首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 625 毫秒
1.
The region of bouncing electron beams in the earth’s magnetosphere can be unstable against a non-resonant electromagnetic lower hybrid instability. The instability is purely growing in the rest frame of the plasma, and can be excited either by the temperature anisotropy or the drift velocity of the bouncing electron beams. The growth rates of the instability decrease with the increase of cold electron density. Consequently the growth rate is maximum at the equator where the cold electron density is minimum. The intense turbulence generated by this instability could broaden the bouncing electron beams thereby explaining the observed wider cone width of the beams at the equator. The instability could generate magnetic pulsations in the frequency range of orderPc 1?Pc3 with typical wavelength ≈ (3–10) km in the, magnetosphere during magnetic storms or substorms.  相似文献   

2.
Our measurements of the arrival-time delays of radio pulses from the Crab pulsar, PSR B0531+21, at low frequencies 111, 63, and 44 MHz revealed additional delays compared to the usual quadratic frequency relation, Δt(v) ∝ v ?2. These additional delays are 65 ms between 63 MHz and 111 MHz—i.e., a factor of two longer than the pulsar’s period, i.e., a factor of five longer than the pulsar period—and cannot be explained by the “twisting” of the magnetic-field lines by the rotation of the pulsar. We suggest the model in which a previously unknown high-density plasma layer with a high electron concentration is present along the line of sight in the Crab nebula, causing an additional frequency-dependent delay of the observed radio pulses at low frequencies due to the contribution of the n e 2 v ?4 term in the dispersion-delay formula. The parameters of this inferred layer have been derived: emission measure EM ? 4 × 106 pc/cm6, electron density n e ? 106 cm?3, depth along the line of sight d ? 4 × 10?6 pc, and electron temperature T e ≥ 2 × 106 K.  相似文献   

3.
Thirty‐three samples, including 22 eclogites, collected from the Dabie ultrahigh‐pressure (UHP) metamorphic belt in eastern China, have been studied for seismic properties. Compressional (Vp) and shear wave (Vs) velocities in three mutually perpendicular directions under hydrostatic pressures up to 1.0 GPa were measured for each sample. At 1.0 GPa, Vp (7.5–8.4 km s?1), Vs (4.2–4.8 km s?1), and densities (3.2–3.6 g cm?3) in the UHP eclogites are higher than those of UHP orthopyroxenite (7.3–7.5 km s?1, 4.1–4.3 km s?1, 3.2–3.3 g cm?3, respectively) and HP eclogites (7.1–7.9 km s?1, 4.0–4.5 km s?1, 3.1–3.5 g cm?3, respectively). Kyanitites (with 99.5% kyanite) show extremely high velocities and density (9.37 km s?1, 5.437 km s?1, 3.581 g cm?3, respectively). The eclogites show variation of Vp‐ and Vs‐anisotropy up to 9.70% and 9.17%, respectively. Poisson’s ratio (σ) ranges from 0.218 to 0.278 (with a mean of 0.255) for eclogites, 0.281–0.298 for granulites and 0.248 to 0.255 for amphibolites. The σ values for serpentinite (0.341) and marble (0.321) are higher than for other lithologies. The elastic moduli K, G, E of kyanitite were obtained as 163, 102 and 253 GPa, respectively. The Vp and density of representative UHP metamorphic rocks (eclogite & kyanitite) were extrapolated to mantle depth (15 GPa) following a reasonable geotherm, and compared to the one dimension mantle velocity and density model. The comparison shows that Vp and density in eclogite and kyanitite are greater than those of the ambient mantle, with differences of up to ΔVp > 0.3 km s?1 and Δρ > 0.3–0.4 g cm?3, respectively. This result favours the density‐induced delamination model and also provides evidence in support of distinguishing subducted high velocity materials in the upper mantle by means of seismic tomography. Such ultra‐deep subduction and delamination processes have been recognized by seismic tomography and geochemical tracing in the postcollisional magmatism in the Dabie region.  相似文献   

