bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pulsed Acceleration in Solar Flares
Authors: Aschwanden Markus J. , Benz Arnold O. , Dennis Brian R. , Kundu Mukul R. 
Published: July 2016

AbstractWe study the nonlinear dynamics of particle acceleration in solar flares by analyzing the time series of various quasi-periodic radio signatures during flares. In particular we present the radio and hard X-ray data of three flares which support the following tentative conclusions:1.Particle acceleration and injection into magnetic structures occurs intrinsically in a pulsed mode (with a typical period of 1-2 s), produced by a single, spatially coherent, nonlinear system, rather than by a stochastic system with many spatially independent components (“statistical flare” produced by a fragmented primary energy release).2.The nonlinear (quasi-periodic) mode of pulsed particle acceleration and injection into a coronal loop can be stabilized by phase locking with an MHD wave (oscillation) mode, if both periods are close to each other.3.Pulsed injection of electron beams into a coronal loop may trigger nonlinear relaxational oscillations of wave-particle interactions. This is particularly likely when the limit cycles of both systems are similar.Subject headings: acceleration of particles— Sun: corona — Sun: flares — Sun: radio radiation — Sun: X-rays, gamma rays
Relativistic Particle Acceleration in Plerions
Authors: Arons Jonathan , Tavani Marco 
Published: July 2016

AbstractWe discuss recent research on the structure and particle acceleration properties of relativistic shock waves in which the magnetic field is transverse to the flow direction in the upstream medium, and whose composition is either pure electrons and positrons or primarily electrons and positrons with an admixture of heavy ions. Particle-in-cell simulation techniques as well as analytic theory have been used to show that such shocks in pure pair plasmas are fully thermalized—the downstream particle spectra are relativistic Maxwellians at the temperature expected from the jump conditions. On the other hand, shocks containing heavy ions which are a minority constituent by number but which carry most of the energy density in the upstream medium do put ~20% of the flow energy into a nonthermal population of pairs downstream, whose distribution in energy space is N(E) ∝ E−2, where N(E)dE is the number of particles with energy between E and E + dE.The mechanism of thermalization and particle acceleration is found to be synchrotron maser activity in the shock front, stimulated by the quasi-coherent gyration of the whole particle population as the plasma flowing into the shock reflects from the magnetic field in the shock front. The synchrotron maser modes radiated by the heavy ions are absorbed by the pairs at their (relativistic) cyclotron frequencies, allowing the maximum energy achievable by the pairs to be γ±m±c2 = mic2γ1/Zi, where γ1 is the Lorentz factor of the upstream flow and Zi, is the atomic number of the ions. The shock’s spatial structure is shown to contain a series of “overshoots” in the magnetic field, regions where the gyrating heavy ions compress the magnetic field to levels in excess of the eventual downstream value.This shock model is applied to an interpretation of the structure of the inner regions of the Crab Nebula, in particular to the “wisps,” surface brightness enhancements near the pulsar. We argue that these surface brightness enhancements are the regions of magnetic overshoot, which appear brighter because the small Larmor radius pairs are compressed and radiate more efficiently in the regions of more intense magnetic field. This interpretation suggests that the structure of the shock terminating the pulsar’s wind in the Crab Nebula is spatially resolved, and allows one to measure γ1, and a number of other properties of the pulsar’s wind. We also discuss applications of the shock theory to the termination shocks of the winds from rotation-powered pulsars embedded in compact binaries. We show that this model adequately accounts for (and indeed predicted) the recently discovered X-ray flux from PSR 1957+20, and we discuss several other applications to other examples of these systems.Subject headings: acceleration of particles — ISM: individual (Crab Nebula) — relativity — shock waves
Radio Emission From Snr 1987A
Authors: Kirk J. G. , Duffy P. , Ball Lewis 
Published: July 2016

