Dissertations / Theses on the topic 'Solar magnetic field'

Abstract This thesis studies the structure and evolution of the large scale heliospheric magnetic field. The work covers the space age, the period when satellite measurements revolutionized our knowledge about space. Now, this period is known to be the declining phase of the grand modern maximum of solar activity. The thesis addresses how the hemispherical asymmetry of solar activity is seen in the photospheric magnetic field and how it appears in the corona and in the heliosphere until the termination shock. According to geomagnetic and heliospheric observations, the heliospheric current sheet has been southward shifted around the solar minima since 1930s. Using Ulysses probe observations, we derive an accurate estimate of 2° for the southward shift of the heliospheric current sheet during two very different solar minimum in the mid 1990s and 2000s. The overall structure of the heliospheric magnetic field has changed significantly now when the grand modern maximum has come to an end. During the present low solar activity the polar fields are weaker and the heliospheric current sheet covered a wide latitudinal range during the previous minimum. When the heliospheric current sheet is wide the asymmetry is less visible at the Earth’s orbit. We extend our study to the outer heliosphere using measurements made by Voyager and Pioneer probes and show that the hemispherical asymmetry in the coronal hole evolution, and the related southward shift of the heliospheric current sheet, are seen until the termination shock. In order to understand the origin of the hemispherical asymmetry, we complete a multipole analysis of the solar magnetic field since 1976. We find that the minimum time southward shift of the heliospheric current sheet is due to the quadrupole component of the coronal magnetic field. The quadrupole term exists because the generation and transport of the magnetic flux in the Sun tends to proceed differently in the northern and southern hemispheres. During this and the following decade the Sun is most likely going to be less active than it has been since 1920s. Therefore it is probable that the hemispherical asymmetry of the heliospheric magnetic field will be less visible in the ecliptic plane in the near future. Now, when the Sun seems to be at the maximum of cycle 24, we are looking forward to see how the polar fields and the heliospheric magnetic field are formed when approaching the following solar minimum. It is possible that, as the activity rises again after the present and future low cycles, the hemispherical asymmetry will be opposite to that of the 20th century and the minimum time heliospheric current sheet would be northward shifted.

Abstract This thesis studies the long-term evolution of the photospheric magnetic field using surface flux transport simulations. The photospheric magnetic field and magnetic activity are tightly connected to space weather, and affect the whole heliosphere including the Earth. However, due to a lack of reliable observations our understanding of the long-term evolution of the photospheric magnetic field is still poor. Surface flux transport models, which are capable of simulating the evolution of the whole surface field from observations of solar activity, can be used to study the field in times when direct observations are not available. In this thesis we validate our surface flux transport model, optimize its parameters and test its sensitivity to uncertainties in parameter values and input data. We find a need to extend the model with a decay term to properly model the deep and long minimum between solar cycles 23 and 24, and simulate the photospheric magnetic field of cycles 21–24 using magnetographic observations as input. We also study consequences of hemispherically asymmetric activity, and show that activity in one hemisphere is enough to maintain polar fields in both hemispheres through cross-equatorial flow of magnetic flux. We develop a new method to reconstruct active regions from calcium K line and sunspot polarity observations. We show that this reconstruction is able to accurately capture the correct axial dipole moment of active regions. We study the axial dipole moments of observed active regions and find that a significant fraction of them have a sign opposite to the sign expected from Hale’s and Joy’s laws, proving that the new reconstruction method has an advantage over existing methods that rely on Hale’s and Joy’s laws to define polarities. We show one example of a long simulation covering solar cycles 15–21, demonstrating that using the active region reconstruction and surface flux transport model presented in this thesis it is possible to simulate the large-scale evolution of the photospheric magnetic field over the past century
Original papers The original publications are not included in the electronic version of the dissertation. Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., Yeates, A., & Mursula, K. (2017). Reconstructing solar magnetic fields from historical observations. II. Testing the surface flux transport model. Astronomy & Astrophysics, 604, A8. https://doi.org/10.1051/0004-6361/201730415 http://jultika.oulu.fi/Record/nbnfi-fe2017103050356 Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., & Mursula, K. (2018). Reconstructing solar magnetic fields from historical observations. III. Activity in one hemisphere is sufficient to cause polar field reversals in both hemispheres. Astronomy & Astrophysics, 616, A134. https://doi.org/10.1051/0004-6361/201732323 http://jultika.oulu.fi/Record/nbnfi-fe201902205813 Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., Bertello, L., Yeates, A., & Mursula, K. (2019). Reconstructing solar magnetic fields from historical observations. IV. Testing the reconstruction method. Astronomy & Astrophysics, 627, A11. https://doi.org/10.1051/0004-6361/201935606 http://jultika.oulu.fi/Record/nbnfi-fe2019091828628 Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., & Mursula, K. (2019) Axial dipole moment of solar active regions in cycles 21-24. Manuscript

This thesis investigates the topology of the magnetic field in the solar corona, due to a variety of source configurations and types. To fully understand the complex behaviour of the Sun's magnetic field, it is important to have a complete description of the features present in its structure. The magnetic topologies due to network source configurations are investigated using both the point source description and the continuous source description. A series of case studies involving an emerging bipole in a hexagonal arrangement to simulate a supergranular cell are studied. This has a particular focus on the behaviour of coronal nulls located in the topology, and a particular case may form the underpinning of a model for polar plumes. A new topological feature, called a null-like point, is defined by relaxing the definition of a magnetic null point. Separatix-like surfaces, originating from null-like points, allow quasi-separatrix layers to be found in magnetic topologies due to continuously distributed sources. The squashing factor, Q, is mapped across the source configuration, highlighting the locations of the quasi-separatrix layers. Finally, an algorithm is developed which automatically detects and classifies magnetic events local to X-ray bright points (XBPs). Significant peaks are identified in the gradients of flux curves (positive, negative and absolute flux) local to XBP footpoints, allowing instances of flux emergence and cancellation to be identified and linked to the onset and demise of the XBPs studied. The algorithm correctly classifies 90% of all emergence and cancellation events related to the studied XBPs.

The main idea behind the work presented in this thesis is to investigate if it is possible to find a mechanism that leads to surface magnetic field concentrations and could operate under solar conditions without postulating the presence of magnetic flux tubes rising from the bottom of the convection zone, a commonly used yet physically problematic approach. In this context we study the ‘negative effective magnetic pressure effect’: it was pointed out in earlier work (Kleeorin et al., 1989) that the presence of a weak magnetic field can lead to a reduction of the mean turbulent pressure on large length scales. This reduction is now indeed clearly observed in simulations. As magnetic fluctuations experience an unstable feedback through this effect, it leads, in a stratified medium, to the formation of magnetic structures, first observed numerically in the fifth paper of this thesis. While our setup is relatively simple, one wonders if this instability, as a mechanism able to concentrate magnetic fields in the near surface layers, may play a role in the formation of sunspots, starting from a weak dynamo-generated field throughout the convection zone rather than from strong flux tubes stored at the bottom. A generalization of the studied case is ongoing.

