An Etymological Dictionary of Astronomy and Astrophysics
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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.

In → magnetohydrodynamic (MHD) treatment of → plasmas, that component of the → Lorentz force which is directed toward the centre of curvature of the → magnetic field lines and thus acts to straighten out the field lines. The Lorentz force can be decomposed into two components orthogonal to the magnetic field: j× B = (B . ∇) B / μ₀ - ∇ (B² / 2μ₀), where j is the → current density, μ₀ is the → magnetic permeability of free space, and B is the → magnetic flux density. The left side term is the Lorentz force, the first term on the right side is the magnetic tension and the second term the → magnetic pressure.

→ magnetic; → tension.

A vector field A defined so that the → magnetic field  B is given by its → curl: B = ∇ x A.

→ magnetic; → vector; → potential.

Radiation emitted by a rotating magnet.

→ magnetic; → dipole; → radiation.

The study of magnetic phenomena, comprising magnetostatics and electromagnetism.

→ magnetic; → -ics.

In the → solar dynamo model, a magnetic field that points from the north to south or south to north.

→ meridional; → magnetic; → field.

An analysis technique applied to some atomic nuclei that have the property to behave as small magnets and respond to the application of a magnetic field by absorbing or emitting electromagnetic radiation. When nuclei which have a magnetic moment (such as ¹H, ¹³C, ²⁹Si, or ³¹P) are submitted to a constant magnetic field and at the same time to a radio-frequency alternating magnetic field, the nuclear magnetic moment is excited to higher energy states if the alternating field has the specific resonance frequency. This technique is especially used in spectroscopic studies of molecular structure and in particular provides valuable information in medicine that can be used to deduce the structure of organic compounds.

→ nuclear; → magnetic; → resonance.

In the context of solar physics, a → magnetic field line when it crosses the solar surface only once, i.e., when it goes from surface to infinity. This is the case at a sufficiently large scale in → coronal holes. This is mostly not the case in → active regions.

→ open; → magnetic; → field; → line.

Relative to or characterized by → paramagnetism.

→ para- + → magnetic

1) In → protoplanetary disk models, the magnetic field whose large-scale lines of force depart away from the → accretion disk, in the direction of the rotation axis.
2) In a → tokamak, the magnetic field produced by a current flowing in the → plasma vertically to the toroid plane. → toroidal magnetic field.

→ pole; → -oid; → magnetic field.

The period of time, about 22 years, after which the magnetic → polarity of the Sun returns to its earlier state. It consists of two consecutive → solar cycles.

→ solar; → magnetic; → cycle.

The Sun's magnetic field which is probably created by the → differential rotation of the Sun together with the movement of charged particles in the → convective zone. Understanding how the solar magnetic field comes about is the fundamental problem of Solar Physics. The solar magnetic field is responsible for all solar magnetic phenomena, such as → sunspots, → solar flares, → coronal mass ejections, and the → solar wind. The solar magnetic fields are observed from the → Zeeman broadening of spectral lines, → polarization effects on radio emission, and from the channeling of charged particles into visible → coronal streamers. The strength of Sun's average magnetic field is 1 → gauss (twice the average field on the surface of Earth, around 0.5 gauss), and can be as strong as 4,000 Gauss in the neighborhood of a large sunspot.

→ solar; → magnetic; → field.

The magnetic moment associated with the → spin angular momentum of a charged particle. The direction of the magnetic moment is opposite to the direction of the angular momentum. The magnitude of the magnetic moment is given by: μ = -g(q / 2m)J, where q is the charge, m is the mass, and J the angular momentum. The parameter g is a characteristic of the state of the atom. It would be 1 for a pure orbital moment, or 2 for a spin moment, or some other number in between for a complicated system like an atom. The quantity in the parenthesis for the electron is the → Bohr magneton. The electron spin magnetic moment is important in the → spin-orbit interaction which splits atomic energy levels and gives rise to → fine structure in the spectra of atoms. It is also a factor in the interaction of atom with external fields, → Zeeman effect.

→ spin; → magnetic moment.

The → magnetic field associated with a star. Magnetic fields are common among stars of solar and lower masses. So far definitive detections of fields in stars with masses ~1.5 Msun have, for the most part, been made for objects having anomalous chemical abundances (e.g., the → chemically peculiar A and B stars). Recently, however, observations of cyclic variability in the properties of → stellar winds from luminous → OB stars have been interpreted as evidence for the presence of large-scale magnetic fields in the surface layers and atmospheres of these objects (→ magnetic massive star). These inferences have been bolstered by the unambiguous measurement of a weak (~ 360 G) field in the chemically normal B1 IIIe star → Beta Cephei. These results suggest that magnetic fields of moderate strength might be more prevalent among → hot stars than had previously been thought. At the present time, the origin of magnetism in massive stars is not well understood. If the magnetic field of a hot star is produced by → dynamo effect in the → convective core, then a mechanism for transporting the field to the stellar surface must be identified. The finite electrical conductivity of the envelope leads to the outward diffusion of any fields contained therein, but only over an extended period of time. Estimates indicate that for stars more massive than a few solar masses, the resistive diffusion time across the radiative interior exceeds the → main sequence lifetime. Another possibility is that dynamo fields are advected from the core to the surface by rotation-induced → meridional circulation (MacGregor & Cassinelli, 2002, astro-ph/0212224).

→ stellar; → magnetic; → field.