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1.
Gary  G. Allen 《Solar physics》2001,203(1):71-86
In this paper, we present a model of the plasma beta above an active region and discuss its consequences in terms of coronal magnetic field modeling. The -plasma model is representative and derived from a collection of sources. The resulting variation with height in the solar atmosphere is used to emphasize that the assumption that the magnetic pressure dominates over the plasma pressure must be carefully employed when extrapolating the magnetic field. This paper points out (1) that the paradigm that the coronal magnetic field can be constructed from a force-free magnetic field must be used in the correct context, since the force-free region is sandwiched between two regions which have >1, (2) that the chromospheric Mgii–Civ magnetic measurements occur near the -minimum, and (3) that, moving from the photosphere upwards, can return to 1 at relatively low coronal heights, e.g., R1.2 R s.  相似文献   

2.
A model of interplanetary and coronal magnetic fields   总被引:5,自引:0,他引:5  
A model of the large-scale magnetic field structure above the photosphere uses a Green's function solution to Maxwell's equations. Sources for the magnetic field are related to the observed photospheric field and to the field computed at a source surface about 0.6 R above the photosphere. The large-scale interplanetary magnetic field sector pattern is related to the field pattern at this source surface. The model generates magnetic field patterns on the source surface that compare well with interplanetary observations. Comparisons are shown with observations of the interplanetary magnetic field obtained by the IMP-3 satellite.  相似文献   

3.
The electromagnetic field produced by a magnetic dipole moment, , which is rotating obliquely surrounded by a corotating plasma sphere, is investigated. This corotating-plasma approximation has the same order of accuracy as the force-free one but has somewhat different physical implications. In the former the effect of non-electromagnetic forces such as the inertial force are included, though in somewhat artificial manner, as a departure from the strict MHD condition and this fact seems to guarantee the existence of physical solutions.Analogous to the relativistic force-free equation, a set of two differential equations (the corotation equation) are derived for the scalar functions associated with the electric and magnetic fields. A self-consistent solution of these equations is given and it is shown that this solution has no singularity, in spite of apparent divergence in the formal solution, on the light cylinder. It is concluded from this solution that, even in the extreme case of the largest possible corotation-radius (i.e.b=r L , wherer L is the light radius), the existence of a corotating plasma does not alter the field structure drastically from the vacuum case. It is also suggested through this treatment that inclusion of the inertial term in generalized Ohm's law might be essential in considering the centrifugal-wind problem.  相似文献   

4.
On the basis of diffusion approach for normal modes, solutions of the radiative transfer problem are obtained and analysed for an optically thick tenuous plasma with a strong magnetic field. The case is considered when the scattering processes without change of photon frequency are dominant. The radiative transfer coefficients as well as spectra, angular dependences and polarization of the outgoing radiation are investigated in detail for a cold plasma,kT emc 2, |–s B|kT e/mc 2 )1/2|cos|, whereT e is the electron plasma temperature, B=eB/mc the electron cyclotron frequency,s=1,2,... the number of cyclotron harmonic and the angle between the magnetic field and wave vector. The effects of electronpositron vacuum polarization are taken into account and shown to be very significant. Simple analytic solutions are obtained for various limiting cases (small and large vacuum polarization; high, low and close to the cyclotron resonance radiation frequencies; different orientations of the magnetic field, etc). The results obtained are necessary for analysing X-ray and gamma-ray radiation from strongly magnetized neutron stars.  相似文献   

5.
An extended current sheet characterized by two peculiarities was formed in a configuration with opposite magnetic fields in a laboratory plasma on a -pinch device. First, development of the small scale turbulence leads to abnormal low sheet conductivity, through-sheet plasma diffusion and establishes the sheet thickness an order greater than the skin thicknessc/ pe ( pe is electron plasma frequency). Second, there develops and quickly stabilizes in a sheet the magnetic force line reconnection. As a result, a stable neutral sheet has the complicated structure of a magnetic field, including closed magnetic loops elongated along the axis of the system. The neutral sheet plasma becomes intensively heated, probably due to ion-sound turbulence, while a group of accelerated electrons, which on the energy spectrum lead to a plateau formation, are observed. The absence of any predominant direction is a typical feature for the motion of accelerated particles. The experimental data, obtained over a broad range of plasma densities and magnetic field values typical for the solar atmosphere, show that the antiparallel magnetic field turbulent dissipation could play an important role in the mechanism of solar energy release. The parameters of accelerated particles (energy 4–12 keV, the energy content being 10–1–102 of all the energy dissipated in a sheet) agree nicely with the data of astrophysical observations.  相似文献   

