High‐altitude gravity waves in the Martian thermosphere observed by MAVEN/NGIMS and modeled by a gravity wave scheme

Abstract

First high‐altitude observations of gravity wave (GW)‐induced CO2 density perturbations in the Martian thermosphere retrieved from NASA's Neutral Gas Ion Mass Spectrometer (NGIMS) instrument on board the Mars Atmosphere Volatile EvolutioN (MAVEN) satellite are presented and interpreted using the extended GW parameterization of Yiğit et al. (2008) and the Mars Climate Database as an input. Observed relative density perturbations between 180 and 220 km of 20–40% demonstrate appreciable local time, latitude, and altitude variations. Modeling for the spatiotemporal conditions of the MAVEN observations suggests that GWs can directly propagate from the lower atmosphere to the thermosphere, produce appreciable dynamical effects, and likely contribute to the observed fluctuations. Modeled effects are somewhat smaller than the observed, but their highly variable nature is in qualitative agreement with observations. Possible reasons for discrepancies between modeling and measurements are discussed.

Traveling Ionospheric Disturbances at Mars

“…NGIMS also measured a change in the composition of the ionosphere, with a local decrease in the density of CO but a slight increase in the density of O + . In addition to decreases in plasma density, NGIMS observed decreases in the density of the ambient neutral atmosphere, consistent with previous MAVEN observations of AGWs (England et al, ; Yiğit et al, ). During the second event, the decrease in neutral and ion densities were prominent and occurred nearly simultaneously.…”

“…The two events presented in Figure are entirely consistent with an in situ encounter with traveling ionospheric disturbances: (1) The events were encountered at an altitude where the primary medium is a partially ionized gas and the ions are blown around by the neutral atmosphere; (2) oscillations in neutral densities are consistent with an AGW (England et al, ; Yiğit et al, ), a primary driver of TIDs at Earth, and consistent with observations at Earth by Earle et al () (see Figure D); (3) simultaneous, and significant, decreases in the density of the ionosphere are consistent with the DMSP‐15 encounters with a TID at Earth (see Figures b and d); (4) simultaneous perturbations in ion velocity are consistent with the bulk motion of the ionosphere due to the underlying gravity waves, exactly as observed inside TIDs at Earth (see Figures c and e); and (5) their wavelengths are consistent with in situ observations of MSTIDs at Earth (a preliminary statistical analysis showing this follows shortly). Thus, we conclude that the most plausible explanation for these observations is an encounter with the Martian equivalent of a traveling ionospheric disturbance.…”

“…With only a single spacecraft, it is not possible to measure the propagation rate of Martian TIDs. However, this does not mean that they are stationary, and indeed, the overwhelming balance of probability is that they do “travel.” For a start, all computer simulations of Martian AGWs predict that they propagate through the atmosphere (“travel” from one place to another; Joshi et al, ; Spiga et al, ; Yiğit et al, ). Second, this is supported by recent in situ MAVEN observations (England et al, ).…”

Ion‐Neutral Coupling in the Upper Atmosphere of Mars: A Dominant Driver of Topside Ionospheric Structure

“…The amplitudes of the density deviations between the measured neutral density profiles and their exponential best fits were between 1% and 94%. These variations are consistent with gravity waves (England et al, ; Grebowsky et al, ; Pilinski et al, ; Seth et al, ; Terada et al, ; Yiğit et al, ), as well as variations due to neutral winds that may regionally affect atmospheric densities (Benna et al, ; Bougher et al, ).…”

Seasonal Variations and Global Wave Distributions in the Mars Thermosphere From MAVEN and Multisatellites Accelerometer‐Derived Mass Densities

“…The annual density variations in the thermosphere are thought to be generated largely by solar radiation and winds, and these processes have been previously parameterized with two parameters, solar zenith angle ( SZA ) and Sun‐Mars distance (Zurek et al, ). Additionally, the MAVEN Neutral Gas and Ion Mass Spectrometer ( NGIMS ) data have been used to investigate thermospheric gravity wave activities, and the vertical structures of the species from 120 to 260 km were described by Bougher, Cravens, et al (), Bougher et al (), and Yiğit et al (). Signatures of IAGWs have been detected in the main species densities of CO 2 , Ar, and N 2 (e.g., England et al, ; ; Terada et al, ).…”