Elsevier

Icarus

Volume 382, August 2022, 115006
Icarus

Research Paper
MAVEN/NGIMS wind observations in the martian thermosphere during the 2018 planet encircling dust event

https://doi.org/10.1016/j.icarus.2022.115006Get rights and content

Highlights

  • Winds in the upper atmosphere of Mars during the 2018 global dust storm are reported.

  • These are the first thermospheric wind measurements during a Mars global dust storm.

  • Mean wind speed increased slightly, but not significantly, near dust storm onset.

  • Orbit-to-orbit variability in velocities increased near the peak of the dust storm.

Abstract

During the 2018 planet encircling dust event (PEDE) at Mars, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft’s Neutral Gas and Ion Mass Spectrometer (NGIMS) was able to take a series of neutral wind observations in the thermosphere at around 150–220 km altitude. These MAVEN observations are the first in-situ measurements of thermospheric wind velocities during a Mars global dust storm. From these measurements, it was found that the observed averaged horizontal thermospheric wind speeds peaked to nearly 200 m/s during the onset of the dust storm before decreasing to about 140 m/s near the peak of the storm. The maximum averaged wind speed observed during the dust storm is not significantly greater than averaged wind speeds at other times, but does represent a local maximum that coincides with the timing of the dust storm. Additionally, throughout the mature and early decay stage of the PEDE, NGIMS wind observations show increased short-scale variability that distinguishes this time period from most other observational periods.

Introduction

Starting in early June 2018, a planet encircling dust event (PEDE) at Mars spread across the planet. Observations from multiple spacecraft have revealed significant changes throughout the entire Martian atmosphere at this time associated with the global-scale dust storm. In particular, this event has provided opportunity for new insight on the impacts of planet encircling dust storms on the upper atmosphere of Mars.

The onset of the storm that would develop into the PEDE began around 30 May 2018 (Ls=184.4), as seen by Mars Reconnaissance Orbiter (MRO)/Mars Color Imager (Cantor and Malin, 2018). The Mars Climate Sounder (MCS) had detected evidence of the storm’s influence in the zonal mean 50 Pa temperatures (25 km) by 3 June (Ls=186.7) (Kass et al., 2020), and the first evidence of an impact by the storm on the upper atmosphere was seen around 8 June (Ls=189.2) by the Mars Atmosphere and Volatile Evolution (MAVEN) mission (Elrod et al., 2020). By 17 June (Ls=194.9), the storm became planet encircling. The PEDE peaked around 7–10 July (Ls=207–208), as determined by zonally averaged temperatures observed by MRO/MCS in the middle atmosphere (Kass et al., 2020). This corresponds to a period where the dust was widespread and optical depth nearly uniformly high. The PEDE remained in a mature stage for nearly a month, until 18 July (Ls=213.3), after which the PEDE began to enter into an early decay stage during which middle atmospheric temperatures and dust column opacities started to decrease. The decay of the storm and return to seasonal conditions occurred gradually, lasting through mid-September or longer (Kass et al., 2020). More details on the development and evolution of the PEDE over time are provided in Kass et al. (2020) and Sánchez-Lavega et al. (2019), among others.

As the dust storm was becoming large scale, changes in upper atmospheric densities and temperatures were seen. Observations from the MAVEN/NGIMS (Neutral Gas and Ion Mass Spectrometer) instrument indicated increased thermospheric CO2 densities at the peak of the storm at 170 km, though not all species measured by NGIMS followed this trend (Elrod et al., 2020). Similarly, an analysis of both NGIMS and Mars Orbiter Mission (MOM)/Mars Exospheric Neutral Composition Analyzer (MENCA) observations found neutral densities in the 150–220 km altitude range increased by a factor of 2–3 (Venkateswara Rao et al., 2020). The effects of atmospheric expansion during the 2018 PEDE were also identified in MAVEN/IUVS (Imaging Ultraviolet Spectrograph) observations of an increase in the altitude of the absorption layer of the Lyman-alpha emission by CO2 near the peak of the storm (Chaufray et al., 2020) as well as through observations of the increased altitude of oxygen dayglow emission peak starting as the dust storm was becoming large-scale (Gkouvelis et al., 2020). In addition, an analysis of IUVS data suggested that thermospheric warming occurred during the PEDE, with an increase of 20 K during late afternoon after the onset of the PEDE (Jain et al., 2020). Furthermore, in the thermosphere, density perturbations driven by gravity waves seen in the NGIMS data were also observed to increase during the peak of the PEDE (Yiğit et al., 2021).

