Dust storm-enhanced gravity wave activity in the Martian thermosphere observed by MAVEN and implication for atmospheric escape
Erdal Yi?it, Alexander S. Medvedev, Mehdi Benna, Bruce Jakosky
mmanuscript submitted to
Geophysical Research Letters
Dust storm-enhanced gravity wave activity in the Martianthermosphere observed by MAVEN and implication foratmospheric escape
Erdal Yi˘git , Alexander S. Medvedev , Mehdi Benna , , Bruce M. Jakosky George Mason University, Department of Physics and Astronomy. Max Planck Institute for Solar System Research, Gø"ttingen, Germany. University of Maryland Baltimore County, Baltimore, MD, USA. Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA. Laboratory for Atmospheric and Space Physics, University of Colorado, CO, USA.
Key Points: • Thermospheric gravity wave activity doubles during the dust storm. • Gravity wave induced density fluctuations in the thermosphere are up to 40% duringthe peak storm phase. • Gravity waves significantly increase Hydrogen escape flux by modulating temperaturefluctuations.
Corresponding author: Erdal Yi˘git, [email protected] –1– a r X i v : . [ phy s i c s . s p ace - ph ] J a n anuscript submitted to Geophysical Research Letters
Abstract
Lower atmospheric global dust storms affect the small- and large-scale weather and variabil-ity of the whole Martian atmosphere. Analysis of the CO density data from the Neutral Gasand Ion Mass Spectrometer instrument (NGIMS) on board NASA’s Mars Atmosphere VolatileEvolutioN (MAVEN) spacecraft show a remarkable increase of GW-induced density fluctu-ations in the thermosphere during the 2018 major dust storm with distinct latitude and localtime variability. The mean thermospheric GW activity increases by a factor of two during thestorm event. The magnitude of relative density perturbations is around 20% on average and40% locally. One and a half months later, the GW activity gradually decreases. Enhanced tem-perature disturbances in the Martian thermosphere can facilitate atmospheric escape. For thefirst time, we estimate that, for a 20% and 40% GW-induced disturbances, the net increase ofJeans escape flux of hydrogen is a factor of 1.3 and 2, respectively. Plain Language Summary
Atmospheric gravity waves play an important dynamical and thermodynamical role incoupling the different atmospheric layers, especially on Earth and Mars. We study the effectsof a planet-encircling major dust storm on thermospheric gravity wave activity and estimatefor the first time a potential influence of gravity waves on atmospheric escape on Mars. Grav-ity activity measured in terms of relative density fluctuations increases by a factor of two dur-ing the peak phase of the storm. We show that larger-amplitude gravity waves facilitate atmo-spheric escape of hydrogen from Mars’ upper atmosphere. For 40% gravity wave-induced rel-ative disturbances of temperature, the net escape rate doubles.
Dust greatly impacts the dynamics and thermodynamics of the entire entire Martian at-mosphere (Haberle et al., 1982; Zurek & Martin, 1993; Bell et al., 2007; Cantor, 2007; Clancyet al., 2010; Heavens et al., 2011; Medvedev et al., 2013; Jain et al., 2020; Wu et al., 2020;Liuzzi et al., 2020). During storms, regolith particles are raised from the surface and modifytemperature by absorbing more solar radiation within the atmosphere and obstructing heatingof the lowermost layers (Gierasch & Goody, 1972; Rafkin, 2009). Once dust is airborne, sed-imentation may take up to several months. Depending on the scale, storms can be regional orglobal with wide-reaching implications for the planetary climate.Dust storms affect circulation at all scales, in particular the atmospheric gravity wave(GW) activity. GWs (or buoyancy waves) are ubiquitous features of all planetary atmospheres(e.g., see recent reviews of Yi˘git & Medvedev, 2019; Medvedev & Yi˘git, 2019). They havebeen extensively studied on Earth since the 1960s, when their essential role in coupling at-mospheric layers was recognized. On Mars, GWs have been observed by a number of satel-lites (Fritts et al., 2006; Tolson et al., 2007; Yi˘git et al., 2015; England et al., 2017; Jesch etal., 2019; Vals et al., 2019; Siddle et al., 2019) and studied with numerical models (Parish etal., 2009; Medvedev et al., 2013; Walterscheid et al., 2013; Imamura et al., 2016; Yi˘git et al.,2018; Kuroda et al., 2019). The main mechanism by which GWs affect the dynamics and stateof the atmosphere is transporting energy and momentum from denser lower levels and deposit-ing them in the thinner upper atmosphere. The latter is also the region where atmospheric es-cape takes place (Walterscheid et al., 2013; Chaffin et al., 2018), however the impact of GWson the escape rate has not been considered observationally before, to the best of our knowl-edge. Thermospheric response to global dust storms (GDS) have been extensively studied dur-ing the major event of 2018. In particular, Jain et al. (2020) and Elrod et al. (2020) charac-terized large-scale thermospheric effects of the GDS. Recently, based on the Ar measurementswith the Neutral Gas and Ion Spectrometer (NGIMS) instrument on board the Mars AtmosphereVolatile Evolution Mission (MAVEN) orbiter, Leelavathi et al. (2020) reported on the increase –2–anuscript submitted to
