
Science
(VeRa)
experiment also provides temperature profiles
but in the mesosphere (below ~90 km and above
~50 km) [Patzold et al. 2007]. Most recently the
Solar Occultation in the InfraRed (SOIR)
instrument, which is mounted on top of SPICAV,
is making measurements of CO2, CO, and other
minor species. From CO2 and CO, temperatures
can be derived [Vandaele et al. 2008]. The on
going observations by VEX are augmenting
previous observations and are slowly revealing
information about Venus’ middle to upper
atmosphere dynamics.
To support the VEX mission and contribute to a
growing climatology of the Venus upper
atmosphere, ground observers have been
making similar measurements; specifically from
O2 IR nightglow measurements, sub-millimeter
measurements
of
CO,
and
heterodyne
spectroscopy
of
CO2.
From
these
measurements, temperatures and winds can be
derived. Bailey et al. 2008 observes the O2 IR
nightglow and derives nightside temperatures.
They have seen ranges of 1 K in an observation
period and have also seen bigger ranges such
as 8 K, see Table 1 for more details.
Furthermore, these warm temperatures generally
coincide with the strongest O2 IR nightglow
intensities [Bailey et al., 2008]. Rengel et al.
2008 made a few preliminary sub-millimeter
measurements of CO and has derived winds and
temperatures from these few measurements, see
Table 1. The CO abundance mostly increases
with altitude and varies from day to night near
100 km. The winds are derived by the analysis
of Doppler shifts of the molecular lines, mainly
the East and West limb positions providing the
morning
and
afternoon
zonal
wind
measurements. The wind speeds derived never
-1
exceeded 100 ms . Clancy et al. 2008 has also
made sub-millimeter measurements of CO. They
derived wind speeds (SSAS and RSZ combined)
for two days across the evening terminator and
are centered around 103 km over 30S – 30N.
The CO profiles vary from day to day and are
dependent on LT; these profiles are in agreement
with Regnel et al. 2008 data (see Table 1). The
temperature also varies depending on the day
and LT. Table 1 shows the observed temperature
values at 100 km for the evening, afternoon, and
the terminator. Another type of measurement is
the ground-based heterodyne spectroscopy of
CO2 at 10 µm wavelength. From this type of
measurement, temperature and winds at a
specific altitude (~110 km) are derived.
Sonnabend et al. 2008 has used this technique
to retrieve temperatures. Their measurements at
the equator are dependent on SZA and they also
show a latitudinal dependence [Sonnabend et
al., 2010]. These values are within the range of
other ground based observations. Sornig et al.
2008 uses the same technique but derives zonal
winds from the observations. They observe
along the limb and have found near the equator
the RSZ winds are minimal, but they increase
near mid-latitudes and decrease at higher
latitudes. Currently the wind values are not
comparable to Clancy et al. 2008 due to
observing different wind components. See Table
1 for values and comparison with other
observations.
Throughout all the observations, ground and
spacecraft, variability is being observed.
Comparing the information provided with 3-D
numerical modeling of Venus’ upper mesosphere
and lower thermosphere can help develop an
understanding of the variable dynamics observed
in Venus’ upper atmosphere.
VTGCM Simulations
The Michigan thermospheric general circulation
model for Venus (VTGCM) produces results that
are comparable to recently obtained VEX data
and ground based observations. The VTGCM is
a three dimensional model that can calculate
temperatures, zonal winds, meridional winds,
vertical winds, and concentration of specific
species. The VTGCM also computes the O2 IR,
and NO UV nightglow intensity distributions.
These parameters have been benchmarked as a
“mean” case and provide a platform to perform
sensitivity tests.
The mean case is a
representation
of
averaged
observed
parameters, such as the O2 IR nightglow
emission map [Gérard et al., 2008], the NO UV
nightglow emission map [Gérard et al., 2010],
and assuming an average nightside temperature
from all the observations.
