bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetic fields in our Milky Way Galaxy and nearby galaxies
Authors: Han JinLin 
Published: July 2013

AbstractMagnetic fields in our Galaxy and nearby galaxies have been revealed by starlight polarization, polarized emission from dust grains and clouds at millimeter and submillimeter wavelength, the Zeeman effect of spectral lines or maser lines from clouds or clumps, diffuse radio synchrotron emission from relativistic electrons in interstellar magnetic fields, and the Faraday rotation of background radio sources as well as pulsars for our Milky Way. It is easy to get a global structure for magnetic fields in nearby galaxies, while we have observed many details of magnetic fields in our Milky Way, especially by using pulsar rotation measure data. In general, magnetic fields in spiral galaxies probably have a large-scale structure. The fields follow the spiral arms with or without the field direction reversals. In the halo of spiral galaxies magnetic fields exist and probably also have a large-scale structure as toroidal and poloidal fields, but seem to be slightly weaker than those in the disk. In the central region of some galaxies, poloidal fields have been detected as vertical components. Magnetic field directions in galaxies seem to have been preserved during cloud formation and star formation, from large-scale diffuse interstellar medium to molecular clouds and then to the cloud cores in star formation regions or clumps for the maser spots. Magnetic fields in galaxies are passive to dynamics.
Interstellar and intergalactic dynamos
Authors: Hanasz M. , Woltanski D. , Kowalik K. 
Published: July 2013

AbstractWe review recent developments of amplification models of galactic and intergalactic magnetic field. The most popular scenarios involve variety of physical mechanisms, including turbulence generation on a wide range of physical scales, effects of supernovae, buoyancy as well as the magnetorotational instability. Other models rely on galaxy interaction, which generate galactic and intergalactic magnetic fields during galaxy mergers. We present also global galactic-scale numerical models of the Cosmic Ray (CR) driven dynamo, which was originally proposed by Parker (1992). We conduct a series of direct CR+MHD numerical simulations of the dynamics of the interstellar medium (ISM), composed of gas, magnetic fields and CR components. We take into account CRs accelerated in randomly distributed supernova (SN) remnants, and assume that SNe deposit small-scale, randomly oriented, dipolar magnetic fields into the ISM. The amplification timescale of the large-scale magnetic field resulting from the CR-driven dynamo is comparable to the galactic rotation period. The process efficiently converts small-scale magnetic fields of SN-remnants into galactic-scale magnetic fields. The resulting magnetic field structure resembles the X-shaped magnetic fields observed in edge-on galaxies.
Small-scale dynamo action in primordial halos
Authors: Schober Jennifer , Schleicher Dominik R. G. , Klessen Ralf S. , Federrath Christoph , Bovino Stefano , Glover Simon , Banerjee Robi 
Published: July 2013

AbstractThe first galaxies form due to gravitational collapse of primordial halos. During this collapse, weak magnetic seed fields get amplified exponentially by the small-scale dynamo - a process converting kinetic energy from turbulence into magnetic energy. We use the Kazantsev theory, which describes the small-scale dynamo analytically, to study magnetic field amplification for different turbulent velocity correlation functions. For incompressible turbulence (Kolmogorov turbulence), we find that the growth rate is proportional to the square root of the hydrodynamic Reynolds number, Re1/2. In the case of highly compressible turbulence (Burgers turbulence) the growth rate increases proportional to Re1/3. With a detailed chemical network we are able to follow the chemical evolution and determine the kinetic and magnetic viscosities (due to Ohmic and ambipolar diffusion) during the collapse of the halo. This way, we can calculate the growth rate of the small-scale dynamo quantitatively and predict the evolution of the small-scale magnetic field. As the magnetic energy is transported to larger scales on the local eddy-timescale, we obtain an estimate for the magnetic field on the Jeans scale. Even there, we find that equipartition with the kinetic energy is reached on small timescales. Dynamically relevant field structures can thus be expected already during the formation of the first objects in the Universe.
Mechanisms of formation ofsolar pores and sunspots
Authors: Kitiashvili Irina N. 
Published: July 2013

