O ciclo solar é um dos fenômenos magnéticos mais interessantes do Universo. Embora ele tinha sido descoberto há mais de 150 anos, até agora permanece um problema em aberto para a Astrofísica. Há diferentes tipos de observações que sugerem que o ciclo solar corresponde a um processo de dínamo operando em algum lugar do interior solar. Parker foi o primeiro a tentar explicar o dínamo solar como um processo hidro-magnético acerca de 50 anos atrás. Desde então, embora tenha havido avanços significativos nas observações e investigações teóricas e numéricas, uma resposta definitiva para o dínamo solar ainda não existe. Acredita-se que no caso do Sol, pelo menos dois processos são necessários para completar o ciclo magnético observado: a transformação de um campo poloidal inicial em um campo toroidal, um processo conhecido como efeito , o qual se deve ao cisalhamento em grande escala ocasionado pela rotação diferencial; e a transformação do campo toroidal em um novo campo poloidal de polaridade oposta ao inicial. Esse segundo processo é menos conhecido e motivo de intensas discussões e pesquisas. Duas hipóteses principais foram formuladas para explicar a natureza deste processo, usualmente conhecido como efeito : a primeira, baseada na idéia de Parker de um mecanismo turbulento onde os campos poloidais resultam de movimentos convectivos ciclônicos operando em tubos de fluxo toroidais em pequena escala. Esses modelos se depararam, no entanto, com um serio inconveniente: na fase não-linear, i.e., quando a reação dinâmica do campo magnético ao fluido torna-se importante, o efeito pode ser amortecido de forma catastrófica, levando a um dínamo pouco efetivo. A segunda hipótese é baseada nas idéias de Babcock (1961) e Leighton (1969) (BL), que propuseram que o campo poloidal forma-se devido à emergência e decaimento posterior das regiões bipolares ativas. Neste modelo a circulação meridional tem um papel fundamental pois atua como mecanismo de transporte do fluxo magnético, de tal forma que a escala de tempo advectivo deve dominar sobre a escala de tempo difusiva. Por essa razão essa classe de modelos é comumente conhecida como modelo de dínamo dominado pelo transporte de fluxo, ou dínamo advectivo. Os modelos de dínamo dominados pelo transporte de fluxo são relativamente bem sucedidos em reproduzir as características em grande escala do ciclo solar, tornando-se populares entre a comunidade de Física solar, no entanto, também apresentam vários problemas amplamente discutidos na literatura. O objetivo principal deste trabalho foi identificar as principais limitações dessa classe de modelos e explorar as suas possíveis soluções. Para tal, construímos um modelo numérico bi-dimensional de dínamo cinemático baseado na teoria de campo médio e investigamos primeiro os efeitos da geometria e da espessura da tacoclina solar na amplificação do dínamo. Depois, consideramos o processo de bombeamento magnético turbulento como um mecanismo alternativo de transporte de fluxo magnético, e finalmente, incluímos a reação dinâmica do campo magnético sobre a difusividade magnética turbulenta, um processo conhecido como amortecimento de . Verificamos que é possível construir-se um modelo de dínamo dominado pelo transporte de fluxo capaz de reproduzir as observações ao considerar-se uma tacoclina de espessura fina localizada abaixo da zona convectiva. Isto limita a criação de intensos campos toroidais não desejados nas altas latitudes. Verificamos também ser importante considerar o bombeamento magnético, pois ele provê advecção do fluxo magnético para o equador e para a base da camada convectiva, o que resulta em uma correta distribuição latitudinal e temporal dos campos toroidais e também permite certa penetração desses campos nas regiões mais estáveis onde podem adquirir maior amplificação. Esse mecanismo é ainda importante para produzir a paridade correta do campo (anti-simétrica) nos dois hemisférios do Sol. Também encontramos que o amortecimento da difusividade magnética é um mecanismo fundamental para a formação de pequenas estruturas de campo toroidal com maior tempo de vida, identificadas com os tubos de fluxo, que acredita-se existirem na base da zona de convecção. Além do mais, os campos magnéticos formados graças ao amortecimento de podem ser até ~2 vezes mais intensos que as estruturas magnéticas formadas sem o seu amortecimento. Por fim, nos últimos anos, alguns trabalhos teóricos vêm chamando a atenção para o papel da conservação da helicidade magnética no processo de dínamo, dando nova vida a modelos de dínamo turbulento, como originalmente proposto por Parker. Com o objetivo de investigar o papel da helicidade magnética e de buscar uma descrição dinâmica mais realista do mecanismo de dínamo, construímos recentemente um modelo numérico de convecção tridimensional (utilizando o código MHD, PLUTO) que tenta reproduzir o cenário natural do interior solar onde teria lugar o processo de dínamo. Exploramos a evolução de um campo magnético semente imposto sobre um estado convectivo estacionário. Os resultados preliminares indicam que a convecção pode facilmente excitar o efeito de dínamo, inclusive em casos sem rotação. Porém, nos casos com rotação, o dínamo parece produzir uma maior quantidade de campo magnético médio com relação aos casos sem a rotação nos quais o campo flutuante é dominante. Estes resultados suportam a existência de um dínamo turbulento y validam a teoria de campo médio, mas uma a análise mais detalhada ainda é necessária. / The solar cycle is one of the most interesting magnetic phenomenon in the Universe. Even though it was discovered more than 150 years ago, it remains until now as an open problem in Astrophysics. There are several observational evidences that suggest that the solar cycle corresponds to a dynamo process operating at some place of the solar interior. Parker, in 1955, was the first to try to explain the solar dynamo as hydromagnetic phenomena. Since then, although there has been important improvements in the observations, theory and numerical simulations, a definitive model for the solar dynamo is still missing. There is common agreement that in the solar case, at least two processes are necessary to close the dynamo loop: the transformation of an initial poloidal field into a toroidal field, the so called Omega effect, which is due to a large scale shear caused by the diferential rotation, and the transformation of the toroidal field into a new poloidal field of opposite polarity, which is still a poorly understood process that has been the subject of intense debate and research. Two main hypotheses have been formulated in order to explain the nature of this effect, usually denominated alpha effect: the first one is based on Parker\'s idea of a turbulent mechanism where the poloidal field results from cyclonic convective motions operating at small scales in the toroidal field ropes. These models, however face an important limitation: in the non-linear regime, i.e. when the back reaction of the toroidal field on the motions becomes important, the alpha effect can be catastrophically quenched leading to an ineffective dynamo. The second hypotheses is based on the formulation of Babcock (1961) and Leighton (1969) (BL), who proposed that the poloidal field is formed due to the emergence and decay of bipolar magnetic regions. In this model the meridional circulation plays an important role by acting as conveyor belt of the magnetic flux, so that the advection time must be dominant over the diffusion time. For this reason these models are often called flux-transport dynamo models. The flux-transport dynamo models has been relatively successful in reproducing the large scale features of the solar cycle, and have become popular between the solar community. However, they also present several problems that have been widely discussed in the literature. The main goal of this work was to identify the main problems concerning the flux-transport dynamo model and to explore possible solutions for them. For this aim, we have built a two-dimensional kinematic numerical model based on the mean-field theory in order to explore first the effects of the geometry and thickness of the solar tachocline on the dynamo amplification. Then, we considered the turbulent pumping as an alternative magnetic flux advection mechanism, and finally, we included the non-linear back-reaction of the magnetic field on the turbulent magnetic diffusivity, a process known as eta-quenching. We have found that it is possible to build a flux-transport dynamo model able to reproduce the observations as long as a thin tachocline located below the convective zone is considered. This helps to prevent the amplification of undesirable strong toroidal fields at the high latitudes. We have also found that it is important to consider the turbulent magnetic pumping mechanism, because it provides magnetic field advection both equatorward and inwards, that results in a correct latitudinal and temporal distribution of the toroidal field and also allows the penetration of the toroidal fields down into the stable layers where they can acquire further amplification. Besides, this mechanism plays an important role in reproducing the correct field parity (anti-symmetric) on both solar hemispheres. We have also found that the eta-quenching may lead to the formation of long-lived small structures of toroidal field which resemble the flux-tubes that are believed to exist at the base of the convection zone. The magnetic fields that are formed thanks to the eta-quenching can be up to ~ twice as larger as the magnetic structures which are developed without this effect. Finally, a number of theoretical works in the last years have called the attention to the role of magnetic helicity conservation in the dynamo processes, giving a new life to the turbulent dynamo model as proposed by Parker. With the aim to study the role of magnetic helicity and explore a more realistic dynamical description of the dynamo mechanism, we have also recently built a 3D convective numerical model (based on the MHD-Goudunov type PLUTO code) where we try to reproduce the natural scenario of the solar interior where the dynamo might take place. We have studied the evolution of a seed field embedded in an initially steady state convection layer. Our preliminary results indicate that convection can easily drive the dynamo action, even in the case without rotation. However, in the rotating cases, the dynamo appears to produce a larger amount of large scale (coherent) magnetic field when compared to the case without rotation where small scale fluctuating fields are dominant. These results support the existence of a turbulent mean field dynamo, but furthermore detailed analysis is still required.

