Constraining Primordial Magnetism

Primordial magnetic fields could provide an explanation for the galactic magnetic fields observed today, in which case they may also leave interesting signals in the CMB and the small-scale matter power spectrum. We discuss how to approximately calculate the important non-linear magnetic effects within the guise of linear perturbation theory, and calculate the matter and CMB power spectra including the SZ contribution. We then use various cosmological datasets to constrain the form of the magnetic field power spectrum. Using solely large-scale CMB data (WMAP7, QUaD and ACBAR) we find a 95% CL on the variance of the magnetic field at 1 Mpc of B_\lambda < 6.4 nG. When we include SPT data to constrain the SZ effect, we find a revised limit of B_\lambda < 4.1 nG. The addition of SDSS Lyman-alpha data lowers this limit even further, roughly constraining the magnetic field to B_\lambda < 1.3 nG.Comment: 12 pages, 9 figure

Waves on the dusk flank boundary layer during very northward interplanetary magnetic field conditions: Observations and simulation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95617/1/jgra18641.pd

Multi-instrument observations of a failed flare eruption associated with MHD waves in a loop bundle

Context. We present observations of a B7.9 class flare that occurred on January 24th, 2015, using SDO/AIA, Hinode/EIS and XRT. The flare triggers an eruption of a dense cool plasma blob as seen in AIA 171Å which is unable to completely break out and remains confined within a local bundle of active region loops. During this process, transverse oscillations of the threads are observed. The cool plasma is then observed to descend back to the chromosphere along each loop strand. At the same time, a larger diffuse co-spatial loop observed in the hot wavebands of SDO/AIA and Hinode/XRT is formed, exhibiting periodic intensity variations along its lenght. Aims. The formation and evolution of magnetohydrodynamic (MHD) waves depend upon the values of the local plasma parameters (e.g., density, temperature, magnetic field) which can hence be inferred by coronal seismology. In this study we aim to assess how the observed MHD modes are affected by the variation of density and temperature. Methods. We combine analysis of EUV/X-ray imaging and spectroscopy using SDO/AIA, Hinode/EIS and XRT. Results. The transverse oscillations of the cool loop threads are interpreted in terms of vertically polarised kink oscillations. The fitting procedure provides estimates for the period of about 3.5–4 min, and the amplitude of ∼ 5 Mm. The oscillations are strongly damped showing very low quality factor (1.5–2), which is defined as the ratio of the damping time and the oscillation period. The weak variation of the period of the kink wave, which is estimated from the fitting analysis, is in agreement with the density variations due to the presence of the plasma blob inferred from the intensity light curve at 171Å. The coexisting intensity oscillations along the hot loop are interpreted as a slow MHD wave with the period of 10 min and phase speed of about 436 km s−1 . Comparison between the fast and slow modes allows for the determination of the Alfvén speed, and consequently magnetic field values. The plasma-β inferred from the analysis is estimated to be around 0.1–0.3. Conclusions. We show that the evolution of the detected waves is determined by the temporal variations of the local plasma parameters, caused by the flare heating and the consequent cooling. We apply coronal seismology to both waves obtaining estimations of the background plasma parameter

Nonlinear dynamics of electromagnetic turbulence in a nonuniform magnetized plasma

By using the hydrodynamic electron response with fixed (kinetic) ions along with Poisson's equation as well as Ampere's law, a system of nonlinear equations for low-frequency (in comparison with the electron gyrofrequency) long-(short-) wavelength electromagnetic waves in a nonuniform resistive magnetoplasma has been derived. The plasma contains equilibrium density gradient and sheared equilibrium plasma flows. In the linear limit, local dispersion relations are obtained and analyzed. It is found that sheared equilibrium flows can cause instability of Alfven-like electromagnetic waves even in the absence of a density gradient. Furthermore, it is shown that possible stationary solutions of the nonlinear equations without dissipation can be represented in the form of various types of vortices. On the other hand, the temporal behavior of our nonlinear dissipative systems without the equilibrium density inhomogeneity can be described by the generalized Lorenz equations which admit chaotic trajectories. The density inhomogeneity may lead to even qualitative changes in the chaotic dynamics. The results of our investigation should be useful in understanding the linear and nonlinear properties of nonthermal electromagnetic waves in space and laboratory plasmas. (C) 1998 American Institute of Physics.5361662