AbstractSpontaneous formation of self-organized magnetic structures, such as sunspots and pores, is one of intriguing and oldest problems, which represents a complicated interaction of convection and magnetic fields on different scales. Observations of sunspots and pores formation reveal a fast process of accumulation of emerging magnetic field into stable long-living magnetic structures. However, the physical mechanisms of the flux accumulation into the compact magnetic structures with high field strength and their stability are not clear. Development of observational capabilities, theory, and realistic-type MHD numerical simulations open a new level of our understanding of the turbulent processes of the magnetic field accumulation. I discuss the recent progress in observations and radiative MHD simulations that provide important clues for possible mechanisms of formation and stability of sunspots and pores, and their links to the dynamo process.
Flux concentrations in turbulent convection
Authors: Käpylä Petri J. , Brandenburg Axel , Kleeorin Nathan , Mantere Maarit J. , Rogachevskii Igor 
Published: July 2013

AbstractWe present preliminary results from high resolution magneto-convection simulations where we find the formation of flux concentrations from an initially uniform magnetic field. The structures appear in roughly ten convective turnover times and live close to a turbulent diffusion time. The time scales are compatible with the negative effective magnetic pressure instability (NEMPI), although structure formation is not restricted to regions where the effective magnetic pressure is negative.
Fractal multi-scale nature of solar/stellar magnetic fields
Authors: Abramenko Valentina I. 
Published: July 2013

AbstractAn abstract mathematical concept of fractal organization of certain complex objects received significant attention in astrophysics during last decades. The concept evolved into a broad field including multi-fractality and intermittency, percolation theory, self-organized criticality, theory of catastrophes, etc. Such a strong mathematical and physical approach provide new possibilities for exploring various aspects of astrophysics. In particular, in the solar and stellar magnetism, multi-fractal properties of magnetized plasma turned to be useful for understanding burst-like dynamics of energy release events, conditions for turbulent dynamo action, nature of turbulent magnetic diffusivity, and even the dual nature of solar dynamo. In this talk, I will briefly outline how the ideas of multi-fractality are used to explore the above mentioned aspects of solar magnetism.
Solar-like differential rotation and equatorward migration in a convective dynamo with a coronal envelope
Authors: Warnecke J. , Käpylä P. J. , Mantere M. J. , Brandenburg A. 
Published: July 2013

AbstractWe present results of convective turbulent dynamo simulations including a coronal layer in a spherical wedge. We find an equatorward migration of the radial and azimuthal fields similar to the behavior of sunspots during the solar cycle. The migration of the field coexist with a spoke-like differential rotation and anti-solar (clockwise) meridional circulation. Even though the migration extends over the whole convection zone, the mechanism causing this is not yet fully understood.
Magnetic helicity transported by flux emergence and shuffling motions
Authors: Zhang Y. , Kitai R. , Takizawa K. 
Published: July 2013

AbstractMagnetic helicity can be transported from sub-photosphere into corona by the emergence of helical magnetic field lines and the shuffling motions of foot-points of pre-existing coronal field lines. Active region NOAA 10930 was observed by SP and NFI of SOT on board Hinode when it pass through the solar meridian. Based on these observations, we calculate magnetic helicity flow of both terms, by regarding Doppler velocity as normal velocity. The results are compared with which calculated by method proposed by Zhang et. al. (2012). Our results show that helicity injection maps calculated by both methods have similar distribution and the integration values have the same magnitude.
On magnetohydrodynamic turbulence and angular momentum transport in accretion disk boundary layers
Authors: Chan Chi-kwan , Pessah Martin E. 
Published: July 2013

AbstractThe physical modeling of the accretion disk boundary layer, the region where the disk meets the surface of the accreting star, usually relies on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity, widely adopted in astrophysics, satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability is inefficient in this inner disk region. I will discuss the results of a recent study on the generation of hydromagnetic stresses and energy density in the boundary layer around a weakly magnetized star. Our findings suggest that although magnetic energy density can be significantly amplified in this region, angular momentum transport is rather inefficient. This seems consistent with the results obtained in numerical simulations and suggests that the detailed structure of turbulent MHD boundary layers could differ appreciably from those derived within the standard framework of turbulent shear viscosity.
Toward understanding the multiscale spatial spectrum of solar convection
Authors: Getling A. V. , Mazhorova O. S. , Shcheritsa O. V. 
Published: July 2013

