La tectonique du socle sous la Manche occidentale d'après les données du magnétisme aéroporté

1975 ◽  
Vol 279 (1288) ◽  
pp. 55-68 ◽  
Keyword(s):  
Magnetic Field ◽  
Central Region ◽  
Positive Anomaly ◽  
First Derivative ◽  
Uniform Magnetization ◽  
Geological Information ◽  
Subsequent Phase ◽  
Anomaly Map ◽  
The Magnetic Field ◽  
Principal Elements

The magnetic field anomaly map compiled by the C. N. R. S. in 1964, which is based on profiles spaced 10 km apart and at a height of 3 km, reveals a strong positive anomaly in the central region of the western Channel. The interpretation of this type of survey is considerably facilitated by the use of transformations which permit us to present two supplementary maps: ( a ) the first derivative of the magnetic field in the vertical sense and ( b ) that of the preceding function reduced to a pole, assuming initially that magnetized bodies are created by pure induction. From these two transformations in which the effect of the magnetic substratum is considerably enhanced, we have drawn contours which represent either the principal elements of the magnetic relief or the principal ranges of magnetization of a relatively flat magnetic substratum. In the subsequent phase of interpretation, namely a comparison with the geological information available, the presence of distinct bodies of uniform magnetization was assumed. Calculations of depth indices have also been made. From the results obtained, which are synthesized in this report, we have been able to formulate a hypothesis concerning the date of the emplacement of the structures responsible for the observed anomalies.

scholarly journals 8.4. Properties of synchrotron emission and magnetic fields in the central region of M31

1998 ◽  
Vol 184 ◽  
pp. 351-352 ◽  
Author(s):  
P. Hoernes ◽  
R. Beck ◽  
E.M. Berkhuijsen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Spectral Index ◽  
Central Region ◽  
Relativistic Electrons ◽  
Synchrotron Emission ◽  
The Magnetic Field

At the centre of M31 the nonthermal spectral index between λ20 cm and λ6 cm is −0.2. It slowly decreases along the southern arm and the northern filaments visible in Hα, but perpendicular to these features it increases much faster. The magnetic field runs along the arm and the filaments. These phenomena suggest the existence of a mono-energetic source of relativistic electrons in the nucleus.

scholarly journals Magnetic Field and Early Evolution of Circumstellar Disks

2016 ◽  
Vol 33 ◽  
Author(s):  
Yusuke Tsukamoto
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Gravitational Collapse ◽  
Central Region ◽  
Early Phase ◽  
Circumstellar Disks ◽  
Early Evolution ◽  
Disk Formation ◽  
Magnetic Braking ◽  
The Magnetic Field

AbstractThe magnetic field plays a central role in the formation and evolution of circumstellar disks. The magnetic field connects the rapidly rotating central region with the outer envelope and extracts angular momentum from the central region during gravitational collapse of the cloud core. This process is known as magnetic braking. Both analytical and multidimensional simulations have shown that disk formation is strongly suppressed by magnetic braking in moderately magnetised cloud cores in the ideal magnetohydrodynamic limit. On the other hand, recent observations have provided growing evidence of a relatively large disk several tens of astronomical units in size existing in some Class 0 young stellar objects. This introduces a serious discrepancy between the theoretical study and observations. Various physical mechanisms have been proposed to solve the problem of catastrophic magnetic braking, such as misalignment between the magnetic field and the rotation axis, turbulence, and non-ideal effect. In this paper, we review the mechanism of magnetic braking, its effect on disk formation and early evolution, and the mechanisms that resolve the magnetic braking problem. In particular, we emphasise the importance of non-ideal effects. The combination of magnetic diffusion and thermal evolution during gravitational collapse provides a robust formation process for the circumstellar disk at the very early phase of protostar formation. The rotation induced by the Hall effect can supply a sufficient amount of angular momentum for typical circumstellar disks around T Tauri stars. By examining the combination of the suggested mechanisms, we conclude that the circumstellar disks commonly form in the very early phase of protostar formation.

scholarly journals Synchrotron Radiation from Energetic Electrons Emitted by AGN: A Probe for Magnetic Fields in External Galaxies

1990 ◽  
Vol 140 ◽  
pp. 399-399
Author(s):  
Matthew G. Baring
Keyword(s):  
Magnetic Field ◽  
Synchrotron Radiation ◽  
Magnetic Fields ◽  
Field Strength ◽  
Central Region ◽  
Low Energy ◽  
X Ray ◽  
The Galaxy ◽  
Energy Cutoff ◽  
The Magnetic Field

