共查询到20条相似文献,搜索用时 31 毫秒
1.
G. Rüdiger 《Astronomische Nachrichten》1974,295(6):275-284
The spectral tensor of turbulent motion in an infinite conductive incompressible medium is given in the case of a uniform magnetic field of any strenght affecting a homogeneous turbulence. With the help of BOCHNER 's theorem we make sure that the trace ui(x, t) ui(x, t) is non-negative. The presence of a weak magnetic field causes a damping of the turbulence, in some cases a strengthening. For strong magnetic fields the norms of the velocity vectors parallel and perpendicular to B approach one and the same value. Compared with the correlation length measured perpendicular to the magnetic field the correlation length measured along the magnetic field increases. Furthermore, our formulas have allowed to calculate the dependence of the α-effect on the magnetic field. 相似文献
2.
Reliable measurements of the solar magnetic field are restricted to the level of the photosphere. For about half a century attempts have been made to calculate the field in the layers above the photosphere, i.e. in the chromosphere and in the corona, from the measured photospheric field. The procedure is known as magnetic field extrapolation. In the superphotospheric parts of active regions the magnetic field is approximately force-free, i.e. electric currents are aligned with the magnetic field. The practical application to solar active regions has been largely confined to constant-α or linear force-free fields, with a spatially constant ratio, α, between the electric current and the magnetic field. We review results obtained from extrapolations with constant-α force-free fields, in particular on magnetic topologies favourable for flares and on magnetic and current helicities. Presently, different methods are being developed to calculate non-constant-α or nonlinear force-free fields from photospheric vector magnetograms. We also briefly discuss these methods and present a comparison of a linear and a nonlinear force-free magnetic field extrapolation applied to the same photospheric boundary data. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
3.
M. K. Das 《Astrophysics and Space Science》1985,114(2):295-302
The nonlinear pseudo-radial mode of oscillation of a rotating magnetic star is studied. It is shown that for a general rotational field, the coupling between magnetic field and rotation tends to reduce the average rotational energy parameterT. This result in a lowering of the maximum pulsation amplitudeq
max, which depends on strength of rotation and magnetic field. The configuration tends, therefore, to a new equilibrium state at lower value ofq
max. The analytic solution of the pulsation equation for the case ofy=5/3 in the presence of rotation and magnetic field has also been derived in the Appendix. 相似文献
4.
Badruddin 《Astrophysics and Space Science》2002,281(3):651-661
Two distinct regions of shock-associated magnetic clouds, (i) magnetically turbulent regions formed due to interaction between
magnetic cloud and ambient magnetic field i.e. turbulent interaction region (TIR), and magnetically quiet region called magnetic
cloud have been considered separately and correlation of interplanetary plasma and field parameters, magnetic field strength
(B) and solar wind speed (V), with cosmic ray intensity (I) have been studied during the passage of these two regions. A good correlation between B and I and between V and I has been obtained during the passage of sheath when the magnetic field is high and turbulent, while these correlation have
been found to be poor during the passage of magnetic clouds when the field is strong and smooth. Further, there is a positive
correlation between enhancement in field strength and its variance in the sheath region. These results strongly support the
hypothesis that most Forbush decreases are due to scattering of particles by region of enhanced magnetic turbulence. These
results also suggest that it will provide a better insight if not the magnetic field enhancement alone but in addition, the
nature of magnetic field enhancement is also considered while correlating the field enhancements with depressions in cosmic
rays.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
Simple analytic models for the passive evolution of arcade-like magnetic fields through a series of force-free equilibria are presented. At the photospheric boundary, the normal magnetic field component is prescribed together with either the longitudinal field component or the photospheric shear. Analytic progress is made by considering either cylindrically symmetric solutions or using the separation of variables technique.
Two distinct cylindrically symmetric force-free fields are obtained that possess the same normal field component and photospheric shear. The scond field contains a magnetic bubble. As the shear increases beyond a critical value, so the magnetic energy of the first configuration exceeds that of the second. The possibility is therefore suggested of an eruption of the first field outwards towards the second. Such an eruptive instability is proposed as the origin of a two-ribbon solar flare.A new analytic solution to the force-free field equations, of separable form, is discovered and it is pointed out that the existence of shear in a magnetic field does not preclude it from being potential.Now at AWRE, Aldermaston, Reading, Berkshire. 相似文献
6.