AbstractConvection is simulated numerically based on two-dimensional Boussinesq equations for a fluid layer with a specially chosen stratification such that the convective instability is much stronger in a thin subsurface sublayer than in the remaining part of the layer. The developing convective flow has a small-scale component superposed onto a basic large-scale roll flow.
Non-linear and chaotic dynamo regimes
Authors: Brandenburg Axel 
Published: July 2013

AbstractAn update is given on the current status of solar and stellar dynamos. At present, it is still unclear why stellar cycle frequencies increase with rotation frequency in such a way that their ratio increases with stellar activity. The small-scale dynamo is expected to operate in spite of a small value of the magnetic Prandtl number in stars. Whether or not the global magnetic activity in stars is a shallow or deeply rooted phenomenon is another open question. Progress in demonstrating the presence and importance of magnetic helicity fluxes in dynamos is briefly reviewed, and finally the role of nonlocality is emphasized in modeling stellar dynamos using the mean-field approach. On the other hand, direct numerical simulations have now come to the point where the models show solar-like equatorward migration that can be compared with observations and that need to be understood theoretically.
Experimental realization of dynamo action: present status and prospects
Authors: Giesecke André , Stefani Frank , Gundrum Thomas , Gerbeth Gunter , Nore Caroline , Léorat Jacques 
Published: July 2013

AbstractIn the last decades, the experimental study of dynamo action has made great progress. However, after the dynamo experiments in Karlsruhe and Riga, the von-Kármán-Sodium (VKS) dynamo is only the third facility that has been able to demonstrate fluid flow driven self-generation of magnetic fields in a laboratory experiment. Further progress in the experimental examination of dynamo action is expected from the planned precession driven dynamo experiment that will be designed in the framework of the liquid sodium facility DRESDYN (DREsden Sodium facility for DYNamo and thermohydraulic studies).In this paper, we briefly present numerical models of the VKS dynamo that demonstrate the close relation between the axisymmetric field observed in that experiment and the soft iron material used for the flow driving impellers. We further show recent results of preparatory water experiments and design studies related to the precession dynamo and delineate the scientific prospects for the final set-up.
Flux transport dynamo coupled with a fast tachocline scenario
Authors: Karak Bidya Binay , Petrovay Kristof 
Published: July 2013

AbstractThe tachocline is important in the solar dynamo for the generation and the storage of the magnetic fields. A most plausible explanation for the confinement of the tachocline is given by the fast tachocline model in which the tachocline is confined by the anisotropic momentum transfer by the Maxwell stress of the dynamo generated magnetic fields. We employ a flux transport dynamo model coupled with the simple feedback formula of this fast tachocline model which basically relates the thickness of the tachocline to the Maxwell stress. We find that this nonlinear coupling not only produces a stable solar-like dynamo solution but also a significant latitudinal variation in the tachocline thickness which is in agreement with the observations.
Solar dynamo model with nonlocal alpha-effect and diamagnetic pumping
Authors: Kitchatinov L. L. , Olemskoy S. V. 
Published: July 2013

AbstractSunspot data and large-scale solar magnetic field data are used to demonstrate the operation of the Babcock-Leighton mechanism on the Sun. A dynamo model is developed that employs jointly a nonlocal alpha-effect of the Babcock-Leighton type and diamagnetic downward pumping. The pumping concentrates magnetic fields to the base of the convection zone. The magnetic cycle period, equatorial symmetry of the generated fields, their meridional drift, and the polar-to-toroidal field ratio obtained in the model agree with observations.
Modelling stellar activity cycles using deep-seated dynamos and surface flux transport
Authors: Işık Emre , Schmitt Dieter , Schüssler Manfred 
Published: July 2013

