Accretion disc–stellar magnetosphere interaction: field line inflation and the effect on the spin-down torque     

Vasso Agapitou  John C. B. Papaloizou 《Monthly notices of the Royal Astronomical Society》2000,317(2):273-288

We calculate the structure of a force-free magnetosphere which is assumed to corotate with a central star and which interacts with an embedded differentially rotating accretion disc. The magnetic and rotation axes are aligned, and the stellar field is assumed to be a dipole. We concentrate on the case when the amount of field line twisting through the disc–magnetosphere interaction is large , and consider different outer boundary conditions. In general the field line twisting produces field line inflation (e.g. Bardou & Heyvaerts), and in some cases with large twisting many field lines can become open. We calculate the spin-down torque acting between the star and the disc, and we find that it decreases significantly for cases with large field line twisting. This suggests that the oscillating torques observed for some accreting neutron stars could be caused by the magnetosphere varying between states with low and high field line inflation. Calculations of the spin evolution of T Tauri stars may also have to be revised in the light of the significant effect that field line twisting has on the magnetic torque resulting from star–disc interactions.  相似文献   

Energetic particle acceleration in a three-dimensional magnetic field reconnection model: the role of magnetohydrodynamic turbulence     

T. Kobak  M. Ostrowski 《Monthly notices of the Royal Astronomical Society》2000,317(4):973-978

The role of magnetohydrodynamic (MHD) turbulence in the cosmic ray acceleration process in a volume with a reconnecting magnetic field is studied by means of Monte Carlo simulations. We performed modelling of proton acceleration, with the three-dimensional analytic model of stationary reconnection of Craig et al. providing the unperturbed background conditions. Perturbations of particle trajectories resulting from a turbulent magnetic field component were simulated using small-amplitude pitch-angle momentum scattering, enabling modelling of both small- and large-amplitude turbulence in a wide wavevector range. Within the approach, no second-order Fermi acceleration process is allowed. Comparison of the acceleration process in models involving particle trajectory perturbations with the unperturbed model reveals that the turbulence can substantially increase the acceleration efficiency, enabling much higher final particle energies and flat particle spectra.  相似文献   

Magnetic flares and the optical variability of the X-ray transient XTE J1118+480     

A. Merloni  T. Di Matteo †  A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2000,318(1):L15-L19

The simultaneous presence of a strong quasi-periodic oscillation, of period ∼10 s, in the optical and X-ray light curves of the X-ray transient XTE J1118+480 suggests that a significant fraction of the optical flux originates from the inner part of the accretion flow, where most of the X-rays are produced. We present a model of magnetic flares in an accretion disc corona where thermal cyclo-synchrotron emission contributes significantly to the optical emission, while the X-rays are produced by inverse Compton scattering of the soft photons produced by dissipation in the underlying disc and by the synchrotron process itself. Given the observational constraints, we estimate the values for the coronal temperature, optical depth and magnetic field intensity, as well as the accretion rate for the source. Within our model we predict a correlation between optical and hard X-ray variability and an anticorrelation between optical and soft X-rays. We also expect optical variability on flaring time-scales (∼tens of ms), with a power-density spectrum similar to that observed in the X-ray band. Finally, we use both the available optical/extreme-ultraviolet/X-ray spectral energy distribution and the low-frequency time variability to discuss limits on the inner radius of the optically thick disc.  相似文献   

Magnetic and optical spiral arms in the galaxy NGC 6946     

P. Frick  R. Beck  A. Shukurov  D. Sokoloff  M. Ehle †  J. Kamphuis 《Monthly notices of the Royal Astronomical Society》2000,318(3):925-937

