TY  - THES
A1  - Kopf, Markus
T1  - Zeeman-Doppler Imaging of active late-type stars
T1  - Zeeman-Doppler Imaging von späten aktiven Sternen
N2  - Stellare Magnetfelder spielen eine wichtige Rolle bei der Entstehung und Entwicklung von Sternen. Leider entziehen sie sich aber, aufgrund ihrer großen Entfernung zur Erde, einer direkten Beobachtung. Dies gilt zumindest für derzeitige und in naher Zukunft zur Verfügung stehende Instrumente. Um aber beispielsweise zu verstehen, ob Magnetfelder durch einen Dynamoprozess generiert werden oder Überbleibsel der Sternentstehung sind, ist es zwingend erforderlich, die Oberflächenstruktur und die zeitliche Entwicklung von stellaren Feldern zu untersuchen. Glücklicherweise haben wir mit der Dopplerverschiebung sowie der Polarisation von Licht Mittel zur Verfügung, um indirekt die Magnetfeldtopologie entfernter Sternen zu rekonstruieren, wenn auch nur die schnell rotierender. Die auf den beiden genannten Effekten basierende Rekonstruktionsmethode ist unter dem Namen Zeeman-Doppler Imaging (ZDI) bekannt. Sie stellt eine leistungsfähige Methode dar, um aus rotationsverbreiterten Stokes Profilen schnell rotierender Sterne Oberflächenkartierungen der Temperatur und Magnetfeldverteilung zu erstellen. Durch das ZDI konnten in den vergangenen Jahren die Magnetfeldverteilungen zahlreicher Sterne rekonstruiert werden. Diese Methode stellt allerdings sehr hohe Anforderungen sowohl an die Instrumentierung als auch an die Rechenleistung und ist deshalb häufig mit zahlreichen Annahmen und Näherungen verbunden. Ziel dieser Arbeit war es, Methoden für ein ZDI zu entwickeln, das darauf ausgelegt ist, zeitaufgelöste spektropolarimetrische Daten von aktiven späten Sternen zu invertieren. Es sollte also insbesondere den komplexen und lokalen Magnetfeldstrukturen dieser Sterne Rechnung getragen werden. Um die Orientierung und Stärke solcher Felder zuverlässig rekonstruieren zu können, sollte die Inversion im Stande sein, alle vier Stokes-Komponenten einzubeziehen. Ferner war vorgesehen auf vollständigen polarisierten Strahlungstransportmodellierungen aufzubauen. Dies ermöglicht eine simultane und selbstkonsistente Temperatur- und Magnetfeld-Inversion, die damit dem komplexen Zusammenspiel zwischen Temperatur und Magnetfeld gerecht wird. Schließlich sollte die Anwendung eines neu zu entwickelnden ZDI Programms auf Stokes I und V Beobachtungen von II Pegasi (kurz: II Peg) erste Magnefeldkarten dieses sehr aktiven Sterns liefern. Um den hohen Rechenaufwand, der mit der Inversionsmethode einhergeht, besser bewältigen zu können, wurde zunächst eine schnelle Approximationsmethode für den polarisierten Strahlungstransport entwickelt. Sie basiert auf einer Hauptkomponentenanalyse (PCA) sowie auf künstlichen Neuronalen Netzen. Letztere approximieren den funktionalen Zusammenhang zwischen atmosphärischen Parametern und den zugehörigen lokalen Stokes Profilen. Inverse Probleme sind potentiell schlecht gestellt und erfordern in der Regel eine Regularisierung. Der entwickelte Ansatz verwendet eine lokale Entropie, die auf die Besonderheiten bei der Rekonstruktion lokalisierter Magnetfeder eingeht. Ein weiterer neuartiger Ansatz befasst sich mit der Rauschreduktion polarimetrischer Beobachtungsdaten. Er macht sich die Hauptkomponentenanalyse zu Nutze, um mit Hilfe einer Vielzahl beobachteter Spektrallinien, einzelne Linien mit drastisch vergrößertem Signal-zu-Rausch-Verhältnis wieder zu geben. Diese Methode hat gegenüber anderen Multi-Spektrallinien-Verfahren den Vorteil, nach wie vor eine Inversion auf der Basis einzelner Spektrallinien durchführen zu können. Schließlich wurde das Inversionsprogramm iMap entwickelt, das die zuvor genannten Methoden implementiert. Detaillierte Testrechnungen demonstrieren die Funktionsfähigkeit und Genauigkeit der schnellen Synthese-Methode und weisen einen Zeitgewinn von nahezu drei Größenordnungen gegenüber der konventionellen Strahlungstransportberechnung auf. Desweiteren untersuchen wir den Einfluss der verschiedenen Stokes Komponenten (IV bzw. IVQU) auf die Zuverlässigkeit, ein bekanntes Magnetfeld zu rekonstruieren. Damit belegen wir die Zuverlässigkeit unseres Inversionsprogrammes und zeigen darüber hinaus auch Einschränkungen von Magnetfeldinversionen im allgemeinen auf. Eine erste Inversion von Stokes I und V Profilen von II Peg liefert zum ersten Mal für diesen Stern simultan Temperatur- und Magnetfeldverteilungen.
