Published online: 26 April 2013

Knowledge of the coronal magnetic field is crucial for understanding (1) the heating mechanism(s) of the solar corona, (2) the acceleration of the fast solar wind, and (3) the structure and dynamics of coronal mass ejections (CMEs). Observation of Faraday rotation (FR) is one of the best remote-sensing techniques for determining plasma properties in the corona and can provide information on the plasma structure of a CME shortly after launch, shedding light on the initiation process. I used the Karl G. Jansky Very Large Array (VLA) to make sensitive Faraday rotation measurements to investigate the general plasma structure of the corona, properties of coronal plasma inhomogeneities and waves, and transients associated with coronal mass ejections. To enhance my measurements of FR transients, I also developed an algorithm in the Common Astronomy Software Applications (CASA) package to mitigate ionospheric Faraday rotation. In August, 2011, I made FR observations at 5.0 and 6.1 GHz of the radio galaxy 3C 228 through the solar corona at heliocentric distances of 4.6 - 5.0 solar radii using the VLA. Observations at 5.0 GHz permit measurements deeper in the corona than previous VLA observations at 1.4 and 1.7 GHz. These FR observations provided unique information on the magnetic field in this region of the corona. My data on 3C 228 provide two lines of sight (separated by 46 arcseconds, 33,000 km in the corona). I detected three periods during which there appeared to be a difference in the Faraday rotation measure between these two closely spaced lines of sight, which I used to estimate coronal currents; these values (ranging from 2.6 to 4.1 GA) are several orders of magnitude below that which is necessary for significant coronal heating (assuming the Spitzer resistivity). I also used the data to determine upper limits (3.3 and 6.4 rad/m⁻²along the two lines of sight) on FR fluctuations caused by coronal waves. These upper limits are comparable to and, thus, not inconsistent with the theoretical models for Alfvén wave heating of the corona by Hollweg et al. (2010). To support the needs of the low frequency radioastronomical community as well as my own research of coronal FR transients, I developed a new calibration algorithm for CASA that uses GPS-based global ionosphere maps of the Total Electron Content (TEC) to mitigate ionospheric Faraday rotation. The Earth's ionosphere introduces direction- and time-dependent effects over a range of physical and temporal scales and so is a major source for unmodeled phase offsets for low frequency radioastronomical observations. It has become common practice to use global ionospheric models derived from the Global Positioning System (GPS) to provide a means of externally calibrating low frequency data. However, CASA, which was developed to meet the data post-processing needs of next generation telescopes such as the VLA and the Atacama Large Millimeter/submillimeter Array (ALMA), did not have the capability to make ionospheric corrections before I implemented this calibration algorithm. I investigated several data centers as potential sources for global ionospheric models and chose the International Global Navigation Satellite System Service data product because data from other sources are generally too sparse to use without additional interpolation schemes. I employed these ionospheric corrections in reducing VLA observations made in August, 2012, at 1 - 2 GHz of a “constellation” of radio sources through the solar corona at heliocentric distances that ranged from 5 - 15 solar radii. Of the nine sources observed, three were occulted by CMEs: 0842+1835, 0900+1832, and 0843+1547. In addition to my radioastronomical observations, which represent one of the first active hunts for CME Faraday rotation since Bird et al. (1985) and the first active hunt using the VLA, I obtained white-light coronagraph images from the LASCO/C3 instrument aboard SOHO to determine the Thomson scattering brightness, BT. BT is proportional to the electron plasma density and provides a means to independently estimate the plasma density and determine its contribution to the observed Faraday rotation. A constant density force-free flux rope embedded in the background corona was used to model the effects of the CMEs on BT and FR. In the case of 0842+1835, the flux rope model underestimated the peak value in BT and did not reproduce the decreasing BT inside the inner cavity region of the CME; however, there was satisfactory agreement between the model and the observed FR. The single flux rope model successfully reproduces both the observed BT and FR profiles for 0900+1832. 0843+1547 was occulted by two CMEs. Therefore, I modeled observations of 0843+1547 using two flux ropes embedded in the background corona. The two flux rope model successfully reproduces both BT and FR profiles for 0843+1547 and, in particular, the two flux rope model successfully replicates the appropriate slope in FR before and after occultation by the second CME and predicts the observed change in sign to FR > 0 at the end of the observing session. I briefly discuss the plasma densities ( 6 - 22 x 10³ cm⁻³) and axial magnetic field strengths (2 - 12 mG) inferred from my models and compare them to the modeling work of Liu et al. (2007) and Jensen et al. (2008), as well as previous CME FR observations by Bird et al. (1985).

Yes / In this work we study the association between eruptive filaments/prominences and coronal mass ejections (CMEs) using machine learning-based algorithms that analyse the solar data available between January 1996 and December 2001. The Support Vector Machine (SVM) learning algorithm is used for the purpose of knowledge extraction from the association results. The aim is to identify patterns of associations that can be represented using SVM learning rules for the subsequent use in near real-time and reliable CME prediction systems. Timing and location data in the NGDC filament catalogue and the SOHO/LASCO CME catalogue are processed to associate filaments with CMEs. In the previous studies which classified CMEs into gradual and impulsive CMEs, the associations were refined based on CME speed and acceleration. Then the associated pairs were refined manually to increase the accuracy of the training dataset. In the current study, a data- mining system has been created to process and associate filament and CME data, which are arranged in numerical training vectors. Then the data are fed to SVMs to extract the embedded knowledge and provide the learning rules that could have the potential, in the future, to provide automated predictions of CMEs. The features representing the event time (average of the start and end times), duration, type and extent of the filaments are extracted from all the associated and not-associated filaments and converted to a numerical format that is suitable for SVM use. Several validation and verification methods are used on the extracted dataset to determine if CMEs can be predicted solely and efficiently based on the associated filaments. More than 14000 experiments are carried out to optimise the SVM and determine the input features that provide the best performance.