A. F. Kholtygin S. N. Fabrika N. A. Drake V. D. Bychkov L. V. Bychkova G. A. Chountonov T. E. Burlakova G. G. Valyavin 《Astronomy Letters》2010,36(5):370-379
Based on an analysis of the catalog of magnetic fields, we have investigated the statistical properties of the mean magnetic
fields for OB stars. We show that the mean effective magnetic field B of a star can be used as a statistically significant characteristic of its magnetic field. No correlation has been found
between the mean magnetic field strength B and projected rotational velocity of OB stars, which is consistent with the hypothesis about a fossil origin of the magnetic
field. We have constructed the magnetic field distribution function for B stars, F(B), that has a power-law dependence on B with an exponent of ≈−1.82. We have found a sharp decrease in the function F(B) for B ⩽ 400 G that may be related to rapid dissipation of weak stellar surface magnetic fields. 相似文献
7.
The effect of a non-uniform magnetic field on the gravitational instability for a non-uniformly rotating, infinitely extending
axisymmetric cylinder in a homogeneous medium has been studied. The Bel and Schatzman criterion of gravitational instability for a non-uniformly rotating medium is modified under the effect of a non-uniform/uniform
magnetic field acting along the tangential and axial directions. As a consequence the stabilizing and destabilizing effect
of the non-uniform magnetic field is obtained, a new criterion for the magneto-gravitational instability is deduced in terms
of Alfven’s wave velocity; and it is also found that the Jeans criterion determines the gravitational instability in the absence of rotation and when the non-uniform/uniform magnetic field acts
along the axis of the cylinder. 相似文献
8.
Z. F. Gao N. Wang D. L. Song J. P. Yuan C.-K. Chou 《Astrophysics and Space Science》2011,334(2):281-292
In this paper, an approximate method of calculating the Fermi energy of electrons (E
F
(e)) in a high-intensity magnetic field, based on the analysis of the distribution of a neutron star magnetic field, has been
proposed. In the interior of a neutron star, different forms of intense magnetic field could exist simultaneously and a high
electron Fermi energy could be generated by the release of magnetic field energy. The calculation results show that: E
F
(e) is related to density ρ, the mean electron number per baryon Y
e
and magnetic field strength B. 相似文献
9.
P. J. Luyten 《Astrophysics and Space Science》1988,141(1):27-43
The oscillations and stability of a homogeneous self-gravitating rotating cylinder in a toroidal magnetic field are investigated. It is assumed that the field is proportional to the distance to the axis of the cylinder. We show the existence of four infinite discreta spectra of magnetic (or rotational) modes. Rotation stabilizes the magneticm=1 instability. The magnetic field decreases the growth rate of rotational instability and reduces the interval of unstable wavenumbers. Ifm=1, instability always occurs with the exception of the equipartition state. Ifm>1, the instability can be suppressed by a sufficiently large magnetic field. Resistivity decreases the growth rate of magnetic instability, but increases the growth rate of rotational instability. For zero wavenumber perturbations secular instability occurs due to the action of resistivity before a neutral point is attained where a second secular instabiliity initiates due to the action of resistivity. 相似文献
10.
We have studied the structure of hot accretion flow bathed in a general large-scale magnetic field. We have considered magnetic parameters , where are the Alfvén sound speeds in three direction of cylindrical coordinate (r,φ,z). The dominant mechanism of energy dissipation is assumed to be the magnetic diffusivity due to turbulence and viscosity in the accretion flow. Also, we adopt a more realistic model for kinematic viscosity (ν=αc s H), with both c s and H as a function of magnetic field. As a result in our model, the kinematic viscosity and magnetic diffusivity (η=η 0 c s H) are not constant. In order to solve the integrated equations that govern the behavior of the accretion flow, a self-similar method is used. It is found that the existence of magnetic resistivity will increase the radial infall velocity as well as sound speed and vertical thickness of the disk. However the rotational velocity of the disk decreases by the increase of magnetic resistivity. Moreover, we study the effect of three components of global magnetic field on the structure of the disk. We found out that the radial velocity and sound speed are Sub-Keplerian for all values of magnetic field parameters, but the rotational velocity can be Super-Keplerian by the increase of toroidal magnetic field. Also, Our numerical results show that all components of magnetic field can be important and have a considerable effect on velocities and vertical thickness of the disk. 相似文献
11.
