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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Satellite Constellation Optimization for In-Situ Sampling and Reconstruction of Tides in the Thermospheric Gap

Lane, Kayton Anne 04 January 2024 (has links)
Earth's atmosphere is a dynamic region with a complex interplay of energetic inputs, outputs, and transport mechanisms. A complete understanding of the atmosphere and how various fields within it interact is essential for predicting atmospheric shifts relevant for spaceflight, the evolution of Earth's climate, radio communications, and other practical applications. In-situ observations of a critical altitude region within Earth's atmosphere from 100-200 km in altitude, a subset of a larger 90 – 400 km altitude region deemed the "Thermospheric Gap", are required for constraining atmospheric models of wind, temperature, and density perturbations caused by atmospheric tides. Observations within this region that are sufficient to fully reconstruct and understand the evolution of tides therein are nonexistent. Certain missions have sought to fill portions of this observation gap, including Daedalus which was selected as a candidate for the Earth Explorer program by the European Space Agency in 2018. This study focuses on the design and optimization of a two-satellite, highly elliptical satellite constellation to perform in-situ observations and reconstruction of tidal features in the 100-200 km region. The model atmosphere for retrieving sample data is composed of DE3 and DE2 tidal features from the Climatological Model of the Thermosphere (CTMT) and background winds from the Thermosphere-Ionosphere-Electrodynamic General Circulation Model (TIEGCM). BoTorch, a Bayesian Optimization package for Python, is integrated with the Ansys Systems Tool Kit (STK) to model the constellation's propagation and simulated atmospheric sampling. A least squares fitting algorithm is utilized to fit the sampled data to a known tidal function form. Key results include 14 Pareto optimal solutions for the satellite constellation based on a set of 7 objective functions, 3 constellation input parameters, and a sample set of n = 86. Four of these solutions are discussed in more detail. The first two are the best and second-best options on the Pareto front for sampling and reconstruction of the input tidal fields. The third is the best solution for latitudinal tidal fitting coverage. The fourth is a compromise solution that nearly minimizes delta-v expenditure, while sacrificing some quality in tidal fitting and fitting coverage. / Master of Science / Earth's atmosphere, the envelope of gaseous material surrounding the planet from an altitude of 0 km to approximately 10,000 km, is a dynamic system with a diverse set of energy inputs, outputs, and transfer mechanisms. A complete understanding of the atmosphere and how various fields within it interact is essential for predicting atmospheric shifts relevant for spaceflight, the evolution of Earth's climate, radio communications, and other practical applications. The atmosphere life breathes on Earth's surface evolves in physical and chemical properties, such as temperature, pressure, and composition, as distance from Earth increases. In addition, the atmosphere varies temporally, with shifts in its properties occurring on several timescales, some as short as a few minutes and some on the order of the age of the planet itself. This thesis project seeks to study the optimization of a satellite system to further understand an important source of atmospheric variability – atmospheric tides. Just as the forces of gravity from the moon and sun cause tides in the oceans, the Earth's rotation and the periodic absorption of heat into the atmosphere from the sun cause atmospheric tides. A model atmosphere with a few tides and a background wind is generated to perform simulated tidal sampling. The latitude, longitude, and altitude coordinates of the satellites as they propagate through the atmosphere are used to model samples of the northward and southward atmospheric winds and determine how well the constellation does at regenerating the input tidal data. The integration of several software tools and a Bayesian Optimization algorithm automate the process of finding a range of options for the constellation to best perform the tidal fitting, minimize satellite fuel consumption, and cover as many latitude bands of the atmosphere as possible.
22

Variações nas marés atmosféricas e nos ventos meteóricos observados em São João Do Cariri-PB e em Cachoeira Paulista-SP. / Variations in atmospheric tides and winds meteoric observed in São João do Cariri-PB and Cachoeira Paulista-SP