Geophysical Research Letters of GW activity during the storm of 2018 in the thermosphere. In our paper, we also quantifythermospheric GW activity during different phases of the planet encircling dust storm that com-menced on 1 June 2018 using NGIMS’ measurements of CO and discuss possible implica-tions for atmospheric escape. For the analysis of the GW activity before and during the planet-encircling global duststorm, we consider data from the NGIMS instrument onboard the MAVEN spacecraft from1 May 2018 till 30 September 2018, corresponding to L s = 167 ◦ − . ◦ in Martian Year(MY) 34. In the analysis to be presented we also compare the dust-storm GW activity in MY34with a low-dust period one Martian year earlier. For this, the low-dust period in MY 33 withsolar longitudes L s = 171 . ◦ − . ◦ (20 June-25 July 2016) is compared with a repre-sentative dust storm period in MY 34, L s = 202 . ◦ − . ◦ (1 July-5 August 2018), whenMAVEN had comparable latitude and local time coverage. The details of the data used andorbital coverage are provided in the supporting information and figures. Calculation of the GW fluctuations requires information on the background field. Forthis, we use a 7-th order polynomial fit to the logarithm of the CO (carbon dioxide) densityprofiles to determine the mean field. Polynomial fit technique has been used in a number ofprevious studies of GWs on Mars (Yi˘git et al., 2015; England et al., 2017; Siddle et al., 2019)and Earth (Randall et al., 2017). In order to calculate the GW-induced fluctuations we sub-tract the background mean density (i.e., the polynomial fit) from the instantaneous measure-ments to determine the GW disturbance as: ρ (cid:48) = ρ − ¯ ρ, (1)where the ¯ ρ is the background (polynomial fitted density) and ρ is the measured (instantaneous)CO density. The relative density perturbation in percentage is then given by dividing the den-sity fluctuations by the background mean, ρ (cid:48) / ¯ ρ , and multiplying by 100. This analysis is usedfor each orbit, including the inbound and the outbound pass.In order to evaluate the variation of the GW activity for the period of one month beforethe onset of the storm to the end phases (1 May 2018 till 30 September 2018), we first orga-nize all 683 orbits in ∼ ∼ ρ (cid:48) / ¯ ρ , are calculated from the average of data points within each bin asa function of altitude, longitude, latitude, and local time as presented in Figure 1, using 5 km, ◦ , ◦ , and 1 hour bins, respectively. For the comparison of MY34 dust-storm period (1 July-5 August 2018) to MY33 low-dust period (20 June-25 July 2016) presented in Figure 2, wefocused on the data between 160 and 200 km, and binned them in terms of 5 km, ◦ lon-gitude, ◦ latitude, and 0.5 hour bins. This for example means that data point at the altitudelevel 160 km represents the average value for the bin from 160-165 km. The typical uncer-tainty in the mean GW activity is about . − . . The details of the GW analysis and theuncertainty are discussed in the supporting material and figures. Variations of the GW-induced CO relative density fluctuations before and during thestorm are shown in Figure 1. The average fluctuations increase from 8-12% before the onset(1 June 2018) and rapidly increase afterwards, peaking with ∼
40% between 1-16 July ( L s =202 . ◦ − . ◦ ) around 190-195 km. The GW activity increases at all thermospheric heights(panel a), but the maximum occurs between ∼ –3–anuscript submitted to Geophysical Research Letters latitude, and local time variations of GW activity during the same period, focusing on the re-gion between 165-185 km. Enhanced activity is systematically seen there in all analyses. Dur-ing this period, MAVEN’s observations sampled low latitudes ( ◦ S − ◦ N) and local night-time (4-6 h). They demonstrate some difference in GW activity with larger values in the low-latitude southern (spring) hemisphere than the low-latitude northern hemisphere. MAVEN’sorbit and coverage change in latitude and local time over the analyzed period (see