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  • 1
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    Structure and composition of the neutral upper atmosphere of Mars from the MAVEN NGIMS investigation (2015)
    Wiley
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    Publication Date: 2015-11-06
    Description: The Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) provides sensitive detections of neutral gas and ambient ion composition. NGIMS measurements of nine atomic and molecular neutral species, and their variation with altitude, latitude, and solar zenith angle are reported over several months of operation of the MAVEN mission. Sampling NGIMS signals from multiple neutral species every several seconds reveals persistent and unexpectedly large amplitude density structures. The scale height temperatures are mapped over the course of the first few months of the mission from high down to midlatitudes. NGIMS measurements near the homopause of 40 Ar/N 2 ratios agree with those reported by the Sample Analysis at Mars investigation and allow the altitude of the homopause for the most abundant gases to be established.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    The spatial distribution of planetary ion fluxes near Mars observed by MAVEN (2015)
    Wiley
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    Publication Date: 2015-11-06
    Description: We present the results of an initial effort to statistically map the fluxes of planetary ions on a closed surface around Mars. Choosing a spherical shell ~1000 km above the planet, we map both outgoing and incoming ion fluxes (with energies 〉25 eV) over a 4 month period. The results show net escape of planetary ions behind Mars and strong fluxes of escaping ions from the northern hemisphere with respect to the solar wind convection electric field. Planetary ions also travel toward the planet, and return fluxes are particularly strong in the southern electric field hemisphere. We obtain a lower bound estimate for planetary ion escape of ~3 × 10 24  s −1 , accounting for the ~10% of ions that return toward the planet and assuming that the ~70% of the surface covered so far is representative of the regions not yet visited by Mars Atmosphere and Volatile EvolutioN (MAVEN).
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Global response of the upper thermospheric winds to large ion drifts in the jovian ovals (2016)
    Wiley
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    Publication Date: 2016-03-25
    Description: We use our fully coupled 3-D Jupiter Thermosphere General Circulation Model (JTGCM) to quantify processes which are responsible for generating neutral winds in Jupiter's oval thermosphere from 20 µbar to 10 -4 nbar self-consistently with the thermal structure and composition. The heat sources in the JTGCM that drive the global circulation of neutral flow are substantial Joule heating produced in the Jovian ovals by imposing high speed anticorotational ion drifts (~3.5 kms -1 ) and charged particle heating from auroral processes responsible for bright oval emissions. We find that the zonal flow of neutral winds in the auroral ovals of both hemispheres is primarily driven by competition between accelerations resulting from Coriolis forcing and ion drag processes near the ionospheric peak. However, above the ionospheric peak (〈0.01 µbar), the acceleration of neutral flow due to pressure gradients is found to be the most effective parameter impacting zonal winds, competing mainly with acceleration due to advection with minor contributions from curvature and Coriolis forces in the southern oval, while in the northern oval it competes alone with considerable Coriolis forcing. The meridional flow of neutral winds in both ovals in the JTGCM is determined by competition between meridional accelerations due to Coriolis forcing and pressure gradients. We find that meridional flow in the lower thermosphere, near the peak of the auroral ionosphere, is poleward, with peak wind speeds of ~0.6 kms -1 and ~0.1 kms -1 in the southern and northern oval, respectively. The corresponding subsiding flow of neutral motion is ~5 ms -1 in the southern oval, while this flow is rising in the northern oval with reduced speed of ~2 ms -1 . We also find that the strength of meridional flow in both auroral ovals is gradually weakened and turned equatorward near 0.08 µbar with wind speeds up to ~250 ms -1 (southern oval) and ~75 ms -1 (northern oval). The corresponding neutral motion in this region is upward, with wind speeds up to 4 ms -1 in both ovals.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    Probing the Martian atmosphere with MAVEN/IUVS stellar occultations (2015)
    Wiley
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    Publication Date: 2015-11-06
    Description: The first campaign of stellar occultations with the Imaging Ultraviolet Spectrograph (IUVS) instrument on board of Mars Atmosphere and Volatile EvolutioN (MAVEN) mission was executed between 24 and 26 March 2015. From this campaign 13 occultations are used to retrieve CO 2 and O 2 number densities in the altitude range between 100 and 150 km. Observations probe primarily the low-latitude regions on the nightside of the planet, just past the dawn and dusk terminator. Calculation of temperature from the CO 2 density profiles reveals that the lower thermosphere is significantly cooler than predicted by the models in the Mars Climate Database. A systematically cold layer with temperatures of 105–120 K is seen in the occultations at a pressure level around 7 × 10 −6 Pa.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 5