N2  - Stellar magnetic fields, as a crucial component of star formation and evolution, evade direct observation at least with current and near future instruments. However investigating whether magnetic fields are generated by a dynamo process or represent relics from the formation process, or whether they show a behavior similar to the sun or something very different, it is essential to investigate their structure and temporal evolution. Fortunately nature provides us with the possibility to indirectly observe surface topologies on distant stars by means of Doppler shift and polarization of light, though not without its challenges. Based on the mentioned effects, the so called Zeeman-Doppler Imaging technique is a powerful method to retrieve magnetic fields from rapid rotating stars based on measurements of spectropolarimetric observations in terms of Stokes profiles. In recent years, a large number of stellar magnetic field distributions could be reconstructed by Zeeman-Doppler Imaging (ZDI). However, the implementation of this method often relies on many approximations because, as an inversion method, it entails enormous computational requirements. The aim of this thesis is to develop methods for a ZDI, designed to invert time-resolved spectropolarimetric data of active late type stars, and to account for the expected complex and small scale magnetic fields on these stars. In order to reliably reconstruct the detailed field orientation and strength, the inversion method is employed to be able to use of all four Stokes components. Furthermore it is based on fully polarized radiative transfer calculations to account for the intricate interplay between temperature and magnetic field. Finally, the application of a newly developed ZDI code to Stokes I and V observations of II Pegasi (short: II Peg) was supposed to deliver the first magnetic surface maps for this highly active star. To accomplish the high computational burden of a radiative transfer based ZDI, we developed a novel approximation method to speed up the inversion process. It is based on Principal Component Analysis and Artificial Neural Networks. The latter approximate the functional mapping between atmospheric parameters and the corresponding local Stokes profiles. Inverse problems, as we are dealing with, are potentially ill-posed and require a regularization method. We propose a new regularization scheme, which implements a local entropy function that accounts for the peculiarities of the reconstruction of localized magnetic fields. To deal with the relatively large noise that is always present in polarimetric data, we developed a multi-line denoising technique based on Principal Component Analysis. In contrast to other multi-line techniques that extract from a large number of spectral lines a sort of mean profile, this method allows to extract individual spectral lines and thus allows for an inversion on the basis of specific lines. All these methods are incorporated in our newly developed ZDI code iMap, which is based on a conjugated gradient method. An in depth validation of our new synthesis method demonstrates the reliability and accuracy of this approach as well as a gain in computation time by almost three orders of magnitude relative to the conventional radiative transfer calculations. We investigated the influence of the different Stokes components (IV / IVQU) on the ability to reconstruct a known synthetic field configuration. In doing so we validate the capability of our inversion code, and we also assess limitations of magnetic field inversions in general. In a first application to II Peg, a K2 IV subgiant, we derived temperature and magnetic field surface distributions from spectropolarimetric data obtained in 2004 and 2007. It gives for the first time simultaneously the temporal evolution of the surface temperature and magnetic field distribution on II Peg.
KW  - Stellare Aktivität
KW  - Magnetfelder
KW  - Polarisation
KW  - Späte Sterne
KW  - Oberflächenstrukturen
KW  - stellar activity
KW  - magnetic fields
KW  - polarization
KW  - late-type stars
KW  - surface features
Y1  - 2008
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-37387
ER  - 
TY  - THES
A1  - Gressel, Oliver
T1  - Supernova-driven turbulence and magnetic field amplification in disk galaxies
T1  - Supernovagetriebene Turbulenz und Magnetfeldverstärkung in Scheibengalaxien
N2  - Supernovae are known to be the dominant energy source for driving turbulence in the interstellar medium. Yet, their effect on magnetic field amplification in spiral galaxies is still poorly understood. Analytical models based on the uncorrelated-ensemble approach predicted that any created field will be expelled from the disk before a significant amplification can occur. By means of direct simulations of supernova-driven turbulence, we demonstrate that this is not the case. Accounting for vertical stratification and galactic differential rotation, we find an exponential amplification of the mean field on timescales of 100Myr. The self-consistent numerical verification of such a “fast dynamo” is highly beneficial in explaining the observed strong magnetic fields in young galaxies. We, furthermore, highlight the importance of rotation in the generation of helicity by showing that a similar mechanism based on Cartesian shear does not lead to a sustained amplification of the mean magnetic field. This finding impressively confirms the classical picture of a dynamo based on cyclonic turbulence.
N2  - Supernovae sind bekanntermaßen die dominante treibende Energiequelle für Turbulenz im interstellaren Medium. Dennoch ist ihre Auswirkung auf die Verstärkung von Magnetfeldern in Spiralgalaxien weitestgehend unverstanden. Analytische Modelle, die auf der Annahme eines unkorrelierten Ensembles beruhen, sagen voraus, dass das erzeugte Feld aus der galaktischen Scheibe herausgedrängt wird bevor eine substantielle Verstärkung erfolgen kann. Mithilfe numerischer Simulationen supernovagetriebener Turbulenz zeigen wir, dass dies nicht der Fall ist. Unter Berücksichtigung einer vertikalen Schichtung und differentieller galaktischer Rotation beobachten wir eine exponentielle Verstärkung des mittleren Magnetfeldes auf einer Zeitskala von 100 Mio. Jahren. Diese selbstkonsistente numerische Bestätigung eines “schnellen Dynamos” erlaubt es, die beobachteten starken Magnetfelder in jungen Galaxien zu erklären. Darüberhinaus stellen wir die Wichtigkeit der Rotation bei der Erzeugung von Helizität heraus, indem wir zeigen, dass ein ähnlicher Effekt basierend auf kartesischer Scherung nicht zu einer Verstärkung des mittleren Magnetfeldes führt. Dies bestätigt eindrucksvoll das klassische Bild zyklonischer Turbulenz.
KW  - Turbulenz
KW  - Magnetfelder
KW  - Magnetohydrodynamik
KW  - Supernovaüberreste
KW  - Galaxien
KW  - turbulence
KW  - magnetic fields
KW  - magnetohydrodynamics
KW  - supernova remnants
KW  - galaxies
Y1  - 2008
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-29094
ER  - 
TY  - THES
A1  - Zhang, Bo
T1  - Magnetic fields near microstructured surfaces : application to atom chips
T1  - Magnetische Felder in der Nähe von microstrukturierten Oberflächen : Anwendung auf Atomchips
N2  - Microfabricated solid-state surfaces, also called atom chip', have become a well-established technique to trap and manipulate atoms. This has simplified applications in atom interferometry, quantum information processing, and studies of many-body systems. Magnetic trapping potentials with arbitrary geommetries are generated with atom chip by miniaturized current-carrying conductors integrated on a solid substrate. Atoms can be trapped and cooled to microKelvin and even nanoKelvin temperatures in such microchip trap. However, cold atoms can be significantly perturbed by the chip surface, typically held at room temperature. The magnetic field fluctuations generated by thermal currents in the chip elements may induce spin flips of atoms and result in loss, heating and decoherence. In this thesis, we extend previous work on spin flip rates induced by magnetic noise and consider the more complex geometries that are typically encountered in atom chips: layered structures and metallic wires of finite cross-section. We also discuss a few aspects of atom chips traps built with superconducting structures that have been suggested as a means to suppress magnetic field fluctuations. The thesis describes calculations of spin flip rates based on magnetic Green functions that are computed analytically and numerically. For a chip with a top metallic layer, the magnetic noise depends essentially on the thickness of that layer, as long as the layers below have a much smaller conductivity. Based on this result, scaling laws for loss rates above a thin metallic layer are derived. A good agreement with experiments is obtained in the regime where the atom-surface distance is comparable to the skin depth of metal. Since in the experiments, metallic layers are always etched to separate wires carrying different currents, the impact of the finite lateral wire size on the magnetic noise has been taken into account. The local spectrum of the magnetic field near a metallic microstructure has been investigated numerically with the help of boundary integral equations. The magnetic noise significantly depends on polarizations above flat wires with finite lateral width, in stark contrast to an infinitely wide wire. Correlations between multiple wires are also taken into account. In the last part, superconducting atom chips are considered. Magnetic traps generated by superconducting wires in the Meissner state and the mixed state are studied analytically by a conformal mapping method and also numerically. The properties of the traps created by superconducting wires are investigated and compared to normal conducting wires: they behave qualitatively quite similar and open a route to further trap miniaturization, due to the advantage of low magnetic noise. We discuss critical currents and fields for several geometries.
