In the present work the vacuum polarization and the circular dichroism of hydrogen-like atoms are studied. We derive equations for the Fourier transforma- tion of the vacuum expectation value of the charge density. We use it to derive Uehling potential and calculate energy shifts caused by it. Then we discuss effects of vacuum polarization in higher orders of α. In second part we define circular dichroism and we express it in terms of reduced matrix elements. Then we derive the formula for parity vilating potential which is generated by weak interaction and together with other results we use it to find the expression for circular dichro- ism in terms of hydrogen radial functions. 1

Buildings consume around half of the UK's total energy consumption and are responsible for almost 50% of UK's total carbon dioxide (CO2) emissions. Use of high thermal resistance insulation in buildings is critical to save the substantial amounts of space heating energy lost through building fabric. Conventional building insulation materials have higher thermal conductivity values ranging from 40 mWm-1K-1 (Glass fibre) - 26 mWm-1K-1 (Polyurethane foam) and require larger thicknesses to achieve stringent building regulation requirements which may not be feasible due to techno-economic constraints. Vacuum Insulation Panel (VIP) is a relatively new insulation for building applications that offers 5-8 times higher thermal resistance and can achieve significant space savings in buildings. VIPs are produced as a rigid panel comprising inner core board laminated in an outer high barrier envelope under evacuated conditions (< 5mbar). However, the main challenge for large scale acceptance of VIPs in building applications is their higher cost. VIPs have been shown to have an approximately 10 times longer payback compared to conventional EPS insulation due to their high initial cost. Expensive materials currently being used for VIP manufacturing such as fumed silica contribute to high cost of VIPs and it is critical to identify alternative low cost materials for VIP components to overcome the challenge of high cost. The aim of this thesis was to develop an alternative low cost material and investigate its suitability for use as VIP core. Expanded perlite, a low cost material was identified as a replacement of expensive fumed silica in a VIP core. Composite samples containing expanded perlite, fumed silica, silicon carbide (SiC) and polyester fibres were developed by dry mixing of the constituents in different mass ratios and their different properties were experimentally measured to identify optimum composition of composite. Gaseous thermal conductivity at different pressures was calculated from the pore size data obtained using Mercury Intrusion Porosimetry (MIP), gas adsorption and electron microscopy. Radiative conductivity of composite samples was measured using Fourier Transform Infrared (FTIR) to ascertain the opacifying effect of expanded perlite and opacifier (SiC). Centre of panel thermal conductivity of core boards of size 100mm x 100mm made of composite material at atmospheric pressure was measured by using a small guarded hot plate device. Average pore diameter values of expanded perlite decreased with the partial filling of fumed silica aggregates and was found to be in the range of 150-300 nm yielding lower gaseous conductivity values of 1.2-2.1 mWm-1K-1 at 100mbar and became negligible upon further decreasing pressures below 10 mbar. Core boards made of optimised composite containing 30% expanded perlite and 50% fumed silica along with SiC and polyester fibres was found to achieve centre of panel thermal conductivity of 28 mWm-1K-1 at atmospheric pressure and the average radiative conductivity of 0.67 mWm-1K-1 at 300K with its gaseous thermal conductivity at 1 mbar being 0.016 mWm-1K-1. According to the results of the thesis VIP prototypes consisting of core made with optimised composite consisting (50 mass% of fumed silica, 30 mass% of expanded perlite along with 8 mass% of fibre and 12 mass% of SiC) yielded centre of panel thermal conductivity of 7.4-7.6 mWm-1K-1 at pressure of 0.53-0.64 mbar. Opacifying properties of expanded perlite were observed and quantified. Expanded perlite reduced the radiative conductivity of the composite requiring smaller quantities of high density opacifiers such as SiC. For sample containing no expanded perlite, average radiative conductivity was calculated to be 1.37 mWm-1K-1 and radiative conductivity values decreased to 1.12 mWm-1K-1, 0.67 mWm-1K-1, 0.63 mWm-1K-1 and 0.50 mWm-1K-1 with mass ratio of expanded perlite 20%, 30%, 40% and 60% respectively. It was concluded that the solid conductivity of prototypes VIPs was 1.8-2 times higher compared to those of commercially available VIPs and is the main reason for higher centre of panel thermal conductivity.

Nesta dissertação, estudamos implicações geradas pela quebra da simetria de Lorentz na Eletrodinâmica Quântica. Analisamos férmions interagindo com um campo eletromagnético nos contextos da mecânica quântica e ao efetuar correções radiativas. Na mecânica quântica, os termos de quebra da simetria de Lorentz foram tratados como perturbações à equação de Dirac, e seus valores esperados no vácuo foram obtidos. Nas correções radiativas, a quebra da simetria de Lorentz foi introduzida nessa interação para que o termo tipo Chern-Simons pudesse ser induzido em (3+1) dimensões. Também discutimos as consequências geradas por este termo sobre as velocidades de propagação de fótons clássicos. / In this dissertation, we study the implications generated by the Lorentz breaking symmetry in quantum electrodynamics. We analyze fermions interacting with an electromagnetic field in the contexts of quantum mechanics and we make radiative corrections. In quantum mechanics, the terms of the Lorentz breaking symmetry were treated as perturbations to the Dirac equation, and their expected values were obtained in a vacuum. In the radiative corrections, the Lorentz breaking symmetry was introduced in this interaction for the Chern-Simons like term could be induced in (3 +1) dimensions. We also discussed the consequences generated by this term on the propagation speeds of classic photons.