At the time of the the doctoral defence the following paper was unpublished and had a status as follows: Paper nr 7: Submitted

The aim of this thesis is the development of numerical models for determining the coronal magnetic field from observations of the magnetic field at the photosphere (coronal magnetic field extrapolation), with an emphasis on numerical methods, computation and code development. Presently, observational methods cannot determine the coronal magnetic field accurately and routinely, motivating numerical modelling. In this thesis we develop improvements to the widely used force-free model of the coronal magnetic field. In Chapter 1 we present motivation for the modelling and its limitations. In Chapter 2 we present free energy estimates for the solar active region AR 11029 derived from a force-free model applied to data from the Hinode Spectropolarimeter and the Synoptic Optical Long-term Investigations of the Sun Vector-Spectromagnetograph. The work illustrates many of the difficulties with modelling discussed in Chapter 1. In Chapter 3 we present a numerical code for solving a magneto-hydrostatic model of the corona which includes pressure forces in the corona. The code is applied to a simple test case to demonstrate its correctness. In Chapter 4 we present a numerical code for solving the nonlinear force-free model in spherical polar coordinates which overcomes the limitations of the Cartesian models. The code is an implementation of the Grad-Rubin method, and is based on a spectral representation of the magnetic field in terms of vector-spherical harmonics. The method is applied to a simple test case to demonstrate the self-consistency of the implementation. In Chapter 5 we derive the vector-spherical harmonic solution to Ampere's law which is presented in Chapter 4 without derivation. This solution forms an important part of the spherical code presented in Chapter 4.

The three-dimensional magnetic structure of solar filament channels and filaments is considered. A simple analytical potential model of a filament channel is setup with line sources representing the overlying arcades and point sources the flux of the filament. A possible explanation of the distinct upper and lower bounds of a filament is given. A more detailed numerical force-free model with discrete flux sources is then developed and the effect of magnetic shear on the separatrix surface explored. Dextral channels are shown to exist for a wider range of negative values of the force-free alpha and sinistral channels for positive values of alpha. Potential models of a variety of coronal structures are then considered. The bending of a filament is modelled and a method of determining the horizontal component of a filament's magnetic field is proposed. Next, the observed opposite skew of arcades lying above switchbacks of polarity inversion lines is shown to be produced by a local flux imbalance at the corner of the switchback. Then, the magnetic structure of a particular filament in a filament channel is modelled using observations from a photospheric magnetogram. It is shown that dips in the filaments magnetic field could result from opposite polarity fragments lying below the filament. Finally, the formation of a specific filament channel and filament is modelled. The formation of the channel is shown to be due to the emergence of new flux in a sheared state. It is shown that convergence and reconnections between the new flux and old remnant flux is required for the filament to form. The field lines that represent the filament form a thin vertical sheet of flux. The changing angle of inclination of the sheet gives the appearance of twist. The method of formation is then generalised to other cases and it is shown that a hemispheric pattern consistent with the results of Martin et al. (1995) can be obtained.

Using the Magnetohydrodynamic model, two problems in the behaviour of magnetic field structures are investigated. Firstly, the stability of tokamak equilibria to coupled tearing modes is calculated. Secondly, the equilibrium structure of a solar coronal loop is examined. The flux co-ordinate method is used to construct toroidal equilibria of the type found in large aspect ratio tokamaks. In such a field configuration, the analysis of tearing modes is complicated by the coupling of different poloidal fourier modes. The effect of coupling through elliptic shaping of plasma surfaces is calculated. For certain current profiles, this effect may cause instability. The response of coronal loops to twisting at their photospheric footpoints is investigated. Long loops are shown to have an essentially 1-D nature. This observation is used to develop a 1-D, line-tied model for such loops. This model is used to conduct an extensive survey of the non-linear twist regime, including the effects of enhanced fluid pressure. The possibility of non-equilibrium, which would provide energy for coronal heating and compact flares, is examined. When the physical variable of footpoint displacement is specified, no loss of equilibrium is found by twisting. Loss of equilibrium is found for high pressures, which we do not, however, expect to find in the corona.

Although the solar corona is one of the most studied areas in solar physics, its activity, such as flares, prominence eruptions and CMEs, is far from understood. Since the solar corona is a low-ß plasma, its structure and dynamics are driven by the magnetic field. The aim of this PhD thesis to study the magnetic field in the solar corona. Unfortunately, high quality direct measurements of the coronal magnetic field are not available and theoretical extrapolation using the observed photospheric magnetic field is required. The thesis is mainly divided in two parts. The first part deals with the comparison between theoretical models of magnetic fields and observed structures in the corona. For any theoretical model, a quantitative method to fit magnetic field lines to observed coronal loops is introduced. This method provides a quantity C that measures how closely a theoretical model can reproduce the observed coronal structures. Using linear force-free field extrapolation, the above field line fitting method is used to study the evolution of an active region. The method is also illustrated when the theoretical magnetic field depends on more than one parameter. The second part of the thesis focuses on the linear force-free field assumption using two different geometric configurations. Firstly a vertical rigid magnetic flux tube is considered. The analytical expression of the magnetic field is obtained as an expansion in terms of Bessel functions. The main properties of this system are discussed and compared with two cylindrically symmetric twist profiles. For the second system, the photosphere is assumed to be an infinite plane. Using translational geometry, the analytical expression of the linear force-free magnetic field that matches a prescribed line of sight magnetic field component is obtained. This solution is compared with the non-linear solution obtained by Roumeliotis (1993).

The magnetic carpet is defined to be the small-scale photospheric magnetic field of the quiet Sun. Observations of the magnetic carpet show it to be highly dynamic, where the time taken for all flux within the magnetic carpet to be replaced is on the order of just a few hours. The magnetic carpet is continually evolving due to the Sun's underlying convection and the interaction of small-scale magnetic features with one another. Due to this, the small-scale coronal field of the magnetic carpet is also expected to be highly dynamic and complex. Previous modelling has shown that much of the flux from the magnetic carpet is stored along low-lying closed connections between magnetic features. This indicates that significant coronal heating could occur low down in the small-scale corona. In this thesis, a new two-component magnetic field model is developed for the evolution of the magnetic carpet. A 2D model is constructed to realistically simulate the evolution of the photospheric field of the magnetic carpet, where many of the parameters for the model are taken from observational studies. The photospheric model contains a granular and supergranular flow profile to describe the motion of the small-scale magnetic features, and includes the processes of flux emergence, cancellation, coalescence and fragmentation. This 2D model then couples to a 3D model as the lower boundary condition, which drives the evolution of the coronal field through a series of non-linear force-free states, via a magnetofrictional relaxation technique. We first apply the magnetofrictional technique to consider the coronal evolution of three basic small-scale photospheric processes: emergence, cancellation and flyby. We consider the interaction of the magnetic features with an overlying coronal magnetic field, and quantify magnetic energy build-up, storage and dissipation. The magnetofrictional technique is then applied to synthetic magnetograms produced from the 2D model, to simulate the evolution of the coronal field in a situation involving many hundreds of magnetic features. We conduct a preliminary analysis of the resultant 3D simulations, considering the magnetic energy stored and dissipated, as well as regions of enhanced velocity and electric current density within the coronal volume. The simulations show that the so-called 'quiet Sun' is not quiet and a significant amount of complex interactions take place.

