Effects of Low Energetic Neutral Atoms on Martian and Venusian Dayside Exospheric Temperature Estimations
Herbert LichteneggerH. LämmerYuri N. KulikovShahin KazeminejadGregorio H. Molina-CuberosR. RodrigoBobby KazeminejadGottfried Kirchengast
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Keywords:
Exosphere
Airglow
Orbiter
Atmosphere of Venus
Atmosphere of Mars
Solar maximum
Exosphere
Atmosphere of Mars
Atmospheric escape
Scale height
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The atmosphere of Venus appears to be deficient in water vapor by a factor of about 104 compared with the total amount of water on Earth. The feasibility of loss of water vapor from the Venus atmosphere is examined, assuming H20 as the sole initial constituent. A steady-state model is constructed, and the photochemistry establishes the distribution of important products in the upper atmosphere. Calculations of exospheric temperatures yield values as high as 100,000K. Such large temperatures result from the large abundance of atomic hydrogen in the exosphere, and imply a dynamic outflow of all constituents from the upper region of the atmosphere. Such an outflow would cause the escape of all hydrogen and some of the oxygen resulting from dissociation of H20. Little loss of CO2 would result due to its low abundance in the upper region permitting its accumulation to the present observed value. It is concluded that if Venus formed from the same mix of materials as Earth, much tectonic activity and fairly rapid outgassing must have occurred during the early phase of its history to account for the loss of water vapor.
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Atmosphere of Venus
Outgassing
Outflow
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Abstract Measurements provided by the Magnetometer and the Extreme Ultraviolet Monitor (EUVM) on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft together with atomic H exospheric densities derived from numerical simulations are studied for the time interval from October 2014 up to March 2016. We determine the proton cyclotron waves (PCWs) occurrence rate observed upstream from Mars at different times. We also study the relationship with temporal variabilities of the high‐altitude Martian hydrogen exosphere and the solar EUV flux reaching the Martian environment. We find that the abundance of PCWs is higher when Mars is close to perihelion and decreases to lower and approximately constant values after the Martian Northern Spring Equinox. We also conclude that these variabilities cannot be associated with biases in MAVEN's spatial coverage or changes in the background magnetic field orientation. Higher H exospheric densities on the Martian dayside are also found when Mars is closer to perihelion, as a result of changes in the thermospheric response to variability in the ultraviolet flux reaching Mars at different orbital distances. A consistent behavior is also observed in the analyzed daily irradiances measured by the MAVEN EUVM. The latter trends point toward an increase in the planetary proton densities upstream from the Martian bow shock near perihelion. These results then suggest a method to indirectly monitor the variability of the H exosphere up to very high altitudes during large time intervals (compared to direct measurements of neutral particles), based on the observed abundance of PCWs.
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Atmosphere of Mars
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Improved Monte Carlo models for the concentration and velocity distribution for hydrogen in the terrestrial exosphere have been formulated for minimum, medium, and maximum solar cycle conditions. Both the plasmaspheric source of hot hydrogen and the classical exobase source have been included, along with solar radiation pressure and photoionization. At solar minimum the hydrogen from charge exchange of hot ions in the plasmasphere exceeds that from the exobase source not only for escape but for the population at geocentric distances greater than two earth radii ( R E ). At about 2 R E the equivalent temperature is about 50% greater than that of the exobase, a situation similar to the “two‐temperature” exosphere observed and calculated for Venus. At solar maximum the exobase source dominates. The concentration at about 2 R E geocentric and above varies little from solar maximum to solar minimum, although the exobase concentration increases by about a factor of 10. The diurnal variation at the exobase is a factor of 2.1 at solar maximum and 2.7 at solar minimum at low latitudes. This variation is mostly smoothed out at 2 R E , but above 10 R E a nighttime enhancement by about a factor of 2 develops, constituting a slight “geotail” of the type found earlier by observation and by calculations of the effects of radiation pressure. The non‐Maxwellian velocity distributions show “flattened” tops compared to Maxwellian distributions for components in the transverse directions between 2 R E and 8 R E which appear to be due to the importance there of satellite particles with high apogees and the depletion of satellite particles with low perigees.
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Solar maximum
Solar minimum
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Earth radius
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Abstract Mars likely lost a significant part of its atmosphere to space during its history. The sputtering of the atmosphere, by precipitating planetary heavy pickup ions accelerated by the solar wind, is one of the processes that could have significantly contributed to this atmospheric escape, in particular since the cessation of its global magnetic field, 4.0–4.1 Gyr ago. We present a 2 year baseline analysis of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of the precipitating flux. We use this measurement to model the expected escape rate and exospheric structure induced by this precipitation. We conclude that sputtering signatures in the dayside exosphere will be difficult to identify by MAVEN, and the induced atmospheric escape of O atoms remains orders of magnitude smaller than the expected rate induced by dissociative recombination of O 2 + in Mars's ionosphere. On the contrary, deep in the nightside, Mars's sputtering might be the main source of the nonthermal part of the exospheric density profiles of species with mass larger or equal to Ar.