6.
Thermal conductivity due to Thomson scattering and free-free absorption of photons is numerically evaluated for a non-relativistic non-degenerate plasma in a magnetic field for a number of values ofb =B/kT 1000. In the case of pure scattering, simple fitting formulae are derived. Atb6, the magnetic field is shown to decrease (by about one order) the characteristic densities above which heat transfer is mainly determined by free-free transitions.  相似文献   

7.
The effect of a strong magnetic field on neutron stars or white dwarfs is calculated for Thomson scattering in a fully ionized collisionless plasma. The Stokes parameters for the scattered radiation are computed explicitly in terms of the state of polarization of the incident wave, the electron-cyclotron frequency, the plasma frequency, the angle of incidence, and the angle of scattering. The effects of the plasma are very insensitive to specific values of ( = 2 p /2,p denotes the electron plasma frequency) so long as 1, whereas the criterion for the magnetic field to substantially affect the Stokes parameters is that the photon frequency be less than the electron-cyclotron frequency. The effects of classical radiation damping and natural line broadening are briefly mentioned.  相似文献   

8.
Makarov  V.I.  Tlatov  A.G.  Callebaut  D.K.  Obridko  V.N. 《Solar physics》2002,206(2):383-399
Lockwood, Stamper, and Wild (1999) argued that the average strength of the magnetic field of the Sun has doubled in the last 100 years. They used an analysis of the geomagnetic index aa. We calculated the area of polar zones of the Sun, A pz, occupied by unipolar magnetic field on H synoptic magnetic charts, following Makarov (1994), from 1878 to 2000. We found a gradual decrease of the annual minimum latitude of the high-latitude zone boundaries, 2m, of the global magnetic field of the Sun at the minimum of activity from 53° in 1878 down to 38° in 1996, yielding an average decrease of 1.2° per cycle. Consequently the area of polar zones A pz of the Sun, occupied by unipolar magnetic field at the minimum activity, has risen by a factor of 2 during 1878–1996. This means that the behavior of the index aa and consequently the magnetic flux from the Sun may be explained by an increase of the area of polar caps with roughly the same value of the magnetic field in this period. The area of the unipolar magnetic field at the poles (A pz) may be used as a new index of magnetic activity of the Sun. We compared A pz with the aa, the Wolf number W and A* -index (Makarov and Tlatov, 2000). Correlations based on `11-year' averages are discussed. A temperature difference of about 1° between the Maunder Minimum and the present time was deduced. We have found that the highest latitude of the polar zone boundaries of the large-scale magnetic field during very low solar activity reaches about 60°, cf., the Maunder Minimum. It is supposed that the 2m-latitude coincides with the latitude where r=0, with (r,) being the angular frequency of the solar rotation. The causes of the waxing and waning of the Sun's activity in conditions like Maunder Minimum are discussed.  相似文献   

9.
As a possible mechanism for particle acceleration in the impulsive phase of solar flares, a new particle acceleration mechanism in shock waves is proposed; a collisionless fast magnetosonic shock wave can promptly accelerate protons and electrons to relativistic energies, which was found by theory and relativistic particle simulation. The simultaneous acceleration of protons and electrons takes place in a rather strong magnetic field such that ce pe . For a weak magnetic field ( ce pe ), strong acceleration occurs to protons only. Resonant protons gain relativistic energies within the order of the ion cyclotron period (much less than 1 s for solar plasma parameters). The electron acceleration time is shorter than the ion-cyclotron period.  相似文献   

10.
Explorer 34 (Imp 4) 2.56 s magnetic data during 131 traversals of the tail current sheet are presented along with simultaneous 2.5 min auroral electrojet indices AE and AL. The normal magnetic field,B , satellite crossing times and positions are tabulated for these 131 crossings.B is defined in the center of the sheet: it is the vector magnetic field at the time of field minimum during the crossing (B x component changes sign). It is remarkable that the only normal components too large in magnitude to be classified as fine structure occur near the time of onset of an AE event. Cases are discussed where the normal component, defined near the plasma sheet edges, has the opposite sign compared to the normal component defined at the sheet center. For quiet times, the current sheet may be only about 1000 km thick within a 3R e (Earth-radii) plasma sheet, and may carry some 10–15% of the total tail current.  相似文献   

