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1.
In this paper, we address the issue of finding velocity fields which conserve magnetic flux or at least magnetic fieldline
connectivity. We start from the basic principles of flux and line conservation and present and discuss the criterion, given
by Newcomb (1958), Stern (1966), and Vasyliunas (1972). In addition, we find a new formulation of the line-conserving velocity
field by solving the system of partial differential equations which corresponds to Newcomb's criterion for line conservation.
This velocity field is given by a correlation between the non-idealness, described by a generalized form of the Ohm's law
and a general transporting velocity, which is fieldline conserving. Our considerations give additional insights into the discussion
on violations of the frozen-in field concept which started recently with the papers by Baranov and Fahr (2003a,b). These authors
analyzed a generalized form of Ohm's law, which is valid for the heliosphere and claimed that the transport velocity for the
magnetic flux may be different from the plasma velocity. We can show that the non-idealness given in the paper by Baranov
and Fahr could not change the magnetic topology and can therefore not be responsible for magnetic reconnection. But we found
that it is in general not clear if the flux-conserving velocity field is identical to the plasma flow or to any species velocity
field. 相似文献
2.
Relationships have been studied between the background magnetic field and the distribution of active regions over the solar surface and time. A series of magnetic-field synoptic maps covering a 20-year period has been cross-correlated with spatio-temporal distributions of three types of active formations (sunspots, calcium plages, and solar flares) used as indicators of the active regions. To make the data analysis more effective, we expanded both the magnetic-field and the active-region distributions in terms of Fourier series in longitude, and then cross-correlated the latitude-dependent Fourier harmonics. Cross-correlation functions calculated from the lower-order Fourier harmonics exhibit prolonged maxima of the amplitude. For the first-order harmonic, the maxima can be tracked throughout a long time interval of at least 13 Carrington rotations, but the time of cross-correlation decreases down to 2 rotations, as the harmonic order increases up to 8. The maxima of the cross-correlation functions indicate moreover a poleward directed drift of the magnetic features that occurred with a velocity of 10–15 m s–1. The cross-correlation functions calculated separately by using the three types of active formations as indicators of the active regions are similar to each other, although they differ in some details of minor significance. The results of the data analysis make it possible to conclude that the cross-correlation between the magnetic-field and the active-region distributions displays long-term evolution of the magnetic features emerged in the photosphere in the form of the active regions, and that the evolution occurs in accordance with Leighton's (1964) concept known at present as the flux transport model. In order to verify this conclusion, we applied the cross-correlation technique to analyze a magnetic field distribution simulated by means of the flux transport equation by using an ensemble of local-scale magnetic bipoles as a source of magnetic flux. Results of the simulated magnetic field analysis exhibit a substantial qualitative agreement with those obtained by examining the observational data. 相似文献
3.
On the particle number conservation inherent in the diffusion approach of the Fokker-Planck equation
Jörn E. Kunstmann 《Astrophysics and Space Science》1978,56(1):81-87
Starting from the Fokker-Planck equation we show that, in a strict sense, the usual diffusive transport of charged particles in weak magnetic field fluctuations is not valid until the space-integrated number of particles per velocity interval has reached its final overall constant value and is conserved. Large anisotropies are thus not compatible with diffusion. Diffusion becomes exactly valid after an infinite time, but has reached a good approximation to the real transport before that. The particle-number conservation holds for quite general mean magnetic field configurations, and is not limited to the case of a constant mean field. 相似文献
4.
For application to studies of the high latitude ionosphere, we have calculated ion velocity distributions for a weekly-ionized plasma subjected to crossed electric and magnetic fields. An exact solution to Boltzmann's equation has been obtained by replacing the Boltzmann collision integral with a simple relaxation model. At altitudes above about 150 km, where the ion collision frequency is much less than the ion cyclotron frequency, the ion distribution takes the shape of a torus in velocity space for electric fields greater than 40 mV m?1. This shape persists for 1–2 hr after application of the electric field. At altitudes where the ion collision and cyclotron frequencies are approximately equal (about 120 km), the ion velocity distribution is shaped like a bean for large electric field strengths. This bean-shaped distribution persists throughout the lifetime of ionospheric electric fileds. These highly non-Maxwellian ion velocity distributions may have an appreciable affect on the interpretation of ion temperature measurements. 相似文献
5.