Abstract I have made a detailed study of the fundamental properties of the solar photospheric magnetic field, which helps in better understanding the Sun’s radiative and particle outputs that affect the Earth’s near-space environment, as well as the entire heliosphere. Photospheric magnetic field is an essential parameter for space weather and space climate. The photospheric magnetic field includes a wide range of large-scale and small-scale structures, but the contribution of weak, small-scale fields to the total flux on the solar surface is dominant. This thesis discusses the spatial-temporal structure and long-term evolution of the solar photospheric magnetic field. Particularly, the thesis presents, for the first time, the spatial distribution of the asymmetry of weak field values and its evolution in solar cycles 21–24. I found that the asymmetry (also called shift) of the distribution of positive and negative weak-field values is a real physical phenomenon. I also found that the shifts are most effectively produced at the supergranulation scale. I studied the asymmetry of the distribution of weak field values separately in the two solar hemispheres. My results show that the shifts of weak-field field distributions in the two solar hemispheres have always the same sign as the new polarity of the polar field in the respective hemisphere and solar cycle. I also found that the hemispheric shifts change their sign in the late ascending to maximum phase of the solar cycle and attain their maximum in the early to mid-declining phase. This evolution of the hemispheric weak-field gives a new signal of the solar magnetic cycle. We also studied the long-term spatial-temporal evolution of the weak-field shift and skewness of the distribution of photospheric magnetic field values during solar cycles 21–24 in order to clarify the role and relation of the weak field values to the overall magnetic field evolution. Our results give evidence for the preference of even the weakest field elements toward the prevailing magnetic polarity since the emergence of an active region, and for a systematic coalescence of stronger magnetic fields of opposite to produce weak fields during the poleward drift of the surge
Original papers Original papers are not included in the electronic version of the dissertation. Getachew, T., Virtanen, I., & Mursula, K. (2017). Structure of the Photospheric Magnetic Field During Sector Crossings of the Heliospheric Magnetic Field. Solar Physics, 292(11). https://doi.org/10.1007/s11207-017-1198-9 http://jultika.oulu.fi/Record/nbnfi-fe201802083259 Getachew, T., Virtanen, I., & Mursula, K. (2019). Asymmetric Distribution of Weak Photospheric Magnetic Field Values. The Astrophysical Journal, 874(2), 116. https://doi.org/10.3847/1538-4357/ab0749 http://jultika.oulu.fi/Record/nbnfi-fe2019061320447 Getachew, T., Virtanen, I., & Mursula, K. (2019). A New Signal of the Solar Magnetic Cycle: Opposite Shifts of Weak Magnetic Field Distributions in the Two Hemispheres. Geophysical Research Letters, 46(16), 9327–9333. https://doi.org/10.1029/2019gl083339 Mursula, K., Getachew, T., & Virtanen, I. (2019). Spatial-temporal evolution of photospheric weak-field shifts in solar cycles 21-24. Astron. Astrophys., submitted

The past few decades of solar observations have seen an increase in both the spatial and temporal resolution of data. The recent launch of the Solar Dynamics Observatory is the next step in a digital era and provides so much data that the satellite has its own Feature Finding Team tasked with creating automated detection algorithms to ease the burden on human analysis. This thesis will present some methods of automated solar feature recognition with the aim of finding a consistent method that can be reliably used on long term datasets (the Michelson Doppler Imager data from 1996-2010 will be used as the example in this thesis). We show methods for detecting sunspots in white light intensity data as well as a method for detecting magnetic fragments in magnetogram data. By applying these methods to a long term dataset we build a sunspot catalogue which is then used to investigate the evolution of sunspot properties over solar cycle 23. We find that the International Sunpot Number does not accurately represent the number of sunspots present on the visible solar disk although the trend does follow the number of sunspots. We also find that the umbral area of sunspots is between 20 and 40% of the total sunspot area and that this exhibits smooth variation over the solar cycle indicating there may be some change in how sunspots are formed at different points in the cycle. We then use the catalogue to investigate the Wilson depression effect and use Monte Carlo simulations along with sunspot models to show that the tau = 1 layer of the photosphere is recessed by 500-1000 km inside sunspots. Next, we examine the magnetic fields inside sunspot umbrae to investigate claims of a long term secular decrease in sunspot magnetic fields that could point to a long term solar minimum spanning many cycles. We do not see evidence of this decrease although we only analyse one cycle of data. Next, five active regions are analysed using an automated magnetic fragment detection and tracking algorithm. We also examine quiet Sun magnetic fields and note that at field strengths of 5 Gauss from the HMI/SDO instrument, the orbital motion of the satellite can be detected as a fluctuation in the measured magnetic field strength with the period of a satellite in geosynchronous orbit. We also calculate the diffusion and drift velocities of fragments in three of the observed active regions and find that our diffusion coefficients are higher than previous studies but our drift speeds are lower than those from the same studies.