The spiral pattern in the nearby spiral galaxy NGC 6946 has been studied using the wavelet transformation technique, applied to galaxy images in polarized and total non-thermal radio emission at λλ 3.5 and 6.2 cm, in broadband red light, in the λ 21.1 cm H  i line and in the optical Hα line. Well-defined, continuous spiral arms are visible in polarized radio emission and red light, where we can isolate a multi-armed pattern in the range of galactocentric distances 1.5–12 kpc, consisting of four long arms and one short spiral segment. The 'magnetic arms' (visible in polarized radio emission) are localized almost precisely between the optical arms. Each magnetic arm is similar in length and pitch angle to the preceding optical arm (in the sense of galactic rotation) and can be regarded as its phase-shifted image. Even details like a bifurcation of an optical arm have their phase-shifted counterparts in the magnetic arms. The average relative amplitude of the optical spiral arms (the stellar density excess over the azimuthal average) grows with galactocentric radius up to 0.3–0.7 at r ≃5 kpc, decreases by a factor of two at r =5–6 kpc and remains low at 0.2–0.3 in the outer parts of the galaxy. By contrast, the magnetic arms have a constant average relative amplitude (the excess in the regular magnetic field strength over the azimuthal average) of 0.3–0.6 in a wide radial range r =1.5–12 kpc. We briefly discuss implications of our findings for theories of galactic magnetic fields.  相似文献   

Stratigraphy of multiple piston-core sediments for the last 30,000 years from Lake Biwa, Japan     

Keiji Takemura  Akira Hayashida  Makoto Okamura  Hiromi Matsuoka  Mohammed Ali  Mohammed Ali  Yoshiki Kuniko  Masayuki Torii  Masayuki Torii 《Journal of Paleolimnology》2000,23(2):185-199

  相似文献   

Statistical analysis and origin of the magnetic susceptibility of drill cuttings from the 9.1-km-deep KTB drill hole   总被引：1，自引：0，他引：1  

Armin Rauen  Heinrich C. Soffel  & Helmuth Winter 《Geophysical Journal International》2000,142(1):83-94

  相似文献   

Coercive force of single crystals of magnetite at low temperatures   总被引：1，自引：0，他引：1  

Özden Özdemir 《Geophysical Journal International》2000,141(2):351-356

The temperature dependence of coercive force H _c was studied on well-characterized and stoichiometric millimetre-sized single crystals of magnetite at a series of 16 temperatures from 300 to 10 K using a SQUID magnetometer. H _c decreases gradually with cooling to the isotropic temperature, T _i = 130 K, where the first magnetocrystalline anisotropy constant K ₁ becomes zero. H _c exhibits a sharp increase at the Verwey transition, T _v = 120 K, where the structure changes from cubic to monoclinic. In crossing the Verwey transition, H _c increases by more than two orders of magnitude, from 20 μT to 2.4 mT, and the shape of the hysteresis loops becomes wasp-waisted.
Observed coercivity between 300 K and 170 K varies with temperature as λ _s / M _s , where λ _s is the magnetostriction constant and M _s is the saturation magnetization, indicating that the coercivity in MD magnetite is controlled mainly by internal stress associated with dislocations or other crystal defects. It seems likely that the stable single-domain-like magnetic memory observed in large MD magnetite crystals is due to magnetoelastically pinned domain walls. The discontinuous change in H _c at the Verwey transition is controlled by abrupt changes in magnetocrystalline and magnetostriction constants due to crystal deformation from cubic to monoclinic structure.  相似文献   

Separation of pedogenic and lithogenic components of magnetic susceptibility in the Chinese loess/palaeosol sequence as determined by the CBD procedure and a mixing analysis   总被引：4，自引：0，他引：4  

Nata&#;a J. Vidic  Jeff D. TenPas  Kenneth L. Verosub  & Michael J. Singer 《Geophysical Journal International》2000,142(2):551-562

