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1.
Results of ourmeasurements of the longitudinal magnetic field B
z
for the young star RWAur A are presented. B
z
measured from the so-called narrow component of the He I 5876 line varies in the range from −1.47 ± 0.15 to +1.10 ± 0.15
kG. Our data are consistent with a stellar rotation period of }~5.6 days and the model of two hot spots with opposite magnetic
field polarities spaced about 180° apart in longitude. Relative to the Earth, the spot with B
z
< 0 lies in the hemisphere above the midplane of the accretion disk, while the spot with B
z
> 0 is below the midplane. The upper limit for B
z
(at the 3σ level) obtained by averaging all observations is 180 G for the photosphere and 220 and 230 G for the Hα and [OI] 6300 line formation regions, respectively. We have also failed to detect a field in the formation region of broad
emission line components: the upper limit for B
z
is 600 G. In two of 11 cases, we have detected a magnetic field in the formation region of the blue absorption wing of the
Na I D doublet lines, i.e., in the wind from RW Aur A: B
z
= −180 ± 50 and −810 ± 80 G. The radial velocity of the photospheric lines in RW Aur A averaged over all our observations
is }~+10.5 km s−1, i.e., a value lower than that obtained by Petrov et al. (2001) ten years earlier by 5.5 km s−1. Therefore, we discuss the possibility that RW Aur is not a binary but a triple system. 相似文献
2.
We studied the behavior of magnetic field, horizontal motion and helicity in a fast emerging flux region NOAA 10488 which
eventually forms a δ spot. It is found that the rotation of photospheric footpoints forms in the earlier stage of magnetic
flux emergence and the relative shear motion of different magnetic flux systems appears later in this active region (AR).
Therefore the emerging process of the AR can be separated into two phases: rotation and shear. We have computed the magnetic
helicity injected into the corona using the local correlation tracking (LCT) technique. Furthermore we determined the vertical
component of current helicity density and the vertical component of induction electric fields Ez = (V× B)z in the photosphere. Particularly we have presented the comparison of the injection rate of magnetic helicity and the variation
of the current helicity density. The main results are as follows: (1) The strong shear motion (SSM) between the new emerging
flux system and the old one brings more magnetic helicity into the corona than the twisting motions. (2) After the maturity
of the main bipolar spots, their twist decreases and the SSM becomes dominant and the major contributor of magnetic non-potentiality
in the solar atmosphere in this AR. (3) The positions of the maxima of Ez (about 0.1 ∼ 0.2 V cm−1) shift from the twisting areas to the areas showing SSMs as the AR evolved from the rotation phase to the shear one, but
no obvious correlation is found between the kernels of Hα flare and Ez for the M1.6 flare in this AR. (4) The coronal helicity inferred from the horizontal motion of this AR amounts to −6 × 1043 Mx2. It is comparable with the coronal helicity of ARs producing flares with coronal mass ejections (CMEs) or helicity carried
away by magnetic clouds (MCs) reported in previous studies (Nindos, Zhang, and Zhang, 2003; Nindos and Andrews, 2004). In
addition, the formation of the δ configuration in this AR belongs to the third formation type indicated by Zirin and Liggett
(1987), i.e., collision of opposite polarities from different dipoles, and can be naturally explained by the SSM. 相似文献
3.
By the test particle method, we have investigated the kinematic characteristics of the electrons in the reconnecting current sheet with a guiding magnetic field Bz after they are accelerated by the supper-Dreicer electric field Ez. Firstly, the influence of the guiding magnetic field Bz on the particle acceleration is discussed under the assumption that Bz is constant in magnitude but different in orientation with respect to the electric field. In this case, the variation of the Bz direction directly leads to the variation of electron trajectories and makes electrons leave the current sheet along different paths. If Bz is parallel to Ez, the pitch angles of the accelerated electrons are close to 180°. If Bz is anti-parallel to Ez, the pitch angles of the accelerated electrons are close to 0°. The orientation of the guiding magnetic field just makes the electric field accelerate selectively the electrons in different regions, but does not change the energy distribution of electrons, and the finally derived energy spectrum is the common power-law spectrum E?γ. In typical coronal conditions, γ is about 2.9. The further study indicates that the magnitude of γ depends on the strengths of the guiding magnetic field and reconnecting electric field, as well as the scale of the current sheet. Then, the kinematic characteristics of the accelerated electrons in the current sheet with multiple X-points and O-points are also studied. The result indicates that the existences of the X-points and O-points have the particles constrained in the accelerating region to obtain the maximum acceleration, and the final energy spectrum has the characteristics of multi-power law spectra. 相似文献
4.