The longitudinal magnetic field measured using the Fe I λ 525 and Fe I λ 524.7 nm lines and global magnetic field of the sun
differ depending on the observatory. To study the cause of these discrepancies, we calculate the H
‖(525)/H
‖(524.7) ratios for various combinations of magnetic elements and compare them with the corresponding observed values. We use
the standard quiet model of the solar photosphere suggesting that there are magnetic fields of different polarities in the
range between zero and several kilogauss. The magnetic element distribution is found as a function of magnetic field strength
and the parameters of this distribution are determined for which the calculated H
‖(525)/H
‖(524.7) ratio agrees with the observed one. The sigma-components are found to be shifted differently for various points of
the Fe I λ 525 nm profile calculated for the inhomogeneous magnetic field. The farther the point is from the line center,
the larger the sigma-components shift. Such a peculiarity of the profiles may be responsible for the discrepancies in the
measured values of the global magnetic field obtained at different observatories. The increase in modulus of the global magnetic
field during the maxima of solar activity can be due to a larger fraction of magnetic elements with kilogauss magnetic fields. 相似文献
12.
We compare the shape and position of some plasma formations visible in the polar corona with the cyclic evolution of the global
magnetic field. The first type of object is polar crown prominences. A two-fold decrease of the height of polar crown prominences
was found during their poleward migration from the middle latitudes to the poles before a polar magnetic field reversal. The
effect could be assigned to a decrease of the magnetic field scale. The second type of object is the polar plumes, ray like
structures that follow magnetic field lines. Tangents to polar ray structures are usually crossed near some point, “a magnetic
focus,” below the surface. The distance q between the focus and the center of the solar disk changes from the maximum value about 0.65 R
⊙ at solar minimum activity to the minimum value about 0.45 R
⊙ at solar maximum. At first glance this behaviour seems to be contrary to the dynamics of spherical harmonics of the global
magnetic field throughout a cycle. We believe that the problem could be resolved if one takes into account not only scale
changes in the global magnetic field but also the phase difference in the cyclic variations of large-scale and small-scale
components of the global field. 相似文献
13.
A. -H. Zhou 《Astrophysics and Space Science》1994,222(1-2):107-111
In this paper a method of estimating the magnetic field strength,B, in a homogeneous microwave burst source with simplified expressions for the synchrotron radiation is presented. An approximate formula of the magnetic field is obtained using the method. Once the magnetic field is estimated the total number of energetic electrons along the line of sightN L can be estimated also. The errors ofB andN L have been given. It is found that this method is useful for semiquantitative investigations of models of radio burst sources. 相似文献
14.
The applications of the spectral analysis methods discovered by Kirchhoff for the investigation of stellar magnetic fields
are considered. The statistical properties of the mean magnetic fields for OBA stars have been investigated by analyzing data
from two catalogs of magnetic fields. It is shown that the mean effective magnetic field ℬ of a star can be used as a statistically
significant characteristic of its magnetic field. The magnetic field distribution functions F(ℬ) have been constructed for B-type and chemically peculiar (CP) stars, which exhibit a power-law dependence on ℬ. A sharp
decrease in F(ℬ) in the range of weak magnetic fields has been found. The statistical properties of the magnetic fluxes for main-sequence
stars, white dwarfs, and neutron stars are analyzed. 相似文献
15.
S. Hubrig M. Schller I. Savanov R.V. Yudin M.A. Pogodin St. tefl Th. Rivinius M. Cur 《Astronomische Nachrichten》2009,330(7):708-716
We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission‐line B‐type star υ Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B‐type stars and He‐strong/He‐weak stars. Nine Be stars have been studied with magnetic field time series obtained over ∼1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi‐mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, μ Cen, o Aqr, and ε Tuc. The strongest longitudinal magnetic field, 〈Bz〉 = 117 ± 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, λ Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star υ Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field 〈Bz〉 = –102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B‐type stars out of the observed fourteen cluster members (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
16.