With changing background atmospheric conditions in the Martian thermosphere in response to the 2018 PEDE, it might also be expected that thermospheric winds would be impacted as well. Direct observations of winds in the upper atmosphere of Mars have not been possible for previous global dust storms. One of the few measurements of upper atmospheric winds available prior to 2016, zonal wind speeds derived from Mars Global Surveyor (MGS) accelerometer and rate data were reported for a regional dust storm in 1997 and found to increase by 200 m/s above the average value prior to the storm (Baird et al., 2007).

Starting in 2016, the MAVEN spacecraft has provided the unique and novel ability to measure neutral thermospheric horizontal velocities in-situ using the NGIMS instrument (Benna et al., 2019). While some knowledge of the Martian global circulation pattern and its variability has been able to be inferred through observations from other orbiting spacecraft such as through temperature observations of the middle and lower atmosphere (e.g. Heavens et al., 2011) as well as from nightglow observations in the middle to upper atmosphere (e.g. Schneider et al., 2020), MAVEN/NGIMS observations are the first in-situ thermospheric wind velocity measurements (Benna et al., 2019). The MAVEN/NGIMS thermospheric wind observations thus provide a new way to examine the potential impacts of the 2018 PEDE in the Martian upper atmosphere. In this analysis, the MAVEN thermospheric wind measurements taken during the 2018 PEDE are examined in terms of campaign-averaged wind speed and orbit-to-orbit variability and how these evolve over the duration of the storm.

Section snippets

Method

The wind velocities in the upper atmosphere of Mars are measured in-situ by the NGIMS instrument onboard MAVEN. NGIMS is a quadrupole mass spectrometer with the primary purpose of characterizing the neutral and ion composition of the Martian upper atmosphere (Mahaffy et al., 2015b, Mahaffy et al., 2015a). However, a new technique was developed in 2016 which allows the instrument to be used to measure horizontal wind velocities at the location of the spacecraft as it passes through the

Results

During the time period of the storm, from its onset to the beginning of the early decay phase (late May through about September), the fastest campaign-averaged wind speed observed was 200 m/s (see Fig. 2). This maximum average speed was from the 9–10 June campaign, which occurred after the onset of the storm, but before the storm became planet encircling. As seen in Fig. 2, averaged speeds gradually decrease after the 9–10 June campaign and reach a local minimum of 140 m/s during the 11–12

Potential impacts of the PEDE on thermospheric wind behavior

As was described in the previous section, variability in wind velocities, as determined by the campaign averaged coefficient of variation, was found to be at its greatest during the time period of the PEDE during the 11–12 July campaign, which occurred close after the peak of the storm, and then gradually diminished until after the 24–25 October campaign. This seems to indicate a substantial change in the amount of variability in the upper atmospheric winds from the development to decay phase

Conclusions

During the time period of the 2018 planet encircling dust event (PEDE), the NGIMS instrument onboard MAVEN completed several observational campaigns to measure neutral thermospheric wind velocities. During this period, a slight (but not statistically significant) increase of campaign-averaged wind speeds was observed coinciding with the time period after the onset of the storm, but prior to it becoming planet encircling. The campaign-averaged speeds reached 200 m/s at this time. Over the next

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments and Data Availability Statement

This work was supported by NASA Headquarters, USA under the NASA Earth and Space Science Fellowship Program — Grant 80NSSC18K1238. Funding was also provided by the MAVEN project, Grant NNH10CC04C. Additionally, the material is based upon work supported by NASA, USA under award number 80GSFC21M0002. The NGIMS dataset used in this study is available on the Planetary Data System ( http://pds-atmospheres.nmsu.edu/data_and_services/atmospheres_data/MAVEN/ngims.html).

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