AbstractWe investigate the relations between tachocline-based dynamos and the surface flux transport mechanisms in stars with outer convection zones. Using our combined models of flux generation and transport, we demonstrate the importance of the buoyant rise of magnetic flux, which physically determines the emergence latitudes and tilt angles of bipolar magnetic regions. The combined effects of the dynamo strength, flux rise, and surface transport lead to various cyclic and non-cyclic time series of total unsigned surface magnetic flux.
Forecasting the solar activity cycle: new insights
Authors: Nandy Dibyendu , Karak Bidya Binay 
Published: July 2013

AbstractHaving advance knowledge of solar activity is important because the Sun's magnetic output governs space weather and impacts technologies reliant on space. However, the irregular nature of the solar cycle makes solar activity predictions a challenging task. This is best achieved through appropriately constrained solar dynamo simulations and as such the first step towards predictions is to understand the underlying physics of the solar dynamo mechanism. In Babcock-Leighton type dynamo models, the poloidal field is generated near the solar surface whereas the toroidal field is generated in the solar interior. Therefore a finite time is necessary for the coupling of the spatially segregated source layers of the dynamo. This time delay introduces a memory in the dynamo mechanism which allows forecasting of future solar activity. Here we discuss how this forecasting ability of the solar cycle is affected by downward turbulent pumping of magnetic flux. With significant turbulent pumping the memory of the dynamo is severely degraded and thus long term prediction of the solar cycle is not possible; only a short term prediction of the next cycle peak may be possible based on observational data assimilation at the previous cycle minimum.
CrH molecule: New diagnostic tool for measuring magnetic fields of cool dwarfs
Authors: Kuzmychov Oleksii , Berdyugina Svetlana V. 
Published: July 2013

AbstractThe quantum mechanical calculations for the A6Σ+ − X6Σ+ system of the CrH molecule were done in order to obtain its energy level structure and transition strenghts both in the absence and in the presence of a magnetic field. Employing this results and solving a set of the radiative transfer equations, we calculated the Stokes profiles for the entire 0-0 band for different magnetic field strengths and orientations. We show that the CrH lines produce a considerable polarization signal (up to 20%) at 0.5 - 10 kG. Furthermore, the polarization signal shows a significant asymmetry (broad-band polarization) that arises from the Paschen-Back effect in the individual CrH lines. An example of the signal, as it can be observed, is provided.
Venus transit, aureole and solar diameter
Authors: Xie Wenbin , Sigismondi Costantino , Wang Xiaofan , Tanga Paolo 
Published: July 2013

AbstractThe possibility to measure the solar diameter using the transits of Mercury has been exploited to investigate the past three centuries of its evolution and to calibrate these measurements made with satellites. This measurement basically consists to compare the ephemerides of the internal contact timings with the observed timings. The transits of Venus of 2004 and 2012 gave the possibility to apply this method, involving a planet with atmosphere, with the refraction of solar light through it creating a luminous arc all around the disk of the planet. The observations of the 2012 transit made to measure the solar diameter participate to the project Venus Twilight Experiment to study the aureole appearing around it near the ingress/egress phases.
Radio imaging-spectroscopy observations of the Sun in decimetric and centimetric wavelengths
Authors: Yan Yihua , Wang Wei , Liu Fei , Geng Lihong , Chen Zhijun , Zhang Jian 
Published: July 2013

AbstractTo address fundamental processes in the solar eruptive phenomena it is important to have imaging-spectroscopy over centimetric-decimetric wave range. The Chinese Spectral Radioheliograph (CSRH) in 0.4-15 GHz range with high time, space and frequency resolutions is being constructed to achieve this goal. The perspectives to open new observational windows on solar flares and CMEs will be achieved by mapping the radio emission from unstable electron populations during the basic processes of energy release. CSRH is located in a radio quiet region in Inner Mongolia of China. The array of CSRH-I in 0.4-2.0 GHz with 40 4.5m antennas has been established and starts test observations. The 60 2m antennas for array of CSRH-II in 2-15 GHz have been mounted and assembled. The progress and current status of CSRH are introduced.
Calibration and data analysis for Chinese Spectral Radioheliograph
Authors: Wang W. , Yan Y. H. , Liu D. H. , Chen Z. J. , Liu F. , Geng L. H. , Chen L. J. , Su C. 
Published: July 2013

