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1.
N. E. Hurlburt A. M. Rucklidge † 《Monthly notices of the Royal Astronomical Society》2000,314(4):793-806
Flux elements, pores and sunspots form a family of magnetic features observed at the solar surface. As a first step towards developing a fully non-linear model of the structure of these features and of the dynamics of their interaction with solar convection, we conduct numerical experiments on idealized axisymmetric flux tubes in a compressible convecting atmosphere in cylindrical boxes of radius up to 8 times their depth. We find that the magnetic field strength of the flux tubes is roughly independent of both distance from the centre and the total flux content of the flux tube, but that the angle of inclination from the vertical of the field at the edge of the tube increases with flux content. In all our calculations, fluid motion converges on the flux tube at the surface. The results compare favourably with observations of pores; in contrast, large sunspots lie at the centre of an out-flowing moat cell. We conjecture that there is an inflow hidden beneath the penumbrae of large spots, and that this inflow is responsible for the remarkable longevity of such features. 相似文献
2.
S. Campana M. Colpi S. Mereghetti L. Stella M. Tavani 《Astronomy and Astrophysics Review》1998,8(4):279-316
Summary. Soft X–ray Transients (SXRTs) have long been suspected to contain old, weakly magnetic neutron stars that have been spun
up by accretion torques. After reviewing their observational properties, we analyse the different regimes that likely characterise
the neutron stars in these systems across the very large range of mass inflow rates, from the peak of the outbursts to the
quiescent emission. While it is clear that close to the outburst maxima accretion onto the neutron star surface takes place,
as the mass inflow rate decreases, accretion might stop at the magnetospheric boundary because of the centrifugal barrier
provided by the neutron star. For low enough mass inflow rates (and sufficiently short rotation periods), the radio pulsar
mechanism might turn on and sweep the inflowing matter away. The origin of the quiescent emission, observed in a number of
SXRTs at a level of , plays a crucial role in constraining the neutron star magnetic field and spin period. Accretion onto the neutron star surface
is an unlikely mechanism for the quiescent emission of SXRTs, as it requires very low magnetic fields and/or long spin periods.
Thermal radiation from a cooling neutron star surface in between the outbursts can be ruled out as the only cause of the quiescent
emission.
We find that accretion onto the neutron star magnetosphere and shock emission powered by an enshrouded radio pulsar provide
far more plausible models. In the latter case the range of allowed neutron star spin periods and magnetic fields is consistent
with the values recently inferred from the properties of kHz quasi-periodic oscillation in low mass X–ray binaries. If quiescent
SXRTs contain enshrouded radio pulsars, they provide a missing link between X–ray binaries and millisecond pulsars.
Received 4 November 1997; Accepted 15 April 1998 相似文献
3.
本文在考虑磁光效应条件下,根据对斯托克斯参数转移方程组求得的数值解,计算了单极太阳黑子的线偏振讯号的单色像,并与美国马歇尔空间飞行中心的观测资料进行了对比,结果表明,径向黑子磁场模型给出与观测相似的单色像,而旋涡形模型导致与观测有显著差异的图像。因此可以认为径向模型更接近于实际情况。 相似文献
4.
Relationship between Rotating Sunspots and Flares 总被引：2，自引：0，他引：2
Active Region (AR) NOAA 10486 was a super AR in the declining phase of solar cycle 23. Dominated by the rapidly rotating positive
polarity of an extensive δ sunspot, it produced several powerful flare-CMEs. We study the evolution and properties of the rotational motion of the major
poles of positive polarities and estimate the accumulated helicity injected by them. We also present two homologous flares
that occurred in the immediate periphery of the rotating sunspots. The main results are as follows: i) anticlockwise rotational motions are identified in the main poles of positive polarities in the AR; the fastest of them
is about 220° for six days. ii) The helicity injection inferred from such rotational motion during the interval from October 25 to 30 is about − 3.0×1043 Mx2, which is comparable that calculated by the local correlation tracking (LCT) method (− 5.2×1043 Mx2) in the whole AR. It is suggested that both methods reveal the essential topological properties of the AR, even if the former
includes only the major poles and the fine features of the magnetic field are neglected. iii) It is found that there is a good spatial and temporal correspondence between the onset of two homologous CME-associated
flares and the rotational motion of sunspots. This suggests that the rotational motions of sunspots not only relate to the
transport of magnetic energy and complexity from the low atmosphere to the corona but may also play a key role in the onset
of the homologous flares.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
5.
