Dynamical simulations of magnetically channeled line-driven stellar winds

We present numerical magnetohydrodynamic simulations of the effect of stellar dipole magnetic fields on line-driven wind outflows from hot, luminous stars. Unlike previous fixed-field analyses, the simulations here take full account of the dynamical competition between field and flow, and thus apply to a full range of magnetic field strength, and within both closed and open magnetic topologies. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless, ‘wind magnetic confinement parameter’, η* = B2eqR2*/Mv∞, which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind. For weak confinement η* ≤ 1, the field is fully opened by the wind outflow, but nonetheless for confinements as small as η* = 1/10 can have a significant back-influence in enhancing the density and reducing the flow speed near the magnetic equator. For stronger confinement η* > 1, the magnetic field remains closed over a limited range of latitude and height about the equatorial surface, but eventually is opened into a nearly radial configuration at large radii. Within closed loops, the flow is channeled toward loop tops into shock collisions that are strong enough to produce hard X-rays, with the stagnated material then pulled by gravity back onto the star in quite complex and variable inflow patterns. Within open field flow, the equatorial channeling leads to oblique shocks that are again strong enough to produce X-rays, and also lead to a thin, dense, slowly outflowing ‘disk’ at the magnetic equator. The polar flow is characterized by a faster-than-radial expansion that is more gradual than anticipated in previous 1d flow-tube analyses, and leads to a much more modest increase in terminal speed (< 30%), consistent with observational constraints. Overall, the results here provide a dynamical groundwork for interpreting many types of observations, e.g., UV line-profile variability; red-shifted absorption or emission features; enhanced density-squared emission; and X-ray emission, that might be associated with perturbation of hot-star winds by surface magnetic fields.

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The Effects of Field-Aligned Rotation on the Magnetically Channeled Line-Driven Winds

Symposium - International Astronomical Union ◽

10.1017/s0074180900196135 ◽

2004 ◽

Vol 215 ◽

pp. 525-526

Author(s):

Asif ud-Doula ◽

Stanley Owocki

Keyword(s):

Magnetic Field ◽

Energy Density ◽

Magnetic Fields ◽

Large Scale ◽

Spatial Scales ◽

Full Range ◽

Magnetic Confinement ◽

Field Energy ◽

Kinetic Energy Density ◽

Mhd Simulations

There is extensive evidence that the radiatively driven stellar winds of OB-type stars are not the steady, smooth outflows envisioned in classical models, but instead exhibit extensive structure and variability on a range of temporal and spatial scales. We examine the possible role of stellar magnetic fields in forming large-scale wind structure. It is based on numerical magnetohydrodynamic (MHD) simulations of the interaction of a line-driven flow with an assumed stellar dipole field.Unlike previous fixed-field analyses, the MHD simulations here take full account of the dynamical competition between field and flow, and thus apply to a full range of magnetic field strength, and within both closed and open magnetic topologies. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless, ‘wind magnetic confinement parameter’, η∗ (= B2eqR2∗/Ṁv∞), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind.We extend these MHD simulations to include field-aligned stellar rotation. The results indicate that a combination of the magnetic confinement parameter and the rotation rate as a fraction of the ‘critical’ rotation now determine the global properties of the wind. For models with strong magnetic confinement, rotation can limit the extent of the last closed magnetic loop, and lead to episodic mass ejections that break through the close loop and are carried outward with a slow, dense, equatorial outflow. Our 2-D numerical simulations indicate that the magnetic fields provide excessive amount of angular momentum to the wind preventing the formation of a Keplerian disk.

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The Measurement of Solar Magnetic Fields

Symposium - International Astronomical Union ◽

10.1017/s0074180900022336 ◽

1971 ◽

Vol 43 ◽

pp. 3-23 ◽

Cited By ~ 16

Author(s):

Jacques M. Beckers

Keyword(s):

Magnetic Field ◽

Energy Density ◽

Kinetic Energy ◽

Magnetic Fields ◽

Solar Atmosphere ◽

Zeeman Effect ◽

Kinetic Energy Density ◽

Solar Magnetic Fields ◽

The Third ◽

The Magnetic Field

The different methods which have been used, or which may be used in the future, to measure solar magnetic fields are described and discussed. Roughly these can be divided into three groups (a) those which use the influence of the magnetic field on the electromagnetic radiation, (b) those which use the influence of the field on the structure of the solar atmosphere (MHD effects), and (c) those which use theoretical arguments. The former include the Zeeman effect, the Hanle effect, the gyro and synchrotron radiations and the Faraday rotation of radiowaves. The second includes the alignment of details at all levels of the solar atmosphere, and the calcium network, and the third makes use, for example, of the assumption of equipartition between magnetic and kinetic energy density.

