Throughout the solar cycle and predominantly during the declining phase, Stream Interaction Regions (SIRs) drive space weather on Earth. SIRs occur when the Sun’s rotation aligns a fast solar wind stream behind a slow solar wind stream. Both fast wind and slow wind are compressed and heated, forming a pressure ridge driven by the dynamic pressure of the fast wind. In the frame advecting with the SIR, the high pressure region is bound by a forward wave, which propagates away from the Sun, and reverse wave which propagates sunwards. The pressure waves steepen into shocks with increasing heliospheric distance, the shocks usually form beyond Earth’s orbit. Located between the waves, the stream interface is a tangential discontinuity separating streams that were originally fast from slow. While the general mechanism for the formation and evolution of SIRs is relatively well known, the implications of the 3D structure in the inner heliosphere have not been well understood, in part due to the sparsity of in situ observations outside of the ecliptic plane. In this dissertation, I have used the heliospheric adaptation of the Lyon-Fedder- Mobarry (LFM-helio) MHD model to simulate both idealized and realistic SIR structures in order to validate the model against in situ measurements and to elucidate which characteristics of the solar wind influence the evolution of SIRs. The LFM-helio is shown to accurately reproduce the solar wind conditions at various heliospheric distances. The simulations produced SIRs which agree with in situ observations. The simulations were used to show that the large scale shape of high speed streams driving SIRs affect the amount of heating, compression, and flow deflection. Further, for even small latitudinal separations, SIR evolution depends on the latitudinal structure of the High Speed Stream driving the SIR. Increasing the temperature at the inner boundary of the LFM-helio results in a solar wind that is globally faster and that produces SIRs exhibiting less compressive heating. Increasing the magnetic field strength uniformly at the inner boundary has an effect on the dynamical evolution SIRs whereas increasing the magnetic field strength in proportion to the solar wind speed latitudinally compresses the extent of the band of slow wind, modifying the global structure of the heliosphere.

Solar wind-magnetosphere-ionosphere (SW-M-I) coupling is investigated with three different computational models that characterize space plasma dynamics on distinct spatial/temporal scales. These models are used to explore three important aspects of SW-M-I coupling. A particle-in-cell (PIC) model has been developed to explore the kinetic scale dynamics associated with the magnetotail dipolarization front (DF), which is generated as a result of magnetotail reconnection. The PIC study demonstrates that the electron-ion hybrid (EIH) instability could relax the velocity shear within the DF via emitting lower hybrid waves. The velocity inhomogeneity driven instability is highlighted as an important mechanism for energy conversion and wave emission during the solar wind-magnetosphere coupling, which has been long neglected before. The Lyon-Fedder-Mobbary (LFM) global magnetohydrodynamic (MHD) model is used to explore the fluid scale electrodynamic response of the magnetosphere-ionosphere to the interplanetary electric field (IEF). It is found that the cross polar cap potential (CPCP) varies linearly with very large IEF if the solar wind density is high enough. With controlled experiments of global MHD modeling driven by observed parameters, the linearity was interpreted as a result of the magnetosheath force balance theory. This study highlights the role of solar wind density in the electrodynamic SW-M-I coupling under extreme driving conditions. The LFM-TIEGCM-RCM (LTR) model, which is the Coupled-Magnetosphere-Ionosphere-Thermosphere (CMIT) model with Ring Current extension, is used to explore the integrated SW-M-I system. The LTR simulation study focuses on the subauroral polarization streams (SAPS), which involve both MHD and non-MHD processes and three-way coupling in the SW-M-I system. The global structure and dynamic evolution of SAPS are illustrated with state-of-the-art first-principle models for the first time. This study has successfully utilized multiscale models to characterize the forefront issues in the space plasma dynamics, which is required by the facts that plasmas have both particle and fluid featured properties and those properties are vastly different across geospace regions. It is highlighted that SW-M-I coupling could be significantly influenced by both microscopic and macroscopic processes. In order for a comprehensive understanding of the SW-M-I coupling, multiscale models and integrated framework of their combinations are critical. / Doctor of Philosophy / Three numerical models are used to explore the processes occurring in the Earth’s space environment from an altitude of ∼ 100 km to 10s Earth radii (R_E). This environment is mainly filled with plasma, the gaseous state of charged particles that collectively behave like a fluid and are also subject to complex electromagnetic interactions. The intrinsic features of plasma determine that the physics on the scale of charged particles and that on the scale of fluids are both very important. On the other hand, considering the vast differences in the plasma properties throughout space, different regions need to be represented by different physically-based models. This dissertation study addresses the processes on three distinct spatial/temporal scales with different models. A particle model that treats plasma as a group of charged particles is used to explore wave generation in the magnetotail (10s R_E in the nightside). It is found that inhomogeneous plasma flow in the sharp boundary layer at the magnetotail (called “dipolarization front”) can excite plasma waves to dissipate the energy originating from the solar wind (high speed plasma ejected from the sun). A magnetohydrodynamic (MHD) model that treats the plasma as a magnetized fluid is used to explore the efficiency of electric field mapping from the solar wind (10s R_E) to the ionosphere (∼ 100 km altitude). The electric field in the ionosphere usually linearly increases with solar wind electric field until it is too strong. An observational event showed that their relationship remains linear for very large driving field. MHD modeling experiments demonstrate that the linearity at large driving field is due to the high solar wind density, which is explained with force balance theory. An integrated model framework is used to explore the system level response of geospace by investigating the enhanced plasma flow in the subauroral ionosphere (called the subauroral polarization streams, SAPS). The generation of SAPS involves driving and feedback processes in different regions (magnetosphere, ring current, ionosphere) that can not be simulated with any individual model. The global structures and dynamic evolution of SAPS have never been explored before with first-principle characterization of the effects from the solar wind to geospace. This integrated modeling represents a state-of-the-art model framework to explore processes in coupled geospace. These studies illustrate that different models are necessary to explore fundamental physics on small and large scales and the coupling processes between different space regions. It is also suggested that incorporating the different models into an integrated framework is necessary to get a comprehensive understanding of the dynamics in geospace.