La convezione rappresenta il meccanismo principale di trasporto dell’energia negli strati sottostanti la superficie solare. La dinamica dei flussi fotosferici associati a tale meccanismo determina la formazione e l’evoluzione del campo magnetico globale e di una grande varietà di strutture presenti nelle regioni più esterne del Sole. In particolare l’interazione tra i flussi di plasma e il campo magnetico determina la configurazione spaziale e l’evoluzione delle regioni attive e degli elementi magnetici superficiali, importanti ad esempio nel determinare la variabilità solare. La convezione solare può essere studiata o mediante lo sviluppo di simulazioni di magnetoconvezione (simulazioni MHD) o attraverso osservazioni spettrali della superficie solare. In questo lavoro il problema della connessione tra campi magnetici solari e dinamica fotosferica è stato affrontato seguendo un approccio sperimentale. In particolare abbiamo lavorato sui sistemi di acquisizione per la spettroscopia solare bidimensionale, sulla pipeline di riduzione di dati spettroscopici solari e infine sull’analisi dei dati. Uno degli strumenti principali della fisica solare sperimentale è la spettroscopia, che permette di derivare informazioni su molti parametri dell’atmosfera solare, quali velocità, temperatura e campo magnetico. Inoltre, l’analisi spettroscopica permette di ricavare la velocità verticale delle strutture emergenti sulla superficie solare. In questo modo, poiché ogni lunghezza d’onda può essere associata ad una determinata quota nell’atmosfera, è possibile trasformare un’immagine bidimensionale in un campo 3D. Al fine di studiare la dinamica dell’atmosfera solare, sono necessarie osservazioni ad alta risoluzione spettrale e spaziale. Inoltre, la rapida evoluzione delle strutture solari osservate richiede monocromatori con un’elevata trasparenza per acquisire spettri multi-riga in un tempo molto breve. Uno strumento che soddisfa tutte queste richieste è IBIS (Interferometric BIdimensional Spectrometer), uno spettrometro bidimensionale installato presso il Dunn Solar Telescope-DST. IBIS produce dati con elevata risoluzione spaziale (0.2” al DST), spettrale (Dl/l~200000) e temporale (tempo di esposizione 10 ms, rate di acquisizione 5 immagini al secondo). Le immagini acuiqiste con IBIS sono registrare da un sensore CCD. Il Capitolo 1 della tesi fornisce un’introduzione alla spettroscopia solare e all’uso delle immagini spettroscopiche per ottenere informazioni sulla dinamica degli strati fotosferici solari. Lo schema dello strumento IBIS, utilizzato in questa tesi per l’acquisizione delle immagini spettroscopiche, è descritto. Nel Capitolo 2 sono riportate le misure e le calibrazioni, effettuate in laboratorio attraverso la Tecnica della Photon Transfer, di due sensori: il sensore CMOS Si-1920-HD e il sensore EMCCD Andor Ixon DV885. Il nostro interesse in questi sensori nasce dalla necessità di sostituire il sensore attualmente installato sul canale spettrale di IBIS, al fine di incrementare l’efficienza di acquisizione dei dati. In particolare, i miglioramenti al sistema di acquisizione di IBIS riguardano diversi aspetti: aumento della sensibilità/efficienza quantica, riduzione del tempo di lettura, incremento della dimensione dell’array e aumento del guadagno del sensore. Nel Capitolo 3 sono descritti i vari passi della pipeline di riduzione dei dati IBIS, che include sia una correzione standard delle immagini sia un software scritto in IDL per l’analisi di immagini solari ad elevata risoluzione. Nel Capitolo 4 riportiamo i risultati scientifici legati allo studio dell’emersione e dell’organizzazione del campo magnetico sulla superficie solare sia come struttura isolata sia come cluster. Tipiche strutture magnetiche isolate sono le macchie solari e i “pore”. E’ stata studiata la dinamica, su piccola scala, di una regione di intenso campo magnetico (pore), con struttura brillante interna. I pore rappresentano una delle tante strutture formate dall’emersione del campo magnetico sulla superficie solare. Essi rappresentano un link tra i più piccoli elementi di flusso e le regioni magnetiche associate alle macchie. I light bridge, in un pore o in una macchia, sono strutture brillanti che dividono la regione di ombra in una strutture più o meno complessa. Comunemente, i light bridge indicano la presenza di un processo in corso all’interno della regione attiva: l’emersione di regioni magnetiche o, al contrario, il disfacimento dell’intera struttura. In entrambi i casi ci si aspetta una riconfigurazione topologica del campo magnetico emergente. Un altro modo per studiare l’interazione del campo magnetico con i moti del plasma consiste nell’andare ad investigare le proprietà oscillatorie della cromosfera solare, sia quieta che attiva, in relazione alla fotosfera sottostante, ponendo particolare riguardo alla topologia del capo magnetico. Nell’atmosfera solare esiste una frequenza di cut-off acustica che produce una riflessione delle onde a bassa frequenza verso gli strati più bassi dell’atmosfera e la regione convettiva. Dunque solo le onde con frequenza maggiori della frequenza di cut-off possono propagarsi verso gli strati più alti dell’atmosfera. Il campo magnetico modifica dunque le proprietà delle oscillazioni acustiche. In particolare, in presenza di un campo magnetico inclinato la frequenza di cut-off acustica si abbassa, permettendo così la propagazione verso l’alto di onde a frequenza maggiore. Questo risultato è stato confermato dalle mappe dei picchi di potenza relative alla fotosfera e alla cromosfera, ottenute utilizzando i dati acquisiti con IBIS. Lo studio della dinamica della fotosfera solare può essere intrapreso anche con metodi statistici, analizzando le proprietà topologiche degli elementi di origine convettiva e magnetica, come la distribuzione spaziale delle strutture presenti nei magnetogrammi. A tal proposito è stato sviluppato un algoritmo in grado di determinare, in maniera automatica, i “vuoti” in una fissata distribuzione di particelle. Questo metodo, applicato ad una serie temporale di magnetogrammi solari con un ampio campo di vista, ha permesso di identificare i vuoti tra strutture magnetiche e di studiarne la distribuzione sulla superficie solare.
Convection is the chief mode of heat transport in the outer envelopes of cool stars such as the Sun. Convective effects are recognizable in large-scale features, such as the global differential rotation and meridional circulation flows, as well as smaller scale phenomena such as granulation, mesogranulation, and supergranulation. Moreover, convective flows widely determine the evolution and organization of tiny magnetic elements observed in the solar surface responsible for small scale irradiance solar variations. Our understanding of the solar convection derives from numerical simulations of compressible convection (MHD approach) and from spectral observations of the solar surface (velocity and center line maps, helioseismological data, etc.). In this work we face the problem of connection between solar magnetic fields and photospheric dynamics through an experimental approach. In particular we worked on acquisitions systems for solar imaging spectroscopy, on a pipeline for the spectroscopic data reduction and on the data analysis. One of the basic tools of observational solar physics is spectroscopy, which allows us to derive information on several physical parameters of solar atmosphere such as velocity, temperature, magnetic field strength etc. Spectroscopic analysis allows us to determine the vertical velocity of solar surface structures. Moreover, as wavelength can be somehow associated to depth in the solar atmosphere, it is possible to transform a bidimensional image in a 3-D field. In order to study solar atmosphere dynamics, observations of adequate spectral purity, together with high spatial resolution to resolve small-scale structures are necessary. Moreover, the rapid evolution of observed solar features requires monochromators with high transparency to acquire multiple-line spectra in a comparatively short time. In order to meet all these requirements, suitable instruments and techniques have to be used. An instrument which satisfies all these constraints is IBIS, an Interferometric Bidimensional Spectrometer, installed at the Dunn Solar Telescope/NSO (Sac Peak, USA). IBIS produces data with high spectral (Dl/l~200000), spatial (0.2’’ at DST telescope) and temporal resolution (exposure time 10 ms; acquisition rate 5 frames s-1). Images acquired with IBIS are currently recorded by a CCD camera. Chapter 1 introduces the reader to the solar spectroscopy and to the use of spectroscopic imaging to retrieve information on solar photospheric layers dynamics. The basic concept and the layout of the IBIS spectrograph, used in this thesis to acquire spectroscopic images, is described. Chapter 2 reports laboratory measurements and calibrations, derived through the application of the Photon Transfer Technique, of two sensors: the SI-1920 HD CMOS sensor and the Andor DV885 EMCCD sensor. Our interest in these sensors is related to the necessity to replace the CCD camera, now installed on the IBIS spectral channel. Improvements in the IBIS camera system concern an increased sensitivity/quantum efficiency, a decreased detector readout time, a larger array size and an increased full well/programmable detector gain. Chapter 3 describes the various steps of the pipeline developed for the IBIS data reduction. The pipeline includes both the standard image processing and a high performance IDL software package written specifically for high resolution solar images. In Chapter 4 we report some results related to the study of the emergence and the organization of the magnetic field on the solar surface both as isolated structures and as clusters. More in detail, typical isolated magnetic features are pores or sunspots. We investigated the small scale dynamics of a strong magnetic field region (pore) with a light bridge inside it, observed with the IBIS spectrometer. An analysis of the intensity and velocity maps revealed the presence, inside the light bridge, of elongated structures showing a kind of reversal in intensity and velocity. More in detail, in the intensity images we observed a narrow central dark lane running along the axis of the light bridge, that we explain proposing an analytical model. Regarding the velocity structure, its topology resembles a convective roll and may indicate a modification of the photospheric convective flows. By adopting the IBIS dataset, we studied the oscillatory properties of the solar atmosphere, in the photosphere and the chromosphere, with particular regard to the influence of the magnetic topology. In particular, we analyzed the propagation of waves in the atmosphere in correspondence of a pore, of a magnetic network area and of a quiet Sun region. Studying the generation and propagation of waves in the solar atmosphere provides information about the atmospheric structure and dynamics and it helps to identify the key mechanism of chromospheric and coronal heating. Finally, by using large FoV MDI magnetograms we analyzed the spatial distribution of reticular clusters of magnetic features, such as the magnetic network. For this purpose, we developed a numerical algorithm able to detect voids between magnetic fragments. We computed Void Probability Functions which describe, in a uniform and objective way, the assessment of the void structure of different magnetic elements distributions.