Magnetic susceptibility variations in the Chinese loess/palaeosol sequences have been used extensively for palaeoclimatic interpretations. The magnetic signal of these sequences must be divided into lithogenic and pedogenic components because the palaeoclimatic record is primarily reflected in the pedogenic component. In this paper we compare two methods for separating the pedogenic and lithogenic components of the magnetic susceptibility signal: the citrate-bicarbonate-dithionite (CBD) extraction procedure, and a mixing analysis. Both methods yield good estimates of the pedogenic component, especially for the palaeosols. The CBD procedure underestimates the lithogenic component and overestimates the pedogenic component. The magnitude of this effect is moderately high in loess layers but almost negligible in palaeosols. The mixing model overestimates the lithogenic component and underestimates the pedogenic component. Both methods can be adjusted to yield better estimates of both components. The lithogenic susceptibility, as determined by either method, suggests that palaeoclimatic interpretations based only on total susceptibility will be in error and that a single estimate of the average lithogenic susceptibility is not an accurate basis for adjusting the total susceptibility. A long-term decline in lithogenic susceptibility with depth in the section suggests more intense or prolonged periods of weathering associated with the formation of the older palaeosols.
The CBD procedure provides the most comprehensive information on the magnitude of the components and magnetic mineralogy of loess and palaeosols. However, the mixing analysis provides a sensitive, rapid, and easily applied alternative to the CBD procedure. A combination of the two approaches provides the most powerful and perhaps the most accurate way of separating the magnetic susceptibility components.  相似文献   

An alternative interpretation of the Cayman trough evolution from a reidentification of magnetic anomalies     

S. Leroy  A. Mauffret  P. Patriat  & B. Mercier de Lépinay 《Geophysical Journal International》2000,141(3):539-557

Magnetic data were collected during the Wilkes (1973) and Seacarib II (1987) cruises to the Cayman trough. A new interpretation of magnetic data is carried out. An isochron pattern is drawn up from our anomaly identifications. An early Eocene age (49 Ma, Ypresian) for Cayman trough opening is proposed instead of the late Oligocene or middle Eocene ages suggested by previous studies. Our plate tectonic reconstruction is simpler and fits the on-land geology (Jamaica and Cuba) and the tectonics. Our reconstruction shows a southward propagation of the spreading centre between magnetic anomalies 8 and 6 (26 and 20 Ma). The trough width increases by 30 km in this period. The southward propagation of the Cayman spreading centre from the Middle Oligocene to the Early Miocene induced the development of the restraining bend of the Swan Islands, the formation of a 1 km high scarp on the eastern trace of the Cayman trough transform fault (Walton fault) and the formation of a pull-apart basin (Hendrix pull-apart). Magnetic anomalies and magnetization maps give information about the deformation and the rocks. The proposed evolutionary model of the Cayman trough from the inception of seafloor spreading to the present configuration is presented in relation to the tectonic escape of the northern boundary of the Caribbean plate from the Maastrichtian to the Present.  相似文献   

Magnetostratigraphy of mid-Cretaceous limestones from the Sierra Madre of northeastern Mexico     

Bradford M. Clement  Ewald Poetisi  Timothy J. Bralower  Emily CoBabe  Jose Longoria 《Geophysical Journal International》2000,143(1):219-229

We present the results of a palaeomagnetic study of four mid-Cretaceous limestone sections exposed in northeastern Mexico. The limestones are weakly magnetized and exhibit two- to three-component magnetizations. These magnetization components appear to be carried by both a sulphide mineral and a magnetite-titanomagnetite mineral. The sulphide mineral carries a reverse polarity overprint that often makes it difficult to isolate definitively the higher-unblocking-temperature component. The high-unblocking-temperature component is well defined in the upper portion of the Santa Rosa Canyon section and in the Cienega del Toro section and passes the fold test. The characteristic remanent magnetization (ChRM) inclinations agree well with predicted mid-Cretaceous inclinations for these sites, although the declinations differ by more than 100°. The relative rotation between these two sites probably occurred as the thrust sheets were emplaced during Laramide deformation. At two of the sections, namely Cienega del Toro and the overturned Los Chorros sections, only normal polarity directions are observed. The La Boca Canyon and Santa Rosa Canyon sections exhibit zones of both normal and reverse polarity magnetization. Correlation of these polarity zones with the geomagnetic polarity timescale provides a time framework for lithostratigraphic and palaeoceanographic studies of these sections.  相似文献