No description available.

Esta tese apresenta um modelo matemático para simulação numérica do escoamento com turbilhonamento (swirl) em tochas de plasma térmico de arco não transferido que operam em corrente contínua, assim como os resultados obtidos com as simulações para estudo de casos. O modelo magneto-hidrodinâmico (modelo MHD) bidimensional permitiu simular a interação entre o escoamento e o arco elétrico usando uma configuração axissimétrica, que abrange as seguintes regiões: entrada do gás; interior da tocha; jato de plasma livre no ambiente. O modelo foi implementado num código numérico baseado no Método dos Volumes Finitos para a solução numérica das equações governantes. Para os estudos foram simulados casos com diferentes condições operacionais (vazão de gás; intensidade de corrente elétrica; gases plasmogênicos: ar e argônio; intensidade de turbilhonamento). A fim de verificar a qualidade do modelo, alguns resultados foram comparados com a literatura e apresentaram boa concordância: a maior diferença obtida entre valores de temperatura experimentais e valores calculados foi -10%, e a média das diferenças obtidas nas comparações foi de aproximadamente ±3,2%. Os perfis de temperatura e de velocidade obtidos para a região do arco e para o jato de plasma resultante permitiram o estudo do comportamento do escoamento na tocha de plasma em diferentes condições. Conclui-se que o modelo desenvolvido é apto à realização de investigações numéricas do escoamento em tochas de plasma e dos efeitos do turbilhonamento na interação arco/escoamento. / This thesis presents a mathematical model for numerical simulation of swirling flow in DC non-transferred arc thermal plasma torches, as well as the results obtained from simulations to case studies. The two-dimensional magnetohydrodynamic model (MHD model) allowed simulate the interaction between the flow and the electric arc using an axisymmetric configuration, covering the following areas: gas inlet; inside the torch; free jet of plasma in the environment. The model was implemented in a computer code based on the Finite Volume Method (FVM) to enable the numerical solution of the governing equations. For the study, cases were simulated with different operating conditions (gas flow rate; electric current intensity; plasmogenic gases: air and argon; swirl intensity). In order to verify the quality of the model, some results were compared with the literature and showed good agreement: the biggest difference between experimental temperature values and calculated values was 10%, and the average of the differences obtained in the comparisons was approximately ±3.2%. The resulting profiles of temperature and velocity obtained for the region of the arc and the plasma jet allowed the study of the flow behavior in the plasma torch in different conditions. It is concluded that the model developed is able to carry out numerical investigations of the flow in plasma torches and the effects of swirl in the interaction arc/flow.

Esta tese apresenta um modelo matemático para simulação numérica do escoamento com turbilhonamento (swirl) em tochas de plasma térmico de arco não transferido que operam em corrente contínua, assim como os resultados obtidos com as simulações para estudo de casos. O modelo magneto-hidrodinâmico (modelo MHD) bidimensional permitiu simular a interação entre o escoamento e o arco elétrico usando uma configuração axissimétrica, que abrange as seguintes regiões: entrada do gás; interior da tocha; jato de plasma livre no ambiente. O modelo foi implementado num código numérico baseado no Método dos Volumes Finitos para a solução numérica das equações governantes. Para os estudos foram simulados casos com diferentes condições operacionais (vazão de gás; intensidade de corrente elétrica; gases plasmogênicos: ar e argônio; intensidade de turbilhonamento). A fim de verificar a qualidade do modelo, alguns resultados foram comparados com a literatura e apresentaram boa concordância: a maior diferença obtida entre valores de temperatura experimentais e valores calculados foi -10%, e a média das diferenças obtidas nas comparações foi de aproximadamente ±3,2%. Os perfis de temperatura e de velocidade obtidos para a região do arco e para o jato de plasma resultante permitiram o estudo do comportamento do escoamento na tocha de plasma em diferentes condições. Conclui-se que o modelo desenvolvido é apto à realização de investigações numéricas do escoamento em tochas de plasma e dos efeitos do turbilhonamento na interação arco/escoamento. / This thesis presents a mathematical model for numerical simulation of swirling flow in DC non-transferred arc thermal plasma torches, as well as the results obtained from simulations to case studies. The two-dimensional magnetohydrodynamic model (MHD model) allowed simulate the interaction between the flow and the electric arc using an axisymmetric configuration, covering the following areas: gas inlet; inside the torch; free jet of plasma in the environment. The model was implemented in a computer code based on the Finite Volume Method (FVM) to enable the numerical solution of the governing equations. For the study, cases were simulated with different operating conditions (gas flow rate; electric current intensity; plasmogenic gases: air and argon; swirl intensity). In order to verify the quality of the model, some results were compared with the literature and showed good agreement: the biggest difference between experimental temperature values and calculated values was 10%, and the average of the differences obtained in the comparisons was approximately ±3.2%. The resulting profiles of temperature and velocity obtained for the region of the arc and the plasma jet allowed the study of the flow behavior in the plasma torch in different conditions. It is concluded that the model developed is able to carry out numerical investigations of the flow in plasma torches and the effects of swirl in the interaction arc/flow.