Nuvens desempenham papel fundamental no balanço radiativo terrestre, e o conhecimento de suas propriedades micro e macrofísicas é importante para o estudo do clima global. O desenvolvimento de nuvens esta ligado à dinâmica da atmosfera, fluxos de energia térmica e radiativa à superfície, e também depende crucialmente do tamanho de seus hidrometeoros. Uma ferramenta importante para o estudo de hidrometeoros em nuvens de grande escala e o sensoriamento remoto por satélite, que representa uma alternativa para se estudar propriedades de nuvens em grandes escalas espaciais. No entanto, atualmente não há estudos para subsidiar análises de propriedades microfísicas de nuvens a partir de plataformas geoestacionárias. Neste trabalho foram comparados dois códigos de transferência radiativa amplamente utilizados pela comunidade científica, SBDART e libRadtran, em simulações considerando variações de propriedades atmosféricas, de superfície, macro e microfísicas de nuvens. Em seguida estudou-se a transferência radiativa em nuvens de água e gelo determinando o impacto simulado de diversas geometrias de iluminação e observação nas radiâncias medidas por sensoriamento remoto. Finalmente, foram desenvolvidas tabelas de referência para a determinação de tamanho de hidrometeoros em nuvens, para fases líquida e solida, a partir de radiâncias medidas por um sensor em satélite geoestacionário. As comparações entre SBDART e libRadtran mostram resultados sistematicamente subestimados pelo libRadtran. Em um céu sem nuvens a diferença entre as refletâncias calculadas em ambos os modelos é inferior a 4% quando consideradas variações em albedo de superfície e coluna de ozônio. Em um céu com nuvens a diferença entre os resultados dos modelos para comprimento de onda de 630 nm e intervalo visível (590 a 660 nm) pode chegar ate 18% para variações de profundidade óptica entre 0 e 20. Para profundidade óptica maior que 20 essas diferenças variam entre 4 e 9%, além de refletâncias constantes para o intervalo entre 3850 e 4000 nm e para comprimento de onda de 3900 nm. A divergência de resultados obtidos pelos dois modelos é atribuída a diferenças estruturais entre os códigos, uma vez que atualmente apenas o libRadtran apresenta atualizações periódicas pela comunidade científica. Assim, este modelo foi utilizado para a construção da tabela de referência simulando medidas de radiâncias em plataforma geoestacionária. A tabela construída compreende 86 valores de raios efetivos de hidrometeoros, variando entre 2 e 59m, com fases termodinâmicas de água e gelo, 16 valores de geometria de observação solar, 3 valores de âgulos de observação, 2 valores de azimute relativo, e condições fixas de profundidade óptica de nuvem de 50, conteúdo integrado de ozônio de 255 DU e 60mm de vapor d água. Essas condições foram escolhidas como representativas para a região Amazônica. Em um teste de aplicação direta da tabela de referência a medidas de radiância realizadas com satélite geoestacionário foram obtidos raios efetivos entre 2 e 30m para gotículas de água e até 24m para cristais de gelo. Os resultados obtidos neste trabalho poderão ser aplicados futuramente a medidas obtidas por plataformas geoestacionárias em estudos de tamanhos de hidrometeoros, tornando possível a análise de sua evolução temporal. / Clouds play a fundamental role in the terrestrial radiative balance, and knowledge of its micro and macrophysical properties is important for the study of global climate. Cloud development is linked to the dynamics of the atmosphere, thermal and radiative energy flows to the surface, and also depends crucially on the size of its hydrometeors. An important tool for the study of large-scale cloud hydrometeors is satellite remote sensing, which represents an alternative to study cloud properties at large spatial scales. However, there are currently no studies to support analyzes of microphysical properties of clouds from geostationary platforms. In this work, two radiative transfer codes widely used by the scientific community, SBDART and libRadtran, were compared in simulations considering variations of atmospheric, surface, macro and microphysical properties of clouds. Next, radiative transference was studied in water and ice clouds, determining the simulated impact of various lighting and observation geometries on radiances measured by remote sensing. Finally, look-up tables were developed for the determination of the size of hydrometeors in clouds, for liquid and solid phases, from radiances measured by a geostationary satellite sensor. The comparisons between SBDART and libRadtran show results systematically underestimated by the libRadtran. In a cloudless sky the difference between the reflectances calculated in both models is less than 4% when considering variations in surface albedo and ozone. In a clouded sky the difference between the model results for wavelengths of 630 nm and the visible range (590 to 660 nm) can reach up to 18% for optical depth variations between 0 and 20. For optical depths greater than 20, these differences range from 4 to 9%, in addition to constant reflections for the range of 3850 to 4000 nm and for a wavelength of 3900 nm. The divergence of results obtained by the two models is attributed to structural differences between the codes, since currently only the libRadtran presents periodic updates by the scientific community. Thus, this model was used for the construction of the reference table simulating measurements of radiances in geostationary platform. The constructed table comprises 86 values of effective radii of hydrometeors, ranging from 2 to 59m, with thermodynamic phases of water and ice, 14 values of geometry of solar observation, 3 values of angles of observation, 2 relative azimuth values, and fixed cloud optical depth conditions of 50, integrated ozone content of 255 DU and 60mm of water vapor. These conditions were chosen as representative for the Amazon region. In a test of direct application of the look-up table to measurements of radiance obtained with geostationary satelite we obtained effective radius up to between 2 and 30 for water droplets and up to 24 for ice crystals. The results obtained in this work can be applied in the future to measurements obtained by geostationary platforms in studies of sizes of hydrometeors, making possible the analysis of their temporal evolution.