Summary. Metallicity is a key parameter that controls many aspects in the formation and evolution of stars and galaxies. In this review
we focus on the metal deficient galaxies, in particular the most metal-poor ones, because they play a crucial r?le in the
cosmic scenery. We first set the stage by discussing the difficult problem of defining a global metallicity and how this quantity
can be measured for a given galaxy. The mechanisms that control the metallicity in a galaxy are reviewed in detail and involve
many aspects of modern astrophysics: galaxy formation and evolution, massive star formation, stellar winds, chemical yields,
outflows and inflows etc. Because metallicity roughly scales as the galactic mass, it is among the dwarfs that the most metal-poor
galaxies are found. The core of our paper reviews the considerable progress made in our understanding of the properties and
the physical processes that are at work in these objects. The question on how they are related and may evolve from one class
of objects to another is discussed. While discussing metal-poor galaxies in general, we present a more detailed discussion
of a few very metal-poor blue compact dwarf galaxies like IZw18. Although most of what is known relates to our local universe,
we show that it pertains to our quest for primeval galaxies and is connected to the question of the origin of structure in
the universe. We discuss what do QSO absorption lines and known distant galaxies tell us already? We illustrate the importance
of star-forming metal-poor galaxies for the determination of the primordial helium abundance, their use as distance indicator
and discuss the possibility to detect nearly metal-free galaxies at high redshift from Ly emission.
Received 19 August 1999 / Published online: 15 February 2000 相似文献
6.
M. Sobotka 《Astronomische Nachrichten》2003,324(4):369-373
Sunspots and pores appear as a consequence of interactions between strong magnetic fields and moving plasma. A wide variety of small‐scale features, presumably of convective origin, are observed in photospheric layers of sunspots and pores: Umbral dots, light bridges, penumbral filaments, and penumbral grains. Each type of features has specific morphological, photometric, spectral, and kinematic characteristics. Spots and pores modify velocity fields in adjacent photosphere and sub‐photospheric layers. Recent high‐resolution spectral, broad‐band, and helioseismic observations of the structure, dynamics, and magnetic fields of sunspots and pores, together with theoretical interpretations, are discussed in this review. 相似文献
7.
Sunspot number, sunspot area, and radio flux at 10.7 cm are the indices which are most frequently used to describe the long‐term solar activity. The data of the daily solar full‐disk magnetograms measured at Mount Wilson Observatory from 19 January 1970 to 31 December 2012 are utilized together with the daily observations of the three indices to probe the relationship of the full‐disk magnetic activity respectively with the indices. Cross correlation analyses of the daily magnetic field measurements at Mount Wilson observatory are taken with the daily observations of the three indices, and the statistical significance of the difference of the obtained correlation coefficients is investigated. The following results are obtained: (1) The sunspot number should be preferred to represent/reflect the full‐disk magnetic activity of the Sun to which the weak magnetic fields (outside of sunspots) mainly contribute, the sunspot area should be recommended to represent the strong magnetic activity of the Sun (in sunspots), and the 10.7 cm radio flux should be preferred to represent the full‐disk magnetic activity of the Sun (both the weak and strong magnetic fields) to which the weak magnetic fields mainly contribute. (2) On the other hand, the most recommendable index that could be used to represent/reflect the weak magnetic activity is the 10.7 cm radio flux, the most recommendable index that could be used to represent the strong magnetic activity is the sunspot area, and the most recommendable index that could be used to represent the full‐disk magnetic activity of the Sun is the 10.7cm radio flux. Additionally, the cycle characteristics of the magnetic field strengths on the solar disk are given. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
8.
Hong-Qi ZhangNational Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2004,4(6):563-577
We analyze the process of formation of delta configuration in some well-known super active regions based on photospheric vector magnetogram observations. It is found that the magnetic field in the initial developing stage of some delta active regions shows a potential-like configuration in the solar atmosphere, the magnetic shear develops mainly near the magnetic neutral line with magnetic islands of opposite polarities, and the large-scale photospheric twisted field forming gradually later. Some results are obtained: (1) The analysis of magnetic writhe of whole active regions cannot be limited in the strong field of sunspots, because the contribution of the fraction of decayed magnetic field is non-negligible. (2) The magnetic model of kink magnetic ropes, supposed to be generated in the subatmosphere, is not consistent with the evolution of large-scale twisted photospheric transverse magnetic field and not entirely consistent with the relationship with magnetic shear in some delta active regions. (3) T 相似文献
9.