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    Mars global ionosphere-thermosphere model (MGITM): Solar cycle, seasonal, and diurnal variations of the Mars upper atmosphere (2014)
    Wiley
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    Publication Date: 2014-12-31
    Description: A new Mars Global Ionosphere-Thermosphere Model (M-GITM) is presented that combines the terrestrial GITM framework [ Ridley et al. , 2006] with Mars fundamental physical parameters, ion-neutral chemistry, and key radiative processes in order to capture the basic observed features of the thermal, compositional, and dynamical structure of the Mars atmosphere from the ground to the exosphere (0-250 km). Lower, middle, and upper atmosphere processes are included, based in part upon formulations used in previous lower and upper atmosphere Mars GCMs. This enables the M-GITM code to be run for various seasonal, solar cycle, and dust conditions. M-GITM validation studies have focused upon simulations for a range of solar and seasonal conditions. Key upper atmosphere measurements are selected for comparison to corresponding M-GITM neutral temperatures and neutral-ion densities. In addition, simulated lower atmosphere temperatures are compared with observations in order to provide a first order confirmation of a realistic lower atmosphere. M-GITM captures solar cycle and seasonal trends in the upper atmosphere that are consistent with observations, yielding significant periodic changes in the temperature structure, the species density distributions, and the large scale global wind system. For instance, mid-afternoon temperatures near ~ 200 km are predicted to vary from ~210 to 350 K (equinox) and ~190 to 390 k (aphelion to perihelion) over the solar cycle. These simulations will serve as a benchmark against which to compare episodic variations (e.g. due to solar flares, dust storms) in future M-GITM studies. Additionally, M-GITM will be used to support MAVEN mission activities (2014-2016).
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    Understanding the variability of nightside temperatures, NO UV and O2 IR nightglow emissions in the Venus upper atmosphere (2011)
    Wiley
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    Publication Date: 2011-08-16
    Description: Venus Express (VEX) has been monitoring key nightglow emissions and thermal features (O2 IR nightglow, NO UV nightglow, and nightside temperatures) which contribute to a comprehensive understanding of the global dynamics and circulation patterns above ∼90 km. The nightglow emissions serve as effective tracers of Venus' middle and upper atmosphere global wind system due to their variable peak brightness and horizontal distributions. A statistical map has been created utilizing O2 IR nightglow VEX observations, and a statistical map for NO UV is being developed. A nightside warm layer near 100 km has been observed by VEX and ground-based observations. The National Center for Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) has been updated and revised in order to address these key VEX observations and to provide diagnostic interpretation. The VTGCM is first used to capture the statistically averaged mean state of these three key observations. This correspondence implies a weak retrograde superrotating zonal flow (RSZ) from ∼80 km to 110 km and above 110 km the emergence of modest RSZ winds approaching 60 m s−1 above ∼130 km. Subsequently, VTGCM sensitivity tests are performed using two tuneable parameters (the nightside eddy diffusion coefficient and the wave drag term) to examine corresponding variability within the VTGCM. These tests identified a possible mechanism for the observed noncorrelation of the O2 and NO emissions. The dynamical explanation requires the nightglow layers to be at least ∼15 km apart and the retrograde zonal wind to increase dramatically over 110 to 130 km.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Pickup Ion Measurements by MAVEN: A Diagnostic of Photochemical Oxygen Escape from Mars (2014)
    Wiley
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    Publication Date: 2014-07-04
    Description: A key process populating the oxygen exosphere at Mars is the dissociative recombination of ionospheric O 2 + , which produces fast oxygen atoms, some of which have speeds exceeding the escape speed and thus contribute to atmospheric loss. Theoretical studies of this escape process have been carried out and predictions made of the loss rate; however, directly measuring the escaping neutral oxygen is difficult, but essential. This paper describes how energetic pickup ion measurements to be made near Mars by the SEP (Solar Energetic Particle) instrument onboard the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft can be used to constrain models of photochemical oxygen escape. In certain solar wind conditions, neutral oxygen atoms in the distant Martian exosphere that are ionized and picked up by the solar wind can reach energies high enough to be detected near Mars by SEP.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 8
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    The structure of the upper atmosphere of mars: In situ accelerometer measurements from mars global surveyor (1998)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1998-03-25