N2  - Mikrotechnologische Oberflächen, sogenannte Atomchips, sind eine etablierte Methode zum Speichern und Manipulieren von Atomen geworden. Das hat Anwendungen in der Atom-Interferometrie, Quanteninformationsverarbeitung und Vielteilchensystemen vereinfacht. Magnetische Fallenpotentiale mit beliebigen Geometrien werden durch Atomchips mit miniaturisierten stromführenden Leiterbahnen auf einer Festkörperunterlage realisiert. Atome können bei Temperaturen im $\mu$ K oder sogar nK-Bereich in einer solchen Falle gespeichert und gekühlt werden. Allerdings können kalte Atome signifikant durch die Chip-Oberfläche gestört werden, die sich typischerweise auf Raumtemperatur befindet. Die durch thermische Ströme im Chip erzeugten magnetischen Feldfluktuationen können Spin-Flips der Atome induzieren und Verlust, Erwärmung und Dekohärenz zur Folge haben. In dieser Dissertation erweitern wir frühere Arbeiten über durch magnetisches Rauschen induzierte Spin-Flip-Ratenund betrachten kompliziertere Geometrien, wie sie typischerweise auf einem Atom-Chip anzutreffen sind: Geschichtete Strukturen und metallische Leitungen mit endlichem Querschnitt. Wir diskutieren auch einige Aspekte von Aomchips aus Supraleitenden Strukturen die als Mittel zur Unterdrückung magnetischer Feldfluktuationen vorgeschlagen wurden. Die Arbeit beschreibt analytische und numerische Rechnungen von Spin-Flip Raten auf Grundlage magnetischer Greensfunktionen. Für einen Chip mit einem metallischen Top-Layer hängt das magnetische Rauschen hauptsächlich von der Dicke des Layers ab, solange die unteren Layer eine deutlich kleinere Leitfähigkeit haben. Auf Grundlage dieses Ergebnisses werden Skalengesetze für Verlustraten über einem dünnen metallischen Leiter hergeleitet. Eine gute Übereinstimmung mit Experimenten wird in dem Bereich erreicht, wo der Abstand zwischen Atom und Oberfläche in der Größenordnung der Eindringtiefe des Metalls ist. Da in Experimenten metallische Layer immer geätzt werden, um verschiedene stromleitende Bahnen vonenander zu trennen, wurde der Einfluß eines endlichen Querschnittsauf das magnetische Rauschen berücksichtigt. Das lokale Spektrum des magnetischen Feldes in der Nähe einer metallischen Mikrostruktur wurde mit Hilfe von Randintegralen numerisch untersucht. Das magnetische Rauschen hängt signifikant von der Polarisierung über flachen Leiterbahnen mit endlichem Querschnitt ab, im Unterschied zu einem unendlich breiten Leiter. Es wurden auch Korrelationen zwischen mehreren Leitern berücksichtigt. Im letzten Teil werden supraleitende Atomchips betrachtet. Magnetische Fallen, die von supraleitenden Bahnen im Meissner Zustand und im gemischten Zustand sind werden analytisch durch die Methode der konformen Abbildung und numerisch untersucht. Die Eigenschaften der durch supraleitende Bahnen erzeugten Fallen werden erforscht und mit normal leitenden verglichen: Sie verhalten sich qualitativ sehr ähnlich und öffnen einen Weg zur weiteren Miniaturisierung von Fallen, wegen dem Vorteil von geringem magnetischem Rauschen. Wir diskutieren kritische Ströme und Felder für einige Geometrien.
KW  - Magnetische Felder
KW  - Atom chip
KW  - Supraleiter
KW  - magnetisches Rauschen
KW  - microstrukturierte Oberfläche
KW  - magnetic fields
KW  - atom chip
KW  - superconductors
KW  - magnetic noise
KW  - microstructured surface
Y1  - 2008
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-28984
ER  - 
TY  - GEN
A1  - Kliem, Bernhard
A1  - Rust, S.
A1  - Seehafer, Norbert
T1  - Helicity transport in a simulated coronal mass ejection
T2  - Postprints der Universität Potsdam : Mathematisch Naturwissenschaftliche Reihe
N2  - It has been suggested that coronal mass ejections (CMEs) remove the magnetic he-licity of their coronal source region from the Sun. Such removal is often regarded to be necessary due to the hemispheric sign preference of the helicity, which inhibits a simple annihilation by reconnection between volumes of opposite chirality. Here we monitor the relative magnetic he-licity contained in the coronal volume of a simulated flux rope CME, as well as the upward flux of relative helicity through horizontal planes in the simulation box. The unstable and erupting flux rope carries away only a minor part of the initial relative helicity; the major part remains in the volume. This is a consequence of the requirement that the current through an expanding loop must decrease if the magnetic energy of the configuration is to decrease as the loop rises, to provide the kinetic energy of the CME.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 569 
KW  - magnetic fields
KW  - MHD
KW  - coronal mass ejections
KW  - magnetohydrodynamics
KW  - sun
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-412907
SN  - 1866-8372
IS  - 569
SP  - 125
EP  - 128
ER  - 
TY  - JOUR
A1  - Green, Luci M.
A1  - Kliem, Bernhard
A1  - Wallace, A. J.
T1  - Photospheric flux cancellation and associated flux rope formation and eruption
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Aims. We study an evolving bipolar active region that exhibits flux cancellation at the internal polarity inversion line, the formation of a soft X-ray sigmoid along the inversion line and a coronal mass ejection. The aim is to investigate the quantity of flux cancellation that is involved in flux rope formation in the time period leading up to the eruption.
 Methods. The active region is studied using its extreme ultraviolet and soft X-ray emissions as it evolves from a sheared arcade to flux rope configuration. The evolution of the photospheric magnetic field is described and used to estimate how much flux is reconnected into the flux rope.
