bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetic Helicity Propagation from Inside the Sun
Authors: Longcope Dana 
Published: March 2016

AbstractModels of twisted flux tube evolution provide a picture of how magnetic helicity is propagated through the solar convection zone into the corona. According to the models, helicity tends toward an approximately uniform length-density along a tube, rather than concentrating at wider portions. Coronal fields lengthen rapidly during active region emergence, requiring additional helicity to propagate from the submerged flux tube. Recent observations of emerging active regions show an evolution consistent with this prediction, and no evidence of helicity concentrating in wider sections.
Generation and Annihilation of Magnetic Helicity in Active Regions
Authors: Kusano K. 
Published: March 2016

AbstractGeneration and annihilation processes of magnetic helicity in solar coronal active regions are investigated based on the observations and the simulations. We first examined the reliability of the numerical techniques, which enable to measure the magnetic helicity flux through the photosphere based on the magnetogram data. Secondly, in terms of the new technique, we found that magnetic helicities of the both signs are simultaneously injected into active regions. Motivated by this result, finally, we investigated the nonlinear process of the magnetic helicity annihilation, using the three-dimensional numerical simulations. The simulations clearly indicated that the helicity reversal can cause the eruption of large-scale plasmoid through the nonlinear process of the resistive instability growing on the helicity inversion layer. From these studies, we point out that the annihilation of magnetic helicity is a key process for the onset mechanism of solar flares.
Transport of Magnetic Helicity and Dynamics of Solar Active Regions
Authors: Georgoulis M. K. , LaBonte B. J. , Rust D. M. 
Published: March 2016

AbstractWe introduce a method to calculate the magnetic helicity density in a given active-region vector magnetogram, and a lower limit of it, based on a linear force-free (Iff) approximation. Moreover, we provide a lower limit of the total magnetic helicity in the active region (AR). A time series of magnetograms can be used to calculate the rate of helicity transport. The results can be then correlated with manifestations of the dynamical activity in ARs, such as flares and filament eruptions.
Solar and Interplanetary Magnetic Helicity Balance of Active Regions
Authors: Mandrini Cristina H. , Démoulin Pascal , van Driel-Gesztelyi Lidia , Dasso Sergio , Green Lucinda M. , Fuentes Marcelo López 
Published: March 2016

AbstractWe analyzed the long-term evolution of two active regions (ARs), NOAA 7978 and 8100, from their emergence through their decay using observations from several instruments on board SoHO (MDI, EIT and LASCO) and Yohkoh/SXT. We computed the evolution of the relative coronal magnetic helicity from one central meridian passage to the next, combining data from MDI and SXT with linear force-free models of the coronal magnetic field. Next, we calculated the injection of helicity by photospheric differential rotation using MDI magnetic maps and a mean differential rotation profile. To estimate the depletion of magnetic helicity we counted all the CMEs of which these ARs were the source, and we evaluated their helicity assuming a one to one correspondence with magnetic clouds (MCs) with an average helicity content; this value was computed for a sample of 18 clouds using a cylindrical linear force-free model. Out of our three helicity estimates (variation of coronal magnetic helicity, injection by differential rotation and ejection via CMEs) the one with the largest uncertainty is the amount of helicity ejected via CMEs. However, we determined, by modeling a particular MC using three different approaches in cylindrical geometry (two force-free models and a non force-free model with constant current), that its magnetic helicity content was nearly independent of the model used to fit in situ field observations (Dasso et al. 2003, in preparation). This result justifies our use of the average magnetic helicity value considering only a single MC model. Comparing the three components in the helicity balance (see Table 1), we find that photospheric differential rotation is a minor contributor to the AR magnetic helicity budget. CMEs carry away at least 10 times more helicity than the one differential rotation can provide. Therefore, the magnetic helicity flux needed in the global balance should come from localized photospheric motions that, at least partially, reflect the emergence of twisted flux tubes. Taking into account the magnetic flux in the ARs and the number of turns that a uniformly twisted flux tube should have to survive its rise through the convection zone, we have found that the total helicity carried away in CMEs is approximately equal to the end-to-end helicity of the flux tubes that formed these two ARs. Therefore, we conclude that most of the helicity ejected in CMEs is generated below the photosphere and emerges with the magnetic flux. Extended versions of this work were published in Demoulin et al. (2002, Astronomy & Astrophys. 382, 650) and in Green et al. (2002, Solar Phys. 208, 43), while in Mandrini et al. (2003, Astrophys. & Space Sci., 290, 319) and van Driel-Gesztelyi et al. (2003, Adv. Space Res., 32, 1855) the helicity computations were revised to include the underestimation of magnetic flux density found in MDI data. After this revision, we confirmed our former results.
A Non-Helical Dynamo — MHD Simulations of Dynamo Action by a Non-Helical Flow
Authors: Archontis V. , Dorch S.B.F. 
Published: March 2016