AbstractDiffusive acceleration of electrons by the blast wave of SNR 1987A is shown to provide a reasonable fit to observations of the radio emission since it reappeared in 1990 July. As well as determining the diffusion coefficient in this model, the observations indicate that the compression ratio of the gas subshock embedded in the blast wave is ~2.7, much lower than the value of 4 expected of a strong shock front. We propose that protons accelerated by the shock front itself cause the weakening of the subshock by both heating and accelerating the upstream plasma, producing a structure with a precursor to the gas subshock. Preliminary calculations using a fluid model for the energetic particle population indicate that these effects can significantly weaken the subshock just a few years after the explosion, if the same rate of particle injection is assumed as in other models of the acceleration of cosmic rays.Subject headings: acceleration of particles — ISM: individual (SNR 1987 A) — radio continuum: ISM — shock waves
The Spectral Turnover of the “Filled-Center” Supernova Remnant 3C 58: Implications for When Acceleration Occurs
Authors: Green D. A. 
Published: July 2016

AbstractIRASobservations the “filled-center” supernova remnant 3C 58 are used to derived upper limits of 0.4, 0.4, 0.8, and 1.5 Jy for its infrared flux density at 12, 25, 60, and 100 μm, respectively. These values imply a break in the spectrum of 3C58 above a few tens of gigahertz—two orders of magnitude below the break in the spectrum of the Crab Nebula—with a change of spectral index across the break for 3C 58 of more than 0.5. This is similar to previous results for another “filled-center” remnant, G74.9+1.2, from radio observations alone. This implies that their emission isnotdominated by a constant injection rate of particles with a power-law distribution. Either the break is due to synchrotron losses in the past, with little particle injection in recent times or it is intrinsic to the acceleration mechanism at work (presumably from a central neutron star).Subject headings: acceleration of particles — ISM: individual (3C 58) — radiation mechanisms: nonthermal — supernova remnants
Particle Acceleration in High-Energy Gamma-Ray Sources
Authors: Eichler David 
Published: July 2016

AbstractMany proficient gamma-ray sources show energy spectra that are consistent with E−2 primary spectra. Such sources include recently identified gamma-ray quasars and some gamma-ray bursts. Assuming thick target conversion, this is consistent with shock acceleration, and the dominance of the gamma rays of the luminosity is also consistent with previous predictions of high production efficiency of fresh cosmic rays in shocks. The spectral cutoffs in the gamma rays may offer clues as to whether the high-energy particles are electrons or protons. Resolution of this matter might have implications for the nature of the sources and for theory of shock accelerated electrons.Subject headings: acceleration of particles — gamma rays: bursts — shock waves
The Origins of High-Energy Cosmic Rays
Authors: Axford W. I. 
Published: July 2016

AbstractOur current understanding of acceleration processes for Galactic and extragalactic cosmic rays is briefly reviewed. Shock acceleration in supernova remnants remains the most favored process for cosmic rays up to the “knee” of the all-particle total energy spectrum at 1014 - 1015 eV. The highest energy particles are almost certainly extragalactic, and the most favored sources are associated with active galactic nuclei in one way or another. The intermediate region between rigidities of 1014 and 1018 V is more difficult to understand, although a galactic origin is preferred at present. The problem of making a smooth join in the spectrum at the knee suggests that these particles should not be considered to be independent of those at lower energies.Subject headings: acceleration of particles — cosmic rays — shock waves
Particle Acceleration and Nonthermal Energy Release as an Effect of Magnetoactive Disk Accretion Onto Gravitating Center
Authors: Ikhsanov N. R. , Pustil’nik L. A. 
Published: July 2016

AbstractObservations of GRO and UHEGR show that a number of Galactic and extragalactic accreting systems release most of their energy in the ultrarelativistic energy range (more than GeV). This result contradicts one of the principal conclusions of the standard models of accretion about a predominantly thermal character of energy release. This contradiction is caused by ignoring in the standard approach the processes of generation and amplification of magnetic field in the case of accretion onto a magnetized gravitating center. A new approach taking into account the processes mentioned above is applied to disk accretion onto a nonmagnetized gravitating center, as well as onto a magnetosphere. It is shown that in both cases the accretion is strictly controlled by the magnetic field, which leads to new conditions of equilibrium and stability and turns on nonthermal processes of energy release. The resulting configuration of the magnetic field, in which the main energy release takes place in both cases, is called “Z-pinch,” and is formed in the polar region of an accreting object. Effective particle acceleration occurs in it owing to the chain of MHD and resistive plasma instabilities, resulting in current discontinuity with the formation of “double layers” and generation of electric fields close to the Dreicer limit in them. The maximum energies of the accelerated particles are limited by the value 10 EeV, that coincides with the results of UHEGR observations.Subject headings: acceleration of particles — accretion, accretion disks — gamma rays: theory — MHD
Bohm Diffusion and Cosmic-Ray-Modified Shocks
Authors: Duffy Peter 
Published: July 2016