A. V. Mordvinov 《Solar physics》2007,246(2):445-456
A comparative analysis of solar and heliospheric magnetic fields in terms of their cumulative sums reveals cyclic and long-term
changes that appear as a magnetic flux imbalance and alternations of dominant magnetic polarities. The global magnetic flux
imbalance of the Sun manifests itself in the solar mean magnetic field (SMMF) signal. The north – south asymmetry of solar
activity and the quadrupole mode of the solar magnetic field contribute the most to the observed magnetic flux imbalance.
The polarity asymmetry exhibits the Hale magnetic cycle in both the radial and azimuthal components of the interplanetary
magnetic field (IMF). Analysis of the cumulative sums of the IMF components clearly reveals cyclic changes in the IMF geometry.
The accumulated deviations in the IMF spiral angle from its nominal value also demonstrate long-term changes resulting from
a slow increase of the solar wind speed over 1965 – 2006. A predominance of the positive IMF B
z
with a significant linear trend in its cumulative signal is interpreted as a manifestation of the relic magnetic field of
the Sun. Long-term changes in the IMF B
z
are revealed. They demonstrate decadal changes owing to the 11/22-year solar cycle. Long-duration time intervals with a dominant
negative B
z
component were found in temporal patterns of the cumulative sum of the IMF B
z
. 相似文献
5.
Alexander V. Turbiner 《Astrophysics and Space Science》2007,308(1-4):267-277
Brief overview of one-two electron molecular systems made out of protons and/or α-particles in a strong magnetic field B≤4.414×1013 G is presented. A particular emphasis is given to the one-electron exotic ions H3++(pppe), He23+(α
α
e) and to two-electron ionsH3+(pppee), He2++(α
α
ee).
Quantitative studies in a strong magnetic field are very complicated technically. Novel approach to the few-electron Coulomb
systems in magnetic field, which provides accurate results, based on variational calculus with physically relevant trial functions
is briefly described.
相似文献
6.
This study follows the numerical results presented in Marsenić & Ševčík (2010) that explored the influence of the critical level position on stability of a system. The model was a horizontal fluid layer between z = ±0.5d rotating with an angular velocity Ω = Ω0 ž about the vertical axis z . The fluid was considered to be inviscid, finitely electrically conducting and incompressible and was permeated by a horizontal magnetic field B 0 = ℬ︁0B0(z) y̌ , where ℬ︁0 was the magnitude of the field and the function B0(z) = tanh [γ (z –z0)]. When γ is large, the field gradient is concentrated near z = z0, the critical level, the field being almost homogeneous elsewhere. In this way it controls the width of the magnetic shear layer associated with the current sheet. It was found that at conditions when the magnetic field gradient was large enough (γ = O (10)) and the critical level was placed close enough to the (bottom) perfectly conducting boundary (z0 < –0.388d for γ = 80), magnetically driven convection appeared localized to a close neighbourhood of the critical level – the so called critical layer. Based on the circumstances of its rise and its properties it was identified with the resistive tearing‐mode instability. This paper presents an analytical treatment of the problem assuming γ ≥ 1. The approach consists in separation of the computational domain into an outer region where the diffusionless limit (Elsasser number Λ → ∞) applies and an inner region (the critical layer) of finite conductivity. According to the tearing‐mode theory in classical systems, the solution in the inner region is sought as long‐wavelength with respect to the width of the critical layer. The obtained solution shows features similar to the one obtained numerically and confirmed relevance of the simplifying physical assumptions made in each region. The convection in the critical layer is strictly conditioned by a sharp magnetic shear. If the shear region is removed by further positioning of the critical level towards the perfectly conducting boundary, the localized convection disappears. It is in compliance with the fact that the system is stabilised by a perfectly conducting boundary with respect to the tearing mode. Stability is then checked numerically in the layer bounded by perfectly conducting boundaries where the critical point of the magnetic field lies on one of them. The existence of a magnetically driven instability is confirmed. Depending on the value γ, it may rise as a stationary convection (for γ < 1.5) or as a wave which for γ > 16 exhibits similarity properties (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
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
8.
It is well known that the parallel cuts of the parallel and perpendicular electric field in electron phase-space holes (electron
holes) have bipolar and unipolar structures, respectively. Recently, electron holes in the Earth’s plasma sheet have been
observed by THEMIS satellites to have detectable fluctuating magnetic field with regular structures. Du et al. (2011) investigated the evolution of a one-dimensional (1D) electron hole with two-dimensional (2D) electromagnetic particle-in-cell
(PIC) simulations in weakly magnetized plasma (Ω
e
<ω
pe
, where Ω
e
and ω
pe
are the electron gyrofrequency and electron plasma frequency, respectively), which initially exists in the simulation domain.