T. Granzer 《Astronomische Nachrichten》2004,325(5):417-423
The possibility of understanding stellar activity as an up‐scaled version of the activity of our Sun is investigated. A theoretical model to explain properties of sunspots is used for explaining observed latitudes of star spots. The model is based on thin‐flux‐tube simulations that study the path of magnetic flux tubes from their origin in a stellar overshoot layer to photospheric layers. A direct comparison of the simulation results with individual stars is given. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
R. Schlichenmaier N. Bello Gonzlez R. Rezaei T.A. Waldmann 《Astronomische Nachrichten》2010,331(6):563-566
The generation of magnetic flux in the solar interior and its transport from the convection zone into the photosphere, the chromosphere, and the corona will be in the focus of solar physics research for the next decades. With 4 m class telescopes, one plans to measure essential processes of radiative magneto‐hydrodynamics that are needed to understand the nature of solar magnetic fields. One key‐ingredient to understand the behavior of solar magnetic field is the process of flux emergence into the solar photosphere, and how the magnetic flux reorganizes to form the magnetic phenomena of active regions like sunspots and pores. Here, we present a spectropolarimetric and imaging data set from a region of emerging magnetic flux, in which a proto‐spot without penumbra forms a penumbra. During the formation of the penumbra the area and the magnetic flux of the spot increases. First results of our data analysis demonstrate that the additional magnetic flux, which contributes to the increasing area of the penumbra, is supplied by the region of emerging magnetic flux. We observe emerging bipoles that are aligned such that the spot polarity is closer to the spot. As an emerging bipole separates, the pole of the spot polarity migrates towards the spot, and finally merges with it. We speculate that this is a fundamental process, which makes the sunspot accumulate magnetic flux. As more and more flux is accumulated a penumbra forms and transforms the proto‐spot into a full‐fledged sunspot (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
11.
We study the relationship between the brightness (I) and magnetic field (B) distributions of sunspots using 272 samples observed at the San Fernando Observatory and the National Solar Observatory,
Kitt Peak, whose characteristics varied widely. We find that the I – B relationship has a quadratic form for the spots with magnetic field less than about 2000 G. The slope of the linear part
of the I – B curve varies by about a factor of three for different types of spots. In general the slope increases as the spot approaches
disk center. The I – B slope does not have a clear dependency on the spot size but the lower limit appears to increase as a function of the ratio
of umbra and penumbra area. The I – B slope changes as a function of age of the sunspots. We discuss various sunspot models using these results. 相似文献
12.
At the solar surface the magnetic field is bundled into discrete elements of concentrated flux, often referred to as magnetic
flux tubes, which cover only a small fraction of the solar surface. Flux tubes span a whole spectrum of sizes, ranging from
sunspots to features well below the best currently obtainable spatial resolution.
Whereas sunspots have been well studied, our knowledge of the true brightness of small-scale magnetic features is hampered
by the insufficient spatial resolution of the observations. A better understanding of the thermal and magnetic properties
of these small-scale features, however, is crucial for an understanding of (climate-relevant) long-term solar irradiance variations. 相似文献
13.
G. J. J. Botha A. M. Rucklidge N. E. Hurlburt 《Monthly notices of the Royal Astronomical Society》2006,369(4):1611-1624
A numerical model of idealized sunspots and pores is presented, where axisymmetric cylindrical domains are used with aspect ratios (radius versus depth) up to 4. The model contains a compressible plasma with density and temperature gradients simulating the upper layer of the Sun's convection zone. Non-linear magnetohydrodynamic equations are solved numerically and time-dependent solutions are obtained where the magnetic field is pushed to the centre of the domain by convection cells. This central magnetic flux bundle is maintained by an inner convection cell, situated next to it and with a flow such that there is an inflow at the top of the numerical domain towards the flux bundle. For aspect ratio 4, a large inner cell persists in time, but for lower aspect ratios it becomes highly time dependent. For aspect ratios 2 and 3 this inner convection cell is smaller, tends to be situated towards the top of the domain next to the flux bundle, and appears and disappears with time. When it is gone, the neighbouring cell (with an opposite sense of rotation, i.e. outflow at the top) pulls the magnetic field away from the central axis. As this happens a new inner cell forms with an inflow which pushes the magnetic field towards the centre. This suggests that to maintain their form, both pores and sunspots need a neighbouring convection cell with inflow at the top towards the magnetic flux bundle. This convection cell does not have to be at the top of the convection zone and could be underneath the penumbral structure around sunspots. For an aspect ratio of 1, there is not enough space in the numerical domain for magnetic flux and convection to separate. In this case the solution oscillates between two steady states: two dominant convection cells threaded by magnetic field and one dominant cell that pushes magnetic flux towards the central axis. 相似文献
14.