4.
Airborne measurements of the number concentration and size distribution of aerosols from 13 to 700 nm diameter have been made at four vertical levels across a coastline at Bhubaneswar (20°25′N, 85°83′E) during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) programme conducted in March–April 2006. The measurements made during the constant-level flights at 0.5, 1, 2 and 3 km altitude levels extend ～100 km over land and ～150km over ocean. Aerosol number concentrations vary from 2200 to 4500 cm?3 at 0.5 km level but are almost constant at ～ 6000 cm?3 and ～ 800 cm?3 at 2 and 3 km levels, respectively. At 1km level, aerosol number concentration shows a peak of 18,070 cm?3 around the coastline. Most of the aerosol size distribution curves at 0.5 km and 1 km levels are monomodal with a maxima at 110nm diameter which shifts to 70 nm diameter at 2 and 3 km levels. However, at the peak at 1 km level, number concentration has a bimodal distribution with an additional maximum appearing in nucleation mode. It is proposed that this maxima in nucleation mode at 1 km level may be due to the formation and transport of new particles from coastal regions.  相似文献   

5.
A sample of Class I methanol masers (MMI) has been surveyed at 1720 MHz to search for possible associations between MMI and 1720-MHz OH masers, which should be formed by the same collisional pumping mechanism. If the model for methanol masers is correct, the sample should contain a statistically significant number of 1720-MHz OH masers at the positions of MMI. The observations were conducted on the 70-meter radio telescope of the National Academy of Sciences of Ukraine (NASU). The results show that ??50% of 72 MMI are associated with OH emission at 1720 MHz. In many sources, strong absorption lines are also observed. In most cases, the OH (1720) lines are narrow (linewidths of <2 km/s) suggesting they may be maser lines. The OH column densities obtained from Gaussian fitting of these narrow OH lines are, on average, 1.5 × 1017 cm?2. TheH2 density in the emitting medium reaches 107 cm?3 if the region of the OH (1720) emission has been subject to interaction with a bipolar-outflow front. This is sufficient to excite MMI, and the presence of narrow, possibly masing OH (1720) lines at the MMI velocities indicates the likely presence of shocks from bipolar outflows in the vicinity of the maser condensations, supporting models in which these molecules are collisionally pumped.  相似文献   

6.
Refraction data taken from ocean bottom seismograph recordings in the western Arafura Sea indicate a continental‐type structure for the region. This structure is characterised by a thin column (2 km) of sediments, with velocities ranging from about to 2 to 4 km s‐1, overlying an essentially two layer crust. The compressional wave velocities in the upper and lower crust are 5.97 and 6.52 km s‐1, respectively, with the boundary between the layers at a depth of 11 km. Very weak mantle‐refracted arrivals with a velocity of about 8.0 km s‐1 were recorded. Large‐amplitude, later arrivals, beginning at distances near 100 and 150 km, have been interpreted to be part of the retrograde branches from the 8.0 and 7.33 km s‐1 layers, respectively. Model studies indicate that a small positive velocity gradient is required between 17 and 30 km, and that the Moho is at a depth of 34 km. A third set of large amplitude, later arrivals starting at a distance near 250 km has been interpreted as most probably multiple refraction‐reflection arrivals from the 5.97 and 6.52 km s‐1 layers. Correlation of this structure with the stratigraphic logs from exploratory oil wells in the Arafura Sea using layer velocities indicates that rocks younger than Jurassic appear to thin towards the east.  相似文献   

7.
Observations of the molecular cloud G1.6-0.025 in the 2K-1K and J0-J?1E series and 5?1-40E line of CH3OH, the (2-1) and (3-2) lines of SiO, and the 7?7-6?6 line of HNCO are described. Maps of the previously observed extended cloud with Vlsr～50 km/s and high-velocity clump with Vlsr～160 km/s, as well as a newly detected clump with Vlsr～0 km/s, have been obtained. The extended cloud and high-velocity clump have a nonuniform structure. The linewidths associated with all the objects are between 20 and 35 km/s, as is typical of clouds of the Galactic center. In some directions, emission at velocities from 40 to 160 km/s and from ?10 to +75 km/s is observed at the clump boundaries, testifying to a connection between the extended cloud and the high-velocity clump and clump at Vlsr～0 km/s. Compact maser sources are probaby contributing appreciably to the emission of the extended cloud in the 5?1-40E CH3OH line. Non-LTE modeling of the methanol emission shows that the extended cloud and high-velocity clump have a relatively low hydrogen density (<104 cm?3). The specific column density of methanol in the extended cloud exceeds 6×108 cm?3s, and is 4×108?6×109 cm?3s in the high-velocity clump. The kinetic temperatures of the extended cloud and high-velocity clump are estimated to be <80 K and 150–200 K, respectively. Possible mechanisms that can explain the link between the extended cloud with Vlsr～50 km/s and the clumps with Vlsr～0 km/s and ～160 km/s are briefly discussed.  相似文献   