The structure of the large-scale background magnetic field evolves in time and space. The large-scale horizontal transport
velocity field of the magnetic flux patterns was inferred over the whole solar photosphere in the course of two solar activity
cycles from year 1976 to 1999. The method of velocity determination and the testing procedures of the velocity accuracy are
presented. The non-axially symmetric component of the horizontal velocity was found and both zonal and meridional velocity
regions were described. The horizontal large-scale transport velocity regions vary in shape and the intensity during different
phases of the 11-year solar activity cycle. The total horizontal transport velocity is characterized by the presence of variable
amounts of the vector field vortices with symmetric orientation relative to the solar equator. The zonal velocity regions,
distributed inside of the zonal belt limited by latitudes ± 35°, are persistent for about 4 Carrington rotations. Recurrent
structures of similar velocity distributions are not coherent over the whole solar photosphere. 相似文献
6.
We consider non-linear transport and drift processes caused by an inhomogeneous magnetic field in a turbulent fluid. The coefficients of magnetic diffusivity and drift velocity are calculated by making use of the second-order correlation approximation. Transport processes in the presence of a sufficiently strong magnetic field become anisotropic with larger diffusion rate and turbulent electrical resistivity across the field than along the field. Non-linear effects also lead to a drift of the magnetic field away from the regions with a higher magnetic energy. 相似文献
7.
Wu Lin-xiang 《Chinese Astronomy and Astrophysics》1992,16(4):427-433
By means of a simple relation between the velocity v of the fluid particle and the velocity vf of the photospheric footpoint of the magnetic field line vz and Bz being respectively the components of v and the magnetic field B normal to the photospheric surface, it is shown formally that through the phtospheric surface the transport of all the quantities attributed to the magnetic field, such as the magnetic flux, the magnetic energy and the helicity, is independent of vz, and vf is the only kinematical quantity on which the transport depends. In addition, in the neighborhood of the neutral line the velocity vl of the moving curve of constant Bz is found to be equal approximately to the component of v or vf in the direction of vl. Since vl can be measured or extimated, so can the components of v and vf near the neutral line. 相似文献
8.
V. A. Urpin 《Astrophysics and Space Science》1981,79(1):11-24
Hydrodynamical equations for a fully ionized hydrogen-helium plasma are derived by the Chapman-Enskog method. The electron and ion transport coefficients are found as the functions of electron and ion temperatures and number densities as well as of the magnetic field strength. The presented equations are needed for describing transport phenomena in laboratory and cosmic plasmas. It is shown that transport phenomena can produce abundance anomalies; e. g., a sound wave propagating through a homogeneous plasma may be accompanied by the oscillations of chemical composition. Various astrophysical consequences of the theory are discussed. 相似文献
9.
Themagnetorotational instability (MRI) in cylindrical Taylor‐Couette flow with external helical magnetic field is simulated for infinite and finite aspect ratios. We solve the MHD equations in their small Prandtl number limit and confirm with timedependent nonlinear simulations that the additional toroidal component of the magnetic field reduces the critical Reynolds number from O (106) (axial field only) to O (103) for liquid metals with their small magnetic Prandtl number. Computing the saturated state we obtain velocity amplitudes which help designing proper experimental setups. Experiments with liquid gallium require axial field ∼50 Gauss and axial current ∼4 kA for the toroidal field. It is sufficient that the vertical velocity uz of the flow can be measured with a precision of 0.1 mm/s.We also show that the endplates enclosing the cylinders do not destroy the traveling wave instability which can be observed as presented in earlier studies. For TC containers without and with endplates the angular momentum transport of the MRI instability is shown as to be outwards. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
S.V. Chalov 《Astrophysics and Space Science》2000,274(1-2):25-33
It is generally accepted that pick-up ions act as a seed population for anomalous cosmic rays originating at the solar wind
termination shock. We believe that the ion pre-acceleration process operating in the heliosphere up to the termination shock
can be very important to inject the ions into the shock acceleration process. The pick-up ions pre-accelerated by solar wind
turbulences have already a pronounced high energy tail when they reach the shock. Some fraction of these ions can experience
further acceleration up to energies of anomalous cosmic rays by means of shock drift and diffusive acceleration. In the present
paper the shock drift acceleration of pick-up ions suffering multiple reflection due to abrupt changes in both the strength
and direction of the magnetic field through the shock is considered. The reflection process operates for high velocity particles
different from the reflection by the electric cross-shock potential. During the first reflection the mean kinetic energy of
pick-up ions increases by approximately a factor of 10. Reflected particles have highly anisotropic velocity distribution.
