GRAVITY WAVE STATISTICS FOR MY34 AND MY35 FROM THE
ACS/TGO MEASUREMENTS
E. D. Starichenko, Space Research Institute of the Russian Academy of Sciences (IKI), Moscow, Russia
(starichenko.ed@phystech.edu), A. S. Medvedev, Max Planck Institute for Solar System Research, Göttingen,
Germany, D. A. Belyaev, A. A. Fedorova, O. I. Korablev, A. Trokhimovskiy, Space Research Institute of the
Russian Academy of Sciences (IKI), Moscow, Russia, F. Montmessin, LATMOS/IPSL, UVSQ Université
Paris-Saclay, UPMC Univ. Paris 06, CNRS, Guyancourt, France

Introduction:
Gravity waves (GWs) are omnipresent in
planetary atmospheres and originate from
displacements of air parcels. Since they re-distribute
energy and momentum between atmospheric layers,
GWs greatly impact atmospheric dynamics. In this
work, we study the activity of GWs in the Martian
atmosphere from solar occultation experiments
conducted by the infrared spectrometers of
Atmospheric Chemistry Suite (ACS) (Korablev et
al., 2018) on board the Trace Gas Orbiter (TGO).
Observations:
ACS is a part of the TGO, which represents the
ESA-Roscosmos ExoMars 2016 collaborative
mission. The instrument consists of three infrared
channels (Korablev et al., 2018): near-IR (NIR,
0.73-1.6 µm) , middle-IR (MIR, 2.3-4.2 µm) , and
thermal-IR (TIRVIM, 1.7-17 µm). In this work, we
use the data obtained from the MIR and NIR
instruments, operating in solar occultation mode
since April 2018. ACS-MIR is a cross-dispersion
echelle spectrometer that allows for retrieving
temperature and density vertical profiles in the
strong 2.7 µm CO2 absorption band covering the
broad altitude range of 20-180 km (Belyaev et al.,
2021, 2022). ACS-NIR, an echelle spectrometer
combined with an acousto-optic tunable filter,
measures the atmospheric structure in the 1.57 µm
CO2 band at altitudes from 10 to 100 km (Fedorova
et al., 2020, 2022). Both ACS channels possess a
high resolving power, exceeding ~25000, signal to
noise ratio more than 1000, and sound the
atmosphere with the vertical resolution of 0.5-2.5
km. During simultaneous occultations, the
instruments lines of sight target identical tangent
points that provide confidential cross validation
between the retrieved atmospheric profiles.
Presently, we report the observations for 1.5 Martian
years (MY), from the middle of MY34 (April 2018)
to the end of MY35 (January 2021), counting ~600
occultations of MIR and ~6200 occultations of NIR.
Figures 1 and 2 show MIR and NIR data coverages
respectively.

Figure 1: Latitudinal coverage of ACS-MIR
occultations versus Solar longitude (Ls) (a) and
Martian longitude (b). Grey area in (a) depicts the
global dust storm (GDS) period in MY34.

Figure 2: The same as in Fig. 1 but for ACS-NIR.

Results:
In order to derive the parameters of GWs, we
use the method described in (Starichenko et al.,
2021). Some of these parameters are shown in
Figure 3. First, from the vertical temperature
profile (Figure 3a, solid black line) we reveal the
background temperature profile (Figure 3a, red
dashed line). From these two, we find their
difference T' and the amplitude (envelope of T') of
GW packets (Figure 3b, solid black and red dashed
lines respectively). Then we calculate the
Brunt-Väisälä frequency, vertical flux of horizontal
momentum and acceleration by GWs (Figure 3c,d).
Also, we find GW potential energy, which we use
as a measure of GW activity in this study.
In many cases we observe the mechanism of
GW saturation and breaking (Figure 3b), where the
wave amplitude increases with altitude and stops
growing at ~110 km. At the same altitude, the
Brunt-Väisälä frequency of the net temperature
tends to zero (Figure 3c), which means that the
temperature gradient is approaching the adiabatic
lapse rate, and dissipation process takes place. This
process, when the GW transfers its energy to the
ambient flow, can be seen in the behavior of the
momentum flux profile, which has steep drops, and
an increase of acceleration (see Figure 3d).
Figure
4
shows
the
latitude-altitude
distributions of GW amplitude, potential energy,
momentum flux and GW drag. The cross-sections
obtained from the ACS-MIR data were averaged
over the period of MY34. It is seen that amplitudes
of temperature fluctuations grow with height. The
latitudinal structure of the GW activity in the
atmosphere is not uniform. For comparison, the