Este trabalho tem como objetivo implementar e testar modelos de gás cinza atualizados na rotina de radiação térmica do software Fire Dynamics Simulator (FDS), além da utilização do próprio modelo de gás cinza disponível no software, para a predição do fluxo de calor radiativo. Os modelos de gás cinza estudados foram o modelo padrão do software FDS (aqui denominado como GC1), e os modelos de gás cinza mais atuais: o GC2, no qual o coeficiente de absorção do meio participante é dado por relações polinomiais, e o GC3, sendo este um modelo de gás cinza que baseia o cálculo do coeficiente de absorção no modelo WSGG. Os novos modelos de gás cinza foram implementados no código fonte do software FDS, o qual é um código aberto, e a verificação da implementação foi realizada através da solução numérica do equacionamento utilizando os valores reportados pelo software. Com os novos modelos de gás cinza já corretamente implementados, passou-se então para a simulação computacional dos casos previamente selecionados. Para todos os modelos de gás cinza, foram simulados incêndios em poças, para diferentes combustíveis (etanol, n-heptano e metanol) em diferentes cenários de incêndio, considerando ou não a presença de fuligem no sistema. Os cenários de incêndio eram: (i) totalmente fechado, (ii) totalmente aberto e (iii) com uma condição intermediária, fechado, porém com uma abertura para o meio externo. Um estudo de análise de malha e de diferentes parâmetros, como o estudo da quantidade necessária de ângulos sólidos discretos, foram realizados para correta padronização dos parâmetros. As simulações computacionais foram validadas para o modelo de gás cinza padrão do FDS através da comparação de resultados com aqueles reportados na literatura específica de cada caso. Com os modelos já validados simulou-se novamente cada cenário de incêndio com os diferentes modelos de gás cinza anteriormente implementados. A partir da análise dos resultados obtiveram-se boas concordâncias para os campos de temperatura, frações molares tanto de CO2 quanto de H2O e para as frações volumétricas de fuligem. Os fluxos de calor radiativos foram corretamente preditos para todos os modelos de gás cinza implementados. O modelo GC2 apresentou resultados com desvios médios na faixa de 15%, o modelo de gás cinza baseado no WSGG (GC3) apresentou os melhores resultados, com erros médios inferiores a 10%, enquanto que o modelo padrão do software, GC1, apresentou resultados intermediários. / This work aims to implement and test updated gray gas models in the thermal radiation routine of the Fire Dynamics Simulator (FDS) software, as well as the use of the gray gas model available in the software to the prediction of radiative heat flux. The gray gas models studied were the default model of the FDS software (determined GC1), and the most current gray gas models: the GC2, in which the absorption coefficient of the participant medium is given by a polynomial relations, and the GC3, which is a gray gas model that was based on the calculation of the absorption coefficient in the WSGG model. The most recently gray gas models were implemented in the source code, which is an open source, and the verification of the implementation was performed by the numerical solution of the equations from the reported values of the software. With the new gray gas models already implemented, the next step was the computational simulation of the previously selected cases. For all the gray gas models, pool fires were simulated different scenarios of fire for different fuels (ethanol, nheptane and methanol), with and without considering soot presence in the system. The fire scenarios were: (i) fully closed, (ii) fully open and (iii) with an intermediate condition, closed but with an opening to the external environment. A study of a mesh analysis and different parameters, such as the study of the required amount of discrete solid angles, were performed to correct the standard parameters. The computational simulations were verified for the default gray gas model of the FDS by comparing the simulations results with those reported in the specific literature of each case. With the models already verified, each fire scenario was simulated with the different gray gas models previously implemented. From the analysis of the results, good agreements were obtained for the fields of temperature, molar fraction of CO2 and H2O and soot volume fraction. The radiative heat fluxes were correctly predicted for all gray gas models early implemented. The GC2 model present results with average deviation in the range of 15%, the gray gas model based on WSGG (GC3) presented the best results, with average deviation lower than 10%, while the default software model (GC1) presented intermediate results.

As nuvens cirrus cobrem uma grande fração das latitudes tropicais e desempenham um papel importante no balanço de radiação da Terra. As propriedades ópticas, altitude, extensão vertical, e cobertura horizontal de nuvens controlam sua forçante radiativa. Além disso, nuvens cirrus tropicais podem influenciar a distribuição vertical do aquecimento radiativo na tropopausa tropical e pesquisas recentes também apontam para um aumento do vapor de água estratosférico ligado principalmente à ocorrência de nuvens cirrus na camada da tropopausa tropical (TTL). Apesar de sua importância, estudos relatando propriedades de nuvens cirrus sobre florestas tropicais como a Amazônia são ainda escassos. A maioria estão baseados em imagens de satélites de órbita polar que não fornecem informações sobre o ciclo diurno, nem sobre a estrutura vertical destas nuvens. Ao mesmo tempo, os estudos com lidar em solo são restritos a poucos estudos de caso, em geral associados a campanhas de campo de curta duração. Este panorama começou a mudar em 2011 com a instalação do sistema lidar do Laboratório de Física Atmosférica do IF-USP próximo à cidade de Manaus, Brasil. Neste trabalho, um conjunto de um ano de dados (de julho de 2011 a junho de 2012) foi utilizado para caracterizar as propriedades macro, microfísicas e ópticas das nuvens cirrus sobre a região amazônica e, posteriormente, calcular o papel que essas nuvens têm no balanço radiativo do planeta. Para tanto, foi desenvolvido um algoritmo automático para detectar as nuvens e para obter as propriedades ópticas, incluindo a correção de múltiplo-espalhamento. As forçantes radiativas foram estimadas com dois modelos diferentes, a partir dos perfis de extinção medido com o lidar e de uma parametrização para estimar o raio efetivo dos cristais de gelo. Nossos resultados mostraram que as nuvens cirrus na alta troposfera foram mais frequentes na Amazônia do que relatado previamente em outras regiões tropicais. A frequência de ocorrência foi de 88 % durante a estação chuvosa e não inferior a 50 % durante a estação seca. O ciclo diurno mostrou um mínimo ao redor do meio-dia local e máximo durante o final da tarde, associado ao ciclo diurno da precipitação. Os valores médios das alturas de topo e base, da espessura e da profundidade óptica da nuvem foram de 14,3 +- 1,9 (desv. pad.) km, 12,9 +- 2,2 km, 1,4 +- 1,1 km e 0,25 +- 0,46, respectivamente. As nuvens cirrus foram encontradas em temperaturas de até -90 degC, com 6 % ocorrendo acima da tropopausa. A distribuição vertical não se mostrou uniforme, e nuvens cirrus finas (0,03 < COD < 0,3) e subvisuais (COD < 0,03) ocorreram mais frequentemente nas proximidades da tropopausa. A razão lidar média foi de 23,3 +- 8,0 sr. Contudo, para as nuvens cirrus subvisuais foi encontrada uma distribuição bimodal com um pico secundário em