 Results. About one third of the active region flux cancels at the internal polarity inversion line in the 2.5 days leading up to the eruption. In this period, the coronal structure evolves from a weakly to a highly sheared arcade and then to a sigmoid that crosses the inversion line in the inverse direction. These properties suggest that a flux rope has formed prior to the eruption. The amount of cancellation implies that up to 60% of the active region flux could be in the body of the flux rope. We point out that only part of the cancellation contributes to the flux in the rope if the arcade is only weakly sheared, as in the first part of the evolution. This reduces the estimated flux in the rope to similar to 30% or less of the active region flux. We suggest that the remaining discrepancy between our estimate and the limiting value of similar to 10% of the active region flux, obtained previously by the flux rope insertion method, results from the incomplete coherence of the flux rope, due to nonuniform cancellation along the polarity inversion line. A hot linear feature is observed in the active region which rises as part of the eruption and then likely traces out the field lines close to the axis of the flux rope. The flux cancellation and changing magnetic connections at one end of this feature suggest that the flux rope reaches coherence by reconnection immediately before and early in the impulsive phase of the associated flare. The sigmoid is destroyed in the eruption but reforms quickly, with the amount of cancellation involved being much smaller than in the course of its original formation.
KW  - Sun: activity
KW  - Sun: coronal mass ejections (CMEs)
KW  - magnetic fields
KW  - magnetic reconnection
KW  - Sun: photosphere
KW  - Sun: magnetic topology
Y1  - 2011
U6  - https://doi.org/10.1051/0004-6361/201015146
SN  - 0004-6361
VL  - 526
IS  - 2
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Arlen, T.
A1  - Aune, T.
A1  - Beilicke, M.
A1  - Benbow, W.
A1  - Bouvier, A.
A1  - Buckley, J. H.
A1  - Bugaev, V.
A1  - Byrum, K.
A1  - Cannon, A.
A1  - Cesarini, A.
A1  - Ciupik, L.
A1  - Collins-Hughes, E.
A1  - Connolly, M. P.
A1  - Cui, W.
A1  - Dickherber, R.
A1  - Dumm, J.
A1  - Falcone, A.
A1  - Federici, S.
A1  - Feng, Q.
A1  - Finley, J. P.
A1  - Finnegan, G.
A1  - Fortson, L.
A1  - Furniss, A.
A1  - Galante, N.
A1  - Gall, D.
A1  - Godambe, S.
A1  - Griffin, S.
A1  - Grube, J.
A1  - Gyuk, G.
A1  - Holder, J.
A1  - Huan, H.
A1  - Hughes, G.
A1  - Humensky, T. B.
A1  - Imran, A.
A1  - Kaaret, P.
A1  - Karlsson, N.
A1  - Kertzman, M.
A1  - Khassen, Y.
A1  - Kieda, D.
A1  - Krawczynski, H.
A1  - Krennrich, F.
A1  - Lee, K.
A1  - Madhavan, A. S.
A1  - Maier, G.
A1  - Majumdar, P.
A1  - McArthur, S.
A1  - McCann, A.
A1  - Moriarty, P.
A1  - Mukherjee, R.
A1  - Nelson, T.
A1  - de Bhroithe, A. O'Faolain
A1  - Ong, R. A.
A1  - Orr, M.
A1  - Otte, A. N.
A1  - Park, N.
A1  - Perkins, J. S.
A1  - Pohl, Martin
A1  - Prokoph, H.
A1  - Quinn, J.
A1  - Ragan, K.
A1  - Reyes, L. C.
A1  - Reynolds, P. T.
A1  - Roache, E.
A1  - Ruppel, J.
A1  - Saxon, D. B.
A1  - Schroedter, M.
A1  - Sembroski, G. H.
A1  - Skole, C.
A1  - Smith, A. W.
A1  - Telezhinsky, Igor O.
A1  - Tesic, G.
A1  - Theiling, M.
A1  - Thibadeau, S.
A1  - Tsurusaki, K.
A1  - Varlotta, A.
A1  - Vivier, M.
A1  - Wakely, S. P.
A1  - Ward, J. E.
A1  - Weinstein, A.
A1  - Welsing, R.
A1  - Williams, D. A.
A1  - Zitzer, B.
A1  - Pfrommer, C.
A1  - Pinzke, A.
T1  - Constraints on cosmic rays, magnetic fields, and dark matter fromgamma-ray observations of the coma cluster of galaxies with veritas and fermi
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - Observations of radio halos and relics in galaxy clusters indicate efficient electron acceleration. Protons should likewise be accelerated and, on account of weak energy losses, can accumulate, suggesting that clusters may also be sources of very high energy (VHE; E > 100 GeV) gamma-ray emission. We report here on VHE gamma-ray observations of the Coma galaxy cluster with the VERITAS array of imaging Cerenkov telescopes, with complementing Fermi Large Area Telescope observations at GeV energies. No significant gamma-ray emission from the Coma Cluster was detected. Integral flux upper limits at the 99% confidence level were measured to be on the order of (2-5) x 10(-8) photonsm(-2) s(-1) (VERITAS, >220 GeV) and similar to 2 x 10(-6) photonsm(-2) s(-1) (Fermi, 1-3GeV), respectively. We use the gamma-ray upper limits to constrain cosmic rays (CRs) and magnetic fields in Coma. Using an analytical approach, the CR-to-thermal pressure ratio is constrained to be < 16% from VERITAS data and <1.7% from Fermi data (averaged within the virial radius). These upper limits are starting to constrain the CR physics in self-consistent cosmological cluster simulations and cap the maximum CR acceleration efficiency at structure formation shocks to be <50%. Alternatively, this may argue for non-negligible CR transport processes such as CR streaming and diffusion into the outer cluster regions. Assuming that the radio-emitting electrons of the Coma halo result from hadronic CR interactions, the observations imply a lower limit on the central magnetic field in Coma of similar to(2-5.5) mu G, depending on the radial magnetic field profile and on the gamma-ray spectral index. Since these values are below those inferred by Faraday rotation measurements in Coma (for most of the parameter space), this renders the hadronic model a very plausible explanation of the Coma radio halo. Finally, since galaxy clusters are dark matter (DM) dominated, the VERITAS upper limits have been used to place constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the DM particles, <sigma nu >.