Cosmic-ray propagation in simulations of cross-helical plasma turbulence

Turbulence is a ubiquitous phenomenon in astrophysical plasmas. Most of these systems exhibit a property called cross helicity, a non-zero correlation between velocity fluctuations and magnetic-field fluctuations. In the presence of a magnetic mean-field, such as in the solar wind or in the interstellar medium, cross helicity is equivalent to an imbalance between Alfven waves co- and counter-propagating with respect to the mean-field direction. Although this imbalance can have a dramatic influence on the heating and scattering rate of charged particles which propagate through the plasma, it is often neglected in computational studies of turbulent particle transport. In an effort to remedy this situation, we present numerical simulations of magnetohydrodynamic turbulence in which we can control the energy and the cross helicity of the system, without injecting kinetic or magnetic helicity as an unwanted side effect. Varying the strength of a magnetic guide-field allows us to determine the degree of anisotropy that the system assumes as a steady-state configuration. Detailed analysis proves that these simulations conform to theoretical models of realistic turbulence. The diffusion of cosmic-ray particles in turbulent plasmas is often calculated using quasilinear theory and a simplified description of the electromagnetic-field spectra. By computing the trajectories of test-particles in dynamically evolving turbulence simulations with non-zero cross helicity, we study whether such quasilinear predictions of the heating rate of charged particles are valid under realistic conditions. Theory and numerical results agree well for particles propagating at the Alfven velocity, unless resistive effects play a dominant role. Furthermore, strongly anisotropic field configurations are used to compare quasilinear pitch-angle diffusion coefficients with measurements of test-particle scattering after one gyroperiod. In particular, we focus on the scaling of the scattering rate with cross helicity. We observe excellent agreement in simulations of both balanced and imbalanced turbulence and explain the role of the magnetic moment, an approximate invariant of charged-particle motion, for pitch-angle scattering on timescales of several gyroperiods.Turbulenz ist in astrophysikalischen Plasmen allgegenwärtig. Viele solche Systeme weisen eine sogenannte Kreuz-Helizität auf, also eine von Null verschiedene Korrelation zwischen Geschwindigkeits- und Magnetfeld-Fluktuationen. In einer anisotropen Magnetfeldgeometrie, z. B. im Sonnenwind oder dem interstellaren Medium, deutet die Kreuz-Helizität auf ein Ungleichgewicht zwischen Alfven-Wellen, die sich in Richtung des gemittelten Feldes ausbreiten, und solchen, die in die Gegenrichtung propagieren, hin. Obwohl dieses Ungleichgewicht die stochastische Beschleunigung und Streuung, die geladene Teilchen in einem Plasma erfahren, dramatisch beeinflusst, wurde es in bisherigen numerischen Studien über turbulenten Teilchentransport gemeinhin außer Acht gelassen. In dieser Arbeit nun werden rechnergestützte Simulationen von magnetohydrodynamischer Turbulenz präsentiert, in denen die Energie und die Kreuz-Helizität kontrolliert werden können, ohne jedoch kinetische oder magnetische Helizität als unerwünschte Nebenwirkung zu erzeugen. Die Stärke des mittleren Magnetfeldes bestimmt dabei die Anisotropie des Gleichgewichtszustandes. Die Simulationen erfüllen in allen Parameterbereichen die Vorhersagen, die theoretische Modelle für realistische Plasmaturbulenz treffen. Die Diffusion kosmischer Strahlung in turbulenten Plasmen wird häufig im Rahmen der quasilinearen Theorie unter Heranziehung eines stark vereinfachten Turbulenzspektrums berechnet. Indem die Trajektorien von Testteilchen in dynamischen Turbulenzsimulationen mit Kreuz-Helizität berechnet werden, lassen sich quasilineare Ergebnisse für die Beschleunigungsrate geladener Teilchen nachprüfen. Theorie und numerische Simulation stimmen für Teilchen mit der Alfven-Geschwindigkeit gut überein, solange resistive Effekte vernachlässigt werden können. Weiterhin werden aus der quasilinearen Theorie berechnete Diffusionskoeffizienten mit numerisch ermittelten Streuraten für Testteilchen nach einer Gyroperiode in stark anisotropen Feldkonfigurationen verglichen, wobei der Schwerpunkt erneut beim Einfluss der Kreuz-Helizität liegt. Für alle verwendeten Werte der Kreuz-Helizität ergibt sich eine exzellente Übereinstimmung zwischen Simulationsergebnis und Vorhersage. Schließlich wird die Rolle des magnetischen Moments, einer adiabatischen Invarianten bei der Bewegung geladener Teilchen in einem Magnetfeld, für die Streuung über Zeitskalen von mehreren Gyroperioden erläutert

