Magnetohydrodynamic oscillations in the solar corona and Earth's magnetosphere : towards consolidated understanding

Magnetohydrodynamic (MHD) oscillatory processes in different plasma systems, such as the corona of the Sun and the Earth’s magnetosphere, show interesting similarities and differences, which so far received little attention and remain underexploited. The successful commissioning within the past ten years of THEMIS, Hinode, STEREO and SDO spacecraft, in combination with matured analysis of data from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI) makes it very timely to survey the breadth of observations giving evidence for MHD oscillatory processes in solar and space plasmas, and state-of-the-art theoretical modelling. The paper reviews several important topics, such as Alfv´enic resonances and mode conversion; MHD waveguides, such as the magnetotail, coronal loops, coronal streamers; mechanisms for periodicities produced in energy releases during substorms and solar flares, possibility of Alfv´enic resonators along open field lines; possible drivers of MHD waves; diagnostics of plasmas with MHD waves; interaction of MHD waves with partlyionised boundaries (ionosphere and chromosphere). The review is mainly oriented to specialists in magnetospheric physics and solar physics, but not familiar with specifics of the adjacent research fields

Magnetohydrodynamic Oscillations in the Solar Corona and Earth's Magnetosphere: Towards Consolidated Understanding

Magnetohydrodynamic (MHD) oscillatory processes in di�erent plasma systems, such as the corona of the Sun and the Earth's magnetosphere show interesting similarities and di�erences, which so far received little attention and remain underexploited. The successful commissioning within the past ten years of SDO, Hinode, STEREO and THEMIS spacecraft, in combination with matured analysis of data from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI) makes it very timely to survey the breadth of observations giving evidence for MHD oscillatory processes in solar and space plasmas, and state-of-the-art theoretical modelling. The paper reviews several important topics, such as Alfv�enic resonances and mode conversion; MHD waveguides, such as the magnetotail, coronal loops, coronal streamers; mechanisms for periodicities produced in energy releases during substorms and solar flares, possibility of Alfv�enic resonators along open �eld lines; possible drivers of MHD waves; diagnostics of plasmas with MHD waves; interaction of MHD waves with partly-ionised boundaries (ionosphere and chromosphere). The review is mainly oriented to specialists in magnetospheric physics and solar physics, but not familiar with speci�cs of the adjacent research �elds

Imaging Electrical Properties Using MRI and In Vivo Applications

University of Minnesota Ph.D. dissertation. November 2015. Major: Biomedical Engineering. Advisor: Bin He. 1 computer file (PDF); viii, 137 pages.Electrical properties, namely conductivity and permittivity, describe the interaction of materials with the surrounding electromagnetic field. The electrical properties of biological tissue are associated with many fundamental aspects of tissue, such as cellular and molecular structure, ion concentration, cell membrane permeability, etc. Electrical properties of tissue in vivo can be used as biomarkers to characterize cancerous tissue or provide useful information in applications involving tissue and electromagnetic field. Among many related electrical-property imaging technologies, electrical properties tomography (EPT) is a promising one that noninvasively extracts the in vivo electrical properties with high spatial resolution based on measured B1 field using magnetic resonance imaging (MRI). In this thesis, advanced EPT methods have been developed to improve the imaging quality of conventional EPT. First of all, a multi-channel EPT framework was introduced to release its dependency on a B1 phase assumption and expand its application under high field strength. Secondly, a gradient-based EPT (gEPT) approach was proposed and implemented, showing enhanced robustness against effect of measurement noise and improved performance near tissue boundaries. Using gEPT, high resolution in vivo electrical-property images of healthy human brain were obtained, and an imaging system for rat tumor models was also developed. As a result of malignancy, increased conductivity was captured in tumors using the in vivo animal imaging system. Thirdly, based on EPT theory, quantitative water proton density imaging was proposed using measured B1 field information, provide a new way for estimating water content in tissue for diagnostic and research purpose

Quantitative MR Imaging of the Electric Properties and Local SAR based on Improved RF Transmit Field Mapping

This work presents three new quantitative methods for magnetic resonance imaging. A method for simultaneous mapping of B1 and T1 (MTM) is developed and validated. Electric Properties Tomography (EPT), a method for quantitative imaging of dielectric properties of tissue, is presented. Based on EPT, separate (phase-based) conductivity and (amplitude-based) permittivity measurements are introduced. Finally, a B1-based method for patient-specific local SAR measurements is presented

Quantitative MR Imaging of the Electric Properties and Local SAR based on Improved RF Transmit Field Mapping

This work presents three new quantitative methods for magnetic resonance imaging. A method for simultaneous mapping of B1 and T1 (MTM) is developed and validated. Electric Properties Tomography (EPT), a method for quantitative imaging of dielectric properties of tissue, is presented. Based on EPT, separate (phase-based) conductivity and (amplitude-based) permittivity measurements are introduced. Finally, a B1-based method for patient-specific local SAR measurements is presented