torno de 44 sr, sugerindo uma composição mista dos cristais de gelo. Não foi encontrada uma dependência da razão lidar com a temperatura da nuvem (altitude), indicando que as nuvens estão verticalmente bem misturadas. A frequência de ocorrência relativa das camadas de nuvens cirrus classificadas como subvisuais foi de 41,6 %, enquanto que 37,8 % foram cirrus finos e 20,5 % de cirrus opacos (COD > 0,3), com uma superposição média de 1,41 +- 0,63 camadas por perfil. Assim, na Amazônia central não ocorre apenas uma alta frequência de nuvens cirrus, mas também uma grande fração de nuvens cirrus subvisuais, o que pode estar contaminando as medidas de fotômetros solares e sensores orbitais. As propriedades medidas foram utilizadas no cálculo da forçante radiativa das nuvens cirrus (CRF) e dos perfis da taxa de aquecimento da atmosfera, em detalhe pelo libRadtran e aproximadamente pelo modelo de Corti e Peter (modelo CP). Com tamanha frequência de ocorrência e residindo tão alto sobre a intocada floresta Amazônica (albedo ~ 0,12), essas nuvens produziram uma CRF líquida no topo da atmosfera e na superfície (TOA e BOA) de +15,3 +- 0,4 e -3,7 +- 0,2 W m-2, respectivamente, muito mais intenso do que o estimado sobre a Europa (0,9 a 1,7 W m-2 no TOA). Cirrus opticamente mais espessas, em geral, apresentaram CRF líquido maior, com CRF instantâneo atingindo valores máximos (mínimos) de 140 (-65) W m-2 para o período noturno (diurno) no TOA. Juntos, os perfis verticais com COD_Coluna > 0,3 foram responsáveis por cerca de 72 % (62 %) do CRF líquido no TOA (BOA), o que significa que uma importante fração do CRF é gerada por cirrus opticamente mais finos (COD_Coluna < 0,3), que são mais difíceis de serem detectados por radares e instrumentos passivos a bordo de satélites. O ciclo diurno da profundidade óptica das nuvens cirrus teve reflexo em sua forçante radiativa. Observamos um ciclo diurno do valor médio da CRF líquida no TOA (BOA), que vaiou entre 1,7 (-23) W m-2 à tarde e 47 (3,1) W m-2 durante a noite. As nuvens cirrus promovem um aquecimento aproximadamente constante de 1,2 K dia-1 no perfil vertical entre 8 e 18 km (dentro da nuvem), mas com valores instantâneos superiores a 10 K dia-1 para porções da nuvem com alto IWC. Acredita-se que esse perfil de aquecimento gerado pelas nuvens cirrus tenha um papel importante na circulação da alta troposfera/baixa estratosfera, gerando um fluxo ascendente médio de massa de ar entre 2 e 15 kg m-2 dia-1 para altitudes entre 13 e 16,5 km, contribuindo para a manutenção da camada de cirrus próximo da tropopausa tropical. / Cirrus clouds cover a large fraction of tropical latitudes and play an important role in the Earth\'s radiation balance. Their optical properties, altitude, vertical extension, and horizontal cover control their radiative effect. In addition, tropical cirrus clouds can influence the vertical distribution of radiative heating near the tropopause, and recent research associate the moistening of the lower stratosphere with the occurrence of cirrus clouds in the tropical tropopause layer (TTL). Despite their importance, studies describing the properties of cirrus clouds over tropical forests like the Amazon are still scarce. Most studies are based on images from polar orbiting satellites, which do not give information on the diurnal cycle nor on the vertical structure of these clouds. At the same time, the studies based on ground-based lidars are restricted to a few case studies, from short-term field campaigns. This panorama started to change in 2011 with the installation of a lidar system from the Laboratory of Atmospheric Physics of IF-USP near the city of Manaus, Brazil. In this study, data from July 2011 to June 2012 was used to characterize the macro, microphysical and optical properties of cirrus clouds over the Amazon region, and then to calculate the role of those clouds in the radiative balance of the planet. An automatic algorithm was developed to detect the cloud layers and to obtain the optical properties, already considering the multiple-scattering correction. Two different models, using as input the measured extinction profiles and a parameterization for the ice crystals effective radius, were used to estimate the cirrus radiative effect. Our results showed that cirrus clouds are more frequent in Amazonia than in other tropical regions. The frequency of occurrence was 88 % during the rainy season and not less than 50 % during the dry season. The diurnal cycle showed a minimum around local noon and a maximum around late afternoon, associated with the diurnal cycle of precipitation. The average values of the top and cloud base heights, thickness, and optical depth were 14.3 +- 1.9 km, 12.9 +- 2.2 km, 1.4 +- 1.1 km, and 0.25 +- 0.46, respectively. Cirrus clouds were found at temperatures as low as -90 degC, with 6 % occurring above the tropopause. The vertical distribution was not uniform, and thin cirrus (0.03 < COD <0.3) and subvisible (COD <0.03) were more frequent in the vicinity of the tropopause. The mean lidar-ratio was 23.3 +- 8.0 sr. However, for the subvisible clouds a bimodal distribution with a secondary peak at about 44 sr was found, suggesting a mixture of ice crystals habits. No dependence of the lidar-ratio with temperature (altitude) was found, suggesting these clouds are well mixed vertically. The relative frequency of occurrence of cirrus layers classified as subvisible was 41.6 %, while 37.8 % were thin cirrus and 20.5 % opaque cirrus (COD > 0.3), with an average overlap of 1.41 +- 0.63 layers per profile. Therefore, in central Amazonia, there is not only a high incidence of cirrus clouds, but also a large fraction of subvisible clouds, which may be contaminating the measurements of sunphotometers and satellite sensors. These measured properties were used for the calculation of the cirrus radiative forcing (CRF) and the heating rate profiles, in detail with libRadtran, and approximately with the model of Corti and Peter (modelo CP). Given their high frequency of occurrence and location at high altitude over the pristine Amazon forest (albedo ~ 0,12), these clouds produced a net CRF at the top and bottom of the atmosphere (TOA and BOA) of +15.3 +- 0.4 and -3.7 +- 0.2 W m-2, respectively. This is greater than what was found over Europe (0.9, to 1.7 W m-2 at TOA). Optically thicker cirrus usually had larger CRF, with instantaneous CRF reaching peak (minimum) values of 140 (-65) W m-2 for the nocturnal (diurnal) period at TOA. The vertical profiles with COD_Column> 0.3 were responsible for about 72 % (62 %) of the net CRF at TOA (BOA), which means that a significant fraction of the cirrus CRF is generated by optically thin cirrus (COD_Column <0.3), which are more difficult to detect by radars and passive instruments on satellites. The diurnal cycle of the cirrus clouds optical depth had influence in its radiative forcing. We observed a diurnal cycle of the mean value of net CRF at TOA (BOA), which ranged from 1.7 (-23) W m-2 in the afternoon to 47 (3.1) W m-2 at night. The heating rates associated with these cirrus clouds were approximately constant, with 1.2 K day-1 from 8 to 18 km (within the cloud), but with instantaneous values that reached values higher than 10 K day-1 for portions with higher IWC. It is believed that this warming profile plays an important role in the circulation of the upper troposphere/low stratosphere, generating an average air mass flux between 2 and 15 kg m-2 day-1 for altitudes between 13 and 16.5 km, a positive feedback for the maintenance of the cirrus layer near the tropical tropopause.