The electron hole is unstable to the transverse instability and broken into several 2D electron holes. They successfully explained
the observations by THEMIS satellites based on the generated magnetic structures associated with these 2D electron holes.
In this paper, 2D electromagnetic particle-in-cell (PIC) simulations are performed in the x–y plane to investigate the nonlinear evolution of the electron two-stream instability in weakly magnetized plasma, where the
background magnetic field (B0 = B0[(e)\vec] x)(\mathbf{B}_{0} =B_{0}\vec{\mathbf{e}} _{x}) is along the x direction. Several 2D electron holes are formed during the nonlinear evolution, where the parallel cuts of E
x
and E
y
have bipolar and unipolar structures, respectively. Consistent with the results of Du et al. (2011), we found that the current along the z direction is generated by the electric field drift motion of the trapped electrons in the electron holes due to the existence
of E
y
, which produces the fluctuating magnetic field δB
x
and δB
y
in the electron holes. The parallel cuts of δB
x
and δB
y
in the electron holes have unipolar and bipolar structures, respectively. 相似文献
9.
This contribution is a follow-up to the recent paper of Kuznetsov et al. (Contrib. Astron. Obs. Skalnaté Pleso
36, 85, 2006) on the ground level enhancement (GLE) on 20 January 2005. We focused on a study of Forbush decrease (FD) of 17 – 18 and
21 – 22 January 2005, respectively. The data from the neutron monitor at Lomnicky Štít (1 min counts) and from the Geomagnetic
Observatory in Hurbanovo, both in Slovakia, were used as the basis for our investigation. The data on magnetic field and solar
wind from GOES 10 and 12, SOHO-CELIAS, ACE and WIND satellites were used for better understanding of the global evolution
of the event. The magnetic field is transformed to the RTN (Radial – Tangential – Normal) system where only the disturbed
part of the field is compared, i.e., daily variations and a constant part are subtracted. The field reduction method is described. Our results are temporal vector
diagrams of variation of all parameters at all positions from where we used the data. The amplitudes of |B| exceed 100 nT and variations during the arrival of the wavefront of CME take place simultaneously at the ground-based station
and at GOES satellites. The character of the variations is as if there would be regions with the dominant electric charge
of opposite signs, or electric currents with different orientations in the CME. On the basis of the values v
p and n
p and using certain assumptions we determined the mass of CME on 17 January and 21 January, respectively, of 1012 kg. A decrease of the cosmic ray level runs suddenly (during 10 minutes), starting, however, about two hours after a sudden
change of the magnetic field. 相似文献
10.
A model for the high-frequency (20–2400 Hz) quasi-periodic oscillations (QPOs) of magnetars based on the representation of
coronal magnetic loops as equivalent electric RLC circuits is proposed. The observed periods of the QPOs and their high Q-factor (Q ≈ 104–105) are explained. It follows from the model that the QPOs can be excited not only in the tail of a flare but also before the
main pulse. The parameters of the QPO source at the “ringing tail” stage of the flare from SGR 1806–20 on December 27, 2004,
have been estimated: electric current I ≈ 3 × 1019 A, minimum magnetic field strength B
min ≈ 1013 G, and electron density n ≈ 2 × 1016 cm−3. 相似文献
11.
An elementary analysis based on Ampére's Law is given to separate the general magnetic field above the photosphere into two parts B=B
1+B
*. The field B
1 is a potential field due to electric currents below the photosphere. The field B
* is produced by electric currents above the photosphere combined with the induced mirror currents. By symmetry, B
* has a set of field lines lying entirely in the plane z = 0 taken to be the photosphere. This set of field lines can be constructed from given vector magnetograph measurements and represents all the information on the electric currents above the photosphere that a magnetograph can provide. Theoretical illustrations are given and the implications for data analysis are discussed. 相似文献
12.
