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1.
The problem of electromagnetic radiation from electric and magnetic line sources interacting with a moving magnetoplasma slab backed by a finitely conducting medium is treated. The local magnetostatic field is aligned parallel with the line source and is perpendicular to the direction of slab motion. For the configuration, theE andH modes are excited independently by a magnetic and an electric line source respectively. Expressions for the far zone radiation fields and the radiation pattern have been obtained for both the line sources. It is found that the radiation due to an electric line source is not affected by the presence of a static magnetic field and the motion of the slab medium. Numerical results for the radiation pattern referring to both the line sources have been presented for a wide range of parameters characterizing the finite magnetostatic field, the conductivity of the medium backing the plasma, the thickness of the slab and the location of the line source. 相似文献
2.
The problem of electromagnetic radiation from electric and magnetic line sources interacting with a moving uniaxially anisotropic dielectric or plasma half-space is treated. The anisotropy of the plasma is due to an infinitely strong magnetizing field impressedparallel to the motion of plasma. The line source is oriented normal to the direction of medium motion. TheE and theH modes are excited independently in the medium by the magnetic and the electric line sources, respectively. Invoking the saddle-point method of integration, the far-zone radiation field and the radiation pattern are obtained for both line sources. It is found that the radiation from an electric line source is not affected by the anisotropy of the moving medium and that a magnetic line source invacuum has finite components of radiation in directions which are parallel and antiparallel to the direction of motion, for the case of a uniaxial plasma in contrast to the results for a uniaxial dielectric. Numerical results for the far-zone radiation pattern, referring to a magnetic line source, are presented for several values of parameters characterizing the non-dimensional velocity, the anisotropy of the medium, the electron-plasma density and the location of the line source. 相似文献
3.
R. -L. Xu 《Astrophysics and Space Science》1988,144(1-2):257-277
4.
Using time dependent MHD simulations, we study the nature of three-dimensional magnetic reconnection in thin quasi-separatrix
layers (QSLs), in the absence of null points. This process is believed to take place in the solar atmosphere, in many solar
flares and possibly in coronal heating. We consider magnetic field configurations which have previously been weakly stressed
by asymmetric line-tied twisting motions and whose potential fields already possessed thin QSLs. When the line-tied driving
is suppressed, magnetic reconnection is solely due to the self-pinching and dissipation of narrow current layers previously
formed along the QSLs. A generic property of this reconnection process is the continuous slippage of magnetic field lines
along each other, while they pass through the current layers. This is contrary to standard null point reconnection, in which
field lines clearly reconnect by pair and abruptly exchange their connectivities. For sufficiently thin QSLs and high resistivities,
the field line footpoints slip-run at super-Alfvénic speeds along the intersection of the QSLs with the line-tied boundary,
even though the plasma velocity and resistivity are there fixed to zero. The slip-running velocities of a given footpoint
have a well-defined maximum when the field line crosses the thinnest regions of the QSLs. QSLs can then physically behave
as true separatrices on MHD time scales, since magnetic field lines can change their connections on time scales far shorter
than the travel-time of Alfvén waves along them. Since particles accelerated in the diffusive regions travel along the field
much faster than the Alfvén speed, slip-running reconnection may also naturally account for the fast motion of hard X-ray
sources along chromospheric ribbons, as observed during solar flares.
Electronic Supplementary Material Supplementary material is available for this article at 相似文献
5.
用时间缓变的非线性无力场模拟超级活动区（弧岛式大型δ黑子）的磁场位形。这个复杂磁场包含了向量磁场的主要观测特征：正负磁流极端不平衡性（正负磁流之比为１：６）,Ｕ形磁反变线,局域磁场的二极子、四极子差异性。模拟结果厅用来解释一些观测结果：（１）大耀斑主要产生在Ｕ形中性线的磁性混杂区或四极子区（２）Ｕ形反变线的准双极性区几乎没有大耀斑很小。（３）活动区内部的大型旋转运动和磁沲运动会导致四极子场磁拓扑分 相似文献
6.
R. I. Kostyk 《Kinematics and Physics of Celestial Bodies》2012,28(4):155-161
Statistical properties of solar granulation in an active region on the solar surface from the photosphere to the lower chromosphere are studied. We use the values of the velocity, intensity, and magnetic field that were obtained at different heights in the solar atmosphere according to the observation data on the VTT telescope at Observatorio del Teide, Tenerife. The changes in the line??s parameters (central depth of the line, halfwidth, equivalent width, and central depth shift) and convective velocity are presented as functions of the value of the magnetic field. We propose a 16-column model of solar granulation depending on the direction of motion of convective elements and on the sign of contrast at two heights??in the continuous spectrum and in the highest layer (h = 650 km). We found that the magnetic field impedes the change in the sign and motion direction of convective elements. 相似文献
7.
