An Etymological Dictionary of Astronomy and Astrophysics
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The imaginary straight line joining the two → poles of a → magnet.

→ magnetic; → meridian.

Any configuration of → magnetic fields used in the containment of a → plasma during controlled → thermonuclear reaction experiments.

→ magnetic; → bottle.

The process whereby a star which loses mass slows down under the action of its → magnetic field. The stellar material follows the → magnetic field lines extending well beyond the stellar surface. The material gain → angular momentum and the underlying object is slowed down. Magnetic braking is an efficient mechanism for removing angular momentum from the the rotating object. See also → disk locking.

→ magnetic; → braking.

The failure of numerical star formation calculations to produce rotationally supported → Keplerian disks because of the → magnetic braking effect, when → magnetic fields of strengths comparable to those observed in → molecular clouds are accounted for. The formation and early evolution of disks is a long-standing fundamental problem in → star formation models. Early work in the field had concentrated on the simpler problem of disk formation from the → collapse of a rotating dense core in the absence of a magnetic field. However, dense star-forming cores are observed to be significantly magnetized. There is increasing theoretical evidence that disk formation is greatly modified, perhaps even suppressed, by a dynamically important magnetic field. This has been found in analytic studies, axisymmetric numerical models and in 3D calculations using → ideal magnetohydrodynamics. By contrast, recent observations suggest the presence of massive, 50-100 AU disks and evidence for associated → outflows in the earliest (→ class 0) stages of star formation around both low and high mass stars. Two primary solutions have been proposed: → turbulence and → non-ideal magnetohydrodynamics. Calculations of the collapse of a massive 100 Msun core have shown that 100 AU scale disk formation in the presence of strong magnetic fields was indeed possible, with some argument over whether this is caused by turbulent reconnection or another mechanism. Studies, using simulations of collapsing 5 Msun cores, have found that turbulence diffuses the strong magnetic field out of the inner regions of the core, and that the non-zero → angular momentum of the turbulence causes a misalignment between the rotation axis and the magnetic field. Both of these effects reduce the magnetic braking, and allow a massive disk to form (Wurster et al. 2016, arxiv/1512.01597 and references therein).

→ magnetic; → braking; → catastrophe.

Same as → synchrotron radiation.

→ magnetic; → bremsstrahlung.

The phenomenon whereby the presence of a → magnetic field can make a portion of → compressible fluid less dense than its surroundings, so that it floats upward under the influence of gravity. This magnetic buoyancy is thought, in fact, to be the mechanism by which magnetic flux tubes rise through the Sun's → convection zone and break at the surface in the form of → sunspots. The Sun's rotation would have a major effect on the rate at which these magnetic flux tubes rise. The rotation substantially lengthen the time taken for the flux tubes to reach the surface (D. J. Acheson, 1979, Nature 277, 41).

→ magnetic; → buoyancy.

A cataclysmic binary in which the white dwarf primary has a strong magnetic field that radically affects the accretion flow in the system. → polar

→ magnetic; → cataclysmic; → binary.

A transient ejection in the → solar wind having an enhanced field, a large and smooth change in field direction, and a low → proton temperature compared to the ambient proton temperature (L. F. Burlaga, 1995, Interplanetary Magnetohydrodynamics, Oxford Univ. Press, 89-114).

→ magnetic; → cloud.

→ compass.

→ magnetic; → compass.

Of magnetic field lines, the condition for them to be connected or the process whereby they become connected or connective.

→ magnetic;→ connectivity.

A physical constant relating mechanical and electromagnetic units of measurement. It has the value of 4π × 10^-7 henry per meter. Also called the permeability of free space, or → absolute permeability.

→ magnetic; → constant.

Thermal → convection modified by the presence of magnetic fields.

→ magnetic; → convection.

In terrestrial magnetism, the difference between → true north (the axis around which the earth rotates) and magnetic north (the direction the needle of a compass will point,→ magnetic pole).

→ magnetic; → declination.

The process whereby the magnetic field tends to diffuse across the plasma and to smooth out any local inhomogeneities under the influence of a finite resistance in the plasma. For a stationary plasma the → induction equation becomes a pure → diffusion equation: ∂B/∂t = D_m∇²B, where D_m = (μ₀σ₀)^-1 is the → magnetic diffusivity.

→ magnetic; → diffusion.

The → diffusion coefficient for a magnetic field. It is expressed as: η = 1/(μ₀σ), where μ₀ is the → magnetic permeability and σ the → conductivity.

→ magnetic; → diffusivity.

In terrestrial magnetism, the angle that a → magnetic needle makes with the horizontal plane at any specific location. The angle of magnetic dip at the → magnetic poles of Earth is 90°. Also called → inclination and → dip.

→ magnetic; → dip.

A system that generates a → magnetic field in which the field is considered to result from two opposite poles, as in the north and south poles of a magnet, much as an → electric field originates from a positive and a negative charge in an → electric dipole. A loop carrying an electric current also acts as a magnetic dipole. Magnetic dipoles experience a torque in the presence of magnetic fields. → dipole moment; → magnetic moment.

→ magnetic; → dipole.

Same as → magnetic moment.

→ magnetic; → dipole; → moment.

Any of several microscopic areas in a → ferromagnetic material that possesses a net → magnetic field, because electron spins are aligned in the same direction. In the absence of an external magnetic field, the directions of the magnetization vectors of the separate domains do not coincide and the resultant magnetization of the whole body may be zero.

→ magnetic; → domain.

The energy stored in a magnetic field. It is the → work that must be done to establish a magnetic field in terms of the → magnetic induction. Magnetic energy varies as the square of the magnetic induction. It can be expressed in several other ways, for example in terms of the current and of the magnetic flux, or in terms of the current density and vector potential.

→ magnetic; → energy.