8.
Atransmission electron microscope (TEM) study of quartz-coesite inclusions in garnet in crustal rocks from the Western Alps is presented. Coesite shows a low dislocation density (<107 cm?2), and quartz a higher density of defects, Brasil twins (104 cm?1) and dislocations (108 cm?2). It is concluded that coesite has been not or only slightly plastically deformed and that the yield strength of coesite is higher than that of quartz. The large scale deformation implications are briefly discussed. TEM observations show no systematic topotactic relationship between the two polymorphs and their boundaries have a scalloped morphology which suggests that growth of quartz from coesite was controlled by a diffusion process.  相似文献   

9.
Comparisons of the brightness distributions of the white corona observed at distances of several solar radii with solar wind velocities derived from interplanetary-scintillation observations, as well as analyses of solar wind data obtained on spacecraft from December 1994 to June 1995, indicate that the fast solar wind can contain plasma with velocities V ≈ 300–450 km/s, approaching those typical for the slow solar wind that flows in the streamer belt and chains of streamers. At the same time, certain other parameters, first and foremost the plasma density N and ratio T/N 0.5 (where T is the temperature), indicate that these two flows differ considerably. The slow solar wind flowing in the streamer belt and chains displays high densities N > 10 ± 2 cm?3 and low T/N 0.5 < 1.7 × 104 K cm3/2 at the Earth’s orbit. The number of slow solar-wind sources observed in chains can be comparable with the number observed in the belt. The fast solar wind flowing from coronal holes always displays low densities N≤ 8 cm?3 and high T/N 0.5 > 1.7 × 104 K cm3/2. These properties probably indicate different origins of the fast and slow solar winds.  相似文献   

10.
P.M Sychev 《Tectonophysics》1973,19(4):343-359
Upper-mantle structure and heat flow and gravity data are considered. It is shown that crustal isostasy disturbance is compensated for by inhomogeneities in the density of the upper mantle; compaction beneath trenches and discompaction under island arcs. On the basis of the available data an upper-mantle density model is constructed which is represented by a seismoactive layer with a surplus density of from 0.05 to 0.1 g/cm3 and a discompacted zone overlying that layer with a density deficiency of 0.05 g/cm3 beneath the island arc and 0.01 g/cm3 in the remaining area. The available data are in poor agreement with the concept of “new global tectonics”, therefore the formation of density inhomogeneities is associated with gravity differentiation of the mantle material. A scheme of this process is proposed according to which heated differentiates of the mantle material rise along deep faults or weakened zones from depths of 700 km and more. Upper-mantle material differentiation occurs mainly in several levels, but mostly at depths down to 150–200 km within the interval of 350–450 km. Separation of heavy components as a result of differentiation and the “zone melting process” causes a high velocity and correspondingly a denser inclined layer. Discompaction of the upper mantle overlying the focal plane is associated with its being heated by high-temperature melts circulating over the inclined fault.  相似文献   

