Vertical dust mixing and the interannual variations in the Mars thermosphere

Abstract

[1] Mars winter polar warming is a phenomenon of the lower thermosphere temperatures and densities that is well documented by in situ accelerometer data taken during spacecraft aerobraking maneuvers. Previous work by Bougher et al. (2006) simulates two specific time periods, corresponding to existing aerobraking data sets, and confirms the existence of winter polar warming structures in the Martian upper atmosphere, using the coupled Mars General Circulation Model-Mars Thermosphere General Circulation Model (MGCM-MTGCM). The present work investigates the underlying mechanisms that drive winter polar warming in three major studies: (1) a systematic analysis of vertical dust mixing in the lower atmosphere and its impact upon the dynamics of the lower thermosphere (100-130 km), (2) an interannual investigation utilizing 3 years of lower atmosphere infrared (IR) dust optical depth data acquired by the Thermal Emission Spectrometer (TES) instrument on board Mars Global Surveyor (MGS), and finally (3) a brief study of the MTGCM's response to variations in upward propagating waves and tides from the lower atmosphere. From the first study, we find that the vertical dust mixing greatly modifies the simulated winter polar warming features in the MTGCM's thermosphere at both solstice seasons. Furthermore, this sensitivity study confirms that an interhemispheric Hadley circulation and the concomitant adiabatic heating rates are the primary causes for these winter polar warming structures. From the second study, we find that as revealed in the lower atmosphere by Liu et al. (2003), the polar lower thermosphere exhibits a high degree of variability at L S = 270, with reduced yet significant variability at L S = 090. Finally, the third numerical experiment indicates, as was found in the lower atmosphere by Wilson (1997), that upward propagating waves and tides allow meridional flows to access the thermosphere's winter polar latitudes. This last investigation also indicates that the Hadley circulation responsible for thermospheric winter polar warming originates in the lower atmosphere and extends high into the upper atmosphere. In summary, these new efforts establish a baseline numerical study upon which more comprehensive model-to-data comparisons may be conducted for the Martian thermosphere.

Neutral Upper Atmosphere and Ionosphere Modeling

“…Figure 13 illustrates coupled MGCM-MTGCM simulations for Ls = 90 (aphelion) and 270 (perihelion) conditions, demonstrating that the basic features of the Martian thermospheric winter polar warming are controlled by seasonal changes in the solar plus tidal forcing, the corresponding variations in the strength of the inter-hemispheric Hadley circulation, and the resulting changes in the magnitude of the adiabatic heating near the winter poles. Calculations of polar warming show that perihelion adiabatic heating can be highly variable from one Mars year to the next, and more than twice as strong as that for aphelion conditions (Bougher et al 2006b;Bell et al 2007). Finally, without the deep inter-hemispheric Hadley circulation made possible using these coupled lower and upper atmosphere simulations, winter polar warming features in the Mars thermosphere would not be reproduced at all (Bell et al 2007).…”

“…Calculations of polar warming show that perihelion adiabatic heating can be highly variable from one Mars year to the next, and more than twice as strong as that for aphelion conditions (Bougher et al 2006b;Bell et al 2007). Finally, without the deep inter-hemispheric Hadley circulation made possible using these coupled lower and upper atmosphere simulations, winter polar warming features in the Mars thermosphere would not be reproduced at all (Bell et al 2007).…”

“…The Mars TGCM (MTGCM) is a finite difference primitive equation model that selfconsistently solves for time-dependent neutral temperatures, neutral-ion densities, and three component neutral winds over the Mars globe (e.g., Bougher et al 1999aBougher et al , 1999bBougher et al , 2000Bougher et al , 2002Bougher et al , 2004Bougher et al , 2006bBell et al 2007). The MTGCM code is adapted from the NCAR TGCM framework (see Sect.…”

New nitric oxide (NO) nightglow measurements with SPICAM/MEx as a tracer of Mars upper atmosphere circulation and comparison with LMD‐MGCM model prediction: Evidence for asymmetric hemispheres

“…[17] The dust distribution in the lower atmosphere has been shown to have an important influence on the dynamics of the upper thermosphere [Bell et al, 2007]. For the simulation used in this study, we use a climatology of the dust as observed by the Thermal Emission Spectrometer on board the Mars Global Surveyor mission between 1999 and 2001, i.e., MY24-25.…”

Neutral Atmospheres

“…While dynamics in the upper atmosphere are thought to be driven primarily by solar EUV heating, studies have suggested that vertical dynamical coupling nevertheless plays an equally significant role on Mars as it does on Venus and Earth, most prominently in the form of strong wave signatures in the thermosphere that are not only formed in situ at those altitudes, but also include effects due to topography . Bougher et al (2006b) and Bell et al (2007) additionally proposed the existence of a deep inter-hemispheric circulation in order to enable thermospheric winter polar warming to operate. While most models available to-date still rely on implementing this coupling to lower altitudes as a lower boundary condition , codes are now appearing for Mars with a vertical range from the ground to the thermosphere (González-Galindo et al 2005), having the advantage of more self-consistently including processes of vertical dynamical coupling.…”