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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.06.034 Byline: T.L. McDunn (a), S.W. Bougher (a), J. Murphy (b), M.D. Smith (c), F. Forget (d), J.-L. Bertaux (e), F. Montmessin (e) Keywords: Mars, Atmosphere; Atmosphere, Structure; Aeronomy Abstract: The objective of this work is to advance the understanding of the structure and dynamics of the middle altitude region of the martian atmosphere. While numerous advancements have been made in the level of scientific understanding of Mars's upper and lower atmospheres over the past several decades, insight into the mechanisms of the middle atmosphere has come at a significantly slower pace due to the small number of datasets available for this region. Over the past decade this has begun to change, with renewed interest by NASA and ESA to send spacecraft to Mars. The result of these recent efforts is a growing database for Mars's middle atmosphere, enabling long-awaited and necessary studies characterizing the middle altitude region. Various numerical models of the martian atmosphere can now be validated and constrained using this database. We utilize the Mars Express/Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (MEX/SPICAM) density and temperature datasets to characterize the middle atmosphere as well as validate and constrain the coupled multi-dimensional Mars General Circulation Model-Mars Thermosphere General Circulation Model (MGCM-MTGCM) at middle altitudes in order to explore the underlying physics controlling the structure and dynamics at these levels. The results of this study stress the importance of proper dust prescription within the MGCM-MTGCM for accurately reproducing the density and thermal structure of the middle and upper atmosphere regions on Mars. Simulations conducted with horizontal dust opacities that are consistent with the SPICAM observation period (i.e. Mars Odyssey/THEMIS opacities) yield modeled densities and temperatures that are closer to the observed values than simulations conducted with 'typical' dust conditions (i.e. Mars Global Surveyor/TES opacities). We show that the MGCM-MTGCM closely reproduces the observed densities during low-dust and high-dust scenarios but displays difficulty during the pre-dust-season 'ramp-up' period (L.sub.s [approximately equal to]120-200[degrees]) during MY27. In addition, we show that the MGCM-MTGCM accurately reproduces the temperature profiles below the mesopause, but, the mesopause altitude is too low and its temperature warmer (5-10K) than observations. This may be related to nightside dynamical heating processes that require further refinement. In addition, CO.sub.2 15-[mu]m cooling rates may be too small, which would be consistent with underestimated atomic O abundances. Author Affiliation: (a) Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109, USA (b) Department of Astronomy, New Mexico State University, P. O. Box 30001, MSC 4500, Las Cruces, New Mexico 88003, USA (c) NASA Goddard Space Flight Center, Greenbelt, MD 20071, USA (d) Laboratoire de Meteorologie Dynamique, Institut Pierre-Simon Laplace, Universite Paris 6, Paris, France (e) Service d'Aeronomie, Institut Pierre-Simon Laplace, Verrieres-le-Buisson, France Article History: Received 24 September 2008; Revised 23 June 2009; Accepted 24 June 2009 more...

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.11.006 Byline: A.A. Fedorova (a), O.I. Korablev (a), J.-L. Bertaux (b)(c), A.V. Rodin (a)(d), F. Montmessin (b)(c), D.A. Belyaev (a), A. Reberac (b)(c) Keywords: Mars; atmosphere; Spectroscopy Abstract: The infrared AOTF spectrometer is a part of the SPICAM experiment onboard the Mars-Express ESA mission. The instrument has a capability of solar occultations and operates in the spectral range of 1-1.7 [mu]m with a spectral resolution of [approximately equal to]3.5 cm.sup.-1. We report results from 24 orbits obtained during MY28 at L.sub.s 130[degrees]-160[degrees], and the latitude range of 40[degrees]-55[degrees] N. For these orbits the atmospheric density from 1.43 [mu]m CO.sub.2 band, water vapor mixing ratio based on 1.38 [mu]m absorption, and aerosol opacities were retrieved simultaneously. The vertical resolution of measurements is better than 3.5 km. Aerosol vertical extinction profiles were obtained at 10 wavelengths in the altitude range from 10 to 60 km. The interpretation using Mie scattering theory with adopted refraction indices of dust and H.sub.2O ice allows to retrieve particle size (r.sub.eff[approximately equal to]0.5-1 [mu]m) and number density ([approximately equal to]1 cm.sup.-3 at 15-30 km) profiles. The haze top is generally below 40 km, except the longitude range of 320[degrees]-50[degrees] E, where high-altitude clouds at 50-60 km were detected. Optical properties of these clouds are compatible with ice particles (effective radius r.sub.eff=0.1-0.3[mu]m, number density N[approximately equal to]10 cm.sup.-3) distributed with variance [nu].sub.eff=0.1-0.2[mu]m. The vertical optical depth of the clouds is below 0.001 at 1 [mu]m. The atmospheric density profiles are retrieved from CO.sub.2 band in the altitude range of 10-90 km, and H.sub.2O mixing ratio is determined at 15-50 km. Unless a supersaturation of the water vapor occurs in the martian atmosphere, the H.sub.2O mixing ratio indicates [approximately equal to]5 K warmer atmosphere at 25-45 km than predicted by models. Author Affiliation: (a) Space Research Institute (IKI), 84/32 Profsoyuznaya, 117997 Moscow, Russia (b) Service d'Aeronomie du CNRS, BP 3, 91371, Verrieres-le-Buisson, France (c) Institut Pierre Simon Laplace, Universite de Versailles-Saint-Quentin, 78 Saint Quentin en Yvelines, France (d) Moscow Institute of Physics and Technology, Institutsky dr. 9, 141700 Dolgoprudnyi, Russia Article History: Received 26 November 2007; Revised 28 July 2008; Accepted 3 November 2008 more...