11.
Romano  P.  Contarino  L.  Zuccarello  F. 《Solar physics》2003,214(2):313-323
In this paper we analyze the eruption of a prominence, characterized by a helical-like structure and by a non-linear rising motion. We approximated the prominence as a cylindrical curved flux tube and estimated the behaviour of several geometrical parameters during the activation and the eruption phases. We determined that, at the onset of the activation, the number N of turns of a magnetic field line over the whole length of the prominence was 5.0, while the value of the ratio P/r 0 between the pitch of the magnetic field lines and the prominence width was 0.45. These values are in good agreement with those predicted by the kink-mode instability. Moreover, we found a decrease of the total twist of one helical thread from 10 to 2 during the prominence eruption, indicating a relaxation of the magnetic field towards a less twisted configuration. We conclude that the prominence was initially destabilized by the kink-mode instability and, not succeeding in finding a new equilibrium configuration, it erupted.  相似文献   

12.
Observation of the adiabatic behaviour of energetic particle pitch-angle distributions in the magnetosphere (Lyons, 1977, and others) in the past indicated the development of pronounced minima or drift-loss cones on the pitch-angle distributions centred at 90° in connection with storm-time changes in magnetospheric convection and magnetic field. Using a model of a drift-modified loss-cone distribution (MLCD) of the butterfly type, the linear stability of electromagnetic whistler or ion-cyclotron waves propagating parallel to the magnetic field has been investigated. The instability is shown to be quenched at high frequencies < m =A/(A+1), whereA is the thermal anisotropy. This quenching becomes stronger the higher are the respective parallel hot particle thermal velocityA h and cold plasma densityn c . Particles around pitch-angles 90° are identified as generating electromagnetic cyclotron waves near the marginally stable frequency m . It is concluded that the absence of electromagnetic VLF and ELF noise during times when MLCD develops is the result of the shift of the unstable spectrum to low frequencies.  相似文献   

13.
Steiner  Oskar 《Solar physics》2000,196(2):245-268
A magnetopause that separates two regimes of different flow, additional to the separation of a magnetic field from a field-free plasma, gives rise to the formation of asymmetric Stokes profiles. Using a simple two-layer model atmosphere, where one layer comprises a magnetic field, the other being field-free, it is shown by analytical derivation that a wide variety of Stokes V profiles can be produced, having amplitude asymmetries a in the range –a. These include two-humped V profiles, which have two lobes of equal sign. For the most simple models, the asymmetry depends on the ratio of continuum intensity to the Planck radiation intensity of the magnetic layer at the wavelength of the spectral line under consideration, and on the line depth. Two-humped profiles (|a|>1) require the temperature of the magnetic layer to surpass the temperature of the line-core forming region, implying a temperature inversion, so that the V profile is partially in emission. The confrontation of this formation scenario with properties of observed one-lobe profiles of quiet-Sun network regions is inconclusive due to insufficient spatial resolution and lack of a sufficient sample of simultaneously recorded Stokes spectral lines of varying line depths. It seems, however, to be in good agreement with the observed frequent occurrence of abnormal V profiles of the very strong Nai D 2 and D 2 spectral line. A possible observational verification for the present formation scenario of abnormal Stokes V profiles and a novel method of Stokes inversion are discussed.  相似文献   

14.
It is shown that, at neutral points of force-free magnetic fields, the electric current density must vanish. This property is independent of whether the neutral points are isolated or (e.g.) fill lines or surfaces. One implication is the fact that in a cold pressureless plasma the formation of neutral current sheets cannot be adiabatically slow. The field-line topology in the neighbourhood of neutral points is discussed. At neutral points of force-free magnetic fields in general three constant- surfaces, defined by the equation ×B=B, with the same value of intersect orthogonally. If, during a time-development, the magnetic field gradient matrix B i/x j becomes singular at a neutral point, the field topology can change qualitatively — in general connected with the merger of two or more neutral points into one and/or the splitting up of one neutral point into several others. This can be interpreted as implying the transition from a quasi-static evolution to a dynamical state in which magnetic energy is released.  相似文献   

15.
Cyclotron waves in the solar wind near 1 AU with frequencies well below the electron cyclotron frequency and wavelengths much larger than the electron cyclotron radius but less than the proton cyclotron radius are considered. The cyclotron radii are defined from parallel thermal velocity of electron component and proton component with respect to the interplanetary magnetic field. No LH cyclotron waves are found to propagate for p < 0, where p 1 –T p/T p is the temperature anisotropy of the proton component with respect to the interplanetary magnetic field. The damping or growth of RH cyclotron waves is found to depend on the frequency range and the temperature anisotropy of the proton component. The RH cyclotron waves are damped in the frequency range r | p | p for p < 0, where p is the proton cyclotron frequency. RH cyclotron instabilities occur in the frequency range | p | p > r > | p | p /(1– r ) for p < 0. The marginal state is at r =| p | p .Abstract presented at theInternational Symposium on Solar-Terrestrial, São Paulo, Brazil, 17–22 June, 1974  相似文献   

