An Etymological Dictionary of Astronomy and Astrophysics
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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.

In → plasma physics, a → quantity that describes the → resistance of a → charged particle to change its direction of motion under the influence of a perpendicular → magnetic field. Rigidity is defined as: R = r_LBc = (pc)/(Ze), where r_L is the → Larmor radius, B is → magnetic induction, c is the → speed of light, p is the → momentum of the particle, Z is → atomic number, and e the → electron charge. Since pc has the dimensions of energy and e the dimensions of charge, rigidity has the dimensions of → volts (a 10 GeV proton has a rigidity of 10 GV). In → cosmic ray studies, the energies of cosmic rays are often quoted in terms of their rigidities, rather than their energies per nucleon.

→ magnetic; → rigidity.

The → counterpart of the → magnetic north pole. It lies near the → geographic north pole.

→ magnetic; → south; → pole.

A process whereby the (internal) → magnetic field of a star modifies the → pulsations by lifting some of its degeneracy. Instead of just one pulsation frequency, a multiplet of frequencies is then observed. This effect was proposed as a possible explanation for the observed frequency pattern of → Beta Cephei. In practice, the magnetic splitting is difficult to observe, because of the very small expected frequency difference between the peaks. However, when unaccounted for, it may lead to a wrong mode identification. The current best candidate to detect magnetic splitting is → HD 43317, since this star displays two close frequency patterns (Buysschaert et al., 2017, astro-ph/1709.02619).

→ magnetic; → splitting.

A star whose → spectral lines show the → Zeeman effect. See also: → stellar magnetic field, → magnetic massive star, → Ap/Bp star.

→ magnetic; → star.

A temporary, worldwide disturbance of the Earth's magnetic field by streams of charged particles from the Sun. Magnetic storms are frequently characterized by a sudden onset, in which the magnetic field undergoes marked changes in the course of an hour or less, followed by a very gradual return to normalcy, which may take several days.

→ magnetic; → storm.

A property of material defined by the ratio of the → magnetization to the → magnetic intensity. In other words, the magnetization per unit magnetic intensity.

→ magnetic; → susceptibility.

A continuous, flexible ribbon impregnated or coated with magnetic-sensitive material on which information (sound, images, data, etc.) may be recorded.

→ magnetic; → tape.