Search

Gravity waves are a key mechanism that facilitates coupling between the lower and upper atmosphere of Mars. In order to better understand the mean, large‐scale impacts of gravity waves on the thermosphere, a modern whole atmosphere, nonlinear, non‐orographic gravity wave parameterization scheme has been incorporated into a three‐dimensional ground‐to‐exosphere Mars general circulation model, the Mars Global Ionosphere‐Thermosphere Model (M‐GITM). M‐GITM simulations utilizing the gravity wave parameterization indicate that significant gravity wave momentum is deposited in the thermosphere, especially within the altitude range of 90–170 km. This impacts the winds in the thermosphere; in particular, M‐GITM simulations show a decrease in speed of the wind maximum in the summer hemisphere by over a factor of two. Gravity wave effects also impact the temperatures above 120 km in the model, producing a cooler simulated thermosphere at most latitudes. M‐GITM results were also compared to upper atmospheric temperature and wind data sets from the MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft. Some aspects of wind data‐model comparisons improved once the gravity wave scheme was added to M‐GITM; furthermore, a cooler temperature profile produced by these new M‐GITM simulations for the MAVEN Deep Dip 2 observational campaign resulted in a closer data‐model comparison, particularly above 180 km. Overall, these modeling results show that gravity waves play an important role for the energy and momentum budget of the Martian thermosphere. Plain Language Summary: Atmospheric gravity waves are an important physical process in the upper atmosphere of Mars. To better understand the average effects of gravity waves on the temperatures and winds above 100 km, a modern numerical scheme designed to represent the relevant physics has been added to a 3‐D general circulation model, M‐GITM (Mars Global Ionosphere‐Thermosphere Model), which extends from the surface to about 250 km. Results from these M‐GITM simulations show that in the upper atmosphere, the wind maximum in the summer hemisphere decreases in speed by over a factor of two once the effects of gravity waves are added to the model. Additionally, above 120 km, the model now produces a cooler upper atmosphere, on average. The new M‐GITM results were also compared to select upper atmospheric temperature and wind data sets from the MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft. Data‐model comparisons in upper atmospheric wind speeds for a January 2017 observational campaign improve with the addition of gravity wave effects, as do data‐model comparisons for upper atmospheric temperatures for a MAVEN Deep Dip campaign. Overall, these model results show that gravity waves can have significant impacts on the winds and temperature structure in the Martian upper atmosphere. Key Points: Large‐scale impacts of gravity waves in the thermosphere are examined using a modern gravity wave scheme in a Mars general circulation modelSignificant gravity wave momentum deposition is found in model simulations from 90 to 170 km altitudeGravity waves which propagate to the upper atmosphere of Mars can have an appreciable impact on thermospheric winds and temperatures [ABSTRACT FROM AUTHOR]

In situ measurements of ionospheric and thermospheric temperatures are experimentally challenging because orbiting spacecraft typically travel supersonically with respect to the cold gas and plasma. We present O2+ ${\mathrm{O}}_{2}^{+}$ temperatures in Mars' ionosphere derived from data measured by the SupraThermal And Thermal Ion Composition instrument onboard the Mars Atmosphere and Volatile EvolutioN spacecraft. We focus on data obtained during nine special orbit maneuvers known as Deep Dips, during which MAVEN lowered its periapsis altitude from the nominal 150 to 120 km for 1 week in order to sample the ionospheric main peak and approach the homopause. We use two independent techniques to calculate ion temperatures from the measured energy and angular widths of the supersonic ram ion beam. After correcting for background and instrument response, we are able to measure ion temperatures as low as 100 K with associated uncertainties as low as 10%. It is theoretically expected that ion temperatures will converge to the neutral temperature at altitudes below the exobase region (∼180–200 km) due to strong collisional coupling; however, no evidence of the expected thermalization is observed. We have eliminated several possible explanations for the observed temperature difference between ions and neutrals, including Coulomb collisions with electrons, Joule heating, and heating caused by interactions with the spacecraft. The source of the energy maintaining the high ion temperatures remains unclear, suggesting that a fundamental piece of physics is missing from existing models of the Martian ionosphere. Plain Language Summary: A small fraction of a planet's atmosphere, the ionosphere, exists as a plasma made of positive ions produced when sunlight removes an electron from a neutral particle. We measured the temperatures of ions in Mars' lower ionosphere for the first time since 1976 using an instrument called SupraThermal And Thermal Ion Composition on the Mars Atmosphere and Volatile EvolutioN spacecraft. In the part of the ionosphere with the most ions, ions were expected to have the same temperature as the neutral atmosphere because ions and neutrals collide with each other many times per second. However, we found that during the day, the ions were more than 100 K hotter than the neutrals. We eliminated many possible sources of energy for the ions, including sunlight, collisions with electrons, chemical reactions, and electric fields in Mars' atmosphere. The source of the energy that keeps the ions hot remains unclear, suggesting that a fundamental piece of physics is missing from existing models of the Martian ionosphere. Key Points: We present ionospheric O2+ temperatures down to 100 K derived from Mars Atmosphere and Volatile EvolutioN SupraThermal And Thermal Ion Composition measurements from 120 to 300 km altitudeIonospheric O2+ is warmer than the neutral gas more than 7 scale heights below the exobase, which requires an active source of ion heatingThe source of the ion heating remains unclear, indicating a gap in our understanding of thermalization in planetary ionospheres [ABSTRACT FROM AUTHOR]

