No description available.

A fundamental problem in astrophysics is the interaction between space plasmas and energetic particles. Plasmas form the vast majority of space and can be found in any astrophysical environment, from the plasma of the solar wind to the interstellar medium for example. When plasmas stream through space, they give rise to turbulent magnetic fields. In addition, space is populated by energetic particles whose origins could be interplanetary, such as solar energetic particles generated by explosions on the surface of the Sun, or Galactic, such as cosmic rays generated by supernova remnants. As these particles propagate through interplanetary or interstellar space, they experience scattering due to magnetic turbulence. Describing these scattering effects, through the calculation of diffusion coefficients, is crucial to understanding several important processes in astrophysics. Such processes include particle acceleration at interplanetary shocks, solar modulation and space weather studies, and the motion of cosmic rays through galaxies. A test-particle code is developed to simulate the interaction of charged particles with turbulent magnetic fields. Diffusion coefficients along and across the mean magnetic field are calculated and compared with analytical theories and space mission measurements. Turbulence models with reduced dimensionality and full three-dimensional corresponding to different space settings are considered, including reproducing the magnetic turbulence in the solar wind and the interstellar medium. Wave propagation, dynamical effects, and unique turbulence setups such as noisy hydrodynamic models are also considered. We show that the influence of turbulence properties and various space regimes on transport parameters is minor and not as strong as originally thought. This points toward a universal transport behaviour of charged particles which motivates the employment of a comprehensive diffusion formula for different space settings. In addition, we were able for the first time to accurately describe the interaction between Galactic cosmic rays and dynamical solar wind turbulence to reproduce observational results of mean free paths. The validity of certain asymptotic limits for the parallel and perpendicular diffusion coefficients are examined. It is shown that only two parameters control those limits, namely the fundamental length scale of magnetic turbulence and the ratio of turbulence strength to the mean field. / February 2017

Under the conditions of quasitransverse propagation of electromagnetic waves in a magnetoionic plasma, the effect analogous to Faraday rotation in the quasilongítudinal case (which we will call Faraday pulsation) can produce a large elliptical polarization of originally linearly polarized radiation. Therefore, the presence of elliptical polarization in the radiation cannot serve as a means for distinguishing between a synchrotron mechanism with high energy electrons and any other type of emission process in radio sources in which conditions suggesting quasi - transverse propagation (large magnetic fields) may take place. The Faraday pulsation has a wavelength dependence through which it can be identified.

Seventy-eight low-altitude passes of MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer data from August to September of 2014 have been applied to produce an approximate map of the crustal magnetic field at a constant altitude of 80 km covering latitudes of 50°–80°N and longitudes of 160°–330°E. The strongest anomalies are located in the westernmost part of the mapped region just north and northeast of the Caloris impact basin. The strongest single anomaly group lies over a smooth plains unit that extends northward from Caloris. A second anomaly lies on the Odin Formation, interpreted as primary Caloris ejecta. A third broad anomaly also lies over a smooth plains unit, Suisei Planitia. Although many smooth plains on Mercury may have a volcanic origin, a subset of these plains is concentrated around Caloris and could have a dominantly impact origin. It is therefore proposed that many of the Mercurian anomaly sources consist of impact basin ejecta materials.

We use a method developed by Roberts. that optimizes the phase angles of an ensemble of plane waves with amplitudes determined from a Kolmogorov-like power spectrum, to construct magnetic field vector fluctuations having nearly constant magnitude and large variances in its components. This is a representation of the turbulent magnetic field consistent with that observed in the solar wind. Charged-particle pitch-angle diffusion coefficients are determined by integrating the equations of motion for a large number of charged particles moving under the influence of forces from our predefined magnetic field. We tested different cases by varying the kinetic energy of the particles (E-p) and the turbulent magnetic field variance (sigma(2)(B)). For each combination of E-p and sigma(2)(B), we tested three different models: (1) the so-called "slab" model, where the turbulent magnetic field depends on only one spatial coordinate and has significant fluctuations in its magnitude (b=root delta B-x(2)(z)+ delta B-y(2)(z) + B-0(2)); (2) the slab model optimized with nearly constant magnitude b; and. (3) the slab model turbulent magnetic field with nearly constant magnitude plus a "variance-conserving" adjustment. In the last case, this model attempts to conserve the variance of the turbulent components (sigma(2)(Bx) + sigma(2)(By)), which is found to decrease during the optimization with nearly constant magnitude. We found that there is little or no effect on the pitch-angle diffusion coefficient D mu mu between. models 1 and 2. However, the result from model 3. is significantly different. We also introduce a new method to accurately determine the pitch-angle diffusion coefficients as a function of mu.

