An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) Same as → magnetic flux density.
2) The production of a magnetic field in a piece of un-magnetized iron or other → ferromagnetic substance when a magnet is brought near it. The magnet causes the individual particles of iron, which act like tiny magnets, to line up so that the sample as a whole becomes magnetized.

→ magnetic; → induction.

Strength of a magnetic field at a point, denoted H. The force which could be exerted on unit north magnetic pole situated at that point. Measured in oersteds. Same as → magnetic field strength.

→ magnetic; → intensity.

A → stellar magnetic field associated with a → massive star. Magnetic fields are detected only for seven to ten percent of all studied massive → OB stars, and the magnetic field occurrence does not depend on the → spectral type. Because these magnetic fields seem to be stable over long time-scales and their strength does not seem to correlate with known stellar properties, it is assumed that they are of fossil origin (→ fossil magnetic field) and are frozen into the → radiative envelope of the stars. The fields are those of the birth → molecular clouds, partly trapped inside the → pre-main sequence star during the cloud → collapse phase, possibly further enhanced by a → dynamo effect in the early fully convective stellar phase. Typically, the polar field strength ranges from about a hundred → Gauss up to several kiloGauss. However, some weaker fields, below 100 G, have recently been detected.
The stellar magnetic field influences many different regions of the star with various effects. In the deep interior of the star, the field influences the internal → mixing of the star and this affects the size of the → convective overshooting region, changing the lifetime of the star by decreasing the amount of fuel for nuclear burning. Magnetic stars can also confine their → stellar winds, due to their strong magnetic fields, into a → magnetosphere, which slows down the → rotational velocity of the star. This → magnetic braking is an efficient mechanism for → angular momentum transport. At the stellar surface, the magnetic fields can create and sustain areas of chemical over- or under-abundances and/or large temperature differences, which are called spots (Buysschaert et al., 2016, astro-ph/1709.02619).

→ magnetic; → massive; → star.

A meridian passing through the Earth's → magnetic poles.

→ magnetic; → meridian.

1) A measure of the strength of a magnet or current-carrying coil. In the case of a bar magnet it is obtained by multiplying the distance between the two magnetic poles by the average strength of the poles. Same as → magnetic dipole moment See also → dipole moment.
2) A measure of the magnetic flux set up by the gyration of an electric charge in a magnetic field.
3) In atomic and nuclear physics, → spin magnetic moment.

→ magnetic; → moment.

A hypothetical particle that carries a single → magnetic pole, in contrast to magnets which are north-south pole pairs. These massive particles (billions of times heavier than the → proton) are required by grand unified theories(→ GUTs) to explain the actual matter content of the Universe, particularly the dominance of matter upon → antimatter. However, their existence contradicts → Gauss's law for magnetism.

→ magnetic; → monopole.

A problem concerning the compatibility of grand unified theories (→ GUTs) with standard cosmology. If standard cosmology was combined with grand unified theories, far too many → magnetic monopoles would have been produced in the early Universe. The → inflation hypothesis aims at explaining the observed scarcity of monopoles. The inflation has deceased their density by a huge factor.

→ magnetic; → monopole; → problem.

A slender → magnet suspended in a magnetic compass on a mounting with little friction; used to indicate the direction of the Earth's → magnetic pole.

→ magnetic; → needle.

A point of the → magnetosphere where the Earth's → magnetic field points vertically downward; in other words it has a 90° → magnetic dip toward the Earth's surface. The magnetic north pole can also be defined as the point toward which the south pole of the → compass needle is directed. The magnetic north pole is different from the → geographic north pole. It is actually hundreds of kilometers south of the geographic north pole. However, this has not always been the case. In the past 150 years it has moved more than 1,000 kilometers. Every 200,000 to 300,000 years the magnetic field of the Earth reverses direction, → magnetic reversal. Since the Earth's magnetic field is not exactly symmetrical, the north and south magnetic poles are not → antipodal.

→ magnetic; → north; → pole.

A region of the → solar corona where the → magnetic field vanishes.

→ magnetic; → null; → point.

The ratio of the → magnetic induction, B, in the substance to the external magnetic field, H, causing the → induction: μ = B/H. It is measured in henry/meter and is known as absolute permeability. The relative permeability is equal to the ratio of absolute permeability to the permeability of the free space. Thus μ_r = μ/μ₀, where μ₀, the permeability of free space has the value 4π x 10^-7 henry/meter.

→ magnetic; → permeability.

1) The region of a magnet toward which the lines of magnetic force converge (south pole) or from which the lines of force diverge (north pole).
2) Either of the two points on the Earth's surface where the magnetic lines of force converge. They are not aligned with the geographical poles, but shift and do not lie exactly opposite of the other. → magnetic north pole, → magnetic south pole, → magnetic reversal.

→ magnetic; → pole.

A → dimensionless quantity used in → magnetohydrodynamics to describe the relative balance of → kinematic viscosity to → magnetic diffusion. It is described by: P_r = σμ₀ν = ν/η, where σ is the → conductivity of the fluid, μ₀ is the → magnetic permeability of the fluid, ν is the kinematic viscosity of the fluid, and η is the → magnetic diffusivity.

→ magnetic; → Prandtl number.

The pressure exerted by a magnetic field on the material that contains the field.

→ magnetic; → pressure.

In atomic physics, a quantum number that denotes the energy levels available within a subshell. Designated by the letter m, it is one of a set of quantum numbers which describe the unique quantum state of an electron.

→ magnetic; → quantum; → number.

In a → plasma, a change of → magnetic connectivity of plasma elements due to the presence of a localized → diffusion region. It allows charged particles to move from one → magnetic field line to another. Magnetic reconnection is an important process transforming magnetic energy into heat or/and kinetic energy. Magnetic reconnection events occur in the Earth's → magnetosphere. The process plays an important role in explosive phenomena in the Sun, such as → coronal mass ejections and → solar flares which heat the → solar corona.

→ magnetic; → re-; → connection.

The process by which a magnetic system relaxes to its minimum energy state over time.

→ magnetic; → relaxation.

A phenomenon exhibited by certain atoms whereby they absorb energy at specific (resonant) frequencies when subjected to alternating magnetic fields.

→ magnetic; → resonance.

A change in the Earth's → magnetic field in which the → magnetic north pole is transformed into a → magnetic south pole and the magnetic south pole becomes a magnetic north pole. There are geological proofs indicating that the Earth's magnetic field has undergone numerous reversals of → polarity in the past. In the last 10 million years, there have been, on average, 4 or 5 reversals per million years. At other times, for example during the → Cretaceous era, there have been much longer periods when no reversals occurred. Over the past two centuries, Earth's magnetic field has weakened by 15%. Risks of a weak magnetic field include more deaths from cancer due to increased radiation, electrical grid collapse from severe solar storms, climate change, and temporary ozone holes. See also → geomagnetic excursion.

→ magnetic; → reversal.

A → dimensionless quantity used in → magnetohydrodynamics to describe the relative balance of → magnetic advection to → magnetic diffusion. It is given by: R_m = σμ₀νLU₀, where σ is the → conductivity of the fluid, μ₀ is the → magnetic permeability of the fluid, L is he characteristic length scale of the fluid flow, and U₀ the characteristic velocity of the flow. A typical value for the Earth is R_m ~ 200.

→ magnetic; → Reynolds number.