The surface climate response to 11-yr solar forcing during northern winter is first reestimated by applying a multiple linear regression (MLR) statistical model to Hadley Centre sea level pressure (SLP) and sea surface temperature (SST) data over the 1880–2009 period. In addition to a significant positive SLP response in the North Pacific found in previous studies, a positive SST response is obtained across the midlatitude North Pacific. Negative but insignificant SLP responses are obtained in the Arctic. The derived SLP response at zero lag therefore resembles a positive phase of the Arctic Oscillation (AO). Evaluation of the SLP and SST responses as a function of phase lag indicates that the response evolves from a negative AO-like mode a few years before solar maximum to a positive AO-like mode at and following solar maximum. For comparison, a similar MLR analysis is applied to model SLP and SST data from a series of simulations using an atmosphere–ocean general circulation model with a well-resolved stratosphere. The simulations differed only in the assumed solar cycle variation of stratospheric ozone. It is found that the simulation that assumed an ozone variation estimated from satellite data produces solar SLP and SST responses that are most consistent with the observational results, especially during a selected centennial period. In particular, a positive SLP response anomaly is obtained in the northeastern Pacific and a corresponding positive SST response anomaly extends across the midlatitude North Pacific. The model response versus phase lag also evolves from a mainly negative AO-like response before solar maximum to a mainly positive AO response at and following solar maximum.