In making this study it is important to search for ways to enhance and, if possible, make detection of MMO signals simpler in order that this technique for obtaining FS measurements may be extended to other materials. This attempt to improve measurement techniques has resulted in a significant discovery: the eddy-current techniques described in detail in a later section which should allow MMO to be observed and sensitively measured in many additional solids. The second major thrust of the study has been to use the newly discovered eddy-current technique in obtaining the first indisputable observation of MMO in zinc.

A thesis submitted to the Department of Physics, University of the Witwatersrand in fulfilment of the academic requirements of the degree of Doctor of Philosophy. Johannesburg, June 17, 2016. / The nature of dark matter remains one of the most significant problems in modern physics. One candidate hypothesis with strong theoretical motivation is a WIMP (Weakly Interacting Massive Particle) in the form of the supersymmetric neutralino. Although this lacks experimental evidence, supersymmetry may manifest in collider experiments and also has observable consequences in the emission of annihilation products from cosmic structures which would play host to large neutralino densities. Historically, interest has been focussed upon the direct detection of WIMPs, as well as indirect observation through -rays produced by pair-annihilation processes. In this work we present a detailed argument for a multi-frequency observational strategy. We motivate this by studying the redshift evolution of radio synchrotron radiation emitted by WIMP annihilation-product electrons. In so doing, we demonstrate the potential power of the SKA to probe the neutralino parameter space, being able to produce constraints several orders of magnitude better than current limits and distinguish between differing neutralino masses and annihilation channels. Furthermore, we motivate the SKA as a machine to study dark matter by discussing its ability to characterise cosmic magnetic fields through rotation measures and polarimetry, as well as resolve between non-thermal emissions produced by dark matter and those resulting from strictly baryonic processes, like star formation. These technical capabilities obviate the central uncertainties in the study of synchrotron radiation. Additionally, we highlight optimal laboratories for neutralino detection via the SKA, based on structures that might produce dark matter emissions with lower background synchrotron radiation. In particular we highlight galaxies and galaxy clusters at redshifts z 1 and local dwarf spheroidal galaxies. Finally, we demonstrate the potential of X-rays, through the ASTRO-H mission, to compliment the SKA in a multi-frequency search. We then demonstrate our multi-frequency approach in the analysis of several prominent claims of signals compatible with dark matter-induced emissions. Showing that models designed to account for the AMS-2/Fermi/PAMELA antiparticle and galactic centre -ray excesses are incompatible with existing data as well as demonstrating that a dark matter interpretation of the Reticulum-2 -ray excess is untenable for a wide range of dark matter particle masses. / LG2017

by Lee Yuk Yan. / Chinese title in romanization: Chang cong xiang ci chang zi you dian zi ji guang fang da qi di jing dian li lun. / Thesis (M.Ph.)--Chinese University of Hong Kong, 1987. / Bibliography: leaves 45-47.

