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Leonard G Tyler - One of the best experts on this subject based on the ideXlab platform.
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structure of the venus neutral atmosphere as observed by the radio science experiment vera on venus express
Journal of Geophysical Research, 2009Co-Authors: S Tellmann, M Patzold, B Hausler, M K Bird, Leonard G TylerAbstract:[1] The European Space Agency Venus Express Radio Science experiment (VeRa) obtained 118 radio occultation measurements of the Venusian atmosphere between July 2006 and June 2007. Southern latitudes are uniformly sampled; measurements in the northern hemisphere are concentrated near the pole. Radial profiles of neutral number density derived from the occultations cover the Altitude range 40–90 km, which are converted to profiles of temperature (T) and pressure (p) versus height (h). Profiles of static stability are found to be latitude-dependent and nearly adiabatic in the middle cloud region. Below the clouds the stability decreases at high latitudes. At an Altitude of 65 km, the VeRa T[p(h)] profiles generally lie between the Venus International Reference Atmosphere (VIRA) and VIRA-2 models; the retrieved temperatures at any given pressure level typically are within 5 K of those derived from the Pioneer Venus Orbiter Radio Occultation experiments. A large equator-to-pole temperature contrast of ∼30 K is found at the 1-bar (1000 hPa) level. The VeRa observations reveal a distinct cold collar region in the southern hemisphere, complementing that in the north. At the latitudes of the cold collars, the tropopause Altitude increases relative to higher and lower latitudes by ≈7 km while the temperature drops roughly 60 K. The observations indicate the existence of a wave number 2 structure poleward of ±75° latitude at Altitudes of 62 km.
G. M. Keating - One of the best experts on this subject based on the ideXlab platform.
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the effects of topographically controlled thermal tides in the martian upper atmosphere as seen by the mgs accelerometer
Icarus, 2003Co-Authors: Paul Withers, S W Bougher, G. M. KeatingAbstract:Abstract Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at Altitudes of 130–160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing Altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both Altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure.
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Winds in the martian upper atmosphere from MGS aerobraking density profiles
2002Co-Authors: Paul Withers, Stephen W. Bougher, G. M. KeatingAbstract:We have used a novel technique to measure the zonal wind in the martian upper atmosphere using MGS Accelerometer aerobraking density profiles. Typical results for the northern hemisphere (NH) at about Ls=40, 115 km Altitude and midafternoon local solar times (LSTs) show a westward speed of 50 to 100 m/s; those for the southern hemisphere (SH) at about Ls=80, 110 km Altitude show an eastward speed of 0 to 50 m/s. Solar activity is moderate for both periods with an F10.7 index of about 140 units. In the NH, wind speed shows no dependence on longitude, decreases as latitude increases poleward, and increases as Altitude increases. In the SH, repeated measurements of wind speed at fixed latitude, Altitude, LST, and longitude during the 8:1 resonance between MGS's orbit and Mars' rotation show a significant dependence of wind speed on longitude. At 20E longitude the typical wind speed is 50 m/s westward, whereas at 335E it is 120 m/s eastward. The dependence of wind speed on latitude and Altitude is difficult to examine, because periapsis Altitude steadily decreased as periapsis precessed poleward. The two variables are strongly correlated. In some longitude regions, eastward wind speeds increase as periapsis moves poleward and downward, but in others the eastward wind speeds stay constant. At 60S latitude and nighttime LSTs, wind speeds differ from their daytime values. Nighttime wind speeds at a given longitude show much less variability than their daytime counterparts. These results will be compared to MTGCM simulations. Other applications of this technique will be suggested.
Franck Montmessin - One of the best experts on this subject based on the ideXlab platform.