Borges, Fabricio Batista 04 April 2010 (has links)
Made available in DSpace on 2015-09-25T12:23:41Z (GMT). No. of bitstreams: 1 Fabricio Batista Borges.pdf: 1546340 bytes, checksum: 1d8be2583a092a34d256d749548325ef (MD5) Previous issue date: 2010-04-04 / Meteor wind measurements obtained from São João do Cariri B and Cachoeira Paulista SP, Brazil, obtained during the period from August 2004 to July 2006 were used to examine Variations in atmospheric tides and winds in the mesopause region of the equatorial and low Latitudes in the southern hemisphere. The results for the mean wind were compared with the HWM 93 and HWM 07 models (Horizontal neutral wind model) and the results of Atmospheric tides with GSWM 02 model (Global Scale Wave Model). From the analysis of These data it was observed that the mean wind, as well as diurnal fluctuations, showed Temporal and in altitude variations for both the zonal and to the south for both the localities investigated. Mean zonal winds for the region of the S. J. do Cariri show a structure that is characterized by a semi annual oscillation, with a flow westward most of the time, in accordance with HWM 07 model. The mean zonal wind at C. Paulista is eastward in the most time and presents a semi annual variation in the 80 90 km altitude range and an annual variation in the altitudes above, in which some aspects are in accordance with HWM 93 and 07 models. The amplitudes of the mean meridional winds were weaker than zonal and present An annual variation for both sites, which are in accordance with HWM 93 model, however the HWM 07 do not reproduce the observed behavior. The meridional diurnal tide amplitudes Showed semi annual variation with maximum of the up to 65 m/s during February April and August September. The vertical wavelength estimated reached values between 21.2 and 27.5 Km at S. J. do Cariri, smaller than the GSWM 02, whereas to C. Paulista the vertical Wavelength were determined between 24 and 31.9 km, near of the GSWM 02 model, but More than the diurnal tide meridional wavelength for Cariri. / Medidas de vento meteóricos obtidos em São João do Cairi-PB e Cachoeira Paulista-SP Brasil, realizadas durante o período compreendido entre agosto de 2004 a julho de 2006, foram usadas para investigar as variações nas marés atmosféricas e nos ventos meteóricos na região da mesopausa equatorial e de baixas latitudes do hemisfério sul. Os resultados obtidos para os ventos médios foram comparados com os modelos HWM 93 e HWM 07 (Horizontal Neutral Wind Model) e os resultados das marés atmosféricas com GSWM 02 (Global Scale Wave Model). A partir da análise destes dados, foi possível observar que os ventos médios, Assim como as oscilações diurnas, apresentaram variação temporal e em função da altura, tanto para a componente zonal como para a meridional para ambas as localidades investigadas. Os ventos médios na direção zonal para região de S. J. do Cariri mostram uma estrutura que é Caracterizada por uma oscilação semi anual, apresentando um escoamento para oeste na maior Parte do tempo, semelhante com o modelo HWM 07. Já o vento médio zonal verificado na Região de C. Paulista é predominante para leste com variação semi anual entre 80 e 90 km e Anual nas altitudes acima, semelhante em alguns aspectos com os modelos HWM 93 e 07. O Escoamento médio na direção meridional apresenta amplitudes menores do que as do zonal e Oscilação anual para ambas as localidades, a qual é semelhante com o modelo HWM 93, Porém o modelo HWM 07 não reproduz o comportamento observado. As amplitudes da maré Diurna para a componente meridional mostraram variação semi anual com máximos de até 65 m/s em fevereiro abril e agosto setembro. Comprimentos de onda vertical para a componente meridional foram estimados entre 21,2 e 27,5 km para S. J. do Cariri, menores do que as observadas pelo modelo GSWM 02, enquanto que para C. Paulista os comprimentos de onda assumiram valores entre 24 e 31,9 km, próximos aos previstos, porém maiores do que os de Cariri.
23

Gravity wave coupling of the lower and middle atmosphere.

Love, Peter Thomas January 2009 (has links)
A method of inferring tropospheric gravity wave source characteristics from middle atmosphere observations has been adapted from previous studies for use with MF radar observations of the equatorial mesosphere-lower thermosphere at Christmas Island in the central Pacific. The nature of the techniques applied also permitted an analysis of the momentum flux associated with the characterised sources and its effects on the equatorial mean flow and diurnal solar thermal tide. An anisotropic function of gravity wave horizontal phase speed was identified as being characteristic of convectively generated source spectra. This was applied stochastically to a ray-tracing model to isolate numerical estimates of the function parameters. The inferred spectral characteristics were found to be consistent with current theories relating convective gravity wave spectra to tropospheric conditions and parameters characterising tropical deep convection. The results obtained provide observational constraints on the model spectra used in gravity wave parameterisations in numerical weather prediction and general circulation models. The interaction of gravity waves with the diurnal solar thermal tide was found to cause an amplification of the tide in the vicinity of the mesopause. The gravity wave-tidal interactions were highly sensitive to spectral width and amplitude. Estimates were made of the high frequency gravity wave contribution to forcing the MSAO with variable results. The data used in the analysis are part of a large archive which now has the potential to provide tighter constraints on wave spectra through the use of the methods developed here. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1352362 / Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 2009
24

Modélisation du rayonnement proche infrarouge émis par la haute atmosphère : étude théorique et observationnelle / Nightglow modelling at high altitude : theoretical and observational study