KW  - cosmic rays
KW  - dark matter
KW  - galaxies: clusters: general
KW  - galaxies: clusters: individual (Coma (ACO 1656))
KW  - gamma rays: galaxies: clusters
KW  - magnetic fields
Y1  - 2012
U6  - https://doi.org/10.1088/0004-637X/757/2/123
SN  - 0004-637X
VL  - 757
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Valori, Gherarod
A1  - Demoulin, Pascal
A1  - Pariat, E.
A1  - Masson, S.
T1  - Accuracy of magnetic energy computations
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. For magnetically driven events, the magnetic energy of the system is the prime energy reservoir that fuels the dynamical evolution. In the solar context, the free energy (i.e., the energy in excess of the potential field energy) is one of the main indicators used in space weather forecasts to predict the eruptivity of active regions. A trustworthy estimation of the magnetic energy is therefore needed in three-dimensional (3D) models of the solar atmosphere, e. g., in coronal fields reconstructions or numerical simulations.
 Aims. The expression of the energy of a system as the sum of its potential energy and its free energy (Thomson's theorem) is strictly valid when the magnetic field is exactly solenoidal. For numerical realizations on a discrete grid, this property may be only approximately fulfilled. We show that the imperfect solenoidality induces terms in the energy that can lead to misinterpreting the amount of free energy present in a magnetic configuration.
 Methods. We consider a decomposition of the energy in solenoidal and nonsolenoidal parts which allows the unambiguous estimation of the nonsolenoidal contribution to the energy. We apply this decomposition to six typical cases broadly used in solar physics. We quantify to what extent the Thomson theorem is not satisfied when approximately solenoidal fields are used.
 Results. The quantified errors on energy vary from negligible to significant errors, depending on the extent of the nonsolenoidal component of the field. We identify the main source of errors and analyze the implications of adding a variable amount of divergence to various solenoidal fields. Finally, we present pathological unphysical situations where the estimated free energy would appear to be negative, as found in some previous works, and we identify the source of this error to be the presence of a finite divergence.
 Conclusions. We provide a method of quantifying the effect of a finite divergence in numerical fields, together with detailed diagnostics of its sources. We also compare the efficiency of two divergence-cleaning techniques. These results are applicable to a broad range of numerical realizations of magnetic fields.
KW  - magnetic fields
KW  - methods: numerical
KW  - Sun: surface magnetism
KW  - Sun: corona
Y1  - 2013
U6  - https://doi.org/10.1051/0004-6361/201220982
SN  - 0004-6361
VL  - 553
IS  - 2
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - GEN
A1  - Driel-Gesztelyi, L. van
A1  - Baker, Daniel N.
A1  - Török, Tibor
A1  - Pariat, Etienne
A1  - Green, L. M.
A1  - Williams, D. R.
A1  - Carlyle, J.
A1  - Valori, G.
A1  - Démoulin, Pascal
A1  - Matthews, S. A.
A1  - Kliem, Bernhard
A1  - Malherbe, J.-M.
T1  - Magnetic reconnection driven by filament eruption in the 7 June 2011 event
T2  - Postprints der Universität Potsdam : Mathematisch Naturwissenschaftliche Reihe
N2  - During an unusually massive filament eruption on 7 June 2011, SDO/AIA imaged for the first time significant EUV emission around a magnetic reconnection region in the solar corona. The reconnection occurred between magnetic fields of the laterally expanding CME and a neighbouring active region. A pre-existing quasi-separatrix layer was activated in the process. This scenario is supported by data-constrained numerical simulations of the eruption. Observations show that dense cool filament plasma was re-directed and heated in situ, producing coronal-temperature emission around the reconnection region. These results provide the first direct observational evidence, supported by MHD simulations and magnetic modelling, that a large-scale re-configuration of the coronal magnetic field takes place during solar eruptions via the process of magnetic reconnection.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 608 
KW  - MHD
KW  - instabilities
KW  - Sun: activity
KW  - magnetic fields
KW  - coronal mass ejections (CMEs)
KW  - filaments
KW  - methods: numerical
KW  - data analysis
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-415671
IS  - 608
SP  - 502
EP  - 503
ER  - 
TY  - JOUR
A1  - Aliu, E.
A1  - Archambault, S.
A1  - Arlen, T.
A1  - Aune, T.
A1  - Behera, B.
A1  - Beilicke, M.
A1  - Benbow, W.
A1  - Berger, K.
A1  - Bird, R.
A1  - Bouvier, A.
A1  - Buckley, J. H.
A1  - Bugaev, V.
A1  - Byrum, K.
A1  - Cerruti, M.
A1  - Chen, Xuhui
A1  - Ciupik, L.
A1  - Connolly, M. P.
A1  - Cui, W.
A1  - Duke, C.
A1  - Dumm, J.
A1  - Errando, M.
A1  - Falcone, A.
A1  - Federici, S.
A1  - Feng, Q.
A1  - Finley, J. P.
A1  - Fleischhack, H.
A1  - Fortin, P.
A1  - Fortson, L.
A1  - Furniss, A.
A1  - Galante, N.
A1  - Gillanders, G. H.
A1  - Griffin, S.
A1  - Griffiths, S. T.
A1  - Grube, J.
A1  - Gyuk, G.
A1  - Hanna, D.
A1  - Holder, J.
A1  - Hughes, G.
A1  - Humensky, T. B.
A1  - Johnson, C. A.
A1  - Kaaret, P.
A1  - Kertzman, M.
A1  - Khassen, Y.
A1  - Kieda, D.
A1  - Krawczynski, H.
A1  - Krennrich, F.
A1  - Lang, M. J.
A1  - Madhavan, A. S.
A1  - Maier, G.
A1  - Majumdar, P.
A1  - McArthur, S.
A1  - McCann, A.
A1  - Meagher, K.
A1  - Millis, J.
A1  - Moriarty, P.
A1  - Mukherjee, R.
A1  - Nieto, D.
A1  - Ong, R. A.
A1  - Otte, A. N.
A1  - Park, N.
A1  - Perkins, J. S.
A1  - Pohl, M.
A1  - Popkow, A.
A1  - Prokoph, H.
A1  - Quinn, J.
A1  - Ragan, K.
A1  - Reyes, L. C.
A1  - Reynolds, P. T.
A1  - Richards, G. T.
A1  - Roache, E.
A1  - Sembroski, G. H.
A1  - Smith, A. W.
A1  - Staszak, D.
A1  - Telezhinsky, Igor O.
A1  - Theiling, M.
A1  - Varlotta, A.
A1  - Vassiliev, V. V.
A1  - Vincent, S.
A1  - Wakely, S. P.
A1  - Weekes, T. C.
A1  - Weinstein, A.