As nuvens cirrus cobrem uma grande fração das latitudes tropicais e desempenham um papel importante no balanço de radiação da Terra. As propriedades ópticas, altitude, extensão vertical, e cobertura horizontal de nuvens controlam sua forçante radiativa. Além disso, nuvens cirrus tropicais podem influenciar a distribuição vertical do aquecimento radiativo na tropopausa tropical e pesquisas recentes também apontam para um aumento do vapor de água estratosférico ligado principalmente à ocorrência de nuvens cirrus na camada da tropopausa tropical (TTL). Apesar de sua importância, estudos relatando propriedades de nuvens cirrus sobre florestas tropicais como a Amazônia são ainda escassos. A maioria estão baseados em imagens de satélites de órbita polar que não fornecem informações sobre o ciclo diurno, nem sobre a estrutura vertical destas nuvens. Ao mesmo tempo, os estudos com lidar em solo são restritos a poucos estudos de caso, em geral associados a campanhas de campo de curta duração. Este panorama começou a mudar em 2011 com a instalação do sistema lidar do Laboratório de Física Atmosférica do IF-USP próximo à cidade de Manaus, Brasil. Neste trabalho, um conjunto de um ano de dados (de julho de 2011 a junho de 2012) foi utilizado para caracterizar as propriedades macro, microfísicas e ópticas das nuvens cirrus sobre a região amazônica e, posteriormente, calcular o papel que essas nuvens têm no balanço radiativo do planeta. Para tanto, foi desenvolvido um algoritmo automático para detectar as nuvens e para obter as propriedades ópticas, incluindo a correção de múltiplo-espalhamento. As forçantes radiativas foram estimadas com dois modelos diferentes, a partir dos perfis de extinção medido com o lidar e de uma parametrização para estimar o raio efetivo dos cristais de gelo. Nossos resultados mostraram que as nuvens cirrus na alta troposfera foram mais frequentes na Amazônia do que relatado previamente em outras regiões tropicais. A frequência de ocorrência foi de 88 % durante a estação chuvosa e não inferior a 50 % durante a estação seca. O ciclo diurno mostrou um mínimo ao redor do meio-dia local e máximo durante o final da tarde, associado ao ciclo diurno da precipitação. Os valores médios das alturas de topo e base, da espessura e da profundidade óptica da nuvem foram de 14,3 +- 1,9 (desv. pad.) km, 12,9 +- 2,2 km, 1,4 +- 1,1 km e 0,25 +- 0,46, respectivamente. As nuvens cirrus foram encontradas em temperaturas de até -90 degC, com 6 % ocorrendo acima da tropopausa. A distribuição vertical não se mostrou uniforme, e nuvens cirrus finas (0,03 < COD < 0,3) e subvisuais (COD < 0,03) ocorreram mais frequentemente nas proximidades da tropopausa. A razão lidar média foi de 23,3 +- 8,0 sr. Contudo, para as nuvens cirrus subvisuais foi encontrada uma distribuição bimodal com um pico secundário em torno de 44 sr, sugerindo uma composição mista dos cristais de gelo. Não foi encontrada uma dependência da razão lidar com a temperatura da nuvem (altitude), indicando que as nuvens estão verticalmente bem misturadas. A frequência de ocorrência relativa das camadas de nuvens cirrus classificadas como subvisuais foi de 41,6 %, enquanto que 37,8 % foram cirrus finos e 20,5 % de cirrus opacos (COD > 0,3), com uma superposição média de 1,41 +- 0,63 camadas por perfil. Assim, na Amazônia central não ocorre apenas uma alta frequência de nuvens cirrus, mas também uma grande fração de nuvens cirrus subvisuais, o que pode estar contaminando as medidas de fotômetros solares e sensores orbitais. As propriedades medidas foram utilizadas no cálculo da forçante radiativa das nuvens cirrus (CRF) e dos perfis da taxa de aquecimento da atmosfera, em detalhe pelo libRadtran e aproximadamente pelo modelo de Corti e Peter (modelo CP). Com tamanha frequência de ocorrência e residindo tão alto sobre a intocada floresta Amazônica (albedo ~ 0,12), essas nuvens produziram uma CRF líquida no topo da atmosfera e na superfície (TOA e BOA) de +15,3 +- 0,4 e -3,7 +- 0,2 W m-2, respectivamente, muito mais intenso do que o estimado sobre a Europa (0,9 a 1,7 W m-2 no TOA). Cirrus opticamente mais espessas, em geral, apresentaram CRF líquido maior, com CRF instantâneo atingindo valores máximos (mínimos) de 140 (-65) W m-2 para o período noturno (diurno) no TOA. Juntos, os perfis verticais com COD_Coluna > 0,3 foram responsáveis por cerca de 72 % (62 %) do CRF líquido no TOA (BOA), o que significa que uma importante fração do CRF é gerada por cirrus opticamente mais finos (COD_Coluna < 0,3), que são mais difíceis de serem detectados por radares e instrumentos passivos a bordo de satélites. O ciclo diurno da profundidade óptica das nuvens cirrus teve reflexo em sua forçante radiativa. Observamos um ciclo diurno do valor médio da CRF líquida no TOA (BOA), que vaiou entre 1,7 (-23) W m-2 à tarde e 47 (3,1) W m-2 durante a noite. As nuvens cirrus promovem um aquecimento aproximadamente constante de 1,2 K dia-1 no perfil vertical entre 8 e 18 km (dentro da nuvem), mas com valores instantâneos superiores a 10 K dia-1 para porções da nuvem com alto IWC. Acredita-se que esse perfil de aquecimento gerado pelas nuvens cirrus tenha um papel importante na circulação da alta troposfera/baixa