Fr.: vide magnétisé
Fr.: magnéto-, magnét-
Combined study of the large-scale → magnetic field (→ magnetometry) and → stellar pulsations (→ asteroseismology). Magneto-asteroseismology provides strong complementary diagnostics suitable for detailed stellar modeling and permits the determination of the → internal structure and conditions within → magnetic massive → pulsators, for example the effect of magnetism on → mixing processes. More specifically, asteroseismology yields information on the → density, → composition, and → chemical mixing in multiple internal layers (depending on the number of studied frequencies). Additionally, when rotationally split pulsation modes are observed, the internal rotation profile can be retrieved. From magnetometry surface properties are determined, related to the → chemical composition, including → starspots, and the magnetic field, such as its geometry, obliquity, and strength. Magnetic studies also provide constraints about the → stellar wind geometry and the → circumstellar environment. Moreover, the stellar → rotation period period and the → angle of inclination toward the observer are also retrieved (Buysschaert et al., 2017, astro-ph/1709.02619).
Fr.: accelération magnetocentrifuge
The acceleration exerted on the plasma particles according to the → magnetocentrifugal model.
Fr.: modèle magnétocentrifuge
A → magnetohydrodynamic model devised to account for the → bipolar jets and → outflows observed around → protostars. Basically, a → poloidal magnetic field is frozen into a rotating → accretion disk. If the angle between the magnetic field lines threading the disk and the rotation axis of the disk is larger than 30°, the plasma can be accelerated out of the accretion disk along the field lines. The field lines rotate at a constant → angular velocity, and as the gas moves outward along the field lines, it is accelerated by an increasing → centrifugal force (magnetocentrifugal acceleration). At some point, when the rotation velocity is about the same as the → Alfven velocity in the gas, the field lines get increasingly wound up by the inertia of the attached gas and a strong → toroidal magnetic field component is generated. The toroidal component is the main agent in collimating the flow into a direction along the → open magnetic field lines. The earliest version of the model was proposed by Blandford & Payne (1982, MNRAS 199, 883). It has two main versions: → X-wind and → disk wind models. See also → magnetorotational instability.
A graphic representation of solar magnetic field strengths and polarity.
Of or relating to → magnetohydrodynamics.
The dynamics of an ionized plasma in the non-relativistic, collisional case. In this description, charge oscillations and high frequency electromagnetic waves are neglected. It is an important field of astrophysics since plasma is one of the commonest forms of matter in the Universe, occurring in stars, planetary magnetospheres, and interplanetary and interstellar space.
Any of a variety of devices used to measure the strength and direction of a magnetic field.
The detection or measurement of a magnetic field, especially its strength and direction. See also → magnetometer.
Fundamental constant, first calculated by Bohr, for the intrinsic magnetic moment of an electron. → Bohr magneton.
The boundary layer between a planet's → magnetosphere and the → magnetic field of the → solar wind. It borders the → magnetosheath and is defined by the surface on which the pressure of the solar wind is balanced by that of the planet's magnetic field. The front point of the Earth's magnetopause, on the sun-ward side of the Earth, is about 10 terrestrial radii, on average. This point can be closer or farther, because the magnetopause contracts or expands depending on the intensity of the solar wind.
From → magneto- + pause "break, cessation, stop," from M.Fr. pause, from L. pausa "a halt, stop, cessation," from Gk. pausis "stopping, ceasing," from pauein "to stop, to cause to cease."
From meqnât-→ magnet + marz "frontier, border, boundary," from Mid.Pers. marz "boundary;" Av. marəza- "border, district," marəz- "to rub, wipe;" Mod.Pers. parmâs "contact, touching" (→ contact), mâl-, mâlidan "to rub;" PIE base *merg- "boundary, border;" cf. L. margo "edge" (Fr. marge "margin"); Ger. Mark; E. mark, margin.
magnetorotational instability (MRI)
Fr.: instabilité magnétorotationnelle
An instability that arises from the action of a weak → poloidal magnetic field in a → differentially rotating system, such as a → Keplerian disk. The MRI provides a mechanism to account for the additional outward → angular momentum transport. To put it simply, the → frozen magnetic field line acts as a spring connecting two radially neighboring fluid parcels. In a Keplerian disk the inner fluid parcel orbits more rapidly than the outer, causing the spring to stretch. The magnetic tension forces the inner parcel to slow down reducing its angular momentum by moving it to a lower orbit. The outer fluid parcel is forced by the spring to speed up, increase its angular momentum, and therefore move to a higher orbit. The spring tension increases as the two fluid parcels grow further apart, and eventually the process runs away. The MRI was first noted in a non-astrophysical context by E. Velikhov in 1959 when considering the stability of → Couette flow of an ideal hydromagnetic fluid. His result was later generalized by S. Chandrasekhar in 1960. The MRI was rediscovered by Balbus and Hawley 1991 (ApJ 376, 214) who demonstrated that this instability does indeed manifest itself in → accretion disks, and could account for the turbulent mixing needed to explain the observed mass → accretion rates.