A1  - Welsing, R.
A1  - Williams, D. A.
A1  - Zajczyk, A.
A1  - Zitzer, B.
T1  - A three-year multi-wavelenght study of the very-high-energy gamma-ray Blazar 1ES 0229+200
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
KW  - BL Lacertae objects: general
KW  - BL Lacertae objects: individual (1ES 0229+200, VER J0232+202)
KW  - diffuse radiation
KW  - galaxies: active
KW  - gamma rays: general
KW  - magnetic fields
Y1  - 2014
U6  - https://doi.org/10.1088/0004-637X/782/1/13
SN  - 0004-637X
SN  - 1538-4357
VL  - 782
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Nishikawa, Ken-Ichi
A1  - Hardee, P. E.
A1  - Dutan, I.
A1  - Niemiec, J.
A1  - Medvedev, M.
A1  - Mizuno, Y.
A1  - Meli, A.
A1  - Sol, H.
A1  - Zhang, B.
A1  - Pohl, Martin
A1  - Hartmann, D. H.
T1  - Magnetic agnetic field generation in core-sheath jets via the kinetic Kelvin-Helmholtz instability
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We have investigated magnetic field generation in velocity shears via the kinetic Kelvin-Helmholtz instability (kKHI) using a relativistic plasma jet core and stationary plasma sheath. Our three-dimensional particle-in-cell simulations consider plasma jet cores with Lorentz factors of 1.5, 5, and 15 for both electron-proton and electron-positron plasmas. For electron-proton plasmas, we find generation of strong large-scale DC currents and magnetic fields that extend over the entire shear surface and reach thicknesses of a few tens of electron skin depths. For electron-positron plasmas, we find generation of alternating currents and magnetic fields. Jet and sheath plasmas are accelerated across the shear surface in the strong magnetic fields generated by the kKHI. The mixing of jet and sheath plasmas generates a transverse structure similar to that produced by the Weibel instability.
KW  - acceleration of particles
KW  - magnetic fields
KW  - plasmas
KW  - radiation mechanisms: non-thermal
KW  - relativistic processes
KW  - stars: jets
Y1  - 2014
U6  - https://doi.org/10.1088/0004-637X/793/1/60
SN  - 0004-637X
SN  - 1538-4357
VL  - 793
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - GEN
A1  - Morel, T.
A1  - Castro, Norberto
A1  - Fossati, Luca
A1  - Hubrig, Swetlana
A1  - Langer, N.
A1  - Przybilla, Norbert
A1  - Schöller, Markus
A1  - Carroll, Thorsten Anthony
A1  - Ilyin, Ilya
A1  - Irrgang, Andreas
A1  - Oskinova, Lida
A1  - Schneider, Fabian R. N.
A1  - Simon Díaz, Sergio
A1  - Briquet, Maryline
A1  - González, Jean-Francois
A1  - Kharchenko, Nina
A1  - Nieva, M.-F.
A1  - Scholz, Ralf-Dieter
A1  - de Koter, Alexander
A1  - Hamann, Wolf-Rainer
A1  - Herrero, Artemio
A1  - Maíz Apellániz, Jesus
A1  - Sana, Hugues
A1  - Arlt, Rainer
A1  - Barbá, Rodolfo H.
A1  - Dufton, Polly
A1  - Kholtygin, Alexander
A1  - Mathys, Gautier
A1  - Piskunov, Anatoly E.
A1  - Reisenegger, Andreas
A1  - Spruit, H.
A1  - Yoon, S.-C.
T1  - The B fields in OB stars (BOB) survey
T2  - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - The B fields in OB stars (BOB) survey is an ESO large programme collecting spectropolarimetric observations for a large number of early-type stars in order to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. As of July 2014, a total of 98 objects were observed over 20 nights with FORS2 and HARPSpol. Our preliminary results indicate that the fraction of magnetic OB stars with an organised, detectable field is low. This conclusion, now independently reached by two different surveys, has profound implications for any theoretical model attempting to explain the field formation in these objects. We discuss in this contribution some important issues addressed by our observations (e.g., the lower bound of the field strength) and the discovery of some remarkable objects.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 821 
KW  - magnetic fields
KW  - stars: early-type
KW  - stars: magnetic fields
KW  - stars: individual (HD 164492C, CPD –57 ◦ 3509, HD 54879, β CMa, ε CMa)
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-415238
SN  - 1866-8372
IS  - 821
ER  - 
TY  - JOUR
A1  - Vafin, Sergei
A1  - Schlickeiser, R.
A1  - Yoon, P. H.
T1  - AMPLIFICATION OF COLLECTIVE MAGNETIC FLUCTUATIONS IN MAGNETIZED BI-MAXWELLIAN PLASMAS FOR PARALLEL WAVE VECTORS. I. ELECTRON-PROTON PLASMA
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - The general electromagnetic fluctuation theory is a powerful tool to analyze the magnetic fluctuation spectrum of a plasma. Recent works utilizing this theory for a magnetized non-relativistic isotropic Maxwellian electron-proton plasma have demonstrated that the equilibrium ratio of vertical bar delta B vertical bar/B-0 can be as high as 10(-12). This value results from the balance between spontaneous emission of fluctuations and their damping, and it is considerably smaller than the observed value vertical bar delta B vertical bar/B-0 in the solar wind at 1 au, where 10(-3) less than or similar to vertical bar delta B vertical bar/B-0 less than or similar to 10(-1). In the present manuscript, we consider an anisotropic bi-Maxwellian distribution function to investigate the effect of plasma instabilities on the magnetic field fluctuations. We demonstrate that these instabilities strongly amplify the magnetic field fluctuations and provide a sufficient mechanism to explain the observed value of vertical bar delta B vertical bar/B-0 in the solar wind at 1 au.
KW  - instabilities
KW  - magnetic fields
KW  - solar wind
KW  - turbulence
KW  - waves
Y1  - 2016
U6  - https://doi.org/10.3847/0004-637X/829/1/41
SN  - 0004-637X
SN  - 1538-4357
VL  - 829
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Archambault, S.
A1  - Archer, A.
A1  - Benbow, W.
A1  - Buchovecky, M.
A1  - Bugaev, V.
A1  - Cerruti, M.
A1  - Connolly, M. P.
A1  - Cui, W.
A1  - Falcone, A.
A1  - Alonso, M. Fernandez
A1  - Finley, J. P.
A1  - Fleischhack, H.
A1  - Fortson, L.
A1  - Furniss, A.