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Atmosphere of Mars
Atmospheric escape
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Abstract The first measurements of the emission brightness of the oxygen atomic exosphere by Mars Atmosphere and Volatile EvolutioN (MAVEN) mission have clearly shown that it is composed of a thermal component produced by the extension of the upper atmosphere and of a nonthermal component. Modeling these measurements allows us to constrain the origins of the exospheric O and, as a consequence, to estimate Mars' present oxygen escape rate. We here propose an analysis of three periods of MAVEN observations based on a set of three coupled models: a hybrid magnetospheric model (LATmos HYbrid Simulation (LatHyS)), an Exospheric General Model (EGM), and the Global Martian Circulation model of the Laboratoire de Météorologie Dynamique (LMD‐GCM), which provide a description of Mars' environment from the surface up to the solar wind. The simulated magnetosphere by LatHyS is in good agreement with MAVEN Plasma and Field Package instruments data. The LMD‐GCM modeled upper atmospheric profiles for the main neutral and ion species are compared to Neutral Gas and Ion Mass Spectrometer/MAVEN data showing that the LMD‐GCM can provide a satisfactory global view of Mars' upper atmosphere. Finally, we were able to reconstruct the expected emission brightness intensity from the oxygen exosphere using EGM. The good agreement with the averaged measured profiles by Imaging Ultraviolet Spectrograph during these three periods suggests that Mars' exospheric nonthermal component can be fully explained by the reactions of dissociative recombination of the O 2 + ion in Mars' ionosphere, limiting significantly our ability to extract information from MAVEN observations of the O exosphere on other nonthermal processes, such as sputtering.
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Atmospheric escape
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Hot atomic oxygen velocity distributions and corresponding profiles of density and temperature have been calculated for the Venus and Mars exospheres by Monte Carlo simulation. The Venus results are realistically based on well‐established models of the Venus atmosphere and ionosphere that were derived from Pioneer Venus orbiter measurements near solar maximum. The confidence level is lower for Mars because the only viable data come from Viking entry measurements in daytime at solar minimum, and the global morphologies of the atmosphere and ionosphere are poorly understood. What is clear is that planetary differences arise because the exothermal velocities of hot oxygen created by dissociative recombination of O 2 + are below satellite speeds on Venus and greater than escape on Mars. As a result, the distribution of velocities on Venus tends to be nearly isotropic, whereas it is grossly anisotropic on Mars, favoring the vertical with increasing distance from the planet. The calculated lower bound for the Martian oxygen escape rate indicates that the lifetime of CO 2 in the Mars atmosphere is probably less than 10 Myr.
Atmosphere of Venus
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Orbiter
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This thesis describes work carried out at Hobart (42.9°S, 147.3°E) during the 1968 - 1970 solar/auroral maximum on the airglow emissions of atomic oxygen particularly the red 6300A line. (This 'line' is actually a doublet : see chapter two). Both the red and the green 5577A lines originate from forbidden transitions of OI; these are the (3P - ¹D) and the (¹D - ¹S) transitions respectively. Until recently the green line was much more studied than the 6300A line, partly because it is usually stronger in the normal night airglow and also because photo-electron detectors are more sensitive in its spectral range. It is really only during the last fifteen years or so that adequately red sensitive detectors have become available to study the weak night airglow. Thus although these OI lines have been known to exist in auroras for over a century (Angstrom 1868, Zollner 1870) the recent solar maximum is only the second available for detailed study of the weak red night emissions. The previous maximum of 1957 - 1959 proved very fruitful for airglow/auroral studies so it was hoped that the recent maximum would be the same. However this maximum was considerably weaker and few opportunities were available to study such novel aurbral features as the 'mantel aurora' (Sandford 1964) or the Stable Auroral Red Arc (SAR-arc) (Barbier 1959), Cole 1965a) both originally detected during Lilo previous maximum. Furthermore the normal night airglow appeared to be less intense by a factor of five during the 1968 -1970 maximum than during the previous one. This is rather puzzling and would not be expected in terms of the solar flux or ionospheric content differences. Possible explanations are examined in various chapters of this thesis.
Airglow
Solar maximum
Line (geometry)
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Atmosphere of Venus
Atmosphere of Mars
Mixing ratio
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Atmosphere of Mars
Atmosphere of Venus
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