AbstractThe Chinese Spectral Radioheliograph (CSRH) is under construction in Mingantu station of NAOC in China. Now, CSRH-I which includes antenna, receiver and correlator in decimetric wave range has been established. This paper introduced calibration on CSRH and present some results of data processing.
Micorwave observations of the Chinese Solar Broadband Radio Spectrometer at Huairou
Authors: Tan Chengming , Tan Baolin , Yan Yihua , Liu Yuying 
Published: July 2013

AbstractThis work presents an overview of the microwave observations of the Chinese Solar Broadband Radio Spectrometer at Huairou (SBRS/Huairou) during 1997-2011. The relationships between the microwave bursts and solar flares and the calibration of spectrometers are also studied.
Helicity transport from solar convection zone to interplanetary space
Authors: Zhang Mei 
Published: July 2013

AbstractMagnetic helicity is a physical quantity that describes field topology. It is also a conserved quantity as Berger in 1984 demonstrated that the total magnetic helicity is still conserved in the corona even when there is a fast magnetic reconnection. It is generally believed that solar magnetic fields, together with their helicity, are created in the convection zone by various dynamo processes. These fields and helicity are transported into the corona through solar photosphere and finally released into the interplanetary space via various processes such as coronal mass ejections (CMEs) and solar winds. Here I will give a brief review on our recent works, first on helicity observations on the photosphere and how to understand these observations via dynamo models. Mostly, I will talk about what are the possible consequences of magnetic helicity accumulation in the corona, namely, the formation of magnetic flux ropes, CMEs taking place as an unavoidable product of coronal evolution, and flux emergences as a trigger of CMEs. Finally, I will address on in what a form magnetic field in the interplanetary space would accommodate a large amount of magnetic helicity that solar dynamo processes have been continuously producing.
Magnetic helicity ejections and coronal activity
Authors: Nindos A. 
Published: July 2013

AbstractMagnetic helicity quantifies the degree of linkage and/or twistedness in the magnetic field. It is probably the only physical quantity which is approximately conserved even in resistive MHD. This makes it an ideal tool for the exploration of the physics of solar eruptions. In this article, I discuss the sources of magnetic helicity injected into active regions and I point out that coronal mass ejections (CMEs) are probably necessary to remove at least part of the excess helicity produced in the Sun. I also discuss the importance of magnetic helicity in the overall coronal evolution that may lead to eruptions.
Radiative Hydrodynamic Simulations of He I 10830 Å
Authors: Cheng J. X. , Ding M. D. , Fang C. 
Published: July 2013

AbstractWe study the properties of the He I 10830 Å line under the bombardment of nonthermal electron beam. Using radiative hydrodynamics method, we obtain the line profiles from different model atmospheres by varying the electron beam fluxes. Below a certain electron flux, the spectra become much more absorptions as nonthemal flux increases. Above the threshold, the spectral intensities increase as the flux goes up. We also investigate the temporal evolution of the spectra under the nonthermal heating. For weak electron flux, the profiles evolve from weak absorptions to strong absorptions. For strong nonthermal heating, the profiles can be significantly changed from absorptions to emissions. The spectra also show red or blue asymmetries.
A statistical study on solar soft X-ray flare and the sources
Authors: Guo Juan 
Published: July 2013

AbstractWe surveyed 28 year of Solar X-ray flares and sunspot group records supplied by the National Geophysical Data Center (NGDC) to study the dependence of different class Xray flares on the magnetic structure of sunspot groups statistically. It is found that 82.5% X-class flare, 51.4% M-class flare, 26.3% C-class flare, and 10.1% B-class flare occurred in the sunspot group with βγδ structure. There are 6.6% X-class flares, 17.5% M-class flare, 39.1% C-class flare, and 60.4% B-class flare occurred out of sunspot group.