The origin of the tropical lower-stratospheric response to 11-yr solar forcing and its possible coupling to a troposphere–ocean response is investigated using multiple linear regression (MLR) analyses of stratospheric ozone and temperature data over the 1979–2009 period and tropospheric sea level pressure (SLP) data over the 1880–2009 period. Stratospheric MLR results, comparisons with simulations from a chemistry–climate model, and analyses of decadal variations of meridional eddy heat flux indicate that the tropical lower-stratospheric response is produced mainly by a solar-induced modulation of the Brewer–Dobson circulation (BDC), with a secondary contribution from the Hadley circulation in the lowermost stratosphere. MLR analyses of long-term SLP data confirm previous results indicating a distinct positive response, on average, during the northern winter season in the North Pacific. The mean response in the Northern Hemisphere resembles a positive Arctic Oscillation mode and can also be characterized as “La Niña–like,” implying a reduction of Rossby wave forcing, a weakening of the BDC, and an increase in tropical lower-stratospheric ozone and temperature near solar maxima. However, MLR analyses of different time periods show that the Pacific SLP response is not always present during every cycle; it was most clearly detected mainly during the ~1938–93 period when 11-yr solar variability was especially strong. During the 1979–93 period, the SLP response was strongly present when the lower-stratospheric responses were large. But during the 1994–2009 period, the SLP response was much less significant and the lower-stratospheric responses were weak, supporting the hypothesis that the lower-stratospheric and surface climate responses are dynamically coupled.

Windmessungen im Mesopausenbereich über Mitteleuropa (Collm, 52°N, I5°E) werden im Hinblick auf Langzeittrends und eine eventuelle Abhängigkeit vom 11-jährigen Sonnenfleckenzyklus hin untersucht. Der Einfluß der solaren Variabilität ist jahreszeitenabhängig; nur in Frühjahr und Sommer wird eine signifikante Korrelation gefunden. Im Sommer ist diese Abhängigkeit mit stärkeren vertikalen Gradienten des mittleren zonalen Grundwindes im solaren Maximum verbunden. Dies weist auf einen Ursprung dieser Abhängigkeit in der Stratosphäre/Mesosphäre sowie auf Koppelungsmechanismen zwischen Stratosphäre, Mesosphäre und unteren Thermosphäre hin. Da die 2-Tage-Welle vom Gradienten abhängt, führt dies zu einer positiven Korrelation zwischen der Amplitude der 2-Tage-Welle und der Sonnenfleckenrelativzahl. / Mesopause winds over Central Europe (Collm, 52°N, I5°E) are analysed with respect to longterm trends and 11-year solar cycle dependencies. The response of the prevailing wind to the solar cycle differs throughout the year. While in winter no significant correlation between the zonal prevailing wind and the solar activity is found, in spring and summer a negative correlation between solar activity and zonal prevailing wind can be seen from the measurements. This is connected with strenger vertical gradients of the zonal prevailing wind during solar maximum than during solar minimum. This hints to a forcing of the mesopause region dynamical reaction on solar activity from the stratosphere/mesosphere and to coupling processes in the stratosphere-mesosphere-lower thermosphere system. Since the amplitude of the quasi twoday wave is dependent on the zonal mean wind gradient, this is connected with a positive correlation between solar activity and quasi two-day wave activity.