In the present study, we investigate the characteristics of helium (He) bulges in the Martian upper atmosphere using He densities and winds measured by the Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. The observations are compared with those predicted by the Mars Global Ionosphere Thermosphere Model (M‐GITM). The results of the present study show that the nightside He bulge is a persistent feature of the Martian upper atmosphere in all seasons. The He densities inside the bulges are 1–2 orders of magnitude greater than those on the dayside. In solstices, the bulges are observed in the winter polar region which is in accordance with the model predictions. In equinoxes, however, the bulges are observed to extend from mid‐latitudes into the southern polar regions (>60°S), which is contrary to the model predictions at mid‐latitudes. These anomalous bulges are predominantly observed in the northern spring equinox and are 10–30 × greater than the modeled ones. During the autumnal equinox, the observed winds depart from the modeled winds. Furthermore, the observed winds point to the southern polar regions where the bulges are observed. Thus, the results of the present study indicate that in equinoxes the regions of local vertical advection, that are responsible for the formation of the bulges, are displaced toward the southern polar regions. The results of the present study point to the need of a larger wind database from NGIMS in southern polar region, particularly during equinoxes. Plain Language Summary: Helium in the upper atmospheres of Earth, Venus, and Mars is known to accumulate on the nightside which is often referred as "He bulge." The upwelling of winds on the dayside and their downwelling on the nightside, combined with large‐scale circulation, is the primary driver of the bulge formation. The densities inside the He bulge are, in general, 1–2 orders of magnitude greater than those on the dayside. In the present study, the He bulge in the Mars upper atmosphere is investigated using the neutral densities measured by the Mars Atmosphere and Volatile EvolutioN spacecraft and those of a global model. In the northern summer, the He bulge occurs in the winter polar nightside which is in accordance with the model predictions. In spring and autumn, however, the observed He bulges extend from mid‐latitudes into the southern polar region whereas the model predicts their presence at mid‐latitudes. The southern polar bulges in autumn are observed in regions where the observed winds point toward the southern pole. Inclusion of the effects of the upward propagating small‐scale waves on the Martian upper atmosphere in the model, along with a larger observational wind database, is likely to address the data‐model differences. Key Points: In equinoxes, helium bulges are observed to extend from mid‐latitudes in to the southern polar regions (>60°S)The observed helium densities in the southern polar regions in equinoxes are 10–30 × greater than the modeled onesThe observed winds seem to support the formation of helium bulge in the southern polar region during the autumnal equinox [ABSTRACT FROM AUTHOR]