AbstractWe illustrate that helicity is not a necessary ingredient for fast dynamo action; we use the stagger-grid method of Galsgaard, Nordlund and others (e.g. Galsgaard & Nordlund 1997, and applied to dynamos by e.g. Dorch 2000): we solve the full MHD equations including a forcing term that keeps the kinetic energy at an approximately constant level. A 3-d flow with no mean helicity (an ABC-like flow without cosines, cf. Galloway & Proctor 1992) is implemented and it turns out that apart from the high growth rate in the linear regime (compared to kinematic dynamo action, cf. Archontis & Dorch 2003a), the dynamo saturates at a level significantly higher that the intermittent turbulent dynamos (cf. Archontis & Dorch 2003b); namely at exact energy equipartition. During the linear regime, several kinematic modes are present, e.g. a sheet/vortex-mode and a mode that resembles the ABC ”double cigar” mode (e.g. Dorch 2000). In the non-linear regime, the magnetic topology is not symmetric, but the initial structure of the velocity field is retained. The presence of helicity is not a requirement for dynamo action but it is rather the stretching ability of the flow that amplifies the magnetic energy in an exponential manner (Archontis & Dorch, in preparation).
Magnetic Neutral Line Rotations in Flare-Productive Regions
Authors: Ishii Takako T. , Asai Ayumi , Kurokawa Hiroki , Takeuchi Tsutomu T. 
Published: March 2016

AbstractWe examined the evolution of active regions and found that rotational motions of the neutral line in the δ-type magnetic configuration are common in the flare-productive active regions.
On a Cyclic Variation of the Hemispheric Helicity Rule
Authors: Pevtsov A. A. , Hagyard M. J. , Blehm Z. , Smith J. E. , Canfield R. C. , Sakurai T. , Hagino M. 
Published: March 2016

AbstractWe report the result of a study of magnetic helicity in solar active regions during 1980-2000. Using the vector magnetograms from four instruments (Haleakala Stokes Polarimeter, Marshall Space Flight Center, Mitaka Solar Flare Telescope and Okayama Observatory Solar Telescope) we calculated the force-free parameter α and computed a slope dα/dφ as the linear fit of α vs. latitude φ, using annual subsets of data. The hemispheric helicity rule can be expressed in terms of this slope as dα/dφ < 0. We find that each instrument exhibits change in sign of dα/dφ for some years. However, we do not see consistency between different instruments in regards to years disobeying the rule. We show that this inconsistency can be attributed to insufficient numbers of active regions in annual subsets of data. We conclude that the present data sets do not allow to make statistically significant inference about possible cyclic variation of the hemispheric helicity rule.For further discussion see Hagyard et al., 2003, ApJ, submitted.
The Search for Correlation between BiSON SMMF Data and CME Events
Authors: Chaplin William J. , Dumbill Andrew M. , Elsworth Yvonne P. , Isaak George R. , McLeod Clive P. , Miller Brek A. , New Roger , Pintér Balázs 
Published: March 2016

AbstractWe present first attempts to compare the Birmingham Solar-Oscillations Network (BiSON) high precision solar mean magnetic field (SMMF) data of four years with the occurrence of CMEs (coronal mass ejections) as recorded by LASCO on board SOHO. The BiSON magnetic measurement technique is given in Chaplin et al. (2003). Particularly interesting results of recent SMMF high-cadence observations have come from studies of correlation between the SMMF determined by MDI and the occurrence of CMEs (Boberg and Lundstedt 2000 and Boberg et al 2002). Two frequency ranges, centered on 13 and 90 minutes, have been identified as possibly correlating with CME occurrence.We have used BiSON SMMF data from two sites to investigate CME related SMMF signals to try to confirm the MDI results. To search methodically through our data set we have developed two correlation techniques suited to short (up to 32 minutes) and long (up to 3 hours) period wavelets, respectively. For short periods we analyzed SMMF data in the vicinity of CMEs, and for long periods we compared SMMF results for days with and without recorded CMEs. In neither period range have we yet clearly identified correlations between SMMF power excesses and CME onsets. For the details of the techniques and the results see Chaplin et al. (2004).
M31’s Disk System of Globular Clusters
Authors: Morrison Heather , Harding Paul , Hurley-Keller Denise , Perrett Kathy 
Published: March 2016

