On the magnetic field diffusion in mhd modeling of the inner coma of comet halley

Abstract. In a system with at least one ignorable spatial dimension charged particles moving in fluctuating fields are tied to the magnetic field lines. Thus, in one-and two-dimensional simulations cross-field diffusion is inhibited and important physics may be lost. We have investigated cross-field diffusion in self-consistent 3-D magnetic turbulence by fully 3-dimensional hybrid simulation (macro-particle ions, massless electron fluid). The turbulence is generated by the electromagnetic ion/ion beam instability. A cold, low density, ion beam with a high velocity stream relative to the background plasma excites the right-hand resonant instability. Such ion beams may be important in the region of the Earth's foreshock. The field turbulence scatters the beam ions parallel as well as perpendicular to the magnetic field. We have determined the parallel and perpendicular diffusion coefficient for the beam ions in the turbulent wave field. The result compares favourably well (within a factor 2) with hard-sphere scattering theory for the cross-field diffusion coefficient. The cross-field diffusion coefficient is larger than that obtained in a static field with a Kolmogorov type spectrum and similar total fluctuation power. This is attributed to the resonant behaviour of the particles in the fluctuating field.

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3D-Numerical Simulations of Magnetic Field Evolution in the Turbulent Interstellar Gas

Symposium - International Astronomical Union ◽

10.1017/s0074180900174510 ◽

1993 ◽

Vol 157 ◽

pp. 405-409

Author(s):

K. Otmianowska-Mazur ◽

M. Urbanik

Keyword(s):

Magnetic Field ◽

Galactic Plane ◽

Kinematic Model ◽

Three Dimensional ◽

Field Structure ◽

Interstellar Gas ◽

Field Diffusion ◽

Parcel Size ◽

Tube Well ◽

The Magnetic Field

A fully three-dimensional kinematic model is applied to simulate the evolution of the magnetic field in a small (120 pc in size) volume of the turbulent interstellar medium (ISM) in the presence of the field diffusion. The turbulent motions arc approximated by a sequence of non-overlapping in time vortices, which have the form of the rotating (with a non-zero hclicity) columns much longer than the parcel size. They moved vertically and at the inclination of ±30° to the galactic plane. The resulting magnetic field structure closely resembles a fragment of a classical twisted flux tube, well-ordered over the whole parcel of the ISM.

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Reconciliating the Vertical and Horizontal Gradients of the Sunspot Magnetic Field

Physics Research International ◽

10.1155/2013/195403 ◽

2013 ◽

Vol 2013 ◽

pp. 1-16 ◽

Cited By ~ 6

Author(s):

Véronique Bommier

Keyword(s):

Magnetic Field ◽

Aspect Ratio ◽

Vertical Gradient ◽

Spectral Lines ◽

Horizontal Gradient ◽

Plasma Anisotropy ◽

Different Depths ◽

Mhd Modeling ◽

Horizontal Flux ◽

The Magnetic Field

In the literature, we found 15 references showing that the sunspot photospheric magnetic field vertical gradient is on the order of 3-4 G/km, with field strength decreasing with height, whereas the horizontal gradient is nine times weaker on the order of 0.4-0.5 G/km. This is confirmed by our recent THEMIS observations. As a consequence, the vanishing of divB→ is not realized. In other words, a loss of magnetic flux is observed with increasing height, which is not compensated for by an increase of the horizontal flux. We show that the lack of spatial resolution, vertical as well as horizontal, cannot be held responsible for the nonvanishing observed divB→. The present paper is devoted to the investigation of this problem. We investigate how the magnetic field is influenced by the plasma anisotropy due to the stratification, which is responsible for an “aspect ratio” between horizontal and vertical typical lengths. On the example of our THEMIS observations, made of two spectral lines formed at two different depths, which enables the retrieval of the three components entering divB→, it is shown that once this aspect ratio is applied, the rescaled divB→ vanishes, which suggests a new methodology for MHD modeling in the photosphere.

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Three-Dimensional MHD Modeling of Interplanetary Solar Wind Using Self-Consistent Boundary Condition Obtained from Multiple Observations and Machine Learning

Universe ◽

10.3390/universe7100371 ◽

2021 ◽

Vol 7 (10) ◽

pp. 371

Author(s):

Yi Yang ◽

Fang Shen

Keyword(s):

Magnetic Field ◽

Machine Learning ◽

Boundary Condition ◽

Solar Wind ◽

Three Dimensional ◽

The Self ◽

Multiple Observations ◽

Self Consistent ◽

Mhd Modeling ◽

The Magnetic Field

Three-dimensional (3-d) magnetohydrodynamics (MHD) modeling is a key method for studying the interplanetary solar wind. In this paper, we introduce a new 3-d MHD solar wind model driven by the self-consistent boundary condition obtained from multiple observations and the Artificial Neural Network (ANN) machine learning technique. At the inner boundary, the magnetic field is derived using the magnetogram and potential field source surface extrapolation; the electron density is derived from the polarized brightness (pB) observations, the velocity can be deduced by an ANN using both the magnetogram and pB observations, and the temperature is derived from the magnetic field and electron density by a self-consistent method. Then, the 3-d interplanetary solar wind from CR2057 to CR2062 is modeled by the new model with the self-consistent boundary conditions. The modeling results present various observational characteristics at different latitudes, and are in better agreement with both the OMNI and Ulysses observations compared to our previous MHD model based only on photospheric magnetic field observations.

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Conditions for jet formation in accreting neutron stars: the magnetic field decay

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310016248 ◽

2010 ◽

Vol 6 (S275) ◽

pp. 309-310

Author(s):

Federico García ◽

Deborah N. Aguilera ◽

Gustavo E. Romero

Keyword(s):

Magnetic Field ◽

Neutron Star ◽

Neutron Stars ◽

Compact Objects ◽

Jet Formation ◽

Field Diffusion ◽

Induction Equation ◽

Strong Surface ◽

Field Decay ◽

The Magnetic Field

AbstractAccreting neutron stars can produce jets only if they are weakly magnetized (B ~ 108 G). On the other hand, neutron stars are compact objects born with strong surface magnetic fields (B ~ 1012 G). In this work we study the conditions for jet formation in a binary system formed by a neutron star and a massive donor star once the magnetic field has decayed due to accretion. We solve the induction equation for the magnetic field diffusion in a realistic neutron star crust and discuss the possibility of jet launching in systems like the recently detected Supergiant Fast X-ray Transients.

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