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COOLING OF RELATIVISTIC ELECTRONS IN TEV BLAZARS: CLUES FROM MULTIWAVELENGTH SPECTRA

International Journal of Modern Physics D ◽

10.1142/s0218271808013194 ◽

2008 ◽

Vol 17 (09) ◽

pp. 1591-1601

Author(s):

R. SCHLICKEISER

Keyword(s):

Magnetic Field ◽

Energy Density ◽

Kinetic Energy ◽

High Energy ◽

Particle Energy ◽

Field Energy ◽

Kinetic Energy Density ◽

Relativistic Electrons ◽

Magnetic Field Energy ◽

The Magnetic Field

In powerful cosmic nonthermal radiation sources with dominant magnetic-field self generation, the generation of magnetic fields at almost equipartition strength by relativistic plasma instabilities operates as fast as the acceleration or injection of ultra-high energy radiating electrons and hadrons in these sources. Consequently, the magnetic field strength becomes time-dependent and adjusts itself to the actual kinetic energy density of the radiating electrons in these sources. This coupling of the magnetic field and the magnetic field energy density to the kinetic energy of the radiating particles changes both the intrinsic temporal evolution of the relativistic particle energy spectrum after injection and the synchrotron and synchrotron self-Compton emissivities.

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Emergence of Twisted Magnetic Flux Related Sigmoidal Brightening

International Astronomical Union Colloquium ◽

10.1017/s0252921100064630 ◽

2000 ◽

Vol 179 ◽

pp. 263-264

Author(s):

K. Sundara Raman ◽

K. B. Ramesh ◽

R. Selvendran ◽

P. S. M. Aleem ◽

K. M. Hiremath

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Magnetic Flux ◽

Ultra Violet ◽

Active Regions ◽

Morphological Properties ◽

Energy Storage And Conversion ◽

The Sun ◽

X Rays ◽

The Magnetic Field

Extended AbstractWe have examined the morphological properties of a sigmoid associated with an SXR (soft X-ray) flare. The sigmoid is cospatial with the EUV (extreme ultra violet) images and in the optical part lies along an S-shaped Hαfilament. The photoheliogram shows flux emergence within an existingδtype sunspot which has caused the rotation of the umbrae giving rise to the sigmoidal brightening.It is now widely accepted that flares derive their energy from the magnetic fields of the active regions and coronal levels are considered to be the flare sites. But still a satisfactory understanding of the flare processes has not been achieved because of the difficulties encountered to predict and estimate the probability of flare eruptions. The convection flows and vortices below the photosphere transport and concentrate magnetic field, which subsequently appear as active regions in the photosphere (Rust & Kumar 1994 and the references therein). Successive emergence of magnetic flux, twist the field, creating flare productive magnetic shear and has been studied by many authors (Sundara Ramanet al.1998 and the references therein). Hence, it is considered that the flare is powered by the energy stored in the twisted magnetic flux tubes (Kurokawa 1996 and the references therein). Rust & Kumar (1996) named the S-shaped bright coronal loops that appear in soft X-rays as ‘Sigmoids’ and concluded that this S-shaped distortion is due to the twist developed in the magnetic field lines. These transient sigmoidal features tell a great deal about unstable coronal magnetic fields, as these regions are more likely to be eruptive (Canfieldet al.1999). As the magnetic fields of the active regions are deep rooted in the Sun, the twist developed in the subphotospheric flux tube penetrates the photosphere and extends in to the corona. Thus, it is essentially favourable for the subphotospheric twist to unwind the twist and transmit it through the photosphere to the corona. Therefore, it becomes essential to make complete observational descriptions of a flare from the magnetic field changes that are taking place in different atmospheric levels of the Sun, to pin down the energy storage and conversion process that trigger the flare phenomena.

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Investigating the Magnetospheres of Rapidly Rotating B-type Stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317002812 ◽

2016 ◽

Vol 12 (S329) ◽

pp. 369-372

Author(s):