supplemen-tary Figure S1). From the pre-storm period toward the peak of the GDS, the spacecraft cov-erage moves from southern midlatitudes ( ◦ S − ◦ S) to equatorial latitudes ( ◦ S − ◦ N)and from local times 9-13 h to 4-6 h. These changes should be accounted for in order to iso-late them from dust-induced effects.For that, we consistently compared the GW activity during the 2018 GDS against mea-surements for low-dust conditions one Martian year earlier. MAVEN’s coverage changed with L s , latitude and local time due to specifics of the orbit. We identified two periods with sim-ilar seasonal and spatial orbit characteristics: 20 June-25 July 2016 (MY 33, L s = 171 . ◦ − . ◦ ) and 1 July-6 August 2018 (MY 34, L s = 202 . ◦ − . ◦ ) (see supplementary in-formation). Figure 2 shows the altitude, longitude, latitude and local time variations of GWactivity during these two periods. Similar to Figure 1, averaging over the height interval 165-185 km has been performed. It is seen that GW activity is about two times larger during thestorm. The southern hemisphere (SH) values are larger than those in the northern hemisphere(NH) for both the low-dust and dusty conditions. Figure 3 shows another perspective of howGW activity increases as a consequence of the GDS, presented in terms of global distributionsof wave-induced density fluctuations during the chosen periods. Here, we binned the night-time (local times 1.5-4.5 h) data between 165-185 km in terms of latitude and longitude. Theeffect of the storm on the GWs is remarkable: the activity is around 8-10% under low-dustconditions and increases to more than 20% globally and even 40% locally. The observed enhancement of GW activity in the upper atmosphere during the dust stormagrees well with the results of Leelavathi et al. (2020), but is quite unexpected. Since the grav-ity wave energy and momentum flux are proportional to the square of the wave amplitude, theincrease in observed amplitude is, in fact, even higher in terms of these dynamically impor-tant quantities. An overall effect of storms on the lower atmosphere is the convective (Figure1c of Kuroda et al., 2020) and baroclinic (Figure 2 of Kuroda et al., 2007) stabilization of thecirculation: smaller lapse rates impede development of convection, and intensified zonal jetsinhibit formation of larger-scale weather disturbances. This effectively suppresses the majormechanisms of GW generation in the lower atmosphere. Observations by Mars Climate Sounderprovided evidence of a reduction of GW activity below 30 km by several times during the 2018GDS (Heavens et al., 2020). Airborne aerosol particles do not rise above ∼
70 km. Why doesthe wave activity in the upper atmosphere increase then?In the absence of other indications favoring in-situ wave generation, a plausible expla-nation is related to changes in the upward propagation of GWs. The latter primarily dependson the background winds and wave dissipation, such as nonlinear breaking and molecular dif-fusion (Hickey & Cole, 1988; Yi˘git et al., 2008; Parish et al., 2009; Hickey et al., 2015). GWharmonics are absorbed by the mean flow, when their horizontal phase velocity approachesthe ambient wind speed. Large local vertical gradients within a wave make harmonics proneto break-down and/or enhanced dissipation. During dust storms, the middle atmosphere cir-culation undergoes substantial changes due to the storm-induced radiative heating, which inturn modulate upward propagation and dissipation of GWs. The observed increase in thermo-spheric GW activity indicates that GW harmonics encounter more favorable propagation con-ditions during the dust storm. High-resolution simulations have demonstrated that the middleatmospheric GW activity increases despite the reduction in the lower atmosphere (Kuroda et –4–anuscript submitted to
Geophysical Research Letters al., 2020), thus supporting this hypothesis. Although the details of this mechanism are not fullyunderstood, it provides evidence for yet another consequence of Martian dust storms: they fa-cilitate vertical coupling between atmospheric layers.The increase of GW activity is even more unexpected in view of the recent finding thatwave amplitudes observed by NGIMS typically decrease in proportion to the upper thermo-spheric temperature (England et al., 2017; Terada et al., 2017; Vals et al., 2019). The mech-anism that likely controls such behavior is wave saturation due to convective instability, whichpermits larger amplitudes when the atmosphere is colder. However, the thermosphere warmsduring the 2018 dust storm event (Jain et al., 2020), which would imply weaker GW activ-ity contrary to our results.