The time-dependent interaction of the granulation velocity field with a magnetic flux tube is investigated here. It is seen
that when a magnetic field line is displaced normal to itself so as to simulate thebuffeting action of granules, a flow of gas is initiated along the field. By choosing a lateral velocity field which is consistent
with observations of granules, it is found that the resulting gas motion is a downward flow with a velocity compatible with
the observed downflow in isolated photospheric flux tubes. It is therefore proposed that the observed photospheric downflow
is a manifestation of the interaction of granules with flux tubes. 相似文献
8.
R. T. Gangadhara 《Astrophysics and Space Science》1995,232(2):327-336
Pulsars accelerate the charged particles moving along their magnetic field lines due to their rapidly spinning motion. Particles gain maximum energy from pulsars within the light cylinder when they are moving along the field lines perpendicular to the rotation velocity. In pulsars with non-aligned rotation and magnetic axes, the production of two intense and sharp pulses (main pulse and interpulse) separated by 180° longitude occur at the two regions near the light cylinder where the rotation velocity is perpendicular to the magnetic field. Since the radiating particles move radially along the relativistically compressed magnetic field lines, the observer in the stationary frame receives beamed and transversely compressed radiation pulse. Near the light cylinder position angle varies smoothly during pulsar rotation in a way as Radhakrishnan and Cook (1969) expect its variation near the magnetic pole, as the field lines experience relativistic compression in the direction of rotation. The motion of two charge species along the field lines produce orthogonal modes at each pulse longitude. 相似文献
9.
The magnetic splitting peculiarities of the absorption lines in the sunspot spectrum are considered. The most common and typical of them is breaking of all Stokes parameter symmetry in regard to the line center. The possible reason of this effect is the macroscopic gas motion with inhomogeneous velocity. Computed contours are given for the line Fe i λ5250 Å with various combinations of magnetic and velocity fields. Magneto-optical effects within the line which are connected with the magnetic and velocity field inhomogeneity are discussed. The observation results are discussed for longitudinal magnetic field zero lines. These observations were carried out for the sunspot and photosphere in two spectral lines Fe i λλ 5250 and 5233 Å. In the sunspot the regular displacement of one zero line with respect to the other zero line takes place whereas in the photosphere the displacements are random. The possible reason of the regular displacement is the change of the magnetic field direction in the different optical layers wherein corresponding spectral lines are formed effectively. The connection between the zero line displacement of a longitudinal magnetic field and the crossover effect is discussed. The computation results are given which agree with observations and illustrate the above-mentioned relationship. The influence of the Stokes parameter asymmetry on the measured magnetic field signals is considered. 相似文献
10.
Numerical studies with a spherical dynamo model have shown two remarkable phenomena. The model consists of a spherical body of an electrically conducting incompressible uid surrounded by free space. In addition to a rotation of the body an inner motion due to a given forcing is considered which satisfies a no–slip condition at the boundary. The full interaction of magnetic field and motion is taken into account. Starting from a fluid motion capable of dynamo action and a very weak magnetic field it was observed that the growing magnetic field destroys the dynamo property of the motion and then decays, and that the system ends up in a state with another motion incapable of dynamo action and zero magnetic field. In another case with a motion unable to prevent small magnetic fields from decay it proved to be possible that stronger magnetic fields deform it so that a dynamo starts to work which enables the system to approach a steady state with a finite magnetic field. 相似文献
11.