AbstractGlobular cluster systems are often thought to be associated with violent formation events such as galaxy mergers or the formation of large bulges. However, formation in relatively ordered regions such as thin disks may also be an important process which has been overlooked.Recent high-quality spectroscopic studies of the M31 globulars show that a significant number of the clusters projected on its disk belong to a rapidly rotating thin disk. This contrasts strongly with the Milky Way system, which is composed of a halo and thick disk system and has no known thin disk globulars. It is also likely that M31 has experienced no minor mergers since the globular cluster formation epoch, as such a merger would have heated the globulars into a thick disk system. The metallicity distributions of the disk and non-disk clusters are quite similar.
Extreme Globular Cluster Systems
Authors: Blakeslee John P. 
Published: March 2016

AbstractThe superior resolution and large format of the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) make it a powerful new tool in the study of extra-galactic globular cluster (GC) systems. We summarize some early results on GC populations from the ACS GTO program, concentrating on the extreme cases of the isolated dwarf NGC 2915 and the core of the massive lensing cluster Abell 1689.
The ACS Virgo Cluster Survey
Authors: Côté Patrick , West Michael J. 
Published: March 2016

AbstractWe describe the ACS Virgo Cluster Survey, an HST imaging survey of early-type galaxies in the Virgo Cluster. Multi-color ACS images for one hundred member galaxies, spanning a range of 450 in luminosity, are being used to study the central regions of these galaxies, their globular cluster systems, and the three-dimensional structure of Virgo itself. In terms of depth, spatial resolution, sample size and homogeneity, this represents the most comprehensive imaging survey to date of early-type galaxies in dense environments.
Stellar Population Models with Variable Element Abundance Ratios
Authors: Thomas Daniel , Maraston Claudia , Bender Ralf 
Published: March 2016

AbstractWe present a comprehensive set of new generation stellar population models of Lick absorption line indices, which for the first time include element abundance ratios different from solar. We computed the 21 Lick indices CN1, CN2, Ca4227, G4300, Fe4383, Ca4455, Fe4531, C24668, Hβ, Fe5015, Mg1, Mg2, Mgb, Fe5270, Fe5335, Fe5406, Fe5709, Fe5782, Na D, TiO1, and TiO2, in the wavelength range 4000 ≲ λ ≲ 6500 Å. Models are provided with: [α/Fe] = 0.0, 0.3, 0.5, [α/Ca] = -0.1, 0.0, 0.2, 0.5, and [α/N] = −0.5, 0.0; ages from 1 to 15 Gyr; total metallicities from 1/200 to 3.5 solar (-2.25 ≤ [Z/H] ≤ 0.67).
Self-Induced Formation of Metal-Rich Globulars in Bulges?
Authors: Kravtsov Valery V. 
Published: March 2016

AbstractTaken together, the latest key observations assume that (i) old metal-rich globular cluster populations (MRGCPs) in bulges were able to form due to an increased self-induced star formation rate (SFR) in their host galaxies, while galaxy merging played an additional role; (ii) massive star cluster populations (MSCPs) in irregulars may be young, less prominent counterparts of the old MRGCPs in spheroids.
Evolution of Globular Cluster Populations in Compact Galaxy Groups
Authors: Leon S. , Perea J. , Del Olmo A. , Athanassoula E. , Bergond G. , Meylan G. , Gomez C. Garcia 
Published: March 2016

AbstractThe redistribution of globular clusters within compact groups of galaxies is followed through N-body simulations. Particular emphasis is given to the globular clusters released in the Intra-Group Medium (IGM) and to the final configuration of the evolution.
Chemical Abundances in the Sagittarius Galaxy: Terzan 7
Authors: Tautvaišienė Gražina , Wallerstein George , Geisler Doug , Gonzalez Guillermo , Charbonnel Corinne 
Published: March 2016