The observed enhancement of GW activity in the upper atmosphere during the MY34GDS has far-reaching implications for the state as well as short- and long-term evolution ofthe Martian atmosphere. Recent ExoMars Trace Gas Orbiter observations reported a suddenincrease of water vapor in the middle atmosphere during the storm, which was delivered therefrom below by the thermally-enhanced meridional circulation (Vandaele et al., 2019; Fedorovaet al., 2020). This finding was further supported by numerical general circulation modeling(Shaposhnikov et al., 2019; Neary et al., 2020). It was suggested that this mechanism has likelygoverned the escape of water to space over geological time scales (Fedorova et al., 2020). Thereported increase of GW activity at the very top of the atmosphere indicates that the wavesnot only contribute to the intensification of the transport, but can also directly boost the es-cape of hydrogen - a product of water photo-dissociation. The dominant process of its losseson Mars – Jeans escape – strongly depends on air temperature, which determines Maxwellianvelocities of molecules.Large density disturbances within the GW field imply similarly large variations of tem-perature: by 50 K on average and 100 K locally, based on relative density fluctuations and 250K exobase mean temperature (Medvedev et al., 2016). In order to illustrate the net increaseof atmospheric losses induced by temperature variations associated with GWs, we considerthe escape flux φ at the exobase. It is given by the expression (Chaffin et al., 2018) φ = n v mp √ π (1 + λ ) e − λ , v mp = (cid:114) kTm , λ = GM mkRT , (2)where n is the exobase density, T is the exobase temperature, v mp is the most probable Maxwell-Boltzmann velocity, λ is the escape or Jeans parameter, k is the Boltzmann constant, R is theexobase radius, m is the mass of the hydrogen atom, M is the planetary mass, and G is theuniversal gravitational constant. The parameter λ ≈ at T = 250 K at the Martian exobase(Lammer et al., 2005). The ratios of fluxes for wave-disturbed and undisturbed temperature φ ( T + δT ) φ ( T ) for sinusoidally varying temperature disturbance δT are shown in Figure 4 for twocharacteristic values: the reported 20% (on average) and 40% (locally). It is seen that the hy-drogen escape flux increases by a factor of more than 2.5 and 5.5 at the peak of the positivephase for 20%- and 40% disturbances of temperature, respectively. The difference grows withthe amplitude of fluctuations. Since the enhancement on the positive phase exceeds the reduc-tion on the negative one, the net flux (integrated over the entire wave phase, the area shownwith shades) also increases. For a 20% and 40% disturbances of temperature, the increase ofthe net escape flux is of 1.3 and 2, respectively. Note that this estimate does not account forwave-induced displacements of air parcels (pressure variations), which also contribute to theescape flux enhancement.Ordinarily, GW activity would be strongest when the thermosphere is coolest and viceversa, limiting escape as one effect canceled the other. However, dust storms reverse this paradigm,enabling larger wave amplitudes in a warmer background thermosphere. If the impact of thevertical water transport is considered, dust storms really represent a triple threat for atmospheric –5–anuscript submitted to Geophysical Research Letters losses. Constraining the role of GWs in both transport and escape can thus help with quan-tifying the processes, which have made Mars a dry planet.