Origin and Evolution of Large-scale Magnetic Fields

Magnetic elds are ubiquitous at all scales in the Universe and have been observed in galaxies and clusters of galaxies via observations of di use radio emission and Faraday Rotation Measures. Despite the observations, the origin and impact of the magnetic elds in these systems is poorly understood. In this thesis we develop a state of the art cosmological Smoothed Particle Magnetohydrodynamics code, GCMHD+, to enable the study of the magnetic elds of the largest bound structures in the Universe. Using a wide range of idealized test problems, we justify our choice of free parameters and demonstrate the performance of the code relative to analytical solutions and the results produced by a grid based MHD scheme. We then used the code to investigate the evolution of a seed magnetic eld due to the formation of structure. By varying the numerical scheme, we demonstrate that the growth of magnetic elds in galaxy clusters are very sensitive to the growth of numerical divergence of the magnetic eld. We nd that amplitude and topology of the cluster magnetic eld are insensitive to the mass or formation history of the cluster. Using high resolution simulations, we show that a primordial seed magnetic eld is capable of reproducing a wide range of observations of large-scale magnetic elds in galaxy clusters. Additionally, we examine the impact of the formation of spiral structure in a disc galaxy on the galactic magnetic eld. We nd that the numerical scheme can become unstable unless the divergence cleaning scheme is limited. We nd that the rotation of the galaxy produces a disc orientated magnetic eld with a spiral structure and large-scale eld reversals. The formation of spiral arms ampli es the ambient G magnetic eld to 20 G, in agreement with the observations of spiral galaxies. We conclude that additional physics is required to produce a more realistic galactic magnetic eld

Multi-modal MHD oscillations in the solar corona, and their use in coronal seismology

The solar atmosphere is a dynamic, inhomogeneous environment which acts as a natural plasma laboratory for a keen community of observers and researchers at the forefront of modern physics. Colossal plasma non-uniformities on the Sun are seen to host a wide variety of magnetoacoustic oscillatory motions, which may be used as probes into the local plasma conditions using the theory of long wavelength, large scale magnetohydrodynamics (MHD): this process is known as coronal seismology. The focus of this thesis is to contribute to the detailed observation of these waves and their use in coronal seismology, particularly the usefulness of observing multiple harmonics and understanding of dispersion. Fast kink-mode oscillations of coronal loops, observed as rapidly decaying transverse displacements, are a well-understood wave mode used for seismology. The simultaneous detection of multiple harmonics can provide more information, allowing one to match the observed dispersion with that predicted by theory. Extreme ultraviolet observations of a coronal loop hosting a standing kink oscillation are analysed using image processing and time series techniques. The presence of two simultaneous harmonics is revealed, a fundamental mode at a period of ∼ 8 minutes and its third harmonic at ∼ 2.6 minutes. The ratio of periods P1/3P3 was found to be ∼ 0.87, whose departure from unity indicates a non-uniform distribution of kink speed through the loop. For all locations, the ratio of damping time to period for the two harmonics were found to agree within error, validating the widely assumed 1d resonant absorption theory used to explain a kink oscillation’s rapid damping. This is the first time a measurement of the signal quality for a higher harmonic of a kink oscillation has been reported with spatially resolved data. One exciting development in coronal seismology is the recent detection of decay-less oscillations, which are a different regime of fast-kink oscillations omnipresent in coronal loops. The first detection of a coronal loop exhibiting multi-modal decay-less oscillations is presented, in which both the loop’s fundamental mode (P1 = 10.3 +1.5 −1.7 minutes) and its second harmonic (P2 = 7.4 +1.1 −1.3 minutes) are detected. To make this detection possible, the observational data was passed through a novel motion magnification algorithm to accentuate transverse oscillations. An illustration of seismology using the ratio P1/2P2 ∼ 0.7 to estimate the density scale height is presented. The existence of multiple harmonics has implications for understanding the driving and damping mechanisms for decay-less oscillations, and adds credence to their interpretation as standing kink mode oscillations. There is a myriad of MHD oscillation modes, and whilst fast-kink modes are observed as transverse displacements of the plasma non-uniformity, slow modes may be observed as intensity enhancements. Analysis of such propagating slow modes observed in a fan of coronal loops above a sunspot is performed. The instantaneous velocities and periods of these intensity enhancements are measured and compared in different temperature passbands and azimuthal angles. The waves seen in the 171˚A channel (∼ 0.6 MK) appeared slower than when observed co-spatially in the 193˚A (∼ 1.58 MK). This contradicts the expectation that the phase speed is approximately the local sound speed, which varies as the square root of the temperature. This discrepancy is resolved by attributing the difference in apparent velocity to different inclination angles, which are estimated to be 9° ± 3° from the vertical for the waves seen in 193 A, and 19° ± 4° when seen in 171 A. This provides some evidence supporting the theory that coronal loops are formed of several distinct, unresolved strands of different temperature. From the theoretical point of view, the dispersion relation governing slow MHD modes in the presence of a wave-induced misbalance between the plasma heating and cooling mechanisms is developed. The thin flux tube approximation is used to account for finite-β effects, and thermal conduction is also included. The dispersion relation in the limits of weak non-adiabaticity and strong non-adiabaticity with finite-β is identified. It is found that the characteristic timescales of this imbalance (e.g. damping time) may be expressed in terms of the partial derivatives of the combined heating/cooling function with respect to constant gas pressure and constant magnetic pressure. Moreover, these characteristic timescales for the thermal misbalance coincide with typical MHD wave periods for a large range of densities and temperatures typical of the corona. Thus in the general case the dispersion on slow waves by the wave-induced thermal misbalance should not be neglected, and its inclusion may resolve some contradictions that have arisen when attributing the rapid damping of slow modes to thermal conduction or compressive viscosity alone. Instability criteria for the slow mode and entropy (thermal) mode are expressed in terms of a parameterisation of the unknown coronal heating function, under this thin flux tube approximation. Finally, noting that observations of slow modes in the corona do not show over-stability, and that the thermal mode does not appear to be unstable in general (with the exception of coronal rain), a new way of constraining the coronal heating function is presented