Subsequent excursion of the particles in the turbulent upstream flow leads to diffusion in pitch-angle space and, as a result,
the particles can return to the shock again suffering, thus, multiple encounters. In order to describe the motion of particles
in the upstream and down streamparts of the flow we solve the Fokker-Plank transport equation for anisotropic velocity distribution
function.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
For application to the auroral ionosphere we have calculated ion velocity distributions for a weakly-ionized plasma subjected to crossed electric and magnetic fields. By replacing the Boltzmann collision integral with a simple relaxation model, we have been able to obtain an exact solution to Boltzmann's equation. This solution has the advantage over a series expansion in that all the higher order velocity moments are inherent in it. The exact solution is particularly advantageous when studying large departures of the distribution from its Maxwellian form because these departures are caused by the higher velocity moments. In general, however, a simple relaxation model can only be used to obtain qualitative information on the distribution function. Consequently, we can determine when the higher order velocity moments affect the ion velocity distribution and the nature of their effect, but we cannot obtain accurate quantitative results. The higher velocity moments have their greatest effect on the distribution function above about 120 km, where the ion-neutral collision frequency is less than the ion cyclotron frequency. As the magnitude of the electric field increases, these higher moments act to decrease the number of ions at the peak of the distribution function. Peak densities are reduced by a few per cent for perpendicular electric fields of about 20 mV m?1. 相似文献
12.
ICME Identification from Solar Wind Ion Measurements 总被引：1，自引：0，他引：1
Interplanetary coronal mass ejections (ICMEs), the interplanetary counterpart of coronal mass ejections (CMEs), are most commonly
identified by their enhanced magnetic field strengths and rotating magnetic field orientation. However, there are other frequent
signatures in the plasma. We use a pair of these signatures, a linearly decreasing plasma bulk velocity and a cool (< 20 km s−1) ion thermal speed, to identify candidate ICMEs. Many ICMEs, identified through their magnetic signatures, are also found
by their ion signatures alone. However, many are not. These missed ICMEs appear not to be expanding, even when they are accompanied
by leading shocks. The ICMEs with both the magnetic and ion signatures appear to be expanding as judged from either set of
observations. The most clearly defined ICMEs have transit times from the Sun and growth times to the observed size that are
equal. These ropes fit the paradigm of compact magnetic structures arising low in the corona and expanding uniformly in time,
as they travel at constant center of mass speed toward 1 AU. 相似文献
13.
The Kelvin-Helmholtz instability in magnetized, dusty plasmas is examined, for both negatively and positively charged dust. The critical shear in the ion velocity along the magnetic field is computed as a function of the charge residing on dust grains. 相似文献
14.
The presence of an imposed vertical magnetic field may drastically influence the structure of thin accretion discs. If the field is sufficiently strong, the rotation law can depart from the Keplerian one. We consider the structure of a disc for a given eddy magnetic diffusivity but neglect details of the energy transport. The magnetic field is assumed to be in balance with the internal energy of the accretion flow. The thickness of the disc as well as the turbulent magnetic Prandtl number and the viscosity, α , are the key parameters of our model. The calculations show that the radial velocity can reach the sound speed for a magnetic disc if the thickness is comparable to that of a non-magnetic one. This leads to a strong amplification of the accretion rate for a given surface density. The inclination angle of the magnetic field lines can exceed the critical value 30° (required to launch cold jets) even for a relatively small magnetic Prandtl number of order unity. The toroidal magnetic fields induced at the disc surface are smaller than predicted in previous studies. 相似文献
15.
Wang Jing-xiu 《Chinese Astronomy and Astrophysics》1995,19(4):480-486
For the active corona—a system consisting of electromagnetic fields and particles, the only open field surface is the solar photosphere. Interaction between the velocity and magnetic fields on the photosphere is the primary cause of the upward transport of magnetic energy and complexity to the corona, with the diffusion process there also playing a role. 相似文献
16.