N. O. Weiss M. R. E. Proctor D. P. Brownjohn 《Monthly notices of the Royal Astronomical Society》2002,337(1):293-304
Three-dimensional non-linear magnetoconvection in a strongly stratified compressible layer exhibits different patterns as the strength of the imposed magnetic field is reduced. There is a transition from a magnetically dominated regime, with small-scale convection in slender hexagonal cells, to a convectively dominated regime, with clusters of broad rising plumes that confine the magnetic flux to narrow lanes where fields are locally intense. Both patterns can coexist for intermediate field strengths, giving rise to flux separation: clumps of vigorously convecting plumes, from which magnetic flux has been excluded, are segregated from regions with strong fields and small-scale convection. A systematic numerical investigation of these different states shows that flux separation can occur over a significant parameter range and that there is also hysteresis. The results are related to the fine structure of magnetic fields in sunspots and in the quiet Sun. 相似文献
15.
16.
Summary. Galactic globular clusters, which are ancient building blocks of our Galaxy, represent a very interesting family of stellar
systems in which some fundamental dynamical processes have taken place on time scales shorter than the age of the universe.
In contrast with galaxies, these clusters represent unique laboratories for learning about two-body relaxation, mass segregation
from equipartition of energy, stellar collisions, stellar mergers, and core collapse. In the present review, we summarize
the tremendous developments, as much theoretical as observational, that have taken place during the last two decades, and
which have led to a quantum jump in our understanding of these beautiful dynamical systems.
Received 3 August 1996 相似文献
17.
L. Colangeli Th. Henning J.R. Brucato D. Clément D. Fabian O. Guillois F. Huisken C. Jäger E.K. Jessberger A. Jones G. Ledoux G. Manicó V. Mennella F.J. Molster H. Mutschke V. Pirronello C. Reynaud J. Roser G. Vidali L.B.F.M. Waters 《Astronomy and Astrophysics Review》2003,11(2-3):97-152
Abstract. Silicate grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality
of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have
shown that the presence of silicates is ubiquitous in space and that their properties vary with environmental characteristics.
Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, silicate grains
cross many environments characterised by different physical and chemical conditions which can induce changes to their nature.
Moreover, the transformations experienced in the interplay of silicate grains and the medium where they are dipped, are part
of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical
and physical changes of silicate grains during their life play a key role in the chemical evolution of the entire Galaxy.
The knowledge of silicate properties related to the conditions where they are found in space is strictly related to the study
in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and
processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate
the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding
of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the
simulation and characterisation of cosmic silicate analogues. The paper describes also laboratory studies of the chemical
reactions undergone and induced by silicate grains. The comparison of available laboratory results with observational data
shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems
that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose
of this paper is to provide an overview of the present stage of knowledge about silicates in space and to provide to the reader
some indication of the future developments in the field.
Received 25 April 2002 / Published online 14 November 2002
Send offprint requests to: L. Colangeli e–mail: colangeli@na.astro.it 相似文献
18.
E.E. Benevolenskaya 《Astronomische Nachrichten》2010,331(1):63-72
The investigation of the dynamics of magnetic fields from small scales to the large scales is very important for the understanding of the nature of solar activity. It is also the base for producing adequate models of the solar cycle with the purpose to predict the level of solar activity. Since December 1995 the Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SOHO) provides full disk magnetograms and synoptic maps which cover the period of solar cycle 23 and the current minimum. In this paper, I review the following important topics with a focus on the dynamics of the solar magnetic field. The synoptic structure of the solar cycle; the birth of the solar cycle (overlapping cycles 23 and 24); the relationship of the photospheric magnetic activity and the EUV solar corona, polar magnetic fields and dynamo theory (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
19.
Magnetic topology has been a key to the understanding of magnetic energy re-lease mechanism. Based on observed vector magnetograms, we have determined the three-dimensional (3D) topology skeleton of the magnetic fields in the active region NOAA 10720.The skeleton consists of six 3D magnetic nulls and a network of corresponding spines, fans,and null-null lines. For the first time, we have identified a spiral magnetic null in Sun's corona.The magnetic lines of force twisted around the spine of the null, forming a 'magnetic wreath'with excess of free magnetic energy and resembling observed brightening structures at extra-ultraviolet (EUV) wavebands. We found clear evidence of topology eruptions which are re-ferred to as catastrophic changes of topology skeleton associated with a coronal mass ejection(CME) and an explosive X-ray flare. These results shed new lights on the structural complex-ity and its role in explosive magnetic activity. The concept of flux rope has been widely used in modelling explosive magnetic activity, although their observational identity is rather ob-scure or, at least, lacking of necessary details up to date. We suggest that the magnetic wreath associated with the 3D spiral null is likely an important class of the physical entity of flux ropes. 相似文献
20.
对十个活动区出现的卫星黑子进行分析,据它们不同的形态、发展状况及在耀斑活动中的作用大致分成三种类型。结果表明,高能耀斑与卫星黑子有密切关系。随着卫星黑子的出现,发展在活动区中可经常产生耀斑。如果卫星黑子是静止的,通常没有耀斑爆发。 相似文献