AbstractA numerical solution to the problem of self-consistent diffusive shock acceleration is presented. The cosmic rays are scattered, accelerated and exert a back-reaction on the gas through their interaction with turbulence frozen into the local fluid frame. Using a grid with a hierarchical spacetime structure the physically interesting limit of Bohm diffusion (к ∝ pv), which introduces a wide range of diffusion lengthscales and acceleration timescales, can be studied. Some implications for modified shocks and particle acceleration are presented.Subject headings: acceleration of particles — cosmic rays — diffusion — shock waves
Taiwan Oscillation Network and Small-Telescope Research at Tsing Hua University
Authors: Chou Dean-Yi 
Published: July 2016

AbstractTwo projects, the Taiwan Oscillation Network (TON) project and the earthshine project, at Tsing Hua University will be discussed. The TON is a ground-based network to measure solar intensity oscillations to study the solar interior. Four telescopes have been installed in Tenerife (Spain), Big Bear (USA), Huairou (PRC), and Tashkent (Uzbekistan). The recent scientific results from the TON data will be briefly discussed. The earthshine project is to measure the brightness of the dark portion of the lunar disk to obtain the Earth’s global albedo. The dark portion of the Moon is lit by the sunlight reflected from the Earth. The global albedo is linked to the global temperature of the Earth. The long-term measurement of earthshine will provide information on the long-term variation of the global temperature. An automated earthshine telescope is being developed at Tsing Hua University. It will be installed at Lulin Mountain in central Taiwan. The ultimate goal is to build a ground-based global network to measure the long-term variation of earthshine to learn about the long-term variation of the global temperature.
The Grid Giant Star Survey for the Space Interferometry Mission
Authors: Patterson Richard J. , Majewski Steven R. , Slesnick Catherine L. , Rhee Jaehyon , Crane Jeffrey D. , Polak Allyson A. , Kundu Arunav , Kunkel William E. , Geisler Douglas , Muñoz Ricardo , Arenas Jose , Seguel Juan , Gieren Wolfgang , Smith Verne V. , Benedict G. Fritz , Johnston Kathryn V. 
Published: July 2016

AbstractNASA’s Space Interferometry Mission (SIM), scheduled for launch in 2009, will determine the positions of thousands of stars as faint as V = 20 to a precision better than 4 microarcseconds (µas). A key part of the mission is the Astrometric Grid, which is a reference frame of several thousand stars with V ≤ 13 against which all relative measurements will be calibrated. To serve as a reliable inertial reference frame, the Grid must be astrometrically stable against photocenter jitter (from planets, binary companions, flaring or spotting) at the ~ 4µas level. Sub–solar metallicity giant stars, by virtue of their intrinsic luminosity, can probe the Galaxy to greater distances than almost any other stellar type at the same apparent magnitude. Thus, distant (> 3 kpc) giants with V < 13 will have proportionately smaller astrometric jitter compared to other potential Astrometric Grid star candidates. The Grid Giant Star Survey is a patchwork all-sky survey to find sub–solar metallicity K giants for the Grid, and to provide a unique database for studies of Galactic stellar populations. We describe here the survey characteristics and give examples of results to date.
Development of the Far-ultraviolet Imaging Spectrograph on KAISTSAT-4
Authors: Han Wonyong , Min Kyoung Wook , Edelstein Jerry , Nam Uk-Won , Seon Jong-Ho , Seon Kwang-Il , Park Jang-Hyun , Yuk In-Soo , Jin Ho , Lee Woo-Baik , Kang Kyungin , Ryu Kwangsun , Rhee Jin-Geun , Lee Dae-Hee , Korpela Eric , Dixon W. Van Dyke 
Published: July 2016