zonal wind distribution simulated with the Max
Planck
Institute
(MAOAM)
MGCM
(https://mars.mipt.ru/) is depicted with the contour
lines. During the time of observation, wind
distribution has changed from equinoctial to
solstitial and back to equinoctial. The result reflects
the largest contribution of the prograde and
retrograde jets during the perihelion solstice. The
regions with peak values of all depicted
characteristics encircle the upper edges of two
midlatitude jets. This is the consequence of intense
filtering of individual harmonics by strong
background winds. This pattern is present in
amplitude distribution and is also obvious in other
GW parameters, because they are the quadratic
functions of amplitude.
Figure 5 is a summary of GW’s observations by
ACS-MIR for the second halves of MY34 and 35.
It is seen that the GW activity is highly intermittent
depending on altitude and on season. One can also
observe the difference between hemispheres during
the perihelion season. GW activity is larger in the
northern winter than in the southern summer.
Figure 6 presents seasonal variations of the
zonal mean temperature in MY35 observed by the
NIR channel. The presented Ls intervals were
chosen to encompass maximal local time difference
between the morning and evening observations,
i.e. ~12h. The temperature for the specified interval
of longitudes was computed as the average of all
background temperature profiles pertaining to this
interval. The difference between morning and
evening measurements (Figure 6c) shows big
structured contrasts that reflect diurnal tidal
fluctuations.

Figure 3: Vertical profiles for the orbit 4926n1: a) The measured (solid black) and fitted mean temperature
(red dashed); b) wave temperature disturbance (solid black) and envelope (red dashed); c) Brunt-Väisälä
frequency calculated for the mean (black) and net temperature (red dashed); d) momentum flux (black) and
mean flow acceleration ("wave drag", upper axis, red). Shading denotes observational uncertainties.

Figure 4: MY34 latitude-altitude cross-sections of the retrieved GW a) amplitudes (in K), b) potential energy
(per unit mass), c) vertical fluxes of absolute horizontal momentum (per unit mass) and d) associated
momentum forcing (GW drag). The size of the employed latitudinal bins is 10 deg. Contour lines present the
zonal wind (in m/s) simulated with the MAOAM MGCM for MY34 (https://mars.mipt.ru/data.php) and
averaged over the same as in Figure 4 period of observations.

Figure 5: First row: Ls-latitude distribution of ACS-MIR occultations (color depicts morning/evening
observations); Second and Third rows: Seasonal-altitude variations of amplitude (in K) and potential energy (per
unit mass) for the second parts of MY34 and MY35. Two columns represent northern and southern hemispheres
respectively.

Figure 6: Latitude-altitude cross-sections of the zonal temperatures from the ACS-NIR data averaged over
four Ls intervals in MY35. First row: morning (0-12h) observations; Second row: evening (12-24h)
observations; Third row: their difference.
Conclusions:
Although small-scale gravity waves represent a
major dynamical mechanism in the Martian
atmosphere, their statistics remain largely
unquantified. The presented ACS measurements
for the first time cover almost the entire atmosphere
from the troposphere to the middle thermosphere
(~160 km) with the vertical resolution sufficient for
deriving GW characteristics. The analysis provides
evidence of systematic seasonal and intraday
variations of the GW activity, whose physical

mechanism is to be studied in detail nowadays.
Acknowledgments:
ExoMars is the space mission of ESA and
Roscosmos. The ACS experiment is led by IKI
Space Research Institute in Moscow. Science
operations of ACS are funded by Roscosmos and
ESA. The analyses of temperature profiles and
wave structures at IKI are funded by the grant
#20-42-09035 of the Russian Science Foundation.

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