A Statistical Study of the Solar Wind Dependence of Multi-Harmonic Toroidal ULF Waves Observed by the Arase Satellite

Toroidal standing Alfvén wave is one of the ultra-low frequency waves that are frequently observed in the terrestrial magnetosphere. They sometimes exhibit multi-harmonic frequency spectra, indicating wide energy range input in the magnetosphere. However, their energy source has not been fully understood due to the lack of statistical studies. Here we used the data of the Arase satellite observations for ∼3.5 years and conducted a statistical analysis of the solar wind dependence of the occurrence rate, wave power, and frequency of the multi-harmonic toroidal waves. We automatically detected the multi-harmonic waves and categorized them into four groups according to the solar wind velocity and the cone angle of the interplanetary magnetic field. We found that the occurrence rate and wave power of the multi-harmonic waves increase with the solar wind velocity on the flank sides. In the noon sector, the occurrence rate of the multi-harmonic waves increases with the decrease of the cone angle. The median frequency of the multi-harmonic waves on the dayside is positively correlated with the upstream wave frequency predicted by the theory of the ion beam instability for a small cone angle. The occurrence rate also increases with the solar wind dynamic pressure fluctuations. Therefore, we suggest that the Kelvin-Helmholtz instability, the upstream waves, and the dynamic pressure fluctuations are possible sources of the multi-harmonic waves. This study sheds light on the activity of the multi-harmonic waves which can affect radiation belt electrons under various solar wind conditions

Cosmic-ray propagation in simulations of cross-helical plasma turbulence

Turbulence is a ubiquitous phenomenon in astrophysical plasmas. Most of these systems exhibit a property called cross helicity, a non-zero correlation between velocity fluctuations and magnetic-field fluctuations. In the presence of a magnetic mean-field, such as in the solar wind or in the interstellar medium, cross helicity is equivalent to an imbalance between Alfven waves co- and counter-propagating with respect to the mean-field direction. Although this imbalance can have a dramatic influence on the heating and scattering rate of charged particles which propagate through the plasma, it is often neglected in computational studies of turbulent particle transport. In an effort to remedy this situation, we present numerical simulations of magnetohydrodynamic turbulence in which we can control the energy and the cross helicity of the system, without injecting kinetic or magnetic helicity as an unwanted side effect. Varying the strength of a magnetic guide-field allows us to determine the degree of anisotropy that the system assumes as a steady-state configuration. Detailed analysis proves that these simulations conform to theoretical models of realistic turbulence. The diffusion of cosmic-ray particles in turbulent plasmas is often calculated using quasilinear theory and a simplified description of the electromagnetic-field spectra. By computing the trajectories of test-particles in dynamically evolving turbulence simulations with non-zero cross helicity, we study whether such quasilinear predictions of the heating rate of charged particles are valid under realistic conditions. Theory and numerical results agree well for particles propagating at the Alfven velocity, unless resistive effects play a dominant role. Furthermore, strongly anisotropic field configurations are used to compare quasilinear pitch-angle diffusion coefficients with measurements of test-particle scattering after one gyroperiod. In particular, we focus on the scaling of the scattering rate with cross helicity. We observe excellent agreement in simulations of both balanced and imbalanced turbulence and explain the role of the magnetic moment, an approximate invariant of charged-particle motion, for pitch-angle scattering on timescales of several gyroperiods.Turbulenz ist in astrophysikalischen Plasmen allgegenwärtig. Viele solche Systeme weisen eine sogenannte Kreuz-Helizität auf, also eine von Null verschiedene Korrelation zwischen Geschwindigkeits- und Magnetfeld-Fluktuationen. In einer anisotropen Magnetfeldgeometrie, z. B. im Sonnenwind oder dem interstellaren Medium, deutet die Kreuz-Helizität auf ein Ungleichgewicht zwischen Alfven-Wellen, die sich in Richtung des gemittelten Feldes ausbreiten, und solchen, die in die Gegenrichtung propagieren, hin. Obwohl dieses Ungleichgewicht die stochastische Beschleunigung und Streuung, die geladene Teilchen in einem Plasma erfahren, dramatisch beeinflusst, wurde es in bisherigen numerischen Studien über turbulenten Teilchentransport gemeinhin außer Acht gelassen. In dieser Arbeit nun werden rechnergestützte Simulationen von magnetohydrodynamischer Turbulenz präsentiert, in denen die Energie und die Kreuz-Helizität kontrolliert werden können, ohne jedoch kinetische oder magnetische Helizität als unerwünschte Nebenwirkung zu erzeugen. Die Stärke des mittleren Magnetfeldes bestimmt dabei die Anisotropie des Gleichgewichtszustandes. Die Simulationen erfüllen in allen Parameterbereichen die Vorhersagen, die theoretische Modelle für realistische Plasmaturbulenz treffen. Die Diffusion kosmischer Strahlung in turbulenten Plasmen wird häufig im Rahmen der quasilinearen Theorie unter Heranziehung eines stark vereinfachten Turbulenzspektrums berechnet. Indem die Trajektorien von Testteilchen in dynamischen Turbulenzsimulationen mit Kreuz-Helizität berechnet werden, lassen sich quasilineare Ergebnisse für die Beschleunigungsrate geladener Teilchen nachprüfen. Theorie und numerische Simulation stimmen für Teilchen mit der Alfven-Geschwindigkeit gut überein, solange resistive Effekte vernachlässigt werden können. Weiterhin werden aus der quasilinearen Theorie berechnete Diffusionskoeffizienten mit numerisch ermittelten Streuraten für Testteilchen nach einer Gyroperiode in stark anisotropen Feldkonfigurationen verglichen, wobei der Schwerpunkt erneut beim Einfluss der Kreuz-Helizität liegt. Für alle verwendeten Werte der Kreuz-Helizität ergibt sich eine exzellente Übereinstimmung zwischen Simulationsergebnis und Vorhersage. Schließlich wird die Rolle des magnetischen Moments, einer adiabatischen Invarianten bei der Bewegung geladener Teilchen in einem Magnetfeld, für die Streuung über Zeitskalen von mehreren Gyroperioden erläutert