AbstractAbundances of 21 chemical elements have been determined in three red giants in Terzan 7 using high-resolution spectra obtained with the UVES spectrograph on the ESO 8.2 m Kueyen telescope. The mean [Fe/H]= –0.61 ±0.07. The relative elemental abundance ratios indicate a close similarity of Terzan 7 to its host galaxy.
Mergers and Non-Thermal Processes in Clusters
Authors: Sarazin Craig L. 
Published: March 2016

AbstractClusters of galaxies generally form by the gravitational merger of smaller clusters and groups. Mergers drive shocks in the intra-cluster gas which heat the intra-cluster gas. Mergers disrupt cluster cooling cores. Mergers produce large, temporary increases in the X-ray luminosities and temperatures of cluster; such merger boost may bias estimates of cosmological parameters from clusters. Chandra observations of the X-ray signatures of mergers, particularly “cold fronts,” will be discussed. X-ray observations of shocks can be used to determine the kinematics of the merger. As a result of particle acceleration in shocks and turbulent acceleration following mergers, clusters of galaxies should contain very large populations of relativistic electrons and ions. Observations and models for the radio, extreme ultraviolet, hard X-ray, and gamma-ray emission from non-thermal particles accelerated in these shocks are described.
A Physicist’s View of Stellar Dynamics: Dynamical Instability of Stellar Systems
Authors: Gurzadyan V.G. 
Published: March 2016

AbstractI argue that the widely adopted framework of stellar dynamics survived since 1940s, is not fitting the current knowledge on non-linear systems. Borrowed from plasma physics when several fundamental features of perturbed non-linear systems were unknown, that framework ignores the difference in the role of perturbations in two different classes of systems, in plasma with Debye screening and gravitating systems with no screening. Now, when the revolutionary role of chaotic effects is revealed even in planetary dynamics i.e. for nearly integrable systems, one would expect that for stellar systems, i.e. non-integrable systems, their role have to be far more crucial. Indeed, ergodic theory tools already enabled to prove that spherical stellar systems are exponentially instable due to N-body interactions, while the two-body encounters, contrary to existing belief, are not the dominating mechanism of their relaxation. Chaotic effects distinguish morphological and other properties of galaxies. Using the Ricci curvature criterion, one can also show that a central massive object (nucleus) makes the N-body gravitating system more instable (chaotic), while systems with double nuclei are even more instable than those with a single one.
Formation of Young Star Clusters
Authors: Elmegreen Bruce 
Published: March 2016

AbstractTurbulence, self-gravity, and cooling convert most of the interstellar medium into cloudy structures that form stars. Turbulence compresses the gas into clouds directly and it moves pre-existing clouds around passively when there are multiple phases of temperature. Self-gravity also partitions the gas into clouds, forming giant regular complexes in spiral arms and in resonance rings and contributing to the scale-free motions generated by turbulence. Dense clusters form in the most strongly self-gravitating cores of these clouds, often triggered by compression from local stars. Pre-star formation processes inside clusters are not well observed, but the high formation rates and high densities of pre-stellar objects, and their power law mass functions suggest that turbulence, self-gravity, and energy dissipation are involved there too.
Apparent Inconsistencies in the Formation of Cometary Matter
Authors: Huebner W. F. , Keller H. U. 
Published: March 2016

AbstractThe Joint Discussion (JD14) is intended to focus attention on several apparent inconsistencies found in comet observations when compared with observations of collapsing interstellar clouds, star-forming regions, and models of the solar nebula. The JD is not intended to resolve these problems, for that a one-day discussion is too short. It rather is intended to draw attention to the inconsistencies to be discussed and resolved in future workshops.
Cometary Silicates: Interstellar and Nebular Materials
Authors: Wooden Diane H. 
Published: March 2016

AbstractEvidence for interstellar material in comets is deduced from IR spectra, in situ measurements of Comet Halley, and chondritic porous interplanetary dust particles (CP IDPs). IR spectra of comets reveal the spectrally active minerals: amorphous carbon, amorphous silicates, and (in some comets) crystalline silicates. Evidence suggests amorphous silicates are of interstellar origin while crystalline silicates are of nebular origin.10 μm spectra of comets and sub-micron amorphous silicate spherules in CP IDPs have shapes similar to absorption spectra through lines-of-sight in the ISM. Thermal emission models of cometary IR spectra require Fe-bearing amorphous silicates. Fe-bearing amorphous silicates may be Fe-bearing crystalline silicates formed in AGB outflows that are amorphized through He+ ion bombardment in supernova shocks in the ISM.Crystalline silicates in comets, as revealed by IR spectra, and their apparent absence in the ISM, argues for their nebular origin. The high temperatures (>1000 K) at which crystals form or are annealed occur in the inner nebula or in nebular shocks in the 5 – 10 AU region. Oxygen isotope studies of CP IDPs show only 1% by mass of the silicate crystals are of AGB origin. Together this suggests crystalline silicates in comets are probably primitive grains from the early solar nebula.
Uncertainties in Stellar Abundance Analyses
Authors: Asplund Martin 
Published: March 2016