A. M. Uralov V. V. Grechnev G. V. Rudenko I. G. Rudenko H. Nakajima 《Solar physics》2008,249(2):315-335
Neutral Line associated Sources (NLSs) are quasi-stationary microwave sources projected onto vicinities of the neutral line
of the photospheric magnetic field. NLSs are often precursors of powerful flares, but their nature is unclear. We endeavor
to reveal the structure of an NLS and to analyze a physical connection between such a source with a site of energy release
in the corona above NOAA 10488 (October/November 2003). Evolution of this AR includes emergence and collision of two bipolar
magnetic structures, rise of the main magnetic separator, and the appearance of an NLS underneath. The NLS appears at a contact
site of colliding sunspots, whose relative motion goes on, resulting in a large shear along a tangent. Then the nascent NLS
becomes the main source of microwave fluctuations in the AR. The NLS emission at 17 GHz is dominated by either footpoints
or the top of a loop-like structure, an NLS loop, which connects two colliding sunspots. During a considerable amount of time,
the emission dominates over that footpoint of the NLS loop, where the magnetic field is stronger. At that time, the NLS resembles
a usual sunspot–associated radio source, whose brightness center is displaced towards the periphery of a sunspot. Microwave
emission of an X2.7 flare is mainly concentrated in an ascending flare loop, initially coinciding with the NLS loop. The top
of this loop is located at the base of a non-uniform bar-like structure visible in soft X-rays and at 34 GHz at the flare
onset. We reveal i) upward lengthening of this bar before the flare onset, ii) the motion of the top of an apparently ascending flare loop along the axis of this bar, and iii) a non-thermal microwave source, whose descent along the bar was associated with the launching of a coronal ejection. We
connect the bar with a probable position of a nearly vertical diffusion region, a site of maximal energy release inside an
extended pre-flare current sheet. The top of the NLS loop is located at the bottom of this region. A combination of the NLS
loop and diffusion region constitutes the skeleton of a quasi-stationary microwave NLS. 相似文献
12.
We develop a simple model to explain the non-radial motion of eruptive solar filaments under solar minimum conditions. The global magnetic field is derived from the first and third components of the spherical harmonic expansion of a magnetic scalar potential. The filament is modeled as a toroidal current located above the mid-latitude polarity inversion line. We investigate the stability of the filament against changes in the filament current and attempt to explain the non-radial motion and acceleration of the eruptive filament. We also discuss the limitations of this model. 相似文献
13.
In this paper we discuss coupling processes between a magnetic field and an unsteady plasma motion, and analyze the features of energy storage and conversions in active region.It is pointed out that the static force-free field is insufficient for a discussion of storage processes, and also the pure unsteady plasma rotation is not a perfect approach. In order to analyze the energy storage, we must consider the addition of poloidal plasma motion. The paper shows that because the unsteady poloidal flow is added and coupling occurs between the magnetic field and both the toroidal and the poloidal plasma flows, an unsteady process is maintained which changes the force-free factor with time. Hence, the energy in the lower levels can be transferred to the upper levels, and a considerable energy can be stored in the active region. Finally, another storage process is given which is due to the pure poloidal flow. The article shows that even if there is no twisted magnetic line of force, the energy in the lower levels may still be transferred to the upper levels and stored there. 相似文献
14.
Yu. N. Mishurov I. A. Acharova M. G. Shevchenko D. A. Tyshlangov 《Astrophysical Bulletin》2014,69(1):21-26
A new mechanism of sweeping out of dust grains beyond galactic disks both in the radial direction along the galactic plane and in the vertical, cross-disk direction is proposed. The mechanism is driven by the interaction of dust grains with the bisymmetric nonstationary magnetic field of the galaxy, whose lines are curved and corotate with the stellar spiral density wave responsible for the arms. We attribute the radial transfer of interstellar dust grains in the plane of galactic disks to the fact that charged dust grains are “glued” to magnetic field lines and are therefore pushed outward because of the rotation of magnetic field lines and their tilt with respect to the radial direction parallel to the disk plane. In addition, dust is swept out vertically in the cross-disk direction because of the drift motion in crossed magnetic and gravitational fields (both are parallel to the galactic plane). Numerical computations of the motion of dust grains in real magneto-gravitational fields with the allowance for the drag force from interstellar gas show that the time scale of dust grain transport beyond galactic disks is on the order of 1 Gyr or shorter. 相似文献
15.
J. &#x;oln 《Astronomische Nachrichten》1999,320(3):141-146
It is at very strong magnetic fields that the helical Čerenkov effect, originating from the electron guiding center for an electron in helical motion in a magnetic field which is superimposed on a dielectric medium, resembles most closely the ordinary Čerenkov effect. In the absence of extremely strong magnetic fields in the laboratory, we turn our attention to the neutron stars (pulsar) and supernovae which can have magnetic fields whose values can easily be in the range of 105 — 109 T. The medium in which these magnetic fields reside is likely to be an ionized medium; that is, a plasma, which, as usual, may be assumed to be dominated by electrons. Here we wish to argue that in such a strong magnetic field dominated medium, at least on a classical level, radiation process associated with the helical Čerenkov radiation could be rather important. 相似文献
16.
