A model of a quadrupole magnetosphere is developed with the aid of conformal mappings. Such a type of magnetosphere might have occurred at the Earth during times of polarity reversals of the dipole field, when the residual field essentially may have had a quadrupole structure. The magnetospheric field-line configuration is calculated for an axial quadrupole directed perpendicular to the solar-wind flow. The model is two-dimensional and leads to analytical formulae. 相似文献
High resolution electron microscope studies have been carried out on ‘zero-age’ (New Flow) basalts from the Juan de Fuca Ridge and on young (< 20 ka) basalts from the axis of the East Pacific Rise at 12°N. Such data lead to characterization of the magnetic minerals, especially those of smaller grain size, which have been hypothesized by Kent and Gee to have undergone grain size-dependent alteration. In addition to larger titanomagnetite grains, abundant submicrometer titanomagnetite has been observed in globules within a glassy matrix. These grains, likely to be single-domain (SD) or superparamagnetic, are associated with apatite, uncommon pyrrhotite and residual glass. The submicrometer titanomagnetite grains have a wide compositional range (0 < x < 0.8), where x is the fraction of ulvöspinel component, whereas the larger, multi-domain (MD)-sized titanomagnetite grains have a narrow composition range of approximately x = 0.6. This variability in Ti content provides a ready explanation for the thermal rock magnetic properties observed by Kent and Gee and eliminates the need to invoke extremely rapid (< 20 ka) alteration of these young basalts. 相似文献
Omnidirectional intensities of electrons with energies Ee > 1·5 MeV detected by a low orbiting polar satellite (GRS-A/AZUR) in the outer radiation belt are examined during disturbed times including the main phase of a very strong geomagnetic storm on 8 March 1970. The particle intensity features are discussed in relationship with proposed magnetospheric processes. It is found that a superposition of the two following effects can explain the particle behavior in the trapping region:(A) Radial diffusion. After the southward turning of the interplanetary field an inward motion of both the energetic electron belt and the plasmapause took place. This effect was observed at L > 3 RE and we attribute it to enhanced magnetospheric electric field fluctuations. Later, a strong interplanetary shock impinged upon the magnetosphere which was related to the triggering of intense magnetospheric substorms; a further inward diffusion occurred at L ? 3 RE, accompanied by an inward movement of the electron slot. A rough estimation of the diffusion coefficient leads to a power spectrum of the electric field fluctuations which seems to be consistent with experimentally determined power spectra (Mozer, 1971).(B) Adiabatic response to ring current changes. Large energetic electron intensity decreases within the outer radiation belt are shown to be adiabatic changes due to ring current variations. The influence of the inflation of the magnetosphere due to the developing ring current is simultaneously observed by the decrease of the solar proton outoff (1·7-2·5 MeV). 相似文献
We follow the interaction of massive stars with their circumstellar gas over their entire life-times by combining hydrodynamic stellar evolution calculations for 35 and 60M stars and one- and two-dimensional gas dynamical calculations for the circumstellar medium. 相似文献
The ca. 13 m long sediment core PG1351, recovered in 1998 from the central part of Lake El’gygytgyn, NE Siberia, was investigated
for lithostratigraphy, water content, dry bulk density (DBD), total organic carbon (TOC), total nitrogen (TN), total sulphur
(TS) and biogenic silica (opal) contents, and for TOC stable isotope ratios (δ13CTOC). The event stratigraphy recorded in major differences in sediment composition match variations in regional summer insolation,
thus confirming a new age model for this core, which suggests that it spans the last 250 ka BP. Four depositional units of
contrasting lithological and biogeochemical composition have been distinguished, reflecting past environmental conditions
associated with relatively warm, peak warm, cold and dry, and cold but more moist climate modes. A relatively warm climate,
resulting in complete summer melt of the lake ice cover and seasonal mixing of the water column, prevailed during the Holocene
and Marine Isotope Stages (MIS) 3, 5.1, 5.3, 6.1, 6.3, 6.5, 7.1–7.3, 7.5, 8.1 and 8.3. MIS 5.5 (Eemian) was characterized
by significantly enhanced aquatic primary production and organic matter supply from the catchment, indicating peak warm conditions.
During MIS 2, 5.2, 5.4, 6.2 and 6.4 the climate was cold and dry, leading to perennial lake ice cover, little regional snowfall,
and a stagnant water body. A cold but more moist climate during MIS 4, 6.6, 7.4, 8.2 and 8.4 is thought to have produced more
snow cover on␣the perennial ice, strongly reducing light penetration and biogenic primary production in␣the lake. While the
cold–warm pattern during␣the past three glacial–interglacial cycles is probably controlled by changes in regional summer insolation,
differences in the intensity of the warm phases and in the degree of aridity (changing snowfall) during cold phases likely
were due to changes in atmospheric circulation patterns.