    Description: The Mars Global Surveyor (MGS) z-axis accelerometer has obtained over 200 vertical structures of thermospheric density, temperature, and pressure, ranging from 110 to 170 kilometers, compared to only three previous such vertical structures. In November 1997, a regional dust storm in the Southern Hemisphere triggered an unexpectedly large thermospheric response at mid-northern latitudes, increasing the altitude of thermospheric pressure surfaces there by as much as 8 kilometers and indicating a strong global thermospheric response to a regional dust storm. Throughout the MGS mission, thermospheric density bulges have been detected on opposite sides of the planet near 90 degreesE and 90 degreesW, in the vicinity of maximum terrain heights. This wave 2 pattern may be caused by topographically-forced planetary waves propagating up from the lower atmosphere.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Keating -- Bougher -- Zurek -- Tolson -- Cancro -- Noll -- Parker -- Schellenberg -- Shane -- Wilkerson -- Murphy -- Hollingsworth -- Haberle -- Joshi -- Pearl -- Conrath -- Smith -- Clancy -- Blanchard -- Wilmoth -- Rault -- Martin -- Lyons -- Esposito -- Johnston -- et -- New York, N.Y. -- Science. 1998 Mar 13;279(5357):1672-6.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉G. M. Keating, R. H. Tolson, G. J. Cancro, S. N. Noll, J. S. Parker, T. J. Schellenberg, R. W. Shane, B. L. Wilkerson, The George Washington University at NASA Langley, MS 269, Hampton, VA 23681, USA. S. W. Bougher and J. M. Babicke, Universi.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/9497278" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 9
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    Dayside thermal structure of Venus' upper atmosphere characterized by a global model (2012)
    Wiley
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    Publication Date: 2012-08-07
    Description: Observations of Venus' dayside thermal structure are being conducted through ground based observatories. These temperature measurements, along with those from several instruments onboard the current Venus Express mission, are augmenting the previous thermal structure data from past missions (e.g., Veneras', Pioneer Venus Orbiter, Pioneer Venus Probes). These recent ground-based and VEx observations reveal the Venus dayside lower thermosphere to be considerably warmer and dynamically important than previously understood. In this study, a three dimensional general circulation model, the Venus Thermospheric General Circulation Model (VTGCM), is used to provide dayside temperature predictions for comparison to these recent ground based observations. Such a comparison serves to identify and quantify the underlying thermal processes responsible for the observed dayside temperature structure. The VTGCM reproduces the dayside temperatures observed near 110 km at noon from 40°S to 40°N very well. In addition, the global winds generated by these warm dayside temperatures are shown to give rise to dayside upwelling (divergence) and nightside subsidence (convergence) resulting in nightside warming near the anti-solar point at ∼104 km. Corresponding nightside temperatures reach ∼198 K, in accord with averaged measurements. This agreement implies (1) it is important for GCMs to include the updated radiative heating and cooling rates presented in Roldán et al. (2000) and (2) the current VTS3 and VIRA empirical models are in-sufficient in representing the warm regions observed in the thermal structure of the dayside lower thermosphere (∼100 to 130 km) and need to be updated.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Incorporation of a gravity wave momentum deposition parameterization into the Venus Thermosphere General Circulation Model (VTGCM) (2013)
    Wiley
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    Publication Date: 2013-02-01
    Description: [1]  The gravity wave-drag parameterization of Alexander and Dunkerton (1999) was implemented into a Venus Thermosphere General Circulation Model (VTGCM) to investigate breaking gravity waves as a source of momentum deposition in Venus' thermosphere. Previously, deceleration of zonal jets on the morning and evening terminators in models was accomplished via Rayleigh friction, a linear drag law that is not directly linked to any physical mechanism. The Alexander and Dunkerton (1999) parameterization deposits all of the momentum of a breaking wave at the breaking altitude and features a spectrum of wave phase speeds whose amplitudes are distributed as a Gaussian about a center phase speed. We did not find a combination of wave parameters (namely, center phase speed, amplitude at center phase speed, and distribution width) to produce sufficient drag in the jet cores that would bring VTGCM density and nightglow emissions into agreement with Venus Express observations. The zonal wind shear from 100 to 120 km altitude is very strong. Gravity waves launched below 100 km either break in the strong shear zones below 115 km or are reflected and do not propagate into the jet core regions where drag is needed. The results we present demonstrate that parameterizations developed for the middle atmosphere do not work in the thermosphere and that appropriate damping mechanisms other than nonlinear breaking/saturation dominate and should be accounted for at these heights.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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