In this thesis, the collective motion of Magnetospirillum magneticum strain AMB-1 is studied. Bacteria have developed various types of mechanism in order to search for the optimum environment, which are called taxis. There are two types of taxis, aerotaxis and magnetotaxis, that are involved in AMB-1. It is commonly believed that the vertical component of the Earth magnetic field helps to align the bacteria and confine them in one dimension, whereas the oxygen gradient determines their swimming velocity. In this way, magnetotaxis is conceived as a passive response, while aerotaxis is active and they work independently. Previous works done by our group may suggest an alternative picture. In this thesis, two experiments were carried out to gain a better understanding of the taxis of AMB-1. In the first experiment, bacterial suspension of density around 10⁸ cells cm⁻³ was injected into a chamber of dimensions 5×5×0.025 mm³. The motion of the bacteria was captured by a video camera mounted in the microscope under darkfield condition. Tracking of the quasi-2D motions under different applied magnetic field generated by two pairs of Helmholtz coils was done. In this way, we could determine the effect of magnetic field on basic physical properties of the bacteria, such as the diffusive motion, the rate of change of the motion direction. In the second experiment, a one dimensional oxygen gradient was achieved inside a microfluidic chamber. An aerotactic band of highly concentrated bacteria was observed under darkfield condition when bacterial suspension of density around 10⁹ cells cm⁻³. It is found that the time for the formation of the band varies under different applied magnetic fields, which suggests that the magnetotaxis in AMB-1 may not be completely passive. / 這篇論文報告關於趨磁螺菌AMB-1在集體運動上的研究。為了在野外尋找最好的生存環境，細菌發展出不同的趨向性機制。AMB-1擁有趨磁性和趨氧性兩個機制。在普遍的認知裡，地球磁場的垂直分量會把牠們的運動局限為一維，氧氣濃度梯度則影響牠們游泳的速度。趨磁性一直被認為是被動反應，而趨氧是主動的，兩者獨立運作。我課題組最近的實驗結果提出了存在另一種機制的可能性。為更了解AMB-1的趨向性機制，我們進行了兩個實驗。 / 在首個實驗中，濃度約為每立方厘米10⁸個的細菌溶液被注入到5×5×0.025立方毫米的空間。我們用攝像機把顯微鏡暗場觀測下的細菌運動記錄下來，在兩對亥姆霍茲線圈產生的不同磁場下，追蹤細菌的準二維運動。這樣，我們可以測定細菌的基本物理性質在不同磁場下的影響，例如其擴散運動、運動方向的轉換頻率等等。在第二個實驗中，我們在一個微流控空間裡建立了一維的氧氣濃度梯度。當細菌懸液濃度達到每立方厘米10⁹個時，在暗場條件下可以一條由高濃度細菌組成的趨氧性帶。我們發現帶形成所需的時間隨不同的外加磁場而改變，顯示AMB-1的趨磁性可能不是一個完全被動的反應。 / Lau, Tsz Ki = 趨磁螺菌AMB-1的集體運動觀測 / 劉子麒. / Thesis M.Phil. Chinese University of Hong Kong 2013. / Includes bibliographical references (leaves 53-58). / Abstracts also in Chinese. / Title from PDF title page (viewed on 12, October, 2016). / Lau, Tsz Ki = Qu ci luo jun AMB-1 de ji ti yun dong guan ce / Liu Ziqi. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.

Massive young stars dynamically modify their surroundings, altering their stellar nurseries and the gas that exists between stars. With my research, I assess the modification of the Galactic magnetic field within HII regions and stellar bubbles associated with OB stars. Because HII regions are plasmas, magnetic fields should be important to the dynamics of the region. Understanding how the magnetic field is modified in these structures is critical for inputs to simulations and for assessing stellar feedback. To obtain information on the properties of the magnetic field, I measure the Faraday rotation of linearly polarized radio waves that pass through the plasma of the HII region. In this thesis, I present results of Faraday rotation studies of two Galactic \HII regions. The first is the Rosette Nebula (l = 206 deg, b = -1.2 deg), and the second is IC 1805 (l = 135 deg, b = 0.9 deg), which is associated with the W4 Superbubble. I measure positive rotation measure (RM) values in excess of +40 to +1200 rad m^-2 due to the shell of the Rosette nebula and a background RM of +147 rad m^-2 due to the general interstellar medium in this area of the Galactic plane. In the area of IC 1805, I measure negative RM values between +600 and --800 rad m^-2 due to the HII region. The sign of the RM across each HII region is consistent with the expected polarity of the large-scale Galactic magnetic field that follows the Perseus spiral arm in the clockwise direction, as suggested by Van Eck et al. (2011, ApJ, 728, 14). I find that the Rosette Nebula and IC 1805 constitute a "Faraday rotation anomaly", or a region of increased RM relative to the general Galactic background value. Although the RM observed on lines of sight through the region vary substantially, the |RM| due to the nebula is commonly 100 -- 1000 rad m^-2. In spite of this, the observed RMs are not as large as simple, analytic models of magnetic field amplification in HII regions (such as by magnetic flux conservation in a swept-up shell) might indicate. This suggests that the Galactic field is not increased by a substantial factor within the ionized gas in an HII region. Finally, these results show intriguing indications that some of the largest values of |RM| occur for lines of sight that pass outside the fully ionized shell of the IC 1805 HII region, but pass through the Photodissociation Region (PDR) associated with IC 1805.