estratosfera, gerando um fluxo ascendente médio de massa de ar entre 2 e 15 kg m-2 dia-1 para altitudes entre 13 e 16,5 km, contribuindo para a manutenção da camada de cirrus próximo da tropopausa tropical. / Cirrus clouds cover a large fraction of tropical latitudes and play an important role in the Earth\'s radiation balance. Their optical properties, altitude, vertical extension, and horizontal cover control their radiative effect. In addition, tropical cirrus clouds can influence the vertical distribution of radiative heating near the tropopause, and recent research associate the moistening of the lower stratosphere with the occurrence of cirrus clouds in the tropical tropopause layer (TTL). Despite their importance, studies describing the properties of cirrus clouds over tropical forests like the Amazon are still scarce. Most studies are based on images from polar orbiting satellites, which do not give information on the diurnal cycle nor on the vertical structure of these clouds. At the same time, the studies based on ground-based lidars are restricted to a few case studies, from short-term field campaigns. This panorama started to change in 2011 with the installation of a lidar system from the Laboratory of Atmospheric Physics of IF-USP near the city of Manaus, Brazil. In this study, data from July 2011 to June 2012 was used to characterize the macro, microphysical and optical properties of cirrus clouds over the Amazon region, and then to calculate the role of those clouds in the radiative balance of the planet. An automatic algorithm was developed to detect the cloud layers and to obtain the optical properties, already considering the multiple-scattering correction. Two different models, using as input the measured extinction profiles and a parameterization for the ice crystals effective radius, were used to estimate the cirrus radiative effect. Our results showed that cirrus clouds are more frequent in Amazonia than in other tropical regions. The frequency of occurrence was 88 % during the rainy season and not less than 50 % during the dry season. The diurnal cycle showed a minimum around local noon and a maximum around late afternoon, associated with the diurnal cycle of precipitation. The average values of the top and cloud base heights, thickness, and optical depth were 14.3 +- 1.9 km, 12.9 +- 2.2 km, 1.4 +- 1.1 km, and 0.25 +- 0.46, respectively. Cirrus clouds were found at temperatures as low as -90 degC, with 6 % occurring above the tropopause. The vertical distribution was not uniform, and thin cirrus (0.03 < COD <0.3) and subvisible (COD <0.03) were more frequent in the vicinity of the tropopause. The mean lidar-ratio was 23.3 +- 8.0 sr. However, for the subvisible clouds a bimodal distribution with a secondary peak at about 44 sr was found, suggesting a mixture of ice crystals habits. No dependence of the lidar-ratio with temperature (altitude) was found, suggesting these clouds are well mixed vertically. The relative frequency of occurrence of cirrus layers classified as subvisible was 41.6 %, while 37.8 % were thin cirrus and 20.5 % opaque cirrus (COD > 0.3), with an average overlap of 1.41 +- 0.63 layers per profile. Therefore, in central Amazonia, there is not only a high incidence of cirrus clouds, but also a large fraction of subvisible clouds, which may be contaminating the measurements of sunphotometers and satellite sensors. These measured properties were used for the calculation of the cirrus radiative forcing (CRF) and the heating rate profiles, in detail with libRadtran, and approximately with the model of Corti and Peter (modelo CP). Given their high frequency of occurrence and location at high altitude over the pristine Amazon forest (albedo ~ 0,12), these clouds produced a net CRF at the top and bottom of the atmosphere (TOA and BOA) of +15.3 +- 0.4 and -3.7 +- 0.2 W m-2, respectively. This is greater than what was found over Europe (0.9, to 1.7 W m-2 at TOA). Optically thicker cirrus usually had larger CRF, with instantaneous CRF reaching peak (minimum) values of 140 (-65) W m-2 for the nocturnal (diurnal) period at TOA. The vertical profiles with COD_Column> 0.3 were responsible for about 72 % (62 %) of the net CRF at TOA (BOA), which means that a significant fraction of the cirrus CRF is generated by optically thin cirrus (COD_Column <0.3), which are more difficult to detect by radars and passive instruments on satellites. The diurnal cycle of the cirrus clouds optical depth had influence in its radiative forcing. We observed a diurnal cycle of the mean value of net CRF at TOA (BOA), which ranged from 1.7 (-23) W m-2 in the afternoon to 47 (3.1) W m-2 at night. The heating rates associated with these cirrus clouds were approximately constant, with 1.2 K day-1 from 8 to 18 km (within the cloud), but with instantaneous values that reached values higher than 10 K day-1 for portions with higher IWC. It is believed that this warming profile plays an important role in the circulation of the upper troposphere/low stratosphere, generating an average air mass flux between 2 and 15 kg m-2 day-1 for altitudes between 13 and 16.5 km, a positive feedback for the maintenance of the cirrus layer near the tropical tropopause.