Gravity wave-induced disturbances of CO density obtained from the NGIMS instrumenton board MAVEN in the Martian thermosphere have been compared for two distinctive pe-riods with the most close orbital coverage around the mid-year equinoxes: one during the dust-less Martian Year (MY) 33 and the other in the midst of the MY34 global dust storm. For thefirst time, the net increase in Jeans escape due to GW-induced fluctuations is estimated dur-ing the storm. The main results are listed below.1. GW activity approximately doubles during the dust storm. This estimate quantitativelyagrees with that of Leelavathi et al. (2020), who considered Ar density fluctuations overa half-year period.2. The magnitude of relative density perturbations is around 20% on average and 40% lo-cally.3. The estimated net increase of Jeans escape flux of hydrogen is a factor of 1.3 and 2 fora 20% and 40% GW-induced disturbances of temperature, respectively.From a technological point of view, large GW-induced thermospheric density disturbancesduring dust storms can endanger spacecraft entries into the atmosphere, similar to aircraft thatencounter bumpiness when flying over hills and mountains, and occasionally due to clear airturbulence. In all these cases, GWs are involved, and their forecasting is important and chal-lenging. Acknowledgments
The NGIMS level 2, version 8 data supporting this article are publicly available at https://atmos.nmsu.edu/data_and_services/atmospheres_data/MAVEN/ngims.html
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Figure 1. (a) Altitude, (b) longitude, (c) latitude, and (d) local time variations of the gravity wave activityin terms of relative carbon dioxide density perturbations ⇢ / ¯ ⇢ before and during the different phases of thedust storm in MY=34 from solar longitudes L s = 167 (1 May-30 September 2018). All data areaveraged over a ⇠ , , and 1 hour binsin (a)-(d), respectively. –10– Figure 1. (a) Altitude, (b) longitude, (c) latitude, and (d) local time variations of the gravity wave activityin terms of relative carbon dioxide density perturbations ρ (cid:48) / ¯ ρ before and during the different phases of thedust storm in MY=34 from solar longitudes L s = 167 ◦ − ◦ (1 May-30 September 2018). All data areaveraged over a ∼ ◦ , ◦ , and 1 hour binsin (a)-(d), respectively. –10–anuscript submitted to Geophysical Research Letters manuscript submitted to
Geophysical Research Letters
Figure 2.
Comparison of gravity wave activity between the low-dust period in MY 33 L s = 171 . . (20 June – 25 July 2016) and dust storm period in MY 34, L s = 202 . . (1 July – 5 August2018). (a) Altitude, (b) longitude, (c) latitude, and (d) local time variations of gravity wave activity under lowdust conditions in 2016 and during the dust storm in 2018. The data is presented in terms of 5 km , ,and 0.5 hour bins in (a)-(d), respectively. –11– Figure 2.
Comparison of gravity wave activity between the low-dust period in MY 33 L s = 171 . ◦ − . ◦ (20 June – 25 July 2016) and dust storm period in MY 34, L s = 202 . ◦ − . ◦ (1 July – 5 August2018). (a) Altitude, (b) longitude, (c) latitude, and (d) local time variations of gravity wave activity under lowdust conditions in 2016 and during the dust storm in 2018. The data is presented in terms of 5 km ◦ , ◦ ,and 0.5 hour bins in (a)-(d), respectively. –11–anuscript submitted to Geophysical Research Letters
Figure 3.
Comparison of the global distribution of the nighttime (1.5- 4.5 h) GW activity averaged within165– 185 km between the low-dust period in 2016 (MY 33, L s = 171 . ◦ − . ◦ , 20 June-25 July 2016)and dust storm period in 2018 (MY 34, L s = 202 . ◦ − . ◦ , 1 July-5 August 2018) presented in Fig 2.The data is binned in ◦ , ◦ longitude-latitude bins.–12–anuscript submitted to Geophysical Research Letters
Gravity wave phase (rad) R e l a t i v e e s c a p e f l u x GW T -disturbance: 20%GW T -disturbance: 40% Figure 4.
Relative escape flux φ ( T + δT ) φ ( T ) as a function of wave phase for the sinusoidally varying temper-ature disturbance δT . Blue and orange lines correspond to 20% and 40% amplitudes of fluctuations of thecharacteristic exobase temperature ( T exo = 250= 250