AbstractThe Far-ultraviolet IMaging Spectrograph (FIMS) is a small spectrograph optimized for the observations of diffuse hot interstellar medium in far-ultraviolet wavebands (900–1150Å and 1335–1750Å). The instrument is expected to be sensitive to emission line fluxes an order of magnitude fainter than any previous missions. FIMS is currently under development and is scheduled for launch in 2002.
Search for δ Scuti Type Pulsating Components in Eclipsing Binary Systems
Authors: Kim S.-L. , Lee J.W. , Youn J.-H. , Moon H.-K. , Choo K.J. 
Published: July 2016

AbstractWe present observational results of four eclipsing binary stars, Y Cam, RZ Cas, AS Eri and RU UMi, which were reported to have δ Scuti type pulsating components.
Distinguishing KBO from Asteroid Occultations in TAOS
Authors: Lemme Claudia , Liang Chyng-Lan 
Published: July 2016

AbstractThe goal of the Taiwan-America Occultation Survey (TAOS) project is to estimate directly the number of Kuiper Belt Objects (KBOs) by measuring the rate of stellar occultations. Occultations of distant stars may also be caused by asteroids and this article describes how both types of occultations can be distinguished on the basis of their shadow ground speed on Earth and the observing angle.
A Simulation of Shell Structures of Comet Hale-Bopp in February 1997
Authors: Tao J. , Qian B.C. 
Published: July 2016

AbstractWe present a simulation based on the morphology of the inner coma of comet Hale-Bopp(C/1995 O1) in February 1997. The synthetic images can fit the observations well, suggesting a simple spin mode. The rotation parameters, obliquity of orbital plane to equator and argument of subsolar meridian at perihelion, are 71 and 81 degrees respectively.
Cometary Polarimetry
Authors: Sen Asoke K 
Published: July 2016

AbstractComets are known to have a high (2-10 percent) polarization, caused mainly due to dust scattering and resonance fluorescence emission. Since near earth comets are generally bright (integrated visual magnitudes of 10 or even brighter) and as they show high polarization near earth-sun location, one can try to perform imaging polarimetry of such objects with small (40 cm or even smaller) telescopes. By using a dichroic polaroid sheet either in front of the telescope tube or before the imaging detector at the Cassegrain plane, one can record cometary images with a good signal to noise ratio. By rotating the dichroic sheet in three discrete steps and then reducing the corresponding three comet images to a single image, one can determine the linear polarization value at each pixel location on the image. The error in polarization will typically be the inverse of the ‘signal to noise ratio’. Such polarization images of a comet help us to determine its dust properties and also to look for possible dust jet activities. This type of work is possible with small telescopes and minor instruments.
Fast Drift-Scan CCD Imaging and Photometry with Small Telescopes: Lunar Occultations and Speckle Interferometry
Authors: Núñez Jorge , Fors Octavi 
Published: July 2016

AbstractIn this paper we show how inherent features of typical CCD cameras can be used for fast (in the order of milliseconds) photometric observations (FPO), even with submeter class telescopes. This is based on a modified drift-scanning technique which we show to be advantageous for such kind of observations. In particular, we successfully carried out some lunar occultations and speckle interferometry observations using this technique. During the year 2000, we registered several occultation events. In particular, we present the occultation of SAO79031. The millisecond photometry of the event allows us subsequent data analysis for stellar diameter estimation and close binary detection. We also show an example of the application of the technique to speckle interferometry observations. Finally, we present the main characteristics of our project to build a new remotely controlled observatory near Barcelona.
Chemical Abundances of the Planetary Nebulae NGC 2392 and NGC 3242
Authors: Wu C.H. , Li J.Z. , Chang Z.W. , Lin C.Y. , Hu J.Y. , Ip W.H. 
Published: July 2016

AbstractPlanetary nebulae represent the end product of the evolution of low mass stars with M < 8M⊙. The central stars have masses between 0.55 and 0.85M⊙. This means a large amount of material must have been distributed in the interiors of the PNe. The observed variations of the chemical compositions and mass distributions therefore carry important information about the nature of the associated AGB superwinds and Red Giant outflows. (Kwok et al, 1978) A program for comprehensive multiwavelength study of PNe has been initiated at NCU. One recent study has to do with the spectrographic observations of the planetary nebulae NGC 2392 and NGC 3242 using the 2.16 m telescope of the Beijing Astronomical Observatory. Some preliminary results are presented here.
Russian/Former Soviet Union Experience in Professional Small Telescope Usage
Authors: Bochkarev N.G. 
Published: July 2016