La nouvelle fenêtre spectroscopique dans le sub-millimétrique, ouverte par l’avènement del’observatoire spatial Herschel, a permis la détection d’espèces azotées simples, les hydruresd’azote NH, NH2 et NH3, dans les enveloppes froides de proto-étoiles. Ces enveloppes sontconstituées de gaz dense et froid caractéristique des conditions physico-chimiques des nuagesmoléculaires. L’observation d’hydrures d’azote dans de tels environnements a donc permis d’apporterde nouvelles contraintes sur la chimie interstellaire de ces nuages, et nous a donné enparticulier l’occasion de ré-explorer la chimie de l’azote.L’objectif de mon travail de thèse a été d’analyser en détail cette chimie interstellaire etprincipalement la formation en phase gazeuse d’espèces polyatomiques simples : les hydruresd’azote. Dans des conditions de gaz dense et froid (n = 104 cm−3, T = 10 K), la chimie de cesderniers est initiée par une chimie lente (la conversion de N en N2 par réactions neutre-neutre),contrairement à celles de ses analogues oxygénés et carbonés. Nous nous sommes particulièrementintéressés à cette étape de la chimie de l’azote, au vu des récents travaux théoriqueset expérimentaux menés par plusieurs équipes d’experts (Bordeaux, Besançon). De plus, lesrécents travaux concernant la conversion ortho-para de l’hydrogène moléculaire et les nouveauxcalculs de rapports de branchement de spins nucléaires pour les voies de production des hydruresd’azote dans leurs configurations ortho et para, menés à l’IPAG, nous ont permis d’entreprendrele calcul auto-cohérent des différentes symétries de spin des hydrures d’azote et de l’hydrogènemoléculaire. Nous avons ainsi pu développer un nouveau réseau chimique de l’azote, bénéficiantdes taux cinétiques les plus à jour pour les réactions critiques impliquées dans la chimie deshydrures d’azote.Ce nouveau réseau est utilisé pour modéliser l’évolution temporelle des abondances desespèces azotées dans des conditions de gaz dense et froid ( 103 < n < 106 cm−3, T ≤ 50 K).Les résultats à l’état stationnaire sont comparés aux observations de NH, NH2 et NH3, dans lesenveloppes froides de proto-étoiles de faible masse, en étudiant l’influence des abondances totalesen phase gazeuse du carbone, de l’oxygène et du soufre. Nos modèles chimiques reproduisent lesabondances des hydrures d’azote observés et leurs rapports pour un rapport C/O élementaire, enphase gazeuse, de ∼ 0.8 et à condition que l’abondance totale de soufre soit déplétée d’au moinsun facteur 2. Les rapports ortho/para prédits par nos modèles, pour NH2 et NH3, respectivement∼ 2.3 et ∼ 0.7, sont compatibles avec les observations de ces derniers dans des nuages diffusfroids. Les abondances des hydrures d’azote, dans des conditions de nuages sombres, sont donccohérentes avec une synthèse purement en phase gazeuse. De plus, nos résultats soulignent lefait que NH provient d’une voie de formation différente de celle de NH2 et NH3. NH vient de larecombinaison dissociative de N2H+ alors que la formation de NH2 et NH3 est principalementdue à la recombinaison dissociative de l’ion ammonium (NH+4 ), lui même molécule fille deN+. Ainsi, NH2 et NH3 procèdent de l’échange de charge dissociatif N2 + He+, tandis que NHprovient de la réaction N2 + H+3 . / The new spectroscopic window opened by the advent of the Herschel Space Observatory,has enabled the detection of simple nitrogen species, the nitrogen hydrides NH, NH2, and NH3,in the cold envelope of protostars. These envelopes are made of dense cold gas characteristicof the physico-chemical conditions of molecular clouds. The observation of nitrogen hydrides insuch environments has brought new constraints on the interstellar chemistry of these kind ofclouds, and gives, in particular, the opportunity to revisit the chemistry of nitrogen.The aim of my thesis was to comprehensively analyse the interstellar chemistry of nitrogen,focussing on the gas-phase formation of the simplest polyatomic species, namely nitrogen hydrides.Under dense, cold gas conditions (n = 104 cm−3, T = 10 K), the chemistry of theselatter is initiated by a slow chemistry (the conversion from N to N2 with neutral-neutral reactions),in contrast to their carbonated and oxygenated analogues. We have investigated andrevisited this specific part of the nitrogen chemistry in the light of recent theoretical and experimentalwork carried out by several expert teams (Bordeaux, Besançon). In addition, recentwork about the ortho-para conversion of molecular hydrogen and new calculations of nuclearspin branching ratios for the production pathways of nitrogen hydrides in their ortho and paraconfigurations conducted at IPAG, enabled us to treat self-consistently the different spin symmetriesof the nitrogen hydrides together with the ortho and para forms of molecular hydrogen.We were able to develop a new network of chemical nitrogen in which the kinetic rates of criticalreactions involved in the nitrogen chemistry have been updated.This new network is used to model the time evolution of the nitrogen species abundancesin dense cold gas conditions (T ≤ 50 K, 103 < n < 106 cm−3). The steady-state resultsare compared to observations of NH, NH2 and NH3 towards a sample of low-mass protostars,with a special emphasis on the influence of the overall amounts of gaseous carbon, oxygen, andsulphur. Our chemical models reproduced the nitrogen hydrides abundances and their ratios fora gas-phase elemental C/O ratio of ∼ 0.8, provided that the total amount of sulphur is depletedby at least a factor of two. Our predicted ortho-to-para ratios for NH2 and NH3, ∼ 2.3 and∼ 0.7 respectively, are in good agreement with the observations towards cold diffuse clouds.Then, in dark gas conditions, the nitrogen hydride abundances are consistent with a pure gasphasesynthesis. Moreover, our results are based on the fact that NH is coming from a differentpathway than NH2 and NH3. NH is the daughter molecule of N2H+, deriving from the reactionN2+H+3 , while NH2 and NH3 proceed from NH+4 , itself daughter molecule of N+, resulting fromthe dissociative charge exchange N2 + He+.