The structure of the magnetic field is often an important factor in many energetic processes in the solar corona. To determine the topology of the magnetic field features such as null points, separatrix surfaces, and separators must be found. It has been found that these features may be preferred sites for the formation of current sheets associated with the accumulation of free magnetic energy. Over the last decade, it also became clear that the geometrical analogs of the separatrices, the so-called quasi separatrix layers, have similar properties. This thesis has the aim of investigating these properties and to find correlations between these quantities. Our goal is to determine the relation between the geometrical features associated with the QSLs and with current structures, sites of reconnection and topological features. With these aims we conduct three different studies. First, we investigate a non linear force free magnetic field extrapolation from observed magnetogram data taken during a solar flare eruption concentrating our attention on two snapshots, one before the event and one after. We determine the QSLs and related structures and by considering carefully how these change between the two snapshots we are able to propose a possible scenario for how the flare occurred. In our second project we consider potential source distributions. We take different potential point source models: two four sources models already presented in the literature and a random distribution of fifteen sources. From these potential models we conduct a detailed analysis of the relationship between topological features and QSLs. It is found that the maxima of the Q-factor in the photosphere are located near and above the position of the subphotospheric null points (extending part way along their spines) and that their narrow QSLs are associated with the curves defined by the photospheric endpoints of all fan field lines that start from subphotospheric sources. Our last study investigates two different flux rope emergence simulations. In particular, we take one case with and one without an overlying magnetic field. Here, we can identify the QSLs, current, and sites of reconnection and determine the relation between them. From this work we found that not all high-Q regions are associated with current and/or reconnection and vice-versa. We also investigated the geometry of the field lines associated with high-Q regions to determine which geometrical behaviour of the magnetic field they are associated with. Those that are associated with reconnection also coincide with topological features such as separators.

Magnetic fields are fundamental to the structure and dynamics of the Sun’s corona. Observations show them to be locally complex, with highly sheared and twisted fields visible in solar filaments/prominences. The free magnetic energy contained in such fields is the primary source of energy for coronal mass ejections, which are important—but still poorly understood drivers of space weather in the near-Earth environment. In this thesis, a new model is developed for the evolution of the large-scale magnetic field in the global solar corona. The model is based on observations of the radial magnetic field on the solar photosphere (visible surface). New active regions emerge, and their transport and dispersal by surface motions are simulated accurately with a surface flux transport model. The 3D coronal magnetic field is evolved in response to these photospheric motions using a magneto-frictional technique. The resulting sequence of nonlinear force-free equilibria traces the build-up of magnetic helicity and free energy over many months. The global model is applied to study two phenomena: filaments and coronal mass ejections. The magnetic field directions in a large sample of observed filaments are compared with a 6-month simulation. Depending on the twist of newly-emerging active regions, the correct chirality is simulated for up to 96% of filaments tested. On the basis of these simulations, an explanation for the observed hemispheric pattern of filament chirality is put forward, including why exceptions occur for filaments in certain locations. Twisted magnetic flux ropes develop in the simulations, often losing equilibrium and lifting off, removing helicity. The physical basis for such losses of equilibrium is demonstrated through 2D analytical models. In the 3D global simulations, the twist of emerging regions is a key parameter controlling the number of lift-offs, which may explain around a third of observed coronal mass ejections.

Development of a self-consistent theoretical model is of fundamental importance to the study of the solar wind. Such a model is necessary to understand the origin of the solar wind as well as observational and theoretical aspects. For instance, a complete description of the acceleration of solar wind particles, intrinsic velocity and magnetic field components, role of magnetic field in the solar wind's angular momentum loss, and so on has not yet been achieved. This thesis presents two data-driven solar wind models to provide more detailed pictures of the solar wind in the equatorial plane, to extract the solar wind plasma quantities from the direct observations at 1 AU, and to describe the underlying physics. It also provides a comprehensive comparison between analytic predictions, observations, and advanced MHD (magnetohydrodynamics) simulation outputs. Chapter 1 provides a short literature review and a brief introduction for the thesis. Chapter 2 develops an analytic, self-consistent, theoretical model for the solar wind that includes conservation of angular momentum, frozen-in magnetic fields, and radial (r) and azimuthal (φ) components of velocity and magnetic field from the source surface/inner boundary to 1 AU. The solar wind model enforces corotation at the source surface (rs) assumes a constant radial speed at all heliolongitude, and applies near the equatorial plane. This model generalises previous models and reproduces the previous models in the appropriate limits. The model calculates the Alfvénic critical radius (ra) using the radial Alfvénic Mach number at 1 AU, and the predicted values agree with some recent observations. The predicted azimuthal velocity, which is only due to corotation is in the sense of corotation, but varies with, heliolongitudes (φ). Observations of the azimuthal velocity at 1 AU are usually much larger than predictions and not always in the corotation direction. These azimuthal velocities can not be explained by conservation of angular momentum alone. The standard interpretation involving stream-stream interactions and dynamical behaviour seems reasonable. Chapter 3 develops an accelerating solar wind model that includes the following: conservation of angular momentum, deviations from corotation, and non-radial velocity and magnetic field components from an inner boundary (or source surface) to beyond 1 AU. The model includes an accelerating solar wind profile using a solution of the time-steady isothermal equation of motion and predicts locations ra for the Alfvénic critical point which agree with recent observations. This model allows the flow velocity v to not always be parallel to magnetic field B in the corotating frame with the Sun, which results an electric field (E′) in the corotation frame. The resulting (E′ × B) drift may lead to enhanced scattering/heating of sufficiently energetic particles. The model demonstrates the existence of non-zero deviations δvφ from corotation at the source surface. These deviations of corotation are analogous to the transverse velocities caused by granulation and supergranulation motions. The abrupt changes in δvφ(rs,φs) are interpreted in terms of converging and diverging flows at the granulation cell boundaries and centers, respectively. Large range of variations of the angular momentum predicted and then are interpreted in terms of an intrinsic source in the solar wind of vorticity and turbulence from near the Sun towards 1 AU and beyond. Chapter 4 presents a comprehensive comparison where the accelerating solar wind model's predictions, observations are compared qualitatively and quantitatively with Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) simulation's outputs for the solar rotation period from November 21 to December 17, 2013. The chapter compares simulation outputs in the ecliptic plane with the analytic model results in the equatorial plane. Comparisons between simulated plasma quantities for long run time and short run time demonstrate that the initial solar wind plasma is entirely swept out by the simulated wind. It appears that high order grid refinement helps the simulation to reach a steady-state MHD system. The current version of the BATS-R-US simulation code treats the solar corona (SC) and the inner heliosphere (IH) separately and discontinuities in simulation outputs remain in the intersection of two modules. Overall, the simulated magnetic fields agree quite well with model predictions, much better than the density, velocity, and temperature. Radial profiles of plasma quantities have some qualitative agreement along a plasma flux tube, but quantitative differences are apparent. Chapter 5 summarizes the results in this thesis and discusses future avenues for research.