FSU astronomers traditionally use small telescopes (⊘ ≤1.5 m, hereafter STs) for both science and education. Russian/FSU experience here is among the largest world-wide. There are only 2 large and moderate-sized facilities in whole Russia: the 6 m telescope of SAO RAS and Russian-Ukrainian 2 m one on the 3100 m high peak Terskol in Central Caucasus.Equipped with good light receivers and handled by skilled observers, STs can produce first class scientific data. Important results are typically yielded by long-time sequences of observations and/or new observational “know how”: good instrument/receiver design, appropriate selection of objects and moments, etc. Examples of what has been done with STs in FSU, within my memory, (in the last ≃ 1/3 century) are listed below, without a list of references, because of lack of space. The author plans to publish a larger article on this subject in Astr.&Aph.Trans.
On Numerical Studies of Solar/Stellar Convection
Authors: Chan Kwing L. , Serizawa Katsuyo 
Published: July 2016

It has become a cliché to say that convection lies at the foundation of the activities, magnetism, and dynamos of the Sun and cool stars – the title of this colloquium. However, it has not been possible, until recently, to study theoretically these phenomena and their connections with quantitative rigor. The new development owes much to the rapid growth in the power of computers and the spread of sophisticated numerical techniques.Since the last decade numerical studies of solar and stellar convection problems thrive. The trend towards a greater use of computers in these studies will likely become stronger. Now is a good time to reflect on the numerical approach itself, so that we may understand it better and make better use of it in the future. Here we look at two aspects of this methodology. First (Section 2), we examine its relationships to the analytical approach and observation, for which we make a differentiation between numerical experimentation and simulation. It is obvious that the ultimate test of any theory, numerical or not, is to confront observations, but not generally realized is that the connection to analytical studies is an important requisite for the healthy development of numerical studies. This point is to be elaborated. Second (Section 3), we discuss the prevalence of pitfalls in the numerical approach; two examples of current interest in convection computations are analysed in detail. In Section 4, we go on to suggest that one of these pitfalls might have caused the discrepancy between the results of numerical modelling of the solar differential rotation and helioseismology.
Magnetoconvection Patterns in Rotating Convection Zones
Authors: Roberts Paul H. 
Published: July 2016

AbstractIn addition to the well-known granulation and supergranulation of the solar convection zone (the “SCZ”), the presence of so-called “giant cells” has been postulated. These are supposed span the entire thickness of the SCZ and to stretch from pole to pole in a sequence of elongated cells like a “cartridge belt” or a bunch of “bananas” strung uniformly round the Sun. Conclusive evidence for the existence of such giant cells is still lacking, despite strenuous observational efforts to find them. After analyses of sunspot motion, Ribes and others believe that convective motions near the solar surface occurs in a pattern that is the antithesis of the cartridge belt: a system of “toroidal” or “doughnut” cells, girdling the Sun in a sequence that extends from one pole to the other. Galloway, Jones and Roberts have recently tried to meet the resulting theoretical challenge, with the mixed success reported in this paper.
Stellar Convection as a Low Prandtl Number Flow
Authors: Massaguer Josep M. 
Published: July 2016

AbstractThermal convection in the Sun and cool stars is often modeled with the assumption of an effective Prandtl number σ ≃ 1. Such a parameterization results in masking of the presence of internal shear layers which, for small σ, might control the large scale dynamics. In this paper we discuss the relevance of such layers in turbulent convection. Implications for heat transport – i.e. for the Nusselt number power law – are also discussed.
The Spatial Structure of Homogeneous Turbulence at High Reynolds Number
Authors: Vincent A. , Meneguzzi M. 
Published: July 2016

AbstractA direct numerical simulation at resolution 2403 is used to obtain a statistically stationary three-dimensional homogeneous and isotropic turbulent field at a Reynolds number around 1000 (Rλ ≈ 150). The energy spectrum displays an inertial subrange extending over more than one decade. In the physical space, the strongest vorticity is organized in very elongated thin tubes. The typical length of a tube is around the integral scale of the flow and its thickness is a few dissipation scales.