11.
Hydrous species and the amount of water (OH? ions and crystal hydrate H2O) in structures of nominally anhydrous rock-forming minerals (olivine, ortho- and clinopyroxenes) were studied with Fourier spectroscopy in peridotite nodules (19 samples) from Cenozoic alkali basalts of the Baikal-Mongolia region (Dariganga Plateau, Taryat Depression, and Vitim Plateau). Single-crystal samples oriented relative to the crystallographic axes of minerals were examined with an FTIR spectrometer equipped with an IR microscope at the points of platelets free from fluid inclusions. FTIR spectra were measured in regions of stretching vibrations of OH? and H2O (3800–3000 cm?1) and deformation vibrations of H2O (1850–1450 cm?1). The water content in mineral structures was determined from integral intensities. To estimate the conditions of entrapment and loss of structural water in minerals, their chemical composition, including Fe2+ and Fe3+ contents, was determined with an electron microprobe analysis and Mössbauer spectroscopy. The bulk chemical composition of some nodules was determined with XRF and ICP MS. The total water content (OH? + H2O) varies from 150 to 1140 ppm in olivines, from 45 to 870 ppm in clinopyroxenes, and from 40 to 1100 ppm in orthopyroxenes. Both water species in the mineral structures are retained down to a depth of 150–160 km in wide temperature and pressure ranges (1100–1500 °C, 32–47 kbar) at the oxygen fugacity of ?1.4 to ?0.1 log units relative to that of the quartz-fayalite-magnetite buffer.  相似文献   

12.
The influence of pressure on the OH-stretching vibration of zoiste has been studied by single crystal high pressure infrared spectroscopy. A band related to the OH-stretching vibration displayed a linear shift from 3170 cm?1 at 1 bar to 2795 cm?1 at 116 kbar. The half-band width increased linearly with respect to pressure from 60 cm?1 at 1 bar to 500 cm?1 at 116 kbar. The strength of the absorption of this band is strongly frequency dependent. The high-energy shift of a band at around 2200 cm?1 on pressure increase indicates that this band is not due to a second OH-stretching vibration as previously suggested by Langer and Lattard (1980).  相似文献   

13.
Results of a study of the shell of Nova V2659 Cyg based on spectrophotometric observations carried out over a year and a half after its eruption are presented. The physical conditions in the nova shell have been studied. The electron temperature (9000 K) and density (5 × 106 cm?3) in the nebular stage have been estimated, together with the abundances of helium, oxygen, nitrogen, neon, argon, and iron. The abundances of nitrogen, oxygen, neon, and argon are enhanced relative to the solar values. The relative abundances are [N/H] = 2.26 ± 0.25 dex, [O/H] = 1.66 ± 0.35 dex, [Ne/H] = 0.78 ± 0.25 dex, and [Ar/H] = 0.32 ± 0.38 dex. The estimated mass of oxygen and total mass of the emitting shell are ≈1 × 10?4M⊙ and ≈3 × 10?4M⊙, respectively. In the period of chaotic brightness oscillations, the maximum velocity of the shell expansion derived from the radial velocities of the absorption components of the HI and FeII line profiles increased by ≈400 km/s 41 days after the maximum, and by ≈200 km/s 101 days after the maximum, reaching 1600 km/s in both cases.  相似文献   

14.
Two sites in the eastern Fram Strait, the Vestnesa Ridge and the Yermak Plateau, have been surveyed and sampled providing a depositional record over the last glacial‐interglacial cycle. The Fram Strait is the only deep‐water connection from the Arctic Ocean to the North Atlantic and contains a marine sediment record of both high latitude thermohaline flow and ice sheet interaction. On the Vestnesa Ridge, the western Svalbard margin, a sediment drift was identified in 1226 m of water. Gravity and multicores from the crest of the drift recovered turbidites and contourites. 14C dating indicates an age range of 8287 to 26 900 years BP (Early Holocene to Late Weichselian). The Yermak Plateau is characterized by slope sediments in 961 m of water. Gravity and multicores recovered contourites and hemipelagites. 14C ages were between 8615 and 46 437 years BP (Early Holocene to mid‐Weichselian). Downcore dinoflagellate cyst analyses from both sites provide a record of changing surface water conditions since the mid‐Weichselian, suggesting variable sea ice extent, productivity and polynyas present even during the Last Glacial Maximum. Four layers of ice‐rafted debris were also identified and correlated within the cores. These events occurred ca at 9, 24 to 25, 26 to 27 and 43 ka, asynchronous with Heinrich layers in the wider north‐east Atlantic and here interpreted as reflecting instability in the Svalbard/Barents Ice sheet and the northward advection of warm Atlantic water during the Late Weichselian. The activity of the ancestral West Spitsbergen Current is interpreted using mean sortable silt records from the cores. On the Vestnesa Ridge drift the modern mass accumulation rate, calculated using excess 210Pb, is 0·076 g cm?2 year?1. On the Yermak Plateau slope the modern mass accumulation rate is 0·053 g cm?2 year?1.  相似文献   