Abstract The Cassini/Huygens mission is a collaborative mission of NASA and ESA to study the Saturnian system. Cassini Plasma Spectrometer (CAPS)is one of the scientific investigations onboard the Cassini orbiter. It consists of three separate spectrometers: Electron Spectrometer (ELS), Ion Mass Spectrometer (IMS) and Ion Beam Spectrometer (IBS). The University of Oulu has a co-investigator status in the CAPS project, and been mainly involved in simulating the structure and scientific performance of the IBS instrument. IBS is a high resolution hemispherical electrostatic analyser aimed to study the solar wind ions. This thesis contains an Introduction and five original papers. Papers I–III contain a detailed description of the simulation process of the IBS instrument and related results. In Paper I the manufacturing tolerances were calculated in order to verify that the high resolution requirements can be achieved using available manufacturing processes. In Paper II the simulations have been further developed and the instrument properties have been studied in more detail. In Paper III the simulation model is used to help the analysis and interpretation of the laboratory calibrations of the IBS flight model. Papers IV and V study the long-term fluctuations in solar wind and interplanetary magnetic field in the period range of 1–2 years (so called mid-term quasi periodicities, MTQP), using the wavelet transformation method to produce dynamic power spectra. In paper IV the MTQP structure in solar wind speed at 1 AU was studied using the longest available series of geomagnetic activity. It was shown that the long-term occurrence MTQP fluctuations roughly follows the long-term solar activity, suggesting that MTQP fluctuations are closely connected with the solar dynamo activity. Moreover, it was also noted that MTQP activity may offer a possibility for a precursory signal which could be used to predict significant changes in long-term solar activity. While Paper IV presents the temporally longest study of MTQP fluctuations, Paper V gives the spatially widest treatment of the same phenomenon. Paper V studies MTQP fluctuations in solar wind and interplanetary magnetic field measured by four probes in the outer heliosphere. It is shown that two MTQP fluctuations of different periods (1.3 and 1.7 years)co existed during solar cycle 22, while during solar cycle 21 only the 1.7-year band existed. This suggests that the solar dynamo acts differently during even and odd cycles. It is also shown that the two MTQP fluctuations during solar cycle 22 are organized latitudinally. While the 1.3-year periodicity originates from equatorial regions, the 1.7-year fluctuations arise at mid-latitudes
Original papers Original papers are not included in the electronic version of the dissertation. Vilppola, J. H., Keisala, J. T., Tanskanen, P. J., & Huomo, H. (1993). Optimization of hemispherical electrostatic analyzer manufacturing with respect to resolution requirements. Review of Scientific Instruments, 64(8), 2190–2194. https://doi.org/10.1063/1.1143958 Vilppola, J. H., Tanskanen, P. J., Huomo, H., & Barraclough, B. L. (1996). Simulations of the response function of a plasma ion beam spectrometer for the Cassini mission to Saturn. Review of Scientific Instruments, 67(4), 1494–1501. https://doi.org/10.1063/1.1146881 Vilppola, J. H., Tanskanen, P. J., Barraclough, B. L., & McComas, D. J. (2001). Comparison between simulations and calibrations of a high resolution electrostatic analyzer. Review of Scientific Instruments, 72(9), 3662–3669. https://doi.org/10.1063/1.1392337 Mursula, K., Zieger, B., & Vilppola, J. H. (2003). Mid-term quasi-periodicities in geomagnetic activity during last 15 solar cycles: Connection to solar dynamo strength. Solar Physics, 212(1), 201–207. https://doi.org/10.1023/a:1022980029618 Mursula, K., & Vilppola, J. H. (2004). Fluctuations of the Solar Dynamo Observed in the Solar Wind and Interplanetary Magnetic Field at 1 AU and in the Outer Heliosphere. Solar Physics, 221(2), 337–349. https://doi.org/10.1023/b:sola.0000035053.17913.26

Kyoto University (京都大学)
0048
新制・課程博士
博士(理学)
甲第19359号
理博第4121号
新制||理||1593(附属図書館)
32373
新制||理||1593
京都大学大学院理学研究科物理学・宇宙物理学専攻
(主査)教授 一本 潔, 教授 柴田 一成, 准教授 野上 大作
学位規則第4条第1項該当

This thesis consists of three main studies: magnetic field effects in thermally activated delayed fluorescent (TADF) organic light emitting diodes (OLEDs), magnetic field effects in bipolar and unipolar polythiophene (P3HT) devices and a study of hybrid organic/inorganic perovskite devices. Spin-dependent transport and recombination processes of spin-pair species have been detected by magnetic field effect (MFE) technique in carbon-based semi- conductor devices. Magneto-electroluminescence (MEL) and magneto-conductivity have been measured as a function of the applied magnetic field, B, in light emitting diodes. TADF materials have been used instead of simple fluorescent materials in OLEDs. We have observed very large magnetic response with TADF materials. The second study is magnetic field effects of regio-regular P3HT based OLED devices. P3HT is a well known semiconducting polymer, and its electrical properties such as magneto-conductance can be affected by an applied magnetic field. P3HT was chosen because it exhibits a sign change in magnetoresistance (MR) as the bias is increased. Unipolar and bipolar devices have been fabricated with different electrode materials to understand which model can be best to explain organic magnetoresistance effect, possibly depending on the operating regime of the device. Transport and luminescence spectroscopies were studied to isolate the different mechanisms and identify their fingerprints. The third study is on hybrid organic-inorganic perovskite devices. With the potential of achieving very high efficiencies and the very low production costs, perovskite solar cells have become commercially attractive. Scanning electron microscopy (SEM) images and absorption spectrum of the films were compared in single-step solution, two-step solution and solution-assisted vapor deposition techniques. Grain size, morphology and thickness parameters of perovskite films were studied within these techniques. Perovskite solar cells were fabricated and their efficiencies were measured.

Terrestrial auroras are visible-light events caused by charged particles trapped by the Earth's magnetic eld precipitating into the atmosphere along magnetic eld lines near the poles. Auroral events are very dynamic, changing rapidly in time and across large spatial scales. Better knowledge of the low of energy during an aurora will improve understanding of the heating processes in the atmosphere during geomagnetic and solar storms. The Auroral Spatial Structures Probe is a sounding rocket campaign to observe the middle-atmosphere plasma and electromagnetic environment during an auroral event with multipoint simultaneous measurements for fine temporal and spatial resolution. The auroral event in question occurred on January 28, 2015, with liftoff the rocket at 10:41:01 UTC. The goal of this thesis is to produce clear observations of the magnetic eld that may be used to model the current systems of the auroral event. To achieve this, the attitude of ASSP's 7 independent payloads must be estimated, and a new attitude determination method is attempted. The new solution uses nonlinear least-squares parameter estimation with a rigid-body dynamics simulation to determine attitude with an estimated accuracy of a few degrees. Observed magnetic eld perturbations found using the new attitude solution are presented, where structures of the perturbations are consistent with previous observations and electromagnetic theory.