Two kinematic models of line-tied reconnection are considered which describe the motion of a magnetic neutral line (NL) during the main phase of a two-ribbon solar flare and during the recovery phase of a magnetospheric substorm in the geomagnetic tail. The models are kinematic in that they use only the magnetic induction equation, which suffices to determine the position and velocity of the NL as functions of time if the rate of reconnection is prescribed. The solar flare model shows that the observed large decrease in the rate at which “post”-flare loops rise upward from the photosphere during the main phase does not require a corresponding decrease in the rate of reconnection. Instead it is found that a constant rate of reconnection can account for the motion of the loops for almost the entire period during which they are observed. By contrast, application of the same procedures to the recovery phase of the magnetospheric substorm in the tail predicts a slightly increasing speed of NL motion if the rate of reconnection is constant. Furthermore, it is found that the motion of the NL relative to the ambient medium may account for much of the observed asymmetry in the magnetic field in the plasma sheet during recovery. Due to this motion, the plasma sheet thickness may be up to 4 times smaller and the normal magnetic field component up to 2 times weaker in the region tailward of the NL than in the corresponding region earthward of the NL. 相似文献
17.
We study the changes of the CaI λ6102.7 Å line profile and the magnetic field structure during the 1B/M2.2 while-light flare of August 12, 1981. The two brightest flare knots located in the penumbra of a sunspot with a δ configuration are investigated. The 1 ± V line profiles are analyzed. The reduction and analysis of our observations have yielded the following results. (1) The line profiles changed significantly during the flare, especially at the time of optical continuum emission observed near the flare maximum. In addition to the significant decrease in the depth, a narrow polarized emission whose Zeeman splitting corresponded to a longitudinal magnetic field strength of 3600 Gs was observed. This is much larger than the magnetic field strength in the underlying sunspot determined from the Zeeman splitting of absorption lines. (2) The largest changes of the CaI λ6102.7 Å line profile observed during the flare can lead to an underestimation of the longitudinal magnetic field strength measured with a video magnetograph by a factor of 4.5, but they cannot be responsible for the polarity reversal. (3) A sharp short-term displacement of the neutral line occurred at a time close to the flare maximum, which gave rise to a reversed-polarity magnetic field on a small area of the active region, i.e., a magnetic transient. This can be interpreted as a change in the inclination of the magnetic field lines to the line of sight during the flare. The short-term depolarization of the CaI λ6102.7 Å line emission observed at the other flare knot can also be the result of a change in the magnetic field structure. (4) These fast dynamic changes of the magnetic field lines occurred after the maximum of the impulsive flare phase and were close in time to the appearance of type II radio emission. 相似文献
18.
Zhang Hongqi 《Solar physics》1993,144(2):323-340
In this paper, the formation and the measurement of the H line in chromospheric magnetic fields are discussed. The evolution of the chromospheric magnetic structures and the relation with the photospheric vector magnetic fields and chromospheric velocity fields in the flare producing active region AR 5747 are also demonstrated.The chromospheric magnetic gulfs and islands of opposite polarity relative to the photospheric field are found in the flare-producing region. This probably reflects the complication of the magnetic force lines above the photosphere in the active region. The evolution of the chromospheric magnetic structures in the active region is caused by the emergence of magnetic flux from the sub-atmosphere or the shear motion of photospheric magnetic fields. The filaments separate the opposite polarities of the chromospheric magnetic field, but only roughly those of the photospheric field. The filaments also mark the inversion lines of the chromospheric Doppler velocity field which are caused by the relative motion of the main magnetic poles of opposite polarities in the active region under discussion. 相似文献
19.
Shi Chang Chun 《Astrophysics and Space Science》1986,125(2):379-390
Some observed astrophysical phenomena, such as the blast of a supernova, suggest the necessity to study the motion of shock waves in a relativistic fluid flow in the presence of a magnetic field. This paper deals with the motion of a special relativistic shock wave which propagates from the center line outwardly after an explosion with the assumption that the magnetic field which has an axial component only. Similarity solutions which depend on the parameter =r/t are constructed. Two special cases are then studied in detail. In the first case, there is an ultrarelativistic fluid in front of the shock and in the second case, there is a cold fluid in front of the shock. 相似文献
20.
Allowed regions for the motion of charged particles in superposed dipole and uniform magnetic fields
We consider a magnetic field that is a superposition of the field of a dipole and a uniform magnetic field, the latter directed parallel to the magnetic moment of the dipole. We study the form of the field lines of that field, and determine the allowed and forbidden regions of the motion of charged particles into it. We support the analysis with diagrams that refer to the motion of high-energy particles. 相似文献