In this paper, we present a study of the correlation between the speed of flare ribbon separation and the magnetic flux density
during the 10 April 2001 solar flare. The study includes the section of the neutral line containing the flare core and its
peripheral area. This event shows clear two-ribbon structure and inhomogeneous magnetic fields along the ribbons, so the spatial
correlation and distribution of the flare and magnetic parameters can be studied. A weak negative correlation is found between
the ribbon separation speed (V
r) and the longitudinal magnetic flux density (B
z
). This correlation is the weakest around the peak of the flare. Spatially, the correlation is also weakest at the positions
of the hard X-ray (HXR) sources. In addition, we estimate the magnetic reconnection rate (electric field strength in the reconnection
region E
rec) by combining the speed of flare ribbons and the longitudinal magnetic flux density. During flare evolution, the time profiles
of the magnetic reconnection rate are similar to that of the ribbon separation speed, and the speeds of ribbon separation
are relatively slow in the strong magnetic fields (i.e., V
r is negatively correlated with B
z
). However, along the flare ribbons, E
rec fluctuates in a small range except near the HXR source. A localized enhancement of the reconnection rate corresponds to the
position of the HXR source. 相似文献
13.
We investigated the structure of magnetic field and vertical electric currents in two active regions through a comparison of the observed transverse field with the potential field, which was computed according to Neumann boundary-value problem for the Laplace equation using the observed H
z
-value. Electric currents were calculated from the observations of the transverse magnetic field.There exist two systems of vertical electric currents in active regions: a system of local currents and a global one. The global current is about 2 × 1012 A. In the leading part of the active regions it is directed upward, and in the tail downward.Flare activity is closely connected with the value and direction of both local and global currents: the flares tend to apear in places with upward currents. The luminosity of H flocculi is also connected with vertical electric currents; the brighter the flocculi, the more frequently they appear in places of upward electric currents.The sensitivity of H emission to the sign of the current suggests that charged particles accelerated in the upper parts of magnetic loops may be responsible for these formations. Joule heating might be important for flocculi, if plasma conductivity is about 5 × 108 c.g.s.e.A model of a flare is suggested based on current redistribution in a system of emerging loops due to changes of loop inductance. 相似文献
14.
V. A. Sheminova 《Solar physics》2009,254(1):29-50
The properties of solar magnetic fields on scales less than the spatial resolution of solar telescopes are studied. A synthetic
infrared spectropolarimetric diagnostic based on a 2D MHD simulation of magnetoconvection is used for this. Analyzed are two
time sequences of snapshots that likely represent two regions of the network fields with their immediate surroundings on the
solar surface with unsigned magnetic flux densities of 300 and 140 G. In the first region from the probability density functions
of the magnetic field strength it is found that the most probable field strength at log τ
5=0 is equal to 250 G. Weak fields (B<500 G) occupy about 70% of the surface, whereas stronger fields (B>1000 G) occupy only 9.7% of the surface. The magnetic flux is −28 G and its imbalance is −0.04. In the second region, these
parameters are correspondingly equal to 150 G, 93.3%, 0.3%, −40 G, and −0.10. The distribution of line-of-sight velocities
on the surface of log τ
5=−1 is estimated. The mean velocity is equal to 0.4 km s−1 in the first simulated region. The average velocity in the granules is −1.2 km s−1 and in the intergranules it is 2.5 km s−1. In the second region, the corresponding values of the mean velocities are equal to 0, −1.8, and 1.5 km s−1. In addition the asymmetry of synthetic Stokes V profiles of the Fe i 1564.8 nm line is analyzed. The mean values of the amplitude and area asymmetry do not exceed 1%. The spatially smoothed
amplitude asymmetry is increased to 10% whereas the area asymmetry is only slightly varied. 相似文献
15.
Anirudh Pradhan Vineet K. Yadav Lallan Yadav A. K. Yadav 《Astrophysics and Space Science》2007,312(3-4):267-273
Cylindrically symmetric inhomogeneous cosmological model for perfect fluid distribution with electromagnetic field is obtained.
The source of the magnetic field is due to an electric current produced along the z-axis. F
12 is the non-vanishing component of electromagnetic field tensor. To get the deterministic solution, it has been assumed that
the expansion θ in the model is proportional to the shear σ. Physical and geometric aspects of the models are also discussed in presence and absence of magnetic field.
相似文献
16.
The evolution of the current sheet in the electric current direction (in the guiding magnetic field direction) is studied
numerically in the 3-D particle-in-cell model with two current sheets and periodic boundary conditions. In the regime with
(where v
D and
are the electric current drift and electron thermal velocities, respectively) the current sheets are unstable owing to the
Buneman and kink instabilities and become strongly fragmented. During their evolution, in addition to an increase of the energy
of the electric field component in the guiding magnetic field direction, the energies of the electric field components in
the perpendicular direction are even more enhanced. In the current sheet the anomalous resistivity (η
anom/η
C∼7×105, where η
C is the classical resistivity) is generated and thus the magnetic field dissipates. Most of the dissipated magnetic energy
is transformed into the electron kinetic energy in the direction of the electric current. The associated electric field accelerates
the electrons from the tail of the distribution function. 相似文献
17.