A thesis submitted to the university of Mumbai for the Ph. D.(Science) degree in Physics under the guidance of Prof. B.M.Pathan.
A brief summary of the important new findings is given below: • The data adaptive filtering technique singular spectrum analysis identifies and extracts trend and period modes of around 27-day, 13-day and 9-day in various solar wind and geomagnetic parameters. The response of the magnetosphere to the solar wind forcing is found to be the most prominent during the declining phases of the solar cycles. However, oscillations of these modes have considerable amplitudes during the entire sunspot cycle.Multi-frequency structures in substorm associated magnetic fluctuations are extracted by the SSA. The study throws light on several features of various modes thus detected, for example, poleward propagation of modes at high latitudes, dip equatorial enhancement. • Geomagnetic substorms, which may have considerably high magnetic disturbance (up to ∼-500 nT) at stations poleward of standard auroral oval, are occasionally missed out in the standard AE indices. However, their low latitude signatures like positive bays, Pi2 bursts are often evident. Signature and strength of such substorms have significant asymmetry in the opposite hemispheres. • This study clearly brings out 24-hour periodicity in the ring current asymmetry during magnetic storms. The asymmetry is observed maximum near dusk hours, whereas it is minimum near dawn hours. This periodicity is attributed to changing local time due to rotation of the Earth. For the first time, we also report clear westward and eastward propagating modes around the globe using ground-based magnetic data. These propagation characteristics are associated with the westward and eastward drifts of energetic ions and electrons, respectively in the ring current region. • This thesis reports various new aspects of substorm associated auroral and low latitude indices. (1) The AU index (supposedly positive), which is expected to represent the maximum intensity of the eastward electrojet during a substorm, turns negative under the conditions when entire auroral oval is dominated by westward electrojet. Such negative AU values result in underestimation of strength of substorm in the AE index (AE = AU − AL). Our study supports the finding of Kamide and Ros toker [2004] that use of AE index should be avoided for identification of a substorm.Rather AL index gives better representation of substorms. (2) Intense and prolonged solar flares generate asymmetric magnetic field at low latitudes. This asymmetry significantly alters non-substorm and substorm time ASY indices. (3) Low latitude ASY indices, often used in relation to substorm activities, are affected by prompt penetration of interplanetary electric field to lower latitudes.

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Secretaria de Educação do Estado de São Paulo
São calculados os níveis de energia para o átomo de hidrogênio sob campo magnético uniforme, utilizando o método das diferenças finitas. Estes resultados, quando multiplicados pelo Rydberg efetivo (que depende da massa efetiva e da permitividade elétrica do meio) correspondem à solução do problema de um elétron ligado a uma impureza doadora rasa em um semicondutor sob campo magnético (caso isotrópico, parabólico, não degenerado). Os valores encontrados, para campo nulo, são comparados com a solução analítica. Para campos magnéticos não nulos as soluções são comparadas com resultados teóricos obtidos mediante o método variacional ou por expansão em séries de potências na direção radial. O efeito do campo magnético sobre os orbitais atômicos é analisado a partir da representação gráfica dos mesmos. Os valores numéricos das energias de transição são comparados com dados experimentais para impurezas doadoras rasas em GaN, GaAs e InP.
The energy levels of the hydrogen atom in a uniform magnetic field are calculated by using the finite difference method. The resulting energy levels, when multiplied by the effective Rydberg (that depends on the effective mass and the electric permittivity of the medium), correspond to the energy levels of an electron bound to a shallow donor impurity in a semiconductor (with non-degenerate, parabolic and isotropic conduction band) subject to a magnetic field. The results in the absence of the magnetic field are compared with the analytical solutions. For finite magnetic-field strengths, the solutions are compared with the results obtained by the variational method or through an expansion in a power series of the radial variable. The effect of the magnetic field on the atomic orbitals is analyzed with the aid of their graphical representation. The calculated transition energies are compared with experimental data for shallow donor impurities in GaN, GaAs e InP.

Orientador: Alexys Bruno Alfonso
Banca: Fábio de Jesus Ribeiro
Banca: André Luiz Malvezzi
O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp
Resumo: São calculados os níveis de energia para o átomo de hidrogênio sob campo magnético uniforme, utilizando o método das diferenças finitas. Estes resultados, quando multiplicados pelo Rydberg efetivo (que depende da massa efetiva e da permitividade elétrica do meio) correspondem à solução do problema de um elétron ligado a uma impureza doadora rasa em um semicondutor sob campo magnético (caso isotrópico, parabólico, não degenerado). Os valores encontrados, para campo nulo, são comparados com a solução analítica. Para campos magnéticos não nulos as soluções são comparadas com resultados teóricos obtidos mediante o método variacional ou por expansão em séries de potências na direção radial. O efeito do campo magnético sobre os orbitais atômicos é analisado a partir da representação gráfica dos mesmos. Os valores numéricos das energias de transição são comparados com dados experimentais para impurezas doadoras rasas em GaN, GaAs e InP.
Abstract: The energy levels of the hydrogen atom in a uniform magnetic field are calculated by using the finite difference method. The resulting energy levels, when multiplied by the effective Rydberg (that depends on the effective mass and the electric permittivity of the medium), correspond to the energy levels of an electron bound to a shallow donor impurity in a semiconductor (with non-degenerate, parabolic and isotropic conduction band) subject to a magnetic field. The results in the absence of the magnetic field are compared with the analytical solutions. For finite magnetic-field strengths, the solutions are compared with the results obtained by the variational method or through an expansion in a power series of the radial variable. The effect of the magnetic field on the atomic orbitals is analyzed with the aid of their graphical representation. The calculated transition energies are compared with experimental data for shallow donor impurities in GaN, GaAs e InP.
Mestre

Electrically detected magnetic resonance (EDMR) is a sensitive spectroscopic technique, which can be used to readout few to single electron spins in semiconductor and carbon nanodevices for applications in solid state quantum information processing (QIP). Since only electrically active defects contribute to the EDMR signal, this technique can be used further to investigate defects and impurities in photovoltaic devices, in which they limit the sunlight-to-energy conversion efficiency significantly. Here, I employ X-band EDMR for semiconductor defect analysis and identify the most important recombination centres in Czochralski silicon with oxide precipitates, which can be intentionally grown to confine detrimental metallic impurities to inactive regions of the wafer in order to serve as a defect-free substrate for modern silicon photovoltaic devices. Those experiments show that oxide precipitation is accompanied by the formation of silicon dangling bonds. Furthermore, I describe a very promising route towards the fabrication and readout of few to single electron spins in carbon nanotube devices, which can be characterised structurally via transmission electron microscopy in order to relate their electrical and spin properties with their structure. Finally, I employ EDMR to read out electron spin states in donor-doped silicon field-effect transistors as a prerequisite for their application in QIP. I report on a novel cryogenic probe head for EDMR experiments in resonant microwave cavities operating at 0.35 T (9.7 GHz, X-band) and 3.34 T (94 GHz, W-band). This approach overcomes the inherent limitations of conventional X-band EDMR and permits the investigation of paramagnetic states with a higher spectroscopic resolution and signal intensity. Both advantages are demonstrated and discussed. I further report on a novel mechanism giving rise to the EDMR effect in donor-doped silicon field-effect transistors, which is capable of explaining why the EDMR signal intensities of the conduction electrons are enhanced by a factor of ∼100, while the donor resonance signals increase by a factor of ∼20 from X- to W-band only. The spin-relaxation and dephasing times are extracted from a series of pulsed-EDMR measurements and confirm this model. The author gratefully acknowledges funding from Trinity College Oxford, Department of Materials, EPSRC DTA, and Konrad-Adenauer-Stiftung e.V. (Begabtenförderung).