The main objective of this thesis is to model the magneticstructure of interplanetary coronal mass ejections (ICME) measuredin-situ from the WIND spacecraft positioned at L1. The modeling isdone by a magnetohydrodynamic reconstruction technique based onthe GS equation with a toroidal geometry. The purpose has been toextend the application of the reconstruction program to real dataand to test its performance when different input parameters arechanged. Two events are presented; 16-17 May 2012 and 15-16 May2005 ICMEs have been successfully reconstructed with this model. The main achievements of the study are that a) the code now worksfor real data b) the important parameters that can be changed fordifferent reconstructions in the code are the number of iterationsused to find the optimal Z-axis, the plasma pressure and the orderof the polynomial fitting of the flux functional, c) if all crosssection reconstructions for different variations of theseparameters strongly resembles each other then this is anindication that the model approximation is good and that the fluxrope exists. The results have been compared and verified withpreviously published studies of these events. Using a toroidal geometry for the GS reconstruction method weobtain very similar results to the one obtained with differentreconstruction techniques.This implies that at L1, the ICMEs haveexpanded so much that a cylindrical geometry is sufficient todescribe the flux rope geometry. The toroidal Grad-Shafranovreconstruction technique is best suited for circular, or slightlyelongated, flux rope cross section profiles but have been provento work for one complex ejecta consisting of two merged fluxropes. The toroidal model might become an important asset in thefuture when data from spacecraft closer to the Sun, such as ParkerSolar Probe and Solar Orbiter, is public. When the major radius ofthe flux rope is smaller the choice of geometry will most likelyhave a larger role than for measurements at L1 and so, thetoroidal Grad-Shafranov reconstruction technique will probably bethe better alternative of the models that exists today.

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.

Observations show that Coronal Mass Ejections (CMEs) are associated with twisted magnetic flux configurations. Conventionally, CMEs are modeled by shearing and twisting the footpoints of a certain distribution of magnetic flux at the solar surface and letting it evolve at the surface. Of course, the surface velocities and magnetic field patterns should ultimately be obtained from realistic simulations of the solar convection zone where the field is generated by dynamo action. Therefore, a unified treatment of the convection zone and the CMEs is needed. Numerical simulations of turbulent dynamos show that the amplification of magnetic fields can be catastrophically quenched at magnetic Reynolds numbers typical of the interior of the Sun. A strong flux of magnetic helicity leaving the dynamo domain can alleviate this quenching. In this sense, a realistic (magnetic) boundary condition is an important ingredient of a successful solar dynamo model. Using a two-layer model developed in this thesis, we combine a dynamo-active region with a magnetically inert but highly conducting upper layer which models the solar corona. In four steps we improve this setup from a forced to a convectively driven dynamo and from an isothermal to a polytropic stratified corona. The simulations show magnetic fields that emerge at the surface of the dynamo region and are ejected into the coronal part of the domain. Their morphological form allows us to associate these events with CMEs. Magnetic helicity is found to change sign in the corona to become consistent with recent helicity measurements in the solar wind. Our convection-driven dynamo model with a coronal envelope has a solar-like differential rotation with radial (spoke-like) contours of constant rotation rate, together with a solar-like meridional circulation and a near-surface shear layer. The spoke-like rotation profile is due to latitudinal entropy gradient which violates the Taylor--Proudman balance through the baroclinic term. We find mean magnetic fields that migrate equatorward in models both with and without the coronal layer. One remarkable result is that the dynamo action benefits substantially from the presence of a corona becoming stronger and more realistic. The two-layer model represents a new approach to describe the generation of coronal mass ejections in a self-consistent manner. On the other hand, it has important implications for solar dynamo models as it admits many magnetic features observed in the Sun. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Manuscript; Paper 6: Manuscript.</p>

No description available.

Observations by spacecraft such as ACE, STEREO, and others show that there are proton spectral "breaks" with energy E-br at 1-10 MeV in some large CME-driven shocks. Generally, a single shock with the diffusive acceleration mechanism would not predict the "broken" energy spectrum. The present paper focuses on two converging shocks to identify this energy spectral feature. In this case, the converging shocks comprise one forward CME-driven shock on 2006 December 13 and another backward Earth bow shock. We simulate the detailed particle acceleration processes in the region of the converging shocks using the Monte Carlo method. As a result, we not only obtain an extended energy spectrum with an energy "tail" up to a few 10 MeV higher than that in previous single shock model, but also we find an energy spectral "break" occurring on similar to 5.5 MeV. The predicted energy spectral shape is consistent with observations from multiple spacecraft. The spectral "break," then, in this case is caused by the interaction between the CME shock and Earth's bow shock, and otherwise would not be present if Earth were not in the path of the CME.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20181号 / 理博第4266号 / 新制||理||1613(附属図書館) / 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 柴田 一成, 教授 一本 潔, 准教授 浅井 歩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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.