Shock acceleration of protons in the central region of active galaxies can energize them to Lorentz factors as high as 108 (Sikora et al., 1987), and these can subsequently produce a host of other relativistic particles including pions, e+e– pairs and neutrons. The luminosities of each species are expected to be of the same order of magnitude. Rapid decay of the pions leads to the secondary production of photons and pairs with energies of around 109 − 1011 MeV. The electrons and positrons can escape the compact central region and interact with the microwave background forming a pair cascade, and can also emit synchrotron radiation in the magnetic field. The neutrons do not interact with the field, and a significant fraction of them can escape the central region of a galaxy (Kirk and Mastichiadis, 1989). They can travel until they decay, producing protons and electrons in outer regions of the galaxy. Their decay time of γnτn gives a typical length for decay of about 1 kpc for the most energetic neutrons. The synchrotron radiation of these decay product electrons is examined in Baring (1989, in preparation), and it produces definite signatures of galactic magnetic fields. Magnetic fields of 1μG imply synchrotron emission in the X-ray and soft gamma-ray range for maximum Lorentz factors of γe = 1010, with a continuum extending down to much lower energies. It is observed that cooler neutrons deposit electrons at smaller radii, and these electrons are cooler (in a decay γe ~ γn). Hence the radiation would be cooler at smaller radii. This provides a diagnostic for the magnetic field: estimates of the field strength are possible from cut-offs that are expected in spectra from galactic halos. The injection of energetic electrons via neutron decay is found to yield a sharp cut-off in the injection distribution at γm = γe ~ r/τnc at radius r. Below this, no electrons are injected since they are produced in decays at smaller radii. This implies a low energy cutoff of ωm = γ2mBmec2 in the spectrum at given radius. Typically for r = 10 pc and a field of 1μG, the cutoff is at 10−8mec2 in the far infra-red. At larger radii, this low energy cutoff rapidly increases to X-ray energies. This cut-off provides a good way to measure the magnetic field strength and obtain its spatial dependence. In practice the situation is complicated by the superposition of different regions within the galaxy along the line of sight. Ways in which the observations could be deconvolved are discussed in Baring (1989, in preparation). A real possibility exists for detecting these signatures of energetic neutron emission from central regions of nearby galaxies and using the electron synchrotron spectra to spatially map galactic fields.

Exact expression for the magnetic field of a finite cylinder with arbitrary uniform magnetization

2018 ◽  
Vol 456 ◽  
pp. 423-432 ◽  
Author(s):  
Alessio Caciagli ◽  
Roel J. Baars ◽  
Albert P. Philipse ◽  
Bonny W.M. Kuipers
Keyword(s):  
Magnetic Field ◽  
Exact Expression ◽  
Finite Cylinder ◽  
Uniform Magnetization ◽  
The Magnetic Field

scholarly journals NGC 4631: The Unusual Star Forming Region Around CM67 — CO Observations with NMA and Pico Veleta

1994 ◽  
Vol 140 ◽  
pp. 363-365 ◽  
Author(s):  
M. Krause ◽  
G. Golla ◽  
K.-I. Morita ◽  
R. Wielebinski
Keyword(s):  
Magnetic Field ◽  
Central Region ◽  
Formation Rate ◽  
Radio Continuum ◽  
Field Orientation ◽  
Hii Region ◽  
Star Forming ◽  
Radio Halo ◽  
Star Forming Regions ◽  
The Magnetic Field

The late-type edge-on spiral galaxy NGC 4631 is known for its high star formation rate and extended radio halo with a uniform magnetic field component ordered predominantly perpendicular to the plane of the galaxy in the inner 6 kpc (assuming a distance of 7.5 Mpc, 1′ = 2.2 kpc) (Hummel et al. 1988; Golla, Ph.D., in preparation). The strongest radio continuum source of NGC 4631 is located at the eastern edge of the central region near a giant HII region complex CM67 (Crillon and Monnet 1969). The magnetic field orientation going out from the central region and especially from the region CM67 as well as the prominent north eastern low frequency radio spur (cf. Hummel et al. 1991) indicate a close connection between the synchrotron emission of the radio halo of NGC 4631 and star forming regions/CM67 in the disk (Golla and Hummel, in preparation). Probably cosmic rays from star forming regions in the disk propagate along the magnetic field Unes into the halo.

scholarly journals The Early Era: How do protostellar discs form?

2013 ◽  
Vol 8 (S299) ◽  
pp. 163-164
Author(s):  
Marc Joos ◽  
Patrick Hennebelle ◽  
Andrea Ciardi ◽  
Sébastien Fromang
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Central Region ◽  
Turbulent Diffusion ◽  
Rotation Axis ◽  
Core Collapse ◽  
Turbulent Environment ◽  
Protostellar Discs ◽  
The Magnetic Field ◽  
Disc Formation

AbstractDiscs are a key element in star and planet formation; however, magnetic fields can efficiently transport angular momentum away from the central region of the collapsing core during the dense core collapse, preventing disc formation. We perform numerical simulations of magnetically supercritical collapsing cores with a misalignment between the rotation axis and the magnetic field (Joos et al. 2012) and in a turbulent environment (Joos et al. 2013). The early formation of massive discs can take place at moderate magnetic intensities if the rotation axis is tilted or in a turbulent environment, because of misalignment and turbulent diffusion.

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