Bellisario, Christophe 10 December 2015 (has links)
Le rayonnement atmosphérique appelé nightglow est un phénomène se produisant à haute altitude (environ 90 km). Il consiste en l’émission d’un rayonnement suite à la désexcitation de certaines molécules et atomes (OH, Na, O2 et O). Il se répartit sur une large gamme spectrale, en particulier dans l’infrarouge et se propage jusqu’au niveau du sol. Le rayonnement nightglow constitue un marqueur important pour la haute atmosphère, permettant de remonter à la température, mais également à de nombreux phénomènes dynamiques comme les marées atmosphériques ou les ondes de gravité. Sa propagation au niveau du sol permet l’éclairage de scène terrestre et ainsi la vision nocturne à l’aide de caméras proche infrarouge. Afin de mieux connaître les fluctuations de ces émissions en fonction du temps à différentes échelles et en différents lieux sur la planète, la thèse s’est axée sur une étude observationnelle et une étude théorique. L’étude observationnelle a produit une climatologie à grande échelle par l’extraction du rayonnement issu des données de l’instrument GOMOS. Les campagnes de mesures réalisées au sol ont quant à elles mis en avant certains aspects dynamiques importants comme les marées et les ondes de gravité. Pour reproduire le rayonnement nightglow, il a été nécessaire de modéliser les réactions chimiques des nombreuses espèces présentes à haute altitude, le chauffage, la photodissociation de certaines molécules par le rayonnement solaire et la propagation du rayonnement vers le sol. Certains processus dynamiques ont été inclus comme la diffusion moléculaire, la diffusion turbulente et une paramétrisation des marées. Enfin, les résultats du modèle sont comparés aux observations satellitaires ainsi qu’au niveau du sol et des tests de sensibilité sont effectués pour estimer la réponse du rayonnement aux différents modules du modèle. / The nightglow is an atmospheric radiation which occurs at high altitude (around 90 km). It comes from the desexcitation of specific molecules and atoms (OH, Na, O2 and O). It spreads over a wide spectral band, especially in the infrared and propagates to the ground level. The nightglow emission is an important mark for the high atmosphere, as it allows the retrieval of the temperature and many dynamic processes such as atmospheric tides or gravity waves. Its propagation to the ground level allows the illumination of terrestrial scene and therefore the night vision with the use of near infrared cameras. In order to have a better knowledge of the emission fluctuations as a function of time for various scales and at various locations, the work is focused on an observational and theoretical study. The observational study produced large scale climatology with the extraction of nightglow emission from GOMOS data. On the other hand, ground measurements highlighted some dynamical aspects such as tides and gravity waves. To model the nightglow emission, it has been necessary to take into account the chemical reactions of the species available at high altitude, the heating, the photodissociation process and the propagation of the emission to the ground. Selected dynamical processes have been included, such as the molecular and turbulent diffusion, and a tide parameterization. Finally, the results of the model are compared to the satellite and ground observations and sensitivity tests are run to estimate the response of the emission to the various modules of the model.
25

Role Of Sea Surface Temperature Gradient In Intraseasonal Oscillation Of Convection In An Aquaplanet Model

Das, Surajit 09 1900 (has links) (PDF)
In this thesis we examine intra-seasonal oscillations (ISO) in the aqua-planet setup of the Community Atmospheric Model (CAM) version 5.1, mainly based on July and January climatological sea surface temperature (SST). We investigate mainly two questions -what should be the SST distribution for the existence of (a) northward moving ISO in summer, and (b) eastward moving MJO-like modes in winter. In the first part of the thesis we discuss the northward propagation. A series of experiments were performed with zonally symmetric and asymmetric SST distributions. The basic lower boundary condition is specified from zonally averaged observed July and January SST. The zonally symmetric July SST experiment produced an inter tropical convergence zone (ITCZ) on both sides of the equator. Poleward movement is not clear, and it is confined to the region between the double ITCZ. In July, the Bay of Bengal (BOB) and West Pacific SST is high compared to the rest of the northern tropics. When we impose a zonally asymmetric SST structure with warm SST spanning about 80 of longitude, the model shows a monsoon-like circulation, and some northward propagating convective events. Analysis of these events shows that two adjacent cells with cyclonic and anticyclonic vorticity are created over the warm SST anomaly and to the west. The propagation occurs due to the convective region drawn north in the convergence zone between these vortices. Zonally propagating Madden-Julian oscillations (MJO) are discussed in the second part of the thesis. All the experiments in this part are based on the zonally symmetric SST. The zonally symmetric January SST configuration gives an MJO-like mode, with zonal wave number 1 and a period of 40-90 days. The SST structure has a nearly meridionally symmetric structure, with local SST maxima on either side of the equator, and a small dip in the equatorial region. If we replace this dip with an SST maximum, the time-scale of MJO becomes significantly smaller (20-40 days). The implication is that an SST maximum in the equatorial region reduces the strength of MJO, and a flat SST profile in the equatorial region is required for more energetic of MJO. This result was tested and found to be valid in a series of further experiments.

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