M. Goossens 《Astrophysics and Space Science》1977,52(1):3-10
The frequencies of the linear and adiabatic oscillations of a gaseous polytrope with a poloidal magnetic field are determined with the aid of a perturbation method. The influence of the poloidal magnetic field on the different types of spheroidal oscillation modes is discussed. The poloidal magnetic field generally strengthens the stability of the oscillation modes and this effect is the largest in the case of the non-radialp-modes. 相似文献
17.
N. Seehafer 《Astronomische Nachrichten》1975,296(4):177-181
It is shown that for a class of force-free magnetic fields, i.e., ∇ × B = α B with α = constant, the magnetic field cannot be determined uniquely from the observed vertical component of the photospheric magnetic field given in an area of limited extent. Then it is proposed how some functions, the additional knowledge of which permits the magnetic field to be determined uniquely, could be chosen as a first approximation. 相似文献
18.
Using polarimetric spectra obtained with the SOFIN spectrograph installed at the Nordic Optical Telescope, we detect a longitudinal magnetic field 〈Bz〉 = –168±35 G in the Of?p star HD 108. This result is in agreement with the longitudinal magnetic field measurement of the order of –150 G recently reported by the MiMeS team. The measurement of the longitudinal magnetic field in the Of?p star HD 191612 results in 〈Bz〉 = +450±153 G. The only previously published magnetic field measurement for this star showed a negative longitudinal magnetic field 〈Bz〉 = –220±38 G, indicating a change of polarity over ∼100 days. Further, we report the detection of distinct Zeeman features in the narrow Ca II and Na I doublet lines for both Of?p stars, hinting at the possible presence of material around these stars. The origin of these features is not yet clear and more work is needed to investigate how magnetic fields interact with stellar wind dynamics (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
19.
Knowledge regarding the coronal magnetic field is important for the understanding of many phenomena, like flares and coronal
mass ejections. Because of the low plasma beta in the solar corona, the coronal magnetic field is often assumed to be force-free
and we use photospheric vector magnetograph data to extrapolate the magnetic field into the corona with the help of a nonlinear
force-free optimization code. Unfortunately, the measurements of the photospheric magnetic field contain inconsistencies and
noise. In particular, the transversal components (say B x and B y) of current vector magnetographs have their uncertainties. Furthermore, the magnetic field in the photosphere is not necessarily
force free and often not consistent with the assumption of a force-free field above the magnetogram. We develop a preprocessing
procedure to drive the observed non–force-free data towards suitable boundary conditions for a force-free extrapolation. As
a result, we get a data set which is as close as possible to the measured data and consistent with the force-free assumption. 相似文献
20.
Coronal mass ejections (CMEs) carry magnetic structure from the low corona into the heliosphere. The interplanetary CMEs (ICMEs)
that exhibit the topology of helical magnetic fluxropes are traditionally called magnetic clouds (MCs). MC fluxropes with
axis of low (high) inclination with respect to the ecliptic plane have been referred to as bipolar (unipolar) MCs. The poloidal
field of bipolar MCs has a solar cycle dependence. We report a cyclic reversal of the poloidal field of low inclination MC
fluxropes during 1976 to 2009. The MC poloidal field cyclic reversal on the same time scale of the solar magnetic cycle is
evident over three sunspot cycles. Approximately 48% of ICMEs are MCs, and 40% of IMCs are bipolar MCs during solar cycle 23.
The speed of the bipolar MCs has essentially the same distribution as all ICMEs, which implies that they are not from any
special type of CMEs in terms of the solar origin. Although CME fluxropes may undergo a number of complications during the
eruption and propagation, a significant group of MCs retains sufficient similarity to the source region magnetic field to
posses the same cyclic periodicity in polarity reversal. The poloidal field of bipolar MCs gives the out-of-ecliptic-plane
field or B
z
component in the IMF time series. MCs with southward B
z
field are particularly effective in causing geomagnetic disturbances. During the solar minima, the B
z
field IMF sequence within MCs at the leading portion of a bipolar MC is the same with the solar global dipole field. Our
finding shows that MCs preferentially remove the like polarity of the solar dipole field, and it supports the participation
of CMEs in the solar magnetic cycle. 相似文献