Mars Express (MEX) does not carry its own magnetometer which complicates interpretation of ASPERA-3/MEX ion measurements. The direction of the interplanetary magnetic field (IMF) is especially important because it, among other things, determines the direction of the convective electric field and orientation of the cross tail current sheet and tail lobes. In this paper we present a case study to show the properties of the magnetic field near Mars in a quasi-neutral hybrid (QNH) model at the orbits where the Mars Global Surveyor (MGS) has made measurements, present a method to derive the IMF clock angle by comparing fields in a hybrid model and the direction of the magnetic field measured by MGS by deriving the IMF clock angle. We also use H+ ring velocity distribution observations upstream of the bow shock measured by the IMA/ASPERA-3 instrument on board MEX spacecraft. These observations are used to indirectly provide the orientation of the IMF. We use a QNH model (HYB-Mars) where ions are modeled as particles while electrons form a mass-less charge neutralizing fluid. We found that the direct MGS and non-direct IMA observations of the orientation magnetic field vectors in non-crustal magnetic field regions are consistent with the global magnetic field draping pattern predicted by the global model. 相似文献
17.
We investigate the properties of intermittency of magnetic turbulence by using magnetic field data collected by the Helios spacecraft in the inner heliosphere. Clear scaling laws for magnetic structure functions are visible in periods where the velocity of the bulk plasma is low. Within these periods we found that intermittency of magnetic turbulence is high with respect to velocity field. A comparison with fluid flows where passive scalars are more intermittent than velocity, yields to consider the magnetic field like a “passive vector”. 相似文献
18.
Yu. V. Vandakurov 《Astronomy Letters》2001,27(9):596-607
In view of the recently discovered time variations in rotation velocity within the solar differentially rotating tachocline (Howe et al. 2000), we study conditions for the equilibrium and excitation of motions in nonrigidly rotating magnetized layers of the radiative zones located near the boundaries of the convection zone. The emphasis is on the possible relationship between quasi-periodic tachocline pulsations and the generation of a nonaxisymmetric magnetic field in the convection zone. This field generation is studied under the assumption that it results from a reduction in the expenditure of energy on convective heat transport. The (antisymmetric about the equator) field is shown to increase in strength if there are both a radial gradient in angular velocity and steady-state axisymmetric meridional circulation of matter. The sense of circulation is assumed to change (causing the sign of the generated field to change) after the maximum permissible field strength is reached. This is apparently attributable to the excitation of the corresponding turbulent viscosity of the medium. It is also important that the cyclic field variations under discussion are accompanied by variations in solar-type dipole magnetic field. 相似文献
19.
Noble P. Abraham Samuel George G. Sreekala Sijo Sebastian Chandu Venugopal G. Renuka 《Earth, Moon, and Planets》2014,111(3-4):115-125
We have studied the stability of the electrostatic electron cyclotron wave in a plasma composed of hydrogen, oxygen and electrons. To conform to satellite observations in the low latitude boundary layer we model both the ionic components as drifting perpendicular to the magnetic field. Expressions for the frequency and the growth rate of the wave have been derived. We find that the plasma can support electron cyclotron waves with a frequency slightly greater than the electron cyclotron frequency ω ce ; these waves can be driven unstable when the drift velocities of both the ions are greater than the phase velocity of the wave. We thus introduce another source of instability for these waves namely multiple ion beams drifting perpendicular to the magnetic field. 相似文献
20.
Starting from the present version of the Riga dynamo experiment with its rotating magnetic eigenfield dominated by a single frequency we ask for those modifications of this set‐up that would allow for a non‐trivial magnetic field behaviour in the saturation regime. Assuming an increased ratio of azimuthal to axial flow velocity, we obtain energy oscillations with a frequency below the eigenfrequency of the magnetic field. These new oscillations are identified as magneto‐inertial waves that result from a slight imbalance of Lorentz and inertial forces. Increasing the azimuthal velocity further, or increasing the total magnetic Reynolds number, we find transitions to a chaotic behaviour of the dynamo (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献