AbstractOver the last half-century quantitative stellar spectroscopy has made great progress. However, most stellar abundance analyses today still employ rather simplified models, which can introduce severe systematic errors swamping the observational errors. Some of these uncertainties for late-type stars are briefly reviewed here: atomic and molecular data, stellar parameters, model atmospheres and spectral line formation.
Evolution of Metals and Stars in Damped Lyman-Alpha Galaxies
Authors: Kulkarni Varsha P. 
Published: March 2016

AbstractDamped Lyman-alpha absorbers in quasar spectra provide a unique tool to directly measure the abundances of elements in galaxies at red-shifts 0 < z < 5, and hence probe the chemical evolution of galaxies over > 90% of the age of the Universe. Since cosmic chemical evolution models predict the global metallicity of galaxies to increase with time, it is of great interest to determine whether DLAs actually show such a trend. We discuss statistical analysis of existing DLA Zn data to examine the metallicity-redshift relation, and a comparison of the observed data with models of cosmic chemical evolution. We also describe efforts to expand the DLA abundance sample at z < 1.5, where the current data are particularly sparse. Finally, we discuss emission-line imaging studies of the absorber galaxies and compare constraints on their star formation rates with models based on the global star formation history.
Iron Kα Spectra from an Atomic Modeling Perspective
Authors: Liedahl Duane A. 
Published: March 2016

AbstractIn order to upgrade the diagnostic capability associated with Fe Kα spectra, we have calculated detailed atomic models that include the effects of inner-shell photoionization, autoionization, collisional excitation at relatively high particle number densities, and photoexcitation in plasmas with high X-ray and/or UV energy densities. I discuss some of the potential uses of these models in the context of compact X-ray sources.
Photorecombination and Photoionization Experiments at Heavy-Ion Storage-Rings and Synchrotron-Light Sources
Authors: Schippers Stefan 
Published: March 2016

AbstractRecent experimental work on the photorecombination and the photoionization of astrophysically relevant atomic ions employing the merged-beams technique at heavy-ion storage-rings and synchrotron-light sources, respectively, is summarized. The resulting absolute photoionization cross sections and recombination rate coefficients benchmark corresponding theoretical calculations and are needed for the accurate determination of ionization equilibria in astrophysical plasmas.
The Ionized Gas and Nuclear Environment in NGC 3783
Authors: Netzer Hagai 
Published: March 2016

AbstractThe 900 ks Chandra spectrum of NGC 3783 was analyzed in various ways to study the properties of the absorbing gas. The main findings are: 1) On a time scale of 20-120 days, the source exhibits two very different spectral shapes with different softness ratios. The absorbing gas is not responding to these continuum variations. 2) Detailed photoionization modeling based on 10 silicon and sulphur lines near 5-7Å produces a satisfactory fit of the entire 3-25 A spectrum of the source. The only discrepancy is in the central wavelength of the iron UTA feature. 3) A combination of three ionized absorbers, each split into two kinematic components, can explain the strengths of almost all the absorption lines and bound-free edges. The three components span a large range of ionization and have a total column of about 4×1022 cm-2. Moreover, all three components are thermally stable and seem to have the same gas pressure.
FUSE and the Quest for High-Resolution Spectroscopy in the Far Ultraviolet
Authors: Moos H. Warren 
Published: March 2016

AbstractThe Far Ultraviolet Spectroscopic Explorer (FUSE) instrument covers the spectral range 912-1187 Å with a resolving power of 15,000 to 20,000. This spectral region provides unique access for the study of many atomic and ionic species found in the interstellar medium, intergalactic medium, stars, and extragalactic objects. This paper summarizes the status of the mission and then discusses the need for higher resolution spectroscopy. Although the FUSE instrumental resolution is sufficient to separate most species, it usually it is not adequate for analyzing the gas velocity structure in detail. Implications for future missions are discussed.