The region between a planet's magnetopause and the bow shock caused by the solar wind.
From → magneto- + sheath, from O.E. sceað, scæð, from P.Gmc. *skaithiz (cf. M.Du. schede, Du. schede, O.H.G. skaida, Ger. Scheide "scabbard").
From meqnât-, → magnet, + niyâm "sheath," from Proto-Iranian *nigāma-, from ni- "down; into," → ni-, + gāma- "to go, to come" (Av. gam- "to come; to go," jamaiti "goes;" O.Pers. gam- "to come; to go;" Mod./Mid.Pers. gâm "step, pace," âmadan "to come;" cf. Skt. gamati "goes;" Gk. bainein "to go, walk, step;" L. venire "to come;" Tocharian A käm- "to come;" O.H.G. queman "to come;" E. come; PIE root *gwem- "to go, come"); cf. Skt. nigamá- "insertion, incorporation."
The region around a celestial body in which the magnetic field of the body dominates the external magnetic field. Each planet with a magnetic field (Earth, Jupiter, Saturn, Uranus, and Neptune) has a magnetopause. The Earth's magnetosphere is a dynamic system that responds to solar variations. It prevents most of the charged particles carried in the → solar wind, from hitting the Earth. Since the solar wind is → supersonic, a → bow shock is formed on the sunward side of the magnetosphere. The solar wind ahead is deflected at a boundary called → magnetopause. The region between the bow shock and the magnetopause is called the → magnetosheath. As the solar wind sweeps past the Earth, the terrestrial magnetic field lines are stretched out toward the night side to form a → magnetotail.
Fr.: queue magnétique
The portion of a planet's → magnetosphere which is pushed away from the Sun by the solar wind. Earth's magnetosphere extends about 65,000 km on the day-side but more than 10 times further.
The factor by which the angular diameter of an object is apparently increased when viewed through an optical instrument to that of the object viewed by the unaided eye.
Verbal noun of → magnify.
A thing or device that magnifies.
From → magnify + suffix -ir.
Bozognemâ, agent noun of bozorg nemudan, → magnify.
bozorg nemudan, bozorgidan
To increase the apparent size of, as a lens does.
From O.Fr. magnifier, from L. magnificare "esteem greatly, extol," from magnificus "splendid," from magnus "great" + root of facere "to make."
From bozorg "large, magnificent, great" + nemudan "to show."
The first element from Mid.Pers. vazurg
"great, big, high, lofty;" O.Pers. vazarka- "great;" Av. vazra-
"club, mace" (Mod.Pers. gorz "mace"); cf. Skt. vájra-
"(Indra's) thunderbolt," vaja- "strength, speed;" L. vigere "be lively, thrive,"
velox "fast, lively," vegere "to enliven," vigil "watchful, awake;"
P.Gmc. *waken (Du. waken; O.H.G. wahhen; Ger. wachen
"to be awake;" E. wake); PIE base *weg- "to be strong, be lively."
The second element nemudan from Mid.Pers. nimūdan, nimây-
"to show," from O.Pers./Av. ni- "down; into,"
→ ni-, + māy-
"to measure," → display.
A lens or lens system that produces an enlarged virtual image of an object placed near its front focal point. According to Enoch (1998, SPIE vol. 3299, p. 424), the earliest lenses identified are from the IV/V Dynasties of Egypt, dating back to about 4,500 years ago (e.g., the eyes of the Louvre statue Le scribe accroupi and other examples located in the Cairo Museum). For more information see → burning sphere.
Zarrebin, from zarré "a minute thing," → particle, + bin "seer; to see" (present stem of didan; Mid.Pers. wyn-; O.Pers. vain- "to see;" Av. vaēn- "to see;" Skt. veda "I know;" Gk. oida "I know," idein "to see;" L. videre "to see;" PIE base *weid- "to know, to see").