Es werden die Ergebnisse der Untersuchungen von Stromverteilungen an Solarzellen und Solarmaterial durch magnetische, elektrische und optische Messmethoden gezeigt. Die neue magnetfeldtopographische Messmethode CAIC wird hierbei erläutert und deren Stromverteilungen mit Ergebnissen der IR-Durchlichtmikroskopie, der LBIC- und der LIT-Methode verglichen und ausgewertet. Auf Basis der durchgeführten Untersuchungen und der Annahme des Vorhandenseins einer Korngrenzendekoration werden die Stromverlaufsmodelle einer elektrisch aktiven Korngrenze für eine Probe mit pn-Übergang sowie für eine Probe ohne pn-Übergang gezeigt. Anhand von CAIC-Messungen können die Position und die Orientierung leitfähiger und oberflächennaher SiC- und Si₃N₄-Ausscheidungen in mc-Silizium ohne pn-Übergang ermittelt werden. Hierfür wird ein Stromverlaufsmodell gezeigt. Weiterhin werden Zellmikrobrüche, Fehler in der Kontaktstruktur und Layoutunterschiede der Kontaktstruktur durch CAIC-Messungen an Solarzellen eindeutig nachgewiesen.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2015.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 251-283).
The first magnetic fields in the solar system were embedded in the ionized gas of the protoplanetary disk itself. Soon after, newly formed protoplanets may have harbored magnetic core dynamos. Paleomagnetic analysis of ancient samples permits direct constraints on these early solar system magnetic fields. Here I present paleomagnetic studies of several classes of meteorites. Experiments on inclusions of chondritic meteorites have led to some of the first constraints on the intensities of protoplanetary disk magnetic fields. Meanwhile, measurements of eucrites, a class of achondrites believed to originate from the asteroid Vesta, suggest that Vesta once hosted a magnetic core dynamo. New techniques developed during the course of these measurements permit ongoing and future investigations of the remanent magnetizations of new meteorites and terrestrial rocks. In support of the paleomagnetic results, I present analytical and numerical modeling of magnetic dust grain dynamics in the solar nebula and of the interior dynamics of differentiated asteroids capable of hosting magnetic dynamos.
by Roger R. Fu.
Ph. D.

Solar plage has been the topic of many studies since its initial description in the mid 19th century, but as of yet it has not been understood to the point where we can reproduce all aspects of these active regions in quasi-realistic numerical models. To a large extent, this is caused by an incomplete understanding of the magnetic structure that drives the activity in these areas. Detailed measurements have been done of the magnetic field configuration of plage in the photosphere since the late 20th century, but only a handful of papers have managed to make any measurements at all in the higher situated chromosphere, despite the fact that the magnetic field vector of plage is important in understanding chromospheric magnetic fields in general, as well as the heating processes of the higher atmosphere. In Pietrow et al. (2020) we add to these measurements by introducing what is to our knowledge the first full Stokes inversion of chromospheric plage, which allowed us to estimate the magnetic field vector at an optical depth of logτ = -3.5. The obtained value is |B| = 440 ± 90 G in the plage with an inclination of 10° ± 16° with respect to the local vertical. Our reported magnetic field strength matches with a recent study by Morosin et al. (2020), but is higher by a factor of two or more compared to previous studies that measured the field using other methods. Additionally we measure an average magnetic field strength of |B| = 300 ± 50 G in a fibrillar region close to the plage. In this thesis we explore the difficulties of measuring this magnetic field vector. Since plage exists in a complex environment, we will begin with a general description of the structure and properties of the solar atmosphere and the layers from which it is composed, as well as review the types of active regions that can be found in the solar atmosphere. Our focus then narrows to the chromosphere, the diagnostic properties of spectral lines that are sensitive to this layer (mainly the \cair line), plage regions, and plage chromospheric magnetic fields. Additionally, we touch upon the theory of radiative transfer and how physical characteristics of the atmosphere can be inferred from polarised light. We also give attention to the observing process with the Swedish 1-m Solar Telescope (SST) and the workings of the reduction pipeline and post-reduction methods as well as the process spectropolarimetric inversions. Finally, once we have understood why and how this project has been done, we summarize our findings and compare them to current literature.

Magnetic fields play a key role in a wide variety of phenomena found on the Sun. One such phenomena is the Coronal Mass Ejection (CME) where a large amount of material is ejected from the Sun. CME’s may directly affect the earth, therefore understanding their origin is of key importance for space weather and the near-Earth environment. In this thesis, the nature and evolution of solar magnetic fields is considered through a combination of Magnetic Flux Transport Simulations and Potential Field Source Surface Models. The Magnetic Flux Transport Simulations produce a realistic description of the evolution and distribution of the radial magnetic field at the level of the solar photosphere. This is then applied as a lower boundary condition for the Potential Field Source Surface Models which prescribe a coronal magnetic field. Using these two techniques, the location and variation of coronal null points, a key element in the Magnetic Breakout Model of CMEs, are determined. Results show that the number of coronal null points follow a cyclic variation in phase with the solar cycle. In addition, they preferentially form at lower latitudes as a result of the complex active latitude field. Although a significant number of coronal nulls may exist at any one time (≈ 17), it is shown that only half may satisfy the necessary condition for breakout. From this it is concluded that while the Magnetic Breakout Model of CMEs is an important model in understanding the origin of the CMEs, other processes must occur in order to explain the observed number of CMEs. Finally, the Magnetic Flux Transport Simulations are applied to stellar magnetic fields and in particular to the fast rotating star HD171488. From this speculative study it is shown that the Magnetic Flux Transport Simulations constructed for the Sun may be applied in very different stellar circumstances and that for HD171488 a significantly higher rate of meridional flow (1200-1400 ms⁻¹) is required to match observed magnetic field distributions.

This thesis investigates the topology of the magnetic field in the solar corona. It is important have an understanding of how the highly complex coronal magnetic field behaves in order to study many fundamental coronal phenomena, such as coronal heating events, solar flares and polar plumes. The magnetic fields due to three or four discrete sources are investigated and the corresponding topological states are found. The locations of these states in parameter space is calculated and the bifurcations between states are analysed. A complete analysis has been undertaken for the three-source case and a selective one for the four-source case in order to identify new non-generic behaviour. The thesis goes on to study the topological behaviour of a coronal bright point. Different phases during the lifetime of the bright point are identified and the responsible topological behaviour due to the movement of the magnetic fragments in the photosphere is discussed.