This is the seventh in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and
its catchment in NE Russia. JulieBrigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue. 相似文献
This study focused on the luminescence dating of sediments from Lake El’gygytgyn, a meteorite impact crater 100 km north of
the Arctic Circle in northeast Siberia, formed 3.58 Ma ago. The sediment is principally eolian deposited in to a lake with
nearly permanently ice. The fine-grained polymineral and quartz extracts taken from nine distinct levels from the upper 12.3 m
of sediment core PG1351 were dated by infrared stimulated (IRSL) and green stimulated luminescence (GSL) using multiple aliquot
additive dose procedures. The veracity of these ages is evaluated by comparing to an age model for the core derived from magnetic
excursions and from correlation of variations of the magnetic susceptibility record to similar magnitude variations in δ 18O in the Greenland Ice core record. The IRSL ages from the upper 9 m of core correspond well with the independent age control
for the past ca. 200 ka. However, sediments deeper in the core at 12.3 m with an inferred age of ca. 250 ka age yield a saturated
IRSL response and therefore a non-finite OSL age. The youngest sediment dated from 0.70 m depth yielded the IRSL age of ca.
11.5 ka, older than the corresponding age of 9.3–8.8 ka, indicating a discrepancy in dating the youngest sediments in the
upper 1 m of core. This study confirms the utility of IRSL by the multiple aliquot additive dose method to date sediments
<200 ka old from eastern Siberia.
This is the sixth in a series of eleven papers published in this special issue dedicated to initial studies of El’gygytgyn Crater Lake and
its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue. 相似文献
Three new Middle–Late Ordovician and two new Early Carboniferous paleomagnetic poles have been obtained from the North Tien Shan Zone (NTZ) of the Ural–Mongol belt in Kyrgyzstan and Kazakhstan. Paleolatitudes for the Carboniferous are unambiguously northerly and average 15.5°N, whereas the Ordovician paleolatitudes (6°, 9°, and 9°) are inferred to be southerly, given that a very large (180°) rotation of the NTZ would be necessary during the middle Paleozoic if the other polarity option was chosen. Thus, the NTZ drifted northward during much of the Paleozoic; east–west drift cannot be determined, as is well known, from paleomagnetic data. In addition, detailed thermal demagnetization analysis reveals two overprints, one of recent age and the other of Permian age, which is a time of strong deformation in the NTZ. The paleolatitude of the combined Permian overprint is 30.5+2°N. The paleolatitudes collectively track those predicted for the area by extrapolation from Baltica very well, but are different from those of Siberia for Ordovician times. This finding is compatible with Sengör and Natal'in's [Sengör, A.M.C., Natal'in, B.A., 1996. Paleotectonics of Asia: fragments of a synthesis. In: Yin A., Harrison, M. (Eds.), The Tectonic Evolution of Asia. Cambridge Univ. Press, Cambridge, pp. 486–640] model of tectonic evolution of the Ural–Mongol belt and disagrees with the models of other researchers. Declinations of the Ordovician and Early Carboniferous results range from northwesterly to northeasterly, and are clearly affected by local relative rotations, which seem characteristic for the entire NTZ, because the Permian overprint declinations also show such a spread. Apparently, the important latest Paleozoic–Triassic deformation involved shear zone-related rotations as well as folding and significant granitic intrusions. 相似文献
X-ray pulsars, which form together with a main sequence star or a giant a close binary system, show strong variations in their pulse period. Epochs of spin-up and spin-down interchange on timescales of some tens up to some thousand days.
We study this phenomenon in the framework of the disk-fed model by Ghosh and Lamb, which we generalize by allowing for an inclination angle χ between the magnetic dipole moment μ and the rotation axis. Moreover, we take into account the relatively small magnetic field induced by currents in the magnetosphere flowing along the dipole field lines. This component, which we calculate numerically, leads to a torque perpendicular to the rotation axis and induces a precession of it. Due to this additional degree of freedom, the calculated pulse period history is much smoother than in the classical Ghosh and Lamb model. Within this extended model we compute, as an example, the pulse period history of Cen X-3 using the observed X-ray flux as a measure for the mass accretion rate. Qualitative accordance with the measured pulse period data (see e.g. Nelson et al., 1997 [ApJ, 488, L117]) is found to be good. 相似文献
A newly developed field distribution measuring system based on a high- T c SQUID has been employed in the study of magnetic mineral distribution in several Permian sedimentary rocks. The instrument consists of a small, 1.4×1.4 mm sized YBaCu-oxide SQUID magnetic field sensor that is operated in a thin-walled dewar, so that the sample's surface, at room temperature, can be scanned at a distance of only ∼1.5 mm. The samples were subjected to a saturation remanence perpendicular to the surface and the scanning measurements in zero field reveal that the magnetization might be carried by only a small part of a sample, in one case associated with secondary oxide phases. High-resolution magnetic scans can aid in the interpretation of the magnetic remanence acquisition process. 相似文献