L'avènement d'une nouvelle génération de lasers ultra-relativistes (d'éclairement supérieur à 10^22 W/cm2), tels le laser APOLLON sur le plateau de Saclay, donnera lieu à un régime d'interaction laser-matière sans précédent, couplant physique des plasmas relativistes et effets électrodynamiques quantiques. Sources de particules et de rayonnements aux propriétés énergétiques et spatio-temporelles inédites, ces lasers serviront, entre autres applications, à la mise au point de nouveaux concepts d'accélérateurs et de diagnostics radiographiques, au chauffage de plasmas denses, comme à la reproduction de configurations astrophysiques en laboratoire. En prévision des futures expériences, les codes particle-in-cell (PIC), qui constituent les outils de référence pour la simulation de l'interaction laser-plasma, doivent être enrichis des processus radiatifs et quantiques propres à ce nouveau régime d'interaction. C'est le cas du code CALDER développé au CEA/DAM, qui modélise désormais l'émission de photons énergétiques et la conversion de ceux-ci en paires électron-positron ; autant d'effets susceptibles d'affecter le bilan d'énergie de l'interaction laser-cible et, plus précisément, le rendement du laser en particules et rayonnements énergétiques. L'objet de ce stage théorique est d'étudier, à l'aide du code CALDER, l'influence de ces processus dans un certain nombre de scénarios physiques en champ extrême (accélération électronique et ionique dans un plasma surcritique, production de rayonnement, génération de choc non-collisionnel…). / Forthcoming multi-petawatt laser systems, such as the French Apollon and European Extreme Light Infrastructure facilities, are expected to deliver on-target laser intensities exceeding 10^22 W/cm^2. A novel regime of laser-matter interaction will ensue, where ultra-relativistic plasma effects are coupled with copious generation of high-energy photons and electron-positron pairs. This will pave the way for many transdisciplinary applications in fundamental and applied research, including the development of unprecedentedly intense, compact particle and radiation sources, the experimental investigation of relativistic astrophysical scenarios and tests of quantum electrodynamics theory.In recent years, most theoretical studies performed in this research field have focused on the impact of synchrotron photon emission and Breit-Wheeler pair generation, both directly induced by the laser field and believed to be dominant at intensities >10^22 W/cm^2. At the lower intensities (≲10^21 Wcm^(-2)) currently attainable, by contrast, photon and pair production mainly originate from, respectively, Bremsstrahlung and Bethe-Heitler/Trident processes, all triggered by atomic Coulomb fields. The conditions for a transition between these two regimes have, as yet, hardly been investigated, particularly by means of integrated kinetic numerical simulations. The purpose of this PhD is precisely to study the aforementioned processes under various physical scenarios involving extreme laser-plasma interactions. This work is carried out using the particle-in-cell CALDER code developed at CEA/DAM which, over the past few years, had been enriched with modules describing the synchrotron and Breit-Wheeler processes.Our first study aimed at extending the simulation capabilities of CALDER to the whole range of photon and positron generation mechanisms arising during relativistic laser-plasma interactions. To this purpose, we have implemented modules for the Coulomb-field-mediated Bremsstrahlung, Bethe-Heitler and Trident processes. Refined Bremsstrahlung and Bethe-Heitler cross sections have been obtained which account for electronic shielding effects in arbitrarily ionized plasmas. Following validation tests of the Monte Carlo numerical method, we have examined the competition between Bremsstrahlung/Bethe-Heitler and Trident pair generations by relativistic electrons propagating through micrometer copper foils. Our self-consistent simulations qualitatively agree with a 0-D theoretical model, yet they show that the deceleration of the fast electrons due to target expansion significantly impacts pair production.We then address the competition between Bremsstrahlung and synchrotron emission from copper foils irradiated at 10^22 Wcm^(-2). We show that the maximum radiation yield (into >10 keV photons) is achieved through synchrotron emission in relativistically transparent targets of a few 10 nm thick. The efficiency of Bremsstrahlung increases with the target thickness, and takes over synchrotron for >2μm thicknesses. The spectral properties of the two radiation processes are analyzed in detail and correlated with the ultrafast target dynamics.Finally, we investigate the potential of nanowire-array targets to enhance the synchrotron yield of a 10^22 Wcm^(-2) femtosecond laser pulse. Several radiation mechanisms are identified depending on the target parameters and as a function of time. A simulation scan allows us to identify the optimal target geometry in terms of nanowire width and interspacing, yielding a ∼10% radiation efficiency. In this configuration, the laser-driven nanowire array rapidly expands to form a quasi-uniform, relativistically transparent plasma. Furthermore, we demonstrate that uniform sub-solid targets can achieve synchrotron yields as high as in nanowire arrays, but that the latter enable a strong emission level to be sustained over a broader range of average plasma density.

Les observations météorologiques depuis les satellites dans le domaine des micro-ondes sont actuellement limitées à 190 GHz. La prochaine génération de satellites météorologiques opérationnels européens (EUMETSAT Polar System-Second Generation-EPS-SG), em- portera un instrument, le Ice Cloud Imager (ICI), avec des fréquences sub-millimétriques jusqu’à 664 GHz, afin d’améliorer la caractérisation globale des nuages de glace. Pour préparer l’exploitation de ces nouvelles données, durant cette thèse, des travaux ont été effectués sur quatre axes complémentaires. Des simulations réalistes de transfert radiatif ont été réalisées de 19 à 700 GHz, pour des scènes météorologiques réelles, couvrant une grande variabilité des nuages en Europe. L’objectif était double : premièrement mieux comprendre la sensibilité des ondes millimétriques et sub-millimétriques à la phase glacée des nuages, deuxièmement créer une base de données robuste pour développer une méthode d’inversion statistique des caractéristiques des nuages de glace. Un code de transfert radiatif atmosphérique (Atmospheric Radiative Transfer Simulator ARTS) a été couplé avec des profils atmosphériques simulés par un modèle méso-échelle de nuage (Weather Research and Forecasting WRF), pour douze scènes européennes aux moyennes latitudes. Les propriétés de diffusion des hydrométéores (glace, neige, graupel, pluie et eau dans le nuage) ont été soigneusement sélectionnées, en particulier pour la phase glace, et la compatibilité avec la microphysique de WRF a été testée : le Discrete-Dipole approximation (DDA) est adopté pour calculer les propriétés diffusantes des particules de neige. Les simulations obtenues ont été systématiquement comparées avec des observations satellitaires coïncidentes d’imageurs et de sondeurs jusqu’à 200 GHz. L’accord entre les simulations et les observations montre la bonne qualité de la base de données, au moins jusqu’à 200 GHz... / The meteorological observations from satellites in the microwave domain are currently limited to 190 GHz. The next generation of European operational Meteorological Satellite (EUMETSAT Polar System-Second Generation-EPS-SG) will carry an instrument, the Ice Cloud Imager (ICI), with frequencies up to 664 GHz, to provide unprecedented measurements in the sub-millimeter spectral range, aiming to improve the characterization of ice clouds over the globe. To prepare this upcoming satellite-borne sub-millimeter imager, during this thesis, scientific efforts have been made on four complementary aspects. Realistic radiative transfer simulations have been performed from 19 to 700 GHz, for real meteorological scenes, covering a large variability of clouds in Europe. The goal was two fold, first to better understand the sensitivity of the microwave to sub-millimeter waves to the cloud frozen phases, and second, to create a robust training database for a statistical cloud parameter retrieval. The Atmospheric Radiative Transfer Simulator (ARTS) is coupled with atmospheric profiles from the Weather Research and Forecasting (WRF) model, for twelve diverse European mid-latitude scenes. The single scattering properties of the hydrometeors (cloud ice, snow, graupel, rain, and cloud water) are carefully selected, especially for the frozen phases, and compatibility with the micro- physics in WRF is tested: the Discrete-Dipole Approximation (DDA) is adopted for snow particles. The resulting simulations have been systematically compared with coincident satellite observations from imagers and sounders up to 200 GHz. The agreement between simulations and observations shows the good quality of the simulated training database, at least up to 200 GHz...