This work investigates changes implied by variability of temperature, zonal wind and ozone mixing ratio during the eleven-year cycle in the stratosphere and lower mesosphere. The analysis is performed by using multiple linear regression of the MERRA reanalysis dataset for the period 1979-2012. Furthermore, we describe possibilities of available dataset, which can be also considered for the study of the upper atmosphere. Results of this study show a qualitative agreement with other related studies in the stratosphere, we find substantial differences especially in the ozone concentrations above the stratopause. It has been shown more intensive sudden stratospheric warming during solar maxima and the associated impact on the Brewer-Dobson circulation in all fields of meteorological variables.

Submitted by Jean Medeiros (jeanletras@uepb.edu.br) on 2016-04-27T12:51:25Z No. of bitstreams: 1 PDF - Juliana Cardoso Herculano.pdf: 2901982 bytes, checksum: 1eb37789ca96855b4c63bd56d9a06682 (MD5) / Made available in DSpace on 2016-04-27T12:51:25Z (GMT). No. of bitstreams: 1 PDF - Juliana Cardoso Herculano.pdf: 2901982 bytes, checksum: 1eb37789ca96855b4c63bd56d9a06682 (MD5) Previous issue date: 2015-05-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Temperature values around 90 km height were used to investigate a possible relationship between the variations of temperature and solar flux during the solar cycle 23. The temperature data were obtained from the meteoric signal decay time in Cachoeira Paulista (22.7 ° S, 45.0 ° W). To find a possible long-term trend and variations induced by solar activity in annual average temperatures, the data sets were analyzed by multiple linear regression. It was observed that over time the temperature obtained by the two methods show variations. The variation appears to be most pronounced for the temperatures obtained by the gradient method, in which the amplitudes are larger, as well as the respective standard deviations. To analyze the behavior of temperature during solar cycle 23, the seasonally adjusted annual averages, after eliminating the influence of solar activity, they were used and the linear trend was estimated. The average temperature calculated by the model pressure tends to increase about 0.035 K / year when the entire period of observation is considered. However, for the period between maximum and solar minimum the average annual temperature has a tendency of increase of 0.166 K / year. The results show that the annual average temperature of seasonally adjusted after removal of the effects due to solar flux obtained by the gradient method would be higher than the original period for the solar decline. It is noted that the annual temperature seasonally adjusted virtually no change when t he time trend is removed, which may indicate that the annual temperature deseasonalized obtained by the gradient method are strongly affected by the solar flux. / Valores da temperatura em torno de 90 km de altura foram usados para investigar uma possível relação entre as variações da temperatura e o fluxo solar durante o ciclo solar 23. Os dados de temperatura foram obtidos a partir do tempo de decaimento do sinal meteórico em Cachoeira Paulista (22,7°S, 45,0°O). Para encontrar uma possível tendência de longo prazo e variações induzidas pela atividade solar nas médias anuais das temperaturas, as séries de dados foram submetidas à análise de regressão linear múltipla. Observou-se que ao longo do tempo as temperaturas obtidas através dos 2 métodos apresentam variações. A variação mostra-se mais notória para as temperaturas obtidas pelo método do gradiente, em que as amplitudes são maiores, assim como os respectivos desvios padrão. Para analisar o comportamento da temperatura durante o ciclo solar 23, as médias anuais dessazonalizadas, após a eliminação da influência da atividade solar, foram utilizadas e a tendência linear foi estimada. A temperatura média calculada pelo modelo de pressão tende a aumentar cerca de 0,035 K/ano quando todo o período de observação é considerado. Contudo, para o período compreendido entre máximo e o mínimo solar a temperatura média anual apresenta uma tendência de aumento de 0,166 K/ano. Os resultados mostram que as médias anuais das temperaturas dessazonalizadas após a remoção dos efeitos devido ao fluxo solar, obtidas pelo método do gradiente, seriam mais elevadas do que as originais para o período de declínio solar. Verifica-se que as temperaturas anuais dessazonalizadas praticamente não mudam quando a tendência temporal é removida, o que pode indicar que as temperaturas anuais dessazonalizadas obtidas pelo método do gradiente são fortemente afetadas pelo fluxo solar.