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.03.007 Byline: M.R. Combi (a), J.T.T. Makinen (b)(c), J.-L. Bertaux (d), E. Quemerais (d) Keywords: Comets; Hyakutake; dynamics; composition; Ultraviolet observations Abstract: Comet 1996 B2 (Hyakutake) displayed strong evidence for break-up, with a prominent antisunward dust spike and fragments traveling antisunward for many days after an eruptive event in late March 1996. Because of its high orbital inclination and rapid southward motion after perihelion, its post-perihelion activity was not well monitored from the ground. The SWAN all-sky Lyman-alpha camera on the SOHO spacecraft was ideally placed for long-term monitoring of the hydrogen coma of Comet Hyakutake both before and after perihelion. The SWAN images were analyzed with a new time-resolved model (TRM) that provides daily averages of the water production rate and an estimate of the hydrogen atom lifetime (dominated by charge exchange with solar wind protons) during extended periods throughout the apparition. A long-term variation of water production rate of 2.7x10.sup.29r.sup.-2.1[+ or -]0.08moleculess.sup.-1, where r is the heliocentric distance in AU was found. The daily average values of the production rate covered the March 19 outburst and two more outbursts seen in the April before perihelion, which had progressively shorter durations at respectively smaller heliocentric distances. The long-term variation of the production rate was found to be consistent with the seasonal effect predicted by the jet rotation model of Schleicher and Woodney [2003. Analyses of dust coma morphology of Comet Hyakutake (1996 B2) near perigee: Outburst behavior, jet motion, source region locations, and the nucleus pole orientation. Icarus 162, 190-213] when added to a more steady source that is about two-thirds of the maximum of the jet source. The seasonal effect in their model found the dust jet source largely not illuminated after perihelion, coinciding with somewhat reduced overall activity and the absence of outbursts and fragmentation. The locations of the dust jets appear to be responsible for the outbursts and fragmentation before perihelion. The erratic behavior of the pre-perihelion jet sources as contrasted with the smoother variation from the rest of the surface after perihelion indicates there is a strong heterogeneity in the physical make-up of active areas on the nucleus. Author Affiliation: (a) Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA (b) Finnish Meteorological Institute, P.O. Box 503, FIN-00101 Helsinki, Finland (c) European Space Agency, Kepleraan 1, SCI-SB, 22-1 AZ Noordwijk ZH, The Netherlands (d) Centre National de la Recherche Scientifique, Service d'Aeronomie, BP3, 91371 Verrieres le Buisson cedex, France Article History: Received 19 April 2004; Revised 2 March 2005 more...

The SWAN all-sky camera on the SOHO spacecraft observed the hydrogen Lyman-alpha coma of Comet Hale-Bopp (1995 O1) from January through May 1997. The observations covered heliocentric distances from 1.75 AU before perihelion to 1.29 AU after, showing the hydrogen coma to vary in brightness, shape, and orientation with the comet's orbital position. The water production rate of the comet was determined from an analysis of these images with a model that accounts for the detailed physics and chemistry. The model incorporates coma dynamics (expansion velocity and kinetic temperatures), which have been shown to be consistent with a variety of other observations. The distribution of brightness in the modeled images reproduces the absolute magnitude and distribution of brightness seen in the observations. We find that the maximum water production rate occurred about 2 weeks after perihelion, and that the magnitude and variation with time are consistent with other published observations which were analyzed in a consistent manner. Key Words: comets; Hale-Bopp; dynamics; composition; ultraviolet observations. more...