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Variations of water vapor and cloud top Altitude in the Venus’ mesosphere from SPICAV/VEx observations
Icarus, 2016Co-Authors: Anna Fedorova, Emmanuel Marcq, Mikhail Luginin, Oleg Korablev, Jean-loup Bertaux, Franck MontmessinAbstract:SPICAV VIS-IR spectrometer on-board the Venus Express mission measured the H2O abundance above Venus’ clouds in the 1.38 µm band, and provided an estimation of the cloud top Altitude based on CO2 bands in the range of 1.4-1.6 μm. The H2O content and the cloud top Altitude have been retrieved for the complete Venus Express dataset from 2006 to 2014 taking into account multiple scattering in the cloudy atmosphere. The cloud top Altitude, corresponding to unit nadir aerosol optical depth at 1.48 μm, varies from 68 to 73 km at latitudes from 40ºS to 40ºN with an average of 70.2±0.8 km assuming the aerosol scale height of 4 km. In high northern latitudes, the cloud top decreases to 62-68 km. The Altitude of formation of water lines ranges from 59 to 66 km. The H2O mixing ratio at low latitudes (20ºS-20ºN) is equal to 6.1±1.2 ppm with variations from 4 to 11 ppm and the effective Altitude of 61.9±0.5 km. Between 30º and 50º of latitude in both hemispheres, a local minimum was observed with a value of 5.4±1 ppm corresponding to the effective Altitude of 62.1±0.6 km and variations from 3 to 8 ppm. At high latitudes in both hemispheres, the water content varies from 4 to 12 ppm with an average of 7.2±1.4 ppm which corresponds to 60.6±0.5 km. Observed variations of water vapor in 2-3 times on the short timescale appreciably exceed individual measurement errors and could be explained as a real variation of the mixing ratio or/and possible variations of the cloud opacity within the clouds. The maximum of water at lower latitudes supports a possible convection and injection of water from lower atmospheric layers. The vertical gradient of water vapor inside the clouds explains well the increase of water near the poles correlating with the decrease of the cloud top Altitude and the H2O effective Altitude. On the contrary, the depletion of water in middle latitudes does not correlate with the H2O effective Altitude and cannot be completely explained by the vertical gradient of water vapor within the clouds. Retrieved H2O mixing ratio is higher than those obtained in 2.56 μm from VIRTIS-H data [Cottini et al., 2015] at Altitudes of 68-70 km which is well consistent with the lower Altitudes of water mixing ratio from the 1.38 µm band. Observations for different solar and emission angles allowed to constrain also the average vertical distribution of H2O mixing ratio in the clouds at Altitudes of 59-66 km with 2 ppm at 66 km and 7-7.5 ppm at 59-61 km. The water vapor latitudinal-longitudinal distribution does not show any direct correlation with the cloud tops. Yet a strong asymmetry of H2O longitudinal distribution has been observed with a maximum of 7-7.5 ppm from -120º to 30º of longitude and shifted to the southern hemisphere (20ºS-10ºN). To the east, the minimum is observed with values not in excess of 6 ppm and over a wide range of longitudes from 30º to 160º. Bertaux et al. (2015) announced a correlation between the zonal wind pattern in the equatorial region and underlying topography of Aphrodite Terra as the result of stationary gravity waves produced at the ground level near the mountains. The water minimum corresponds to the Aphrodite Terra highlands and can be also associated with the influence of Venus topography. No prominent long-term on the time scale of 8.5 years nor local time variations of water vapor and the cloud top Altitude were detected.
D Pisa - One of the best experts on this subject based on the ideXlab platform.
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emic waves observed by the low Altitude satellite demeter during the november 2004 magnetic storm
Journal of Geophysical Research, 2015Co-Authors: D Pisa, M Parrot, O Santolik, J D MeniettiAbstract:This paper presents an analysis of ULF (0–20 Hz) waves observed by the low-Altitude satellite Detection of ElectroMagnetic Emissions Transmitted from Earthquake Regions (DEMETER) during the magnetic storm of November 2004. Since these ULF waves are measured by both electric and magnetic antennas, they may be identified as electromagnetic ion cyclotron (EMIC) waves. While EMIC waves have been previously observed in the low-Altitude ionosphere, this is the first time that they are observed for such extensive time periods and at such high frequencies. A common feature of these emissions is that their observation region in the low-Altitude ionosphere extends continuously from the high-latitude southern trough in one side up to the high-latitude northern trough. The analysis of wave propagation points to the possible source region placed in the inner magnetosphere (L∼2–3). Observed wave frequencies indicate that waves must be generated much farther from the Earth compared to the satellite orbit. Exceptionally high frequencies of about 10 Hz can be explained by the source region placed in the deep inner magnetosphere at L ∼2.5. We hypothesize that these waves are generated below the local helium gyrofrequency and propagate over a large range of wave normal angles to reach low Altitudes at L ∼1.11. In order to investigate this scenario, a future study based on ray tracing simulations will be necessary.
S Tellmann - One of the best experts on this subject based on the ideXlab platform.
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structure of the venus neutral atmosphere as observed by the radio science experiment vera on venus express
Journal of Geophysical Research, 2009Co-Authors: S Tellmann, M Patzold, B Hausler, M K Bird, Leonard G TylerAbstract:[1] The European Space Agency Venus Express Radio Science experiment (VeRa) obtained 118 radio occultation measurements of the Venusian atmosphere between July 2006 and June 2007. Southern latitudes are uniformly sampled; measurements in the northern hemisphere are concentrated near the pole. Radial profiles of neutral number density derived from the occultations cover the Altitude range 40–90 km, which are converted to profiles of temperature (T) and pressure (p) versus height (h). Profiles of static stability are found to be latitude-dependent and nearly adiabatic in the middle cloud region. Below the clouds the stability decreases at high latitudes. At an Altitude of 65 km, the VeRa T[p(h)] profiles generally lie between the Venus International Reference Atmosphere (VIRA) and VIRA-2 models; the retrieved temperatures at any given pressure level typically are within 5 K of those derived from the Pioneer Venus Orbiter Radio Occultation experiments. A large equator-to-pole temperature contrast of ∼30 K is found at the 1-bar (1000 hPa) level. The VeRa observations reveal a distinct cold collar region in the southern hemisphere, complementing that in the north. At the latitudes of the cold collars, the tropopause Altitude increases relative to higher and lower latitudes by ≈7 km while the temperature drops roughly 60 K. The observations indicate the existence of a wave number 2 structure poleward of ±75° latitude at Altitudes of 62 km.