The mean condition provides a dayside
temperature near the exobase around 240 K
while at 100 km the temperature is ~181 K at the
equator. The nightside warm area is at 105 km
with a temperature of 197 K, see figure 1. The
winds at the evening terminator near 120 km are
-1
247 ms and on the morning terminator they are
-1
201 ms , see figure 2. The model produces CO
density profiles that change over LT. At 80 km
the range is 46.3 ppm to 52 ppm and at 100 km
the range is 144 ppm to 874 ppm. The maximum
near these altitudes occurs at midnight, see
figure 3. The temperatures and winds also
contribute to producing the O2 IR nightglow layer
at 98 km near the equator with a hemispheric
averaged intensity of ~0.4 MR and the NO UV
nightglow layer is near 107 km with a
hemispheric averaged intensity of ~0.4 kR
[Bougher et al., 2010; Brecht et al., 2010].
This study will examine in detail the modeled
temperatures, winds, and the CO density. There
will be maps and profiles created to specifically
be compared to VEX and ground based
observations. Furthermore, starting with the
mean case, sensitivity tests are performed on the
nightside temperature and CO density with two
adjustable parameters: the maximum nightside
eddy diffusion and the global wind system.
These tests show how sensitive the nightside
temperature distributions and CO density
distributions are to these specific parameters.
Conclusion
The VTCM displays results which are in
agreement with the ranges VEX and ground
base have observed.
The winds, night
temperatures, and CO distribution are also with
in the ranges of observations. Sensitivity tests
have been conducted with the two nightglows
and show the two parameters, the maximum
eddy diffusion coefficient and the global wind
system, do provide variability in the atmosphere
[Brecht et al., 2010]. Specifically, the NO UV
nightglow was mainly controlled by the vertical
velocity and the O2 IR nightglow was controlled
by the eddy diffusion. Overall both parameters
impacted the night airglow. The eddy diffusion
impacted the nightglow peak layer height and
intensity, while the wind system only impacted
the intensity of the nightglows. We will show
similar variations and controllers for the CO
density
distribution
and
the
nightside
temperatures. Furthermore, we want to show the
variations we simulate are within ranges the
observations provide. With direct comparison
with observations and sensitivity tests, our goal
is to provide an interpretation of the varying
dynamics in Venus’ middle to upper atmosphere.
CO Abundances
80 km
80 km (afternoon to
evening near the equator)
100 km
100 km (afternoon to
evening near the equator)
50 ppm
[Rengel et al., 2008]
60 ppm
[Clancy et al., 2008]
300 ppm – 500 ppm
[Rengel et al., 2008]
600 ppm - >1000 ppm
[Clancy et al., 2008]
Wind Speeds
~ 90 km - ~120 km
103 km (30S – 30N)
110 km (0° lat; mid-lats;
high-lats)
< 100 m/s
[Rengel et al., 2008]
195 ± 70 m/s and 235 ±
70 m/s (SSAS+RSZ)
[Clancy et al., 2008]
~3 m/s; ~32 m/s south;
~18 m/s South, 23 m/s
North (RSZ)
[Sornig et al., 2008]
Fig 2: Venus winds (m/s) near the equator for
different LT modeled by the VTGCM
Temperatures
97 km (night)
100 km (night)
100 km (night)
100 km (afternoon)
100 km (terminator)
110 km (SZA 3° - 83°
near the equator)
110 km (terminator near
southern pole, sub-solar
point)
195 – 196 K, 181 – 190 K
[Bailey et al., 2008]
185 K
[Rengel et al., 2008]
170 – 175 K
[Clancy et al., 2008]
176 – 192 K
[Clancy et al., 2008]
172 – 186 K
[Clancy et al., 2008]
258 – 122 K
[Sonnabend et al., 2008]
160 K, 250 K
[Sonnabend et al., 2010]
Table 1: Summary table of ground observations
for CO abundances, Winds, and Temperatures
Fig 3: Venus CO density (vmr) near the equator
for different LT modeled by the VTGCM
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