The time-dependent modulation of galactic cosmic rays in the heliosphere is studied by computing intensities using a two-dimensional, time-dependent modulation model. The compound approach of Ferreira and Potgieter (2004), which describes changes in the cosmic ray transport coefficients over a solar cycle, is improved by introducing recent theoretical advances in the model. Computed intensities are compared with Voyager 1 and 2, IMP 8 and Ulysses proton observations in search of compatibility. It is shown that this approach gives realistic cosmic ray proton intensities on a global scale at Earth and along both Voyager spacecraft trajectories. The results show that cosmic ray modulation, in particular during the present polarity cycle, is not just determined by changes in the drift coefficient but is also dependent on changes in the diffusion coefficients. Furthermore, a comparison of computations to observations along the Voyager 1 and Voyager 2 trajectories illustrates that the heliosphere is asymmetrical. Assuming the latter, E > 70 MeV and 133-242 MeV cosmic ray proton intensities along Voyager 1 and 2 trajectories are predicted from 2012 onwards. It is shown that the computed intensities along Voyager 1 can increase with an almost constant rate since the spacecraft is close to the heliopause. However, the model shows that Voyager 2 is still under the influence of temporal solar activity changes because of the relatively large distance to the heliopause when compared to Voyager 1. Along the Voyager 2 trajectory the intensities should remain generally constant for the next few years and then should start to steadily increase. It is also found that without knowing the exact location of heliopause and transport parameters one cannot conclude anything about local interstellar spectra. The effect of a dynamic inner heliosheath width on cosmic ray modulation is also studied by implementing a time-dependent termination shock position in the model. This does not lead to improved compatibility with spacecraft observations so that a time-dependent termination shock along with a time-dependent heliopause position is required. The variation of the heliopause position over a solar cycle is found to be smaller compared to that of the termination shock. The model predicts the heliopause and termination shock positions along Voyager 1 in 2012 at 119 AU and 88 AU respectively and along Voyager 2 at 100 AU and 84 AU respectively. / Thesis (PhD (Space Physics))--North-West University, Potchefstroom Campus, 2013

The time-dependent modulation of galactic cosmic rays in the heliosphere is studied by computing intensities using a two-dimensional, time-dependent modulation model. The compound approach of Ferreira and Potgieter (2004), which describes changes in the cosmic ray transport coefficients over a solar cycle, is improved by introducing recent theoretical advances in the model. Computed intensities are compared with Voyager 1 and 2, IMP 8 and Ulysses proton observations in search of compatibility. It is shown that this approach gives realistic cosmic ray proton intensities on a global scale at Earth and along both Voyager spacecraft trajectories. The results show that cosmic ray modulation, in particular during the present polarity cycle, is not just determined by changes in the drift coefficient but is also dependent on changes in the diffusion coefficients. Furthermore, a comparison of computations to observations along the Voyager 1 and Voyager 2 trajectories illustrates that the heliosphere is asymmetrical. Assuming the latter, E > 70 MeV and 133-242 MeV cosmic ray proton intensities along Voyager 1 and 2 trajectories are predicted from 2012 onwards. It is shown that the computed intensities along Voyager 1 can increase with an almost constant rate since the spacecraft is close to the heliopause. However, the model shows that Voyager 2 is still under the influence of temporal solar activity changes because of the relatively large distance to the heliopause when compared to Voyager 1. Along the Voyager 2 trajectory the intensities should remain generally constant for the next few years and then should start to steadily increase. It is also found that without knowing the exact location of heliopause and transport parameters one cannot conclude anything about local interstellar spectra. The effect of a dynamic inner heliosheath width on cosmic ray modulation is also studied by implementing a time-dependent termination shock position in the model. This does not lead to improved compatibility with spacecraft observations so that a time-dependent termination shock along with a time-dependent heliopause position is required. The variation of the heliopause position over a solar cycle is found to be smaller compared to that of the termination shock. The model predicts the heliopause and termination shock positions along Voyager 1 in 2012 at 119 AU and 88 AU respectively and along Voyager 2 at 100 AU and 84 AU respectively. / Thesis (PhD (Space Physics))--North-West University, Potchefstroom Campus, 2013

Thesis (Ed.D.)--Teachers College, Columbia University, 1995. / Typescript; issued also on microfilm. Sponsor: Warren E. Yasso. Dissertation Committee: O. Roger Anderson. Includes bibliographical references (leaves 95-109).

Thesis (Ph.D. (Physics & Electronics)) - Rhodes University, 2009 / A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science