A1  - Griffin, S.
A1  - Hutten, M.
A1  - Hervet, O.
A1  - Holder, J.
A1  - Humensky, T. B.
A1  - Johnson, C. A.
A1  - Kaaret, P.
A1  - Kar, P.
A1  - Kieda, D.
A1  - Krause, M.
A1  - Krennrich, F.
A1  - Lang, M. J.
A1  - Lin, T. T. Y.
A1  - Maier, G.
A1  - McArthur, S.
A1  - Moriarty, P.
A1  - Nieto, D.
A1  - Ong, R. A.
A1  - Otte, A. N.
A1  - Pohl, M.
A1  - Popkow, A.
A1  - Pueschel, Elisa
A1  - Quinn, J.
A1  - Ragan, K.
A1  - Reynolds, P. T.
A1  - Richards, G. T.
A1  - Roache, E.
A1  - Rovero, A. C.
A1  - Sadeh, I.
A1  - Shahinyan, K.
A1  - Staszak, D.
A1  - Telezhinsky, Igor O.
A1  - Tyler, J.
A1  - Wakely, S. P.
A1  - Weinstein, A.
A1  - Weisgarber, T.
A1  - Wilcox, P.
A1  - Wilhelm, Alina
A1  - Williams, D. A.
A1  - Zitzer, B.
T1  - Search for Magnetically Broadened Cascade Emission from Blazars with VERITAS
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present a search for magnetically broadened gamma-ray emission around active galactic nuclei (AGNs), using VERITAS observations of seven hard-spectrum blazars. A cascade process occurs when multi-TeV gamma-rays from an AGN interact with extragalactic background light (EBL) photons to produce electron-positron pairs, which then interact with cosmic microwave background photons via inverse-Compton scattering to produce gamma-rays. Due to the deflection of the electron- positron pairs, a non-zero intergalactic magnetic field (IGMF) would potentially produce detectable effects on the angular distribution of the cascade emission. In particular, an angular broadening compared to the unscattered emission could occur. Through non-detection of angularly broadened emission from 1ES 1218 vertical bar 304, the source with the largest predicted cascade fraction, we exclude a range of IGMF strengths around 10(-14) G at the 95% confidence level. The extent of the exclusion range varies with the assumptions made about the intrinsic spectrum of 1ES. 1218+304 and the EBL model used in the simulation of the cascade process. All of the sources are used to set limits on the flux due to extended emission.
KW  - BL Lacertae objects: general
KW  - galaxies: active
KW  - gamma rays: galaxies
KW  - magnetic fields
Y1  - 2017
U6  - https://doi.org/10.3847/1538-4357/835/2/288
SN  - 0004-637X
SN  - 1538-4357
VL  - 835
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Lopez-Barquero, Vanessa
A1  - Xu, S.
A1  - Desiati, Paolo
A1  - Lazarian, Alex
A1  - Pogorelov, Nikolai V.
A1  - Yan, Huirong
T1  - TeV Cosmic-Ray Anisotropy from the Magnetic Field at the Heliospheric Boundary
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
KW  - cosmic rays
KW  - magnetic fields
KW  - magnetohydrodynamics (MHD)
KW  - solar wind
KW  - Sun: heliosphere
Y1  - 2017
U6  - https://doi.org/10.3847/1538-4357/aa74d1
SN  - 0004-637X
SN  - 1538-4357
VL  - 842
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Zhang, Heshou
A1  - Yan, Huirong
A1  - Richter, Philipp
T1  - The influence of atomic alignment on absorption and emission spectroscopy
JF  - Monthly notices of the Royal Astronomical Society
N2  - Spectroscopic observations play essential roles in astrophysics. They are crucial for determining physical parameters in our Universe, providing information about the chemistry of various astronomical environments. The proper execution of the spectroscopic analysis requires accounting for all the physical effects that are compatible to the signal-to-noise ratio. We find in this paper the influence on spectroscopy from the atomic/ground state alignment owing to anisotropic radiation and modulated by interstellar magnetic field, has significant impact on the study of interstellar gas. In different observational scenarios, we comprehensively demonstrate how atomic alignment influences the spectral analysis and provide the expressions for correcting the effect. The variations are even more pronounced for multiplets and line ratios. We show the variation of the deduced physical parameters caused by the atomic alignment effect, including alpha-to-iron ratio ([X/Fe]) and ionization fraction. Synthetic observations are performed to illustrate the visibility of such effect with current facilities. A study of Photodissociation regions in rho Ophiuchi cloud is presented to demonstrate how to account for atomic alignment in practice. Our work has shown that due to its potential impact, atomic alignment has to be included in an accurate spectroscopic analysis of the interstellar gas with current observational capability.
KW  - magnetic fields
KW  - submillimetre: ISM
KW  - ultraviolet: ISM
KW  - ISM: abundances
KW  - ISM: lines and bands
KW  - techniques: spectroscopic
Y1  - 2018
U6  - https://doi.org/10.1093/mnras/sty1594
SN  - 0035-8711
SN  - 1365-2966
VL  - 479
IS  - 3
SP  - 3923
EP  - 3935
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Vafin, Sergei
A1  - Rafighi, Iman
A1  - Pohl, Martin
A1  - Niemiec, Jacek
T1  - The Electrostatic Instability for Realistic Pair Distributions in Blazar/EBL Cascades
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - This work revisits the electrostatic instability for blazar-induced pair beams propagating through the intergalactic medium (IGM) using linear analysis and PIC simulations. We study the impact of the realistic distribution function of pairs resulting from the interaction of high-energy gamma-rays with the extragalactic background light. We present analytical and numerical calculations of the linear growth rate of the instability for the arbitrary orientation of wave vectors. Our results explicitly demonstrate that the finite angular spread of the beam dramatically affects the growth rate of the waves, leading to the fastest growth for wave vectors quasi-parallel to the beam direction and a growth rate at oblique directions that is only a factor of 2-4 smaller compared to the maximum. To study the nonlinear beam relaxation, we performed PIC simulations that take into account a realistic wide-energy distribution of beam particles. The parameters of the simulated beam-plasma system provide an adequate physical picture that can be extrapolated to realistic blazar-induced pairs. In our simulations, the beam looses only 1% of its energy, and we analytically estimate that the beam would lose its total energy over about 100 simulation times. An analytical scaling is then used to extrapolate the parameters of realistic blazar-induced pair beams. We find that they can dissipate their energy slightly faster by the electrostatic instability than through inverse-Compton scattering. The uncertainties arising from, e.g., details of the primary gamma-ray spectrum are too large to make firm statements for individual blazars, and an analysis based on their specific properties is required.