15.
An efficient method for the detection and estimation of the parameters of the coronas of isolated white dwarfs possessing magnetic fields of about 107 G is tested. This method is based on the detection of thermal radiation of the coronal plasma at harmonics of the electron gyrofrequency, which is manifest as a polarized infrared excess. The Stokes parameters for the thermal cyclotron radiation from the hot corona of a white dwarf with a dipolar magnetic field are calculated. A new upper limit for the electron density, 1010 cm?3, in a corona with a temperature of ?106 K is found for the white dwarf G99-47 (WD 0553+053). This limit is a factor of 40 lower than the value derived earlier from ROSAT X-ray observations. Recommendations for subsequent infrared observations of isolated magnetic white dwarfs aimed at detecting their coronas or deriving better constraints on their parameters are presented.  相似文献   

16.
Room temperature and low temperature Mössbauer and optical absorption spectroscopic data on six natural chloritoids characterized by means of electron microprobe and X-ray powder diffraction techniques are presented. Two narrow quadrupole doublets with widths of 0.25–0.29 mm/s assigned to Fe2+ in a relatively large octahedral site and Fe3+ in a smaller octahedral site, are observed in the Mössbauer spectra. Polarized optical absorption spectra reveal three main absorption bands. A broad absorption band at 16,300 cm?1, which is strongly polarized in E‖X and E‖Y and shows a linear increase in integral absorption with increasing [Fe2+] [Fe3+] concentration product, is assigned to a Fe2++Fe3+→Fe3++Fe2+ charge transfer transition. This band displays also a temperature dependence different from that of single ion d?d transitions. Two absorption bands at 10,900 cm?1 and 8,000 cm?1 are, on the basis of compositional dependence and energy, assigned to Fe2+ in the large M(1B) octahedra of the brucite-type layer in chloritoid. Combined spectroscopic evidence and structural and chemical considerations support a distribution scheme for ferrous and ferric iron which orders the Fe2+ ions in the M(1B) octahedra and the Fe3+ ions in the small M(1A) octahedral sites. Both types of octahedra are found in the brucite type layer of chloritoid.  相似文献   

17.
This paper examines the possibility of using the energy spectra of accelerated solar cosmic-ray ions and features formed by Coulomb losses to study the solar plasma (the power-law index S for the scattering turbulence, particle number density N, and temperature T of the background medium). For an individual solar flare, Coulomb losses can be manifest to different degrees in the spectra of different ions, providing a means to determine S. A comparison of theoretical spectra for H, He, C, O, and Fe ions with observed spectra for the prolonged solar flare of October 20, 1995 yields S≈3, N≈5×109 cm?3, and T≈106 K, assuming that the characteristic time scale over which these particles gain energy is about a second.  相似文献   