16.
Positrinium atom is considered in a strong magnetic field in a vector-potential gauged asA x =–By. The energy spectrum is obtained including its dependence on the centre-of-mass wave vector across the magnetic field. The pole-like contributions into the photon polarization operator coming from the positronium states are calculated and dispersion curves of joint photon-positronium states are obtained as trajectories of poles of the photon Green function in momentum space.When propagating in a strong magnetic field (B0.1B cr 4×1012 G) with curved lines of force, a photon is canalized along the magnetic field by adiabatically transforming into a bound electron-positron pair, which is a stronger effect than the analogous photon capture by transforming into an unbound pair at the edge of the continuum discussed previously by the authors. The effect of bound pair formation by-quanta in a strong magnetic field may be important near pulsars,-burst sources, powerful X-ray sources in close binary systems and other astronomical objects, recognized as magnetic neutron stars.  相似文献   

17.
The system of a self-gravitating scalar field is frequently used in inflationary cosmological models. In the present paper we study a more complicated system containing an extra linear tensor field ik=ki with minimal coupling. We determine five of the six free parameters that occur in the most general expression for the actionS of this field. In doing so we assume that in flat space-time the field ik must be invariant under gauge transformations. In a special case theS found becomes a known expression for the action of a massless tensor field ik. We compute the metric energy-momentum tensor that determines the contribution of ik to the Einstein equations. We also exhibit the equations of motion of ik in curved space-time.Translated fromAstrofizika, Vol. 38, No. 3, 1995.  相似文献   

18.
Previous global models of coronal magnetic fields have used a geometrical construction based on a spherical source surface because of requirements for computational speed. As a result they have had difficulty accounting for (a) the tendency of full magnetohydrodynamic (MHD) models to predict non-radial plasma flow out to r 10r and (b) the appreciable magnitude, 3, of B r , (the radial component of B) consistently observed at r 1 AU. We present a new modelling technique based on a non-spherical source surface, which is taken to be an isogauss of the underlying potential field generated by currents in or below the photosphere. This modification of the source surface significantly improves the agreement between the geometrical construction and the MHD solution while retaining most of the computational ease provided by a spherical source surface. A detailed comparison between the present source-surface model and the MHD solution is made for the internal dipole case. The resulting B field agrees well in magnitude and direction with the coronal B field derived from the full MHD equations. It shows evidence of the slightly equatorward meridional plasma flow that is characteristic of the MHD solution. Moreover, the B field obtained by using our non-spherical source surface agrees well with that observed by spacecraft in the vicinity of the Earth's orbit. Applied to a solar dipole field with a moment of 1 G-r 3 , the present model predicts that B r at r 1 AU lies in the range of 1–2 and is remarkably insensitive to heliomagnetic latitude. Our method should be applicable also to more general (i.e., more realistic) configurations of the solar magnetic field. Isogauss surfaces for two representative solar rotations, as calculated from expansions of observed photospheric magnetic-field data, are found to show large and significant deviations from sphericity.  相似文献   

19.
A quantum mechanical treatment of an electron plasma in a constant and homogeneous magnetic field is considered, with the aim of (a) defining the range of validity of the magnetoionic theory (b) studying the deviations from this theory, in applications involving high densities, and intense magnetic field. While treating the magnetic field exactly, a perturbation approach in the photon field is used to derive general expressions for the dielectric tensor . The properties of are explored in the various limits. Numerical estimates on the range of applicability of the magnetoionic theory are given for the case of the one-dimensional electron gas, where only the lowest Landau level is occupied.  相似文献   

20.
A numerical method is developed for solving the force-free magnetic field equation, × B = B, with spatially-varying . The boundary conditions required are the distribution of B n (viz. normal component of the field on the photosphere) as well as the value of in the region of positive (or negative) B n . Examples of calculations are presented for a simple model of a solar bipolar magnetic region. It is found that the field configuration and the energy stored in the field depend crucially on the distribution of . The present method can be applied to a more complex configuration observed on the Sun by making use of actual magnetic field measurements.On leave of absence from Department of Astronomy, University of Tokyo.  相似文献   

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