A.L. Taktakishvili A. Greco P. Veltri G. Zimbardo L.M. Zelenyi A.V. Milovanov 《Astrophysics and Space Science》2001,277(1-2):71-79
The ion dynamics in the Earth's magnetotail is studied in the case when a cross tail electric field E
0 and reconnection-driven magnetic turbulence are present in the current sheet. The magnetic turbulence observed by the Interball
spacecraft is modeled numerically by a power law magnetic fluctuation spectrum. A test particle simulation is performed for
the ions, and the distribution function moments are obtained as a function of the magnetic fluctuation level, δB/B
0, and of the value of the normal component B
n. It appears that even in the presence of magnetic turbulence, the normal component has a marked influence on particle dynamics:
the ion bulk velocity along E
y and ion temperature are almost inversely proportional to B
n. The magnetic turbulence causes the current to split in two layers, and the level of magnetic fluctuations needed to have
splitting is roughly proportional to B
n. It appears that in the relevant range of parameters, B
n and δB/B
0 have opposite effects on the current structure and on ion heating.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
We analyze the influence of neutrino helicity conversion, ν
L → ν
R, on the neutrino flux from a supernova attributable to the interaction of the Dirac neutrino magnetic moment with a magnetic
field.We show that if the neutrino has a magnetic moment in the interval 10−13μB < μν < 10−12μB and provided that a magnetic field of ∼1013–1014 G exists in the supernova envelope, a peculiar kind of time evolution of the neutrino signal from the supernova attributable
to the resonance transition ν
L → ν
R in the magnetic field of the envelope can appear. 相似文献
19.
The inner disk rotation of NGC 6946 and the Milky Way is dominated by gravity but magnetism is not negligible at radii where the rotation curve becomes flat, and indeed could become dominant at very large radii. Values of the order of 1 μG, or even less, produce a centripetal force when the absolute value of the slope of the curve [B φ , R ] (azimuthal field strength versus radius) is less than the slope of a B φ ‐profile proportional to R –1. The ∝ R –1‐profile is here called the critical profile. From the hypothesis of magnetically driven rotation curves, the following is to be expected: at large radii, a “subcritical” profile (slope flatter than R –1); at still larger radii a B φ ‐profile becoming asymptotically critical as the density becomes asymptotically vanishing. Recent observations of magnetic fields in NGC 6946 and the Milky Way are in very good agreement with these predictions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
20.
D.V. Erofeev 《Solar physics》2001,198(1):31-50
To investigate the relationship between solar activity and the large-scale axisymmetric magnetic field of the Sun, we inferred from sunspot data over the period 1964–1985 a latitude–time distribution of magnetic field associated with active regions. This has been done allowing for both bipolar structure of the active regions and inclination of their axes to parallels of latitude, so the inferred magnetic field characterizes latitudinal separation of magnetic polarities which might be related to the large-scale magnetic field of the Sun according to the Babcock–Leighton model. The inferred magnetic field, A
z, is compared with the longitude-averaged (zonal) magnetic field of the Sun, B
z, derived from series of magnetograms obtained at Mount Wilson Observatory in the years 1964–1976, and at Kitt Peak National Observatory during the period from 1976 to 1985. The inferred magnetic field, A
z, exhibits a complex structure distribution of magnetic polarities with respect to latitude and time. Apart from concentration of the different polarity magnetic fields inside the high- and low-latitude portions of the sunspot belts, bipolar active regions produce an intensive, shorter-scale component of the magnetic field which varies on the time scale of about 2 years. Such a short-term variation of A
z reveals substantial correlation with the short-term component of B
z which has the form of the poleward-drifting streams of magnetic field. Most significant correlation takes place between the short-term variations of A
z occurring at latitudes below 20° and those of the large-scale magnetic fields occurring at middle latitudes of 40–50°. Moreover we analyze harmonic coefficients a
l and b
l obtained by expanding A
z and B
z into series in terms of the spherical harmonics. Power spectra of the time-dependent harmonic coefficients indicate that both A
z and B
z reveal a number of resonant modes which oscillate either with the 22-year period in the case of the anti-symmetric (odd-l) modes or with periods of about 2 years in the case of the symmetric (even-l) modes, but the resonant modes of A
z have significantly larger values of the spherical harmonic degree l (and, hence, smaller spatial scales) as compared to those of B
z. It is found that there is a close relationship between the harmonic coefficients b
l and a
m for which either m–l16 (even l=4,...,10) or m–l=4 (odd l=5,...,15). 相似文献