KW  - gamma rays: general
KW  - instabilities
KW  - magnetic fields
KW  - relativistic processes
KW  - waves
Y1  - 2018
U6  - https://doi.org/10.3847/1538-4357/aab552
SN  - 0004-637X
SN  - 1538-4357
VL  - 857
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Poppenhäger, Katja
T1  - How stars and planets interact
BT  - A look through the high-energy window
JF  - Astronomische Nachrichten = Astronomical notes
N2  - The architecture of exoplanetary systems is often different from the solar system, with some exoplanets being in close orbits around their host stars and having orbital periods of only a few days. In analogy to interactions between stars in close binary systems, one may expect interactions between the star and the exoplanet as well. From theoretical considerations, effects on the host star through tidal and magnetic interaction with the exoplanet are possible; for the exoplanet, some interesting implications are the evaporation of the planetary atmosphere and potential effects on the planetary magnetism. In this review, several possible interaction pathways and their observational prospects and existing evidence are discussed. A particular emphasis is put on observational opportunities for these kinds of effects in the high-energy regime.
KW  - magnetic fields
KW  - planet-star interactions
KW  - stars
KW  - activity
KW  - X-rays
Y1  - 2019
U6  - https://doi.org/10.1002/asna.201913619
SN  - 0004-6337
SN  - 1521-3994
VL  - 340
IS  - 4
SP  - 329
EP  - 333
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Vafin, Sergei
A1  - Deka, Pranab Jyoti
A1  - Pohl, Martin
A1  - Bohdan, A.
T1  - Revisit of Nonlinear Landau Damping for Electrostatic Instability Driven by Blazar-induced Pair Beams
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We revisit the effect of nonlinear Landau (NL) damping on the electrostatic instability of blazar-induced pair beams, using a realistic pair-beam distribution. We employ a simplified 2D model in k-space to study the evolution of the electric-field spectrum and to calculate the relaxation time of the beam. We demonstrate that the 2D model is an adequate representation of the 3D physics. We find that nonlinear Landau damping, once it operates efficiently, transports essentially the entire wave energy to small wave numbers where wave driving is weak or absent. The relaxation time also strongly depends on the intergalactic medium temperature, T-IGM, and for T-IGM << 10 eV, and in the absence of any other damping mechanism, the relaxation time of the pair beam is longer than the inverse Compton (IC) scattering time. The weak late-time beam energy losses arise from the accumulation of wave energy at small k, that nonlinearly drains the wave energy at the resonant k of the pair-beam instability. Any other dissipation process operating at small k would reduce that wave-energy drain and hence lead to stronger pair-beam energy losses. As an example, collisions reduce the relaxation time by an order of magnitude, although their rate is very small. Other nonlinear processes, such as the modulation instability, could provide additional damping of the nonresonant waves and dramatically reduce the relaxation time of the pair beam. An accurate description of the spectral evolution of the electrostatic waves is crucial for calculating the relaxation time of the pair beam.
KW  - gamma rays: general
KW  - instabilities
KW  - magnetic fields
KW  - relativistic processes
KW  - waves
Y1  - 2019
U6  - https://doi.org/10.3847/1538-4357/ab017b
SN  - 0004-637X
SN  - 1538-4357
VL  - 873
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Rüdiger, Günther
A1  - Schultz, Manfred
T1  - Large-scale dynamo action of magnetized Taylor-Couette flows
JF  - Monthly notices of the Royal Astronomical Society
N2  - A conducting Taylor-Couette flow with quasi-Keplerian rotation law containing a toroidal magnetic field serves as a mean-field dynamo model of the Tayler-Spruit type. The flows are unstable against non-axisymmetric perturbations which form electromotive forces defining a effect and eddy diffusivity. If both degenerated modes with m = +/- 1 are excited with the same power then the global a effect vanishes and a dynamo cannot work. It is shown, however, that the Tayler instability produces finite alpha effects if only an isolated mode is considered but this intrinsic helicity of the single-mode is too low for an alpha(2) dynamo. Moreover, an alpha Omega dynamo model with quasi-Keplerian rotation requires a minimum magnetic Reynolds number of rotation of Rm similar or equal to 2000 to work. Whether it really works depends on assumptions about the turbulence energy. For a steeper-than-quadratic dependence of the turbulence intensity on the magnetic field, however, dynamos are only excited if the resulting magnetic eddy diffusivity approximates its microscopic value, eta(T) similar or equal to eta. By basically lower or larger eddy diffusivities the dynamo instability is suppressed.
KW  - dynamo
KW  - instabilities
KW  - MHD
KW  - magnetic fields
Y1  - 2020
U6  - https://doi.org/10.1093/mnras/staa293
SN  - 0035-8711
SN  - 1365-2966
VL  - 493
IS  - 1
SP  - 1249
EP  - 1260
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Rüdiger, Günther
A1  - Küker, Manfred
T1  - Angular momentum transport by magnetoconvection and the magnetic modulation of the solar differential rotation
JF  - Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO)
N2  - In order to explain the variance of the solar rotation law during the activity minima and maxima, the angular momentum transport by rotating magnetoconvection is simulated in a convective box penetrated by an inclined azimuthal magnetic field. Turbulence-induced kinetic and magnetic stresses and the Lorentz force of the large-scale magnetic background field are the basic transporters of angular momentum. Without rotation, the sign of the magnetic stresses naturally depends on the signs of the field components as positive (negative) B theta B phi transport the angular momentum poleward (equatorward). For fast enough rotation, however, the turbulence-originated Reynolds stresses start to dominate the transport of the angular momentum flux. The simulations show that positive ratios of the two meridional magnetic field components to the azimuthal field reduce the inward radial as well as the equatorward latitudinal transport, which result from hydrodynamic calculations. Only for B theta B phi>0 (generated by solar-type rotation laws with an accelerated equator) does the magnetic-influenced rotation at the solar surface prove to be flatter than the nonmagnetic profile together with the observed slight spin-down of the equator. The latter phenomenon does not appear for antisolar rotation with polar vortex as well as for rotation laws with prevailing radial shear.
KW  - magnetic fields
KW  - Sun
KW  - rotation
KW  - convection
Y1  - 2021
U6  - https://doi.org/10.1051/0004-6361/202039912
SN  - 1432-0746
VL  - 649
PB  - EDP Sciences
CY  - Les Ulis
ER  -