C. L. Fletcher ◽

V. Petit ◽

Y. Nazé ◽

G. A. Wade ◽

R. H. Townsend ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Centrifugal Force ◽

Point Of View ◽

Closed Field ◽

Theoretical Point ◽

X Rays ◽

Rapid Rotation ◽

X Ray ◽

Surface Magnetic Field

AbstractRecent spectropolarimetric surveys of bright, hot stars have found that ~10% of OB-type stars contain strong (mostly dipolar) surface magnetic fields (~kG). The prominent paradigm describing the interaction between the stellar winds and the surface magnetic field is the magnetically confined wind shock (MCWS) model. In this model, the stellar wind plasma is forced to move along the closed field loops of the magnetic field, colliding at the magnetic equator, and creating a shock. As the shocked material cools radiatively it will emit X-rays. Therefore, X-ray spectroscopy is a key tool in detecting and characterizing the hot wind material confined by the magnetic fields of these stars. Some B-type stars are found to have very short rotational periods. The effects of the rapid rotation on the X-ray production within the magnetosphere have yet to be explored in detail. The added centrifugal force due to rapid rotation is predicted to cause faster wind outflows along the field lines, leading to higher shock temperatures and harder X-rays. However, this is not observed in all rapidly rotating magnetic B-type stars. In order to address this from a theoretical point of view, we use the X-ray Analytical Dynamical Magnetosphere (XADM) model, originally developed for slow rotators, with an implementation of new rapid rotational physics. Using X-ray spectroscopy from ESA’s XMM-Newton space telescope, we observed 5 rapidly rotating B-types stars to add to the previous list of observations. Comparing the observed X-ray luminosity and hardness ratio to that predicted by the XADM allows us to determine the role the added centrifugal force plays in the magnetospheric X-ray emission of these stars.

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Finite size scaling in the solar wind magnetic field energy density as seen by WIND

Geophysical Research Letters ◽

10.1029/2001gl014587 ◽

2002 ◽

Vol 29 (10) ◽

pp. 86-1-86-4 ◽

Cited By ~ 34

Author(s):

B. Hnat ◽

S. C. Chapman ◽

G. Rowlands ◽

N. W. Watkins ◽

W. M. Farrell

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Energy Density ◽

Finite Size ◽

Field Energy ◽

Finite Size Scaling ◽

Size Scaling ◽

Magnetic Field Energy ◽

Solar Wind Magnetic Field ◽

Field Energy Density

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Measuring T Tauri star magnetic fields

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309030713 ◽

2008 ◽

Vol 4 (S259) ◽

pp. 345-356 ◽

Cited By ~ 3

Author(s):

Christopher M. Johns–Krull

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Scaling Laws ◽

Main Sequence ◽

Critical Role ◽

X Rays ◽

Stellar Rotation ◽

T Tauri ◽

Dipole Component ◽

Stellar Magnetic Fields

AbstractStellar magnetic fields including a strong dipole component are believed to play a critical role in the early evolution of newly formed stars and their circumstellar accretion disks. It is currently believed that the stellar magnetic field truncates the accretion disk several stellar radii above the star. This action forces accreting material to flow along the field lines and accrete onto the star preferentially at high stellar latitudes. It is also thought that the stellar rotation rate becomes locked to the Keplerian velocity near the radius where the disk is truncated. This paper reviews recent efforts to measure the magnetic field properties of low mass pre-main sequence stars, focussing on how the observations compare with the theoretical expectations. A picture is emerging indicating that quite strong fields do indeed cover the majority of the surface on these stars; however, the dipole component of the field appears to be alarmingly small. The current measurements also suggest that given their strong magnetic fields, T Tauri stars are somewhat faint in X-rays relative to what is expected from simple main sequence star scaling laws.

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Mössbauer study of the Ni–Zn ferrite system

Canadian Journal of Physics ◽

10.1139/p70-054 ◽

1970 ◽

Vol 48 (4) ◽

pp. 381-396 ◽

Cited By ~ 189

Author(s):

J. M. Daniels ◽

A. Rosencwaig

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Full Range ◽

Zinc Content ◽

Zero Magnetic Field ◽

Nickel Zinc ◽

Zn Ferrite ◽

Superparamagnetic Clusters ◽

Ferrite System ◽

Nuclear Magnetic

Mössbauer spectra of 57Fe in the nickel–zinc ferrite system (ZnO)x(NiO)1−xFe2O3 have been obtained, at room temperature and at 77 °K, in zero magnetic field and also in a longitudinal magnetic field of 13.5 kG, covering the full range of zinc content. The dependence of the isomer shifts, line widths, quadrupole interactions, and nuclear magnetic fields of 57Fe3+ ions in both tetrahedral and octahedral sites has been determined. The principal results of this study are (a) the confirmation of the determination of the nuclear magnetic fields by Abe et al. using NMR, and the extension of these measurements to higher zinc concentrations, (b) an indication that the hypotheses of paramagnetic centers, proposed by Gilleo, and superparamagnetic clusters, proposed by Ishikawa, are not applicable to the nickel–zinc ferrites, (c) evidence of canted spin structures, first proposed by Yafet and Kittel, (d) various effects of chemical disorder in the nickel-zinc ferrites, and (e) an observation of the effect on the Mössbauer spectrum of relaxation in a magnetically ordered iron system.

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