The effects of solar flares on the upper atmosphere of Mars are large but poorly constrained by observations. These effects are an important aspect of the response of Mars to space weather events and may also influence the escape of volatiles from Mars, particularly in the solar system's early history. Here we report the effects of the X8.2 flare on 10 September 2017 on the density and composition of the thermosphere of Mars. This analysis uses neutral number densities of He, O, N2, CO, Ar, and CO2 from the MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS). From these observations, we investigate how the enhanced solar irradiance during the flare produced changes in the neutral upper atmosphere of Mars due to atmospheric heating and photochemistry. Flare‐produced photochemical changes in the neutral thermosphere of Mars have been previously implied but not quantified. We find that at fixed altitudes, the number densities of all species barring He increase and the O/CO2 ratio decreases by ∼15–45%, indicating thermal expansion. However, when viewed at fixed total number densities, the densities of CO2 and Ar decrease, and the O/CO2 ratio increases by up to a factor of ∼3. The photodissociation of CO2 is one photochemical process that produces changes resembling those identified. The quantified changes will help to constrain the shifting chemical makeup of the upper atmosphere due to these impactful flare events and may aid modeling of flare behavior and the impact of flares on the evolution of the Martian climate. Key Points: Mars thermospheric densities and composition are affected by this solar flareThermospheric responses at fixed altitude (heating‐based changes) and fixed number density (chemistry‐based changes) differThe O/CO2 ratio nearly triples at low number densities during this flare [ABSTRACT FROM AUTHOR]

The onset of the planet encircling dust event (PEDE‐2018) started around 1 June 2018 as observed by Mars Reconnaissance Orbiter/Mars Color Imager, peaking around 7–10 July and persisting through mid‐October 2018. After the onset of the event, the upper atmosphere underwent significant changes in density and thermal structures. Mars Atmosphere and Volatile Evolution‐Neutral Gas and Ion Mass Spectrometer (MAVEN NGIMS) had a good opportunity to observe these changes from the first detection in the upper atmosphere and throughout the duration of the PEDE. The compositional changes included increased density at a constant altitude for CO2 and Ar, while the O decreased from the peak throughout the decay of the bulk of the PEDE. Plain Language Summary: From June through October 2018 Mars experienced a planet encircling dust event (PEDE‐2018), a fairly rare event last observed in 2007. The dust storm grew from a local event to cover the entire planet and was opaque enough that so little sunlight reached the surface that the solar‐powered opportunity rover ceased operations and all attempts to re‐establish contact with it were unsuccessful. Meanwhile, the orbiter Mars Atmosphere and Volatile Evolution (MAVEN) was able to observe changes in the upper atmosphere in the composition as a result of this globally extensive PEDE. MAVEN observed increases in both the CO2 and Ar while also observing an unexpected reduction in the O densities. Key Points: MAVEN/NGIMS observed increased of CO2 and Ar densities observed in the upper atmosphere corresponding to the peak of the dust eventUnexpected decrease in O densities in the upper atmosphere (160–250 km) was simultaneously observedComparisons between model and data results show good agreement with scale height and temperatures, further M‐GITM model revisions needed to capture circulation effects [ABSTRACT FROM AUTHOR]

Electron temperatures are a key parameter for understanding the ionosphere‐thermosphere system. This study uses in situ data from the MAVEN mission to investigate the level of coupling between the Martian ionospheric electrons and the neutral thermosphere below the altitude of 190 km. Such coupling is responsible for significant diurnal variations and dawn/dusk asymmetries in both the neutral and plasma portions of the ionosphere‐thermosphere system. Earlier theoretical studies suggested that neutral species controlled the electron temperature by collisional cooling thus implying a diurnal trend in electron temperatures due to the diurnal variability of the neutrals. This hypothesis is evaluated across the dayside ionosphere in the present work. Observations of neutral densities confirm that neutral‐electron collisions are the primary process controlling the cooling of electrons in the Martian ionosphere below ~190 km. The consequence of this is that electron temperatures can be anticorrelated with the extreme ultraviolet (EUV) irradiance due to enhanced electron cooling brought about by an expansion of neutral densities. The results also indicate that in the afternoon sector, neutral densities are less responsive to changes in EUV. This might be associated with enhanced winds leading to dynamic cooling that mitigates the thermal expansion of the thermosphere in the afternoon region. Subsequently, the electron temperature as a function of EUV is relatively uncorrelated in the afternoon sector. Plain Language Summary: The upper atmospheres of planets are partially composed of an ionized plasma. The plasma component is important to understanding the behavior of this region as well as how radio transmissions travel through it. Plasma temperatures are especially important as they determine the rates of many chemical reactions in the upper atmosphere. This analysis confirmed that at Mars, plasma temperatures are made cooler by collisions with carbon dioxide. The upper atmosphere expands when the amount of solar ultraviolet light increases leading to increased amounts of carbon dioxide at any given altitude. Therefore, more solar ultraviolet radiation generally leads to cooler plasma in the upper atmosphere. We find that there is significant variation to this phenomenon across the planet's dayside with dawn, noon, and dusk each having a different atmospheric and plasma temperature response to solar ultraviolet light. Key Points: Electron temperature in the Martian ionosphere is anticorrelated with ultraviolet irradiance; this is consistent with neutral collisional coolingThe change in electron temperatures with ultraviolet irradiance is strongest near the morning and noon sectorsElectron temperatures exhibit little response to ultraviolet irradiance in the afternoon [ABSTRACT FROM AUTHOR]