18.
Seismic velocities under confining pressures to 10 kbar have been measured for rocks of the Ivrea—Verbano and Strona—Ceneri Zones of northern Italy, a metamorphic complex thought to represent a cross-section of the continental crust and crust—mantle boundary. Laboratory-determined compressional wave velocities for schists and gneisses of the amphibolite facies found in the upper levels of the section (having an average density of 2.74 g/cm3) average 6.45 km/sec at pressures between 6 and 10 kbar. These increase with depth to values greater than 7.1 km/sec for amphibolites and rocks of the amphibolite—granulite facies transition and to 7.5 km/sec. (average density 3.06 g/cm3) in intermediate and mafic granulite facies rocks near the base of the section. Compressional wave velocities then abruptly increase to 8.5 km/sec in ultramafic complexes near the Insubric Line. Regional geophysical surveys show that Pg is 6.0 km/sec (density of 2.7 g/cm3), P* is 7.2–7.4 km/sec (density of 3.1 g/cm3) and Pn is 8.1 km/sec, values which are in agreement with the laboratory data when effects of temperature are taken into consideration. Estimated thicknesses of exposed rock units are in reasonable agreement with thicknesses determined for crustal layers in seismic refraction experiments. The agreement between the regional crustal structure and the laboratory-determined values of velocity and density provides strong evidence for the hypothesis that the rocks of this metamorphic complex represent a cross-section of the continental crust of the Po Basin.Using the Ivrea—Verbano and Strona—Ceneri sequence as a model of the continental crust, the crust of northern Italy is shown to consist of a thick series of metamorphic rocks with greenschist facies rocks occupying the uppermost levels. These grade downward into amphibolite facies gneisses and schists with occasional granitic intrusives. The Conrad discontinuity is marked by a change from silicic and intermediate amphibolite facies gneisses to intermediate and mafic granulite facies rocks in which hydrous minerals diminish in abundance and thus represents a distinct transition in terms of both composition and metamorphic grade. The lower crust is dominated by a heterogeneous series of mafic and metapelitic rocks in the granulite facies. Importantly, metasedimentary rocks of intermediate silica content found in the complex can have compressional wave velocities equivalent to velocities in mafic rocks suggesting that the lower continental crust everywhere is not necessarily mafic in composition. Ultramafic complexes near the Insubric Line may represent the upper mantle of the continent and their setting suggests that the continental crust-upper mantle boundary is sharp and is not isochemical.  相似文献   

19.
We have calculated a model for the dust envelope of the protoplanetary nebula IRAS 18062+2410 using observations from the ultraviolet to the far infrared. We assume that the envelope is spherically symmetrical and consists of identical silicate grains with a radius of 0.10 micron, and with the number density of the grains inversely proportional to the square of the distance. The optical depth of the envelope, whose inner boundary is 1.40×10?3 pc from the center of the star, is 0.050 at 10 microns. At the inner envelope boundary, the temperature of the dust grains is 410 K and their density is 2.7×10?7 cm?3. Using calculations of stellar evolution at the stage following the exit from the asymptotic giant branch, we estimate the dust envelope’s expansion velocity to be 12 km/s. The mass-loss rate of the star preceding the ejection of the envelope was about 4.5×10?6 M ⊙/yr. The observed excess of far-IR flux is not associated with the continuum radiation of the nebula, and may provide evidence for the presence of dust ejected by the star in earlier stages of its evolution.  相似文献   

20.
The paper proposes pioneering data on the polarized optical absorption spectra of Li-Fe micas: intermediate members of the siderophyllite-zinnwaldite-polylithionite and annite-protolithionite-zinnwaldite-trilithionite series with variable Fe and Li proportions and Li- and Fe-bearing muscovite (phengite). Based on the analysis of structural data, the complicated structure of the Fe2+ → Fe3+ charge transfer band in the mica structures is explained, and arguments are presented to justify the ascribing of its shortwave component (CTB-1, ν = 17200-14900 cm?1) to charge transfer in the pair Fe2+(M2) → Fe3+(M2) and the longwave component (CTB-2, ν = 14200-13600 cm?1) to charge transfer in the pair Fe2+(M1) Fe3+(M2). It is demonstrated that the anomalous shift of the superposition of two-component CTB toward the shortwave region, to 17000 cm?1, results from a decrease in the length of oxygen edges between adjacent M2M2 and M1M2 tetrahedrons when Li is accommodated in the mica structure. The first data are presented on the spectrum of Fe2+ ions in large distorted M3(M1) tetrahedrons (OAC Fe2+ II) in hetero-octahedral Li micas (zinnwaldite), and the behavior of the corresponding absorption bands at 11400 and 8000 cm?1 is determined. It is proved that characteristics of the optical spectra of Fe2+ ions can be used as an indicator of the structure of the octahedral layer in the mica structures. Results of the comparative analyses of spectral parameters of the Fe2+ → Fe3+ charge transfer, crystal field spectra of Fe2+ ions, and the crystal-chemical characteristics controlling them in micas of the polylithionite-siderophyllite series are completely consistent with the character of cation ordering in the crystal structures of these micas determined by X-ray diffraction analysis.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号