Μαγνητικοί πύργοι σε αστροφυσικούς δίσκους προσαύξησης

Ο σκοπός της εργασίας είναι η μελέτη της δημιουργίας και επιτάχυνσης αστροφυσικών πιδάκων πλάσματος. Ειδικότερα, μελετώνται οι μαγνητικοί πύργοι σε αστροφυσικούς δίσκους προσαύξησης, οι οποίοι δημιουργούνται όταν λόγω της διαφορικής περιστροφής στο δίσκο αυξάνεται το μαγνητικό πεδίο και η μαγνητική πίεση εκτοξεύει το πλάσμα έξω από το δίσκο. Οι εκροές αυτές εστιάζονται μέσω της πίεσης από το περιβάλλον. Αυτού του είδους οι πίδακες έχουν προταθεί από τον Lynden-Bell και το κύριο χαρακτηριστικό τους είναι ότι περιέχουν αντίθετης πολικότητας μαγνητικά πεδία τα οποία συνδέονται με τον δίσκο. Η εργασία χωρίζεται σε δύο μέρη. Στο πρώτο μέρος χρησιμοποιείται η πλήρης μορφή του μοντέλου του Lynden-Bell όπου ο πίδακας θεωρείται ότι είναι μια αλληλουχία στατικών δομών και ότι μέσα στον πίδακα κυριαρχούν τα μαγνητικά πεδία, δηλαδή ισχύει η προσέγγιση της μηδενικής μαγνητικής δύναμης (force-free). Μέσα από την διατήρηση της ενέργειας και κάποιων προσεγγίσεων για τα μαγνητικά πεδία βρίσκεται πως αλλάζει με τον χρόνο το σχήμα του πίδακα. Επίσης παρουσιάζονται κάποιες αναλυτικές λύσεις που προκύπτουν από την επίλυση της εξίσωσης ορμής. Στο δεύτερο μέρος της εργασίας με την βοήθεια του κώδικα PLUTO προσομοιώνονται μη σχετικιστικοί μαγνητικοί πύργοι για διάφορες τιμές του λόγου πυκνοτήτων πίδακα-περιβάλλοντος και της αρχικής ταχύτητας.he purpose of this project is to study the driving and acceleration of astrophysical jets. We study in particular, magnetic towers in accretion disks, which are formed when from a differential rotating disk the magnetic pressure increases and as a result plasma is launched outside the disk. These outflows are collimated through the environmental pressure. Lynden-Bell studied this kind of jet whose main characteristic is the opposite polarity of the magnetic field. The project has two parts. In the first part we use Lynden-Bell&apos;s model so we consider a sequence of static configurations adopting the force-free approximation. Using energy conservation and appropriate approximations on the magnetic fields we find how the shape of the jet changes with time. In the second part we use the PLUTO code to numerically simulate nonrelativistic magnetic towers for different values of the jet-environment density ratio and the initial velocity of the jet