Dedicated in situ measurements of Mars' exospheric Ar density by Neutral Gas Ion Mass Spectrometer/Mars Atmosphere and Volatile Evolution (NGIMS/MAVEN) highlight profiles displaying two slopes altitude variation. Below 230 km, the Ar density is characterized by scale height of 13.5 ± 1.7 km and above 400 km by scale height of 156 ± 25 km. A comparison with the model HELIOSARES suggests that the most probable origin of the Ar nonthermal component is the collisional interaction, below the exobase, between the atmospheric Ar and the nonthermal O atoms produced by the dissociative recombination of O2+. These measurements therefore provide a new approach to constrain one of the main sources of Mars' atmospheric escape. They also lead to set up an upper limit on the efficiency of pickup ion sputtering at Mars. The comparison with HELIOSARES suggests that NGIMS detection sensitivity might allow a positive detection of this mechanism if more measurements at high altitude deep in the nightside are performed. Key Points: We present the first in situ detection of Mars' nonthermal exosphereThis detection provides a new constrain on Mars' atmospheric escapeThis detection suggests that sputtering might be observable deep in the nightside [ABSTRACT FROM AUTHOR]

Abstract: On 10 September 2017, the Mars Atmosphere and Volatile EvolutioN mission observed a particularly strong X‐class flare. This paper will focus on observations made by Neutral Gas and Ion Mass Spectrometer (NGIMS) and the flare response detected by extreme ultraviolet monitor. We focus the data to the region of the upper atmosphere from 160 to 300 km and to 10 orbits before and after the flare. The flare peaked near 16:12 UTC with the closest periapsis pass from 17:30 to 17:54 UTC (Lee et al., , https://doi.org/10.1029/2018GL079162). NGIMS measured a significant enhancement in the neutral densities above 195 km for the flare. This enhancement stands out for the major species measured by NGIMS (Ar, CO2, CO, O, and N2). The correlation of the flare and the enhancement in density and temperature in the upper atmosphere indicates that solar flare heating is most likely the main driver and has important implications for the effects of space weather events on terrestrial atmospheres. [ABSTRACT FROM AUTHOR]

Abstract: We report the response of the Martian upper atmosphere to a strong X‐class flare on 10 September 2017 as observed by the Imaging Ultraviolet Spectrograph (IUVS) instrument aboard the Mars Atmosphere Volatile EvolutioN (MAVEN) mission. The solar flare peaked at 16:24 hr UT, and IUVS dayglow observations were taken about an hour after the flare peak. Retrieved temperatures from IUVS dayglow observations show a significant increase during the flare orbit, with a mean value of ∼270 K and a maximum value of ∼310 K. The retrieved temperatures during the flare orbit also show a strong latitudinal gradient, indicating that the flare‐induced heating is limited between low and middle latitudes. During this event IUVS observed an ∼70% increase in the observed brightness of CO 2 + ultraviolet doublet and CO Cameron band emission at 90 km, where high‐energy photons (< 10 nm) deposit most of their energy. [ABSTRACT FROM AUTHOR]